PHYSICS OF PLASMAS

On the edge of matter

One of a series of plasma balls on show at the Microcosme teaching and experimental exhibition put together by CERN and permanently open to the public in Geneva.© CERN
One of a series of plasma balls on show at the Microcosme teaching and experimental exhibition put together by CERN and permanently open to the public in Geneva.
© CERN
Oxygen plasma produced at low pressure.© Thierry Visart (ULB)
Oxygen plasma produced at low pressure.
© Thierry Visart (ULB)
One of the cold plasma torches used by researchers working on the Nano2Hybrids project at the ULB. This working group vaporises the metallic nanoparticles onto the carbon nanotubes exposed to the plasma (blue light).© FUNDP/ SAVÉ/Daniel Van Acker
One of the cold plasma torches used by researchers working on the Nano2Hybrids project at the ULB. This working group vaporises the metallic nanoparticles onto the carbon nanotubes exposed to the plasma (blue light).
© FUNDP/ SAVÉ/Daniel Van Acker
Atmospheric plasma reactor in operation at the Department of Science and Materials Analysis (SAM) of the Centre de recherche Gabriel Lippmann in Luxembourg.© Ali Mansour/SAM Luxembourg
Atmospheric plasma reactor in operation at the Department of Science and Materials Analysis (SAM) of the Centre de recherche Gabriel Lippmann in Luxembourg.
© Ali Mansour/SAM Luxembourg
One of the cold plasma torches used by researchers working on the Nano2Hybrids project at the ULB. This working group vaporises the metallic nanoparticles onto the carbon nanotubes exposed to the plasma (blue light).
One of the cold plasma torches used by researchers working on the Nano2Hybrids project at the ULB. This working group vaporises the metallic nanoparticles onto the carbon nanotubes exposed to the plasma (blue light).

Plasmas constitute 99% of known and visible matter. The object of extensive research and the basis of a host of applications, they are invading our living rooms, changing our plastics and revolutionising our laboratory analysis tools.

Plasmas are rare on Earth, even though they form the bulk of the visible universe. The fourth state of matter, plasma lies behind the incredible energy generated by the Sun and stars. On Earth, it is manifested in lightning and generates phenomena like the aurora borealis (Northern Lights) or St. Elmo’s Fire (see box) that fascinated our ancestors.

Mastery of plasmas has given us the neon tube, and more recently, extra-large, ultra-flat TV screens (see box Plasmas and TV). But that’s not all. From laboratories to industry, plasmas are on the cutting edge of analysis technology and manufacturing pro cesses. But what is a plasma?

Polymorphous portrait

Back to the classroom. States of matter are defined by the cohesive force between atoms. It is not the atoms themselves that are liquid, solid or gaseous, but the entire structure that they form. Under the effect of an energy source, most solids become liquid and even gaseous if sufficient energy is applied.

The plasma state is one notch higher. The amount of energy to which the matter is submitted is such that some atoms destructure and lose one or more electrons. These collide with other atoms or molecules and impart energy to them, one of the main effects of which is to shift electrons from one orbit to another. Once these electrons fit back into place, they release energy that can take the form of photons. Hence the first characteristic of plasmas: they glow. Ions, that is electrically charged atoms that have lost or gained one or more electron(s), may also strike other atoms or molecules, thereby contributing to the general disorder that is characteristic of plasma.

In short, plasma is an ionised gas. And since not all atoms of this gas release electrons, plasma represents a jumbled mass of molecules, atoms, ions and electrons. The advantage of this protean avatar of matter lies in its increased properties of conductivity and reactivity, because of its charged particles that can react with an electric or magnetic field and interfere with other materials to change their structures and properties.

Serve hot or cold

Two large families of plasmas – hot plasmas and cold plasmas – exist side-by-side, distinguished by their degree of ionisation. “In hot plasma, the electrons gain so much energy from the electric field they are able to redistribute it by numerous collisions with the other forms of plasma. This translates into much greater heat”, says Riccardo d’Agostino, professor of chemistry at the University of Bari (IT) and co-editor of the journal Plasma Processes and Polymers.

This heat can reach that of the Sun, the plasma of which approaches 10 million degrees through the fusion of hydrogen atoms within it. With the discovery of nuclear fusion in the 1930s, controlling this fundamental energy of the stars has been hugely and universally cove ted. In the field of military research, this powerful principle has allowed the development of the H-bomb, fortunately never used in conflict. In the field of civil research, fusion plasmas are at the centre of an unprecedented scientific effort aimed at providing mankind with energy that is both green and inexhaustible. Such is ITER (1), a huge international research project which aims to build the first pilot nuclear fusion facility.

Theoretically, the principle is simple: when two nuclei of light atoms fuse, the resulting nucleus can attain a stable condition only by ejecting particles to evacuate excess energy. The fusion of deuterium (D) and tritium (T), for example, two hydrogen isotopes having one and two neutrons respectively, ejects a helium nucleus (two protons and two neutrons) and a fast neutron. It is more particularly the kinetic energy of the latter, the most important, that scientists wish to exploit.

Uncovering the secret of the stars

Putting theory into practice is much less obvious. First of all, the fusion plasma reaches becomes so hot that no material can contain it. “We therefore confine it inside a Tokamak, a sort of huge cylindrical ring in which a magnetic field keeps the plasma away from the walls”, explains Phil Morgan, plasma physicist at the Joint European Torus ( JET), the largest experimental Tokamak in the world, based at the Culham Science Centre (UK). It is at JET that most of the experiments prior to the launch of ITER are being carried out.

