Continuous large area cold atmospheric pressure plasma sheet source

The present disclosure is generally directed to a plasma sheet source and methods of using same. The plasma sheet source includes a cylindrical electrode having a conductive cylindrical core surrounded by a dielectric material, a plurality of channels configured to direct gas from a gas inlet to the electrode, and a plasma outlet positioned below the electrode. Gas is introduced to the plasma sheet source and directed toward the electrode, which when powered by pulsed direct current, produces plasma as the gas ionizes. The produced plasma is then directed out of the plasma outlet to a specimen for treatment of the specimen. Notably, the plasma exiting the plasma outlet is in the form of a plasma sheet that is at approximately room temperature.

TECHNICAL FIELD

The present disclosure is directed to a large area plasma sheet source for the treatment of a specimen surface.

RELATED ART

Atmospheric pressure plasma is utilized in many industries as a means of activating, cleaning, or decontaminating surfaces, or changing surface chemistry. Further, as the plasma that interacts with a specimen surface may be at or near room temperature, it is compatible with the treatment of living tissues and objects. For example, a noble gas “working gas” mixed with nitrogen has been used to produce plasma, which in turn is used to treat seeds. The treated seeds have been shown to have enhanced germination and seedling growth, as well as decontaminated surfaces. Other applications for atmospheric pressure plasma include water treatment, bacteria inactivation, surface activation, adhesion enhancement, plasma cleaning, and surface modifications.

Atmospheric pressure plasma is typically utilized as plasma jets, which are thin jets that have diameters ranging from microns to millimeters and lengths of a few centimeters. The effective treatment area of each jet is very small due to its small diameter, leading to the production of arrays of plasma jets that can treat larger areas. However, these jet arrays have the disadvantage of creating non-uniformly treated surfaces since the jet arrays have discontinuous plasma production. The present disclosure is directed to a plasma sheet source, which produces atmospheric pressure plasma and applies the plasma to a specimen as a long continuous plasma sheet. In this way, specimen treatment area is expanded from that of an individual jet without the heterogeneity of j et arrays.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to a plasma sheet source and methods of treating a specimen using the plasma sheet source. In one aspect of the disclosure, there is provided a plasma sheet source with a gas inlet for receiving gas and an electrode with a conductive core covered with a dielectric material, and the electrode is configured to generate an electrical field. The plasma sheet source further includes a body having a plurality of channels for directing gas from the gas inlet through the electrical field generated by the electrode to an elongated outlet of the body, so that the electrical field converts the gas to a plasma sheet output from the elongated outlet.

In some embodiments, the gas includes a noble gas and the dielectric material is quartz. In some embodiments, the body has a first cavity for receiving gas from the gas inlet and a second cavity in which the electrode is located, and each of the channels extends from the first cavity to the second cavity. In some embodiments, the plurality of channels includes at least a first channel and a second channel, and the first channel is parallel to the second channel. In some embodiments, the electrode is elongated and has a longitudinal axis that is parallel with the elongated outlet. In some instances, the electrode is electrically connected to a power source configured to transmit pulsed direct current to the electrode. In some embodiments, the plasma sheet is at room temperature and the plasma sheet source is configured to form the plasma at atmospheric pressure.

In another aspect of the disclosure, there is provided a system for generating a plasma sheet. The system includes at least one gas source, a power source, and a plasma sheet source connected to the gas source. The plasma sheet source has an electrode and a plurality of channels for receiving gas from the at least one gas source and directing the gas past the electrode and through an electrical field generated by the electrode. The electrode is connected to the power source and the power source is configured to apply power to the electrode for generating the electrical field at a strength sufficient for converting the gas into the plasma sheet that egresses the plasma sheet source through the elongated outlet. The electrode has a conductive core covered with a dielectric material.

In some embodiments, the gas includes a noble gas and the dielectric material is quartz. In some embodiments, the plasma sheet source has a first cavity for receiving the gas and a second cavity in which the electrode is positioned, and the plurality of channels includes at least a first channel and a second channel, and each of the first channel and the second channel extends from the first cavity to the second cavity. In some instances, the first channel is parallel to the second channel. In some embodiments, the electrode is elongated and has a longitudinal axis that is parallel with the elongated outlet. In some embodiments, the power source is configured to apply pulsed direct current to the electrode. In some embodiments, the plasma sheet source is configured to form the plasma at atmospheric pressure.

