Abstract:
Electron charging of particles, as may be associated with electrographic and/or xerographic image forming devices, powder coat finishing devices and/or guns, or the like, is performed by subjecting a stream of particles to electron bombardment from at least one electrode overcoated with nanotubes, such as carbon nanotubes. An alternating electric field may be employed to reduce the possibility of charged particles being deposited on opposing electrodes in an electron charging zone defined by the electrodes. Particles of varying sizes, and of irregular shapes, may be uniformly charged while required input voltages to the system are reduced based on efficiencies gained through nanotube technology.

Description:
BACKGROUND 
     This disclosure is directed to systems and methods for charging particles with electron emission from carbon nanotubes and other nanotube variants, such as systems and methods for uniformly charging toner particles for use in document image forming, copying and printing devices, and/or pigment particles for use in electrostatic powder coat finishing applications. 
     Electrostatically charged particles are used as coloring agents in a number of different practical applications. These applications include electrophotographic and/or xerographic production and reproduction of documents throughout the electrophotographic industry, and electrostatic powder coat finishing processes employed in the painting and finishing industries. The advantages of using powder toners and/or coatings, when compared to liquid inks and other related coatings, include: ease of cleaning; reduction in the use of solvents; ability to collect overspray for potential reuse; and better control of uniformity of image or film thicknesses. In typical applications, electrostatically charged toner particles in image forming devices vary in size in a range of 3–15 μm in diameter, while electrostatically charged pigment particles used for powder coating have an average particle size in the range of approximately 30–40 μm in diameter. In either application, typically the particles are transported from a hopper to an apparatus in which an electrostatic charge is imparted to the particles prior to being further brought into contact with a charged or grounded surface upon which the particles are to be deposited and affixed. 
     Traditionally, triboelectric, or frictional electric, charging is typically the phenomenon and/or process which is used to impart the electrostatic charge to the particles. U.S. Patent Publication No. US 2004/0184840 A1 to Hays, published on Sep. 23, 2004 (hereinafter referred to as the “&#39;840 Publication”), which is commonly assigned and the entire disclosure of which is incorporated herein by reference, describes the process of triboelectric charging and catalogs some of the advantages and disadvantages of triboelectric charging, specifically of toner particles. 
     The &#39;840 Publication teaches that triboelectric charging is widely used in the eletrophotographic industry to charge toner particles. Triboelectric charging is performed by rubbing two dissimilar materials together, and in the course of such rubbing, a charge is transferred from one body to the other. A disadvantage is that the triboelectric charging phenomenon is very sensitive to surface conditions of the particle as well as temperature and humidity. Charge deposited on particles, particularly irregularly surfaced particles, can easily become non-uniform, resulting in localized areas of higher charge concentration on the particles. This increases the localized adhesion state of such particles to any surface, including the internal structural surfaces of the devices within which the electrostatically charged particles are manipulated. In electrophotographic and/or xerographic image forming devices, for example, this unwanted particle adhesion results in electrostatically charged particles such as, for example, toner particles, being “captured” along the pathways leading to an image on an image-carrying member, such as, for example, paper. This unwanted adhesion decreases the efficiency of the device because less than all of the available toner reaches the photoreceptor and subsequently the image-carrying medium. Reducing these adhesion forces results in delivering more toner to the image-carrying medium, thereby increasing the efficiency of transfer of the toner to the medium and the quality of the images produced. Additionally, there is a concomitant reduction in a requirement to clean adhered toner from the inside of the device and/or in a requirement to clean residual toner held by the photoreceptor. 
     For electrostatic powder coating applications, both tribocharging charging and ion charging guns are employed to charge the powder. For a tribocharging gun, an electrostatic charge is generated by friction between pigment particles and the gun barrel. When, for example, individual particles of epoxy powder used in powder coating move through a powder gun barrel, the particles rub against the inner surfaces and are electrostatically charged. For this reason, typical triboelectric powder guns used for powder coating have a long and tortuous barrel in order to increase the inside surface area, thereby increasing the triboelectric charging of the particles. A drawback of the tribocharging gun is that the types of powder materials that are compatible with the triboelectric charging process are limited. 
