US2904431A

US2904431A - Electrographotographic charging means

Info

Publication number: US2904431A
Application number: US452243A
Authority: US
Inventors: Alexander J Moncrieff-Yeates
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1954-08-26
Filing date: 1954-08-26
Publication date: 1959-09-15
Anticipated expiration: 1976-09-15

Description

Sept. l5, 1959 A. J. MoNcRlEFF-YATES I' 2,904,431 I EL TR c EC O RAPHOTOGRAPHIC CHARGING BEANS Filed Aug. 26, 1954 /E il United States Patent ELECTROGRAPHOTGGRAPHIC CHARGING MEANS Alexander I. Moncriei-Yeates, Granbury, NJ., assignor to Radio Corporation of America, :a corporation of Delaware Application August 26, F1954, Serial No. '452,243

2 Claims. k(Cl."'96`1) This invention relates to`improve`d methods -of and means for producing relectric connection to solid bodies of semiconductors and dielectrics, and to improved methods of and means forelectrostatic printing.

Ordinarily, it is diilicult to provide an electrical connection between an electrode `and a body of semiconductive Vor dielectric material. Poor contact vmay be caused by barrier layers at the surfaces of the electrode and the body, or byimproper physical contact between the body andthe electrode, such as by discontinuous physical contact Vbetween the surface ofthe body and the surface ofthe electrode.

-Prior methods for providing electric contact to such bodies include: ionizing the Agas inthe space Vbetween the electrode and the surface ofthe semiconductive or dielectric body which requires either high voltages or high electric currents and is ineflicient; pyroelectric methods 'which are inconvenient; iand triboelectric methods which are difficult to-control.

Another method for providing-anfele'ctric contact is to apply an electrode to the stu'face ofthe body-of semiconductive Sor dielectric material and `then 'to Vapply a known voltage with respect to the body to thefelectrode. While small amounts ofcharge are transferred `Viby this method, the transfer isunequal'across ithe electrode surface, even when highvoltages are applied.

It has 'been lproposed to sandwich a 'thin sheet of pliable semiconducting*material such'as'rubbenmaper or plastic sheets between the electrode andthe bo'clyin order to improve the connection. While this method is an improvement over the structure `having no `-such semiconducting sheet, still the required yvoltages 'are `high andthe transfer is uneven.

Itfhas been further proposedto improve the connection between an electrode and abody ofserniconductingor dielectric material by interposing therebetween a thin layer of yan electrically-conducting -1iquid. 'While this method :provides irnproved' electric contacts between the electrode `and the-body, it has "the disadvantage t that the liquid must be-insulated from adjacenttconductors. rFurthermore, the liquid layeriseonductive laterally along its length as well as transversely through its thickness. Where it is desired to pass a pattern of electric-charge or .current, -this method dissipates .the pattern laterally.

It-isan-object of this inventionto provide improved electrical connections to ibodies yof vsemiconductors Yand v dielectrics.

or dielectric material which comprises closely spacingthe surface of an electrode from the surface of the 'body to which connection is to be made, with a ilm forming liquid such as lpropyl alcohol which is electrically-insulating in bulk. When a voltage with respect to the body is applied to the electrode, the electric field across the liquid unexpectedly causes the liquid to behave as a conductor transversely throughthe layer while continuing'to behave as an insulatorin the lateral direction. By semiconductive or dielectric material is meant 1a` material having a resistivity greater than 106 ohm-cm.

The invention includes a method of Velectrically charging the surface of a .body of semiconductive or dielectric material which ineludesthe above described step *for providing an electrical connection in combination with the subsequent steps of applyinga voltage withrespe'ct to the body to the electrode, removing said electrode from said body, removingthe voltage from said lectrode and then removing vthe liquid'remaning on the surface of said body.

The invention includes improved vmethods andmeans for electrostatic printing including the kimproved lelectrical connections and the improved methods of charging.

The invention will be described in greater detail vby reference to theaccompanying drawing inwhich:

Figure 1 is a sectional, elevational view of animproved means for making an electrical connection betweena conductor and a dielectric.

"Figure 2 is a sectional, elevational view'of 'animproved means for making an electrical connection between a conductor and a semiconductor, and

Figure 3 is a sectional, elevational viewof an -improved means for ymaking an electrical connectionbetween a semiconductor and a dielectric.

