Processes for preparing chalcogenide alloys

Abstract: A process for the preparation of chalcogenide alloys which comprises crystallizing a chalcogenide alloy, grinding and pelletizing the crystallized product, and evaporating the alloy on, to for example, a support substrate to form a photoreceptor.

Description: The present invention relates to processes for preparing and controlling the fractionation of chalcogenide alloys. More specifically, the present invention relates to processes for controlling the fractionation of chalcogenide alloys during the vacuum deposition thereof. In one embodiment, the present invention is directed to a process for controlling and suppressing the fractionation of chalcogenide alloys, and especially selenium alloys, which comprises the crystallization including the partial crystallization of said alloy, grinding and forming pellets thereof, and subsequently vacuum evaporating thereby enabling, for example, desirable homogeneous products. Another embodiment of the present invention comprises the provision of chalcogenide alloy pellets, the crystallization or partial crystallization of said alloy pellets, grinding and forming second pellets thereof, and subsequently vacuum evaporating. The products resulting from the process of the present invention can be selected as photoconductors in electrophotographic imaging systems, including xerographic imaging and printing processes. Chalcogens and chalcogenide alloys, and their use in electrophotographic processes, are known. Generally, the aforementioned photoconductors are prepared by vacuum deposition, flash evaporation, and chemical vapor deposition methods. These methods possess disadvantages in some instances. Thus, for example, with vacuum deposited chalcogenide alloys, the products obtained lack controllable reproducibility in their homogeneity thereby adversely affecting the electrophotographic electrical characteristics thereof. As the components selected for vacuum deposition usually have different vapor pressures, such components tend to separate during the vacuum deposition process, causing undesirable inhomogeneity, or fractionation thereof. Also, in the vacuum deposition processes the components or species with, for example, high selenium content tend to evaporate more rapidly during the initial stages of deposition primarily, it is believed, because of their higher partial vapor pressure, thus resulting in uncontrolled, undesirable fractionation. Accordingly, for example, the final photoconductor will contain less selenium on the top surface, which adversely affects the electrical characteristics thereof, and adds significantly to the cost of the process. With the processes of the present invention, these and other disadvantages are avoided. Electrophotographic photoconductive imaging members containing amorphous selenium can be modified to improve panchromatic response, increase speed and to improve color copyability. These improved members are usually comprised of chalcogenide alloys, such as alloys of selenium with tellurium and/or arsenic. The selenium imaging members may be fabricated as single layer devices comprising a selenium-tellurium, selenium-arsenic, selenium antimony, or selenium-tellurium-arsenic alloy layer which functions as a charge-generation and charge-transport medium. The selenium electrophotographic imaging members may also comprise multiple layers such as, for example, a selenium alloy transport layer and a continuous selenium alloy generator layer. One known process for the preparation of photoconductors comprises the vacuum deposition of a selenium alloy on a supporting substrate such as of aluminum. Tellurium can then be incorporated therein as an additive, primarily for the purpose of enhancing the spectral sensitivity thereof. Also, arsenic can be incorporated as an additive for the primary purpose of improving wear characteristics, passivating against crystallization and improving electrical properties of the resulting photoconductor. Generally, the tellurium addition can be accomplished as a thin selenium-tellurium alloy layer deposited over a selenium arsenic alloy layer to achieve the benefits of the photogeneration and transport characteristics of these respective layers. Fractionation of chalcogenide alloys, such as tellurium and/or arsenic composition, during the vacuum evaporation processes, results in an undesirable concentration gradient in the deposited photoconductor. Accordingly, there results inhomogeneities (fractionation) in the stoichiometry of the vacuum deposited thin film alloys. More specifically, fractionation occurs, it is believed, as a result of differences in the partial vapor pressure of the molecular species of the solid and liquid phases of binary, ternary and other multicomponent alloys. An important aspect in the generation of chalcogenide alloy imaging members is controlling the fractionation of alloy components, one of the advantages of the process of the present invention, such as tellurium and/or arsenic during the evaporation of source alloys. More specifically, tellurium and/or arsenic fractionation control is particularly important since the tellurium and/or arsenic concentration at the extreme surface of the resulting photoreceptor effects xerographic sensitivity, charge acceptance, dark discharge, copy quality, photoreceptor wear, yield, crystallization resistance, and the like. For example, in single layer low arsenic selenium alloy photoreceptors, arsenic enrichment at the surface caused by fractionation can also cause severe reticulation of the evaporated film. Further, in single-layer tellurium selenium alloy photoreceptors, tellurium enrichment at the surface because of fractionation can cause undue sensitivity enhancement, poor charge acceptance and enhancement of dark discharge. Also, in two-layer or multilayer photoreceptors where low arsenic alloys may be incorporated as a transport layer, arsenic enrichment at the interface with the layer above