W/VO₂ Nanoparticles for Thermochromic Polymer Films—A Comparative Study Between Bead Milling of Powders and Hydrothermal Synthesis

Vanadium dioxide displays a structural phase transition (SPT) from monoclinic VO₂ (M) to rutile VO₂ (R) at 68°C, which is reversible [1]. This SPT is accompanied by a metal-insulator transition, which makes this material interesting for application in energy efficient windows. VO₂ (M) is an insulator and transmits solar infrared (s-IR) radiation, VO₂ (R) is metallic and reflects and/or absorbs s-IR light. For application in energy efficient windows, the switching temperature needs to be lowered from 68°C to 15-25°C, which is typically achieved by doping with metal ions such as W⁶⁺. To introduce thermochromic W/VO₂ in windows, we aimed at preparing thermochromic PVB polymer films comprising W/VO₂ nanoparticles of a size below 100 nm to avoid scattering of visible light. These should then be incorporated into PVB via a masterbatch, and the resulting nanocomposite polymer films should be applied to laminate glass and produce insulating glass units with a laminated outboard. Based on the temperature of the outer glass pane and the nanocomposite film, the resulting window should then switch from a s-IR transmissive to a blocking state and <i>vice versa</i>. This contributes to a reduction in energy consumption and costs for heating and cooling of buildings in intermediate climates.<br/>To prepare VO₂and W/VO₂ nanoparticles, we selected two different routes: a hydrothermal synthesis starting from a mixture of organometallic V(IV) and W(VI) complexes, and a top-down processing route based on bead milling of VO₂and W/VO₂ powders. We successfully prepared thermochromic VO₂ and W/VO₂ nanoparticles using both routes. We characterized their composition, size and shape using X-ray diffraction, laser diffraction, dynamic light scattering, scanning and transmission electron microscopy. Furthermore, we comparatively studied their functional performance focusing on the thermodynamics and kinetics of the SPT [2]. For that purpose, we performed differential scanning calorimetry at various heating and cooling rates. The switching kinetics were determined using Friedman's differential isoconversional method. For undoped VO₂ (M), we found that the activation energy (<i>E_A</i>) of the SPT decreases with increasing difference between the actual temperature of the material and its switching temperature. Furthermore, the activation energy decreases with progressive milling, and kinetic asymmetry is induced. For milled particulate materials, │<i>E_A</i> │is lower for the switch from VO₂ (R) to VO₂ (M) than for the opposite switch. For hydrothermally synthesized nanoparticles, <i>E_A</i> is in the same order of magnitude, albeit with inverse switching asymmetry. Latter may result from different defects that are introduced during both preparation techniques.<br/>W/VO₂ nanoparticles obtained by bead milling were successfully introduced into PVB via a masterbatch, and the resulting nanocomposite polymer material was processed into films. These were then applied for lamination of glass plates up to 1 square meter in size. Subsequently, pilot scale windows were prepared comprising these laminated glass plates. We started testing these in demo houses to gather information on the performance of the innovative energy efficient window in real environment. Initial results show that the smart window transitions from an s-IR transparent to blocking state as soon as direct sunlight hits the window and ambient temperatures are above 20°C. The transition back to the s-IR transparent state usually happens over night when the glass surface cools down. The window is optimized to reduce energy consumption in moderate climates with cold winters and warm summers, such as in the Netherlands, which can lead to additional energy and cost savings of up to 8% and 23.70 € per square meter per year when compared to state of the art HR++ windows.<br/><br/>[1] L. Calvi <i>et al.</i>, <i>Solar Energy Materials & Solar Cells</i> <b>2021</b>, <i>224</i>, 110977.<br/>[2] L. Calvi <i>et al.</i>, <i>Solar Energy Materials & Solar Cells</i> <b>2022</b>, <i>242</i>, 111738.