Final Report - HIPER-ACT (Novel technology for high-performance piezoelectric actuators)
Objective
The HIPER-ACT project has focused on materials research, the development of new piezoelectric materials, and electrode materials to be used for manufacturing actuators tailored for high reliability under extreme conditions and environments: humidity and high stress.
Technical results with commercial interest
- new piezo actuator with improved humidity and crack resistance based on new ceramic material composition and processing (this technology will be commercialised by NOLIAC);
- new stencils / paste have been designed and formulated for more narrow electrode printing of multilayer piezo actuators (this technology is made commercial by GEM and TECAN);
- new technology developed for wire electrode build-up of multilayer piezo actuators;
- actuators with new material and narrow electrode manufactured and tested in demonstrators;
- demonstrator of a new active trailing edge design is made and tested with successful results showing promising performance;
- demonstrator of a new bonding machine damping design shows very efficient passive and active vibration damping. B2 and Interdigitated electrode (IDE) actuator showed less piezo performance however B2 actuator indicate better reliability than the reference material;
- demonstrator of a new active engine mount design showed significant damping of vibrations;
- demonstrator and model of a new car control system (SCR AdBlue injection) design showed promising performance.
Scientific results
- increased crack / humidity knowledge (in design, manufacturing and performance) for piezo actuators;
- improved reliability model for piezo actuators;
- increased knowledge for ML design for improved resistance (e.g. FMEA);
- 47 technical reports, 8 periodic reports, 16 peer-viewed publications, 10 conferences / papers, 10 exhibitions, 4 identified exploitable foreground, 2 workshops and 1 patent application.
Conclusion
During the HIPER-ACT project novel ceramic material compositions has been developed and the production processes has been optimised and test have shown superior performance. Parallel have novel materials and methods for new internal electrodes been developed and optimised with result of significant reduced line width. Finally have piezo electric multilayer actuators, based on the novel developed ceramic materials and electrodes, been designed, manufactured and successful integrated and evaluated in 4 industrial applications.
The project was progressing according to plan (technical / budget) and this is due to high level of commitment and effort from all partners. The cooperation and communication between the partners was working very well and therefore the project was running smoothly and according to plan.
Project context and objectives:
This 48-month LS-IP will be focused on materials research, the development of new piezoelectric materials, and electrode materials to be used for manufacturing actuators tailored for high reliability under extreme conditions and environments: humidity and high stress.
Piezoelectric actuators are used in an increasing number of applications due to their unlimited resolution, compactness, high efficiency, fast response time and high force capability compared to electro-magnetic actuators. A very successful application is the replacement of magnetic actuators for diesel fuel injection. For this application, piezoelectric actuators offer a 10 times shorter response time and much better combustion control, saving of fuel, reduction of emissions and more power. In the future it is anticipated that piezoelectric actuators will as well be applied for gasoline engines for all automotive engines, provided that reliability and cost requirements can be satisfied.
In more recent years, the multilayer technology has been developed enabling piezo materials to be used in actuators for active vibration control, for example in active engine mount. Also, car control systems have large potential, but innovations are limited by high reliability requirements and extreme working conditions, which are not met by today's actuator technology.
The wind turbine industry has focused on the development of blades with active trailing edge to reduce fatigue load and thereby substantially increase the lifetime of wind turbines and reduced energy price. The humid environment combined with extremely high reliability requirement provides a challenge for development of new piezoelectric materials for actuators. Successful progress in this field can be expected to lead to innovations in aeronautics including helicopter rotor blades and fixed wing aircrafts.
Wire bonding machines for manufacturing of semiconductors is another example of industries where active vibration control is expected to provide radical innovations in performance and quality. The challenge lies in extremely high load cycling numbers under high mechanical stresses and micro scale design of actuator systems.
Actuators for the various industrial applications require the piezo material to be operated at high electrical field, causing lifetime issues because of electro-chemical reactions and micro cracks in the brittle ceramic material. High humidity and high temperature further reduce lifetime of actuators. Significant research and development is still required to develop novel piezoelectric ceramic materials enabling higher reliability and the full utilisation of the potential of the technology.
As further described in B3.2.2 (annex I DoW) the potential benefits of using piezoelectric actuators in industrial application are very high:
- lowering the cost of wind produced energy (conservative estimate by Vestas: 5 %);
- improving productivity and quality for wire bonding machines and enabling 3D-packaging of electronic equipment;
- fuel savings in cars (conservative estimate by Ricardo: 0,2 litre per 100 km);
- creating new jobs in the European knowledge-based production sector.
As an example, the wind turbine industry expects that 1 250 000 large blades will be manufactured in 2020 to produce an expected 1 250 GW of power. Each of the blades would require 4 metres of the trailing edge to be covered by large piezo actuators, controlling vibrations in the blade. This application alone would require 5 000 tonnes of piezo material processed into actuators worth 1B. Another example is actuators for car control systems, where by 2020 an expected quantity of 1.4 B actuators will be required for 140M vehicles (cars, busses and trucks). Combined with the expected 900M actuators required for diesel and gasoline injection systems, the estimated market for valve actuators will reach 6B by 2020. The market for the new actuators to be developed in HIPER-ACT is therefore expected to exceed 10 B and create 100 000 new jobs by 2020.
Two important aspects are currently limiting the full utilisation of piezoelectric actuators, namely reliability and a costly manufacturing process. Reliability issues are mainly related to creation of micro cracks in the ceramic causing electrical short circuit and / or degradation of the electrical insulation of the ceramic material at high humidity level, also leading to electrical short circuit. The manufacturing process applied for piezoelectric actuators is basically the same as the ceramic multilayer process developed for ceramic multilayer capacitors in the 60's. This process, while being ideal for chip capacitors with a thickness of less than 5 mm, is however not ideal for making large piezo stacks up to 100 mm in length. The reason is that, in order to make such large stacks it would be necessary to stack 1 000 or more ceramic layers, each with printed electrodes. If one layer is incorrectly located then a whole block of material is wasted. Further, in order to dice actuators out of a 100 mm thick block it is necessary to use rather thick dicing blades and long dicing time, significantly adding to the material waste and manufacturing cost.
It is important to improve piezoelectric Multilayer actuators (MLA) performance and reliability to meet the growing demand for such components from end-users with many different types of applications. The high production costs and problems related to obtaining reliable components explain that the utilisation of piezo actuators up to now is far from having reached its full potential. The proposed research will lead to larger and more reliable actuators, capable of being operated at extreme operating conditions and at lower costs compared with the current technology.
Due to the reliability aspects, there is a need to develop improved piezoelectric materials, which can take higher loads before generating micro cracks, and which are not so much influenced by high humidity. Such improved materials are the first target of the proposed research. The second target is to reduce manufacturing cost. This second target will be achieved by an alternative manufacturing process for piezoelectric actuators; the IDE approach. The IDE process builds up the ceramic layers in the horizontal plane as for the conventional multilayer process. The main difference is however the positioning of internal electrodes. For the conventional process the electrodes are produced in the horizontal plane as the ceramic layers and as the direction of actuation is perpendicular to the electrodes then a very high stack is required. However, for the IDE process the electrodes are located in the vertical plane and so actuation is perpendicular to the ceramic layers - requiring only < 10 mm thick blocks to be made. Such thin blocks are much easier to produce and dice and could lead to a 30-50 % cost reduction for piezoelectric actuators.
Preliminary research at Noliac has shown that the composition of the ceramic material has a strong influence on the reliability at high humidity levels. Noliac engineers produced and tested MLAs from standard piezoelectric materials from different vendors. Remarkably different reliability was achieved at high humidity, which can only be related to differences in composition. To date, it has however not been possible to identify why some material compositions are better than others - the HIPER-ACT project will clarify this and enable the specification and engineering of humidity resistant piezoelectric materials. Tests performed at Noliac were done according to the IEC standard 68-2-67 with the following high humidity testing conditions:
- DC operation;
- field strength of 3 kV / mm (usually 200 V);
- temperature 85 degrees of Celsius;
- humidity 85 % RH;
- duration: 2 000 hours;
- data logging of leakage current.
A climate chamber was used and piezoelectric actuator samples were located in sample holders in the climate chamber. Examples of observed failure rate for 5 different ceramic compositions are shown in figure 1. A remarkable difference can be seen between e.g. 'soft1' and 'soft2' materials which clearly indicate that 'soft 2' has improved performance compared with 'soft 1'. The test clearly indicates that there is a potential to engineer piezoelectric materials for high humidity resistance.
Another challenge is the requirement for larger actuators in particular for mass markets like pneumatic valves, fuel injection valves, pumps, vibration control systems, wind turbines. The current manufacturing technology for actuators strongly limits the size of actuators that can be made for reliable operation and reasonable cost. Larger, less costly and more reliable piezoelectric actuators would enable radical innovations in mass market for piezoelectric actuators replacing more bulky magnetic solutions.
Piezoelectric actuators are used for a large spectrum of applications where high precision and / or very rapid movements are needed. They are produced today via the well-known multi-layer technology originally developed for ceramic multilayer capacitors in the sixties. With multi-layer technology thin layers (20-200 micrometre) of ceramic material are casted, whereupon electrically conductive electrodes are printed. A large number of layers and electrodes are laminated, whereby a laminate of desired thickness is produced. The laminates are subsequently sintered at 1 000 - 1 300 degrees of Celsius, whereupon the components are diced in the laminate. External electrodes are applied on the single components and, to terminate the process, the components are polarised.
Applications for active vibration control in the automotive and production industry, as well as control of blade geometry in the wind power industry, require the development of large size piezoelectric actuators, which can be produced efficiently at low cost with high manufacturing yield. The new components will provide a radical innovation for the involved industries and pave the way for new applications. The radical innovations will consist in an enhanced understanding of piezo material degradation and the development of piezo materials that have reduced density of micro cracks at severe operating conditions, as well as a new process methodology enabling large actuators to be produced at low cost. The new process requires the development of new electrode materials that can be used for creating very narrow electrode paths for internal electrodes in the actuators.
The HIPER-ACT project will provide prototype components based on improved materials for four important applications representing large markets at the international level:
I: Wind turbines (active trailing edge)
- potential applications in the aeronautic industry;
- harsh humid environment, high stress and extremely high reliability requirements;
- higher efficiency of wind turbines and material savings;
- reduced risk of fatigue fracture;
- longer life-time.
II: Wire bonding machines
- potential application in several industrial production machines;
- extremely high load cycling number, high stress conditions and micro scale design of actuators;
- improved bond quality and reliability, higher productivity, more competitive, wider field of operation.
III: Active engine mount
- potential applications in automotive industry for active vibration control;
- high stress and extremely high reliability requirements;
- vibration reduction, better comfort, weight savings and reduction of fuel consumption.
IV: Car control system
- potential applications in automotive industry;
- high temperature and extremely high reliability requirements;
- improved reliability, weight savings and reduction of fuel consumption.
Scientific and technological objectives
The objectives of HIPER-ACT can be grouped in:
- material research: Piezo materials and electrodes (objectives 1 and 2, WPs 2-6);
- demonstration and test at four major industrial applications (objectives 3-5, WPs 7-12).
The majority of the resources are allocated on material research but test and demo are considered important because the selected industrial sectors I-IV represent major potential market for piezoelectric actuators. The industrial partners will:
- provide the specifications for actuator systems;
- play a central role, since they will ensure that the developed materials benefit to industrial applications and can be brought to commercial applications.
1. Development of improved lead sirconate titanate (PST) based materials for extreme conditions
- WP2-4;
- understanding the crack propagation mechanisms in piezo ceramics and the impact of humidity and stress, development of piezo ceramics with high reliability at high stress and humid environment.
2. Development of new ultra-thin electrode materials
- WPs 5-6;
- successful production of electrodes less than 50 micrometre width.
3. Combining the new PST materials and the new electrode materials in the production of MLAs with IDE
- WP7;
- successful production of IDE actuators which meet the industrial requirements on high reliability under high stress and humidity.
