US 3541329 A
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Description (OCR text may contain errors)
Nov. 17, 1970 0TH 3,541,329
W. R NEGATIVE CORONA DEVICE WITH MEANS FOR PRODUCING A REPELLING ELECTROSTATIC FIELD Filed Dec. 1, 1966 FIG. Fla. 2
FIG. 3 FIG. 4
WALTER ROTH United States Patent US. Cl. 250-495 7 Claims ABSTRACT OF THE DISCLOSURE Apparatus for depositing uniform negative charge on a surface and including an electrode positioned on the opposite side of a corona wire from the surface and operated at a negative potential equal to or higher than that on the corona wire.
In general, the present invention relates to a discharge device and more specifically to a negative corona charging device.
It is well known that corona charging devices find numerous applications in xerography, electrostatic coating, and other electrostatic processes. In electrostatic imaging processes it is commonly necessary to charge an insulator or photoconductor with a uniform positive or negative charge pattern. The distinction between positive and negative corona is particularly significant in the prior art because each of these corona processes appears to proceed by a separate and distinct mechanism which has a significant effect upon the nature of the discharge and the results of the charging of an insulating surface.
A brief discussion of the mechanism of the positive and negative corona discharge phenomena is necessary to provide an understanding of the prior art problems which the present invention solves.
As a result of naturally occurring ionization processes (cosmic ray bombardment), a small number of free electrons and positive ions are normally present in the air. When a sufiiciently high positive potential is applied to a conductive wire, the surrounding free electrons are caused to move toward the wire with sufiicient velocity to ionize some of the neutral gas molecules which they may strike while traveling towards the positive wire. In this manner additional positive ions and electrons are produced. The newly createal electrons are themselves accelerated towards the corona wire and may in the process of their travel collide with other neutral gas molecules producing still more ions and electrons. As a result of the above avalanching process the wire becomes surrounded by a sheath of electrons and positive ions. The positive ions are repelled by the positive potential on the corona wire. Some of these positive ions will strike a nearby insulating surface having a ground plane behind it thereby giving the surface a positive electrostatic charge. In the above process the corona wire itself plays essentially no part in the corona generating process other than providing the necessary electric field. Variations in wire diameter will, of course, according to the laws of electrostatics, vary the surrounding electric field strength and thus the properties of the corona discharge. Isolated points or other surface imperfections in the wire will create locally high electric fields near the wire. However, these points produce field anomalies which have their primary effect close to the wire surface, while the corona generating process occurs in a sheath extending relatively some distance from the wire. The above description of positive corona is supported by the observation of a bluish-white sheath over the entire surface of the wire of uniform intensity. Because the positive corona produced is relatively independent of the exact nature of the corona wire by which it is generated, it is Patented Nov. 17, 1970 possible to get relatively uniform positive corona emission from a wire of commercial grade.
The mechansm of negative corona is apparently entirely different. The rate and pattern of electron emission in negative corona is observed to be very much a characteristic of the wire material and the exact state of the wire surface. Such factors as dirt spots, areas of oxidation, variations in the crystal structure of the wire, the degree of smoothness, and the like have been observed to have pronounced effects on the uniformity of negative corona. Whatever the true causes or theoretical explanations, it is observed that, as the negative voltage on a small wire is increased, corona discharge commences and there appears discontinuous, discrete, and approximately periodic light emission points seen as reddish tufts of glowing gas at points along the wire. On a polished conductor, these glowing points are approximately uniformly spaced along the wire. As the voltage is further increased the glowing points on a negative corona wire move closer and closer together, and their number increases with the current, however in the range of practical potentials the corona never becomes adequately uniform. The use of higher voltage is undesirable because of increased ozone production. An increase in ozone is objectionable since it is toxic. causes damage to other components of a copy apparatus such as rubber belts and the like, and, in general, acts as a strong oxidizer. Further, higher voltages create the problem of potential damage to the material being charged by high energy electrons or ion impact.
