Abstract:
An ionizer emitter electrode is ideally formed of or at least partially coated with a carbide material, wherein the carbide material is selected from the group consisting of germanium carbide, boron carbide, silicon carbide and silicon-germanium carbide. Alternatively, a corona-producing ionizer emitter electrode is substantially formed of silicon carbide. Alternatively, a corona-producing ionizer emitter electrode is formed of an electrically conductive metal base that is at least partially coated with silicon carbide. Alternatively, a corona-producing ionizer emitter electrode ionizes gas when high voltage is applied thereto, and the emitter electrode is formed substantially of silicon carbide and has a resistivity of less than or equal to about one hundred ohms-centimeter (100 Ω-cm).

Description:
BACKGROUND OF THE INVENTION 
   The present invention is directed to emitter electrodes for gas ionizers and, more specifically, to a gas ionizer emitter electrode formed of or coated with a carbide material such as silicon carbide. 
   Ion generators are related generally to the field of devices that neutralize static charges in workspaces to minimize the potential for electrostatic discharge. Static elimination is an important activity in the production of technologies such as large scale integrated circuits, magnetoresistive recording heads, and the like. The generation of particulate matter by corona-producing electrodes in static eliminators competes with the equally important need to establish environments that are free from particles and impurities. Metallic impurities can cause fatal damage to such technologies, so it is desirable to suppress those contaminants to the lowest possible level. 
   It known in the art that when metallic ion emitters are subjected to corona discharges in room air, they show signs of deterioration and/or oxidation within a few hours and the generation of fine particles. This problem is prevalent with needle electrodes formed of copper, stainless steel, aluminum, and titanium. Corrosion is found in areas under the discharge or subjected to the active gaseous species NO X . NO 3  ions are found on all the above materials, whether the emitters had positive or negative polarity. Also, ozone-related corrosion is dependent on relative humidity and on the condensation nuclei density. Purging the emitter electrodes with dry air can reduce NH 4 NO 3  as either an airborne contaminant or deposit on the emitters. 
   Surface reactions lead to the formation of compounds that change the mechanical structure of the emitters. At the same time, those reactions lead to the generation of particles from the electrodes or contribute to the formation of particles in the gas phase. 
   Silicon and silicon dioxide emitter electrodes experience significantly lower corrosion than metals in the presence of corona discharges. Silicon is known to undergo thermal oxidation, plasma oxidation, oxidation by ion bombardment and implantation, and similar forms of nitridation. Some have tried to improve silicon emitters by using 99.99% pure silicon that contains a dopant such as phosphorus, boron, antimony and the like. For example, U.S. Pat. No. 5,650,203 (Gehlke) discloses silicon emitters containing a dopant material. However, even such high purity doped silicon emitters suffer from corrosion and degradation. 
   Another approach is to form emitter electrodes from nearly pure germanium or from germanium with a dopant material. For example, U.S. Pat. No. 6,215,248 (Noll), the contents of which are incorporated by reference herein, discloses germanium needles or emitter electrodes for use in low particle generating gas ionizers and static eliminators. While such germanium emitter electrodes have proven to be less susceptible to corrosion and degradation than metallic emitter electrodes and silicon emitter electrodes with a dopant, there is a need for an emitter electrode that produces or causes even less metallic and/or non-metallic contamination with enhanced resistance to erosion. 
   BRIEF SUMMARY OF THE INVENTION 
   Briefly stated, in one embodiment, the present invention comprises an ionizer emitter electrode formed of or coated with a carbide material, wherein the carbide material is selected from the group consisting of germanium carbide, boron carbide, silicon carbide and silicon-germanium carbide. The present invention also comprises a corona-producing ionizer emitter electrode substantially formed of silicon carbide. In another aspect, the present invention is a corona-producing ionizer emitter electrode formed of an electrically conductive metal base, the metal base being coated at least partially with silicon carbide. In yet another aspect, the present invention is a corona-producing ionizer emitter electrode that ionizes gas when high voltage is applied thereto, and the emitter electrode is formed substantially of silicon carbide with the necessary dopant to achieve a resistivity of less than or equal to about one hundred ohms-centimeter (100 Ω-cm). 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention and its applications are not limited to the precise arrangements and instrumentalities shown. 
