US2836751A - Cathode ray tube manufacture - Google Patents

Description

May 27, 1958 J. c. TURNBULL ETAL CATHODE RAY TUBE'MANUFACTURE Filed Aug. 2, 1954 INVENTUR Jv/ew C TUF/waa ,s How/f0.2?, 5MM/V56 United States Patent 2,836,751. CATHODE RAY TUBE MANUFACTURE John C. Turnbull and Richard D. Faulkner, Lancaster,

Pa., assignors to Radio Corporation of America, a corporation of Delaware Application August 2, 1954, Serial N o. 447,250

l Claims. (Cl. 313-64) This invention is directed to electron discharge tubes, and more particularly to cathode ray picture tubes used for viewing of television pictures and to the method of manufacturing the same.

The usual television picture tube is one consisting of an evacuated tubular envelope having a large conical or frusto-pyramidal tubular portion, which is closed at its large end by a face plate of glass. The smaller end of the cone normally has fixed thereto a tubular glass portion, which is used as a housing or support for an electron gun. .The inner surface of the glass face plate is covered with a phosphor layer known as the fluorescent screen which produces a luminescence when struck by electrons from the electron gun. The electron emission of a cathode in the gun is formed into an electron beam which is brought to a sharp focus on the surface of the fluorescent screen. Normally, magnetic fields are used to scan the electron beam over the surface of the fluorescent screen to form a rectangular shape raster. Incoming video signals are applied between the cathode and control grid of the electron gun to modulate the electron beam so that the luminescence produced on the fluorescent screen varies from point to point and produces a light distribution or picture on the fluorescent screen.

The trend in television tube developemnt is one directed toward larger picture tubes requiring large evacuated containers for housing the large fluorescent screen. One form of large tube development has been that of utilizing metal containers or shells, to which the face plate and the tubular glass gun housing are sealed. The use of metal shells has the advantage of providing greater over-all strength to withstand the high stresses due to atmospheric pressure. Also the use of a metal shell enables the tube to have a greater safety factor in case of implosion. If the metal of the shell can be made suiiiciently thin, an added advantage is the fact that metal tubes have inherently less weight than comparable glass tubes. In making large metal shells, it is desirable to use a minimum amount of metal and one of low cost. The development of large metal shells for television picture Atubes has progressed from the use of high chromium Vcontent-iron alloys to the use of low cost, low carbon steels, such as SAE 1010 steel, for example. Such low `carbon steels are known as iron, mild steel, and cold rolled steel. I Many problems arise in the use of such low carbon steels for the metal shell portion of a television picture tube. Normal processing of a metal tube includes heating the metal shell in air at high temperatures during ythe formation of the glass-to-metal seals and during 'annealing of the metal and glass parts. Such processing of low carbon steel tubes results in undesired oxidation Ior corrosion of the metal surfaces, which produces scaling. The scaling is objectionable, for one thing, because 'it gives the shell a bad appearance. A more serious objection is that in the process of forming the uorescent 2,836,751 Patented May 27, 17958 screen, loose scale particles on the inside of the shell fall onto the fluorescent screen and produce holes and tears in the fluorescent layer. Also, during manufacturing, problems of rusting or corrosion of the low carbon steel shell are encountered after bulb washing and phosphor settling which include the use of water.

According to the prior art methods, the problems of metal shell corrosion and rusting with consequent scaling have been overcome by coating the metal shell with vitreous enamel. However, this expedient is not entirely satisfactory for several reasons. The enamelcoating is an insulator and therefore will not readily conduct away secondary electrons which leave the liuorescent screen during tube operation and impinge on the shell. lt is not easy to mix conductive material with the enamel constituents to render the enamel coating conductive, so the usual practice is to apply a layer of conductive material such as graphite over the enamel coating, which adds to the manufacturing cost.

Another problem that arises with the employment of a vitreous enamel coating is the difliculty -of achieving a mat black surface on the enamel coating to reduce internal light reflections during tube operation and thus not interfere with picture contrast.

Furthermore, the enameling operation itself is costly, and the elimination of such an operation would reduce considerably the cost of manufacturing tubes if some other and simpler method could be devised to afford protection to the metal shell during tube processing and at the same time permit normal desired tube operation.

