22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Macroscopic kinetic description of plasma processing of graphite micropowder
V.R. Giampietro1, C. Roth1, V. Wood2, M. Gulas3 and Ph. Rudolf von Rohr1
1
ETH Zürich, Institute of Process Engineering, Zurich, ZH, Switzerland
ETH Zürich, Institute of Integrated Systems, Zurich, ZH, Switzerland
3
Imerys Graphite & Carbon, Bodio, TI, Switzerland
2
Abstract: An empirical correlation between flowability factor of plasma-processed graphite micropowder and user-set plasma parameters based on a quasi-Arrhenius equation is put
forward and interpreted within the macroscopic kinetic description of plasma polymerization.
Keywords: plasma polymerization, micropowder, flowability
1. Introduction
The powder flow behaviour depends on the balance between gravitational and interparticle forces, namely the
Van der Waals attractive forces in case of dry micropowders1. The flow behaviour critically influences powder
handling and processing, as cohesive and sticking powders can cause pipe and hopper clogging as well as difficulties in mixing and sieving. Improvement of powder
flow behaviour is achieved by plasma deposition, which
is implemented in a tubular plasma reactor to perform a
fast deposition of a non-continuous coating of nanoparticles on the powder particles2. By roughening the particle
surface, the nanoparticles can improve the powder flow
behaviour, which is quantified by the flowability factor
(ff) with values ranging from below 1 (non-flowing powder) to above 10 (free-flowing powder)2. The ff increment
for a given powder is related to the characteristics of the
plasma coating, which in turn depend on the plasma process conditions. We therefore postulate that the ff increment is a function of the plasma deposition rate, which is
in turn a function of the plasma process parameters (equation [1]). The dependency of the deposition rate on the
plasma parameters was previously studied by Hegemann
et al. on the basis of a model proposed by Park et al.3. In
this model, the deposition rate is expressed as a quasiArrhenius function of the plasma composite parameter
W/F, (i.e. the ratio of plasma powder W to feed-gas flow
rate F) corresponding to the specific energy delivered by
plasma (equation [2])3. Such a model provides a macroscopic kinetic description of a plasma-polymerization
process, and was proved to be effective in describing
several depositing plasma systems3. Therefore in the
present study, by assuming that the dependency of the ff
increment on the deposition rate is linear, the ff variation
of a plasma-processed micropowder is measured and
investigated with respect to the plasma composite parameter W/F on the basis of a quasi-Arrhenius function
(equation [3]).
2. Experimental method
A tubular inductively-coupled RF plasma reactor (Fig.
1) fed with a mixture of monomer and argon was used to
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ignite a glow-discharge plasma for the fast plasma processing of micropowders. Graphite was chosen as micropowder to modify because it is an important raw material for industry with a cohesive flow behaviour. A ring
shear tester (RST-XS) was used to measure the ff of the
processed graphite, while scanning electron microscopy
(Zeiss Leo 1530 SEM) was used to observe the morphological characteristics of the powder surface.
Fig. 1. Tubular inductively-coupled RF plasma
reactor for fast powder-surface modification (RF:
radio-frequency generator; MN: impedance
matching network ; PIC: pressure indicator controller; FIC: flow indicator controller. Image taken from Reference 4).
3. Results
The results show that the plasma-deposition process
improves the flowability of graphite. The ff of graphite
modified in a plasma fed with 250 sccm of monomer and
250 sccm of argon over the power range 100 to 700 W
ranges from 4.3 ± 0.1 to 7.9 ± 0.0 for acetylene and from
3.7 ± 0.1 to 5.8 ± 0.0 for hexamethyldisiloxane (HMDSO)
monomers, while the ff of the native unprocessed graphite
is 3.4 ± 0.1 (Fig. 2). If the graphite is processed with only
1
argon (no monomer) the flow behaviour remains the same
as the native particles, which indicates that the observed
ff increase is not a plasma-related artifact.
Plotting the flowability data in a quasi-Arrhenius graph,
we find linear relations where the slope of the best-fit
curves can be interpreted as activation energies (E a ) of
the plasma-polymerization of the monomers utilized (Table 1). The values of E a obtained for acetylene (0.76 ±
0.04 W/sccm) and HMDSO (0.98 ± 0.06 W/sccm) are in
good agreement with those obtained by Hegemann et al.
in previous studies with thin film plasma deposition5,6.
The comparison of these energies with the dissociation
energies required to break the monomer chemical bonds
permits investigation of the reaction mechanism of the
processes occurring in the plasma zone5.
The SEM photos of the processed powder confirm the
presence of a nanoparticle distribution on the particle
surface (Fig. 4).
Table 1. Best-fit parameters of the quasi-Arrhenius plots in Figure 3.
Plot
-Slope (E a )
[W/sccm]
Intercept
Correlation
coefficient
(R2)
Ar + C2H2
0.76 ± 0.04
5.51 ± 0.05
0.9900
Ar + HMDSO
0.98 ± 0.06
5.00 ± 0.08
0.9868
Fig. 2. Flowabiliy-factor trend for graphite
processed with different plasma feed gases.
Fig. 4. SEM photos of graphite processed at 700
W of plasma power (SEM conditions: 10.0 KV
electron-acceleration voltage, 30 µm aperture
size, in-lens image mode, 250 kx magnification)
Fig. 3. Flowabiliy-factor increment for graphite
processed with mixtures of a monomer and argon
as a quasi-Arrhenius function of the composite
plasma parameter W/F.
2
4. Conclusion
Plasma deposition performed in tubular inductivelycoupled RF plasma reactor improves the flow behaviour
of graphite micropowder. A quasi-Arrhenius relation
between flowability-factor increment and plasma composite parameter W/F is found. Just as the deposition rate in
the plasma processing of thin film can be used to study
the kinetics of the plasma-deposition processes, we show
that flowabilty can be used to study plasma processing of
powders, where deposition rate can be difficult to quantify.
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5. Acknowledgements
The Scientific Center for Optical and Electron Microscopy of the ETH Zürich is gratefully acknowledged for
providing SEM-facility access and training. The Claude &
Giuliana Foundation (Switzerland) is also gratefully
acknowledged for the financial support.
6. Equations
[1] ff − ff 0
=
f ( R (W , F , p, G ))
(ff 0 : flowability factor of the unprocessed powder;
W: plasma power; F: monomer flow rate;
p: plasma pressure; G: reactor geometrical factor)
R
F
=
[2]

Ea 
G exp  −

 W /F 
(R: plasma deposition rate;
E a : activation energy)
[3]

Ea 
ff − ff 0 = CR = CGF exp  −

 W /F 
(C: empirical constant)
7. References
[1] I. Zimmermann, M. Ebner, K. Meyer, Z. Phys.
Chem. 2004, 218, 51
[2] A. Spillmann, A. Sonnenfeld, P. Rudolf von Rohr,
Plasma Processes and Polymers 2007, 4 (S1), S16.
[3] D. Hegemann, Comprehensive Materials
Processing 2014, 4, 201
[4] C. Roth, Z. Künsch, A. Sonnenfeld, P. Rudolf von
Rohr, Surface & Coating Technology 2011, 205,
S597
[5] D. Hegemann, M.M. Hossain, E. Koerner, D.J.
Balazs, Plasma Processes and Polymers 2007, 4,
229
[6] D. Hegemann, U. Schütz, A. Fischer, Surface &
Coating Technology 2005, 200, 458
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