Control of nanoparticle size in RF thermal plasma synthesis
of silicon oxide starting from solid and liquid precursors
M. Boselli2, V. Colombo1,2, E. Ghedini1,2, M. Gherardi1, F. Lo Iacono1, F. Rotundo2, P. Sanibondi1, E. Traldi1
Alma Mater Studiorum-Università di Bologna
Department of Industrial Engineering (D.I.N.)
2
Industrial Research Centre for Advanced Mechanics and Materials (C.I.R.I.-M.A.M.)
Via Saragozza 8, 40123 Bologna, Italy
1
Abstract: An inductively coupled plasma torch (ICPT) system was used to synthesize
nanosilica particles starting both from glass powders from fluorescent lamps and tetraethyl
orthosilicate (TEOS) as precursors. The effect of the curtain gas on nanoparticles size,
morphology and composition was studied by the use of BET, SEM and EDS analysis.
Keywords: RF Plasma synthesis, fluorescent lamps, TEOS, nanosilica, quenching gas.
1. Introduction
Nanomaterials, i.e. materials that show nanoscale
morphology, have become increasingly important in
the last decades due to their particular characteristics in
comparison with bulk materials. Nanoparticles have
showed to be the starting point of many “bottom-up”
processes to realize nanomaterials for several
applications since their addition can strongly improve
the physical properties of almost all bulk materials
types. This improvement is given primarily by the
nanometric size, which leads to a surface to mass ratio
(order of several tens of m2/g) much higher than that of
micro-sized materials. For example, the improvement
of mechanical properties has been proved to be a
function of particle size [1]. Furthermore in the
nanoscale range the physical properties of
nanoparticles are changed, with respect to the bulk
material properties, due to quantum effects such as the
Surface Quantum Effect and Quantum confinement
size effect [2] that makes them very promising for
electronic and magnetic applications. In particular,
ceramic nanopowders like nanosilica and nanoalumina
(nano SiO₂, nano Al₂ ) have being studied and used
in many applications as medicine, concrete science,
nano-reinforcement, fuel cells [3-6]. As a consequence
of the growing request for these materials, the interest
for large-scale industrial systems for their synthesis is
strongly increased in the last years.
Between the different nanoparticles production
methods, thermal plasma synthesis is one of the most
promising, having potential for high production rates
(more than 1 kg/h) [7]. Moreover, in a plasma system
low valuable material can be used as precursors while
other processes, like flame pyrolysis and sol-gel, need
more expensive precursors. However plasma synthesis
shows a poor control of the nanoparticles composition
and size. Among all the plasma sources, the radio
frequency inductively coupled plasma torches are
characterized by higher operating conditions flexibility,
higher discharge stability and higher precursor
residence time into the core of the plasma discharge if
compared with the most widespread DC torches.
In this work, the synthesis of nanosilica powders has
been carried out using an inductively coupled plasma
torch (ICPT) system, starting from two different
precursors. In particular, micrometric glass powders
obtained from milled fluorescent lamps and a solution
of tetraethyl orthosilicate (TEOS) has been tested as
solid and liquid precursors, respectively. Since glass
from fluorescent lamps is categorized as waste, its
treatment in an ICPT would be of great economic
interest, going in the direction of producing high value
material from waste. On the other hand TEOS has been
chosen as precursor since it’s easier to vaporize and
can provide higher purity with respect to the previous
glass precursor.
The aim of this work is to identify the main process
parameters that influence the synthesis and the
characteristics of SiO2 nanopowders.
2. Experimental setup
As reported in Figure 1, the plasma system consists of
an inductively coupled plasma torch (Tekna PL-35)
powered by a 35 kW generator working at 3 MHz,
equipped with a reaction chamber designed for the
production of metallic nanoparticles and metallic
oxides with low deposition on chamber walls. The
chamber is composed of a conical part with two curtain
gas injectors, the first at 5 mm and the second at 50
mm downstream the torch nozzle and a cylindrical part
with a lateral outlet connected to a sampling filter,
where nanoparticles can be collected.
Table 1. Process parameters and filter yield for different
tests with solid precursor.
Name of
1A
the test
Precursor
A
type
Upper
0
curtain gas
(m³/h)
Lower
0
curtain gas
(m³/h)
Feed rate
6
(g/min)
Filter yield
N.A.
