| Sodium bismuth titanate-based ceramic, ((Na0.5Bi0.5)TiO3, NBT), is a kind of perovskite-type ferroelectric, which has been considered to be one of promising candidates for lead-free ceramics. Volatilization of Bi3+ and Na+ during sintering process can cause point defects in NBT- based ceramics. In this dissertation, NBT-based ceramics were prepared via a conventional solid state reaction method. Effects of nonstoichiometry of Na+、Bi3+、Ti4+ and sintering atmosphere on crystallite structure, microstructure, ferroelectric, dielectric and piezoelectric properties were studied in detail. The main results are as following:(i) (Bi0.5Na0.5)0.94Ba0.06TiO3- xwt.% Na2CO3 (BNBT6-xNa, x= 0,0.5,1,2) ceramics were prepared via a conventional solid state reaction method. Effects of Na+ excess on crystallite structure, microstructure, ferroelectric, dielectric and piezoelectric properties were studied. X-ray diffraction (XRD) results show that all ceramics have pure perovskite structure without secondary phase. Scanning electronic microscopy (SEM) results show that Na+ excess decrease the grain size. Compared with the ceramic with x=0, the ceramics with excess Na+ have decreased maximum dielectric constant, Curie temperature and depolarization temperature. The ceramic with x= 1 shows the lowest volatilization ratio (mass loss≈0.41%), highest relative density (96.5%) and highest piezoelectric constant (d33= 79 pC/N).The coercive filed of the ceramic decreases and the maximum polarization does not show obvious change. The results show that the appropriate excess Na2CO3 can compensate volatilization of Na+, which can improve densification and electrical properties of the ceramics.(ii) Based on the results of (i), (Bi0.5Na0.5)0.94Ba0.06TiO3- 1wt.% Na2CO3-xwt.% Bi2O3 (BNBT6- 1Na- xBi, x= 0,0.5,1,2) ceramics were prepared via a conventional solid state reaction method. Effects of Bi3+ excess on crystallite structure, microstructure, ferroelectric, dielectric and piezoelectric properties were studied. XRD results show that the ceramics with x= 0,0.5,1 have pure perovskite structure; while the ceramic with x= 2 shows a secondary phase. SEM results show that Bi3+ excess does not cause obvious change in grain size except the ceramic with x= 2. The appropriate excess Bi2O3 (x= 0.5) can improve dielectric and ferroelectric properties of the ceramics. The ceramic with 0.5 wt.% excess Bi2O3 shows the lowest volatilization ratio (mass loss≈0.07%) and high dielectric and ferroelectric properties:depolarization temperature Td= 81℃, Curie temperature Tc-270℃, maximum dielectric constant εm= 6292, remanent polarization Pr= 28.4 μC/cm2, coercive field Ec= 33.0 kV/cm. Compared with the ceramic of BNBT6-1Na-OBi, excess Bi2O3 can improve piezoelectric constant of the ceramics.(iii) (Na0.54Bi0.5)0.94Ba0.06Ti1+x O3 (x=-0.02,0,0.02,0.04) ceramics were prepared via a conventional solid state reaction method. Effects of Ti4+ nonstoichiometry on crystallite structure, microstructure, ferroelectric, dielectric and piezoelectric properties were studied. XRD results show that the ceramics with x=-0.02 and 0 have pure perovskite structure; while the ceramic with x= 0.02 and 0.04 shows a secondary phase. The rhombohedral-tetragonal morphotropic phase boundary exists in all samples and the relative amount of the tetragonal phase changes with the increase in the Ti4+ amount from deficiency to excess. Size and shape of grains in the ceramics are unaffected by the change in the Ti4+ nonstoichiometry. Dielectric, ferroelectric and piezoelectric properties of the ceramics are close related to the Ti4+ amounts. Compared with the ceramic with x= 0, the ceramics with the Ti4+ nonstoichiometry have higher depolarization temperature, lower maximum dielectric constant, decreased piezoelectric constant and ferroelectric properties.(iv) 0.97[(Na0.5Bi0.5)1-x Lax]Ti1-0.25xO3- 0.03BaTiO3 (x= 0,0.01) were prepared via a conventional solid state reaction method. The sintering atmospheres were chosen as air and nitrogen, respectively. Effects of the sintering atmospheres on crystallite structure, microstructure, ferroelectric, dielectric and piezoelectric properties were studied. XRD results show that all ceramics have pure perovskite structure and high relative density (>93%). The presence of La3+ facilitates small grain size and fine and homogeneous microstructure. All samples exhibit two dielectric anomalies in dielectric constant-temperature curves. The maximum dielectric constant and piezoelectric constant of the ceramics sintered in air atmosphere increase; while coercive field decreases. The doping of La3+ improves piezoelectric constants and remanent polarization, and decreases depolaration temperature, maximum dielectric constant, mechanical quality factor and coercive field of the ceramics. Compared with the ceramics sintered in air, the ceramics sintered in nitrogen demonstrate decreased activation energies. |
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