04/728224

10/26/1971

05/10/1968

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Inc, Honeywell

430/60

430/65, 205/201

G03G5/07; G03G5/082; G03G5/10; G03G5/00

117/215,230,119,219,222 96/1.5,1,1.8 204/38.1,5

3017285	Method of writing on anodized aluminum	January 1962	Wainer
3026255	Method of protecting metal surfaces	March 1962	Riou et al.
3032435	Process for improving the corrosion resistance of pieces of light metals and light metal alloys	May 1962	Michel
3449705	PHOTOCONDUCTIVE MATRIX SHEET	June 1969	Chamberlin

Leavitt, Alfred L.

Weiffenbach C. K.

What is claimed is

1. An electrographic plate for receiving and retaining electrostatic images applied thereto, comprising an aluminum substrate material having an electrically conductive surface and a layer of aluminum oxide on said surface having a plurality of like micropores uniformly distributed thereacross and extending therethrough to said conductive surface, with an electrographic imaging material having a dark resistivity on the order of 10⁹ to 10¹⁰ ohm-cm. and an exposed resistivity on the order of 10⁴ to 10⁵ ohm-cm. disposed in said pores and contacting said conductive surface such that the oxide material is completely impregnated with said imaging material, but said imaging material not completely covering the outer surface of said oxide material.

2. An electrographic plate for receiving and retaining electrostatic images applied thereto, comprising a substrate material having an electrically conductive surface and a layer of porous refractory material on said surface having a plurality of like micropores uniformly distributed thereacross and extending therethrough to said conductive surface, wherein said porous material is taken from the class consisting of aluminum oxide, copper oxide, magnesium oxide and manganese oxide, with an electrographic imaging material having a resistivity of sufficient value to receive and retain an electrostatic image disposed in said pores and contacting said conductive surface such that the porous material is completely impregnated with said imaging material, but said imaging material not completely covering the outer surface of said porous material.

3. The electrographic plate of claim 2 wherein said substrate material is aluminum and said porous refractory material is an aluminum oxide.

4. The electrographic plate of claim 2 wherein said imaging material is taken from a class of material selected to have a dark resistivity on the order of 10⁹ to 10¹⁰ ohm-cm. and an exposed resistivity on the order of 10⁴ to 10⁵ ohm-cm.

5. The electrographic plate of claim 2 wherein said porous material shows a resistivity of not less than 10⁸ ohms-cm.

BACKGROUND, NOVEL FEATURES

In the arts of electrography, it is relatively conventional to form an electrostatic image on an imaging surface comprised of an electrical conductor material coated with a prescribed dielectric imaging material, such as an aluminum substrate coated with imaging material such as selenium, sulfur, etc., and, in some cases, with an intermediate oxide layer. Such features, and others relating to this invention, are indicated in standard texts, such as "Xerography and Related Processes" edited by J. Dessauer and H. Clark or "Electrophotography" by R. Schaffert (both from Focal Press, New York). The present invention relates to an improved electrographic surface and methods for providing this wherein imaging material is deeply embedded in, and completely through, an insulating layer (or "matrix") on a conductive substrate so as to comprise a homogeneous array or imaging "pillars" dispersed homogenously across an imaging plane (on the substrate) and in electrical isolation from one another.

Of course, certain types of imaging plates have been known to comprise a conductive plate on which is superposed a relatively insulative perforate matrix, the perforations of which are filled with imaging material such as photosensitive sulfur, selenium or the like. However, such "perforation layers" are to be contrasted with coatings which are made microscopically (not macroscopically) perforate, i.e. "cross-sectionally porous" (order of billion pores/cm. ² .) especially with those which comprise an oxide coating formed directly on a conductive substrate, such as the invention teaches.

