铌酸钠基陶瓷的相界调控及储能性能

Phase Boundary Modulation and Energy Storage Properties of NaNbO_(3)-Based Ceramics

铌酸钠(NaNbO_(3))基无铅陶瓷具有超高的储能密度,在高功率介电储能电容器领域引起了广泛关注。然而,纯NaNbO_(3)陶瓷在室温表现为矩形形状的类铁电电滞回线,并伴随着大的回滞和高的剩余极化强度,往往导致大的能量损耗。为优化NaNbO_(3)的储能性能,基于相界调控策略采用传统固相法制备了(1–x)NaNbO_(3)–xCaTiO_(3)(0.08≤x≤0.18)陶瓷,并使用CuO作为助烧剂。结果表明,该体系在0.09≤x≤0.16范围内存在因晶体结构改变而引起的多晶型相界区域,其中相界组分0.84NaNbO_(3)–0.16CaTiO_(3)在590 kV/cm时的最大极化强度为42.2μC/cm^(2),可逆储能密度达到6.1 J/cm^(3),储能效率为77%。同时该陶瓷表现出良好的频率和温度稳定性,以及优异的放电性能。为铌酸钠基高性能储能电容器材料的开发提供了新思路。

Introduction Lead-free sodium niobate(NaNbO_(3))based ceramics with a superior energy storage density have attracted recent attention in high power dielectric energy storage applications.However,a pure NaNbO_(3)(NN)ceramic exhibits a square-like square hysteresis loop associating with a large hysteresis and a high remnant polarization at room temperature,leading to a high energy dissipation.In general,stabilizing the antiferroelectricity in NN ceramic is one of the effective measures to solve the problems above.However,despite efforts are made on this measure,the large hysteresis is still kept due to the existence of antiferroelectricferroelectric phase transition.The compositions in the phase boundary region usually exhibit abnormally enhanced electrical properties and relaxation characteristics,thus constructing a phase boundary region in NN through composition modulation is another promising method.Moreover,sodium element volatilizes inevitably during the high-temperature and long-time sintering process,giving rise to a poor sintering quality in NN system,which is not beneficial to achieving superior energy storage performance.In this paper,(1–x)NaNbO_(3)–xCaTiO_(3)(NN–CT100x,0.08≤x≤0.18)ceramics were prepared by a conventional solid-state method with CuO as a sintering aid.The phase structure and polymorphic phase boundary in NN–CT100x system were investigated,and the energy storage performance of compositions in the phase boundary region was analyzed.Materials and method For the synthesis of NN–CT100x powder,Na2CO_(3)(≥99.8%,in mass fraction,the same below),Nb2O5(≥99.9%),CaCO_(3)(≥99.0%),TiO_(2)(≥98.0%)and CuO(≥99.0%)were used as raw materials(Shanghai Sinopharm Chemical Reagent Co.,Ltd.,China).The raw materials were weighed/mixed according to the stoichiometric ratio and ground with anhydrous ethanol in a ball mill with zirconia balls for 12 h.After dried and sieved,the powder was calcined at 900℃for 6 h.The calcined powders were mixed with 1.5%(in mole fraction)of CuO as a sintering aid and further ground for 12 h.The ground powder was hand-pressed into discs with the diameter of 7 mm and the thickness of 1.2 mm and then further densified by cold isostatic pressing at 300 MPa for 30 min.The NN–CT100x discs were sintered at 1050–1250℃for 2 h.Results and discussion Based on the results by X-ray diffraction,Raman spectroscopy and dielectric behavior,an‘antiferroelectric P phase(orthorhombic,Pbma)–ferroelectric INC phase–paraelectric CT phase(orthorhombic,Pbnm)’polymorphic phase boundary(PPB)region exists as 0.09≤x≤0.16 when CaTiO_(3)content increases.The polarization exhibits an enhancement in compositions within this PPB region.The optimum comprehensive energy storage performance is obtained as x=0.16,having the energy storage density(W_(rec))of 3.04 J/cm^(3) and efficiency(ƞ)of 84.4%at 300 kV/cm,respectively.The difference between the maximum polarization(Pm)and the remnant polarization(Pr)of the NN–CT16 ceramic increases as the electric field increases,thus obtaining an admirable energy storage performance.This composition is in the PPB region and contains a few antiferroelectric phases.These antiferroelectric phases are easily transformed into ferroelectric phases at high electric fields,whichƞreduces from 92.4%to 77.0%.The NN–CT16 composition has a high Pm of 42.2μC/cm^(2),a high W_(rec) of 6.1 J/cm^(3) andƞof 77%at 590 kV/cm.For the NN–CT16 ceramic,W_(rec) andƞboth exhibit a frequency stability at 1–300 Hz,with minimal variations of<10%and<8%,respectively.Furthermore,the W_(rec) andƞalso demonstrate temperature-insensitive characteristics at 25–160℃,with small variations of<10%and<5%,respectively.The discharge energy density(Wdis)of the NN–CT16 ceramic increases and reaches a plateau in a short time.The t0.9(time at which 90%of the stored energy is released)is less than 8μs.Moreover,the t0.9 decreases with the increasing electric field or temperature,indicating the discharge rate accelerates,which is probably ascribed to domains respond quickly and switch more easily at high electric fields or high temperatures.Conclusions Based on the phase boundary modulation strategy,the NN–CT100x(0.08≤x≤0.18)ceramics were designed and prepared via introducing CaTiO_(3)into NN with CuO as a sintering aid.The polarization of the compositions in the phase boundary region was enhanced.The NN–CT16 ceramic exhibited a high maximum polarization of 42.2μC/cm^(2),a high energy storage density of 6.1 J/cm^(3) and an energy efficiency of 77%at 590 kV/cm.It also exhibited a good frequency/temperature stability and an excellent discharge performance.This ceramic could be used as a promising dielectric energy storage material.