A nonthermal plasma, cold plasma or non-equilibrium plasma is a plasma which is not in thermodynamic equilibrium, because the electron temperature is much hotter than the temperature of heavy species (ions and neutrals). As only electrons are thermalized, their Maxwell-Boltzmann velocity distribution is very different from the ion velocity distribution. [1] When one of the velocities of a species does not follow a Maxwell-Boltzmann distribution, the plasma is said to be non-Maxwellian.
A kind of common nonthermal plasma is the mercury-vapor gas within a fluorescent lamp, where the "electron gas" reaches a temperature of 20,000 K (19,700 °C ; 35,500 °F ) while the rest of the gas, ions and neutral atoms, stays barely above room temperature, so the bulb can even be touched with hands while operating.
In the context of food processing, a nonthermal plasma (NTP) or cold plasma is specifically an antimicrobial treatment being investigated for application to fruits, vegetables and meat products with fragile surfaces. [2] These foods are either not adequately sanitized or are otherwise unsuitable for treatment with chemicals, heat or other conventional food processing tools. While the applications of nonthermal plasma were initially focused on microbiological disinfection, [3] newer applications such as enzyme inactivation, biomolecule oxidation, protein modification, prodrug activation, and pesticide dissipation are being actively researched. [4] [5] [6] [7] Nonthermal plasma also sees increasing use in the sterilization of teeth [8] [9] and hands, [10] in hand dryers [11] as well as in self-decontaminating filters. [12]
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 Plasma (physics) § Temperature. 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. "Cold plasmas" may also loosely refer to weakly ionized gases (degree of ionization < 0.01%).
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 pencil", "plasma needle", "plasma jet", "dielectric barrier discharge", "piezoelectric direct discharge plasma", 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.
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 |
References | [16] [17] [18] [19] [20] | [21] [22] [23] [24] [17] [18] |
An emerging field adds the capabilities of nonthermal plasma to dentistry and medicine. Cold plasma is used to treat chronic wounds. [25]
Magnetohydrodynamic power generation, a direct energy conversion method from a hot gas in motion within a magnetic field was developed in the 1960s and 1970s with pulsed MHD generators known as shock tubes, using non-equilibrium plasmas seeded with alkali metal vapors (like caesium, to increase the limited electrical conductivity of gases) heated at a limited temperature of 2000 to 4000 kelvins (to protect walls from thermal erosion) but where electrons were heated at more than 10,000 kelvins. [26] [27] [28] [29]
A particular and unusual case of "inverse" nonthermal plasma is the very high temperature plasma produced by the Z machine, where ions are much hotter than electrons. [30] [31]
Aerodynamic active flow control solutions involving technological nonthermal weakly ionized plasmas for subsonic, supersonic and hypersonic flight are being studied, as plasma actuators in the field of electrohydrodynamics, and as magnetohydrodynamic converters when magnetic fields are also involved. [32]
Studies conducted in wind tunnels involve most of the time low atmospheric pressure similar to an altitude of 20–50 km, typical of hypersonic flight, where the electrical conductivity of air is higher, hence non-thermal weakly ionized plasmas can be easily produced with a fewer energy expense.[ citation needed ]
Atmospheric pressure non-thermal plasma can be used to promote chemical reactions. Collisions between hot temperature electrons and cold gas molecules can lead to dissociation reactions and the subsequent formation of radicals. This kind of discharge exhibits reacting properties that are usually seen in high temperature discharge systems. [33] Non-thermal plasma is also used in conjunction with a catalyst to further enhance the chemical conversion of reactants or to alter the products chemical composition.
Among the different application fields, there are ozone production [34] at a commercial level; pollution abatement, both solid (PM, VOC) and gaseous (SOx, NOx); [35] CO2 conversion [36] in fuels (methanol, syngas) or value added chemicals; nitrogen fixation; methanol synthesis; liquid fuels synthesis from lighter hydrocarbons (e.g. methane), [37] hydrogen production via hydrocarbons reforming [38]
The coupling between the two different mechanisms can be done in two different ways: two-stage configuration, also called post-plasma catalysis (PPC) and one-stage configuration, also called in-plasma catalysis (IPC) or plasma enhanced catalysis (PEC).
In the first case the catalytic reactor is placed after the plasma chamber. This means that only the long-lived species can reach the catalyst surface and react, while short-lived radicals, ions and excited species decay in the first part of the reactor. As an example, the oxygen ground state atom O(3P) has a lifetime of about 14 μs [39] in a dry air atmospheric pressure plasma. This means that only a small region of the catalyst is in contact with active radicals. In a such two-stage set-up, the main role of the plasma is to alter the gas composition fed to the catalytic reactor. [40] In a PEC system, synergistic effects are greater since short-lived excited species are formed near the catalyst surface. [41] The way the catalyst is inserted in the PEC reactor influence the overall performance. It can be placed inside the reactor in different ways: in powder form (packed bed), deposited on foams, deposited on structured material (honeycomb), and coating of the reactor walls
Packed bed plasma-catalytic reactor are commonly used for fundamental studies [33] and a scale-up to industrial applications is difficult since the pressure drop increase with the flow rate.
In a PEC system, the way the catalyst is positioned in relation to the plasma can affect the process in different ways. The catalyst can positively influence the plasma and vice versa resulting in an output that cannot be obtained using each process individually. The synergy that is established is ascribed to different cross effects. [42] [43] [38] [44] [45]
Catalyst effects on plasma are mostly related to the presence of a dielectric material inside the discharge region and do not necessarily require the presence of a catalyst.
