US3288602A - Xerographic plate and method - Google Patents

Description

Nov. 29, 1966 c. SNELLING ET A1. 3,288,602

XEROGRAPHIC PLATE AND METHOD Filed April 4, 1962 2 Sheets-Sheet l Y l f' 'I' 3 F l L- T 91 ff @HO-*W1 F/G. l l

i; NWT/l "L orma., d ArToR/VEV Nov. 29, 1966 C. SNELUNG ET AL 3,288,602

`XERC)GRAPHIC PLATE AND METHOD 2 Sheets-Sheet 2 Filed April 4, 1962 INVENTORS G V1 WH E LC EV M LATP O LI-To T ND H T SNO. A R mmRmOi El. TGL/ TMDOHH. www HR C Y am B United States Patent O 3,288,602 XERGGRAPHIC PLATE AND METHOD Christopher Snelling, Penfield, Robert W. Gundlach, Victor, and George R. Mott and William D. Hope, Rochester, N.Y., assignors to Xerox Corporation, Rochester,

NX., a corporation of New York Filed Apr. 4, 1962, Ser. No. 185,051 18 Claims. (Cl. 96-1) This invention relates in general to Xerography and in particular to an improved xerographic plate and improved methods of xerographic plate sensitization, image formation, development and transfer.

In the art of Xerography as originally disclosed in U.S. Patent 2,297,691 to Carlson and later related patents an electrostatic latent image is formed on a photoconductive insulating layer with a conductive backing and is developed through the deposition thereon of finely divided electroscopic material which is later transferred and fixed to a sheet of copy material. According to these inventions the photoconductive insulating layer with a conductive backing in the form of a fiat plate or cylindrical drum is first charged, to sensitize it, and is then exposed to the image or pattern to be reproduced. This exposure renders the illuminated areas of the photoconductor conductive allowing the charge in those areas to be dissipated while the charge in the nonilluminated areas is retained. Since there is very little or no lateral conduction in the nonilluminated photoconducting material, charge is retained in image configuration.

Since the time of the original Carlson patent noted above the original plate charging and adhesive transfer techniques disclosed in that patent have been largely superseded by more uniform and reproducible electrostatic techniques. For example, xerographic plate charging is now generally accomplished by the method disclosed in U.S. Patent 2,588,699 to Carlson which involves the use of an ion producing filament or filament arrays operating on corona discharge principles. This type of corona discharge electrostatic technique is also utilized at the present time for the transfer of the electroscopic particles representing a developed image to a sheet of copying material as disclosed in U.S. Patent 2,576,047 to Schaffert. U.S. Patent 2,945,434 to Eichler discloses the use of both of these techniques in a xer-ographic ofiice copying machine. Although these electrostatic charging and transfer techniques have been used in xerographic copiers to produce copies of high quality they require the use of relatively -large and delicate equipment employing relatively high voltages.

Although other charging and transfer techniques have been proposed from time to time it has been found for one reason or another that they are not commercially feasible.

In addition to problems arising in applying charge to the xerographic plate and transferring the developed image to the copying material, selection of the materials for the plates has presented somewhat of a problem because the photoconductive layer must not only be a good photoconductor but also a relatively good dielectric so that it will retain the applied charge until it is exposed. This has restricted to some extent the number of materials which may be used as photoconductors on xerographic plates.

Another problem of Xerography is developing relatively large uniformly charged areas. The lines of force of these large fields extend outward of the plate surface at their edges or peripheries whereas they tend to run into the plate towards the conductive plate backing at their centers. Since the field lines attract the developing material this has resulted in large concentrations of the charged electroscopic developing particles at the edges of 3,258,602 Patented Nov. 29, 1966 ICC these large charged areas and relatively small amounts of developer in the centers of these areas making the centers look washed out in the final copy.

Accordingly, it is an object of this invention to define novel xerographic plates.

It is a furtherk object of this invention to define novel xerographic plates in which the dielectric and photoconductive functions of the plate are separated.

It is another object of this invention to define novel methods and apparatus of sensitizing xerographic plates.

A further object of this invention is to define novel and improved methods and apparatus of electrostatic image formation.

It is also an object of this invention to define novel xerographic apparatus and methods for the transfer of the developed xerographic images to a copy surface.

It is yet another object of this invention to define novel xerographic plates and techniques of Xerography which result in good developer coverage of large charged areas on the exposed xerographic plate.

It is also an object of this invention to describe a novel xerographic copying method and device which may be easily and quickly switched from positive to positive copying to positive to negative copying.

The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of specific embodiments of the invention, especially when taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a cross section View of a plate according to this invention.

FIGURE 2 is atop view taken along section lines 2-2 of FIGURE l and including some external connections not shown in FIGURE l.

FIGURE 3 is a cross sectional View of a xerographic plate comprising a second embodiment of this invention.

FIGURE 4 is a top view taken along section lines 3-3 of FIGURE 3 and showing one possible -grid configuration for that embodiment.

FIGURE 5 is a top view showing an embodiment of this invention for use in a cylindrical rotatable xerographic drum.

FIGURE 6 is an end view of xerographic apparatus including a drum of the type shown in FIGURE 5.

FIG. 6A is a sectional view of FIG. 6 along line 6A.

FIGURE 7 is a side sectional view of a further -improved modification of the plate embodiments of this invention.

