US2241215A - Luminescent screen - Google Patents

Description

May 6, 1941- L. MjMYl-:Rs

LUMINESCENT SCREEN INVENTOR LEONARD MORRIS MYERS ATTORNEY Filed Nov. 26, 1938 Patented May 6, 1941 UN'i'r s7 LUMINESCENT SCREEN Leonard Morris Myers,

Middlesbrough, Yorkshire, England, assigner to Radio Corporation of America, a corporation of Delaware Roseberry,

Application November 26, 1938, Serial No. 242,501 In Great Britain December 1, 1937 (ci. 25o-27.5)

Claims.

. This invention relates to the method of preparing luminescent screens as well as to provide improved screens for use, for example, in cathode ray tubes suitable for use for television reproduction on large screens and more particulally to television reproducer tubes in which reproduced pictures are built up on a temperature radiating screen by which expression is meant a picture reproducing screen which is scanned by a picture signal modulated cathode ray, elemental areas on the screen being so heated by the scanning bombarding ray, as to be raised to incandescence the pictures being thus obtained in the form of light from the heated elemental areas.

This invention embodies the new and improved screen in a television cathode ray reproducer tube with a temperature radiating screen comprising a highly refractory metallic layer supporting a layer of a highly refractory heater insulator which in turn carries a layer of a highly refractory metal in powdered form such as for example, a thin layer of lamp black on which is deposited a thin layer of fine tungsten powder. Now, as will be apparent, if ionized gas atoms are present in any quantity in the tube, the picture images will be adversely afiected by these atoms bringing about a positive space charge with consequent gas focusing and it is, therefore, important that the degassing of the tube shall be as complete as possible. Lampblack is, however, extremely diilicult to degas because of the large amount of absorbed gases it contains. The usual method of degassing the anode ln an evacuated tube, is to bombard it with electrons (while pumping is proceeding) until the anode is hotter than any temperature it will reach in ordinary use. If, however, it is sought to apply this method to the above described temperature radiating screen, such large quantities of gas are emitted from the lampblack that, on bombardment at a high potential, serious cathodic sputtering occurs While, further, the anode or screen tends to disintegrate under the bombardment probably owing to oxidation of the carbon with the emitted oxygen from within its content. This is particularly noticeable in the case of the tungsten deposit which soon oxidizes during bombardment and forms the well known greenish tungstic acid.

The present invention overcomes these diiiiculties and according to the said invention, a cathode ray tube with a temperature radiating screen comprising a layer of lampblack refractory oxide or other material of poor heat conductivity and large gas content '1. characterized in that the said layer is carried upon a carrier member of tantalum or similar metal which, during degassing, has an electric current passed therethrough so that it acts as a heater heating, by conduction, the material carrier thereon.

'I'he invention is illustrated in the accompanying schematic drawing, in which Figs. 1 to 3 showin diierent views one embodiment of the invention, while Figs. 4 and 5 show elevation and plan views respectively of a modication of the embodiment of the invention shown in Figs. l-3.

Referring iirst to Figures l to 3 which illustrate one way of carrying out this invention a temperature radiating screen is made of composite construction as follows: A tantalum carrier sheet is roughened in potassium hydroxide (KOH) and then slitted with a plurality of slits which start from two opposite edges of the sheet, each slit starting from one edge but stopping short of the other. In this way a series, zigzag electrical path is created in the sheet this path consisting of a series of strips which are side by side and lie in a plane. The slitted sheet is represented in Fig. 1 in which the carrier sheet as a whole is designated CS and the slits are designated CSI the strips between the slits being marked CS2. The two extreme end strips are left as shown at CS3 to extend well beyond the rest of the opposite edges to facilitate electrical connection. Two mica plates MPI and MP2 each with a large central rectangular window, are placed one each side of the tantalum carrier CS the edges of these plates corresponding with the general edges of the tantalum carrier. Two nickel plates NPI, NP! shaped like the mica plates each with a rectangular window and of the same surface dimensions are placed outwardly of the mica discs. Bolt or rivet holes are provided in the nickel, mica, and tantalum members and these members are then clamped up in a pile by bolts B or rivets through these holes, the tantalum (CS) being between two micas (MPI and MP2) and each mica being between the-tantalum (CS) and one nickel (NPI or NPZ). 'Ihe bolts B or rivets should be insulated. The result is a frame in which the tantalum will then be seen through the rectangular windows, the extended strip ends CS3 of the tantalum projecting well beyond the micas and nickels. The tantalum in the windows is then carbonized on both sides with lampblack by holding the frame kin a suitable flamepreferably a controlled benzine iiame since this is found to give very pure lampblack. After thus coating with lampblack, tungsten powder is then shaken on through a muslin sieve. In place of using tungsten powder, a powder of other similar material-for vexample tantalum powdercould be deposited.