It is therefore an object of -this invention to provide a new and improved method of manufacturing a cathode ray tube;

It is another object o'f this invention to provide a novel metal envelope structure for an electron discharge device;

It is a further object of this invention to provide a new and improved method of processing an envelope portion formed from a readily oxidizable metal;

It is a further object of this invention to provide a new and improved method of manufacturing a metal shell having a surface characterized by its resistance to corrosion, rusting and scaling, is mat black, and electrically conductive;

It is a further object to provide for an electron discharge device a new and improved metal envelope portion having a protective coating which is resistant to corrosion, rusting, and scaling, is mat black, and electrically conductive;

It is a further object to provide a new and improved cathode ray tube including a metal envelope portion having a surface which is resistant to corrosion, rusting, and scaling, and which is mat black and electrically conductive.

The above and related objects are achieved in accordance with this invention by thoroughly cleaning the readily oxidizable metal shell, and Sandblasting the interior surface of the shell preparatory to the formation of a protective coating. The Sandblasting provides a clean rough surface of bare metal free of surface contaminants, on which a tightly adherent mat black oxide coating will form during later processing steps which includes heating the metal shell during sealing and annealing in air or au oxidizing atmosphere. The oxide coating gives protection to the metal shell during tube processing, will not interfere with tube processing, anu enhances finished tube performance.

The single ligure shows a partially cut away View in perspective of a television picture tube made in accordance with the invention.

i The television picture tube shown consists of a frustopyramidal portion 10 closed at its large end by a glass sheet orYV face plate 12 and at-its smaller end by a glass" tubular portion or cone 14, which is ared at one end, as

' is shown at 16, and is sealed to the metal shell 10. The

inner surface of the glass'face` plate 12 is coated with a fluorescent layer 18 of a-phosphor material. The phos# phor .materiali isY onel which'upon" bombardment by electrous 'luminesceswith' a substantially vWhite light. Y

Within the neck portion 14- ofthe envelope, there isl mounted an electron gun 2)A consisting of a source of'el'ectrons or cathode electrodeV 22, mountedbchind a control grid 24. Spaced from the grid 24 are acceleratingelec# trodes. 26v and 28 for acceleratingy and forming the electronsernittedY from cathode `22 into an electron beam directed at the phosphor screen V1S. The electron beam is focussed toV a fine spot on the'tluorescent screen 18 by a magnetic field` produced by a focussing magnet 34B arranged aroundA the neck of the tube. Accelerating'electrode'2S is connected by spring spacers 151V tothe usual conductive graphite coating on the adjacent inner surface of the tube neck l, with the conductive coating extended in the direction of screen 18 to makeV contact with the metal shell l0. Potential is applied to accelerating electrode 28 during tube operation by connecting shell 10 to a source of potential, not'shown, by a lead 33. The other electrodes of the tubeY are also supplied respectively with operating potentials by connecting the electrodes in Va conventional way by leads extending outside of the tube through Vthe base 35 and base pins 37. t

lThe details of the gun structure and its operation are well known and do not constitute a part of this convention. Theelectron beam, produced by gun 20eduring-`tujbe operation, is normally scanned in a substantially rectangular raster overV the inner surface of the Fluorescent screen v18 by vn'tagnetic iieldsproduced by coils Within Vagyoke member 32 aroundthe' neck i4 of the tube envelope. These scanning coils are conventional and well known.Y

incoming television signals are noramlly applied between control grid 24 and cathode 22 of the electron gun to provide a modulated electronrbeam. As the modulated beam is scanned over the surface of screenV i8, it will produce a'pattern of light or picture corresponding to the incoming electrical signals; SinceY the uorescent screen is normally insulating, the electrons striking the screen cause secondary electron emission from the screen, and the secondary electrons impinge on'the side walls of the tube envelope. Y i Y In accordance with this' invention the metal'shell 10 Y it is believed that the presence of any surface contaminants on the metal. shell will result inV scaling. ofthe. suhse` quent oxide coating. blasting also aids in promoting adherence of thev oxide coating, and since the oxide coating to be formed conforms to the surface irregularities, a non-reilective surface will be formed. Sandblasting of the seal areas also provides clean surfaces for the application of vitreous enamel K frit. The vitreousk enamel frit is preferably applied to the small end ofthe shell l0, shown generally at 42. The

Y ness ot 4-7 mils is satisfactory Vfor sealing to glass.

