Acktar’s Metal Velvet

The study demonstrates the potential of VO₂-based thermochromic emitters for spacecraft thermal control by varying their heat rejection according to varying environment conditions or spacecraft heat load, with a sharp increase in radiative power during the VO₂ phase transition.

The objectives of the study were to investigate the thermal radiative properties of VO₂ (Vanadium Dioxide) thin films and a nanophotonic VO₂ variable emitter, examining the hysteresis behavior of it under partial heating and subsequent cooling, and demonstrating the potential of VO₂-based thermochromic emitters for spacecraft thermal control.

The study uses a cryothermal vacuum experiment to simulate a cold space environment, with a liquid nitrogen-cooled cryostat and a vacuum pressure. A nanophotonic coating was also used. It passively adjusts its infrared emission according to temperature, enabling efficient radiative cooling in space without requiring electrical power. The nanophotonic coating showed the ability to dynamically change its thermal properties.

The experimental setup featured surfaces covered with Acktar’s metal velvet, this coating was used to model deep space conditions and minimize stray radiation, as it is an ideal IR coating. This material provided a uniform background which is critical for accurate thermal emission measurements. Acktar’s coating contribution was crucial to maintain a reliable radiative environment and ensure stray light suppression during the tests.

The results show that the nanophotonic variable-emittance sample exhibits behavior similar to the aluminum mirror at low temperatures, but as the temperature increases beyond the onset of the VO2 phase transition, the trend shifts, and the emittance starts to increase abruptly. The steady-state temperature achieved by each sample is used to derive the radiative heat rejection for each sample to the cold finger as a function of steady-state temperature.

The darkest black background allowed precise observation of the variable emitter’s behavior, showing excellent thermal stability within a wide temperature range without performance degradation. Moreover, the research introduced several models for hysteresis prediction effects during partial heating and cooling. Those are relevant for spacecraft operations where surface temperatures may vary dynamically

S. Taylor, N. Boman, J. Chao, and L. Wang, “Cryothermal vacuum measurement of thermochromic variable-emittance coatings with heating/cooling hysteresis for spacecraft thermal management,” Applied Thermal Engineering, vol. 199, Article 117561, 2021, doi: 0.1016/j.applthermaleng.2021.117561