CERMET AS A DIELECTRIC IN A DIELECTRIC BARRIER DISCHARGE

DEVICE

BACKGROUND The present invention relates to a discharge device for generating a plasma.

More particularly, the present invention relates to using a cermet as a dielectric material in a dielectric barrier discharge (DBD) device.

Dielectric barrier discharge (DBD) devices are well-known for use as plasma generators. A plasma is a highly ionized gas composed of ions, electrons, radicals and ozone. Plasmas are commonly used for air purification and various other applications.

In an exemplary design, a DBD device consists of two electrodes separated by a gap. A dielectric layer is attached to each electrode such that an electrical charge builds up on surfaces of the dielectric layers and is discharged as streamers, which form the plasma. The dielectric layer is typically a nonconductive, insulating material, such as glass or ceramic. In the example in which the plasma is used for air purification, air is passed through the gap separating the two electrodes. The plasma attacks contaminants in the air stream, and converts the contaminants to less harmful or harmless by-products.

SUMMARY The present invention relates to a discharge device for producing a plasma of electrons, radicals, other ions and ozone. In an exemplary embodiment, the discharge device is a dielectric barrier discharge (DBD) device that includes a first electrode and a second electrode configured to receive current from a power supply. A first cermet dielectric layer is attached to the first electrode. A second cermet dielectric layer is attached to the second electrode such that the first cermet dielectric layer and the second cermet dielectric layer face one another and are separated by a gap. The cermet dielectric layers may include at least one of copper, aluminum, nickel and molybdenum. In preferred embodiments, the metal content in the cermet dielectric layers is controlled in order to remain below the percolation threshold. In an exemplary embodiment, the DBD device is used in an air purification system to remove contaminants from an air stream. To accommodate larger volumes of airflow, in some embodiments, the DBD device may include one or more additional pairs of electrodes, which have cermet dielectric layers and may be stacked on the first and second electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of a dielectric barrier discharge (DBD) device. FIG. 2 is a schematic of an existing DBD device using glass or ceramic as the dielectric material.

FIG. 3 is a schematic of a DBD device of the present invention, which uses a cermet as the dielectric material.

FIG. 4 is a plot of current as a function of time comparing the DBD devices of FIGS. 2 and 3. FIG. 5 is an exploded view of a portion of the DBD device of FIG. 3.

FIG. 6A is a schematic of a DBD device of the present invention having multiple pairs of electrodes stacked on one another, and designed for use in an air handling system.

FIG. 6B is an exploded view of a portion of the DBD device of FIG. 6A to better illustrate how the electrode pairs are stacked.

FIG. 7 is a schematic of an air handling system that includes a DBD device of the present invention inside a duct of the air handling system.

FIG. 8 is a schematic of the air handling system of FIG. 7, which includes alternative or additional locations for a DBD device. FIG. 9 is an alternative design of the air handling system of FIG. 7 in which the DBD device is located in a duct by-pass.

It is noted that the figures are not to scale.

DETAILED DESCRIPTION A system and method is described herein for using a cermet layer as a dielectric in a dielectric barrier discharge (DBD) device used for generating a plasma. A cermet is a composite of ceramic and metal, and thus exhibits some conductivity. In existing designs, the dielectric material is commoniy formed from an insulating, nonconductive material, such as glass or ceramic. The dielectric discharges electric current as microstreamers into an air stream and the microstreamers ionize the air, thus forming the plasma. A cermet dielectric layer, although it has a lower dielectric strength than a ceramic layer, results in a discharge of more microstreamers, while using less current. An improved plasma is formed using a more efficient DBD device. Although the cermet dielectric is

described herein as being used in a dielectric barrier discharge device, it is recognized that the system and method of using a cermet as a dielectric layer may apply to other plasma generating devices that use a dielectric, such as a resistive barrier discharge device.

