st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Nanostructured Particle Surfaces by PECVD for Improved Powder Processing
C. Roth1, G. Oberbossel1, D. Butscher1, P. Rudolf von Rohr1
1
ETH Zurich, Institute of Process Engineering, Zurich, Switzerland
Abstract: The surface properties of fine-grained and temperature-sensitive lactose powder
were modified to adjust the powder’s flowability and wettability. For this, the lactose particles
were coated with SiOx nanoparticles in a tubular plasma reactor. The originally cohesive
powder showed a free-flowing behaviour after the surface modification and depending on the
used organosilicon monomer, either hydrophobic or hydrophilic powders were obtained.
Keywords: Tubular Plasma Reactor, Particle Surface Modification, Nanoparticle Deposition.
1.
Introduction
Fine-grained powders have a very high surface-to-volume ratio and their macroscopic behaviour
depends mainly on the surface structure and chemistry of
the single powder particles. Therefore, powder properties
as the flow behaviour, compactibility, wettability, or dissolution rate can be tailored by a plasma-assisted surface
modification of the single particles [1].
A major problem throughout industry is the handling,
conveying, and dosing of fine-grained powders, which is
caused by the high attractive van der Waals forces FvdW in
between the single powder particles [2].
The force between two spherical particles with diameter
R calculates according to the Hamaker law [3] as given in
equation (1). A is the Hamaker constant and a material
property and H is the interparticle distance.
(1)
The classic process to improve the flowability of such
bulk solids is the batch-wise admixture of nanoparticles,
which act as spacers between the much larger substrate
particles and increase the interparticle distance. This reduces the attractive van der Waals forces [4, 5] and leads
to a higher powder flowability. However, this process is
highly empirical, time-consuming and the achievable flow
behaviour is limited by the distribution of the spacers on
the single powder particles.
In contrast, nanoparticles can also be formed and uniformly deposited onto substrate particles in a plasma
process within a very short residence time in the order of
0.1 s [6, 7]. For this, the particles need to be mixed with
argon, oxygen and an organosilicon monomer and pass a
non-equilibrium discharge. In the plasma, SiO x nanoparticles are synthesized out of an organosilicon monomer [8]
and directly deposited onto the surface of the much larger
substrate particles, as illustrated in Fig. 1. In doing so,
both, the flowability and wettability of the powders can be
adjusted as a function of the applied plasma conditions
and the choice of precursor [1, 9].
Therefore, the plasma-assisted deposition of such spacers, is seen as convenient alternative to improve the flow
behaviour of fine-grained and temperature-sensitive
powders.
Fig. 1: Principle sketch of the simultaneous formation and deposition of nanoparticles onto a substrate particle [9].
Within the scope of this paper, the influence of the substrate powder feed rate on the resulting powder properties
is studied for the two organosilicon monomers hexamethyldisiloxane (HMDSO) and tetraethyl orthosilicate
(TEOS).
2.
Experimental
In the applied tubular plasma reactor (TPR), lactose
particles passed a non-equilibrium discharge with high
velocity (~10 m/s) and their surfaces were modified during their transit of the plasma zone. A schematic diagram
of the tubular plasma reactor including powder storage
tank, nozzle, reactor tube, and separation unit is shown in
Fig. 2.
The substrate powder is fed by a metering screw from
the storage container and mixed with the process gases in
a converging-diverging nozzle. The flow rates of argon,
oxygen, and the organosilicon monomer are adjusted by
flow controllers and the monomer is evaporated prior
feeding. The plasma chamber itself consists of an insulating tube with an inner diameter of 40 mm. An RF signal is
applied at the helical coil, which is wrapped around the
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
tubular reactor. The generated electromagnetic field penetrates into the low-pressure environment within the tube
and sustains the discharge.
Table 1: Standard process parameters.
Parameter
Plasma power
System pressure
Argon flow rate
Oxygen flow rate
Monomer flow rate
Value
300 W
200 Pa
950 sccm
500 sccm
50 sccm
Lactose monohydrate powder (GranuLac 200, Meggle,
Germany) with a median particle diameter of 28 m was
chosen as test substrate due to its similarity to industrially
relevant products. The flow behavior of the powders was
classified according to the so-called flow function coefficient ffc, which is defined as the ratio between the consolidation stress σ1 and the unconfined yield strength σc
within a powder.
(2)
Fig. 2: Tubular plasma reactor (TPR) concept for
powder surface modification [1].
A fluid element with dispersed particles passes the discharge in several tens of milliseconds, depending on the
total gas flow rate and pressure. Downstream the plasma
zone, the particles are separated from the gas by a downcomer, cyclone, and filter unit. A constant pressure in the
reactor is maintained during the process by a butterfly
control valve in front of a double stage vacuum pump.
The constant process conditions of the experiments are
given in Table 1. Only the powder feed rate was varied
between approximately 0.4 and 6 kg/h.
The flow behavior is classified according to Schulze
as not flowing for ffc < 1, very cohesive for 1 < ffc < 2,
cohesive for 2 < ffc < 4, easy flowing for 4 < ffc < 10 and
as free-flowing for 10 < ffc [10]. The flow factor was
measured with a ring shear tester (RST-XS, Schulze
Schüttguttechnik, Germany) and a shear cell volume of
30 ml. The preshear stress was set to 5000 Pa and shear
stresses of 1000, 2500 and 4000 Pa were applied to determine the yield locus and thus, the value of the flow
factor. Details about the ring shear tester, the method and
the connected theory are well described by Schulze [11].