Deterioration of such selenium-embedded coatings and the like can commonly occur due to such mechanical abrasion as is typically involved in a xerographic developing process or (subsequent) cleaning process where, for instance, a rapidly rotating fur brush may sweep a powder-toned selenium xerographic imaging surface to remove all residual developer material adhering there (after a powder-image-transfer step). Additionally, such abrading friction can, of course, also generate heat in this delicate selenium imaging surface. And, of course, any substantial exposure of the imaging material, or of the substrate carrying it (e.g. as cracks resulting from extreme abrasion, overheating, fabrication imperfections, etc.) can permit entry of contaminants like air and other environmental gases. These can quickly and deleteriously oxidize, corrode, etc. both the imaging material and the underlying substrate. By contrast, the aforedescribed oxide matrix taught serves as a protective or mechanical package, protecting electrostatic imaging ("esi") material from such cracking and resultant contamination, as well as also acting to seal the surface of the underlying substrate electrode from contamination. Thus, such a composite oxide-filler surface would be a functional substitute for such prior art expedients as "protective over-coatings," supercoated on imaging layers, or resinous carrier-media for particles of imaging material, or the like (for instance, some workers have proposed using an insulating binder with photoconductive insulating materials dispersed therein to form a different kind of composite xerographic plate).

Of course, workers in various arts have, for various purposes, provided porous oxide coatings and filled the pores to at least some extent with various kinds of imaging material, such as in the photographic arts by provision of a porous oxide coating anodized on an aluminum base with photosensitive material provided in the pores. Such photographic film layers are unlike electrographic imaging surfaces. That is, they are sensitized simply by spectral radiations (without reference to deposition and migration of imaging ions or other charged particles), are not adapted for mechanical abrasion environments (such as involved in toning of xerographic plates and subsequent cleaning thereof) and are not adapted for reuse of the imaging surface, involving structures and operational characteristics which are completely unrelated to those of an electrographic imaging surface. Accordingly, such films have not heretofore been suitable for use in electrography.

By contrast, the invention describes an electrographic imaging structure, and methods whereby dielectric of a prescribed uniform porosity and insulative characteristics is provided with the pores extending completely through the coating and having prescribed electrographic imaging material injected therein. Such an arrangement will be seen to perform a number of novel advantageous functions such as sealing the porous oxide coating (from intrusion of contaminants, etc., protecting the relatively soft imaging material from mechanical abrasion or other abuse, electrostatically isolating a great number of individual imaging pillars across the imaging surface, etc. Moreover, the invention teaches a particularly attractive method of implementing such an imaging arrangement according to a modification of a popular aluminum anodizing technique coupled with a particular evacuating filling technique.

Workers in the art will recognize advantageous characteristics of such improved imaging plate structures as those described below as well as significant advantages in the preferred fabrication methods taught. For example, as taught in the aforementioned texts by Dessauer and Schaffert, it is commonly desired to form xerographic imaging surfaces using the photoconductive insulating properties of highly purified selenium. This selenium has a particularly high photographic speed, but it also presents certain difficulties associated with its preparation. For example, it is unfortunately necessary to provide an extremely clean and uniform substrate, as well as a particularly fussy vacuum evaporation techniques in preparing it. This, together with the high cost of selenium itself, has lead to the use of selenium xerographic plates repetitively, over and over, to save costs. That is, such selenium plates are processed in repeating cycles to be reused many times and keep the cost per image-copy reasonable. For instance, under optimum use conditions, a vitrious selenium plate can be used to prepare on the order of 100,000 copies before deteriorating to the point of unsatisfactory image formation although under less optimum conditions, far fewer copies can be made. A typical xerographic selenium plate must be cleaned approximately every few thousand copy cycles and replaced after about 100,000 cycles. At any rate, workers in the art are well familiar with the mechanical stress that such repeated processing subjects imposes on an imaging plate. For instance, the selenium imaging surface is characteristically toned during each imaging cycle by deposition of toning powder thereon, this powder being mechanically transferred to a copy medium and the untransferred residue thereafter removed, such as by wiping processes or the like. It is quite easy to imagine the great danger of abrasion that such hundreds of thousands of applications of toner powder impose on an imaging plate, such that it is customarily problematical to provide some kind of protection against mechanical abrasion for the soft selenium image coating. The invention meets this need by improved composite imaging surfaces like those aforementioned.

It is, therefore, an object of this invention to provide an electrographic imaging film having the aforementioned structural and operational advantages without the aforedescribed problems. It is another object to provide an electrographic imaging plate and associated fabrication method involving electrostatic imaging material having utility in reusable electrographic surfaces. Yet another object is to provide such a plate having a composite imaging layer comprising a uniformly and microscopically porous insulating material, the pores of which extend completely through the material cross section and are injected with prescribed imaging material.