FIG. l is a schematic of dielectric barrier discharge device 10 which includes power supply 12, first electrode 14, second electrode 16, first dielectric 18, and second dielectric 20. First dielectric 18 is attached to first electrode 14, second dielectric 20 is attached to second electrode 16, and gap G exists between first and second dielectrics 18 and 20. First and second electrodes 14 and 16 are formed of a conductive material, commonly metal. Power supply 12 supplies an alternating current to first and second electrodes 14 and 16, which are oppositely charged relative to one another.

Depending on the cycling of current from power supply 12, either first dielectric 18 or second dielectric 20 builds up electric charge from first electrode 14 or second electrode 16, respectively. Assuming current first flows to second electrode 16, a negative electric charge from electrode 14 builds up on first dielectric 18, since electrons flow in an opposite direction to electric current. A positive charge from second electrode 16 accumulates on second dielectric 20. First dielectric 18 builds up and temporarily stores negative electric charge and thus acts as a capacitor. Dielectric 18 discharges electric current from the surface in the form of microstreamers that start from dielectric 18 and travel towards second dielectric 20. The streamers of electricity ionize the air in gap G and form a plasma of charged electrons, positive and negative ions, long-lived and short-lived radicals, and ozone. The plasma is effective at attacking volatile organic compounds (VOCs) and other contaminants that may be in an air stream that passes through gap G.

As current flows to second electrode 16, a difference in electric potential between first and second electrodes 14 and 16 (i.e. voltage) continues to increase. As the voltage increases, charge builds on first dielectric 18, and microstreamers continue to be released from a surface of dielectric 18. However, once the voltage across electrodes 14 and 16 reaches a maximum value, the microstreamers arc all the way across gap G. Arcing results in an electrical breakdown and a sharp decrease in voitage. Once arcing occurs, the microstreamers are no longer able to generate the plasma. In order to minimize arcing in device 10, power supply 12 is an alternating current (AC) power supply such that a path of the current frequently reverses between first electrode 14 and second electrode 16. Power supply 12 may operate, for example, at 60 Hertz. Once the voltage increases to a certain level, power supply 12 reverses the flow of

current and the current flows to first electrode 14. Electric charge then builds up on second dielectric 20 and microstreamers are discharged from a surface of dielectric 20 to form the plasma in gap G. Device 10 is designed to increase the voltage to a high enough value such that electrical discharge occurs (i.e. microstreamers are released) without arcing (i.e. electrical breakdown). By using alternating current, device 10 minimizes arcing and maximizes a build up and release of electric charge from dielectrics 18 and 20.

FIGS. 2 and 3 are schematics of DBD devices similar to device 10 of FIG. 1. FIG. 2 is a prior art design in which the dielectric layers are formed from glass or ceramic. FIG. 3 is a DBD device of the present invention, which uses a cermet dielectric layer. DBD device 50 of FIG. 2, similar to device 10 of FIG. 1, includes first electrode 54, second electrode 56, first dielectric 58 and second dielectric 60. The electrical power source is not shown in FIG. 2, but it is recognized that an AC power supply is used in device 50 to apply alternating current to electrodes 54 and 56. Microstreamers 62 are shown in gap G and, as described above, represent electrical discharge from dielectrics 58 and 60. Depending on the direction in which the current is flowing, microstreamers 62 travel either from surface 58a of dielectric 58 towards dielectric 60 or from surface 60a of dielectric 60 towards dielectric 58. Given the high frequency with which the current alternates, the direction of flow of microstreamers 62 within gap G is not necessarily distinguishable. In FIGS. 2 and 3, microstreamers 62 and 82, respectively, are shown across the entire gap G between the first and second dielectric layers. However, it is recognized that microstreamers 62 and 82 do not travel all the way across gap G, unless arcing occurs. As described above, with the use of alternating current, arcing is minimized in a DBD device. During the time in which the plasma is being generated in DBD devices 50 and 70, microstreamers 62 and 82 only travel part way into gap G between the dielectrics.

