4.2 Inert-Gas Condensation Process 47
4.2
Inert-Gas Condensation Process
Historically most important, the first process for synthesizing nanoparticles in the
gas phase is the inert-gas condensation process (also called evaporation and condensation process) [3], applying thermal evaporation of a metal within a vacuum
chamber filled with a small amount of inert gas. To produce metal particles, for
example, gold nanoparticles, gold is evaporated in a boat at sufficiently high temperature. The atoms of the gold vapor emanated from the boat are thermalized
and lose energy by collisions with the atoms of the inert gas. Collisions of gold
atoms with others lead to nucleation and subsequently to particle formation. The
particles formed in the gas phase move by thermophoresis (this process is also
called thermodiffusion) to a cold finger, where they are collected. Figure 4.7 displays the general layout of such equipment.
At the end of a production cycle, the metal particles are carefully scraped from
the cold finger. If necessary, this design allows further processing of the product
without breaking the extremely pure conditions. To obtain oxides, before scraping
the powder from the cold finger, minor amounts of oxygen are introduced into
Box 4.5 Collision of Electrically Charged Particles
Two particles with diameters d 1 and d 2 at a distance r carrying electric charges
Q 1 and Q 2 of equal sign repel each other with the force F:
F
Q Q
r
r d d
=
=
1 2
2
2
2
1 2
1
κ
.
(4.10)
This force results in an deceleration
Q Q
mr
1 2
2
reducing speed and changing the
direction of the path of the particles. This reduces the collision volume passed
in the time interval Δt. To a first approximation, the reduction of the
collision volume is given by the factor
1
1
1 2
1 2
Q Q
d d
∝
. Inserting this factor into
Eq. (4.7) leads to the following modified collision parameter:
p
p p
T d d
d d
T d d
1 2
1 2
1 2
0 5
1 2
1 2
0 5
1
−
−
=
=
=
const
const
(
)
(
) .
.
.
(4.11)
As a consequence of the introduction of a repulsion term, Eq. (4.11) now
describes a reduced collision probability with increasing particle size. This
limits particle growth by coagulation and agglomeration, especially for large
particles, as the probability for collision gets smaller with increasing particle
size. The temperature dependence remains unchanged as linear. The collision
parameter for charged (d 1 d 2 )
−0.5 particle is inversely to that for neutral
particles.
4.2
Inert-Gas Condensation Process
Historically most important, the first process for synthesizing nanoparticles in the
gas phase is the inert-gas condensation process (also called evaporation and condensation process) [3], applying thermal evaporation of a metal within a vacuum
chamber filled with a small amount of inert gas. To produce metal particles, for
example, gold nanoparticles, gold is evaporated in a boat at sufficiently high temperature. The atoms of the gold vapor emanated from the boat are thermalized
and lose energy by collisions with the atoms of the inert gas. Collisions of gold
atoms with others lead to nucleation and subsequently to particle formation. The
particles formed in the gas phase move by thermophoresis (this process is also
called thermodiffusion) to a cold finger, where they are collected. Figure 4.7 displays the general layout of such equipment.
At the end of a production cycle, the metal particles are carefully scraped from
the cold finger. If necessary, this design allows further processing of the product
without breaking the extremely pure conditions. To obtain oxides, before scraping
the powder from the cold finger, minor amounts of oxygen are introduced into
Box 4.5 Collision of Electrically Charged Particles
Two particles with diameters d 1 and d 2 at a distance r carrying electric charges
Q 1 and Q 2 of equal sign repel each other with the force F:
F
Q Q
r
r d d
=
=
1 2
2
2
2
1 2
1
κ
.
(4.10)
This force results in an deceleration
Q Q
mr
1 2
2
reducing speed and changing the
direction of the path of the particles. This reduces the collision volume passed
in the time interval Δt. To a first approximation, the reduction of the
collision volume is given by the factor
1
1
1 2
1 2
Q Q
d d
∝
. Inserting this factor into
Eq. (4.7) leads to the following modified collision parameter:
p
p p
T d d
d d
T d d
1 2
1 2
1 2
0 5
1 2
1 2
0 5
1
−
−
=
=
=
const
const
(
)
(
) .
.
.
(4.11)
As a consequence of the introduction of a repulsion term, Eq. (4.11) now
describes a reduced collision probability with increasing particle size. This
limits particle growth by coagulation and agglomeration, especially for large
particles, as the probability for collision gets smaller with increasing particle
size. The temperature dependence remains unchanged as linear. The collision
parameter for charged (d 1 d 2 )
−0.5 particle is inversely to that for neutral
particles.