CA1167511A - Bistable storage target having an admixture of two phosphor particles and a layer of nonluminescent material - Google Patents

Abstract

TWO COLOR WRITE-THROUGH
DIRECT-VIEWING STORAGE TUBE

Abstract The present invention relates to a storage target for a cathode-ray tube. The target is comprised of a sub-strate of electrically insulative material and a collector supported by the substrate for collecting secondary electrons. A storage body of phosphor material is provided on one side of the substrate. The phosphor material is comprised of a substantially uniform admixture of phosphor particles, including particles of a first phosphor capable of bistable storage of charge images and particles of a second phosphor having a color emission different from that of the first phosphor. The particles of the second phosphor have a layer of nonluminescent material chemically bonded to the surfaces thereof in an amount sufficient to inhibit the luminescence of the second phosphor in response to low velocity electron bombardment.

Description

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This is a division of copending Canadian Patent Application Serial No. 356,104 which was filed on ~uly 14, 1980.
Background of the Invention It is known to peovide a direct viewing bistable storage t~be that displays write-through informat;on in a color different from that of stored information. The target for such a "color write~through" CRT has a storage dielectric of admixed phosphor materials. The admixture incl~des two phosphors having different color emission characteristics - one a storage phosphor/ the other a phosphor that emits light of a different color and that has a lower light output efficiency at low ~flood gun3 voltages. In operation, the perceived color of stored images (which are displayed by flood gun illumination alone) is mainly determined by the storage phosphor because of its greater relative efficiency at low voltages. Unstored write-through images, which are produced by high energy electrons from the writing gun, are displayed in a contrasting color resulting from the combined emissions of both phosphors.
S~ch a target could be produced using an admixture of a storage phosphor with two other phosphors having different color emission characteristics. Although this would make it possible to display write-through images in either of two different colors, the necessarily reduced percentage of storage phosphor particles would make stored image displays undesirably dim and diffic~lt to discern.
Summary of the Invention The present invention relates to cathode ray storage t~bes and more particularly to a direct-viewing bistable storage tube that displays write-through inform-ation in either of two colors, both of them different from the color o~ the stored information.
In accordance with an aspect of the invention there is provided a storage target for a cathode-ray tube, comprising: a substrate of electrically insulative material/ means supported by said substrate for collecting .~

- la -secondary electrons, and a storage body of phosphor material on one side of said substrate, said phosphor material comprising a substantially uniform admixture of phosphor particles, includiny particles of a first phosphor capable of bistable storage of charge images and particles of a second, rare earth phosphor having nonluminescent material chemically bonded to the surfaces thereof in an amount sufficient to increase the operating level of said storage body and thereby increase the brightness of stored light images~
An embodiment of the present invention can be realized by a direct-viewing storage CRT target having a storage dielectric layer o admixed phosphor material. The admixed material includes two phosphors having different color emission characteristics; one a low voltage luminescent phosphor and the other a phosphor that emits light of a color different at a higher voltage, preferably distinctly different, from that of the low voltage luminescent phosphor~ An array of collector electrodes extends through the storage dielectric layer for collecting secondary electrons that are emitted from the charge image stored by the storage phosphor and these col:Lector electrodes are formed from a conductive non-storing phosphor having a color different from the phosphors in the admixed dielectric layer.

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One of the admixed phosphors has a dead layer which eliminates low voltage lwninance and this dead layer enables the operating level of the target to be raised thereby enabling the luminance of the stored information to be increased. The collector phosphors also have a dead layer and are conductive thereby collecting secondary electrons but they do not luminesce unless excited by higher voltage electrons at which time they luminesce a different color than the admixed phosphors~
A primary object of the present invention is to provide a direct-viewing bistable storage tube in which write-through information may be displayed in two colors.
Another object of the present invention is the provision of a bistable storage target for use with a direct-viewing bistable tube which will display write-through information in two colors and stored information in a color different from the write~through inormation.
A further object of the present invention is to provide a bistable storage target that will display stored information at a h~gher luminance while simultaneously displaying nonstored information in two different colors which are different from the color of the stored information.
An additional object of the present invention is the provision of a bistable storage target that contains an array of collector electrode members which are formed from conductive phosphor particles having a dead layer surrounding each phosphor particle.
Still a further object of the present invention is to provide a bistable storage target that has an admix of two different phosphor particles that emit different colors forming the storage phosphor layer; one of the phosphor particles being storage dielectric particles to emit light of its color, the other phosphor particles having a dead layer surrounding each of the particles to inhibit low voltage luminance and to raise the operating level of the target thereby enabling the stored information to be displayed with a higher luminance.
Still an additional object of the present invention is the provision of a bistable storage target ~3~3 -- 3 ~

