Thermal Plasma Processes and Nanomaterial Preparation
81
3.4 Transferred Arc Plasma and Nonmaterial
Preparation—System Details
As described in Sect. 4.3 plasma is created between cathode and anode, wherein
anode is the material of interest—for example, for producing nanoparticles of aluminium or aluminium compounds (aluminium oxide, aluminium nitride etc.), elemental aluminium metal is used as anode; for preparing zinc or zinc compound
nanomaterials, elemental zinc is used as anode and so on. The elemental metal (in
any form: blocks, powder, pellets, scraps etc.) is placed in a graphite or copper crucible and connected to the anode potential of the power supply. The entire electrode
assembly is housed inside a double-walled stainless steel chamber with multiple
ports for electrode housing, powder collection, visual monitoring of the process and
other necessary functions.
3.4.1 Nanomaterial Synthesis System
Inside the plasma zone both the evaporated material as well as the ambient gas/air
would be in ionised state. As shown in Fig. 3 these charged particles/metal vapour
move away from the plasma zone forming neutral atoms and then onto molecules and
clusters. Depending on the heat energy available at the clustering zone, the cluster size
could increase from few atoms/molecules to tens of thousands of atoms/molecules
resulting in the formation of either small nanoclusters of few nanometres or bigger
clusters of tens or hundreds of nanometres. The higher the temperature, the larger is
the cluster size. So to obtain nanometre size small clusters, it is essential to reduce the
ambient temperature inside the production chamber to a minimum. This is achieved
by the water circulation between the two walls of the synthesis chamber.
As shown in the figure, the nucleation and growth of the atoms/molecules
(cluster formation) occurs in the gas phase itself—homogeneous nucleation process—followed by agglomeration of the various clusters. These nanosized clusters
(nanoparticles) deposit itself on the inner surface of the synthesis chamber walls.
In short, the single-step process consists of evaporating a material from its solid
state and then recondensing it into small nanosize clusters. For evaporating the material, thermal plasma which has high enthalpy is used; and for an accelerated recondensation the chamber ambient is maintained at a low temperature. Higher chamber
ambient temperature would lead to continued growth of clusters into micron size.
The high temperature within the central plasma zone and low temperatures outside of
it create a sharp temperature gradient that is at the crux of the nanomaterial synthesis.
The process and the apparatus can be used to produce varied nanomaterials. A
schematic of a typical nanomaterial synthesis chamber is shown in Fig. 4. When the
entire process of evaporation, nucleation and growth is done in air or oxygen medium,
the metal vapours react with them to form metal oxide nanostructures. If, on the other
hand, the air in the synthesis chamber is evacuated and filled with argon or helium
81
3.4 Transferred Arc Plasma and Nonmaterial
Preparation—System Details
As described in Sect. 4.3 plasma is created between cathode and anode, wherein
anode is the material of interest—for example, for producing nanoparticles of aluminium or aluminium compounds (aluminium oxide, aluminium nitride etc.), elemental aluminium metal is used as anode; for preparing zinc or zinc compound
nanomaterials, elemental zinc is used as anode and so on. The elemental metal (in
any form: blocks, powder, pellets, scraps etc.) is placed in a graphite or copper crucible and connected to the anode potential of the power supply. The entire electrode
assembly is housed inside a double-walled stainless steel chamber with multiple
ports for electrode housing, powder collection, visual monitoring of the process and
other necessary functions.
3.4.1 Nanomaterial Synthesis System
Inside the plasma zone both the evaporated material as well as the ambient gas/air
would be in ionised state. As shown in Fig. 3 these charged particles/metal vapour
move away from the plasma zone forming neutral atoms and then onto molecules and
clusters. Depending on the heat energy available at the clustering zone, the cluster size
could increase from few atoms/molecules to tens of thousands of atoms/molecules
resulting in the formation of either small nanoclusters of few nanometres or bigger
clusters of tens or hundreds of nanometres. The higher the temperature, the larger is
the cluster size. So to obtain nanometre size small clusters, it is essential to reduce the
ambient temperature inside the production chamber to a minimum. This is achieved
by the water circulation between the two walls of the synthesis chamber.
As shown in the figure, the nucleation and growth of the atoms/molecules
(cluster formation) occurs in the gas phase itself—homogeneous nucleation process—followed by agglomeration of the various clusters. These nanosized clusters
(nanoparticles) deposit itself on the inner surface of the synthesis chamber walls.
In short, the single-step process consists of evaporating a material from its solid
state and then recondensing it into small nanosize clusters. For evaporating the material, thermal plasma which has high enthalpy is used; and for an accelerated recondensation the chamber ambient is maintained at a low temperature. Higher chamber
ambient temperature would lead to continued growth of clusters into micron size.
The high temperature within the central plasma zone and low temperatures outside of
it create a sharp temperature gradient that is at the crux of the nanomaterial synthesis.
The process and the apparatus can be used to produce varied nanomaterials. A
schematic of a typical nanomaterial synthesis chamber is shown in Fig. 4. When the
entire process of evaporation, nucleation and growth is done in air or oxygen medium,
the metal vapours react with them to form metal oxide nanostructures. If, on the other
hand, the air in the synthesis chamber is evacuated and filled with argon or helium