| The preparation of submicron BaTiO3 superfine powder and the dielectric properties of high dielectric constant & high voltage X7R MLC ceramic were researched in this dissertation, of which the principles and process is researched by XRD, TEM, EDX, SEM, on the basis of crystal chemistry, physics & chemistry, surface and interface chemistry. 1.The mechanism of high dielectric constant of fine-grain BaTiO3 was fully analyzed, i.e. if the grain size in submicron BaTiO3 ceramic locates below 1μm, the chance rate of 90( domain will be limited. Because of the high inner stress in the grains, the dielectric constants at temperature spots below Curie are increased; this is called Fine-grain Effect,which is very important to improve the temperature characteristics of the capacitance of BaTiO3 ceramic. The dielectric constant reaches maximum value when the grain size locates in 0.7μm-1μm. 2.The qualify BaTiO3 superfine powder was synthesized by hydrothermal and co-precipitation methods. The average size of particles is below 0.8μm, with narrow distribution, without hard agglomeration. It is suitable to be high dielectric constant MLC ceramic.The effects of different heat treatment temperatures on microstructure and dielectric properties of co-precipitation synthesized BaTiO3 has been investigated. The dielectric constant of BaTiO3 can be improved by preparing tetragonal BaTiO3.3.In high dielectric constant X7R MLC ceramic system, Grain inhibitor Nb2O5, Broadening Cure peak addition MgO and various Rare-earth oxides were added to the major material of submicron BaTiO3. The optimal microstructure with dense fine grains, high dielectric constant and other excellent dielectric properties has been obtained. It was testified that diffusion rate of Nb2O5 was low in BaTiO3 grains by TEM,EDAX analysis, so the core-shell structure was eventually formed. The dielectric properties of the system were determined by ferroelectric BaTiO3 core and paraeletric BaTiO3 doped Nb5+. The curve of capacitance vs. temperature showed double peaks, so the temperature characteristics of the capacitance of the system improved. The temperature change rate of capacitance (TCC) in negative temperature region was reduced by adding MgO. Cooperated with Nb2O5, it can strengthen the shell action in the grains, enhance the inner stress and improve Nb2O5. The curve of capacitance vs. temperature became flat.There were two types in BaTiO3 ceramic modified chemically by rare-earth oxides, i.e. Chemical inhomogeneity and chemical homogeneity. La2O3, Sm2O3 and Nd2O3 present homogeneity. TCC in high temperature region was improved by adding La2O3 and the temperature dependence of such property showed single peak. Sm2O3 depressed Curie peak intensively and shift it to low temperature. Nd2O3 was superior to former additions. With the increase of the content, the curve of ε-T became flat and the intensity of the base peak was elevated. Pr6O11,Gd2O3 present inhomogeneity. The perfect dielectric properties of X7R ceramic material were obtained with high dielectric constant & performance.The dielectric constant of the resultant X7R ceramic material was above 4700. This provided the foundation for preparing MLC with high volume efficiency of the capacitance and thin dielectric layers. 4．In high voltage X7R MLC ceramic system, CaZrO3, ZnO, SrCO3, MgO and rare-earth oxides (Ho2O3,Er2O3或Y2O3) were added to the major material of submicron BaTiO3.By doping smaller ion (Ho2O3, Er2O3, Y2O3), the hysteresis loops of BaTiO3 became narrow obviously and the remanent polarization decreased ,so the dissipation was reduced and the insulation resistance was increased. The reliability working at high voltage was enhanced and the breakdown voltage was improved. MgO, CaZrO3 can significantly broaden the curie peak of BaTiO3, εpeak was reduced and the curve of ε-T became flat. ZnO had low melt point,it aggregated at grain boundary , promote sintering and inhibited the growth of grains to realize densification and improve the wi...
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