Thermochemistry of volatile metal hydroxides and oxyhydroxides at elevated temperatures
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FOCUS ISSUE
UNDERSTANDING WATER-OXIDE INTERFACES TO HARNESS NEW PROCESSES AND TECHNOLOGIES
Thermochemistry of volatile metal hydroxides and oxyhydroxides at elevated temperatures Dwight L. Myers1 , Nathan S. Jacobson2,a) Elizabeth J. Opila4
, Charles W. Bauschlicher, Jr.3
,
1
Department of Chemistry and Physics, East Central University, Ada, Oklahoma 74820, USA Materials and Structures Division, NASA Glenn Research Center, Cleveland, Ohio 44135, USA 3 Thermal Protection Materials Branch, NASA Ames Research Center, Moffett Field, California 94035, USA 4 Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 6 July 2018; accepted: 22 October 2018
A principal mode of corrosion in combustion or fuel cell environments is the formation of volatile hydroxides and oxyhydroxides from metal or oxide surfaces at high temperatures. It is important to determine the degree of volatility and accurate thermodynamic properties for these hydroxides. Significant gaseous metal hydroxides/ oxyhydroxides are discussed, along with available experimental and theoretical methods of characterizing species and determining their thermodynamic properties.
Introduction Reaction of metals, alloys, or oxides with water vapor at high temperatures The reaction of water vapor with metal oxide surfaces is an extremely important area of research for a variety of engineering applications. Metal oxides can react with water vapor to produce gaseous metal hydroxides or oxyhydroxides according to the following equation: 1 Mp Oq ðs; lÞ þ mH2 OðgÞ þ nO2 ðgÞ ¼ MOqpþmþ2n H2m ðgÞ : p ð1Þ At higher partial pressures of H2O, the quantity of metal hydroxide or oxyhydroxide vapor can be a significant source of material transport. Reviews by Glemser and Wendlandt [1] and Hastie [2] reported results for a variety of metallic and nonmetallic hydroxides. A previous review by Meschter et al. [3] covered an extensive portion of the periodic table. This review will focus on selected hydroxides of particular importance in water vapor/oxide interface environments such as turbomachinery and fuel cells and on the determination of reliable thermodynamic data for these species. However, basic thermodynamic data are important in many applications beyond turbomachinery and fuel cells, such as
ª Materials Research Society 2019
nuclear systems, materials processing, and chemical plant environments.
Combustion environments Combustion of hydrocarbon fuels under optimal operating conditions results in a combusted gas stream containing ;3% oxygen and ;10% water vapor as well as carbonbearing species. Thermochemistry predicts ;10% water vapor [4]; however, kinetic factors may reduce this amount to some extent [5]. Modern gas turbines may operate at higher than 1 bar total pressure, and thus, several percent water vapor would constitute a significant partial pressure. Conditions are thus conducive to water vapor/oxide reactions, whic
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