US3808154A

US3808154A - Glass used for face panels of color television picture tubes

Info

Publication number: US3808154A
Application number: US00199377A
Authority: US
Inventors: T Omori
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1970-11-16
Filing date: 1971-11-16
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30

Abstract

ALKALI METAL-SILICATE-GLASS COMPOSITIONS CONTAINING zNO AND/OR WO3 IN COMBINATION WITH BAO WILL PREVENT THE TRANSIMISSION OF X-RAYS AND DISCOLORATION IN COLOR TELEVISION PICTURE TUBE FACE PANELS. THE AMOUNT OF THESE CONSTITUENTS IN THE GLASS SHOULD BE BETWEEN 15% TO 25% BY WEIGHT.

Description

United States Patent US. Cl. 252-478 6 Claims ABSTRACT OF THE DISCLOSURE Alkali metal-silicate-glass compositions containing ZnO and/or W0 in combination with BaO' will prevent the transmission of X-rays and discoloration in color television picture tube face panels. The amount of these constituents in the glass should be between 15% to 25% by weight.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a novel glass composition and more particularly to a glass composition which is characterized by a low degree of X-ray transmission and which has a very low tendency toward discoloration when subjected to X-ray radiation or electron bombardment.

Description of prior art It is well known that X-rays and electrons are frequently emitted from the face of television picture tubes during operation, and these emissions can be harmful to the human body. With the advent of color picture tubes and the advent of larger diameter tubes, higher operating voltages have been necessitated and the quantity and intensity of the X-rays and electrons being emitted have increased considerably. In order to protect the television viewer from the ill effects of such emissions, it is now conventional to insert a protective transparent face plate between the viewer and the tube in order to try to absorb as much of these harmful emissions as possible.

One difficulty which has been encountered, however, is that when conventional glasses have been used for forming these face plates, they tend to discolor and give a brownish tinge to the picture, upon X-ray and electron bombardment. While the actual discoloration mechanism which occurs when electrons are impinged upon the glass is somewhat different from that caused by the effects of X-ray emission, the end result is the same, i.e., loss of picture brightness and disruption of color balance. Moreover, the higher the voltage, or the higher the intensity of the emission, the greater will be the degree of discoloration of the protective plates. Color tubes which usually require high voltages for operation, therefore, will characteristically cause greater face plate discoloration than will the lower voltages used in black and white tubes.

Another difliculty with conventional glasses for use as television face plates, is that although some will protect the viewer from X-ray emissions, they have little or no effect in protecting against higher energy electron bombardment.

A need exists, therefore, for a protective face plate which will not readily discolor upon electron or X-ray impingement, and which will have a higher degree of absorptive capacity for such emissions.

PbO containing glasses have been used for this purpose because of their excellent electrical characteristics and high X-ray absorbing capacity. However, lead glass is 3,808,154 Patented Apr. 30, 1974 quite likely to become objectionably discolored, by either electron bombardment or by X-ray irradiation. It has, therefore, been proposed to add cerium oxide to the glass composition. While this expedient has reduced the degree of discoloration caused. by X-ray irradiation, it has been found to have little or no effect in reducing discoloration due to electron bombardment.

Another prior art suggestion has been to substitute some of the PhD with BaO. Although glasses of this type have succeeded in reducing discoloration and have proven to be characterized by good absorptive characteristics for reducing the degree of transmission of X-rays, and electrons, it only displays these advantageous characteristics at lower voltages as are used in operating black and white picture tubes.

Still another expedient suggested in the prior art is the use of Ce0 which while being quite effective, is quite high in cost, and hence is not economically satisfactory.

X-rays originate from a target being bombarded by means of high velocity electrons emitted from a cathode and accelerated at high voltage. Such X-rays have a white radiation spectrum and a characteristic radiation spectrum characterized in the material itself used as the target.

The wave length of the shortest wavelength A shown by an angstrom unit (A.), can he expressed as follows:

where V is the acceleration voltage, the wavelength of the maximum radiation A is shown in angstrom units (A.) in the equation:

where A is the shortest wave length mentioned above.

The following table shows the wave-length A and A being calculated in the range of 20 kv. to 40 kv. of the acceleration voltages.

