US3713822A

US3713822A - Pyroelectric photoconductive elements and method of charging same

Info

Publication number: US3713822A
Application number: US00068067A
Authority: US
Inventors: H Kiess
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1970-08-31
Filing date: 1970-08-31
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30
Also published as: JPS4931659B1; BE771909A; CA945423A; GB1306665A; CH542465A; FR2104360A5; AU459574B2; NL7111935A; DE2141918A1; AU3229871A; SE361746B

Abstract

The recording element comprises an electrically conductive substrate and a layer of a photoconductive, pyroelectric compound on the substrate. The layer comprises one or more wafers of the pyroelectric compound, each wafer being similarly oriented on the substrate so that its polar c-axis is transverse to the opposite large surfaces of the layer. The novel electrophotographic recording element is charged by merely changing the temperature of the layer a few degrees.

Description

United States Patent 1 1 Kiess 1 1 Jan. 30, 1973 PYROELECTRIC PHOTOCONDUCTIVE ELEMENTS AND METHOD OF CHARGING SAME Inventor: Helinut Gustav Kiess, l-lightstown,

Assignee: RCA Corporation Filed: Aug. 31,1970

Appl. No.: 68,067

us. (:1. "9.6/1.5, 96/1 R, 96/1 c,

96/] .8, 317/262 A, 317/262 AE 1111. c1. ..G03g 5/02, 003 13/02 Field of Search ..96/l, 1.5 1.8;23/147, 148

References Cited UNITED STATES PATENTS 5/1971 Williams ..96/1 .8 1/1967 Goffe ..96/1.8

OTHER PUBLICATIONS Berlincourt et al., Electroplastic Properties of the Sulfides, Selenides, and Tellurides of Zinc and Cadmium, Phys. Rev., Vol. 129, No. 3, Feb. 1963, pp.

Minkus, Temperature Dependence of the Pyroelec-' tric effect in Cadmium Sulfide, Phys. Rev., Vol. 138, No. 4A, May, 1965, pp. 1277l287.

Heiland 'et al., Pyroelectricity of Zinc Oxide," Solid State Comm, Vol.4, 1966, pp. 353-356.

Grigas, Pyroelectric effect in SBSI," C'.A., Vol. 69, 1968, pp. 910828. I 5

Primary ExaminerGeorge F. Lesmes Assistant ExaminerRoland E. Martin, Jr. Att0rneyGlenn H. Bruestle [57] ABSTRACT 11 Claims, 6 Drawing Figures LPATENTEDJMOIQB 3.713.822

. INVENTOR. Helmut 6. Kiess v ATTORNEY P YROELECTRIC PI-IOTOCONDUCTIVE ELEMENTS AND METHOD OF CHARGING SAME BACKGROUND OF THE INVENTION This invention relates generally to an electrophotographic recording element and a method of charging same. More particularly, the invention relates to a novel electrophotographic plate of a pyroelectric compound and a novel method of charging the electrophotographic plate by changing its temperature. The novel electrophotographic plate and method of charging it are particularly useful in an electrophotographic process wherein an image developed on the plate is to be transferred from the plate to a transfer sheet.

It has been proposed to charge an electrophotographic recording element, comprising a photoconduc- I tive layer, by subjecting the photoconductive layer to a corona discharge from a corona discharge device. While this prior-art method is the most conventional one for charging an electrophotographic recording element, it requires a relatively expensive, high-voltage power supply, capable of generating at least 5,000 volts, and a corona discharge device that must be carefully shielded and insulated to protect an operator or maintenance personnel from electric shock. Also, unless the high-voltage equipment is suitably shielded, it can cause annoying radio and television interference.

The novel electrophotographic recording element and method of charging it make it possible to carry out an electrophotographic process without a high voltage power supply, thereby eliminating the inherent disadvantage thereof.

SUMMARY OF THE INVENTION The novel electrophotographic recording element comprises a layer of a photoconductive pyroelectric material on a relatively electrically conductive substrate and in electrical contact therewith. The pyroelectric material of the layer is oriented so as to produce electrostatic charges on the opposite surfaces of the layer when the temperature of the layer is changed.

The novel method of charging the novel recording element comprises changing the temperature of the layer to produce an electrostatic charge between the opposite surfaces of the layer.

