LASER SYNTHESIS OF NANOPOWDERS
V.V. Osipov, M.G. Ivanov, Yu.A. Kotov, V.V. Platonov, O.M. Samatov
Institute of Electrophysics, Ural Division of RAS, 620016, Ekaterinburg, Russia,
E-mail: plasma@iep.uran.ru
Corresponding Author’s e-mail: max@iep.uran.ru
SUMMARY: The design and characteristics of a setup for producing oxide nanopowders
are reported. Y2O3 – stabilized ZrO2 (YSZ), Al2O3+YSZ and CeGdO nanopowders are
prepared by target evaporation with a pulse-periodic CO2-laser. Average laser radiation power
is 600 W, pulse power ~ 10 kW. The output rates of YSZ and Al2O3+YSZ nanopowders are
15-20g/h, and CeGdO nanopowder - 55-60g/h. The grain mean size in the powders is 15nm.
Data for the powder characteristics, as well as results of X-ray phase and structure analysis,
are reported. The results of investigation of unstable behavior of plasma plume produced by
the long-pulse laser irradiation of the targets are reported as well. The mushroom-like shape
of the glowing area is believed to be determined by the Richtmyer-Meshkov instability of the
plasma-air interface and formation of nanoparticles in the plasma expanding into the buffer
gas.
KEYWORDS: nanopowders, laser-assisted evaporation, laser synthesis
INTRODUCTION
A great number of chemical and physical methods are available today to produce
nanopowders [1]. Chemical methods are highly productive. Chemically synthesized powders
are relatively cheap, but are strongly agglomerated. Physical methods give weakly
agglomerated nanopowders, but the productivity is smaller and the nanopowders are more
expensive. The most productive methods among physical techniques include plasma methods,
wire explosion and laser synthesis. The laser synthesis provides powders with the narrowest
particle size distribution, but the cost of the powders is high. Therefore, the choice of method
depends on application of nanopowder.
Laser-synthesized powders are most favorable for preparation of complex nanoceramics
having crystallites less than 100 nm in size, whose density is equal to or approaches the
theoretical density. Such ceramics have been used for preparation of mid-temperature
electrolyte of fuel cells, Nd:YAG ceramic lasers, and so on.
It has been known for a long time [2] that nanopowders can be synthesized in the plume of
material evaporated with laser radiation. However, only in 1995 Müller et al. [3] proved
competitiveness of this method as compared to other techniques. Using a continuous CO2
laser providing output radiation up to 4 kW, they produced ZrO2 nanopowders with particles
~60 nm in diameter on the average. The productivity was 130 g/hour and the energy
consumption amounted to 25 Wh/g. Later they achieved more impressive results using a
continuous CO2 laser for production of nitrides [4]. To raise the intensity, the laser was
switched to the pulse mode with either mechanical chopper or Fabry-Perot interferometer. In
this case the output radiation had typical duration 120 ns (full width at half maximum), peak
power up to 100 kW, and repetition rate 3.5 kHz. The particle diameter decreased to 15 nm,
the productivity dropped to 11 g/h, and the energy consumption increased greatly. Therefore,
this mode showed little promise.
We believe the pulse-repetitive mode must provide not only finer particles, but also
decrease the energy consumption for production of nanopowders thanks to a smaller loss of
energy taken away by heat conduction.
EXPERIMENT
The given above considerations were confirmed with the help of experimental setup, its
block diagram is shown in Fig. 1.
Fig.1. Experimental installation for nanopowder production
The laser radiation was focused on the target 2 through the lens 8, which also served as
the inlet window of the chamber 3. A special drive l rotated target 2 and moved it linearly in
the horizontal plane that the laser beam traversed the target surface at a constant speed and the
target evaporated uniformly. As the target surface was wearing out, the target was moved
axially so the focal spot remained on the target surface. The focal distance of the KCl lens
was 10 cm. The focal spot was 0.45 mm in diameter. The scan rate of the beam over the target
surface was 20 cm/s that assured displacement of the target for 0.43 mm during the pulse-topulse time. The fan 4 blew air through the evaporation chamber 3, which carried the powder
to the cyclone 5 and the electric filter 6 where the powder was trapped. Air was cleaned
additionally in the mechanical filter 7 and was returned with the fan to the chamber. The gas
flow rate over the target surface was ~15 m/s.
The targets in form of tablets were compacted and sintered of raw powder with particle
110 m in size.
The nanopowders were analyzed with help of DRON-4 X-ray diffractometer, JSM-T220A
scanning electron microscope, GC-1 chromatograph for the BET method, and Q-1500
thermogravimeter.
The most complex unit of the setup for nanopowders production was a pulse-repetitive
CO2 laser, which was excited by a combined discharge. It turned out later that some
capabilities of the laser predetermined small dimensions of particles and a relatively small
energy consumption for their production. Therefore let's discuss briefly this our device.
Earlier N.G. Basov et al. (e.g., [5]) showed that the pulse-repetitive mode is more efficient
for treatment of materials than continuous mode.
However, until now the world industry has not produced high-power pulse-repetitive (PR)
CO2 lasers for this purpose. First of all it is due to the lack of high-power fast and long-life
switches and the tendency to excite the whole of gas medium traversing the discharge zone.
