High Temperature Heterogeneous Reaction Kinetics and Mechanisms of Tungsten Oxidation
Open Access
- Author:
- Sabourin, Justin Leo
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 03, 2010
- Committee Members:
- Richard A Yetter, Dissertation Advisor/Co-Advisor
Richard A Yetter, Committee Chair/Co-Chair
David Lawrence Allara, Committee Member
Grant Alexander Risha, Committee Member
Vigor Yang, Committee Member
Matthew M Mench, Committee Member - Keywords:
- rocket nozzles
thermogravimetry
kinetics
oxidation
tungsten
metal oxidation
graphite - Abstract:
- Tungsten, a material used in many high temperature applications, is limited by its susceptibility to oxidation at elevated temperatures. Although tungsten has the highest melting temperature of any metal, it corrodes at much lower temperatures since volatile oxides are formed during oxidation. Understanding heterogeneous oxidation and vaporization processes may allow for the expansion and improvement of tungsten applications. Tungsten oxidation was thoroughly studied in the past, and today there is a good phenomenological understanding of these processes. However, as the design of large scale systems increasingly relies on computer modeling, there becomes a need for improved descriptions of oxidation. Thermochemical parameters that cannot be measured experimentally, may now be determined theoretically, a tool that was previously unavailable to scientists and engineers. Additionally, chemical kinetic modeling software is now available for both homogeneous and heterogeneous reactions. This study takes advantage of these new theoretical chemical modeling tools, as well as a thermogravimetric (TG) flow reactor developed as part of this study, to learn about mechanisms and kinetics of tungsten oxidation. Reaction rates were studied with various oxidizers (O2, CO2, H2O) using helium as an inert carrier gas. Isothermal rates were determined at temperatures up to 1970 K, and oxidizing species partial pressures up to 64.6 torr. Kinetic parameters such as activation energies, frequency factors, and pressure exponents were determined for each reactive system. An important contribution of this work was quantifying the effects of CO on the CO2 reaction, and H2 on the H2O reaction. In both cases the non-oxidizing species significantly reduced oxidation kinetics. Results have led to new interpretations and thought processes for limiting nozzle erosion in rocket motors. Combined with the TG analysis, as well recent theoretical interpretations of reaction thermodynamic and kinetics, a new mechanism for tungsten and O2 oxidation has been developed using a one-dimensional model of the TG flow reactor. Important chemical processes and species are also identified for reaction systems involving H2O and CO2. In the future, additional studies are needed to improve our understanding of these chemical species and processes so that more advanced kinetic mechanisms may be developed.