The enamel fritisthenV fired down or glazed, preferably by radioy frequency induction heatingrofV the `smallrend of the metal shell locally. Local heating ofthe small end of the. shell Il!)` is carried out for 3 to 5 minutes at a temperaturer of about 850 toA 95.0YC., during which process,V

some slight amount of the desired Aoxidizing occursY in the Yvicinity. of the'red. enamel.

Enamel frit is then applied, by sprayingfor example,

to Ythe. inside sealing vareas of thelarge endof the shell 10, shown generally at 44, to a depth of 'at least inch below the sealing ange shoulder 46.

The glass cone 16 isthen sealed to the enameled fsmll Y.

end of theyshelllliV at a temperature of about 925 to 105.0. C. duringwhich time a slightzformation of black oxide occurs adjacent to the seal., v Y Y t .Next :the face plate 12 issealed tothe large end thevshell 10, preferably withoutrpreiring V.the enamel.V

This may be accomplished at a sealing temperature of about925" C; to 1050? C. for about 8-10 minutes, dur-v ing which time, there Yoccurs some formation of black of a television picture tube is formed of'a readilyox Y,

dizable Acorrosive metal such as a low carbon steel. The

' corrosive metal shell is provided as described below with a mat black protective coating of iron oxide. As shown in VFigure l, the black oxide coating'34 extends over the interior surface of the lmetal except the areas to which e operation. Coating 34, as formed according to the inven` tion, is electrically conductive and thus permits electrical conduction of secondary electrons fromthe screen 18, through the coating 34, to the metal shell 10, connected by lead 33 to an external source of potential.

The method of applying the protective coatingr34 to the interior surface ofV the metal shell 10V is as follows. Y

First the .shell it? is prepared for reception of the Viron oxide coating 34 and an enamel frit in the seal areas by an alkaline degreasing operation to clean the metal shell free of lubricants, rust, and dirt;Y Next the entire interior surface and the sealing areas of the metal shell are sandblasted, with silica sand or Alundum, for example, to provide a clean rough surface: Sandblasting is continued untilV there is obtained a uniform dull metallic finish, free of anyV stains or oxide spots onthe .entireinterior of the metal shell and the sealing areas. This insuresV a clean bare metal surface to which anyiron oxide subsequently formed by theapplication ofheatwill firmly adhere, since oxide on the shell adacent to the face plate seal. f However, thus far there has Vbeen `no appreciable black oxide formed on the major inside surface'areas of the shell.

. lf desired,the enameling and sealing steps may beV varied somewhat. VPor example, enamel frit may be apf plied to each end of the shell and the enamel may then be tired by local heating of each end prior to sealing.

The sealedassembly is then transferred to an annealing oven, wherein the face plate'and metal shellare maintained at a temperature within the annealing range of the face plate(` glass, or slightly higher, for about 8 to 10 minutes. U Y Y The exact temperature at which annealing is carried out depends on the specific composition `of, the face plate glassv used, and will generally fall in thel range of about425 to 575 C. for commercial high expansion glasses suitable for sealing to low carbon steel shells. .Annealing is carried out in airror 'some other oxidizing atmosphere. During this step,. a strongly adherent iron Yoxide coatingis `formed'on the sandblasted interior surface of the metal shell. The assembly is then cooled down to the ambient temperature.

If desired,.the oxide coating may be formedV on Ythe shell prior to sealing.V .As indicated previously, both ends of the metal shell may be enameled by uniformly heating the entire shell to the enameling temperature whichis about 850 to 950 C. At this high temperature,r the thicknessofthe oxide coating which is formed `isy metalrshell' by local heating, and then heat theshell uniformly toa temperaturei preferably of about 500,1 to i The roughening caused by Vsand- A 600 C. for about 3-10 minutes to form the oxide coating. A somewhat lower temperature may be used, but consistently good results may be obtained at temperatures substantially below 400 C.

The iron oxide, which is formed on the whole interior surface of the metal shell by heating the shell in air or in an oxidizing atmosphere, has a mat black appearance. This mat black surface reduces internal light reections which would ordinarily detract from picture contrast during tube operation.

