Low emissivity thin film coating on glass fiber reinforced plastic used for cryogenic application

Glass Fiber Reinforced Plastics (GFRP) are extensively used as internal structural supports in large cryogenic systems due to their low thermal conductivity, thermal expansion coefficient, and high specific strength and stiffness. However, its thermal emissivity (above 0.86) is high, so it acts as a radiative heat source to the surrounding cryogenic components working at a relatively lower temperature. During operation, this radiative heat transfer from the GFRP support significantly contributes to the static heat-in-leak to the cryogenic system. This heat-in-leak can be minimized by reducing the emissivity of GFRP. In this study, aluminum thin films of 1.11 ± 0.09 µm were coated on a GFRP substrate using thermal evaporation. The emissivity of aluminum-coated GFRP was measured to be 0.02 ± 0.01, and its sheet resistivity was 0.60 ± 0.02 Ω/square. Thus, aluminum serves as an efficient low emissivity coating for GFRP. The emissivity was also calculated from the measured sheet resistivity using the Hagen and Rubens correlation. The experimentally measured emissivity of aluminum-coated GFRP is consistent with the estimated value. The difference between the two results can be attributed to the assumption of a perfectly smooth surface and ideal defect-free material in the case of the Hagen-Rubens approximation.