US3758802A

US3758802A - Improved cathode ray tube having a glass envelope coated with crystallized glass

Info

Publication number: US3758802A
Application number: US00107900A
Authority: US
Inventors: T Kubo; M Itazu; H Yamashita
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1970-01-23
Filing date: 1971-01-20
Publication date: 1973-09-11
Anticipated expiration: 1990-09-11
Also published as: GB1300503A; FR2076908A5; JPS5023591B1; DE2102587A1; DE2102587B2; NL7100543A

Abstract

A cathode ray tube including an electron gun enclosed by a glass envelope including a neck portion is disclosed in accordance with the teachings of the present invention wherein arcing between electrodes of the cathode ray tube is minimized by providing a coating of crystallized glass formed by heat-treatment of sealing glass, on the surface of the inner wall of said neck portion and coextensive with said electron gun, and a coating of conductive material, spaced from said coating of crystallized glass, on the surface of the inner wall of said neck portion. A method of treating a cathode ray tube to minimize arcing between the electrodes thereof is also disclosed.

Description

IMPROVED CATI-IODE RAY TUBE HAVING A GLASS ENVELOPE COATED WITll-ll CRYSTALLIZED GLASS Inventors: Tomoichi Kubo; Masakatsu Itazu;

I-Iirofumi Yamashita, all of Tokyo, Japan Assignee: Nippon Electric Company, Limited,

Minato-ku, Tokyo, Japan Filed: Jan. 20, 1971 Appl. No.: 107,900

Foreign Application Priority Data Jan. 23, 1970 Japan 45/6644 US. Cl 313/64, 313/220, 313/317 References Cited UNITED STATES PATENTS 9/1956 Amdursky et a1. 220/21 A 1451 Sept. 11, 1973 Primary Examiner-Rudolph V. Rolinec Assistant ExaminerWilliam H. Punter Att0rney--Marn & .langarathis [5 7] ABSTRACT A cathode ray tube including an electron gun enclosed by a glass envelope including a neck portion is disclosed in accordance with the teachings of the present invention wherein arcing between electrodes of the cathode ray tube is minimized by providing a coating of crystallized glass formed by heat-treatment of sealing glass, on the surface of the inner wall of said neck portion and coextensive with said electron gun, and a coating of conductive material, spaced from said coating of crystallized glass, on the surface of the inner wall of said neck portion. A method of treating a cathode ray tube to minimize arcing between the electrodes thereof is also disclosed.

2 (Ilaims, 1 Drawing Figure PATENTED 1 I973 3.758.802

INVENTORS Tomoichi Kubo Masukotsu ltozu Hirofumi Yomoshito ATTORNEYS IMPROVED CATl-EODE RAYTUBE HAVllNG A GLASS'ENVELOPE COATED WITH .CRYSTALLIZED GLASS This invention relates to cathode ray tubes and more particularly to cathode ray tubes wherein arcing between electrodes thereof is minimized.

The cathode ray tube adapted for'television and especially color television reception requires high voltage for the effective operation'thereof, which voltage attains a value up to 27,000 volts. Such cathode ray tubes usually consist of a glass envelope having a neck portion and a viewing screen disposed at opposite ends of said envelope. Within the neck portion is an electron gun structure for generating and focusing a beam of electrons upon the viewing screen. This electron gun structure includes a cathode which serves as the source for the beam of electrons and a plurality of electrodes. One or several of these electrodes may comprise accelerating anodes and others of said electrodes may comprise control grids which cooperate with the accelerating anodes to focus the beam of electrons onto the viewing screen with the proper intensity. In addition, the surface of the inner wall of the neck portion of the glass envelope may be coated with conductive material such as graphite which acts as an additional anode to provide a final focus to the beam of electrons.v

During operation of the cathode ray tube, the application of the aforementioned high voltage to the electrodes thereof often results in a vacuum discharge phenomenon commonly known as arcing, which may damage the display screen, the glass envelope and portions of the electron gun. Arcing may be attributed to roughness of, projections from, and irregularities in the electrodes of the cathode ray tube. Accordingly, during the manufacture of cathode ray tubes it is preferred to employ a process commonly known as high voltage treatment" or spot knocking whereina voltage greater than the normal operating voltage of the cathode ray tube is applied to the electrodes thereof. The applied voltage is usually equal to twice the normal operating voltage or about 60,000 volts. During such high voltage treatment arcing occurs to induce enough current to flow between the electrodes such that the contaminations and irregularities thereof are fused or melted off, thereby minimizing the probability of arcing duringnormal operation of the cathode ray'tube.

