Synthesis and Characterization of Nonsubstituted and Substituted Proton-Conducting La_6–xWO_12–y

Abstract

Mixed proton-electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic-electronic conductivity. Lanthanum tungstate La(6-x)WO(12-y) (with y = 1.5x + δ and x = 0.5-0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La(6-x)WO(12-y) and for all substituted La(6-x)WO(12-y) compounds was measured in the temperature range of 400-900 °C in wet (2.5% H2O) and dry mixtures of 4% H2 in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H2 at 800 °C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM.

Hydrogen separation membranes based on dense ceramic composites in the La27W5O55.5–LaCrO3 system

“…750 1C and mixed protonic electronic conductivity at higher temperatures [25][26][27][28][29][30][31][32][33]. Lanthanum tungstate is found to be single phase for La/W ratios in the range 5.3-5.7 [29], owing to a stabilizing effect of substituting W on La sites within the fluorite related structure [34].…”

Luminescence Properties of Phosphate Phosphor: Barium Tungstate Doped with Dy

“…It can be seen that the XRD diffraction peaks of the samples matched well with the standard card of BaWO 4 (PDF 43-0646), and the diffraction peak intensity were high without any other additional peak of Dy 2 O 3 [18]. Therefore, the as-prepared BaWO 4 samples were crystallization with pure phase that is tetragonal phase structure, belonging to the I41/a (88) space group.…”

Mechanical properties of tape casted Lanthanum Tungstate for membrane substrate application

“…These structures, in particular Lanthanum tungstates (LaWO) with general formula La 28-x W 4+x O 54+δ v 2-δ where x = 0.74÷1.08 (La/W = 5.75÷5.3) and δ = 1.11÷1.62 (δ = 3x/2) [ 11 , 12 ] exhibit sufficient elevated temperature electron-proton conductivity in hydrogen containing atmospheres [ 13,14 ] and acceptable stability in CO 2 containing atmospheres [ 15 , 16 ].…”

“…Moreover, due to the operation at high temperatures, thermal expansion mismatch (between the dense layer and the porous substrate) can result in significant residual stresses and bending or fracture of the layered structures [ 28 , 7 , 29 ]. Furthermore, the thermally-induced cation diffusion can lead to undesired grain boundary segregations and secondary phase formation [ 14 ], which may decrease the material/membrane performance and its integrity [ 30 ]. Thermal expansion coefficients of different membrane materials including LaWO and substrate material were compared in [ 30 ], while chemical compatibility data were reported in [ 31 ].…”