(g/h)
N.A. = Not Available
Figure 1. Schematic of the plasma nanoparticles synthesis
system.
A Tekna Plasma Systems powder feeder PF400 has
been used for the synthesis of silica nanopowders
starting from a solid glass precursor, injecting powders
in the plasma torch trough an injection probe. An
electrical oven has been used to pre-heat precursors
before injection.
A Tekna Plasma Systems suspension feeder SF-300
has been used for the liquid precursor injection using
an atomization probe sending atomized precursor
droplets in the core of the plasma discharge by means
of a carrier gas (Ar).
3. Nanoparticle and precursor characterization
Size, morphology and composition of synthetized
powders have been analysed. Specific surface area
(SSA) analysis was carried out using a NOVA 2200e
analyser (Quantachrome Instruments), based on BET
theory [8]. Before nitrogen adsorption, samples were
dried at 300°C and degassed. A mean diameter of
nanoparticles can be evaluated specific surface area
assuming spherical and dense particles and using the
following relation:
(1)
where D is the mean diameter and ρ is the density
(≈2300 kg/m³ for the glass of fluorescent lamps, ≈2650
kg/m³ for pure silica). A scanning electron microscope
(EVO 50 from ZEISS) was used to study the size and
morphology of precursor particles and of the produced
nanoparticles; analysis of the chemical composition of
the particles was carried out using Energy-Dispersive
X-ray Spectroscopy (EDS).
2B
3A
4A
5A
6A
B
A
A
A
A
3
3
10
0
0
0
0
0
0
12
6.5
2.3
1
N.A.
N.A.
21
25
25
5
21
4. Nanosilica synthesis from solid precursors
Micrometric glass powders used as precursor were
obtained from grinded fluorescent lamps with different
meshes (average diameter: precursor A = 38-75 μm
precursor B = 75-125 μm). The mercury content has
been removed from the glass powders before the
plasma treatment, for safety reasons. EDS analysis of
powders showed that they are mainly composed by
SiO₂ and Na₂O with small fraction of other elements as
metallic oxides (O 52.40%, Na 10.87%, Mg 1.50%, Al
1.14%, Si 30.19%, K 0.90%, Ca 2.99% by weight, with
only slight variations in different samples). The
samples were heated at a temperature of 180°C for one
hour in an oven to increase the powder flowability
during injection. Different tests for the production of
silica nanoparticles have been performed keeping fixed
plasma operating conditions and changing the curtain
gas flow conditions, precursor feed rate and precursor
type, as reported in Table 1. As a comparison between
different tests, the amount of nanoparticles collected in
the sampling filter has been measured and normalized
with respect to the duration of the test, thus obtaining a
mean collection rate in the sampling filter that will be
called “filter yield” in the next sections. Argon has
been used as carrier gas and plasma gas with a flow
rate of 6 slpm and 13 slpm, respectively. Air was used
for sheath gas, injected with a flow rate of 60 slpm. Air
has also been used as curtain gas in the reaction
chamber, with conditions reported in Table 1. Total
power of the RF system was set to 30 kW and the
reaction chamber operating pressure was set to 70 kPa.
Only one test has been done with precursor B since its
low vaporization efficiency induced the formation of
millimetric SiO₂ crystals in the chamber and near the
torch outlet.
Tests with precursor A have been carried out for
different conditions of the curtain gas. As can be seen
in Table 1 comparing test 5A with other ones, an
Table 2. Results of BET analysis
Table 3. Composition (w%) of samples collected in the filter
Name of
the test
Specific Surface
Area (m2/g)
Mean Diameter
(nm)
Test
O
Na
Mg
Al
Si
K
Ca
2B
51
22
0
0.6
24.2
1.4
0.6
1A
9.4
278
3A
51
18
0.6
0
27.9
1.2
0.7
2B
17.8
146
3A
18.6
140
Figure 2. SEM image of nanosilica powders synthesized in
test 3A
Figure 3. Crystalline particle in the sample of test 3A
increase in curtain gas flow rate results in a higher
amount of powders collected in the filter per unit time.
