The elastic properties of 0, 10, 15, and 20 mol% yttrium-doped barium zirconate (BZY0, BZY10, BZY15, and BZY20) at the operating temperatures of protonic ceramic fuel cells were evaluated. The proposed measurement method for low sinterability materials could accurately determine the sonic velocities of small-pellet-type samples, and the elastic properties were determined based on these velocities. The Young’s modulus of BZY10, BZY15, and BZY20 was 224, 218, and 209 GPa at 20 °C, respectively, and the values decreased as the yttrium concentration increased. At high temperatures (>20 °C), as the temperature increased, the Young’s and shear moduli decreased, whereas the bulk modulus and Poisson’s ratio increased. The Young’s and shear moduli varied nonlinearly with the temperature: The values decreased rapidly from 100 to 300 °C and gradually at temperatures beyond 400 °C. The Young’s modulus of BZY10, BZY15, and BZY20 was 137, 159, and 122 GPa at 500 °C, respectively, 30–40% smaller than the values at 20 °C. The influence of the temperature was larger than that of the change in the yttrium concentration.
To clarify the fundamental issues in the elastic moduli of all-solid-state electrochemical devices such as all-solid-state lithium ion batteries and solid oxide fuel cells, the elastic moduli of the component solid-state-ionics materials: 8 mol% yttria stabilized zirconia (8YSZ) and the composites of lithium cobaltite (LCO) and lithium borate (LBO) were evaluated using the ultrasonic method. For 8YSZ porous materials, variations of up to 10% between the reported Young's modulus were investigated, and it was suggested that the variation was not due to the influence of the measurement method or porosity, but originated from the elastic properties of the sample. In addition, the porosity dependence of Young's modulus in 8YSZ porous materials was evaluated and compared with reported models, and a model applicable to porous materials fabricated by solid-phase sintering was obtained. For LCO-LBO composites, the model obtained based on the 8YSZ porous materials was examined for applicability. The Young's modulus of the composites showed similar porosity dependence to the model based on that of 8YSZ, which was more consistent with the model calculated based on the microstructure consisting of solid spheres. Because the melting point of LBO was close to the sintering temperature, and it was considered to be half melted during sintering, the microstructure of the LCO-LBO composites was likely to be different from that prepared by the solid-phase sintering method. Therefore, the difference in microstructure was suggested to have affected the difference in porosity dependence. From these results, it was confirmed that it was necessary to consider not only the porosity but also the microstructure characteristics to correctly evaluate the elastic modulus of porous materials in the field of solid-state electrochemical devices.
Elastic properties are fundamental knowledge for structural designing. Under the development of proton-conducting ceramics fuel cells (PCFCs), there is no doubt about the importance of elastic properties. However, the systematic study about component materials of PCFCs has been insufficient, and obtained elastic properties cannot cover the entire operation conditions. Low sinterability and processability of solid electrolytes prevent from adopting conventional measuring method for high temperature metals. We therefore modified an ultrasonic transmission method for pellet type specimens and measured elastic properties in 10, 15, 20 mol% yttrium doped barium zirconate from room temperature to 700ºC. The results indicated that elastic moduli such as Young’s modulus and shear modulus gradually decrease with temperature rising to approximately 200°C and the slope became moderate over 400°C. Elastic moduli in BZY decreased by 30-40% from room temperature to 500°C. Observed temperature dependences are important basic properties for the structural designing of PCFCs.