The novel recording element and method employ the principles of pyroelectricity and obviate the need for relatively expensive high-voltage power supplies, corona discharge devices, and the necessary safety precautions required therefore. The novel method is also relatively free from conditions that cause interference with radio and television reception.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a hexagonal, singlecrystal structure of a photoconductive, pyroelectric material, zinc oxide," exhibiting itspolar c-axis;

3 FIG. 2 is a fragmentary plan view of one embodiment of anovel electrophotographic recording element in the form of a plate, employing elemental bodies of the single-crystal material illustrated in FIG. 1;

FIG. 3 is a fragmentary cross-sectional view of the electrophotographic plate shown in FIG. 2, taken along the line 3-3, and viewed in the direction indicated by the arrows;

FIG; 4 is a side elevational view of the novel recording element and means, illustrated symbolically, for charging it in accordance with the novel method; and,

FIGS. 5 and 6 are side elevational views of the novel electrophotographic plate in different steps of forming an electrophotographic image thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I of the drawing, there is shown a single-crystal, photoconductive, pyroelectric material, such as a compound 10, of the type used to make the novel electrophotographic recording element. The compound 10 may be a single-crystal, hexagonal structure of zinc oxide, for example, having opposite polar (0001)

surfaces

12 and 14 that are different from each other with respect to the direction of a c-axis 16, the surface 14 being designated as the Zn surface and the surface 12 being designated as the 0 surface. While the novel recording element will be described with respect to the pyroelectric compound 10 of zinc oxide, it is within the contemplation of the present invention to use other pyroelectric compounds, such as CdS, SbSI, or CdSe, for example.

If the temperature of the pyroelectric compound 10 is changed a voltage is produced between the

opposite surfaces

12 and 14 that are transverse to the c-axis 16. For example, if the temperature of the compound I0 is raised about 5C, the surface 12 acquires a negative electrostatic charge with respect to the surface 14, the surface 14 acquiring a positive electrostatic charge. The polarities of the electrostatic charges on the

surfaces

12 and 14 depend upon whether the c-axis 16 is pointing upwardly, in the direction of the arrow 18, or downwardly in the direction 20 with respect to the

surfaces

12 and 14, looking at FIG. 1. The amplitude of the voltage produced by the change in temperature is proportional to the thickness of the compound 10 between the

opposite surfaces

12 and 14.

One method of determining the polarities of the electrostatic charges produced on the

faces

12 and 14 of the pyroelectric compound 10 is to etch one of the prism (1010) faces 22 with 40 percent hydrofluoric acid. A plurality of etch pits 24 (only one shown in FIG. 1) are produced which can indicate the direction of the polar c-axis. Each etch pit 24 has an arrowhead-type shape that points in the positive direction of the c-axis. Hence, the etch pit 24, in FIG. 1, indicates that the polar axis 16 is positive in the direction of the arrow 20 and negative in the direction of the arrow 18.

The pyroelectric compound 10 of zinc oxide may be grown by any suitable method known in the art or it may be purchased from commercial vendors, as from Litton, Inc., Airtron Division, Morris Plains, New Jersey; or from the Dielectric Materials and System Division of The 3M Company, St. Paul, Minnesota. The zinc oxide crystal compound 10 may be purchased either in the uncut crystal form or in the form of wafers 28 cut from the crystal, for example, along the dashed line 26'parallel to the

faces

12 and 14. Uncut crystals are commercially available in diameters of 1mm through 5 mm and in varying lengths of 2 mm to 10 mm, depending on their diameter. Square wafers of zinc oxide compound 10 having a side of 10 mm and a thickness of at least about 5 mm are also available commercially. The zinc oxide compound 10 is preferably doped with a suitable dopant, such as copper (concentration of about l-l0 cm' or lithium (concentration of about 5.l0 cm' to compensate the zinc oxide with P-type carriers so as to provide it with a suitable resistivity. When doped, the dark resistivity of wafers 28 of the compound 10 is at least l0ohm cm, and usually much higher. The doped single-crystal, zinc oxide compound 10 possesses an illuminated resistivity of approximately 10 ohm cm.