Thus, in contrast to the continuous, uniform glow of positive corona, the glow of negative corona is inherently non-uniform, consisting of discontinuous, discrete, approximately periodic glows. The nonuniformity of negative corona may be reduced somewhat by operating the corona wire at a potential well above that at which negative corona commences and by using an extremely clean, smooth, uniform wire or a wire uniformly pitted by a process such as sandblasting; however, it is virtually impossible to eliminate the effects of the discrete glow points sufliciently for the uniform charging of, for example. a xerographic plate by any of the above methods.
There have been numerous attempts to overcome the problem of nonuniformity in the negative corona. For example, it has been suggested that if the distance between the plate to be charged and the corona wire is increased sufficiently there will be the equivalent of a defocusing or radial spreading effect and thus the plate will be charged more uniformly. This procedure results in some loss of efficiency and imposes severe limitations on the charging speeds obtainable. Further as the distance is increased, higher voltages are required and dielectric breakdown becomes a serious problem. Others have suggested that the corona wire may be sandblasted or twisted into ropes to provide a plurality of many emitting points. The use of radioactive sources has been suggested to lower the voltages at which ionization takes place, thus providing a shift in the relative control voltages. The addition of an AC signal applied in series and superimposed on a high negative DC charging voltage has been attempted to eliminate the nonuniformity in negative corona. It has also been proposed that the wire be oscillated or rotated while it is charged and producing the negative corona to counteract the effect of the nonuniformity of the negative corona. However, such methods become prohibitive for mechanical reasons as higher charging speeds are desired. Similarly, the air or gas surrounding the corona wire may be moved or vibrated so as to effectively alter the fiow of ions and electrons from a particular conductor area. While each of these prior art methods has had some limited success each has had a corresponding disadvantage in efficiency or complexity.
Because of the problem of the nonuniformity of the negative corona the prior art has tended to concentrate its efforts on the solution of the less difficult problems related to positive corona. In order to control the positive corona to a high degree electrostatic shields and electrodes have been employed, however these elements have been directed toward the control of the electrons and ions at some distance after they have left the corona wire. Since the prior art electrodes and shields are generally directed toward the solution of problems associated with positive corona they have neither the proper potential or configuration to establish a suitable field in the vicinity of a negative corona wire as it is proposed in the present invention. Indeed, the prior art has failed to recognize the problem toward which the present invention is directed, namely, the establishment of electrostatic conditions in the vicinity of the negative corona Wire which will allow the glow discharge points to move closer together at a .given potential, thus improving uniformity of charging.
Accordingly it is an object of this invention to provide a new, simple, and highly efiicient device and method for correcting negative corona nonuniformity which overcomes the deficiencies of the prior art as described above.
It is a further object of this invention to provide a negative corona device capable of greater uniformity than heretofore obtained.
Another object of this invention is to establish electromagnetic field conditions favorable for the uniform deposition of negative corona charges.
It is an additional object of this invention to provide for greater uniformity in the deposited charge on a surface to be charged.
Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawings.
The present invention overcomes the deficiencies of the prior art and achieves its objectives by providing a suitable field in the vicinity of the wire through the use of an electrode plate positioned on the opposite side of the corona wire from the surface to be charged and operated at a potential relative to the potential of the corona wire 'itself, so that the divergence of the effective beam of negative charges produced by each emission point is reduced, thus allowing the glow discharge or emission points to move closer together at a given potential, improving the uniformity of charging produced. The above is accomplished in particular by the establishment of electromagnetic field conditions in the region surrounding the negative corona wire which will have a focusing effect on the cone of charges produced by each emission point along the negative corona wire.
In order to facilitate understanding of the present invention, reference will now be made to the appended drawings of a preferred embodiment of the present invention. The drawings should not be construed as limiting the invention but are exemplary only. In the drawings:
FIG. 1 is a schematic representation of the present invention.
FIG. 2 is a cross sectional representation of the apparatus of the present invention.