The particle surfaces were characterized by scanning
electron microscopy (SEM). For the SEM analysis, a
small amount of powder was fixed to conductive polycarbonate stickers with admixed graphite (G3347, Plano,
Germany) and placed on aluminum sample holders. The
prepared samples were coated with an approximately
2 nm thick platinum layer in a sputter coater (MED 010,
Bal-Tec, Liechtenstein) to reduce charge accumulation on
the samples during analysis. The microscope (Gemini
1530, Zeiss, Germany) was operated at an acceleration
voltage of 2.0 kV and using a secondary electron detector.
3.
ff
Results and Discussion
The lactose powder feed rate influenced directly the
available specific surface area in the TPR. To assess its
impact on the achievable flowability, the powder feed rate
was varied for the monomers HMDSO and TEOS. All
other process conditions remained constant as specified in
Table 1.
The flowability improvement with respect to untreated
powder as a function of the powder feed rate is shown in
Fig. 3. The obtained flowability decreased with rising
powder feed rate and approached the flow behaviour of
untreated lactose asymptotically. At low powder feed rates,
a plateau in the achieved ffc is cognizable, from which on
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
a further reduction of the powder feed rate did not improve the flowability anymore.
rate. In contrast to the particle growth model for argon:oxygen:monomer discharges in such kind of reactors
[8], also substrate particles were present in the TPR and
provided a large specific surface area. The formed SiOx
clusters and primary particles can either grow to larger
structures by coagulation and agglomeration or directly
deposit onto the lactose particles. With rising specific
lactose surface area, the direct deposition became more
probable so that large spacer structures and agglomerates
were hardly formed.
Fig. 3 Flowability of lactose powder as a function
of the lactose feed rate.
The formed spacer structures were distributed onto a
larger surface, if more substrate powder was fed per time
unit. Assuming that the nanoparticle production rate was
approximately constant, the fractional surface coverage
reduced with rising lactose feed rate so that the obtained
surface roughness decreased.
The levelling out of the ffc at low powder feed rates can
be understood as well. At low powder feed rates, the substrate particles were covered intensively with SiOx
nanoparticles so that an additional coating with similar
nanoparticles could no longer increase the surface roughness of the particles and a plateau in the obtained flowability was reached. This situation is schematically illustrated in Fig. 4 c).
Fig. 4: Two particles a) without flow agents b)
with highly dispersed spacer structures and c) fully
covered with nanoparticles.
This interpretation was qualitatively confirmed by SEM
images of coated lactose particles, shown exemplarily for
the series with TEOS as precursor in Fig. 5. At the lowest
powder feed rate of 0.4 kg/h, the lactose surface was intensively covered with SiOx structures. With rising powder feed rate, the spacer structures were distributed onto a
higher specific surface and at the maximum powder feed
rate of 5.6 kg/h, many surfaces appeared nearly uncoated.
It is hypothesized that the powder feed rate influenced
the different particle growth phases as well, so that larger
primary particles and more agglomerated structures were
found on the lactose surfaces with decreasing powder feed
Fig. 5: SEM images of lactose surfaces coated with
SiOx structures from the monomer TEOS. The
powder feed rate is indicated in the images.
All lactose powders coated with TEOS-derived nanostructures were wettable with water, whereas no sample
from the HMDSO series featured a water contact angle
below 90°. Hence, more polar groups are expected on the
powders that were processed in the TEOS series.
Since the Hamaker constant A is proportional to the
surface free energy, the van der Waals force (1) increases
with rising surface free energy too [12]. As a result, the
flowability decreases with rising surface free energy.
Hence, the different surface free energy in combination
with the varying nanoparticle production rate (2.24 g/h for
HMDSO and 1.57 g/h for TEOS [8]) could have caused
the difference between the two measurement series,
shown in Fig. 3.
Overall, the flowability is assumed to increase for each
parameter combination and organosilicon monomer by
lowering the substrate powder feed rate. At a certain
powder feed rate, a maximum flowability ffc,max is reached
and a further reduction of the available substrate surface
area does not improve the flowability anymore, since additional spacer structures either keep the surface roughness at a constant high level or even decrease the roughness again due to the development of a coherent particle-like layer.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
4.
Conclusions
Cohesive lactose powder with a median particle diameter of 28 m was used to investigate a non-equilibrium
plasma process for flowability improvement. Since the
dispersed substrate powder provided a large specific surface area in the discharge, the growing primary nanoparticles did not agglomerate but deposited directly onto the
lactose surface. These formed spacers on the particle surfaces decreased the interparticle van der Waals forces and
thus improved the powder flowability.
The originally cohesive lactose powder reached flow
function coefficients that are classified as easy- or
free-flowing after the surface modification in the TPR.
However, only lactose powders coated with
HMDSO-derived nanoparticles were wettable with water,
which was related to the higher surface free energy of
TEOS-based depositions.
The fractional coverage of the lactose particles with
SiOx structures rose with decreasing lactose feed rate.
Consequently, high flowabilities were measured if the
powder was processed at low powder feed rates. However,
below a specific powder feed rate, the resulting flow
function coefficient remained approximately constant. A
further reduction of the powder feed rate did not improve
the flowability anymore, since the surfaces were already
intensively covered with spacer structures and the attractive forces in between the deposited glidants partially
compensated the reduced van der Waals interactions in
between the lactose particles.
Overall, the plasma-assisted process for flowability improvement is much faster than the classic method of
nanoparticle admixture and the intrinsic dispersion of
particles in the plasma enables a uniform and homogeneous surface modification. In addition, no segregation can
occur, since the nanostructures are directly bond to the
substrate material. Hence, it is highly recommended to
apply this process in industry for the flowability improvement of fine-grained, cohesive, and potentially
heat-sensitive powders.
5.
References
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6.
Acknowledgements
We would like to thank the Foundation Claude &
Giuliana for the financial support and we acknowledge
the support of the Electron Microscopy Centre of the ETH
Zurich (EMEZ).