A review on recent progress of thermionic cathode
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Tungsten (2020) 2:289–300 https://doi.org/10.1007/s42864-020-00059-1
www.springer.com/42864
REVIEW PAPER
A review on recent progress of thermionic cathode Jun‑Yan Gao1 · Yun‑Fei Yang1 · Xiao‑Ke Zhang1 · Shi‑Lei Li1 · Peng Hu1 · Jin‑Shu Wang1 Received: 28 June 2020 / Revised: 26 July 2020 / Accepted: 27 July 2020 / Published online: 16 September 2020 © The Nonferrous Metals Society of China 2020
Abstract As the performance of vacuum electron devices is essentially governed by the properties of their cathodes, developing efficient and durable thermionic cathode is necessary and highly desired to meet the boosting requirements of vacuum electron devices. This review summarized the progress made in the past decades with a detailed discussion on the occurred various thermionic cathodes and their features, and the understandings of the correlation between the emission properties and the composition, where structure and synthesis method are well illustrated. Furthermore, dispenser cathodes with novel structures and emission mechanism are highlighted to indicate the recent achievement in this area of research, and Sc-cathode is considered as a promising candidate for the next-generation vacuum electron devices due to the greatly improved efficiency. However, challenges still exist to meet the ever-growing demands of thermionic cathode with collaborative requirement of high performance, easy fabrication and inadequate reproducibility. Keywords Thermionic cathodes · Performance · Emission mechanism · Ion bombardment · Work function · Tungsten
1 Introduction As the electron sources for vacuum electron devices (VEDs), thermionic cathodes have been continuously developed and applied in traveling wave tubes (TWTs) [1, 2], magnetrons, communication satellites, medical applications, cathode ray tubes (CRT) [3, 4] and klystrons [5]. Generally, thermionic cathodes use heat to expel electrons from a solid, which is different from cold cathodes using high electric fields to excite electrons out of the solid. The fundamental understanding of the electron emission of thermionic cathodes is that when the activating energy is injected into the cathode metal matrix with increased temperature, the electrons at the Fermi level could capture enough energy to overcome the energy barrier to escape from the cathode surface into the vacuum. According to the Richardson equation [6], energy barrier could be expressed by work function of the cathode
* Peng Hu [email protected] * Jin‑Shu Wang [email protected] 1
Key Laboratory of Advanced Functional Materials, Education Ministry of China, Faculty of Materials and Manufacturing, Beijing University of Technology, Beijing 100124, China
materials, and decreasing the work function of cathode surface is a feasible and effective strategy to improve the emission performance. Up to now, many methods to decrease the work function have been developed and thus derived various types of cathodes. Over the past century, significant progresses have been achieved on thermionic cathodes, and various stra
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