The above and other objects, features and advantages of the invention will become apparent upon consideration of the following material detailed disclosure especially when taken in conjunction with the accompanying drawings wherein like reference characters denote like elements:

FIG. 1 representing, very schematically and in cross section, the condition of a representative aluminum conductive substrate for an electrostatic imaging plate embodiment typical of the invention;

FIG. 2 representing this substrate as oxidized in a preferred manner;

FIG. 3 representing this oxidized surface in an evacuated condition;

FIG. 4 representing the super-position of a layer of imaging material; and

FIG. 5 representing the completed imaging plate with the imaging material residing in the pores of the evacuated oxide layer; and

FIG. 6 representing an imaging plate like that in FIG. 4, but rendered by a modified alternate fabrication method.

IN GENERAL

FIGS. 1 through 5 are intended to represent, rather schematically, successive stages in the fabrication of an electrographic imaging plate and various plate layers involved, according to the invention. To characterize these layers, rather broadly: layer S comprises a conductive metal substrate, such as an aluminum plate, having an imaging surface is and a contact surface cs. According to the invention, an oxide layer so of specified structure and characteristics is grown (or otherwise provided) on imaging surface is of substrate S and is thereafter substantially completely evacuated of all air and other environmental substances (at so-e). Layer so, preferably, comprises an oxide layer grown on substrate S and having pores which are uniformly distributed across the imaging surface, which have a prescribed relatively uniform microscopic diameter and which extend completely through layer so into communication with imaging surface is. These evacuated pores are then injected with imaging filler material, such as a xerographic photoconductive insulator like selenium, in various forms. This filler may, for instance, be embedded in layer so-e by depositing a layer if on the surface of so-e (kept evacuated), and then heating (or otherwise driving in) the if material to fill the pores of so-e and thereby provide a finished, composite "oxide/imaging" (oxide matrix, containing imaging material) layer so-ef will include an imaging surface iso, adapted to be imaged conventionally. As workers in the art well realize, the resistivity and like electrographic characteristics of the oxide so and filler material if will be selected so as to, together, provide specific electrographical properties for the contemplated imaging operations. For instance, some of these properties, such as resistivity, may be verified by impressing a test voltage Vt between imaging surface iso and substrate contact surface cs as indicated in FIG. 5.

TYPICAL USE

Before turning to the details of the invention and of embodiments illustrating the structure and fabrication techniques of imaging plates according to the invention, a brief reference will now be made to how such an imaging plate would typically be used in the electrographic arts. An illustrative electrographical plate according to the invention would be similar to plate IP in FIG. 5, comprising a xerographic plate XP (not shown) constructed to be operated with prescribed electrographic treating stations, such as (in order of treatment): an electrographic charging station, a xerographic imaging station, a toning station, a transfer station and a cleaning station, as known in the art. Accordingly, plate XP would first be brought into operative relation with a charging unit in the charging station, that is, typically be brought into spaced relation with one or more corona-charging wires, connected to a source of high potential and adapted to charge the imaging plate surface nonselectively according to the well-known corona discharge effect. More particularly, the substrate conductor of plate XP is typically connected to ground so that application of a high potential to the corona wires adjacent the imaging surface will act to initiate a corona discharge (ionization and subsequent transfer of charge particles) onto this surface, thus depositing a specific (e.g. positive) charge thereon, to raise its level of electrostatic potential above ground a prescribed amount. Next, the so-charged surface is presented at the imaging station where the charged surface is exposed to the image to be copied so that the radiant image sectors drop the resistivity of the imaging material where they impinge. With the conductive substrate connected to ground these irradiated portions will thus be selectively discharged and the "dark," unirradiated portions left relatively undischarged (or the reverse, as known in the art).

