US3441940A - Process for electro-junction thermography - Google Patents

Description

nited States Patent 3,441,940 PROCESS FOR ELECTRO-JUNCTION THERMOGRAPHY Roy George Salaman and Robert J. Taylor, Boulder, Colo., assignors, by mesne assignments, to Phonocopy, Inc., Wilmington, Del., a corporation of Delaware Filed Sept. 15, 1966, Ser. No. 579,772 Int. Cl. G01d 15/10 U.S. Cl. 346-1 11 Claims ABSTRACT OF THE DISCLOSURE A process for producing visual images from electrical signals employs a localized junction contact between two electrical conductors for generating heat by passing current across the junction and using the elevated temperature to activate a thermally responsive marking system directly opposed to the junction. As used for a particular facsimile printing system a conductive scanning probe is energized with signal current and directly contacts one surface of a thin metallic foil which has the opposite side coated with thermotransfer compound.

This invention relates to the process for production of visual images from electrical signals and more particularly to an improved process which utilizes the junction contact between two metals to generate heat in a controlled manner for the production of high resolution black on whi e images. This process is herein referred to as electrojunction thermography.

Various arrangements for the production of images in accordance with an electrical signal are known in the prior art including a wide variety of arrangements which utilize an electrically conductive stylus for producing a visible mark on a suitable medium in accordance with electric signals coupled to the conductive stylus. One of the most common arrangements of this type utilizes a recording medium which is made up of a conductive paper base where the electrical conductivity of the paper is achieved by distributing carbon black or fine metallic particles throughout the paper when it is manufactured and in which the surface of the specially conductive paper is coated with a dielectric layer usually of a contrasting color. When such a composite sheet is scanned by being in contact with an electrically conductive probe and sufficient voltage is applied between the conductive base paper and the probe to break down the dielectric that separates the probe from the conductive base, a mark is achieved by arcing through the dielectric and burning it away thus exposing the darker background color through the contrasting light dielectric coating. The patent to Newman 2,713,822 shows a system of this type with the transfer of a wax coating to make a lithographic master.

Printing processes of this prior art type have a number of disadvantages. One disadvantage is the requirement of a special type paper on which the permanent record is to appear which paper is generally objectionable as to appearance and feel relative to the ordinary bond papers which are used in business. Another objection is the odor which results from the operation of the printing process due to the vaporization of the dielectric layer when the electric arc burns away the dielectric to make the subjacent portion of the contrasting conductive layer visible.

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Perhaps the most serious objection to recording processes of this type, however, is the inability of such processes to produce the desired fidelity of reproduction due to the nature of the process. In particular the process utilizes a voltage high enough to rupture the dielectric layer that coats the specially prepared paper. This voltage of necessity is higher than the voltage which would be required if conductive contact were made in order to achieve the same current flow and the initiation of current flow is only obtained by a voltage breakdown process in the dielectric. This process is accompanied by a changing current flow and a somewhat uncontrolled disassociation of the dielectric layer in the immediate neighborhood of the initial are that ruptures the dielectric. Since the process of puncturing a dielectric with an electric arc is inherently unstable and progresses in a random fashion due to the carbonization of the material vaporized by the arc, the exact pattern of the removal of dielectric cannot be controlled and hence the resolution relative to the stylus which is conducting current to the region is not well defined. Due to the same process these papers also have some grey scale response since the magnitude of current flow generally can be related to the amount of dielectric which is vaporized and therefore a light are will vaporize a small amount of dielectric whereas a heavy discharge will vaporize a large quantity of dielectric with the resultant variation in the amount of the darker substrate showing through the dielectric. This ability to reproduced a limited grey scale is relied upon to some extent in facsimile systems but requires a high degree of control of all the variables in order to be realized. In both the reproduction of black on white and half tone copy the essential instability of the process which converts the electric circuit conditions from a dielectric stressed under relatively high voltage into a conductive arcing high current low voltage phenomenon at the precise point at which the image is to be generated produces an ultimate limit on the fidelity that can be realized with such systerns.

The present invention utilizes what is essentially a stable electro-thermal process for producing a visible thermographic image. In this process a circuit which is essentially completely conductive throughout the image generating system is utilized with the properties of a metallic contact junction relied upon to produce thermal response in the marking medium. Because the circuit is essentially ohmic the voltage and current relations are normally in steady state condition and faithfully follow the electric signal from which they are generated. In combination with this stable electrical system the process of the present invention utilizes a thermal system which preserves the electrical fidelity inherent in the electrical system and in fact utilizes a configuration which can enhance the resolution of the overall system.

