| Microwave dielectric ceramics with high dielectric constant (r) and high quality factor(Q) and small temperature coefficient of the resonance frequency(f), are a kind of functional ceramics used as dielectric at microwave circuits. Microwave dielectric ceramics have been developed for near 30 years, and are key material used in microwave components such as resonators and filters. Civil microwave components are used at higher microwave frequency. Microwave dielectric ceramics requires not only excellent microwave properties but also low price. Ba(Mg 1/3 Nb 2/3)O3 ceramics own good microwave dielectric properties and low price. However, its properties are not good with properties of Ba(Mg 1/3 Ta 2/3)O3 and Ba(Zn 1/3 Ta 2/3)O3 ceramics. Based on the theories and experimental results of research works on microwave dielectric ceramics, principles used to modify Ba(Mg 1/3 Nb 2/3)O3 ceramics are suggested. With the principles' guidance, effect of preparation processing, composition defects, doping and substitution on microwave dielectric properties are studied systematically.Based on the theories and experimental results of research works on microwave dielectric ceramics, the methods of modifying Ba(Mg 1/3 Nb 2/3)O3 ceramics are put forward. The origin of difference between calculated dielectric constant from Clausius-Mosotti equation and experimental dielectric constant was analyzed. The modified Clausius-Mosotti equation was proposed. It was found that intrinsic and extrinsic losses were the main factor of effecting quality factor. Improving B-site 1:2 ordering and eliminating defects are effectively methods for improving Q value. Based on theoretical analysis, tuning Tf can base on the electronegativity of B-site ions. The criterion was suggested by author.Effect of preparation processing on microstructure and microwave dielectric of Ba(Mg 1/3 Nb 2/3)03 ceramics was studied. Pure Ba(Mg 1/3 Nb 2/3)03 powders could be synthesized by molten salt method at 900 C. However, powders synthesized by conventional solid reaction method could not get pure Ba(Mg 1/3 Nb 2/3)03 powders even at 1400 C. Powders synthesized by molten salt method had better activity and finer particle. The sintering temperature could lower about 50'C. The microwave dielectric properties of Ba(Mg 1/3 Nb 2/3)03 ceramics prepared by molten salt methodwas improved. Sintering temperature, soaking time and annealing time also have great influence on Qf value of Ba(Mg 1/3 Nb 2/3)03 ceramics prepared by conventional solid reaction method.The effect of A-site and B-site nonstoichiometry on the microstructure and microwave dielectric properties of Ba(Mg 1/3 Nb 2/3)O3 ceramics was studied systematically. An improvement of the degree of 1:2 ordering and sinterability was found in Ba-deficient Ba(Mg 1/3 Nb 2/3)03 because there existed Ba vacancies. Excessive Ba inhibits the sintering and the degree of 1:2 ordering. We concluded that excessive BaO existed in grain boundary and inhibited the movement of grain boundary. Therefore, the sintering was restrained. An improvement of the degree of 1:2 ordering and sinterability was found in Nb- excessive Ba(Mg 1/3 Nb 2/3)03. We thought that excessive Nb2Os maybe reacted with BaO and produced Ba vacancies. So the sinterability was improved. Whereas deficient Nb inhibits the sintering and the degree of 1:2 ordering, because free BaO and MgO existed in grain boundary and inhibited the movement of grain boundary. Ba-deficiency had little effect on dielectric constant of Ba(Mg 1/3 Nb 2/3)03 ceramics. With the Nb excessiveness increasing, the dielectric constant increased. With Ba excessiveness and Nb deficiency increasing, Qf value decreased drastically.The influence of low molten point additives MnO2 and BaWO4 on the sintering behavior, microstructure and microwave dielectric properties of Ba(Mg 1/3 Nb 2/3)03 ceramics was studied. MnO2 can effectively low the sintering temperature from 1550 C to 1400 C and promote the growth of grain. MnO2 addition improved the microwave dielectric properties of Ba(Mgi/3Nb2/3)O3 ceram...
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