Study on evaporation from alloys used in microwave vacuum electron devices
Authors: Liu Yan-Wen  , Wang Xiao-Xia  , Lu Yu-Xin  , Tian Hong  , Zhu Hong  , Meng Ming-Feng  , Zhao Li  , Gu Bing  ,   The Institute of Electronics, Chinese Academy of Science, Beijing 100190, China
Published: December 2020

The development of modern satellite communication technologies is imposing higher demands on the lifetime and reliability of the microwave vacuum electronic devices, which directly depend on the evaporation properties of the extensively used Monel and stainless steel. Therefore, it is of vital importance to study the evaporation properties of these two types of metallic materials. For the first time, as far as we know, this paper proposes to study the evaporation properties of metallic materials using time-of-flight mass spectrometer (TOFMS). The components and the contents of the vacuum background, the evaporants from the Monel and from the stainless steel have been measured using the TOFMS, respectively. After the pressure of the measurement chamber is below 4.010-8 Pa, the TOFMS is used for the metallic materials working at different temperatures. They are respectively acquired when the Monel and stainless steel are at room temperature on operate between 750 to 900 ℃ under a pressure of 1.010-6 Pa. The measurements are carried out rapidly and in high sensitivity. As disclosed by the measurements, Mn and Cu began to evaporate when the Monel and the stainless steel are heated to 800 ℃, which is still far below the melting points of the two alloys (1243 ℃ and 1080 ℃). When the Monel and the stainless steel are further heated to 900 ℃, the evaporation of Mn, Cu, and Cr becomes quite considerable. Once the evaporated Mn, Cu, or Cr deposit on the ceramics for the insulation in an electron gun, its insulation will be deteriorated. Hence, the Monel and the stainless steel are not suitable to be use as the components in cathode electron guns, especially those used in the devices that are to work a long lifetime in high vacuum. Moreover, the Monel and the stainless steel are not suitable for used as the components that are often under the electron bombardment, e.g., anodes and collectors, either. The SEM images and XRD of the heat treated surface structures of the Monel and the stainless steel in ultrahigh vacuum (1.010-6 Pa) have also been studied. On heating at 900 ℃ for 30 and 120 min the surface structure and composition change remarkably and a significant reduction in Mn and Cr is visible, and also a large number of holes and crystal boundaries emerge on the surfaces of the two metallic alloys. With increasing heating time, the boundaries will grow larger and larger. As a result, the strength of the two metallic materials becomes weaker and gas permeation and leakage even occur. Therefore, it can be concluded that the components made from Monel and stainless steel, especially those with thin walls, should not be heated to high temperatures in ultrahigh vacuum for a long time. The above phenomena are analyzed in detail theoretically and the proper and feasible application methods of the metallic materials are explored in device design and technological process control. These works are expected to contribute to the prolonging of the lifetimes of the satellites, and will lead to tremendous economic benefits.