resumo
The total conductivity of oxygen-hyperstoichiometric YBaCo4O7 + delta is predominantly p-type electronic at oxygen partial pressures from 5x10(4) Pa down to the phase decomposition limit, 10(-11)-10(-4) Pa at 973-1223 K. The ion transference numbers, determined by the oxygen permeation and faradaic efficiency measurements at 1073-1223 K, vary in the range 3x10(-5)-2 x10(-4) and increase with temperature. The oxygen permeability of YBaCo4O7 + delta ceramics, with overall level similar to that of K2NiF4-type cuprates, is mainly limited by the bulk ionic conduction. Heating above 1050-1100 K and redox processes under oxidizing conditions lead to a first-order transition accompanied with extensive oxygen losses from the lattice, resulting in decreasing total oxygen content from 8.5 down to approximately seven atoms per unit formula. Except for the variations associated with this transition, the electron-hole conductivity and Seebeck coefficient are essentially p(O-2)-independent within the phase stability limits. The use of different synthesis methods, namely the standard ceramic technique and the glycine-nitrate process, has no significant effect on the properties of YBaCo4O7 + delta ceramics. The thermal expansion coefficients averaged at 600-1100 K in air are (7.3-7.6) x10(-6) K-1. Porous YBaCo4O7 - delta based cathodes show a very high electrochemical activity in contact with LaGaO3-based solid electrolyte at 873- 1073 K.
palavras-chave
OXYGEN-TRANSPORT; CERAMIC MICROSTRUCTURE; SURFACE MODIFICATION; IONIC-CONDUCTION; OXIDE; MEMBRANES; NONSTOICHIOMETRY; PEROVSKITES; ELECTRODES; REACTORS
categoria
Electrochemistry
autores
Tsipis, EV; Kharton, VV; Frade, JR; Nunez, P
nossos autores
Jorge Ribeiro Frade
Professor(a) Jubilado(a)