Electrical Conductivity of Ce-Nd Co-doped SnO2 Based on the First-Principle
Yu Shuangmiao Wang Jingqin Chen Ling Liu Zhou
State Key Laboratory of Reliability and Intelligence of Electrical Equipment,Hebei University of Technology
Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability of Hebei Province,Hebei University of Technology
Abstract:
The new AgSnO2 material is the most suitable electrical contact material for replacing AgCdO material. But AgSnO2 electrical contact would precipitate SnO2 crystals on its surface during use. The conductivity of SnO2 is extremely poor,which affects the use of electrical contacts seriously. How to improve the electrical conductivity of SnO2 has become the main direction of scholars' research.This research adopted the first principle based on density functional theory,and aimed to simulate and analyse the effects of co-doping Ce and Nd with SnO2 on energy band diagrams,state densities,charge distribution and electrical conductivity. The results showed that both single-doping and co-doping could reduce the band gap of SnO2,meanwhile tightening the energy bands. Moreover,the performance of co-doping samples was noticed significantly better than that of single-doping samples. The 4 f state of Nd atoms and Ce atoms during single doping introduced a new impurity level at the bottom of the SnO2 conduction band. The conduction band bottom moved toward the lower energy level. On the other hand,when Ce and Nd were co-doped,more energy levels were introduced to the conduction band bottom and the valence band top of SnO2,which further shortened the band gap. Furthermore,by the assistance of layout charge analysis,it was identified that the ionicity of the co-doped sample was stronger than that of the single-doped sample. Most importantly,both single doping and co-doping could contribute to the increase of electrical conductivity of SnO2 electrical conduction materials,and the effectiveness of co-doping was significantly higher than those of single doping.
Keyword:
electrical contact materials;first principles;SnO2;rare earth element co-doping;electrical properties;
Received: 2019-04-08
理想的电触头材料应具有良好的导电性、导热性、抗熔焊性、抗材料转移、抗电弧侵蚀能力以及低而稳定的接触电阻
[1]。长久以来人们都使用Ag Cd O作为触头材料的主要成分,因为其在耐电弧侵蚀性、抗熔焊性、耐腐蚀、耐磨性等方面具有优良的性能,曾一度被称为“万能”的触头材料。但是近年来,人们对环境保护和安全的重视程度越来越高,而Ag Cd O触头材料在使用过程中产生的Cd蒸汽是有剧毒的,同时也会对环境产生严重的污染,因此Ag Cd O材料正在逐渐被淘汰
[2,3]。
本征Sn O2和各个掺杂体系Sn O2的能带结构图如图2所示。由图2(a)可知本征Sn O2和各个掺杂后的Sn O2模型的导带底和价带顶均位于布里渊区的G点,故它们都是直接带隙半导体。其中本征Sn O2的带隙计算值为1.1 e V,但是该计算值明显小于实验值3.6 e V
[17],这是由于计算模块所使用的广义梯度近似存在一定局限性,主要是低估了Sn的5s、5p态与O的2p态之间的相互排斥作用,使得计算值会偏小。由于本实验考虑对比各个掺杂体系的带隙相对大小,故实验结论不会受到影响。
经过计算,稀土掺杂Sn O2的带隙分别为:CeSn O2为0.805 e V,Nd-Sn O2为0.736 e V,Ce-Nd-Sn O2为0.701 e V。通过计算结果可知,单掺杂稀土元素后Sn O2带隙宽度均有所减小,而Ce,Nd共掺杂Sn O2体系的带隙宽度进一步减小,在一定程度上可以表明Ce,Nd共掺杂Sn O2的导电性更强。从能带图2(b~d)可以直观地看出掺杂后导带和价带都变得更加紧密,且出现了新的价带,导带整体向下移动。而共掺杂体系的能带图最为紧密且平滑,其导带整体向下移动成都要强于单掺体系,导带底更加靠近费米能级,而价带也整体向上移动,导带和价带处的能带都十分密集,并且其价带顶穿过费米能级,半导体发生简并,呈现更强的金属性,使导电性增强。
图3所示为计算所得的本征Sn O2,Ce单掺杂,Nd单掺杂和Ce,Nd共掺杂Sn O2体系的总态密度图和分态密度图。综合图3(a~d)可分析得知:对于本征Sn O2,其导带位于0~20 e V间,主要由Sn原子的5s,5p态共同提供,而价带部分有两个峰值,上价带位于-10~0 e V,由Sn原子的5s,5p态和O原子的2p态共同提供,下价带位于-20~-15 e V,由O原子的2s态和Sn原子的5s,5p态提供。当Ce元素单掺杂时:导带部分位于0~10 e V,导带由Ce原子的4f态和Sn原子的5s态共同组成,其中Ce的4f态为主要组成成分,Sn的5s,5p态向低能带端移动,而价带部分位于-10~0 e V,远离费米能级的部分对费米能级处影响可忽略不计,价带主要由Sn原子的5s,5p态和O原子2p态共同提供。当Nd元素单掺杂时:导带位于0~10 e V,导带由Nd原子的4f态和Sn原子的5s,5p态共同提供,其中Nd的4f态为主要组成成分,Sn的5s,5p态向低能端移动,而价带位于-10~0 e V,远离费米能级的部分忽略不计,价带主要由Sn原子的5s,5p态,O原子的2p态和Nd原子的4f态共同提供。当Ce,Nd共掺杂时:导带位于0~5 e V,导带由Ce原子的4f态,Nd原子的4f态和Sn原子的5s态共同提供,其中Ce原子的4f态和Nd原子的4f态为主要组成成分,Sn原子的5s,5p态均明显向低能端移动,而价带位于–10~0 e V,远离费米能级部分忽略不计,价带由Sn原子的5s,5p态,O原子的2p态和Nd原子的4f态共同提供,其中O原子的2p态为主要成分。