US2908838A - Aperture mask treatment to prevent cathode poisoning - Google Patents

Description

Q 1959 M. EQNORDBERG APERTURE MASK TREATMENT TO PREVENT CATHODE POISONING Filed Feb. v 25, 1957 APERTURE MASK TREATMENT TO PREVENT CATHODE POISONING Martin E. Nordberg, Corning, N.Y., assignor to Corning Glass Works, Corning, N.Y., acorporation of New York Application February 25, 1957, Serial No. 642,073 15 Claims. (Cl. 313-85) 7 This invention relates to an improved perforate mask for assembly in a cathode ray tube of the type used in multicolor television receivers and to a cathode ray tube embodying such mask.

Certain types of multicolor television systems operate on the general principle of emitting a stream of electrons from one or more electron guns mounted in the neck of a cathode ray tube and directing such stream United States Patent a plurality of electron beams passing through the apertures or perforations in the mask to the patternof phosphors on the face. of the tube. Multicolor television systems of this sort are described in detail in the patent litice being used in cathode ray tubes for absorption of electrons.

By way of illustration the present invention is described with reference to the accompanying drawings in which,

Fig. l is a schematic representation, largely in section, of a cathode ray tube and,

Fig. 2 is an enlarged fragmentary view in section of a perforate mask in accordance with the invention.

-The cathode ray tube, schematically represented in Fig. 1 and designated by the numeral 10, has an electron gun 12 mounted in the tube neck 14 and a viewing screen '16 composed of a phosphor coating 18, of suitable composition and pattern formulticolor televisingjapplied to the inside surface of transparent face plate or panel 20 which forms the opposite, enlarged end of the tube. The phosphor coating is applied in any known manner in a dot, stripe or other type. of pattern suitable for the purpose. Perforate glass mask 2 2 is mounted intermediate electron gun 12 and screen v16, by any suitable conventional means, not specifically shown.

In Fig. 2 a fragment of mask 22 is shown greatly exaggerated to better illustrate chemically perforated apertures 26 which are too small and too numerous to be satisfactorily shown in a full viewsuch as Fig. 1.. In accordance with the exemplary mask specifications, referred to earlier, the diameter of apertures 26 is 10 mils and they are spaced 28 mils apart center to center. For purposes of illustration mask 22 is shown fiat, and apertures 26, as shown, are formed normal to the electron intercepting surface. As a practical matter mask 22 will frequently have imparted to it a definite curvature and apertures 26 may be formed at angles up to 25 or so from normal depending on the location and geometry of the tube components. The angle of each aperture will be such as to provide alignment of electron gun, apertures and screen elements for proper screen scanning.

dimensions being held to a tolerance of 10.5 mils. The

inherent difficulties. in producing and handling such a mask are quite readily apparent. Furthermore it is frequently necessary to sag or otherwise form such a mask sequent handling. It has been recognized that a mask.

composed of ceramic material such as glass should be highly desirable, but, until discovery of the chemical method of machining glass described in United States Patent No. 2,628,160 issued to S. D. Stookey, no satisfactory method existed for producing such masks. It was naturally assumed that, if a mask of glass or other ceramic could be satisfactorily produced, its use in a tube should present no particular problem since glass is used in construction of the tube envelope itself and other ceramics are widely used in electronic tubes.

Once glass masks were successfully made by the Stookey method, it was quite unexpectedly found that tubes embodying such masks in their assembly tended to fail- In accordance with my present invention an electron absorbing, perforate mask, for assembly and use in a cathode ray tube, is produced by firing a chemically perforated glass mask in areducingatmosphere containing free hydrogen. The efiectiveness of this treatment indicates that the poisoning agent released during electron bombardment of an untreated glass mask is oxygen, and that preliminary firing ofthe glass in the presence of hydrogen, that is in accordance with the present invention, causes such oxygen to combine with the hydrogen or to otherwise be rendered unavailable for subsequent release. This theory is supported by tests in which gases evolved from various glasses under electron bombardment have been collected and analyzed, such test glasses including both hydrogen-fired and untreated glass capable of being chemically perforated.