FIG. 3 is a pictorial representation in which 3a represents the glow pattern with corona applied alone and 3b represents'the glow pattern with a suitable field applied.
FIG. 4 is a pictorial representation showing the peripheral negative charges of the cone of charges emitted at each glow point being effectively bent back and attracted to a field plate.
A preferred embodiment of the present invention is shown in FIGS. 1 and 2 in which a surface to be charged 4 is shown for purposes of illustration as composed of a photoconductive insulating layer 6 adhered to a grounded electrically conductive backing support plate 8 such as is commonly utilized in xerographic processes. This plate 8 serves as a ground plane so that charge may be retained on the insulating layer 6. Plate 8 may also be biased on either side of ground with corresponding alterations in the other potentials required. In the alternative the bottom of insulating layer 6 may be subjected to positive corona and plate 8 omitted. The photoconductive insulating layer 6 has sufiiciently good insulating properties to retain an electrostatic charge for a reasonable length of time in a practical system which implies a resistivity of at least on the order of 10 ohm-centimeters, in a conventional xerographic system.
At some distance from the surface to be charged 4 is one or more corona charging electrodes 10. Corona charging electrodes 10 may be in any structural configuration which is suitable for the production of corona, such as round wires, a knife edge or the like. The distance of corona electrode 10 from the surface to be charged 4 for optimum charging of surface 4 is determined by the characteristics of the electrode, its dimensions, the relative voltages applied, and the environmental conditions. In the preferred embodiment any suitable noncorrosive material having a uniform exterior and capable of corona discharge may be employed as the corona electrode within a wide range of dimensions and shapes. A typical corona wire array is composed of a smooth 0.0035 inch stainless steel wire mounted at their ends in end walls including a polystyrene insulating support block (not shown) or other good insulating material with a wire-towire spacing of 0.5 inch in a single plane (if more than one corona wire is employed). The corona wires need not be made of a particularly good conductor but are nevertheless preferably made out of a metal for mechanical strength. The minimum diameter of corona Wire 10 is determined by considerations of mechanical strength. The maximum diameter of corona wire 10 is determined by the fact that the voltage required for corona discharge increases with increasing wire diameter and approaches that required for sparking. Because of the corrosive nature of the corona discharge which forms ozone, oxides of nitrogen and in the presence of moisture nitric acid, the corona wire is preferably corrosion resistant.
Within reasonably broad limits the separation between corona wire 10 and the surface to be charged 4 is not critical, however, a typical separation providing satisfactory charging for a stainless steel wire of 3.5-mil diameter with a negative potential of 4-11 kilovolts applied between the backing plate 8 and the corona electrode 10 in an air atmosphere is a /2 inch.
As shown in FIG. 2, a negative source of potential 14 is electrically connected to corona electrode 10 to provide for the emission of negative corona. A typical voltage for potential source 14 is 6,000 volts. The range of potentials appropriate for corona wire dimensions of the order discussed above required to generate a useful corona discharge is from approximately 4,000 volts to 11,000 volts, with a preference for potentials between 6,000 and 8,000 volts. Smaller wires require lower voltages and larger wires require higher voltages.
On the side of the corona electrode 10 opposite from the surface to be charged 4 is a negatively charged metallic plate, screen or field electrode 12 which serves to produce a suitable field which results in a continuous emission pattern as compared with the spotty emission pattern of the negative corona in the absence of such a suitable field. While a negatively charged metallic plate is referred to throughout as the preferred embodiment of the structure 12 for producing the field in the vicinity of corona wire 10 any other suitable structure for producing a suitable electromagnetic field in the vicinity of corona electrode 10 may be used. For example, parallel strands of conductive wire; a woven wire screen; a planar, polygonal, cylindrical or other curved continuous conductive member; an apertured grid; a continuous insulating surface with appropriately spaced and connected conductors embedded therein; or a plate of any rigid conducting material such as steel or aluminum may be.
utilized to produce the field in response to the application of an appropriate potential. The geometry of members producing the electromagnetic field in the vicinity of corona electrode are such that at the voltage em.- ployed they do not emit corona themselves. In general, the plate electrode 12 has a width from 10 to 100 times the wire diameter or the smallest dimension of the corona electrode to insure that the electromagnetic field of the plate electrode 12 is felt at all points of the emitted charge cone. The separation of the plate electrode 12 from the corona charging electrode 10' is sufficient to avoid dielectric breakdown. This distance depends upon the applied potentials. A typical separation distance between plate electrode 12 and corona electrode 10 is on the order of /2 inch.
