Contact conductance can be improved through the use of appropriate filler materials between the two plates. Such materials fill the microscale voids present because of surface roughness, and some materials can also fill the macroscale voids resulting from flatness deviation. For the microscale voids, because the dimensions are small, even a low-conductivity material, if thin enough, may provide an improvement over the radiative heat transfer that existed before filling. However, care must be exercised in the use of fillers. Fletcher et al. ^[8.45] show in Fig. 8.26 that a wide variety of fillers have a lower heat-transfer coefficient than an unfilled bare aluminum joint. Such fillers are thermal insulators and may be useful for applications where thermal isolation is required. As a rule of thumb, for a given thickness, filler thermal performance is proportional to thermal conductivity divided by hardness. For convenience, fillers can be divided into three classes: greases, gaskets, and cured-in-place room-temperature-vulcanized (RTV) silicone compounds. Greases and gaskets are available from many suppliers in a wide variety of materials, and the offerings are summarized in Appendix C, "Summary of Thermally Conductive Filler Materials and Suppliers."

Figure 8.26: Heat-transfer coefficients of selected interstitial materials. ^[8.45]

Use of fillers can create problems not present with bare interfaces. These include interference with unit grounding, inability to remove a unit for rework (or difficulty in doing so), structural loads, contamination, and outgassing the last two problems being particularly important in spacecraft applications. Other considerations in the use...