Electrochromics for Thermal Control on Spacecraft

Abstract

Orbital spacecrafts face the problem of high thermal gradients or different thermal loads mediated by differential illumination from the sun. As spacecrafts are limited in weight there is a growing interest in heat transfer solutions, which are lightweight and economical. Variable electrochromic radiation heat transfer is a promising approach as it offers a better mass budget by saving heater power in the coldest boundary conditions. The focus of the present work is on the engineering and the material science of such electrochromic radiators, which are based on electrochemical cells and the intercalation of Li +-Ions into transition-metal-oxides. Samples out of soda lime glass, Kapton and high thermal conductive pyrolytic graphite foil, with different absorption modulation, are prepared by the use of reactive magnetron sputtering and electrochemical intercalation. The purpose of this work is represented by maximizing the Δα and Δε, feasibility of oppositional α/ε modulation, different substrate testing and relate the results with preliminary on-orbit simulations. Several concepts of electrochromic radiators are investigated in the last years. They are made out of full solid state materials or implemented with electrolyte-polymers and a Δε up to 0.5 and still high absorptivity. We found a modulation range of-0.14 up to 0.23 for the infrared emissivity and 0.43 up to 0.58 for transmittance in the spectral range of 300 to 1000 nm in the case of single layers. Aluminized Kapton shows twice as high absorption modifications as aluminized graphite foil, which is caused by the intercalation of Li + in the graphite-structure instead of tungsten oxide. The modulation is not yet sufficient in range and it's oppositional α/ε. The pyrolytic graphite foils are not suitable for direct deposition of the electrochromic stack and should be used for thermal distribution only. According to the experimental data, a simple stack with a Δε in the range of 0.21 to 0.29 seems to be realizable.

Multifunctional Nanostructures with Controllable Band Gap Giving Highly Stable Infrared Emissivity for Smart Thermal Management

“…It has been shown that, with this approach, it is possible to achieve power savings nearing 100%. 32 However, designing a material that actively adjusts α S and ε IR according to spacecraft requirements is challenging. 25 − 27 Thin-film optical structures are often vulnerable to degradation or delamination on exposure to the harsh environment of space.…”

“…33 − 35 Furthermore, these structures are usually delicate and difficult to handle and deteriorate when manually handled. 32 − 41 The CTE mismatch is especially exacerbated for carbon-fiber polymer (CFRP), a material that is often a choice for fabricating platforms, support structures, optical benches, telescope tubes, and parabolic reflectors among other satellite components due to its strength and light weight. Here a MFNS ( Figure 2 b) that consists of a plasma-deposited poly( p -xylylene) (PECLP) buffer layer and high-density diamondlike-carbon (DLC) superlattice is used to create a mechanically and environmentally ultrastable platform.…”

“…One approach to overcome the high power/mass budgets typically required by thermal control systems of satellites is to provide a selective adjustment of both solar absorptivity (α S ) and infrared emissivity (ε IR ) smart coatings in order to facilitate radiative cooling, while harvesting the electromagnetic radiation in space (Figure a). It has been shown that, with this approach, it is possible to achieve power savings nearing 100% . However, designing a material that actively adjusts α S and ε IR according to spacecraft requirements is challenging. − Thin-film optical structures are often vulnerable to degradation or delamination on exposure to the harsh environment of space.…”

“… 25 , 28 , 29 This has been shown and rejected for the Sentinel missions under the Copernicus Earth Observation Programme. 30 − 32 …”

“…This further added extra mass and complexity. − Various smart electrochromic and thermochromic materials that are designed to operate over a selected range of wavelengths have also been investigated as a means of achieving variable thermal control, which would allow seamless operation. However, these materials usually require heavy equipment and they only vary the thermal emittance in the infrared spectrum (e.g., architectures based on vanadium dioxide (VO 2 ) films with an emissivity change from 0.13 to 0.68). − Furthermore, they are sensitive to thermal stresses and impacted by changes in levels of moisture. ,, This has been shown and rejected for the Sentinel missions under the Copernicus Earth Observation Programme. − …”