Research On Dielectric Property Of Barium Strontium Calcium Titanate-based Ferroelectric Ceramics

Barium strontium titanate ceramics possess series of advantages such as the high tunability and changable Cure-temperature. However, the relatively high dielectric constent and worse dielectric temperature stability above Cure-temperature limit its extensive application in microwave tunable devices. In this work, Ba_0.55Sr_0.4Ca_0.05TiO₃ (BSCT) was prepared as the main composition. Bi_1.51Mg_0.76Nb_1.51O_6.8(BMN) and CaTiSiO₅(CTS) were chosen as the secondary composition due to the relatively lower dielectric constant of BMN and the positive dielectric constant temperature coefficient of CTS. The influence of the two additives on the structure and the property of the BSCT was discussed.Firstly, the Bi_1.51Mg_0.76Nb_1.51O_6.8(BMN) phase was synthesized and it was found that the ratio of raw materials could influence the structure and the dielectric character. The pyrochlore phase was obtained only when the mole ratio of MgO﹕Bi₂O₃﹕Nb₂O₅ was setted at 2﹕1.35﹕1. The tanδis 7×10^-4 when sintered at 900℃. Then the compound ceramics was prepared using BMN and BSCT as raw meterials. The BMN phase could not be found in the material because of its solution in the lattice of BSCT. The addition of BMN shifted the cure-temperature of BSCT to a lower temperature.The Cure-temperature of the ceramics with 5wt% BMN addition is shifted from 9℃to -49℃. The tanδs is decreased from 10×10^-3 to 7×10^-3 at a frequeny of 1MHz.The tunability is decreased from 22.5% to 3.8% under a DC field of 1kV/mm. The density of the samples decreased as sintering temperature decreased. But the tunability shows little change. Secondly, another compound of CaTiSiO₅(CTS) was synthesized using CaCO₃, TiO₂ and H2SiO₃ as the raw meterials with the mole ratiol of 1﹕1﹕1.1. Then the synthesized CTS was also added to BSCT. The grain size as well as densityof BSCT increased with the increasing of CTS dopant. Ba2TiSi₂O₈ phase emerged When 10wt% CTS was added. The material showed dielectric constant of 2100 and dielectric loss of 2.03x10^-3 when measured at 1MHz. The tunability is 11.7% when a DC field of 1 KV?mm-1 was installed. Since most of CTS soluted in lattice of BSCT, It showed indistinguishable effect on the temperature stability of dielectric constant.