Plasma generator

Details Plasma generator Plasma generator

2002-01-08

2004-07-20

McDonald, Rodney G. (Department: 1753)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

With means applying electromagnetic wave energy or...

C422S186180, C422S186190, C422S186200, C422S186290, C118S7230MP, C118S7230ER, C118S7230ER, C204S298070, C204S298330, C156S345290, C156S345330, C156S345340, C156S345380

Reexamination Certificate

active

06764658

ABSTRACT:

FIELD OF THE INVENTION
This disclosure concerns an invention relating generally to plasma generators, and more specifically to “cold” plasma generators and/or plasma generators operating at atmospheric pressure.
BACKGROUND OF THE INVENTION
Plasma, the fourth state of matter, consists of gaseous complexes in which all or a portion of the atoms or molecules are dissociated into free electrons, ions, free radicals, and neutral particles. On earth, plasma occurs naturally in lightning bolts, flames, and similar phenomena, or may be manufactured by heating a gas to high temperatures, or by applying a strong electric field to a gas, the more common method. The latter type of plasma, often referred to as an electrical discharge plasma, can be further sub-classified as a “hot” plasma, i.e., dissociated gas in thermal equilibrium at high temperatures (
~
5000K), or “cold” plasma, i.e., nonthermal plasma wherein the dissociated gas is at low temperatures but its electrons are at high temperature (i.e., in a state of high kinetic energy).
The usefulness of plasma for manufacturing and other applications is best understood by reviewing common applications for cold plasma. As an example, common cold plasma processing methods are commonly used to alter the surface properties of industrial materials without affecting the bulk properties of the treated material. The most common cold plasma surface treatments may be generally categorized as cleaning, activation, grafting, and deposition processes, each of which will now be briefly reviewed.
Plasma cleaning processes typically utilize inert or oxygen plasmas (i.e., plasmas generated from inert or oxygen-based process gases) to remove contaminants (generally organic contaminants) on a material surface subjected to vacuum. The contaminants are exposed to a plasma stream, and they undergo repetitive chain scission from the plasma until their molecular weight is sufficiently low to boil away in the vacuum.
Plasma activation is used when a material (generally a polymer or elastomer) is subjected to a plasma generally produced from an inert or non-carbon gas, and results in the incorporation of different moieties of the process gas onto the surface of the material being treated. For example, the surface of polyethylene normally consists solely of carbon and hydrogen. However, if subjected to an appropriate plasma, the surface may be activated to contain a variety of functional groups which enhance the adhesion and permanence of coatings later applied to the surface. As an example, a surface can be treated to greatly enhance its ability to bond with adhesives.
Deposition, which is exemplified by a process referred to as plasma-enhanced chemical vapor deposition (PECVD), utilizes a complex molecule as the process gas. The process gas molecules are decomposed near the surface to be treated, and recombine to form a material which precipitates onto and coats the surface.
Grafting generally utilizes an inert process gas to create free radicals on the material surface, and subsequent exposure of the radicalized surface to monomers or other molecules will graft these molecules to the surface.
The foregoing cold plasma processes have numerous practical applications, including sterilizing of medical equipment, application of industrial and commercial coatings, etching computer chips, semiconductors, and circuits, and so forth. Hot plasma might be used for generally the same types of applications as cold plasma. However, hot plasma applications are limited since most organic matter cannot be treated under the high temperatures required for hot plasmas without severe degradation. Additionally, hot plasma technology is energy and equipment intensive, making it expensive and difficult to work with. In contrast, cold plasma may be used at temperature ranges as low as room temperature (or lower), making it significantly easier to handle. However, cold plasma processes have the disadvantage that they generally need low pressure conditions to operate (generally a vacuum), and consequently need large, static (i.e., immobile) equipment with a low-pressure treatment chamber to operate. This causes significant manufacturing constraints since the need to treat items within an enclosed chamber makes it inherently difficult to process the items continuously in assembly-line fashion, as opposed to processing the items in batches.
Some of these difficulties have been overcome with further developments in dielectric barrier discharge (DBD) plasma production processes. These processes, which may take place at room temperature and non-vacuum conditions, space a pair of electrodes apart across a free space, with one or more dielectric layers also being situated between the electrode. When an alternating high voltage electrical current is applied to the plates, “microbursts” of plasma are generated from the gas(es) in the free space. DBD apparata are sometimes used to generate ozone by ionizing oxygen passing through the free space of the apparatus, or to break apart volatile gaseous organic compounds passing through the free space. However, conventional DBD plasma generation apparata are not well suited for surface treatment of workpieces because of the difficulty in transporting the workpieces through the free space without the plasma's interference with the transport mechanism; for example, one generally cannot run a conveyor through the free space. Plasma processes using DBD are further limited by the size constraints that the free space imposes on the workpieces. Since the free space is relatively small, the size range of workpieces that can be treated is correspondingly small, which greatly limits usage.
Thus, it would be useful to have available methods and apparata of generating cold plasma at low pressures (including at and/or around atmospheric pressure) while alleviating or eliminating the disadvantages of prior cold plasma equipment and methods.
SUMMARY OF THE INVENTION
The invention involves a plasma generator which is intended to at least partially solve the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions of the plasma generator. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
The plasma generator includes several plasma sources distributed in an array for plasma treatment of surfaces. Each plasma source includes spaced first and second conductive electrodes between which plasma will be generated. Each second electrode has a gas passage defined therein, and one of the first electrodes is situated within the gas passage in spaced relation from the second electrode, with the gas passage thereby constituting the free space for plasma generation. As an example, each second electrode may be formed as a hollow cylinder having an interior gas passage, and each first electrode may be formed as a rod which is concentrically situated within a second electrode's gas passage spaced from the gas passage walls. An insulating layer is interposed between the first and second electrodes, as by providing a ceramic coating on the surfaces of the first electrodes and/or upon the gas passage walls of the second electrodes, to facilitate plasma formation via dielectric barrier discharge (DBD) in the gas passages between the first and second electrodes.
The first electrodes may be provided on a common bed so that they protrude therefrom, with their bases affixed to the common bed and their tips being spaced from the common bed. This monolithic or integrally affixed first electrode structure, wherein the common bed may take the form of a plate having the first electrodes extending therefrom as groups of adjacently-spaced protrusions, therefore effectively connects the first electrodes togeth

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