Acceleration The applied voltage of the usual color television tube is generally in the order of about 25 kv. Accordingly, the A is about 0.75 A. (from the table). The glasses used for the face panels have been composed mainly of oxides which will absorb most of the X-rays having a wavelength in the order of 0.75 A.

On the contrary, the applied voltage of one recently developedwolor tube is up to 40 kv., in an emergency,

so that. the wavelength of X-rays irradiated must be estimated at about 0.3 A., (in order to provide an adequate reserve). A need exists, therefore,for a face panel absorbing glass which will absorb such short wavelengths.

SUMMARY OF THE INVENTION Accordingly, it is one object of the present invention to provide a glass which can be used for the face panels of television picture tubes, and particularly those operating at higher voltages, such as in color television tubes.

It is another object of this invention to provide a glass which has a high degree of absorptive capability for X-ray radiation and electron bombardment, and which will not deleteriously discolor upon exposure to either, or'both, types of emissions.

These and other objects have now been attained by providing a glass containing ZnO and/ or W in combination with -Ba0. This type of glass has been found to be especially effective for face panel applications for both black and white and color type picture tubes.

DETAILED DESCRIPTION OF THE INVENTION It has now been found that the substitution of a portion of Si0 and/or BaO, in barium glass, with either ZnO and/or W0 will result in low X-ray transmission for X-rays having wavelengths of between 0.3 A. to 0.9 A., and will not discolor when bombarded by unusually highly accelerated electrons. Moreover, the addition of ZnO will enhance the chemical durability of the glass. It has also been found that the addition of a small quantity of CaO and TiO to the glass will further enhance the resistance of the glass to discoloration caused by either X-ray irradiation or electron bombardment.

Although the larger the content of BaO, ZnO and/or W0 the more superior will be the absorptive capability of the glass, so far as X-rays are concerned, too large an amount can deleteriously affect such other characteristics as transition point, expansion coeflicient, or the melting or firing properties of the glass.

Accordingly, it is desirable that the glasses have the compositions Other optional oxides may be included, such as Li O, CeO TiO As O SbO and F, so long as the total sum of such optional oxides does not exceed 5% and the total sum of BaO, ZnO and/or W0 is between and 25%.

(1) When the total sum of BaO, ZnO and/or W0 is in a weight percent of less than 15%, the X-ray absorbing capability of the glass will not be sufiicient to inhibit the transmission of X-rays. When the content is more than 25%, other desirable characteristics of the glass will be adversely affected. The capability of the glass for preventing X-ray transmission and discoloration will be adequate for picture tubes operated at a maximum of 40 kv., when the glass contains 15 to 25% of the total sum of the three components.

(2) The ZnO component must be contained in amounts of more than 4% in order to provide sufiicient capability 4 to prevent the transmission of X-rays and to prevent discoloration. More than 15%, however, will adversely affect the melting and firing properties of the glass, and/ or its other desirable qualities.

(3) W0 has a high X-ray absorbing capability, which is similar to PhD, but it is diflicult to contain more than 5% due to its easy sublimability during fusing. It is advisable, therefore, to use CaWO as the raw material.

(4) The strain point of the glass will be too high if it contains more than S102, so that processing will be diflicult. If the glass contains 50% or less SiO the expansion coefficient will be so high that there is a risk that the glass could break when sealed to the cone of a television tube.

(5) The addition of 6% or more A1 0 to the glass composition may cause the generation of stride or cord. If the glass contains 0.5% or less A1 0 it is apt to cause devitrificaiton.

(6) If the glass contains Na O or K 0 in an amount of more than the aforementioned range, the expansion coefficient will be too high, and the electrical resistivity and chemical durability characteristics of the glass may be injured. Less than the specified amount may increase the strain point and also make it difficult to fuse the glass.

(7) It is desirable to use CaO or MgO in order to control the strain point. The use of 6% or more CaO, however, is apt to cause devitrification, and the addition of 3% or more MgO will tend to worsen the melting and fusing properties of the glass.

It is desirable that the transition point of the glass be between 470 C. to 510 C. for use as a face panel. When the temperature is below this range, face panel deformation during frit sealing of the glass to the tube cone can occur. If the temperature is higher than this range, the strength of the seal between the face plate and the tube may be quite poor.

