Pyrolytic graphite film thermal straps: Characterization testing

摘要

This paper reports on the experimentally-measured conductance, stiffness, and particulate contamination of pyrolytic graphite film thermal straps. This work was aimed at assessing the feasibility of replacing standard aluminum foil in thermal straps with graphite film, which is more conductive and lighter. Four different U-shaped straps with similar cross-sections and terminals were tested in the study. Three of the straps had a three-inch long flexible section. One of these was made from aluminum 1100 foil, and two were made from Pyrovo pyrolytic graphite film (PGF). One of the PGF straps was fabricated with an aluminized mylar blanket that was sealed at the terminals. The last strap was made from PGF, was blanketed, and was six inches long. The conductance of each strap was measured as a function of mean strap temperature ranging from 60 K to 300 K. The peak measured conductance of the three-inch PGF and aluminum straps were 1.0 W/K at 162 K and 0.28 W/K at 64K, respectively. The conductance of all straps converged to around 0.3 W/K as the mean strap temperature approached 60 K. In addition, the peak conductance of the six-inch PGF strap was 0.83 W/K at 150 K. The fact that its peak conductance was near the conductance of the three-inch PGF strap indicated that the thermal resistance of the terminals in the PGF straps was significant. For a given temperature, the conductance varied by as much as 15% for two units of the same strap design. One of the straps was thermally cycled from 300 K to 60 K ten times. Its conductance was unchanged by the thermal cycling. Furthermore, one of the six-inch long PGF straps was subjected to random vibration. The random vibration spectrum was designed so that one terminal achieved a maximum displacement of 0.25 in. from its neutral position in three orthogonal axes while the other was held stationary. The conductance of this strap was unaffected by the random vibration test. The straps were also tested for the level of contamination introduced to the environment. The bare aluminum and bare PGF straps had equal particulate cleanliness levels while the encapsulated PGF strap had a lower one. Finally, the dynamic stiffness of one of the six-inch strap was measured to be less than 0.5 lb/in. in all directions for temperatures between 200 and 300 K. (C) 2016 Elsevier Ltd. All rights reserved.