Referring now to FIGS. 2 and 3, there is shown a novel recording element in the form of an electrophotographic plate 30 suitable for use in an electrophotographic process for reproducing images. The plate 30 comprises an electrically conductive substrate 32, such as a sheet of aluminum, copper, or stainless steel, and a photoconductive layer 34 of one or more wafers 28 of the pyroelectric compound 10. As shown in FIGS. 2 and 3, the photoconductive layer 34 is a composite of a plurality of the pyroelectric elemental bodies, such as wafers 28. The wafers 28 are adhered to the substrate 32 by any suitable electrically conductive paste 36 or solder. While the wafers 28 of zinc oxide,illustrated in FIGS. 2 and 3, are shown as being hexagonal, they may be in other shapes, such as rectangular or square. The layer 34, however, should have its opposite (large)

major surfaces

38 and 40 substantially continuous; that is, the wafers 28 should abut each other as closely as possible so as to provide substantially continuous, smooth, opposite

major surfaces

38 and 40. The lower surfaces of the wafers 28 that are in electrical contact with the substrate 32 may be coated with a very thin coating of indium so as to make a good electrical contact with the substrate 32 through the conductive paste 36. To provide a layer 24 of a composite of wafers 28, each wafer 28 should preferably have a thickness of at least about 0.5 mm, one major surface area of at least 2.5 mm, and a resistivity of at least ohm-cm in the dark.

All of the wafers 28 of the electrophotographic plate 30 are oriented so that their c-axes are transverse to, and preferably perpendicular to, the opposite

major surfaces

38 and 40 of the lay 34. Since the photoconductor of zinc oxide in most electrophotographic processes is charged negatively, the zinc oxide wafers 28 of the compound 10 are disposed in the layer 34 so that their c-axes point outwardly, as in the direction 18 of FIG. 1, from the upper major surface 38 of the layer 34. Thus, a negative electrostatic charge is produced upon the upper major surface 38 of the layer 34 when the temperature of the layer 34 is raised, as will be explained hereinafter. Conversely, the upper major surface 38 of the photoconductive layer 34 will exhibit a positive electrostatic charge if the temperature of the layer 34 is suddenly lowered from its ambient.

The upper surface 38 of the photoconductive layer 34 is preferably coated with a monolayer of one or more sensitizing dyes, such as fluorescence, cyanine dyes, rose bengal, or erythrosin, for example, to make the photoconductive layer responsive to visible light. In the absence of such dye sensitization, the photoconductive layer 34 of zinc oxide wafers 28 can be exposed by ultraviolet light to which it is particularly sensitive.

Referring now to FIG. 4, means are shown to electrostatically charge the novel electrophotographic plate 30 by changing its temperature, in darkness, in accordance with the novel method. To this end, a heater 42, such as a hot plate, is disposed adjacent to the electrophotographic plate 30. The heater is connected in series with a suitable voltage source 44 and a switch 46. Thus, to charge the electrophotographic plate 30, the switch 46 is closed to energize the heater 42. The voltage developed pyroelectrically across the photoconductive layer 34 of the electrophotographic plate 30, in darkness, is proportional to the thickness of the layer 34; that is, the thickness of the wafers 28, and to the change in the temperature of the layer 34. A thickness of the photoconductive layer 34 of about 0.7 mm will provide a voltage thereacross of about 250 volts when the temperature of the layer 34 is raised about 5C. If the c-axes of the wafers 28 are pointing outwardly from the upper surface 38 of the photoconductive layer 34, the upper surface 38 of the photoconductive layer 34, the upper surface 38 is charged negatively and the lower surface 40 is charged positively, as indicated symbolically in FIG. 4. If the c-axes of the wafers 28 were pointing downwardly from the upper surface 38, the charges on the

surfaces

38 and 40 of the photoconductive layer 34 would be reversed.

A change in temperature of the photoconductive layer 34 of between about 5C and 30C from the ambient has been found suitable to charging the novel electrophotographic plate 30 for processing in an electrophotographic process. A change in temperature of less then'5C may not develop a sufficient charge on a very thin layer 34, while a change in temperature of more than 30C may render the layer 34 too conductive to retain the charge for a desired period.

Although a preferred method of charging the novel electrophotographic late 30 is by raising its temperature from the ambient, it is within the contemplation of the novel method to charge the electrophotographic plate 30 by cooling it between about 5C and 30C from the ambient. This may be accomplished by blowing refrigerated air onto the surface 38 of the layer 34, from any suitable source, through a plurality of jets 47 disposed above the photoconductive layer 34, as shown in FIG. 4. Any other means of cooling, such as by Peltier elements, for example, may also be used.