The present invention relies for its operation upon the pressure contact between an electrically conductive stylus and an electrically conductive sheet which extends over the area of the record to be made. In an ordinary facsimile application of the process the electrically conductive sheet will be coextensive with the copy to be reproduced and the stylus will scan successively in closely spaced lines over the desired areas. In the preferred embodiment of this arrangement the electrically conductive sheet is a thin metallic foil of aluminum which has a thickness dimension substantially smaller than the diameter of the conductive contact which makes presssure contact therewith. Since the stylus and the aluminum foil are relatively good electrical conductors the primary resistance in the circuit is the contact resistance across the junction between the stylus and the aluminum foil against which the stylus is pressed with a predetermined pressure. This pressure is generally such that the aluminum oxide which is inherent on the surface of the aluminum foil is penetrated at least over a portion of the contact and hence the circuit can be considered to be entirely conductive including the region of the junction. The theory of the resistivity of such a metal junction contact is not entirely clear but it is generally recognized that such a junction presents a higher resistance point than the solid conductors which exist on either side of such a junction and hence for given current flow the maximum heat generation occurs at the junction as in an ordinary resistive load.

The heat which is generated at the junction flows in accordance with the ordinary laws of thermal conductivity and due to the dimensions of the probe or stylus and the foil the temperature rise tends to be concentrated within the area of the stylus in contact with the foil. The reason for this is apparently due to the extreme thinness of the foil which permits conductive heat flow through the thickness dimension to the opposite surface of the foil within the subtended area of the stylus before any appreciable heat flow radially outward from the periphery of the stylus can occur. The aluminum foil in the region outside of the area of the stylus acts as a relatively good heat sink and radiator and the axial dimension of the stylus itself represents a long path relative to the thickness of the foil so that the temperature front that propagates as a result of the given heat energy increment generated at the junction results in a rapid temperature rise on the opposite surface of the foil directly below the junction without a corresponding temperature rise in regions adjacent the stylus. In actual practice it has been found that the system is capable of reproducing visible marks in response to this temperature increment which are substantially smaller than the actual dimensions of the stylus contact with the foil thereby indicating that the edge effect of radiatoin by the foil effectively reduces the marking size of the stylus to an area somewhat smaller than the area of contact with the foil. While this theory has not been experimentally verified and the condition may in fact be due to a partial junction effect resulting from an uneven contact under the stylus, it has nevertheless been found that well defined marks approximately to mils in diameter can be obtained from a round stylus which is 20 mils in diameter.

The locally elevated temperature on the side of the foil opposite the area in contact with the stylus can be utilized to produce a visible mark in any desired manner. One arrangement for producing such marks is to press a thermally responsive marking paper into contact with the undersurface of the foil so that the locally elevated temperature can produce a thermal response mark on the paper as the elevated temperatures occur during the scan of the stylus. While this arrangement produces satisfactory results as far as fidelity is concerned it does require the use of a specially prepared thermal responsive paper which in some cases is undesirable from the standpoint of appearance and feel and hence has some of the disadvantages of the prior art spark discharge type electric marking systems.

The preferred embodiment of the present invention em ploys a coating of thermal transfer material on the surface of the aluminum foil opposite the surface which is contacted by the conductive stylus. The aluminum foil is coated with a substance which initially provides good adhesion to the aluminum but when melted and solidified in contact with paper forms a stronger bond to the paper. Thus any such paper can be employed for making the record which is to be reproduced merely by pressing the paper into contact with the coated surface of the aluminum foil during the scanning process. High quality papers and printed letterheads and the like can be used, if desired,

and the finished copy is a black on white replica of the original with high definition and good quality both as to the paper and the image produced thereon.

The principal object of the present invention accordingly is the provision of an improved process for thermographic reproduction employing an electro-junction as set forth herein.

Referring now to the drawings:

FIG. 1 is the View partially schematic representing the scanning process with respect to a metal foil in accordance with the preferred embodiment of the invention;

FIG. 2 is a greatly enlarged view of the assembly of FIG. 1 showing the stylus in contact with the foil;

FIG. 3 is a view showing the reproduced image resulting from the process as practiced in FIGS. 1 and 2;

FIG. 4 is a view like FIG. \2 of a modification employing thermo-responsive paper for the production for the image; and

FIG. 5 is a partial sectional view of a further modification;

