Reactive flash sintering has been demonstrated as a method to rapidly densify and synthesize ceramic materials, but determining the extent of chemical reactions can be complex since the maximum temperature reached by the sample may be brief in time. The black body radiation (BBR) model has been shown to accurately predict the sample temperature during the steady state of flash (stage III). This work demonstrates situations where the BBR model alone does not accurately predict when a phase transformation will occur. We examine the model reactions of CuO reduction to Cu2O during stage II and Mn(2)O(3)reduction to Mn(3)O(4)in stage III. In CuO, highly resistive samples result in initially localized current flow, a stochastic process resulting in inhomogeneous heating and error in the BBR model during stage II. CuO reduction does not occur in constant heating rate experiments with 6.25 V/mm fields, even though the sample temperature momentarily exceeds the phase transformation temperature. Increased furnace heating to 950 degrees C before application of a field is required to drive the transition. In Mn2O3, the calculated sample temperature of the gauge is less than the transformation temperature, but localized heating at the contact will exceed the transformation temperature, causing the transformation to propagate away from the electrode during stage III. This work demonstrates two forms of inhomogeneity (local, stochastic current flow, and local contact resistance) that result in a complex thermal profile of the sample. This profile should be interrogated to understand reaction kinetics, and can be beneficial when engineered.

New Na-ion conductors Na0.5La0.5ZrO3 and Na0.25La0.25Ba0.5ZrO3 with expanded lattices were developed based on perovskite LLTO, successfully increasing the conductivities vs. unexpanded (Na,La)TiO3 by up to ∼50×.

Flash sintering has been established as a method to rapidly densify or induce phase transformation in materials by applying an electric field. Understanding rapid transformations is challenging in a laboratory setting, where phase and defect populations may change with sub-second time scales. In this work, we apply in situ Raman spectroscopy to investigate defect complexes and phase transformations during flash sintering. We demonstrate that the oxygen vacancy complexes in Ce0.85Gd0.15O1.925 and the evolving phases in a SrCO3 + V2O5 reaction can be probed with acquisition times sufficient to monitor stage II of flash sintering. We establish how thermal effects can be separated from effects of the electric field and determine the resolution of the defect concentration. The fast characterization we demonstrate here can be combined with well-developed models of thermal and phase evolution to elucidate the mechanism of densification during flash sintering.

The grand challenge of synthesizing materials by predictive design remains outstanding because controlling all necessary thermodynamic and kinetic factors quickly becomes intractable, even for well-known systems. Nevertheless, predictions that are strengthened by large amounts of quality data should have well-defined rates of success. Here, we show that density functional theory calculations highlight four chemical landscapes in alkali-zinc-chalcogenide ternary systems that appear to be densely populated by new phases. For such 3d(10) systems, the total energy calculations are so accurate that a majority of the newly predicted ground-state phases are synthesized experimentally. Nine new ternary phases are presented, compared to the two that were previously known. The compounds Na2Zn2S3, Na6ZnSe4, Na2ZnSe2, Na2Zn2Se3, K6ZnS4, K2ZnS2, K2Zn3S4, K2ZnSe2, and K2Zn3Se4 are all semiconductors with Zn-S connectivity ranging from zero- to two-dimensional. Their anisotropic structures lead to potential applications in birefringence and UV absorption. Even for relatively common combinations of elements, the potential for computationally informed material discovery remains high.

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 degrees C in electronic conductors.

We report on changes in the thermal conductivity of solid-state synthesized MnxM Ge (M = Co, Ni, 0.98 < x < 1.02) alloys through their temperature-induced martensitic structural transition. T ...