The atmosphere in which the glass is fired prefe rably contains at least 25% hydrogen. While smaller amounts may be effective the amount of time required is usually too long for practical purposes. Theelfectiveness of the hydrogen firing increases with temperature and it is therefore desirable to employ the maximum firing temperature' possible without distortion of the glass. This maximum temperature will depend in large measure on the viscosity characteristics of the glass and will generally be on the order of C. below the glass softening point.

However higher temperatures .may be employed Where,

filed October 10, 1955, in the name of Stanley D. Stookey and assigned to the assignee of the present application.

By way of specifically illustrating the present invention and its effectiveness, reference is made to a test conducted on glass test pieces approximately ofan inch square by 50 mils thick and prepared from a glass composed essentially of 79.5 %'SiO 10% Li O, K 0,

4% A1 0 and 1% ZnO and containing minor amounts of Ag, Au and Ce0 as photosensitizing agents and Sb O as a firing agent. These glass test pieces were given the standard thermal treatment which precedes etching of this of each type was then placed in a special electron bombardment apparatus, designed to permit collection and analysis of gas evolved as well as a record of cathode operation, and was subjected to electron bombardment under cathode ray tube operating conditions of 25 kilovolts. In the course of this test it was observed that, when a test piece which had not been fired in hydrogen was being bombarded, there was a steady evolution of gas, including a substantial proportion of oxygen, and a corresponding steady diminishing of cathode effectiveness. In contrast to this, no appreciable evolution of oxygen was observed during bombardment of a test piece previously fired in hydrogen, and operation of the cathode remained normal during such bombardment, thus illustrating the effectiveness of hydrogen treatment on this glass.

The commercial significance of these results was confirmed in a subsequent actual tube test. In this test a cathode ray tube, having mounted therein a chemically perforated glass mask fired in a hydrogen containing atmosphere, was still successfully operating at the conclusion of an arbitrary thousand-hour test period, whereas similar tubes containing identical glass masks, except for the hydrogen treatment, fail at intervals from a bundred to five hundred hours with an average life around three hundred hours.

While the invention has been described essentially with relation to what is currently known as a shadow mask, it will be appreciated that it is not restricted to a mask having the specific size and pattern of perforations in such mask, but is equally applicable to other types of perforate masks as well regardless of perforation pattern 7 or size.

What is claimed is:

1. A mask adapted to absorb electrons in a cathode ray tube and having a plurality of perforations extending therethrough which transmit electrons to form a corresponding plurality of electron beams, said mask consisting of a thin'sheet of perforated glass having a surface incapable of evolving cathode poisoning gas upon being subjected to electron bombardment.

2. In a cathode raytube assembly including an image reproducing screen, an electron emission means, and a perforate mask interposed between the emission means and screen to form a plurality of electron beams by selective electron absorption, a perforated glass mask having a surface incapable of evolving cathode gas upon being subjected to electron bombardment.

3. A method of treating a perforate glass mask for incorporation into a cathode ray tube which includes firing such mask in a reducing atmosphere composed in part at least of hydrogen and forming a surface on said glass which is capable of absorbing electrons without appreciable evolution of cathode poisoning gas.

4. The method of claim 3 wherein the reducing atmosphere contains at least 25% hydrogen.

5. In the production of a glass mask adapted to absorb electrons in a cathode ray tube, the method which comprises treating the mask in a hydrogen-containing atmosphere at an elevated temperature and forming thereon a surface capable of absorbing electrons without appreciable evolution of cathode poisoning gas.

References Cited in the file of this patent UNITED STATES PATENTS 2,279,168 Kalischer Apr. 7, 1942 2,347,982 Bachman May 2, 1944 2,392,099 Pollard Ian. 1, 1946 2,777,084 Lafierty Jan. 8, 1957 2,805,360 McNaney Sept. 3, 1957

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