| The relationships between properties and structures for piezoelectric secondary phase toughened multiphase ceramics and composite dielectric ceramics have been investigated in this dissertation. Sr2Nb2O7/3Y-TZP, Sr2Nb2O7/ZTA, BaTiO3/8Y-FSZ, Nd2Ti2O7/8Y-FSZ, CaTiO3/LaSrAlO4 and Sr2Nb2O7/LaAlO3 multiphase ceramics have been prepared and the mechanical and/or dielectric properties have been analyzed together with the microstructures. The following primary conclusions have been drawn.In the Sr2Nb2O7/3Y-TZP (3mol% yttria-doped tetragonal zirconia polycrystal) system, the piezoelectric secondary phase could co-exist with 3Y-TZP matrix, and the transformable tetragonal phase in 3Y-TZP ceramics is affected by the content of secondary phase and the sintering condition. The fracture toughness is enhanced significantly with incorporating Sr2Nb2O7 into 3Y-TZP ceramics, and the highest fracture toughness, 13.7MPa-ml/2 is achieved in 0.005Sr2Nb2O7/0.995(3Y-TZP) ceramics sintered at 1500癈 for 6h, which is twice that of the matrix. Moreover, it should be emphasized that a high KIC value of 10.0MPa-m1/2 is achieved in the present composite ceramics even where the transformation toughening mechanism is inactive and the piezoelectric secondary phase toughening plays the primary role in this composite.In the Sr2Nb2O7/ZTA (zirconia toughened alumina, 45ZrO2-55Al2O3) system, the piezoelectric secondary phase will react with alumina in ceramic matrix to form platelet SrAl|2O19 phase. Combining the effects of grain bridging, transformation toughening and microcracking toughening, the highest fracture toughness, l0.0MPa-m1/2 is achieved in 0.005Sr2Nb2O7/0.995ZTA ceramics sintered at 1600℃ for 6 hours.In the BaTiO3/8Y-FSZ (8mol% yttria-doped fully stabilized zirconia) system, the introduction of secondary phase could significantly decrease the sintering temperature of 8Y-FSZ ceramics. The secondary phase is rhombohedral Ba(Ti1-xZrx)O3 ferroelectric phase for the composite with 5mol% additive, while for those with higher additive content, the secondary phases change to cubic Ba(Ti1-xZrx)O3 phase. The highest fracture toughness reaches 6.08 MPa-m1/2 for 0.05BaTiO3/0.95(8Y-FSZ) sintered at 1475癈 for 3h, where the piezoelectric secondary phase toughening plays an important role.In the Nd2Ti2O7/8Y-FSZ system, the secondary phase could co-exist with the matrix and it also could significantly decrease the sintering temperatures of 8Y-FSZceramics. The fracture toughness of composite ceramics increases firstly to a maximum value and then decreases with increasing the content of secondary phase and the sintering temperature. The ferroelectric domain has been observed in the composite sintered at 1525℃ with 15mol% additive, which is the key point of piezoelectric secondary phase toughening. Combination of the effects of piezoelectric secondary phase toughening, crack deflecting and transformation toughening, the highest fracture toughness, 7.97MPa-m1/2 is achieved in this composite, which is 2.5 times that of the matrix.In the CaTiO3/LaSrA104 system, CaTi03 and LaSrAlO4 could co-exist when the content of CaTiO3 is no more than 30mol%, while LaAlO3 and CaO phases are observed in the present composite ceramics with higher CaTiO3 content. The dielectric constant of the present composite ceramics increases with increasing the additive content, and it varies slightly with the sintering conditions. The dielectric loss generally increases with increasing the additive content for the high dielectric loss of minor phases, and it changes significantly with the sintering conditions. The temperature coefficient of dielectric constant decreases with increasing the additive content firstly, and then it increases unconventionally when x equals to 40mol%, finally it decreases with increasing the additive content further.In the Sr2Nb2O7/LaAlO3 system, the grain size decreases significantly with the additive, and Sr2Nb2O7 reacts with the LaAlO3 matrix to form a LaNbO4 secondary phase. The dielectric constant varies slightly, while the...
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