February 5-6, 2004 1

Plasma Processes, Inc.

Engineered Tungsten for IFE Engineered Tungsten for IFE Dry Chamber WallsDry Chamber Walls

HAPL Program MeetingGeorgia Institute of Technology

Scott O’Dell, PPIR. Raffray and J. Pulsifer, UCSD

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Plasma Processes, Inc.

IntroductionIntroduction

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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Plasma Processes, Inc.

Demonstrate the Feasibility of Producing Demonstrate the Feasibility of Producing Nanoporous W ArmorNanoporous 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 ActivationFerritic Steel

Dense W Functionally Graded to Ferritic Steel

Porous W

SEM backscattered image of submicron tungsten starting powder

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Plasma Processes, Inc.

Porous Tungsten Deposits on Steel SubstratesPorous 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%

SEM backscattered image of a porous tungsten deposit on a steel substrate

Steel substrate

Porous W

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Plasma Processes, Inc.

TEM Analysis of Porous StructureTEM 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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Plasma Processes, Inc.

Permeability TestingPermeability 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.

K=Qd/A(P2-P1)

K is the permeabilityQ is the leak rate

A is the aread is the thickness

P2 is the pressure on the helium inlet sideP1 is the pressure on the leak detector side

Leak Detector

P1 Gauge

P2 Gauge

MechanicalPump

He inlet

Sample

Double SidedCF Flange

Epoxy Seal

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Plasma Processes, Inc.

Permeability Test Set-up and ResultsPermeability 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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Plasma Processes, Inc.Porous Structure Dimension Needed for Diffusion and Porous Structure Dimension Needed for Diffusion and

Release of Implanted Helium Between ShotsRelease of Implanted Helium Between Shots

Diffusion: average square displacement<R2>=6DtRef for D: Wagner and Seidman Phys Rev Lett 42, 515 (1979)

1.00E-16

1.00E-15

1.00E-14

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

0 500 1000 1500 2000 2500 3000

Temperature (K)

dip

lace

men

t (m

)

t=0.1 t=0.01 t=0.001 t=0.0001 t=0.00001 t=0.000001

•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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Plasma Processes, Inc.

SummarySummary

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

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Plasma Processes, Inc.

Future WorkFuture 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