Chemical and thermal properties of VO 2 mechanochemically derived from V 2 O 5 by co-milling with paraffin wax.

A novel mechanochemical reduction process of V 2 O 5 to VO 2 was established by milling with paraffin wax (PW, average molecular weight 254-646), serving as a reductant. The reduction progressed with increasing milling time and mass ratio V 2 O 5  : PW (MRVP). The mechanochemically derived VO 2 became phase pure after milling for 3 h with an MRVP of 30 : 1 and exhibited a reversible polymorphic transformation between tetragonal and monoclinic phases at around 53-60 °C and 67-79 °C during heating and cooling, respectively. The latent heat was above 20 J g -1 in both processes, being superior to those of commercial VO 2 . Doping of starting V 2 O 5 with Cr, Mo or W at 1 at% in the form of oxide did not increase the latent heat. This is another difference from the conventionally prepared doped VO 2 . These anomalous heat storage properties of mechanochemically derived VO 2 were discussed mainly on the basis of X-ray photoelectron spectroscopy V 2p3/2 peaks combined with ion etching. The observed relatively high heat storage capacity of undoped VO 2 is primarily ascribed to the abundance of V 4+ ionic states introduced during milling with PW, which were stabilized with simultaneously introduced structural degradation throughout the entire particles. The possible role of a remaining small amount of PW was also discussed.