US3791546A

US3791546A - Cathode-ray tube having conductive internal coating comprised of iron oxide and graphite

Info

Publication number: US3791546A
Application number: US00202428A
Authority: US
Inventors: J Maley; D Bartch
Original assignee: RCA Corp
Current assignee: RCA Licensing Corp
Priority date: 1971-11-26
Filing date: 1971-11-26
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12
Also published as: JPS5238713B2; GB1401338A; DE2257497A1; IT969626B; DE2257497B2; BE791817A; JPS4861070A; NL175565C; FR2161000A1; NL7215969A; AU463352B2; CA979477A; DE2257497C3; FR2161000B1; AU4868172A; NL175565B

Abstract

A cathode-ray tube having a conductive internal coating comprised of iron oxide, graphite, and an alkali silicate on the interior walls of the tube.

Description

United States Patent 1191 'Maley et al.

' [451 Feb. 12,1974

CATHODE-RAY TUBE HAVING CONDUCTIVE INTERNAL COATING COMPRISED OF IRON OXIDE AND GRAPHITE Inventors: James John Maley, Lancaster;

Donald Walter Bartch, Columbia, both of Pa.

Assignee: RCA Corporation, New York, NY.

Filed: Nov. 26, 1971 Appl. No.: 202,428

us. 01. 220/2.1 A, 117/33.3, 117/335 c, 117/54,117/124 A, 117/211, 117/226,

v 313/39, 313/92 .'1m.c1. H05b 33/23, c09 1/44 Field of Search... 117/54, 37 R, 124 A, 33.5 c, 117/333, 226, 211; 252/313 s; 313/89, 92; 220/2.1 A

Primary Examiner-Alfred L. Leavitt Assistant Examiner-l4. F. Es o sito Attornej Agent, or F irm-G. H. Bruestle; L. Greenspan ABSTRACT A cathode-ray tube having a conductive internal coating comprised of iron oxide, graphite, and an alkali silicate on the interior walls of the tube.

7 Claims, 1 Drawing Figure CATHODE-RAY TUBE HAVING CONDUCTIVE INTERNAL'COATING COMPRISED OF IRON OXIDE AND GRAPHITE BACKGROUND OF THE INVENTION This invention relates to an improved cathode-ray tube having an internal coating comprised of iron oxide and graphite on the interior walls of the tube/ Cathode-ray tubes usually have a conductive internal coating on the interior walls of the tube. This'coating is used to carry a high potential of the order of to 30 kilovolts, which is applied at the anode button. The most common internal coating consists essentially of particulate graphite and an alkali silicate binder. The coating is applied to the walls of the tube by spraying and/or brushing as an aqueous .coating composition, then dried, and then baked in air at about 400C for one hour. After baking, this prior-art coating has a point-to-point resistance of about 50 to 90 ohms per inch. During normal tube processing and in subsequent handling, graphite particles are released from the coating and may cause excessive arcing and electrical leakage in the electron gun during the operation of a finished tube. Excessive arcing and electrical leakage are causes of poor high voltage stability of the tube.

SUMMARY OF THE INVENTION ent to glass, releases fewer graphite particles both because there is a lower content of graphite particles present and because the coating is more cohesive, and is more scratch resistant. And, surprisingly, loose'iron oxide particles have been shown not to be detrimental to the high voltage stability of the tube. The coating has an adequate conductivity and is otherwise compatible in performance with prior coatings. In addition, the internal coating may be applied from an aqueous coating composition by presently used techniques.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a partially broken-away longitudinal view of a novel cathode-ray tubeof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Example The cathode-ray tube illustrated in the sole FIGURE is an apertured-mask-type kinescope. The tube includes an evacuated envelope designated generally by the numeral 21, which includes a neck 23 integral with a funnel 25, and a faceplate or panel 27 joined to the funnel 25 by a seal 29, preferably of a devitrified glass. There is a luminescent layer 31 of a phosphor material on the interior surface of the faceplate 27. There is a light-reflecting metal coating 33, as of aluminum metal, on the luminescent layer 3I. The luminescent layer 31, when suitably scanned by an electron beam or beains from a gun in a mount assembly 35 located in the neck, 23, is capable of producing a luminescent image which may be viewed through the faceplate 27.

There is an electrically-conductive internal coating 37, consisting essentially of an alkali silicate binder material, iron oxide particles, and carbon particles, on a portion of the interior surface of the funnel 25 between the mount assembly 35 and the seal 29. Three metal fingers 39 space the mount assembly 35 from the neck wall and connect the forward portion of the mount assembly 35 with the internal coating 37.

