Speaker
Description
Passive thermal control is vital in space, especially for extended missions involving cryogen storage needing protection from sunlight. Long-term cryogen storage in space can be possible when the exterior thermal coating can reflect most of the incident sunlight. Additionally, high emissivity in the longwave infrared wavelengths allows passive cooling by taking advantage of low-temperature conditions in deep space. Such desirable radiative properties minimize cryogen loss and could obviate the need for active cooling strategies to store cryogens. This study aims to develop a thermal coating with wavelength-selective characteristics using polymeric materials that scatter sunlight with minimal absorption, resulting in high solar reflectance. Furthermore, these coatings allow emission in infrared wavelengths, making them suitable for deep-space missions requiring extended storage. The study chose polymers and surface morphologies suitable as solar reflectors and are lightweight, compact, and easily manufacturable. This study focuses on multilayered nanofibers of polytetrafluoroethylene (PTFE), and polyethylene-oxide (PEO) made using the electrospinning technique. Since electrospinning offers control over fiber geometry, this study determines how fiber geometry affects solar reflectance and infrared emittance.
We analyze light propagation through several layers of electrospun nanofibers to determine the spectral reflectance, absorptance, and transmittance by solving the Maxwell equations. While focusing on the size effect, we consider two-dimensional (circular) features to resemble the randomly arranged cylindrical nanofibers achieved by electrospinning. The analysis involves solving Maxwell’s equations for a given fiber geometry and arrangement using the constituent material’s wavelength-dependent refractive index. As light propagates through the random structure, it is reflected, absorbed, or transmitted by interacting with the multiscale fibers. The amount of incident light transmission, reflection, and absorption are then obtained using the electromagnetic field distribution across the material. Consequently, this predictive model determines the fiber geometry most suitable for thermal coating for a chosen material and within fabrication constraints. The model is validated by measuring the spectral reflectance and transmittance of the electrospun nanofibers using spectrophotometers interfaced with integrating spheres.
Among all geometric parameters, the spectral hemispherical reflectance of PTFE films shows a strong dependence on thicknesses. The experiments and simulations indicate that the reflectance increases with increasing thickness. The transmittance, on the other hand, drops with increasing thickness, and the film becomes opaque. In addition to the thickness effect, the model also determines the effect of other geometric parameters, such as fiber diameter and spacing, indicating the scope for design optimization. Finally, both model and experiments indicate that an average solar reflectance of 0.988 can be achieved using electrospun PTFE:PEO (90:10) nanofibers of thickness 1 mm. On the other hand, spectral emittance at a wavelength of 8 μm was found to be 0.9. Based on these observations, we believe the PTFE/PEO films are promising thermal coatings for cryogen storage.
Acknowledgments:
The authors acknowledge the support of the National Aeronautics and Space Administration under Grant No. 80NSSC21K0072 issued through the Space Technology Research Grants.
