22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
HMDSO dissociation and nanoparticle formation in the afterglow of a dielectric
barrier discharge
R. Wallimann, G. Oberbossel, C. Probst and Ph. Rudolf von Rohr
Transport Processes and Reactions Laboratory, Institute of Process Engineering, ETH Zürich
Abstract: The dissociation of HMDSO and nanoparticle formation in the afterglow of a
dielectric barrier discharge plasma is investigated. FTIR showed dissociation of HMDSO
which was directly injected into the plasma zone. The setup is ready to investigate
nanoparticle production in the afterglow when HMDSO is injected into the plasma zone
and to optimize process conditions for optimal nanoparticle yield.
Keywords: dielectric barrier discharge, FTIR, HMDSO dissociation
1. Introduction
Powders are the predominant product form in
pharmaceutical and chemical industry. However, handling
and processing of most powders is difficult due to poor
wettability and flowability. Fine-grained particles tend to
stick to surfaces and clog in equipment, leading to plant
malfunction and downtime. To benefit from the high
potential of fine powders, methods to improve their
flowability are of high interest. Standard procedure is the
admixture of nanoparticles to fine-grained powders [1].
Thus, the interparticle van der Waals force is reduced
resulting in an increased powder flowabiliy. However,
dispersing nanoparticles in powder is a difficult and timeconsuming task. Different mixing processes have been
investigated, but mixing times lower than several minutes
have never been achieved. Another approach to enhance
flowability of powders was shown by Spillmann [2]. He
used low- pressure plasma to deposit nanoparticles onto
the powder surface. Powders fall through a RF-plasma
zone containing argon, oxygen and an evaporated
monomer. Silica nanoparticles are formed and directly
deposited onto the powder surface [3]. Treatment time
was reduced to tenths of a second. However, low pressure
plasma requires high investment costs for pumps and
vacuum equipment. A transition from low pressure to
atmospheric pressure plasma is necessary to reduce
process costs. A first study was carried out by Sonnenfeld
et al. [4] using a dielectric barrier discharge cell. Here, the
influence of process gas composition on the activation of
HDPE powder and on the production of silica
nanoparticles in the afterglow of the discharge was
investigated. The monomer was injected into the plasma
afterglow. Si wafers were placed downstream to collect
nanoparticles. Deposited nanoparticles were investigated
using scanning electron microscopy (SEM) and fourier
transform infrared spectroscopy (FTIR) on the wafers.
Deposition time was in the range of 10 to 30 minutes. The
FTIR measurements showed a clear trend to desired Si-OSi groups using argon and TMS as process gas and
precursor, respectively. In the presented pictures, the
characteristic scale of the surface structure produced was
P-II-7-33
larger than the 3 nm which is beneficial for the
improvement of flowability.
In this study, the aim is to investigate the dissociation of
different monomers in the plasma and its afterglow.
Hexamethyldisiloxane (HMDSO) instead of tetraethyl
orthosilicate (TMS) was used to compare the results to the
study from Sonnenfeld. The influence of injecting
precursor directly into the plasma compared to afterglow
injection is investigated. Additionally, the setup was
slightly altered to also investigate monomer dissociation
along the DBD channel.
2. Experimental
The electric circuit of the experimental setup is
presented in figure 1. A custom-made power supply is
used to generate the high voltage required to ignite the
plasma.. All plasma measurements were obtained at 7
kHz with 9.8 kV.
Fig 1. Electric circuit showing high-voltage supply
(dashed) with the measuring setup (C m =1.49nF) [4]
A DBD cell as shown in Fig 2 is used. The barrier is
made from PMMA and is 1 mm in height and 21 mm in
width. The aluminum electrodes are 15 mm in width and
cover an area of 600 mm2. As process gas argon (Pangas
4.6, purity >99.996%) is used. The DBD cell is placed in
a chamber which is purged with nitrogen (Pangas 4.5,
purity >99.995%). A Bruker Equinox 55 FTIRspectroscope was used to obtain in-situ measurements. All
spectra were measured with a resolution of 0.5 cm-1
starting from 4000 cm-1 to 500 cm-1. Each measurement
consisted of 10 separate and consequently averaged scans.
1
The detector unit was a liquid nitrogen cooled HgCdTe
detector (InfraRed Associates, MSL-8).
