Reactivity of Vanadium Nanoparticles with Oxygen and Tungsten

Abstract

A mechanistic study was carried out on the optimal methods of fabrication of products containing higher loads of thermochromic VO2(M1) fabricated by thermal treatments of V nanoparticles in air, that, once achieved, are more stable than other commercial products upon natural aging or reiterated reheating. At the best temperatures for single runs, 55% of VO2 can be attained by the reactions of a limited number of the species initially formed in a process, that, if not stopped, can degrade the product by solid state reactions of oxidations and reductions without O2 consumption. This fact supports the use of two-step treatments at lower temperatures and faster cooling rates that reach 65% of VO2; such reactions should, ideally, take place in the 550-625 °C temperature range. The impregnation of V with a tungstate salt is an ideal and simple doping platform that can decrease the energy of activation of the 2-cycle process, allowing higher yields and enthalpies of transformation (71% of VO2, 26 J/g) than undoped counterparts or trademarks. A good balance is reached for 1% at. of W, with a reduction in Tc of 20 °C not significantly resenting the enthalpy of the reversible metal-to-insulator transition. For higher W amounts, the appearance of tetragonal VO2, and W alloyed V3O7 and V2O5, decrease the fractions of increasingly and effectively doped M1-VO2 achieved till 2% of W, a concentration for which Tc attains the stimulating values of 35 °C on heating and 25 °C on cooling.