| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系 === 88 === Commercial ceramic capacitors for application , briefly to discriminate Y5V , Z5U , X7R for EIA . Especially, it is the severest for X7R specification. Following the devel-opment and progress of multilayer ceramic capacitors technique (MLCCs). It is a ten-dency to design the elements that thinning and multilayers are inevitable. following , it is gradually severe to the demands of dielectrics design for high capacitance and micro-grain (~1μm ).Our research use different doping processes for Nb5+ and Co3+ to BaTiO3 dielectrics to study the relation between the microstructure and dielectric constant vs temperature stability and to find the process to fit in with X7R specification . The result shows having semiconducting phenomenon.
In A process, when the dopant amount Nb5+/Co3+ ratio nearly to or equal to 2. Di-electric peak mainly shift to low temperature and showing a broaden diffuse phase tran-sition behavior when having a Nb5+ increment. This process is impossible to get a flat dielectric vs temperature curve. In B process ,because two compositions have similar chemical properties and melting point so that it is easily to become solid solution.We can not use low diffusivity of Nb5+ in BaTiO3 to hold the dielectric property of high cu-rie temperature phase and low curie temperature phase individually.This result can not get as a result as low firing temperature X7R materials of Pb- relaxor - BaTi(1-x)ZrxO3 composites . Nevertheless, we also found semiconducting phenomenon in this process.
In C process,because we additionally doped ZrO2 and MnO2,we resolve semicon-ducting problems and get small grain .The dielectric constant have probably 2500 which dielectric constant vs temperature curve can keep fairly flat characteristic from 10℃ to 80℃.But ,the microstructure has a obvious change when increase soaking time and sintering temperature.The microstructure changes from uniform grains to Bimodal grains.This phenomenon explain abnormal grain growth to change dielectric constant severely.Moreover, a dielectric peak in about 80℃ gradually raises.Only in D proc-ess ,we can meet X7R specification which dielectric constant is about 2800~3000.The microstructure in this process ,we can obtain smaller than 1μm grain size and so-call core-shell grain strcuture.However,Our research confirm that core-shell grain structure really not cause the main reason of X7R,probably other mechanism.
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