HMDSO (Sigma-Aldrich, purity > 98.5%) was used as
precursors in the reactor.
Fig. 2. DBD cell setup with ZnSe windows (yellow), high
voltage electrode (dark red) and ground electrode (green).
Monomer injection points were either directly into the
afterglow or together with the process gas into the plasma
zone, as shown in figure 3.
Fig. 3. DBD cell side view with Si wafer (yellow), high
voltage electrode (red), PMMA channel (black), plasma
zone (light gray) and ground electrode (green).
3. Results and discussion
In figures 4 and 5, already discussed results of
Sonnenfeld et al. are shown. A process time of 1 minute
in the afterglow already led to nanoparticles in the range
of 20 nm which is already larger than the aspired particle
size of about 3 nm that have been found beneficial for
flowability improvement [5]. In figure 5, argon showed a
much higher Si-O-Si yield than helium and is therefore
more suitable for SiO x nanoparticle production as shown
in figure 5. Helium yielded in larger molecular structures
e.g. Si-(CH 2 )-Si and structures containing C=O bonds as
shown in figure 5.
Fig. 4: Scanning electron micrographs of SiO x deposits
on Si wafers from (a) an Ar-O 2 -TMS mixture after a
process time of 10 min with 20000 fold magnification and
(b) a He-O 2 -TMS mixture after a process time of 1
minute with 50000 fold magnification[4]
2
Fig. 5. Silicon wafer FTIR spectra of SiO x coatings from
Ar-O 2 -TMS and He-O 2 -TMS mixtures on Si wafers
deposited at positions different positions in the
afterglow[4]
In figure 6, two FTIR measurements in the afterglow of
Ar plasma with HMDSO injected into the plasma zone
are presented. The HMDSO to Ar ratio was kept constant
at 0.01g HMDSO per litre Ar. The IR beam was focused
directly at the outlet of the DBD cell. The curves show the
deviation spectra between ignited and not ignited plasma.
Three large peaks can be seen at 852, 1072, 1260 and
2913 cm-1 which correspond to Si-(CH 3 ) x , Si-O-Si, Si(CH 3 ) x and CH 3 , respectively. For both flow rates
HMDSO was dissociated in the plasma zone. The
deviations for the lower flow rate are higher for all four
described peaks. We assume, this is due to the longer
residence time of HMDSO in the plasma zone and
therefore longer exposure consequently would lead to this
difference. A higher dissociation is beneficial since a
higher percentage of HMDSO could react to form solid
nanoparticles. An increase in other gaseous products and
radicals was not detected yet.
Fig. 6. Deviation spectra of 0.3g/h (blue) and 0.9g/h
(green) HMDSO and 0.5l/min and 1.5l/min Ar,
respectively. Measurements were taken in the afterglow.
P-II-7-33
4. Conclusion
With the designed setup, precursor decompostion as
well as nanoparticle production was detected. We will
conduct further studies to see whether we produced
nanoparticle in the afterglow when HMDSO is injected
into the plasma zone using SEM and FTIR, compare our
results to the study from Sonnenfeld and optimize the
process conditions to meet requirements for flowability
improvement of powders. Moreover, the dissociation
along the plasma channel will be investigated.
5. Acknowledgements
The authors gratefully acknowledge the support by the
Scientific Center for Optical and Electron Microscopy of
ETH Zurich (SCOPEM) and the financial support from
Swiss Claude & Giuliana Foundation. Furthermore, the
authors would like to thank Bruno Kramer and Peter
Feusi for their support in setup construction.
6. References
[1] J. Yang, A. Sliva, A. Banerjee, N. Rajesh, R. Pfeffer,
Powder Technology 158, 1-3 (2005)
[2] A. Spillmann, PhD Thesis No. 17927 ETH Zurich
(2008)
[3] M. Ricci, J. Dorier, C. Hollenstein, P. Fayet, Plasma
Processes and Polymers, 8, 2 (2011)
[4] A. Sonnenfeld, V. Papageorgiou, P. Reichen, L.
Körner and Ph. Rudolf von Rohr, Journal of Physics D:
Applied Physics, 44 (2011)
[5] K. Zimmermann, I. Zimmermann, Powder
Technology, 139, 1 (2004)
P-II-7-33
3