Synthesis of nanopowders under high frequency electric discharge assisted mechanical milling

Article abstract of DOI:10.1016/j.jallcom.2006.08.271

Electric discharge assisted mechanical milling can be applied to the synthesis of a range of fine powder products, including nanocrystalline agglomerates and individual nanoparticles and nanofragments. Processing variables include: starting powder sample size; electric arc parameters such as arc length and arc voltage/current; mechanical milling parameters; gas atmosphere and ionized gas species present. We describe results of an initial experimental program underway to investigate phase transformations and/or particle fragmentation during discharge milling using a new pulsed power supply working at frequencies in the kHz range. The aims were to determine processing parameters required for the synthesis of potentially useful high surface area particles, nanostructural powders and nanoparticles and to compare products with those synthesised by Hz frequency discharge milling. Microstructural, morphological and phase changes induced by kHz discharge milling were characterised by X-ray diffractommetry and transmission electron microscopy. Results were found to depend on the often competing processes of fragmentation into nanoparticles, agglomeration of powder particles, particle melting and/or sintering and chemical reaction induced by mechano-processing in the presence of a particular type of plasma. Discharge milling of graphite under Ar/4% H₂ resulted in a range of products including: graphite nanostructures, carbon nanotubes and other exotic nanofragments. It was found that, compared with processing at 50 Hz, high frequency (kHz) electric discharge assisted mechanical milling of graphite resulted in higher yields of carbon nanofragments. Discharge milling of hematite resulted in partial reduction to magnetite and FeO and the formation of nanostructural oxide nanorods and nanorod clusters. Discharge milling of Co-WC resulted in products including: micron and submicron fracture products, nanostructural regions of Co and WC and carbon rich nanorods.

Full text of DOI:10.1016/j.jallcom.2006.08.271

Journal of Alloys and Compounds 434–435 (2007) 463–466
Synthesis of nanopowders under high frequency electric
discharge assisted mechanical milling
A. Calka∗, D. Wexler, A.Y. Mosbah
Department of Materials Engineering, Faculty of Engineering, University of Wollongong, Northfield Avenue,
Wollongong, NSW 2522, Australia
Available online 13 October 2006
Abstract
Electric discharge assisted mechanical milling can be applied to the synthesis of a range of fine powder products, including nanocrystalline
agglomerates and individual nanoparticles and nanofragments. Processing variables include: starting powder sample size; electric arc parameters
such as arc length and arc voltage/current; mechanical milling parameters; gas atmosphere and ionized gas species present. We describe results
of an initial experimental program underway to investigate phase transformations and/or particle fragmentation during discharge milling using a
new pulsed power supply working at frequencies in the kHz range. The aims were to determine processing parameters required for the synthesis
of potentially useful high surface area particles, nanostructural powders and nanoparticles and to compare products with those synthesised by Hz
frequency discharge milling.
Microstructural, morphological and phase changes induced by kHz discharge milling were characterised by X-ray diffractommetry and trans-
mission electron microscopy. Results were found to depend on the often competing processes of fragmentation into nanoparticles, agglomeration
of powder particles, particle melting and/or sintering and chemical reaction induced by mechano-processing in the presence of a particular type of
plasma. Discharge milling of graphite under Ar/4% H2 resulted in a range of products including: graphite nanostructures, carbon nanotubes and
other exotic nanofragments. It was found that, compared with processing at 50 Hz, high frequency (kHz) electric discharge assisted mechanical
milling of graphite resulted in higher yields of carbon nanofragments. Discharge milling of hematite resulted in partial reduction to magnetite and
FeO and the formation of nanostructural oxide nanorods and nanorod clusters. Discharge milling of Co–WC resulted in products including: micron
and submicron fracture products, nanostructural regions of Co and WC and carbon rich nanorods.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials; Mechanochemical processing; Transmission electron microscopy; Nanofabrications
1. Introduction
cation of high voltage, low current electrical discharges during
materials synthesis and processing. Within the mill, the elec-
trical discharges cause molecular breakdown of the controlled
atmosphere, the formation of monatomic gases and, depending
on the species present and discharge conditions, formation of
specific types of plasma in the proximity of powder particles.
The power supplies employed for this work allowed high
voltage 50 Hz discharges to occur each time physical and elec-
tricalcontactwasmadeandbrokenduringeachmillingcycle, the
contact being between the conducting chamber, powder particles
and conducting rod or balls, depending on the device used. In
the current series of experiments a high voltage high frequency
(kHz range), pulsed ac power supply was used instead of the
50 Hz ac power supples employed in previous investigations.
Preliminary investigations were performed involving discharge
milling of small samples under an Ar/4% H atmosphere. Starting
powder samples investigated included: (i) high purity graphite,
known to transform under 50 Hz discharge processing, into a
Electric discharges have been extensively used in many mate-
rials processing techniques; including plasma coating, plasma
synthesis of fine powders, waste processing, sintering of pow-
ders, metallurgy and, recently, for the synthesis of nanomaterials
[1]. Present research efforts concentrate on new approaches for
the synthesis of nanometre size structures the future growth of
this technology will depend on development of new processing
methods with better control of the particle size, size distribution,
reproducibility and better process efficiency. Therefore, novel
reactor designs are of great interest.
In previous publications we have described application of
novel electric discharge assisted mechanical milling materials
processing method [2–7]. This new technique involves appli-
∗
Corresponding author. Tel.: +61 2 4221 4945; fax: +61 2 4221 3112.
E-mail address: andrzej calka@uow.edu.au (A. Calka).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.271

