Synthesis of cobalt-doped bismuth vanadate by combustion-synthesis: Influence of fuel on phase content and morphology
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Cobalt (15 at.%) doped bismuth vanadate, Bi4(V0.85Co0.15)2O11-d (BICOVOX0.15), is known to have high oxygen ion conduction in the medium temperature range (400– 600 C). Small grain size may be important in stabilizing the highly conductive and disordered g-phase at lower temperature. In this article, we report for the first time the synthesis of highly porous nanoscale BICOVOX powders by a solution combustion technique. The effects of fuel-to-oxidizer ratio, and postcombustion heat treatment temperature and time on the phase content and microstructure of the powders were investigated. As-combusted powders were revealed to be a mixture of Bi2O3, BiVO4, and g-BICOVOX phases that were converted to phase pure g-BICOVOX during heat treatment. I. INTRODUCTION
BIMEVOX represents a group of solid oxide electrolytes with potential to be used in different industrial applications such as fuel cells,1–4 water electrolyzers,3 oxygen sensors,1–3 cathodes in rechargeable batteries,5 high transition temperature ferroelectrics,5 catalysts,6,7 gas separation membranes,2,3 and high temperature heating elements.3 BIMEVOX is derived from the parent compound Bi4V2O11 (BIVOX) by partially substituting V5+ with iso- and/or aliovalent metallic ions (Me = Li+, Cu2+, Co2+, Ni2+, Zn2+, Fe3+, Al3+, Ti4+, Zr4+, Ge4+, Sn4+, Pb4+, Nb5+, etc.). The subvalent substitutions may create additional oxygen vacancies in the highly distorted vanadate octahedral layers causing higher oxygen mobility. BIVOX shows three main polymorphs: a (monoclinic, ˚ ), b (orthorhoma ffi 5.533, b ffi 5.611, and c ffi 15.288 A ˚ bic, a ffi 11.06, b ffi 5.61, c ffi 15.28 A), and g (tetragonal, ˚ ), with space group I4/mmm, a ffi 3.92 and c ffi 15.5 A sequential phase transition temperatures shown below8,9: 450 C
570 C
890 C
a ! b ! g ! Liquid
:
ð1Þ
Among these three, the higher temperature g phase shows the highest O2 ion conductivity due to higher oxygen vacancy disorder in O-V polyhedra. To stabilize the highly conductive g phase of BICOVOX under 500 C, the amount of cobalt substitution should be approximately 10 to 15 at.%.10,11 The highest O2 conductivity of BICOVOX0.15 is observed to be 101 S/cm at 600 C, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0392
3078
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 10, Oct 2009 Downloaded: 28 Mar 2015
which is two orders of magnitude higher than that for yttriastabilized zirconia electrolytes at the same temperature.10 Due to the fact that the g-BIMEVOX phase has an Aurivillius-type crystal structure—alternate stacking of (Bi2O2)2+ layers and [(V/ME)O3.5□0.5)]2 perovskite sheets with intrinsic oxygen ion vacancies (□) to conduct oxygen ions, the conductivity of the materials is anisotropic in nature.3 For both single crystal and highly textured pellet samples (uniaxially pressed from the powder, textured by applying magnetic field), conductivity parallel to the (Bi2O2)2+ layer (ab plane) has been observed to be a few orders of magnitude higher than tha
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