US2660686A

US2660686A - Fluorescent screen

Info

Publication number: US2660686A
Application number: US34060A
Authority: US
Inventors: Charles W Putnam
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1948-06-19
Filing date: 1948-06-19
Publication date: 1953-11-24
Anticipated expiration: 1970-11-24
Also published as: GB669584A; BE515306A; BE515307A

Description

Nov. 24, 1953 c. w. PUTNAM 2,660,686

FLUORESCENT SCREEN Filed June 19, 1948 I I 5/? I 1 .5

WITNESSES: INVENTOR 4%! Charles W Putnam iatcnted Nev. 24,1953

illilTED STATES PATENT OFFlCE FLUDRESCEN'E SCREEN Application June 19, 1948, Serial No. 34,066

13 Claims. l

My invention relates to radiation-emitting screens and, in particular, it relates to a process for making light-emitting screens which produce a visible replica of the intensity distribution in an X-ray field. One use for my invention is in electr cal discharge tubes in which electron images corresponding to a light image generated on a fluorescent screen by X-rays are first produced, and then a second light image which is an intensified replica of the initial light image is produced by incidence of the electron ima e on a second fluorescent screen. Certain subject matter herein disclosed is being claimed in my application Serial No. 146,373, filed February 25, 1950, for a Fluorescent Screen.

One example of an electrical discharge device this type is shown and claimed in Lloyd P. Hunter and Richard Longini ?atent 2,555,5 l5 for an Image intensifier, issued June 5, 1951. In the device just mentioned, an electron image is produced which is a replica of an X-ray picture on a fluorescent screen; the electrons are then accelerated to high velocity and into incidence upon a screen comprising a cathode ray phosphor, and the light image produced on the latter is observed. In the electrical discharge tube just described, the photo-electric surface comprises materials such as cesium which. are active chemically and hav a vapor of a substantial pressure and which likely to react chemically with the components in the fluorescent screen during processing. It is, accordingly, necessary to provide some protective coating for the fluorescent material which is capable of preventing this reaction.

One object of my invention is, accordingly, to provide a novel type of fluorescent screen in which the fluorescent material is provided with a coating which is substantially transparent to incident light and is capable of preventing chemical reaction between the phosphor of the screen and cesium vapor.

Another object of my invention is to provide a protective coating for the particles comprised in a fluorescent screen which shall prevent chemical reaction between them and vapors present in the space around them.

Still another object of my invention is to provide an improved form of fluorescent screen in which chemical reaction between the fluorescent materials and an adjacent layer of chemically active material is prevented.

A still more specific object of my invention is to provide a method of protecting zinc sulphide from chemical reaction with photo-electrically active materials.

A still further object of my invention is to provide a method of preventin chemical reaction between fluorescent zinc sulphide and th components of cesiated antimony located in a common container therewith.

Still another object 01" my invention is to provide a fluorescent screen which is much less difficult to outgas than are prior art screens in which the individual fine particles of fluorescent material are exposed to the atmosphere of an evacuated container, and which contains a minimum of adsorbed vapors likely to gradually exude into such atmosphere during operative life of the tube.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawing, in which:

Figure 1 shows a view in longitudinal section of an electrical discharge tube containing a fluorescent screen protected in accordance with the principles of my invention;

Fig. 2 is an enlarged view in section of the screen forming a wall portion of 1; and

Fig. 3 is a longitudinal section of a container in which the plate 5 is placed at an intermediate point in the fabrication.

Referring in detail to the drawing, the fluorecent screen, in accordance with my invention, comprises a layer 5A made up of particles of a fluorescent material or phosphor, such, for example, as zinc sulphide or silver-activated zinc sulphide, which is embedded in glass plate 5 positioned within a container 4 of glass or other suitable material. The container i may be a vacuum-tight enclosure, such as the tube l in the above-mentioned Hunter and Longini application. As is illustrated in said application, the enclosure t may contain during the process of its manufacture a substantial pressure of the vapor or some photo-electric material, such, for example, as cesium used to produce a layer 8 of cesiated antimony deposited in close contiguity to the layer 5A. There is also located on another portion of the wall 6 of the container 4, a layer 9 of an electron phosphor such as zinc sulphide, which may be provided with chemical protection such as is described and claimed in the Patent 2,586,304 of J. W. Coltman and R. L. Longini for Protection of Electron Phosphors, issued February 19, 1952. As has been stated above, there is a likelihood of chemical reaction between the fluorescent material of the layer 5A and photo-electric material just mentioned, and

to prevent such undesired chemical reaction, the fluorescent layer 5A is embedded in the glass of the plate 5 by a procedure which will now be described.

Referring to the drawing, the final desired product is a glass plate 5 with a layer 5A of fluorescent material or phosphor sunk into the upper surface until all the phosphor particles are completely enclosed in the glass. The method of achieving this result is as follows:

A glass must be used for the plate 5 which will not be detrimental to the phosphor. The glass must not dissolve the phosphor, and it must not contain substances that will react with or unfavorably activate the phosphor. For example, the phosphor to be used maybe a silver-activated zinc sulphide which fluoresces blue under excitation. This material as readily purchasa ble upon the market is notably subject to adverse activation by very minute quantities of impurities.

The glass for plate 5- is up from chemicals containing impurities less than 0.001%. The glass contains the following components: K20, ZnO, B203. To give one instance, I have found a mix of 31 percent by weight of potassium oxide (K20) with 69 percent by weight of boric oxide (B203) to be a satisfactory one. However, I found mixes within the range of O to 40 percent K20, to 60 percent ZnO and 100 to 40 percent B203 to be satisfactory for many purposes. The constituents were fused in a platinum crucible at 1000 C. and poured into an aluminum mold. Besides their purity, these glasses are relatively low-melting, and the compositions mentioned above had a considerable range of thermal expansion coefficients.