After this discharging exposure (and thus after the generation of an electrostatic image on the imaging surface plate XP, the plate is next presented to the developing unit, whereat charged toner particles are selectively deposited on either the positive or negative (charged) image surface portions (to thus develop the electrostatic image) by any one of many developing techniques well known in the art. For instance, in a well-known cascade type development a supply of toner material is cascaded over the imaged surface. Such a toner mixture may comprise known composite developer particles, including finely divided, colored marking particles (the toner dye) and grossly larger carrier beads, which serve to deagglomerate the toner and to charge it (e.g. by virtue of their relative rank in the triboelectric series, etc.). Thus, when the carrier beads with toner particles clinging to them are cascaded over the imaged surface, the electrostatic field from the charged pattern pulls toner particles off the carrier bead (e.g. to cling to the positive charged negative (dark) portions of the imaged surface (assuming the beads are charged relatively negative--though the polarities may be reversed, of course).

The toned image on the image surface is then presented to the image transfer station, whereat the toner pattern may be transferred to copy media. Typically, this will involve translating the image surface past a contact-transfer locus, together with an adjacent strip of copy paper, and pressing the two into light contact therewhile, as known in the art. A known transfer charge unit is typically used also, being similar to the aforementioned corona-charging arrangement whereby the copy paper web is charged to a polarity opposite that of the toner particles so as to pull them away from the imaging surface to reside on the copy paper (overcoming the attraction of the electrostatic image on plate XP etc.). Thereafter, the copy web may be fixed, such as with a heat fuser, a solvent vapor or the like. The image surface will typically retain a residue of some nontransferred toner particles, which may be removed at a downstream cleaning station. Such a removal typically involves lightly wiping this surface with a charged cleaning implement, such as a fur brush or the like (and also neutralizing any residual image charge).

To those skilled in the art, the above imaging cycle or representative electrographic imaging plate P will be very familiar. A useful plate is expected to undergo hundreds of thousands of such cycles without radical improvement. The delicate imaging surface, will, of course, typically be subjected to a number of possibly damaging mechanical contacts during each imaging cycle, such as when it is impacted by the toner, when it and the toner are impacted by the copy medium, and when the toner is thereafter removed, such as by wiping away. Thus, any improvement in the resistance of such an electrographic surface to deterioration by mechanical contact will be highly prized by workers in this art.

PREFERRED MATERIALS, METHODS--IN GENERAL

Before turning to the specific examples of electrographic surfaces according to the invention, general introductory mention will be made of materials suitable for use with the invention and the associated fabrication methods. Of course, the selection of materials depends to a great degree upon the intended use of the electrographic copying plate, as well as upon the selected fabrication methods. For instance, substrate S (FIG. 1) will be understood as functioning, in general, as an electrical conductor as well as a mechanical carrier for the imaging layer; the imaging material (filler) will be understood as selected to develop an electrostatic image under the imaging conditions contemplated (e.g. xerographic, purely electrostatic, etc.), and the dielectric matrix (surface layer) will be seen as furnishing mechanical protection, e.g. by providing a housing matrix for uniformly dispersing filler pores and mechanically protecting the filler (imaging) material therein, being mechanically and chemically stable enough for this purpose, and by providing suitable electrical properties (e.g. dielectric) so as not to interfere with the imaging process, to electrically isolate filler areas, etc. Of course, these materials must be adherent and compatible, e.g. the imaging material must be able to be adhered properly to the dielectric matrix (pore walls) and the matrix, in turn, must be firmly affixed on the substrate imaging surface.

More particularly, the fabrication processes will usually dictate a more specific selection of materials. For instance, it will typically be desirable (for convenience, improved performance, etc.) to provide the dielectric matrix by anodizing the substrate surface (or by similar processes); hence, a substrate metal must, of course, be selected which will lend itself to this process, and moreover which, when so anodized, can efficiently produce a suitable dielectric matrix (having the proper electrostatic properties, structure, thickness, etc.). Known aluminum anodizing methods will be seen as especially useful here. Alternatively, one may deposit a porous dielectric matrix directly onto a conductor surface, such as by fastening a porous ceramic wafer or by "sinter-spraying" ceramic oxide onto metal (or otherwise building up a porous dielectric film thereon). On the other hand, it may be preferable to fabricate the dielectric matrix separately from the substrate and then join these. For example, it will be seen that one may provide a thin porous dielectric strip such as ceramic wafer etched to the proper porosity, and mount this on the surface of a suitable substrate conductor, filling the pores of the ceramic with imaging material, either before or after mounting. Alternatively, it may be preferable to provide a relatively nonporous dielectric layer on a substrate surface (e.g. by treating the substrate surface or by attaching a separate dielectric strip) and thereafter etch the pores therethrough (completely through the dielectric) to provide the prescribed porosity (e.g. pore dimensions, communication with underlying conductor, etc.).

