High-performance nickel wick development for loop heat pipes

In the present investigation, high-performance capillary pump has been developed and evaluated for using inside loop heat pipes with 500 W heat transfer capability up to distance of 250 mm. Wick structure is one of the most critical components of the loop heat pipe that provides the necessary capillary pumping, liquid-vapour phase separation and heat leak barrier from evaporation section to the compensation chamber. In order to fabricate wick structure with appropriate physical characteristics, highly pure (>; 99.5%) nickel powders with average particle sizes of 2, 10, 12 and 75 μm were selected for sintering experiment. It was established that nickel powder can be effectively sintered when maintained at 750 - 850 °C temperature for one hour. Out of four nickel powders, NM-12 powder with average particle diameter of 12 μm was able to provide most qualified porous structure, sintered at 850 °C for one hour, with high porosity (>; 72 %), high permeability (>; 2 × 10^-13 m²), finer pore radius (>; 7.2 μm), low shrinkage (<; 22%), good axial straightness and acceptable strength. The main issues faced in sintering trials and remedies to avoid them are explained in detail. Center rod extraction was the major problem faced in the sintering experiment which was rectified by replacing carbon rod with high strength, lubricious center rod made from stainless steel with boron nitride coating. The methodologies development in this study can be used for the fabrication of high performance capillary wicks for miniature to large-scale loop heat pipes.