Microwave Dielectric Properties And Low Temperature Sintering Of CaTiO₃-(Li_0.5Sm_0.5)TiO₃ Ceramics

With the development of communication technology,especially the information technology which is sweeping across the globe,A number of researchers are trying to reduce the size of resonant circuits to meet the requirements of portability,integration,lightweight and miniaturization of communication devices and information terminals.Thus,research and development of the microwave dielectric ceramics with high dielectric constant,low loss,good thermal stability and low sintering temperature has become a research hotspot in recent years.At the microwave frequencies,CaTiO3 ceramics have a high dielectric constant(?r?170)while a large ?f value(?f? +800 ppm/?),which hinders the industrial application.In contrast,the A-site complex perovskite materials of Li 1/2Ln1/2TiO3(Ln = La,Pr,Nd,Sm),which have oxygen octahedral with strong internal electric field and polarization ability,have a higher dielectric constant and a large negative ?f value.Therefore,it is expected that the,combination of a high dielectric constant and a near-zero temperature coefficients of resonant frequency can be achieved in the composite of the two materials(i.e.,CaTiO3-(Li0.5Sm0.5)TiO3(CLST)).In this paper,the CLST microwave dielectric ceramics were prepared by the traditional solid-state reaction method.The microwave dielectric properties of the CLST ceramics with various compositions were investigated,the relationship between the vibrational modes of A-site ions and microwave dielectric properties were established,and the base composition for the optimum microwave dielectric properties of the CLST ceramics was obtained.In addition,by doping Mg and Nd into the A-site of the CLST ceramics,the effects of different A-site doping ions on the phase composition,microstructure,Raman spectra and microwave dielectric properties of the CLST ceramics were investigated in detail.Finally,Li2CeO3 was used as a sintering aid to lower the sintering temperature of the CLST ceramics.The main work and results are as follows:(1)(1-x)CaTiO3-x(Li0.5Sm0.5)TiO3(0.7 ? x ? 0.8,CLST)microwave dielectric ceramics with a pure orthorhombic perovskite phase were prepared by the conventional solid-state reaction.The microwave dielectric properties of the CLST ceramics with various compositions were investigated,and the results showed that the dielectric constant and Qxf value decrease,and the ?fvalue changes from a positive to a negative value with an increase in the x value.The relationship between the vibrational modes of A-site ions and microwave dielectric properties was revealed by Raman and infrared spectra analyses.The Raman results showed that with an increase in the x value,the disorder distribution of A-site ion lattice vibration increases,disrupting the ordered arrangement of the charge and resulting in an increase in dielectric loss,as a result the Qxf value decreased.According to the fitting analysis of infrared reflectance spectra,the dielectric constant is strongly dependent on the lattice vibration modes of A-site ions at low frequencies,and this dependence was weakened as the x value increases.The optimal microwave dielectric properties with ?r = 109.4,Q×f?4657 GHz,?f?1.6 ppm/? were obtained for 0.22CaTiO3-0.78(Li0.5Sm0.5)TiO3 ceramic sintered at 1240?.(2)To further study the effects of A-site ions on the microwave dielectric properties of the CLST ceramics,Mg and Nd were doped into the A-site of the CLST ceramics respectively.By using the traditional solid-state reaction method,0.22Ca1-xMgxTiO3-0.78(Li0.5Sm0.5)Tio3(x=0.05,0.10,0.30,0.50,CMxLST)ceramics and 0.22CaTiO3-0.78(Li0.5Sm.5-xNdx)TiO3(x ?0.05,0.10,0.25,0.50,CLSNxT)ceramics were prepared,respectively.The effects of different A-site doping ions on the phase structures,microstructures,Raman spectra and microwave dielectric properties of the CLST ceramics were compared.When the A-site was doped with Mg,the dielectric loss of the CLST ceramics was reduced,and the Q×f value increased.And when A-sites was doped with Nd,the dielectric constant of the CLST ceramics improved.The results indicated that CMxLST(x = 0.10)ceramics sintered at 1280? exhibited the optimal microwave dielectric properties:?r = 100.2,Q×f= 5261 GHz,?f=-3.1 ppm/?.And CLSNxT(x = 0.05)ceramics sintered at 1220? achieved the optimal microwave dielectric properties:?r= 115.1,Q×f= 4570 GHz,?f=-1.2 ppm/?.(3)In order to reduce the sintering temperature of the CLST microwave ceramics,Li2CeO3 was introduced as a sintering aid.The preparation of sintering aids of Li2CeO3 were also studied.And the effects of Li2CeO3 addition on the sintering behaviors,phase structures,microstructures and microwave dielectric properties of the CLST microwave dielectric ceramics were investigated.The results suggested that Li2CeO3 can effectively promote the densification and lower the sintering temperature of the CLST ceramics without deteriorating their microwave dielectric properties remarkably.In detail,adding 0.5 wt%Li2CeO3 reduced the sintering temperature of CLST ceramics from 1240°C to 1140°C,and the microwave dielectric properties are:?r = 100.5,Q×f= 4545 GHz,?f-5.8 ppm/?.