Similarly, the characteristics of the imaging material selected will influence, or depend upon, the selection of substrate material, dielectric material and processing methods. For instance (as indicated in FIGS. 4 and 5), the imaging material may be laid over an (exhausted) porous dielectric layer and then vapor-impregnated (cf. Example I), such as by heating; hence, the dielectric selected will obviously have to be unaffected by the attendant heating temperatures. As to methods of fabricating an imaging surface according to the invention, workers in the art will particularly appreciate the novel vacuum impregnation method (described below example I) whereby the atmosphere in, and surrounding, a porous dielectric matrix (e.g. as an anodized surface of an aluminum substrate) is effectively evacuated to remove all air from the pores, and then a portion of imaging material is introduced into the pores (such as by heating a superposed layer of filler to vaporize it) to create a satisfactory image pillar therein. Besides such vapor impregnation, the filler may be injected by other methods, such as by capillarity in a vacuum (cf. example II) or the like.

As suggested above, the dielectric matrix may also be formed on a conductor substrate surface, such as by depositing the dielectric material (e.g. by sputtering, vapor disposition or other techniques), rather than being formed as a separate strip of dielectric material to be affixed to the substrate. Similarly, although it is more convenient to define matrix properties (e.g. porosity) according to how the fabrication is controlled (e.g. during aluminum anodizing as aforementioned) these two steps may be separated, with porosity and similar characteristics being separately defined either before oxidizing (e.g. prepitting the substrate), during it (e.g. by masking out microscopic pore-loci and depositing nonporously elsewhere) or after oxidizing (e.g. by etching through a nonporous dielectric deposit). Thus, for instance, if one fabricates the matrix from a thin ceramic wafer (or porous plastic film; e.g. porous Nylon, etc.) of prescribed dielectric characteristics, uniform porosity (pores extending completely through the wafer cross section) and dense distribution of microscopic pores (e.g. order of a billion pores per cm. ² ), etc., he might then lay the wafer over a pumping orifice, inside a bell jar or other evacuation chamber, pump down the wafer pores, evacuate them and superpose imaging material on the exposed wafer surface to thereafter inject it (e.g. impregnation) into the pores to thus form the desired composite dielectric imaging layer separate from the conductor substrate. Likewise, a corresponding conductor substrate may be also separately provided, such as by depositing metal on one face of this pore-filled wafer (e.g. by plating etc.) and then braze, or otherwise join, this metallized wafer surface to a conductor plate or the like. Similarly and alternatively, one may select (or fabricate) a metal substrate of prescribed fine porosity; deposit similarly porous dielectric oxide on one surface thereof (as a matrix with pores registering); pump down the so-formed dielectric metal pores and fill them with imaging material, and then attach a continuous conductor plate to the other substrate surface.

PARTICULAR MATERIALS

The substrate material used in preparing electrographic imaging plates may function to physically support the composite imaging (dielectric filler) layer and also provide electrical ground (e.g. during the formation of an electrostatic image on the composite layer; and also subsequently, during toning, transferring and cleaning the imaged surface). Such a plate will be seen as essentially comprising any suitably planar (flat or curved) surface, such as on a platform, a drum, or the like. It will be evident to those skilled in the art that a number of conductive metals may be used for such plates, usually depending upon the selected method of fabricating the associated composite layer. For instance, a suitably oxidizable metal like aluminum (or magnesium, manganese, or copper--e.g. heating copper in air) may be selected (or copper sulfide, etc.). In most cases, these may be satisfactorily oxidized by processes (e.g. anodizing) which are, for the most part, well known. Under certain conditions one might also use conductor-coated dielectrics such as aluminum-coated glass, metallized plastic, or the like. In general, a substrate may act as a ground plane for the electrographic processes and still exhibit a surprisingly high resistivity, such as 10 ⁶ or 10 ⁸ ohm-cm. The substrate structure must, of course, be stable when subjected to any heating involved in forming the dielectric matrix, or in injecting the imaging material. It must also remain stable under other processing conditions (e.g. not break down under mechanical abrasion; not corrode, e.g. after anodizing or like oxidizing). The selected dielectric must also be thermally compatible with the imaging and substrate materials. For instance, if, during fabrication, or use, it may be subjected to a particular degree of heating (in some cases it might be quite close to a copy-fusing station, etc.) one must, of course, match its thermal response parameters (such as the heat expansion coefficient) to the substrate and filler (e.g. so they do not separate in use).