FIGURE 8 is a showing of simplified appartus illustrating image formation in accordance with an embodiment of this invention.

FIGURE 9 is a showing of simplified apparat-us illustrating developer transfer in accordance with this invention.

Referring now to FIGURE l of the drawings there is illustrated a plate 1 comprising `a supporting base 2 bearing a number of slender conductors 4, 5, '6, and 7 which run in parallel lines across the Width of the plate. ASince as will be explained hereinafter it is desired to maintain electrical separation between the conductors 4, 5, 6, and 7 the supporting base 2 should be a good insulator land of adequate strength to act as a support for the plate 1. Since base 2 is mainly for support purposes its use is 0ptional and it may be eliminated if support is unnecessary or if other plate elements make up a self supporting element. The conductors on the supporting base are very slender and thin and are uniformly spaced so that there are from to 350 of them per inch of plate although less or more conductors per inch may also be used. These conductors make up approximately 50% of the area of the plate. The conductors may be placed on the supporting base by means of photoresist and etch or engraving techniques as explained in connection with the construction examples which follow. The conductors are covered with a layer of dielectric 3 adequate to prevent dielectric breakdown when the plate is employed in accordance with the method described below. Materials with a fairly high dielectric constant are preferably used lhere so as to allow a thin dielectric layer however a Mylar layer with 1a relatively high dielectric constant of 3.4 was successfully utilized.

Above the dielectric 3 there is a photoconductive layer 8 which may for example be any of the known photoconductive insulating layers employed in xerography such as photoconductive pigments in an insulating binder, vitreous selenium, yanthracene, and the like. In this instance, however, the photoconductor need not be an exceptionally good insulator in its non-activated condition since in the plate of this invention the dielectric layer 3 rather than the photoconductive layer is used to trap charge representing the latent electrostatic image remaining after exposure to the original to be reproduced. This allows the use of binder plates employing binders of less insulating qualities, if desired, as Well as other photoconductors with relatively poor charge retaining properties. Alternate conductors in the plate such as 4 and 6 are interconnected beyond the plate periphery while conductors such as 5 and 7 separating the alternate conductors are also interconnected beyond the plate periphery, both sets of conductors being kept electrically separated. Alternatively, the conductor sets may be mutually perpendicular if they are placed at different levels in the dielectric or in the insulating base. If such a base is utilized, the only requirement is that the conductor sets be electrically separated.

Thus, it may be seen that each set of adjacent conductors together with the dielectric between them makes up a small capacitor in the plate. The capacitors may be charged by the application of a potential. Preferably, the dielectric layer 3 should be no thicker than the center to center spacing of the conductors so as to maximize the capacitor induced fringing elds reaching the photoconductive layer 8.

FIGURE 1 also shows :a switch 9 for connecting the conductor sets either across a potential source or 58 or connecting them together through ground.

Exposure of the plate may be through the back or support or to the outer surface. In the event that it is desired to expose the photoconductor through the supporting base it would be necessary to make the base and the dielectric layers of a transparent insulating material such as glass, Mylar, or the like. Since the conductors only cover about half of the area ofthe plate as explained above these conductors could be opaque and still allow :approximately half of the exposure entering through the back of.

the plate to reach the photoconductive layer resulting in a halftone line pattern of the image on the photoconductor. Alternatively, the conductors could be fabricated from a transparent material such as a thin layer of tin oxide so that all of the light from the exposure would reach the photoconductive layer.

FIGURE 2 is a top-sectional View taken through section lines 2-2 of FIGURE 1 and `showing an embodiment of conductors 4, 5, 6, 7, etc., embedded in the plate 1 and connected to an outside potential source 10.

The plate section shown in FIGURE 3 is basically another embodiment of the concept disclosed in connection with the description of FIGURE 1 above. This embodiment comprises a dielectric layer 12 disposed on a conductive backing 11 which is connected to ground. A conducting screen or grid structure made up of conductors 14, 15, 16, 17, etc., is placed above the dielectric layer 12 and the upper surface is then overcoated with a photoconductive layer 13 similar to that described in connection with the FIGURE vl embodiment. In this embodiment, the built-in capacitor is made up of the conductive grid 14-17 etc., electrically separated from the conductive backing 11 by dielectric material 12. Since all of the parallel conductors 14-17 are maintained at the same polarity in this embodiment there is no problem with inadvertent connections between adjacent parallel conductors as might arise with the FIGURE 1 embodiment where adjacent conductors are held at opposite polfarities. The grid structure 14-17 etc., may be connected through switch 18 either to potential source 19, or 55, or to ground. As in the FIGURE l embodiment the photoconductor and dielectric layer are rather thin so as to maximize the effects of the fringing iields from the built-in capacitor. In fact, certain alternative grid placements may be used so as to maximize this eld. For example, the grid may be placed ush with the t-op surface of the photoconductive layer away from the remainder of the plate. With this. modication, devolpment of the plate will take place close to the electric field source where the ield is relatively strong, thus providing high density images and improved sensitivity.

FIG. 4 shows conductors 14, 15, 16, and 17 interconnected with other conductors to make up the grid structure of the FIGURE 2 embodiment of the plate. By fabricating the grid in this closed screenpattern the effects of a break in any one conductor will be minimized because potential will be applied through perpendicular conductors and carried to each side of the break.