Closely spaced from the metal coating 33 toward the mount assembly 35 is a metal mask 41 having a multiplicity of apertures therein. The mask 41 is welded to a metal frame 43 which is supported by springs 47, which are attached to the frame 43, on studs 45 integral with the panel 27. Inasmuch as the invention is con cerned primarily with the conductive internal coating 37, a detailed description of the components and parts normally associated with the neck and

faceplate

23 and 27 is omitted or shown schematically.

The tube of this example may be fabricated by methods known in the art. The mask 41, frame 43 and springs 47 are assembled. The luminescent layer 31 and the metal coating 33 are deposited on the inner surface of the faceplate 27. The conductive internal coating 37 is applied to the interior surface .of the funnel 25 and an adjacent portion of the neck 23 as indicated in the sole FIGURE. The internal coating 37 is applied from an aqueous suspension in two steps. First, a portion is brushed on in the neck 23 and the adjacent portion of the funnel 25, and then a portion is sprayed on in the funnel 25, so that the sprayed-on portion overlaps the brushed-on portion. A typical coating formulation (44478/36) is as follows:

, 100 grams graphite percent below l0 microns) 220 grams ferric oxide Fe O (90 percent below 10 microns) 300 grams aqueous solution containing 54 weight percent sodium silicate (Na ozSiO is about l:2.0)

2.2 grams dispersant 500 grams deionized water This formulation has a viscosity of about 20.5 seconds. After application, the coating is dried in air.

Next, a bead of devitrifying glass frit is deposited on the seal land of the funnel 25. The frame 43 with the mask 41 and springs 47 attached thereto is mounted on the studs 45. The seal land of the panel 27 is placed against the bead of frit on the seal land of the funnel 25,

and the assembly is heated at about 400C until the frit melts and devitrifies and forms a vacuum-tight seal 29 between the panel 27 and the funnel 25. During the heating to form the seal 29, the heat also bakes the funnel coating 37, driving out moisture and rendering the funnel coating 37 electrically-conductive and chemically-stable to the atmosphere. After cooling to room temperature, the funnel coating 37 has, by the tests described below, a hardness of about l,200 grams, a scratch resistance of about 6,968 particles, mostly iron oxide, and an electrical resistance of about 200 ohms per inch.

The metal fingers 39, the electron gun and the convergence assembly are assembled on a glass stem producing the mount assembly 35. The metal spacer fingers 39 are depressed and inserted into the neck 23 and slid into the tube until the mount assembly 35 is at the desired position. The fingers 39 slide on a portion of the coating 37 during this step and may abrade some of the coating material therefrom. Next, the glass stem is sealed to the neck 23. Finally, the entire tube is baked at about 440C, the tube exhausted and then sealed.

GENERAL CONSIDERATIONS The internal coating may be used in any cathode-ray tube including picture tubes, display tubes, oscilloscopes, camera tubes and storage tubes. The coating is particularly suitable where an adherent conductive coating is desired on the interior glass surfaces of the tube. The conductive internal coating is prepared by applying to the interior surface of the glass funnel 25 a coating of an aqueous suspension ofa desired compo sition, drying the coating and then baking the coating at about 200 to 450C. The suspension comprises about 200 to 600 weight parts iron oxide particles, 100 weight parts powdered graphite and about 50 to 150 weight parts alkali silicate solids as an aqueous solution. Funnel coatings for use in kinescopes according to the invention preferably consist essentially of about 100 weight parts graphite, about 200 to 220 weight parts dehydrated ferric oxide and about 125 to 150 weight parts sodium silicate solids. A dispersant and additional water are added to adjust the coating qualities of the coating formulation as desired. The iron oxide is in anhydrous form and may be any oxide of iron, such as ferric oxide Fe O ferrous oxide FeO, or ferrosoferric oxide Fe O The preferred form is ferric oxide Fe O The particle-size range of the iron oxide is such that 90 percent is below microns. The graphite may be any of the varieties that can be used for making conductive coatings. The particle-size range of the graphite is such that 90 percent of the graphite is below 10 microns. The alkali silicate solids are in an aqueous solution of about 32 to 63 weight percent solids. The alkali is preferably sodium, but may be potassium or lithium. Where sodium silicate is used, the ratio of alkali to silica in the silicate may be in the range of 1:1.6 to 1:3.8.

The constituents of the suspension are mixed together preferably with about 0.1 to 0.3 weight percent of dispersant such as Marasperse, marketed by American Can Company, New York, New York. The constituents of the coating formulation are mixed together and then ball milled for a period of time, for example about 6 hours. Changing the proportions of iron oxide, graphite and alkali silicate affects the electrical resistance of the final coating. Increasing the proportions of iron oxide increases the electrical resistance of the finished coating. Increasing the proportions of alkali silicate increases the electrical resistance and scratch resistance of the finished coating. Increasing the proportions of graphite decreases the electrical resistance and decreases the scratch resistance of the finished coating. However, the internal coatings described herein provide a set of practical compromises of the electrical and physical properties needed for their use in cathode-ray tubes.