The dielectric material must, in general, be a good electrical insulator (higher resistivity than imaging material--e.g. order of 10 ¹² ohm-cm. is typical). It should also be mechanically hard and inert so as to be unaffected by contemplated abrasive or corroding environments, and to protect the imaging material therein. Most importantly, perhaps, it should also exhibit a prescribed cross-sectional fine porosity ("swiss cheese" configuration). Also, if the dielectric layer is not grown directly on a substrate surface, it must, of course, be capable of being firmly joined (in ohmic contact) thereto, such as by brazing or the like. It is also very important that the dielectric be completely inert to the imaging material and the imaging environment, e.g. so as not to "poison" the imaging filler embedded therein. It must be relatively hard mechanically so as to match the mechanical properties of the wiping or contacting elements such as the toner beads, the paper copy, the cleaning brushes, etc. For instance, it obviously must be hard enough to resist all but a slight abrasion by these; yet it must not be so hard as to, itself, cut into or abrade and damage the contacting elements (e.g. the paper, the brushes, etc.). Moreover, since it is expected that the dielectric will be somewhat abraded and worn away, however gradually, it must be rigid enough so as not to substantially deform, or "flow," under wiping contact and thereby cover or obstruct the pores and imaging material therein.

The imaging material useful in the instant invention can comprise virtually any electrographic imaging material known in the art (see aforementioned texts by Dessauer and by Schaffert) consistent with its compatibility with the substrate and dielectric materials, with selected fabrication processes and with the electrostatic imaging application contemplated. For instance, if one intends to use a xerographic type of electrography, he would typically select an imaging material comprising a photoconductor showing a relatively high "dark resistivity" (e.g. order of dielectric resistivity, or above about 10 ⁹ ohms-cm., typically about 10 ¹¹ ohm-cm.), with a substantially lower "exposed resistivity" (e.g. order of 10 ⁴ , 10 ⁵ ohm-cm., when exposed to light). Such photoconductors may be theoretically characterized as having electrons in the nonconductive energy level (valence band) which are activatable by illumination to a different energy level (conduction band) whereby an electric charge is free to migrate under an applied electric field (on the order of 10 ³ volts per cm. or more). Classic xerographic imaging materials generally include the sulfides of zinc, mercury, antimony, arsenic, indium, cadmium and calcium-strontium; the selenides of cadmium, gallium and arsenic; the oxides of zinc, lead, mercury, titanium, zinc-magnesium; or others such as zinc titanate, zinc silicate, etc. Common photoconductors used in xerography are those metallic-ion-containing inorganic compounds termed as "phosphors" which exhibit photoluminescence when subjected to light. Any good electrostatic imaging receptor which will not react with the dielectric matrix, and yet will adhere thereto, will be preferable as a filler. For instance, ordinary photoresist materials may be used in certain cases.

PARTICULAR METHODS

An important feature of this teaching relates to some preferred techniques for fabricating electrographic plates; particularly to techniques, such as aluminum anodizing or like oxidizing, where the composite dielectric-imaging layer is convenient to form. Such methods are described in the following illustrative examples which are merely exemplary and not to be taken as limiting.