can lead to residual cycle-up problems. Moreover, in two-layer or multilayer photoreceptors where tellurium alloys may be incorporated as a generator layer, tellurium enrichment at the surface of the tellurium alloy layer can result in similar undue sensitivity enhancement, poor charge acceptance, and enhancement of dark discharge. A specific method of preparing selenium alloys for evaporation comprises the grinding of amorphous selenium alloy shot (beads) and compressing the ground material into pellet agglomerates, typically from about 150 to about 300 milligrams in weight and having an average diameter of about 6 millimeters. The pellets are then evaporated from crucibles in a vacuum coater with a time/temperature crucible program designed to minimize the fractionation of the alloy during evaporation. One disadvantage of the aforementioned vacuum deposited photoconductors, such as a selenium-tellurium alloy layer, is the crystallization of the selenium-tellurium alloy at the surface of the layer when exposed to heat. To retard premature crystallization and extend photoreceptor life, the addition of up to about 5 percent arsenic to the selenium-tellurium alloy can be beneficial without impairment of xerographic performance. Also, in deposited layers of selenium-tellurium alloys the amounts of surface tellurium present can cause excessively high photosensitivity. This photosensitivity is variable and changes as the surface of the layer wears away. Surface injection of corona deposited charge and thermally enhanced bulk dark decay involving carrier generation cause the toner images in the final copies to exhibit a washed out, low density appearance. Excessive dark decay causes loss of high density in solid areas of toner images and a general loss of image density. For example, when the photoreceptor comprises a single-layer selenium arsenic alloy, about 1 to about 2.5 percent by weight arsenic based on the weight of the entire layer at the surface of the alloy layer, there is provided protection against surface crystallization. When the concentration of arsenic is greater than about 2.5 percent by weight, electrical instability risks increase. One known method for attempting to control fractionation is the selection of shutters for incorporation over the evaporation crucibles. The tellurium or arsenic-rich material originating from the crucible deposits on the shutter rather than on the photoreceptor substrate. However, in planetary coating systems, installation of shutters is complex, difficult and expensive. Further, after one or more coatings, it may be necessary to clean the surface of the shutters, and the resulting debris can cause defects to occur in subsequently-formed photoreceptor layers. Accordingly, a problem encountered in the fabrication of chalcogenide alloy photoconductors, such as selenium alloy photoreceptors, is the fractionation or preferential evaporation of an alloy component whereby the resulting film composition is not equivalent to the source alloy. Thus, the deposited film or layer does not have a uniform composition extending from one surface to the other. For example, when tellurium is the dopant, the tellurium concentration is high at the free surface and can approach zero at the surface in contact with the substrate of the vacuum deposited layer. This problem is also observed for alloys of Se-Te, Se-As, Se-As-Te, Se-As-Te-Cl, or other halogens, mixtures thereof, and the like. US-A-4,780,386 illustrates a process wherein the surfaces of large particles of an alloy comprising selenium, tellurium and arsenic, the particles having an average particle size of at least 300 micrometers and an average weight of less than about 1,000 milligrams, are mechanically abraded while maintaining the substantial surface integrity of the large particles to form between 3 to 20 percent by weight dust particles of the alloy based on the total weight of the alloy, prior to mechanical abrasion. The alloy dust particles are substantially uniformly compacted around the outer periphery of the large particles of the alloy. The large particles of the alloy may be beads of the alloy having an average particle size of between 300 and 3,000 micrometers, or pellets having an average weight between 50 and 1,000 milligrams, the pellets comprising compressed finely ground particles of the alloy having an average particle size of less than about 200 micrometers prior to compression. In one embodiment, the process disclosed in this patent comprises mechanically abrading the surfaces of beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between 300 and 3,000 micrometers while maintaining the substantial surface integrity of the beads to form a minor amount of dust particles of the alloy, grinding the beads and the dust particles to form finely ground particles of the alloy, and compressing the ground particles into pellets having an average weight between 50 and 1,000 milligrams. In another embodiment, mechanical abrasion of the surface of the pellets after the pelletizing step may be substituted for mechanical abrasion of the beads. The process includes providing beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between about 300 micrometers and about 3,000 micrometers, grinding the beads to form finely ground particles of the alloy having an average particle size of less than about 200 micrometers, compressing the ground particles into pellets having an average weight between about 50 milligrams and about 1,000 milligrams, and mechanically abrading the surface of the pellets to form alloy dust particles while maintaining the substantial surface integrity of the pellets. Of particular interest with respect to the invention of the present application is copending EP-A-0,337,683 which describes and claims a process for crystallizing particles of an alloy of selenium comprising providing pellets of an alloy comprising amorphous selenium and an alloying component of tellurium, arsenic, or mixtures thereof, the