US3005731A

US3005731A - Method of applying an electroconductive film to a vitreous surface

Info

Publication number: US3005731A
Authority: US
Inventors: Jr Paul D Payne
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1958-02-11
Filing date: 1958-02-11
Publication date: 1961-10-24
Anticipated expiration: 1978-10-24

Description

Ocvt. 24, 1961 P. METHOD OF' APPLYING AN ELE FILM TO A VITREOUS SURFACE D. PAYNE,

J R CTROCONDUCTIVE Filed Feb. l1, 1958 Fail.

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INVENTOR. Pam. PAyA/f, Je.

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United States Patent O 3,005,731 METHOD F APPLYlNG AN ELECTRGCONDUC- TIVE FILM T0 A VHTREOUS SURFACE Paul D. Payne, Jr., Chalfont, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania n Filed Feb. 11, 1958, Ser. No. '714,467 Claims. (Cl. 117-211) This invention relates to methods for producing an electrically conductive layer on a substrate, and in particular to improved methods for producing on vitreous and ceramic bodies thin, electrically conductive layers which are transparent to radiation of a selected wavelength.

There are many cases in which it is desired to produce a conductive and/ or transparent layer on a given substrate. A number of these applications may be found in the electronic and allied industries in which it is desired that glass or similar vitreous substance be so coated that the surface thereof is rendered electrically conductive yet transmissive of light. For example, there are many types of electrolnminescent devices which require the formation of a sandwich consisting of an electro-luminescent substance placed between two electrodes, at least one of which must be transparent.

Another example of the use of conductive coatings is in the construction of photo-index type cathode ray tubes used in the reproduction of images in color. A tube of this type is described, for example, in U.S. Patent No. 2,749,449 issued to W. E. Bradley andD. E. Sunstein on June 5, 1956. ln one form of this type of cathode ray tube a set of indexing elements is disposed in a predetermined spatial relation to certain elements of a iiuorescent screen on which a multi-colored image is reproduced in response to the scanning of an electron beam thereupon. When the electron beam strikes the indexing elements light of a predetermined wavelength is emitted which passes through a window formed in the envelope of the tube behind the screen to a photosensitive device which thereupon produces electrical indexing signals indicative of the beam position, these signals thereupon being used to effect coordination between the modulation of the beam and its position.

in the past, this window has consisted simply of a portion of the tubes glass funnel, or flared portion, which has not been coated with the customary opaque, conductive second anode coating. It has been found, however,

'that leaving a portion of the funnel uncoated permits that portion of the glass to acquire spurous electrical charges thereby distorting the electric field in the funnel. This distortion, in turn, produces distortion in the raster, and inthe beam spot. It has therefore been proposed that the window be not only transparent to the indexing radiation but also conductive (as are other adjacent areas) so as not to disturb the desired electric field within the funnel portion of the tube.

Still another example of the use to which conductive coatings may be put in the electronics industry is the depositionvof an annular layer of conductive material which acts as an attenuator on the internal surface of traveling wave tubes.

Hitherto, Vapplication of transparent conductive layers or iilms to vitreous or ceramic surfaces has required the application (usually by spraying 'or vaporous diffusion) of a halide such as stannous chloride to the vitreous object which has previously been heated to a point in the neighborhood of the temperature at which it begins to soften, i.e., Aabout 500 C. in the case of glass. The stannous 'chloride becomes converted into a transparent conductive ice layer of stannic oxide which is closely bounded to the substate.

In the manufacture of photo-index Windows for cathode ray tubes for color television of the type described the use of this known method has been found extremely inconvenient and time consuming. This inconvenience results from the fact that if the faceplate of the tube has already been joined to the funnel portion of the tube, an opening of only about 4 diameter is available in which to insert the spray gun into the funnel. Even if it were otherwise feasible to insert the spray gun this method requires critical handling of the tube since the latter necessarily has to be heated to a high temperature before spraying begins. Furthermore, if the fluorescent screen has previously been deposited on the inner surface of the faceplate, heating of the Whole tube to a high temperature such as the softening temperature of glass may damage the structure or the alignment of the screen itself. Such heating also requires that the glass be carefully Vanneale to avoid the production of undue strains therein.V

0n the other hand, if only the portion of the glass Where the window was to be located was heated to the required high temperature great care would be required in the annealing process to equalize the strains produced in that and other portions of the glass. In either event, it is expensive, in production, to heat the glass up to the softening temperature of the glass.

Accordingly, it is an object of my invention to provide a new and improved method of applying an electrically conductive coating to a given substrate. Y

lt is another object of my invention to provide a new and improved method of applying to a given object a conductive coating which is substantially transparent to a selected form of radiation.

Still another object of my invention is to provide an improved method for producing a conductive and transparent coating on vitreous and ceramic bodies.

Another aim of the invention is to provide a simplified method for applying a conductive and transparent coating to vitreous and ceramic bodies.

A further object of my invention is to provide a cheaper method for producing conductive and transparent coatings on vitreous and ceramic bodies.

