High-temperature polymer matrix composites (PMCs) based on high thermal conductivity (~1000 W/mK) pitch-based carbon fibers (e.g., K1100, Cytec Industries Inc., and K13D2U, Mitsubishi Chemical Corp.) are being considered for facesheet/fin structures in next-generation large-area space radiators such as the heat-rejection system for the Jupiter Icy Moon Orbiter and the Service Module radiator for the Crew Exploration Vehicle (CEV) (refs. 1 and 2). Replacement of conventional metallic structures with PMC structures could lead to significant weight reductions along with improved thermal performance, tailorability and manufacturability, and structural integrity. However, several challenges, including establishing the reliability and space durability of these PMC structures, must be addressed. Various commercial resin systems including cyanate esters (e.g., RS-9D, YLA, Inc.), bismaleimides, and polyimide were selected for this application because of their high-temperature capability and processability. To improve the thermal conductivity (TC) and microcracking resistance of PMCs, nanocomposites modified with vapor-grown carbon nanofibers (VGCNFs) and/or exfoliated graphite flakes (ExGfs) were evaluated at the NASA Glenn Research Center (see the following figure).
Typical scanning electron microscope micrographs of RS-9D/K13D2U PMC that was modified with vapor-grown carbon nanofibers (VGCNF) and/or exfoliated graphite flakes (ExGF), showing fiber/filler distribution, connectivity, and defects.
Long description of figure 1.
PMC laminates were designed to match the thermal expansion coefficient of various metal heat pipes or tubes. Large, thin composite panels were successfully fabricated after the cure conditions for the brittle, highly anisotropic fibers were optimized. The space durability of these PMCs was assessed by accelerated thermal aging tests in high vacuum (1 to 3×10-6 torr) at 227, 277,and 316 °C for up to 10,000 hr (see the photographs). Various thermal, physical, and mechanical properties were monitored systematically as a function of aging time and temperature to determine potential degradation mechanisms. High-quality nanocomposites were fabricated and evaluated for TC and durability. TC was measured in both in plan and through the thickness. The effects of microcracking on TC also were investigated in collaboration with Prairie View A&M University. Laminates were subjected to up to 1250 thermal spiking conditioning cycles (from 177 to -196 °C) to generate microcracks, and TC is being measured. The radiation resistance of PMCs including nanocomposites was evaluated by electron beam irradiation testing at -20, 200, and 400 Mrad. Weight changes after irradiation were minimal regardless of material, but more extensive residual property measurements are being conducted to ascertain any sign of degradation.
Long-term vacuum thermal aging test setup for PMC facesheet candidates.
Long description of figure 2.
A joint program with the NASA Johnson Space Center for the Advanced Technology Demonstration Radiator project was initiated to develop and demonstrate a PMC radiator for the CEV. In this project, the fabrication of prototype substructures based on feasible material-design-fabrication concepts for the PMC-heat tube attachment such as the PMC facesheet design; the heat tube type, geometry, and radius; and heat tube-PMC bonding mechanisms are being pursued; and the structural integrity of the substructures prepared from these methods is being evaluated. A commercial CE film adhesive (RS-4A, YLA Inc.) rated for up to 230 °C cure was used to bond heat tubes to PMC facesheets, along with higher use temperature CE adhesives modified with conductive nanofillers to improve TC. A full-scale, a single quarter-cylinder panel based on the results of these studies and representing one-fourth of a CEV radiator is planned to be fabricated and tested in fiscal year 2007.
Find out more about this research:
Glenn’s Polymers Branch:
http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/polymers/
Glenn’s Advanced Metallic Branch:
http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/metallics/
Glenn’s Thermal Energy Conversion Branch:
http://www.grc.nasa.gov/WWW/TECB/
Last updated: December 15, 2007
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