The charged electrophotographic plate 30 can now be exposed to a light image to be reproduced, whereby to discharge the electrophotographic plate 30 selectively and to provide an electrostatic latent image on the photoconductive layer 34. This operation in the electrophotographic process is illustrated in FIG. 5. A transparency 48 of the image to be reproduced, as a frame of a motion picture film 50, for example, is projected through a light projector 52 to the upper surface 38 of a charged (previously heated) photoconductive layer 34. The exposure step should be carried out as soon as possible after the charging step to prevent a loss of the charge with a change in temperature. Those porby any suitable developing means known in the electrophotographic art. For example, the surface 38 may I be brushed with a magnetic brush comprising a magnet 54 and a triboelectric mixture 56 of iron and toner particles, as taught in U.S. Pat. No. 2,874,063, issued on February 17, 1959, to H. G. .Greig for Electrostatic Printing. Thetoner particles 58 are positive. in the triboelectric mixture 56 and are attracted to the negative charges of the latent electrostatic image, thereby developing the latent image, as shown in FIG. 6., The

developed image may be fixed, as by heating, on the surface 38 of the photoconductive layer 34, or, if desired, the unfixed image may be transferred to a transfer sheet by any suitable means and methods well known in the art for transferring an unfixed image from an electrophotographic plate to a transfer sheet.

Although the improved electrophotographic plate has been shown and described as comprising singlecrystal, photoconductive, pyroelectric wafers of zinc oxide which is white, the photoconductive layer of the plate may comprise other pyroelectric, photoconductive compounds. While some of the pyroelectric compounds, such as CdS, SbSI, or CdSe, are not white, they are quite suitable for producing unfixed images on the novel electrophotographic plate for transfer purposes to transfer surfaces. The novel method of charging the novel electrophotographic plate eliminates the need of the high voltage power supply of the prior art and the attendant disadvantages inherent herein. The dimensions of the embodiments of the electrophotographic plate and conditions of the method of charging it described herein are merely illustrative and are not intended to be considered in a limiting sense.

I claim: 1. An electrophotographic recording element comprising;

a substrate of electrically conductive material, and a layer of photoconductive, pyroelectric material comprising a composite of similar crystalline elemental bodies, said layer being in electrical contact with said substrate, and said elemental bodies of said material being similarly oriented with respect to their crystal axes to produce electrostatic charges of the same polarity on an exposed surface of said recording element when the temperature of said layer is changed. 2. An electrophotographic recording element as described in claim 1, wherein said material is a compound selected from the group consisting of single-crystal ZnO, CdS, CdSe, and

SbSI, and

5. An electrophotographic recording element as described in claim 1, wherein said material is single-crystal zinc oxide, said layer comprises one or more bodies of said single-crystal zinc oxide, and each of said bodies has a c-axis pointing toward said substrate.

6. An electrophotographic recording element as described in claim 4, wherein each of said wafers has a thickness of at least 0.5 mm, a large surface with an area of at least 2.5 mm, and a resistivity of at least lO ohm-cm in the dark. 7. In an electrophotographic method of the type wherein a photoconductive layer of a recording element is sequentially electrostatically charged, selectively discharged with a light image to provide an electrostatic latent image, and said latent image is developed with an electroscopic toner to provide a visible image on said layer, the improvement of charging said layer comprises one or more elemental bodies of said compound. 3. An electrophotographic recording element as described in claim 3, wherein each of said elemental bodies has a c-axis, and said caxes extend in the same direction and are substantially perpendicular to said surface of said layer. 4. An electrophotographic recording element as described in claim 1, wherein said material is single-crystal ZnO, said layer comprises a plurality of wafers of said single-crystal ZnO, and each of said wafers has a c-axis extending outwardly from said exposed surface and said layer.

