Final Report Summary - BISM (Bio-inspired structural materials)
One of the major scientific challenges for the 21st century is the development of new stronger and tougher lightweight structural materials to support advances in strategic fields as diverse as building, transportation, energy or healthcare. Applications such as high performance bearings, turbocharged rotors, fuel igniters, or the next power generation turbines demand structures capable to provide exceptional mechanical performance at high temperatures and in corrosive environments. Similarly, orthopaedic implants that are safer, less bulky and longer lasting will have a tremendous human and economic impact. In these applications, like in many others, current materials are approaching their performance limits. The superalloys used at high temperature are reaching their ceiling (~1150oC) and the performance of current metallic implants is hampered by wear and mismatch in mechanical properties with tissue. The goal of this project is to develop a new family of hybrid, high-performance and light-weight structural materials by combining novel processing approaches with structural and mechanical characterization.
The project has been based on the development of freeze casting (also known as ice templating) as a technique for the fabrication of porous ceramic scaffolds with complex architectures. These scaffolds can be subsequently filled with a second “softer” phase (e.g. a metal or a polymer) to fabricate composites whose structure resembles to some extent that of nacre, a natural composite that exhibit fracture toughness that are orders of magnitude greater than either of its constituent phases. Freeze casting is based on the directional freezing of ceramic suspensions. As it grows, the ice expels the ceramic particles. Subsequently the water is sublimated to form a porous ceramic. The porosity is the replica of the ice crystals and by controlling the composition of the suspension and the freezing conditions it is possible to create layered scaffolds with controlled pore and layer thickness (from 5 to 60 µm). In this project we have built a freeze casting set-up and have studied systematically the freezing of ceramic suspensions to determine the best conditions for the fabrication of scaffolds. Our work has focused on aluminium oxide as a model ceramic although studies have also been performed with silicon carbide.
The second step is the fabrication of composites through the infiltration of the scaffolds with a second phase. For example, we have developed a process to form carbon/ceramic materials by the infiltration of pitch solutions or by the in situ growth of carbon nanofibres inside the porous ceramics. In this respect, we have established the best conditions for growth (temperature, atmosphere and scaffold pore size). The carbon infiltrated materials are then consolidated by spark plasma sintering to create composites in which ceramic blocks are separated by thin carbon films. In parallel, we have developed other approaches to infiltrate the scaffold with polymer or metals and create dense composites. Mechanical characterization has shown that these materials can exhibit very high fractures resistances (up to 10 times that of the ceramic in terms of MPa.m1/2) that result from the activation of multiple extrinsic toughening mechanisms. These results have opened the opportunity for the fabrication novel structural ceramic-based composites and point directions for future research in the development of the materials.
In parallel we have extended the freeze casting process towards the fabrication of highly porous graphene-based cellular networks. The process allows the manipulation of the structure at multiple levels from the densities (between 1 and 200 mg/cm3), cell size (from 5 to 70 µm) and shape (lamellar to isotropic). As a result it is possible to tune properties like, surface area (up to 400 m2/g), elasticity, specific strength, energy loss coefficient and conductivity. This opens up new opportunities to explore applications in numerous fields like in energy damping, compression tolerant supercapacitors or catalyzers or any application where separation, absorption or filtration is required. We have also extended the templating approach to the use of soft-templates, in particular emulsions, to create complex structures. In this respect, we have created responsive ceramic particles that can assemble and disassemble in an emulsion as a response to an external stimulus. The process can be used to form ceramic scaffolds that can be strong (up to hundreds of MPa in compression), stiff and highly porous. The structure can be manipulated to control pore size (from less than ten to hundreds of microns) and interconnectivity.
