Influence of the forming method on flash sintering of ZnO ceramics

This work aimed to investigate different forming techniques used in the ceramic industry when applied to flash sintering. The way the density gradient interferes with the final densification and microstructural heterogeneity development of ZnO specimens was studied. For this purpose, ZnO cylindrical specimens were formed by uni-axial pressing, isostatic pressing, and slip casting. All experiments were conducted isothermally at 800 degrees C in an adapted tube furnace, with an applied electric field of 60 V/cm and maximum current density of 200 mA/mm2. Samples formed by isostatic pressing showed better control in the development of microstructural homogeneity. The ones formed by slip casting had lower apparent densities and longer incubation times among all the forming methods. Also, slip casting samples showed the largest detachment of average grain size in the central region compared to the other regions. One outstanding result is related to differences between the regions close to the upper and lower electrodes in the samples formed by uniaxial pressing, even when using alternating current mode. These results showed that the density distribution along the green specimen should be considered a decisive factor when evaluating flash sintered samples. The chosen forming technique causes significant differences in incubation time, densification, and the development of microstructural heterogeneity.