Abstract
In this work, a simple, fast and dry method for the fabrication of a thermochromic product with a high load of VO2(M1) consisting of the controlled heat treatment of pure vanadium nanoparticles in air is presented. After a complete design of experiments, it is concluded that the most direct way to attain the maximum transformation of V into VO2(M1) consists of one cycle with a fast heating ramp of 42 °C s−1, followed by keeping 700 °C for 530–600 seconds, and a subsequent cooling at 0.05 °C s−1. Careful examination of these results lead to a second optimum, even more suitable for industrial production (quicker and less energy‐intensive because of its lower temperatures and shorter times), consisting of subjecting V to two consecutive cycles of temperatures and times (625 °C for 5 minutes) with similar preheating (42 °C s−1) but a much faster postcooling (∼ 8 °C s−1). These green reactions only use the power for heating a tube open to atmosphere and a vanadium precursor; without assistance of reactive gases or catalysts, and no special vacuum or pressure requirements. The best products present similar thermochromic properties but higher thermal stability than commercial VO2 particles. These methods can be combined with VO2 doping.
Highlights
Vanadium is a transition element with an incomplete electronic structure and a consequent availability of multiple valences involving a complex and rich chemistry.[1]
The influence of key process parameters was verified by an exhaustive design of experiments (DOE)
These tables collect the sample label A/B/C/D# for which # indicates the run order in the arbitrary sequence of experiments, the plateau temperatures (400 to 900 °C) and times (1 to 1000 s), plus the smaller or bigger average heating (4 to 42 °C sÀ 1) and cooling rate (0.05 to 2.42 °C sÀ 1) on each thermal recipe, and the area under the endothermic differential scanning calorimetry (DSC) peaks at ~ 67 °C, indicative of the enthalpy of the M1 to R phase transformation during heating
Summary
Vanadium is a transition element with an incomplete electronic structure and a consequent availability of multiple valences involving a complex and rich chemistry.[1]. Layers of vanadium oxy-nitride nanotubes were developed by wet electrochemical anodization of V sheets,[18] while using O plasma to treat V strips promoted the formation of mixes of the dioxide and the pentoxide between 200 and 600 °C.[19] It is worth noting the recent achievement of VO2 microtubes on a V2O5 substrate by a fast method of thermal oxidation of vanadium foils that needed a special heating source for reaching over 1700 °C.[20] In an attempt to reach an economical and environmentally friendly solution on this matter, the present work describes a simple, fast, dry, costeffective, safe and clean method to get thermochromic VO2 particles from a metallic V precursor that is commercially available at a cheaper price than vanadium dioxide nanoparticles,[21] and which can be obtained massively by economic routes, as for example, by high-energy ball milling.[22] Note that if needed for modulating the MIT temperature, this method is compatible with doping, for example by a preimpregnation of V with diluted cation salts (W, Mo, Nb, Ge.), and in this way, tungsten doping is demonstrated. This method can be applied to nanoporous films, dense thin films, or deposited nanoparticles of V
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