Various other screen or grid configurations and placements will occur to those skilled in the art such as placingV the screen or grid in the dielectric so that it-s top layer is Hush with the surface of the dielectric thereby giving a uniform thickness of photoconductive material or making the screen in the form of a spiral with or without inter secting Iradial conductors and the like.

Although any one of the novel plates described .above could be charged or sensitized with conventional high voltage corona discharge techniques the following novel methods of plate sensitization and image formation are contemplated for use with these plates. Essentially these novel methods involve operating the plate as a capacitor while using the photoconductive layer of the plate as a light responsive switch and the dielectric layer of the plate as a charge trap.

When battery potential is applied across alternate conductors of the FIGURE 1 embodiment by placing switch 9 in the position shown in the figure a field is set up between the alternate conductors through the dielectric material and the insulating base 2. If the plate is not illuminated and consequently the photoconductive layer 8 is in its relatively insulating state the iield will also be set up in this layer. Thus, lines representing this eld would appear similar to lines 20 as shown in FIGURE 1.

With potential applied the plate is then exposed to an image to be reproduced. This renders the illuminated areas of the photoconductive layer 8 conductive. As is well known an electric eld when applied to a conductor causes the free electrons within it to move in such a way as to make the interior of the conductor a tield free, equipotential volume. In this case when the area of the photoconductive layer lying above conduct-ors 4 and 5 is illuminated, or subjected to other activating electromagnetic radiation, the electric field resulting from an application of potential across the conductors 4 and 5 for example, causes the free electrons within this conducting area of layer 8 to move towards the positive conductor 5 leaving the area above the negative conductor 4 relatively positive. In this case the charge -moves through the illuminated photoconductive layer until it reaches the interface of dielectric layer 3. Since the material 3 remains in an insulating condition regardless of illumination the charge is stopped at this interface. Thus, at the interface of dielectric layer 3 and photoconductive layer 8 above conductor 4, positive charge is stopped while at the interface above-conductor 5 negative charge is stopped, and when the illumination is shut off, these char-ges are trapped at the interface because photoc-onductor 8 reverts to its insulating state thereby limiting charge mobility. Owing to this rearrangement of charge within the photoconductor 8 while in its conducting state and the later trapping of the charge in its new position when the illumination source is shut off the electric field previously created by conductors 4 and 5 no longer exists since the photoconductor is equipotential. Assuming that the area of the photoconductor 8 -overlying conductors 6 and 7 is not illuminated and consequently is still in its relatively insulating state an electric field emanating from conductors 6 and 7 still exists in the photoconductor because the in- -sulating character `of the photoconductor in this area limits charge mobility thereby preventing this area from becoming equipotential.

Triboelectrically charged electroscopic developing particles are then used to develop the fringing fields emanating from the exposed plate. These particles are attracted by the electric field which still exist through the photoconductive layer in the unexposed areas. It is interesting to note that either positively charged developing particles, negatively charged developing particles or a combination of both may be used to develop this plate in this condition. Either polarity of electroscopic developing parti-cles hereinafter referred to as toner, or a combination of both polarities may be used to develop the plate because negative toner is attracted t-o the area above positive conductor 7 while positive toner is attracted to the area above conductor 6. Thus, if only positive toner is used only every other unexposed conductor area is developed. This also holds true for the use of negative toner alone. However, if a combination of positive and negative toner is used all unilluminated areas above the conductors are developed. Owing to the close spacing of the c-onductors within the dielectric layer it is only necessary to use the two polarities of toner for devel-oping Iin the event that continuous tone qual-ity is desired. Since most electrophotographic developers in use today contain a't least some toner of each polarity while only toner of one polarity is attracted to the conventional latent electrostatic image during development the invention of a novel plate and method utilizingtoner of both polarities is of signicant value. If desired, developer containing substantial amounts of both polarities of toner may be deliberately produced for use with this invention. For example, when a two element developer consisting of carrier beads and toner particles is used the toner may consist of a mixture of two different types of particles selected so that one type is above and one below the carrier beads in the triboelectric series. This would result in the carrier imparting positive charge to one type of .toner and negative to the other. This and other methods of producing two polarity toner, any of which may be used in connection with this invention are more fully described in U.S. Patent 3,013,890 to Bixby.

Operation of the plate in the manner explained above results in a positive image from the projection of a positive original. This plate may also be operated so that a negative -image results from a projection of a positive original. This result is obtained by moving switch 9 to the grounded position after charging and exposure as outlined above while maintaining the plate in total darkness. This shorts out the alternate conductors through ground while the whole photoconductive surface 8 is relatively insulating. Since potential no longer exists across alternate conductors these conductors no longer produce an electric field in any part of the plate. Thus, the electric field collapses in those areas of the photoconductor which have been exposed to light. However in those areas of the photoconductor which have been illuminated and consequently were conducting, charges moved so that a large number of negative charges reside at the interface of the photoconductor and the dielectric above the positive conductor and a large number of positive charges reside at the interface of the photoconductor and the dielectric above the negative conduct-or. These charges are trapped at the interface when the plate illumination is shut off because the photoconductor reverts to its insulating condition. These trapped charges then set up electric fields of their own of a polarity opposite to that of the original fields through the relatively insulating photoconductive layer. These lields may now be developed resulting `in a negative image in a photographic sense of the original light pattern.

