CHEN Jie1, ZHANG Xin1, LIU Hongliang1, XIAO Yixin1, LI Fan1, FENG Qi1, ZHAO Weikang1, LIU Yanqin1, ZHANG Jiuxing1,2
1 Key Laboratory of Advanced Functional Materials of Ministry of Education, College of Materials Science and Engineering,Beijing University of Technology, Beijing 100124, China 2 School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China
Abstract: In 2002, researchers at the Tokyo Institute of Technology synthesized a new inorganic mayenite electride—12CaO·7Al2O3:2e- (hereinafter C12A7:e-), as a potential transparent conductive oxide, which opened a new chapter in electride synthesis and utilization. Compared with other organic electrides, it has better chemical stability and can be stable in air below 450 ℃.This thermally stable electride has a number of potentially useful properties, such as air-stability, low work function, excellent performance in resisting poisoning, and metallic conductivity. Thermal stability and low reactivity with air make this material has a broad application prospect in electron emitter, optical device, catalyst support and other fields. Especially, C12A7:e- has been found to have certain emission performance at low temperature and is very suitable as a electron emission material for low-power vacuum electronic devices. Compared with commercialized cathode materials, such as Ni(5.0 eV), Mo(4.6 eV) and LaB6(2.67 eV), C12A7:e-exhibits low work function (~2.4 eV). On the other hand, the strong chemical bond force in C12A7 crystal makes it have better ion bombardment resistance. The sputtering effect of ion bombardment on the cathode at work has an impact influence on the service life. Furthermore, the search for other inorganic electrides is a very promising area of study. Due to the prolonged synthesis cycle and special encapsulation, in the past decade, substantial efforts have been devoted to seeking simple and efficient route to acquire the electride in desire forms. So far, various forms of mayenite electride have been realized, mainly including single crystal, thin film and polycrystalline. These researchers have been working to make full use of the advantages of C12A7:e- applications while significantly reducing the preparation cycle and increasing the electron concentration. Now, the electron concentration of C12A7:e- can reach 2.3×1021 cm-3, which is close to the theoretical electron concentration of 2.33×1021 cm-3.The conductivity at room temperature also jumped from 0.3 S·cm-1 in 2002 to 1 380 S·cm-1. Since Hayashi found that C12A7:H- can be changed from an insulator to an electrical conductor C12A7:e- after being exposed to ultraviolet light, many scholars have been working on developing more efficient and rapid preparation methods. In 2003, Matsuishi et al. prepared C12A7:e- with high electron concentration (2×1021 cm-3) by Ca metal vapor reduction, but this method would form a dense CaO film on the sample surface and hinder the continuation of reduction, resulting in too long reduction time. After that, Hosono effectively reduced reduction time by replacing Ca metal with Ti, but metal vapor reduction could not reduce C12A7:e- film. In 2006, the CO/CO2 atmospheric reduction solved this problem, but the sample prepared had a low electron concentration (Ne~1.4×1019 cm-3).Recently developed by our team of SPS combinate with Ti metal vapor reduction method and in-situ aluminothermic reduction process, can manufacture the high electron concentration (Ne~2.3×1021 cm-3) C12A7:e- block within 0.5 h. These two methods need the shortest reduction time, lowest cost, and are the easiest way to mass production in many preparation methods, they overcome many problem, such as the long preparation period, large energy consumption and low electron concentration, which laid a foundation for its large-scale application. However, those methods still has great limitations, because they can only be used to prepare C12A7:e- polycrystalline block. This paper summarizes the preparation methods of C12A7:e- single crystal, thin film and polycrystalline, compares the advantages and disadvantages of each preparation method, analyzes the problems faced by the preparation of the material and prospects its application, in order to provide reference for finding a more rapid and effective preparation method.
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2020, Vol. 34 