pellets having an average weight between 50 and 1,000 milligrams, exposing pellets to an ambient temperature of between 114°C and 190°C until an exothermic reaction occurs in the pellets at between about 104°C and about 180°C, carrying reaction through to substantial completion, grinding the pellets into powder having an average particle size of less than about 200 mm, and compressing the powder into fresh pellets having an average weight between 50 and 1,000 milligrams. With the present invention, in one important embodiment the crystallized source alloy is initially subjected to grinding and then pelletizing. A more complex process is, for example, disclosed in EP-A-0,337,683 since one has initially to prepare the pellets, and heat at higher deexotherm temperatures. In the application, it is indicated that vacuum-deposited layers formed directly from pellets that are heat treated by exposure to temperatures of between about 110 and 190°C until an exothermic reaction occurs in the pellets at between about 104 and about 120°C, are characterized by unacceptably high photoreceptor sensitivity, whereas vacuum-deposited layers formed from pellets subjected to the same heat treatment followed by grinding and repelletizing are characterized by photoreceptor sensitivity within the specified values. With the process of the present invention low crystallization temperatures are preferably selected, for example, from about 85 to about 100°C, and preferably 95°C, thereby enabling an alloy product with a different microstructure, presumably smaller crystallite size, which permits more desirable thermal properties, including thermal conductivity, and superior heat transfer from a alloy product particle to another alloy product particle providing for superior control of fractionation. Prior art identified in the EP application includes US-A- 4,205,098; 4,609,605; 4,297,424; 4,554,230; 3,524,745; 4,710,442; 4,583,608; 4,585,621; 4,632,849; 4,484,945; 4,414,179; 4,015,029; 3,785,806 and 3,911,091, CH-A-656 486 and 172-346 and 57-91567. There is illustrated in US-A-4,513,031 a process for the formation of an alloy layer on the surface of a substrate, which for example comprises forming in a vessel a molten bath comprising at least one vaporizable alloy component having a higher vapor pressure than at least one other vaporizable alloy component in the bath; forming a thin substantially inert liquid layer of an evaporation retarding film on the upper surface of the molten bath, the liquid layer of the evaporation retarding film having a lower or comparable vapor pressure than both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component; covaporizing at least a portion of both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component whereby the evaporation retarding film retards the initial evaporation of the vaporizable alloying component having a higher vapor pressure; and forming an alloy layer comprising both the vaporizable alloying component having a higher vapor pressure and the other vaporizable alloying component on the substrate, see column 3, lines 33 to 54, for example. Examples of vaporizable alloying components include selenium-sulfur and the like; and examples of vaporizable alloying components having relatively low vapor pressures which include tellurium, arsenic, antimony, bismuth, and the like are illustrated in column 4, reference for example lines 41 to 50. Examples of suitable evaporation retarding film materials are outlined in column 4, at line 54, and continuing onto column 5, line 36, such materials including long chain hydrocarbon oils, inert oils, greases or waxes at room temperature which readily flow at less than the temperature of detectable deposition of the vaporizable alloying components having higher vapor pressures in the alloying mixture. Examples of retarding materials include lanolin, silicone oils such as dimethylpolysiloxane, branched or linear polyolefins such as polypropylene wax and polyalpha olefin oils, and the like, see column 5. According to the teachings of this patent, optimum results are achieved with high molecular weight long chain hydrocarbon oils and greases generally refined by molecular distillation to have a low vapor pressure at the alloy deposition temperature, see column 5, lines 32 to 36. It is believed with the aforementioned process that the levels of organics which are incorporated into the resulting alloy film may be sufficiently high causing negative adverse effects in the electrical properties of the resulting photoreceptor, for example, residual potential, dark decay, charge trapping, and cyclic stability are adversely effected. These and other problems are avoided with the process of the present invention. Moreover, there is no teaching in this patent with respect to initially crystallizing, or partially crystallizing the source component, and thereafter vacuum evaporating this component in the presence of a hydrocarbon, silicones, lanolins, which are a mixture of polyalcohols, polyesters, and fatty acids, especially the addition of these materials at low levels, that is from about 10 to about 30 parts per million thereby enabling photoreceptors with lower concentrations of organic materials thus permitting the advantages of the present invention, and avoiding those disadvantages as illustrated in the prior art such as the '031 patent, including, it is believed, the ineffective control of fractionation when low levels of organic additives, such as, for example, from about 10 to about 30 parts per million are selected. More specifically, there is described in US-A-4,205,098 a process wherein a powdery material of selenium alone or at least with one additive is compacted under pressure to produce tablets, the tablets being degassed by heating at an elevated temperature below the melting point of the metallic selenium and thereafter using the tablets as a source for vacuum deposition. The tablets formed by compacting the powdery selenium under pressure may be