Materials exhibiting pronounced metal−insulator transitions such as VO 2 have acquired great importance as potential computing vectors and electromagnetic cloaking elements given the large accompanying reversible modulation of properties such as electrical conductance and optical transmittance. As a first-order phase transition, considerable phase coexistence and hysteresis is typically observed between the heating insulator → metal and cooling metal → insulator transformations of VO 2 . Here, we illustrate that substitutional incorporation of tungsten greatly modifies the hysteresis of VO 2 , both increasing the hysteresis as well as introducing a distinctive kinetic asymmetry wherein the heating symmetryraising transition is observed to happen much faster as compared to the cooling symmetry-lowering transition, which shows a pronounced rate dependence of the transition temperature. This observed kinetic asymmetry upon tungsten doping is attributed to the introduction of phase boundaries resulting from stabilization of nanoscopic M 2 domains at the interface of the monoclinic M 1 and tetragonal phases. In contrast, the reverse cooling transition is mediated by point defects, giving rise to a pronounced size dependence of the hysteresis. Mechanistic elucidation of the influence of dopant incorporation on hysteresis provides a means to rationally modulate the hysteretic width and kinetic asymmetry, suggesting a remarkable programmable means of altering hysteretic widths of an electronic phase transition.
A temperature-responsive PVA gel is achieved that reversibly holds fluid lithium nitrate trihydrate and releases it in response to temperature for easy gelling in-place and later removal from heat-exchange modules.
Lithium nitrate trihydate (LNH) is a promising latent heat energy storage material with one of the largest specific and volumetric enthalpies of fusion of all near‐room temperature melting phase change materials. However, the understanding of corrosion rates and mechanisms of common metals and polymers in LNH, which is an extremely high salt content solution (wH2O < 0.50), remains relatively limited. Here, we report corrosion rates and mechanisms resulting from 6 months immersion corrosion studies on 9 common polymeric materials and 12 metallic alloys in molten LNH along with likasite, a common nucleation agent. Copper alloys were observed to corrode in liquid LNH, experiencing both uniform surface corrosion and localized pitting corrosion. Aluminum alloys experienced localized corrosion in liquid LNH, which was more severe in cases with likasite present, in which cases Cu dissolved from likasite enhanced corrosion pitting rates. Thus, in the absence of effective corrosion inhibitors, aluminum, and copper are not viable as heat exchanger materials for LNH.
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