Flash sintering of ZnO, TiO2 and a few other oxide systems has been investigated. A quantitative model has been developed to forecast the thermal runaway conditions. The predicted thermal runaway temperatures from the measured conductivities are in excellent agreements with the observed onset flash temperatures for at least 15 cases with different base materials, doping and surface treatments, particle sizes, and sintering atmospheres, attesting that the “flash” starts as a thermal runaway.
Specifically, using ZnO as a model system, a strong dependence of the onset flash sintering temperature on the atmosphere has been discovered. In a set of optimized conditions, ZnO specimens have been sintered to >97% relative densities in ~30 s at furnace temperatures of <120 °C in Ar + 5 mol. % H2, with uniform microstructures and fine grain sizes of ~1 um. The enhanced conductivities of ZnO powder specimens in reduced atmospheres are responsible for the substantial decreases of the onset flash sintering temperatures.
More recently, we have investigated the effects of joule heating and high heating ramp rate on fast densification in flash sintering of ZnO, as well as the electrical field/current effects on densification and microstructural developments. Mimic heating profile in flash sintering by rapid thermal annealing (RTA), which excludes the electric filed effects and has the similar heating ramp rate as flash sintering up to 200 °C per second. The specimens after RTA at similar estimated temperature as flash sintering could reach almost the same density and grain size.
Observation of an unusual case of triple-line wetting by a gas phase is another project I involved during my graduate study. The Bi vapor penetrates along the triple lines in the electrodeposited Ni to form open channels at 800 and 900 °C. This is interpreted as a case of triple-line wetting by a gas phase, which has never been reported before. This unusual wetting phenomenon is related to the formation of a bilayer complexion and grain boundary embrittlement in the Ni-Bi system [Science 333: 1730 (2011)]. Further controlled experiments using high-purity Ni specimens with and without S doping suggest that the presence of S impurities is essential for the occurrence of this wetting phenomenon. This discovery has practical importance for understanding and controlling the microstructural stability and corrosion resistance.