Proton conducting ceramics (PCC) cells are promising energy conversion devices that enable high efficiency energy conversion at lower temperature range, solving the challenge of conventional solid oxide cells (SOCs) due to the high operating temperature. Electrochemical performance and chemical stability of PCC electrolyte has been investigated in recent studies, suggesting that rare-earth doped Ba(Zr,Ce)O3 perovskite-type ceramics are optimal materials exhibiting high proton conductivity and chemical stability during operations. On the contrary, mechanical stability of these PCC electrolyte materials has not been evaluated despite the fact that the mechanical properties are critically important for achieving long-term stable operation as fuel cells or electrolyser cells. For the development of conventional SOCs, mechanical stability during high temperature operation was one of the most significant challenges to deal with, which was attained as a result of detailed studies on in-situ elastic properties of composing materials such as oxygen ion conducting electrolytes and residual stresses. Similarly, for PCC cells, mechanical properties of cells and composing materials have been of significant interest in order to achieve mechanically stable long-term operation, even though PCC cells operate at lower temperature than SOCs. Furthermore, the metal-supported (MS) structure which provides superior mechanical robustness compared to anode-supported (AS) structure is expected to be applied effectively to PCC cells, which are called proton conducting ceramics – metal-supported cells (PCC-MSCs), leading to greater necessity of the mechanical evaluation of the cells and composing materials. Electrolyte is the most crucial component in an electrochemical cell and must be mechanically stable because ion transport and gas tightness made by electrolyte determines electrical performance. However, there has been important concern that larger thermal stresses might be introduced in PCC cells compared to SOCs, resulting from the thermal expansion coefficient (TEC) mismatch between the electrode and electrolyte and from the chemical expansion by the hydration that occurs in a certain temperature range. The PCC electrolyte is highly in need of investigation on in-situ mechanical properties, especially on elastic properties. In this study, elastic properties of electrochemically promising PCC, Y-doped Ba(Zr,Ce)O3 perovskite-type ceramics, were investigated under high temperature conditions. Elastic moduli such as Young’s modulus and Poisson’s ratio were measured by the method that we previously developed for elastic investigation in high temperature conditions using ultrasonic waves. This method enables highly accurate and repetitive examination of elastic properties at high temperatures in materials with poor sinterability including PCC by measuring ultrasonic sound velocities in pellets typically fabricated for electrochemical tests. Pellets of BaZr1-xYxO3-δ (BZY) with different concentrations of doped yttrium, BaZr0.9Y0.1O3-δ (BZY10), BaZr0.85Y0.15O3-δ (BZY15), and BaZr0.8Y0.2O3-δ (BZY20), were fabricated. Additionally, pellets of BaZryCe1-yY0.1O3-δ (BZCY) with different ratio of Ce to Zr, BaZr0.7Ce0.2Y0.1O3-δ (BZCY721) and BaZr0.8Ce0.1Y0.1O3-δ (BZCY811) were fabricated. Powders of PCCs above were consolidated to be thick rounded shape and sintered in air. Each prepared sample was set in an electric furnace in laboratory air atmosphere and sound velocities were measured with the sample slowly heated up to 700 °C and subsequently cooled down to room temperature to calculate elastic moduli at each measuring point. In the first series of heating and cooling measurements for as-sintered samples, hysteresis on elastic moduli in intermediate temperature range was observed. We repeatedly conducted a series of heating and cooling measurements several times, and then the hysteresis was not observed any further. Fig.1 shows final state Young’s modulus of BZCY721, BZCY811, and BZY10 (BZCY901) without hysteresis. Elastic moduli at room temperature have not changed through multiple heating and cooling measurements, and crystal structures and lattice parameters were also confirmed to remain constant by x-ray diffraction (XRD) analysis. The hysteresis found in a specific temperature range suggests that elastic moduli were influenced presumably by a change in defect structure of PCC caused by hydration or defect association of oxygen vacancies and dopants. At room temperature, Young’s modulus decreased with the increment of Ce concentration by 16 % from BZY10 to BZCY721. When materials have the same crystalline structure, Young’s modulus generally decreases as mean atomic volume of the base crystal increases. Because BaCeO3 has larger mean atomic volume than BaZrO3, this observation is qualitatively reasonable. However, in high temperatures, the difference became significant only for BZCY721, Young’s modulus decreased by 30 % from that at room temperature in BZCY721. These results suggest that Ce substitution causes different high temperature dependences. Figure 1
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