Preparation And Modification Of CLST Microwave Dielectric Ceramics

With the rapid development of communication technology,microwave dielectric ceramics,with the characteristics of easy to realize miniaturization and integration,light weight,and with excellent microwave dielectric properties have become a hotspot in the field of communication technology in recent years.Generally,microwave dielectric ceramics with high dielectric constant exhibit moderate Q×f value and near-zeroτ_f,and have been widely used in mobile phones,navigation and civilian communication equipment.As we know,CaTiO₃ microwave dielectric ceramics show higher dielectric constant（～170）,but have rarely been put into practical application due to its highly positiveτ_f（～+800ppm/℃）;while Li_1/2Sm_1/2TiO₃materials possesses negativeτ_f.Through the recombination of two materials mentioned above,theτ_f of CaTiO₃-Li_1/2Sm_1/2TiO₃（CLST）microwave dielectric ceramics can be expected to be near-zeroe,and at the same time high dielectric constant and excellent microwave dielectric properties are expected.In this thesis CLST ceramics were prepared by traditional solid-state reaction method.First,the effects of the non-stoichiometric ratio of Li⁺ion on the phase composition,density,micromorphology,and microwave dielectric properties of CLST ceramics were studied,and the optimal ceramic composition and microwave dielectric properties were obtained.In addition,the doping of Sm Al O₃（SA）materials improved the Q×f value of CLST ceramics,and the correlation between microwave dielectric properties and microstructure characteristics of ceramics such as ionic polarizability,atomic packing density,A-site bond valence,was studied;Finally,to improve the dielectric constant of the ceramics we replaced the Sm³⁺ion with Nd³⁺in CLST ceramics,established the relationship between microstructure characteristic parameters and microwave dielectric properties,and also introduced Raman spectrum and infrared reflection spectrum to the research on lattice vibration.Through the above research,the following conclusions are drawn:1.0.22CaTiO₃-0.78(Li_0.5+xSm_0.5)TiO_3+x/2(x=-0.01,0,0.01,0.02,0.03,C_0.22L_1+xST)microwave dielectric ceramics were prepared by the traditional solid-state reaction method,and the influence of Li⁺ion’s dosage on microwave dielectric properties of C_0.22L_1+xST ceramic was studied.The results show that when x≤0.02,the unkown second phase will appear in the ceramic samples due to the influence of Li⁺volatilization.As the content of Li⁺ion increases,the content of the second phase gradually decreases and disappears at x=0.02;When x>0.02,a small amount of liquid phase will appear in the ceramic sample.With the further increase of Li⁺content,the microwave dielectric properties of the samples will increase at first and then decrease.When x=0.02,the C_0.22L_1.02ST ceramics exhibit the best microwave dielectric properties:ε_r=110.6,Q×f=4685.3 GHz,τ_f=14.1 ppm/°C after being sintered at 1260℃for 4h.2.In order to improve the Q×f value,the SA material with higher Q×f value was selected to dope into C_0.22L_1.02ST ceramics.0.22CaTiO₃-0.78(Li_0.52Sm_0.5)TiO_3.01+x Sm Al O₃(x=0,0.02,0.04,0.06,0.08,0.10,C_0.22L_1.02ST+x SA)microwave dielectric ceramics were prepared by the traditional solid-state reaction route.The phase composition,density and micro-morphology of ceramics were studied,and the correlation between microstructure characteristic parameters and microwave dielectric properties was established.The results show that,SA material successfully solid-dissolved into the C_0.22L_1.02ST ceramic and did not affect the phase composition of the ceramic;in addition,its relative density and micro-morphology did not change significantly.With the increase of the doping amount of SA,the Q×f value of the sample gradually increases with the increase of the atomic packing density;and the dielectric constant gradually reduced due to the influence of polarizability;the decrease ofτ_f is mainly related to A-site bond valence and dielectric constant.At x=0.04,C_0.22L_1.02ST+0.04SA ceramics obtain the best dielectric properties:ε_r=98.7,Q×f=5667.7GHz,τ_f=-1.5 ppm/℃after being sintered at 1260℃for 4h.3.In order to improve the dielectric constant of ceramics,Nd³⁺ion with similar properties and larger ionic polarizability to Sm³⁺ion was selected to replace ceramics.0.2CaTiO₃-0.8(Li_0.52Sm_0.5-xNd_x)TiO_3.01(x=0,0.05,0.1,0.25,0.5,C_0.2L_1.02SN_xT)microwave dielectric ceramic were prepared by solid-state reaction method.We studied the correlation between ceramic microstructure characteristic parameters and the microwave dielectric properties,as well as the lattice vibration modes through Raman spectrum and infrared reflection spectrum.The results show that,with the increase of Nd³⁺,the order of A-site cations decreased,the degree of rotation between oxygen octahedrons decreases,the stretching vibration among Ti-O is strengthened,and the rigidity of oxygen octahedral was weakened.In addition,the dielectric properties of the sample were deduced by fitting the reflection spectrum of the sample in the infrared band,and the conclusion that the dielectric constant of the material was mainly dominated by the phonon vibration mode at low frequencies can be obtained.With the increase of Nd³⁺ions,the dielectric constant andτ_f of the material increased,and the Q×f value decreased.At x=0.1,C_0.2L_1.02SN_0.1T ceramics exhibit the optimal microwave dielectric properties:ε_r=118.4,Q×f=4365.9 GHz,τ_f=1.9ppm/°C after being sintered at 1260℃for 4h.