The coating formulation may be applied by any convenient process. It is preferred to brush the coating onto the interior wall of the neck 23 so that a sharply defined edge is produced opposite the mount assembly 35. It is preferred to spray the coating formulation onto the interior surface'of the funnel 25, overlapping the brushed-on portion. Spraying is a rapid process for covering the relatively large funnel area. Either air or airless spraying can be used.

The physical and electrical properties of the iron oxide-graphite internal coating employed in the novel tube and similar prior all-graphite coatings used in prior tubes have been measured many times. The results are quite consistent. A typical set of comparative data, along with the formulations, is shown below in the Table. The particular materials used are those mentioned in the Example.

When a prior-art all-graphite coating is abraded (as with bulb spacers or with the getter housing), graphite particles are released which have been shown to be detrimental to high voltage stability, such as causing exces-v sive arcing or electrical leakage. Far fewer particles are generated from the iron oxide-graphite coating described herein. This fact in itself results in better high voltage stability. In addition, since the coating is mainly iron oxide, most of the particles which are released from abrasion are iron oxide. Unlike graphite particles, loose iron oxide particles have been shown not to be detrimental to the high voltage stability of the tube. For these reasons, much better high voltage electrical stability is obtained with the novel tube. Thus, the presence of iron oxide in the internal coating provides adequate electrical properties, improved abrasion and scratch resistance and, when particles are released, produces less arcing and electrical leakage.

In a color kinescope, for example, electrons are projected to the screen and are then returned to the high voltage supply through the anode button. If the resistance of the internal funnel coating is too high 1 X 10 ohm), there will be a significant voltage drop between the applied voltage at the anode button and the screen. This will be observed as a dull picture on the screen. It may be argued that a high resistance coating 1 X 10 ohm) may still be used with a conducting band between the screen and the anode button to prevent voltage drop. In principle,.this is true, but it is difficult to produce a reproducible coating with suitable propertiesin practice. An additional coating must be used, which is undesirable in production. Also, when a high resistance coating is used over the interior surface of the funnel, the resistance of the coating must be uni form. If isolated patches of insulating coating are present, there will be nonuniform charging of the funnel coating. This will have an undesirable effect on the electron beam and will probably be observed as convergence drift or dynamic convergence errors. High resistance internal coatings with uniform resistances are difficult to prepare. The electrical resistance of the iron oxide-graphite coating described herein is about 200 to 2,000 ohms per inch, and preferably about 200 to 300 ohms per inch. Consequently none of the problems associated with high resistance coatings are present. In the novel tube, the resistance of the oxide-graphite coating is low enough so that no other coating is required, and no electrical problems result from the use of this coating. In addition, the high voltage stability of the kinescope is greatly enhanced.

TABLE Fe o -Graphite All-Graphite Iron Oxide 200 to 600 grams 0 Graphite lOO grams l00 grams Silicate Solids 50 to I50 grams lOO grams Dispersant 1.0 to 4.4 grams 3.2 grams Water 400 to 1,500 grams 275 grams Viscosity 20 to 222 secs. l7.5 vto 20 secs. Hardness 1,000 to 1,250 25 to 50 grams grams Coating Particle 2.5 to 3.0 p. 3.0 .1. Size (Median) Adherence No visible par- Particles on over ticles 50% of the tape Scratch Resis- X =7,000 par- Y 20,000 partance ticles (mostly ticles (all iron oxide) graphite) Electrical Rc- 200 to 2,000 50 to 90 ohms] sistance ohms/inch inch l. Hardness is measured by a Hoffman Method test using a weighted cylindrical stylus. The reading obtained is the weight necessary to scratch the coating so that the substrate is visible. This indicates the amount of coating which may be scraped off when the coating is abraded; e.g., when the mount assembly is inserted into the neck. Generally, the harder the coating, the better it is for fabrication and operation.

2. Adherence is measured by a Scotch Tape Test. A 2000-gram cylinder is rolled over a piece of tape on the surface of the funnel coating, pressing the pressure-sensitive adhesive against the funnel coating. The tape is then removed. and a visual inspection is made of the particles on the tape. The number of particles on the time is a relative measure of particles loosely adhereing to the coating surface. These particles may be removed from the funnel coating without abrasion during the processing or life of the tube. Generally, the fewer the number of particles on the tape, the better the coating is for fabrication and operation.

