A thin polymer/metal laminate has been developed for use as an envelope material for lightweight heat pipes and heat-pipe radiators that operate at temperatures up to 360 K. The material is flexible enough to make it possible to roll or fold heat pipes compactly for transport and to unfurl them for use. The material is durable enough to withstand folding or rolling without incurring leaks. In addition, it is heat-sealable and thus an attractive alternative to metal heat-pipe envelope materials, which must be welded or brazed at temperatures much greater than those needed for heat sealing.
A polymer/metal laminate was selected for development because neither a metal nor a polymer foil exhibits the required properties, whereas the combination of materials could be expected to exhibit those properties. A metal foil can serve as a leak-proof pressure boundary for containing a heat-transfer fluid, but it fatigues easily and fails through growth of cracks and/or pinholes at the highly stressed tips of wrinkles that form during flexing. A polymer film has the required flexibility and does not develop cracks or pin holes when flexed; however, air and the vapor of the heat-transfer fluid can diffuse through a polymer film. When the two materials are bonded together in a laminate, the metal foil serves as the fluid-containment and pressure boundary, while the polymer film supports the metal foil, preventing large localized stresses and thus increasing the flex-fatigue resistance of the metal foil. With the proper selection of laminate layers, a laminate can be designed to exhibit such desired characteristics as flex-fatigue resistance, high emissivity, and resistance to ultraviolet radiation.
The present laminate contains six plies. It is an improved derivative of four-ply polymer/metal laminates that are used commercially to package foods and medicines and that do not have the strength or the high-temperature stability required for the heat-pipe application.
The table describes the six layers. The two copper layers provide the hermetic seal; in contrast, all commercial laminates use only single metal layers. The use of two metal layers in this laminate affords redundancy for protection in the event that a leak arises in one metal layer. The innermost layer is made of a heat-sealable polymer to ease fabrication. The outer polyvinyl fluoride layer provides the optical properties needed for efficient radiation of heat.
This work was done by John D. Cornwell of Johnson Space Center and John E. Fale and Nelson J. Gernert of Thermacore, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.