The adsorption and diffusion properties of scandium atom on the surfaces of tungsten and noble metals

Significance 

There has been significant development in high-performance thermionic cathodes. For instance, scandate cathodes have attracted the significant interest of researchers owing to their efficient properties such as low operating temperature and high electron emission density, suitable for high power devices. Unfortunately, it has been difficult to achieve the high-density emission scandate cathodes due to numerous challenges including the nonuniformity of electron emission, poor surface contamination resistance among others.

Generally, several methods like chemical vapor deposition and plasma sintering have been developed to fabricate high-density emission scandate cathodes. However, understanding the ion bombarding insensitivity, which is the greatest challenge, will be a remarkable step towards realizing scandate cathodes. On the other hand, preventing surface contamination and depletion of the scandate are also key considerations. Contamination results from the adsorption of water, oxygen, and other organic compounds, especially when exposed to air while depletion of the scandate is caused by the destruction of the Bs-Sc-O layers due to evaporation and ion bombarding. To this end, this compromises the application of scandate cathodes and thus researchers have been looking for an alternative to supplement the depleted scandate to allow efficient electron emission recovery. Therefore, the use of the first principle calculations in predicting the scandate cathode properties have been recently identified as a promising solution.

A group of researchers at Beijing University of Technology: Dr. Chen Lai, Professor Jinshu Wang, Fan Zhou, Wenyuan Zhou, Yazhou Hua, and Feifei Wang investigated the adsorbability and diffusivity properties of the scandate atom on the surface of different noble metals. They purposed to use the research results in fabricating high-density emission scandate cathodes. The authors used the first principle calculation methods and the density functional theory method. Eventually, the obtained properties for the different metals were compared to each other. Their research work is currently published in the research journal, Applied Surface Science.

From the experimental results, the research team observed that the ion bombarding insensitivity was influenced by the adsorption energies. For instance, the scandate cathodes fabricated using the Ir, Os, Ru and Re metals exhibited significantly high adsorption energies and ion bombarding insensitivity. On the other hand, the adsorption energies of the scandate atom on metals surfaces decreased as a result of the W-doping. Furthermore, adsorbabilities and diffusivities properties of the Sc atom on the Ru and Re surfaces are far much higher than that of the W surfaces.

The study by Beijing University of Technology scientists successfully investigated the vital properties of the scandate cathodes as opposed to the initial works. According to the authors, the results indicate the potential of the Re and Ru materials to be used as the coating in the scandate cathodes to enhance the cathode electron emission recovery and the ion bombarding insensitivity. Therefore, it provides a path towards the realization of a high ion bombarding insensitivity and electron emission recovery which will further advance future thermionic cathodes used for various applications.

The adsorption and diffusion properties of scandium atom on the surfaces of tungsten and noble metals - Advances in Engineering
Fig. 1 The possible adsorption sites of Sc atom (in pink color) on (a) W(011), (b) Ir(112(—)1), and (c) Os(0001), Re(0001) and Ru(0001). The blue, green and red color represents the top, 2nd and 3rd layer atoms, respectively.
The adsorption and diffusion properties of scandium atom on the surfaces of tungsten and noble metals - Advances in Engineering
Fig. 2 The diffusion pathways of Sc atom (in pink color) on (a) W(011), (b) Ir(112(—)1), (c) Os(0001), (d) Re(0001), (e) Ru(0001), and (f) the corresponding diffusion barrier energies.

About the author

Mr. Chen Lai is a PhD candidate at the College of Materials Science and Engineering, Beijing University of Technology (BJUT) under the supervision of Prof. Jinshu Wang. His research focuses on novel rare earth doped cathode materials for thermionic emission.

Email address: [email protected] (C. Lai).

About the author

Dr. Jinshu Wang is a Professor of Materials Science and Engineering at Beijing University of Technology (BJUT). She received her PhD degree from the College of Materials Science and Engineering, BJUT, in 1999. From 2002 to 2004, she has worked as a postdoctoral researcher in Tohoku University, Japan.

Her research interests include refractory metal-based electronic emission materials, renewable energy and environmental materials. She has published over 200 peer reviewed journal papers (Adv. Funct. Mater., Appl. Catal. B :Environ., J. Mater. Chem. A etc.) and 2 monographs. She has authorized over 40 invention patents of China and 2 American invention patents. She obtained the National Nature Science Fund for Distinguished Young Scholars in 2012.

She was awarded the Distinguished Professor of Chang Jiang Scholars Program by the Ministry of Education, China in 2015. She is a senior committee member of Vacuum Electron Society, Chinese Electronics Society, a committee member of Chinese Corrosion & Protection Society, a member of Society of Metallurgical Physiochemistry, and a member of Photocatalysis Industry Association of China. She is also an editor of journals of Powder Metallurgy Technology, Tungsten, Vacuum electronics Technology, etc.

Email address: [email protected] (J. Wang).

Reference

Lai, C., Wang, J., Zhou, F., Zhou, W., Hua, Y., & Wang, F. (2018). The adsorption and diffusion properties of scandium atom on the surfaces of tungsten and noble metals. Applied Surface Science, 457, 1057-1063.

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