48 4 Gas-Phase Synthesis of Nanoparticles
the system to oxidize the metal powder slowly. Careful oxidation is essential, as
otherwise the product is overheated and sintered. As the formation of the particles
is a purely random process, typically, the inert-gas evaporation process leads to a
product with broad particle-size distribution; an example is given in Figure 4.4.
4.3
Physical and Chemical Vapor Synthesis Processes
The basic principle of the inert-gas condensation process led to many variants.
These systems differ in how the metal is introduced into the system and evaporated. Technical upscaling of the inert-gas condensation process needs elements
to limit particle size growth to avoid the formation of the long tail of the size
distribution function on the side of the large particles. There are two possible
measures to control particle size and particle-size distribution: (i) Reduction of the
residence time of the particles in the reaction zone and (ii) quenching, rapid
cooling of the particles after leaving the reaction zone. Both measures disrupt the
originally diffusion-controlled process. To bring these measures into action, it is
necessary to replace the transport by thermal diffusion by transport using a carrier
Figure 4.7 Typical setup for synthesis using
the inert-gas condensation process. In a
vacuum vessel, filled, at reduced pressure,
with an inert gas, a metal is evaporated. In
the gas atmosphere, the metal vapor is
thermalized. The metal atoms lose thermal
energy by collision with the inert gas atoms,
and form nanoparticles. In a temperature
gradient, the product moves to a liquidnitrogen-cooled finger by thermophoresis
and is collected from the surface of the cold
finger.
Vacuum vessel
Liquid-nitrogencooled finger
Aerosol of metal
parƟcles
Metal for
evaporaƟon
Heated boat
To pumping system
and gas inlet
the system to oxidize the metal powder slowly. Careful oxidation is essential, as
otherwise the product is overheated and sintered. As the formation of the particles
is a purely random process, typically, the inert-gas evaporation process leads to a
product with broad particle-size distribution; an example is given in Figure 4.4.
4.3
Physical and Chemical Vapor Synthesis Processes
The basic principle of the inert-gas condensation process led to many variants.
These systems differ in how the metal is introduced into the system and evaporated. Technical upscaling of the inert-gas condensation process needs elements
to limit particle size growth to avoid the formation of the long tail of the size
distribution function on the side of the large particles. There are two possible
measures to control particle size and particle-size distribution: (i) Reduction of the
residence time of the particles in the reaction zone and (ii) quenching, rapid
cooling of the particles after leaving the reaction zone. Both measures disrupt the
originally diffusion-controlled process. To bring these measures into action, it is
necessary to replace the transport by thermal diffusion by transport using a carrier
Figure 4.7 Typical setup for synthesis using
the inert-gas condensation process. In a
vacuum vessel, filled, at reduced pressure,
with an inert gas, a metal is evaporated. In
the gas atmosphere, the metal vapor is
thermalized. The metal atoms lose thermal
energy by collision with the inert gas atoms,
and form nanoparticles. In a temperature
gradient, the product moves to a liquidnitrogen-cooled finger by thermophoresis
and is collected from the surface of the cold
finger.
Vacuum vessel
Liquid-nitrogencooled finger
Aerosol of metal
parƟcles
Metal for
evaporaƟon
Heated boat
To pumping system
and gas inlet