Beyond flash sintering in 3 mol % yttria stabilized zirconia

Abstract

A flash sintering experiment can be carried out by applying an electric field and heating the specimen at a constant rate. The flash event occurs at a specific temperature that depends on the strength of the electric field. Alternatively, the furnace can be held at a constant temperature and the voltage applied as a step function; after an incubation time there is a highly non-linear rise in conductivity. This incubation step is called Stage I. The non-linearity is constrained by switching the power supply to current control. This short transient, during which the sample sinters nearly instantaneously, is the second stage. Under current-control, the (essentially dense) sample remains in a highly excited state indefinitely, which we call Stage III. In this state, the samples are often brightly electroluminescent emitting a green glow; unusual phase transformations occur and the rate of chemical reactions is greatly enhanced. We infer that these manifestations are evidence of a defect catastrophe that includes unusual generation of electrons, holes and point defects, which can produce sintering, electronic conductivity, electroluminescence, and phase transformations, all at the same time. We hypothesize that both Joule heating and electric field are necessary for this catastrophe.

Flash sintering of ceramic materials

“…However they do not agree that the temperatures rise sufficiently in the sample to enable the molten grain boundary mechanism proposed [73]. These defects are thought to aid chemical diffusion, and also cause electroluminescence and non-equilibrium phase transformations observed for flash sintering of 3Y-TZP particularly during Stage III of the process where Joule heating considerations no longer apply [73].…”

“…Rishi Raj and co-workers [20,34,37,39,73,129] likewise attribute the effect of flash sintering to a combination of Joule heating from power dissipation and the contribution of these defectmediated processes, with different mechanisms occurring in different stages of the flash sintering process, and have explicitly stated that Joule heating alone cannot completely account for the flash sintering mechanism [130]. However they do not agree that the temperatures rise sufficiently in the sample to enable the molten grain boundary mechanism proposed [73].…”

“…Samples have been flash sintered in dogbone [8,20,22,23,39,41,[71][72][73] and pellet form [26,31,36,37]. The apparatus used includes a vertical tube furnace with camera to detect shrinkage in dogbone samples [20,22,39,41,71,72], a box furnace with front window for use of a camera [8], a commercial spark plasma sintering machine used without the usual graphite mould [31], a vertical furnace adapted to hold pellets by the use of alumina supporting bars with [26,37] or without [36] significant applied load, and a modified system holding suspended dogbones designed for in situ X-ray diffraction studies [23,73].…”

“…In the introduction to their 2016 paper, Jha et al [73] It should also be noted in considering the validity of proposed flash sintering theories that the switching of the power supply from voltage control to current control is an essential step in avoiding entering an uncontrollable state of electrical runaway which would eventually lead to the melting of the sample. By switching to current-control, the power becomes constant and the sample enters a steady state condition as the voltage and current are both stable.…”

“…Jha et al state that Joule heating causing thermal runaway is insufficient to fully explain the flash sintering process, arguing that linearization of parameters during the numerical analysis which accounts for thermal runaway is unlikely to be valid for a non-linear phenomenon such as the increases in sample conductivity seen in flash sintering [73]. However it should be noted that a recent analysis by Pereira da Silva et al uses a non-linear approach but also concludes that the critical condition for flash sintering can be determined from thermal runaway rather than being defect mediated [126], a clear contradiction in findings which is not yet resolved.…”