| With the rapid development of electronic information technology, electronic products are getting lighter, thinner, shorter and smaller. Multilayer ceramic capacitor (MLCC) as a basic electronic components subsequently are developing for miniaturization, large capacity, high volume, high voltage, high frequency, anti-jamming, multi-function and low power consumption. In recent years, with the working temperature of MLCC has become higher and higher, research on permittivity and temperature-stability of MLCC has become imperative. Barium titan ate ceramic has high dielectric constant, proper Curie temperature (Tc), and adjustability in permittivity and Tc by doping, so it is the best material to manufacture ceramic capacitors with high permittivity and stability.In order to improve the temperature stability of the capacitor materials, this paper proposed the synthesis route of multilayer structure grain using the atmospheric liquid phase technology and hierarchical classification growth technologies, based on the traditional core/shell structure prepared by solid-phase method. Every dielectric layer in the grain had different Curie temperature. When the grain grew layer by layer, its performance would be the stack results of all the dielectric layers according to the Lichtenecher’s law and multimodal effect. Distinct from traditional method, each dielectric layer in the multilayer structure was ferroelectric phase, so the dielectric constant could be increased greatly through proper doping. Through experimental studies, the optimum conditions for the synthesis was as followed:the reaction time of3h, reactant concentration of0.2mol/L and [OH-] of0.5mol/L. The particle size is about300nm after surface-coating.Core and shell both have important influence on the properties. In order to study the effects of core in this method, the article chose materials with high Tc by comparing (Bi0.5Na0.5)TiO3and (Bao.985Bi0.01)TiO3as grain cores respectively, BaTi0.9Zr0.1O3as shell with low Tc growing on the surface of both grain cores. The results showed that (Bi0.5Na0.5)TiO3obviously improved the Curie temperature. But the low dielectric constant of (Bi0.5Na0.5)TiO3led to poor properties of the material. However, the dielectric constant and temperature stability of Ba0.985Bi0.01TiO3-BaTi0.9Zr0.1O3ceramic were superior to that of single phase materials. Its highest dielectric constant was close to6,000, temperature coefficients of capacity were12.0%,14.1%and8.3%at-55℃,130℃and160℃, respectively, which met X8R specification, dielectric loss was less than0.1. It was suggested that the nice properties of powders prepared by liquid-phase method were attributed to the ferroelectric phase.Based on the stability of Ba0.985Bi0.01TiO3-BaTi-xZrxO3ceramics, the microstructure and dielectric properties of Ba0.985Bi0.01Ti03-BaTi1-xZrxO3ceramics sintered at two different conditions were studied. When holding time was1h, with the increase of Zr content, dielectric constant declined, Curie point moved to lower temperature, and the temperature stability was improved. When x=0.1, temperature coefficients of capacity met X8R specification. When holding time is2h, ceramic materials with chemical uniformity were obtained. When x=0.001, the material had high permittivity of55,678, the temperature coefficient of capacity was less than21%in the temperature range of20～140℃, and the maximun dielectric loss was less than0.1.In order to research the effcts of shell on the material properties, Bao.985Bi0.01TiO3-BaTi1-xSnxO3ceramic materials with core/shell structure were prepared. The influences of Sn content and sintering condition on phase constitution and dielectric properties were discussed. With the amount of Sn increased, the peak value of permittivity increased before dropping, as well as temperature stability. When x=0.02, the temperature coefficients of capacity were-15%,14.4%and15%at-55℃,120℃and170℃, respectively, which was close to X9R specification, and the dielectric loss was less than0.5. Two-step sintering method can inhibit grain growth effectively. Samples sintered by this method had a high permittivity above22000and a falling dielectric loss, less than0.6. Its temperature coefficient of capacity was less than10%in the range of40～140℃.Due to the obvious effects of rare earth element on dielectric performance of barium titanate, Ba0.985Bi0.01Ti03-Ba1-1.5XLaxTiO3ceramic powders with core/shell structure were prepared. The influences of La content, addition of MgO and sintering temperature on the phase constitution and dielectric properties were discussed. When La content is0.010, dielectric constant and temperature stability were the most outstanding. Permittivity was higher than6000, dielectric loss was less than0.35, and the temperature coefficient of capacity was in the range of-15%～10%. Due to the existence of MgO, grains grew sufficiently and could complete densification in low temperature. Dielectric loss at room temperature was significantly lowered. Due to evaporation and agglomeration of Bi2O3in sintering process, samples sintered in different temperatures were very different from each other in microstructure and density. The permittivity and dielectric loss were also influenced. The maximum dielectric constant decreases first, and then increases before dropping again, mimicking the change of dielectric loss. Samples sintered at1250℃and1270℃had the best temperature stability, which meet X8R requirements. The permittiveity was near to9500, dielectric loss was less than0.27, and the temperature coefficient of capacity was in the range of-12.8%～13.8%. |
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