     For ion charging guns, a corona from a high voltage coronode wire is used to generate the electrostatic charge. Typical powder ion charging guns employed in powder coating applications require very high voltage power supplies of as much as 100 kV. Some of the ions are captured on the powder whereas the majority of ions are collected on the article to be coated. High ion charging of the coating is not desired since back ionization can occur that causes coating defects. 
     The &#39;840 Publication proposes an ion-charging device for toner particles seeking to overcome disadvantages of conventional triboelectric charging and ion charging methods. The &#39;840 Publication teaches a device for electrostatically charging toner particles by exposing the toner particles to unipolar gas ions emitted from opposing scorotrons that each consist of a coronode with pins of corona emitting points in combination with an electrically biased screen. The coronodes and screens are connected to power supplies through a network of high voltage diodes and resistors. A high voltage, e.g., in a range of 8 kV, AC power supply connected to the opposing scorotons through the diode and resistor network causes ions of a single polarity to be alternately generated by each scorotron during each half cycle in the region between the scorotrons where toner particles are entrained in an air stream. Thus, the toner particles accumulate charge of a single polarity as they flow through the zone between the scorotrons. The alternating electric field in the charging zone prevents deposition of toner on the coronodes and screens of the opposing scorotrons. The invention disclosed in the &#39;840 Publication includes an apparatus for charging particles prior to being delivered to a development delivery device. 
     The &#39;840 Publication explains that toner particle charging with ions has a number of advantages including insensitivity to toner material surface properties thereby making the proposed device and/or process adaptable to particles with irregular surfaces. The &#39;840 Publication points out that the disclosed device more uniformly charges irregular or spherical-shaped toner particles with further advantages of a reduction in toner adhesion to surfaces. The toner charging device disclosed in the &#39;840 Publication may be viewed as a separable component of the disclosed image forming devices and is, in fact, described as an “interface” between various methods for supplying toner to such a device and developing an electrostatic image with ion-charged toner. The &#39;840 Publication describes that the disclosed charging devices and methods improve electrophotographic development, electrostatic transfer and the ability of the overall image-forming device to be cleaned. 
     SUMMARY 
     It would be advantageous to continue to increase the efficiency of an electrostatic charging process by developing devices which may provide uniform charging of particles while enabling reduced particle adhesion. Further, were efficiencies obtainable which would allow a reduction in input voltages from current levels of, for example, approximately 8 kV for charging toner particles in image forming device and up to approximately 100 kV to generate a corona in an ion powder coat spray gun, simplified and more compact devices may be made available that may provide electrostatic particle charging capabilities across a broad spectrum of applications. 
     Nanotube technologies provide the opportunity to achieve such efficiencies. Carbon nanotubes, for example, represent a new molecular form of carbon in which a single layer of atoms is rolled into a seamless tube that is on the order of, e.g., 1 to 10 nanometers in diameter and up to hundreds of micrometers in length. Multi-walled nanotubes were first discovered by NEC Labs in 1991. Two years later, single-walled nanotubes were discovered. Nanotubes exhibit extraordinary electrical, mechanical and thermal conductivity properties. The thermal conductivity, for example, is much higher than that of copper. Nanotubes can be fabricated by a number of methods including carbon arc discharge, pulsed laser vaporization, chemical vapor deposition (CVD) and high-pressure carbon monoxide vaporization. It should be appreciated that other material variants of carbon nanotubes can be used for charging device such as those disclosed herein. Examples of nanotube material variants include boron nitride, bismuth and metal chalcogenides. 
     In simplest terms, a carbon nanotube, on a microscopic scale, appears like a hexagonally shaped poultry wire mesh formed of hexagonal carbon rings. Associated with each ring, or with each junction in the mesh, there is an unbound electron. These unbound electrons are free to move about allowing the entire mesh to act like a good conductor. Therefore, each carbon nanotube exhibits relatively high electrical conductivity because the electrons are not bound, but rather are free to move throughout the mesh. When a voltage is applied to create a high voltage electric field at the tip, electrons can tunnel out of the tube end to become field-emitted electrons. 
