ZHANG Rui, YANG Chen, TENG Hang, MAO Kaidi, GUO Xue

(School of Materials Science and Engineering, Shandong University of Technology, Zibo 255000, Shandong, China)

Extended abstract:

[Background and purposes] In oxide-ion-conducting solid oxide fuel cell (SOFC) components, the electrolyte should have sufficiently high ionic conductivity to ensure the stable operation of high-performance fuel cells. YSZ nanopowders with high activity are the key to achieving high ionic conductivity and improving the sintering performance of the electrolyte. Commonly used methods for synthesizing YSZ nanopowders include combustion method, sol-gel method, co-precipitation method and so on. Although YSZ nanopowders can be synthesized through the above methods, these approaches suffer from drawbacks, such as complicated procedures, high cost and emission of toxic and harmful gases. However, the traditional solid-state reaction method offers advantages, such as wide availability of raw material sources, precise control of component composition, simple operation process and low environmental pollution. As a heterogeneous reaction, however, the solid-state reaction usually requires relatively high calcination temperature and long holding time. Additionally, the produced powders tend to have coarse crystal grains and are prone to agglomeration.

[Methods] YSZ nanopowders were synthesized using sand mill solid-phase method with zirconium carbonate (ZrCO₄) and yttrium oxide (Y₂O₃) as raw materials with designed stoichiometric proportions. Anhydrous ethanol was used as the grinding medium, while zirconia grinding beads (0.2 mm in diameter) were added at a ball-to-powder ratio of 4:1. The mixture was subjected to sand grinding at 3000 r·min⁻¹ for different durations (10 min, 20 min, 30 min, 40 min, 50 min and 60 min) using a sand mill, followed by calcination and secondary sand grinding. For comparison, YSZ powders were also prepared via the traditional solid-phase method, involving planetary ball milling at 400 r·min⁻¹ for 12 h. The phase composition, particle size distribution and microstructure of the powders were characterized by using X-ray diffraction (XRD), laser particle size analysis and scanning electron microscopy (SEM). The optimal calcination temperature was determined by testing powders calcined at 1000 ℃, 1100 ℃ and 1200 ℃. The synthesized powders were pressed into pellets with 4 wt.% PVB and then sintered at 1400 ℃ and 1500 ℃ for 6 h. Ionic conductivity of the sintered samples was measured by using an electrochemical workstation in humid air (over 550–800 ℃ and frequency range of 0.1 Hz–0.1 MHz, with AC amplitude of 10 mV).

[Results] According to XRD, laser particle size analysis and SEM results, it is showed that the optimal sand grinding time was 50 min for both primary and secondary grinding. The YSZ powders synthesized under these conditions were pure cubic-phase solid solutions, without monoclinic ZrO₂ or Y₂O₃. Insufficient degree of grinding (≤40 min) led to incomplete mixing of the raw materials and residual impurities. When the sand grinding time was 40 min, the particle size reached the minimum (254 nm), but exhibited uneven mixing, leading to residual monoclinic ZrO₂ after calcination. Sand grinding for 50 min ensured uniform mixing of the raw materials and hence pure cubic-phase YSZ powders were obtained. The optimal calcination temperature for the sand grinding solid-phase method was 1100 ℃, which was 200 ℃ lower than the that (1300 ℃) required for the traditional solid-phase method, while the holding time was shortened from 15 h to 6 h. The secondary sand-ground YSZ powders had an average particle size of 186 nm, significantly smaller than that of the micron-scale particles produced by using the traditional solid-phase method. The SS 1500 (sintered at 1500 ℃) sample had higher densification rate, fewer pores and more continuous grain boundaries than the CS 1500 one. Ionic conductivity of the SS 1500 sample reached 3.785×10⁻³ S·cm⁻¹ at 550 ℃ and 0.177 S·cm⁻¹ at 800 ℃, which were much higher than those of the CS 1500 (2.653×10⁻⁴ S·cm⁻¹ at 550 ℃ and 0.0407 S·cm⁻¹ at 800 ℃) one. Excessive sand grinding (≥60 min) caused lattice defects and increased surface energy, leading to particle agglomeration and coarsening. Comparatively, the sand grinding solid-phase method-prepared YSZ electrolytes exhibited significantly higher ionic conductivity.

[Conclusions] The sand grinding solid-phase method has been successfully used to synthesize ultra-fine YSZ nanopowders with optimal primary and secondary sand grinding times of 50 min and calcination temperature of 1100 ℃. This method not only retains the advantages of the traditional solid-phase method (simple operation, low environmental pollution, wide raw material availability) but also overcomes the limitations of high calcination temperature, long holding time, coarse powder grains and poor sinterability. The prepared YSZ electrolytes have high densification rate and excellent ionic conductivity at both medium and high temperatures, outperforming those prepared by using the traditional solid-phase method and other conventional synthesis routes. The sand grinding solid-phase method has reduced energy consumption, simplified process and improved product performance, showing great potential for large-scale industrial production and application in intermediate-temperature SOFCs. This is an efficient and environmentally friendly approach for the synthesis of high-performance SOFC electrolytes and offers valuable references for the development of advanced ceramic materials.

Key words: YSZ; traditional solid phase method; sand grinding solid phase method; electrolyte