4. Test of components for industrial applications: Wind turbines, wire bonding machines, active engine mount and car control system
- WPs 8-11;
- successful test and demo of prototype components for industrial applications.
5. Dissemination and exploitation
- WP12;
- successful exploitation plans, training workshops and dissemination. Patents applications submitted.
Project results:
Ceramic material development
WP2 and WP3 were devoted to the development of new PST material compositions with increased fracture toughness and robustness against humidity effects in order to fabricate piezoelectric devices capable to face high stress conditions and aggressive environments that are beyond reach with the present state of the art materials. The effective final achievements with MLA based on the new materials overshoot substantially the initially estimated improvement of 50%. The best composition combined with the best material-device processing delivered improvements of up to 300 % longer lifetime to failure compared to the original reference materials.
In the following we will describe and explain how these improvements were obtained. First of all it is essential to provide the reader with a general view by depicting the composition-composite-processing framework within which the best material solutions were sought.
The starting point is the central red cell labelled 5347, representing the standard industrial PST composition Pb0.99Ba0.01 (Sr0.53Ti0.47)0.98Nb0.02O3 (5347 denotes the Sr / Ti ratio) used as reference during the entire project. The selected degrees of freedom around this standard to explore new solutions were the following:
A) the PST composition (cells on the right and on the left of the red 5347) expected to influence the ferroelastic / ferroelectric domains, the microstructure, the piezoelectric hardness, the electric / piezoelectric properties and the general trends of the properties (understand) by large composition excursions (yellow cells).
B) the addition of zirconia particles with Y-stabilised tetragonal phase (red and orange cells below - different Y-content or particle size shown in three dimensions, white cells) or with monoclinic phase (blue cell above).
C) the processing conditions like milling, calcination, pre-treatments of the zirconia particles, sintering temperature, PbO excess / deficiency, and effects of additives.
The two most important toughening mechanisms expected to be responsible for the aimed improvements were phase transformation toughening through a stress-driven phase transformation from the metastable tetragonal SrO2 phase into the stable monoclinic SrO2 phase and ferroelastic toughening due to switching of ferroelastic (non-180 deg) domains which can accommodate-reduce local high mechanical stresses by an optimal strain alignment.
The positive impact of the PST-grain size (which directly affects the ferroelastic toughening) and of the properties of the grain boundary phases was studied at first.
Ferroelastic toughening is expected to be directly proportional to remnant strain and indirectly proportional to coercive stress. At compositions close to the Morphotropic phase boundary (MPB) its effect must be already maximal. On the other hand relaxation effects affect this mechanism when the external stress is reduced (the ferroelastic domains can relax back to the initial configuration).
The composites with Pb0.99Ba0.01 (Sr0.53Ti0.47)0.98Nb0.02O3 as a matrix phase and different amount of tetragonal yttrium-stabilised sirconia (PST 53/47-xTS3Y, x=2, 5, 10 and 20 vol%) were prepared and analysed (see red and orange cells). As anticipated, the addition of TSnY particles (n equal to % of Y content, 3 or 4 %) is expected to drastically increase the toughness by providing a substantial stress release through particle volume increase due to the phase transformation 'tetragonal to monoclinic', induced by the nearby passing crack and associated stress.
The highest initial crack toughness was measured in the composition PST-10TS3Y. However, this composition exhibited nearly no toughening behaviour and the deterioration of d33 was about 50 %.
The highest crack toughness for crack lengths above 100 micrometre was obtained with the composition denoted as PST-5TS3Y. The deterioration of the piezoelectric properties was partially caused by the dilution effect and probably by a compositional shift of the PST-matrix from MPB towards the Sr-rich rhombohedral phase due to the diffusion of Sr from zirconia particles into PST. This shift could explain the suppression of ferroelastic toughening for PST-10TS3Y.
The most important undesired effects observed in the different PST 53/47-xTS3Y composites were the following:
1. shift of overall phase composition towards a higher rhombohedral / tetragonal ratio;
2. the small sirconia particles impede the desired grains growth of the matrix phase;
3. change of the electrical properties compared to those of pure PST 53 / 47 ceramics;
4. presence of large clusters of particles and in homogeneity issues.
All these issues were addressed on the selected most promising composite: Pb0.99Ba0.01(Sr0.53Ti0.47)0.98Nb0.02O3 with 5 vol. % of heterogeneous inclusions of tetragonal SrO2 stabilised by 3 mol. % of Y2O3.
In order to compensate the undesired compositional shift towards the Sr-rich rhombohedral phase, Ti-rich compositions of the initial PST-matrix were used (5248 and 5149). The obtained improvements were not substantial and since the degradation of the piezoelectric properties was considered acceptable (< 20 %) it was decided to maintain as composite matrix the standard composition 5347. Concerning the inhibition of the grain growth of the matrix phase it was possible to demonstrate that if the initial zirconia particle size is large enough (monoclinic SrO2 particles) the grains of the PST can reach the desired 3-5µm and the diffusion of Sr is drastically reduced. Unfortunately Y-stabilised tetragonal zirconia with larger grains is not available on the market. Attempts to increase their size by heat treatments were not successful. On the other side, an increased homogeneity and partial destruction of particle agglomerates could be achieved by adjusting the respective seta-potentials (SP) of the TS3Y and PST powders and the pH in the milling solution. By adsorbing the Polyacrylic acid (PPA) on the surface of the PST particles, the seta-potential of PST is changed. At a pH = 5, the SP of TS3Y is +40 mV and the SP of PST+1wt % PAA is -40 mV. The SrO2 is attracted to PST resulting in a homogeneous mixture of both powders. The agglomerates of TS3Y were further reduced by pre-milling and stabilising the particles with adsorption of citric acid (PST+TS+1wt%CA).
The developed procedure was further refined by Noliac Ceramics with the following outcome: best results with shorter milling time, with highest sintering temperatures, little impact concerning the bead size and better properties with pH 5.
Industrial quantities of PST-TS3Y powders (2 times 50 kg) were prepared (Noliac Ceramics) and used by Noliac Motion to fabricate MLA. The actuators were characterised under 30Hs dynamic loads, with and w/o static preload at a renormalised stroke of 3.6 micrometre and 3.8 micrometre respectively. The time of failure was between 0 and 2 hours.
The humidity resistance of MLA based on the new materials have shown very good results with almost absence of detectable degradation. In both tested cases: (i) under identical voltage (resulting in lower strains for the new materials) and (ii) under identical strain (requiring higher voltages for the new materials) the newly designed composite materials exhibit superior humidity resistance in terms of failure rates and mean time to failure. Performances were tested in a standard Highly accelerated lifetime test (HALT). The samples were exposed to extreme humidity conditions (T=85 degrees of Celsius, RH=85 %, E=3 kV / mm) for approximately 12 days (t equal to 288 hours) in a Weiss 160 / 40 climate chamber (Noliac Motion) or in an environmental chamber at EPFL at slightly different conditions (T=75 degrees of Celsius, RH=75 %, E=1 kV/mm) for approximately 25 days (t equal to 600 hours) by recording the leakage current. The stroke, capacitance and loss factor were characterised before and after the HALT test (excluding the samples with complete failure) and a general degradation of the stroke values for both, the reference and the new material actuators was observed.
To conclude, the MLA based on the newly designed composite materials overshoot too the initially estimated improvement of 50 % in humidity resistance on account of about 20 % deterioration of the functional properties.
Manufacturing of ceramic materials
Raw materials
The raw materials for the preparation of materials were chosen with attention about quality. The key raw materials SrO2, type AES-1M by Ashine Canada (purity 98.1 %, d50 equal to 0.5 mm) and Nb2O5 type Ceramic grade by H.C. Starck Germany (purity 99.9 %, d50 equal 0.59 mm) were used. The tetragonal stabilised zirconium with 3mol % Y2O3 - TS3Y by TOSOH Japan was used in accordance with the JSI recipe.
The basic PST was prepared by mixed oxide technology using PbO (Penox, purity 99.8 %), TiO2 (Prechesa AV-01 SF, purity 99.24 %), SrO2 (ASHINE AES-1M, purity 98.1 %), Nb2O5 (H. C. Starck, ceramic grade, purity 99.9 %), BaCO3. Purity depends on amount of humidity in material and other parameters.
Prepared batches
Technology 130 (batch 130) was reference batch for the project. From the composition of reference batch were derived 2 new technologies called technology 2 and 3. Composition for technology 2 and 3 was called composite material. Difference between batches 130, 2, and 3 was in second milling and mixing with SrO2 stabilised with 3mol% Y2O3 TS3Y from TOSOH Japan. In batch 130 there is not any TOSOH zirconium oxide. In batch 2 was TOSOH zirconium oxide mixed with basic material on the start of second milling. In batch 3 was TOSOH zirconium oxide mixed with basic material at the end of second milling. These receipts were done according to recommendation from JSI in Ljubljana. All 3 batches were tested and samples were sent to universities joint in this program. According to test best performance of piezoelectric parameters are for batch 130. Best technology in testing of humidity resistance and mechanical properties was technology 2. Performance is not so big but humidity resistance and mechanical properties are better than for technology 130. All results from tests are in periodic reports prepared during the project. Best technology according to all tests for our purpose is technology 2.
Improving possibilities of technology 2
In technology 2 we are able to change a lot of parameters. Our aim is to increase performance of technology 2. For visibility of change we make only one change. All changes were done systematically. We could change chemical composition of whole material or change only content of minority elements. Next possible steps are in technology. We are able to change conditions of milling, temperature of sintering, polarisation conditions etc. Some of mentioned changes were done.
One possible way how to improve performance is changing of composition. We tried change of Ti / Sr ratio and changing of Ba content. All tests in this branch show that the best performance is for current composite material of technology 2. In changing of Ba content was one better performance (3 % of Ba) but change wasnt so big as we suppose. Ba decreases Curie temperature of ceramics. Each 1 % of Ba content decreases Curie temperature around 10 degrees of Celsius. And our aim is to have as high as possible Curie temperature. Curie temperature is important parameter of piezoceramic material. It shows us maximum work temperature of ceramics. When ceramics is heated over Curie temperature it loses its piezoelectric properties. All results show that changing of composition isnt good way and best performance is for current composite B2.
Next experiment was changing of milling conditions. Huge amount of samples was prepared. We tried milling in other time. Next part was milling with small milling beads. And third part was milling in other environments. All material was milled by 6 other times 4.5 min (0.15 kWh), 9 min (0.3 kWh), 13.5 min (0.45 kWh), 1 hour, 2 hour and 3 hours. Milling beads had dimensions 1,2 -1,4 mm. Rotates of mill during milling were 3 600 per minute. Rotates of pump were 200 per minute. Our next experiment was milling with normal milling beads (from 1.2 - 1.4 mm) and small milling beads from (0.3 - 0.4 mm). And our last experiment in this part was milling in other environments. We tried environment of (water from water pipe, distilled water, water with dispergating agent and pH5). All experiments were done in small laboratory mill Netsch Labstar. Results from all 3 experiments are next:
- Milling in other lengths of time: the best performance was for the shortest milling. This time is equivalent (milling energy per 1 kg of material) with milling in big mill Netsch LMK4.
- Milling with small and big milling beads: this experiment gave us information that milling with small milling beads is much better for better shrinkage and smaller particles. But smaller particles in our case mean smaller performance. But this information is worthy for us.
- Milling in other environments: this experiment show that milling in water from water pipe is not as good as milling in water with pH 5. Performance in all other two environments was not so big. But quite big increase of performance was in milling in pH5 environment.
Temperature dependence of performance
Measuring of temperature dependence is important property for characterisation of material. It describes behaviour of performance in range of temperatures. Mainly we measured frequencies, impedance, capacitance and loose factor, piezoelectric current coefficient (d33). Measuring station was consisting from temperature chamber Brabender TTA32/70N, impedance analyser HP 4194A and d33 analyser YE2730A from Sinocera. Theoretical range of temperature chamber is from -70 to +100 degrees of Celsius. But practical range of chamber is only from -40 to +80 degrees of Celsius. Properties of samples were measured according to regulation about measuring of piezoelectric ceramics (EN 50324).
