| Field Assisted Sintering Technique(FAST) including a series of novel sintering methods has been studied for more than 50 years and some commercial models have been developed. Compared with non-press sintering, hot press sintering and other conventional sintering methods, FAST has many advantages, such as low sintering temperature, short process time and high material performance. For example, it is possible to use FAST to sinter nano powder into a material with theoretical density. In recent years, researchers have discovered a new type of FAST process named flash sintering. In 2011, Indian-American Professor Raj from the University of Colorado at Boulder found that 3YSZ could be sinter ed within a few seconds at 850 °C under an electrical field of 120 V/cm and the application of electrical field depressed the growth of grains.All the above mentioned studies are either theoretical or focused on single bulk materials other than composite materials. Flash sintering is a newly developed field, and only a few ceramic materials were studied for the purpose of densifying. Considering the unique advantages, we believe that FAST or flash sintering could play a key role in the preparation of composite materials or devices, such as solid oxide fuel cells(SOFCs). Based on this idea and the previous research, the phenomena of FAST/flash sintering and related mechanisms were studied, and FAST/flash sintering was further applied to SOFC sintering process in this thesis with the following aspects:(1) Research on the effect of applied field on the sintering behavior of gallium doped ceria(GDC). The GDC powder was compressed into rectangle bar and the Pt wires were used as electrodes to connect the GDC specimen to the power supply. The GDC specimen was placed in a thermal expansion analyzer, where temperature was raised at a certain rate, to obtain the shrink curves. It was found that the GDC specimen could be sintered in a very short time at a lower temperature than traditional method s. In addition, higher applied voltage lowered the sintering temperature. SEM analysis showed that higher voltage prohibited the grain growth and finally the grain size was equal to that of raw powder. This result suggested that FAST/flash sintering could be beneficial to increase the conductivity and mechanical strength of GDC electrolyte in SOFCs.(2) Research on the flash sintered 3YSZ samples. It was found that 1-3 nm Pt nanoclusters were dispersed in the 3YSZ matrix after flash sintering as a result of electron wind effect. This is helpful to further understand the mechanism of flash sintering. We also studied the structural and chemical properties of 3YSZ. It is found that the crystal structure is partially reorganized, and the valence state of Y element was changed too. In addition, the grain boundary area got wider, which would influence the co ncentration and movement of electron- hole pairs. This could explain the increase of ion conductivity. The 3YSZ supported Pt nanoclusters suggest flash sintering is potentially a novel method to synthes ize metal/ceramic composite materials, which could be applied in electro-catalysis, sensors and energy devices.(3) Research on the application of FAST in the sintering process of normal SOFC. Anode supported SOFC was prepared with tape-casting and sintered with FAST other than the conventional sintering process. A perfect single cell with dense 8YSZ electrolyte was obtained in a very short time at 800 °C. All of the open circuit voltages were over 1 V at 650, 700 and 750 °C, which were close to the theoretical value. The maximum output power densities were 0.8, 1.1 and 1.4 Wcm-2, which were higher than those of SOFC obtained by the traditional way. Through a SEM analysis of the electrolyte, it was found that 8YSZ grain sizes were 440 nm, smaller than those from the conventional method(4.06 μm). We also studied the impedance of the cell and simulated the grain grow process of flash sintering.(4) Reaseach on the FAST preparation of La0.2Sr0.7TiO3-Ni/YSZ functional gradient anode(FGA) supported SOFCs. To improve the coke resistance of SOFC, we utilized a novel co-tape casting approach to fabricate a LST-Ni/YSZ FGA supported SOFC. FAST was applied to sinter the LST-Ni/YSZ FGA supported SOFC in order to avoid the distortion and delamination between layers in anode, which were hardly avoided in the traditional sintering process. This LST-Ni/YSZ FGA supported SOFC exhibited high output performance and long-term stability when working with dry methane. Raman spectroscopy showed that no obvious carbon deposition occur after 70 hours work in methane. The present work demonstrated that LST-Ni/YSZ FGA supported SOFC could be compatible with hydrocarbon fuels. |
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