Hexagonal VO2 particles: synthesis, mechanism and thermochromic properties

Phase-Tailoring Wx V1-x O2 Meta-Nanofiber Enables Temperature-Editing Energy Control

Room-temperature phase change materials (RTPCMs) exhibit promise to address challenges in thermal energy storage and release, greatly aiding in numerous domains of human existence and productivity. The conventional RTPCMs undergo inevitable volume expansion, structural collapse, and diffusion of active ingredients while maintaining desirable phase change enthalpy and ideal phase change temperature. Here, a sol-gel 1D-induced growth approach is presented to fabricate meta nanofibers (Meta-NFs) comprised of vanadium dioxide with monoclinic crystal structure, and further achieve the editable phase change temperature from 68 to 37 °C through W-doping, which allowed for tailored length variation of the zigzag V-V bond. Subsequently, Meta-NFs are assembled into 3D aerogels with self-standing architecture, thereby enabling the independent use of the RTPCMs. The obtained metamaterials demonstrate not only the temperature-editing solid-solid phase transition, but also the stiffness of the ceramic matrix, exhibiting the thermal energy control capability at room temperature (37 °C), thermal insulation properties, temperature resistance, and flame retardancy. The effective creation of these fascinating metamaterials might offer new insights for next-generation and self-standing solid-solid RTPCMs.

Reactivity of Vanadium Nanoparticles with Oxygen and Tungsten

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 T_c 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 T_c attains the stimulating values of 35 °C on heating and 25 °C on cooling.

A Novel Route for the Easy Production of Thermochromic VO2 Nanoparticles

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.

Hydrothermal epitaxy growth of self-organized vanadium dioxide 3D structures with metal–insulator transition and THz transmission switch properties

The hydrothermal method is an effective approach for the synthesis of VO₂ films with unique crystallites morphology and sharp electrical and optical switch properties.