High-temperature proton-conducting lanthanum ortho-niobate-based materials. Part II: Sintering properties and solubility of alkaline earth oxides

The sintering properties and microstructure of La(1-x)A(x)NbO(4) powders (x=0, 0.005, and 0.02 and A=Ca, Sr, and Ba), prepared by spray pyrolysis have been investigated. Dense materials (> 97%) were obtained by conventional sintering at 1200 degrees C and by hot pressing (25 MPa) at 1050 degrees C, respectively. Homogeneous materials were obtained and the average grain size obtained by the two densification methods was similar to 2.0 and similar to 0.4 mu m, respectively, for the 2% doped materials. Pure lanthanum ortho-niobate (LaNbO4) showed a higher degree of grain growth. In the acceptor-doped materials, secondary phases were observed to inhibit grain growth at 1200 degrees C. At 1400 degrees C or higher, molten secondary phases in the Ba-doped materials resulted in severe grain growth, causing microcracking during cooling due to crystallographic anisotropy. A low solubility of AO (A=Ca, Sr, and Ba) in LaNbO4 is inferred from the presence of secondary phases, and 1 mol% solubility of SrO in LaNbO4 was found by electron microprobe analysis. The electrical conductivity in wet hydrogen of the materials demonstrated that the main charge carrier was protons up to 1000 degrees C and reached a maximum value of similar to 8.10(-4) S/cm at 900 degrees C.