Freeze/Thaw and Thermal Cycling
Heat pipes utilize a wick structure to transport the liquid working fluid from the condenser to the evaporator. When properly made, the working fluid fully saturates the wick without making a puddle of excess fluid. With the fluid completely contained within the wick, it is not able to bridge the gap across the inside diameter of the heat pipe. This allows multiple freeze thaw cycles to occur without heat pipe deformation. A variety of working fluids may be used which directly affects the freezing temperature of the heat pipe.
ACT routinely subjects heat pipes to thermal cycling to meet customer requirements. Typical freeze thaw tests are conducted from temperatures ranging from -20 to +20°C and -45 to +125°C. ACT has tested heat pipes up to 1,200 cycles, but 50-300 cycles are a more standard practice. Heat pipes may be thermally cycled prior to installation into assemblies. Heat pipe assemblies are also thermally cycled in assembled units to assure system level performance. Below are three examples:
- Heat Pipes – ACT conducted tests to collect data on heat pipe thermal cycling survivability. The data set for these experiments used both fabricated flattened and bent 4mm heat pipes as well as 0.25” diameter copper water heat pipes. Heat pipes were exposed to as many as 1200 freeze thaw cycles without deformation or performance degradation
- AlSiC HiK™ Plates – This project developed an innovative low-CTE heat spreader by embedding heat pipes into AlSiC plates. These plates showed similar effective thermal conductivity before and after 100 freeze/thaw cycles from -55°C to 125°C.
- Aluminum HiK™ Plates – In this project, copper water heat pipes are soldered into aluminum plates. Prior to fabrication, the heat pipes are screened by being exposed to 300 cycles from -20°C to +20°C. Once the assemblies were fabricated, the plates were exposed to an additional 50 cycles from -40°C to +75°C in two different orientations (100 cycles total) to assure freeze/thaw survivability. Figure 5 shows the temperature profile of plates exposed to thermal cycling. This is a 100% test requirement which all assemblies must pass prior to shipping. All plates are checked for any signs of thermal or mechanical degradation.