Interstitial Materials for Low Thermal Resistance Joints in Avionic Equipment 891441
IMPROVED AVIONIC THERMAL CONTROL can reduce avionic failure rates and enhance aircraft reliability and supportability. This paper discusses methods to improve thermal control with the use of Interstitial Materials (IM) for low thermal resistance joints in avionic equipment.
The overall temperature difference from an avionic circuit module component to the Environ mental Control System (ECS) coolant is an indicator of the thermal resistance to heat transfer and, consequently, of the efficiency of the heat exchange circuit. Reduction of the temperature difference (i.e., thermal resistance) lowers the component operating temperature (with associated reliability gain) or reduces the ECS requirement. The overall temperature difference is the sum of the differences across the four distinct heat transfer paths shown in Figure 1. Studies have shown that, because of the thermal resistances, the temperature differences across “mechanical” interfaces (path segments C and D) account for an inordinate proportion of the overall temperature difference from the avionic component to the coolant. The problem is compounded as the thermal resistances increase (resulting in increased temperature differences) upon contamination by airborne particulates during flight line maintenance. Low thermal resistance joints which reduce the temperature difference across these interfaces, yet remain unaffected by contaminants, can be developed.
Different methods to reduce the thermal resistance at the interfaces have been studied. These methods include the use of thermal greases, soft metal foils, compliant materials, semi solids, and liquid metals. Thermal greases and soft metal foils effectively reduce the interface thermal resistance but need to be re-applied periodically. In an effort to develop a durable IM, MCAIR studied compliant materials, semi solids, and liquid metals. The discussion will detail our development of IM candidates that can reduce the interface thermal resistance (and corresponding temperature difference) by 96% in comparison to a bare metallic interface. It will present the approaches evaluated, the advantages and disadvantages of each, the selection of the most promising, and the effort remaining to establish the IM as a viable method of reducing interface thermal resistance.