The Preparation And Properties Of High-Temperature Ceramic Capacitor Materials Based On Sodium Bismuth Titanate

Capacitors are widely used in every electronic device because they can serve a host of functions,such as bypassing,filtering,coupling,decoupling,pulse discharge,dc blocking,and so on.Comparing with some other kinds of capacitors,ceramic capacitors have excellent temperature-dependent capacitance properties,frequency-impedance characteristics,and low cost,which makes them a type of capacitor with the largest production and demand at present.With multiple developments of advanced electronic devices in the past few decades,the demands on high-performance capacitors are increasing rapidly.Nowadays,in many industry fields,such as aviation and aerospace,down-hole drilling,automobile electronic,national defense,and so on,the operating temperature of electronic devices can reach200°C or even 300°C,which puts forward higher requirements for the temperature stability of ceramic capacitor dielectrics.Hence,it is extremely essential and necessary to investigate dielectric materials of high-temperature ceramic capacitors.In this work,we analyzed the relaxor behaviors of material systems based on modified sodium bismuth titanate(Bi_0.5Na_0.5TiO₃,BNT)ceramics,and gradually optimized their temperature stability of dielectric properties both at high temperature（≥200°C）and low temperature（<25°C）,and successfully designed capacitor dielectric materials with superior temperature stability of capacitance and low loss in the range of-55—400°C.In the meantime,tentative MLCC chips were also fabricated.This work is not only useful for the research of high-temperature dielectrics,but also promotes the practical application of BNT-based materials for high-temperature capacitors.Firstly,the effect of antiferroelectric NaNbO₃ on composition control in BNT-BT solid solutions was deeply investigated.And the material system（1-x）（0.94BNT-0.06BaTiO₃）-x NaNbO₃（BNT-BT-xNN）withdifferentdesigned compositions was built successfully.The decreased polar rhombohedral phase shown in XRD and the weakened macroscopical polarization in P-E test indicate that NN addition can dilute polar nanoregions（PNRs）and weaken their mutual coupling,which is very conducive to flattening the curves of temperature dependent dielectric permittivity at the expense of a small reduction in dielectric permittivity value.And dielectric materials with relatively high permittivity and wonderful temperature stability in high temperature range were finally obtained.Secondly,to deal with some limitations of BNT-BT-NN system,such as strong dependence on temperature and frequency at low temperature,and high dielectric loss at high temperature,the paraelectrics CaZrO₃,which is consist of weak polarization ions,was introduced into BNT matrix.The new system（1-x）[0.94（0.75BNT-0.25NN）-0.06BT]-xCaZrO₃（CZ100x）was designed and its bulk samples were produced by solid state reaction method.The influence of CaZrO₃ contents on dielectric temperature stability,loss and insulation characteristics at high temperature was basically discussed in this study.The results show that polarization of PNRs was weakened further by higher random fields and local strain effects with CaZrO₃addition,leading to greatly improved temperature and frequency stability of dielectric properties.Moreover,the activation energy E_a and the resistivityρof materials can be enhanced by the regulation of CaZrO₃ as well.Among all samples,the composition of CZ10 shows the optimum electric properties,including the small deviation in capacitance(ΔC/C_25°C≤±15%)across the ultra-wide temperature ranging from-58°C to 400°C,low loss（tanδ≤0.02）between-60°C and 300°C,accompanied by high DC resistivity with 10⁷Ω·m at 300°C.Eventually,based on the excellent electrical characteristic of CZ10 bulk samples,the tentative MLCC chips of the same component was manufactured in order to promote BNT-based high-temperature materials in real application better.The microstructure of MLCCs was observed and analyzed and differences of dielectric properties,temperature stabilities,and energy storage properties between MLCC components and bulk samples were discussed.The variation trend of permittivity,dielectric loss andΔC/C_25°C of MLCCs resembles those of bulk samples,which suggests that the overall properties of the bulk samples are well maintained in the process of transferring to MLCC chips.SmallΔC/C_25°C（<±15%）in the temperature range of-60—385°C and low loss factor（tanδ≤0.02）between-55°C and 340°C were achieved.The temperature stability of the energy storage density is also maintained at a relatively high level.