     In the drawings: 
       FIG. 1  is a side elevational view of an emitter electrode formed or coated with a carbide material in accordance with some preferred embodiments of the present invention; 
       FIG. 2A  is a schematic view of a point-to-plane corona producing apparatus in accordance with a first preferred embodiment of the present invention; 
       FIG. 2B  is a schematic view of a point-to-point corona producing apparatus in accordance with a second preferred embodiment of the present invention; 
       FIG. 2C  is a schematic view of a wire-to-plane corona producing apparatus in accordance with a third preferred embodiment of the present invention; 
       FIG. 2D  is a schematic view of a wire to cylinder corona producing apparatus in accordance with a fourth preferred embodiment of the present invention; 
       FIG. 2E  is a schematic view of a point-to-room corona producing apparatus in accordance with a fifth preferred embodiment of the present invention; and 
       FIG. 3  is a schematic diagram of a gas ionizer which utilizes the preferred embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Certain terminology is used in the following detailed description for convenience only and is not limiting. The words “right,” “left,” “lower” and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the described device and designated parts thereof. The terminology includes the words above specifically mentioned, derivatives thereof and words of similar import. Additionally, the word “a,” as used in the claims and in the corresponding portions of the specification means “one” or “at least one.” 
   Referring to the drawings in detail, wherein like numerals represent like elements throughout, there is shown in  FIG. 1  an emitter electrode  12  formed or coated with a carbide material, such as silicon carbide (SiC), in accordance with some preferred embodiments of the present invention. The emitter electrode has a generally cylindrically-shaped body and a generally conically-shaped tip  18  ending with a rounded end  17 . Alternatively, the rounded end  17  is sharply tapered or pointed. The rear end has a chamfer  19 . The shape of the emitter electrode  12  of  FIG. 1  is merely exemplary and should not be construed as limiting to this invention. Other shapes, sizes or proportions may be utilized without departing from the present invention. 
   Pure and ultra-pure SiC has been found, by experimentation, to outlast other electrode materials such as metallic, doped silicon and even pure germanium electrodes. SiC has been found to have superior chemical, plasma and erosion resistance with phenomenal thermal properties as compared to the other mentioned electrode materials. Chemical vapor deposition (CVD) manufacturing produces chemical vapor deposition (CVD) SiC that is highly pure and is commercially available. For example, purities of about 99.9995% CVD SiC can be obtained by CVD manufacturing. Because of the high purity of CVD SiC, the potential for unwanted metallic and non-metallic contamination is drastically reduced and nearly eliminated in gas ionization applications. CVD SiC emitter electrodes  12  also exhibit greater mechanical strength and reduced breakage as compared to similarly designed semiconductive counterparts. Experimentation has demonstrated that SiC, particularly CVD SiC, emitter electrodes are cleaner—with respect to fine particulates—than polycrystalline germanium emitters and single crystal silicon emitter electrodes. Other carbide materials exhibiting physical properties may be utilized such as germanium carbide, boron carbide, silicon carbide, silicon-germanium carbide and the like. 
   Preferably, the emitter electrode  12  is formed of at least 99.99% pure silicon carbide. Preferably, the silicon carbide is chemical vapor deposition (CVD) silicon carbide. Preferably, the emitter electrode  12  is a corona-producing ionizer emitter electrode  12  that is substantially formed of silicon carbide. 
   Doping of the carbide material may be necessary to achieve the desired conductivity. For example, in the case of silicon carbide, nitrogen is typically introduced to control the conductivity (resistivity). Preferably, the carbide material is doped to achieve predetermined conductivity characteristics. 
   Alternatively, the emitter electrode  12  is a corona-producing ionizer emitter electrode  12  formed of an electrically conductive metal base that is at least partially coated with silicon carbide. The metal base may be formed of copper, stainless steel, aluminum, titanium and the like, so long as silicon carbide material coats at least a substantial portion or all of the tip  18 . Preferably, silicon carbide material coats all of exposed surfaces of the metal base to reduce the potential for corrosion and degradation. 
