| Bismuth layer-structured piezoelectric ceramics had a wide application prospect in the high-temperature devices field, owing to their high Curie temperature (Tc), low temperature coefficients of dielectric (εr), low dielectric loss (tanδ), low aging rate, strong anisotropic electromechanical coupling factors (Qm) and good high temperature frequency stability. However, the piezoelectric property of bismuth layer-structured piezoelectric ceramics is low because the rotation of the spontaneous polarization is restricted to the a-b plane. In this thesis, we engaged in the improvement of the piezoelectric, dielectric and ferroelectric properties without producing a large decrease in their high Tc. The systems Bi4Ti3O12, Na0.25K0.25Bi2.5Nb2O9 and Na0.5Bi2.5Nb2O9 were chosen in the thesis due to their high Tc (-675 ℃,650 ℃ and 780 ℃ respectively). To obtain their more excellent performance with relatively high Tc and to improve their practical value, co-doped bismuth layer-structured piezoelectric ceramics were fabricated by the conventional solid-state reaction method.Fistly, bismuth layer-structured high temperature piezoelectric ceramics of Bi4Ti2.92Nb0.08O12.04-0.5xwt%CeO2-0.5xwt%SrCO3 (x= 0.0-1.5) were fabricated by solid-state reaction method. The effects of Ce and Sr co-doping on the microstructure and electrical properties of the ceramics were investigated. The results manifested that all the ceramic samples had a single phase of bismuth oxide layered structure with m= 3. The piezoelectric activity and ferroelectric properties of the ceramics was found significantly enhanced by the modification of Ce and Sr. The ceramic exhibited the excellent electrical properties with the doped content x= 0.9:d33= 29 pC/N, kp= 8.77%, tanδ= 0.13%, Pr= 15.87 μC/cm2 and Tc= 631 ℃. In addition, the d33 of the component ceramic sample still remained over 20 pC/N after annealing at 500 ℃, which demonstrated that the ceramic possessed excellent thermal stability.Secondly, the A-site and B-site of Na0.25K0.25Bi2.sNb2O9 ceramics were co-doped by Ce and Cr. Bismuth layer-structured high temperature piezoelectric ceramics of Na0.25K0.2sBi2.5Nb2O9-0.4wt%Cr203-xwt%CeO2(x= 0.00-1.00) were fabricated. The effects of Ce addition on the microstructure and electrical properties of the ceramics were investigated. It is found that all the ceramic samples had a single phase of bismuth oxide layered structure with m= 2. The size of grain becomed small and the density got improved by an appropriate amount of Ce addition. After Ce doping, the piezoelectric, ferroelectric and mechanical properties of the ceramics were significantly improved, and the dielectric loss decreased. The ceramic exhibited the optimum electrical properties with the doping content x= 0.50:d33= 27 pC/N, tanδ= 0.09%, kp= 7.97%, Qm= 2637, Tc= 656℃, Ec= 46 kV/cm and Pr= 4.4 μC/cm2.Lastly, the A-site of Nao.25Ko.2sBi2.5Nb209 ceramics were co-doped by Nd and Ba. Bismuth layer-structured high temperature piezoelectric ceramics of Ba0.09(Na0.5Bi0.5)0.9iBi2Nb2O9-xwt%Nd2O3 (x= 0.0-2.0) were fabricated. The influence of Nd addition on the microstructure and electrical properties of the ceramics were investigated. It was found that the ceramic samples had a single phase of bismuth oxide layered structure with m-2. The electrical conductivity and dielectric loss decreased after Nd doping. The piezoelectric, ferroelectric and mechanical properties of the ceramics were significantly enhanced by an appropriate amount of Nd addition. The ceramic exhibited the excellent properties with the doping content x= 1.0:d33-23 pC/N, tanδ= 0.050%, Qm=2095, kp= 5.46%, Tc= 713 ℃, Pr= 8.2 μC/cm2 and Ec= 43.5 kV/cm, which demonstrated that the ceramic would have a good perspective on high temperature applications. |
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