DBD device 50 is a known, existing dielectric barrier discharge device which utilizes a nonconductive, insulating material for dielectric layers 58 and 60. Common dielectric materials include glass and ceramic. Insulating materials are used as the dielectric because they are poor conductors of electricity and thus provide resistance to the flow of electrons. As such, electric charge builds up in the dielectric and the dielectric acts as a capacitor. At a certain point, there is a discharge of current from the dielectric, resulting in microstreamers 62 which form the plasma. The dielectric strength of dielectrics 56 and 58 refers to an amount of charge that the materials are able to store before electrical discharge.

In the case in which dielectric layers 58 and 60 are ceramic, a resistance of the dielectrics may depend, in part, on a porosity of the ceramic material since the electrons travel through holes in the ceramic.

DBD device 70 of FIG. 3 represents the present invention and is configured similarly to DBD device 50 to include first electrode 74, second electrode 76, first dielectric 78, and second dielectric 80. In an alternative embodiment, a DBD device may be used in which only one of the electrodes includes a dielectric layer. DBD device 70 uses an alternating current and microstreamers 82 flow from surfaces 78a and 80a of dielectrics 78 and 80 to form the plasma in gap G. In contrast to DBD device 50 and previously used designs, dielectric layers 78 and 80 are formed from a cermet.

A cermet is a ceramic impregnated with metal particles. Cermets may be classified as dielectrics, but they have a lower dielectric strength than commonly used dielectrics. Due to the metal content, cermets also have higher conductivity than standard dielectrics. When a current is supplied, electrons from first electrode 74 pass to first dielectric 78 such that an electric charge builds on first dielectric 78. Because cermet dielectric 78 contains metal particles, electrons from electrode 74 may pass through dielectric 78 by hopping between metal particles. This is known as electron tunneling or quantum tunneling. (See Likalter, A.A., Hopping conductivity in granular metals near the insulator-metal transition, Physica A (2001), pp. 144-158.) The electrons preferentially contact the metal particles in the cermet. Provided that the metal particles are generally dispersed evenly through the ceramic material, electrical conductivity of dielectrics 78 and 80 is uniform through layers 78 and 80. Consequently, a more diffuse pattern of microstreamers 82 is discharged from surfaces 78a and 80a. As shown in FIG. 3, more microstreamers are discharged from cermet dielectrics 78 and 80, as compared to the number of microstreamers 62 discharged from ceramic or glass dielectrics 58 and 60 of FIG. 2.

A more diffuse pattern of microstreamers 82 results in a greater volume of active plasma, thus making the plasma more effective for its intended purpose, which may be, for example, to remove contaminants from an air stream. If DBD device 70 is used for air purification, DBD device 70 may have a higher single pass efficiency (SPE) compared to DBD device 50.

FIG. 4 is a plot of current as a function of time for DBD device 50 of FIG. 2 and DBD device 70 of FIG. 3. As shown in FIG. 4, the current alternates between the two

electrodes at the same frequency in both devices. However, because the cermet material has a lower dielectric strength than a ceramic material, electric discharge occurs at a lower voltage in DBD device 70. Microstreamers 82 in device 70 of FIG. 3 have a lower charge compared to microstreamers 62 in device 50 of FIG. 2. The plasma is produced using less voltage across electrodes 74 and 76 of device 70, and thus the power supply of device 70 is able to supply less current (i.e. less electricity). This is illustrated in FIG. 4. The maximum current for DBD device 70, which uses a cermet dielectric, is lower than the maximum current for DBD device 50. Therefore, device 70 is more efficient than device 50 at generating a plasma. FIG. 5 is an exploded view of a portion of DBD device 70 of FIG. 3, including second electrode 76 and second dielectric 80. In the exemplary embodiment of FIG. 5, adhesive layer 86 is used to attach dielectric 80 to second electrode 76. Second electrode 76 and second dielectric 80 of DBD device 70 are discussed in detail in reference to FIG. 5. It is recognized that the same features apply to first electrode 74 and first dielectric 78 of FIG. 3.