3. In the scratch re si tag tg tg, a gleanlqu nt is inserted into a coated glass neck to a distance of 1 inch. The mount is withdrawn, and the coated neck and the gun are washed with freon. The freon is filtered, and the particles are collected on the filter paper. The particles are then counted by aid of a microscope. This test is a measure of the number of particles that may be released from the funnel coat ing during mount insertion. In tube construction, a significant number of these particles fall on the mount and cause arcing and electrical leakage. The probability of arcing and electrical leakage in the tube increases with the relative increase in the number of particles produced during this ten the coating consists essentially of about [00 weight pans graphite particles, 200 test, using the same mount.

4. in the electrical resistance test. two -inch-diameter metal balls mounted a known distance apart in'an insulating plate are applied against the coating, and the DC resistance between them is measured. 7

We claim:

1. ha cathode-ray tube comprising an evacuated envelope including a funnel, the improvement comprising a conductive coating adhered to at least a portion of the interior walls of said funnel, said coating consisting essentially of graphite particles, iron oxide particles and an alkali silicate binder, the weight ratio of iron oxide particles to graphite particles being in the range of 2 to l and 6 to I, said coating being of such thickness as to provide a point-to-point electrical resistance of about 200 to l0,000 ohms per inch.

2. The tube defined in claim 1 wherein the coating consists essentially of about weight parts graphite particles, 200 to 600 weight parts iron oxide particles and about 50 to 150 weight parts alkali silicate binder.

3. The tube defined in claim 2 wherein said alkali silicate binder is sodium silicate.

4. In a television picture tube comprising an evacuated glass envelope including a neck for housing an electron gun, a faceplate panel opposite the neck, and a funnel providing a closed transition between said neck and said panel, the improvement comprising a coating adhered to at least a portion of the interior walls of saidfunnel and neck, said coating consisting essentially of about 100 weight parts graphite particles, about 200 to 600 weight parts iron oxide particles and about 50 to 150 weight parts alkali silicate binder, said coating being of such thickness as to provide a point-topoint electrical resistance of about 200 to 2,000 ohms per inch.

5. The tube defined in claim 4 wherein said electrical resistance is about 200 to 300 ohms per inch and the iron oxide is ferric oxide.

6. The tube defined in claim 5 wherein said coating consists essentially of about 100 weight parts graphite particles, about 200 to 220 weight parts iron oxide particles, and about to weight parts alkali silicate binder.

7. The tube defined in claim 6 wherein said alkali silicate is sodium silicate.

UNITED STATES PA'IENT OFFICE (IERTIFICA'IE ()F CORRECTION 'PA'ILN'I NO. 3, 791, 54 DATED February 12, 1974 INVENT()R(S) 1 James John Marley and Donald Walter Bartch H is certified that error appears in the above identified patent and that said Letlers Patent are hereby corrected as shown below:

Column 6, line 9 change "10,000" to -2,000

Signed and sealed this 13th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents and Trademarks RUTH C. MASON Attesting Officer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Q Patent No. 3,791,546 Dated February 12, 1974 Inven-ltofls) James John Maley and Donald Walter Bartch It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 24 after "of' change "a" tow-thew Column 5, line 26 change .tpae" to -tape+ Column lines 38 remove "ten the eo ati ng consi sts essentiand 39 ally of abOut ljOO weight. parts graphite particle's, ZOO" Signed and sealed this 18th day of June 1971;.

(SEAL) Attest:

EDWARD MQFLETCHERJRQ I c'. MARSHALL DANN I Attesting Officer I v Commissioner of Patents FORM F'O-IOSO (10-69) I sc -nc 503754359 e vs. sovznuuzm PRINTING OFFICE: 19' o-ass-a 4

Claims

2. The tube defined in claim 1 wherein the coating consists essentially of about 100 weight parts graphite particles, 200 to 600 weight parts iron oxide particles and about 50 to 150 weight parts alkali silicate binder.
3. The tube defined in claim 2 wherein said alkali silicate binder is sodium silicate.
4. In a television picture tube comprising an evacuated glass envelope including a neck for housing an electron gun, a faceplate panel opposite the neck, and a funnel providing a closed transition between said neck and said panel, the improvement comprising a coating adhered to at least a portion of the interior walls of said funnel and neck, said coating consisting essentially of about 100 weight parts graphite particles, about 200 to 600 weight parts iron oxide particles and about 50 to 150 weight parts alkali silicate binder, said coating being of such thickness as to provide a point-to-point electrical resistance of about 200 to 2,000 ohms per inch.
5. The tube defined in claim 4 wherein said electrical resistance is about 200 to 300 ohms per inch and the iron oxide is ferric oxide.
6. The tube defined in claim 5 wherein said coating consists essentially of about 100 weight parts graphite particles, about 200 to 220 weight parts iron oxide particles, and about 125 to 150 weight parts alkali silicate binder.
7. The tube defined in claim 6 wherein said alkali silicate is sodium silicate.