1.College of Engineering, Bohai University, Jinzhou 121013, China 2.College of New Energy, Bohai University, Jinzhou 121013, China 3.State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11404032, 11404034), the Foundation of the Science and Technology Department of Liaoning Province, China (Grant Nos. 20170540014, 20170540009), and the Foundation of the Education Department of Liaoning Province, China (Grant No. LJ2019013).
Received Date:26 April 2019
Accepted Date:29 July 2019
Available Online:01 October 2019
Published Online:20 October 2019
Abstract:In this paper, the grain and grain boundary characteristics of pure rutile TiO2 under pressure are investigated by electrochemical impedance spectroscopy equipped with diamond anvil cell (DAC). Only one semi-circle can be detected under each pressure in a range of 1.4–11.5 GPa. With the pressure increasing, the shape of semi-circle is unchanged, while the size of semi-circle gradually decreases, which can be attributed to the decrease of bulk resistance due to the reduction of band gap under pressure. The absence of grain boundary characteristic in the impedance spectra signifying that Schottky barrier is not present at the grain boundaries. With further increasing pressure, an interesting phenomenon can be observed above 12.7 GPa. The shape of semi-circle is distorted, and exhibits two overlapping semi-circles. The first semi-circle (high frequency) originates from the contribution of bulk, and the second one (low frequency) can be ascribed to the effect of grain boundary. The occurrence of grain boundary semicircle indicates that the aggregation of space charges at the grain boundary. In this case, the phase transformation from rutile to baddeleyite structure occurs, the electric transport mechanism is changed, and new lattice defects are formed. Also, two discontinuous points (11.5 and 15.4 GPa) can be detected in the resistance curve. The remarkable change of resistance occurs at 12.7 GPa which is corresponding to the phase transition from rutile to baddeleyite phase. The occurrence of phase transition leads the new interfacial energy to occur, the total energy of system to increase, and the movement of carriers to impede. Thus, the resistance increases significantly, and the maximum value occurs at 15 GPa. Further analysis indicates that the space charge potential is modified with pressure increasing, implying that the electrical transport properties of TiO2 are related closely to phase transition. With the pressure increasing from 12.7 to 25.2 GPa, the irregular change of space charge potential can