Experimental protocols for the assessment of redox thermodynamics of nonstoichiometric oxides: A case study of YMnO3–δ
[1];
[2];
- Northwestern University, Evanston, IL (United States); Northwestern University
- Northwestern University, Evanston, IL (United States)
- Northwestern University, Evanston, IL (United States); Raytheon Technologies Research Center, East Hartford, CT (United States)
- Northwestern University, Evanston, IL (United States); University of Messina (Italy)
Here, the thermodynamics of reduction in variable valence oxides is important in a vast number of fields in which point defects, and in particular oxygen vacancies, control material functionality. Here we present, by way of an example measurement of the material YMnO3-δ, for which the vacancy formation energy has not been previously reported, best practices for material characterization. Sample mass is recorded by thermogravimetric analysis at temperatures from 600 to 1500 °C under five different gas atmospheres, with oxygen partial pressure ranging from 0.081 to 7.66 × 10-5 atm. Because YMnO3-δ, displays relatively small nonstoichiometry, less than 0.09, over the range of conditions examined, a large sample was required to record the modest mass changes. In the more oxidizing conditions equilibrium behavior was recorded using a finite heating rate of 10°C min-1. In the more reducing conditions, absolute mass changes with temperature were large such that the evolved oxygen increased the oxygen partial pressure in the sample vicinity, precluding equilibration under finite heating. Accordingly, a temperature-stepped protocol with long isothermal holds was employed. Analysis of the results from these stepped measurements required interpolation, which was carried out on the ln(δ) vs 1/T plane. In the non-polar, centrosymmetric phase (P63/mcm and δ ≥ 0.016), the enthalpy of reduction was found to be 304 ± 4 kJ (mol-O)-1, as averaged over the δ range 0.020 to 0.035. While YMnO3-δ is known to incorporate oxygen excess in its ambient temperature, polar phase (P63cm), the conditions leading to such behavior were not included in this study.
- Research Organization:
- Northwestern University, Evanston, IL (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- Grant/Contract Number:
- EE0008089
- OSTI ID:
- 1985154
- Journal Information:
- Journal of the American Ceramic Society, Journal Name: Journal of the American Ceramic Society Journal Issue: 6 Vol. 105; ISSN 0002-7820
- Publisher:
- American Ceramic SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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