The FIGURE 3 embodiment of this invention may be operated in the manner described above in connection with FIGURE l; however, a second mode of operation which `may be used with either plate embodiment is described below in connection with the FIGURE 3 embodiment.

The plate is first flooded or uniformly exposed to light so as to put its photoconductive layer 13 is in its relatively conductive state. During this lighted period the conductive grid made up of members 14, 15, 16, '17, etc., is connected through switch 18 to a source of potential such as battery 19, the other end of which is connected through ground to the conductive plate back 11. If, for example, the grid structure is connected to the positive pole of the potential source the grid is made positive and the conductive backing of the plate is made negative. Thus, the applied potential acts to charge the capacitor made up of grid 14-17 dielectric 12 and plate 11. For illustrative purposes an electric fringe field ernanating from this charged capacitor is shown only above conductors 16 and f17. When potential is first applied to the grid structure a field such as that shown above conductors 16 and 17 exists through the illuminated photoconductive layer 13 and the dielectric layer 12. Fringe fields such as the one shown do not exist for very long across the whole surface of the illuminated plat while it remains in its conducting state since the field causes rearrangement of the charge or free electrons in the photoconductor resulting in rapid field collapse as the photoconductor approaches its equipotential condition. In this case the field causes electrons to move towards the top of the positive conductors and renders the interface of the photoconductive layer and the dielectric layer Ipositive to offset the negative charge on the conductive plate 11. The induced negative charge is drained off" through the conducting screen or grid While the positive charge is trapped at the photoconductor-dielectric interface when the light source is turned off and the photoconductor reverts to its relatively insulating state. Switch 18 is then connected to ground, draining charge from the capacitor but leaving the trapped charge at the photoconductor-dielectric interface. The result is a positively charged plate. With the screen or grid structure ystill connected to ground the plate is then exposed to the subject to be copied. Those areas of the photoconductive layer which are exposed to light from the projectedl subject again become conductive allowing the trapped charge in those areas to move laterally through the photoconductor to the grid structure and thence to ground effectively discharging those areas while leaving the nonilluminated areas charged. Alternatively, the grid structure may be connected to a negative potential source rather than to ground so as to move charge from the illuminated areas more quickly thus making the plate more sensitive. Thus, either plate may be used to trap charge at the interface under uniform illumination put in the dark, grounded and exposed to the subject so as to form a latent electrostatic image or either plate may have potential applied during exposure and placed in the dark so that the ield emanating from the conductors in areas which had not been exposed may be developed.

The method outlined just above produces positive prints from the exposure of a positive original. In order to produce image reversal or a negative print from the exposure of a positive original, the steps of the method outlined just above are carried out e.g. uniformly illuminate, apply potential, place in the dark ground and expose in that order and-then the same potential is reapplied during development so that the eld set up in non-equipotential or unexposed areas is developed. Thus, either method of image reversal copying may also be used with either plate.

Any one of the plates described heretofore may be exposed through its back if it is provided with transparent base and dielectric layers. The conductors must also be transparent if the whole area of the plate is to be exposed to the image to be reproduced. However, even if these conductors are opaque a halftone line pattern of exposure will get through to the photoconductive layer on exposure to the plate backing. In some instances this method of exposure to the subject is advantageous because it increases the plate speed. The advantage of using rear exposure with a plate such as that shown in FIGURES 3 and 8 will be clear when the -basic operation of a photoconductor is considered.

When the energy of a photon of light is absorbed by the photoconductor it produces a hole-electron pair making the photoconductive layer conductive. According to present theory this hole-electron pair is formed within the first few microns of the exposed surface of a photoconductor such as selenium. Thus, with rear exposure of the plate as shown in FIGURE 8 using a projector 39 hole-electron pair formation occurs close to the photoconductor-dielectric interface and in the region of maximum field strength due to the closeness of the grid. With front exposure of this plate a weaker field would exist at the region of hole-electron pair formation since this region would -be relatively far from the grid. In addition, path lengths for discharge would be increased since the charge would have to move from the top surface of the photoconductor all the way through to its rear surface and the interface with the dielectric of the grid.

In order to incorporate the basic concepts of this invention into an automatic, rotary copying machine utilizing a cylindrical, hexagonal or other polygonally shaped xerographic plate, also referred to as a drum, proper switching circuits for plate segments are required. By way of example, FIGURE shows a very small segment or arc of a cylindrical drum surface viewed from outside the cylinder. This iigure shows exemplary conductor placement and switches needed for automating the FIG- URE 3 plate embodiment. The drum includes a number of transverse conductors 2,1, A22, and 23 parallel to the longitudinal drum axis. Perpendicular to, `and intersecting these transverse conductors are additional conductors such as 24, 25, and 26. Each of these perpendicular conductors intersects only one transverse conductor and is interleaved between two adjacent perpendicular conductors on the adjacent transverse conductor. Each of the transverse conductors is connected to a contact at the drum end such as 27. It should be noted that these contacts may be placed a either or both ends of the drium. During rotation of the drum the contacts such as 27, etc., intermittently make connection with fixed external contacts such as 28, 29, etc. Each of these iixed external contacts is separate from the other external contacts and is connected to various circuit elements such as the potential source 30, a reverse potential source 85, etc. Thus, that portion of the plate in a charging zone is connected to the positive potential source, that portion of the plate in an exposure zone is connected directly to ground, that portion of the plate in a transfer zone is connected to a potential source of a polarity opposite to that of the source 30, etc. In adapting the FIGURE l plate embodiment for use in an automatic rotary copying machine every other conductor in a plate zone would be connected to a contact at one end of the drum while the conductors between these alternate conductors in a plate area would be connected to a contact at the opposite end of the drum. The FIGURE 3 e'mbodiment could also be adapted for use in a continuous automatic copying machine by making the photoconductive grid and dielectric layers in the form of an endless belt and using a number of separated base plates like plate 11 of FIGURE 3. Then different potentials may be applied to these plates each having a different effect on the belt portion above it.

