The Studies On The Preparation And Modification Mechanism Of MLCC Dielectric Materials With High Temperature Stability Over Wide Range

Multilayer ceramic capacitors(MLCC) are important fundamental components of electronic information technology.To meet the requirements of miniaturization, lightweight and integration of electronic system,MLCC are developing rapidly towards the direction of high frequency,wide temperature and large capacitance.The key materials of MLCC are R(â–³C/C≤±15%) system materials composed of BaTiO₃ based ferroelectric ceramics.The current researches are focused on investigating and developing X8R,X9R,even ultra-temperature MLCC materials because the present X7R materials can not afford stable dielectric properties above +125℃.The target of this thesis is to enhance the dielectric temperature stability of BaTiO₃ system ceramics on the basis of the core-shell structure formation mechanism.High temperature resistant MLCC materials have been developed by means of shifting the Curie point(Tc) towards higher temperature with the addition of Bi_0.5Na_0.5TiO₃(BNT).Following are the main results:1.The effective ways to prepare MLCC dielectric materials are enhancing the T_C and flattening the Curie peak of BaTiO₃ ceramics.The additions of BNT and BiNbO₄ to BaTiO₃ were investigated.It has been found that BNT addition can shift the T_C of BaTiO₃ to higher temperature,flatten the Curie peak,and suppress the capacitance variation at high temperature simultaneously,that is helpful to improve the high temperature stability of dielectric properties of BaTiO₃ ceramics.In the case of BiNbO₄ doping,the Curie peak is markedly suppressed and broadened and the temperature-capacitance coefficient curves become flattened.2.The formation of core-shell structure is the key to achieve flat capacitance temperature characteristic.The effects of Nb₂O₅ and ZnO doping on the dielectric properties and mierostructure of BaTiO₃ ceramics were studied in detail.The formation mechanism of core-shell structure in BaTiO₃-Nb₂O₅-ZnO system is brought forward. Core-shell structure is observed in single Nb or single Zn-doped samples.The diffusion of Nb into BaTiO₃ grains will be promoted by increasing either Nb content or sintering temperature,resulting in a decrease of the fraction of grain core and an increase of the fraction of grain shell.The grain shell of Zn-doped samples is thinner compared with that of Nb-doped samples,due to a rather low solid solubility and diffusion rate of Zn element in BaTiO₃.Core-shell structure is also found in Nb-Zn-codoped samples.Nb plays the leading role to form core-shell structure.On the other hand,the grain growth can be effectively inhibited by Zn,contributing to the formation of core-shell structure. Moreover,the diffusion of Nb into BaTiO₃ grains is promoted by the liquid produced by the eutectic compound during sintering in Nb-Zn-codoped samples,which is in favor of the formation of core-shell structure.3.The engendering mechanism of internal stress in BaTiO₃ ceramics has been summarized.It is proposed that internal stress originates from inhomogeneous thermal expansion,ferroelectric phase transition,grain boundaries,secondary phase and mismatch between grain core and grain shell.The computation module of internal stress that is resulted from the difference between thermal expansion coefficients of grain core and grain shell is established.Moreover,the microstrain in Nb₂O₅-doped and MnNb₂O₆-doped BaTiO₃ ceramics is calculated taking advantage of XRD data and Maud software.The results indicate that the change tendency of T_C versus doping content is similar to that of microstrain.Therefore,the mechanism governing the shift of T_C is proposed as follows:internal stress in ceramics is the predominant factor determining the shift of T_C.T_C moves to higher temperature as internal stress increases. Moreover,higher internal stress will be produced either by the mismatch in core-shell structure or by the expansion difference between secondary and BaTiO₃ phases,leading to an increase in T_C.4.Based on BT-BNT system,high performance and temperature stable MLCC materials have been prepared.Novel X8R(-55℃～150℃,â–³C/C_{25℃≤±15%} material suitable for firing in air is obtained in BaTiO₃-BNT-BiNbO₄ system.In the case of 3.0～4.0 mol%BiNbO₄ addition,the X8R specification is satisfied and a room temperature dielectric constant of 2780 is achieved.Novel X9R(-55℃～175℃,â–³C/C_25℃≤±15%) material suitable for firing in air is obtained in BaTiO₃-BNT-Nb₂O₅-ZnO system,with dielectric properties ofε25℃≈1800, tanδ_25℃≤1.5%andρ_v≥10¹¹Î©Â·cm.Dielectric material with ultra-temperature stability (-55℃～200℃,â–³C/C_{25℃≤±15%} is also obtained in BNT-BaTiO₃-CaCO₃ system, which is applied to sinter in air.With the addition of≥24 mol%CaCO₃,dielectric properties ofε_25℃≈1000,tanδ_25℃≤1.5%andρ_v≥10¹¹Î©Â·cm are obtained.