Plasma -The fourth state of matter: a treasure of applications

Credit: aa.washington.edu/research

Plasma, designated as a fourth state of matter, is an existence of charged particles (electrons, ions and neutral particles) together under some conditions and said to occupy naturally almost 99% of our universe.  Plasma is a state of matter in which an ionized sold/liquid/gaseous substance becomes highly electrically conductive to the point that long-range electric and magnetic fields dominate the behaviour of the matter.  Neon signs and lightning  re examples of partially ionized plasmas.

The Earth's ionosphere is a plasma and the magnetosphere contains plasma in the Earth's surrounding space environment. The interior of the Sun is an example of fully ionized plasma, along with the solar corona and stars. Plasma is used in television, neon signs and fluorescent lights. Stars, lightning, the Aurora, and some flames consist of plasma. There are various examples of plasma sources  like: lightning, aurorae, the excited low-pressure gas inside neon signs and fluorescent lights, solar wind, welding arcs, the Earth's ionosphere, the tail of a comet. Plasmas are generated by applying electromagnetic power across a gas volume. At sufficiently high power, the field has the power to strip electrons from some of the gas molecules/atoms and a highly reactive dynamic mixture consisting of electrons, ions, radicals, etc., is obtained. Plasma has already been put to its science and technological applications in various fields. Making use of plasma fusion (taking place at all stars including the Sun) as an economical, pollution free and unlimited source of energy generation was the main cause leading for the study of plasma.  Therefore, the most important practical applications of plasmas lie in the future, largely in the field of power production. The major method of generating electric power has been to use heat sources to convert water to steam, which drives turbogenerators. Recently, plasma is gaining interest as a source of energy to thrust vehicles into space known as plasma space engines, plasma thrusters etc. Plasma technology is commonly used in many industries, including in the automotive, microelectronics, packaging and medical device industries. There are many other applications of plasma and some of them are summarized here.

Plasma applications:

  • Fusion:Nuclear fusion is the process of recombining nuclei to form different nuclei and release vast amounts of energy. This is the process that powers the sun. If we can harness it, nuclear fusion has the potential to provide us with nearly limitless amounts of clean energy. There are three conditions necessary for nuclear fusion: high temperatures ( to about 107 K), high density, and prolonged stability. The high temperature requirement places us in the regime of plasmas. While experiments have attained these high temperatures, the primary difficulty is in achieving a sufficiently high combination of density and stability.
  • Plasma processing of material surfaces has been in use for almost half a century now. Use of low pressure plasma in the microelectronics industry, initially and later for surface treatment of many other surfaces like metals, ceramics, polymers, etc., has been well established. Plasma treatments have been used mainly for modification of surface properties like wettability, adhesion, functionality for improved reactivity, etc. Use of plasma processing in textiles has its potential as an important processing technique and is well recognized and currently it is attracting a lot of interest amongst researchers.The action of the plasma on a substrate surface can lead to the chemical and physical modification of the top layers of the textile material. Reactive sites can be generated on an otherwise inert surface. Longer treatment may lead to material removal by volatilization. Cleaning of surfaces by removal of surface dirt/contamination is another aspect which has been in use for some time now for improving ink adhesion on polymer surfaces.
  • Thermal plasmas (TPs): TPs are characterized by equilibrium between all species in the plasma in terms of energy or temperature (~ 104 K) and the ionization degree approaches 100%. Due to their high energy, these plasmas have very high energy contents and are generally used to remove, fuse or fragment (cutting, welding of metals) a substrate.
  • Cold (non-thermal) plasmas: Since energy transfer from electrons to heavy molecules and atoms is not efficient, a lack of equilibrium exists between electrons (at high temperature, 1–10 eV) and gas molecules at room temperature. Both work at low and atmospheric pressure. These types of plasmas can be safely used for treatment of textile materials. These systems operate typically at low pressures, but the vacuum brings its own set of problems. Hence there is now an attempt to develop plasma processes which can operate at atmospheric pressure and retain the properties of low pressure plasma. Examples include fluorescent illuminating tube discharges (neon), DC and RF discharges, DBD, etc.
  • Plasma cleaning: Plasma cleaning is capable of eliminating oils and grease down to the nanoscale. It can also reduce various risks of contamination much more efficiently than conventional cleaning processes. Plasma cleaning generates a spotless surface, suitable for bonding or additional processing, without producing damaging waste material. Ultraviolet light produced in the plasma is very efficient at the breaking the organic bonds of common surface contaminants, including those in oils and greases. Energetic oxygen species in the plasma also perform cleaning actions, reacting with contaminants to create primarily water and carbon dioxide. A plasma cleaning process for easily-oxidised materials like silver will use inert gases like argon or helium. In this cleaning process, the plasma-activated ions blast away organic contaminants, breaking them down so they can be vaporized and removed from the chamber.
  • Plasma coating: A plasma coating process creates a nanoscale polymer layer over the surface of an object. The process requires only a few minutes to produce a coating less than 1/100th the width of a human hair. Attached at the atomic scale, these coatings are typically clear, odourless and otherwise undetectable. Plasma coatings are currently a hot topic in many scientific fields because they have massive potential in a broad range of applications.
  • Biological decontamination: Plasma applications in medicine are one of hottest topics in plasma physics and plasma chemistry applications. Among them, the great interest is given for the sterilization or biological decontamination of various materials. There are different levels of killing living cells. Besides sterilization, which means the full decomposition of cells with cell membrane rupture and the leak of the inner cell content, apoptosis is probably more important and also easily achievable. This means that the cell itself is in a latent state and its replication ability is at zero level. With respect to cultural heritage objects this can have an advantage because such cell films can play a role as the protecting layer on the surface. To damage cells or their apoptosis more plasma-generated species can be applied. It is possible to use oxidizing species like oxygen atoms, metastable oxygen molecules, ozone, OH radicals, and others as well as UV and VUV radiation. Nonthermal plasma was recently tested for the decontamination of surfaces and liquids (wastewater, ready-to-eat food, food packaging materials), including the inactivation of bacteria, fungi, virus, biomolecules, or biofilm.
  • Propulsion in Space: Plasmas also have applications in the propulsion of spacecraft. Plasma jets may one day propel aircraft. Plasma thrusters could help jet planes fly without fossil fuels. Now researchers have created a prototype thruster capable of generating plasma jets with propulsive forces comparable to those from conventional jet engines, using only air and electricity. A variety of spacecraft generate plasma from gases such as xenon for propulsion. However, such thrusters exert only tiny propulsive forces, and so can find use only in outer space, in the absence of air friction. Since, it requires no externally applied magnetic field, the weight and size requirements of a vehicle can be drastically lower than other configurations would require. Such a thruster could achieve an Isp of 1,000,000 s, and a thrust on the order of 105N (similar to a Boeing 747). The experimental setup, except that the inner electrode is hollow and the end wall flares out. Most of it flows out the back (to the right), propelling the space craft forward (to the left). Some flows back into the inner electrode to be used to provide power to the spacecraft’s control systems. The thruster could operate in either a steady-state or long-pulse mode. An air compressor forces high-pressure air at a rate of 30 liters per minute into an ionization chamber in the device, which uses microwaves to convert this air stream into a plasma jet blasted out of a quartz tube. Plasma temperatures could exceed 1,000 °C.

If air plasma jets ever become practical, they could reduce fossil fuel use and greenhouse gas emissions. Researchers think that within five years one could use a scaled-up plasma engine to power small pilotless airplanes or heavy-duty drones to carry cargo for shipping. For an air-plasma engine to power a large jumbo jet, it would require a large array of megawatt microwave sources, high-power turbine compressors, and an extremely high electric energy storage capability and the development could take another decade. The scientists are currently focused on scaling up the power of the system. If they can build a megawatt-strength plasma engine capable of driving a real airplane, they will then pay attention on ways to reduce weight and size. The plasma engines of today consume less propellant than chemical combustion rockets, enabling them to carry out lighter space missions, and as such, less costly ones. However, there have complexity and durability problems: in order to operate they need metallic electrodes in contact with the plasma, which over time erode to the point that the device stops working. This limits its efficiency and flexibility, since modifying the point of operation without affecting the electrodes is very complex. But now, researchers at Universidad Carlos III de Madrid (UC3M) have patented a new spatial plasma-fueled engine capable of satellite and spacecraft propulsion, with magnetic field geometry and configuration that would minimize losses on walls and their erosion, thereby resolving issues of efficiency, durability, and operating restrictions of engines that are currently in orbit.

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