Variable-thermal-conduction devices containing shape-memory-alloy (SMA) actuators have been proposed for use in situations in which it is desired to switch on (or increase) thermal conduction when temperatures rise above specified values and to switch off (or decrease) thermal conduction when temperatures fall below those values. The proposed SMA thermal-conduction switches could be used, for example, to connect equipment to heat sinks to prevent overheating, and to disconnect the equipment from heat sinks to help maintain required operating temperatures when ambient temperatures become too low. In comparison with variable-conductance heat pipes and with thermostatic mechanisms that include such components as bimetallic strips, springs, linkages, and/or louvers, the proposed SMA thermal-conduction switches would be simple, cheap, and reliable.
The basic design and principle of operation of an SMA thermal-conduction switch is derived from an application in which thermal conduction from hot components to a cooling radiator takes place through the contact area of bolted joints. The thermal conductance depends on the preload in each joint. One could construct an SMA thermal-conduction switch by simply mounting an appropriately designed SMA washer under the bolthead. As the temperature falls below (or rises above) the SMA transition temperature, the SMA washer would contract (or expand) axially by an amount sufficient to unload (or load) the bolt, thereby shutting off (or turning on) most of the thermal conduction through the joint contact area. SMA washers with various transition temperatures can be made to suit specific applications.
This work was done by Virginia Ford and Richard Parks of Caltech for NASA's Jet Propulsion Laboratory.