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012-On-line Measurement of the Size of Silicon Carbide (SiC) Nanoparticles
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On-line Measurement of the Size of Silicon Carbide (SiC) Nanoparticles
Introduction
Silicon carbide (SiC) has physical and chemical properties suited to the most demanding industries. Its
use is therefore very widespread, not just in electronics on account of its wide band semi-conductor
performance, but also in the nuclear field where it is used for its excellent resistance to ionising radiation
and its chemical and thermal stability.
This material may be synthesised using conventional powder metallurgy techniques. More recently, the production of SiC nanoparticles on an industrial scale has been carried out by dry process by means of
laser or plasma pyrolysis.
Figure 1: Laser pyrolysis nanoparticles synthsis
Laser pyrolysis
Laser pyrolysis consists in radiating a gas, a liquid or
a suspension by a CO2 laser beam in order to form
nanoparticles (figure 1).
This process is based on a step of absorption of the
infrared radiation of the laser by the molecular
compounds (gas, liquid or suspension) followed by
the homogenous nucleation and growth of the
nanoparticles.
The production of nanoparticles using this technique
enables yields greater than several hundreds of grams per hour to be attained.
Argon Argon
ArgonPrecursors
Laser beam
The syntheses of SiC nanoparticles by laser
pyrolysis developed at the CEA (French Commision
for Atomic Energy) are conducted using a silane, for
example SiH4, and acetylene, C2H4, as precursor
reagents.
This synthesis process produces nanoparticles in
the form of aggregates.
The input flows of the precursor gases thus makes it
possible to obtain structures organised into more or
less compact strings (figure 2).
Synthesis of SiC nanoparticles
200 nm
Figure 2: Aggregate morphology as a function of
synthesis parameters
012
On-line measurement of the size of aggregates
Within the scope of the SAPHIR (Safe NanoManufacturing) Project, which aims to produce and
characterise nanoparticles in complete safety for both the user and the environment, the CEA
synthesises SiC nanoparticles by laser pyrolysis, which are then dispersed in a Liquid Recovery System.
CILAS has developed an on-line particle size analyser for this installation in order to be able to control in
real time both the agglomeration and any drift in the size of the synthesised particles (figure 3).
Nanoparticles injection
Liquid Recovery System
Dispersing agent
Remplissage
automatiqueStirring device
Ultrasonic transducer
Analyzer cell
Drain
Recovering
Circulatingpump
Purging
Ultrasound probeCirculating pump
Sampling pump
Figure 3: Diagram of the on-line CILAS particle size analyser on the CEA Liquid Recovery System
This project has made it possible to validate the concepts of installing on-line particle size analysers and
also to validate the sampling, dilution, measurement and cleaning phases in complete safety.
Laser diffraction is an optical technique that is particularly well suited to measuring the size of particles,
particularly in liquid processes.
To enable this measurement to be carried out, an optimisation of the dispersion conditions of the powder
in the carrier liquid is often necessary to break down the agglomerates. To do this, the operator has to
make judicious choices as regards:
Measuring the size of SiC aggregates by laser diffraction
- The carrier liquid,- The chemical dispersant, - The physical dispersion and the use of ultrasounds.
In collaboration with the CEA, the dispersion of the SiC powder aggregated in water has been optimised
by using suitable experimental conditions, particularly the choice of the dispersant and the duration and
the power of the ultrasound treatment.
Figure 4: Effect of ultrasound treatment on the
agglomeration state of SiC aggregates
Particle size (µm)
Vo
lum
e (
%)
0
20
40
60
80
100
0.10 1.00 10.00
With ultrasounds treatment
Without ultrasounds treatment
Choice of the dispersant
Dispersion tests by chemical process have led
to the choice of polyethylenimine (PEI), a
cationic polymer, which absorbs on the surface
of the nanoparticles thus enabling a
stabilisation by steric effect.
Ultrasound conditions
The power and the duration of the ultrasound
treatment have been optimised in order to
disperse the nanoparticles as efficiently as
possible in aqueous medium.
A power of 200 W for 20 minutes makes it
possible to obtain a good dispersion, without
short term precipitation. The effect of ultrasounds makes it possible to improve the
dispersion of the nanoparticles and to obtain a
particle size distribution centred around 800
nm.
Conclusion
References
[1] Application of the laser pyrolysis to the synthesis of SiC, TiC and ZrC pre-ceramics nanopowders
Journal of Analytical and Applied Pyrolysis
Volume 79, p. 465–470, (2007)
[2] Role of the initial degree of ionization of polyethylenimine in the dispersion of silicon
carbide nanoparticles
Journal of the American Ceramic Society
Volume 86, Issue 1, p. 189–91, (2003)
[3] Dispersing SiC powder and improving its rheological behaviour
Journal of the European Ceramic Society
Volume 21, p. 2447–2451, (2001)
The characterisation of the size of nanoparticles represents a real difficulty and the
measurement of this parameter during the synthesis process itself is a key consideration,
both for properly controlling the synthesis process and also limiting the exposure of
operators to the nanoparticles.
The optimisation of the dispersion conditions of the nanoparticles leading to de-
agglomeration is a vital step in enabling the particles to be measured.
In the case of aggregated particles, as is the case in dry synthesis processes,
suspending the particles in liquid medium enables the size of the particles to be
measured by laser diffraction.