FIGURE 6 shows an end view of a hexagonal plate according to this invention utilized in a rotary copying machine. The hexagonal plate 40 is made up of six photoconductive faces 41, 42, 43, 44, 45, and 46 each including conductors as shown in the FIGURE l or FIGURE 3 embodiments. These photoconductive faces are over a dielectric layer 47 mounted on the conductive drum backing 48. At the end of the drum an insulating disc-ring 49 supports slip rings 41a, 42a, 43a, 44a, 45a, and 46a which are connected to the conductorsin the corresponding plate faces.

This figure also indicates how the drum would be moved through the reproduction steps. When using this hexagonal plate configuration the plate is operated in a stop and go manner being indexed around to each successive processing station as explained below. Each of these processing operations is controlled by the indexing of the drum. The indexing control may also include a counter so as to allow multiple projections of the same original. With lthe plate stopped face 46 of the plate is charged from a potential source such as a battery through slip ring 46a and an external fixed contact while face 41 is being exposed to an original to be reproduced by projector 50. Alternatively the exposure device may comprise a cathode ray tube or other image source. At the same time face 42 having previously been charged and exposed is in the process of being developed by cascade developer 51 while face 44 is in the transfer zone. The developed image on face 44 is in the process of being transferred to copy sheet 52 backed up by `a grounded conductive web holder 65. It is later fixed by heating element 53. Since the copy sheet must be backed up by a grounded conductor during image transfer as explained elsewhere in this specification in connection with FIG- URE 9 and since the copy sheet must be moved away from the plate when it is indexed around to prevent image destruction due to rubbing between the hexagonal plate and the copy surface a grounded conductive web holder 65 is provided.- Both the copy web and the web holder are wider than the length of the plate and the holder is provided with short overhanging lips `so it can pull the web away from the plate after the image transfer is cornpleted. These lips are short enough so that they dont contact the plate when the web holder pushes the web against the plate prior to powder image transfer. The web holder 65 is reciprocated by a piston and cylinder arrangement 66 which is operated by the plate indexing control. A sectional view along line 6A of FIG. 6 through face 43 of hexagonal plate 40, more clearly illustrating the relationship between web holder 65, copy sheet 52, and piston-cylinder arrangement 66 is shown in FIG. 6A. In this case transfer is achieved by the application of a potential of a polarity opposite to that used in charging the plate. This potential may be applied through an external fixed contact to the internal plate conductors touching slip ring 44a, this method of transfer being more fully explained in connection with FIGURE 9 below. Face 45 is in the process of being cleaned of any residual developing material by spring mounted brush 54 so that it may begin a new cycle. In addition to being hexagonal, the drum shown in FIGURE 6 could be of almost any other 'polygonal shape or cylindrical in which case it might be continually rotated through the processing steps. In place of the contacts 27-29 shown in connection with FIGURE 5, and the broken slip ring 41a through 46a shown in connection with FIGURE 6, other various modifications for making sliding intermittent contact between the conductors in selected plate areas and external sources may be utilized. For example, concentric slip rings, commutator segments separated by insulators and other mechanisms familiar to those skilled in the art of rotating electrical machinery might be utilized.

FIGURE 7 shows a modified version of the plate disclosed in FIGURE 3. This modified plate has a photoconductive layer 31 and a grid structure 32, 33, 34, etc., both of which are the same as the photoconductive layer and grid structure of the FIGURE 3 embodiment. Underlying these is a dielectric layer 35 which is also similar to the FIGURE 3 embodiment except for the fact that the dielectric is filled with electroluminescent phosphor of the type which may be excited to luminescence by the application of A.C. potential or pulsating D.C. Any one of the standard luminescent phosphors may be used such as Blue AQ 62-2861 manufactured by E. I. du Pont de Nemours Co. Below this dielectric phosphor layer is a conductive base 36 similar to the conductive base of the FIG- URE 3 embodiment. By applying `an A.C. potential source such as 3'7 across the dielectric-phosphor layer through the grid structure and conductive backing by closing a switch such as 38 during the exposure step a bootstrap effect is achieved. As soon as the A.C. potential is applied the phosphor directly below the conductive grid structure will begin to glow, however since the conductors in this embodiment are relatively opaque most of the light produced directly below the conductors will not reach the photoconductive layer. Since all of the plate layers are quite thin including the layer containing electroluminescent phosphor, luminescing of the phosphor below these conductors has relatively little effect on the photoconductive layer between the conductors. But when incident light strikes the area of the photoconductor between adjacent conductors this area will become conductive allowing A C. potential from the source 37 to be applied across the phosphor dielectric through the grid and laterally across the photoconductor between the adjacent conductors. This causes the phosphor between adjacent conductors to glow, reinforcing or further exposing the photoconductive layer above these phosphors. Thus, a bootstrap effect is achieved and even when the plate is exposed to relatively low light level images it will become quite conductive due to the glowing'of the phosphor below the photoconductive layer after the initial exposure to incident light. Since the conductors are opaque only areas between conductors are bootstrap exposed.

