Scandate cathode surface characterization: Emission testing, elemental analysis and morphological evaluation
Scandate cathode surface characterization: Emission testing, elemental analysis and morphological evaluation
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DOI：
10.1016/j.matchar.2018.12.013
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发表时间：
2019-02-01
影响因子：
4.7
通讯作者：
Balk, T. John
中科院分区：
文献类型：
--
作者：
Liu, Xiaotao;Zhou, Qunfei;Balk, T. John
本文介绍了一种调查的钪酸盐阴极，表现出良好的发射特性，在近距离二极管测试的表面。用电子显微镜的形态学评价表明，构成阴极表面的微米尺寸的钨颗粒是刻面的，并装饰有纳米级的Ba/BaO点，沿着与较大的Sc2 O3和BaAl 2 O 4颗粒。根据Wulff对钨晶体形状的分析，确定钨晶粒的晶面为{100}、{110}和{112}。此外，{112}面是普遍的Wulff形状，这与实验观察到的小面的表面积。提出了一种假设，以解释在操作温度下覆盖阴极表面的金属Ba的量，并且其在发射测试后阴极冷却期间转变，以形成在空气暴露时氧化的纳米级半球形颗粒。根据计算，在阴极操作期间Ba原子的覆盖对应于0.5（+ 0.05/-0.12）单层，其代表可用钨表面的完全覆盖。这项调查使用一系列的材料表征技术提供了一个图片的结构和组成的钪酸盐阴极表面。值得注意的是，这些结果不支持存在Ba-Sc-O层占据顶部10-100 nm的钨晶粒表面。
This paper describes an investigation of the surfaces of scandate cathodes that exhibited good emission characteristics during close-spaced diode testing. Morphological evaluation with electron microscopy showed that micron-sized tungsten particles constituting the cathode surface are faceted and decorated with nanoscale Ba/BaO dots, along with larger Sc2O3 and BaAl2O4 particles. The facets of tungsten grains were determined to be {100}, {110} and {112}, based on Wulff analysis of the tungsten crystal shape. Moreover, {112} facets are prevalent in the Wulff shape, which agrees with the experimentally observed facet surface areas. A hypothesis is proposed to account for the amount of metallic Ba that covers the cathode surface at operating temperatures, and which transforms during cooling of the cathode after emission testing, to form nanoscale hemispherical particles that oxidize upon air exposure. According to computation, the coverage of Ba atoms during cathode operation corresponds to 0.5 ( + 0.05/ - 0.12) monolayer, which represents full coverage of the available tungsten surface. This investigation using a range of materials characterization techniques has provided a picture of the structure and composition of scandate cathode surfaces. It is noted that these results do not support the existence of a Ba-Sc-O layer occupying the top 10-100 nm of tungsten grain surfaces.