Low-temperature plasma. Basic concepts, properties, application.
|Plasma is the quasineutral gas of charged and neutral particles, the concentration of which is sufficient to ensure that the space charge created by them limits their movement.
To fulfill this condition, it is necessary that the characteristic dimension of plasma L0 be much larger than the characteristic screening size (Debye radius) rD – the distance, at which quasineutrality is violated due to the thermal motion of particles. If, during a chaotic motion with thermal velocities, the charges are separated by a distance greater than rD, the quasineutrality of the medium is restored by the action of electric fields of the space charge.
The term «quasineutral plasma» means that the number of uncompensated charges in the volume L03 is much less than the total number of charges.
|Depending on the plasma parameters, there are different methods of describing it.
There is weakly ionized, highly ionized, and fully ionized plasma, distinguished by the degree of ionization, which is determined by the relative concentration of electrons ne ⁄ n0, with n0 being the number of heavy particles – i.e. atoms and ions.
|The plasma used in technological processes is mainly weakly ionized plasma. In weakly ionized plasma, the chemical kinetics, transport, collisions, and other elementary processes are determined by the individual properties of neutral atoms and molecules. Thus, there are plasma-forming media, for example, inert gases, hydrogen, nitrogen, oxygen, air, water vapor, etc.
|When used as a plasma-forming medium for molecular gases, the energy from the external source is first transferred to the electronic component of the plasma, then the electron gas transfers the energy received to heavy particles during the heating process, excitation of internal degrees of freedom of atoms and molecules, ionization, and dissociation with the formation of a large number of reactive particles. It is these plasma states in combination with thermodynamic, optical and transport properties that determine its chemical and catalytic activity, that is, the ability to produce targeted chemical changes in the initial gas medium or in a substance placed in the plasma, in the implementation of various chemical transformations in technological processes. At the same time, the electronic component of the plasma has a defining role.
Note. The processes of mass, charge, momentum, and energy transfer in the plasma, caused by the inhomogeneity of its parameters, that is, the presence of spatial gradients of density, velocity, and temperature, as well as the presence of external fields (for example, electric and magnetic fields), are collectively called transfer (transport) phenomena.
|The devices called low-temperature plasma generators or plasma torches allow to obtain plasma with a temperature of 7,000-30,000 K, realize its stabilization in space (actually used plasma always has finite dimensions), and practically use it in all gaseous media.
Plasma torches are divided into four large groups by the type of electrical discharge, the frequency of the supply current, and the type of power source: arc plasma torches (direct or alternating current), high-frequency plasma torches (induction or capacitive), ultrahigh-frequency plasma torches, and hybrid plasma torches.
|The maximum values of plasma temperature in different plasma-forming gases are different, but close in magnitude, and depend on the power per unit volume of plasma formation.
Technologies for the application of low-temperature plasma are based on the interaction of plasma with condensed substance (solid, liquid or dispersed), gas, and electromagnetic field. At present, low-temperature plasma applications belong to a field of technology, which is hardly foreseeable. To note, it concerns only those that are directly or indirectly related to the activities of PLAZARIUM company. These are various applications of low-temperature plasma as a working fluid in systems of conversion of thermal energy into electrical energy; creation of devices and plasma generators, in which the electric arc is used to create plasma jets; the application of low-temperature plasma as an active medium; plasma energy technologies of fuel processing; the use of plasma for the production of dispersed materials, the processing of powders, and the production of coatings, the modification of polymeric materials surfaces; the use of plasma for solving various problems of applied chemistry (the term “plasmochemistry” is often used); and the application of low-temperature plasma in ecology.
|This section is intended for non-specialists who want to better understand the role of plasma methods in modern technology and their development in the short term. For those who wish to communicate with the primary sources of information on this section: Encyclopedia of low-temperature plasma. Introductory volume 1-5. – Nauka, 2000.