We solved this problem.
Figure 2 presents the schematic circuit diagram of the pulse-repetitive CO2 laser excited by
combined discharge [6].
Fig.2. The schematic circuit diagram of the pulse-repetitive CO2 laser.
A generator of short high-voltage pulses is on the left (DC source 5, inductances 6 and 7,
thyratron 8, capacitors 15 and 16) and a DC voltage source (DC source 4, inductances 12,
capacitors 5 and 14) maintaining a non-self-sustained discharge is on the right. A high-voltage
pulse generator initiated a self-sustained discharge in the gas (gap between 1 and 2
electrodes). After the self-sustained discharge, the main part of energy was inputted into the
gas at the stage of the plasma decay with the DC voltage source.
The waveforms of self-sustained and non-self-sustained discharge current are shown in
Fig. 3. It is seen that the duration of self-sustained and non-self-sustained discharges were 100
ns and 100 s respectively. When the current dropped to a certain level, a self-sustained
discharge was initiated again and the process repeated.
a) Self-sustained discharge current
b) Non-self-sustained discharge current
Fig.3. Waveforms of current
The energy, that was inputted into the gas at the stage of the self-sustained discharge and
passed through the thyratron, amounted to only 1-2% of the non-self-sustained discharge
energy. This circumstance predetermined the possibility of developing a technological PR
CO2 laser.
Fig.4 Waveforms of output radiation
Changing the number of pulses in a burst could vary the radiation duration. If the number
of burst pulses was three or less, the duration of the radiation pulse didn’t change and only the
shape of radiation pulse was changed (Fig. 4). It is seen that the peak power was 10 kW,
although the average power was about 1 kW. The pulse shape depends on the delay between
high-voltage pulses. When we used the radiation to prepare the nanopowders, the best results
were obtained at the delay of 75 s and two pulses in the burst.
The laser has the following characteristics:
Mean radiation power
665-800 W
Peak radiation power
10-11.2 kW
Diameter of the light beam in the outlet window
35 mm
Radiation pulse length
140-250 s
Pulse repetition frequency
400-450 Hz
Efficiency
8.3-10%
Power consumption
8 kW
The characteristics are averaged over 2.5 hours operation in the sealed off regime. These
characteristics were averaged since the output power decreased during the laser operation due
to leaking of atmospheric air into the gas chamber.
RESULTS AND DISCUSSION
This laser was used to prepare the following nanopowders:
1. 2.8 YSZ, 4.1 YSZ, and 9.8 YSZ
- 8 kg
(The numerals mean the mole percentage of Y2O3 in ZrO2.
The output rate was 15-20 g/h, Q = 30-40 Wh/g)
2. A41.1 + 1.45 YSZ and A88.8 + 1.1 YSZ 11.2 mixtures
- 1.5 kg
(A = Al and the suffix numerals mean the weight
percentage of the component. The output rate was 20 g/h,
Q
3.
= 30 Wh/g)
Ce0.8Gd0.2Q2 corresponds to the mass ratio CeO2:Gd2O3 =
= 0.792:0.208
- 1.2 kg
The output rate was 60 g/h, Q = 10 Wh/g
A typical photograph of an YSZ nanopowder after sedimentation is given in Figure 5.
Fig.5. Typical picture of the YSZ powder nanofraction after sedimentation.
Ninety-eight percent of particles had a size smaller than 40 nm. It is to be noted that as
compared to the initial material the prepared nanopowders were always deficient of the
component having a higher boiling point.
The size distribution of YSZ nanoparticles after sedimentation is shown in Fig.6. The
average size of the nanoparticles was 15 nm.
Fig.6. The grain size distribution
The results of analysis of the nanopowder and the target material before and after laser
irradiation, including the concentration of Y2O3 in ZrO2, the specific surface, the phase
composition and lattice parameters are given in Table 1.
Table 1
No
Raw material for target
Powder after sedimentation
preparation
1
Powder mixture: ZrO2, S = 20 m2/g;
3.1Y2O3, S=4.5m2/g
2.8YSZ: S = 68 m2/g; T: a = 5.106 Å and c = 5.1638 Å,
grain size D = 19 nm, weight of volatiles m(V) = 2.7 wt %
sediment 3.4YSZ: T: a = 5.1084 Å and c = 5.1674 Å; D =
26 nm. 6% of M phase. Melted ... of target 5.2YSZ
2
Powder mixture: ZrO2, S = 20
m2/g, T= 45 and M = 55 wt %;
4.5Y2O3, S = 4.5 m2/g
4.15YSZ:5 = 64.4m2/g, T:a = 5.115 Å and c = 5.161 Å,
D = 17 nm, m(V) = 2.6 wt %. Sediment 4.35YSZ; C-a =
5.13Å, D = 25 nm,. 7% of M phase.
3
Powder mixture: ZrO2, 5=51
m2/g, T = 60 and M = 40 wt %;
9.1 Y2O3, S = 4.5 m2/g
8.6YSZ: 5= 86 m2/g; C-a = 5.1405 Å
D = 17 nm, m(V) = 2.4 wt %
Sediment 8.9YSZ: C -a = 5.144 Å, D = 25 nm.