EXAMPLE I

An aluminum plate (any common commercial alloy) is hand anodized in any conventional aqueous sulfuric acid/oxalic acid anodizing bath, sufficient to form a (uniformly) porous aluminum oxide coating of up to about 1 mil (.001 inch) on an image surface portion of the plate. This coating may be colored, if desired, with any conventional dye. The coating is then washed thoroughly in distilled water, sufficient to remove all anodizing bath, and other, residue (including, it is believed, all residue in the coating pores). The coating is dried thoroughly and placed in a container, then a layer of colloidal sulfur is placed on the coating, sufficient to cover the desired image area, using about 1 gm. sulfur per 5 square inches of surface. The sulfur-aluminum sandwich is then inserted into an evacuation oven and evacuated to about 1 mm. Hg total atmospheric pressure. The oven is then operated to heat the contents to about 135° C., sufficient to produce a small amount of sulfur vapor for impregnating the pores (at this under-pressure) and kept so for about three hours or more. This may be characterized as "vapor-impregnating" sulfur into the matrix pores. After the oven has cooled, the plate is finished and may be removed. The matrix pores are believed to thus be coated, at least on their base, with a sulfur film or its reaction product (e.g. an aluminum oxide or the like, such as a very thin film sealing the pore base and walls). These films are believed to be much thinner than the dielectric and not fill the pores, although neither their structure nor theoretical explanation is, as yet, completely understood). The image surface should be thoroughly washed with clean water or any nonpolar solvent to remove condensates or other minor contaminants. The dielectric-separated sulfur pillars have "electret" properties and may be characterized as "quasi-electrets."

The resultant composite image plate will be observed to have advantageous properties of the type aforementioned. For instance, when image-charged, the image surface will be observed to decay (charge-leakage) at about 1 percent of the rate of the untreated surface portions. Surface leakage (and consequent loss of image resolution) is not excessive either, and applied electrographic images will be quite satisfactorily rendered.

EXAMPLE II

An aluminum substrate plate is anodized, cleaned and dried as in example I. The intended image surface area of the oxide coating is then coated with Stycast No. 35 epoxy resin, catalyzed with No. 9 catalyst (a low-viscosity resin by Emerson-Cumming Co. of the type conventionally used to "pot" components, form circuit boards, etc.). The plate is then placed in the vacuum oven, which is evacuated to approximately 1 mm. Hg and heated to bring the contents up to about 140° F. to be kept at this vacuum and temperature for about 10 minutes or more. The oven is then cooled and returned to atmospheric pressure, with the plate remaining therein until the resin has set (about 11/2 hours or more). The plate is then removed and its resin-filled imaging surface machined uniformly down to the oxide layer, grinding away all plastic above this hard coat surface. A portion of the filled oxide may also be lapped down to render a smoother finish, if desired. The decay characteristic and other electrographic properties will be formed comparable to that of example I, indicating that a like matrix of quasi-electrets formed in the pores of the oxide matrix.

Unlike the vapor impregnation techniques of example I, injection here is effected evidently by a simple mechanical (probably by capillary) action whereby the filler liquid replaces the atmosphere being sucked completely out of the pores. Care should be taken to evacuate the pores thoroughly, leaving no air pockets, etc., to interrupt the cross-sectional integrity of the quasi-electret pillars. Other like electrostatically compatible resins and other organics may be similarly applied, such as those of examples III and IV below. However, certain apparently equivalent organics have been found to give unsatisfactory electrographic results when used as such fillers. For instance, paraffin and carnauba wax were unsatisfactory (e.g. very unsatisfactory surface leakage and image resolution for some reason), as were certain oils. Of course, materials that are too conductive (low resistivity, etc.) like glycerin, cannot be used. It is believed that low viscosity and ability to flow into the pores (by capillary action) are important constraints (e.g. molecule size less than pore diameter). Other like materials should be generally applicable as fillers for such injection; such as other low-viscosity nonconductors (for the contemplated conditions). Such nonconductors (dielectrics) may also be applicable for vapor impregnation (as with the sulfur in example I) if sufficiently volatile (under the contemplated conditions), such as cadmium sulfide or like photoconductors.

The plate was further tested by machining the resist-filled image surface down to the original pure aluminum substrate surface level in stages, testing its electrographic response at each reduction stage. The results showed that the imaging photoresist was still present and in a useful, electrographically active, condition essentially all the way down to the original substrate surface level. This graphically indicated that the photoresist filler appeared to have been embedded entirely through the cross section of the dielectric matrix and filled the pores therein (or at least the pore walls lined) substantially completely.

