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.
What is a microplasma?
There are 4 states of matter: solid, liquid, gas, and plasma. Plasmas make up more than 99% of the visible universe. In general, when energy is applied to a gas, internal electrons of gas molecules (atoms) are excited and move up to higher energy levels. If the energy applied is high enough, outermost electron(s) can even be stripped off the molecules (atoms), forming ions. Electrons, molecules (atoms), excited species and ions form a soup of species which involves many interactions between species and demonstrate collective behavior under the influence of external electric and magnetic fields. Light always accompanies plasmas: as the excited species relax and move to lower energy levels, energy is released in the form of light. Microplasma is a subdivision of plasma in which the dimensions of the plasma can range between tens, hundreds, or even thousands of micrometers in size. The majority of microplasmas that are employed in commercial applications are cold plasmas. In a cold plasma, electrons have much higher energy than the accompanying ions and neutrals. Microplasmas are typically generated at elevated pressure to atmospheric pressure or higher.
Successful ignition of microplasmas is governed by Paschen's Law, which describes the breakdown voltage (the voltage at which the plasma begins to arc) as a function of the product of electrode distance and pressure,
where pd is the product of pressure and distance, and and are the gas constants for calculating Townsend's first ionization coefficient and is the secondary emission coefficient of the material. As the pressure increases, the distance between the electrodes must decrease to achieve the same breakdown voltage. This law is proven to be valid at inter-electrode distances as small as tens of micrometers and pressures higher than atmospheric. However, its validity at even smaller scales (approaching debye length) is still currently under investigation.
Generating microplasmas While microplasma devices have been studied experimentally for more than a decade, understanding has been spurred in the past few years as the result of modelling and computational investigations of microplasmas.
Confinement to small spaces When the pressure of the gas medium in which the microplasma is generated increases, the distance between the electrodes must decrease to maintain the same breakdown voltage. In such microhollow cathode discharges, the product of pressure and distance ranges from fractions of Torr cm to about 10 Torr cm. At values below 5 Torr cm, the discharges are called "pre-discharges" and are low intensity glow discharges. Above 10 Torr cm the discharge can become uncontrollable and extend from the anode to random locations within the cavity. Further research by David Staack provided a graph of ideal electrode distances, voltages, and carrier gases tested for microplasma generation.
Dielectric materials Dielectrics are poor electrical conductors, but support electrostatic fields and electric polarization. Dielectric barrier discharge microplasmas are typically created between metal plates, which are covered by a thin layer of dielectric or highly resistive material. The dielectric layer plays an important role in suppressing the current: the cathode/anode layer is charged by incoming positive ions/electrons during a positive cycle of AC is applied which reduces the electric field and hinders charge transport towards the electrode. DBD also has a large surface-to-volume ratio, which promotes diffusion losses and maintains a low gas temperature. When a negative cycle of AC is applied, the electrons are repelled off of the anode, and are ready to collide with other particles. Frequencies of 1000 Hz or more are required to move the electrons fast enough to create a microplasma, but excessive frequencies can damage the electrode (~50 kHz). Although dielectric barrier discharge comes in various shapes and dimensions, each individual discharge is in micrometer scale.
Pulsed power AC and high frequency power are often used to excite dielectrics, in place of DC. Take AC as an example, there are positive and negative cycles in each period. When the positive cycle occurs, electrons accumulate on the dielectric surface. On the other hand, the negative cycle would repel the accumulated electrons, causing collisions in the gas and creating plasma. During the switch from the negative to positive cycles, the above-mentioned frequency range of 1000 Hz-50,000 Hz is needed in order for a microplasma to be generated. Because of the small mass of the electrons, they are able to absorb the sudden switch in energy and become excited; the larger particles (atoms, molecules, and ions), however aren't able to follow the fast switching, therefore keeping the gas temperature low.
Radio frequency and microwave signals Based on transistor amplifiers low power RF (radio frequency) and microwave sources are used to generate a microplasma. Most of the solutions work at 2.45 GHz. Meanwhile, is a technology developed which provide the ignition (high voltage generation) on the one hand and the high efficient operation (matching of the plasma and the wave guide impedance) on the other hand with the same electronic and impedance transformer network.
Laser induced With the use of lasers, solid substrates can be converted directly into microplasmas. Solid targets are struck by high energy lasers, usually gas lasers, which are pulsed at time periods from picoseconds to femtoseconds (mode-locking). Successful experiments have used Ti:Sm, KrF, and YAG lasers, which can be applied to a variety of substrates such as lithium, germanium, plastics, and glass.
