Note: Descriptions are shown in the official language in which they were submitted.
22 Background of the Invention
23 Field of the Invention
24 The present invention relates to the field of
electrographic imaging and more particularly to an improved
26 electrode for generating a negative corona discharge for
27 electrographic imaging.
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1 Description of the Prior Art
2 Electrodes for generating corona discharges for
3 use in electrographic imaging are known in the prior art.
4 Examples of such prior art references are U. S. Patent
3,566,-108, issued February 23, 1971 to J. W. Weigl et al;
6 U. S. Patent 3,612,864, issued October 12, 1971 to Y. Tamai;
7 and U. S. Patent 3,075,078, issued January 22, 1963 to
8 R. G. Olden.
9 These references are cited to illustrate the
environment of applicants' invention, however, applicants'
11 invention is distinct over the prior art in that the prior
12 art does not show corona discharge electrodes having a
13 uniform high resistivity coating and a self-healing feature.
14 SummarY of the Invention
Electrographic image copier systems employ negative
16 discharge corona electrodes to produce a negative charge on
17 a photoconductive surface. The electrode is normally a
18 conductive wire which inherently produces a non-uniform
19 corona discharge along its length resulting in streaks and
other imperfections in the resultant visible copy.
21 An object of the present invention is to provide
22 an electrode for producing a uniform homogeneous negative
23 corona discharge.
24 Another object of the present invention is to
provide a negative corona discharge which is self-healing
26 in the even~ that cracks and imperfections occur.
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1 Still another object of the present invention
2 is to provide a negative corona discharge electrode which
3 includes a valve metal having a uniform high resistivity
4 coating.
The foregoing and other objects, features and
6 advantages of the invention will be apparent from the
7 following more particular description of the preferred
8 embodiments of the invention, as illustrated in the
9 accompanying drawings.
Brief Descriptlon of the Drawings
11 FIG. 1 is a schematic drawing illustrating the
12 field distribution along the length of a typical prior art
13 corona electrode wire which is suspended above a ground
14 plane.
FIG. 2 schematically shows an embodiment of the
16 present invention wherein a,uniform, high resistivity
17 coating is disposed around a valve metal to form a corona
18 electrode.
19 FIG. 3 shows a plan view wherein the electrodes
of FIG. 2 are arranged to form a structure which may be
21 employed in an electrophotographic copying machine.
22 Description of the Preferred Embodiments
23 As previously mentioned, negative corona discharge
24 is used in rnany types of electrographic image copying
machines. The negative corona discharge is used to apply a
26 negative charge pattern on a photoconductive surface to form
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1 an electrostatic latent image. The latent electrostatic
2 image is used in combination with the deposition of electro-
3 scopic material to form a visible image. A problem with this
4 technology is that the corona around the discharge electrode
S is often inhomogeneous along the length of the electrode wire
6 due to non-uniformity of the wire. This in turn results in
7 an inhomogeneous corona and non-uniform charging of the
8 photoconductive surface and produces streaks and imperfections
9 in the final visible copy.
More particularly, the non-uniformity of the corona
11 discharge results from distortions of the electric field
12 around the electrode wire caused by charge clouds. The
13 discharge is initiated by the field-induced injection of
14 electrons from the wire into space.
Referring to FIG. 1, a schematic drawing is shown
16 illustrating the field distribution along the length of a
17 typical prior art corona electrode wire 10 which is suspended
18 above a ground plane 12. The electrons, positive ions and
19 negative ions are represented as indicated in the drawing.
The negative ions formed by the discharge drift slowly from
21 wire 10 to the collecting electrode (ground plane 12) as
22 represented in FIG. 1. Explicitly, a negative ion cloud 14
23 forms an electrostatic shield covering a length of the wire 10.
24 Corona glow does not appear over most of the shielded region
because of a reduced surface field at the wire. Although
26 the equipotential lines are distorted as shown in FIG. 1, a
27 plasma glow is found at the point of electron injection into
28 the corona. The field free region of the plasma glow,
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1 therefore, acts to enhance the field at the point of electron
2 injection and to continue the injection at that point. Hence,
3 this regenerative process produces corona discharge at several
4 small points along the wire with dark spaces between them
as indicated by the designations "high field" and "low field".
6 The points of corona migrate along the wire until they stabi-
7 lize at regions where conditions on the wire surface facilitate
8 discharge.
