Tech Briefs Magazine - October 2021 - 42a

RESEARCH SPOTLIGHT
» TOUGHER THAN
TUNGSTEN
Tungsten also has downsides.
It is usually brittle, and
coupled with exposure to
neutron bombardment and
overheating, it can experience
even further embrittlement
over the operational lifetime
of a fusion reactor. One
solution to its brittleness is
to produce a material called
tungsten-fiber-reinforced
tungsten (Wf
/W), a tougher
FIGURE 2     
     
material that, through its
composite structure, offers
crack-dissipating mechanisms
that give it a pseudoductile
composite behavior, as in a
fiber-reinforced ceramic.
When producing Wf
/W,
FIGURE 3     

one of the current methods
of choice is chemical vapor
deposition (CVD), also a
popular production process in
the semiconductor industry.
In this process, gas molecules
adsorb on the surface of,
and then react in, a reaction
chamber that contains a
heated substrate (Figure 2).
Their interaction causes a
thin, highly pure material film
(here, W) to deposit onto the
substrate. To ensure that the
/W produced by this process
can be used in a fusion reactor,
the CVD process itself needs to
be optimized to ensure that
the material produced has the
right relative density and fiber
volume fraction. Researchers
from Forschungszentrum Jülich
GmbH (FZJ), Institute for Energy
and Climate Research, and
Max Planck Institute for Plasma
Physics in Germany aimed to
investigate this process and
how it could be optimized.
Wf
» DEVELOPING A
COMPLETE MODEL
FOR CVD PRODUCTION
OF WF
/W
One of the key factors of
the CVD process for Wf
/W
FIGURE 4       
           
42a COMSOL NEWS
Intro
Cov
ToCToC
+
-
A
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production is the tungsten
deposition rate, which
depends on the temperature
and partial pressures involved.
The tungsten deposition rate
is hard to predict because
it involves a lot of different
parameters, including the
surface temperature and
partial pressure at the
reaction sites, which depend
on the reactor geometry,
heater temperature, gas flow
rates, and gas composition.
One important motivation
for predicting the CVD process
is to avoid the formation of
pores in the tungsten material
(Figure 3). During the CVD
process, gas flows through the
fiber substrate and tungsten
is deposited between fibers.
The area between the fibers is
supposed to be filled up with
the solid W; however, some
gaseous domains can become
isolated from fresh reactants
when the path from the bulk
of the gas phase is closed, or
obstructed, by the W deposits.
In other words, the pores do
not have access to the reactants
needed to fill them with
tungsten, thus they remain
pores throughout the process.
In order to reduce or avoid
material strength-reducing
pore formation, the substrate
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Tech Briefs Magazine - October 2021

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