US3312548A - Xerographic plates - Google Patents

Description

2 Sheets-Sheet l Filed Dec. 27, 1965 Se -As-I SENSTIVITY R.. m N E W 5 I & m m %m mm 55 www 5 5 nam maa 5%m H sm 5 m. 5%0 omo m m w m m OO VIRGIL E. STRAUGHAN A TTORNEVS 2 Sheets-Sheet 2 Filed Dec. 27, 1965 NVENTOR. VIRGIL E. STRAUGHAN 1.4 TTORNE VS United States Patent Ofilice &312548 Patented Apr. 4, 1967 3,312,548 XEROGRAPHEC PLATES Virgil E. Straughan, Euclid, Ohio, assignor, by mesne assignments, to Xerox Corporation, Rochester, N.Y., a Corporation of New York Filed Dec. 27, 1965, Ser. No. 516529 15 Claims. (Cl. 96--1.5)

This application is a continuation-in-part of my copending application Ser. No. 293,357, filed July 8, 1963, now abandoned.

This invention relates t-o Xerography and more specifically, to a system utilzing an improved photosensitive plate.

In the art of xerography, it is usual to form an electrostatic latent image on a member or plate which comprises a substantially electrically conductive backing member such as for example, a paper or a metallic member, having a photoconductive insulating surface thereon. It has previously been found that a suitable plate for this purpose is a metallic member having a layer of vitreous seleniurn. Such a plate is characterized by being cap-able of receiving a satisfactory electrostatc charge and selectively dissipating such charge when exposed to a light pattern and, in general, is largely sensitive to light in the blue and blue-green spectral range.

In the usual form of xerography, the electrostatic charge pattern formed by the selective dissipation of charge noted above is converted into a visible image through the selective attraction of marking particles by known methods of image development. The Xerographic plate is accordingly of great importance in the xerographic process as it is the element responsible for the Creation of the charge pattern. Other forms of xerographic plates are known including, for example, sheets of paper coated with a photoconductive mixture of zinc oxide particles in an insulating resn. However, the vitreous selenium xero- 'graphic plate remains the most widely used because it is capable of holding an electrostatic charge for long periods of time when not exposed to light, because it is relatively sensitive to light compared with other xerographic plates, and because it has sufcient strength and stability to be reused hundreds or even thousands of times. The selenium plate, ho wever, is susceptible to deleterious crystal growth when the plate is heated during operation. This growth of crystals in the selenium layer destroys the photoconductive insulating properties of the selenium, and places a limit upon the effective life of the selenium plate.

At the same time, imp rovements in the light sensitivity and response to longer wavelengths are much desired. A significant contribution was made by O. S. Ullrich in the U.*S. Patent 2,803,542, which disclosed that the addition of arsenic to selenium causes a general increase in the light sensitivity of the Xerographic plate and also causes the plate to be sensitive to longer wavelentghs of light.

There is still, however, a continuing need for plates which require still shorter exposu re times and yield a `wider range of reproducible colors.

It is, therefore, an object of this invention to provide an improved selenium-arsenic xerographic plate and an improved method for preparing a selenium-arsenic xerographic plate which overcomes the above noted disadv antages.

It is another object of this invention to provide a selenium-arsenic xerographic plate 'having increased light sensitivity.

It is a further object of this invention to provide a selenium-arsenic xerographic plate having a broadened range of spect ral response.

It is yet -a further object of this invention to provide an improved method of making selenium-arsenic xerographic plates having increased light sensitivity and a broadened range of spectra l response.

It is another object of this invention to provide a selenium-arsenic Xerographic plate having improved thermal stability.

The foregoing objects and others are accomplished in accordance with this invention by preparing a Xerographic plate containing selenium, arsenic, and up to 10,000 parts per million (p.p.m.) of at least one member of the halogen family. In the preparation of this plate suitable quantities of selenium, arsenic, and a ha logen are sealed in a container and reacted at an elevated temperature to form a homogeneous m-iXture of these elements. The alloy is then cooled and 'applied to a suitable conductive supporting base by vacu-um evaporation. When the evaporation process is completed, a 'finished plate is removed from the vacuum chamber.