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A. Calka et al. / Journal of Alloys and Compounds 434–435 (2007) 463–466
range of products, with minor fractions of nanofragments and
nanotubes [6], (ii) high purity hematite, also known to transform
under 50 Hz discharge processing into reduced oxide species and
pure iron [7] and (iii) Co–WC hardmetal composite, crushed into
powder form.
plastic bags connected to the milling cell gas outlet. Milling times used in these
experiments were within the range, 1–15 min. Powders were first analysed by
X-ray diffraction (XRD), using Cu K␣ radiation and graphite monochrometer.
The shape and size of nanostructural particles was investigated by transmission
electron microscopy (TEM) using a JEOL 2011, 200 keV analytical instrument
with JEOL EDS (energy dispersive X-ray spectroscopy) analysis system. For
bright field TEM examination, powders were dispersed on carbon or lacy carbon
support films.
2. Experimental details
2.1. Device description
3. Results and discussion
A vibrational laboratory rod mill was modified in order to a produce milling
mode involving repeated impact of a hardened curved rod end on particles placed
on a vibrating stainless steel hemispherical container under a controlled electri-
cal discharge in a controlled atmosphere. During milling, gaps, of up to ∼3 mm,
between the vibrating mill base, the powder particles and the loosely suspended
conducting plunger occur, as illustrated in Fig. 1a. A particular type of electrical
discharge was generated within those gaps. The experiments were performed
using a custom built power supply, designed to generate kHz frequency range
pulsedarcdischarges, ofvariableenergyandduration, superimposedonanather-
mal or glow discharge. The aim of the arc discharge is to locally heat up powders
which are subsequently subjected to complex ac plasma treatment (Fig. 1a). The
pulse current and pulse duration time (in the 20–100 ms range) can be manually
adjusted to create a range of electrical conditions, as illustrated in Fig. 1b.
The main electric parameters varied in this study are the current (10–800 mA
range) and impulse duration time, as shown in Fig. 1b. The power supply used
in this study was connected through an ac high voltage transformer, generating
typically up to 3 kV and 10–800 mA impulses within the kHz range. A digital
camera image of discharge in the vicinity of milled particles is shown in Fig. 1c.
In the past few years there has been a growing interest in a
new class of materials called nanocrystalline, nanophase, nanos-
tructured or ultrafine-grained materials [8–10], consisting of
nanoscaleparticlesorgrains(1–100 nm). Thesematerials, which
have a very high surface or interfacial areas, exhibit exiting prop-
erties that can be utilized in many applications [11–13]. In the
following we describe milling results involving the synthesis of
nanopowders in graphite, hematite and cemented WC.
3.1. Graphite
XRD examination of graphite, discharge milled in Ar/H2
revealed peaks corresponding to amorphous carbon, graphite
and additional peaks which corresponded to carbon nanofrag-
ments. TEM examination confirmed the presence of graphite
and amorphous carbon, and revealed a significant fraction of
carbon nanofragments, some of which are believed to be carbon
nanotubes, as indicated in Fig. 2 which shows low magnifica-
tion image of a region containing tube-like features and high
magnification inset showing what appears to be the end of a
hollow tube, with outer diameter less than 2.5 nm. High resolu-
tion imaging is currently underway to determine the nature of
the nanofragments formed; however, they appear to include (i)
thin ribbons of graphite sheets and (ii) multi-walled nanotubes,
often associated with the presence of small remelted particles
of stainless steel. For example, the dark round particles, such as
those marked P, were confirmed by EDS analysis to be rich in
Fe, and containing Ni and Cr. Whether these Fe-rich particles
play a role in nanotube formation within the discharge mill is yet
2.2. Sample preparation
The following powders were processed; high purity hematite; high purity
graphite and cemented WC–10 wt.% Co, crushed under liquid nitrogen into
starting powder (<300 mesh). Samples were placed in the milling cell and sub-
jected to mechanical milling with electrical impulses. Milling was performed
with and without gas flow (Ar4% H2). The small amount of hydrogen was
included in the gas mixture, both to increase arcing temperature thus improv-
ing efficiency of the process, and to ensure a partially reducing environment.
Throughout all experiments the Ar/H2 gas pressure with no gas flow was main-
tained at near atmospheric pressure. In flow mode the flow rate at 1 atm was
maintained at about 2 cm3 min−1. Nanoparticles formed by the processing com-
bination of local arc heating of the powder target and plasma treatment were
collected from milling cell after completion of the milling process or, in the case
of flow mode, samples were immediately introduced with flow of the gas into
Fig. 2. Bright field TEM image of graphite discharge milling products. Spherical
particles, such as those marked P, were confirmed by EDS analysis to contain
Fe, Ni and Cr. Regions believed to be multi-walled carbon nanotubes are also
marked, including the possible end-on nanotube in the high magnification inset.
Fig. 1. Schematic of (a) electrical discharge milling cell, (b) spectra of pulsed
discharges used in this work and (c) photograph of plunger base, perspex shield
and mill base during discharge milling.

