Abstract:
An electrode for atmospheric corona discharge apparatus provide a passive conductor whose surface is decorated with nanostructures such as carbon nanotubes. The nanotubes provide for a lower corona discharge initiation voltage and raise the possibility for reduced ozone production on corona discharge devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application 60/641,858 filed Jan. 6, 2005 hereby incorporated by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   BACKGROUND OF THE INVENTION 
   The present invention relates to atmospheric corona discharge devices, and in particular, to an improved electrode for corona discharge devices. 
   Atmospheric, direct current (DC), corona discharge is used to provide a unipolar ion source for a variety of electrical devices including air cleaners, in which the ions charge particulates to draw them to a collector plate, and photocopiers and laser printers, in which the ions charge a photosensitive drum. 
   Atmospheric corona discharge, as its name suggests, employs a discharge electrode surrounded by air. A steep electrical gradient at the discharge electrode produces a plasma of ionized atoms or molecules near the discharge electrode. Some ions escape from the plasma region to form charge carriers that migrate to a second electrode. Atmospheric corona discharge is readily distinguishable from devices that provide a stream of electrons such as field emission devices and thermionic emission devices, each of which normally operate in a near or complete vacuum. 
   The plasma region in which the ions are generated may convert atmospheric oxygen (O 2 ) to ozone (O 3 ), the latter being a reactive gas that in high concentrations can be a health concern. Ozone can be reduced by using a positive voltage at the discharge electrode. Ozone can also be reduced by limiting discharge current, but at the cost of reducing the number of ions generated, and thus reducing the effectiveness of the associated equipment. Air temperature and air velocity are not major factors in the control of ozone creation for most indoor applications. 
   The ionization of air by the discharge electrode is influenced by the sharpness (radius of curvature) of the discharge electrode such as increases the gradient of the electrical field about the discharge electrode. This relationship is captured in the empirically derived Peek&#39;s equation. Experimental data for different electrode radii as low as 10 micrometers also indicate a reduced ozone production for a given surface current density as the electrode radius decreases. 
   For these reasons, commercial corona devices have employed wire electrodes as small as one micrometer in radius. Such wires provide a small radius of curvature, reducing ozone production and discharge voltage (and thus discharge power consumption) while maintaining an acceptable ion production rate. 
   The ability to further decrease the wire size is limited by practical considerations of wire strength and durability in the typical operating environment of an atmospheric corona discharge device. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the problem of producing a robust discharge electrode with a small radius of curvature by coating a conductive substrate such as a metal wire or plate with nanostructures, for example, carbon nanotubes. The small radius of curvature of the nanotubes provides for a high electrical field strength that may reduce power consumption for a given ion production rate by lowering the necessary voltage needed to produce a given current flow. It is also believed that the small radius of curvature, by reducing the volume of the corona plasma region, will further reduce the interaction of the plasma with oxygen molecules and thus the production of ozone. 
   Specifically then, the present invention provides a corona discharge electrode having a conductive support adapted to be exposed to the air and to receive an electrical voltage. A plurality of conductive nanostructures is attached to, and in electrical communication with, the conductive support. The nanostructures are arranged to provide electrode tips positioned to extend into the surrounding air and having radii less than 100 nanometers to ionize the air at the nanostructure with the electrical voltage. 
   Thus it is an object of at least one embodiment of the invention to provide for extremely small electrode radii using nanostructures, which include nanotubes, nanowires, nanorods, and nanoparticles. 
   The nanostructures may be carbon nanotubes having first ends attached to the conductive support, and second ends extending outward from the conductive support. 
   It is thus another object of at least one embodiment of the invention to provide an nanostructure configuration that can significantly increase the ionization area of the substrate. 
   The carbon nanotubes may be preselected according to whether they are metallic. 
   Thus, it is an object of at least one embodiment of the invention to select nanotubes for improved electrode operation and resistance to erosion. 
   Alternatively, the nanostructures may be carbon nanotubes having a side attached to the conductive support. 
   Thus, it is an object of at least one embodiment of the invention to provide for a simple fabrication technique in which nanotubes are arrayed over a substrate without alignment. 
   The substrate may be either a plate or a wire. 
   Thus, it is an object of at least one embodiment of the invention to provide a flexible electrode design that may match well with the particular application requiring atmospheric corona discharge. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a corona discharge device such as may make use of the present invention using a wire discharge electrode; 
       FIG. 2  is a cross-sectional view through the discharge electrode of  FIG. 1  showing a plasma region that would be expected based on the radius of the wire; 
       FIG. 3  is an enlarged cross-sectional view of the wire of  FIG. 2  showing the endwise attachment of carbon nanotubes to provide for small radius discharge electrodes providing small volume plasma regions; 
       FIG. 4  is an alternative embodiment of the electrode of  FIG. 3  showing a plate electrode having carbon nanotubes attached on their sides to the plate; and 
       FIG. 5  is a graph showing an experimental measurement of the VI curve using the embodiment of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , an atmospheric corona discharge device  10  may provide for a discharge electrode  12  connected to one terminal of a voltage source  14 , the other terminal of which is connected to a return electrode  16 . 