In general, the effective range of arsenic in the selenium layer is about 0.5 to 50 percent by weight With the prefenred range being about 1 to 25 percent. The lower limit of 'about 1 percent is dctated by the fact that arsenic in amounts as low as 0.5 percent raises the crystal-lization temperature and 1 percent practi-cally eliminates crystallization. The upper limit of 25 percent is chosen because this amount of arsenic in combination with halogen doping will achieve essentially the same degree of light sensitivity and broadened spectral response as As Se o r 38.5 percent arsenic, without introducing the high dark discharge property of As Se The upper end of the preferred range, from 15 to 20 percent arsenic, would be more desirable from the standpoint of obtaining the optimum light sensitivity.

The effective range of the halogen addition is from about 10 to 10,000 parts per million with about to 5,000 parts per million being preferred. The sensitivity for a given amount of arsenic increases to a certain degree with increased amounts of the halogen. Although amounts of 10 parts per million do exhibit an increased sensitivity a more desirable sensitivity value can be obtained with greater amounts, such as at least 100 parts of the halogen. Similarly amounts as high as 10,000 parts per million (1 percent) are effective, but are unnecessary in most cases, in that there is no significant change over the use of 5000 parts per million.

The seleni-um-arsenic-halogen composition may comprise the entire insul'ation layer or be present as -a thin outer layer overlaying a base layer of pure selenium.

The advantages of the improved xerographic plate and the method for pr-oducing said rplate will 'become more apparent upon consideration of the 'following disclosure of the invention; especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is' a schematic sectional View 'of one `form of erographic plate according to the invention;

FIG. 2 is a schematic sectional View of a second form of xerographic plate according to the invention;

FIG. 3 is a bar graph showing the relative sensitivity of various xerographic plates;

FIG. 4 shows the relative sensitivity of a selenium plate with increasing amounts of arsenic, with and without the addition of a halogen.

The halogen is primarily illustrated by the use of iodine in FIGURES 1 to 4.

FIGURE 1 shows a first form of improved Xerographic plate according to the invention. Reference character 10 designates an electrically conducti-ve mechanical support member. This is conventionally a metal plate such as -brass, aluminum, gold, platinurn, steel or the like. The support member may be of any convenient thickness, rigd or flexible, in the form of a sheet, a web, a cylinder, or the like, and may be coated with a thin layer of plastic.

It may also comprse such other materials as metallized paper, plastic sheets covered with a thin coating of aluminum or Copper iodide, or glass coated with a thin layer of chr-omiurn or tin oxide. An important consideration is that the member be at least somewhat electrically conductive or have a somewhat conductive surface and that it be strong enough to permit a certain amount of handling. Member 11 may even be dispensed with entirely in some cases. Reference character 12 designates the photocon ductive insulating layer which is coated on member 11. As shown in the figure, this layer is comprised of yitreous selenium together with lesser amounts of arsenc and a halogen consisting of iodine, bromine, chlorine or fiuorine. As shown in the Xerogr-aphic art, this layer may be as thin as about 1 micron or as thick as about 300 microns or more, 'but for most commercial applications the thickness will -generally lie between about 20 to 80 microns. This range of 20 to 80 is preferred in that the thi-cker layers (ie. those approaching 300 microns) show some signs less desirable adherence to the support member than lesser thicknesn A suitable method of making the plate of FIGURE 1 is described below for illustrative purposes only.

