VO2 thermochromic coatings show great potential as the main component in smart radiator devices (SRD) for spacecraft, most notably for micro- and nano-satellites. Indeed, the inherent metal to ...insulator transition (MIT) of VO2 allows such a coating to act as a lightweight thermal regulator, eliminating the need for heavy and failure-prone mechanical louvers. However, spacecraft industry standards require an emissivity variation Δε of at least 60%, a value which, to our knowledge, has yet to be demonstrated. To reach and surpass this value, we first apply a modeling approach to optimize the optical properties and thicknesses of the individual constituent films of a typical SRD with the following architecture: mirror | dielectric resonant cavity | VO2. This study then highlights various possible avenues to enhance the performance of the devices, one of these being the use of an infrared transparent ultralow refractive index dielectric material for the resonant cavity. This theoretical prediction is then confirmed by the deposition of various prototype devices implementing a CaF2 cavity layer with n @ 10 μm = 1.17 and with measured values of Δε in excess of 60% for the full 3 to 25 μm wavelength range. In fact, we demonstrate a prototype device with a maximum Δε value of 66%, thus bringing this technology one step closer to implementation.
•An optical modeling approach is implemented to optimize the emissivity variation.•A thin VO2 film and ultralow refractive index cavity are shown to be beneficial.•A CaF2 cavity with n = 1.17 is shown to broaden the absorption at high temperature.•Prototype devices with emissivity variations of up to 66% are demonstrated.
Thermochromic VO2 windows have the potential of managing heat transfer in an efficient way. However, several problems such as a high transition temperature ~68°C and high deposition temperatures ...(over 400°C) limit their applicability. We present a novel approach for the fabrication of thermochromic VO2 films in which High Power Impulse Magnetron Sputtering (HiPIMS) is used for deposition. Indeed, HiPIMS is known for its high ionization degree of sputtered species that allows one to control the evolution of the film microstructure by ion bombardment. The optical and other physical properties of the obtained HiPIMS VO2 coatings are first presented. Based on spectrophotometry, ellipsometry, AFM, SEM, TOF-SIMS, Raman spectroscopy and XRD results, we show that it is possible to deposit dense stoichiometric crystalline VO2 films at lower substrate temperatures (300°C) compared to other approaches. These films exhibit a high infrared modulation (ΔT2500nm of 61% between 30°C and 90°C), low surface roughness (Rrms under 10% of total thickness for films approximately 100nm thick), and lower transition temperatures than the bulk material (Tc down to 50°C for thicker films).
•Thermochromic VO2 coatings with a high infrared modulation are fabricated by HiPIMS.•Dense homogeneous stoichiometric crystalline VO2 films are obtained.•A lower deposition temperature is demonstrated when compared to other approaches.•We observe a lower transition temperature than the bulk material.
Smart windows offer an opportunity to reduce energy consumption. However, the use of multiple optical elements, such as low emittance coatings and electrochromic devices, is detrimental to the ...luminous transmittance of these high performance windows. Although the addition of antireflective coatings has helped to reduce this problem, some elements, such as high index of refraction materials still give rise to loss of light. We show that replacing the single WO
3 active coating, the main component of an electrochromic device, by an appropriately designed electrochromic interference filter can significantly increase the transmittance. This active filter is based on a stack of dense and porous WO
3 layers. We first study the effect of porosity on the physical and electrochromic properties of WO
3 prepared by radio frequency magnetron sputtering. We demonstrate that the overlying dense coating does not inhibit the coloration of the underlying porous coating. The best performing films are combined into a 27 layer quarter-wave interference filter which is shown to cycle between bleached and colored states, while providing attractive transmission. Finally, we discuss various filter designs which can increase the transmission of an electrochromic device in its bleached state, as well as the potential use of active filters for optical security devices possessing two levels of authentication.
Counterfeiting of products and important documents is at an all-time high and is costing the world economy hundreds of billions of dollars yearly as well as posing significant safety and health ...hazards through the production of uncertified goods, e.g., pharmaceutical products. To limit these effects, interference-based optical security devices offering an angular color shift are still widely in use. Unfortunately, commercial iridescent materials are now readily available and represent a potential source of counterfeiting.