sintered at a temperature between about 100°C and about 220°C. Typical examples of sintering conditions include 210°C for between about 20 minutes and about 1 hour, and about 1 to about 4 hours at 100°C depending upon compression pressure. Additives mentioned include Te, As, Sb, Bi, Fe, Tl, S, I, F, Cl, Br, B, Ge, PbSe, CuO, Cd, Pb, BiCl₃, SbS₃, Bi₂S₃, Zn, CdS, CdSeS and the like. Furthermore, in US-A-4,609,605 there is illustrated a multilayered electrophotographic imaging member wherein one of the layers may comprise an amorphous selenium-tellurium-arsenic alloy prepared by grinding selenium-tellurium-arsenic alloy beads, with or without halogen doping, preparing pellets having an average diameter of about 6 millimeters from the ground material, and evaporating the pellets in crucibles in a vacuum coater; and in US-A-4,297,424 there is described a process for preparing a photoreceptor wherein selenium-tellurium-arsenic alloy shot is ground, formed into pellets and vacuum evaporated. Further, in US-A-4,780,386 there is illustrated a process for the preparation of electrophotographic imaging members which comprises providing large particles of an alloy of selenium, tellurium, arsenic, mechanically abrading the surfaces of the large particles whereby there is formed a certain amount of dust particles based on the total weight of the alloy prior to abrasion. There is illustrated in US-A-4,554,230 a method for fabricating a photoreceptor wherein selenium-arsenic alloy beads are ground, formed into pellets and vacuum evaporated. Also, the following US patents are mentioned: US-A-3,524,754 directed to a process for preparing a photoreceptor wherein selenium-arsenic-antimony alloys are ground into fine particles and vacuum evaporated; US-A-4,710,442 relating to an arsenic-selenium photoreceptor wherein the concentration of arsenic increases from the bottom surface to the top surface of the photoreceptor such that the arsenic concentration is about 5 weight percent at a depth of about 5 to mm from the free surface of the photoreceptor, and is about 30 to 40 weight percent at the free surface of the photoreceptor, which photoreceptor can be prepared by heating a mixture of selenium-arsenic alloys in a vacuum in a step-wise manner such that the alloys are sequentially deposited on the substrate to form a photoconductive film with an increasing concentration of arsenic from the substrate interface to the free surface of the photoreceptor. In one specific embodiment, a mixture of three selenium-arsenic alloys is maintained at an intermediate temperature in the range of from about 100° to 130°C for a period sufficient to dry the mixture; and US-A-4,583,608 relating to the heat treatment of single-crystal superalloy particles by using a heat treatment cycle during the initial stages of which incipient melting occurs within the particles being treated. During a subsequent step in the heat treatment process, substantial diffusion occurs in the particle. In a related embodiment, single crystal particles which have previously undergone incipient melting during a heat treatment process are prepared by a heat treatment process. It is an object of the present invention to provide processes for the preparation of chalcogenide alloys. Accordingly the present invention provides a process for preparing and controlling the fractionation of chalcogenide alloys as claimed in the appended claims. More specifically, the present invention is directed to a process which comprises providing a chalcogenide alloy source material, such as a selenium alloy, including the specific alloys as illustrated in copending EP-A-0 337 683, crystallizing the source material; grinding the crystallized product; pelletizing it; and vacuum evaporating the source material from the pellets, enabling the formation of a photoconductor with improved characteristics as illustrated herein. In one specific embodiment of the present invention, there is provided a process which comprises providing a chalcogenide alloy, such as an alloy containing selenium, including selenium arsenic and selenium tellurium alloys; crystallizing the alloy, preferably at low temperatures, such as for example 85 to 100°C; grinding the crystallized product; pelletizing; and thereafter vacuum evaporating the alloy by heating at a temperature of from 250°C to 350°C, and depositing on a support substrate the resulting desired chalcogenide alloy with reduced fractionation. In another specific embodiment of the present invention, there is provided a process which comprises providing a chalcogenide alloy, such as an alloy containing selenium, selenium arsenic, and selenium tellurium alloys; subjecting the aforesaid alloy to crystallization at a temperature of from 85 to 100°C, preferably 95°C; grinding the crystallized product; pelletizing; subjecting the resulting components to evaporation by heating at a temperature of from 250°C to 350°C and depositing on a support substrate the desired chalcogenide alloy with reduced fractionation, and containing from about 0.3 to about 2 percent by weight of arsenic or tellurium, wherein the loss of the selenium-rich component is avoided. Grinding and pelletizing can be accomplished by methods as illustrated, for example, in US -A-4,780,386 or by the process as illustrated in the appropriate copending applications mentioned herein. In one embodiment of the present invention, subsequent to crystallization of the alloy beads of, for example, 0.5 to about 2 mm diameter, and preferably 1 millimeter diameter, they are added to a grinder such as a hammer mill grinder which grinds the beads to a powder mixture wherein the average particle size diameter is from 5 to 50 µm, preferably from 15 to 25 µm, and more preferably from 8 to 10 µm. The resulting powder particles usually have a narrow small particle size distribution, low electrostatic charge thereon, so that substantially no clumping results as is the situation with amorphous selenium alloys. As a result, less