Growth and Faceting of Tungsten and Oxides in Scandate Cathode Particles during In Situ Heating in the Scanning Electron Microscope

Abstract

Tungsten-based scandate dispenser cathodes are promising next-generation thermionic electron sources for vacuum electron devices, due to their excellent emission performance at temperatures lower than those required for conventional cathodes. There has been a significant recent effort to understand scandate cathode performance and to characterize the tungsten and other materials on the emitting surface, primarily via the study of cathodes before and after emission testing. Moreover, these scandate cathodes have typically been characterized at room temperature only. In situ observations of scandate cathodes is challenging, as these devices are thermionic emitters that operate in a high-vacuum environment, and because the sizes of relevant material features range from the micron (2.0 µm) to the nanometer (<50 nm diameter) length scales. In the current study, a series of in situ heating experiments was conducted on un-activated impregnated scandate cathode fragments, utilizing a micro-electro-mechanical system-based heater chip in a scanning electron microscope, enabling the real-time observation of cathode material evolution at elevated temperature (up to 1200 °C) under a pressure of 10−6 to 10−7 mbar. This study revealed how impregnant materials grow and migrate within the cathode matrix at elevated temperatures, and these observations are key to a thorough understanding of the behavior of scandate cathode materials. It also enabled direct observation of the incipient faceting of tungsten surfaces at high temperature while surrounded by impregnant materials. These are the first in situ observations of scandate cathode material evolution in relevant environmental conditions and at sufficiently high resolution to provide insights into the morphological and phase changes that occur in the near-surface regions of scandate cathodes.