Nonthermal plasma
Encyclopedia
A nonthermal plasma is in general any plasma
which is not in thermodynamic equilibrium
, either because the ion temperature is different from the electron temperature, or because the velocity distribution of one of the species does not follow a Maxwell-Boltzmann distribution.
, a nonthermal plasma (NTP) is specifically an antimicrobial
treatment being investigated for application to fruits, vegetables and other foods with fragile surfaces.
These foods are either not adequately sanitized or are otherwise unsuitable for treatment with chemicals, heat or other conventional food processing tools. The term cold plasma has been recently used as a convenient descriptor to distinguish the one-atmosphere
, near room temperature
plasma discharges from other plasmas, operating at hundreds or thousands of degrees above ambient (see here for more on plasma temperatures). Within the context of food processing the term "cold" can potentially engender misleading images of refrigeration requirements as a part of the plasma treatment. However, in practice this confusion has not been an issue.
Nonthermal plasma also sees increasing use in the sterilization of teeth and hands.
", "plasma needle", "plasma jet", "dielectric barrier discharge", etc.), while other names are more generally descriptive, based on the characteristics of the plasma generated ("one atmosphere uniform glow discharge plasma", "atmospheric plasma", "ambient pressure nonthermal discharges", "non-equilibrium atmospheric pressure plasmas", etc.). The two features which distinguish NTP from other mature, industrially applied plasma technologies, is that they are 1) nonthermal and 2) operate at or near atmospheric pressure. An upcoming technology will add the capabilities of nonthermal plasma to dentistry
.
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
which is not in thermodynamic equilibrium
Thermodynamic equilibrium
In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. The word equilibrium means a state of balance...
, either because the ion temperature is different from the electron temperature, or because the velocity distribution of one of the species does not follow a Maxwell-Boltzmann distribution.
Applications
In the context of food processingFood processing
Food processing is the set of methods and techniques used to transform raw ingredients into food or to transform food into other forms for consumption by humans or animals either in the home or by the food processing industry...
, a nonthermal plasma (NTP) is specifically an antimicrobial
Antimicrobial
An anti-microbial is a substance that kills or inhibits the growth of microorganisms such as bacteria, fungi, or protozoans. Antimicrobial drugs either kill microbes or prevent the growth of microbes...
treatment being investigated for application to fruits, vegetables and other foods with fragile surfaces.
These foods are either not adequately sanitized or are otherwise unsuitable for treatment with chemicals, heat or other conventional food processing tools. The term cold plasma has been recently used as a convenient descriptor to distinguish the one-atmosphere
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
, near room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
plasma discharges from other plasmas, operating at hundreds or thousands of degrees above ambient (see here for more on plasma temperatures). Within the context of food processing the term "cold" can potentially engender misleading images of refrigeration requirements as a part of the plasma treatment. However, in practice this confusion has not been an issue.
Nonthermal plasma also sees increasing use in the sterilization of teeth and hands.
Nomenclature
The nomenclature for nonthermal plasma found in the scientific literature is varied. In some cases, the plasma is referred to by the specific technology used to generate it ("gliding arc", "plasma pencilPlasma pencil
The plasma pencil is a dielectric tube where two disk-shaped electrodes of about the same diameter as the tube are inserted, and are separated by a small gap. Each of the two electrodes is made of a thin copper ring attached to the surface of a centrally perforated dielectric disk...
", "plasma needle", "plasma jet", "dielectric barrier discharge", etc.), while other names are more generally descriptive, based on the characteristics of the plasma generated ("one atmosphere uniform glow discharge plasma", "atmospheric plasma", "ambient pressure nonthermal discharges", "non-equilibrium atmospheric pressure plasmas", etc.). The two features which distinguish NTP from other mature, industrially applied plasma technologies, is that they are 1) nonthermal and 2) operate at or near atmospheric pressure. An upcoming technology will add the capabilities of nonthermal plasma to dentistry
Dentistry
Dentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders and conditions of the oral cavity, maxillofacial area and the adjacent and associated structures and their impact on the human body. Dentistry is widely considered...
.
Technologies
| NTP Technology Class | |||
|---|---|---|---|
| I. Remote treatment | II. Direct treatment | III. Electrode contact | |
| Nature of NTP applied | Decaying plasma (afterglow) - longer lived chemical species | Active plasma - short and long-lived species | Active plasma - all chemical species, including shortest lived and ion bombardment |
| NTP density and energy | Moderate density - target remote from electrodes. However, a larger volume of NTP can be generated using multiple electrodes | Higher density - target in the direct path of a flow of active NTP | Highest density - target within NTP generation field |
| Spacing of target from NTP-generating electrode | Approx. 5 – 20 cm; arcing (filamentous discharge) unlikely to contact target at any power setting | Approx. 1 – 5 cm; arcing can occur at higher power settings, can contact target | Approx. ≤ 1 cm; arcing can occur between electrodes and target at higher power settings |
| Electrical conduction through target | No | Not under normal operation, but possible during arcing | Yes, if target is used as an electrode OR if target between mounted electrodes is electrically conductive |
| Suitability for irregular surfaces | High - remote nature of NTP generation means maximum flexibility of application of NTP afterglow stream | Moderately high - NTP is conveyed to target in a directional manner, requiring either rotation of target or multiple NTP emitters | Moderately low - close spacing is required to maintain NTP uniformity. However, electrodes can be shaped to fit a defined, consistent surface. |
| Examples of technologies | Remote exposure reactor, plasma pencil | Gliding arc; plasma needle; microwave-induced plasma tube | Parallel plate reactor; needle-plate reactor; resistive barrier discharge; dielectric barrier discharge Dielectric barrier discharge Dielectric-barrier discharge is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Originally called silent discharge and also known as ozone production discharge or partial discharge, it was first reported by Ernst Werner von Siemens in 1857.- Process... |
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