grinding time, for example from about 20 minutes, is needed and pelletization is simpler. Further, the resulting pellets are dense primarily because of the aforementioned small particle size. Pelletizing can be accomplished by any appropriate process. In one embodiment of the present invention, pelletizing is effected by feeding the ground alloy powder into a pelletizer. Usually, the average diameter of the resulting pellets is from 4 to 10 mm, and preferably from 5 to 7 millimeters. The thickness of the pellets can vary generally, from 2 to 6 mm, and preferably from 3 to 4 millimeters thick. Pellet weight is in one embodiment from 100 to 1,000 mg and preferably from 300 to 400 milligrams. Also, as the pellets are comprised of small particles with narrow particle size distribution, the heat transfer from one particle to another particle during the vacuum evaporation is very efficient thus avoiding or minimizing early undesirable losses of selenium-rich species, thereby preventing or controlling fractionation. In another specific embodiment of the present invention, there is initially prepared the source alloy reactant product by melt quenching methods. According to this method, the desired elements in appropriate amounts are mixed in a quartz vessel and are melted at temperatures of from 400 to 600°C depending, for example, on the elements selected and other reaction parameters. Effective mixing is usually accomplished by purging the aforementioned molten mass with a stream of a dry inert gas, such as nitrogen, argon, or mixtures thereof. Subsequent to heating and mixing the alloy components for an extended period, for example, from 2 to 72 hours, the molten mass resulting is shotted, for example, by pouring into cold water through a screen. There results alloy shot, for example, an alloy shot of selenium-tellurium with about 70 to 80 percent by weight of selenium, and about 30 to 20 percent by weight of tellurium; or selenium arsenic with about 95 to 99.8 percent by weight of selenium, and about 5 to 0.2 percent by weight of arsenic. Also, the resulting diameter of the alloy shot is dependent on the screen size, and the shot is amorphous at this stage as determined by X-Ray diffraction techniques (XRD). The exact composition of the alloy is usually determined by X-Ray Fluorescence Spectroscopy (XRF). Examples of shot produced in this manner include selenium arsenic alloy with about 0.5 or 2 weight percent arsenic content; SeTe alloys with 20, 12, 14.5, 25, or 30 weight percent tellurium; SeAsTe with Te, 8 weight percent, As, 2 weight percent, Te, 10 weight percent, or As, 4 weight percent. Also, the diameter of the shots usually varies between about 0.5 millimeter to about 2.5 millimeters. The resulting shot is then subjected to crystallization. Crystallization may be accomplished by a variety of means as indicated herein, and detailed in the aforementioned copending applications. For example, in one embodiment with regard to a selenium tellurium alloy containing about 10 weight percent of tellurium, and about 90 percent by weight of selenium, although other amounts, and other alloys may be selected as indicated herein, for example, the alloy shots are initially abraded for about 1 hour to generate surface nucleation sites. The abraded alloy is then added to open trays, for example metal or pyrex trays, and the trays are then placed into an oven. Subsequently, the oven temperature is increased to about 62°C and the alloys maintained at this temperature for about 1 hour during which time the trays are shaken periodically. Thereafter, the temperature in the oven is increased to about 67°C and maintained at this temperature for about 1 hour. The trays are then shaken again, and the temperature in the oven then further increased to about 75°C and maintained at this temperature for about 2 hours followed by increasing the temperature in the oven to about 85°C, and maintaining this temperature for about 1 hour. Subsequently, the temperature in the oven is raised to about 100°C and held at this temperature for about 8 hours. During the aforesaid heating steps, the trays were periodically shaken by hand, usually an average of once every 30 minutes. There results crystalline alloy shot as determined by XRD. Thereafter, the resulting crystallized pelletized alloy source material can be added to crucibles in abell jar coater, which material was substantially evenly spread within the crucibles and coating on a substrate such as aluminum accomplished in about 20 minutes. For this coating, which is known in the art, the crucible temperature is usually maintained at about 350°C, the vacuum was maintained at 4 x 10⁻⁵ Torr, the substrate temperature was maintained at 55°C, and the preferred aluminum substrate thickness was 25 µm and generally up to about 2.5 mm in thickness. There resulted a photoreceptor film with a preferred thickness of 50 µm. The aforementioned process embodiment is also applicable to the preparation of other photoreceptors not specifically described herein, which photoreceptors, it is believed, possess the advantages illustrated herein. These photoreceptors can be comprised of known binary, ternary, and quaternary alloys including selenium arsenic, selenium tellurium, selenium antimony, selenium arsenic antimony, and the like, with or without halogen doping as illustrated herein, which alloys can be selected for the process of the present invention. The substrate in a thickness of up to 2.5 mm, and preferably from 25 µm to 1.25 mm, selected for the deposited chalcogenide product, may be opaque or substantially transparent and may comprise numerous materials having the desired mechanical properties. The entire substrate may comprise the same material providing an electrically-conductive surface, or the electrically-conductive surface may merely be a coating on the substrate. Typical electrically-conductive materials include, for example, aluminum, titanium, nickel, chromium, brass, stainless steel, copper, zinc, silver