Similar reference ycharacters are applied to similar elements throughout the drawing.

YExample .1

-.Referring to Figure l, Janimproved velectrical .connection means "between a vconductor and a dielectric comprises Va sheet 21 of dielectric material-sandwiched fbetween an upper .electrode 41 and alower electrode 43. The-dielectric 'material y21 may 'be for example .a =sheet of polystyrene 0.0005 to0.0l0finchthi'ck preferably about 0.003 inchfthick. The electrodesfi-land 43may'lbeofiany conducting material, for example, brass.

In order to improve the electrical connection between theupper electrode 41 and'thedielectric sheet`21, athin liquid layer 31 of propyl alcohol'isinterposed'therebetween, The -layer'rnay be about 0.00002 to10.003 yinch thick,.preferably0:0001 inch thick.

A uniform electric charge is .formed on the upper surface of the4 dielectric sheet "21 by applying between about '100 :and 2000 -volts to le'a'ds 45 fatta'ched to fthe electrodes 41 and 43. Forthe preferred lthicknesses of elements,.about v200'volts is preferred. `Withthicker elements thehigher voltages areipreferred.

In order'to producea similar charge on'the upper surface of the dielectric sheet 21 Without'the liquidlayer 31, tbetween about 1000 'and 30,000 volts are required. For vthe ,preferred thicknesses of felements, `about 7000 volts .arerequired `Furthermore;'the resulting charge on the surfacezof the dielectric sheet isnot asuniformas'the charge formed vby themethod'of `Example l. IThis lack of uniformity VVmay be dueto surfaceirregularities of th'e surfaces ofthedielectricsheet or Vthe electrode or both resulting rin discontinuous physical contact therebetween.

If a thin sheet of a pliable, semiconductor, suchas paper about 0.002 inch thick is substituted furthe-liquid layer 31, about -1000to 7000 volts is required-to charge the surfaceof the dielectricsheet '21. For thepreferred thicknessabout'5000\voltsi -isfrequired The charged surface may be uncovered by removing the electrodes 41 and 43 and evaporating the propyl alcohol remaining on the surface of the dielectric sheet 21. The charged surface may then be used for any desired purpose.

Example 2 The electrical connecting means of the invention may be used in an electrostatic printing process to provide an improved method of blanket charging. An electrostatic printing process is that type of process for producing a visible image, reproduction or copy which includes, as an intermediate step, converting a light image or electric signal to an electrostatic charge pattern on an electricallyinsulating base. The process may include the conversion of the charge pattern into a visible image which may be a substantial reproduction of an original except that it may be different in size. A detailed description of the steps of such a process may be found in U.S Patent 2,297,691, issued October 6, 1942, to C. F. Carlson.

In a typical electrostatic printing process, a layer of photoconductive material disposed upon an electricallyconductive backing is provided with a blanket electrostatic charge. A light image is then focused upon the photoconductive layer. 'Ihe electrical resistance of the illuminated areas of the photoconductive layer is reduced, allowing an electrostatic image corresponding to the light image to form on the photoconductive layer.

The blanket electrostatic charge may be provided triboelectrically, pyroelectrically, by irradiation with electrons or ions or by physical contact with a charged electrode. Each of these processes has disadvantages as described above.

Referring to Figure 2, a layer 23 of a photoconductive material such as selenium supported upan an electrically conducting backing 25, such as paper, is sandwiched between an upper electrode 41a and a lower electrode 43a. The backing 25 rests upon the lower electrode 43a. The electrodes may be of any conductive material such as brass.

In order to improve the electrical connection between the upper electrode 41a and the photoconductive layer 23, a thin liquid layer 31a of gasoline is interposed therebetween. The liquid layer may be about .0001 to .002 inch thick but is preferably .0005 inch thick.

A uniform electric charge is formed on the upper surface of the photoconductive layer 23 by applying between 100 and 500 volts preferably about 300 volts to leads 45a attached to the electrodes 41a and 43a. The structure is then disassembled and the gasoline remaining on the surface of the photoconductive layer 23 is evaporated leaving a dry surface having a uniform electrostatic charge thereon. The surface may then be exposed photographically and developed by conventional means.

