Final Report Summary - HITS (High temperature sensors)
The automotive, aerospace, and power generating industries require piezoelectric sensors that can operate at high temperatures. The maximum operating temperature of piezoelectrics is limited by their Curie point and high temperature piezoelectric stability.
(i) Materials and characterisation - Bismuth layer-structured ferroelectrics (BLFS)
Bi2WO6 is the simplest member (n=1) of the BLSFs family, and has been considered as a high temperature piezoelectric materials because of its high Curie point (above 900 degrees Celsius) and the high piezoelectric coefficients of single crystals (d33~40 pC/N). Bi2WO6 single crystals do not exhibit good saturated P-E hysteresis loops because of a high leakage current. To our knowledge, no piezoelectric coefficient and saturated P-E hysteresis loops have been reported for Bi2WO6 ceramics.
Single phase bismuth tungstate (Bi2WO6) ceramics with high relative density (> 99 %) were fabricated by spark plasma sintering (SPS). Ferroelectric, dielectric and piezoelectric properties of Bi2WO6 ceramics were investigated. Almost saturated polarisation-electric field (P-E) hysteresis loops with a remnant polarisation (Pr) of ~16.1 degrees Celsius / cm2 and a coercive field (Ec) of 3.7 kV/mm were obtained. Curie point and second phase transition temperatures were observed at 937 degrees Celsius and 665 degrees Celsius, respectively. The average piezoelectric constant (d33) of this high Curie point ceramic is 15.0 pCN-1.
(ii) Materials and characterisation - B site doping
A2B2O7-type ferroelectrics belongs to the perovskite-like layered structure (PLS) family, which may therefore be good candidates for high-temperature piezoelectric and microwave dielectric applications because of their super-high Tc and low loss, respectively.
PLS ferroelectrics ceramics have high coercive fields, which restrict the ability to pole them and, therefore, achieve their optimum piezoelectric properties. Just as for perovskite structured ferroelectrics, the addition of donor dopants reduces their coercive field. The Nd2Ti1.95Nb0.05O7 and Nd2Ti1.9Nb0.1O7 were fabricated to investigate the effect of Nd-doping on Nd2Ti2O7.
( iii) High temperature properties - thermal depoling and ageing
Point defects formed by vacancies or substituted ions tend to interact with the domain structure, which produce aging effects and inhibit the movement of domain walls at low temperature. However, there is very limited information on the effect of point defects on the thermal depoling of ferroelectrics.
Effect of point defects on the thermal depoling behaviour of bismuth layer-structured ferroelectric Bi2WO6 (BW) and Sr2Bi4Ti5O18 (SBT) ceramics was investigated. Point defects in BW ceramics formed defect dipoles that interacted with the ferroelectric domain structure. These defect dipoles produced pinched P-E hysteresis loops and an irreversible reduction in d33 after annealing below 200 degrees Celsius. They became decoupled and randomised above 200 degrees Celsius, and the d33 of BW became stable with increasing temperature from 200 degrees Celsius up to its Curie point. SBT ceramics with low defect concentration showed symmetric P-E hysteresis loops and good piezoelectric stability with increasing temperature.
(iv) Spark plasma sintering-texturing ceramics
Spark plasma sintering (SPS) involves rapid heating (up to 500 degrees Celsius / min) combined with pressure (up to 100 MPa) during sintering. One of the advantages of the equipment is that it allows you to accurately control the temperature-pressure schedule to achieve highly textured ceramics in Aurivillius ceramics by a processing of dynamic forging.
Two step method was used to fabricate high textured ceramics by SPS. In order to understand the texture mechanisms during second step, creep behaviour of ferroelectric ceramics was investigated using SPS. Exponent stress of ferroelectric ceramics at 1300 degrees Celsius, 1350 degrees Celsius and 1375 degrees Celsius are all above 1.8 which indicated that the creep of ferroelectric ceramics during second step is not only dominated by diffusion.
(i) Materials and characterisation - Bismuth layer-structured ferroelectrics (BLFS)
Bi2WO6 is the simplest member (n=1) of the BLSFs family, and has been considered as a high temperature piezoelectric materials because of its high Curie point (above 900 degrees Celsius) and the high piezoelectric coefficients of single crystals (d33~40 pC/N). Bi2WO6 single crystals do not exhibit good saturated P-E hysteresis loops because of a high leakage current. To our knowledge, no piezoelectric coefficient and saturated P-E hysteresis loops have been reported for Bi2WO6 ceramics.
Single phase bismuth tungstate (Bi2WO6) ceramics with high relative density (> 99 %) were fabricated by spark plasma sintering (SPS). Ferroelectric, dielectric and piezoelectric properties of Bi2WO6 ceramics were investigated. Almost saturated polarisation-electric field (P-E) hysteresis loops with a remnant polarisation (Pr) of ~16.1 degrees Celsius / cm2 and a coercive field (Ec) of 3.7 kV/mm were obtained. Curie point and second phase transition temperatures were observed at 937 degrees Celsius and 665 degrees Celsius, respectively. The average piezoelectric constant (d33) of this high Curie point ceramic is 15.0 pCN-1.
(ii) Materials and characterisation - B site doping
A2B2O7-type ferroelectrics belongs to the perovskite-like layered structure (PLS) family, which may therefore be good candidates for high-temperature piezoelectric and microwave dielectric applications because of their super-high Tc and low loss, respectively.
PLS ferroelectrics ceramics have high coercive fields, which restrict the ability to pole them and, therefore, achieve their optimum piezoelectric properties. Just as for perovskite structured ferroelectrics, the addition of donor dopants reduces their coercive field. The Nd2Ti1.95Nb0.05O7 and Nd2Ti1.9Nb0.1O7 were fabricated to investigate the effect of Nd-doping on Nd2Ti2O7.
( iii) High temperature properties - thermal depoling and ageing
Point defects formed by vacancies or substituted ions tend to interact with the domain structure, which produce aging effects and inhibit the movement of domain walls at low temperature. However, there is very limited information on the effect of point defects on the thermal depoling of ferroelectrics.
Effect of point defects on the thermal depoling behaviour of bismuth layer-structured ferroelectric Bi2WO6 (BW) and Sr2Bi4Ti5O18 (SBT) ceramics was investigated. Point defects in BW ceramics formed defect dipoles that interacted with the ferroelectric domain structure. These defect dipoles produced pinched P-E hysteresis loops and an irreversible reduction in d33 after annealing below 200 degrees Celsius. They became decoupled and randomised above 200 degrees Celsius, and the d33 of BW became stable with increasing temperature from 200 degrees Celsius up to its Curie point. SBT ceramics with low defect concentration showed symmetric P-E hysteresis loops and good piezoelectric stability with increasing temperature.
(iv) Spark plasma sintering-texturing ceramics
Spark plasma sintering (SPS) involves rapid heating (up to 500 degrees Celsius / min) combined with pressure (up to 100 MPa) during sintering. One of the advantages of the equipment is that it allows you to accurately control the temperature-pressure schedule to achieve highly textured ceramics in Aurivillius ceramics by a processing of dynamic forging.
Two step method was used to fabricate high textured ceramics by SPS. In order to understand the texture mechanisms during second step, creep behaviour of ferroelectric ceramics was investigated using SPS. Exponent stress of ferroelectric ceramics at 1300 degrees Celsius, 1350 degrees Celsius and 1375 degrees Celsius are all above 1.8 which indicated that the creep of ferroelectric ceramics during second step is not only dominated by diffusion.