In this short review, we first describe the basic principles behind passive interference security image structures (ISIS) and the qualities which have resulted in their integration into most important documents. Various features which have been added to ISIS in order to make them harder to duplicate yet simpler to authenticate are also presented (metamerism, magnetic materials, diffraction, etc.). We then address the implementation of active materials, mainly electrochromic WO3 as a means of generating two-level authentication devices. Finally, we discuss some general considerations to keep in mind when developing features for security applications.
•We review Fabry–Perot-like metal-dielectric filters used in optical security.•We discuss/demonstrate recent additions: metamerism, magnetism and diffraction.•We demonstrate a feature based on the use of thin metallic mirrors.•We cover recent developments in the use of active materials.•We demonstrate an electrochromic feature with two levels of authentication.
Brownian thermal noise as a result of mechanical loss in optical coatings will become the dominant source of noise at the most sensitive frequencies of ground-based gravitational-wave detectors. ...Experiments found, however, that a candidate material, amorphous Ta2O5, is unable to form an ultrastable glass and, consequently, to yield a film with significantly reduced mechanical loss through elevated-temperature deposition alone. X-ray scattering PDF measurements are carried out on films deposited and subsequently annealed at various temperatures. Inverse atomic modeling is used to analyze the short and medium range features in the atomic structure of these films. Furthermore, in silico deposition simulations of Ta2O5 are carried out at various substrate temperatures and an atomic level analysis of the growth at high temperatures is presented. It is observed that upon elevated-temperature deposition, short range features remain identical, whereas medium range order increases. After annealing, however, both the short and medium range orders of films deposited at different substrate temperatures are nearly identical. A discussion on the surface diffusion and glass transition temperatures indicates that future pursuits of ultrastable low-mechanical-loss films through elevated temperature deposition should focus on materials with a high surface mobility, and/or lower glass transition temperatures in the range of achievable deposition temperatueres.
Abstract
Sensitivity in instruments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) is limited by a noise originating from fluctuations linked to internal mechanical ...dissipation (IMD) in the amorphous thin films of their Bragg reflectors. We investigate the correlation between IMD, characterized by a loss angle, and the hydrogen concentration in a thin film made of Zr-doped tantalum oxide after annealing at different temperatures. The film was deposited by magnetron sputtering and the loss angle measured by gentle nodal suspension. The concentration of heavier elements was obtained by Rutherford backscattering spectrometry (RBS). The hydrogen concentration in the as-deposited and annealed samples was obtained by elastic recoil detection (ERD). We observe that the hydrogen atomic concentration gradually decreases from 2.0 ± 0.1% down to the detection limit at 0.2 ± 0.1% as we anneal to 650°C. We also find that the loss angle decreases by a factor of two over the same annealing temperature range, suggesting that the loss angle is correlated with the hydrogen concentration. However, the loss angle remains relatively high even when most of the hydrogen is desorbed. We conclude that the presence of hydrogen is not the main limiting factor for further reducing the IMD.
A comparative study of the environmental stability of high power impulse magnetron sputtered (HiPIMS) and radio frequency magnetron sputtered (RFMS) thermochromic vanadium dioxide thin films in a ...highly oxidizing environment has been performed. We observe that RFMS-deposited VO2 films quickly transform into V2O5 at 80°C and 100% relative humidity while HiPIMS-deposited films retain their thermochromic behaviour at least three times longer. Following a thorough analysis of the films, this increase in performance is ascribed to the films’ higher density and larger average grain size. The results indicate that the HiPIMS process provides sufficient durability for VO2 films without the need for a diffusion barrier used in typical everyday conditions. Still, when a 35nm thick SiNx diffusion barrier is added on top of the HiPIMS VO2 films, the thermochromic device shows less than 5% loss in thermochromic performance following extended accelerated ageing.
•HiPIMS and RFMS deposition of thermochromic VO2.•Good durability of HiPIMS deposited VO2 in humid conditions.•SiNx demonstrate oxidation-protection of RFMS and HiPIMS coatings.
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