Tech Briefs Magazine - October 2021 - 43a

RESEARCH SPOTLIGHT
(Ref. 1), he validated these
models successfully by
comparing experiments for
the deposition rate, pore
structure, and relative densities
of the CVD process of Wf
/W
(Figure 6). In a third step, the
multifiber model was used
for a CVD process parameter
optimization to successfully
improve the simulated and
later also experimental
material density.
FIGURE 5          
                
» DEVELOPING
AND VALIDATING A
MULTIPHYSICS MODEL
The single-fiber setup to
develop the new models for the
chemical vapor deposition rate
of tungsten is shown in Figure
4, including a preheater and a
main heater. The researchers
wanted to see how fast the
tungsten would grow and how
this rate of growth was affected
by the temperature and partial
pressure. They then adjusted
the tungsten hexafluoride
(WF6
FIGURE 6    
        
      
geometry and the parameters of the CVD process need to be
carefully adjusted.
The goal of the FZJ research was to reduce porosity in Wf
/W. For
this, Leonard Raumann, material engineer at FZJ, needed to find
the W deposition rate equation as a first step. Existing literature
about CVD for tungsten is controversial and incomplete because
the equations and values for tungsten deposition kinetics often
contradict each other from study to study. Raumann found a new
rate equation for the CVD process, putting the smaller pieces from
literature together as a whole (Ref. 1). But how?
He designed an experimental single-fiber setup with
very well-known boundary conditions. With the help of the
COMSOL Multiphysics®
software and a parameter study, he
found the rate equations. He then used the equations to
model the Wf
/W production with multiple fibers. For this
Raumann conducted a parameter optimization. The resulting
parameters were also applied in reality with success.
» SCALING UP
FUSION RESEARCH
The FZJ-IPP team is currently
planning to apply the
validated model to a 3D
geometry to scale up Wf
/W
) reaction order between
one and zero, depending on
the temperature and the WF6
partial pressure. To do so, they
used numerical modeling to
study the fluid dynamics of
the gas mixtures, heat transfer
of the thermal losses, and
chemistry and rate equations
for the chemical reactions at the
deposition surface.
A macroscaled CVD
reactor model returned the
partial pressures as input
for microscale transient
simulations. For this, Raumann
modeled the W coatings
growing onto multiple
adjacent W fibers as well
as the surface-to-surface
contact of the W coatings
and the corresponding
potential pore formation.
In Raumann's dissertation
production even further.
They aim to develop a
new approach that would
involve one coil delivering
the W fabric (CVD substrate)
to another, with one coil
unbound and the other coiled
and heated up. This allows
the fabric layer stacking to
take place with the chamber
closed, so that all layers can be
deposited in one CVD process
(there is also a lower risk of
contamination this way).
Scaling up the production
process for tungsten-fiberreinforced
tungsten means
new possibilities for fusion
power. Before this research,
producing one layer of the
tungsten material took around
5 hours, but by optimizing the
CVD process parameters, it can
take just 30 minutes to produce
one layer of Wf
/W - which is
10 times faster! By optimizing
production processes for highperformance
materials for
fusion reactors, we can ensure
that fusion power is both
possible and cost efficient.
REFERENCE
1. L. Raumann, Modeling
and validation of chemical
vapor deposition for tungsten
fiber reinforced tungsten,
dissertation, Energy &
Environment, Schriften des
Forschungszentrums Jülich, 2020.
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Tech Briefs Magazine - October 2021

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