An attendant disadvantage of the high voltage treatment as heretofore utilized by the prior art is the production of a potential gradient along the surface of the inner wall of the neck portion of the glass envelope due to the application of the high voltage between the cathode and the graphite coating on the glass envelope. Since the graphite coating is supplied with the high anode voltage, a portion of the surface of the inner wall of the neck portion of the glass envelope is maintained at a relatively high voltage potential. Consequently the glass wall undergoes a concentrated electron bombardment from an electrode supplied with a relatively low voltage potential. The concentrated electron bombardment produces a heating of the glass wall resulting in punctured and cracked portions of the glass envelope. Accordingly, the magnitude of the voltages applied to the electrodes of the cathode ray tube during thehigh voltage treatment and the time duration of such high voltage treatment must be strictly limited to avoid damage of the glass envelope. Unfortunately, such limitations may forestall the successful completion of the high voltage treatment such that the aforementioned irregularities which contribute to arcing are not removed. I

Therefore, it is an object of the present invention to provide an improved cathode ray tube that is particularly adapted to withstand complete high voltage treatment.

It is another object of this invention to provide a cathode ray tube wherein arcing between the electrodes thereof is substantially minimized during normal operation.

A further object of the present invention is to provide a cathode ray tube which is adapted to receive a higher operating voltage then heretofore utilized by the prior art.

Yet another object of this invention is to provide a method of treating a cathode ray tube whereby arcing between the electrodes thereof is minimized during normal operation. of a Various other objects and advantages of the invention will become clear from the following detailed description of an exemplary embodiment thereof and the novel features will be particularly pointed out in connection with the appended claims.

In accordance with this invention an improved cathode ray tube including an electron gun enclosed by a glass envelope is provided wherein a film of crystallized glass formed by heat-treatment of sealing glass, is coated on the surface of the inner wall of said glass envelope, the coating being coextensive with said electron gun. A cathode ray tube treated in accordance with the method of the present invention exhibits minimal arcing between the electrodes thereof during normal operation.

The invention will be more clearly understood by reference to the following detailed description of an exemplary embodiment thereof in conjunction with the accompanying drawing which is a cross sectional view of a portion of a cathode ray tube in accordance with the present invention.

Referring now to the drawing, there is illustrated a portion of a cathode ray tube which may be utilized in a color television receiver and includes a unipoten'tial electron gun I comprised-of a cathode assembly 2, a

first electrode 3, a'second electrode 4, a third electrode 5, a fourth electrode 6 and a fifth electrode 7. The electron gun assembly 1 is sealingly enclosed in the neck portion 8 of an evacuated glass envelope. It is understood that a display screen, although not shown, is provided at one end of the glass envelope oppositely disposed from stem portion 11 of the neck 8. The cathode assembly 2 serves as the source for the beam of electrons and may comprise a conventional cathode such as an indirectly heated oxide coated cathode. The various electrodes 4 through 7 may comprise control grids, accelerating anodes and focusing electrodes, well known in the prior art, the-description of which need not be set forth in detail herein for a complete understanding of the present'invention. A portion of the surface of the inner wall of neck 8 is coated with conductive material 9 such as a graphite layer, which coating may extend from the mid-portion of neck 8 into the bulb portion of the glass envelope, not shown. It is recognized by those skilled in the cathode ray tube art that conductive coating 9 may act as an accelerating anode or a focusing electrode.

During the manufacture of'the cathode ray tube one or more of the electrodes 3 through 7 as well as the conductive coating 9 may be provided with irregularities such as contaminations, roughness or protrusions, which may result in deleterious arcing effects during normal operation of the cathode ray tube. Accordingly, it is preferable to provide high voltage treatment of the assembled cathode ray tube to remove such irregularities from the electrodes. In the high voltage treatment of cathode ray tube as illustrated in the drawing, a high voltage of about twice the magnitude of the normal operating voltage is applied to electrodes 5 and 7-and a low potential such as ground potential is applied to

electrodes

3, 4 and 6. The high voltages applied to the aforementioned electrodes results in arcing and electrons will be emitted from electrode 6 to bombard the electrodes 5 and 7- oppositely disposed thereto. it is clear however that the electrons emitted from electrode 6 will also bombard that portion of the surface of the inner wall of neck portion 8 that is oppositely disposed from electrode 6. The concentrated bombardment of the glasswall of neck 8 may produce a heating of the glass wall causing thermal breakdown thereof resulting in electrical-puncture or breakage of theglass.