A. Calka et al. / Journal of Alloys and Compounds 434–435 (2007) 463–466
465
Fig. 3. (a) Nanoparticle clusters and (b) individual, isolated nanoparticles, observed in bright field TEM images of hematite, discharge milled in Ar/H2.
to be determined. The yield of nanofragments and/or nanotubes
appeared much higher than in graphite or activated carbon dis-
charge milled using a 50 Hz power supply [6].
Fig. 4 shows the results kHz frequency milling of hematite
with relatively low electric discharge energy: 100 mA and
0.17 kV. This condition led to the formation of particles believed
to be magnetite nanorods. By varying the current under these
low energy milling conditions, both the length of nanorods, and
the number of joined nanorods were observed to increase with
increasing current. The tendency for formation of clusters of
larger, parallel-joined nanorods (Fig. 4b) also increased with
increasing current. However, above around 200 mA current, no
nanorods were detected in the product. There is much interest
in the synthesis of magnetic iron oxide nanorods [14] and other
magnetic nanoparticles, as these have great potential for sev-
eral applications in range of areas including: magnetic recording
material, pigments, catalysts, sensors and biomedical tracing
particles. A more detailed TEM investigation of oxide nanorod
formation during discharge milling is currently underway.
3.2. Hematite
XRD analysis of hematite discharge milled under high fre-
quency for 5 min in Ar/H2 indicated partial reduction of the
starting powder, with the major product, magnetite and minor
product, FeO. This result was consistent with a previous investi-
gation, where the product of extended discharge milling (15 min
in Ar/H2) at 50 Hz frequency was magnetite, FeO and pure iron
[7]. TEM examination of the product the kHz discharge milling
experiments revealed a range of morphologies. Fig. 3 shows
TEM micrographs of hematite milled for 5 min in ArH2, using
the electric condition; 250 mA, 0.50 kV and impulse duration
time 0.02 ms. This milling condition was found to produce pre-
dominantly particles ∼20 nm in size (Fig. 3a and b).
3.3. WC–Co
These results, and those obtained for milled graphite, indicate
that during milling a thermal plasma condition is created which
is a source of very high temperatures and steep temperature gra-
dients, which leads to the synthesis of fine powders down to the
nanometer size range. The condensation of supersaturated vapor
is believed to provide the driving force for particle condensation
leading to the formation of nanoparticles by homogenous nucle-
ation. Thermal plasmas suitable for the synthesis of fine powders
are primarily produced by means of high-intensity arcs (ac or
dc) and high-frequency discharges (RF and microwave).
Sintered WC–Co based hardmetals have formed a major
basis of metal cutting, wire drawing, fitting and turning, and
rock drilling industries for the past century. Unfortunately,
this material is difficult to recycle directly by crushing and
re-sintering due to chemical reaction during initial processing,
leading to dissolution of carbon in the Co binder and decar-
burisation of the WC particles. It was decided to investigate
fragmentation of this material under kHz discharge assisted
milling. Preliminary TEM investigations (Fig. 5) revealed mixed
Fig. 4. Iron oxide nanorods observed in hematite, discharge milled in Ar/H2.

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A. Calka et al. / Journal of Alloys and Compounds 434–435 (2007) 463–466
Fig. 5. (a) Carbon rich nanofragments observed in Co–WC hardmetal, first pulverised under liquid nitrogen and then discharge milled in Ar/H2, features marked R
in (a) and (b) are believed to be carbon nanorods while the object marked G in (a) is either graphite or carbon nanorods.
product, including large particles of WC and Co, regions of
ultrafine and/or nanostructural product comprising a mixture of
Co and WC and, surprisingly, a range of carbon rich nanotubes
or graphite ribbons (marked G in Fig. 5a) and thicker, carbon
rich nanorods (marked R in Fig. 5a and b). Further work is
underway to investigate the discharge milling products, and the
mechanism of formation of the carbon nanotubes and nanorods
in Co–WC. However, conditions of decarburisation under the
extremes of thermal plasma, in the presence of either Co or Fe
catalyst (from the stainless steel milling chamber) are likely
candidates to promote carbon nanotube and nanorod formation
in Co–WC.
These results confirm that, depending on the starting pow-
der, electric discharge mechanical milling under high frequen-
cies (kHz) in an Ar/H2 atmosphere can promote conditions
favourable for the synthesis of a range of nanofragments,
nanorods and nanotubes.
Acknowledgement
This investigation was supported by funding from the Aus-
tralian Research Council (Grant No. A00103022).
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Electric discharge assisted mechanical milling of graphite
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of carbon nanofragments than that resulting from discharge
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