High efficiency thermochromic VO2(R) resulting from the irreversible transformation of VO2(B)

The metastable VO2(B) transforms irreversibly into thermochromic VO2(R) between 400 and 500 degrees C. Differential scanning calorimetry (DSC) of this transition shows a broad transformation starting with a first endothermic step at 420 degrees C followed by a second exothermic step at 455 degrees C. During this process, the initial platelet like VO2(B) morphology leads to a submicronic VO2(R) powder. This low temperature thermal treatment avoids the grain growth commonly observed in the case of classical routes at higher temperatures. This transformation with constant mass and high increases of density has been studied by XRD, IRTF spectroscopy, thermal analyses and electronic microscopies. The structure and properties of the VO2(R) issued from this irreversible transition are very similar to those of the VO2(R) obtained by the powder metallurgy process, except the optical properties. Indeed, this thermochromic VO2(R) powder shows an unexpected transition contrast in the IR spectral range during the insulator-metal transition at 68 degrees C. Experimental observations by scanning electron microscopy (SEM) correlated with calculations could explain this high efficiency by the unusual morphology of the submicronic particles. (C) 1998 Elsevier Science S.A. All rights reserved.