硅酸盐学报

质子陶瓷燃料电池(PCFC)是新一代固体氧化物燃料电池(SOFC)技术的重要发展方向。钇掺杂锆酸铈钡BaZr_1–x–yCe_xY_yO_3–δ(BZCY)是常用的质子导体电解质材料。然而，BZCY电解质致密化通常需要在超过1 700℃的高温烧结5 h以上，这不仅会消耗大量能源和时间成本，而且，由于制备温度高，当电解质冷却到室温时，残余应力可能会导致试样后期力学破坏，进而影响其力学性能。本研究探索了质子导体电解质BaZr_0.7Ce_0.2Y_0.1O_3–δ (BZCY721)的微波烧结工艺及研究了其电化学和力学性能。结果表明：相对于传统烧结方式，微波烧结可以将BZCY721烧结温度降低100～150℃。试样在1 500℃微波烧结40 min，致密度可以达到97%以上。试样在650℃测试温度下，晶粒阻抗为34.81?·cm²，晶界阻抗为89.98?·cm²，电导率为6.41×10^–4 S?cm^–1。力学性能测试结果表明，微波烧结的电解质具有与传统烧结结果相当的弹性模量(190.8 GPa)，以及更优的硬度(12.379 GPa)和断裂韧性(0.359 MPa·m^1/2)。本研究结果为在低温下制备致密BZCY721电解质的进一步研究提供了重要参考。

Introduction Solid oxide fuel cell(SOFC), as an efficient and environmentally friendly energy conversion device, has attracted much attention due to its numerous advantages, i.e., high electrical efficiency, low emissions, and high fuel flexibility, compared to other power generation technologies. To address the degradation issues faced by SOFCs operating at high temperatures, developing proton-conducting electrolytes as an alternative to oxygen-conducting electrodes is recognized as an effective strategy to lower the operating temperature of SOFCs. BZCY721(BaZr_0.7Ce_0.2Y_0.1O_3–δ), a proton-conducting electrolyte material used in protonic ceramic fuel cells(PCFCs), exhibits both high proton conductivity and excellent chemical stability at lower temperatures. However, sintering the BZCY material typically requires high temperatures exceeding 1 700 ℃ for over 4–5 h, resulting in significant energy consumption and residual stress induced failures of components. It is thus imperative to explore advanced sintering technology to fabricate the BZCY electrolyte materials. Microwave sintering is a field-assisted sintering technique that utilizes the heat generated by the interaction between the specific microwave band and the microstructure of the material to heat the entire material, thereby achieving the desired density. This method effectively reduces the sintering temperature and significantly shortens the sintering time. Methods Commercial BZCY721 powder and NiO powder were mixed at a mass ratio of 20:1, and ground in a ball mill for 24 h to obtain a homogeneous mixed powder. Subsequently, the mixed NiO-BZCY721 powder was blended with a 10% PVA solution at a mass ratio of 10:1 for wet granulation. The granulated powder of 0.8 g was weighted and pressed into a die with a diameter of 15 mm at 350 MPa for 30 s. This process yielded circular specimens with the diameter of 15 mm and the thickness of approximately 1 mm. The experiment was repeated to produce several sets of green samples. The samples were categorized into two groups for traditional sintering and microwave sintering experiments, respectively. The samples from the traditional sintering group were placed directly into a box furnace for debinding and sintering. The debinding process was carried out at a rate of 1 ℃/min to 400 ℃, for 2 h, then heating at 5 ℃/min to 1 600 ℃ for 6 h, and finally cooling at 5 ℃/min to 500 ℃ before naturally cooling to room temperature. In contrast, the samples from the microwave sintering group underwent the same debinding process in the box furnace but were then transferred to a microwave sintering furnace for sintering at different temperatures and holding time. The mechanical properties and electrochemical properties of the samples prepared using two sintering methods were investigated. Results and discussion Based on the results by the Archimedes method, the density of samples prepared by both traditional sintering and microwave sintering exceeds 96%. The scanning electron microscopy images reveal that the samples both exhibit a high degree of density. The grain size distribution is relatively uniform. The statistical analysis of the grain size distribution demonstrates that as the microwave sintering temperature increases, the overall grain size of the resulting sample decreases, becoming more uniform. The average grain size of the sample sintered by microwave method at 1 500 ℃ is 0.49 μm, whereas the average grain size of the sample sintered by traditional method is 0.53 μm. The impedance analysis reveals that there is a minimal variance in the impedance of the BZCY electrolyte produced by both sintering techniques when the operating temperature remains below 500 ℃. However, once the temperature surpasses 500 ℃, the impedance of the sample sintered by microwave method exceeds that of the sample sintered traditional method. This indicates that the microwave sintered sample exhibits a greater sensitivity at different operating temperatures, resulting in higher impedance values at elevated temperatures. Among the samples, the conductivity peaked in the sample sintered by traditional method at 1 600 ℃ for 6 h reaches a maximum value of 1.01×10^–3 S·cm^–1 at 650 ℃. Meanwhile, the sample sintered at 1 500 ℃ for 40 min exhibits a conductivity of 6.41×10^–4 S·cm^–1 at 650 ℃. Note that the activation energy associated with microwave sintering conductivity is only 1.071 e V. The sample sintered by microwave method at 1 500 ℃ for 40 min exhibits the maximum hardness(i.e., an average value of 12.379 GPa). The sample sintered by traditional method at 1 600 ℃ for 6 h demonstrates a slightly lower hardness(i.e., 11.521 GPa), which is still higher than the sample sintered at 1 450 ℃. In terms of elastic modulus, the sample sintered by traditional method at 1 600 ℃ for 6 h displays the maximum elastic modulus(i.e., 194.68 GPa), slightly surpassing that of the sample sintered by microwave method at 1 500 ℃. According to the Evans and Niihara empirical formulas, the sample sintered by traditional method at 1 600 ℃ exhibits a fracture toughness of 0.280 MPa·m^1/2 and 0.305 MPa·m^1/2, respectively. Also, the sample sintered by microwave method at 1 500 ℃ has a fracture toughness of 0.359 MPa·m^1/2 and 0.347 MPa·m^1/2, respectively. Conclusions The density of the BZCY electrolyte with 5%(in mass) NiO sintering aids sintered by microwave method at 1 500 ℃ for 40 min was comparable to that of the sample sintered by traditional method at 1 600 ℃ for 6 h, achieving a density of over 96%. The grain size obtained by microwave sintering was smaller. The impedance and conductivity of the sample produced by microwave sintering exhibited minimal differences, compared to those obtained by traditional sintering. The grain impedance at 650 ℃ measured was 34.81 ?·cm², and the conductivity was 6.41× 10^–4 S·cm^–1. The grain boundary impedance of microwave-sintered sample decreased as the operating temperature increased. This decrease could be attributed to the smaller grain size achieved by microwave sintering since a smaller grain size resulted in a greater number of grain boundaries, thereby increasing the impact on the grain boundary impedance. Furthermore, the activation energy for conductivity in microwave-sintered samples was only 1.071 e V. The average hardness of the samples sintered by microwave sintering at 1 500 ℃ for 40 min reached 12.379 GPa, which was higher than that obtained by conventional sintering at 1 600 ℃ for 6 h. The average elastic modulus of the microwave-sintered sample reached 190.8 GPa, which was similar to that of traditional sintered samples. According to the statistics of indentation and crack length in SEM images, the crack type was radial crack. Based on the Evans and Niihara formulas, the results of microwave-sintered samples were 0.359 MPa·m^1/2 and 0.347 MPa·m^1/2 respectively, which were higher than those of conventionally-sintered samples.