The oxide coating, by virtue of its being tightly adherent to the sandblasted surface of the metal shell, is non-scaling and resistant to rusting, so that bulb washing and formation of the uorescent screen 18 may be carried out Without danger of contaminating the screen. The screen 18 is normally formed by settling the phosphor coating from a suspension of the phosphor in an aqueous solution, which is poured into the envelope through the tubular portion 14 before the gun is sealed in. Using steel or iron for the metal portion of the envelope results in excessive corrosion of the metal surfaces by the solutions used in the screening process. Corrosion of the metal surfaces results in the formation of loose scale, which falls on the phosphor surface and contaminates the phosphor screen 18. The oxide coating, however, on the steel shell interior surface eliminates such corrosion and enables the formation of the phosphor screen without undue diiculty.

The oxide coating, moreover, is electrically conductive, and thus provides suicient conductivity through the coating to the metal shell to prevent the build up of a charge by secondary electrons which are emitted from the uorescent screen during the operation. The charge would affect scansion of the beam and distort the picture.

ln the specification and claims low carbon steel will be taken to include those ferrous metals known in the art as SAE 1010 steel, mild steel, low carbon steel or cold rolled steel.

What is claimed is:

l. The method of applying a mat black, rust-resistant, non-scaling, electrically conductive coating to the interior of a low carbon steel envelope portion for an electron discharge device, said method comprising removing suicient metal from the interior surface of said envelope portion to impart a dull metallic finish thereto, sealing a glass face plate and a glass funnel portion to said steel envelope portion to form a composite sealed assembly, annealing said sealed assembly in an oxidizing atmosphere and in the temperature range of about 425-575 C. to form a tightly adherent iron oxide coating on the interior of said steel envelope portion, and cooling said sealed assembly to the ambient temperature, said iron oxide coating being mat black, rust-resistant, non-scaling, and electrically conductive.

2. T he method of applying a mat black, rust-resistant, non-scaling, electrically conductive coating to the interior of a low carbon steel envelope portion for an electron discharge device, said method comprising degreasing said steel envelope portion, Sandblasting the interior surface of said envelope portion to impart a dull metallic finish thereto, sealing a glass funnel portion to one end of said shell, sealing a glass face plate to another end of said shell to form a composite sealed assembly, annealing said sealed assembly, in an oxidizing atmosphere and in the temperature range of about 425-575 C. to form a tightly adherent iron oxide coating on the interior of said steel envelope portion, and cooling said sealed assembly to the ambient temperature, said iron oxide coating being mat black, rust-resistant, non-scaling and electrically conductive.

3. The method of applying a mat black, rust-resistant, non-scaling, electrically conductive coating to the interior of a low carbon steel shell for an electron discharge device, said shell having a large sealing flange at one end, and a small sealing flange at the other end, said method comprising degreasing said shell, Sandblasting said sealing anges and the interior surface of said shell to obtain a uniform dull metallic nish, applying vitreous enamel coating to said sealing anges, sealing a glass funnel to said small sealing liange, sealing a glass face plate to saidlarge sealing flange to form a composite assembly, annealing said sealed assembly in air atmosphere in the temperature range of about 425-575 C. for about 8-10 minutes to produce a tightly adherent iron oxide coating on said sandblasted interior surface, and cooling said assembly to the ambient temperature, said iron oxide coating being mat black, rust-resistant, non-scaling, and electrically conductive.

4. The method of applying a mat black, rust-resistant, non-scaling, electrically conductive coating to the interior of a low carbon steel shell for an electron discharge device, said steel shell comprising a hollow frustum of a cone having a large sealing ange at one end and a small sealing ange at the other end, said method comprising degreasing said shell, Sandblasting said sealing flanges and the interior surface of said shell to obtain a uniform dull metallic finish, applying vitreous enamel coating to said sealing ilanges, sealing a glass funnel to said small sealing flange, sealing a glass face plate to said large sealing flange to form a composite envelope assembly, transferring said envelope assembly to an oven having an air atmoshpere and maintained at a temperature Within the range of about 425-575 C., holding said envelope assembly in said oven for about 8-10 minutes to form a tightly adherent iron oxide coating on the interior of said shell, and cooling said envelope assembly to the ambient temperature, said iron oxide coating being mat black, rust-resistant, non-scaling, and electrically conductive.

5. An envelope portion for an electron discharge device comprising a low carbon steel shell and a tightly adherent iron oxide coating on the interior of said shell, said coating being mat black, rust-resistant, non-scaling, and electrically conductive.

6. An electron discharge device comprising an envelope including a low carbon steel shell and a tightly adherent iron oxide coating on the interior surface of said shell, said coating being mat black, rust-resistant, non-scaling, and electrically conductive.

