Self-healing Electrode For Uniform Negative Corona

Abstract

The present invention relates to electrodes used for charging electrophotographic image surfaces in copying machines. More particularly, the disclosure is directed to the negative corona discharge electrodes which produce a negative charge that is applied to the photoconductive surface exposed to the corona discharge. In the present invention, the electrode structure includes a combination of a wire of valve metal with a high resistivity coating spread uniformly over the surface of the wire. The valve metal, one example being tantalum, may serve as the electrode wire itself or may surround an inner wire such as stainless steel. By providing an electrode for corona discharge having a uniform high resistive coating, the plasma glow produced will spread uniformly along the length of the wire. By using a valve metal, which forms a hard oxide under the high resistivity coating, the electrode is self-healing in that if cracks or imperfections occur in the coating, the exposed valve metal will oxidize and fill in the cracks and imperfections.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of electrographic imaging and more particularly to an improved electrode for generating a negative corona discharge for electrographic imaging.

2. Description of the Prior Art

Electrodes for generating corona discharges for use in electrographic imaging are known in the prior art. Examples of such prior art references are U. S. Pat. No. 3,566,108, issued Feb. 23, 1971 to J. W. Weigl et al.; U. S. Pat. No. 3,612,864, issued Oct. 12, 1971 to Y. Tamai; and U. S. Pat. No. 3,075,078, issued Jan. 22, 1963 to R. G. Olden. Others are U. S. Pat. application Ser. No. 317,038, filed Dec. 20, 1972 of Bingham et al. entitled "Corona Charging Device" assigned to the assignee hereof, and U. S. Pat. application Ser. No. 211,542, filed Dec. 23, 1971 entitled "Corona Generator," a counterpart of which was published in Germany.

These references are cited to illustrate the environment of applicants' invention, however, applicants' invention is distinct over the prior art in that the prior art does not show corona discharge electrodes having a uniform high resistivity coating and a self-healing feature.

SUMMARY OF THE INVENTION

Electrographic image copier systems employ negative discharge corona electrodes to produce a negative charge on a photoconductive surface. The electrode is normally a conductive wire which inherently produces a non-uniform corona discharge along its length resulting in streaks and other imperfections in the resultant visible copy.

An object of the present invention is to provide an electrode for producing a uniform homogeneous negative corona discharge.

Another object of the present invention is to provide a negative corona discharge which is self-healing in the event that cracks and imperfections occur.

Still another object of the present invention is to provide a negative corona discharge electrode which includes a valve metal having a uniform high resistivity coating.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating the field distribution along the length of a typical prior art corona electrode wire which is suspended above a ground plane.

FIG. 2 schematically shows an embodiment of the present invention wherein a uniform, high resistivity coating is disposed around a valve metal to form a corona electrode.

FIG. 3 shows a plan view wherein the electrodes of FIG. 2 are arranged to form a structure which may be employed in an electrophotographic copying machine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As previously mentioned, negative corona discharge is used in many types of electrographic image copying machines. The negative corona discharge is used to apply a negative charge pattern on a photoconductive surface to form an electrostatic latent image. The latent electrostatic image is used in combination with the deposition of electroscopic material to form a visible image. A problem with this technology is that the corona around the discharge electrode is often inhomogeneous along the length of the electrode wire due to non-uniformity of the wire. This in turn results in an inhomogeneous corona and non-uniform charging of the photoconductive surface and produces streaks and imperfections in the final visible copy.

More particularly, the non-uniformity of the corona discharge results from distortions of the electric field around the electrode wire caused by charge clouds. The discharge is initiated by the field-induced injection of electrons from the wire into space.

Referring to FIG. 1, a schematic drawing is shown illustrating the field distribution along the length of a typical prior art corona electrode wire 10 which is suspended above a ground plane 12. The electrons, positive ions and negative ions are represented as indicated in the drawing. The negative ions formed by the discharge drift slowly from wire 10 to the collecting electrode (ground plane 12) as represented in FIG. 1. Explicitly, a negative ion cloud 14 forms an electrostatic shield covering a length of the wire 10. Corona glow does not appear over most of the shielded region because of a reduced surface field at the wire. Although the equipotential lines are distorted as shown in FIG. 1, a plasma glow is found at the point of electron injection into the corona. The field free region of the plasma glow, therefore, acts to enhance the field at the point of electron injection and to continue the injection at that point. Hence, this regenerative process produces corona discharge at several small points along the wire with dark spaces between them as indicated by the designations "high field" and "low field." The points of corona migrate along the wire until they stabilize at regions where conditions on the wire surface facilitate discharge.

In accordance with the present invention, by providing an electrode wire with a uniform resistive coating, it is possible to cause the plasma glow to spread uniformly along the length of the wire. The resistive coating acts as a limiting resistor which decreases the surface field at the points of high current injection. If the coating has a sufficiently high resistivity, any point of high injection current will be less favorable to corona discharge than the surrounding dark regions. Therefore, the corona glow 16 will spread to cover the entire wire uniformly. This mechanism is illustrated in FIG. 2.

Referring to FIG. 2, there is schematically shown a field distribution along the length of a corona wire 18 which is uniformly coated with a material 20 of high electrical resistance. An electrical field across coating material 20 at the point of injection lowers the surface field at that point.

