Preparation Of BNT-based Ceramics By Solid Phase/cold Sintering/SPS And Their Dielectric And Energy Storage Properties

In recent years,with the rapid development of electronic science and technology,ceramic capacitors are widely used because of their high power density and fast charging and discharging process.However,the development of integration and miniaturization puts forward higher requirements for capacitor energy storage performance.Ferroelectric ceramics have excellent energy storage performance,among which lead-based energy storage materials have the best performance.However,the volatility and toxicity of lead are harmful to the environment and human,the development of lead-free energy storage materials with comparable energy storage performance has become hot.Na0.5Bi0.5TiO3（BNT）-based ceramic materials are widely studied.The polarization is high and the hysteresis loop will become long and thin with the increase of temperature,which is conducive to improvement of electric energy storage.However,the residual polarization and coercive field of BNT are high at room temperature,and the relatively full hysteresis loop is not conducive to energy storage.In general,the purpose of modification of BNT-based materials is to ensure relatively large saturation polarization,reduced residual polarization,and increased breakdown strength,so as to obtain better energy storage density and energy storage efficiency.This present research focuses on improving energy storage and high temperature dielectric stability of BNT-based ceramics via doping of NaNbO3;then using different synthesis methods including cold sintering process and spark plasma sintering to prepare the BNT-based ceramics;discussing influence of synthesis parameters on structure and electrical properties of the ceramics.Finally,thick film and multilayer ceramics were prepared by means of tape casting,and their structure,dielectric and ferroelectric properties were studied.The lead-free ceramics（1-x）(0.94(Bi_0.47Na_0.47Ba_0.06)TiO₃-0.06BiAlO3)-xNaNbO3 were prepared by means of a traditional solid-state sintering method.All ceramics have pure perovskite structure and average grain sizes around 1 μm.The NaNbO3-doped ceramics display drastically reduced temperature coefficient of capacitance（TCC）compared to the ceramics without doping.The samples with x＝0.01 show good energy storage characteristics,with the energy storage density reaching 1.68 J/cm3 and the energy storage efficiency reaching 62%.Dielectric properties remained stable between 66℃ and 266℃.The samples with x=0.02 exhibit high permittivity and excellent temperature stability between 67 0C and 380℃,meeting the requirement of TCC≤±15%.The ceramics show good energy storage behavior with excellent frequency stability from 0.5 Hz to 100 Hz,great temperature stability from 25℃ to 130℃,and excellent fatigue resistance within 105 fatigue cycles.0.99(0.94(Bi_0.47Na_0.47Ba_0.06)TiO₃-0.06BiAlO3)-0.01NaNbO3 ceramics were prepared by means of cold sintering（CS）.After the CS samples being annealed,a secondary phase appears,which was mainly related to Na0.5Bi4,5Ti4O15 with the bismuth layered structure.The CS samples were annealed at temperatures between 1000 and1150℃.The grain sizes of the samples increase from 0.4 μm to 1 μm with increasing the annealing temperatures,and the relative densities are above 95%.The results about dielectric and ferroelectric behavior of the samples annealed at different temperatures demonstrate that the different annealing temperatures have little effect on the performance of the samples.The maximum permittivity of the samples annealed at different temperatures fluctuated around 1800 and the dielectric loss was 0.05.After being annealed at 1000℃,the samples show the energy storage density of 1.03 J/cm3 and energy storage efficiency of 74.3%.The results show that the cold sintering processing not only broadens the temperature window for obtaining dense ceramics,but also lowers synthesis temperature.BNBTA-0.01NN ceramics were obtained by spark plasma sintering.Sintering temperatures were chosen as 900℃,925℃,950℃,and 975℃.The relative densities of all samples are above 95%.The density increases gradually with the increase of sintering temperature.XRD results show that all samples are pure phase and no secondary phase is found.The average grain size is around 80 nm,which is significantly smaller than that of the samples via the conventional solid-phase sintering and cold sintering.The values of maximum permittivity of all samples fluctuate around 2000,and the frequency dispersion always exists in the test temperature range.The energy storage density of the samples is 1.24 J/cm3.The sintering temperatures have little effect on dielectric and energy storage characteristics of the samples.Compared to the samples via the traditional solid-state sintering,the values of dielectric constant decrease obviously.The breakdown strength is improved up to 160 kV/cm.(Bi_0.47Na_0.47Ba_0.06)TiO₃（BN6BT）thick film was prepared by means of tape-casting and lamination processing.The pH of the slurry was determined to be 9,the concentration of dispersant was 43%,and the proportion of adding powder was 1%.The thickness of blank is about 100 μm,and the thickness of the laminated ceramic body is about 2 mm.The ceramic samples sintered at 1100℃show good densification.No obvious holes were found in the samples and cubic grains exist in the samples.The permittivity values of the samples change dramatically with the measurement temperature,especially at 1 kHz.With increasing temperature,the values of dielectric loss continue increasing,which are far greater than those of the samples prepared via the.conventional sintering method.