US3518488A

US3518488A - Corona discharge charging of particles wherein a porous insulator is disposed between the corona electrodes

Info

Publication number: US3518488A
Application number: US694996A
Authority: US
Inventors: Michael Michalchik
Original assignee: Fairchild Camera and Instrument Corp
Current assignee: Fairchild Semiconductor Corp
Priority date: 1968-01-02
Filing date: 1968-01-02
Publication date: 1970-06-30
Anticipated expiration: 1987-06-30
Also published as: DE1815440A1

Description

June 30, 1970 M. MICHALCHIK 3 5 CORONA DISCHARGE CHARGING 0F PARTICLES WHEREIN A POROUS INSULATOR IS DISPOSED BETWEEN THE CORONA ELECTRODES Filed Jan. 2, 1968 lxvavrola. MMMEL M0944 ov/K United States Patent US. Cl. 317-3 15 Claims ABSTRACT OF THE DISCLOSURE Apparatus and method for electrostatically charging particles. Spaced apart electrodes adaptable to set up corona discharge therebetween are provided and porous insulator is interposed between said electrodes, insulator being of sufficient area to effectively separate corona charge distribution in aerial portions adjacent the opposite surfaces of insulator.

This invention relates to electrostatically charged particles, and in particular to a method and apparatus for applying a uniform electrostatic charge to particles and the like.

In the past, various methods and means have been utilized to electrostatically charge particles. Some prior art methods utilized the phenomena of triboelectrification to electrostatically charge the particles; these methods usually entailed the rubbing of the particles to be charged against a surface or other particles of a dissimilar substance so that each becomes oppositely electrified.

In one prior art method, triboelectrification is carried out by mixing the particles to be charged with a slightly more coarsely-divided material commonly referred to as a carrier. Upon agitation of this mixture, the frictional contact between the particles and the carrier charge the particles due to the triboelectric effect. This method, however, has been found undesirable for many reasons, notably the lack of uniformity of the charge placed on the particles, the difficulty of sustaining the charging process for appreicable periods of time, and the difficulty of separating the particles from the carrier after the particles have been charged.

In another prior art method, triboelectrification is carried out by mixing the particles in a stream of gas and forcing the resultant mixture through a capillary structure having interior walls constructed of poorly conducting material so that the particles rub against the side walls and acquire an electrical charge. This method has also been found unsatisfactory since the particles are not uniformly charged and since no means are available to efiiciently and properly compensate for the various particle sizes and types utilized and the various conditions of humidity normally attendant such processes.

In comparison to the above described methods which utilize the triboelectric effect, other prior methods electrostatically charge particles 'by passing the same through a corona discharge system. A corona discharge system usually consists of two spaced apart electrodes, at least one of which may be a very thin wire. Upon the application of a very high voltage difference between the spaced apart electrodes, a corona discharge is set up between the electrodes due to the resultant ionization of the gaseous medium surrounding the electrodes. Thus, by passing the particles through this discharge, the particles are electrostatically charged by the medium.

The corona method of electrostatically charging particles has been found desirable since no carriers are utilized and this obviates the difficulties attendant the men 3"5l8,488 Patented June 30, 1970 tioned systems which utilized carriers and since the corona discharge density can be eifectively regulated in order to compensate for variation in particle size and type as well as changing conditions of humidity. The corona discharge method as practiced heretofore, however, like the other mentioned prior art methods, failed to adequately and uniformly charge the particles as they passed through the corona discharge. Moreover, it has been found that undesirable particle precipitation and clumping occur about the corona electrodes.

The present invention substantially alleviates the abovernentioned difiiculties of the prior art methods and provides a method and apparatus which applies a substantially uniform charge to particles without the use of carriers and yet permits compensation for varying particle size and type as well as changing humidity conditions. Thus, in accordance with the present invention, the beneficial features attendant prior art corona discharge systems are retained but substantially uniform charge distribution is achieved.