Referring now to FIG. 1 the representation of the invention as it is employed in a scanned facsimile mode will be described. The printing sheets comprise a thin foil 11 of metal such as aluminum on the undersurface of which is coated a layer of thermo-transfer substance 12 and in the process this coated aluminum sheet is assembled adjacent a sheet of paper 13 on which the facsimile copy is to be reproduced. The assembled sheets 11, 13 are suitably supported so that a stylus 14 can be pressed thereagainst with a predetermined force indicated schematically at 15. The relative motion between the stylus 14 and the aluminum sheet 11 is arranged to follow a regular scanned pattern in any suitable manner and, as indicated diagrammatically, is under the control of a scanning mechanism 16. The force 15 on the stylus 14 is determined by the exact nature of the mechanics involved depending on such factors as the hardness of the foil 11, the nature of the point contact that the stylus 14 makes with the surface of the foil 11 and the pattern scanned. One suitable mechanical arrangement for scanning is shown in the application of Salaman and Taylor entitled Facsimile Scanner-Printer, Serial No. 579,584, filed of even date with this application. In that application a 2% thoriated tungsten rod 20 mils in diameter is employed as the stylus and a spring force of 4 to 15 grams is used to urge the stylus 14 against the surface of the foil 11 with the assembly 11, 13 supported on a platen supporting the undersurface of the paper 13.

The stylus 14 is fed current impulses from a current signal generator 21 which responds in accordance with binary signals from a suitable source 22. As indicated the current signal generator 21 supplies a square wave type current impulse that varies from a value of zero to a predetermined fixed value depending respectively upon whether the stylus is not to print or to print. The current supplied from the signal generator 21 is returned from the foil 11 by making electrical contact therewith in any suitable manner as at 23. In actual practice the foil and paper 13 will be moving past the scanning station and the contact indicated at 23 can be made by a metallic platen or roller which contacts the surface of foil 11 as it moves past the scanning station.

Referring now to FIG. 2 the mechanism of electro-junction thermography will be described as presently understood.

As indicated in FIG. 2 the diameter of the area of contact of the stylus 14 with the surface of the foil 11 is substantially larger than the thickness dimension of the foil 11. Generally the diameter of the probe 14 in the vicinity of this contact junction will approximate the resolution desired in the facsimile copy. In other words, with the probe stationary and a pulse of current passed from the probe 14 to the foil 11 the dot which is produced by the process should be as small as the smallest dot that is to be resolved by the system when it is reproducing copy. As previously mentioned the process produces somewhat smaller dots than the diameter of the contact area so that in general the probe 14 which is 20 mils in diameter will resolve dots mils or less in diameter in the transfer of substance 12 to the paper sheet 13. This is indicated in FIG. 3 wherein the transferred substance 12 is indicated at 17 and has dimensions smaller than the diameter of contact of probe 14 shown in FIG. 2.

As further indicated in FIG. 2 the pressure on the probe 14 slightly embosses the surface of foil 11 so that in an ordinary line scanning process a succession of line indentations are made in the surface of the foil 11. This pressure is adequate to assure good electrical contact between the probe 14 and the foil 11 by mechanically penetrating at least over a portion of the area of contact any aluminum oxide that may be on the surface of the foil. Thus the occurrence of the current pulse will start current flow across an ohmic contact which in the abovedescribed example typically requires two volts across the junction with a current flow of 7 amperes. The heat generated by this current is confined by the stylus 14 so that it flows by thermal conduction through the thickness of the foil 11 to melt the corresponding area of thermoplastic thermotransfer compound 12 directly beneath the probe 14. The existance of cool metal foil in all directions horizontally on the foil 11 from the point of contact represents a concentric heat sink for the point location of heat generation and this effect is aided by the radiation and convection cooling which the free surfaces area 18 adjacent the probe 14 provides. Thus the temperature rise in the region 18 is limited by the ability of the system to dissipate heat which flows by conduction latterly through the foil 11 whereas the dissipation of the system for the heat that flows transversely through the foil 11 to the region of the thermotransfer substance 12 is limited due to the insulating effect to the paper layer 13.

The heat transferred through the thickness of the foil 11 melts the thermotransfer substance 12 and in its melted state the substance has an affinity for the relatively porous surface of the paper 13. In this connection coated papers can be used to advantage to assure perfectly fiat smooth surface contact with the substance 12 providing the coating on the surface of the paper 13 does not reject the thermotransfer compound 12 in its molten state. Two papers which have been used with statisfactory results include the Oxford Paper Co.s Scanamaster and Dependoweb papers.

As the probe 14 passes along the scan line the molten thermotransfer substance 12 resolidifies and in so doing produces a preferential bond to the surface of the paper 12 relative to the foil 11. When the facsimile copy is completed and the foil 11 is stripped from the paper 12 the thermotransfer substance 12 in the areas where current has flowed results in the retention of the image elements 17 on the paper 13 as indicated in FIG. 3.