According to the present invention, this portion of the glass tube wall surface is provided with the abovementioned kind of protective coating film 10 made of materials different from that of the glass tube so as to encircle the electrodes. Then, the electrons would be reflected or absorbed by this protective coating film, so that the direct effect of the electrons does not reach up to the glass wall. If the protective coating 10 admits of a rough surface then the increase in surface area is effective to decrease the concentration of electrons per unit area communicated thereto, resulting in a further improvement of the protective qualities of the coating The protective coating 10 is preferably comprised of a film of material such as sealing glass manufactured by the Corning Glass Works. A complete description of the method of making sealing glass as well as the composition and attributes thereof may be found in U.S.

Pat. No. 2,889,952 which issued to SA. Claypoole on June 9, 1959. It will suffice therefore, for a complete understanding of the present invention to note that the protective coating 10 is comprised of a film of crystallized glass formed by heat-treatment of sealing glass.

The manner in which the protective coating 10, having a rough surface, may beattained will now be described. 200 grams of crystalline glass powder such as sealing glass powder, having a particle size of less than 100 mesh is suspended in a mixture of 125 c.c. of acetone and 0.5 grams of nitrocellulose. The suspension is then deposited on the surface of the inner wall of neck portion 8 such as by spraying or by applying the suspension with a brush. It should be noted that the thickness of the protective coating 10, although critical, admits Consequently the thickness of the protective coating 10 is preferably within the range of to 800 microns. The suspension, after being deposited on the surface of the inner wall of the neck portion 8, is subjected to natural drying and is then heated in air at a temperature of about 450C to rigidly secure the protective coating to the glass wall.

A cathode ray tube provided with a film of protective coating 10 as described hereinabove is particularly adapted for high voltage treatment to thereby minimize arcing between electrodes thereof during normal operation. it has been found that the high voltage potential applied to the electrode of a cathode ray tube of the present invention during high voltage-treatment thereof may exceed the voltage potentials hereinbefore applied by the prior art by approximately 10,000 to 20,000

volts. Experimentation has proven that the damage to cathode ray tubes prepared in accordance with the present invention during high voltage treatment thereof has been substantially reducedeven when voltages in the'range of 60,000 to 80,000 volts have been applied. The following table provides an impressive indication v of the improved results achieved by the present inven- Rate of damage of tubes caused by the high voltage treatment Voltage used in the high voltage treatment While the present invention has been particularly described with reference to only a single electron gun included in a cathode ray tube it is readily apparent that a plurality of electron guns may be employed in accordance with this invention and the cathode ray tube may comprise a conventional three gun cathode ray tube conventionally utilized in the color television art. In addition the particular application of the cathode ray tube is not limited to television and may include'other uses such as an oscilloscope, a'radar screen, an optical read out device or the like. Therefore, it will be obvious to those skilled in the art that the foregoing and various other changes and modifications in form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims be interpreted as including all such changes and modifications.

What is claimed is:

1. In a cathode ray tube including an electron gun en closed'by' a glass envelope, the improvement comprisofa wide range. If the thickness of the coating 10 is too thin then proper protection is not afforded the glass wall of neck portion 8 and electron bombardment during high voltage treatment may produce sufficient heat to damage the wall. Conversely, if the thickness of the coating 10 is too thick, the coating will readily peel off.

ing a film consisting of crystallized glass formed by heat-treatment sealing glass coated on the surface of the inner wall of said glass envelope and coextensive with said electron gun.

2. The improvement of claim 1 wherein said film consisting of crystallized glass includes a rough surface.

I t t i

Claims

1. In a cathode ray tube including an electron gun enclosed by a glass envelope, the improvement comprising a film consisting of crystallized glass formed by heat-treatment sealing glass coated on the surface of the inner wall of said glass envelope and coextensive with said electron gun.