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 (DS C) of this transition shows a broad transformation starting with a fir st endothermic step at 420 degrees C followed by a second exothermic s tep at 455 degrees C. During this process, the initial platelet like V O2(B) morphology leads to a submicronic VO2(R) powder. This low temper ature thermal treatment avoids the grain growth commonly observed in t he case of classical routes at higher temperatures. This transformatio n 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 irreversi ble transition are very similar to those of the VO2(R) obtained by the powder metallurgy process, except the optical properties. Indeed, thi s thermochromic VO2(R) powder shows an unexpected transition contrast in the IR spectral range during the insulator-metal transition at 68 d egrees C. Experimental observations by scanning electron microscopy (S EM) correlated with calculations could explain this high efficiency by the unusual morphology of the submicronic particles. (C) 1998 Elsevie r Science S.A. All rights reserved.