that can be operatcd to display ref reshed multicolor infcrmation simultaneously with stored information.
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The present invention taken in conjunction with the invention disclosed in copending Canadian Patent Application Serial No. 356 ,104 will be described in detail hereinbelow wi'ch the aid of the accompanyiny drawings, in which:
Brie_escri~t~on of the Drawing ] ,~ FIG. 1 is a diagrammatic view of a direct-viewing storage cathode ray tube in accordance with the present invention, together with associated circuitry; and FIG. 2 is a fra~mentary cross-sectional view taken along line 2-2 of FIG. 1, showing on an enlarged scale a preferred embodiment of a stora~e target in accordance with the present invention.
Detailed Description of Preferred Embodiment Referring first to FIG. 1, a direct-viewing bistable storage tube 10 includes an evacuated envelope 12 having a transparent faceplate 14 at one end.
Supported by faceplate 14 is a storage target 16 that includes a conductive target electrode 18 and a storage dielectric layer 20. Mounted in the opposite end of the tube is a writing gun 22 comprising a cathode 24, a control grid 26, and a focusing and accelerating anode structure 28 for formin~ a beam 30 of highvelocity electrons directed toward target 16. Beam 30 is deflected by signals applied to horizontal deflection plates 32 and vertical deflection plates 34 by conventional deflection circuits 36. Storage tube 10 is additionally`provided with one or more flood guns 3g for bombarding the storage dielectric uniformly with low velocity elec~rons. The cathodes of the flood guns are connected ~o a low voltage, suitably ground potential (0 volts).
A plurality of electrodes i5 disposed on the inner surface of envelope 12 intermediate flood guns 38 and target 16. These electrodes preferably are provided as spaced coatings, or bands, of a conductive material such as silver, graphite, or the like. A first wall band electrode 40 functions primarily as a focusing electrode for the flood electrons emitted by guns 38. It is connected to a suitable positive voltage, about +250 volts, for example. A
second wall band electrode 42 spaced from electrode 40 and connected to a less positive voltage) e.~., about ~150 volts, functions as a focusing and collimating electrode. A third wall band electrode 44 spaced from electrode 42 is connected to a still less positive voltage, e.g., about ~125 volts, and also functions as a focusing and collimating electrode. A fourth wall band electrode 46 is located intermediate and spaced from electrode 44 and storage target 16 Electrode 46 4 _ i~ connected to a still less positive voltage (about +75 volts) and functions as a focusing and collimating electrode, but may also act as an auxiliary collector for secondary electrons e~itted by the storage target at its periphery. As a result of the collimating action of the wall ~and electrodes, flood gun electrons are ~ubstantially uniformly distributed over the surface of target 16.
It should be noted that a conventional resistive coating 48, such as Aquadag (trade rnark), is provided on the interior of the funnel portion of envelope 12 and is electrically connected to an isolation shield (not shown) in writing gun 22. Coating 48 thus serves as an extension of the writing gun's second anode (not shown)~ The voltages applied to wall band electrodes 40, 42, 44, and 46 are suitably adjusted to provide optimum focusing and collimation of the flood gun electrons, and the specific values given herein and shown on the drawing are by way of example only.
Target electrode 18 is suitably connected to the rnidpoint of a voltage divider consisting of resistors 50 and 52. Resistor 50 is adjusted so that a potential of about +200 to +300 volts is applied So the target electrode.
The cathode of writing gun 22 is connected to a high negative D. C.
potential, suitably about -6000 volts via the movable contact of switch 23. The control grid 26 is connected to the movable contact of a double pole, double throw switch 53. In the STORE position of switch 53 and switch 23 in connection with -6KV, grid 26 is connected to a negative D. C. potential -VGI to provide a suitable grid-to-cathode reverse bias to cause writing beam 30 to bombard target16 with hi~h Yelocity electrons. ~hen struck by the writing beam, dielectric layer 20 emits secondary electrons, which are then collected by electrode 18.
The written area of the layer is driven positive by the secondary emission, and retained at a relatively positive potential after beam 30 has passed by low energy electrons emitted by flood guns 38. In this well known manner a stored char~e image is formed on the dielectric layer. In the Y/. T. or write-through mode of operation of switch 53, the control grid is connected to the output of a rectangular pulse generator 54, which applies positi~e-going voltage pulses 56 to the grid. Pulses 56 have a maximum voltage level equal to -VGI, and a minimum (more negative) voltage level sufficient to turn off the writing gun. Switching the writin~ beam off for a portion of the time it is bombarding a particular area of the target allows the charge image formed in the storage dielectric to he dischar~ed by the flood electrons. The write-through image is thus prevented frorn being stored. A more compJete description of pulsed write-through operation may he had by reference to V. S. Patent No. 3,430,093 to Winningstad.
ReferlinK now to FIG. 2, there is illustrated in cross section the storage target incorporated in storage tube 10. Target 16 includes a transparentsubstrate body in the form of faceplate l4, which is provided with a thin, conductive tin oxide film 60. A multiplicity of raised "dots" 62 of a surface-killed conductive phosphor material is distributed in a regular pattern over the exposed surface of film 60. The conductive phosphor material is preferably P-22 blue (ZnS:Ag) coated with a cobalt sulfide precipitate that is diffused into thephosphor particles by heat treatment according to the teaching in U. S. Patent Nos. 3,664,862; 3,767,459 and 3,826,679. The dots, which suitably have a generally cylindrical or conical configuration, are electrically connected to the tin oxide film. Thus, conductive film 60 and phosphor dots 62 together form collector or target electrode 18. Disposed on conductive film 60 and surroundingdots 62 is an at least semi-continuous storage dielectric layer 20. Dots 62 extend through layer 20 and have their outer ends exposed and they collect secondary electrons that are emitted from adjacent positively-charged char~e images. Due to the dead layer surrounding each of the phosphor particles of do~s 62, they will not luminesce except at -8KV or above dependin~ on the thickness of th`e dead layer.
According to the present invention, layer 20 comprises an admix-ture of phosphor particles, including parlticles 64 of a phosphor capable of bistable storage of charge Images, and particles of another phosphor 66.
Phosphor 66 is a surface-killed~phosphor and it is chosen to have a color emission different, and preferably substantially different, from that of phosphor 64 when- bombarded by hi~h energy electrons, and to have a substantially lower light output efficiency when bombarded by low energy flood gun electrons. ~le sur~ace-killed phosphQr particles 66 present in layer 2~ ~ill ra~se ~he o~erating level of the storaae target, which will increase the luminance of the stored information. Suitable phosphors meeting these criteria include the red-emitting P-22R phosphors, such as Y2O2S:Eu, Y2O3:Eu, Gd2O2S:Eu, Gd2O3:Eu and YVO4:Eu. The red phosphor particles 66 have their surfaces killed to providea dead layer of a desired thickness. One way in which the dead layer can be obtained surrounding each of the red phosphor particles of the oxysulfides 7 5 1 ~