     In ion-charging devices for electrophotography, corotron wires are typically approximately 70 μm in diameter, and voltages of approximately 7 kV are required. In the case of carbon nanotubes, because they are relatively so much smaller, the voltages required to obtain the high fields resulting in electron emission are considerably less, for example reduced in many applications by an order of magnitude (or a factor of 10). Such voltage reductions may not only result in simpler and potentially safer devices but may also aid in reducing and/or eliminating undesirable byproduct gaseous components produced by an ion gas discharge in ion-charging devices. Such byproducts include, but are not limited to, ozone and/or nitrogen oxide. 
     With the CVD fabrication method discussed above, when used with a substrate catalyst, the nanotubes tend to be single-walled nanotubes and orient perpendicular to the substrate. Such structure lends itself to potential application in any manner of electron field emission devices such as, for example, displays. Japanese Patent Publication Nos. 2002-268328 to Toshihiro et al. and 2002-279855 to Akishige describe carbon nanotube electron field emission devices for the direct charging of photoreceptors in electrophotographic devices. These direct charging devices are concerned with operational stability of carbon nanotube field emission currents when operating a carbon nanotube charging device under atmospheric conditions, thereby leading to proposals for systems and methods to mitigate these concerns. 
     It may be advantageous to improve an electrostatic charging device of the &#39;840 Publication by replacing the scorotrons, and diode and resistor network, with nanotube-overcoated plates or electrodes. For example, carbon single-walled nanotube fields could be formed perpendicular to a substrate thereby creating electron-emissive plates. Voltage requirements for such an electrostatic charging device may be significantly reduced while advantages of being able to impart a substantially uniform electrostatic charge to irregularly-shaped particles while minimizing particle adhesion are maintained. Such a device may find advantageous application not only in electrophotographic or electrographic image reproduction devices but also in devices concerned with particles of various and larger particle size such as, for example, electrostatic powder coat finishing devices and spray guns. 
     Exemplary embodiments of disclosed systems and methods may provide electron charging of electrostatic particles, particularly toner particles used for development of electrophotographic images in electrophotographic and/or xerographic image forming devices, and/or pigment particles used to apply paint-like finishes in powder coat finishing devices and/or spray guns. 
     Exemplary embodiments of disclosed systems and methods may provide electron emissions by applying an alternating voltage, such as, for example, a square-wave alternating voltage, to opposing electrodes overcoated with nanotubes, such as, for example single-walled carbon nanotubes. Such electrodes may be formed as plates and arranged substantially parallel to, and opposing, one another to form an electron-charging zone between the plates. Electron emission occurs alternately from each nanotube-overcoated electrode during a half cycle of the applied alternating voltage. 
     Exemplary embodiments of disclosed systems and methods may employ carbon nanotubes as the basic elements of which each of a plurality of electrode plates are formed. 
     Exemplary embodiments of disclosed systems and methods may employ electron emissions to charge particles flowing between a pair of electron-emissive electrodes and/or electrode plates subjected to an alternating voltage in order to impart increasing levels of electrostatic charge to the particles while decreasing the potential for particle deposition within an electron-charging zone within a device. 
     Exemplary embodiments of disclosed systems and methods may employ reduced input voltages, compared to those required in conventional ion-charging devices, in order to impart the same electrostatic charge to the same-sized particle. 
     Exemplary embodiments of disclosed systems and methods may find utility in a broad array of applications for electrostatically charging particles to include, but not be limited to, electrophotographic and/or xerographic image forming devices, and/or electrostatic powder coat finish applying devices and/or spray guns. 
     Exemplary embodiments of systems and methods according to this disclosure may reduce and/or eliminate undesirable byproducts produced by an ion gas discharge in ion-charging devices. Such byproducts include, but are not limited to, ozone and/or nitrogen oxide. 
