Chemical and thermal properties of VO₂ mechanochemically derived from V₂O₅ by co-milling with paraffin wax

A novel mechanochemical reduction process of V₂O₅ to VO₂ 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 V₂O₅ : PW (MRVP). The mechanochemically derived VO₂ 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 VO₂. Doping of starting V₂O₅ 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 VO₂. These anomalous heat storage properties of mechanochemically derived VO₂ 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 VO₂ is primarily ascribed to the abundance of V⁴⁺ 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.