Thermal stability of nanometer-sized NiO and Sm-doped ceria powders
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Toshiya Doi Department of Electrical and Electronics Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan
Yoshihiro Hirataa) Department of Applied Chemistry and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan (Received 18 February 2002; accepted 3 June 2002)
A fine NiO powder and a Sm-doped ceria powder with a composition of Ce0.8Sm0.2O1.9 were synthesized by heating the oxalate precursors at 300–1200 °C in air to produce a cermet (anode material) for solid oxide fuel cell. A 0.2 M Ni(NO3)2 solution and a 0.2 M Ce(NO3)3–Sm(NO3)3 solution were mixed with 0.4 M oxalate solution, respectively, to produce the oxalate precursors. Only the cubic phase of Ce0.8Sm0.2O1.9 was formed in the calcined powders from the Sm-doped cerium oxalate. However, the mixed phases of NiO and Ni were produced in the NiO precursor after the calcination at 300–600 °C. At higher temperatures, only NiO was detected. The primary particle sizes, which were determined from the Brunauer-Emmett-Teller analysis surface areas, were 60 nm for NiO and 10 nm for Ru/Sm-doped ceria (SDC) after the heat treatment at 400 °C. The oxalate precursors of SDC and NiO provided 433 and 259 kJ/mol of the activation energy, respectively, for sintering/grain growth in the temperature range from 600 to 1200 °C. As-produced SDC precursor formed platelike secondary particles of 0.5–2-m length by the heating at 800 °C. Heating of Ni oxalate at 800 °C produced isotropic fine NiO secondary particles of 0.5–2-m sizes.
I. INTRODUCTION
Solid oxide fuel cells (SOFCs) have been developed as highly efficient and clean electric generators. A metal/ ceramic composite (cermet) of mixed conductors of electrons and oxide ions can be applied to an anode material consisting of SOFCs. The candidate cermet materials are as follows: Ni/Y2O3-stabilized ZrO2 (YSZ) cermet;1– 4 Ru/Sm-doped ceria (SDC) cermet;5 Ni/CeO2 cermet;6 Ni/Gd-doped ceria (GDC) cermet;7 Ni/SDC cermet.8 –11 The important functions of the YSZ particles in the Ni/ YSZ cermet are to (i) supply oxide ions to the triple phase boundary,4 (ii) suppress the sintering of Ni, and (iii) match the thermal expansion coefficient of the cermet with that of YSZ electrolyte. The electrical conductivity of Ni/YSZ cermet increases about three orders of
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J. Mater. Res., Vol. 17, No. 9, Sep 2002 Downloaded: 18 Mar 2015
magnitude at the Ni contents above 30 vol% by a percolation model.1 The Ni/YSZ cermet is usually made from YSZ and NiO mixed powders. The mixed powders are deposited or printed on a solid electrolyte and heated to make a porous structure over 1100 °C and then exposed to a hydrogen gas to reduce NiO to Ni metal. The prevention of sintering of Ni particles in a long-term operation of SOFC at 800–1000 °C is an important research program. The degradation of cell performance is caused by the decrease in the active surface of Ni particles.1 T
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