said recording element comprising the steps of:

providing said photoconductive layer in theform of one or more similar crystalline elemental bodies of a photoconductive, pyroelectric material on an electrically conductive substrate, each of said elemental bodies being similarly oriented with respect to their crystal axes to produce electrostatic charges of opposite polarity on the opposite large surfaces, respectively, of said layer when the temperature of said layer is changed, and changing the temperature of said layer to produce said electrostatic charges, whereby to charge said layer. 8. In an electrophotographic method of the type described in claim 7, wherein the step of changing the temperature of said layer to produce said electrostatic charges comprises raising the temperature of said layer between 5C and 30C. 9. In an electrophotographic method of the type described in claim 7, wherein the step of changing the temperature of said layer to produce said electrostatic charges comprises lowering the temperature of said layer between 5C and 30C. 10.In an electrophotographic method of the type described in claim 7, wherein the step of providing said photoconductive layer comprises providing a layer of one or-more wafers of single-crystal, photoconductive zinc oxide on an electrically conductive substrate, each of said wafers of zinc oxide having a c-axis that is pointing in the same direction and oriented substantially perpendicularly to opposite large surfaces of said layer, and the step of changing the temperature of said layer comprises heating or cooling said layer at least about 5C. 11. In an electrophotographic method of the type describedin claim 10, wherein said layer has one surface in electrical contact with anelectrically conductive substrate, and said c-axes of said wafers of zinc oxide point outwardly from the opposite large surface of said layer.

t t l t UNITED STATES PATENT oFFIcE CETIFICATE 0F Patent No. 3,713 ,822 Dated January 30 1973 Inv t Helmut Gustav Kiess It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 3, Col. 5, line 55 "Claim 3" should be Claim 1-- Claim 4, Col, 5, line 65 Cancel "and" and insert -of- Column 4, line '36 Cancel "late" and insert --plate Column 5, line 29 Cancel "herein" and insert thereinfiigned and Scaled this twenty-seventh D ay OF April 1 9 76 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (mnmissinncr oj'lulenls and Trademarks

Claims

1. An electrophotographic recording element comprising; a substrate of electrically conductive material, and a layer of photoconductive, pyroelectric material comprising a composite of similar crystalline elemental bodies, said layer being in electrical contact with said substrate, and said elemental bodies of said material being similarly oriented with respect to their crystal axes to produce electrostatic charges of the same polarity on an exposed surface of said recording element when the temperature of said layer is changed.

2. An electrophotographic recording element as described in claim 1, wherein said material is a compound selected from the group consisting of single-crystal ZnO, CdS, CdSe, and SbSI, and said layer comprises one or more elemental bodies of said compound.

3. An electrophotographic recording element as described in claim 3, wherein each of said elemental bodies has a c-axis, and said c-axes extend in the same direction and are substantially perpendicular to said surface of said layer.

4. An electrophotographic recording element as described in claim 1, wherein said material is single-crystal ZnO, said layer comprises a plurality of wafers of said single-crystal ZnO, and each of said wafers has a c-axis extending outwardly from said exposed surface and said layer.

6. An electrophotographic recording element as described in claim 4, wherein each of said wafers has a thickness of at least 0.5 mm, a large surface with an area of at least 2.5 mm2, and a resistivity of at least 109ohm-cm in the dark.

7. In an electrophotographic method of the type wherein a photoconductive layer of a recording element is sequentially electrostatically charged, selectively discharged with a light image to provide an electrostatic latent image, and said latent image is developed with an electroscopic toner to provide a visible image on said layer, the improvement of charging said recording element comprising the steps of: providing said photoconductive layer in the form of one or more similar crystalline elemental bodies of a photoconductive, pyroelectric material on an electrically conductive substrate, each of said elemental bodies being similarly oriented with respect to their crystal axes to produce electrostatic charges of opposite polarity on the opposite large surfaces, respectively, of said layer when the temperature of said layer is changed, and changing the temperature of said layer to produce said electrostatic charges, whereby to charge said layer.

8. In aN electrophotographic method of the type described in claim 7, wherein the step of changing the temperature of said layer to produce said electrostatic charges comprises raising the temperature of said layer between 5*C and 30*C.

9. In an electrophotographic method of the type described in claim 7, wherein the step of changing the temperature of said layer to produce said electrostatic charges comprises lowering the temperature of said layer between 5*C and 30*C.

10. In an electrophotographic method of the type described in claim 7, wherein the step of providing said photoconductive layer comprises providing a layer of one or more wafers of single-crystal, photoconductive zinc oxide on an electrically conductive substrate, each of said wafers of zinc oxide having a c-axis that is pointing in the same direction and oriented substantially perpendicularly to opposite large surfaces of said layer, and the step of changing the temperature of said layer comprises heating or cooling said layer at least about 5*C.