7. An electron discharge device comprising an envelope including a low carbon steel shell substantially in the form of a frustum of a cone, a glass face plate sealed to one end of said shell, a glass funnel sealed to the other end of said shell, and a tightly adherent iron oxide coating on the interior surface of said shell, said coating being mat black, rust-resistant, non-scaling and electrically conductive.

8. An electron discharge device comprising an envelope including a low carbon steel shell having sealing anges at opposing ends thereof, vitreous enamel coating on each of said sealing lianges, a glass face plate sealed to one of said enamel coated flanges, a tubular gun housing sealed to the other of said enamel coated anges, and a tightly adherent iron oxide coating on the interior of said shell, said oxide coating being mat black, rustresistant, non-scaling, and electrically conductive.

9. A cathode-ray tube comprising an envelope including a low carbon steel shell, a glass face plate sealed to one end of said shell, a luminescent phosphor screen on the interior surface of said face plate, a tubular gun housing sealed to the other end of said shell, said housing containing an electron gun for producing and directing an electron beam on said phosphor screen, and a tightly adherent iron oxide coating on the interior surface of said shell, said iron oxide coating being mat black, rustresistant, non-scaling, and electrically conductive.

l0. A cathode-ray tube comprising an envelope including a low carbon steel shell substantially in the form of a frustum of a cone, said shell having a large sealing ange at one end and a small sealing flange at the other end, vitlfeous` enamel coating sealedY to said flanges, a glass face plate Vsealed to .said large sealing ange, a luminescent phosphor screen on `the interior surface of saidrfarcre plate, a'glass funnel sealed'to saidtsmall sealing flange, electron beam producing means within said glass funnel, and a tightly adherent iron oxide coating on the interior surface of said shell, said iron oxide coating being mat black, rustresistant, non-scaling, andelectrically conductive.' Y' 1 t u'References Cited in the le of this patent UNTED STATES PATENTS Y 1,637,033 Basch July 26, V1927 Stareck V Oct. 27, 19,36V yBowie I Vuly 25, 1939 Curtin Aug. 6, V1940Y VScheaclei1 ,Ian. 2l, 1941 Power Aug; 26, 1.941 Gage f Y Feb. 1,v 1944 -Schabacker May 9, 1944 Pratt L V (9ct. 15, 1946 McCarthy June V17, 1947 Raeuber et al. Y June 1, 1954 UNITED STATES PATENT @ETTEE CERTIFICATE @E @@ERRECHN Patent Noo 238365,751

John Co et elo It is hereby certified that error appears in the printed specification of the above number d e patent requiring correction and that the said Letters Patent should read as corrected belowu Column 3*9 line 3G, for "convention" reed i 5 line 3y after may" insert ee not me; line 339 for "the", second occurrence, read n tube w; seme line, for "The cbewe" am This charge Signed end seele-: this 6th ay of Jennery 195% (SEAL) Attest;

ROBERT C. WATSGN Attesting Oficer Commissioner of Patents

Claims (1)

1. THE METHOD OF APPLYING A MAT BLACK, RUST-RESISTANT, NON-SCALING, ELECTRICALLY CONDUCTIVE COATING TO THE INTERIOR OF A LOW CARBON STEEL ENVELOPE PORTION FOR AN ELECTRON DISCHARGE DEVICE, SAID METHOD COMPRISING REMOVING SUFFICIENT METAL FROM THE INTERIOR SURFACE OF SAID ENVELOPE PORTION TO IMPART A DULL METALLIC FINISH THERETO, SEALING A GLASS FACE PLATE AND A GLASS FUNNEL PORTION TO SAID STEEL ENVELOPE PORTION TO FORM A COMPOSITE SEALED ASSEMBLY, ANNEALING SAID SEALED ASSEMBLY IN AN OXIDIZING ATMOSPHERE AND IN THE TEMPERATURE RANGE OF ABOUT 425-575*C. TO FORM A TIGHTLY ADHERENT IRON OXIDE COATING ON THE INTERIOR OF SAID STEEL ENVELOPE PORTION, AND COOLING SAID SEALED ASSEMBLY TO THE AMBIENT TEMPERATURE, SAID IRON OXIDE COATING BEING MAT BLACK, RUST-RESISTANT, NON-SCALING, AND ELECTRICALLY CONDUCTIVE.

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