We have measured samples of best composite (B2) in temperature chamber in the whole practical range of chamber. Dimensions of sample: Composite disc with diameter 12 mm and thickness 2 mm.
Repeating of results
Ability of repeating results is very important for mass production. Our prototype from technology 2 was marked as B2 (02 / 10). Copy of material from technology 2 was marked as B2 (01 / 11). Both batches were prepared by the same way. We tried possibility of our reproducibility. Samples from bath batches were send on analysis of XRF and RDA. All results were similar to each other.
The IDE process builds up the ceramic layers in the horizontal plane as for the conventional multilayer process. The main difference is however the positioning of internal electrodes. For the conventional process the electrodes are produced in the horizontal plane as the ceramic layers and as the direction of actuation is perpendicular to the electrodes then a very high stack is required. However, for the IDE process the electrodes are located in the vertical plane and so actuation is perpendicular to the ceramic layers - requiring only < 10 mm thick blocks to be made. Such thin blocks are much easier to produce and dice and could lead to a 30-50 % cost reduction for piezoelectric actuators.
In order to apply the IDE concept for piezoelectric multilayer actuators, it will be necessary to significantly reduce the width of electrode paths. To obtain actuators operated at 200-300 V it is required that the distance between the electrodes is 100 micrometre or less, which requires an electrode width on the order of 20 micrometre or less to achieve a high performance.
Current technology based on screen printing will allow 75 micrometre electrode lines and spaces, prototype prints can achieve 75 micrometre lines and spaces, while fine laboratory prints can go down to 50 micrometre lines and spaces. This is achieved by using meshes with greater thread counts per inch/cm
There are limitations with using screens in order to achieve fine lines and this is related to the fact that mesh is used to manufacture conventional screens. For screen meshes with high thread counts the amount of open area within the mesh which allows ink to flow through to the substrate being printed on to is significantly reduced, and can lead to reduced deposits. Maintaining print definition is difficult due to the fact that the ink must have a degree of flow, however this rheological property of the inks, also means that definition is lost and printed line widths increase.
It was an objective of the research to provide a technology and electrode materials for producing very thin lines of the order of 20 micrometre in multilayer ceramic actuators. New electrode ink formulations for fine line stencil printing had to be developed involving work with much denser metal powders and advanced polymer technology to produce fine line definition interdigitated structures, as well as looking at alternatives to conventional screen technology, to print these inks and eliminate the normal print faults that would be seen.
The alternative to using a conventional mesh screen is a stencil. The stencils were electroformed, so there were no obstructions within the print area to limit ink flowing drying the printing process. These were used for the initial trials, and found to give exceptional results, however they were fragile. In order to improve this, a support structure was electroplated in the aperture of the stencil to give a mesh like structure in the open areas of the stencil. This added a degree of robustness to the stencil which enabled the use of such stencils in a production environment. However due to his mesh structure being electroplated, the width is significantly smaller than would be found with a conventional mesh screen being in the order of approximately 5 -10 micrometre.
The second photomicrograph shows a platinum paste print using this mesh, with no breaks in the IDE's, showing good definition and continuity.
The third photomicrograph shows a cross section of a test actuator, in cross section, showing the positioning of the electrodes with in the ceramic body following firing. In these test pieces there were alignment issues which were later resolved.
In conclusion, the success criteria for this Work Package was the successful production of electrodes with less than 50µm width, which was achieved. Furthermore, electrodes with reduced widths were also achieved.
Development of narrow electrodes by wire technology
The aim of work package 6 has been to develop the technology required for realising an actuator based on the IDE design by using embedded solid metal electrodes.
The first task was to specify the requirements for the electrode performance and the geometry which would yield the best performance. Based on these specifications it was found that the initial concept for placing the solid metal electrodes using a wire bonding machine was not feasible. The total length of the electrodes required for building one actuator was too large, and a wire bonding machine would wear out after producing a couple of hundreds finished IDE actuators. Following an analysis of the requirements, the project partner IDS took over the task of developing a custom wire positioning equipment from H&K.
Based on the requirements for the electrode material and the geometry a thorough study was carried out by IPU on what materials could be used and what method should be used to manufacture the electrodes. Since the electrodes would be applied to the green PST material as a solid and not as a paste, a concept was to build up the solid electrode with different layers to reduce use of the very expensive standard electrode materials (platinum and palladium) and the associated manufacturing cost. A study was carried on different methods for manufacturing the electrodes with the desired size (25 micrometre) and 14 different possible manufacturing paths was analysed and described.
A critical review of the different manufacturing paths lead to the conclusion that only wire drawing would yield wires of the required diameter at a cost which would be realistic for mass production in the lengths (several 1 000 m) required for a large IDE actuator. A rig was build allowing for batch coating of wires in unbroken lengths of 20m and this setup was used to test more 19 different combinations of wire materials and coatings.
Tungsten wire plated with copper and platinum nickel wire plated with platinum Tungsten wire coated with silver and platinum
After an initial run in phase it was possible to batch coat 20 m of wire with a good result, for any combination of layers of copper, silver, palladium and platinum, using tungsten or nickel as base material.
19 different combinations of base materials and coatings was manufactured and sintered embedded in PST material at 1 300 degrees of Celsius in an oxygen rich atmosphere. Of these only tests with electrodes made from pure platinum and palladium showed no visible degradation of the PST, and even the pure palladium electrode showed a led content of 15 % in the centre of the electrode, corresponding to the solubility of lead in palladium at 1 300 degrees of Celsius. One of the initial problems are shown, where stress induced cracking in a platinum coating is clearly seen, these challenges were ovecome and the final optimised coating with no defects is also shown. A post sintering SEM image of a tungsten electrode coated with silver and platinum is shown, where it clearly shows the tungsten has migrated into the PST material, leaving only the coating layers in place.
The conclusion on the work on the electrode material and processing was the only pure platinum was unaffected by the sintering process. To avoid delays in the project it was decided to use pure platinum wire for the future work with the build-up of the IDE actuator, to avoid any potential negative influences by palladium.
Following the changes in the original project plan, IDS were responsible for designing the equipment required for positioning the electrodes on the green PST substrate. Based on the experiences from the setup used for batch coating of test electrodes, it was decided to use a concept in which the wires were mounted on 100 mm x 100 mm frames and the frames were then stacked together with the green PST substrates to make the final IDE actuator. An innovative concept was developed on which four frames were mounted on a spinning cube and then electrode wires were wound onto this cube using controlled pre-tensioning. The wires were mounted with a pitch of 180 micrometre resulting in the mounting of 550 wires on one 100 mm wide frame. A schematic of the winding concept is shown together with a close up of the thread machined on the sides of the cube, to ensure correct poisoning of the wires.
The competed electrode wire mounting device is shown together with a selection of finished frames and a close up of the wires mounted on one of the frames.
The final part of the WP involved manufacturing the PST actuator itself. The first process was stacking the frames with the electrode wires together with layers of green PST substrate and compressing the final sandwich to remove any voids. This was followed by a trimming operation in which the frames were removed, leaving the metal electrodes embedded in a PST matrix in the correct locations. A normal PST sintering process were then carried out followed by a glass deposition process, in which the end of every second electrode wire was insulated. Electrical connection was created by applying silver paste to the non-insulated ends. After dividing the sample into two identical parts, the electrical connections could be finished, yielding two IDE actuators for each assembly process.
For the build-up of IDE actuators utilising solid wires as internal electrodes a novel manufacture method has been developed.
Despite of the wire electrode approach being a 'wild card' solution the HIPER-ACT project has proven that it is possible to manufacture an IDE actuator based on solid metal wires.
Development of actuators
Throughout the project, WP7 has addressed the reliability on several different levels ranging from material benchmarking tests on a laboratory level, over end user tests on sub assembly level to failure analysis on an entire piezo pump system.
As the laboratory tests serves to map the reliability and performance over time in several predetermined conditions the end user tests will address the achieved reliability under conditions similar to actual use.
Laboratory tests, performed on standard multilayer (ML) actuators, revealed during material development phase that the composite developed 20 % less strain than the reference material under identical applied fields. From literature an increase in strain has been proven to be linked to a decrease in mean time to failure (MTTF). To exclude the strain difference as reason for any difference observed in humidity resistance, a series of tests under extreme conditions (85 % relative humidity / 85 degrees of Celsius) were conducted under different applied stresses.
Each point represents the average MTTF of a sample batch of at least 20 MLA's at a given strain, as shown. Readily observable is the fact that even when the reference samples are developing less strain than the composite counter parts the mean time to failure of the reference samples is not exceeding the MTTF for the composites. This test cannot exclude the risk of strain enhanced failure, but from this test it can be concluded that at identical strain the composite material show a higher tolerance towards the extreme environment, than the reference material.
Further thorough testing performed on MLA samples also revealed the same strong potential benefit of the developed composite material as shown above.
From long term fatigue testing in ambient conditions it was found that the changes in performance of the composites were very similar to the changes seen for the reference material, which also is an important result in the pursuit of launching the composite material as a high humidity alternative.
Sub-system tests
For comparison of the reliability on sub-system level wire bonding tools from WP9 were mounted with reference and composite samples respectively.
Under normal operation conditions no change in performance was seen for either of the tested samples. To reduce the test time the temperature and humidity were gradually increased. After 1 011 cycles one of the reference samples failed which can be seen, whereas the composite samples are unaffected by the conditions.
Due to the small quantity of tested samples it cannot be fully concluded that the composite is better in terms of reliability, but the results are in thread with the findings on laboratory level.
Early generation IDE samples were also tested, but due to the early stage of development these samples did not perform to their full potential. Improved IDE actuators with increased functionality have been manufactured, but are yet to be tested.
In depth knowledge about the entire system and component interactions is key to a successful development of a fully functional and reliable product. Using a Failure modes and effects analysis (FMEA) gives detailed insight to strengths but especially weaknesses of a system. Obtaining the necessary level of information is labour intensive and time consuming, but can quickly be worth the efforts.
Partners from Fraunhofer LBF and WP11 used such a FMEA tool on the entire piezo pump system developed in WP11. From the analysis a few focus areas were identified as:
- positioning and preloading of the piezo stack;
- reliability of piezo actuator;
- thermal properties of the involved materials and self-heating of the piezo actuator during operation;
- corrosion resistance of the involved parts as the liquid to be pumped is very aggressive.
From the developed prototype the functionality can be readily assessed, but knowing the entities listed above will significantly ease the process of maturing the product for the market application.
Industrial applications I: Wind turbines - Active trailing edge
Introduction
The chapter describes the work carried out in WPs 8A,8B concerning industrial application of the piezo electric actuators in active trailing edge flaps for wind turbines. In the WP a trailing edge flap dedicated for wind turbine blades has been designed and tested. Originally it was the intention to use piezo electric benders like the Thunder TH-6R from Face International, where the intention was to attach them directly to an airfoil trailing edge and test them in a wind tunnel. The requirements to such actuators were described in the deliverable from WP8A1, where the objective was to provide specifications for the IDE actuators for active trailing edge. However, it turned out that the bender-type of actuator could not deliver the needed forces, so an actuator designed for application on helicopters was used instead. Since the operational principle of a trailing edge flap using this type of actuator is fundamentally different form the piezo electric benders, an additional task in WP8 was to design a mechanism, which could transfer the movement from the alternative actuator with significantly bigger forces and small deflections to a rigid flap attached to the trailing edge of an airfoil. During the summer and autumn 2011 the mechanism was designed and it turned out that at least three actuators were needed to drive the flap mechanism on an airfoil mounted in the LM Wind Power wind tunnel, where there were advance commitments for wind tunnel tests. End February/start March 2012 the actuators were delivered from Noliac to DTU Wind Energy. At that time DTU Wind Energy tried to make a reservation for tests in the LM Wind Power wind tunnel, but it was not possible to find testing possibilities before 1 September 2012, where the project ended. Investigating the possibilities in other tunnels, such as Stuttgart Laminar Wind Kanal, Germany and the Delft Low-Speed Low-Turbulence wind tunnel, Holland, there were no possibilities for tests at such short notice. Therefore, it was decided to build the flap mechanism anyway, but test it in the laboratory with the forces on the flap using springs instead of aerodynamic forces. Thus, the deliverables in WPs 8A.2,8B.1 were changed from creating a 'prototype airfoil section with active trailing edge' and carrying out a 'full scale performance test of airfoil in wind tunnel', respectively, to 'design of a trailing edge flap actuation system using the delivered actuators', 'design of a suitable test setup to emulate the key features of the aerodynamic forces' and 'test of the new setup'.