9 In accordance with the present invention, by pro-
viding an electrode wire with a uniform resistive coating,
11 it is possible to cause the plasma glow to spread uniformly
12 along the length of the wire. The resistive coating acts
13 as a limiting resistor which decreases the surface field at
14 the points of high current injection. If the coating has a
sufficiently high resistivity, any point of high injection
].6 current will be less favorable to corona discharge than the
17 surrounding dark regions. Therefore, the corona glow 16
18 will spread to cover the entire wire uniformly. This mecha-
19 nism is illustrated in FIG. 2.
Referring to FIG. 2, there is schematically shown
21 a field distribution along the length of a corona wire 18
22 which is uniformly coated with a material 20 of high
23 electrical resistance. An electrical field across coating
24 material 20 at the point of injection lowers the surface
field at that point.
26 More particularly, what is shown in FIG. 2 is a
27 set of equipotential surfaces around a point of high current
28 injection. Potential drop across the resistive coating 20
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1 at the corona point (the corona glow is indicated by reference
2 numeral 22) lowers the surface field a~ that point of the
3 electrode wire. The coating 20 must be uniform and free of
4 cracks and imperfections to function properly. In the
present invention, if any cracks or imperfections occur,
6 self-healing of the cracked areas is produced by a plasma
7 enhanced oxidation of the chemically active valve metal
8 which is found under the resistive coating 20. The metallic
9 wire underlayer 18 will plasma oxidize when exposed to the
corona discharge to form a resistive patch in the coating
11 layer. The resistive surface coating should have a high
12 resistivity, for example, greater than 103 ohms per centi-
13 meter. Also, the resistive surface 20 should be initially
14 amorphous and crack resistant. The resistive coating 20
should also be a material that will not sputter easily, so
16 that the coating will not be eroded during operation.
17 The electrode wire 18 should be an active valve
18 metal such that a self-healing oxide will form in any cracks,
19 imperfections or damaged areas which may occur in resistive
coating 20 in order to restore uniformity.
21 In one embodiment of the present invention, the
22 corona electrode may be as shown in FIG. 2 wherein the
23 electrode wire 18 is a valve metal selected from the repre-
24 sentative group including tantalum, niobium, zirconium,
hafnium, bismuth, tungsten and antimony, and any other
26 hard, active valve metals which plasma oxidize to produce
27 a resistive oxide for self-healing purposes. The aforesaid
28 valve metals may be used separately or in combination.
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1 In the embodiment of FIG. 2, as conceived and
2 fabricated according to the present invention, the corona
3 electrode is formed by selecting the vaIve metal, i.e.,
4 tantalum, for wire element 18 which may have a diameter
S in the order of .005 inches. The tantalum wire 18 is then
6 anodized to form an oxide (Ta2Os) of thickness in the order
7 of 1000 Angstroms using an anodize-etch repeat technique
8 wherein the tantalum is placed under tension in a suitable
9 electrolyte with a potential applied between the wire and
a cathode to produce the oxide. The resultant oxide is
11 removed by etching and then the anodizing-etching steps
12 are repeated until an oxide surface is formed on the tantalum
13 wire having desired uniformity. The final anodization of the
14 tantalum wire is achieved by connecting the two electrodes of
the electrolytic cell (the wire and the cathode) through a
16 constant current source to achieve the desired final thickness
17 of the hi~h resistive oxide coating 20.
18 Although not depicted in FIG. 2, the electrode
19 wire 18 may also consist of an inner core of stainless steel,
hardened steel, or tungsten surrounded by one of the aforesaid
21 valve metals such as tantalum, to provide a three-layer
22 structure.
23 In the previous description, it was explained
24 how a self-healing corona electrode could be fabricated
with a uniform high resistive coating wherein the coating
26 is an oxide of the interior electrode wire formed by
27 anodization. In another embodiment of FIG. 2, the corona
28 electrode structure may be composed such that the uniform
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1 high resistivity coating 20 is formed of amorphous, semi-
insulating layers of silicon nitride (Si3N4) or silicon
dioxide (SiO2) deposited on a wire with a valve metal
surface, such as tantalum or aluminum for instance. If
cracks occur in the resistive layer, the electrode will
self-heal by the formation of the growth of Ta2O5 or A12O3.
Referring to FIG. 3, a plan view is shown illus-
trating 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 there-
of, 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.
What is claimed is:
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