     These and other features and advantages of various disclosed embodiments are described in, or apparent from, the following detailed description of various exemplary embodiments of systems and methods according to this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of systems and methods according to this disclosure will be described, in detail, with reference to the accompanying figures, wherein: 
         FIG. 1  illustrates a simple schematic diagram of an exemplary electron-charging device for charging electrostatic particles; and 
         FIG. 2  illustrates an exemplary embodiment of a device for delivering, charging, and collecting particles, including an exemplary electron-charging device for charging particles according to the systems and methods of this disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The following detailed description of various exemplary embodiments of systems and methods for electron charging of electrostatic particles may alternatively refer to specific applications wherein toner particles are charged to support the production of images in electrophotographic and/or xerographic image forming devices, or wherein pigment particles are charged to support finish applications using electrostatic powder coat finishing devices in the painting and/or finishing industries. These references are included in order to enhance clarity of this disclosure and to provide ease of description and depiction of a readily available set of applications into which exemplary systems and methods according to this disclosure may be incorporated. It should be appreciated, however, that the principles disclosed and described herein may apply to any system or device that performs electrostatic charging of particles. 
       FIG. 1  illustrates a simple schematic diagram of an exemplary electron-charging device  100  for charging electrostatic particles. As shown in  FIG. 1 , an exemplary electron-charging device  100  includes at least one electrode  120 , shown here as a plurality of electrodes  120 , 140  which are overcoated with, or otherwise formed of, nanotubes  125 , 145 . A nanotube forming process whereby the nanotubes  125 , 145  are formed so as to have the principle axis of the nanotubes perpendicular to the substrate which comprises the electrode  120 , 140  is preferred. It should be appreciated that other irregular structures of nanotubes can also be used for the purposes of charging devices such as those disclosed herein. 
     Exemplary nanotube-overcoated electrodes  120 , 140  are connected to at least one power supply  160  which supplies voltage, e.g., alternating voltage, to each of the electrodes  120 , 140 . An objective of supplying voltage to the electrodes is to promote electron emission from at least one electrode into an electron-charging zone based on an electrical potential generated in the electron-charging zone between the at least one electrode and an electrical ground. In the embodiment shown in  FIG. 1 , two electrodes  120 , 140  are arranged substantially parallel to, and opposing, one another to form the electron-charging zone  170  between the electrodes  120 , 140 . In this configuration, alternating voltage, and preferably a square-wave alternating voltage, may be applied to each of the electrodes  120 , 140  in such a manner that the voltages applied respectively to the electrodes  120 , 140  are 180 degrees out of phase with one another. 
     Particles  180  that are intended to be electrostatically charged are conveyed, in some manner, such as, for example, in an-air or gas fluidized stream, into the electron-charging zone  170 , being caused to move through that zone generally in a direction A. When the particles  180  enter the electron-charging zone  170 , electron emission from the nanotubes  125 , 145  of electrodes  120 , 140  bombard the particles  180  alternately, from opposing sides of the electron-charging zone  170 , with electrons emitted alternately from the opposing electrodes  120 , 140  in response to the alternating voltage being applied by the power supply  160 . This mechanism of alternate bombardment of the particles  180  by electrons alternately emitted from opposing electrodes  120 , 140  imparts electrostatic charge to the particles  180  and also tends to keep the particles  180  centered as they are axially conveyed through the electron-charging zone  170 . 
     It should be appreciated that although the electron-charging zone  170  depicted in  FIG. 1  comprises two nanotube-overcoated electrodes  120 , 140  to which a power  160  supplies alternating voltage, a single nanotube-overcoated electrode opposing an uncoated electrode, or any number of multiple electrodes, may be appropriately configured to form an electron-charging zone  170  for imparting electrons from a nanotube overcoating in response to an applied alternating voltage. Further, it should be appreciated that direct current voltages may be applied in embodiments as well provided the particle conveyance with a gas stream is sufficient for the particles to exit the charging zone  170 . 
       FIG. 2  illustrates an exemplary embodiment of a device  300  for delivering, charging, and collecting particles including an exemplary electron-charging device  100  for electrostatically charging particles according to the systems and methods of this disclosure. As shown in  FIG. 2 , a blower  310 , or other air forcing device, may be available to generate an air or gas supply in order to entrain particles  380  in an airborne stream. A dispenser  320  may be provided with an agitation device  325  (shown as an auger type agitator in this embodimenta) attached to, or housed in, the dispenser  320  in order to agitate or otherwise move the particles  380  toward an exit from the dispenser  320  in order that the particles may be more easily entrained in the airborne stream produced by the blower  310 , or may be otherwise conveyed from the dispenser  320 . 
