Engineered Tungsten for IFE Dry Chamber Walls

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Engineered Tungsten for IFE Dry Chamber Walls
Scott ODell, PPI R. Raffray and J. Pulsifer,
UCSD

• HAPL Program Meeting
• Georgia Institute of Technology

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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.

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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

Porous W
Dense W Functionally Graded to Ferritic Steel
Low Activation Ferritic Steel
SEM backscattered image of submicron tungsten
starting powder
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Porous Tungsten Deposits on Steel Substrates

• Samples with and without HfC additions have been
  produced on steel substrates (25mm x25mm x 5mm)
• Coating thickness 0.1-1.5mm
• Porosity values 10-25

Porous W
Steel substrate
SEM backscattered image of a porous tungsten
deposit on a steel substrate
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TEM Analysis of Porous Structure

• Bulk density is 80
• Distance between pores is 500nm
• Pore sizes less than 200nm have been observed

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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 HNO3 solution.
• The coupons were then bonded to double sided
  conflat flanges using a vacuum compatible epoxy.

KQd/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
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Permeability Test Set-up and Results
Sample ID Description Condition Density Permeability (m2/s)
V2-03-450 W(0.5)-HfC As-sprayed 80 3.1x10-6
V2-03-453 W(0.5) As-sprayed 80 5.7x10-6
V2-03-450-HT W(0.5)-HfC Heat treated TBD TBD
V2-03-453-HT W(0.5) Heat treated TBD TBD
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Porous Structure Dimension Needed for Diffusion
and Release of Implanted Helium Between Shots
For a temperature of 1000-1500K 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 20-100 nm, or lower. These results
need to be confirmed through detailed modeling
and experiments
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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 lt100nm microstructure dimension has
  been identified to allow release of implanted He

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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 (lt500nm) 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