Experimental study of the generation of metal nanoparticles and carbon
nanostructures in pulse electric discharge in liquid dielectric
D. Medvedev1, S. Korobtsev1, B. Potapkin1, D. Sapunov1, and V. Petyaev1
1
National Research Center “Kurchatov Institute”, Moscow, 123182,
Russia
Abstract: Current work is concentrated on the investigation of a pulsed electric discharge
in liquid dielectric with the current restricted by means of ballast inductivity. By controlling
the parameters of the discharge in a liquid aromatic dielectric at the “slow” stage optimal
conditions for effective generation of carbon nanostructures were determined. New types of
carbon nanomaterials (lace-like carbon materials, tetrapods) were obtained.
Keywords: carbon nanoparticles, liquid suspension, electric discharge.
1.Introduction
Discharges in liquid dielectrics were of a great interest for
many groups of investigators after the pioneer works of
Macfarlane [1] at the end of the 19th century. Since then,
many works have been published concerning different
aspects of discharge phenomenon. For many years there
were several mainstreams in this area of research such as:
investigation of breakdown properties of dielectric liquids in
civil engineering applications (capacitors, oil transformers
etc.), theoretical study of possible mechanisms of discharge
propagation and development and others. Extensive
theoretical analysis of the processes occurring during
discharge in liquid dielectric has been performed in [2].
After the works of Bacon [3] and Igima [4] great interest has
arisen in the field of nanoparticles production and use in
different applications. Shortly after those works, it was
shown [5, 6] that nanoparticles can be produced by means of
electric discharges in liquid dielectrics. This method proved
to be an efficient instrument for production [7] of different
carbon nanoparticles, metal nanoparticles and their mixtures
not only in dielectric media.
2. Experimental setup
The setup used in the experiments is shown on fig.1.
The discharge chamber was filled with dielectric liquid
(xylol) and a two-electrode “point to plate” system was
immersed in it. A pack of 10 wire screens with different wire
diameters was used as a high voltage electrode. High voltage
was applied to the electrode system.
High voltage generator
IC control system
HV
R
Motor-reducer
Fig.1 Scheme of the installation used in the
experiments.
One of the electrodes was attached to a screw with
motor drive to maintain constant interelectrode gap
while high voltage electrode erosion takes place. In
addition, a pyrolysis gas sampling system was used to
take samples of outgoing gases.
3. Discharge stages
High voltage pulsed electric discharge consisted of two
main stages. The “fast” stage is shown in fig.2 (upper
graph).
hydrogen formed was
inductivities L (fig.3).
obtained
for
different
Fig.3. Energy inputs per mole of hydrogen at different
inductivities.
Fig.2. Voltage and current profiles for“fast” (upper graph)
and “slow” (lower graph) stages of the pulsed high voltage
electric discharge in liquid dielectric (L = 530 μH).
During this stage the discharge of parasite conductance of
the system occurs. The discharge current is limited only by
the intrinsic inductivity of the electrode system. It rapidly
rises and reaches its peak at the time when the applied
voltage is still high. Electric power of the process is very
high, i.e. about few megawatts. Though the duration of this
stage is short, energy input is sufficient and is comparable
with the summary energy input in the process.
Estimation of temperature of the plasma channel was
carried out assuming the process to be stationary
(energy input in each gas layer is equal to the output
due to the thermal conductivity). The temperature of
the plasma channel is extremely high (over 10 5 K).
Such high temperatures obtained on the assumption of
the stationary process show that during the discharge
stationarity is never achieved. If the thermal
conductivity has little influence on the heat balance
more reliable assumption should be to neglect losses
associated with the heat exchange with the walls of the
channel and to assume that all the energy is used for
chemical processes, liquid vapourization, and gas
heating.
Plasma temperatures were estimated assuming that
specific heat is determined by rotational and
transitional degrees of freedom for all inductivities and
thus discharge currents (fig.4).
After the “fast” stage, the discharge of the main capacitor
occurs. The discharge current rises relatively slowly. The
voltage during this stage is about several hundred volts and
thus the power is lower (about 10 kW). After the current
reaches its peak the voltage crosses “zero” and the capacitor
is cut from the discharge system. During this stage the
energy stored in inductivity is dissipated in the system. The
current drops exponentially (fig.2, lower graph).
4. Experimental results and discussion.
Preliminary experiments have showed, that the main
outgoing gas in the process is hydrogen.
Fig.4. Temperatures of plasma channel for different
inductivities.
Using this fact and measuring the energy input in the slow
stage, dependence of specific energy input per mole of
It can be seen that the estimated temperature in the
region of low inductivities is significantly higher than
in the high inductivity region, so one should expect that
plasmochemical processes will lead to other results in low
inductivity experiments
Types of carbon material in experiments with high and low
inductivity were observed. For all types of liquid dielectrics
(xylol, benzene, toluol) similar regularities were observed,
but experiments with xylol gave most peculiar results.
In the experiments with small ballast inductivity L= 16 μH
rather large (fig.5) soot particles were formed. In regimes
with bigger inductivities products drastically changed.
Fig.5. Soot formation in experiments with low ballast
inductivity.
When larger inductivities were used the formation of carbon
nanotubes and sheets of graphene was observed. After
increasing the inductivity above 100 μH soot particles
disappeared from the products and all the products were
nanostructured. Besides in some regimes new, rather exotic
nanomaterials were observed (such as nanostroctured
graphene-like material (fig.6, upper graph) and faceted
nanorods and tetrapods from those nanorods (fig.6, lower
graph).
Faceted structures were observed in [5], where it was shown
that these structures are multiwall and multifaceted
nanotubes.
The ratio of facets and the diameter of these rods is in
conformity of those observed in [5]. This fact allows one to
suggest that these structures are also multiwall nanotubes.
Tetrapods from these tubes have not been seen before.
It should be noted that the analysis by means of “Source of
synchrotron radiation” at Kurchatov Institute also showed
this graphene-like structure. The appearance of these
structures in the regimes with the current limited by ballast
inductivity L > 100 μH is likely due to the organized growth
of crystaline graphene-like structures and can be explained
by lower temperatures in the plasma channel.
Fig.6. Graphene-like material (upper graph) and
nanorods (lower graph).
At lower temperatures benzene rings could undergo
only partial dissociation and instead of the regular soot
growth from atomic carbon, graphene sheets and
nanostructures based on these sheets can be formed by
means of polymerization of fragments of aromatic
molecules.
5. Results and conclusions.
Extensive analysis of all stages of the discharge has
been performed. It was shown that the discharge
consists of two main stages: “fast” and “slow”, having
comparable energy inputs. It was also shown, that
during the “fast” stage of the electric discharge, the
parasite capacity of electrode system and circuit
occurs. Estimations of the temperature of the plasma
channel during the “slow” stage depending on the
ballast inductivity have been conducted. Rapid
decrease of the average temperature of plasma channel
in experiments with ballast inductivity exceeding 20 μH has
been shown. Investigation of the process of generation of
nanosuspensions of metals and carbon nanostructures during
the described discharge has been conducted. Experimental
investigation of the generation of nanosuspensions of metal
and carbon material in liquid dielectric has been carried out.
The effect of generating carbon nanoparticles with high
yield during experiments with ballast inductivity higher than
100 μH has been shown.
References
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Berlin), 2007.
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