Another technical challenge is heating the D-T mixture beyond 100 million degrees in order to reach the ignition point of the plasma, i.e., the state where enough heat is released for the fusion reaction to become self-perpetuating. “Several technologies are combined to achieve such temperatures”, Phil Morgan explains. “Initially we pass an electric current through the D-T mixture to excite the charged particles into colliding, creating heat in the process. But the more the heat increases, the weaker this reaction becomes, and we have to switch to heating by injecting new particles that in turn collide with the plasma particles and generate heat. Lastly, the plasma is subjected to very high frequencies, enabling it to finally achieve the optimal fusion temperature.”

Fusion plasmas offer immense potential. So much so, as Riccardo d’Agostino stresses, that this often obscures the applications of the other hot plasmas. “These are used in industry to cut delicate materials such as ceramics. In analytical chemistry they are causing a revolution. The hot plasma torch (Inductively Coupled Plasma – ICP) allows us to decompose a sample at the atomic scale and simultaneously identify almost all its component elements.”

From nanotechnologies…

But the most immediate revolution comes undoubtedly from the cold plasmas, which are a lot more manageable because of their low temperature. Here we are taking advantage of their reactivity to edit the properties of materials. “Originally, we could generate cold plasmas only at low pressure. At times this made them too expensive, because of the need to isolate the material in a vacuum chamber”, Riccardo d’Agostino explains. “But since about 10 years ago it has been possible to produce them at atmospheric pressure. They are increasingly being used by industry, for example replacing the chemicals used in the past to increase the paint adhesion qualities of automotive plastics, leading to the widespread use of painted bumpers.”

In laboratories, cold plasmas are fascinating nanotechnology researchers, enabling them to generate nanomaterials with specific properties. This technology is of particular interest to the researchers of Nano2Hybrids, a European project to develop mini-gas sensors from carbon nanotubes. “As the sensor pro perties of carbon are too low, we graft metallic nanoparticles onto their surface using cold plasmas”, says François Reniers, the director of the General Chemistry Laboratory of the Université Libre de Bruxelles (ULB) (BE) , one of the partners in Nano2Hybrids. “For this different techniques are used. At the ULB and the Gabriel Lipmann Public Research Centre (LU), we are working with plasmas at atmospheric pressure while our colleagues at the Université de Namur (BE) are studying the effectiveness of low-pressure deposition. The objective is to determine the best way to functionalise the surfaces of the nanotubes by modifying a set of parameters, such as gas composition, pressure, exposure time or the type of metal particles.”

…to biomedical sciences

Other researchers are planning to use plasmas to sterilise medical instruments. At least this is the technology that was closely examined by participants in the European project Biodecon – Decontamination of biological systems using plasma discharges, which was completed in early 2009. Biodecon has demonstrated the feasibility and the benefits of plasma sterilisation. Traditional methods – ultraviolet (UV) treatment, high temperature and/or oxidation using chemicals – call for very stringent control and can at times damage the medical equipment. There are also occasions when they are ineffective, like UV when the bacteria are grouped into biofilms, a kind of germ cluster that can be up to several millimetres thick. And none of the traditional sterilisation methods is effective for removing prions, the biomolecules behind Creutzfeldt-Jakob disease.

“Cold plasma sterilisation would not only simplify the traditional procedures but also improve efficiency”, explains Achim von Keudell, Biodecon project coordinator for the Institut für reaktive Plasmen of the University of Bochum (DE). “Indeed, a hydrogen-based plasma attacks biofilms more effectively and eradicates all the biomolecules, including prions, while reducing the risk of damaging the instruments.”

Further upstream on the basic research side, other scientists are examining the effects of the fourth state of matter on living tissue. “Teams are working on the use of cold plasmas for wound disinfection or for dental scaling”, says Achim von Keudell. “In the future, plasmas could even help treat certain cancers”, adds Riccardo d’Agostino. “But this is still very hypothetical, because we still have no idea of how plasma impacts human tissues”, Achim von Keudell cautions. One thing is for sure, plasmas are far from having said their last word.

Julie Van Rossom(2)

  1. See ‘ITER emerges from the Earth’, Research*eu 61.
  2. With the kind collaboration of Nicolas Vandencasteele, PhD in chemistry and researcher at the ULB.


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When physics and myth join hands

During storms, the sailors of yesteryear sometimes saw the tips of their masts light up. This so-called “St. Elmo’s Fire”, considered at the time as a divine manifestation, is in fact an example of the natural generation of plasmas. Before a storm, the air becomes electrically charged, and the electrical field that tends to concentrate around pointed objects is sufficient to ionise the surrounding air, creating the faint blue or violet light characteristic of St. Elmo’s Fire.

Plasmas are also the source of the aurora borealis or Nothern Lights. In this case, magnetic storms generated by the Sun produce an inflow of charged particles that collide with the atoms of the ionosphere to generate, depending on altitude, nitrogen, oxygen or hydrogen plasmas.

More recently scientists have observed plasmas lasting no more than 5 milliseconds when powerful lightning occurs in the upper atmosphere. Depending on the type of plasma, the researchers have given them epic-sounding names like elves, leprechauns, red sylphs… The Lightning and Sprites Observations experiment by the International Space Station is still trying to better understand the origin of these transient phenomena.

www.esa.int



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Plasmas and TV

© Shutterstock
© Shutterstock

The advent of plasma screens in the late ‘90s signed the death warrant of our good old cathode ray tubes. Gone are the small television sets discreetly placed on furniture. Now the TV stands in the middle of the living room and polarises everyone’s attention.

Hundreds of thousands of small cells connected to electrodes make up these televisual giants. Each cell contains a mixture of argon (90 %) and xenon (10 %), so that when the electrodes are activated, these gases ionise to form a plasma. At the plasma state, the photons emitted by argon and xenon give off ultraviolet light. The front of each cell is coated with luminophores, substances that excite on contact with the UV and glow blue, red or green, depending on their type.



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