Structure And Property Of BNT-based Wide-temperature Energy-storage Dielectric Ceramics

Dielectric capacitors are key electronic components in pulse power system,automobile,aerospace exploration,defense technology.With the continuous integration and miniaturization of electronic components,as well as their diversified application fields,higher performance is required for dielectric materials in both high-temperature properties and energy-storage properties.Bi_0.5Na_0.5TiO₃?BNT?based solid solutions are relaxor ferroelectrics.The temperature dependent permittivity curves possess double peaks,which is in favor of hight-temperature dielectric stability.Besides,the hysteresis loops show pinched shape,which is in favor of high energy-storage density.BNT-based ceramic systems are promising in both high-temperature capacitors and energy-storage capacitors.In this thesis,we focused on different BNT-based solid solutions and discussed their structure,dielectric and ferroelectric properties,trying to develop a new kind of high-temperature and high energy-storage density dielectric material system.The research was started from BNT-based binary solid solution,which has a relatively simple composition.Pure BNT was selected as the matrix,NaNbO₃?NN?as the modifying member.A series of wide-temperature dielectric stable BNT-NN ceramics were successfully prepared by solid state reaction method.The compositions with NN content of 25 mol%³5 mol%show excellent dielectric temperature stability with TCC_150°C50°C lower than 10%in the range of-60⁴00°C,dielectric constant?_{r 150°C} 150°C of 960-1240 and dielectric loss tan?_150°C50°C of 0.02,which is superior to many other temperature-stable dielectric materials.However,being restricted by the ferroelectricity of the matrix,the energy-storage properties of BNT-NN binary system were not largely improved.Therefore,in the second part of work,a new matrix BNT-BT was adopted,which possesses comparable P_m but much smaller P_r to pure BNT.A series of BNT-BT-RN?R=Na,Li,K,Ag?ternary solid solution dielectrics with both favourable high-temperature stability and high energy-storage density were obtained.For the composition 0.90?BNT-BT?-0.10NN,the working temperature range satisfying TCC_150°C?±15%reached 260°C with the dielectric constant?_{r 150°C} 150°C of³000 and dielectric loss tan?_150°C50°C of⁰.003,the maximum energy-storage density reached1.81 J/cm³ and kept stable in the temperature range of 25¹50°C.The failure behavior at high temperature and high voltage in BNT-BT-NN ceramics were also investigated.Based on the experimental results,a mean-time-to-failure empirical formula was established.It was predicted by this formula that at room temperature and electric field of 7 kV/mm,the mean-time-to-failure for BNT-BT-NN?⁰.5 mm in thickness?is 6.03×10⁶h.When temperature rise up to 150°C,the mean-time-to-failure reduces to 19 h.This part of work further supplement the high-temperature performance of BNT-based ceramics and is supposed to be an early exploration for failure behavior investigation in BNT-based capacitors.In both BNT-based binary system and ternary system,the energy-storage properties as well as the high-temperature dielectric stability were largely improved by the introduction of the second or third member.On the one hand,the improvement in energy-storage density was attributed to the reduction of ferroelectricity and the consequently pinched P-E loops.However,the energy-storage density failed to move forward a step due to the restriction of the low breakdown strength.According to literature reports,the insulativity of ceramics is closely related with breakdown strength.Thus,improve the insulativity of the samples would contribute to favourable energy-storage properties.Essentially,it is important to explore the conduction mechanism in BNT-based ceramics.We selected BNT-BT-NN ternary system as the research object in this part.Through analyzing the ac-impendence spectroscopy,the bulk conductivity at different measuring temperature was obtained,which followed the Arrhenius law.The activation energy of conduction E_a was estimated to be 1.24–1.55 eV.We propose that the electronic conduction may be responsible for the conduction process in BNT-BT-NN ceramics.On the other hand,the improvement in the high-temperature dielectric stability was attributed to the largely depressed temperature dependent permittivity curves,which resulted from the dielectric relaxation behavior.Based on the theory that BNT-based ceramics are relaxors,two kinds of polar–nano–regions?PNRs?with different symmetry exist in BNT-based ceramics and they would transform with temperature.However,the correlation between the structural changes and the dielectric relaxation behavior is not clear.In this part,dielectric relaxation behavior in BNT-BT-NN ternary system were investigated.The alternating-current impedance analysis revealed that dielectric relaxation is related to PNRs response.Two characteristic temperatures,T_f and T_B were calculated.A correlation between the PNRs evolution and the relaxation behavior was constructed.