Plasma Processes, Inc. February 5-6, Engineered Tungsten for IFE Dry Chamber Walls HAPL Program Meeting Georgia Institute of Technology Scott O’Dell, PPI R. Raffray and J. Pulsifer, UCSD

Plasma Processes, Inc. 2 Introduction Tungsten is an ideal material for armoring IFE dry chamber walls Techniques are needed to prevent premature armor failure due to helium entrapment. A nanoporous structure would allow helium to migrate to the surface eliminating premature failures. PPI and the UCSD are currently working on a Phase I STTR to demonstrate a nanoporous W structure with interconnected porosity is feasible.

Plasma Processes, Inc. 3 Demonstrate the Feasibility of Producing Nanoporous W Armor Vacuum Plasma Spray (VPS) forming techniques have been used. Submicron tungsten starting powder (~0.5μm) HfC additions to pin grain boundaries and prevent grain growth, i.e., prevent removal of the nanoporous structure Low Activation Ferritic Steel Dense W Functionally Graded to Ferritic Steel Porous W SEM backscattered image of submicron tungsten starting powder

Plasma Processes, Inc. 4 Porous Tungsten Deposits on Steel Substrates Samples with and without HfC additions have been produced on steel substrates (25mm x25mm x 5mm) Coating thickness: mm Porosity values: % SEM backscattered image of a porous tungsten deposit on a steel substrate Steel substrate Porous W

Plasma Processes, Inc. 5 TEM Analysis of Porous Structure Bulk density is ~ 80% Distance between pores is ~500nm Pore sizes less than 200nm have been observed

Plasma Processes, Inc. 6 Permeability Testing Tests using a helium leak detector were conducted to determine permeability. To facilitate testing, 9.5mm diameter coupons were EDMed from the 25x25mm samples. The steel substrates were chemically removed using a dilute HNO 3 solution. The coupons were then bonded to double sided conflat flanges using a vacuum compatible epoxy. K=Qd/A(P2-P1) K is the permeability Q is the leak rate A is the area d is the thickness P2 is the pressure on the helium inlet side P1 is the pressure on the leak detector side

Plasma Processes, Inc. 7 Permeability Test Set-up and Results Sample IDDescriptionConditionDensityPermeability (m 2 /s) V W(0.5)-HfCAs-sprayed~80%3.1x10 -6 V W(0.5)As-sprayed~80%5.7x10 -6 V HTW(0.5)-HfCHeat treatedTBD V HTW(0.5)Heat treatedTBD

Plasma Processes, Inc. 8 Porous Structure Dimension Needed for Diffusion and Release of Implanted Helium Between Shots For a temperature of ≈ K over a time of 0.1 s, the characteristic He diffusion dimension ≈10-50 nm. Higher temperature would help but shorter times would hurt. From these initial results, the goal should be to have interconnected porosity and microstructure of dimension ≈ nm, or lower. These results need to be confirmed through detailed modeling and experiments

Plasma Processes, Inc. 9 Summary Using 500nm starting W powder, submicron porous W deposits have been produced with porosity levels between 10-25%; thus, demonstrating VPS forming as a viable technique for producing nanoporous W deposits He permeability tests have demonstrated the porosity is interconnected Minimize porosity levels in the porous region to minimize the W armor temperature (~20% porous) A goal of <100nm microstructure dimension has been identified to allow release of implanted He

Plasma Processes, Inc. 10 Future Work Near Term Heat treat porous W deposits and test to determine the effect of elevated temperatures on the porous W structure and permeability Phase II Evaluate finer W starting powders (<500nm) for producing smaller pore sizes and a smaller microstructure dimension (distance between pores) Optimize the fabrication techniques to produce a uniform porous structure Continue working with UCSD to optimize VPS W armor for IFE dry walls (porous layer, dense layer, compliant layer, substrate) Determine critical properties (e.g. thermal conductivity) of porous and dense tungsten deposits produced on LAF steel substrates Produce samples for testing at DOE sponsored laboratories Demonstrate scale-up of the process on medium scale components