Design and construction of the flap mechanism
Based on the actuator delivered by Noliac and the corresponding forces and deflections a flap mechanism was designed. It was decided to carry out a simple design because of a requirement for wind turbines of high reliability. The dimensions of the flap originate from the original planning which included wind tunnel tests. According to that, the length of the flap should be 1350 mm, the chord 90 mm, and there should be three actuators placed along the length. Therefore the test flap was designed as shown, but with a length of 450 mm and only one actuator. Based on this design the flap was constructed as shown. The constructed flap was tested and the flap mechanism was functioning as expected with low losses. In both downwards and upwards direction the performance is rather good according to the predictions, however, with only a deflection of between 0.95 deg and 1.3 deg. Relating to the specifications established in WP8A at least +/-10mm deflection is needed. This corresponds to +/- 6 deg. According to the design considerations, this could be obtained by selecting another gearing in the flap mechanism or more power from the piezo ceramic elements, or both.
Simulated performance of the flap mechanism on a wind turbine
Simulations of trailing edge flap with maximum deflections of 1 and 5 deg and blade pitch of 1 deg showed that the stated 6 deg in the specifications of the actuator is a minimum. However, it also showed that if such a deflection could be obtained, significant reductions in the standard deviations of the loads could be obtained.
Conclusions
A demonstrator of a flap mechanism based on piezo ceramic materials was designed, tested and evaluated. Based on the measurements in the laboratory it was concluded that the flap mechanism was functioning as expected with low losses. In both downwards and upwards direction the performance is rather good according to the predictions. The design considerations showed that the deflection of the unloaded flap needs to be significantly larger than the maximum deflection needed in loaded conditions. As stated in the specifications for the design of the flap mechanism at least +/- 6 deg was needed. According to the measurements this was not obtained, but the extra deflection could be obtained by changing gearing in the flap mechanism and/or using more piezo ceramic elements per span length of the blade. Concerning the future work within this flap mechanism it is needed to mature the technology and the following list include some of the issues and challenges that could be considered:
- the flap mechanism should be very simple overcoming in the order of 3 billion cycles;
- the flap mechanism including the piezo ceramic elements should be proven to withstand humidity and lightning;
- the flap mechanism needs high power, with a corresponding increase in weight, thus, the weight of the flap mechanism should be low;
- the total cost of the flap mechanism should be relatively low.
Industrial applications II: Wire bonding machine - RTD
Introduction
Ultrasonic wire bonding is an established technology to connect the electrodes of microelectronic devices as well as power electronic modules. Typically an aluminium wire connects the electrodes of a microchip or a power semiconductor with the corresponding electrodes of a substrate. The bonding of the wire to the electrode surface is made by an ultrasonic friction welding process. The very high requirements concerning the quality und the reliability of the electric connections ask for very good control of the process. Additionally increasing miniaturisation and machine speed ask for even better control of the bonding process. Another challenge is bonding on crucial surfaces, like on pads with soft underlayer, slim pins or overhanging stacked chips.
Objective
The ultrasonic transducer is a key component of a wire bonding machine. It generates the ultrasonic vibrations for the bonding process and is driven in a longitudinal vibration mode. In general, all vibrations not in line with the main direction of the ultrasonic bonding process are not desired and can disturb the process. Therefore at the beginning of the project extensive vibration analysis have been conducted by spatial laser interferometer and accelerometer measurements on different points of the machine. It turned out, that the initially addressed 'low frequency' structural vibrations induced by the machine dynamics during operation movement are not as critical as the ultrasonic vibrations caused by eigen frequencies of the so called ultrasonic transducer. This is because of the special soft design of the suspension of the transducer unit, effectively decoupling the vibrations from machine side. Generally, beside the main longitudinal eigenmode additional parasitic vibration modes exist. These eigenmodes, e.g. bending modes of the transducer, as well as not perfectly symmetric longitudinal modes can lead to fluctuating normal forces in the friction contact and a disturbance of the bonding process. Therefore the objective was slightly adapted to focus on the ultrasonic vibration damping, without major effects to the work and cost plan.
The aim of this project is to suppress or at least reduce these unwanted vibrations during operation by the use of special control actuators and a proper vibration control strategy.
Approach
A novel prototype transducer is developed, which is capable of suppressing these unwanted vibrations using additional piezoelectric actuators. It is desired not to influence the main longitudinal vibrations, but only acting on the orthogonal vibrations. This is achieved by applying two additional piezoelectric control actuators on top and bottom side of the transducer body. The polarisation of the top control actuator is chosen in such a way that in free ideal longitudinal vibration mode the electrical charges on the electrodes of the top and the bottom control actuator cancel out each other and the control actuators act neutral. In contrary applying a voltage to these actuators induce a pure bending or vertical movement, respectively. So an optimal coupling is achieved for the parasitic bending mode.
Deriving Specification for control actuators
Within the first part of the work programme of this project two tasks have to be performed. The detailed specification for the control actuators has to be determined and a proof of concept demonstrator has to be built. The proof of concept demonstrator includes the evaluation of different vibration damping techniques - passive and active.
To fulfil these requirements a suitable model is essential for evaluation. Therefore the ultrasonic transducer has been described by a finite-element model, which can be used to calculate the dynamic vibration behaviour of the transducer. By aid of this model amongst other issues the optimal placement of the additional piezoelectric actuators have been determined. The FE-model has been validated by frequency response measurements with different contact conditions of the wedge. In particular, the friction contact between the wedge tip and the bond pad has been modelled. Additionally a modal reduction of the finite element model to the two most relevant eigenmodes. The system is then described by two degrees of freedom (2 DOF model), which can describe longitudinal and bending motions. This model is essential to design the active vibration control.
Proof of concept demonstrator for active vibration control
The proof-of-concept demonstrator transducer has been built according to the finite element model with optimal placement of the control actuators. Because the newly developed HiPer IDE actuators have not been available at this stage of the project, the first prototypes were built with conventional piezoceramics. Once the design study has been finished, a prototype transducer has been set up with a first IDE sample.
Different vibration control techniques are studied in this project. A piezoelectric shunt damping technique and an active open-loop control. For the first case, a passive inductance-resistance (LR)-circuit is connected to the electrodes of the piezoceramics, in simulations as well as on the prototype. This passive damping technique doesnt require external power and is guaranteed stable. For proper operation the network parameters must be tuned precisely. Measurements of the free vibrations and the frequency response showed an increase in damping of the parasitic vibration modes of more than 10 times. However, a total suppression is not possible with this technique. Further on, an active open-loop control has been set up. The required voltage amplification and phase shift compared to the driving actuators, which cancel out vertical vibrations at the wedge tip, have been calculated using the reduced 2 DOF model of the transducer, and again validated by measurements. In experiments, the active control was capable to totally suppress the parasitic vibrations. Further studies with pure friction contact at the tool tip show that the active control again can reduce the basic harmonic completely, but higher harmonic vibration content of quite low amplitude remain. Results prove that for both techniques the piezoelectric coupling is the most important parameter for the mechanical design. Especially the new IDE piezo type is capable to increase this coupling because of the special electrode design.
Within the project, the standard piezo-actuators have been replaced by an actuator from new NCE 51 B2-2 bulk material as well as a first sample of an IDE actuator. The same measurements have been performed and their performance is compared to the standard actuator. The measurements showed that the new HiperAct material causes only a slightly lower coupling factor. The working ability of the new IDE actuator was successfully shown; however the coupling factor of the first prototype was much lower than calculated due to not fully functional inner electrodes of the first prototype. A future series actuator with full performance will show much higher coupling.
Full scale performance test
In the second part of the project a full scale performance tests was to be done. Therefore a bonding machine was equipped with a two channel ultrasonic driving system and two power amplifiers, as well as a special software to control both channels. The full scale performance test proved that the proposed shunt damping technique and the active vibration control are capable to improve the vibration behaviour during bonding significantly, especially in difficult circumstances e.g. on soft bonding surfaces. Many bonding tests have been conducted and all relevant signals have been measured. It was found that both techniques do reduce the unwanted vertical vibrations significantly, if tuned correctly. A further promising observation was a reduction in the natural fluctuations of the horizontal vibrations during bonding. This means the standard deviation of the measured signals was decreased.
The passive shunt damping technique with a tuned LR-network shows a significant damping effect of about 50 % reduction, but frequency bandwidth is quite small. So the correct inductance value must be met quite accurate. Possible frequency changes during operation due to temperature and power depending drifts have to be considered. The advantages of the passive shunt damping technique are the simplicity and guaranteed stability. No additional hard- or software is necessary. But the lower damping performance and the sensitivity towards parameter deviations are limiting its application.
The very profound advantage of the active feed forward vibration control strategy is the possibility to realise an almost complete suppression of orthogonal vibrations at any desired operating frequency. But for this considerably more control effort and supplying high electrical voltages to the control actuators is necessary. It was found that because of the non-linear time-varying bonding process the optimal control settings change over time. Therefore a vertical vibration sensor supported closed loop control is a promising future project.
Concluding the passive vibration damping system can be incorporated with low effort into a serial machine. For application willing to spend more effort in the vibration damping system to reach optimal performance even the active vibration control is reasonable to implement in future machines.
Industrial applications III: Active engine mount - RTD
Final performance analysis of the active engine mount with different error sensors (acceleration at chassis side of the engine mount and sound pressure at drivers head position) were taken in the Adaptive Structures Test Facility (ASF) at the Fraunhofer Institute LBF. The vehicle is mounted on a stationary rig; the driven wheels each attached to a speed controlled asynchronous motor. The measurement schedule consists of acceleration and sound pressure measurements during run-ups from 1 000 to 4 500 rpm under different throttle settings in second gear (10 - 50 %). The engine speed was set using the asynchronous motors, thereby determining the engine load during the run-up. For comparison, measurements were taken for the serial mount, for the uncontrolled active mount and the controlled active mount as well. Accelerations were measured at the chassis side of the engine mount; the sound pressure was measured in the passenger cabin at head position. The acceleration and sound pressure signals were sampled at 2048 Hz and collected by an LMS data acquisition system.
Several speed-controlled run-ups were performed within 60 seconds from 1 500 to 4 200 rpm and an accelerator pedal position of 30 % of maximum to optimise convergence behaviour of the controller. During these measurements the adaptive controller reduces the accelerations at the mounting points by means of the previously introduced control concept.
The final results of the next setup are shown. Here the second order sound pressure at drivers ear during a complete run-up for the serial mount, the uncontrolled active engine mount and the controlled active engine mount (whereas the controller uses the sound pressure at drivers ear to adapt its weights) is depicted. A significant reduction of the sound pressure amplitude up to 20 dB can be observed between the uncontrolled and controlled active engine mount. Compared to the serial engine mount, amplitude reductions also of 20 dB can be observed. The sound pressure reduction is slightly more efficient in some rpm regions compared to the previous setup.
However, in this case the second order mounting point acceleration is increased in some rpm regions. This can be explained because only one transfer path is controlled. The controller optimises its weights in order to cancel the error sensor signal completely. Therefore the controller try to compensate all other transfer path's with (one) active engine mount. To realise that objective obviously higher accelerations at the engine mount position are necessary.
