Author ORCID Identifier

https://orcid.org/0000-0003-0097-3286

Date Available

8-1-2024

Year of Publication

2024

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Chemical and Materials Engineering

Advisor

Thomas John Balk

Abstract

Scandate cathode presents a great potential of being used as electron emitters in vacuum electron device due to its excellent emission performance. However, it is burdened by several issues, including poor emission uniformity, inadequate reproducibility, and limited lifetime, which have impeded its further industrial application. Therefore, extensive efforts and studies need to be conducted to obtain a more comprehensive understanding of scandate cathode.

W particles, as the base material, play a fundamental role in scandate cathode. Most recent experimental research has reported that the highly faceted crystallography of W grains contributes to the properties of scandate cathode. In order to fully understand the mechanism of W particles surface faceting, both nanoscale and bulk W were investigated in the current study.

Nano scale W particles were fabricated utilizing PVD technique. A pre-heating step was applied to these nanoscale particles in a vacuum chamber. It is found that the deposited nanoscale W network transformed into individual, highly faceted nanoparticles when first pre-heated at an intermediate temperature, followed by annealing at 1100 °C, under a pressure of 10-7 Torr. Wulff analysis indicated that these well-developed W particles are dominated by {110} faces.

Meanwhile, surface faceting of bulk W pellets was investigated by in situ heating in environmental scanning electron microscope (ESEM), which allows direct, high-fidelity observation of the morphology changes during high temperature annealing in low vacuum. Here in situ Environmental SEM heating was performed on W pellets with a surrounding pressure of 0.8 Torr. For comparison, another group of samples was annealed in an Ultra High Vacuum chamber (UHV). Surface faceting was observed during ESEM annealing, while UHV heating will not cause any faceting of W surface. With the combination application of EBSD, stereo imaging, as well as the Slice and View technique, it proves that the faceting of bulk W surface is {110} faces dominated, which aligns with nano W particle faceting.

Additionally, more characterization of scandate cathode was performed in this project. A series of in situ heating experiments were conducted on as-received un-activated impregnated scandate fragments by using a MEMS based heater chip in a modern SEM under a pressure of 10-6 -10-7 mbar. The results demonstrated the faceting, growth and migration of impregnates in cathodes at elevated temperatures, which are favorable for better understanding the behavior of scandate cathode. It was the first time to report the real time observations of scandate cathode with a high-resolution technique in a high vacuum chamber close to its actual operate condition, which will provide an insight on the morphology evolvement of impregnated materials.

Furthermore, to gain more deep understanding of cathodes, a cross-section analysis through entire cathode thickness was performed for the first-ever time by Broad ion beam milling technique, which provides a view of the full-length cathode cross section, including emitting surface and bottom of a scandate cathode. With the full-length cathode cross section available, distribution of impregnated materials and tungsten particle faceting along the cross section were observed and analyzed by SEM and EDS. The results also extensively reveal the influence of impregnated materials on W faceting, and provide a new method to evaluate the property of cathodes.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2024.316

Funding Information

This work was financially supported by the Defense Advanced Research Projects Agency (DARPA) Innovative Vacuum Electronics Science and Technology (INVEST) program, under grant number N66001-16-1-4041.

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