Second electrode 76 is a conductive material, and is most commonly formed of metal. In an exemplary embodiment, second electrode 76 is a metal plate. Dielectric 80 is a cermet layer. Suitable metals that may be impregnated into the ceramic to form cermet dielectric 80 include, but are not limited to, copper, aluminum, nickel and molybdenum. The amount of metal in the cermet affects the conductivity of the cermet, and consequently affects a performance of the cermet as a dielectric. As described above, electric charge travels through the cermet due to electron tunneling or electron hopping between metal particles. The metal content in cermet dielectric 80 should be below the percolation threshold, which is defined as the point in which the metal particles have continuous conduction channels. If the cermet composition is below the percolation threshold, the metal particles in the ceramic are non-contiguous and the only conduction process in the cermet is electron tunneling. In contrast, if the cermet had enough metal such that continuous conduction channels did exist, then the cermet would essentially operate as a conductor. For effectiveness as a dielectric, the cermet should have some conductivity, while maintaining some resistivity.

The percolation threshold of the cermet depends on the porosity of the cermet, the type of metal or metals impregnated into the ceramic, and the particle sizes of the metal. Smaller particles of metal may lower the percolation threshold. If the weight

percent of the metal is constant, the total surface area of the metal increases as the particle size decreases. This increases the probability of metal particles contacting one another, which in turn lowers the percolation threshold. To remain below the percolation threshold, an appropriate range of the metal content in cermet dielectric 80 is between approximately five and 25 weight percent. In preferred embodiments, the metal content is between approximately five and 20 weight percent. Reference is made to Xie, N., Electrical Conductivity of Inhomogeneous Cu2θ-10CuAlO?-xCu Cermets, Journal of American Ceramic Society 2005, volume 88, pp. 2589-2593.

Cermet dielectric layer 80 may be formed using known techniques, such as, for example, hot pressing. In an exemplary embodiment, cermet dielectric layer 80 has a thickness T between approximately 5 and 10 millimeters. Dielectric layer 80 may be bonded to electrode 76 using adhesive 86. In preferred embodiments, adhesive 86 is an electrically conductive adhesive. Examples of suitable adhesives include, but are not limited to, cyanoacrylate glue and an epoxy, such as a silver filled epoxy. In order to effectively form the plasma, a gap between the electrodes in a dielectric barrier discharge device is minimized. As such, when the DBD device is used for air purification, it is preferred that a low volume of air be passed through the gap in the DBD device in order to maintain a sufficient SPE. Otherwise, it may be necessary to circulate an air stream through a DBD device multiple times. Alternatively, multiple pairs of electrodes may be stacked on top of one another to accommodate larger airflows.

FIG. 6A is a schematic of dielectric barrier discharge device 100 of the present invention which includes multiple pairs 102 of electrodes stacked on top of one another. Individual electrodes and dielectric layers are not shown in FIG. 6A. However, FIG. 6B shows the components or layers that are contained within each electrode pair 102. Device 100 is configured for use in an air handling system and includes dielectrics formed from a cermet, as described above. In the exemplary embodiment shown in FIG. 6A, device 100 includes six pairs 102a- 102f of electrodes. It is recognized that more than six or less than six pairs of electrodes may be used. The number of pairs of electrodes may be determined, in part, based on, a volume of air passing through device 100 and/or an expected contamination level of the air. DBD device 100 has inlet end 104 and outlet end 106. A contaminated air stream flowing into device 100 through inlet end 104 is split such that a fraction of the air flows through a gap created between each pair 102 of electrodes. The air then exits DBD device 100 at outlet end 106.

Each pair 102 of electrodes is connected to an AC power supply (not shown) and operates similarly to DBD device 70 of FIG. 3. Each electrode pair 102 is configured to produce microstreamers 108, which form a plasma that is well suited for attacking contaminants in an air stream. Because DBD device 100 includes multiple electrode pairs 102, a larger volume of contaminated air may pass through DBD device 100, as compared to DBD device 70. As described above, the microstreamers in a DBD device do not span an entire gap between the two dielectric layers, unless arcing occurs. Microstreamers 108 are shown in FIGS. 6A and 6B as covering all of the gap between an electrode pair. However, it is recognized that, during generation of the plasma, microstreamers 108 travel into the gap separating the dielectrics, but not completely across the gap.

