Abstract:

Ceramics often require high processing temperatures, but recently a new method has been discovered which applies external electric field to reduce furnace temperature requirements and shorten sintering times to seconds instead of hours. Since the discovery of this method, termed flash sintering, a wide range of primarily oxide ceramics have been densified. While the potential is clear, the understanding of this process has been constantly debated: is it simple Joule heating or an effect of electric field? Through use of in situ energy diffraction, verified with other tools, an understanding of the dynamic structural changes during the flash process is developed to substantiate the role of high heating rate and applied field. Particularly highlighted is work on flash sintering of 8YSZ electrolyte which has been determined to generate a metastable phase of zirconium monoxide along the grain boundaries. Phase field modeling supports our hypothesis of oxygen depletion in the grain boundary core region while non-stoichiometry induced residual stress contours were revealed using high resolution TEM. Application of alternating current (AC) electric field resulted in sintering without metastable phase as high frequency prevents the build-up of significant chemical potential. Peak profiles and lattice expansion under flash conditions were correlated to a conventionally annealed standard.

In another study, energy dispersive X-ray diffraction (EDD) was used to understand the spatial stress-strain relation of various components of a prismatic NMC battery during operation. The change in lattice parameters was tracked for the NMC cathode, graphite anode, and current collectors during the charge-discharge cycle. Irregular cycling behavior was determined to result from shear stress at the cathode-current collector interfaces, which may result in long term plastic deformation. The role of design and current discharge rate were analyzed in this deterioration mechanism. An analytical model is described to understand the problem and possible solutions to improve the existing battery design.