Another object of my invention is to provide a new method for producing in certain types of cathode ray tubes improved photo-indexing Windows which do not disturb the electric field and do transmit the selected indexing radiation. According to my invention a solution of stannous chloride may be applied to the vitreous substrate While the latter is at room temperature. Thereafter, the temperature of the substrate itself is raised to a temperature of at least about 250 C.

In a preferred form of the invention the stannous chlorider is dissolved in an ester such as n-propyl acetate and applied to the vitreous substrate at room temperature. The ester evaporates almost immediately leaving stannous chloride crystals on the substrate. The substrate is then heated at about 415 C. for about l0 or 15 minutes at the least.

In another form ofthe invention the stannous chloride is dissolved in an alcohol such as glycerol and applied to thev substrate which is then heated at about 300 C. for about one half hour or, alternatively at about 415 C. for 10 or l5 minutes. In this form of the invention, care is talcento keep the stannous chloride solution from drying before the maximum temperature is reached.

FIGURE 1 is block diagram depicting the general method of forming a transparent conductive coating on a vitreous substance in accordance with the invention;

FIGURE 2 represents one form of the method according to the present invention; and

yabout a half hour.

FIGURE 3 represents another form of the method ac cording to my invention.

Referring to FIG. l, itfis seen that, in essence, my invention comprises the application, at room temperature, 'of a stannous chloride-organic compound solution to the vitreous material to be coated, and thereafter heating the 'vitreousV material to'about 415 C. for at least about l minutes, although, as will be seen below, a lower ltemperature'may bcemployed for a longer time to give a 'result' equivalenty thereto. The'invention will now be Vexplained in several of its 'alternative forms.

The preferred form of my invention, as shown in FIG. 2, involves the deposition of a solution of SnClz 'dissolved in an ester such as n-propyl acetate. This solution may be applied by any conventional method, i.e.,

brushing, swabbing, spraying, flowing on, etc. The solu` tion is prepared by grinding, t0 a fine powder, an anhydrous form of SnCIZ, which may be obtained, for example, under the trade name Stannochlor NCNSC-56" -as marketed'by the Metal and Thermit Corporation of Rehwayg/New Jersey. First, a concentrated solution is Vmade consisting of 50 grams oi the anhydrous SnClg and 150 ml. n-propyl acetate. This solution is then poured into a small screw neck bottle. rIhe bottle cap :isV preferably lined 'with a piece of polyethylene sheet 'material and screwed tightly onto the neck. This concentrated solution is then rolled for about 8 hours to make sure that all the stannous chloride is dissolved. The solution is then ltered through Whatman paper #'42 until clear to produce a stock solution fromrwhich a diluted solution for the process may be obtained. I have found that as little as about 1 gram or as much as about 12 grams of anhydrous SnCl to 100 ml. of n-propyl acetate will produce acceptable results for the fabrication of windows for cathode ray tubes of the type previously described.' Windows with conductivities per square of 5000 ohms toseveral megohms may be achieved depending on the thickness of the layer and the concentration of the solution used. Where resistivities outside ,this range maybe required, quantities of SnClg outside the range 1f-l2 grams can correspondingly be employed.

After the dilute solution has been prepared it is applied, by brushing or swabbing, on the desired appropriate substrate. In the Acase of color television cathode ,ray tubes of the type previously described it is most convenient to apply the solution to the inside surface of the glass funnel of the tube by brushing it onto the desired area. The organic solvent dries almost immediately upon Aapplication leaving the SnClz in contact Ywith* the glass lSubstrate. The temperature of the substrate is then raised to about 415 C. for lO'or 15 minutes. desired, the glass need be raised to only about 250 C.-

Or, if

300 C., and that temperature maintained for at least Theiilm thus produced has approximately the same characteristics as that produced by the higher temperature process. It should be noted that in neither case is the glass heated to the temperature at which it begins to soften. Y t Y After the tube has beenallowed to cool gradually there will be found deposited upon the selected area an iridescent `conductive coating, consisting primarily of stannic oxide,

which transmits visible light and ultraviolet light with 'high e'iciency, the percentage of ultraviolet radiation passing through it being as high as 80 percent. As a matter of fact, the deposition of the transparent and electrically -conductive film in accordance with this process produces ka. window which transmits more ultraviolet light than a window consisting merely of uncoated glass because the iilrn reduces specular reflection of such light.

If anhydrous stannous chloride of the qualityY desig- 'nated by the trade name Stannochlor is not available, I have achieved equally good results by processing stannous-chloride dihydrate, SnCl2.2H2O to obtain it. This `chemical may be procured commercially, under the desigy`nation Reagent Grade, Catalogue T-142, from the Fisher Scientific Company. E This stannous chloride* di- 110 C. for twenty-four hours.