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
A plasma afterglow is the radiation emitted from a plasma after the source of ionization is removed. The external electromagnetic fields that sustained the plasma glow are absent or insufficient to maintain the discharge in the afterglow. A plasma afterglow can either be a temporal, due to an interrupted (pulsed) plasma source, or spatial, due to a distant plasma source. In the afterglow, plasma-generated species de-excite and participate in secondary chemical reactions that tend to form stable species. Depending on the gas composition, super-elastic collisions may continue to sustain the plasma in the afterglow for a while by releasing the energy stored in rovibronic degrees of freedom of the atoms and molecules of the plasma. Especially in molecular gases, the plasma chemistry in the afterglow is significantly different from the plasma glow. The afterglow of a plasma is still a plasma and as thus retains most of the properties of a plasma.
Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target which deposits it as a thin film on a substrate. This process can occur in ultra high vacuum or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films.
Thermogravimetric analysis or thermal gravimetric analysis (TGA) is a method of thermal analysis in which the mass of a sample is measured over time as the temperature changes. This measurement provides information about physical phenomena, such as phase transitions, absorption, adsorption and desorption; as well as chemical phenomena including chemisorptions, thermal decomposition, and solid-gas reactions.
The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:
An atmospheric pressure discharge is an electrical discharge in air or another gas at atmospheric pressure.
Plasma activation is a method of surface modification employing plasma processing, which improves surface adhesion properties of many materials including metals, glass, ceramics, a broad range of polymers and textiles and even natural materials such as wood and seeds. Plasma functionalization also refers to the introduction of functional groups on the surface of exposed materials. It is widely used in industrial processes to prepare surfaces for bonding, gluing, coating and painting. Plasma processing achieves this effect through a combination of reduction of metal oxides, ultra-fine surface cleaning from organic contaminants, modification of the surface topography and deposition of functional chemical groups. Importantly, the plasma activation can be performed at atmospheric pressure using air or typical industrial gases including hydrogen, nitrogen and oxygen. Thus, the surface functionalization is achieved without expensive vacuum equipment or wet chemistry, which positively affects its costs, safety and environmental impact. Fast processing speeds further facilitate numerous industrial applications.
Dielectric-barrier discharge (DBD) is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Originally called silent (inaudible) discharge and also known as ozone production discharge or partial discharge, it was first reported by Ernst Werner von Siemens in 1857.
A microplasma is a plasma of small dimensions, ranging from tens to thousands of micrometers. Microplasmas can be generated at a variety of temperatures and pressures, existing as either thermal or non-thermal plasmas. Non-thermal microplasmas that can maintain their state at standard temperatures and pressures are readily available and accessible to scientists as they can be easily sustained and manipulated under standard conditions. Therefore, they can be employed for commercial, industrial, and medical applications, giving rise to the evolving field of microplasmas.
The Wingless Electromagnetic Air Vehicle (WEAV) is a heavier than air flight system developed at the University of Florida, funded by the Air Force Office of Scientific Research. The WEAV was invented in 2006 by Dr. Subrata Roy, plasma physicist, aerospace engineering professor at the University of Florida, and has been a subject of several patents. The WEAV employs no moving parts, and combines the aircraft structure, propulsion, energy production and storage, and control subsystems into one integrated system.
Plasma is one of four fundamental states of matter characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly in stars, but also dominating the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.
Plasma actuators are a type of actuator currently being developed for aerodynamic flow control. Plasma actuators impart force in a similar way to ionocraft. Plasma flows control has drawn considerable attention and been used in boundary layer acceleration, airfoil separation control, forebody separation control, turbine blade separation control, axial compressor stability extension, heat transfer and high-speed jet control.
Plasma medicine is an emerging field that combines plasma physics, life sciences and clinical medicine. It is being studied in disinfection, healing, and cancer. Most of the research is in vitro and in animal models.
In electromagnetism, a streamer discharge, also known as filamentary discharge, is a type of transient electric discharge which forms at the surface of a conductive electrode carrying a high voltage in an insulating medium such as air. Streamers are luminous writhing branching sparks, plasma channels composed of ionized air molecules, which repeatedly strike out from the electrode into the air.
Piezoelectric direct discharge (PDD) plasma is a type of cold non-equilibrium plasma, generated by a direct gas discharge of a high voltage piezoelectric transformer. It can be ignited in air or other gases in a wide range of pressures, including atmospheric. Due to the compactness and the efficiency of the piezoelectric transformer, this method of plasma generation is particularly compact, efficient and cheap. It enables a wide spectrum of industrial, medical and consumer applications.
Techniques have been developed to produce carbon nanotubes (CNTs) in sizable quantities, including arc discharge, laser ablation, high-pressure carbon monoxide disproportionation, and chemical vapor deposition (CVD). Most of these processes take place in a vacuum or with process gases. CVD growth of CNTs can occur in a vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth are making CNTs more commercially viable.
In materials science, vertically aligned carbon nanotube arrays (VANTAs) are a unique microstructure consisting of carbon nanotubes oriented with their longitudinal axis perpendicular to a substrate surface. These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled. VANTAs are consequently widely useful in a range of current and potential device applications.
Praseodymium(III,IV) oxide is the inorganic compound with the formula Pr6O11 that is insoluble in water. It has a cubic fluorite structure. It is the most stable form of praseodymium oxide at ambient temperature and pressure.
Heinz Artur Raether was a German physicist. He is best known for his theoretical and experimental contributions to the study of surface plasmons, as well as for Kretschmann-Raether configuration, a commonly-used experimental setup for the excitation of surface plasmon resonances.
Plasmalysis is a electrochemical process that requires a voltage source. On the one hand, it describes the plasma-chemical dissociation of organic and inorganic compounds in interaction with a thermal/non-thermal plasma between two electrodes. On the other hand, it describes the synthesis, i.e. the combination of two or more elements to form a new molecule. Plasmalysis is an artificial word made of plasma and lysis.
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