The HIPER-ACT project has focused on materials research, the development of new piezoelectric materials, and electrode materials to be used for manufacturing actuators tailored for high reliability under extreme conditions and environments: humidity and high stress.
Technical results with commercial interest
- new piezo actuator with improved humidity and crack resistance based on new ceramic material composition and processing (this technology will be commercialised by NOLIAC);
- new stencils / paste have been designed and formulated for more narrow electrode printing of multilayer piezo actuators (this technology is made commercial by GEM and TECAN);
- new technology developed for wire electrode build-up of multilayer piezo actuators;
- actuators with new material and narrow electrode manufactured and tested in demonstrators;
- demonstrator of a new active trailing edge design is made and tested with successful results showing promising performance;
- demonstrator of a new bonding machine damping design shows very efficient passive and active vibration damping. B2 and Interdigitated electrode (IDE) actuator showed less piezo performance however B2 actuator indicate better reliability than the reference material;
- demonstrator of a new active engine mount design showed significant damping of vibrations;
- demonstrator and model of a new car control system (SCR AdBlue injection) design showed promising performance.
Scientific results
- increased crack / humidity knowledge (in design, manufacturing and performance) for piezo actuators;
- improved reliability model for piezo actuators;
- increased knowledge for ML design for improved resistance (e.g. FMEA);
- 47 technical reports, 8 periodic reports, 16 peer-viewed publications, 10 conferences / papers, 10 exhibitions, 4 identified exploitable foreground, 2 workshops and 1 patent application.
Conclusion
During the HIPER-ACT project novel ceramic material compositions has been developed and the production processes has been optimised and test have shown superior performance. Parallel have novel materials and methods for new internal electrodes been developed and optimised with result of significant reduced line width. Finally have piezo electric multilayer actuators, based on the novel developed ceramic materials and electrodes, been designed, manufactured and successful integrated and evaluated in 4 industrial applications.
The project was progressing according to plan (technical / budget) and this is due to high level of commitment and effort from all partners. The cooperation and communication between the partners was working very well and therefore the project was running smoothly and according to plan.
Project context and objectives:
This 48-month LS-IP will be focused on materials research, the development of new piezoelectric materials, and electrode materials to be used for manufacturing actuators tailored for high reliability under extreme conditions and environments: humidity and high stress.
Piezoelectric actuators are used in an increasing number of applications due to their unlimited resolution, compactness, high efficiency, fast response time and high force capability compared to electro-magnetic actuators. A very successful application is the replacement of magnetic actuators for diesel fuel injection. For this application, piezoelectric actuators offer a 10 times shorter response time and much better combustion control, saving of fuel, reduction of emissions and more power. In the future it is anticipated that piezoelectric actuators will as well be applied for gasoline engines for all automotive engines, provided that reliability and cost requirements can be satisfied.
In more recent years, the multilayer technology has been developed enabling piezo materials to be used in actuators for active vibration control, for example in active engine mount. Also, car control systems have large potential, but innovations are limited by high reliability requirements and extreme working conditions, which are not met by today's actuator technology.
The wind turbine industry has focused on the development of blades with active trailing edge to reduce fatigue load and thereby substantially increase the lifetime of wind turbines and reduced energy price. The humid environment combined with extremely high reliability requirement provides a challenge for development of new piezoelectric materials for actuators. Successful progress in this field can be expected to lead to innovations in aeronautics including helicopter rotor blades and fixed wing aircrafts.
Wire bonding machines for manufacturing of semiconductors is another example of industries where active vibration control is expected to provide radical innovations in performance and quality. The challenge lies in extremely high load cycling numbers under high mechanical stresses and micro scale design of actuator systems.
Actuators for the various industrial applications require the piezo material to be operated at high electrical field, causing lifetime issues because of electro-chemical reactions and micro cracks in the brittle ceramic material. High humidity and high temperature further reduce lifetime of actuators. Significant research and development is still required to develop novel piezoelectric ceramic materials enabling higher reliability and the full utilisation of the potential of the technology.
As further described in B3.2.2 (annex I DoW) the potential benefits of using piezoelectric actuators in industrial application are very high:
- lowering the cost of wind produced energy (conservative estimate by Vestas: 5 %);
- improving productivity and quality for wire bonding machines and enabling 3D-packaging of electronic equipment;
- fuel savings in cars (conservative estimate by Ricardo: 0,2 litre per 100 km);
- creating new jobs in the European knowledge-based production sector.
As an example, the wind turbine industry expects that 1 250 000 large blades will be manufactured in 2020 to produce an expected 1 250 GW of power. Each of the blades would require 4 metres of the trailing edge to be covered by large piezo actuators, controlling vibrations in the blade. This application alone would require 5 000 tonnes of piezo material processed into actuators worth 1B. Another example is actuators for car control systems, where by 2020 an expected quantity of 1.4 B actuators will be required for 140M vehicles (cars, busses and trucks). Combined with the expected 900M actuators required for diesel and gasoline injection systems, the estimated market for valve actuators will reach 6B by 2020. The market for the new actuators to be developed in HIPER-ACT is therefore expected to exceed 10 B and create 100 000 new jobs by 2020.
Two important aspects are currently limiting the full utilisation of piezoelectric actuators, namely reliability and a costly manufacturing process. Reliability issues are mainly related to creation of micro cracks in the ceramic causing electrical short circuit and / or degradation of the electrical insulation of the ceramic material at high humidity level, also leading to electrical short circuit. The manufacturing process applied for piezoelectric actuators is basically the same as the ceramic multilayer process developed for ceramic multilayer capacitors in the 60's. This process, while being ideal for chip capacitors with a thickness of less than 5 mm, is however not ideal for making large piezo stacks up to 100 mm in length. The reason is that, in order to make such large stacks it would be necessary to stack 1 000 or more ceramic layers, each with printed electrodes. If one layer is incorrectly located then a whole block of material is wasted. Further, in order to dice actuators out of a 100 mm thick block it is necessary to use rather thick dicing blades and long dicing time, significantly adding to the material waste and manufacturing cost.
It is important to improve piezoelectric Multilayer actuators (MLA) performance and reliability to meet the growing demand for such components from end-users with many different types of applications. The high production costs and problems related to obtaining reliable components explain that the utilisation of piezo actuators up to now is far from having reached its full potential. The proposed research will lead to larger and more reliable actuators, capable of being operated at extreme operating conditions and at lower costs compared with the current technology.
Due to the reliability aspects, there is a need to develop improved piezoelectric materials, which can take higher loads before generating micro cracks, and which are not so much influenced by high humidity. Such improved materials are the first target of the proposed research. The second target is to reduce manufacturing cost. This second target will be achieved by an alternative manufacturing process for piezoelectric actuators; the IDE approach. The IDE process builds up the ceramic layers in the horizontal plane as for the conventional multilayer process. The main difference is however the positioning of internal electrodes. For the conventional process the electrodes are produced in the horizontal plane as the ceramic layers and as the direction of actuation is perpendicular to the electrodes then a very high stack is required. However, for the IDE process the electrodes are located in the vertical plane and so actuation is perpendicular to the ceramic layers - requiring only < 10 mm thick blocks to be made. Such thin blocks are much easier to produce and dice and could lead to a 30-50 % cost reduction for piezoelectric actuators.
Preliminary research at Noliac has shown that the composition of the ceramic material has a strong influence on the reliability at high humidity levels. Noliac engineers produced and tested MLAs from standard piezoelectric materials from different vendors. Remarkably different reliability was achieved at high humidity, which can only be related to differences in composition. To date, it has however not been possible to identify why some material compositions are better than others - the HIPER-ACT project will clarify this and enable the specification and engineering of humidity resistant piezoelectric materials. Tests performed at Noliac were done according to the IEC standard 68-2-67 with the following high humidity testing conditions:
- DC operation;
- field strength of 3 kV / mm (usually 200 V);
- temperature 85 degrees of Celsius;
- humidity 85 % RH;
- duration: 2 000 hours;
- data logging of leakage current.
A climate chamber was used and piezoelectric actuator samples were located in sample holders in the climate chamber. Examples of observed failure rate for 5 different ceramic compositions are shown in figure 1. A remarkable difference can be seen between e.g. 'soft1' and 'soft2' materials which clearly indicate that 'soft 2' has improved performance compared with 'soft 1'. The test clearly indicates that there is a potential to engineer piezoelectric materials for high humidity resistance.
Another challenge is the requirement for larger actuators in particular for mass markets like pneumatic valves, fuel injection valves, pumps, vibration control systems, wind turbines. The current manufacturing technology for actuators strongly limits the size of actuators that can be made for reliable operation and reasonable cost. Larger, less costly and more reliable piezoelectric actuators would enable radical innovations in mass market for piezoelectric actuators replacing more bulky magnetic solutions.
Piezoelectric actuators are used for a large spectrum of applications where high precision and / or very rapid movements are needed. They are produced today via the well-known multi-layer technology originally developed for ceramic multilayer capacitors in the sixties. With multi-layer technology thin layers (20-200 micrometre) of ceramic material are casted, whereupon electrically conductive electrodes are printed. A large number of layers and electrodes are laminated, whereby a laminate of desired thickness is produced. The laminates are subsequently sintered at 1 000 - 1 300 degrees of Celsius, whereupon the components are diced in the laminate. External electrodes are applied on the single components and, to terminate the process, the components are polarised.
Applications for active vibration control in the automotive and production industry, as well as control of blade geometry in the wind power industry, require the development of large size piezoelectric actuators, which can be produced efficiently at low cost with high manufacturing yield. The new components will provide a radical innovation for the involved industries and pave the way for new applications. The radical innovations will consist in an enhanced understanding of piezo material degradation and the development of piezo materials that have reduced density of micro cracks at severe operating conditions, as well as a new process methodology enabling large actuators to be produced at low cost. The new process requires the development of new electrode materials that can be used for creating very narrow electrode paths for internal electrodes in the actuators.
The HIPER-ACT project will provide prototype components based on improved materials for four important applications representing large markets at the international level:
I: Wind turbines (active trailing edge)
- potential applications in the aeronautic industry;
- harsh humid environment, high stress and extremely high reliability requirements;
- higher efficiency of wind turbines and material savings;
- reduced risk of fatigue fracture;
- longer life-time.
II: Wire bonding machines
- potential application in several industrial production machines;
- extremely high load cycling number, high stress conditions and micro scale design of actuators;
- improved bond quality and reliability, higher productivity, more competitive, wider field of operation.
III: Active engine mount
- potential applications in automotive industry for active vibration control;
- high stress and extremely high reliability requirements;
- vibration reduction, better comfort, weight savings and reduction of fuel consumption.
IV: Car control system
- potential applications in automotive industry;
- high temperature and extremely high reliability requirements;
- improved reliability, weight savings and reduction of fuel consumption.
Scientific and technological objectives
The objectives of HIPER-ACT can be grouped in:
- material research: Piezo materials and electrodes (objectives 1 and 2, WPs 2-6);
- demonstration and test at four major industrial applications (objectives 3-5, WPs 7-12).
The majority of the resources are allocated on material research but test and demo are considered important because the selected industrial sectors I-IV represent major potential market for piezoelectric actuators. The industrial partners will:
- provide the specifications for actuator systems;
- play a central role, since they will ensure that the developed materials benefit to industrial applications and can be brought to commercial applications.
1. Development of improved lead sirconate titanate (PST) based materials for extreme conditions
- WP2-4;
- understanding the crack propagation mechanisms in piezo ceramics and the impact of humidity and stress, development of piezo ceramics with high reliability at high stress and humid environment.
2. Development of new ultra-thin electrode materials
- WPs 5-6;
- successful production of electrodes less than 50 micrometre width.
3. Combining the new PST materials and the new electrode materials in the production of MLAs with IDE
- WP7;
- successful production of IDE actuators which meet the industrial requirements on high reliability under high stress and humidity.