Various substances can be used for the thermotransfer compound 12, the preferred compounds being disclosed in the application of Salaman, Taylor and Bohner, entitled Electro-Junction Thermographic Transfer Sheet, Serial No. 579,605, filed Sept. 15, 1966. As therein set forth one of the preferred compounds comprises a coating made up of 10 parts ethyl cellulose, 3 parts sudan black and 150 parts solvent, such as emil acetate and ethyl alcohol. This coating is uniformly applied to one surface of the aluminum foil at a rate of 1 pound per ream and the aluminum foil preferably is 0.00035" thick (Reynolds 1235 H 18 for example).

Many foil and stylus combinations may be employed successfully but the materials and dimension should be selected in accordance with the principles herein set forth. The thinness of the foil is essential since it has been found that foil much thicker than 1 mil produces generally poor results. The foil also should preferably be lower in thermal conductivity than copper since copper foil conducts the heat rapidly enough to adversely affect the resolution obtainable. Thus stainless steel and aluminum have been found to give good results with thermal conductivity approximately half that of copper. The material for the stylus may likewise be selected from various materials such as carbon, titanium, tungsten carbide, vitreous carbon copper tungsten and 2% thoriated tungsten. The latter material is typically obtainable as welding electrode in very small diameters and when such a stylus is used it presents the additional advantage that the rod is cylindrical and does not change the area of contact as it wears away. Other conductive materials may be employed for the stylus and, as previously indicated, when a metal scanning stylus is used it is pressed against the foil and paper assembly with sufficient pressure to slightly emboss the surface of the aluminum along the scan line.

Referring now to FIG. 4 a modification of the invention is shown in which the conductive probe 14 contacts the surface of the foil 11 to form the heat generating junction when current flows. Instead of using a thermotransfer substance, however, the foil 11 may alternately be supported in intimate contact with a heat responsive sheet 19. Many forms of heat responsive papers are available in the prior art and typically the type used with ordinary Thermofax copying machines represents one class of such papers. With this arrangement the operation of the process produces enough heat on the surface of foil 11 directly beneath the probe 14 whenever a current pulse passes through the probe 14 to the foil 11 to produce the desired mark on the thermoresponsive paper 19. Depending upon the characteristic of the ther-moresponsive paper 19 that is employed, the desired resolution can be obtained. The Thermofax type paper referred to produces resolutions as good as that obtained with a thermotransfer compound but has the disadvantage that the finished copy is not bond quality paper.

FIG. 5 indicates a modification in which the assembly of sheets which is placed underneath the stylus 14 comprises the foil 11, the paper 13 on which the final copy is to appear and an intermediate thin sheet comprising a layer of tissue 31 or Mylar or some other thin film that will not provide significant heat insulation. The tissue or film 31 is coated on the undersurface with a thermotransfer compound 12 which may be a wax transfer compound such as used in thermotransfer copying machines of the prior art. This arrangement has the advantage of permitting some selection of the thermotransfer compound 12 to accommodate different requirements such as different colors and the like without requiring that the aluminum foil 11 be stocked in various colors or other characteristics of the coating thereon. The arrangement also permits the practice of the invention with a permanent sheet of foil 11. When scanning is employed the permanent foil 11 is preferably stainless steel so that it can withstand repeated scanning by the stylus 14 with the only consumable material required being the coated tissue 31 and the final copy sheet 13. In the practice of the invention by scanning a permanent foil surface 11 care must be taken to guide the stylus accurately so that in scanning it is not deflected by the embossed previous scan line that is inherent in the surface after the foil has been once used. If the probe contact is in the form of a matrix or other junction contact with no sliding motion between the foil and probe, a permanent foil of any suitable material can be used.

From the present description it will be appreciated that the invention can be practiced in a variety of modes, the application to a particular repetitive scan pattern as used in facsimile transmission being given by way of example only. In such applications the speed of response is adequate to permit scanning rates of 34 linear inches per second which will permit 5 mil dots to be transferred corresponding to the successful resolution of 4000 cycles per second signals. This is far superior to the ordinary requirements for narrow band facsimile transmission and hence the system of the present invention is capable of utilizing and responding to any improvement in the signalling system which can be made in a given system. Since the melting point of the thermotransfer substance 12 is a fixed temperature and the various parameters can be fixed so that adequate temperature rise occurs for current flow and no melting of the transfer substance occurs when current does not flow and because no melting occurs in areas adjacent to the stylus even when the substance directly beneath the stylus is melted, the system presents improved characteristics with respect to printing only black on white. This feature is particularly useful in utilizing the unique improvements in black on white facsimile transmission such as described in the application of Salaman and Picchiottino, Serial No. 579,591, filed Sept. 15, 1966, entitled Binary Facsimile System.

