Chemical Expansion of Yttrium‐Doped Barium Zirconate and Correlation with Proton Concentration and Conductivity

Abstract

Y‐doped BaZrO3 (BZY) is a promising candidate as an electrolyte in fuel cells, and attracts increasing attention. In this work, a systematic investigation was performed on microstructure, proton concentration, proton conductivity, and hydration induced chemical expansion in Y‐doped BaZrO3. The results revealed that the bimodal microstructure in BaZr0.85Y0.15O3−δ was composed of large grains with composition close to the nominal value, and fine grains with large compositional discrepancy. This property is considered to be one of the evidences of phase separation at lower temperature than sintering temperature (1600°C), which hinders the grain growth. Thermal expansion coefficient of BZY was measured for various dopant level, and was determined to be around 10−5 K−1 in wet and dry argon atmosphere. In addition, chemical expansion effect due to hydration was confirmed by HT‐XRD in dry and wet Ar atmospheres, and suggests an interesting relationship between the lattice change ratio and proton concentration, in the BZY system with different Y content. The change ratio of lattice constant due to hydration increased obviously with the proton concentration for the sample containing the Y content of 0.02 and 0.05, but only changed slightly when the Y content was increased to 0.1 and 0.15. However, when the Y content was further increased over 0.2, the change ratio of lattice constant due to hydration starts to increase obviously again. Such results indicate a high possibility that the stable sites of protons in BZY changed with the variation in Y content.

Regular Network of Misfit Dislocations at the BaZr0.8Y0.2O3−x/NdGaO3 Interface and Its Role in Proton Conductivity

“…From the position of the BZY peaks of Figure a, a lattice parameter value of 4.203 ± 0.006 Å can be calculated. This is slightly larger than 4.1973 Å, the bulk BaZrO 3 lattice parameter, and slightly smaller than the value of 4.224 Å reported for BZY with the same concentration of Y …”

Transport properties of proton conductive Y‐doped BaHfO3 and Ca or Sr‐substituted Y‐doped BaZrO3

“…However, from the insets of Figure , in which the (211) diffraction peak was shown as an example, one can see that the (211) diffraction peak of BaHf 0.9 Y 0.1 O 3‐δ is broad in shape, although the diffraction peak of BaHf 0.8 Y 0.2 O 3‐δ is relatively sharp, and composed of two sub‐peaks generated from Cu K α1 and Cu K α2 radiations. Microstructure as shown in Figure H and I indicates that the grain size of BaHf 0.8 Y 0.2 O 3‐δ is relatively uniform with the size close to 1 μm, but the sample of BaHf 0.9 Y 0.1 O 3‐δ appears to have a bimodal microstructure composed of large grains and fine grains, similar to the case of BaZrO 3 doped with 10 mol% rare earth elements, for example, Y and Tm . Our recent work revealed that these fine grains and large grains in such bimodal microstructure both have perovskite structures, but are different in the dopant content .…”

“…Microstructure as shown in Figure H and I indicates that the grain size of BaHf 0.8 Y 0.2 O 3‐δ is relatively uniform with the size close to 1 μm, but the sample of BaHf 0.9 Y 0.1 O 3‐δ appears to have a bimodal microstructure composed of large grains and fine grains, similar to the case of BaZrO 3 doped with 10 mol% rare earth elements, for example, Y and Tm . Our recent work revealed that these fine grains and large grains in such bimodal microstructure both have perovskite structures, but are different in the dopant content . Therefore, an analogue case is supposed for BaHf 0.9 Y 0.1 O 3‐δ here; that is, the fine and large grains might have different Y content.…”

“…Nowadays, protonic ceramic fuel cells (PCFCs) which use proton conductive oxides as electrolytes are attracting increasing attention, due to their availability to operate at intermediate temperature around 600°C, and no dilution of fuels by byproducts of water. Referring to the candidate for electrolytes of PCFCs, Y‐doped BaZrO 3 (BZY), especially BaZr 0.8 Y 0.2 O 3‐δ (BZY20), is currently regarded as the most promising one, as it shows high proton conductivity in wet atmosphere and significant chemical stability against CO 2 . Although the ionic (mainly protonic) transport number ( t ion ) of BZY is almost unity in wet reducing atmosphere, it deviates obviously from unity by exposing to wet oxidizing atmosphere at the temperature higher than 500°C, due to the generation of hole conduction .…”

Detrimental Effect of Sintering Additives on Conducting Ceramics: Yttrium‐Doped Barium Zirconate

“…Currently,2 0mol %Y -doped BaZrO 3 (BaZr 0.8 Y 0.2 O 3Àd ,B ZY20) attracts the mosta ttention [3][4][5][6] owing to its high proton conductivity (e.g., > 0.01 Scm À1 at 450 8C), [7] almost pure proton conduction in ah umid hydrogen atmosphere, [8,9] and excellent chemicals tability against reaction with CO 2 . [10,11] However,t he refractory nature of BZY20 requires ah ighs intering temperature (at least 1600 8C), which leads to as ignificant challenge for its implementation in fuel cells· [12] Adding sintering addi-tives, such as NiO, [13][14][15] CuO, [15,16] or ZnO, [12,[17][18][19] can effectively reduce the sintering temperature to approximately 1400 8C, and is already widely adopted to process BZY20.…”