Abstract
The safe operation of power equipment largely depends on the overvoltage protection level of the arrester. The ZnO varistors are widely used as the core components of the arresters in power systems because of the excellent nonlinear volt-ampere characteristics. In order to study the electrical properties of ZnO varistors under different external electric fields from the microstructure, the method of first-principles based on density functional theory (DFT) is used, and structure of ZnO/<i>β</i>-Bi<sub>2</sub>O<sub>3</sub> interface containing zinc interstitial (Zn<sub>i</sub>) and oxygen vacancy (V<sub>o</sub>) defects is built. The results show that the V<sub>o</sub> defect migrates after full relaxation. The Zn<sub>i</sub> shifts to the interface under an external electric field. The interface energy increases rapidly after the electric field intensity has exceeded 0.1 V/Å, which means that the interaction force between the interfaces becomes larger, the distance between ZnO and <i>β</i>-Bi<sub>2</sub>O<sub>3</sub> layers decreases, and the conductivity increases rapidly. The differential charge density, work function and Bader charge analysis method are used to calculate the barrier height at the interface, which proves that the built-in electric field is an important cause ingredient responsible for the non-linear volt-ampere characteristics of ZnO varistors. The effects of atomic orbital energy level, trap energy level and energy gap on the macroscopic conductivity of ZnO varistors are analyzed by using the method of density of states analysis. In this work are analyzed the different electrical parameters of the ZnO/<i>β</i>-Bi<sub>2</sub>O<sub>3</sub> interface with aggregation defects by adjusting the intensity of the external electric field, and a new idea is provided for learning the electrical characteristics of ZnO varistors.
Highlights
The results show that the Vo defect migrates after full relaxation
Xia Yu (College of Electrical and New Energy, Three Gorge University, Yichang 443002, China) ( Received 6 April 2021; revised manuscript received 6 May 2021 )
Summary
由 (8) 式可知, ZnO 和 bBi2O3 的 Fermi 能级分别向禁带中央移动, 使得两 者的 Fermi 能级之差减小, 内建电场强度降低. 综 合以上对肖特基势垒和界面功函数的分析, 可知界 面中内建电场的存在是 ZnO 压敏电阻具有良好非 线性特性的关键因素. 图 9(a) 绘制的是未加外电场 ZnO/b-Bi2O3 界 面的总态密度和分波态密度, 从图中可以看出, 价 带分布在–19.89—0 eV 能级区间, 其中第一价带位 于–19.89—–16.07 eV 价带区域, 该峰宽度较窄, 局 域性很强, 主要由 O 2s 轨道贡献, 属于深能级处. 第二价带位于–11.54—–8.02 eV, 主要由 Bi 6s 轨道以及 O 2p 轨道贡献, 从分波态 密度可以看出 Bi 6s 轨道与 O 2p 轨道存在态密度 “共振”, 说明该区域存在 s-p 杂化现象, 属于成键 能级区域. 在–7.86—0 eV 能量区域内存在两个高 密度态分布, 峰值对应的能量分别为–5.68 和–2.16 eV, 该区域主要由 Zn 3d 轨道与 O 2p 轨道贡献, 且根 据分波态密度图来看, Zn 3d 轨道与 O 2p 轨道同 样存在态密度的“共振”现象, 有较强的成键性, 且 存在 p-d 杂化. 由于缔合缺陷的引入, 与纯 ZnO/ b-Bi2O3 界面的态密度相对比 [16], 存在缺陷界面的 态密度曲线在带隙之间出现了陷阱能级, 在 Fermi 能级附近出现了由 Zn 4s 轨道和 O 2p 轨道贡献的 施主能级, 距导带底约 0.33 eV. 处于导带区域的 O 2p 轨道与 Bi 6p 轨道向右移动, 使得共价键成键增 强, 能级增大, 价带顶与导带底之间的距离增大.
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