Thermochromic Coatings and Laminates for Smart Windows Comprising VO2 Nanopigments
HUPPERETZ J. 1,2, CALVI L. 3,4, YEUNG C. 1,2, HABETS R. 1,2, LEUFKENS L. 1,2, VAN ZANDVOORT R. 1,2, WOLTERS D. 1,2, ELEN K. 3,4, HARDY A. 3,4, VAN BAEL M. 3,4, MEULENDIJKS N. 1,2, MANN D. 1,2, BUSKENS P. 1,2,3
1 The Netherlands Organisation for Applied Scientific Research (TNO), Eindhoven, Netherlands; 2 Brightlands Materials Center, Geleen, Netherlands; 3 Design and Synthesis of Inorganic Materials (DESINe), Institute for Materials Research, Hasselt University, Diepenbeek, Belgium; 4 IMEC vzw, IMOMEC Associated Laboratory, Diepenbeek, Belgium
The building sector consumes approximately one third of the total energy worldwide. A large part of this energy is used for heating and cooling of buildings, which can be drastically reduced through application of energy-efficient windows. Here, we studied vanadium dioxide (VO2), which is a promising optical material for this purpose because of its thermochromic properties. Based on its structural phase transition from the semiconductive monoclinic to the conductive rutile structure at a defined switching temperature, it switches from a solar infrared transmissive to blocking state. This switch is reversible and the switching temperature can be adjusted via doping, which makes this material interesting for application in thermochromic windows that regulate solar heat gain. Recently, we have reported the preparation and characterization of thermochromic coatings consisting of VO2 and SiO2, the switching kinetics of thermochromic powders, and the impact of thermochromic windows on energy savings in intermediate climates.
Here, we report the development of thermochromic coatings and laminates comprising W-doped VO2 nanoparticles as functional pigments. We obtained these pigments on pilot-scale via top-down synthesis using a vanadium precursor, a tungsten complex for doping, calcination via rotary tube furnace and size reduction via bead milling. DSC and XRD analysis confirmed the high degree of purity of VO2 as well as the reduced switching temperature of 22°C. We applied the nanopigments in both polymer films and coatings for window applications. The impact of nanoparticle size and concentration on the resulting optical properties of the laminate, viz. visible transmission and solar modulation, was studied in detail. The highest solar modulation obtained was 9.4% at a visible transmission of 55%. Furthermore, we monitored the effect of decreasing particle sizes (1000-100 nm) on both the optical and thermodynamic switching performance. Lastly, we installed a pigmented laminated window (1x1 m2) and a reference window in a test building and demonstrated the real life performance of a thermochromic laminated window over a time span of one year.
 Yeung et al. Solar Energy Materials and Solar Cells 2021, 230, 111238.
 a) Calvi et al. Sol. Energy Mater. Sol. Cells 2021, 224, 110977; b) Calvi et al. Sol. Energy Mater. Sol. Cells 2022, 242, 111783.
 Mann et al. Energies 2020, 13, 2842.
 Calvi et al. Submitted manuscript to Solar Energy Materials and Solar Cells.