Study On Doped And Low-temperature Sintered Ba_xSr_1-xTiO₃ Ceramic Materials

With the rapid development of electrommunication businesses and surface mounting technology, it has been strongly required that multi-layer ceramic capacitor (MLCC) becomes small-sized, high-capacitable, low-cost, lead-free, middle-high voltage, high-functional, and usable at higher frequency range. Middle-high voltage MLCC needs to have excellent breakdown performance, and to fulfill the requirement of low temperature co-fired ceramics (LTCC) technology in order to cofire with low-cost content electrode. Therefore, the development of middle-high voltage ceramics was mainly focused on excellent breakdown performance, low sintering temperature and anti-reduction.Ba_xSr_1-xTiO₃ (BST) ceramics is of adjustable dielectric properties and excellent breakdown performance, which is the research focus of middle-high voltage MLCC materials. Presently, the investigation on improving dielectric properties of BST ceramics was mostly focused on preparation technique, addition of lead, and sintering at high temperature, which was not suitable for the requirements of MLCC for low-cost and lead-free. Based on these requirements, ceramic system of Ba_xSr_1-xTiO₃ (x=0.1) was studied by using low pure raw materials and solid reaction in this dissertation. Furthermore, middle-high voltage MLCC materials with lead-free and good dielectric properties were prepared through substituting B site Ti by (Nb_3/4Li_1/4), (Nb_2/3Zn_1/3) and (Nb_1/2Sm_1/2). Based on these results, ZnO-B₂O₃-SiO₂ glass (ZBS) was used to decrease the sintering temperature of Ba_xSr_1-xTiO₃ ceramics in order to cofire with metal Ag electrode and study the application technology of Ba_xSr_1-xTiO₃ ceramics.The main research results are as follows:1. The sintering characteristics and dielectric properties of Ba_0.9Sr_0.1TiO₃ ceramics were investigated. The perovskite structural solid solutions with tetragonal system were formed when the ceramic samples were sintered at 1225℃～1425℃. With the increase of sintering temperature from 1225℃to 1325℃, the compactness of ceramic samples was enhanced, which caused the improvement of dielectric constant, breakdown electric field and dielectric loss. However, Ti³⁺ was formed to deteriorate the dielectric properties of samples sintered at 1375℃～1425℃. Ba_0.9Sr_0.1TiO₃ ceramics sintered at 1325℃for 2h obtained the best dielectric properties ofε_r=2354, Eb=10.22kv/mm, tanδ=0.01345, p=7.0×10⁹Ω·cm. 2. The B site ion Ti⁴⁺ was substituted by donor-acceptor co-ion to improve the properties ofBa_0.9Sr_0.1TiO₃ ceramics. (Nb_3/4Li_1/4), (Nb_2/3Zn_1/3) and (Nb_1/2Sm_1/2) substituted the B site Ti respectively to modify Ba_0.9Sr_0.1TiO₃ ceramics. Both of them could form perovskite structural solid solutions. Comparing with (Nb_3/4Li_1/4) and (Nb_2/3Zn_1/3), a certain amount of (Nb_1/2Sm_1/2) substituting Ti improved the breakdown electric field and resistance, and decreased the dielectric loss.ε_r=6168, tanδ=0.0024, E_b=14.3kv/mm, p=3.0×10¹²Î©Â·cm were obtained when Ba_0.9Sr_0.1(Nb_1/2Sm_1/2)_0.04Ti_0.96O₃ ceramics were sintered at 1510℃for 2h.3. The sintering temperature of Ba_0.9Sr_0.1TiO₃ ceramics was decreased markedly by adding ZBS glass sintering additive. The sintering temperature of Ba_0.9Sr_0.1(Nb_1/2Sm_1/2)_0.04Ti_0.96O₃ (BSNST) ceramics was decreased to 900℃with ZBS glass addition. The dielectric properties of BSNST ceramics decreased slightly with the increase of ZBS glass addition. The BSNST ceramics with 5wt% ZBS glass sintered at 925℃for 2h exhibited excellent dielectric properties ofε_r=2695, E_b=10.19kv/mm, tanδ=0.01265,ρ=1.0×10¹¹Î©Â·cm. Besides, the low-fired BSNST ceramics had good compatibility with Ag electrode, and no element diffusion was observed. It can be a promising ceramic material for LTCC technology.