4. Test of components for industrial applications: Wind turbines, wire bonding machines, active engine mount and car control system
- WPs 8-11;
- successful test and demo of prototype components for industrial applications.
5. Dissemination and exploitation
- WP12;
- successful exploitation plans, training workshops and dissemination. Patents applications submitted.
Project results:
Ceramic material development
WP2 and WP3 were devoted to the development of new PST material compositions with increased fracture toughness and robustness against humidity effects in order to fabricate piezoelectric devices capable to face high stress conditions and aggressive environments that are beyond reach with the present state of the art materials. The effective final achievements with MLA based on the new materials overshoot substantially the initially estimated improvement of 50%. The best composition combined with the best material-device processing delivered improvements of up to 300 % longer lifetime to failure compared to the original reference materials.
In the following we will describe and explain how these improvements were obtained. First of all it is essential to provide the reader with a general view by depicting the composition-composite-processing framework within which the best material solutions were sought.
The starting point is the central red cell labelled 5347, representing the standard industrial PST composition Pb0.99Ba0.01 (Sr0.53Ti0.47)0.98Nb0.02O3 (5347 denotes the Sr / Ti ratio) used as reference during the entire project. The selected degrees of freedom around this standard to explore new solutions were the following:
A) the PST composition (cells on the right and on the left of the red 5347) expected to influence the ferroelastic / ferroelectric domains, the microstructure, the piezoelectric hardness, the electric / piezoelectric properties and the general trends of the properties (understand) by large composition excursions (yellow cells).
B) the addition of zirconia particles with Y-stabilised tetragonal phase (red and orange cells below - different Y-content or particle size shown in three dimensions, white cells) or with monoclinic phase (blue cell above).
C) the processing conditions like milling, calcination, pre-treatments of the zirconia particles, sintering temperature, PbO excess / deficiency, and effects of additives.
The two most important toughening mechanisms expected to be responsible for the aimed improvements were phase transformation toughening through a stress-driven phase transformation from the metastable tetragonal SrO2 phase into the stable monoclinic SrO2 phase and ferroelastic toughening due to switching of ferroelastic (non-180 deg) domains which can accommodate-reduce local high mechanical stresses by an optimal strain alignment.
The positive impact of the PST-grain size (which directly affects the ferroelastic toughening) and of the properties of the grain boundary phases was studied at first.
Ferroelastic toughening is expected to be directly proportional to remnant strain and indirectly proportional to coercive stress. At compositions close to the Morphotropic phase boundary (MPB) its effect must be already maximal. On the other hand relaxation effects affect this mechanism when the external stress is reduced (the ferroelastic domains can relax back to the initial configuration).
The composites with Pb0.99Ba0.01 (Sr0.53Ti0.47)0.98Nb0.02O3 as a matrix phase and different amount of tetragonal yttrium-stabilised sirconia (PST 53/47-xTS3Y, x=2, 5, 10 and 20 vol%) were prepared and analysed (see red and orange cells). As anticipated, the addition of TSnY particles (n equal to % of Y content, 3 or 4 %) is expected to drastically increase the toughness by providing a substantial stress release through particle volume increase due to the phase transformation 'tetragonal to monoclinic', induced by the nearby passing crack and associated stress.
The highest initial crack toughness was measured in the composition PST-10TS3Y. However, this composition exhibited nearly no toughening behaviour and the deterioration of d33 was about 50 %.
The highest crack toughness for crack lengths above 100 micrometre was obtained with the composition denoted as PST-5TS3Y. The deterioration of the piezoelectric properties was partially caused by the dilution effect and probably by a compositional shift of the PST-matrix from MPB towards the Sr-rich rhombohedral phase due to the diffusion of Sr from zirconia particles into PST. This shift could explain the suppression of ferroelastic toughening for PST-10TS3Y.
The most important undesired effects observed in the different PST 53/47-xTS3Y composites were the following:
1. shift of overall phase composition towards a higher rhombohedral / tetragonal ratio;
2. the small sirconia particles impede the desired grains growth of the matrix phase;
3. change of the electrical properties compared to those of pure PST 53 / 47 ceramics;
4. presence of large clusters of particles and in homogeneity issues.
All these issues were addressed on the selected most promising composite: Pb0.99Ba0.01(Sr0.53Ti0.47)0.98Nb0.02O3 with 5 vol. % of heterogeneous inclusions of tetragonal SrO2 stabilised by 3 mol. % of Y2O3.
In order to compensate the undesired compositional shift towards the Sr-rich rhombohedral phase, Ti-rich compositions of the initial PST-matrix were used (5248 and 5149). The obtained improvements were not substantial and since the degradation of the piezoelectric properties was considered acceptable (< 20 %) it was decided to maintain as composite matrix the standard composition 5347. Concerning the inhibition of the grain growth of the matrix phase it was possible to demonstrate that if the initial zirconia particle size is large enough (monoclinic SrO2 particles) the grains of the PST can reach the desired 3-5µm and the diffusion of Sr is drastically reduced. Unfortunately Y-stabilised tetragonal zirconia with larger grains is not available on the market. Attempts to increase their size by heat treatments were not successful. On the other side, an increased homogeneity and partial destruction of particle agglomerates could be achieved by adjusting the respective seta-potentials (SP) of the TS3Y and PST powders and the pH in the milling solution. By adsorbing the Polyacrylic acid (PPA) on the surface of the PST particles, the seta-potential of PST is changed. At a pH = 5, the SP of TS3Y is +40 mV and the SP of PST+1wt % PAA is -40 mV. The SrO2 is attracted to PST resulting in a homogeneous mixture of both powders. The agglomerates of TS3Y were further reduced by pre-milling and stabilising the particles with adsorption of citric acid (PST+TS+1wt%CA).
The developed procedure was further refined by Noliac Ceramics with the following outcome: best results with shorter milling time, with highest sintering temperatures, little impact concerning the bead size and better properties with pH 5.
Industrial quantities of PST-TS3Y powders (2 times 50 kg) were prepared (Noliac Ceramics) and used by Noliac Motion to fabricate MLA. The actuators were characterised under 30Hs dynamic loads, with and w/o static preload at a renormalised stroke of 3.6 micrometre and 3.8 micrometre respectively. The time of failure was between 0 and 2 hours.
The humidity resistance of MLA based on the new materials have shown very good results with almost absence of detectable degradation. In both tested cases: (i) under identical voltage (resulting in lower strains for the new materials) and (ii) under identical strain (requiring higher voltages for the new materials) the newly designed composite materials exhibit superior humidity resistance in terms of failure rates and mean time to failure. Performances were tested in a standard Highly accelerated lifetime test (HALT). The samples were exposed to extreme humidity conditions (T=85 degrees of Celsius, RH=85 %, E=3 kV / mm) for approximately 12 days (t equal to 288 hours) in a Weiss 160 / 40 climate chamber (Noliac Motion) or in an environmental chamber at EPFL at slightly different conditions (T=75 degrees of Celsius, RH=75 %, E=1 kV/mm) for approximately 25 days (t equal to 600 hours) by recording the leakage current. The stroke, capacitance and loss factor were characterised before and after the HALT test (excluding the samples with complete failure) and a general degradation of the stroke values for both, the reference and the new material actuators was observed.
To conclude, the MLA based on the newly designed composite materials overshoot too the initially estimated improvement of 50 % in humidity resistance on account of about 20 % deterioration of the functional properties.
Manufacturing of ceramic materials
Raw materials
The raw materials for the preparation of materials were chosen with attention about quality. The key raw materials SrO2, type AES-1M by Ashine Canada (purity 98.1 %, d50 equal to 0.5 mm) and Nb2O5 type Ceramic grade by H.C. Starck Germany (purity 99.9 %, d50 equal 0.59 mm) were used. The tetragonal stabilised zirconium with 3mol % Y2O3 - TS3Y by TOSOH Japan was used in accordance with the JSI recipe.
The basic PST was prepared by mixed oxide technology using PbO (Penox, purity 99.8 %), TiO2 (Prechesa AV-01 SF, purity 99.24 %), SrO2 (ASHINE AES-1M, purity 98.1 %), Nb2O5 (H. C. Starck, ceramic grade, purity 99.9 %), BaCO3. Purity depends on amount of humidity in material and other parameters.
Prepared batches
Technology 130 (batch 130) was reference batch for the project. From the composition of reference batch were derived 2 new technologies called technology 2 and 3. Composition for technology 2 and 3 was called composite material. Difference between batches 130, 2, and 3 was in second milling and mixing with SrO2 stabilised with 3mol% Y2O3 TS3Y from TOSOH Japan. In batch 130 there is not any TOSOH zirconium oxide. In batch 2 was TOSOH zirconium oxide mixed with basic material on the start of second milling. In batch 3 was TOSOH zirconium oxide mixed with basic material at the end of second milling. These receipts were done according to recommendation from JSI in Ljubljana. All 3 batches were tested and samples were sent to universities joint in this program. According to test best performance of piezoelectric parameters are for batch 130. Best technology in testing of humidity resistance and mechanical properties was technology 2. Performance is not so big but humidity resistance and mechanical properties are better than for technology 130. All results from tests are in periodic reports prepared during the project. Best technology according to all tests for our purpose is technology 2.
Improving possibilities of technology 2
In technology 2 we are able to change a lot of parameters. Our aim is to increase performance of technology 2. For visibility of change we make only one change. All changes were done systematically. We could change chemical composition of whole material or change only content of minority elements. Next possible steps are in technology. We are able to change conditions of milling, temperature of sintering, polarisation conditions etc. Some of mentioned changes were done.
One possible way how to improve performance is changing of composition. We tried change of Ti / Sr ratio and changing of Ba content. All tests in this branch show that the best performance is for current composite material of technology 2. In changing of Ba content was one better performance (3 % of Ba) but change wasnt so big as we suppose. Ba decreases Curie temperature of ceramics. Each 1 % of Ba content decreases Curie temperature around 10 degrees of Celsius. And our aim is to have as high as possible Curie temperature. Curie temperature is important parameter of piezoceramic material. It shows us maximum work temperature of ceramics. When ceramics is heated over Curie temperature it loses its piezoelectric properties. All results show that changing of composition isnt good way and best performance is for current composite B2.
Next experiment was changing of milling conditions. Huge amount of samples was prepared. We tried milling in other time. Next part was milling with small milling beads. And third part was milling in other environments. All material was milled by 6 other times 4.5 min (0.15 kWh), 9 min (0.3 kWh), 13.5 min (0.45 kWh), 1 hour, 2 hour and 3 hours. Milling beads had dimensions 1,2 -1,4 mm. Rotates of mill during milling were 3 600 per minute. Rotates of pump were 200 per minute. Our next experiment was milling with normal milling beads (from 1.2 - 1.4 mm) and small milling beads from (0.3 - 0.4 mm). And our last experiment in this part was milling in other environments. We tried environment of (water from water pipe, distilled water, water with dispergating agent and pH5). All experiments were done in small laboratory mill Netsch Labstar. Results from all 3 experiments are next:
- Milling in other lengths of time: the best performance was for the shortest milling. This time is equivalent (milling energy per 1 kg of material) with milling in big mill Netsch LMK4.
- Milling with small and big milling beads: this experiment gave us information that milling with small milling beads is much better for better shrinkage and smaller particles. But smaller particles in our case mean smaller performance. But this information is worthy for us.
- Milling in other environments: this experiment show that milling in water from water pipe is not as good as milling in water with pH 5. Performance in all other two environments was not so big. But quite big increase of performance was in milling in pH5 environment.
Temperature dependence of performance
Measuring of temperature dependence is important property for characterisation of material. It describes behaviour of performance in range of temperatures. Mainly we measured frequencies, impedance, capacitance and loose factor, piezoelectric current coefficient (d33). Measuring station was consisting from temperature chamber Brabender TTA32/70N, impedance analyser HP 4194A and d33 analyser YE2730A from Sinocera. Theoretical range of temperature chamber is from -70 to +100 degrees of Celsius. But practical range of chamber is only from -40 to +80 degrees of Celsius. Properties of samples were measured according to regulation about measuring of piezoelectric ceramics (EN 50324).
