Effect of Liquid Phase on the Spark Plasma Sintering Process of ZnO Ceramic

ZnO ceramics were fabricated via spark plasma sintering (SPS) technology. The effect of acetic acid solution on the SPS was mainly investigated. The results indicate that with adding about 10wt% liquid phase (2 mol/L acetic acid solution) during SPS, the shrinkage and densification of ZnO ceramic samples are activated at 52 and 115 degrees C, respectively. The relative density of ZnO ceramic samples can reach the value of more than 95% and then the samples are fully densified when the sintering temperature further increases to 160 and 200 degrees C. The grain grows from 200 nm of the raw particle to 600 nm after sintering at 250 degrees C for 5 min. The X-ray diffraction results show that no obvious impurity phase can be observed, and the grain growth presents a clear anisotropic orientation, preferentially growing along the vertical direction of applied pressure. With assistance of liquid phase in SPS, the grain growth activation energy was determined at only 78.8 kJ/mol, a third as large as that sintered in the conventional SPS. Impedance of ZnO ceramic samples was measured at room temperature. It is found that the grain boundary resistance decreases from 9.82x10(6)Omega to 2.75x10(3)Omega with the increasing sintering temperature from 120 degrees C to 250 degrees C.