In yet another aspect of the disclosure, there is provided a method for generating a plasma sheet. The method includes receiving gas within a first cavity of a plasma sheet source having a plurality of channels, generating an electrical field with an electrode in a second cavity of the plasma sheet source, directing the gas from the first cavity through the plurality of channels to the second cavity so that the electrical field converts the gas into the plasma sheet, and emitting the plasma sheet from the plasma sheet source through an elongated cavity.

In some embodiments, the method further includes directing the plasma sheet to a specimen for treating the specimen. In some embodiments, the electrode is elongated and has a longitudinal axis that is parallel with the elongated outlet and the electrical field is generated using pulsed direct current. In some embodiments, the plurality of channels includes at least a first channel and a second channel, and each of the first channel and the second channel extends from the first cavity to the second cavity, and the first channel is parallel to the second channel.

A further understanding of the nature and advantages of the present invention will be realized by reference to the remaining portions of the specification and the drawings.

DETAILED DESCRIPTION

The present disclosure is generally directed to plasma sheet sources and methods of using same for the treatment of specimen surfaces. The plasma sheet source allows plasma to be applied to a surface of a specimen with a larger application area than typical plasma jets afford. The plasma sheet source additionally improves upon plasma jet arrays in that it results in a generally uniform treatment across the sheet. Furthermore, while plasma jet arrays require multiple devices with multiple power sources, the present plasma sheet source may utilize a single device powered by a single power source. The use of a dielectric-coated electrode produces plasma that is at or near room temperature, so that the presently disclosed plasma sheet sources are compatible with biological specimen treatment.

As used herein and known in the art, the term “plasma” refers to a gas comprised of ions and/or free electrons. Plasma may be partially or fully ionized and may be formed by heating a neutral gas or subjecting a neutral gas to an electrical field.

As used herein and known in the art, the term “atmospheric pressure plasma” refers to plasma that is maintained at a pressure approximately equal to atmospheric pressure. No vacuum or pressurized containers are required to maintain atmospheric pressure plasma.

A plasma sheet source10is shown inFIG.1. Plasma sheet source10includes a gas inlet23through which a gas9(FIG.6) enters a body27of plasma sheet source10. The body27has channels18(FIG.3) through which gas9flows and is directed toward an electrode20(FIG.2) within the body27, at which point the gas9is ionized by the electrical field produced by electrode20and becomes plasma8. As plasma8exits plasma sheet source10, it is directed into the form of a plasma sheet at a plasma outlet25. Plasma8is then configured to treat the surface of a specimen26. In the embodiment depicted byFIG.1, the gas inlet extends from one side of the body27, and the opposite side of the body27has an outlet25(FIG.3) through which the plasma8exits the body27. The outlet25may be an elongated slit extending along the side of the body27, though other configurations of the gas inlet23, body27, and outlet25are possible in other embodiments.

Plasma sheet source10may be manufactured using additive manufacturing techniques or injection molding. In some instances, plasma sheet source10is 3D printed and comprises polylactic acid (PLA), though other non-conductive materials and manufacturing techniques may be used. For instance, the plasma sheet source10may be composed of polyethylene terephthalate (PET), high-density polyethylene (HDPE), polyvinyl alcohol (PVA), nylon, and acrylonitrile butadiene styrene (ABS). Components or features of plasma sheet source10are in some instances produced as a unitary construction, though in other instances not depicted the components or features are manufactured separately and attached using commercially available attachment means.

As shown in the block diagram ofFIG.2, plasma sheet source10includes a plurality of channels18and at least one electrode20. Different embodiments of plasma sheet source10with different features are described below. Channels18are formed directly into the material of body27and are configured to direct gas9toward electrode20. The gas flows over and past the electrode20toward the outlet25, and the electrical field generated by the electrode20converts the gas9to plasma8as it flows through the electrical field. The dimensions and number of channels18may be selected and scaled based on several factors, including the desired size of plasma sheet source10and flow rate of gas9. One embodiment, shown inFIG.3, shows channels18as angled slots leading to electrode20. In other embodiments, such as that depicted inFIG.4, channels18are slots that are angled more toward the center of plasma sheet source10than those inFIG.3. In some embodiments shown below, channels18consist of repeated features of varying shapes and sizes.

As shown byFIG.3, the body27has a cavity51that receives gas9flow through the gas inlet23. Pressure from at least one gas source12forces the gas9from the cavity51through the channels18into a cavity52in which the electrode20is positioned. The electrical field generated by the electrode20converts the gas9into plasma8, which is forced out of the cavity52through the outlet25by the pressure from the gas source12and flow of gas9into the cavity52. As shown byFIG.3, the channels18may be parallel which helps to keep the plasma sheet formed in the cavity52uniform and homogeneous.

