Design and reactive magnetron sputtering of thermochromic coatings

Abstract

Thermochromic coatings based on vanadium dioxide exhibit great potential in various fields, including smart energy-saving windows with temperature-dependent transmittance in the infrared at preserved transmittance in the visible. However, these promises come with challenges concerning the low-temperature preparation of high-quality crystalline VO2-based films by industry-friendly techniques and the simultaneous optimization of all coating characteristics, such as thermochromic transition temperature, luminous transmittance, and modulation of solar energy transmittance. This Perspective outlines these challenges, highlights the recent progress in the field of design and reactive magnetron sputtering of thermochromic coatings, explains the physics that allowed this progress, and provides ideas and recommendations for future research. A combination of the advantages of controlled high-power impulse magnetron sputtering with the not yet fully experimentally utilized multilayered designs constitutes the main reason why further progress is anticipated in the future.

Low Solar Absorptance, High Emittance Performance Thermochromic VO2-Based Smart Radiator Device

“…More recently, Kolenaty et al [13] demonstrated that when an HiPIMS-deposited VO 2 film was doped with W, multilayer smart windows with a thermochromic transition temperature of around 20-21 • C could be fabricated. Doping with W involved the use of DC sputtering concurrently with HiPIMS, offering more flexibility in terms of controlling the doping content in VO 2 films, and the processing temperature of the W-doped VO 2 was 330 • C. These results are important not only for the preparation of smooth films needed in optical devices, as demonstrated by Kolenaty et al, but also because magnetron sputtering is a well-established method in the industry for the preparation of films over large-surface substrates, including temperature-sensitive ones (i.e., scaling up at a low cost) [8]. Several reports can be found in the literature regarding the control of the emittance of VO 2 -based structures for applications such as smart radiator devices.…”

“…As for the materials used for the individual layers, I decided to consider SiO 2 and a-Si deposited by PECVD not only because they are both readily available as they are widely used in technological applications, but also due to the important contrast between their refractive indices, allowing the achievement of the desired reflection over a large bandwidth [8]. In addition, SiO 2 acts as a protective barrier layer between VO 2 and a-Si [32].…”

“…High-valence cations, for example Nb 5+ , Mo 6+ and W 6+ [2,6], act as donors (n-type dopants), and when VO 2 is doped with these elements, the transition temperature decreases. VO 2 is therefore an excellent material for many applications, particularly in ultra-fast switching [7] and smart windows [8].…”

Measurement of Mechanical Properties of VO2 Films by Nanoindentation

“…The phase transition is always accompanied by a structural transition from the monoclinic (M) phase to the rutile (R) phase, which has potential applications in infrared switches [ 3 ], smart thermal control devices [ 4 ] and smart windows [ 5 ]. In previous studies, researchers have paid more attention to the optimization of thermal, optical and electrical properties after phase transition [ 6 ], while mechanical properties have been almost ignored [ 7 ]. In fact, phase transition will cause the lattice to expand by 1% along the c R -direction, and to shrink by 0.6% and 0.1% along the a R -axis and b R -axis in the meanwhile [ 8 , 9 ].…”

Effect of Substrate Temperature on the Structural, Optical and Electrical Properties of DC Magnetron Sputtered VO2 Thin Films

“…Regardless of the method, high growth temperatures (450–800 °C) are employed to assure its stoichiometry [ 16 , 17 , 18 , 19 ], which is crucial in the synthesis of VO 2 thin films that display a sharp phase transition. However, instrumentation that delivers high substrate temperatures ( T S ) is costly and not industry-friendly [ 20 ]. For this reason, there have been numerous efforts to bring the T S down to more practical temperatures [ 21 , 22 , 23 , 24 , 25 , 26 , 27 ].…”