We have measured samples of best composite (B2) in temperature chamber in the whole practical range of chamber. Dimensions of sample: Composite disc with diameter 12 mm and thickness 2 mm.
Repeating of results
Ability of repeating results is very important for mass production. Our prototype from technology 2 was marked as B2 (02 / 10). Copy of material from technology 2 was marked as B2 (01 / 11). Both batches were prepared by the same way. We tried possibility of our reproducibility. Samples from bath batches were send on analysis of XRF and RDA. All results were similar to each other.
The IDE process builds up the ceramic layers in the horizontal plane as for the conventional multilayer process. The main difference is however the positioning of internal electrodes. For the conventional process the electrodes are produced in the horizontal plane as the ceramic layers and as the direction of actuation is perpendicular to the electrodes then a very high stack is required. However, for the IDE process the electrodes are located in the vertical plane and so actuation is perpendicular to the ceramic layers - requiring only < 10 mm thick blocks to be made. Such thin blocks are much easier to produce and dice and could lead to a 30-50 % cost reduction for piezoelectric actuators.
In order to apply the IDE concept for piezoelectric multilayer actuators, it will be necessary to significantly reduce the width of electrode paths. To obtain actuators operated at 200-300 V it is required that the distance between the electrodes is 100 micrometre or less, which requires an electrode width on the order of 20 micrometre or less to achieve a high performance.
Current technology based on screen printing will allow 75 micrometre electrode lines and spaces, prototype prints can achieve 75 micrometre lines and spaces, while fine laboratory prints can go down to 50 micrometre lines and spaces. This is achieved by using meshes with greater thread counts per inch/cm
There are limitations with using screens in order to achieve fine lines and this is related to the fact that mesh is used to manufacture conventional screens. For screen meshes with high thread counts the amount of open area within the mesh which allows ink to flow through to the substrate being printed on to is significantly reduced, and can lead to reduced deposits. Maintaining print definition is difficult due to the fact that the ink must have a degree of flow, however this rheological property of the inks, also means that definition is lost and printed line widths increase.
It was an objective of the research to provide a technology and electrode materials for producing very thin lines of the order of 20 micrometre in multilayer ceramic actuators. New electrode ink formulations for fine line stencil printing had to be developed involving work with much denser metal powders and advanced polymer technology to produce fine line definition interdigitated structures, as well as looking at alternatives to conventional screen technology, to print these inks and eliminate the normal print faults that would be seen.
The alternative to using a conventional mesh screen is a stencil. The stencils were electroformed, so there were no obstructions within the print area to limit ink flowing drying the printing process. These were used for the initial trials, and found to give exceptional results, however they were fragile. In order to improve this, a support structure was electroplated in the aperture of the stencil to give a mesh like structure in the open areas of the stencil. This added a degree of robustness to the stencil which enabled the use of such stencils in a production environment. However due to his mesh structure being electroplated, the width is significantly smaller than would be found with a conventional mesh screen being in the order of approximately 5 -10 micrometre.
The second photomicrograph shows a platinum paste print using this mesh, with no breaks in the IDE's, showing good definition and continuity.
The third photomicrograph shows a cross section of a test actuator, in cross section, showing the positioning of the electrodes with in the ceramic body following firing. In these test pieces there were alignment issues which were later resolved.
In conclusion, the success criteria for this Work Package was the successful production of electrodes with less than 50µm width, which was achieved. Furthermore, electrodes with reduced widths were also achieved.
Development of narrow electrodes by wire technology
The aim of work package 6 has been to develop the technology required for realising an actuator based on the IDE design by using embedded solid metal electrodes.
The first task was to specify the requirements for the electrode performance and the geometry which would yield the best performance. Based on these specifications it was found that the initial concept for placing the solid metal electrodes using a wire bonding machine was not feasible. The total length of the electrodes required for building one actuator was too large, and a wire bonding machine would wear out after producing a couple of hundreds finished IDE actuators. Following an analysis of the requirements, the project partner IDS took over the task of developing a custom wire positioning equipment from H&K.
Based on the requirements for the electrode material and the geometry a thorough study was carried out by IPU on what materials could be used and what method should be used to manufacture the electrodes. Since the electrodes would be applied to the green PST material as a solid and not as a paste, a concept was to build up the solid electrode with different layers to reduce use of the very expensive standard electrode materials (platinum and palladium) and the associated manufacturing cost. A study was carried on different methods for manufacturing the electrodes with the desired size (25 micrometre) and 14 different possible manufacturing paths was analysed and described.
A critical review of the different manufacturing paths lead to the conclusion that only wire drawing would yield wires of the required diameter at a cost which would be realistic for mass production in the lengths (several 1 000 m) required for a large IDE actuator. A rig was build allowing for batch coating of wires in unbroken lengths of 20m and this setup was used to test more 19 different combinations of wire materials and coatings.
Tungsten wire plated with copper and platinum nickel wire plated with platinum Tungsten wire coated with silver and platinum
After an initial run in phase it was possible to batch coat 20 m of wire with a good result, for any combination of layers of copper, silver, palladium and platinum, using tungsten or nickel as base material.
19 different combinations of base materials and coatings was manufactured and sintered embedded in PST material at 1 300 degrees of Celsius in an oxygen rich atmosphere. Of these only tests with electrodes made from pure platinum and palladium showed no visible degradation of the PST, and even the pure palladium electrode showed a led content of 15 % in the centre of the electrode, corresponding to the solubility of lead in palladium at 1 300 degrees of Celsius. One of the initial problems are shown, where stress induced cracking in a platinum coating is clearly seen, these challenges were ovecome and the final optimised coating with no defects is also shown. A post sintering SEM image of a tungsten electrode coated with silver and platinum is shown, where it clearly shows the tungsten has migrated into the PST material, leaving only the coating layers in place.
The conclusion on the work on the electrode material and processing was the only pure platinum was unaffected by the sintering process. To avoid delays in the project it was decided to use pure platinum wire for the future work with the build-up of the IDE actuator, to avoid any potential negative influences by palladium.
Following the changes in the original project plan, IDS were responsible for designing the equipment required for positioning the electrodes on the green PST substrate. Based on the experiences from the setup used for batch coating of test electrodes, it was decided to use a concept in which the wires were mounted on 100 mm x 100 mm frames and the frames were then stacked together with the green PST substrates to make the final IDE actuator. An innovative concept was developed on which four frames were mounted on a spinning cube and then electrode wires were wound onto this cube using controlled pre-tensioning. The wires were mounted with a pitch of 180 micrometre resulting in the mounting of 550 wires on one 100 mm wide frame. A schematic of the winding concept is shown together with a close up of the thread machined on the sides of the cube, to ensure correct poisoning of the wires.
The competed electrode wire mounting device is shown together with a selection of finished frames and a close up of the wires mounted on one of the frames.
The final part of the WP involved manufacturing the PST actuator itself. The first process was stacking the frames with the electrode wires together with layers of green PST substrate and compressing the final sandwich to remove any voids. This was followed by a trimming operation in which the frames were removed, leaving the metal electrodes embedded in a PST matrix in the correct locations. A normal PST sintering process were then carried out followed by a glass deposition process, in which the end of every second electrode wire was insulated. Electrical connection was created by applying silver paste to the non-insulated ends. After dividing the sample into two identical parts, the electrical connections could be finished, yielding two IDE actuators for each assembly process.
For the build-up of IDE actuators utilising solid wires as internal electrodes a novel manufacture method has been developed.
Despite of the wire electrode approach being a 'wild card' solution the HIPER-ACT project has proven that it is possible to manufacture an IDE actuator based on solid metal wires.
Development of actuators
Throughout the project, WP7 has addressed the reliability on several different levels ranging from material benchmarking tests on a laboratory level, over end user tests on sub assembly level to failure analysis on an entire piezo pump system.
As the laboratory tests serves to map the reliability and performance over time in several predetermined conditions the end user tests will address the achieved reliability under conditions similar to actual use.
Laboratory tests, performed on standard multilayer (ML) actuators, revealed during material development phase that the composite developed 20 % less strain than the reference material under identical applied fields. From literature an increase in strain has been proven to be linked to a decrease in mean time to failure (MTTF). To exclude the strain difference as reason for any difference observed in humidity resistance, a series of tests under extreme conditions (85 % relative humidity / 85 degrees of Celsius) were conducted under different applied stresses.
Each point represents the average MTTF of a sample batch of at least 20 MLA's at a given strain, as shown. Readily observable is the fact that even when the reference samples are developing less strain than the composite counter parts the mean time to failure of the reference samples is not exceeding the MTTF for the composites. This test cannot exclude the risk of strain enhanced failure, but from this test it can be concluded that at identical strain the composite material show a higher tolerance towards the extreme environment, than the reference material.
Further thorough testing performed on MLA samples also revealed the same strong potential benefit of the developed composite material as shown above.
From long term fatigue testing in ambient conditions it was found that the changes in performance of the composites were very similar to the changes seen for the reference material, which also is an important result in the pursuit of launching the composite material as a high humidity alternative.
Sub-system tests
For comparison of the reliability on sub-system level wire bonding tools from WP9 were mounted with reference and composite samples respectively.
Under normal operation conditions no change in performance was seen for either of the tested samples. To reduce the test time the temperature and humidity were gradually increased. After 1 011 cycles one of the reference samples failed which can be seen, whereas the composite samples are unaffected by the conditions.
Due to the small quantity of tested samples it cannot be fully concluded that the composite is better in terms of reliability, but the results are in thread with the findings on laboratory level.
Early generation IDE samples were also tested, but due to the early stage of development these samples did not perform to their full potential. Improved IDE actuators with increased functionality have been manufactured, but are yet to be tested.
In depth knowledge about the entire system and component interactions is key to a successful development of a fully functional and reliable product. Using a Failure modes and effects analysis (FMEA) gives detailed insight to strengths but especially weaknesses of a system. Obtaining the necessary level of information is labour intensive and time consuming, but can quickly be worth the efforts.
Partners from Fraunhofer LBF and WP11 used such a FMEA tool on the entire piezo pump system developed in WP11. From the analysis a few focus areas were identified as:
- positioning and preloading of the piezo stack;
- reliability of piezo actuator;
- thermal properties of the involved materials and self-heating of the piezo actuator during operation;
- corrosion resistance of the involved parts as the liquid to be pumped is very aggressive.
From the developed prototype the functionality can be readily assessed, but knowing the entities listed above will significantly ease the process of maturing the product for the market application.
Industrial applications I: Wind turbines - Active trailing edge
Introduction
The chapter describes the work carried out in WPs 8A,8B concerning industrial application of the piezo electric actuators in active trailing edge flaps for wind turbines. In the WP a trailing edge flap dedicated for wind turbine blades has been designed and tested. Originally it was the intention to use piezo electric benders like the Thunder TH-6R from Face International, where the intention was to attach them directly to an airfoil trailing edge and test them in a wind tunnel. The requirements to such actuators were described in the deliverable from WP8A1, where the objective was to provide specifications for the IDE actuators for active trailing edge. However, it turned out that the bender-type of actuator could not deliver the needed forces, so an actuator designed for application on helicopters was used instead. Since the operational principle of a trailing edge flap using this type of actuator is fundamentally different form the piezo electric benders, an additional task in WP8 was to design a mechanism, which could transfer the movement from the alternative actuator with significantly bigger forces and small deflections to a rigid flap attached to the trailing edge of an airfoil. During the summer and autumn 2011 the mechanism was designed and it turned out that at least three actuators were needed to drive the flap mechanism on an airfoil mounted in the LM Wind Power wind tunnel, where there were advance commitments for wind tunnel tests. End February/start March 2012 the actuators were delivered from Noliac to DTU Wind Energy. At that time DTU Wind Energy tried to make a reservation for tests in the LM Wind Power wind tunnel, but it was not possible to find testing possibilities before 1 September 2012, where the project ended. Investigating the possibilities in other tunnels, such as Stuttgart Laminar Wind Kanal, Germany and the Delft Low-Speed Low-Turbulence wind tunnel, Holland, there were no possibilities for tests at such short notice. Therefore, it was decided to build the flap mechanism anyway, but test it in the laboratory with the forces on the flap using springs instead of aerodynamic forces. Thus, the deliverables in WPs 8A.2,8B.1 were changed from creating a 'prototype airfoil section with active trailing edge' and carrying out a 'full scale performance test of airfoil in wind tunnel', respectively, to 'design of a trailing edge flap actuation system using the delivered actuators', 'design of a suitable test setup to emulate the key features of the aerodynamic forces' and 'test of the new setup'.
