Thermal Plasma Processes and Nanomaterial Preparation
75
sustained heating leads to the separation of H 2 O molecule to its constituent atoms
of H and O. These atoms on further heating lead to the removal of electrons from
H and O atoms, resulting in the generation of negatively charged electrons (e
− ) and
positively charged O
+ and H
+ atoms. This collection of positive and negative charges
is defined as plasma state. As can be seen, as one move from solid state to plasma
state the temperature (thermal energy) increases steadily with plasma state having
the highest enthalpy.
Since opposite charges attract each other and gets annihilated, one needs to pump
in energy to maintain the charge separation and sustain the plasma state. There are
a few prerequisites for a group of negatively and positively charged particles to be
certified as plasma state—which are not part of this chapter.
2.1 Plasma and Material Processing
The interaction of the fourth state (plasma) with the first state (solid) has wideranging common industrial as well as societal applications (Fauchais et al. 2008):
plasma cutting/welding, plasma pyrolysis (Huang and Tang 2007), plasma smelting
and so on. It also has bio-medical applications in the fields of dentistry, integumentary
system and so on. Modifications like etching, grafting of various surfaces are also
used to impart functionalities, like hydrophobicity, hydrophylicity and so on, to the
surface.
Based solely on the heat content/temperature, plasmas can be broadly classified
into two categories: high temperature/thermal plasmas and low temperature/cold
plasmas. The thermal energy content of high-temperature plasmas can go from hundreds of degree centigrade to tens of thousands of degree centigrade. All stars (including sun) consist of thermal plasmas. On the other hand, interstellar space plasmas,
plasmas found in fluorescent bulbs are all examples of low-temperature plasmas.
Plasmas can also be classified depending on the process by which it is generated. For example, creation of plasma using microwave source is microwave plasma,
inductively heated source leads to inductively coupled plasma, dielectric barrier discharge plasma and so on. Here in this chapter we would only discuss the properties
and applications of plasmas classified on the basis of heat content; that is thermal
and non-thermal plasmas.
The heat content in plasma is decided by the ambient pressure: Low pressure (few
mbar or lower) plasma results in non-thermal/cold plasmas, whereas high pressure
(few hundred mbar or more) would result in high temperature thermal plasmas. In
the high pressure scenario, the particle (both charged as well as neutral) density
would be higher resulting in higher collision between particles and consequent rise
in temperature. On the other hand, in a low pressure system the particles would be
able to move without much or significantly reduced number of collisions, and thereby
having a higher kinetic energy rather than having heat energy.
As can be deduced, thermal plasma is used for heat-intense applications, like
welding, cutting, pyrolysis, smelting and so on and non-thermal plasmas are used
75
sustained heating leads to the separation of H 2 O molecule to its constituent atoms
of H and O. These atoms on further heating lead to the removal of electrons from
H and O atoms, resulting in the generation of negatively charged electrons (e
− ) and
positively charged O
+ and H
+ atoms. This collection of positive and negative charges
is defined as plasma state. As can be seen, as one move from solid state to plasma
state the temperature (thermal energy) increases steadily with plasma state having
the highest enthalpy.
Since opposite charges attract each other and gets annihilated, one needs to pump
in energy to maintain the charge separation and sustain the plasma state. There are
a few prerequisites for a group of negatively and positively charged particles to be
certified as plasma state—which are not part of this chapter.
2.1 Plasma and Material Processing
The interaction of the fourth state (plasma) with the first state (solid) has wideranging common industrial as well as societal applications (Fauchais et al. 2008):
plasma cutting/welding, plasma pyrolysis (Huang and Tang 2007), plasma smelting
and so on. It also has bio-medical applications in the fields of dentistry, integumentary
system and so on. Modifications like etching, grafting of various surfaces are also
used to impart functionalities, like hydrophobicity, hydrophylicity and so on, to the
surface.
Based solely on the heat content/temperature, plasmas can be broadly classified
into two categories: high temperature/thermal plasmas and low temperature/cold
plasmas. The thermal energy content of high-temperature plasmas can go from hundreds of degree centigrade to tens of thousands of degree centigrade. All stars (including sun) consist of thermal plasmas. On the other hand, interstellar space plasmas,
plasmas found in fluorescent bulbs are all examples of low-temperature plasmas.
Plasmas can also be classified depending on the process by which it is generated. For example, creation of plasma using microwave source is microwave plasma,
inductively heated source leads to inductively coupled plasma, dielectric barrier discharge plasma and so on. Here in this chapter we would only discuss the properties
and applications of plasmas classified on the basis of heat content; that is thermal
and non-thermal plasmas.
The heat content in plasma is decided by the ambient pressure: Low pressure (few
mbar or lower) plasma results in non-thermal/cold plasmas, whereas high pressure
(few hundred mbar or more) would result in high temperature thermal plasmas. In
the high pressure scenario, the particle (both charged as well as neutral) density
would be higher resulting in higher collision between particles and consequent rise
in temperature. On the other hand, in a low pressure system the particles would be
able to move without much or significantly reduced number of collisions, and thereby
having a higher kinetic energy rather than having heat energy.
As can be deduced, thermal plasma is used for heat-intense applications, like
welding, cutting, pyrolysis, smelting and so on and non-thermal plasmas are used