Suitable quantities of selenium, arsenic, and iodine are sealed in a reaction vessel and reacted at 525 C. for

three or four hour in a rocking furnace. After the selenium mixture is cooled and removed from the reaction vessel, it is applied to a suitable support member 10, such as a sheet of polishe d brass, by a vacuum evaporation process. The mixture is placed in a crucible within a bell jar and the brass plate is supported about 12 inches- The sensitivities of a 100 percent selenium plate and a u selenium-arsenc pl-ate prepared under similar conditions are also included for reference. sensitivity was measured by elec'trostatically charging the various Xerographic plates beneath a corona discharge element and then exposing the plates to light for of a second, measuring the relative dissipation of charge by means of an electrometer, and comparing' this relative -discharge with a selenium control plate. It is apparent from FIGURE 3 that the li-ght sensitivity of xerographic *plates prepared with arsenc and iodine eXceed those of a control plate made with ordinary selenium. The 'sensitivity 'of plates made `under similar conditions with 5, 17.5 and 25 percent arsenc and no iodine are included for purposes of comparison. It can be seen that the plates containing iodine 'exhibits a very substantial increase in sensitivity as compared with the plates lacking iodine. This increase sensitivity may not become apparent until several days after the plate is made. The plate containing the bromine addition shows a sensitivity comparable to plates containing iodine.

As shown in FIGURE 2 in a further embo diment of the invention, it has been found advantageous to -deposit the selenium-arsenic-iodine mixture in a thin surface layer on the Xerographic plate onto a layer of substantially pure selenium. A Xerographic plate can be constructed in accondance with this embodiment by including a second evaporation source, such as a molybdenum boat, in the vacuum 'bell jar. The -prin cipal evaporation source is loaded with pure selenium or selenium with iron powder while the selenium-arensic-iodine mixture is placed in the molybdenum boat which comprises a second evapo-ration source. The selenium is first evaporated onto a suitthe sensitivity of a 100 percent selenium plate.

able support member 11 exactly as described previously. As soon as the selenium evaporation is completed and without breaking the vacuum in the bell jar, the seleniumarsenic iodine mixture is eva'porated onto the selenium. By confining the arsenc and iodine to a thin surface layer, smaller quantities of these materials can be emplcyed. In addition, the coating process is simplified because the alloy is evaporated in a very short time and there is less concern about non-uniforrn distribution of the arsenc and iodine in the plate.

Layer 14 may be of any convenient thickness. Layers between about 0.1 and 0.5 micron have been found satisfactory. There is no upper limit on this thickness, since as layer 14 becomes very thick, the embodiment of FIG- URE 2 simply becomes the embodiment of FIGURE 1.

It is apparent that the layered embodiment is capable of producng substantially the same increase in speed as the plate incorporating' arsenc and selenium throughout the bulk of the selenium. There is an additional benet in employing the embodiment of FIGURE 2. All xerographic plates tend to exhibit an increased rate of charge dissipation in darkness after being exposed to bright light. High speed plates tend to eXhibit this undesirable property to a greater degree. It has been found that layered plates corresponding to FIGURE 2 exhibit this effect, known as fatigue, to a lesser degree than those corresponding to FIGURE l. Thus, after exposure to 'bright light, a selenium plate containing 10 percent arsenc and 100 p arts per million of iodine throughout its bulk lost over percent of its charge in 30 seconds. When the same arsenic-selenium-iodine mixture was confined to a one-half micron surface layer the plate was able to retain more than half its charge after the previous exposure to bright light. It may be noted that these two plates have nearly the same sensitivity.

It has been discovered that the spectral response of xerographic plates in accordance with the present invention is broadened in p'roportion to the amount of arsenc present, as taught in U.S. Patent 2,803,542 referred to previously. The addition of a halogen on the other hand, increases sensitivity without afiecting the spectral response.

The increase in sensitivity for a given selenium plate with increasing amounts of arsenc, with and without a halogen, is shown in FIGURE 4. In FIGURE 4, curve A represents the light sensitivity of a selenium plate with increasing amounts of arsenc, with no halogen additive. The light sensitivity of 1 at 0 percent arsenc would -be As can be seen from curve A, the sensitivity of a selenium-arsenc plate does not show improved sensitivity over 'a percent selenium plate until the arsenc is added in amounts upward of about 13 percent. On the other hand, curve B shows that the addition of 1000 parts per million of iodine increases the light sensitivity of the seleniumarsenic plate at any percentage of arsenc, reaching a maximum of about 6 times the sensitivity of a 100 percent selenium plate at about 18 percent arsenc.