The novel transfer process utilized in connection with the new plate of this invention is shown most clearly in FIGURE 9. For purposes of illustration this transfer method is shown in connection with the FIGURE 3 plate embodiment; however, it can be used equally well with the FIGURE l embodiment plate as will be explained below. This plate which has previously been positively charged by applying the positive side of a potential source to conductors 14, 15, 16, 17, etc., exposed and developed with negative electroscopic ydeveloping particles 56 is shown in this view with the potential source reversed. This is accomplished by changing switch 18 of the apparatus of FIGURE 3 to a position so that it connects potential source 55 across the plate thereby applying negative potential to the grid structure 14-17 etc. This negative potential serves to repel the negatively charged developing particles 56 away from the plate and to the copying surface 57, which is backed up by a grounded conductive plate 60 to prevent charge buildup. Actually this copying surface must be very close to or touching the plate at the time of transfer and the wide gap between the plate and the copying surface is shown here only for purposes of illustration as are the greatly enlarged toner particles. When this transfer method is used with the FIGURE l embodiment the switch 9 is moved so as to connect conductors 4, 6, etc., to potential source 58. In this way a potential opposite in polarity to that used to charge the plate is applied to transfer the toner to the copy surface.

The voltage used in charging any one of the plates of this invention is not critical, but as a general rule, increases in this voltage will result in a more dense image. Thus, in order to secure more dense images the voltage applied to the plate may be `increased up to the point where breakdown occurs in the dielectric layer. In testing these plates it was found that voltages of from 300- 600 volts produced satisfactory images with the images becoming denser as the voltages were increased. In view of the relatively low current drain needed to operate the plates of the invention ordinary B or photo-flash batteries may be used as a power source. In testing these plates both Eveready 300 volt #493 and Burgess 300 volt model number U-200 batteries measuring 2% x 29/32 x 3%" were effectively used, with two connected in series to get 600 volts.

One of the most gratifying results yof this invention was that large solid dark image areas could be very uniformly developed. Conventional xerographic plates have suffered somewhat in this respect because the large solid black image areas are represented on a plate by large retained charge patterns which set up on electric field. As is well known to those familiar with development in xerography large electric field areas are more effective at their peripheries or fringe areas than at their centers and in xerographic plates the weak central field lines go in towards the conductive plate backing. This results in electroscopic developing particles being more strongly attracted to these peripheral areas leaving the centersy of these large areas relatively washed out or undeveloped. By using the screen or grid pattern as one of the conductive members or plates in the built-in capacitor of this novel xerographic plate these large area fields are broken up into many small areas. This occurs when the field is caused by charge trapped at the dielectric-photoconductor interference because the field can only exist in the `areas between the conductors since an electric field will decay when crossing a conductor. When the plate is operated so that the field emanates fro-m the conductor itself-the field is broken up because a small separate field emanates from each small conductor which is `separated from its adjacent conductors by a small layer-of dielectric material. Thus, regardless of how the plate is operated or which plate embodiment isr used any large charged area will be a composite of small discrete electric fields thereby eliminating the solid area peripheral field effect.

The examples below are of the FIGURE 3 embodiment.

EXAMPLE 1 A two mil thick Mylar sheet was coated with an opaque layer of copper by evaporation. The copper coated Mylar was then flow coated with Kodak Photo- Resist hereinafter referred to as KPR. This was air dried and exposed to a parallel line pattern of approximately l0() lines per inch lwhich was 50% transparent. The KPR was then developed using a standard KPR developer which hardened the exposed portion of the KPR. The unexposed portion of the KPR was then washed away and the copper which was then exposed was etched away, leaving a conductive copper line pattern on the Mylar. The completed base was then coated with a l2 micron layer of amorphous selenium by evaporation. The plate was then taped to an aluminum plate base fo use in the tests which produced good quality prints.

EXAMPLE 2 An aluminum plate was flow coated with a contact cement and allowed to dry. A 1A mil Mylar sheet was bonded to the plate with pressure. The Mylar layer was then coated with copper and KPR in the manner explained in connection with Example 1 above. The KPR was then exposed to a line per inc-h pattern and developed as explained in Example l above. Etching of the copper 4was also carried out in the manner explained in -connection with Example l above. 4This completed base was then coated with a layer of amorphous selenium having a thickness of approximately 5 microns. Voltages of 300 and 600 volts D.C. were used to charge the plate. Both voltages produced acceptable images, however the higher voltage produced prints of higher maximum density. These voltages were secured from one Burgess U-200 300 volt battery and two such batteries connected in series. A second mode of operation was also used with this particular plate. This involved applying the bias voltage only during subject exposure as explained above in connection with positive to negative copying with the FIGURE 3 embodiment. This resulted in good quality negative images of the subject.

