Electrical & Computer Engineering, Department of
Date of this Version
Spring 4-6-2012
Document Type
Article
Abstract
Laser-induced multi-energy processing (MEP) introduces resonant vibrational excitations of precursor molecules to conventional chemical vapor deposition methods for material synthesis. In this study, efforts were extended to explore the capability of resonant vibrational excitations for promotion of energy efficiency in chemical reactions, for enhancement of diamond deposition, and for control of chemical reactions. The research project mainly focused on resonant vibrational excitations of precursor molecules using lasers in combustion flame deposition of diamond, which led to: 1) promotion of chemical reactions; 2) enhancement of diamond growth with higher growth rate and better crystallizations; 3) steering of chemical reactions which lead to preferential growth of {100}-oriented diamond films and crystals; and 4) mode-selective excitations of precursor molecules toward bond-selective control of chemical reactions.
Diamond films and crystals were deposited in open air by combustion flame deposition through resonant vibrational excitations of precursor molecules, including ethylene (C2H4) and propylene (C3H6). A kilowatt wavelength-tunable CO2 laser with spectral range from 9.2 to 10.9 µm was tuned to match vibrational modes of the precursor molecules. Resonant vibrational excitations of these molecules were achieved with high energy efficiency as compared with excitations using a common CO2 laser (fixed wavelength at 10.591µm). With resonant vibrational excitations, the diamond growth rate was increased; diamond quality was promoted; diamond crystals with lengths up to 5 mm were deposited in open air; preferential growth of {100}-oriented diamond films and single crystals was achieved; mode-selective excitations of precursor molecules were investigated toward control of chemical reactions.
Optical emission spectroscopy (OES), mass spectrometry (MS), and molecular dynamic simulations were conducted to obtain an in-depth understanding of the resonant vibrational excitations. Species concentrations in flames without and with laser excitations under different wavelengths were investigated both experimentally and theoretically. Detection of C2, CH, and OH radicals, as well as CxHy species and their oxides (CxHyO) (x=1, 2; y=0~5) using OES and MS, together with reaction pathway simulations, were used to explain the effect of vibrational excitations of precursor molecules on chemical reactions and on diamond depositions. Adviser: Yongfeng Lu
Comments
A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering (Electrical Engineering), Under the Supervision of Professor Yongfeng Lu. Lincoln, Nebraska: May, 2012 Copyright (c) 2012 Zhiqiang Xie