These results open new paths towards the large scale fabrication of complex and lightweight porous structures and composites with a wide range of materials. The combination of material selection and structural control results in unique properties that will create new opportunities in a wide range of applications, from light weight structural composites, filters, interconnected scaffolds or ceramic coatings with graded porosity for tissue engineering, bulk thermal shock-resistant structures, thermal barrier ceramic coatings or temperature control membranes for auto thermal reforming, to mention a few. The work has also provided new fundamental information on the behavior of colloidal suspensions and emulsions, the growth of carbon nanofibers, or the parameters that control the mechanical response of complex hierarchical structures. The results will therefore be of importance for materials scientists and engineers, particularly those working on materials manufacturing and the ceramic industry.
The project has been based on the development of freeze casting (also known as ice templating) as a technique for the fabrication of porous ceramic scaffolds with complex architectures. These scaffolds can be subsequently filled with a second “softer” phase (e.g. a metal or a polymer) to fabricate composites whose structure resembles to some extent that of nacre, a natural composite that exhibit fracture toughness that are orders of magnitude greater than either of its constituent phases. Freeze casting is based on the directional freezing of ceramic suspensions. As it grows, the ice expels the ceramic particles. Subsequently the water is sublimated to form a porous ceramic. The porosity is the replica of the ice crystals and by controlling the composition of the suspension and the freezing conditions it is possible to create layered scaffolds with controlled pore and layer thickness (from 5 to 60 µm). In this project we have built a freeze casting set-up and have studied systematically the freezing of ceramic suspensions to determine the best conditions for the fabrication of scaffolds. Our work has focused on aluminium oxide as a model ceramic although studies have also been performed with silicon carbide.
The second step is the fabrication of composites through the infiltration of the scaffolds with a second phase. For example, we have developed a process to form carbon/ceramic materials by the infiltration of pitch solutions or by the in situ growth of carbon nanofibres inside the porous ceramics. In this respect, we have established the best conditions for growth (temperature, atmosphere and scaffold pore size). The carbon infiltrated materials are then consolidated by spark plasma sintering to create composites in which ceramic blocks are separated by thin carbon films. In parallel, we have developed other approaches to infiltrate the scaffold with polymer or metals and create dense composites. Mechanical characterization has shown that these materials can exhibit very high fractures resistances (up to 10 times that of the ceramic in terms of MPa.m1/2) that result from the activation of multiple extrinsic toughening mechanisms. These results have opened the opportunity for the fabrication novel structural ceramic-based composites and point directions for future research in the development of the materials.
In parallel we have extended the freeze casting process towards the fabrication of highly porous graphene-based cellular networks. The process allows the manipulation of the structure at multiple levels from the densities (between 1 and 200 mg/cm3), cell size (from 5 to 70 µm) and shape (lamellar to isotropic). As a result it is possible to tune properties like, surface area (up to 400 m2/g), elasticity, specific strength, energy loss coefficient and conductivity. This opens up new opportunities to explore applications in numerous fields like in energy damping, compression tolerant supercapacitors or catalyzers or any application where separation, absorption or filtration is required. We have also extended the templating approach to the use of soft-templates, in particular emulsions, to create complex structures. In this respect, we have created responsive ceramic particles that can assemble and disassemble in an emulsion as a response to an external stimulus. The process can be used to form ceramic scaffolds that can be strong (up to hundreds of MPa in compression), stiff and highly porous. The structure can be manipulated to control pore size (from less than ten to hundreds of microns) and interconnectivity.
These results open new paths towards the large scale fabrication of complex and lightweight porous structures and composites with a wide range of materials. The combination of material selection and structural control results in unique properties that will create new opportunities in a wide range of applications, from light weight structural composites, filters, interconnected scaffolds or ceramic coatings with graded porosity for tissue engineering, bulk thermal shock-resistant structures, thermal barrier ceramic coatings or temperature control membranes for auto thermal reforming, to mention a few. The work has also provided new fundamental information on the behavior of colloidal suspensions and emulsions, the growth of carbon nanofibers, or the parameters that control the mechanical response of complex hierarchical structures. The results will therefore be of importance for materials scientists and engineers, particularly those working on materials manufacturing and the ceramic industry.