Synthesis Of Perovskite Structure Microwave Dielectric Nono-Ceramics By Sol-gel Method

Sol-gel method was widely used in synthesizing the ceramics for it is easy to obtain nanoparticles with monodisperse distribution and small size which can make the precursors react at the molecular level.Microwave dielectric ceramics is an important functional ceramics which attracted many researchers interests for the widely use in communication field.The dielectric ceramics with high dielectric constant have potential application in the electronic device with small size and integration.We found that the perovskite structure ceramics with high dielectric constant are high in dielectric loss and sintering temperature.Therefore,to decrease the sintering temperature and improve the Q×f value are the crucial problems to solve in application.The thesis was based on CaTiO₃ with A-site substituted by Nd or Li.Sol-gel method was used to synthesize the nanoparticles with small crystal size and monodisperse distribution and then the ceramics were prepared by the nanoparticles.The effects on the dielectric properties and microstructure were studied.Compared with the ceramics synthesized by the traditional solid state method,the ceramics prepared by sol-gel method have some advantages as follows:1.The(Ca_0.61,Nd_0.26)TiO₃ ceramics were synthesized by the nanoparticles with diameter size about 5-10 nm,which enjoyed great improvement on dielectric properties and decreased the sintering temperature by 200℃compared with the ceramics synthesized by traditional solid state method.We found that the nanoparticles with monodisperse distribution and the crystal size about 5-10 nm were obtained by the dried gel heat-treated for 2h at 500℃.The (Ca_0.61,Nd_0.26)TiO₃ ceramics prepared by the nanoparticles were sintered at 1200℃,for 2h and the dense ceramics phase were obtained with the crystal size about 1-3μm.At 1200℃,very dense ceramics with uniform grains and good dielectric properties with a dielectric constant of 90.2 and a Q×f value of 25200GHz,were achieved.However,the Q×f value of ceramics synthesized by solid state method at 1400℃is only 17200 GHz.2.The ceramics doped with Li-Cu-B can decrease the sintering temperature of(Ca_0.61, Nd_0.26)TiO₃ ceramics efficiently and can improve the dielectric properties.The nanoparticles with small size and good distribution were obtained at 300℃.The major phase of(Ca_0.61, Nd_0.26)TiO₃ are obtained at 300℃for 2 h.The dense ceramics synthesized by the nanoparticles with the size of 10-15 nm could be achieved at low-temperature of 1100℃.The SEM images showed that the dense microstructures are obtained which mainly consist of hexagonal-shaped crystals with a crystal size about 100-200nm.The excellent dielectric properties are exhibited at 1100℃with dielectric constant of 89.53,a Q×f value of 17148GHz,andÏ„_f value of +239 ppm℃^-1.Compared with the solid-state method,sol-gel method with impurities doped at sol stage could greatly decrease the sintering temperature and improve the dielectric properties.3.Perovskite structure(Nd_0.55,Li_0.35)TiO₃ ceramics with A-site ordering have great effects on the dielectric properties.Perovskite structure(Nd_0.55,Li_0.35)TiO₃ was traditionally synthesized with phase separation into superlattice of nanodomains comprising(Nd_0.5, Li_0.5)TiO₃ and Nd_2/3TiO₃ separated by zigzagging boundaries,which would cause the major loss of dielectric properties.We designed a kind of sol-gel method to synthesize(Nd_0.55,Li_0.35)TiO₃ ceramics at 1150℃with pure single phase to avoid the phase separation caused by cations ordering in A-site in nanometer scale.Permutations for the lattice ordering under high-resolution TEM demonstrated that the neodymium,lithium and vacancy in A-site under the same periodical ordering range,which caused the phase separation in nanometer scale. Therefore,great improvement on dielectric properties was obtained with a Q×f value of 8256 GHz and dielectric constant(ε) of 86.52.4.The resonant frequency temperature coefficient of the(1-x)(Ca_0.61,Nd_0.26)TiO₃—x(Nd_0.55,Li_0.35)TiO₃ composites is achieve to zero with the x value at 0.6.The nanoparticles of the composites are prepared via recombination of the two kinds of complex oxides at sol stage. The ceramics synthesized by the nanoparticles are sintered at 1150℃for 2h.The high positive resonant frequency temperature coefficient of(Ca_0.61,Nd_0.26)TiO₃ ceramics are successfully compensated by the(Nd_0.55,Li_0.35)TiO₃ ceramics and the excellent dielectric properties are obtained with Q×f=7114.5GHz,ε_r=93.43.