Iodine is the preferred halogen additive, in that it can be Conveniently added as a solid in weighed amounts to arsenc and selenium in a Pyrex vial just prior to evacuation and sealing. As previously described, the vial is then heated in a rocking furnace to insure proper mixing and homogenization.

Inasmuc-h as the other members of the halogen family are either liquid or gaseous at room temperature, additional precautions should be taken to insure that they are properly combined with the selenium and arsenc.

Bromine is added as liquid drops from a burette to the arsenc and selenium which is precooled in a glass tube by a Dry Ice-acetone mixture. This procedure is important in order to prevent a complete loss of the bromine during evacuation since the melting point of bromine is -7 C.

Chlorine may be added by slightly different procedure. In this procedure the chlorine gas is admitted to an evacuating tube containing gram quantities of arsenc and selenium. The remaining standard amounts of arsenic and selenium are added to the tube and cooled in Dry Ice-acetone mixtures prior to scaling under vacuum. Both the bromne and chlorine are now 'sufl'iciently blended with the arsenic and selenium and the mixing and homogenizati'on process is then carried out as set forth in the description using iodine as an additive. It has been found that bromine gives effects similar to iodine when used in the plates of this invention.

The following examples further specifically define the present invention with respect to the method of making the halogen-doped selenium-arsenic plates. The parts and percentages in the disclosure, examples, and clams are 'by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of making a halogen-doped selenium-arsenic plate.

Example I Amixture of about 17.5 percent arsenic, about 82.5 percent selenium, plus about 1000 p.p.m. of iodine are sealed in a Pyrex vial and reacted at about 52S C. for about three hours in a rocking furnace. The mixture is then cooled to about room temperature, removed from the Pyrex vial, and placed in a quartz crucible within a bell jar. An alumin-um plate is supported about 12 inches above the crucible and maintained at a temperature of about 70 C. The bell jar is then evacuated to a pressure of about 5 10- torr and the quartz crucible is heated to a temperature of about 380 C. to evaporate the selenium mixture onto the brass plate. The crucible is kept at the evaporation temperature for approximately 30 minutes. At the end of this time the crucible is permitted to cool and the finished plate is removed from the bell jar.

Example II A mixture of about 17.5 percent arsenic and about 82.5 percent selenium are placed in a Pyrex vial. Bromine is added to this mixture in a concentraton of about 500 p.p.m. as liquid drops from a burette to the arsenic and selenium mixture which is precooled in the glass vial by a Dry Ice acetone mixture. The Pyrex vial containing the resulting mixture is then evacuated and sealed. The sealed Pyrex vial is then treated in the same manner as the iodine-doped mixture set forth in Example I.

Example III A mixture of about 15 percent arsenic and about 85 percent selenium is mixed with chlorine by first placing one gram each of arsenic and selenium in an evacuated tube. Chlorine gas is then admitted to the evacuated tube to produce a concentraton of chlorine of about 2,000 p.p.m. The chlorine reacts with the arsenic and selenium as evidenced by the evolution of heat. The remaining amounts of arsenic and selenium are then added to the tube and cooled in a Dry Ice-acetone mixture prior to scaling in the Pyrex vial under vacuum. The sealed Pyrex vial is then treated in the same manner as the iodine-doped mixture set forth in Example I.

The plates made in accordance with the present invention are normally used in a xerographic process including at least the three basi-c steps of charging, exposing, and developing. A plate, which has preferably been stored in darkness, is given a surface electrostatic charge by being passed under a corona disch arge device or the like. A positive potential or charge on the order of several hundred volts is typical. The plate is then exposed to a pattern of light and shadow, as in a camera. This selectively dissipates the charge previously applied and the remaining charge forms a charge pattern conforming to the light pattern. By using the plates of the present invention, substantial shorter exposure times are possible and a wider range of colors may be reproduced due to the increase sensitivity and broadened spectral response of the plates. Finally, the electrostati-c pattern is made into a visible reproduction of the light pattern through selective electrostatically controlled deposition of marking material. Apparatus and materials for carrying out these basic xerographic steps are well-known in the art and need not be further described here.

