Date of Award
Bachelor of Science
Materials constructed of carbon are widely used in applications requiring high emissivity at temperatures above 1300°C. The ability to transmit heat efficiently makes these materials an excellent choice for use with x-ray tubes. However, heating carbon substrates to high temperatures also leads to thermal desorption of surface bound species. The release of these species increases the total vapor pressure inside sealed x-ray tubes. Our present study considers the outgassing of these carbon substrates as a function of temperature, as a result of their reactivity towards oxygen, carbon monoxide, carbon dioxide, and water vapor. By modeling the factors that influence outgassing performance in vacuum environments, we can account for and minimize thermal desorption during an x-ray tube's operation. Work completed to date has uncovered a significant source of outgassing from the surface of the graphite at temperatures approaching 1600°C. The release of carbon monoxide, which occurs readily at these elevated temperatures, corresponds to a binding energy of 450-500 kJ/mol. To explain the desorption of the more strongly bonded molecules, we propose the existence of a carbon-oxygen-carbon bridge between two basal planes of graphite, which had been previously modeled, but not directly observed. Using the results from our own modeling studies, we can suggest a mechanism for this desorption and predict the binding energies associated with the carbon-oxygen-carbon structure. Understanding of the high temperature reaction of carbon "ill ultimately lead to better materials processing during the manufacturing of x-ray tubes.
Tavernier, Philip Ross, "Comparison of outgassing and surface reactivity of graphic substrates" (1997). Honors Theses. 2064.