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What is a Plasma?
Plasmas are said to be the fourth state of matter and are among the most widely
studied subjects in modern science. Stated simply, a plasma is an ionized gas that conducts electricity.
Plasmas come in many forms,
ranging from a welding torch to a fluorescent light bulb. As show in the
diagram above, they may be identified by their relationship between the
neutral gas temperature and the electron temperature in the plasma (1 eV
~11,000 °C). In a torch, an arc forms between the electrodes resulting
in complete ionization of the gas and a temperature rise >4,000 °C.
Atmospheric plasma spray guns take advantage of the high temperatures
found in arcs. These devices are able to vaporize many materials, and
coat them at high speed onto surfaces.
The fluorescent light bulb, by contrast, contains a low pressure glow discharge. The
vacuum inside the bulb is essential, because it allows the gas to be weakly ionized with low power input. Such weakly ionized plasmas
are not at thermal equilibrium, i.e., the neutral molecules are close to 25 °C, while the free electrons are above 10,000 °C. The
high-energy electrons collide with the neutral molecules, dissociating them into atoms and radicals, thereby yielding a reactive gas
mixture. Most plasmas used for materials processing at low temperature are weakly ionized and produced in a vacuum.
Using plasmas for materials processing requires expertise in chemistry. The Atomflo™
atmospheric plasma is a source of atoms and radicals that are carefully selected for surface treatment, cleaning, etching, or depositing
thin coatings. For example, a hydrogen plasma may be used to cure polymer films, whereas a nitrogen plasma may be combined with silane
to deposit a silicon nitride plasma coating. The key in all these applications is combining the right hardware with the right chemistry
to achieve the desired results.
Hardware
One must apply an electrical field to the gas to generate a plasma. In a vacuum chamber,
where the ions and electrons have long lifetimes, it is relatively easy to do this. Radio frequency power can be applied
to two metal plates immersed in the vessel creating a capacitive discharge. Alternatively, RF power may be deposited into a coil mounted
on the chamber walls, thereby producing an inductively coupled plasma. The gas also may be ionized by application of microwave power at
2.45 GHz to a specially designed cavity or horn.
In an atmospheric plasma, a variety of power supplies may be used from DC to RF. Here, the challenge is to design the
electrodes and gas flow to yield intimate contact between the reactive gases and the substrate. At high pressure, the atoms and radicals
are quickly consumed by collisions in the gas, so the transit time from the plasma to the surface must be short. If one wishes to achieve
low temperature operation with atmospheric plasmas, then special precautions must be taken to prevent arcing between the electrodes.
A wide variety of plasma tools are available today, reflecting the broad spectrum of materials applications being
tackled by these remarkable products. To the end user, the choices can be bewildering, making the task of product selection challenging
at best. We encourage you to contact Surfx Technologies so that our engineers may assist you with the selection process.
Technology Breakthrough
The Atomflo™ and Plasma Flow Systems™ are a technology breakthrough! They provide a powerful
plasma for materials processing at atmospheric pressure and near room temperature. These products are well suited for many applications,
particularly for the surface treatment of thermally sensitive materials such as polymers and composites.
The advantages of Surfx's atmospheric plasma sources are: (1) high density of reactive species at low temperature; (2)
ions and electrons confined to the source, avoiding substrate damage; (3) fast, uniform treatment of large areas; (4) operates with wide
range of gas chemistries; (5) easily integrated into other process equipment; (6) extremely reliable; and (7) safe and easy to use.
To see the Atomflo™ atmospheric plasma system in action, visit our
multimedia page. See the Applications section for a discussion of the possible uses of our atmospheric
plasma products. We offer a complete line of atmospheric plasma products, and are sure that we can meet your
surface treatment needs. Please contact us if you have any further questions.
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