Low Temperature Sintering And Dielectric Properties Of 0.4 CaTiO₃-0.6(Li_1/2Nd_1/2) TiO₃ Ceramics

With the rapid progress in mobile telecommunication, it has been strongly required that the related components become small-sized, usable at higher frequency range, integration, and low cost. The primal method was to use high dielectric ceramics and the method has obvious limit.The design of multilayer structure based on the low temperature cofired ceramics technology(LTCC for short) can reduce the size of components effectively, which is the significant way to realize components'miniaturization, integration, high raliabilties and low costs. The key technology of LTCC is preparation of microwave dielectric ceramics which can be co-fired with high-conductivity metal electrode such as Ag or Cu. Even though several types of dielectric materials with good microwave dielectric properties have been reported, most of them could only be well sintered at above 1300°C. Therefore, it is important to lower the sintering temperatures of microwave dielectric materials to less than the melting points of Ag(961°C) or Cu(1083°C) without deteriorating the microwave dielectric properties.(1-x)CaTiO₃-x(Li_1/2Nd_1/2)TiO₃(CTLNT for short) ceramic has an orthorhombic perovskite structure, and is widely studied due to its good dielectric properties. Liu et al. found that CTLNT ceramics sintered at 1300°C had good microwave dielectric properties: relative dielectric constant (ε_r =112.6), mechanical quality factor plus resonant frequency (Q·f =4480GHz), temperature coefficients of the resonance frequencyÏ„_f =8.2ppm/°C, when x=0.6. Obviously, the sintering temperature is too high in practical application.This paper looks back the development and the study of low temperature sintering of microwave dielectric ceramics. The CTLNT ceramic was selected as the research object in this paper. The different sintering aids were used to reduce its sintering temperature. The effects of different sintering aids on sintering characteristics, phase compositions, microstructures and dielectric properties of CTLNT ceramic were studied.The relation among phase, microstructures and dielectric properties of CTLNT ceramic was explained. The sintering mechanism and densification process of low temperature fired CTLNT ceramics were also studied.1. Effects of BaCu(B₂O₅)(BCB for short) CTLNT ceramics with different amounts of BCB addition was prepared by the conventional solid-state reaction method. It was found that a small amount of BCB additive lowered the sintering temperature of CTLNT ceramics effectively from 1300°C to 1050°C. The adding of BCB has no effect on the perovskite structure of CTLNT ceramic. CTLNT ceramics with 4wt% BCB sintered at 1050°C for 2h showed the optimum dielectric properties:ε_r =96.5, tgδ=0.017, andÏ„_f =-13.6ppm/°C.2. Effects of ZnO-B₂O₅(ZB for short)glass ZB glass was prepared by molten quenching method. It was found that a small amount of ZB glass lowered the sintering temperature of CTLNT ceramics effectively from 1300°C to 950°C. The adding of ZB glass has no effect on the perovskite structure of CTLNT ceramic. The CTLNT ceramics with 3wt% ZB glass sintered at 950°C for 2h showed the optimum dielectric properties:ε_r =91.1, tgδ= 0.013, andÏ„_f =10.5 ppm/°C.3. Effects of composite sintering additive The effects of BCB and ZB glass composite sintering additive on sintering behavior, phase composition, micromorphology and dielectric properties of CTLNT microwave dielectric ceramics were investigated. The sintering temperature of CTLNT ceramics was effectively lowered from 1300oC to 925oC by adding 3wt%ZB and 5wt%BCB. The dielectric constant (ε_r) increased initially and then decreased slightly with increasing BCB content. The dielectric loss (tgδ) value decreased and reached saturation with the addition of BCB. The temperature coefficients of resonant frequency (Ï„_f) value were closer to zero. The CTLNT ceramics with 3wt% ZB glass and 5wt% BCB sintered at 925oC for 2h showed the optimum dielectric properties:ε_r =83.2, tgδ= 0.013, andÏ„_f =35.7 ppm/oC,which meets the requirements of the high dielectric constant materials for multilayer microwave components.4. The densification mechanism of low temperature fired CTLNT ceramics The CTLNT ceramics with BCB or ZB glass or composite addition densified through liquid phase sintering.