Plasma Processes, Inc.
Engineered Tungsten for IFE Dry
Chamber Walls
Scott O’Dell, PPI
R. Raffray and J. Pulsifer, UCSD
HAPL Program Meeting
Georgia Institute of Technology
February 5-6, 2004
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Plasma Processes, Inc.
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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Plasma Processes, Inc.
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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Plasma Processes, Inc.
Porous Tungsten Deposits on Steel Substrates



Samples with and
without HfC additions
have been produced on
steel substrates (25mm
x25mm x 5mm)
Coating thickness: 0.11.5mm
Porosity values: 1025%
Porous W
Steel substrate
SEM backscattered image of a porous tungsten
deposit on a steel substrate
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Plasma Processes, Inc.
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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Plasma Processes, Inc.
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.
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
Mechanical
Pump
P2
Gauge
Double Sided
CF Flange
P1
Gauge
Sample
Epoxy Seal
He inlet
Leak Detector
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Plasma Processes, Inc.
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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Plasma Processes, Inc.
Porous Structure Dimension Needed for Diffusion and
Release of Implanted Helium Between Shots
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Diffusion: average square displacement<R >=6Dt
Ref for D: Wagner and Seidman Phys Rev Lett 42, 515 (1979)
t=0.1
t=0.01
t=0.001
t=0.0001
t=0.00001
•For a temperature of ≈10001500K over a time of 0.1 s, the
characteristic He diffusion
dimension ≈10-50 nm.
t=0.000001
1.00E-06
1.00E-07
•Higher temperature would help
but shorter times would hurt.
1.00E-08
diplacement (m)
1.00E-09
•From these initial results, the
goal should be to have
interconnected porosity and
microstructure of dimension ≈20100 nm, or lower.
1.00E-10
1.00E-11
1.00E-12
1.00E-13
•These results need to be
confirmed through detailed
modeling and experiments
1.00E-14
1.00E-15
1.00E-16
0
500
1000
1500
2000
2500
3000
Temperature (K)
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Plasma Processes, Inc.
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
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Plasma Processes, Inc.
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
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