Study On Electrical Properties And High-temperature Stability Of BiFeO₃-BaTiO₃Based High Temperature Lead-free Piezoelectric Ceramics

The high-temperature piezoelectric materials have been widely applied in vibration and impact measurement of nuclear/electrical, aerospace industry, automobile industry, petrochemical and metallurgical industry. The perovskite-type (1-x)BiFeO3-xBaTiO3[(1-x)BF-xBT] lead-free piezoelectric ceramics have excellent piezoelectric properties, together with high-temperature thermal stability, and is expected to applied widely in high-temperature piezoelectric sensors. In the present thesis, the sintering processing, composition, microstructure, electrical properties and high-temperature stability were investigated systematically, and the modification of microstructure and high-temperature properties was also carried out.The effects of sintering processing on phases and crystalline, microstructure, dielectric and piezoelectric properties were studied systematically. The results indicated that sintering temperatures had an obvious effect on microstructure and electrical properties. The grain size, relative density, piezoelectric properties increased with increasing sintering temperatures, and decreased slowly after maximum values. The optimum sintering temperatures varied significantly with BT content due to the large sintering temperature difference between BF and BT components. The results showed that the optimum sintering temperatures increased linearly with increasing BT content.The compositions, microstructure, electrical properties, Curie temperature(Tc) and depolarization temperature(Td) for (1-x)BF-xBT ceramics system sintering at optimum sintering temperature were investigated systemically. Its phase diagram shown a morphortropic phase boundary (MPB) with the coexistence of rhombohedral and pseudo-cubic phases for the composition around0.275≤x≤0.30. The MPB composition exhibited optimal piezoelectric properties of d33=136pC/N and kp=0.312at room temperature due to lowest anisotropic poling. The Curie temperature(Tc) decreased linearly with increased tolerance factor t. The ceramics with rhombohedral phases show more excellent thermal stability than that of pseudo-cubic phases, indicating Tc=485, Td=420℃at MPB composition. The internal bias field in rhombohedral phase would stabilize the remnant polarization and favor the thermal stability.The morphotropic phase boundary (MPB) composition of0.71BF-0.29BT from (1-x)BF-xBT high-temperature piezoelectric ceramics was selected as basic components. MnO2was chosen as a modifier to be ntroduced nto the0.71BF-0.29BT ceramics(BFBT-Mnx%). The addition of Mn can induce combinatory "hard" and "soft" piezoelectric characteristics due to aliovalent substitutions. Mn-doped BFBT-Mnx%showed a decreased loss tgδ, but also increased piezoelectric properties, which shown optimum piezoelectric properties of d33=142pC/N,kp=0.308, Qm=56with0.6%Mn doping. The minimum dielectric loss tgδ=3.16%was obtained with0.9%Mn doping.No phase transition, but grain growth, was observed with excess Bi2O3addition for MPB composition of0.71BF-0.29BT(BFBT-xBi). Both the densification and piezoelectric properties were enhanced by adding a small amount of excess Bi2O3. The Curie temperature was increased with a small amount of excess Bi2O3doping. The Bi-compensated BFBT-Bix ceramics at the composition of0.04Bi2O3showed the highest piezoelectric properties of d33=142pC/N and kp=0.302. An evident variation of crystalline structure and microstructure took place for the non-stoichiometric Ti/Ba ratio BFBT-xTi ceramics with x. The rhombohedral distortion degree increased for titanium deficiency BFBT-xTi ceramics. The piezoelectric constant d33and planar electromechanical coupling factor kp increased first and then decreased with increasing x, giving the maximum values of d33=152pC/N and kp=0.301for x=0.99BFBT-xTi ceramic. The depolarization temperature(Td) decreased monotonically with increasing Ti/Ba ratio x, and moderation titanium deficiency was favorable to improve the temperature stability of piezoelectric properties.The lattice distortion degree increased for CuO doped MPB composition of0.71BF-0.29BT(BFBT-xCuO). CuO is an effective sintering aid, which promotes sintering processing and grain growth. The remnant polarization Pr and coercive field Ec and piezoelectric constant d33and planar electromechanical coupling factor kp and maximum phase angle θmax first increased and then decreased with increasing content of CuO, reached peak values of Ec=35.0kV/cm, Pr=18.8μC/cm2, d33=166pC/N and kp=33.2%for x=0.4%BFBT-xCuO sample. Meanwhile, the x=0.4%BFBT-xCuO sample displayed good high-temperature stability with Tc=450℃and Td=420℃.The compositions, microstructure, electrical properties, Curie temperature(Tc) and depolarization temperature(Td) of MnO2and CuO co-doped (1-x)BF-xBT ceramics were investigated systemically. The composition of morphortropic phase boundary (MPB) shifted toward high BT content, and the MPB with the coexistence of rhombohedral and pseudo-cubic phases in the system was about x=0.325. The MnO2and CuO co-doped ceramics exhibited improved Curie temperature Tc, together with increased piezoelectric properties. The maximum value of kp was obtained for x=0.325ceramic, and d33increased monotonically with increasing x, reached peak value of d33=170pC/N with x=0.35. The Curie temperature(Tc) and depolarization temperature(Td) increased sharply with MnO2and CuO co-doped ceramics. The depolarization temperature(Td) varied insignificantly with increasing BT content of x, which was kept almost constant of500℃, indicating excellent thermal stability.