be attributed to the rutile and baddeleyite phase coexisting. When the pressure is higher than 25.2 GPa, the space charge potential is a constant (about 30 mV). According to the investigations, the TiO2 grain boundary space charge potential under pressure is mainly contributed from two parts: the electrostatic interaction and the elastic interaction. Keywords:high pressure/ TiO2/ impedance spectroscopy/ grain boundary
全文HTML
--> --> --> 1.引 言在环境压力下, TiO2存在三种不同的相结构: 金红石相、锐钛矿相和板钛矿相. 在自然界中, TiO2主要以金红石相存在, 金红石相TiO2是天然岩石的主要成分之一, 与二氧化硅有着相同的结构. 研究表明, 金红石相TiO2在高压下相结构等行为变化与地幔中某些矿物质的行为变化非常接近, 这使得金红石相TiO2高压下的性质变化在地球科学研究领域有着十分重要的研究价值[1-3]. 到目前为止, 对TiO2高压下性质变化的研究方法包括高压X-光散射技术[4,5]、高压Raman光谱技术[6-9]以及相关的理论模拟工作[9,10]等. 高压电学实验技术与地球物理理论计算相结合, 为研究地球内部物质的性能变化提供了可选择的方法. 同时作为其他研究方法的补充手段, 可以获得地球内部物质的各种物理状态等信息, 从而进一步了解地幔的相关性质. 在过去几十年里, 地球内部物质的电学性质研究主要集中在高压电阻率的测量、高压霍尔效应测量、高压阻抗谱的测量等[11-14]. 其中交流阻抗谱技术能够从不同角度例如材料的晶界、极化现象, 以及频率或时域等方面评价并区分材料的电学性能. 通过对TiO2测量阻抗谱的拟合, 可以得到TiO2晶界电阻、势垒、晶界空间电荷势等一系列参数, 对以后的高压地学研究提供一定的参考数据. 通过对材料晶界性质的测量和分析, 可以获得材料在高压环境下的众多性质信息. 金红石结构TiO2高压下的相变次序与地幔中丰富的物质(例如SiO2)相类似, 但是有着相对较低的相变压力点[15-17]. TiO2的晶界电学性质研究可对地球科学领域提供一定的理论依据, 因此有着深远的意义. 本文主要利用阻抗谱法对金红石相TiO2高压下电学性质的变化进行了系统研究. 得到了高压下TiO2的晶粒和晶界电阻随压力的变化关系, 研究了高压下TiO2的界面变化行为. 通过系列实验对空间电荷势的变化进行了深入的分析. 结果表明: 高压下金红石相TiO2的电学性质变化与结构相变有着密切相关的联系, 高压下晶界空间电荷势主要来自于静电相互作用和弹性相互作用两方面的贡献. 2.实 验本文研究用金红石相多晶TiO2 (Alfa Aesar 公司, 纯度99.9%)进行高压原位阻抗谱测量. 利用金刚石对顶砧装置(DAC, Mao-Bell型)产生高压. 实验中所用金刚石砧面直径为425 μm. 采用预压缩的T301不锈钢片作为封压垫片, 初始厚度和预压后厚度分别为250 μm和40—50 μm. 在垫片压痕中心处钻一个直径为150 μm的小孔, 作为样品室. 以红宝石荧光法对压力进行校准和标识, 实验中并不添加任何传压介质. 阻抗谱的测量采用两极法, 其测量方法和电极构造详见文献[18]. 图1给出了金刚石对顶砧表面电路结构以及剖面设计图. 首先采用射频磁控溅射法在压砧上沉积一层Mo薄膜(厚度为0.3 μm)作为电极. 采用同样的方法在垫片表面制备一层Al2O3薄膜(厚度为0.5 μm)以提高其绝缘性, 再用化学法去掉金刚石砧面中央的Al2O3, 形成探测窗口. 阻抗谱测量采用Solartron 1260阻抗谱测试仪连接1296介电分析仪, 实验中所采用的交流信号的输出频率在10 MHz—0.1 Hz范围内, 交流电压信号的幅值保持为0.1 V. 将仪器与计算机相连, 由计算机自动完成测量并通过计算给出最后数值. 在实验过程中, 为减小测量误差应尽可能保持室内环境稳定. 图 1 (a) 金刚石砧面上微电路结构示意图, 1-钼电极, 2-在钼膜上沉积的Al2O3绝缘层, 3-沉积到金刚石砧面上的Al2O3, 4-裸露的金刚石砧面, A和B为微电路的接触端; (b) 金刚石对顶砧的剖面示意图 Figure1. (a) The configuration of a complete microcircuit on a diamond anvil: 1-the Mo electrodes, 2-the Al2O3 layer deposited on the Mo film, 3-the Al2O3 layer deposited on the diamond anvil, 4-exposed diamond anvil, A and B are the contact ends of the microcircuit; (b) the cross section of the designed diamond-anvil-cell.
图 1 (a) 金刚石砧面上微电路结构示意图, 1-钼电极, 2-在钼膜上沉积的Al2O3绝缘层, 3-沉积到金刚石砧面上的Al2O3, 4-裸露的金刚石砧面, A和B为微电路的接触端; (b) 金刚石对顶砧的剖面示意图
图 2 金红石相TiO2在0—12 GPa压力范围内阻抗谱变化示意图 (a) 1.4 GPa; (b) 4.2 GPa; (c) 8.5 GPa; (d) 11.5 GPa
图 3 TiO2在高压环境下(12.7—39.9 GPa)测量的阻抗谱变化曲线
图 4 TiO2总电阻随压力的变化关系
图 5 TiO2晶界空间电荷势(