FIG. 6B is an exploded view of a portion of DBD device 100 of FIG. 6A and includes electrode pair 102b, as well as a portion of pair 102a and pair 102c. Although electrode pair 102b is described in detail, it is recognized that the remaining electrode pairs 102 contain the same components and operate similarly to pair 102b. Note that, for clarity, a bonding layer (like adhesive 86 of FIG. 5) is not shown in FIG. 6B between an electrode and its corresponding dielectric layer.

Electrode pair 102b includes first electrode 1 14b having first dielectric 1 18b attached thereto and second electrode 116b having second dielectric 120b attached thereto. First and second electrodes 1 14b and 1 16b are similar to electrodes 74 and 76 of FIG. 3. First and second dielectrics 1 18b and 120b are similar to dielectric layers 78 and 80 of FIG. 3 and are formed of a cermet material.

First electrode 1 14b of pair 102b is placed under second electrode 1 16a of pair 102a. Insulator 122 is placed in between electrodes 1 14b and 1 16a to prevent a flow of electrons between them. Insulator 122 is also placed between second electrode 1 16b and first electrode 1 14c of pair 104c. Any material with a high dielectric strength, such as ceramic, may be used to form insulator 122.

As stated above, DBD device 100 may be used in an air handling system for purifying an air stream. FIG. 7 is a schematic of heating, ventilation and air conditioning (HVAC) system 200 for space 202. Space 202 may be an inside of any type of building (for example, a hospital) or an enclosed part of a building. In other embodiments, space 202 may be an enclosed space within a vehicle or another type of transportation device, including, but not limited to, spacecraft, aircraft, land vehicles, trains, cruise lines and other types of marine craft.

System 200 includes DBD device 100, air handling unit (AHU) 204, power supply 206, and ducts 208 and 210. Air handling unit 204 may be used for heating and/or cooling space 202. It is recognized that air handling unit 204 is not required in system 200. In the embodiment shown in FIG. 7, air handling unit 204 is located downstream of DBD device 100. In other embodiments, air handling unit 204 may be omitted from system 200 or located upstream of DBD device 100.

As shown in FIG. 7, outside air 214 enters duct 208 and passes through DBD 100 and then through AHU 204. Conditioned air 216 then travels through supply duct 208 to space 202. Return duct 210 removes air 218 from space 202, at which point a first portion 218a of air 218 is recycled back through system 200 and a second portion 218b of air 218 is exhausted from system 200. Recycled air 218a passes through DBD device 100 on its way back to space 202. DBD device 100 may include a blower for drawing air stream 214 and 218a into DBD device 100. Alternatively, a blower which is part of AHU 204 may be used to draw air into DBD device 100 and then through AHU 204. As described above, dielectric barrier discharge (DBD) device 100 is used to create a plasma of short-lived and long-lived reactive species, including ozone, that may react with volatile organic compounds (VOCs) and other contaminants, and remove the contaminants from the air. In the exemplary embodiment of FIG. 7, device 100 is placed upstream of air handling unit 204 and is used to purify an air stream that includes outside air 214 and recycled air 218a.

FIG. 8 is a schematic of air handling system 200 of FIG. 7 illustrating alternative or additional locations for a dielectric barrier discharge (DBD) device of the present invention. As shown in FIG. 8, in addition to DBD device 100, system 200 includes DBD devices 230, 232, 234, and 236, each of which may include a power supply (not shown) similar to power supply 206. Alternatively, power supply 206 may also be used to deliver power to more than one DBD device.

