Variable emissivity through MEMS technology

Abstract

All spacecraft rely on radiative surfaces to dissipate waste heat. These radiators have special coatings that are intended to optimize performance under the expected heat load and thermal sink environment. Typically, such radiators will have a low absorptivity and a high infrared-red emissivity. Given the dynamics of the heat loads and thermal environment it is often a challenge to properly size the radiator. In addition, for the same reasons, it is often necessary to have some means of regulating the heat rejection rate of the radiators in order to achieve proper thermal balance. The concept of using a specialized thermal control coating which can passively or actively adjust its emissivity in response to such load/environmental sink variations is a very attractive solution to these design concerns. Such a system would allow intelligent control of the rate of heat loss from a radiator. Variable emissivity coatings offer an exciting alternative that is uniquely suitable for micro and nano spacecraft applications. This permits adaptive or “smart” thermal control of spacecraft by varying effective emissivity of surfaces in response to either a passive actuator (e.g., a bi-metallic device) or through active control from a small bias voltage signal. In essence the variable emittance surface would be an “electronic louver.” It appears possible to develop such “electronic louvers” through at least three different types of technologies: Micro Electro-Mechanical Systems (MEMS) technology, Electrochromic technology and Electrophoretic technology. This paper will concentrate on the first approach using both MEMS and Micromachining technology to demonstrate variable emissivity.