Example 3 The electrical connecting means of the invention may be used to provide an improved electrostatic printing process. In another typical electrostatic printing process, an electric iield is applied through a layer of photoconductive insulating material; for example, by contacting a rst electrode with one side of the photoconductive layer, spacing a second electrode from the other side of said photoconductive layer and applying voltage between said electrodes. When a light image is focused on the photoconductive layer, the electric resistance of the illuminated areas of the photoconductive layer is reduced allowing an electrostatic image to form in the photoconductive layer. The electrostatic image is conducted to a dielectric surface and then developed in the usual way.

Referring to Figure 3, a sheet 27 of dielectric material such as a polystyrene resin is sandwiched between a lower electrode 43h, for example, brass, and an upper transparent electrode 41b having a layer 29 of photoconductive material on the inner surface thereof. The transparent electrode may comprise a sheet of glass 39 having a turface treated with tin chloride at about 400 C. to produce a conductive layer 41h thereon. The photoconductive layer 29 may be cadmium sulphide bonded in 20% by weight of a silicone resin, but is preferably photoconductive selenium about .001 inch thick.

In order to improve the electrical connection between the photoconductive layer 29 and the dielectric sheet 27, a thin liquid film 3117 of cyclohexanol is interposed therebetween. The layer may be about .0005 to .003 inch thick but preferably about .002 inch thick.

A negative voltage of about 400 volts is applied to the upper electrode 41b with respect to the lower electrode 43b lby leads 45b attached thereto. About 2000 volts is required to obtain the same result if the liquid film of the invention is not used. Simultaneously, a light image is focused upon the photoconductive layer 29, for example with a lens 71. The electrical resistance in the illuminated areas of the photoconductive layer is reduced, allowing an electric charge to pass therethrough and to deposit in the dielectric sheet 27 thereby forming an electrostatic image thereon corresponding to the light image focused on the photoconductive layer.

The voltage and the light image are removed and the apparatus disassembled. The liquid remaining on the dielectric sheet 27 is evaporated and the electrostatic image thereon is developed by conventional methods. The use of liquid film 31h not only improves the electrical connection between the photoconductive layer 29 and the dielectric sheet 27, but also the electric charge pattern is transferred with a minimum dissipation of the pattern laterally along the contact surface.

Poor electrical connection is ordinarily encountered where the contact is between conductor and semiconductor, conductor and dielectric, semiconductor and semiconductor and semiconductor and dielectric. In each of the above described examples electrical connection is improved by interposing a thin lm of a liquid which is ordinarily electrically-insulating in bulk. When such a material is reduced to a thin ilm or layer, it behaves as if it is a conductor or a semiconductor. Thus, the film will conduct electric charges through the lm but remains electrically-insulating laterally along the length of the contact. The practice of the invention affords the equivalent of optimum physical contact between the bodies and a minimum in electric barriers to the flow of electricity.

The liquid used to produce the improved electrical connection of the invention is preferably a film-forming material so as to provide a continuous layer over the surface. The liquid has preferably, a bulky resistivity greater than 109 ohm-cm. so as to minimize the lateral ilow of electric charge along the length of the contact. 'I'he liquid has, preferably, a dielectric `constant greater than 5 so as to obtain a minimum drop in voltage across the liquid layer. Ihe liquid should be of a material that is substantially non-ionizing in order to avoid hydrolysis. The liquid should be chemically stable for the duration of its use and should not attack the electrode or the contacted body.

Examples of other suitable liquids are propyl alcohol, gasoline, cyclohexanol, cyclohexane, amylacetate propylene glycol, acetone, polystyrene dissolved in acetone, polyvinyl chloride in diethylene chloride, water soluble cellulose, distilled water and kerosene.

The electrode is preferably a good conductor of electricity but may be a body having a resistivity as high as 1010 ohm-cm. The body to which connection is made may be a dielectric or semi-conductor and preferably has a resistivity greater than l06 ohm-cm.

There have been described improved electrical connection means to semiconductors and dielectrics. These connection means may be used to provide a uniform electrostatic charge on the surface of a body of semiconduc- 5 tive or dielectric material, and to provide improved methods and means of electrostatic printing.