and tin. Any suitable material such as nickel may be employed, which conductive layer may vary in thickness depending on the desired use of the electrophotoconductive member. Accordingly, the conductive layer may be of a thickness of from 5 to 500 nm. Generally, the substrate may be comprised of known materials, including organic and inorganic materials. Examples of substrate materials include insulating nonconducting materials, such as various resins known for this purpose, including polyesters, polycarbonates, polyamides, polyurethanes, and the like. The coated or uncoated substrate, which may also be comprised of conductive components as indicated herein, may be flexible or rigid and may have any number of configurations such as, for example, a plate, a cylindrical drum, a scroll or, an endless flexible belt. The outer surface of the substrate preferably comprises a metal oxide such as aluminum oxide, nickel oxide, titanium oxide, and the like. In some situations, intermediate adhesive layers between the substrate and subsequently-applied layers may be desirable to improve adhesion. Preferably, these layers have a dry thickness between about 0.1 micrometer to about 5 micrometers. Examples of adhesive layers include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrrolidone, polycarbonate, polyurethane, polymethylmethacrylate, and the like, and mixtures thereof. Any suitable photoconductive chalcogenide source alloy may be selected for the process of the present invention as indicated herein including binary, ternary, quaternary alloys, and the like, in effective amounts of, for example, from about 40 grams to about 55 grams when preparing a 50 to 55 µm thick, 100 X 150 mm alloy thin film to enable the formation of the vacuum-deposited photoconductive layer. Preferred alloys include alloys of selenium with tellurium, arsenic, or tellurium and arsenic with or without a halogen dopant. Specific examples of alloys of selenium selected for the process of the present invention include selenium-tellurium, selenium-arsenic, selenium-tellurium-arsenic, selenium-tellurium-chlorine, selenium-arsenic-chlorine and selenium-tellurium-arsenic-chlorine alloys. Generally, the selenium-tellurium alloy may comprise between 5 and 40 percent by weight tellurium and a halogen consisting of up to about 70 parts per million by weight of chlorine, or up to about 140 parts per million by weight of iodine, all based on the total weight of the alloy, with the remainder being selenium. The selenium-arsenic alloy may, for example, comprise between 0.01 and 35 percent by weight arsenic and a halogen consisting of up to 200 parts per million by weight of chlorine, or up to about 1,000 parts per million by weight of iodine. The selenium-tellurium-arsenic alloy may comprise between about 5 percent by weight and about 40 percent by weight tellurium, between about 0.1 percent by weight and about 5 percent by weight arsenic, and a halogen consisting of up to about 200 parts per million by weight of chlorine, or up to about 1,000 parts per million by weight of iodine. The expressions 'alloy of selenium' and 'selenium alloy' are intended to include halogen-doped alloys as well as alloys not doped with halogen. The thickness of the aforementioned photoconductive chalcogenide alloy layer is generally between about 0.1 micrometer and about 400 micrometers. Other thicknesses may be used. Also, dopants may include metals such as thallium, iron, manganese, and the like in place of halogen. The aforementioned dopants are generally present in an amount of from about 10 to about 500 parts per million, and with halogen preferably in an amount of 10 to 200 ppm, and preferably 10 to 100 parts per million. Vacuum evaporation involves heating the source component in a crucible, at a temperature of from 250°C to 350°C. Selenium-tellurium and selenium-tellurium-arsenic alloy photoconductive layers are frequently employed as a charge generation layer in combination with a charge transport layer. The charge transport layer is usually positioned between a support substrate and the charge- generating selenium alloy photoconductive layer. Generally, a selenium-tellurium alloy may comprise from 60 to 95 percent by weight selenium, and from 5 to 40 percent by weight tellurium, based on the total weight of the alloy. The selenium-tellurium alloy may also comprise other components such as less than about 35 percent by weight arsenic to minimize crystallization of the selenium, and less than about 1,000 parts per million by weight halogen. In a more preferred embodiment, the photoconductive charge generating selenium alloy layer comprises between 5 and 25 percent by weight tellurium, between about 0.1 and 4 percent by weight arsenic, and a halogen consisting of up to about 100 parts per million by weight of chlorine, or up to about 300 parts per million by weight of iodine, with the remainder being selenium. Elevated levels of arsenic in some applications, above about 4 percent by weight, can result in high dark decay, problems in cycling stability, and problems with reticulation of the photoreceptor surface. The resistance of amorphous selenium photoreceptors to thermal crystallization and surface wear begins to degrade as the concentration of arsenic drops below about 1 percent by weight. As the chlorine content rises above about 70 parts per million by weight chlorine, the photoreceptor begins to exhibit excessive dark decay. Any suitable selenium alloy transport layer may be utilized in the aforementioned layered imaging member. Examples of these layers include pure selenium, selenium-arsenic alloys, selenium-arsenic-halogen alloys, selenium-halogen and the like. Preferably, the charge transport layer comprises a halogen-doped selenium arsenic alloy. Generally, about 10 to 200 parts by weight per million of halogen is present in a halogen- doped selenium charge transport layer. When the halogen-doped transport layer free of arsenic is utilized, the halogen content should normally be less than about 20 parts by weight per million. Inclusion of high levels of halogen in a thick halogen-doped selenium charge transport layer free of arsenic may cause excessive dark decay. Imaging members containing high levels of halogen in a thick halogen-doped selenium charge transport layer free of arsenic are described, for example, in US-A-3,635,705 and 3,639,120, and JP-A-5 61 42-537. Generally, halogen-doped selenium arsenic alloy charge transport layers comprise between 99.5 to 99.9 percent by weight selenium, 0.1 to 0.5 percent by weight arsenic, and between about 10 to 200 parts per million by weight of halogen, the latter halogen concentration being a nominal concentration. 