It is desirable that the total sum of the components S102, A1203, N320, K20, CaO, BaO, Z110, W03, be at least of the glass, and the sum of all other components, such as LiO CeO TiO AS203, Sb O or F be 5% or less.

Either Sb O or As O may be added to the glass to improve its firing characteristics, and Li O and F may be added to enhance fusibility. It is preferred, however, that any Sb O AS203 or Li O be less than 1%, and any F be less than 2%.

The following Table 1 represents the composition of nine examples of glasses of this invention. These glasses were compared with certain glasses of the prior art. The quantity of metal oxide is indicated in weight percent; linear expansion coefiicient is shown as the mean value calculated within the range of C. to 300 C.; the softening point is defined as the temperature at which the viscosity of the molten glass is 4.5 X10 poises.

TABLE 1 Sample number 1 2 3 4 5 6 7 8 9 10 Elements:

Expansion coefficient (X10 cm./cm./ C.) 102 102 101 100 102 101 100 102 101 102 Trans tion ppmt C.) 490 480 490 500 4 490 500 490 500 480 softenlng point C.) 600 670 680 690 682 685 690 68 693 676 Absorption coefficient at wavelength of 0.6 A 16. 40 19. 96 21. 68 23. 42 25. 18 21. 20 23. 50 20. 80 22. 10 22.30

Glass No. 1 is a prior art glass which contains no ZnO or W Glasses 2 through 10 are glasses falling within the scope of this invention.

X-ray transmission coefficient is calculated using the absorbing coeflicient in Table 1, by Lamberts law:

T=I I 6" where,

T is X-ray transmission coeflicient,

I is intensity of incident X-rays,

I is intensity of transmitted X-rays,

t is the thickness of a glass specimen, and )t is an X-ray absorbing coefiicient.

It can clearly be seen from the equation that the X-ray absorbing capability increases as the X-ray coefficient is increased.

As mentioned above, the glasses according to this invention have excellent X-ray absorbing capabilities, yet they retain all of the desirable characteristics necessary for adaptation of the glass for use in face panels for color television tubes. None of the prior art glasses have been shown to have as high a capacity as those glasses of the present invention.

The use of the present glass in the formation of television tube face panels enables a brighter picture, low X-ray emission, good color balance and long operation durability.

Moreover, because of the excellent melting, firing and moldability properties of the glass, it is easy to mass-produce this glass without defects, by ordinary conventional methods and apparatus.

When this glass is used for face plates for television tubes, it has been found to be quite desirable. For one, it is low in weight, and does not contain large quantities of CeO For example, it has a high degree of absorptive capabilities for absorbing X-ray radiation and electron bombardment, It also will not seriously discolor under either of these emissions.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention.

What is claimed as new and desired to be secured by letters patent is:

1. A glass composition consisting essentially by weight of:

6 wherein the total amount of BaO, ZnO, and W0 is from 15-25%.

2. The glass composition of claim 1, which further contains other components in amounts of less than 5% by weight selected from the group consisting of Li O,

5 C502, Tiog, AS203, 513 0 and F.

3. The glass composition of claim 1, wherein the transition points is from 470 C. to 510 C.

4. The glass composition of claim 2, wherein the amount of Sb O As O and Li O is less than 1% and the amount of F is less than 2% 5. The glass composition of claim 2, consisting essentially of:

Percent SiO 52.0 A1 0 3.6 N320 K 0 7.4 Li -O 0.5

Geo 3.7 BaO 13.5

ZnO 9.5 C602 0.3 Ti0 0.5

AS203 0.3 Sb O 0.4

6. The glass composition of claim 2, consisting essentially of:

Percent Si0 57.2 A1 0 3.1 Na 0 8.0 K 0 6.0

CaO 2.0

MgO 1.0 BaO 14.

ZnO 5.0 W0 1.0

CeO 0.3 Ti0 0.5 AS203 0.3 Sb O 0.4

References Cited UNITED STATES PATENTS 3,149,234 9/1964 Hood et a1. 252478 X 3,369,961 2/1968 Dalton et a1. 252478 X 3,464,932 9/1969' Connelly et a1. 252478 3,619,219 11/1971 Bromer et al 10647 R 3,627,549 12/1971 Grouw 10652 STEPHEN L. LECHERT, JR., Primary Examiner US. Cl. XJR. 106-52