   In a xerographic system such as a copier or printer, the return electrode  16  may be a xerographic plate attracting toner after it has been charged by the atmospheric corona discharge device  10  and photo exposed. In a filtration system, return electrode  16  may be a collector plate for collecting charged dust particles charged by the atmospheric corona discharge device  10 . In a gas chromatograph-mass spectrometer, the return electrode  16  may be an accelerating or analyzing electrode. 
   The high radius of curvature of the discharge electrode  12  produces a region of high gradient electrical field causing electrical disassociation of the atmosphere gases about the discharge electrode  12  producing a plasma region  15  of ions some of which escape as charge carriers  18 . The charge carriers are unipolar ions of the same polarity as the discharge electrode. The charge carriers  18  may impart a charge to the return electrode  16  or react with other particles such as dust to charge the dust and cause it to collect on return electrode  16 . Oxygen passing into the plasma region  15  may become ozone  20 . 
   Referring now also to  FIG. 2 , the electrode  12  may be a wire  22  having a radius  24  typically as small as one micrometer. In commercial devices using the wire  22  alone as a discharge electrode  12 , a relatively large plasma region  15 ′ will be created that promotes the formation of ozone  20 . 
   Referring now to  FIG. 3 , in the present invention, the wire  22  is provided with a surface coating of nanostructures  26 . In this case, single or multi walled carbon nanotubes  28  are arranged with one end of the nanotubes  28  attached to the outer periphery of the wire  22 , and the other end extending radially therefrom. It is believed that the nanotubes  28  may be grown directly off the wire  22  in upright configuration and with a controlled separation. Alternatively, the nanotubes  28  may be attached to the wire  22  after fabrication by their sidewall in a “layed down” configuration. 
   The extremely small radius  17  of the nanotubes  28 , less than 100 nm and typically on the order of a few nanometers, produces an extremely small volume of plasma region  15  in proportion to a discharge area (such as defines the current flow into the plasma region  15 ). Accordingly, dependent in part on the orientation, spacing and length of the carbon nanotube  28 , the discharge area may be controlled independently of the volume of the plasma region  15  to decrease the formation of ozone while maintaining a high production of charge carriers. 
   Generally, the radius  17  is smaller than the mean free path of charge carriers  18  in the plasma region  15 . 
   Peek&#39;s equation generally predicts that the higher radius of curvature of the nanotubes will also decrease the voltage necessary to produce atmospheric corona discharge, decreasing the power needed for corona discharge. However, it was not known whether Peek&#39;s equation breaks down for very small radii because Peek&#39;s equation is empirically based. One possibility is that an increase in field emission for small radii may cause early initiation of a negative corona preventing advantageous production of positive coronas for reduced ozone production. As will be described below, however, the present inventor has determined that the decrease in radius of carbon nanotubes does result in a decrease in corona initiation voltage. 
   Referring now to  FIG. 4 , wire  22  may be replaced with a plate  30  which may have upwardly extending nanotubes per  FIG. 3  or may have nanotubes  28  that are laid down against a surface  32  of the plate  30  providing a substantially simpler fabrication technique that similarly produces a small volume plasma region  15  relative to discharge area. Again, the nanotubes  28  may be grown directly off the plate  30  in upright configuration or distributed and adhered by electrostatic techniques to coat the surface. 
   The nanostructures  26  may alternatively be other nanostructures that provide for conduction such as are well known in the art. Nanoparticles can be produced with chemical vapor deposition (CVD) and may be grown on the substrate or placed after growth by dispersion. 
   When the nanostructures are single walled nanotubes, they may be preselected for use depending on whether they are metallic or semiconducting. Generally, one-third of nanotubes will be metallic, and two-thirds semiconductor in a random sample, but they may be separated according to their metallic and semiconducting properties according to empirically determined efficiency and resistance to erosion. 
   The improved corona discharge may be useful in charging nanostructures themselves, and thus may be used for the production of the electrodes according to the present invention. 
   EXAMPLE I 
   A discharge electrode  12  was prepared by coating a commercial transmission electron microscope (TEM) copper grid with multi-walled carbon nanotubes about 40 nanometers in diameter and dispersed in methanol and commercially available from Buckey USA of Houston, Tex., U.S.A. As a comparison, an identical TEM grid electrode, a TEM grid electrode and tungsten wire electrode about three millimeters long and 200 micrometers in diameter, were also studied. 
   Referring to  FIG. 5 , the voltage current (VI) plot of the grids with the nanotubes and with the tungsten wire are shown. Plot  34  shows the tungsten wire grid and plot  36  shows the carbon nanotube grid. For the TEM grid with the nanotube, a corona discharge was initiated at 2.4 kV with a current of 1,531 nanoamps at a voltage of 2.6 kV. For the tungsten electrode, the corona initiated at about 3.8 kV and around 230 nanoamps for a maximum voltage of 4.1 kV. In comparison, for the TEM grid only, a maximum current of 20 nanoamps was obtained for a maximum voltage of four kV. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.