More particularly, what is shown in FIG. 2 is a set of equipotential surfaces around a point of high current injection. Potential drop across the resistive coating 20 at the corona point (the corona glow is indicated by reference numeral 22) lowers the surface field at that point of the electrode wire. The coating 20 must be uniform and free of cracks and imperfections to function properly. In the present invention, if any cracks or imperfections occur, self-healing of the cracked areas is produced by a plasma enhanced oxidation of the chemically active valve metal which is found under the resistive coating 20. The metallic wire underlayer 18 will plasma oxidize when exposed to the corona discharge to form a resistive patch in the coating layer. The resistive surface coating should have a high resistivity, for example, greater than 10.sup.3 ohms per centimeter. Also, the resistive surface 20 should be initially amorphous and crack resistant. The resistive coating 20 should also be a material that will not sputter easily, so that the coating will not be eroded during operation.

The electrode wire 18 should be an active valve metal such that a self-healing oxide will form in any cracks, imperfections or damaged areas which may occur in resistive coating 20 in order to restore uniformity.

In one embodiment of the present invention, the corona electrode may be as shown in FIG. 2 wherein the electrode wire 18 is a valve metal selected from the representative group including tantalum, niobium, zirconium, hafnium, bismuth, tungsten and antimony, and any other hard, active valve metals which plasma oxidize to produce a resistive oxide for self-healing purposes. The aforesaid valve metals may be used separately or in combination.

In the embodiment of FIG. 2, as conceived and fabricated according to the present invention, the corona electrode is formed by selecting the valve metal, i.e., tantalum, for wire element 18 which may have a diameter in the order of 0.005 inches. The tantalum wire 18 is then anodized to form an oxide (Ta.sub.2 O.sub.5) of thickness in the order of 1,000 Angstroms using an anodize-etch repeat technique wherein the tantalum is placed under tension in a suitable electrolyte with a potential applied between the wire and a cathode to produce the oxide. The resultant oxide is removed by etching and then the anodizing-etching steps are repeated until an oxide surface is formed on the tantalum wire having desired uniformity. The final anodization of the tantalum wire is achieved by connecting the two electrodes of the electrolytic cell (the wire and the cathode) through a constant current source to achieve the desired final thickness of the high resistive oxide coating 20.

Although not depicted in FIG. 2, the electrode wire 18 may also consist of an inner core of stainless steel, hardened steel, or tungsten surrounded by one of the aforesaid valve metals such as tantalum, to provide a three-layer structure.

In the previous description, it was explained how a self-healing corona electrode could be fabricated with a uniform high resistive coating wherein the coating is an oxide of the interior electrode wire formed by anodization. In another embodiment of FIG. 2, the corona electrode structure may be composed such that the uniform high resistivity coating 20 is formed of amorphous, semiinsulating layers of insulating polymers, silicon nitride (Si.sub.3 N.sub.4) or silicon dioxide (SiO.sub.2) deposited on a wire with a valve metal surface, such as tantalum. If cracks occur in the resistive layer, the electrode will self-heal by the formation of the growth of Ta.sub.2 O.sub.5 or Al.sub.2 O.sub.3.

Referring to FIG. 3, a plan view is shown illustrating a structure wherein a plurality of corona discharge electrodes as illustrated in FIG. 2 are arrayed in parallel to form an apparatus which may be used in an electrostatic copying machine.

What has been described is an improved corona discharge electrode wherein a valve metal is surrounded by a uniform, high resistivity coating, the electrode being self-healing in the event cracks or imperfections occur.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. An electrode means adapted for producing a corona discharge comprising an inner core wire of valve metal surrounded by an outer uniform coating of the oxide of said valve metal wherein said valve metal is selected from the group consisting of tantalum, niobium, zirconium, hafnium, bismuth, and antimony.

2. An electrode means according to claim 1 wherein said valve metal is tantalum and said high resistivity material of said outer coating comprises anodized tantalum oxide.

3. An electrode means according to claim 1 wherein said valve metal surrounds a high tensile strength core wire.

4. An electrode means according to claim 1 wherein said electrode means is adapted to connect to a potential source to produce a negative corona discharge for use in electrographic imaging.

5. An electrode means according to claim 1, wherein said outer coating comprises a film about 1,000 Angstroms thick.

6. An electrode means adapted for producing a corona discharge comprising an inner core wire of valve metal surrounded by an outer uniform coating of high resistivity material, said outer coating being selected from the group consisting of an oxide of said valve metal, silicon nitride, silicon dioxide and an insulating polymer, wherein said valve metal is selected from the group consisting of tantalum, niobium, zirconium, hafnium, bismuth, and antimony.

7. An electrode means according to claim 6 wherein said inner core metal is tantalum and said uniform coating of high resistivity material is an amorphous semi-insulating coating selected from the group consisting of silicon nitride Si.sub.3 N.sub.4, silicon dioxide SiO.sub.2, and insulating polymers.

8. An electrode means adapted for producing a corona discharge comprising an inner core wire of tantalum surrounded by an outer uniform coating of the oxide of tantalum wherein said tantalum of said outer coating comprises an anodized tantalum oxide film about 1,000 Angstroms thick.

9. An electrode means adapted for producing a corona discharge comprising an inner core wire of valve metal surrounded by an outer uniform coating of high resistivity material, said outer coating being selected from the group consisting of a silicon nitride, silicon dioxide and an insulating polymer, wherein said valve metal is selected from the group consisting of tantalum, niobium, zirconium, hafnium, bismuth, and antimony.