'Halogen' includes fluorine, chlorine, bromine, and iodine. Chlorine is preferred primarily because of its stability. Transport layers are described, for example, in US-A-4,609,605 and 4,297,424. The first layer of multiple layered photoreceptors, such as a transport layer, may be deposited by any suitable conventional technique, such as vacuum evaporation. Thus, a transport layer comprising a halogen- doped selenium-arsenic alloy comprising less than about 1 percent arsenic by weight may be evaporated by conventional vacuum coating devices to form the desired thickness. The amount of alloy to be employed in the evaporation boats of the vacuum coater will depend on the specific coater configuration and other process variables to achieve the desired transport layer thickness. Chamber pressure during evaporation may be of the order of about 4 x 10⁻⁵ Torr. Evaporation is normally completed in about 15 to 25 minutes, with the molten alloy temperature ranging from about 250°C to about 325°C. Other times, temperatures and pressures outside these ranges may be used. It is generally desirable that the substrate temperature be maintained in the range of from 50°C to 70°C during deposition of the transport layer. Additional details for the preparation of transport layers are disclosed, for example, in US-A-4,297,424. The photoreceptors of the present invention can be selected for known imaging and printing processes, reference for example US-A- 4,265,990; 4,544,618; 4,560,635 and 4,298,672. In some of these processes, latent images formed on the alloy photoreceptor product with the process of the present invention are developed, transferred to a suitable substrate such as paper, and fixed thereto by, for example, heating. In another embodiment of the present invention, there is provided a process for the preparation of chalcogenide alloys which comprises providing a chalcogenide alloy source component, crystallizing the source component, grinding the resulting product and evaporating the source component. Further, in yet another embodiment of the present invention there is provided a process for controlling the fractionation of selenium alloys, which comprises providing a selenium alloy, crystallizing the alloy, grinding and pelletizing, and evaporating the alloy component. The following examples are being submitted to define various species of the present invention further. These examples are intended to be illustrative only and are not intended to limit the scope of the present invention. Also, parts and percentages are by weight unless otherwise indicated. EXAMPLE I (for comparison) Amorphous selenium-tellurium (Se-Te), 12 weight percent of tellurium, and 88 weight percent of selenium alloy shots, were prepared by mixing 39.9 kg of high-purity (99.999) selenium and 5.4 kg of high-purity (99.999) tellurium in a quartz vessel. The contents of the vessel were then heated at 450°C, at which temperature a molten mass resulted. The agitation of the molten mass was accomplished by sparging it with nitrogen gas. The sparging and heating were continued for 8 hours after which time the molten mass was poured into cold water (25°C) through a stainless steel wire gauge. The resulting product of alloy shots was collected and dried. These alloy shots were of an average diameter of about 2,500 µm as determined by standard sieve analysis. An XRF (X-Ray Fluorescence) analysis indicated the composition to be comprised of a selenium-tellurium alloy (Se-Te), 12 percent by weight of tellurium and 88 percent by weight of selenium. XRD (X-Ray diffraction) analysis indicated the above alloy shots to be amorphous. Fifty (50) grams of the above shots were loaded into the crucibles of a 450 mm bell jar coater and a 50 µm alloy product thick film (88/12 Se-Te) was coated on an aluminum substrate, about 25 µm thick, by maintaining the crucible temperature of 350°C, the substrate temperature at 55°C, and a vacuum of 4 x 10⁻⁵ Torr. The alloy product film thickness of 50 µm was determined by Permascope. The EMPA (electron microprobe analysis) indicated that the average concentrations of tellurium on the top 0.1 µm and 0.5 µm surface of the alloy film product were 36.9 percent and 33.2 percent, respectively. Also, EMPA analysis indicated that the distribution of tellurium concentration varied greatly from the top (36.9, 33.2) to the bottom (2.5, 1.1) of the alloy film product, thus indicating severe fractionation. EXAMPLE II (for comparison) 9.1 kg of the amorphous selenium-tellurium, 12 weight percent of tellurium, 88 weight percent of selenium, amorphous alloy shots obtained by the process of Example I was tumbled in a mechanical Munsen blender for about 0.5 to about 1 hour whereby selenium-tellurium alloy dust particles having an average particle diameter of about 10 µm resulted. The blender and subsequent steps are more specifically described in US-A-4,780,386. Rotation of the cylinder in the blender caused the alloy beads or shots to be moved upward by vanes until the beads tumbled downward because of gravity. Less than about 20 percent of the selenium tellurium alloy beads were fractured. The resulting selenium tellurium alloy, 88/12, was then ground into a powder having an average particle diameter of about 30 µm as determined by a Coulter