Preparation, Microstructure And Properties Of Dielectric Ceramics For High-temperature-stable MLCC Application

This dissertation focuses on the preparation of barium titanate baseddielectric materials for high-temperature-stable multilayer ceramic capacitors（MLCC） application, especially for the MLCCs whose upper temperature limitreaches200oC. The domestic technology in this field is promoted. Therelationship among the composition, microstructures and properties wassystematically studied and the related mechanism was also discussed.BaTiO_3-Nb₂O₅-Co₃O₄was chose as the system for air-sinteredtemperature-stable dielectrics. The ceramics which satisfy EIA X8Rspecification （55to150°C, ΔC/C25≤±15%） could be obtained by the co-dopingof Re₂O₃and Bi₂O₃（Re represents a kind of rare-earth element with smallerionic radius）. The effects of Re₂O₃and Bi₂O₃on the Curie temperature wereinvestigated and Re was proved to play a major role in Tcshift. A “core-shell”interface model was proposed and the effect of the internal stress on Tcwaselucidated. The above-mentioned X8R material was successfully applied in thefabrication of MLCCs with Ag-Pd inner electrodes. The degradationcharacteristic, the dielectric behavior under a direct current （DC） bias field andthe impedance spectra demonstrated the high reliability of X8R-type MLCCs.The elements interdiffusion in the vicinity of ceramic/electrode interface wasstudied by HRTEM.With a proper amount of CaZrO₃addtion, the BaTiO₃-MgO-Y₂O₃-MnO₂nonreducible system was found to fulfill EIA X8R specification. The effects ofCaZrO₃content on the sintering process, the Curie temperature, dielectricproperties and degradation behaviors were investigated and the modificationmechanism was discussed. The site occupancy behavior of amphoteric rare-earthoxide Y2O3was also investigated, which had a significant effect on dielectricand electrical properties. Fine-grained X8R materials with an average grain sizeof less than200nm were prepared. Fine-grained ceramics showed much betterperformance under a DC bias field and the explanation was provided.The solid solutions （1-x）BaTiO_3–x(Bi_0.5Na_0.5)TiO₃（x≤0.12） synthesize by solid-state reactions exhibited a high Curie temperature as well as a diffusephase transition （DPT）. The temperature coefficient of capacitance （TCC）curves of0.9BaTiO₃–0.1（Bi0.5Na0.5）TiO₃system were largely improved with thedoping of Nb₂O₅. As a result, air-sintered X9R-type （55to200°C,ΔC/C25≤±15%） dielectric materials were successfully obtained. The dielectricproperties were further enhanced via two step sintering and chemical coatingmethod. The newly developed air-sintered X9R materials showed an excellentoverall performance with the dielectric constant of1900²000and the dielectricloss of1.7%².1%, which reached the international leading edge. The energydispersive spectrometry was used to analyse the elements distribution inmicroregions. The results revealed that the temperature stability of permittivityoriginated from the heterogeneous distribution of niobium. The evolution ofmicrostructures could be explained by the “solution-precipitation” mechanism.