DBD device 230, as shown in FIG. 8, is placed downstream of AHU 204. In that case, DBD device 230 may likely be used as an alternative to DBD device 100. instead of receiving a mixture of unconditioned outside air 214 and recycled air 218a from space 202, as is the case with DBD device 100, DBD device 230 receives a conditioned air stream from AHU 204. Thus, in some cases, the air stream entering DBD device 230 may be at a lower humidity, as compared to air entering DBD device 100. The DBD device may operate more efficiently if air entering the DBD device contains less humidity.

DBD device 232 is placed within space 202 and, as such, may operate as a stand alone unit. In that case, DBD device 232 may include its own blower. In some embodiments of system 200, DBD device 232 may be used in combination with DBD device 100. DBD device 100 may be used to remove contaminants from outside air 214 and recycled air 218a, which is then delivered to space 202 as clean air 216 through duct 208. DBD device 232 may be used to remove contaminants from air contained with space 202. The combination of DBD devices 100 and 232 facilitates a faster purification of the air contained within system 200.

DBD device 234 is shown inside return duct 210 at a position where exhaust air 218b has already been removed to outside, and recycled air 218a is being returned to supply duct 208. DBD device 234 may be used, similarly to DBD device 232, to remove contaminants from air coming from space 202. In those cases in which it is known that outside air 214 is essentially clean and does not need to be purified, then DBD device 234 may be used instead of DBD device 100. In that case, a lower flow rate may be used, since only recycled air 218a is passing through device 234. A lower flow rate of air through the DBD device results, in some cases, in a higher efficiency of the DBD device.

Finally, DBD device 236 is shown in FIG. 8 near an entrance to duct 208. DBD device 236 may be used alone or in combination with one of the other DBD devices of FIG. 8 when it is known that outside air 214 contains a high level of contaminants. In that case, recycled air 218a from space 202 does not pass through DBD device 236.

FIG. 8 illustrates that a single DBD device or multiple DBD devices may be used within system 200. It is recognized that multiple DBD devices may provide increased purification of air circulating through space 202; however, in some situations, it may not be cost effective to operate more than one DBD device within system 200. As shown in FIG. 8, a DBD device may be located within the duct work of system 200 or as a stand-alone unit within space 202. The DBD devices that are shown in the duct work in FIGS. 7 and 8 may be mounted inside the duct work as a semi-permanent fixture, or they may be portable units that are easily added, moved around, or removed from the ducts, as needed.

FIG. 9 illustrates an alternative embodiment of system 200 in which DBD device 238 is used in a duct by-pass configuration. As shown in FIG. 9, flow diverter 240 may be used to direct a portion of air flowing through duct 242 into duct by-pass 243. Air going through by-pass 243 then passes through DBD device 238. As shown in FIG. 9, DBD device 238 includes blower 244.

The embodiment shown in FIG. 9 may be used in a scenario where it is not necessary to purify all of the air passing through duct 242. Moreover, it is recognized that flow diverter 240 may be modified such that more or less air passes through by-pass duct

243. FIG. 9 further illustrates that the DBD device may be configured in a number of different ways within an HVAC system for air purification.

As described herein, an improved dielectric barrier discharge device utilizes a cermet as the dielectric material. The cermet results in a more diffuse discharge of microstreamers, compared to a standard dielectric, such as ceramic. A more diffuse set of microstreamers results in a greater volume of plasma. Due to electron tunneling, the cermet dielectric has a lower resistance and thus discharges at a lower voltage and requires less current. As such, the DBD device of the present invention utilizes less electricity, resulting in a more effective and more economical device. Although the dielectric barrier discharge (DBD) device of the present invention is described herein in the context of using the plasma for air purification, it is recognized that the DBD device may be used in other applications which utilize a plasma.

In addition to dielectric barrier discharge devices, a cermet dielectric layer may also be used in other plasma generating devices that use electric discharge to produce the plasma. An example of an alternative device is a resistive barrier discharge device, which may include a resistive material on one of the two electrodes and a dielectric material on the second electrode. The cermet layer described herein may be used as the dielectric in the resistive barrier discharge device, which may operate in a direct current (DC) mode or an alternating current (AC) mode.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.