Research On Temperature-stable MLCC Dielectrics And Modification Mechanisms

Multilayer ceramic capacitors(MLCC) are important electric components which are used in almost all areas of electronics.Barium titanate(BaTiO₃,BT) was the most widely developed and used perovskite ferroelectrics.The reason behind a wide range of applications is that the barium titanate boasts of high specific permittivity and long lifetime of insulation resistance.Currently,the development of MLCC has focused on the BT ceramics.Moreover,present application of MLCC required preferable temperature characteristics.However,domestic low-frequency PME-MLCC military used X7R series of ceramic materials almost entirely dependent on imports,X8R PME-MLCC materials research is still at the fledgling stage,development for the higher temperature resistance PME-MLCC ceramic materials is almost blank.It is for this reason that the goal of this study was to investigate temperature stable PME-MLCC dielectrics based on BaTiO₃.The main works and conclusion are summarized as follows:1.The influence of rare earth dopants upon dielectric properties of BT-Nb₂O₅-ZnO system are investigated in detail.The experiment results have revealed that,according to the effects on the low temperature peak(at about 40℃) and the high temperature peak(at about 127℃) of the variation rate curve of capacitance of BT ceramics,the doping rare earth oxides can be divided into three categories.The dielectric properties and micro-properties of Gd/Ce Co-doped and Gd/Nd Co-doped BaTiO₃ are discussed. The vary of the dielectric constant at room temperature can be explained by the occupational sites of ions in perovskite structure.Low porosity and high density was observed in Gd/Ce Co-doped BaTiO₃ ceramics.The high temperature variation rate of capacitance for Gd/Nd co-doped BT ceramic is found to be only governed by the effect of Gd.Nano-scale rare earth dopant(Nd-Li-B-Si) was prepared by sol-gel processing to develop high performance dielectrics.The use amount of Nd in nano dopant have considerable effect to BaTiO₃,quite the contrary,the effect of Nd₂O₃ on BaTiO₃ dielectric properties is not obvious when nano dopant exist.The non-linear regression equation y=b₀+∑b_ix_i+∑b_iix_i²+∑b_iiix_i³ regarding the relationship between the dosage of different dopants and the dielectric properties were established based on the experimental data to develop the environmental friendly X7R MLCC ceramics successfully.X7R type PME-MLCC sintered at 1104℃prepared from our composite with independent intelligence possessed excellent dielectric properties:K=2950±100, dielectric loss＜1.5%,insulation resistance＞5×10¹⁰Ω,â–³C/C＜±10%(-55℃～125℃), average breakdown voltage＞1500V/mil.2.Calcium borosilicate(CBS) glass ceramic have been doped into BT-Nb₂O₅-ZnO system to obtain the X8R materials.Experiment showed that CBS can significantly both reduce the sintering temperature and flat the temperature characteristics of BaTiO₃.The influence of CaZrO₃ on the dielectric properties and microstructures of BaTiO₃-based ceramics has been investigated.Comparison of the experimentally measured temperature coefficient of capacitance(TCC) at 125℃and refined microstrain of BaTiO₃ ceramics with various concentrations of CaZrO₃ gave the indication that the high temperature peak of TCC was dependent on the microstrain of the sample in a manner.The high performance dielectrics satisfying X8R are achieved in the BT-Nb₂O₅-ZMT new system.The influence of Nb₂O₅ and ZMT addition on phase transition,microstructure,TCC,temperature dependence of permittivity and Curie temperature have been studied profoundly.Regression analysis based on uniform design was introduced as a new approach of designing BT-Nb2Os-ZMT based X8R ceramics. By using partial differentiation,trend analysis,surface/contour plots and the established regression model,optimized proportion conditions were obtained at given ranges to achieve lead-free X8R ceramics with various dielectric constants ranged from 1500 to 3300.X8R type PME-MLCC sintered at 1110℃prepared from our composite with independent intelligence had good dielectric properties:K=2200±100,dielectric loss＜1.6%,insulation resistance＞5×10¹⁰Ω,â–³C/C＜±7.5%(-55℃～125℃),average breakdown voltage＞1050V/mil.3.The effects of tetragonality and secondary phase on the Curie point of BaTiO₃ ceramics were investigated in BaTiO₃-MnNb₂O₆ system using differential scanning calorimetry(DSC),X-ray diffraction(XRD) and scanning electron microscope(SEM). The study of BaTiO₃ ceramics sintered with MnNb₂O₆ additions up to 0.50 mol% reconfirms the hypothesis that the Curie point decreased as the room temperature tetragonality decreased.Additions of MnNb₂O₆ to BaTiO₃ above 1.0 mol%were found to result in the formation of the secondary phase of Ba₂Ti₃Nb₄O₁₈ and shift the Curie point to higher temperature by means of alters the internal stress system of BT ceramics.The presintering process on Curie point in BaTiO₃-BiNbO₄ ceramics was studied in detail from microstructure consideration.Shift of Curie point significantly increased when presintering technology was introduced to BaTiO₃ ceramics.TCC of BT ceramics containing titania and silica additives is found to be strongly affected by Mn²⁺ concentration.The reduction of the volume fraction of the secondary phase obtained from SEM and XRD analysis proved that the introduction of Mn²⁺ can restrain the crystallization of TiO₂/SiO₂ glass in BT ceramics.The secondary phase altered the internal stress system,and in consequence TCC improved a lot.The new starting powder BTBNT with high Curie point was synthesized by conventional solid-state reaction.Several effective peak suppressors have been doped into BTBNT-Nb₂O₅-ZMT system in order to obtain the X9R materials.4.Using high purity BaTiO₃ powders of various particle size,we investigated the effects of grain size on the room temperature crystal structure,microstrain and Curie point.As the initial particle size of BaTiO₃ powders increased,the ceramic grain size increased,room temperature tetragonality decreased,microstrain decreased and Curie point shifted toward lower temperature,respectively.Based on a geometric model depicting the state of stress in core-shell structure,only existence of compressive stress but not exist tensile stress was observed in Nb doped BaTiO₃ ceramics.According to our model,Landau-Devonshire theory was applied to explain the change of the room-temperature permittivity successfully.The effects of Mn²⁺ concentration on dielectric properties and microstructure of barium titanate without/with CBS glass doping were investigated.The SEM and XRD analysis showed that Mn²⁺ can induce the crystallization of CBS glass.The formation of the strip-shaped secondary phase of Ca₄Mn₄Si₈O₂₄,which alters the internal stress system of BT ceramics,is presumed to be the factor that improves the TCC of barium tianate.