History
In 1857, Werner von Siemens, a German scientist, originated ozone generation using a dielectric barrier discharge apparatus for biological decontamination. His observations were explained without the knowledge of "microplasmas", but were later recognized as the first use of microplasmas to date. The early electrical engineers, such as Edison and Tesla, were actually trying to prevent the generation of such "micro-discharges", and used dielectrics to insulate the first electrical infrastructures. Subsequent studies have observed the Paschen breakdown curve as being the prime cause of microplasma generation in an article published in 1916.
Subsequent articles during the course of the 20th century have described the various conditions and specifications that lead to the generation of microplasmas. After Siemens' interactions with microplasma, Ulrich Kogelschatz was the first to identify these "micro-discharges" and define their fundamental properties. Kogelschatz also realized that microplasmas could be used for excimer formation. His experiments spurred the rapid development of the microplasma field.
In February 2003, Kunihide Tachibana, a professor of Kyoto University held the first international workshop on microplasmas (IWM) in Hyogo, Japan. The workshop, titled "The New World of Microplasmas", opened a new era of microplasma research. Tachibana is recognized as one of the founding fathers as he coined the term "microplasma". The Second IWM was organized in October 2004 by Professors K.H. Becker, J.G. Eden, and K.H. Schoenbach at Stevens Institute of Technology in Hoboken, New Jersey. The third international workshop was coordinated by the Institute of Low Temperature Plasma Physics alongside the Institute of Physics of Ernst-Moitz-Arndt-University in Greifswald, Germany, May 2006 (K.D. Weltmann). Topics discussed were inspiring scientific and arising technological opportunities of microplasmas. The fourth IWM was held in Taiwan in October 2007 (C.C. Chao and J.E. Chang), the fifth in San Diego, California in March 2009 (J.G. Eden and S.-J. Park), and the sixth in Paris, France in April 2011 (V. Puech). The next (seventh) workshop was held in China in approximately May 2013 (Y.K. Pu).The 8th International Workshop on Microplasmas (IWM 2015) was organized by Jose L. Lopez from May 11–15, 2015 hosted at Seton Hall University in Newark, New Jersey, USA. Subsequently, the 9th International Workshop on Microplasmas (IWM 2017) was held in Garmisch-Partenkirchen, Germany from June 6–9, 2017. The tenth installment of the International Workshop on Microplasmas in Kyoto, Japan from May 20–24, 2019 (O. Sakai and T. Ito).
Applications The rapid growth of applications of microplasmas renders it impossible to name all of them within a short space, but some selected applications are listed here.
Plasma displays Artificially generated microplasmas are found on the flat panel screen of a plasma display. The technology utilizes small cells and contains electrically charged ionized gases. Across this plasma display panel, there are a millions of tiny cells called pixels that are confined to form a visual image. In the plasma display panels, X and Y grid of electrodes, separated by a MgO dielectric layer and surrounded by a mixture of inert gases - such as argon, neon or xenon, the individual picture elements are addressed. They work on the principle that passing a high voltage through a low-pressure gas generates light. Essentially, a PDP can be viewed as a matrix of tiny fluorescent tubes which are controlled in a sophisticated fashion. Each pixel comprises a small capacitor with three electrodes, one for each primary color (some newer displays include an electrode for yellow). An electrical discharge across the electrodes causes the rare gases sealed in the cell to be converted to plasma form as it ionizes. Being electrically neutral, it contains equal quantities of electrons and ions and is, by definition, a good conductor. Once energized, the plasma cells release ultraviolet (UV) light which then strikes and excites red, green and blue phosphors along the face of each pixel, causing them to glow.
Illumination (Light Source)
The team of Gary Eden and Sung-Jin Park are pioneering the use of microplasmas for general illumination (as well as UV source). Their apparatus uses many microplasma generators in a large array, which emit light through a clear, transparent window. Unlike fluorescent lamps, which require the electrodes to be far apart in a cylindrical cavity and vacuum conditions, microplasma light sources can be put into many different shapes and configurations, and generate heat. This is opposed to the more commonly used fluorescent lamps which require a noble gas atmosphere (usually argon), where excimer formation and resulting radiative decomposition strikes a phosphor coating to create light. Excimer light sources are also being produced and researched. The stable, non-equilibrium condition of microplasmas favors three-body collisions which can lead to excimer formation. The excimer, an unstable molecule produced by collisions of excited atoms, is very short lived due to its rapid dissociation. Upon their decomposition, excimers release different kinds of radiation when electrons fall to lower energy levels. One application, which has been pursued by the Hyundai Display Advanced Technology R&D Research Center and the University of Illinois, is to use excimer light sources in flat panel displays. The technology moved further to the technology of a compact, flat source of UV and vacuum UV wavelengths for various applications.
Destruction of volatile organic compounds (VOC's)