In a corona discharge system, each of the electrodes producing the corona discharge charges its surrounding gaseous medium with electrically opposite charges, and there is thus produced a mixture of both positive and negative charges in the corona discharge zone. This charge mixture reduces the uniformity of the charge applied to the particles in the zone, and also is the main contributing factor which causes particle precipitation and clumping. These latter factors are problems with many prior art corona devices.

In accordance with the present invention, a porous in sulator is interposed between the electrodes producing the corona discharge. It is selected to be of sufficient area to effectively separate the corona charge distribution in the aerial portions adjacent its opposite surfaces. Thus, the insulator serves to separate the above noted opposite charges produced by each of the corona electrodes and also acts to neutralize any charge attempting to permeate it so that as a result a zone of uniformly charged gas is produced on the opposite sides of the insulator. Accordingly, by passing the particles to be charged in a stream of air or neutral gas through one zone of the segregated corona discharge, the particles will acquire a substantially uniform charge.

According to another aspect of the present invention, the insulator may be positioned in the corona discharge closer to one electrode than the other in order to increase the working volume through which the particles to be charged may pass and acquire the desired charge. Also, in order to prevent clogging of the pores of the insulator, one side thereof is preferably maintained at a higher pressure than the other.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the present invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent constructions as do not depart from the spirit and scope of the invention.

Certain specific applications of the invention have been chosen for purposes of illustration and description, but the invention should not be limited thereto. These specific applications are shown in the accompanying drawing and p CC are described in the following portions of the specification.

In the drawing:

FIG. 1 is a diagrammatic illustration of one suitable form of the present invention;

FIG. 2 is a diagrammatic illustration of a tubular em.- bodiment of the present invention.

Referring to FIG. 1, the present invention comprises two spaced apart electrodes, one being a corona wire electrode and the other being a plate electrode 12 of opposite polarity. A porous insulator 11 is interposed between

electrodes

10 and 12; the porous insulator may comprise sintered glass with openings ranging from 230 microns. In the preferred embodiment, the insulator 11 is situated further from the corona electrode 10 than from the electrode of opposite polarity 12 so as to increase the working volume through which the particles pass. As shown in FIG. 1, the porous insulator 11 is preferably separated from electrode 12 by insulating members 13. At a small distance above corona electrode 10, a protective shield 15 is provided; the shield provides protection from the corona wire 10 and may form a closure for the particles as described hereinafter.

In order to electrically charge the particles, a powder cloud of the particles is generated by any conventional means (not shown). The suspended particles are then passed between the corona electrode 10 and the porous insulator 11 in the direction indicated by the arrows shown in FIG. 1. Simultaneously, a direct current voltage generated by any conventional means (not shown), is applied between the

electrodes

10 and 12 so as to ionize the surrounding air medium and set up a corona discharge. The effect of the porous insulator 11 is to separate any oppositely charged gaseous particles created by the electrode 12 from the charged gaseous particles created by the corona electrode 10. Thus, a zone designated by numeral 16 of uniform charge is created and as the particles pass through zone 16, they acquire a substantially uniform charge.

In order to compensate for the various physical properties of the particles being charged and changing humidity conditions, the amount of voltage applied to corona electrode 10 and the electrode 12 may be regulated.

In order to prevent any clogging of the pores in insulator 11 by the toner particles, a slight pressure is maintained in zone 14. The pressure is developed by any conventional means (not shown) and directed into zone 14 by piping (not shown).

FIG. 2 shows a alternate embodiment of the present invention. As shown therein, the corona electrode 20 is completely surrounded by the porous insulator 21 which is, in turn, surrounded by a metal shield 22 serving as the opposing electrode. The porous insulator 21 is separated from electrode 22 by stand off insulating members 23. In this embodiment, the particles are blown while suspended in air through zone 26 while pressure is maintained in zone 24 to keep the toner powder from clogging the porous insulator 21.

A preferred material suitable for use as a

corona electrode

10 or 20 is a very thin molybdenum and gold coated copper wire; those skilled in the art, however, will appreciate that other materials are also acceptable for use as corona electrodes. It should also be noted that the opposing electrode 12 or 22 may take several forms depending on the size and proposed use of the apparatus. Thus, the electrode 12 may be a metal plate, or wires placed in succession while electrode 22 may be a metal tube or the like. The only limiting factor is that these electrodes must be of an opposite polarity than the

corona electrode

10 or 20.