Although special Components and proportions have been stated in the above description of the preferred embodiments of the selenium xerographic plate, other suitable materials, as listed above, may be used with similar results. In addition, other materials may be added to the mixture to synergize, enhance, or otherwise modify its properties. For example, the halogen may be convenient ly added as a compound of arsenic or selenium. For example, sodium hypochlorite could well be a source of chlorine. This allows all the halogens to be :added to the selenium-arsenic mixture as solids, notwithstanding the fact that some halogens occur in their elemental form as a gas or liquid at room temperature.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. A xerographic plate having a photoconductive insulating layer, said layer comprising a composition of selenium, arsenic and a halogen, said halogen being present in a concentraton of about 10 to 10,000 parts per million.

2. The plate of claim 1 wherein the halogen is iodine.

3. The plate of claim 1 wherein the halogen is bromine. 4. The plate of claim 1 wherein the halogen is chlorne.

5. The plate of claim 1 wherein the arsenic and halogen are substantially uniformly distributed throughout the photoconductive insulatng layer.

6. The plate of claim 1 wherein the arsenic and halogen are conned to a thin surface layer of the photoconductive insulatng layer.

7. A xerographic plate having an electrically conductive support member and a photoconductive insulatng layer thereon comprising a composition of selenium, arsenic, and a halogen, the thickness of said insulatng layer being about 1 to 300 microns, the arsenic constituting between about 05% to 50% of said layer, and the halogen present therein in an amount between about 10 parts per million to about 10,000 parts per million.

8. The plate of claim l wherein the halogen is iodine.

9. The plate of claim 7 wherein the halogen is bromine.

10. A xerographic plate having a photoconductive insulating layer, said layer comprising a composition having about 820% selenium, 180% arsenic and about 1000 parts per million of iodine.

11. The plate of claim 10 wherein the thickness of the insulatng layer is about 20 to microns.

12. A xerographic plate having an electrically conductive support member, a layer of substantially pure Vitreous selenium coated thereon, and a photoconductive layer coated over said selenium layer, said photoconductive layer comprising a mixture of vitreous selenium, about 0.5% to 50% arsenic, and between about 10 to 10,000 parts per million of a halogen.

13. The plate of claim 12 where-in the halogen is iodine.

14. A method of forming a latent image on a Xerographic plate which comprises applying an electrostatic charge to a photoconductive layer comprising selenium,

arsenic, and a halogen, said halogen being present in a concentraton of about 10 to 10,000 parts per million, and exposing said charged layer to a pattern of activating electromagnetic radiation thereby forming a latent electrostatic image on said layer.

15. The method of claim 14 wherein the latent electrostatic image is developed to make the pattern Visible.

(References on following page) References Cited by the Examner UNITED STATES PATENTS Hart 96-1 Paris 96-1 Ullrich 96-1 Bardeen 96-1 Blakney et al. 96--1 Stern et al. 252-501 OTHER REFERENCES I. TRAVIS BROVJ N, Acting Pr'mary Exam'ner.

NORMAN G. TORCHIN, Exam'ner.

C. E. VAN HORN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,312 ,548 April 4 l967 Virgil E. Straughan It is Certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as show-n below:

column 6, line 47, claim reference numeral "l" should read 7 Signed and sealed this 24th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents

Claims (1)

1. A XEROGRAPHIC PLATE HAVING A PHOTOCONDUCTIVE INSULATING LAYER, SAID LAYER COMPRISING A COMPOSITION OF SELENIUM, ARSENIC AND A HALOGEN, SAID HALOGEN BEING PRESENT IN A CONCENTRATION OF ABOUT 100 TO 10,000 PARTS PER MILLION.

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