Pressure-induced negative capacitance and enhanced grain boundary conductivity in nanocrystalline solid electrolyte BaZrO3

Abstract

Proton-conducting BaZrO3-doped electrolytes are considered as potential high temperature proton conductors due to their high ionic conductivity and electrical efficiency in the operating temperature range of solid oxide fuel cells. However, doping leads to a decrease in grain boundary conductivity and greatly limits its applications. Here, the charge transport properties of sub-micro and nano-BaZrO3 electrolytes were studied by in situ high-pressure impedance measurements and first-principles calculations. Mixed ionic-electronic conduction was found in both samples in the whole pressure range. Pressure-induced negative capacitance in the tetragonal phase of nano-BaZrO3 was observed, which was related to the space charge layer of grain boundaries as well as the electrostrictive strain of grains. The enhanced electrostrictive effect was attributed to the existence of polar nano-domains in nano-BaZrO3. Furthermore, the coincident imaginary part of impedance and modulus peaks on the frequency scale indicated a non-localized carrier conduction in the tetragonal phase of nano-BaZrO3. The grain boundary conductivity of nano-BaZrO3 was enhanced by four orders of magnitude, and the impedance response changed from a constant phase element to an ideal capacitance, which was accompanied by the cubic to tetragonal phase transition. At a switching frequency of 0.1 Hz, the real part of the dielectric function of nano-BaZrO3 increases sharply with frequencies from negative to positive values, exhibiting a plasma-like Drude behavior. Our results provide insight into the optimization and application of BaZrO3-based proton conductors in solid oxide fuel cells.

Effects of defects on electrical transport properties of anatase TiO₂ polycrystalline under high pressure: AC impedance measurement

“…间的对应关系以及不同结构之间的相变机制， 对于拓宽 TiO 2 的应用领域有着重要 的意义． 不同于传统的合成途径，压力被认为是一种"清洁"的调控手段，可以更好 地揭示材料的结构与性能之间的关系 [7] ．基于此，在过去几十年里，人们对 TiO 2 体材料和纳米材料展开了大量的研究工作． 研究表明 TiO 2 体材料和纳米材料的高 压相变存在着很大 的差异，受到样品尺寸 、形貌及生长晶向等 因素的影响 [8][9][10][11][12][13][14] ．TiO 2 体材料的高压相变次序比较清晰，锐钛矿相 TiO 2 在高压下经历由锐钛 矿到 a-PbO 2 到斜锆石的结构相变 [15][16][17][18][19][20][21][22] ．而对于 TiO 2 纳米材料而言，其高压相变 十分复杂，锐钛矿 TiO 2 纳米晶在高压下可能观察不到 a-PbO 2 结构 [15,23,24] ，也可 能在压力作用下发生压制无定形相变 [25] ．Li 等人 [26] 对直径为 50-200 nm 的 TiO 2 纳米线进行了高压研究，认为其相变顺序与尺寸大于 10 nm 的 TiO 2 纳米晶相 似．此外，对掺杂 TiO 2 纳米粒子 [8] 和 TiO 2 纳米片 [27] 的高压研究中也观察到了独 特的高压特性． 但是， 对于 TiO 2 体材料和纳米材料的高压研究工作多集中于高压下的结构相 变，而对压力作用下材料性质变化的研究较少，尤其是针对高压下锐钛矿 TiO 2 的晶界性质、晶体中缺陷行为等方面的研究更是鲜有报道．随着科技的发展，高 压原位阻抗谱测量技术已广泛应用于体材料和纳米材料晶界行为的相关研究 [28][29][30][31][32][33] 检测窗口，详细的金刚石砧面微电路的集成过程参照我们前期的工作 [30] ．为了测 量电学及样品内部界面信息，实验中没有使用任何传压介质，实验过程中压力的 标定采用红宝石进行标压 [34] ． 交流阻抗谱电学测量的频率范围为 10 MHz-0. [35] ．如果在阻抗谱的低频区域，主弧后面观察到一个"尾链"，表明存在电极效 应 [36,37] ．在复阻抗平面图中，如果晶界弛豫时间和晶粒弛豫时间相差两个数量级 以上，则可以很好地分辨出两条半圆弧 [38] ．如果在谱图中没有观察到两条完整半 圆弧(如只有一个不规则圆弧等)，则说明晶粒和晶界的弛豫时间相差小于两个…”