FIG.2depicts an exemplary embodiment of a system29that uses a plasma sheet source10to direct plasma toward a specimen26. Gas9is provided to plasma sheet source10from at least one gas source12. Gas source12may contain the gas9or a component of gas9in a pressurized setting. As an example, the gas source12may be a pressurized tank that holds the gas9until it is dispensed from the tank. The flow of gas9from gas source12may be modulated or otherwise controlled using a controller14with a valve16that at least partially opens to allow gas flow towards plasma sheet source10. In this regard, when plasma is to be generated, the controller14controls the valve16so that it at least partially opens to allow gas to flow from the gas source12to the plasma sheet source10. The rate of gas flow may be controlled by the extent to which the valve16is opened based on the pressure under which the gas9is contained in the gas source12.

Note that the controller14may be implemented in hardware or any combination of hardware, software, and/or firmware. As an example, the controller14may be implemented as a field programmable gate array (FPGA) or an application-specific integrated circuit (ASIC). In some embodiments, the controller14may comprise one or more processors programmed with software or firmware to perform the control functionality described herein. The controller14also may have one or more user interfaces (not shown), such as buttons, dials, switches, keypads, display screens, or other types of devices for receiving or providing user inputs or outputs. As an example, when the plasma sheet source10is to be used, a user may provide a user input that causes the controller14to open the valve16to allow gas9to flow to the plasma sheet source10. When use of the plasma sheet source10is no longer desired, a user may provide another user input that causes the controller14to close the valve16, thereby stopping the flow of gas9to the plasma sheet source10. Other techniques for controlling the flow of gas9are possible in other embodiments.

Gas9is ideally a noble gas, such as helium, neon, argon, or krypton, to facilitate ionization of the gas9by the electrical field from the electrode20. That is, less energy is generally required to ionize noble gases relative to other types of gases. However, if desired, other types of gases9may be used in other embodiments. In some instances, the gas9may be a mixture of gasses that include at least one noble gas as the “working gas” and optionally one or more additional gasses that are application-specific. For instance, when seeds are to be treated, nitrogen gas may be mixed with a noble gas to form gas9. In some embodiments, gas9is output at 2-6 L/min, though other output speeds and flow rates may be used. In some embodiments, gas flow rates may be similar to those used for conventional plasma jets. Gas9may be conveyed from gas source12to gas inlet23through tubing (not specifically shown inFIG.2), such as one or more tubes that connect the gas source12to the valve16and the valve16to the gas inlet23of the plasma sheet source10. Gas inlet23leads to channels18, and thus allows gas to enter plasma sheet source10and reach electrode20.

In some embodiments, the electrode20is elongated and cylindrical in shape, as shown byFIG.5, though other electrode shapes and designs are possible in other embodiments. InFIG.5, cylindrical electrode20is depicted in cross section. The electrode20has a conductive core32that is electrically connected to a power source24(FIG.2), and the conductive core32generates an electrical field when powered by the power source24. InFIG.2, the power source24is electrically connected to the electrode20by one or more wires22, but other techniques and components may be used to electrically connect the power source to the electrode20. In some embodiments, the power source24provides pulsed direct current at about 1-6 kHz, though other frequencies are possible. In some embodiments, the current supplied is similar to that used in typical plasma jets. In some embodiments, the frequency of the power signal from the power source24is about 6 kHz at a voltage of about 6 to 10 kV, though other voltages and frequencies are possible. In addition, other types of current may be applied to the electrode for generating the electrical field in other embodiments.

The conductive core32is at least partially covered by dielectric covering34that prevents the flowing plasma8from contacting the core32. In the embodiment shown byFIG.5, the dielectric covering34completely surrounds the core32. The material of dielectric covering34may be quartz, though other types of dielectric materials may be used in other embodiments, such as glass, ceramic, or a polymer. Using dielectric material for the covering34prevents plasma8from forming an arc with the core32and allows plasma8to remain at a lower temperature, which may be close to room temperature, or about 20° C., in some instances. Dielectric covering34acts as a barrier that prevents the plasma8from flowing to the core32, yet the dielectric properties of the covering34permit the electrical field generated by the core32to pass without significant attenuation.

