Study On The Temperature Stability Of Dielectric Constant Of Barium Titanate-Based Dielectric Ceramics

Multilayer Ceramic Capacitor（MLCC）has been one of the most important chip passive components in modern electronic products due to its merits such as small size,high performance,low cost and easy surface mounting.In recent years,with the gradual expansion of its application fields,the use environment of MLCC becomes harsh increasingly.For example,the control and sensing systems of automotive and aerospace vehicles are required to operate normally at a high temperature more than 200℃,thus promoting the development of X9R MLCC(X9R:the MLCC capacitance change rate△C/C_25℃≤±15%are within the temperature range of-55–200℃).Ba Ti O₃（BT）possesses many advantages,such as high permittivity,low dielectric loss,low cost,and environmental protection,making BT become one of the most popular and potential dielectric materials for MLCC.However,the permittivity（ε_r）of BT mutates abruptly near its Curie temperature（T_C）and cannot meet the temperature stability requirements for the capacitors.In our study,the peak shifting effect produced by La₂O₃ and the broadening effect brought by Bi_0.5(Mg_0.5Ti_0.5)O₃（BMT）were used to regulate the permittivity temperature stability of BT.We intended to prepare MLCC dielectric ceramic materials meeting the X9R application standard,by adjusting the content of La₂O₃ and BMT,and optimizing the process of preparing ceramics.The effect of La₂O₃ on the structure and dielectric properties of BT was studied.The results showed that La₂O₃ could form a stable solid solution Ba(1-_x)La_2x/3Ti O₃ with BT,and with the increase of La doping content,Ba(1-_x)La_2x/3Ti O₃ gradually changed from a tetragonal structure to a pseudo-cubic structure.Doping La had no obvious influence on the valence state of Ti,but had a great influence on the existence state of O.With the increase of doping amount of La,the concentration of lattice oxygen gradually decreased,while the concentration of adsorbed oxygen gradually increased.La could bring about the grain refinement,increase the density of ceramics,and effectively improve the sintering performance of BT ceramics.In terms of dielectric properties,La produced very obvious"peak shifting effect"on Ba(1-_x)La_2x/3Ti O₃ ceramics,and the effect of"peak shifting effect"was approximately linear variation with the change of its doping amount.The permittivity of Ba(1-_x)La_2x/3Ti O₃ ceramics was increased by increasing the incorporation of La appropriately,which was mainly aroused by the increased density of ceramics.A small amount of La significantly reduced the dielectric loss.However,with the further increase of La doping amount,the grain size increased along with the increase of oxygen vacancy content,leading to the increase of the dielectric loss in the high temperature.The effect of BMT on the structure and dielectric properties of BT was studied.The results showed that BMT could form a stable solid solution with BT,and with the increase of BMT doping,BT–BMT gradually changed into a pseu-docubic structure.The increase of BMT content made no difference on the valence of Ti,but it could reduce the concentration of lattice oxygen and increase the concentration of adsorbed oxygen.BMT could improve the sintering performance of BT ceramics and was benefit to obtain dielectric ceramics with good density at a lower sintering temperature.In terms of dielectric properties,BT–BMT exhibited obvious relaxation ferroelectric characteristics,which were confirmed by the improved Curie-weiss law.Compared with pure BT,the dielectric stability of BT–BMT was greatly improved.When y=0.25,0.75BT–0.25BMT possessed the optimal permittivity temperature stability:1030±15%（50～272℃）.A series of 0.75Ba(1-_x)La_2x/3Ti O₃–0.25Bi(Mg_0.5Ti_0.5)O₃（x=0～0.2）ceramics were prepared successfully.The results showed that within the whole doping concentration range,La₂O₃ could form a stable solid solution with perovskite structure with 0.75BT–0.25BMT.The incorporation of La₂O₃ could reduce the long-term ordering of 0.75BT–0.25BMT,improve the density of ceramics,and promote grain growth.Doping La₂O₃could enhance the permittivity frequency dispersion and the dispersion of T_m near the relaxation peak of 0.75BT–0.25BMT,and effectively improve the permittivity temperature stability of 0.75BT–0.25BMT.Among all of samples,0.75Ba_0.8La_0.4/3Ti O₃–0.25Bi(Mg_0.5Ti_0.5)O₃ met the requirements of X9R.Its temperature stability range of dielectric constant was-70～238℃,and the room temperature dielectric constant was 572.The preparation process of ceramics affected the density and grain size of the ceramics,as well as the permittivity temperature stability range of the ceramics.0.75Ba_0.8La_0.4/3Ti O₃–0.25Bi(Mg_0.5Ti_0.5)O₃,pre-sintered at 900℃ for 2h,finally sintered at 1200℃ for 2h,possessed the optimum permittivity temperature stability:571±15%（-76～244℃）.