Developed images were transferred from the plate to a paper copy sheet using both conventional corona discharge transfer equipment and by reversing the bias applied to the plate in the charging step. The bias reversal transfer was carried out as follows: A sheet of paper was laid fiat on the developed image of the plate. A grounded aluminum plate was laid on top of the paper thus forming a sandwich. A potential of a polarity opposite to that used in the charging step was applied to the plate resulting in a successful transfer to the copy sheet.

EXAMPLE 3 A plate was fabricated in accordance with the steps of Example 2 except the transparent materials were used for the conductive base and the dielectric layers. In this case NESA glass was used as a base. This material which is available from the Pittsburgh Plate Glass Company of Pittsburgh, Pennsylvania, is believed to be a glass base covered with a thin conducting layer of tin oxide. This conductive base material was then fiow coated with an epoxy resin solution which was then allowed to harden. The conductive line pattern and the selenium coating were applied in the same manner as explained in connection with Example 2. A selected front exposure of this plate produced underexposed prints, however, the same exposure when applied to the rear of the plate through the transparent backing and dielectric layers resulted in an overexposed print. This confirms the theory proposed in connection with rear exposure of the FIGURE 3 embodiment.

It should be recognized that many alternate materials and configurations might be utilized in constructing a xerographic plate or drum in accordance with the concept of this invention. For example, conductive base materials might include aluminum, brass, copper, NESA glass, etc., while dielectric materials might include Mylar, glass, Bakelite, certain epoxy resins, etc. Alternate photoconductive insulating materials may also be used such as amoprhous selenium, zinc oxide in an insulating binder, or cadmium sulfide. In addition to the copper coatingphotoresist-etching technique for applying the line or grid pattern, this pattern can be engraved on a glass base using a mechanical engraver after which the grooves produced are filled with a conductive composition such as finely divided metal or graphite powder in a suitable binder.

The pattern of the conductors and their concentraiton may be varied from about 50 lines per inch to about 300 or 400 lines per inch depending on the desired resolution. In fact, even plates outside these limits could be used if this was found to be desirable. In view of the many possible modifieations of this invention, some of which have been illustrated above, it is intended that all matter contained in the above description be limited only as defined in the appended claims.

The term xerographic plate as used in this specification and the appended claims should be understood to refer to a device capable of taking and holding an electrostatic charge, and of dissipating portions of said charge in accordance with an image of activating radiation to which it is later exposed without regard to its shape. In other words, the term xerographic plate includes flexible or rigid members, fiat plates, cylindrical drums, spheres, or other surfaces t of whatever configuration.

The term screen as used in this specification and the appended claims should be read in its broadest sense. For example, the screen might include `such diverse configurations as a perforated plate, Ia meshed fabric similar to the common window screen, a spiral with closely spaced adjacent convolutions, a number of narrow closely spaced parallel members, etc.

The term image reversal as used in this disclosure shall be understood to refer to that term as it is generally used in the photographic arts. The effect of image reversal is a negative copy where the original is positive and vice versa.

What is claimed is:

1. A xerographic sensitizing process comprising applying a potential across two dielectrically separated conductive members one of which is a fine conductive screen, said screen being contiguous with a photoconductive layer so as to set up electric fields through said photoconductive layer, said two dielectrically separated conductive members lying on one side of a surface of said photoconductive layer, with at least a portion of said dielectric being in contact with said photoconductive layer, while uniformly subjecting said photoconductive layer to activating radiation, whereby competing electric fields are established in the radiation struck areas, removing said source of activating radiation and removing said potential source from said conductive members whereby charge is trapped at the photoconductive dielectric interface.

2. A method according to claim 1 further including grounding said conductive members and exposing said sensitized plate to an image to be copied with a source of activating radiation and then developing said plate with finely divided electroscopic material.

3. A method according to claim l further including grounding said conductive members and exposing said sensitized plate to an image to be copied with a source of activating radiation and then developing said plate with finely divided electroscopic material.

4. A xerographic method comprising applying a potential source across two dielectrically separated conductive members, one of said conductive members comprising a fine conductive screen continguous with a photoconductive layer so as to set up electric fields through said photoconductive layer, said two dielectrically separated ccnductive members lying on one side of a surface of said photoconductive layer, with at least a portion of said dielectric being in contact with said photoconductive layer, while exposing the photoconductive layer to an image to be copied with -a source of activating radiation, whereby competing electric fields are established in the radiation struck areas, removing the source of activating radiation and developing said plate with finely divided electroscopic material.

5. A method according to claim 4 further including grounding said conductive members prior to development whereby a reversed image will be developed.

6. A xerographic apparatus comprising two conductive members, a dielectric material separating said two conductive members, at least one of said conductive members being a fine screen, a photoconductive layer contiguous to said conductive screen, said conductive members and said dielectric material all lying on one side of a surface of said photoconductive layer with at least a portion of said dielectric layer being in contact with said photoconductive layer, 'and means to apply an electric potential to said screen capable of creating a significant potential difference within said screen, whereby electric fields are established through said photoconductive layer.

7. A xerographic apparatus according to claim 6 in which said fine conductive screen is made up of a group of closely spaced slender conductors.

8. A xerographic apparatus according to claim 7 including a second group of closely spaced slender conducaassoa tors intersecting said first closely spaced slender conductors of the conductive member so as to form an electrically continuous grid-like structure.

