More on Plasma

"The term "plasma" covers a huge range of systems whose density and temperature vary by many orders of magnitude. Some plasmas are very hot and all their microscopic species (ions, electrons, etc.) are in approximate thermal equilibrium through collisions including flame based plasmas.  Other plasmas, however, particularly those at low pressure (eg 100 Pa) where collisions are relatively infrequent, have their constituent species at widely different temperatures and are "non-thermal equilibrium" plasmas."

"Electrons have temperatures of many thousands of Kelvin (K) whilst neutral and ionic species remain cool.  Because the free electrons have almost negligible mass, the total system heat content is low and the plasma operates close to room temperature thus allowing the processing of temperature sensitive materials, such as plastics or polymers, without imposing a damaging thermal burden. The hot electrons create, through high energy collisions, a rich source of radicals and excited and/or unstable species with a high chemical potential energy capable of profound chemical and physical reactivity. It is this combination of low temperature operation plus high reactivity which makes non-thermal equilibrium plasma technologically important and a very powerful tool for manufacturing and material processing as it is capable of achieving processes which, if achievable at all without plasma, would require very high temperatures or noxious and aggressive chemicals."

"Because of their potential in industrial applications atmospheric pressure plasma (APP) systems are of particular interest to industry. APP includes atmospheric pressure non-thermal equilibrium plasma which is typically generated between two parallel electrodes that vary in size and configuration but which need to be within several millimeters distance from each other. Depending on the electrical circuitry and on system configurations, atmospheric pressure glow discharge (APGD) and/or dielectric barrier discharge (DBD) plasmas are generally produced. Advantageously, when compared to many plasma-based systems currently available, APP operates at about atmospheric pressure and at low temperatures (<200.degree. C. and preferably <100.degree. C.). However, limitations exist with respect to system geometry, because the plasma is produced in a plasma region between parallel electrodes with very small gaps between electrodes. It is ideally suited to treat flat, thin and flexible substrates like plastic films, textile webs, etc."

"In the case of powders using APGD type processes, one problem regarding the geometry of the system is that during the generation of powders, other species such as particles, by-products, reactants, and/or treated particles, may deposit on the electrodes, thereby negatively affecting the electrical and chemical properties of the plasma and potentially the duration of usefulness of the electrodes. Furthermore, the use and/or preparation of electrically conducting particles using APGD is difficult as such particles would interact with the electrical field and create filaments or local discharges and potentially adhere to electrode surfaces."

"New plasmas have been produced using gases passing between coaxial electrodes at high flow rates.  These gases form excited  gas mixtures at atmospheric pressure and are free of electrically charged ions.  This "atmospheric pressure post plasma discharge" has some of the physical characteristics of low pressure glow discharge but higher thermal energy, absence of boundary walls eg no electrodes, substantial absence of electrically charged species, large choice of gases and mixture of gases and large flow rate."

Bunce, Parbhoo and Chevalier of Dow Corning, US Patent 7,758,928, 7/20/2010