Thermo-Optical Property Degradation of ITO-Coated Aluminized Polyimide Thin Films Under VUV and Low-Energy Proton Radiat
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TRODUCTION
MATERIALS certified for use in space are characterized by their exceptional properties (for instance lightweight, resistance to ionizing radiation, multifunctional capabilities, self-healing capabilities and outstanding thermal stability) so that they can survive in an environment that combines, among others, ionizing radiation, extreme temperatures, micrometeorites, and deep vacuum. Many space applications require a coating that is applied to the surface of the material to protect it or change its properties. Both the materials and their coatings used for spacecraft applications must be easy to apply and should have low outgassing and stability in space environment. However, despite the unique characteristics, space is a harsh environment for the materials used on spacecraft, especially on their exterior surfaces. Most of these materials exhibit some degree of degradation due to different external factors, including ultraviolet and particle damage. One of the crucial aspects in spacecraft design is the thermal control system whose function is to keep the temperature of spacecraft systems within their operating ranges. The absolute temperature of a given area of the spacecraft in interplanetary space far away MARTA DEMBSKA is with Data Management and Analysis, DLR Institute of Data Science, Ma¨lzerstrabe 3, 07745 Jena, Germany. Contact e-mail:[email protected] THOMAS RENGER and MACIEJ SZNAJDER are with the Mechanics and Thermal Systems, DLR Institute of Space Systems, Robert-Hooke-Str. 7, 28359 Bremen, Germany. Manuscript submitted September 30, 2019. Article published online July 19, 2020 4922—VOLUME 51A, SEPTEMBER 2020
from Earth’s atmosphere is directly determined by the ratio between solar absorptance as and thermal emittance et ,[1] whereby as is defined as the fraction of incident solar radiant flux that is absorbed by a surface and et as the ratio of the radiant intensity of the surface to that which would be emitted from a black body radiator.[2] Thus, any change in thermo-optical properties of the surface materials affect the operation of the thermal control system and—directly or indirectly—the space mission. Thermal control of spacecraft is mainly achieved by isolating it from the harsh environment by means of the multi-layer insulation (MLI) that prevents the heat from the Sun entering the spacecraft and restricts the loss of internally generated heat from the spacecraft. Typically, MLI consists of many (from several to few tens) layers of usually lightweight low-emittance polyimide films that are aluminized on one or both sides, separated by low-conductance spacers that reduce the heat transfer between the aluminized polyimide that acts as a radiation shield. Passive thermal control is also achieved by the application of various coatings with known optical properties, for example, high emittance (to improve the heat radiation characteristics) or low emittance (to minimize the radiative coupling). An example of materials widely used in space applications is Kapton, a polyimide film developed by D
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