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https://hdl.handle.net/2142/21598
Description
Title
New understandings of surface diffusion
Author(s)
Allen, Clark Eldon
Issue Date
1996
Doctoral Committee Chair(s)
Seebauer, Edmund G.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Engineering, Chemical
Physics, Atomic
Language
eng
Abstract
Surface diffusion is of fundamental importance in a variety of industrial kinetic processes, including surface reactions, crystal growth and sintering. However, surface diffusion parameters are rarely used to model industrial processes, resulting from three deficiencies in the literature. First, of the significant amount of work done in the laboratory, only a fraction has been conducted under real industrial temperatures. Second, no sufficient predictive models existed previously to estimate surface diffusion parameters. Third, for chemical reactions at surfaces, no simple method existed to determine whether a reaction might be diffusion-limited. This work attacks these deficiencies from experimental, theoretical and computational points of view.
Experimentally, mass transfer diffusion of antimony and indium on Si(111) was studied under real industrial temperatures using the newly developed technique second harmonic microscopy (SHM). Activation energies and pre-exponential factors for the two systems are presented in terms of a mass transfer diffusion mechanism. Also discussed are recent improvements to the SHM technique, including signal-enhancing (factor of 2500) and resolution-enhancing (factor of 2) procedures.
Second, a new physical picture and empirical model have been developed to predict diffusivities of adsorbates on metallic, semiconducting and insulating substrates for both the intrinsic and mass transfer diffusion regimes. The empirical model is based upon the most comprehensive review in the literature, compiled expressly for this purpose. Theoretical considerations are employed in order to approximate from first principles the energies and entropies associated with the different diffusion mechanisms. The new physical picture for mass transfer diffusion was verified computationally by the molecular dynamics study of Ge/Si(111) in conjunction with experiments conducted concurrently and independently in this laboratory.
Finally, both theoretical and computational methods were employed to develop a potential laboratory experiment to allow one to determine whether a recombinative surface reaction is diffusion-limited. Monte Carlo simulations, in conjunction with a finite difference approach to Fick's Law, prove that diffusion-limited desorption leads to an average radial distribution of adsorbate about any reference atom which is history dependent not only on time, but also on the initial concentration. Thus, for a particular coverage, one should measure different reaction rates dependent only upon the initial coverage.
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