     It should be appreciated that any configuration in which particles are removed from a dispenser  320  or reservoir and conveyed through an electron-charging zone  270 , as shown in  FIG. 2 , is contemplated. Many and widely diverse variations in configuration exist based on the specific application to which the electrostatically charged particles are intended to be used. For example, the dispenser  320  may represent a toner bottle containing toner particles for use in forming images in an electrophotographic and/or xerographic image forming device. Alternatively, the dispenser  320  may represent a powder coat powder reservoir attached to a powder coat spray gun. 
     Particles entrained in an air stream are transported via a channel  330  to an exemplary electron-charging device  100 . 
     It should be appreciated that the channel  330  for conveying particles entrained in an air stream, or otherwise, from a dispenser  320  to an electron-charging zone in an exemplary electron-charging device  100  may include any of a rigid pipe and/or standpipe, and/or a flexible or semi-flexible hose depending on the required configuration by which the particles should be conveyed to the electron-charging device  100 . 
     The electron-charging device  100  may subject the particles  380  conveyed through the electron-charging device  100  to alternate electron emission from either side of an electron-charging zone. Once the particles  380  are substantially uniformly charged with electrons, the particles may be deposited on an exemplary collection device  340 . Such a collection device  340 , when present, may, for example, represent an electrostatically-charged photoreceptor, or an electrostatically-charged image receiving medium in an electrophotographic or xerographic image forming device. Alternatively, the collection device  340  may represent a reservoir from which pigment particles may be otherwise conveyed to any manner of powder coat delivery system. Additionally, uncollected particles may be supplied separately to some manner of collection chamber  350  which may include a vacuum plenum type device to further aid circulation of the particles  380  through the system and on to any manner of delivery device. Particles which do not adhere to the collection device  340  and/or are not otherwise conveyed to a delivery device, may be collected or otherwise filtered in the collection chamber  350  for potential recirculation and/or reuse. 
     It should be appreciated that once the particles  380 , appropriately air entrained or otherwise conveyed through the electron-charging device  100 , become electrostatically charged, conventional conveying devices for transporting the particles  380  for use may be employed. In the case of electrophotographic or xerographic reproduction, the particles may be transported by a conventional device or system of devices to a photoreceptor and/or an image receiving medium in order to be deposited on an image receiving medium for forming an image in the electrophotographic and/or xerographic image forming device. For powder coat finishing applications, the electrostatically charged particles may be picked up in a second stream of high pressure air to be dispensed via an electrostatic powder coat spray gun nozzle, or through a dispensing apparatus in a powder coat spray booth, resulting in dispersion of the electrostatically charged particles as a powder coat cloud to be deposited on a structure intended to be powder coat finished. 
     Incorporation of disclosed systems and methods for electron charging of particles in conventional electrostatically-charged particle applications are contemplated. Because the range of devices within which such an electron-charging device may be installed is widely varied, yet conventionally known, individual depictions and/or descriptions of the details of those devices are not required. 
     It should be appreciated that, while disclosed systems and methods have been described in conjunction with exemplary electrophotographic and/or xerographic image forming devices, or alternatively in conjunction with electrostatic powder coat finish applying devices and/or guns, systems and methods according to this disclosure are not limited to such applications. Exemplary embodiments of systems and methods according to this disclosure may be advantageously applied to virtually any device, machine or equipment wherein electrostatically charged particles may be advantageously charged by electron emissions from electrodes formed of or overcoated with nanotubes, particularly carbon nanotubes. Advantages of the use of disclosed exemplary systems and methods may include: reduced input voltages for electrode plates which define an electron-charging zone of a device; substantially uniform charge distribution to varying sizes of possibly irregularly shaped particles; reduced deposition of particles within the charging zone; and reduced particle adhesion to surfaces. 
     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.