7% of M phase
4
Powder 10.15YSZ, S= 6.1 m2/g,
C - a = 5.1448 Å
9.85YSZ: 5= 79 m2/g, C-a = 5.1459 Å,
D = 18 nm, m(V) = 2.8 wt %.
Sediment 10.4YSZ: C- a = 5.15 Å, D = 41 nm
Note: In raw material mixtures, the mole percentage of Y2O3 powder is indicated. M, T, and C
stand for monoclinic, tetragonal, and cubic lattice, respectively.
Item data were obtained for powders of Al2O3 (Table 2) and Gd2O3-doped CeO2.
Table 2
Target composition
Powder mixture
A40+1.65YSZ60
Powder mixture
A85+M.635YSZ15
Target material
Al2O3 : S = 74 m2/g;  =
20 wt %;  = 80 wt %
1.65 YSZ: S = 7.76 m2/g.
M=58 and T=42wt%. Grain
size D = 70 nm.
-- // --
Powder after sedimentation
A41.1+1.45YSZ58.9, S= 80.6 m2/g, m(V) =
4.9 wt %
Composition and structure:
1.45YSZ: T-31 wt %: a = 5.095 Å and c =
5.156 Å, D = 11 nm.
K - 28 wt %: a = 4.924 Å, D = 6 nm.
 = Al 2 O 3 -20wt%, D= 10nm.
Amorphous Al2O3 - 21wt %
A88.8+1.15YSZ 11.2, S= 86 m2/g, m(V) =
4.8 wt %.
Preliminary composition and structure:
cubic and tetragonal 1.15YSZ and -Al2O3,
amorphous Al2O3; diffraction 2 pattern
interpretation is being continued
Fig.7. Photograph of Gd2O3-doped CeO2
particles
Fig.8. Particle size distribution in the
Ce0.78Gd0.22O2- powder after sedimentation.
Remarkably, the shape of particles in these nanopowders changed from spherical to cubic
(Fig. 7) and the average size of particles decreased to 9.4 nm (Fig. 8).
To understand why the nanoparticles obtained in the pulse-repetitive mode were 4 times
smaller than similar ones prepared in the continuous mode, we analyzed the dynamics of the
laser plume. Graphite and YSZ targets were used. The radiation was directed at the target at
an angle of 45 degrees.
Fig.9. The sequence of frame of the laser plume from the graphite target.
Figure 9 presents a sequence of frame of the laser plume from a graphite target. The
exposure time was 1 s. The analysis of these photographs leads to important facts:
1. The plume had a mushroom shape. We related this phenomena with RichtmeierMeshkov's instability of plasma-gas boundary, which arises when a heavy liquid is braked in
a light liquid, i.e. is determined by the same processes as those accompanying explosion of an
atomic bomb.
2. After 323 s the laser plume decomposed to the laser focus zone and a luminous cloud.
Long-time illumination of the cloud seemed to be caused by chemiluminescence, which was
unduced by growing of nanoparticles.
Formation of the vortices in the pulsed mode would seem to explain the small size of the
nanoparticles formed within the vortices, but this guess was disproved by the fact that YSZ
nanoparticles had the small size too, though the laser plume from YSZ target had absolutely
different shape (Fig. 10) and looked like a needle.
Duration and intensity of luminescence and the height of the plasma plume were
proportional to the laser radiation power. The plume cross-section was at first smaller and
then a little larger than focal spot size of the radiation.
Fig.10. The sequence of frame of the laser plume from YSZ target.
We believe this behavior was due to either clusters evaporation or formation of
nanoparticles very close to the target that could decrease the vapor pressure within the plume,
and thus prevented from expansion of the plume. Coalition of particles did not change their
direction and speed, while the mass of the particles increased. It is the coalition of particles
that leads to reduction of the rate of energy transmitted to the buffer gas and can explain the
larger height of the plume from YSZ target.
Thus, these obtained results suggested that formation of nanosized particles has a strong
influence on the dynamics of the laser plume. However, they did not explain the process of
small nanoparticles formation. Our new experiments when we are going to use a new laser
providing output radiation up to 20 kW and variable radiation duration are believed to give
the explanation.
This work was supported by the INTAS (Project No.03-51-3332).
Dr. M. Ivanov thanks the Russian Science Support Foundation and the Ural Department of
RAS for financial support.
CONCLUSIONS
1. A technology for production of pure complex composition nanopowders with a narrow
particle size distribution has been developed.
2. The experimental setup, which is based on an original pulse-repetitive CO2 laser, has the
following advantages:
– low energy consumption (25-10 Wh/g);
 small size of particles (9-15 nm), which is 4 times smaller than the size of particles
prepared with continuous lasers;
 most particles have sizes from 2 to 40 nm.
3. Nanopowders prepared with the pulse-repetitive CO2 laser, are weakly agglomerated and
the agglomerates can be easily disintegrated.
4. Formation of nanoparticles within the laser plume changes radically the plume shape,
dimensions and luminescence.
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