Near-net-shape vacuum plasma spray (VPS) forming techniques were developed to produce advanced components with internal features such as smart heat pipes and crucibles. The initial results demonstrated the ability to incorporate features such as channels and a porous layer within the wall of a smart crucible.
The Safe Affordable Fission Engine (SAFE) is a heat pipe power system (HPS) that is designed to provide electricity for use in space exploration. Recent experiments of SAFE designs using sodium stainless steel heat pipes and resistance heating to simulate heat from radioactive fuel pins have been very successful (SAFE- 100). The SAFE-100 is designed to provide 100 kWt to an energy conversion system at an operating temperature of 700 ºC. In contrast to the SAFE-100 reactor that provides 100 kWt to an energy conversion system, the SAFE-400 is a 400-kWt reactor that is designed to be coupled with a 100-kWe Brayton power system. The power produced by an HPS is limited by the maximum operating temperature and the thermal conductivity of the materials used to fabricate the components. Therefore, to achieve the higher power output required by the SAFE-400 reactor, improved heat pipe materials are needed. For these reasons, molybdenum and molybdenum alloys are the leading candidates for making the SAFE-400 heat pipe modules.
Potential benefits of the vacuum plasma spray forming process for fabricating the molybdenum heat sink are reduced fabrication time; the ability to produce long, closed-end refractory metal tubes; incorporation of the wick/capillary structure as an integral part of the heat sink wall; and potential incorporation of getter materials. Using advanced VPS techniques, the wick structure is first deposited on a removable mandrel to the desired thickness. An outer close-out layer of dense Mo or Mo-Re alloy is then applied.
An investigation was conducted to develop the VPS processing techniques necessary for producing the porous molybdenum wick structure and the dense molybdenum-rhenium closeout. In addition, electron beam welding techniques were developed to enable attachment of end caps and fill-tubes to the heat pipe. These techniques were then used to produce smart heat pipes with integral wick structures.
Testing showed VPS Mo-Re deposits can have properties equivalent to or better than the properties of Mo-Re materials produced with conventional powder metallurgy processing techniques. Molybdenum wick structures with high porosity levels can be produced using VPS processing techniques. Permeability testing has shown that the pores of the VPS molybdenum wick structures are interconnected. Electron beam welding techniques were developed for welding VPS Mo-Re heat pipes. Smart heat pipes comprising an integral molybdenum wick and a dense Mo-Re closeout layer were produced with Mo end caps and a Mo fill tube. Inspections of the heat pipes showed they were leak tight, i.e., having a leak rate less than 10–6 sccs of helium.