Electrode20may pass through holes31(FIGS.3and4) in the sides of plasma sheet source10, as shown inFIG.12. Such holes31may be dimensioned such that the electrode20snugly fits within holes31, and the electrode20may be held in place by friction between the electrode20and the walls of the holes31. In other embodiments, other techniques for securing the electrode20to the body27are possible. The position of electrode20is such that channels18are above electrode20, while plasma outlet25is below electrode20. Note that electrode20positioning closer to channels18generally increases plasma neutralization before exiting through plasma outlet25, while electrode20positioning closer to plasma outlet25generally reduces the chemical changes that are made to the surface of specimen26. Thus, the position of electrode20between channels18and plasma outlet25may be selected to balance the above considerations for optimization of specimen treatment. The longitudinal axis along the length of the cylinder of electrode20may be parallel to the elongated plasma outlet25to help keep the generated plasma sheets more homogenous across the width of the outlet25.

FIGS.6and7depict computational fluid dynamics simulations of gas9flowing through channels18of embodiments of plasma sheet source10toward electrode20. After reaching electrode20, gas9is ionized and becomes plasma8, which then flows to exit plasma sheet source10at plasma outlet25.

FIG.8shows plasma8exiting plasma outlet25and treating specimen26. Plasma sheet source10is distanced between 0-1 cm from specimen26for treatment with plasma8in the form of a plasma sheet. In the embodiment shown, plasma outlet25has length of approximately 2 inches and a width of approximately 0.5 inches. However, other lengths and widths are possible in other embodiments. In some embodiments, the specimen26is kept stationary during treatment or application of the plasma sheet, with plasma sheet source10moved along the surface of specimen26. However, in other embodiments, the specimen26may be moved (e.g., on a conveyor belt or otherwise moved) to allow treatment by a stationary plasma sheet source10. Plasma sheet source10is handheld in some embodiments, while in other embodiments it is mounted and/or automated in movement and positioning.

InFIG.9, a graphical representation of chemical emission from treatment along a long edge of plasma sheet source10is shown. The length of plasma outlet25is 2.5 inches inFIG.9, with higher emission intensities for OH and N2* in the center of the long edge, where coverage is generally homogenous. Near edges, treatment results in lower emissions of OH and N2*. Similarly, inFIG.10, a graphical representation of chemical emission from treatment along a short edge of plasma sheet source10is shown. The width is shown from 0 to 0.6 inches, with more homogenous emissions of OH and N2* across the width relative to the homogeneity of the treatment across the length.

FIG.11is a block diagram depicting a system40that uses more than one gas9in the production of plasma8by plasma sheet source10. The system40depicted inFIG.11includes a first gas source11and a second gas source13, which store different gasses. In some cases, one gas source11stores a noble gas “working gas” and the other gas source13stores an application-specific gas, such as a gas that changes the surface properties of specimen26. First and second gas sources11,13contain gas in pressurized tanks. First gas source11provides gas using a first controller15with a first valve19. Similarly, second gas source13provides gas using a second controller17with a second valve21. As an example, the gas sources11,13may be pressurized tanks that hold the gas until it is dispensed from the tank. The flow of gas from gas sources11,13may be modulated or otherwise controlled using controllers15,17with valves19,21that at least partially open to allow gas flow towards plasma sheet source10. In this regard, when plasma is to be generated, controllers15,17control valves19,21so that they at least partially open to allow gas to flow from gas sources11,13to the plasma sheet source10. The rate of gas flow may be controlled by the extent to which valves19,21are opened based on the pressure under which gas is contained in gas sources11,13. The flow rates from first gas source11and second gas source13are the same in some instances and differ in other instances. Gas from each source is directed by tubing to a Y connection28that combines the individual gasses to one gas stream, which is to enter plasma sheet source10as gas9. Gas9is directed from Y connection28to gas inlet23by tubing30. Note that controllers15,17may be implemented in hardware or any combination of hardware, software, and/or firmware, as described in the single gas source system above in greater detail.

Prior to entry into gas inlet23, gas9is in some instances mixed so that individual component gasses are uniformly distributed. This mixing may be enhanced or facilitated using turbulence introduced by features in the interior of tubing30or using friction from roughened inner walls of tubing30. Additionally, while the block diagram inFIG.11shows two gas sources, more than two gas sources may be used in other embodiments. In the case where multiple gas sources are used, gas streams from each may be combined into a mixture to form gas9.

Regardless of the number of gas components making up gas9, it is directed toward electrode20by channels18within plasma sheet source10, as described above. After plasma8is produced, it may be directed to plasma outlet25and applied to specimen26as a plasma sheet.

As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.