The frame, thus prepared, is then mounted in the tube in which it is to serve as screen and the tube then put on the pump and, as in the normal present day practice, subjected to a preliminary baking while on the pump in order to remove water vapor from the glass envelope. While the oven isstill hot and the tube is on the pump. a heating cu'rrent is passed through the tantalum strip, this current being gradually increased until the maximum tantalum temperature-this should not exceed about 2,000 K. at which the lampblack will begin to evaporate-is reached. 'I'he tungsten or other surface of the screenwill now glow duily owing to the high temperature gradient in the lampblack and if this procedure is continued the surface temperature `will gradually increase until there is about.1,000 K. difference between it and the tantalum temperature. Maintenance of this condition for a few hours while under the pump will cause the lampblack to lose a great deal of its gas. Electron bombardment is then commenced, care being taken to ensure that the temperature ofthe lampblack does not exceed about 2200 K. as otherwise, owing to high vapor pressure. the carbon will rapidly evaporate. The final stage inthe degassing is anode bombardment during which the screen surface is raised up to and nally beyond, maximum working temperature. Thisfinal stage will not now present any serious diiiiculties because, owing to the preliminary degassing operations, most of the gas has already been removed and there is therefore, no fear of deleterious cathode sputtering. 'I'his final stage, should, however, be accomplished with the tantalum strip carrier at its highest temperature in order that gas shall not be driven into the body of the screen.

The thickness of the illms should be chosen with care and are somewhat critical. If the thicknesses are too great. there will be insufficient heat from the tantalum reaching the outer surfaces; if they are too small. heat developed on the outermost surfaces by electron bombardment when the tube is in use will spread too easily to the tantalum and', by conduction therein, degrade the picture definition. A lampblack deposit of 0.25 mm. (or somewhat less) thick and a tungsten layer just thick enough to cover the lampblack are recommended.

The object of the lampblack is, of course, to keep the heat developed by electron bombardment when the tube is in use, in the tungsten or other surface material in which it is developed. In place of lampblack other vsuitable material, such as a refractory oxide, may be used.

' Suitable refractory oxides must possess .high

vention. In these figures MC' is a molybdenumcarrier on which is a layer of tungsten powder TP and which is mounted in close juxtaposition to a tungsten heater wire TW coated with aluminum silicate. This heater wire is between the Lcarrier MC and a molybdenum sheet MS which is in turn carried by a support SP. Here the heating current is passed through the tungsten heater wire to facilitate the degassing operation.

I claim:

i. A. luminescent screen comprising a thin planar sheet of high melting point metal, a layer of heat-insulating material deposited upon laid planar sheet, said material having high melting point and low vapor pressure, and a layer of highly refractory powdered metal supported by said heat insulating material.

2. A luminescent screen comprising an electrical heating element, a layer of highly refractory heat insulating powder upon said element,

and a planar layer'of highly refractory metallic powder supported upon' said heat insulating' powder.

3. In. the process of preparing a luminescent screen' wherein is provided an electrical heating element, 4the steps of carbonizing the heating element to provide a highly refractory heat insulating layer thereon and depositing a highly refractory metallic powder upon the carboniaed element.

4. In the process of preparing a luminescent screen wherein is provided an electrical heating element, the steps of carbonizing the heating element, depositing a highly refractory metallic powder upon the carbonized element, placing the carbonized element with its deposited powder within an envelope, evacuating the envelope, simultaneously passing current through the 'element to degas the carbonized element, and sub sequently electronically bombarding the deposited powder to complete the degassing of the carbonizetl element. 5. In the process of preparing a luminescent screen, the steps of forming a planar electrical heating element coating the formed element with a layer of refractory heat-insulating material, and depositing a substantially planar layer of refractory metallic powder upon the layer of said material.

6. In the process of preparing a luminescent screen wherein is provided an electrical heating element, the steps of depositing a layer of refrac- 'tory heat-insulating material up'on the element. depositing a layer of refractory metallic powder upon the deposited layer of said material, placing the element with said deposited layer within an envelope, evacuating said envelope, simultaneously electrically heating said element and deposited layers, and subsequently electronically bombarding the deposited metallic layer to elevate the temperature of said deposited material to just below the temperature at which evaporation of said material takes place.

'1. A luminescent screen comprising a highly refractory metal heater element, a layer of carbon on said element and a layer of highly refractory metallic powder supported upon said layer of carbon.

8. A, luminescent screen comprising a planar tantalum sheet, a layer of carbon supported upon said tantalum sheet, and a layer of tantalum powder supported upon said layer of carbon.

9. A luminescent screen comprising a planar tantalum sheet. a'layer of carbon supported upon said-tantalum sheetv and a layer of finely divided tungsten powder supported upon said layer of carbon. I

10. A luminescent screen comprising a highly refractory metal heater element, a layer of highly refractory oxide on said heater element, and a substantially planar' layer of finely divided highly refractory powder supported upon said layer of refractory oxide.

' LEONARD Monats urnas.

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