Ph.D. Thesis Defense by Chad Duty
Friday, November 9, 2001

(Dr. Jack Lackey, advisor)

"Design, Operation, and Heat and Mass Transfer Analysis of a Gas-Jet Laser Chemical Vapor Deposition System"

Abstract

Laser chemical vapor deposition (LCVD) is a new manufacturing process that holds great potential for the production of small and complex metallic and ceramic parts.  However, the field of LCVD is still in its infancy and the deposition process is not well understood, especially as it relates to the manufacture of three-dimensional objects.  Therefore, my graduate research has focused on (1) designing and developing an advanced gas-jet LCVD system, (2) understanding and modeling the effects of the gas-jet on the deposition process, and (3) using the LCVD system to deposit a ceramic (boron nitride) and a metal (molybdenum).

A major portion of the research was the design and development of a flexible and sophisticated LCVD system.  However, successful operation of the LCVD system demanded that the fundamentals of the process were well understood.  Since CVD is a thermally activated process, the most important process variable is temperature.  Therefore, a thermal model was developed for the LCVD system, accounting for Gaussian beam laser heating and gas-jet convection cooling.  Various substrates and reagent mixtures were evaluated as well as the contribution of natural convection.

The deposition rate for a given material is not only affected by temperature, but can also be limited by the mass transport of reagent gases to the deposition zone.  The gas-jet reagent supply for the LCVD system was designed to remove this limitation, but the need and impact of such a system has been debated.  Therefore, a two-dimensional mass transport model was developed to estimate the effects of a gas-jet with respect to local concentration variations and reaction rates.

In the final stage of research, the gas-jet LCVD system was used to deposit materials needed for the fabrication of a thermionic emitter.  Molybdenum and boron nitride were successfully deposited in various forms.