Connection between oxygen-ion conductivity of pyrochlore fuel-cell materials and structural with composition and temperature.

Journal Article · Fri Mar 31 23:00:00 EST 2000 · Solid State Ionics

DOI:https://doi.org/10.1016/S0167-2738(99)00320-3· OSTI ID:942833

Wuensch, B J; Eberman, K W; Heremans, C; Ku, E M; Onnerud, P; Yeo, E M. E.; Haile, S M; Stalick, J M; Kingon, A I; NIST,

Oxides are believed to assume the A{sub 2}B{sub 2}O{sub 7} pyrochlore structure type for a specific range of ratios of the cation radii, R{sub A}/R{sub B}. Substitution of a larger B' ion in solid solution for B can progressively drive the system to complete disorder as in Y{sub 2}(Zr{sub y}Ti{sub 1-y}){sub 2}O{sub 7}, producing an oxygen-ion conductivity, {sigma}, greater than 10{sup -2} S/cm at 1000{sup o}C, comparable to the values of 10{sup -1} S/cm found for M{sup 3+}-stabilized cubic zirconias. Rietveld analyses of neutron and X-ray powder diffraction data have been employed to obtain structural data for the related systems Y{sub 2}(Sn{sub y}Ti{sub 1-y}){sub 2}O{sub 7}, Y{sub 2}(Zr{sub y}Sn{sub 1-y}){sub 2}O{sub 7}, Gd{sub 2}(Sn{sub y}Ti{sub 1-y}){sub 2}O{sub 7} and (Sc{sub z}Yb{sub 1-z}){sub 2}Ti{sub 2}O{sub 7} to test whether the state of disorder and attendant ionic conductivity are indeed determined by R{sub A}/{l_angle}R{sub B'},R{sub B}{r_angle}. This was not the case for the Sn-Ti solid solutions: they retained an ordered pyrochlore structure for all values of y. The slight variation of ionic conductivity (less than one order of magnitude with a maximum in {sigma} at intermediate y) was successfully explained by the structural data. The behavior of Y{sub 2}(Zr{sub y}Sn{sub 1-y}){sub 2}O{sub 7} solid solutions was very similar to that of the Zr-Ti phases. Neutron powder diffraction profiles were recorded as fully-ordered Y{sub 2}Sn{sub 2}O{sub 7} and highly-disordered Y{sub 2}(Zr{sub 0.6}Ti{sub 0.4}){sub 2}O{sub 7} were heated in situ at temperatures in the range 20-1500{sup o}C. The structure of Y{sub 2}Sn{sub 2}O{sub 7} steadfastly remained fully-ordered over this temperature range. The principal change in the structure was increase in the positional coordinate, x, for O(1), corresponding to increased distortion of the oxygen-ion array as temperature was increased, a consequence of greater thermal expansion of the A{sup 3+}-O bond relative to change in the B{sup 4+}-O separation. The highly-disordered cation arrangements in Y{sub 2}(Zr{sub 0.6}Ti{sub 0.4}){sub 2}O{sub 7} remain unchanged up to 1250{sup o}C when the oxygen array began to undergo further disorder. The same anion site occupancies were observed during heating and cooling cycles suggesting that their distribution does represent an equilibrium state.

• Research Organization: Argonne National Laboratory (ANL)
• Sponsoring Organization: SC
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 942833
• Report Number(s): ANL/MSD/JA-35037
• Journal Information: Solid State Ionics, Journal Name: Solid State Ionics Journal Issue: Apr. 2000 Vol. 129; ISSN 0167-2738; ISSN SSIOD3
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

30 DIRECT ENERGY CONVERSION
ANIONS
CATIONS
FUEL CELLS
IONIC CONDUCTIVITY
MATERIALS
ORDER-DISORDER TRANSFORMATIONS
OXIDES
OXYGEN
PYROCHLORE
SOLID SOLUTIONS
TEMPERATURE RANGE
THERMAL EXPANSION