   Referring to  FIG. 3 , a typical gas ionizer  100  is schematically shown which utilizes the preferred embodiments of the present invention. Gas ionizers  100  typically deliver ionized gas to a clean room, such as a Class 10 clean room or other high cleanliness mini-environment. A high-voltage power supply  22  is electrically coupled to the emitter electrode  12 . A corona is produced by application of high voltage to the electrode  12 . The gas ionizer  100  may comprise a plurality of emitter electrodes  12  all connected to an AC voltage for generating both positive and negative ions (not shown). Alternatively, the gas ionizer  100  comprises two separately connected sets of electrical emitter electrodes  12  used in conjunction with bipolar DC voltage that allows one set of emitter electrodes  12  to be operated at a positive voltage and a second set of emitter electrodes  12  to be operated at a negative voltage for generating positive and negative ions (not shown). 
   The high-voltage power supply  22  is typically supplied with electrical power conditioned at between about seventy (70 V) and about two hundred forty (240 V) volts AC at between about fifty (50 Hz) and about sixty (60 Hz) hertz. The high-voltage power supply  22  can include a circuit (not shown in detail), such as a transformer, capable of stepping up the voltage to between about three thousand (3 KV) and ten thousand (10 KV) volts AC at between about fifty (50 Hz) and about sixty (60 Hz) hertz. Alternatively, high-voltage power supply  22  can include a circuit, such as a rectifier that includes a diode and capacitor arrangement, capable of increasing the voltage to between about five thousand (5 KV) and ten thousand (10 KV) volts DC of both positive and negative polarities. Alternatively, the high-voltage power supply  22  is supplied with electrical power conditioned at about twenty-four (24 V) volts DC. The high-voltage power supply  22  can include a circuit, such as a free standing oscillator or switching type arrangement that is used to drive a transformer whose output is rectified, capable of conditioning the voltage to between about three thousand (3 KV) and ten thousand (10 KV) volts DC of both positive and negative polarities. Other power supplies using other voltages may be utilized without departing from the present invention. 
     FIG. 2A  is a schematic view of a point-to-plane corona producing apparatus in accordance with a first preferred embodiment of the present invention. The emitter electrode  12  is arranged in a point geometry and a counter-electrode  20  is arranged in a plane geometry. The power supply  22  is electrically coupled to the emitter electrode  12  to generate a corona. The counter-electrode  20  may be connected to ground (i.e., Earth ground) in the case of high voltage AC or to an opposite polarity of the power supply  22  than the emitter electrode  12  in the case of high-voltage DC. 
     FIG. 2B  is a schematic view of a point-to-point corona producing apparatus in accordance with a second preferred embodiment of the present invention. Two or more emitter electrodes  12  are arranged in a point geometry where the electrodes have opposite voltage polarity. The power supply  22  is electrically coupled to each emitter electrode  12  to generate a corona. 
     FIG. 2C  is a schematic view of a wire-to-plane corona producing apparatus in accordance with a third preferred embodiment of the present invention. A wire electrode  23  formed of SiC is arranged in a thin-wire geometry and a counter-electrode  20  is arranged in a plane geometry. The power supply  22  is electrically coupled to the emitter electrode  12  to generate a corona. The power supply  22  is electrically coupled to the emitter electrode  12  to generate a corona. The counter-electrode  20  may be connected to ground in the case of high voltage AC or to an opposite polarity of the power supply  22  than the emitter electrode  12  in the case of high-voltage DC. 
     FIG. 2D  is a schematic view of a wire to cylinder corona producing apparatus in accordance with a fourth preferred embodiment of the present invention. The wire electrode  23  formed of SiC is arranged in a thin-wire geometry and the counter-electrode  21  is arranged in a plane geometry. The power supply  22  is electrically coupled to the emitter electrode  12  to generate a corona. The power supply  22  is electrically coupled to the emitter electrode  12  to generate a corona. The counter-electrode  21  may be connected to ground in the case of high voltage AC or to an opposite polarity of the power supply  22  than the emitter electrode  12  in the case of high-voltage DC. 
     FIG. 2E  is a schematic view of a point-to-room corona producing apparatus in accordance with a fifth preferred embodiment of the present invention. The emitter electrode  12  is arranged in a point geometry and there is no counter-electrode  20 ,  21 . The power supply  22  is electrically coupled to the emitter electrode  12  to generate a corona. The power supply  22  is also connected to ground (i.e., Earth ground). 
   From the foregoing, it can be seen that the present invention comprises an emitter electrode formed or coated with silicon carbide (SiC) or CVD SiC for use with gas ionizers. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.