A suitable electromagnetic field can be provided by applying a negative potential from potential source 16 to a metallic palte 12 having an absolute value equal to or greater than that potential on the negative corona wire 10. 'For example, a potential source 14 supplying approximately a negative 6,000 volts relative to the surface to be charged 4 may be applied to negative corona wire 10. It is preferred that potential source 16 apply a negative potential between approximately 7,000 and 8,000 volts to metallic plate 12 under these conditions to establish a sufiicient repelling field to produce a continuous glow pattern at the surface to be charged 4 although higher voltages up to approximately 11,000 volts but below the level at which dielectric breakdown occurs and lower voltages down to approximately 4,000 volts are also effective. Variations in the applied voltage from potential source 16 applied to plate 12 will obviously be required to provide optimum effectiveness in response to alterations in the potential 14 applied to corona wire 10 as other parameters of the system are altered.
In operation, when sufficient negative potential 14 is applied to corona wires 10 a corona discharge results consisting of discontinuous, discrete, approximately periodic glows 22 around the corona wire 10. While it is not intended to limit the invention to any specific theory of operation it is presently believed that the reason for the periodic structure of the negative corona glows is that emission starts at a high point on the wire where the fields are the highest. The resulting negative charges and related fields retard emission from neighboring points along corona electrode 10 which lie within a strong interaction distance of the initial high field points. The next emitting point must therefore be the next high field point just outside the strong interaction distance of the initial high field points. The strong interaction distance is apparently increased by scattering and defocusing of electrons and negative ions in the gas. The interaction distance is minimized by providing a repelling field in the vicinity of the corona wire 10 by applying a negative potential equal to or higher than that on the corona electrode 10 to metallic plate 12. The reduction of the interaction distance apparently occurs, at least in part, because the repelling field from plate 12 narrows the radial spread of the emitted electrons as indicated by the dotted lines in FIG. 1. The repelling field thus decreases the effect of the emitted electrons and negative charges on neighboring high points and consequently the interaction distance between glow points 22 on corona wire 10 is decreased. The reduction of the interaction distance between the glow points 22 on corona wire 10 allows the glow points 22 to move closer together and eventually merge and thus increase the uniformity of charging by producing a continuous glow pattern at all distances equal to and greater than the distance to the surface to be charged 4 from the corona electrode 10. The above described effect is represented pictorially in FIG. 3 in which FIG. 3a shows the charge pattern consisting of a series of discontinuous, discrete, approximately periodic charge patterns 18 at a surface to be charged 4, when the corona electrode 10 is energized at approximately 6,000 volts without a repelling field. When the repelling field plate 12 was energized at approximately 7,500 volts a continuous line charge pattern 20 as shown in FIG. 3b was obtained at the surface to be charged 4.