What is claimed is:

1. A method of electrostatic printing comprising the steps -of coating the surface of a photoconductive insulating material on a conductive backing member with a thin lm of a liquid having a bulk resistivity of at least 109 ohm-centimeters, closely spacing an electrode which is conductive in bulk from said photoconductive insulating material and in area contact with the surface of said film of liquid, establishing an electric potential between said electrode and said backing member of a magnitude sui cient to produce an electrostatic charge at the surface of said photoconductive insulating material, removing said electrode from said closely spaced relationship, removing the liquid remaining on the surface of said photoconductive insulating material, photographically exposing the surface of said photoconductive insulating material thereby producing an electrostatic latent image on the surface of said photoconductive insulating material and then developing said electrostatic latent image with an electrostatically attractable substance.

2. A method of electrostatic printing comprising the steps of coating the surface of a photoconductive insulating material on a conductive backing member with a thin lm of a liquid having a bulk resistivity of at least 109 ohm-centimeters, closely spacing an electrode which is conductive in bulk from said photoconductive insulating material and in area contact with the surface of said lm of liquid, establishing an electric potential between said electrode and said backing member of a magnitude sucient to produce an electrostatic charge at the surface of said photoconductive insulating material, photographieally exposing the surface of said photoconductive insulating material thereby producing an electrostatic latent image on the surface of said photoconducting insulating material, then removing said electrode from said closely spaced relationship and removing the liquid remaining on the surface of said photoconductive insulating material and then developing said electrostatic latent image with au electrostatically attractable substance.

References Cited in the ile of this patent UNITED STATES PATENTS 779,190 Thomson Jan. 3, 1905 1,977,263 Meissner Apr. 9, 1935 2,181,946 Lindenblad Dec. 5, 1939 2,277,013 Carlson Mar. 17, 1942 2,297,691 Carlson Oct. 6, 1942 2,438,959 Bartelheim et al. Apr. 6, 1948 2,464,405 Knauf Mar. 15, 1949 2,538,554 Cherry Ian. 16, 1951 2,618,551 Walkup Nov. 18, 1952 2,689,830 Sadowski et al Sept. 21, 1954 2,692,948 Lion Oct. 26, 1954 2,740,184 Thomas Apr. 3, 1956 2,758,939 Sugarman Aug. 14, 1956 2,774,921 Walkup Dec. 18, 1956 2,825,814 Walkup Mar. 4, 1958 OTHER REFERENCES Wainer: Photographic Engineering, vol. 3, No. 1, pages 12-22 (1952). (Copy in Scientiiic Library.)

Claims

1. A METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF COATING THE SURFACE OF A PHOTOCONDUCTIVE INSULATING MATERIAL ON A CONDUCTIVE BACKING MEMBER WITH A THIN FILM OF A LIQUID HAVING A BULK RESISTIVITY OF AT LEAST 10**9 OHM-CENTIMETERS, CLOSELY SPACING AN ELECTRODE WHICH IS CONDUCTIVE IN BULK FROM SAID PHOTOCONDUCTIVE INSULTATING MATERIAL AND IN AREA CONTACT WITH THE SURFACE OF SAID FILM OF LIQUID, ESTABLISHING AN ELECTRIC POTENTIAL BETWEEN SAID ELECTRODE AND SAID BACKING MEMBER OF A MAGNITUDE SUFFICIENT TO PRODUCE AN ELECTROSTATIC CHARGE AT THE SURFACE OF SAID PHOTCONDUCTIVE INSULATING MATERIAL, REMOVING SAID ELECTRODE FROM SAID CLOSELY SPACED RELATIONSHIP, REMOVING THE LIQUID REMAINING ON THE SURFACE OF SAID PHOTOCONDUCTIVE INSULATING MATERIAL, PHOTOGRAAPHICALLY EXPOSING THE SURFACE OF SAID PHOTOCONDUCTIVE INSULATING MATERIAL THEREBY PRODUCING AN ELECTROSTATIC LATENT IMAGE ON THE SURFACE OF SAID PHOTOCONDUCTIVE INSULATING MATERIAL AND THEN DEVELOPING SAID ELECTROSTATIC LATENT IMAGE WITH AN ELECTROSTATICALLY ATTRAACTABLE SUBSTANCE.