Microstructure of high-performance thermochromic ZrO2/V0.984W0.016O2/ZrO2 coating with a low transition temperature (22 °C) prepared on flexible glass

Abstract

We report on the microstructure of high-performance thermochromic ZrO2/V0.984W0.016O2/ZrO2 coating deposited on ultrathin flexible glass using a pulsed magnetron sputtering without any substrate bias voltage and post-deposition annealing. The coating combines a low transition temperature of 22 °C, an integral luminous transmittance approaching 50% and a modulation of the solar energy transmittance over 10%. The microstructure of the coating was studied mainly by high-resolution transmission electron microscopy and selected-area electron diffraction. The active V0.984W0.016O2 layer, prepared at a relatively low temperature of 330 °C, is ~70 nm thick. It is composed of closely packed vertically aligned single-crystalline W-doped VO2 nanocolumns that directly attach to the bottom and the top ZrO2 antireflection layer. The majority of the VO2 nanocolumns have a height of ~70 nm (equal to the thickness of the layer) and a lateral size of ~15 nm to 40 nm. The nanocolumns are formed predominantly by the high-temperature thermochromic tetragonal VO2(R) phase with only minor presence of the metastable non-thermochromic orthorhombic VO2(P) phase, which might be stabilized by the used low W-doping. An array of amorphous, low-density nanodomains of a size ~2–4 nm originated particularly along the interface between the V0.984W0.016O2 layer and the top ZrO2 layer. The V0.984W0.016O2 layer is well adhered to both ZrO2 layers. The ZrO2 layers, prepared at a very low temperature (<60 °C), consist of the monoclinic and the tetragonal ZrO2 phase. The bottom ZrO2 layer (~180 nm thick) exhibits an adherent and seamless interface with the flexible glass substrate. The top ZrO2 layer (~170 nm thick) is sufficiently compact to provide mechanical protection and environmental stability for the thermochromic coating. The results are important for further improvement of the performance of multilayer thermochromic coatings designed for smart-window applications.