resumo
The total conductivity of monoclinic La(2)Ti(2)SiO(9) is mixed oxygen-ionic and n-type electronic, and increases on reduction of the oxygen partial pressure down to 10(-21) atm at 973-1223 K. The Substitution of Ti(4+) with Nb(5+) decreases both contributions to the conductivity, whilst Pr doping and reducing p(O(2)) have opposite effects. The oxygen ion transference numbers of La)Ti(2)SiO(9-delta),5, LaPrTi(2)SiO(9 +/-delta) and La(2)Ti(1.8)Nb(0.2)SiO(9+delta) ceramics, measured by the faradaic efficiency and e.m.f. methods, vary in the range 0.15-0.32, increasing when temperature decreases. In air, the activation energies for the ionic and electronic transport are 1.23-1.40 and 1.59-1.74eV, respectively. Protonic contribution to the conductivity in wet atmospheres becomes significant at temperatures below 1000 K. The experimental data and the results of atomistic computer Simulations suggest that the oxygen-ionic and electronic transport is primarily determined by processes involving TiO(6) octahedra. The ionic conduction may occur via both the vacancy and interstitial migration mechanisms, but the former is more favorable energetically and should dominate, at least, in reducing atmospheres. The average thermal expansion coefficients of La(2)Ti(2)SiO(9)-based ceramics, calculated from dilatometric data in air, are (8.7-9.5) x 10(-6) K(-1) at 300-1373 K. The lattice of lanthanum titanate-silicate is almost intolerant with respect to A-site deficiency and to doping with lower-valence cations, such as Sr and Fe. (C) 2007 Elsevier Inc. All rights reserved.
palavras-chave
OXIDE FUEL-CELLS; CRYSTAL-STRUCTURE; CONDUCTORS; CONDUCTIVITY; DEFECT; TECHNOLOGY; STABILITY; MIGRATION; ND; SM
categoria
Chemistry
autores
Pivak, YV; Kharton, VV; Naumovich, EN; Frade, JR; Marques, FMB
nossos autores
Fernando Manuel Bico Marques
Professor Catedrático
Jorge Ribeiro Frade
Professor Catedrático