72 4 Gas-Phase Synthesis of Nanoparticles
in the system. The design using needle electrodes will come to its full efficiency
at field strengths where the electron emission is so high that all particles that
carried a positive charge in the flame are negatively charged.
There is a broad variation of different designs for flame synthesis. An important
variation applies a liquid fuel, having the advantage that the precursor can be dissolved in the liquid fuel Typical examples are a solution of water-free chlorides in,
for example, acetonitrile, CH 3 CN or solutions of acetylacetonates, for example,
(C 5 H 8 O 2 ) 3 Al in appropriate organic solvents. An important criterion in selecting
organic liquid fuels the avoidance of soot formation, which is often observed in
the case of benzene, C 6 H 6 . On the other hand, mixtures of benzene with ethanol
have been applied successfully, too.
4.7
Synthesis of Coated Particles
Many applications of nanoparticles and nanomaterials need nanocomposites. The
required nanocomposites may be either multifunctional particles or bulk materials. In the case of multifunctional particles, one selects materials with different
properties for the core and the coating of the particles. Looking at bulk nanocomposites, in most cases it is necessary to obtain optimal properties such that the
particles are not touching. Both requirements can be fulfilled only with coated
particles. A series of possible applications were already explained in Chapter 2. To
synthesize coated particles, the process must fulfill a series of requirements. Most
important is the demand that the particles must not touch each other; otherwise,
one coats clusters of particles and not particles. Furthermore, in many instances,
the coating consists of organic compounds. Therefore, the temperature must be
so low that these compounds are not altered or even destroyed. The first process
succeeding in synthesizing coated nanoparticles and, until now, the only gas-phase
process fulfilling these challenges is the microwave plasma process [19, 20]. Figure
4.33 displays such an equipment, which is lastly a doubling of the design as presented in Figure 4.16.
The microwave plasma device to synthesize ceramically coated ceramic nanoparticles consists essentially of two microwave plasma devices working consecutively. It is important to ensure that the distance between these reactors is as small
as possible; otherwise, too large a number of the particles produced in the first
step will lose their positive electrical charge stemming from the synthesis process.
The risk of losing the positive charges is relatively high, as, to maintain electrical
neutrality, there are, also outside of the plasma zone, free electrons with low
energy in the system. This increases the risk of agglomeration. Losing the electrical charges of the particles can be minimized by using conditions with a long free
path length of the particles to reduce the probability of collisions. At each collision,
a free electron will lose energy in the range of 10 to 15 eV. Assuming an energy
of the free electron of approximately 1 keV, there are, at maximum, 100 collisions
allowed. Electrons with low energy will neutralize the particles and attach at the
in the system. The design using needle electrodes will come to its full efficiency
at field strengths where the electron emission is so high that all particles that
carried a positive charge in the flame are negatively charged.
There is a broad variation of different designs for flame synthesis. An important
variation applies a liquid fuel, having the advantage that the precursor can be dissolved in the liquid fuel Typical examples are a solution of water-free chlorides in,
for example, acetonitrile, CH 3 CN or solutions of acetylacetonates, for example,
(C 5 H 8 O 2 ) 3 Al in appropriate organic solvents. An important criterion in selecting
organic liquid fuels the avoidance of soot formation, which is often observed in
the case of benzene, C 6 H 6 . On the other hand, mixtures of benzene with ethanol
have been applied successfully, too.
4.7
Synthesis of Coated Particles
Many applications of nanoparticles and nanomaterials need nanocomposites. The
required nanocomposites may be either multifunctional particles or bulk materials. In the case of multifunctional particles, one selects materials with different
properties for the core and the coating of the particles. Looking at bulk nanocomposites, in most cases it is necessary to obtain optimal properties such that the
particles are not touching. Both requirements can be fulfilled only with coated
particles. A series of possible applications were already explained in Chapter 2. To
synthesize coated particles, the process must fulfill a series of requirements. Most
important is the demand that the particles must not touch each other; otherwise,
one coats clusters of particles and not particles. Furthermore, in many instances,
the coating consists of organic compounds. Therefore, the temperature must be
so low that these compounds are not altered or even destroyed. The first process
succeeding in synthesizing coated nanoparticles and, until now, the only gas-phase
process fulfilling these challenges is the microwave plasma process [19, 20]. Figure
4.33 displays such an equipment, which is lastly a doubling of the design as presented in Figure 4.16.
The microwave plasma device to synthesize ceramically coated ceramic nanoparticles consists essentially of two microwave plasma devices working consecutively. It is important to ensure that the distance between these reactors is as small
as possible; otherwise, too large a number of the particles produced in the first
step will lose their positive electrical charge stemming from the synthesis process.
The risk of losing the positive charges is relatively high, as, to maintain electrical
neutrality, there are, also outside of the plasma zone, free electrons with low
energy in the system. This increases the risk of agglomeration. Losing the electrical charges of the particles can be minimized by using conditions with a long free
path length of the particles to reduce the probability of collisions. At each collision,
a free electron will lose energy in the range of 10 to 15 eV. Assuming an energy
of the free electron of approximately 1 keV, there are, at maximum, 100 collisions
allowed. Electrons with low energy will neutralize the particles and attach at the