Domain Structure Of LiTaO₃ And Its Effect On Properties Of Al₂O₃/LiTaO₃ Composite Ceramics

Pure LiTaO₃ ceramic and Al₂O₃/LiTaO₃ (denoted as ALT) composite ceramics were fabricated using hot-press sintering in N2 atmosphere. The influences of LiTaO₃ grain status, volume fraction of Al₂O₃ and their grain sizes on domain structures, type and quantities in LiTaO₃ grains have been studied. The theoretical model and formation mechanism for the non-180o domain in ALT ceramic composites have been proposed. In addition, the effect of LiTaO₃ domain structure, Al₂O₃ content, decarborization and polarization treatment on microstructure, mechanical properties and dielectric properties of the composite have been investigated systematically.TEM observation revealed that only 180o domain structure existed in a single crystal LiTaO₃, but both 180o domain and non-180o domain presented in the ALT composite ceramics. The quantity of the 180o domain gradually decreases while that of non-180o domain structure increased obviously with increase of Al₂O₃ particles. Domain structures in LiTaO₃ grains seem does not change according to grain sizes of original powders. Non-180o domain structures formed in LiTaO₃ grains in ALT ceramic composite mainly due to mechanical restriction and stress retained from materials fabricated process, which is not the case for LiTaO₃ single crystal. This non-symmetry domain microstructure belongs to ferroelectric twinning with {1012} twin composition plane due to that the deformation occurs by twinning rather than by slipping for hexagonal LiTaO₃. According to spontaneous polarization direction of schematic (c-axis), it can be confirmed that the non-180o domain is 67o.It was found that the relative density of pure LiTaO₃ ceramic hot pressed at 1300oC/25MPa was low (only 91.5%), and it reached 97% for the sample hot-pressed under a pressure of 35MPa. The relative density and mechanical properties of ALT composite ceramics are significantly improved. Al₂O₃ particles dispersed homogeneously in LiTaO₃ matrix and well bonded with the matrix and a little amount of Ta2O5 is observed at the two phase boundaries. The flexural strength, fracture toughness, elastic modulus and Vickers hardness increased with the increase of Al₂O₃ content, and the strengthening and toughening effect is great for the lower incorporation of Al₂O₃, i.e. the flexural strength and the fracture toughness of 5vol% Al₂O₃/LiTaO₃ were 4 and 5 times higher than that of pure LiTaO₃ ceramic, respectively. The LiTaO₃ matrix of the ALT ceramic composite fractured in a intragranular type, in contrast, crack tended to propagated along the Al₂O₃/LiTaO₃ interface when Al₂O₃ particles is met, thus crack deflection effect is shown.The relative density and decarbonization treatment of ALT composite ceramics greatly influenced on dielectric properties. The dielectric constant and dielectric loss of the composites increased with increasing Al₂O₃ content from 40Hz to 1MHz, but the dielectric constant of 5vol% Al₂O₃/LiTaO₃ increased, and dielectric loss decreased. The Curie temperature of the ALT composite ceramic increased till 5vol% and then decreased with the increase of Al₂O₃ content(<30vol%). The dielectric constant of ALT ceramic composite at different temperature exhibited a behavior of frequency dispersion.Piezoelectric constant (d33) of the pure LiTaO₃ and ALT ceramic composites is about 2×10-12C/N when polarizied at 200oC and an electric field of 6000V/mm for 5min. The highest piezoelectric constant of ALT ceramic composites reaches 5×10-12C/N after the treatment is carried at 220oC and an electric field of 7000V/mm for 5min and is approximately 50% of the corresponding value for the LiTaO₃ single crystal.