9. A Xerographic apparatus according to claim 6 including means to switch the conductive members from the potential source to ground whereby they may be discharged.

10. A Xerographic apparatus according to claim 6 including means to reverse the polarity of the applied charging potential and a grounded conductive plate near the phtoconductive surface layer whereby a reversal of potential Ipolarity may be used to transfer electr-oscopic particles from a developed Xerographic plate to a copy surface between the grounded conductive plate and the Xerographic plate.

11. A xerographic apparatus according to claim 6 in which the conductive screen is embedded in the photoconductive layer.

12. A xerographic apparatus according to claim 6 in which the conductive screen is embedded 'in the dielectric layer and separated from the other conductive member by at least -a portion of said dielectric layer.

13. Apparatus according to claim 7 in which both conductive members are ine screens, and the conductors of one screen are interleaved between the conductors of the other screen, said screens being separated from each other by at least a portion of said dielectric material.

14. Apparatus according to claim 13 in which said screens are separated from said photoconductive layer by at least a portion of said dielectric.

15. A Xerographic apparatus according to claim 6 in which both of the conductive members are dielectrically separate-d screens and in which the photoconductive layer is in contact with the dielectric material and electrically separated from the conductive screens and in which the potential applying means comprises means to apply potential of opposite polarity to each of the screens.

16. A xerographic apparatus according to claim 6 including a llin-g of electroluminescent material in said dielectric layer and means to apply an alternating current potential across the two conductive members on opposite sides of said filled dielectric, the lscreen shaped conductive member being at the interface of the photoconductive 4layer and the dielectric layer.

References Cited by the Examiner UNITED STATES PATENTS 2,277,013 3/119442 Carlson 96-1 2,808,328 10/1957 Jacob 96-1 2,836,766 5/1958 Halsted 96-1 2,892,709 6/1959 Mayer 96-1 2,909,971 l10/1959 Barber 95-1.7 2,917,385 12/1959 Byrne 96-1 2,946,682 7/1960 Lauriello 96-1 2,947,625 8/ 1960 Bertelsen 96-1 2,968,553 1/'1961 Gundlach 96-1 2,984,163 5/1961 Giaimo 95-1.7 3,000,735 9/1961 Gunning et al. 96-1 3,003,869 10/196'1 Schaffert 96-1 3,005,707 10/1961 Kallrnan et al 96-1 3,062,110 11/1962 Shepardson et al 95-1.7 3,137,762 6/1964 Baumgartner et al. 96-1 X NORMAN G. TORCHIN, Primary Examiner.

A. L. LIBERMAN, D. PRICE, Assistant Examiners.

Claims (2)

1. A ZEROGRAPHIC SENSITIZING PROCESS COMPRISING APPLYING A POETENTIAL ACROSS TWO DIELECTRICALLY SEPARATED CONDUCTIVE MEMBERS ONE OF WHICH IS A FINE CONDUCTIVE SCREEN, SAID SCREEN BEING CONTIGUOUS WITH A PHOTOCONDUCTIVE LAYER SO AS TO SET UP ELECTRIC FIELDS THROUGH SAID PHOTOCONDUCTIVE LAYER, SAID TWO DIELECTRICALLY SEPARATED CONDUCTIVE MEMBERS LYING ON ONE SIDE OF A SURFACE OF SAID PHOTOCONDUCTIVE LAYER, WITH SAID PHOTOCONDUCTIVE LAYER, WHILE UNIFORMLY CONTACT WITH SAID PHOTOCONDUCTIVE LAYER TO ACTIVATING RADIASUBJECTING SAID PHOTOCONDUCTIVE LAYER TO ACTIVATING RADIATION, WHEREIN COMPETING ELECTRIC FIELDS ARE ESTABLISHED IN THE RADIATION STRUCK AREAS, REMOVING SAID SOURCE OF ACTIVATING RADIATION AND REMOVING SAID POTENTIAL SOURCE FROM SAID CONDUCTIVE MEMBERS WHEREBY CHARGE IS TRAPPED AT THE PHOTOCONDUCTIVE DIELECTRIC INTERFACE.

6. A XEROGRAPHIC APPARATUS COMPRISING TWO CONDUCTIVE MEMBERS, A DIELECTRIC MATERIAL SEPARATING SAID TWO CONDUCTIVE MEMBERS, AT LEAST ONE OF SAID CONDUCTIVE MEMBERS BEING IN A FINE SCREEN A PHOTOCONDUCTIVE LAYER CONTIGUOUS TO SAID CONDUCTIVE SCREEN, SAID CONDUCTIVE MEMBERS AND SAID DIELECTRIC MATERIAL ALL LYING ON ONE SIDE OF A SURFACE OF SAID PHOTOCONDUCTIVE LAYER WITH AT LEAST A PORTION OF SAID DIELECTRIC LAYER BEING IN CONTACT WITH SAID PHOTOCONDUCTIVE LAYER, AND MEANS TO APPLY AN ELECTRIC POTENTIAL TO SAID SCREEN CAPABLE OF CREATING A SIGNIFICANT POTENTIAL DIFFERENCE WITHIN SAID SCREEN, WHEREBY ELECTRIC FIELDS ARE ESTABLISHED THROUGH SAID PHOTOCONDUCTIVE LAYER.

Images