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Title page for ETD etd-07212008-174836


Type of Document Dissertation
Author George, Mekha Raichie
URN etd-07212008-174836
Title Studies of ultra high temperature ceramic composite components: synthesis and characterization of HfOxCy and Si oxidation in atomic oxygen containing environments
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Dr Bridget Rogers Committee Chair
Keywords
  • Pulsed laser deposition
  • Silicon -- Oxidation
  • Ultrahigh temperature ceramics
  • hafnium oxycarbide
  • hafnium carbide
  • atomic oxygen
  • Hafnium compounds
  • Ceramic metals
Date of Defense 2008-06-18
Availability unrestricted
Abstract
This dissertation focuses on two aspects of UHTC development: material synthesis and understanding its behavior in various oxidizing environments. Materials system design and modeling challenges are also highlighted.

The first part of the work addresses two thin film synthesis routes of hafnium carbide. The annealing of hafnium film on graphite does not induce the formation of HfC through solid state reactions due to the affinity of the hafnium to dissolve oxygen and the carbon diffusion from bulk to surface. The next segment details the pulsed laser deposition of HfC using two types of lasers. More particulates were formed when the longer wavelength laser was used. Physically blocking the plume and altering the plume chemistry were shown to reduce these defects, but did not lead to the desired film composition. Oxygen contaminates the target, the surface and bulk of the film during and post deposition. For the range of energy fluence studied, the films were compromised of hafnia and free carbon particulates. A small percentage of Hf-C bonds were present when a 248 nm KrF laser was used for ablation. The last segment investigates the effectiveness of hafnium oxycarbide as an oxidation barrier. The main conclusions were: (i) free carbon was removed upon oxidation, and (ii) bound carbon remains in film if a solid carbon source was used for synthesis, even after oxidation at high temperatures. These results are contradictory to results established in previous work. These three studies indicate that the properties that make HfC attractive as a barrier coating are difficult to achieve using the various processing techniques available currently.

The last part focuses on the behavior of silicon in partially dissociated oxygen (O2, O) at 910-1150 °C. A new parallel oxidation model that incorporates the transport and reaction of neutral oxide point defects was developed. Two scenarios were explored: (i) no interaction between the defects, and (ii) recombination of defects to produce more O2 within oxide. The computations helped confirm that the predominant defect theory generally used to explain the oxidation behavior cannot adequately described the enhanced oxide growth rate observed. A more rigorous, computationally intensive dual-vacancy-interstitial mechanism model may better describe this oxidation process.

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