High precursor feed rates don’t ensure great
improvements on filter yield (test 2B and 3A) No
significant variation of filter yield has been obtained
increasing the upper curtain gas flow rate from 3 to 10
m3/h while reducing the precursor feed rate from 2.3 to
1 g/min (tests 3A and 4A). The use of the lower curtain
gas inlet is less effective than the upper one, as shown
from filter yield obtained in tests 4A and 6A. Results
from specific surface area (SSA) analysis and the
corresponding mean diameter have been reported in
Table 2. The use of curtain gas induces an increase of
SSA and a reduction of the mean diameter of particles
collected in the sampling filter (see Table 2).
Table 4. Operating parameters for different tests with liquid
precursor
Test
Sheath Gas
(slpm)
Curtain Gas
(m3/h)
Feed rate
(g/min)
T1
T2
T3
60 air
60 air
60 air
6 U/D
3 U/D
6 U/D
2.5
2.5
5
SEM images of the test 3A are reported in Figure 2 and
3, respectively. Though most of the particles were
spheroidized and nanometric, even with diameter
smaller than 40 nm (Figure 2), some crystalline
particles reached the filter (Figure 3). These crystalline
particles are micrometric and they reduce the value of
the specific surface area. Their presence could be the
effect of a not completed vaporization of the precursor
in the plasma torch. An excess or an inhomogeneous
feed rate could be the reasons leading to unevaporated
precursor. The EDS analysis of samples collected in
the filter from tests 2B and 3A (see Table 3) shows the
decrease of Si weight fraction and the increase of Na
and K after plasma treatment. The different boiling
points of Na and Si oxides (1950°C and 2230°C
respectively) lead to an easier nucleation of Na₂O
nanoparticles, penalizing the formation of SiO₂ and
then the quality of the final product. Also the presence
of N₂ may lead to this effect, reducing the partial
pressure of gaseous SiO and the nucleation of nano
SiO2 [9].
5. Nanosilica synthesis from liquid precursor
Synthesis tests have been carried out with liquid
precursor. The influence on the process of the liquid
feeding rate (regulated directly on the suspension
feeder) and of the curtain gas (injected from two
positions in the reaction chamber) has been evaluated.
Process parameters for different tests have been
reported in Table 4, whereas results for filter yield and
specific surface area are shown in Table 5. An increase
of the process yield can be observed for the cases with
higher injection of curtain gas (cases T1 and T3). Case
T1 reaches 130% of improvement of filter yield with
respect to case T2. As can be seen in Table 5,
synthetized powders collected in the filter have a lower
diameter with respect to that of powders obtained with
solid precursors, with BET values of 70-90 m2/g.
Table 5. Results of tests with liquid precursor
Test
Filter
yield (g/h)
SSA
(m2/g)
T1
T2
T3
3.8
1.44
3.89
90
70
60
Mean
diameter
(nm)
29
37
43
Figure 4. SEM image of nanosilica powders from filter
Table 6. Composition (w%) of samples collected in the filter
Test
C
O
Si
Au
T2
5.35
50.96
35.32
8.36
T3
3.74
48.95
37.78
9.52
In Figure 4, a SEM picture shows that all particles
collected are nanometric and that they show a tendency
to form agglomerates. It can be noticed from EDS that
a higher purity of product can be obtained from TEOS
with respect to that of powders obtained from solid
precursors (see Table 6). Comparison of results of test
T1 and test T3 (see Table 5) show that increasing the
feed rate the filter yield slightly changes and sampled
nanoparticles are characterized by slightly lower SSA.
6. Conclusion
Results show that the ratio between the filter yield and
the precursor feed rate is higher for solid than for liquid
precursors. However, non-vaporized precursor powders
have been collected in the filter in case of solid
precursor, whereas with liquid precursor only nanosized particles have been obtained. Further
simulative/experimental studies will be conducted to
determine operating conditions for the silica synthesis
process that guarantee optimal evaporation rate of solid
precursors as dependent from their dimension and mass
flow rate, also taking into account loading effects.
Moreover, it has been shown that the use of curtain
gases generally gives the possibility to collect a higher
amount of powders in the filter and at the same time
leads to a reduction of their mean diameters.
To better understand the effects of the curtain gases
composition on the chemical-physical process of
nanosilica synthesis further studies on the use of a
quenching gas need to be conducted.
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