In the alternative it has also been found that the application of a relatively positive field (that is, the application of a negative potential less than that applied to the corona electrode 10) to field plate 12 will produce a similar effect to that described above. For example, with approximately 6,000 volts applied to corona electrode 10, the application of a negative potential from approximately 4,000 volts upward produces a continuous charge pattern. It is believed that the explanation for this fact is that peripheral negative charges of the cone of charges emitted at each glow point are effectively bent back and attracted to the field plate as shown in FIG. 4. Further, it is believed that the relatively higher velocities of the charged particles close to the axis of the emitted cone in the direction normal to the plate prevents any significant spreading of those particles even when the peripheral charges are stripped from the cone of charge by the applied field. This process, also, narrows the radial spread of the emitted charge cone and decreases its effect on neighboring high points. Consequently the interaction distance between glow points is decreased and they move closer together producing an increase in the uniformity of charging obtainable. This process of stripping the charge cone of unwanted peripheral negative charges by means of a stripping electrode is dependent upon such factors as the distance of the plate 12 from electrode 10 and the relative dimensions of these elements. The width of plate 12 is typically several orders of magnitude larger than the diameter of electrode 10 to insure an effective field in the area of electrode 10. In the special case where the potential on plate 12 and electrode 10 are the same the relative dimensions of the field provide an effective tangential component of force which serves to provide a focusing effect on the emitted cone of charge.
From the above description it is obvious that any other known means which are capable of establishing a suitable field in the vicinity of negative corona electrode 10 may be employed in lieu of field plate 12 within the scope of the present invention.
Therefore, in operation, an electromagnetic field is applied in the vicinity of the negative corona wire 10 to minimize the interaction between adjacent glow points of the negative corona thereby causing the glow points to move closer together and merge; thus, increasing the uniformity of the corona pattern at a surface to be charged. The interaction distance between adjacent glow or emission points of the negative corona is decreased by decreasing the divergence of the cone of charges produced at each emission point by either applying a repelling field, a tangential force on the charges, or by causing the peripheral charges to 'be directed out of the effective cone by application of an attractive field which is slightly relatively positive with respect to the corona potential. For the sake of clarity of disclosure and simplicity of explanation the invention has been herein described above in terms of the theory of operation as presently understood although it is to be clearly understood that the theory is illustrative only and is not intended to be interpreted in limitation of the scope of the invention.
Although a specific preferred embodiment of the invention has been described in the detailed description above, the description is not intended to limit the invention to the particular forms or embodiments disclosed herein, since they are to be recognized as illustrative rather than restrictive and it will be obvious to those skilled in the art that the invention is not so limited. The invention is declared to cover all changes and modifications of the specific example of the invention herein disclosed for purposes of illustration, which do not constitute departure from the spirit and scope of the invention.
What is claimed is:
1. A corona discharge device for depositing uniform negative electrical charge on a surface to be charged, said device comprising:
(1) a corona discharge electrode,
(2) first potential means for supplying a negative corona generating potential to said electrode,
(3) an electrostatic field electrode producing a repelling electrostatic field in the vicinity of said corona electrode to focus each of the cone of charges emitted from said corona discharge electrode so as to decrease the interaction distance between said cones of charges, said electrostatic field electrode being located on the opposite side of said corona discharge electrode from said surface to be charged, and
(4) second potential means for supplying a potential to said electrostatic field electrode of a magnitude at least as high as that applied to said corona electrode.
2. The device of claim 1 wherein said corona electrode is approximately equal distance between said surface to be charged and said electrostatic field electrode.
3. The device of claim 1 wherein said electrostatic field electrode has an effective extent several orders of magnitude greater than the diameter of said corona electrode.
4. The device of claim 1 wherein said electrostatic field electrode is biased negatively in the range of from about 4-11 kilovolts.
5. The device as set forth in claim 1 wherein said electrostatic field electrode has a width at least several times the smallest dimension of said corona electrode.
6. The device of claim 1 wherein said repelling electrostatic field electrode is a conductive metallic plate and maintained at a negative potential having an absolute value not less than said negative potential maintained on said corona electrode.
7. The device of claim 6 wherein said corona generating potential is approximately 6,000 volts negative potential and said repelling electrostatic field is produced by maintaining said electrostatic field electrode at a negative potential greater than 7,000 volts.
References Cited UNITED STATES PATENTS 2,856,533 10/1958 Rosenthal 250-495 3,075,078 1/1963 Olden 250-495 WILLIAM F. LINDQUIST, Primary Examiner U.S. Cl. X.R.