, lumps, and stored in a tightly-closed bottle. This anhydrous stannous chloride is then used to make a solution in the manner previously described. c t

if coatings of higher resistivity and less eiiicient ultraviolet radiation transmission are Vdesired they may be made by applying stannous chloride solutions of n-butyl acetate which produces iilms that are somewhat brownish in color. The preparation of the solution and its method of application is otherwise identical to the n-propyl acetate process previously described.

In another form of the'invention, as shown in FIG. 3, the conductive coating is made by applying a solution of stannous chloride in an alcohol such as glycerol, ethylene glycol or triethylene glycol. This solution is applied to the subtrate at room temperature in the usual manner and the subtrate is heated up to about 415 C.

In a preferred version of this form of the invention the solvent used is glycerol which has a high boiling point, care being taken to insure that the stannous chloride re mains in solution until the .maximum temperature to be applied is attained. VIthas been found that if the stannous chloride does not remain in solution until the temperature of the substrate has been raised sufficiently high so that the glass forms a bond with the stannic oxide, little opaque islands of a relatively spongy material cloud the film and lessen its conductivity. These Yislands are formed because glycerol is normally very hygroscopic and the SnClz hydrolyzes somewhat in the water contained therein forming SnOHCl and Sn(OH),. Best results are obtained if the glycerol is first dehydrated to the maximum extent possible and maintained in that State.

Other alcohols having somewhat lower boiling points than glycerol but less ainity for water may alternatively be used in this form of the invention. However, since the stannous chloride is not likely to remain in solution in such lower boiling point solvents until the temperature of the glass attains its maximum value there is a slightly greater possibility that the above-mentioned nonconductive islands of spongy material may be formed with a consequent decrease in the conductivity and light transmissivity of the iilxn formed.

While the invention has been described in terms of the application of a conductive lm to glass as a subtrate,

it should be appreciated that it may also be used with otheryitreous substances such as porcelain. It is even possible toA use the invention to Vdeposit conductive films on surfaces of `ceramic bodies.

I claim:

1. The method of forming an adherent electrically conductive film on a vitreous body which comprises applying to said body at approximately room temperature a coating solution of substantially anhydrous stannous chloride and a volatile solvent thereof selected from the group consisting of glycerol, ethylene glycol, triethylene glycol 'n-propyl acetate` and n-butyl acetate and thereafter heating said body to a temperature of at leest 250 C. Y

2. The method according to claim 1 wherein said ternis maintained at the temperature to which it is heated for at least about l0 minutes.

4. The method according to claim l wherein said object is maintained at the temperature to which it is heated for at least about one half hour.

-S`. In a method of forming an adherent electrically conductive lm on a surface of a vitreous subtrate, the steps of applying to said surface a solution consisting essentially of substantially anhydrous stannous chloride and a solvent selected from the group consisting of n-propyl acetate, n-butyl acetate, glycerol, ethylene glycol and triethylene glycol while said surface is at a temperature below that at which an adherent electrically conductive lm is formed vthereon by said solution, and thereafter heating said surface to a temperature at least as high as said temperature at which said adherent electrically conductive ilm is formed thereon.

6. A method -according Ito claim 5 in which said solvent is n-propyl acetate.

7. In a method of forming an adherent, substantially transparent, electrically conductive ilrn on `a surface of a glass subtrate, the steps of applying a solution consisting essentially of substantially anhydrous stannous chloride and n-propyl acetate to said surface While it is at a ltemperature below that at which an adherent electrically conductive film is formed thereon by said solution and thereafter heating said surface to a higher temperature between about 250 C. and the softening temperature of said glass subtrate.

8. A method according to claim 7, Awherein said stannous chloride has a concentration in said solution of between about 1 and about 12 grams per 100 milliliters of said n-propyl acetate.

9. A method according yto claim 8, wherein said heating step includes the additional step of maintaining said surface at said higher temperature for at least about a half hour.

10. A method according to claim 8, wherein said higher temperature is at least about 415 C. and said heating step includes the additional step of maintaining said surface at said higher temperature for at least about 10 minutes.

References Cited in the file of this patent UNITED STATES PATENTS 2,559,969 Kennedy July 10, 1951 2,613,302 Gurewitsch Oct. 7, 1952 2,614,944 Lytle Oct. 21, 1952 2,617,745 Raymond etal. Nov. 11, 1952 2,772,654 Herkart Dec. 4, 1956 2,839,424 Labino lune 17, 1958

Claims

1. THE METHOD OF FORMING AN ADHERENT ELECTRICALLY CONDUCTIVE FILM ON A VITREOUS BODY WHICH COMPRISES APPLYING TO SAID BODY AT APPROXIMATELY ROOM TEMPERATURE A COATING SOLUTION OF SUBSTANTIALLY ANHYDROUS STANNOUS CHLORIDE AND A VOLATILE SOLVENT THEREOF SELECTED FROM THE GROUP CONSISTING OF GLYCEROL, ETHYLENE GLYCOL, TRIETHYLENE GLYCOL N-PROPYL ACETATE AND N-BUTYL ACETATE AND THEREAFTER HEATING SAID BODY TO A TEMPERATURE OF AT LEAST 250*C.