Synthesis And Properties Of LaAlO₃ Based Ceramics

LaAlO3 with a perovskite-type structure has widely used in high-temperature fuel cells, electronics, catalysis, sewage treatment, ceramics, substrate materials and other fields, because of its high dielectric constant (GHz frequency terms), low dielectric loss, and negative temperature coefficient and many other advantages. In addition, transformation of crystal structure of doping LaA103 makes it have different physical and chemical properties. BiAlO3 is developed rapidly in recent years a new type of high temperature perovskite structure lead-free piezoelectric material and its the Curie temperature is higher than 520℃. However, Study of microstructures and properties for BiA103 doped LaAlO3 has been rarely ever reported.Pure and BiAlO3 doped LaAlO3 have prepared using solid-phase method and sol-gel synthesis, respectively. And their phase evolution, microstructures, dielectric-temperature characteristics and impedance analysis have been investigated. Moreover, the effect of dopping concentration on properties of LaAlO3 ceramics has studied. Their results are as follows:(1) The minimum calcining temperature is 1300℃ for pure LaAlO3 prepared using solid-phase method while pure LaAlO3 is obtained calcined at 800℃ using the sol-gel method. And pure LaA103 ceramics are sintered at 1500℃ using both methods. At the same time, the bulk density and the real part of permittivity of the ceramics reaches the maximum.(2) (1-x) LaA103+xBiA103 powders are prepared by the solid-phase method and the sol-gel method, respectively. Impurity phase (Bi2Al4O9) occurs after the powders were calcined at 1300℃ using solid-phase method. But when the doping is greater than 0.2 mol%, Impurity phase (Bi2Al4O9) appears for the powders calcined at 800℃ using the sol-gel method.(3) Microstructures have been investigated for the samples prepared by both methods. For the sample prepared using solid-phase method, when the doping amount is low 0.2 mol%, the uniform grain size decreases with increasing doping concentration, and then with increasing doping concentration, both grain boundary fusion and grain growth occur. But for the samples prepared using sol-gel method, when x>0.1mol%, uniform grain size decreases with increasing doping concentration..(4) Dielectric properties have been improved with increasing doping concentration for the samples prepared using both methods. When the frequency is 500MHz, the 0.2mol% doped sample prepared using the solid phase method has the best dielectric properties. At the same time, the maximum real part of permittivity is 40.6 and increased 89% compared with x= 0 (the real part of permittivity is 21.5). The 0.2 mol% doped sample prepared using the sol-gel method has also the best dielectric properties. At the same time, the maximum real part of permittivity is 54.4 and increased 131% compared with x= 0 (the real part of permittivity is 23.6).(5) For (1-x) LaAlO3+xBiAlO3 ceramic, hysteresis loop analysis revealed that pure LaAlO3 ceramics has conventional loop shape of dielectric material while the BiAlO3 doping makes hysteresis loop ferroelectricity improved. This can be attributed to the substitution Bi2+ of for La3+ ion. Since the second phase BiaAl4O9 has space group Pbam, it does not have ferroelectricity.(6) In order to model the electrical response and extract resistance and capacitance of each electroactive element, an equivalent circuit was used to fit the experimental data for (1-x)LaAlO3+xBiAlO6 ceramics. An excellent agreement between the experimental and fitting data indicates that the equivalent circuit is reasonable. For the samples prepared using solid phase method, the grain resistance increases with increasing doping while the grain boundary resistance also increases for x is low 0.2mol%. But for the samples prepared using sol-gel method, both grain and grain boundary resistance increase with increasing doping concentration.