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Thermodynamics of Silicon-Hydroxide Formation in H2O Containing AtmospheresThe formation of volatile silicon-hydroxide species from SiO2 in water containing atmospheres has been identified as a potentially important mode of degradation of Si-based ceramics. Availability of thermodynamic data for these species is a major problem. This study is part of an ongoing effort to obtain reliable, experimentally determined thermodynamic data for these species. The transpiration method was used to measure the pressure of Si-containing vapor in equilibrium with SiO2 (cristobalite) and Ar + H2O(g) with various mole fractions of water vapor, X(sub H2O), at temperatures ranging from 1000 to 1780 K. Enthalpies and entropies for the reaction, SiO2(s) + 2H2O(g) = Si(OH)4(g), were obtained, at X(sub H2O) = 0.15 and 0.37, from the variation of lnK with 1/T according to the 'second law method'. The following data were obtained: delta(H)deg = 52.9 +/- 3.7 kJ/mole and delta(S)deg = -68.6 +/- 2.5 J/mole K at an average temperature of 1550 K, and delta(H)deg = 52.5+/-2.0 kJ/mole and delta(S)deg= -69.7 +/- 1.5 J/moleK at an average temperature of 1384 K, for X(sub H2O)= 0.15 and 0.37, respectively. These data agree with results from the literature obtained at an average temperature of 1600 K, and strongly suggest Si(OH)4(g) is the dominant vapor species. Contradictory results were obtained with the determination of the dependence of Si-containing vapor pressure on the partial pressure of water vapor at 1187 and 1722 K. These results suggested the Si-containing vapor could be a mixture of Si(OH)4 + SiO(OH)2. Further pressure dependent studies are in progress to resolve these issues.
Document ID
20020025451
Document Type
Preprint (Draft being sent to journal)
Authors
Copland, Evan (NASA Glenn Research Center Cleveland, OH United States)
Myers, Dwight (East Central Univ. Ada, OK United States)
Opila, Elizabeth J. (Cleveland State Univ. Cleveland, OH United States)
Jacobson, Nathan S. (NASA Glenn Research Center Cleveland, OH United States)
Date Acquired
September 7, 2013
Publication Date
January 1, 2001
Subject Category
Fluid Mechanics and Thermodynamics
Meeting Information
199th Electrochemical Society Meeting(Washington, DC)
Funding Number(s)
PROJECT: RTOP 708-90-43
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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