We claim:

1. The method of graphic reproduction comprising the steps of (a) making direct electrically conductive junction contact between an electrode and a thin metal foil, the dimensions of said electrode contact with said foil corresponding approximately to the resolution desired in said reproduction and the thickness of said foil being less than the dimension of the elemental areas resolved in said reproduction;

(b) urging said electrode against said thin metal foil with force sufiicient to penetrate any intervening resistive material and maintain said direct electrically conductive junction contact;

(c) selectively passing a current across areas of contact between said electrode and said foil corresponding to the desired reproduction, said current of magnitude sufficient to cause an appreciable temperature rise at the selected areas of contact; 7

(d) controlling the time duration of said current flow across the selected areas of contact to permit heat flow through said foil to raise the temperature a predetermined amount on the side of said foil opposite said selected areas of contact without raising the temperature of said foil appreciably in adjacent areas; and

(e) utilizing the predetermined temperature rise on the opposite side of said foil to produce a visible reproduction.

2. In the method of facsimile reproduction wherein electrical signals representative of the scan of a record to be reproduced are generated and produced at the printing station with corresponding control of a scanning printer to operate in synchronism with the signals to reproduce the record, the improvement comprising:

(a) applying the electrical signals to a conductive scanning probe which is in electrically conductive junction contact with a thin metallic sheet, the area of contact between said probe and said sheet being of the order of the desired resolution for the copy produced by the system;

(b) urging said scanning probe against said thin metallic sheet with force sufiicient to penetrate any intervening resistive material and maintain said electrically conductive junction contact;

() scanning said metal sheet with said probe in electrical contact therewith;

(d) generating localized heat in said area of contact directly beneath said probe due to current flow between said probe and said sheet in accordance with said electrical signals;

(e) conducting the heat generated under said probe through said metallic sheet with the thickness of said sheet small enough to permit the temperature on the side of said sheet opposite said area of contact to rise appreciably before a corresponding temperature rise occurs in adjacent areas of said sheet; and

(f) utilizing the localized elevated temperature of said side of said sheet opposite said area of contact throughout the scan of said probe to produce a visible mark pattern which is a reproduction of said record.

3. The method of claim 2 in which said electrical signals are binary waves having one state that produces a predetermined magnitude current between said probe and said metallic sheet suflicient to produce said visible mark pattern With no visible mark produced by the other state of said binary signal.

4. The method of claim 2 in which said metal sheet has a thermal conductivity less than 0.85 cal./sec./sq. cm./ C./cm.

5. The method of claim 4 in which said metallic sheet has a thickness less than 0.001 inch.

6. The method of claim 5 in which said metallic sheet is coated on one side with a thermotransfer substance which melts at a temperature slightly below said localized elevated temperature.

7. The method of claim 6 in Which a sheet of paper is pressed into contact with the coated surface of said metallic sheet under said area of contact of said probe, said substance when melted in contact with said paper and subsequently solidified forming a stronger bond with said paper than with said metallic sheet to produce said visible mark pattern on said paper.

8. The method of claim 7 in which said electrical signals are binary waves having One state that produces a current between said probe and said metallic sheet sufficient to make said localized elevated temperature higher than the melting point of said thermo-transfer substance, the other state of said binary signal producing no melting of said substance and no transfer of said substance from said metallic sheet to said paper.

9. The method of claim 2 in which said visible mark pattern is produced on a thermoresponsive sheet pressed into contact with said metallic sheet under said area of contact of said probe, said thermoresponsive sheet producing visible marks in response to said localized elevated temperature.

10. The method of claim 2 in wihch said visible mark pattern is produced on a sheet of paper pressed against said metallic sheet under area of contact of said probe with a layer of thermoresponsive material between said paper and metallic sheets, said thermoresponsive material having a melting point slightly below said localized elevated temperature.

11. The method of claim 10 in which said material is carried as a coating on a thin heat transfer sheet that is interposed between said metallic and paper sheets with said coating in contact with said paper, the melted and resolidified coating having a stronger bond to said paper than to said heat transfer sheet.

References Cited UNITED STATES PATENTS 12/1953 Dalton 346-X 7/1955 Newman 34676X US. Cl. X.R 17894; 346*76, 135

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