Propagation of the contact‐driven reduction of Mn₂O₃ during reactive flash sintering

Abstract

As the field of flash sintering expands, more diverse flash processes are emerging that exhibit complex mechanisms and kinetics. Reactive flash sintering studies have been performed using precursor oxides and have yet to explore redox reactions. We show that Mn2O3 transforms into Mn3O4 during stage III of flash sintering via a moving reaction front, propagating from an electrode if sufficient energy is supplied. The power density and sample temperature increases as the transformation progresses due to the lower resistivity of Mn2O3 vs Mn3O4, a secondary thermal runaway effect, further confirming the presence of a transformation front. Additionally, in many studies, the contact resistance is accounted for, but not utilized. The energy for the transformation may either be supplied by the contact resistance–induced Joule heating or the furnace. Room‐temperature impedance measurements demonstrate that Pt electrodes provide substantial contact resistance while Ag electrodes do not. The impedance study demonstrates that it is critical to select the appropriate electrode material to maximize or minimize contact resistance. The contact resistance may be used to create a hot spot and propagate a transformation front in any endothermic reduction reaction that occurs below 950°C in electronic conductors.

Predicting transformations during reactive flash sintering in CuO and Mn2O3

“…Localized current flow from applying an electric field that is too large has been established in a variety of systems. 7,26 The local breaking due to a current density that is too large has been demonstrated here in both CHR and ISO flashes of CuO. The failure can occur either at the electrode or along the gauge of the sample.…”

“…The high-frequency (low Z′) arc is due to the sample resistance and the low-frequency (high Z′) arc is due to the contact resistance. 7 The contact resistance is approximately 6300 Ω. The impedance measurement of CuO seems to indicate that contact resistance is very low compared to that of Mn 2 O 3 .…”

“…In our previous work, we have demonstrated that the reduction of Mn 2 O 3 to Mn 3 O 4 is the result of localized heating due to contact resistance at the electrodes. 7 The next section will focus on how the inhomogeneity caused by the contact resistance will affect the results of comparing the calculated sample temperature to the transformation temperature. Impedance measurements demonstrate that Mn 2 O 3 experiences substantial contact resistance when Pt electrodes are used while CuO does not.…”

“…Systems that transform during stage III include Al 2 O 3 + TiO 2 → Al 2 TiO 5 and our previous work on the reduction of Mn 2 O 3 → Mn 3 O 4 . 7,10 The fact that complete reactions can be performed in either stage is due to the difference in the energy supplied. Stage II applies a large amount of power, but only over a few seconds while stage III applies a lower power, but can be applied indefinitely.…”

“…Preparation of the Mn 2 O 3 impedance pellets is described in our previous work. 7 The BBR model was used to calculate the sample temperature during flash sintering. The equation was determined by following the procedure laid out by Yang and Raj, but here emissivity, ε, is maintained in the equation rather than combining it into a correction factor, α.…”