The glass melt is evacuated in the molten state to remove all occluded gases, formed into a thin plate, annealed, and cooled. The layer A of fluorescent material is settled onto one surface of this plate from a solution containing a suitable binder.

Although a number of binders, such as borates, boro-phosphates or silicates of the alkali metals could probably be used, the one developed for this process has certain desirable features. The solution used was approximately 5% by weight potassium borate in water. The potassium borate was roughly of the formula: K2O.3B2O3 or KB305 and very pure. tion is rapidly dried, it first forms an elastic him. which. on further drying becomes hard. Upon heating to fusion, this material forms a glass which will not devitrify under any heat treatment anticipated in this process, and it is a glass which is readily miscible with the glass used for the backing of the phosphor.

A homogeneous suspension of phosphor in the 5 percent potassium borate solution is placed in a container with the glass plate in the bottom. The amount of the phosphor mixture poured into the container will, of course, determine'the thickness of the resulting layer. A layer thickness when dried of 12 mils will be found suitable for many purposes.

The glass plate with superimposed phosphor layer is then baked at approximately 125 C. to dry the binder.

The glass plate is then placed, phosphor layer up, on a support of such a substance as graphite which, when the glass is heated to a softened condition, will maintain the glass shape, and Will not stick to it. This support may have an integral heater, or it should be thin enough and of a material which will readily conduct heat.

When a thin layer of this solu- The glass and support are then placed in a container which can be evacuated. The chamber is closed and evacuated. The temperature is gradually raised while under vacuum to a point just below the softening point of the glass. The purpose of this treatment is to complete the removal of water from the binder and to outgas the phosphor and the surface of the glass. The vacuum is then broken, allowing the chamber to return to atmospheric pressure. The potassium borate binder protects the phosphor against oxidation. Zinc sulphide ordinarily oxidizes at 600 C. in air.

The temperature is then raised above the softening point of the glass (approximately 900 C'.). This temperature is maintained while the molten glass diffuses up through the phosphor layer until the upper surface of the bonded unit is a continuous, though slightly irregular, glass surface. At this point the glass should be quickly reduced to annealing temperature, annealed and cooled.

The cesiated antimony layer may be applied to the surface of the finished plate by methods known in the art. Patent 2,456,968 of R. L. Longini for a Process for Outgassing Photocells Containing Antimony, issued December 21, 1948, shows one such method.

I claim as my invention:

1. A vacuum-tight container containing a glass support having a fluorescent material completely submerged beneath its surface and a coat ing of photo-electric material on said support.

2. A vacuum-tight container containing a glass support having a layer .of zinc sulphide completely submerged beneath its surface and coated with photo-electric material.

3. A vacuum-tight container enclosing an atmosphere containing a substantial pressure of cesium during manufacture and having a screen comprising a glass support having a layer of fluorescent material completely submerged beneath its surface.

4. A vacuumtight container enclosing an atmosphere containing a substantial pressure of cesium during manufacture and having a screen comprising a glass support having a layer of zinc sulphide completely submerged beneath its surface.

5. A vacuum-tight container enclosing an atmosphere containing a substantial pressure of cesium during manufacture and having a screen comprising a support consisting of 0% to 40% potassium oxide, 0% to 60% zinc oxide, and to 40% boric oxide, said support having a layer of zinc sulphide submerged within its surface.

6. A vacuum-tight container having a screen comprising a support consisting of 0% to 40% potassium oxide, 0% to 60% zinc .oxide, and 100% to 40% boric oxide, said support having a layer of zinc suiphide submerged within its surface.

'7. A vacuum-tight container enclosing an atmosphere containing a substantial pressure of cesium and having a screen comprising a support consisting of 0% to 4.0% potassium oxide, 0% to 60% zinc oxide, and 100% to 40% boric oxide, said support having a layer of zinc sulphide submerged within its surface.

8. A vacuum-tight container enclosing an atmosphere containing a substantial pressure of cesium and having a screen comprising a glass support having a layer of fluorescent material completely submerged beneath its surface.

9. A vacuum-tight container enclosing an atmosphere containing a substantial pressure of 5 cesium and having a screen comprising a glass support having a layer of zinc sulphide completely submerged beneath its surface.

10. A vacuum-tight container containing a glass support having a layer of zinc sulphide complete- 1y submerged beneath its surface and coated with cesium.

11. An image screen comprising a support consisting of to 40% potassium oxide, 0% to 60% zinc oxide, and 100% to 40% boric oxide, said support having a layer of zinc sulphide submerged within its surface.

12. An image screen comprising a support consisting of 0% to 40% potassium oxide, 0% to 60% zinc oxide, and 100% to 40% boric oxide, said support having a layer of phosphor submerged beneath its surface.

13. A vacuum-tight container enclosing an atmosphere containing a substantial pressure of a component which is chemically active toward phosphors and having a screen comprising a support consisting of 0% to 40% potassium oxide, 0% to 60% zinc oxide, and 100% to 40% boric oxide, said support having a layer of at least one of said phosphors submerged beneath its surface.

CHARLES W. PUTNAM.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Berger June Leverenz Mar. 2, Hartmann Aug. 3, Winninghoff Oct. 5, Germer July 19, Schleede Nov. 15, Fairbrother Apr. 4, Massa et all. Oct. 31, Bruche et al. Nov. 7, Langmuir Apr. 23, Steadman Aug. Bode Apr. 8, Scott et al. Apr. 7, Roberts Oct. 13, Hood et a1. Mar. 30, Hooley May 14, Szegho July 8,