Heavy‐Water (D₂O) Take‐Up‐Induced Lattice Expansion in the High‐Temperature Proton Conductor Ba₃Ca_1.18Nb_1.82O_9−δ

Abstract

Dimensional changes of the high‐temperature proton conductor Ba3Ca1.18Nb1.82O9−δ, when exposed to D2O vapor, were investigated using length‐change and precision density measurements. Such information is essential for possible applications of proton conductors in solid oxide fuel cells and humidity and hydrogen sensors. A linear increase of the sample lengths with increases in the deuterium content was observed. Comparison of the present D2O data with those that were previously obtained for H2O showed that there was a small isotope effect in the lattice expansion. The fact that the length‐change‐versus‐hydrogen‐isotope‐concentration curves were almost isotope independent supported the validity of the take‐up reaction H2O(g) + VOö+ OxO is an ⇄ 2OHO. In regard to the latter equation, it was found that small islands of silver, palladium, nickel, and platinum act as catalytic promoters of the reaction and lead, for given heavy‐water‐steam exposure conditions, to an increase of water absorption of up to 100%, when compared with samples without the catalytic surface layers.

Chemical lattice strain in nonstoichiometric oxides: an overview

“…If, as a first approximation, the cation radii decrease linearly with CN, thenwhere the coefficients a , m , b and n are obtained by fitting straight lines to the Shannon radii data. 6,81 Sereda et al 6 demonstrated that the model based on eqn (32) and (36) describes satisfactorily the experimental data on chemical expansion caused by hydration for a number of proton-conducting perovskites, such as BaCa (1+ y )/3 Nb (2– y )/3 O 3– δ · x H 2 O with y = 0–0.5 (BCN), 150–152 BaZr 1– y Y y O 3– δ (BZY), 65,67–69,153,154 BaZr 1– y Sc y O 3– δ (BZS), 154–158 BaCe 1– y Y y O 3– δ (BCY) 67,159,160 and La 0.9 Sr 0.1 ScO 3– δ . 161…”

“…6 demonstrated that the model based on eqn(32) and(36) describes satisfactorily the experimental data on chemical expansion caused by hydration for a number of protonconducting perovskites, such as BaCa (1+y)/3 Nb (2-y)/3 O 3-d $xH 2 O with y ¼ 0-0.5 (BCN),[150][151][152] BaZr 1-y Y y O 3-d (BZY),65,[67][68][69]153,154 BaZr 1-y Sc y O 3-d (BZS),[154][155][156][157][158] BaCe 1-y Y y O 3-d (BCY)…”

“…6 For the typical proton-conducting oxide hydration process, expressed asif Shannon radii 81 are used and hypothesized, direct application of eqn (32) yields negative chemical expansion, contradicting the experimental data. 6,65,67–69,150–161 Instead of making within the model smaller than , the authors 6 adopted another approach – it was assumed that the formation of oxygen vacancy decreases the coordination numbers of neighboring cations. Basically, taking and assuming the CN of cations to be nonstoichiometry-dependent are two somewhat equivalent 72 ways of describing the same phenomenon – the crystal lattice relaxation around the oxygen vacancy.…”

Entry and exit of water vapor in bulk ceramic proton conductors

“…4.6. Consequences for the preparation/loading of bulk samples with high proton content Quite often, bulk proton conductor samples are needed with various proton contents even approaching the theoretically possible limits [8,9]. Such samples are valuable in the study of frozen-in proton concentrations by impedance techniques and for the study of mechanical properties.…”

“…The present results suggest that it is advantageous to: (i) expose the sample first at elevated temperatures (450-600 8C) to humidity dfloodingT the sample with a relatively high inventory of protons; (ii) keeping the sample at that temperature for several hours to allow chemical diffusion to proceed; (iii) lower the temperature by~50 8C to increase the thermodynamic driving force and repeat steps (i) and (ii). The temperature can be successively lowered to as low as 250 8C, where a holding time of several weeks may be required [8].…”