EXAMPLE III

The process of example II is repeated using Kodak KPR (or other suitable photoresist resin, e.g. Kodak KTFR) as a filler, applied in the manner of the epoxy in example II.

EXAMPLE IV

The process of example II is repeated using a methacrylate (or plexiglass, or other suitable polyester), as a filler, applied in the manner of the epoxy in example II.

Workers will recognize that other substrate materials may be similarly porously oxidized (e.g. magnesium, copper or manganese). Of course, as mentioned it will often be especially convenient to specify a conventional oxide substrate, such as an aluminum alloy plate that may be suitably anodized as in example V, or the like.

EXAMPLE V

The process of example I is repeated; however, using a commercial anodic coating of an aluminum plate for the dielectric image surface. In particular, a thick anodic hard coating is specified (order of a few mils added thickness); and preferably of the open-pore type, since this admits a greater portion of filler (though the closed pore coating may be used). Such an aluminum hard coating may be characterized as the type satisfying U.S. Government Specification "Mil Std 171," or the equivalent. Such anodizing typically employs a bath with a major portion of sulfuric acid (e.g. 10-25; sometimes with oxalic acid or other additives) and kept moderately cool (e.g. 20°-50° f.), with a fairly high current density (e.g. typically anodize at rate of 1 mil in 15-40 min. up to a maximum of 6-15 mils total thickness). The anodizing can be used on most aluminum alloys (those with more than a few percent copper or silicon are often disfavored though) such as most common wrought and ingot alloys. The anodizing mechanism will typically comprise 50 percent thickness buildup and 50 percent surface penetration (e.g. a specified 2-mil coating will add only 1 mil surface thickness) and will follow substrate porosity (rather than filling in pores). However, porosity should be ample for purposes of the invention with any commercial anodizing (e.g. typically exhibiting an order of one billion pores/cm. ² ). Masking may be specified for substrate surfaces not to be anodized and the substrate must be provided with adequate racking capabilities (i.e. with rack contact points, leaving a void where firm electrical and mechanical contact is made to each piece). The resultant corrosion resistance (especially when filler-sealed), mechanical hardness (e.g. resistance to abrasion or galling), and excellent electrical insulation properties e.g. breakdown voltage on order of a few thousand volts DC) of such anodizing will be found very apt for application as an electrographic dielectric matrix.

EXAMPLE VI

The process of example V is repeated; however, filling the oxide matrix with epoxy as in example II. Resultant electrographic properties are satisfactory here, as in example V.

To summarily characterize the dielectric oxide formed in the examples above, it will be understood that it is typically porous, is a particularly hard refractory material and is quite stable both mechanically (e.g. vs wear) and chemically (e.g. being relatively inert, noncorrosive and nonreactive with other substances, such as the imaging material). Further, sealing the oxide pores by injecting imaging material according to the invention appears to protect the oxide and the underlying metal substrate most effectively against further corrosion, etc. Such a sealing evidently forestalls any further oxide growth or other change in dielectric layer.

Plates according to the present invention are very durable under mechanical abrasion, as well as having other superior physical properties. In many cases they also have particularly desirable electrical properties, such as a tailoring of the composite resistivity of an electrographic imaging layer. That is, one may tailor the electrostatic characteristics of an electrographic plate, and especially its dielectric constant resistivity according to the "lumped" composite resistivities of the dielectric and of the filling (a function of porosity, in turn). In particular, the techniques of the present invention enable a convenient, relatively economical production of electrographic plates including composite dielectric-imaging filler surfaces. Of course, a composite according to the invention may include a dielectric matrix and/or a filling imaging material other than those particularly described. However, the use of aluminum anodizing is particularly convenient, being well-known and offering an unusually high degree of control of the dielectric (e.g. porosity and other properties). Of course, in some cases, different methods than those described may be used to fabricate the dielectric composite using the described or other related materials and with or without conventional additive materials. Other modifications will occur to those skilled in the art, both as to the structure described and to the methods for fabricating it, upon consideration of the foregoing disclosure. Such are generally intended to be encompassed within the scope of the present claimed invention.