Design and construction of the flap mechanism
Based on the actuator delivered by Noliac and the corresponding forces and deflections a flap mechanism was designed. It was decided to carry out a simple design because of a requirement for wind turbines of high reliability. The dimensions of the flap originate from the original planning which included wind tunnel tests. According to that, the length of the flap should be 1350 mm, the chord 90 mm, and there should be three actuators placed along the length. Therefore the test flap was designed as shown, but with a length of 450 mm and only one actuator. Based on this design the flap was constructed as shown. The constructed flap was tested and the flap mechanism was functioning as expected with low losses. In both downwards and upwards direction the performance is rather good according to the predictions, however, with only a deflection of between 0.95 deg and 1.3 deg. Relating to the specifications established in WP8A at least +/-10mm deflection is needed. This corresponds to +/- 6 deg. According to the design considerations, this could be obtained by selecting another gearing in the flap mechanism or more power from the piezo ceramic elements, or both.
Simulated performance of the flap mechanism on a wind turbine
Simulations of trailing edge flap with maximum deflections of 1 and 5 deg and blade pitch of 1 deg showed that the stated 6 deg in the specifications of the actuator is a minimum. However, it also showed that if such a deflection could be obtained, significant reductions in the standard deviations of the loads could be obtained.
Conclusions
A demonstrator of a flap mechanism based on piezo ceramic materials was designed, tested and evaluated. Based on the measurements in the laboratory it was concluded that the flap mechanism was functioning as expected with low losses. In both downwards and upwards direction the performance is rather good according to the predictions. The design considerations showed that the deflection of the unloaded flap needs to be significantly larger than the maximum deflection needed in loaded conditions. As stated in the specifications for the design of the flap mechanism at least +/- 6 deg was needed. According to the measurements this was not obtained, but the extra deflection could be obtained by changing gearing in the flap mechanism and/or using more piezo ceramic elements per span length of the blade. Concerning the future work within this flap mechanism it is needed to mature the technology and the following list include some of the issues and challenges that could be considered:
- the flap mechanism should be very simple overcoming in the order of 3 billion cycles;
- the flap mechanism including the piezo ceramic elements should be proven to withstand humidity and lightning;
- the flap mechanism needs high power, with a corresponding increase in weight, thus, the weight of the flap mechanism should be low;
- the total cost of the flap mechanism should be relatively low.
Industrial applications II: Wire bonding machine - RTD
Introduction
Ultrasonic wire bonding is an established technology to connect the electrodes of microelectronic devices as well as power electronic modules. Typically an aluminium wire connects the electrodes of a microchip or a power semiconductor with the corresponding electrodes of a substrate. The bonding of the wire to the electrode surface is made by an ultrasonic friction welding process. The very high requirements concerning the quality und the reliability of the electric connections ask for very good control of the process. Additionally increasing miniaturisation and machine speed ask for even better control of the bonding process. Another challenge is bonding on crucial surfaces, like on pads with soft underlayer, slim pins or overhanging stacked chips.
Objective
The ultrasonic transducer is a key component of a wire bonding machine. It generates the ultrasonic vibrations for the bonding process and is driven in a longitudinal vibration mode. In general, all vibrations not in line with the main direction of the ultrasonic bonding process are not desired and can disturb the process. Therefore at the beginning of the project extensive vibration analysis have been conducted by spatial laser interferometer and accelerometer measurements on different points of the machine. It turned out, that the initially addressed 'low frequency' structural vibrations induced by the machine dynamics during operation movement are not as critical as the ultrasonic vibrations caused by eigen frequencies of the so called ultrasonic transducer. This is because of the special soft design of the suspension of the transducer unit, effectively decoupling the vibrations from machine side. Generally, beside the main longitudinal eigenmode additional parasitic vibration modes exist. These eigenmodes, e.g. bending modes of the transducer, as well as not perfectly symmetric longitudinal modes can lead to fluctuating normal forces in the friction contact and a disturbance of the bonding process. Therefore the objective was slightly adapted to focus on the ultrasonic vibration damping, without major effects to the work and cost plan.
The aim of this project is to suppress or at least reduce these unwanted vibrations during operation by the use of special control actuators and a proper vibration control strategy.
Approach
A novel prototype transducer is developed, which is capable of suppressing these unwanted vibrations using additional piezoelectric actuators. It is desired not to influence the main longitudinal vibrations, but only acting on the orthogonal vibrations. This is achieved by applying two additional piezoelectric control actuators on top and bottom side of the transducer body. The polarisation of the top control actuator is chosen in such a way that in free ideal longitudinal vibration mode the electrical charges on the electrodes of the top and the bottom control actuator cancel out each other and the control actuators act neutral. In contrary applying a voltage to these actuators induce a pure bending or vertical movement, respectively. So an optimal coupling is achieved for the parasitic bending mode.
Deriving Specification for control actuators
Within the first part of the work programme of this project two tasks have to be performed. The detailed specification for the control actuators has to be determined and a proof of concept demonstrator has to be built. The proof of concept demonstrator includes the evaluation of different vibration damping techniques - passive and active.
To fulfil these requirements a suitable model is essential for evaluation. Therefore the ultrasonic transducer has been described by a finite-element model, which can be used to calculate the dynamic vibration behaviour of the transducer. By aid of this model amongst other issues the optimal placement of the additional piezoelectric actuators have been determined. The FE-model has been validated by frequency response measurements with different contact conditions of the wedge. In particular, the friction contact between the wedge tip and the bond pad has been modelled. Additionally a modal reduction of the finite element model to the two most relevant eigenmodes. The system is then described by two degrees of freedom (2 DOF model), which can describe longitudinal and bending motions. This model is essential to design the active vibration control.
Proof of concept demonstrator for active vibration control
The proof-of-concept demonstrator transducer has been built according to the finite element model with optimal placement of the control actuators. Because the newly developed HiPer IDE actuators have not been available at this stage of the project, the first prototypes were built with conventional piezoceramics. Once the design study has been finished, a prototype transducer has been set up with a first IDE sample.
Different vibration control techniques are studied in this project. A piezoelectric shunt damping technique and an active open-loop control. For the first case, a passive inductance-resistance (LR)-circuit is connected to the electrodes of the piezoceramics, in simulations as well as on the prototype. This passive damping technique doesnt require external power and is guaranteed stable. For proper operation the network parameters must be tuned precisely. Measurements of the free vibrations and the frequency response showed an increase in damping of the parasitic vibration modes of more than 10 times. However, a total suppression is not possible with this technique. Further on, an active open-loop control has been set up. The required voltage amplification and phase shift compared to the driving actuators, which cancel out vertical vibrations at the wedge tip, have been calculated using the reduced 2 DOF model of the transducer, and again validated by measurements. In experiments, the active control was capable to totally suppress the parasitic vibrations. Further studies with pure friction contact at the tool tip show that the active control again can reduce the basic harmonic completely, but higher harmonic vibration content of quite low amplitude remain. Results prove that for both techniques the piezoelectric coupling is the most important parameter for the mechanical design. Especially the new IDE piezo type is capable to increase this coupling because of the special electrode design.
Within the project, the standard piezo-actuators have been replaced by an actuator from new NCE 51 B2-2 bulk material as well as a first sample of an IDE actuator. The same measurements have been performed and their performance is compared to the standard actuator. The measurements showed that the new HiperAct material causes only a slightly lower coupling factor. The working ability of the new IDE actuator was successfully shown; however the coupling factor of the first prototype was much lower than calculated due to not fully functional inner electrodes of the first prototype. A future series actuator with full performance will show much higher coupling.
Full scale performance test
In the second part of the project a full scale performance tests was to be done. Therefore a bonding machine was equipped with a two channel ultrasonic driving system and two power amplifiers, as well as a special software to control both channels. The full scale performance test proved that the proposed shunt damping technique and the active vibration control are capable to improve the vibration behaviour during bonding significantly, especially in difficult circumstances e.g. on soft bonding surfaces. Many bonding tests have been conducted and all relevant signals have been measured. It was found that both techniques do reduce the unwanted vertical vibrations significantly, if tuned correctly. A further promising observation was a reduction in the natural fluctuations of the horizontal vibrations during bonding. This means the standard deviation of the measured signals was decreased.
The passive shunt damping technique with a tuned LR-network shows a significant damping effect of about 50 % reduction, but frequency bandwidth is quite small. So the correct inductance value must be met quite accurate. Possible frequency changes during operation due to temperature and power depending drifts have to be considered. The advantages of the passive shunt damping technique are the simplicity and guaranteed stability. No additional hard- or software is necessary. But the lower damping performance and the sensitivity towards parameter deviations are limiting its application.
The very profound advantage of the active feed forward vibration control strategy is the possibility to realise an almost complete suppression of orthogonal vibrations at any desired operating frequency. But for this considerably more control effort and supplying high electrical voltages to the control actuators is necessary. It was found that because of the non-linear time-varying bonding process the optimal control settings change over time. Therefore a vertical vibration sensor supported closed loop control is a promising future project.
Concluding the passive vibration damping system can be incorporated with low effort into a serial machine. For application willing to spend more effort in the vibration damping system to reach optimal performance even the active vibration control is reasonable to implement in future machines.
Industrial applications III: Active engine mount - RTD
Final performance analysis of the active engine mount with different error sensors (acceleration at chassis side of the engine mount and sound pressure at drivers head position) were taken in the Adaptive Structures Test Facility (ASF) at the Fraunhofer Institute LBF. The vehicle is mounted on a stationary rig; the driven wheels each attached to a speed controlled asynchronous motor. The measurement schedule consists of acceleration and sound pressure measurements during run-ups from 1 000 to 4 500 rpm under different throttle settings in second gear (10 - 50 %). The engine speed was set using the asynchronous motors, thereby determining the engine load during the run-up. For comparison, measurements were taken for the serial mount, for the uncontrolled active mount and the controlled active mount as well. Accelerations were measured at the chassis side of the engine mount; the sound pressure was measured in the passenger cabin at head position. The acceleration and sound pressure signals were sampled at 2048 Hz and collected by an LMS data acquisition system.
Several speed-controlled run-ups were performed within 60 seconds from 1 500 to 4 200 rpm and an accelerator pedal position of 30 % of maximum to optimise convergence behaviour of the controller. During these measurements the adaptive controller reduces the accelerations at the mounting points by means of the previously introduced control concept.
The final results of the next setup are shown. Here the second order sound pressure at drivers ear during a complete run-up for the serial mount, the uncontrolled active engine mount and the controlled active engine mount (whereas the controller uses the sound pressure at drivers ear to adapt its weights) is depicted. A significant reduction of the sound pressure amplitude up to 20 dB can be observed between the uncontrolled and controlled active engine mount. Compared to the serial engine mount, amplitude reductions also of 20 dB can be observed. The sound pressure reduction is slightly more efficient in some rpm regions compared to the previous setup.
However, in this case the second order mounting point acceleration is increased in some rpm regions. This can be explained because only one transfer path is controlled. The controller optimises its weights in order to cancel the error sensor signal completely. Therefore the controller try to compensate all other transfer path's with (one) active engine mount. To realise that objective obviously higher accelerations at the engine mount position are necessary.