Counter in a hammer mill grinder for about 20 minutes. Thereafter, the resulting ground alloy powder was compressed into pellets, resulting in an average weight of the aforesaid alloy of 300 milligrams. The pellets were of a thickness of about 3 millimeters, and had a diameter of about 6 millimeters. Fifty grams of the above pellets were then placed in two crucibles of a 450 mm bell jar coater and a 50 µm thick film alloy product, 88/12, selenium-tellurium alloy was then deposited on an aluminum substrate of 25 µm thickness by repeating the procedure of Example I. EMPA analysis indicated that the average concentrations of tellurium on the top 0.1 µm and 0.5 µm of the vacuum-deposited selenium tellurium alloy film were 15.23 percent and 15.6 percent, respectively. Also, the EMPA analysis indicated that the alloy product film had inhomogeneous distribution of the tellurium throughout the resulting alloy film product, and thus considerable tellurium fractionation. EXAMPLE III A. 22.7 kg of the amorphous Se-Te (12 percent by weight of Te, and 90 percent by weight of selenium) alloy shots prepared by the process of Example I were fully crystallized by abrading for 1 hour and heating, by placing the resulting alloy shot into open metal trays, and the trays were heated in an oven under ambient air flow at 67°C for 1 hour. Within this time, the surface of the shots turned shiny. The temperature of the oven was then increased to 67°C and the shot maintained at 67°C for 30 minutes. The trays containing these shot were shaken 3 times during this 30 minute period. The temperature was then raised to 75°C and the shot maintained at 75°C for 1 hour. The temperature was then increased to 85°C for 1 hour, followed by increasing to 93°C for 30 minutes. The temperature was then increased to 95°C for 8 hours. XRD (X-ray diffraction) indicated that the resulting selenium-tellurium (88/12) alloy shots were completely crystalline. The crystallized product was then subjected to grinding and pelletizing by repeating the procedure as detailed in Example I. B. Fifty (50) grams of the above crystalline ground, pelletized alloy shots were then loaded into a crucible of a bell jar vacuum coater. These shots were spread evenly on the bottom of the crucible with a spatula. An aluminum sheet, which was 25 µm thick, was used as a substrate. A thin Se-Te (88/12) film was vacuum deposited on the substrate at 350°C crucible temperature, 4 x 10⁻⁵ Torr pressure, and 55°C substrate temperature. After the coating was completed, the thickness of the selenium-tellurium (88/12) alloy film photoreceptor was found to be 50 µm. EMPA (electron microprobe analysis) indicated the average concentrations of tellurium at the top 0.1 and 0.5 µm of the alloy film product (90/10 Se-Te) to be 13.7 percent and 13.6 percent, respectively. In addition, EMPA analysis also indicated that the alloy film product had a relatively homogeneous distribution of tellurium throughout the film product, thus fractionation was reduced. EXAMPLE IV A. 18.1 kg of amorphous Se-Te alloy (12 percent by weight of Te, and 90 percent by weight of selenium), alloy shots prepared by the process of Example III, were ground into a fine powder with an average particle size diameter of about 10 mm in a hammer mill, followed by forming pellets with a thickness of 3 millimeters and a diameter of 6 millimeters by repeating the procedure of Example II. B. Fifty (50) grams of the above crystalline alloy pellets were then loaded into a crucible of a bell jar vacuum coater. These shots were spread evenly on the bottom of the crucible with a spatula. An aluminum sheet, 25 µm thick, was used as a substrate. A thin Se-Te (88/12) film was vacuum deposited on the substrate at 350°C crucible temperature, 4 x 10⁻⁵ Torr pressure, and 55°C substrate temperature. After the coating was completed, the thickness of the selenium-tellurium (88/12) alloy film photoreceptor was found to be 50 µm. EMPA (electron microprobe analysis) indicated the average concentration of tellurium at the top 0.1 and 0.5 µm of the alloy film product (88/12 Se-Te) to be 12.4 percent and 12.0 percent, respectively. In addition, EMPA analysis indicated that the alloy film product had a relatively homogeneous distribution of tellurium throughout the film product, thus fractionation was reduced. With further respect to the present invention, although not usually preferred, alloy pellets may be formed first, followed by crystallization, grinding, pelletizing and evaporating.

Claims: A process for the preparation of chalcogenide alloys which comprises: crystallizing a chalcogenide alloy source component; grinding and pelletizing the crystallized alloy product, and evaporating the pellet material in vacuo. A process as claimed in claim 1, in which the alloy component is crystallized at a temperature of from 85 to 100°C. A process in accordance with claim 1 or 2, wherein the ground crystalline alloy particles have an average diameter of from 5 to 50 µm. A process in accordance with claim 1 wherein the average diameter of the pellets formed is from about 4 to about 10 millimeters. A process as claimed in any preceding claim, wherein a halogen is added to the alloy. A process as claimed in claim 5, wherein the halogen is present in an amount of from 10 to 200 parts per million by weight. A process as claimed in claim 5 or 6, wherein the halogen is chlorine. A process as claimed in any preceding claim, wherein the chalcogenide alloy comprises selenium with a component of tellurium, arsenic, antimony, bismuth, or mixtures thereof.

Inventors: BADESHA SANTOKH S, ELDER FREDERICK A, HORDON MONROE J, KOWALCZYK LAWRENCE E, LAFORCE ROGER W, LEES BARRY A, STERLING STEVEN M, ZUKOSKI PAUL F, BADESHA, SANTOKH S., ELDER, FREDERICK A., HORDON, MONROE J., KOWALCZYK, LAWRENCE E., LAFORCE, ROGER W., LEES, BARRY A., STERLING, STEVEN M., ZUKOSKI, PAUL F.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1990
Main classification: C01B19
IPC codes: C01B19, G03G5, C23C14