The present invention has a multiplicity of applications such as the electrostatic charging of toner utilized in electrostatic printing and reproduction techniques, the separation and classification of particle materials, and antiair pollution devices which electrostatically charge particles in smoke and air and precipitate the particles in a cloud chamber.

In one embodiment of the present invention adaptable for electrostatically charging toner particles for electrostatic reproduction, good results were obtained by separating the

electrodes

10 or 20 from electrodes 12 or 22 between 0.5 and 2.0 inches while maintaining the distance between the

corona electrode

10 or 20 and porous insulator 11 between 0.6 and 0.9 of the distance between the electrodes 10 or 20- and 12 or 22, respectively.

There exists in the art certain types of electrostatic printers wherein a printing drum is rotated in such a manner as to require intermittent deposition of the toner particles on the electrostatic latent image thereon; the latter is usually utilized in color reproduction techniques. The present invention may advantageously be utilized in such printers since by pulsing the corona voltage in conjunction with pulsing the toner flow, the intermittent toner deposition can be accomplished in one simple operation.

Thus, it may be seen from the above that the present invention overcomes most of the difiiculties attendant prior art electrostatic charging devices and provides a method and apparatus which applies a substantially uniform charge to particles without the use of carriers while permitting compensation for variation in particle size and type as well as changing humidity conditions.

What is claimed is:

1. An apparatus for electrically charging particles comprising spaced apart electrodes adaptable to set up a corona discharge therebetween and a porous insulator interposed between said electrodes, said insulator being of sufiicient area to effectively separate the corona charge distributions in the aerial portions adjacent the opposite surfaces of said insulator.

2. The apparatus according to claim 1 wherein said insulator is interposed between said spaced apart electrodes in a position closer to one electrode than the other.

3. The apparatus according to claim 1 wherein one of said spaced apart electrodes includes at least one corona wire electrode and the other of said spaced apart electrodes includes a metal plate electrode.

4. The apparatus of claim 3 wherein said metal plate electrode is a cylindrically shaped tube and said corona wire is centrally located therein and extends along the longitudinal axis thereof.

5. The apparatus according to claim 1 wherein both of said spaced apart electrodes includes at least one corona wire.

6. The apparatus of claim 1 wherein said porous insulator comprises sintered glass.

7. An apparatus for electrical charging particles comprising means defining a chamber, spaced apart electrodes positioned in said chamber and adaptable to set up a corona discharge therebetween, a porous insulator interposed between said electrodes, said insulator dividing said chamber and efiectively segregating the corona charge distribution in the chamber portions adjacent the opposite surfaces of said insulator whereby a substantially uniform charge distribution is produced in at least one of said portions.

8. The apparatus according to claim 7 wherein the portion of said chamber providing said uniform charge distribution includes particle input and output means, and a particle cloud generator means is connected to said input means for directing particles through said uniform charge distribution to said output means.

9. The apparatus according to claim 8 wherein the other of said portions includes means for maintaining the pressure therein higher than that in the said portion providing said uniform charge distribution.

10. The apparatus according to claim 9 wherein said insulator comprises sintered glass.

11. The apparatus accordina to claim 7 wherein said insulator is interposed between said spaced apart electrodes in a position closer to one electrode than the other.

12. The apparatus according to claim 7 wherein one of said spaced apart electrodes includes a metal cylinder and the other electrode includes a centrally located thin corona Wire extending along the longitudinal axis of said metal cylinder.

13. The apparatus according to claim 12 wherein said metal cylinder defines said chamber.

14. The apparatus according to claim 13 wherein said porous insulator is cylindrical in shape and is interposed concentric with said metal cylinder.

15. A process for electrically charging particles comprising passing said particles through a segregated portion UNITED STATES PATENTS 1/ 1969 Topper et al 317-262 WALTER STOLWEIN, Primary Examiner 10 A. L. BIRCH, Assistant Examiner US. Cl. X.R.