(Gd2O25:Eu and Y2O2S:Eu) is ~o air bake the target at a temperature between 450C and 550C . See J. W. Haynes and JO J. ~rown, "Preparation and Luminance of selected Eu+3-activated rare earth-oxygen sulfur-compounds", Journal Electrochem Society, Vol. 115, p 1060 (1969). This produces an oxysulfate dead layer having a thickness depending on the time of bake at the prescribed temperature. Another way to obtain the dead layer at the surface of each of the red phosphor particles for each of the above rare earth oxides or oxysulfides is to slurry the red phosphor particles in a phosphoric acid solution at a controlled temperature between room temperature and ~0C and also controlled phosphor-to-solution concentration so that the pH of the solution should be less than 4.5. This forces a surface reaction to produce a rare earth phosphate dead layer having a thickness depending upon the foregoing variables.
See U. S. Patent Nos. 3,607,371 and 3,927,240 for phosphate coating of phosphors. Phosphor 64 suitably is a green-ernittlng storage phosphor such as P-l (Zn2SiO4:Mn).
The two types of phosphor particles are uniformly admixed, either dry or in slurry form, and deposited on conductive film 60 of target electrode 18 in a known manner, for example using the procedure outlined in U. S. Patent No.
3,956,662 to MeTeague, et al. The ratio of the two types of phosphor in the admixture may range from about 10% to about 80% by weight of phosphor 64, with the balance being phosphor 66. A preferred composition comprises about 24% by weight P-l, about 56% dead layer red phosphor (or a mixture of P-22R
phosphors) of the rare earth type mentioned above and about 20% of P-22 blue as the collector electrodes 62. A storage target provided with a dielectric layer of the preferred composition will exhibit a green display of stored charge images and an oran~e and a blue display of write-through charge images. The admixed layer contains about 3396 by wei~ht of P-l phosphor and about 67% by weight of P-22R phosphor.
1n one mode of write-through operation, switch 23 is connected with -6KV while switch 53 remains at its W.T. position. This will result in the stored information being displayed in green color, if information has been stored, and the write-through information being displayed in an orange color produced by the combination of the red and green phosphors.
In another mode of write-through operation, switch 23 is connected to -8KV while switch 53 is at its W.T. position. If stored information is displayed 1 :~67S 1 ~

in its green color, write-through information will be displayed in a blue color due to emission from phosphors 66, 64 plus dots 62. Thus, a second write through col~r is produced that is diff erent from the write-through information when switch 23 is in the -6KV position and also different from the stored information.
In a further mode of write-through operation, switch 23 is switched between -6KV and -8KY while switch 53 is at its W.T. position. If stored information is displayed in its green color, write-through information will be displayed in the orange and blue colors that are diffcrent from ~he stored information. The writing gun voltages can be less ~han -61CV or greater than -8KV depending upon instrument design.
Of course, the tube can operate in a stored mode only or in one or both color write-through modes as desired. Thus, the CRT can be operated as a refresh multicolor monitor with bistable stora~e capability.
It will be understood that color coding of write-through information is not restricted in application to the storage tube target structure exemplified herein. Other suitable target structures include those descrlbéd in U. S. PatentNos. 3,293,474 to Gibson, Jr.; 3,401,293 to Morris; 3,531,675 to Frankland; and 3,614,820 to Morris.
About 20% of each of the collector electrode members 62 can be metallic powder instead of 100% conductive phosphor to insure proper operating range of the CRT because resistance of the phosphor dot collectors may be too high thereby decreasing the collection efficiency of the secondary electrons andlowering the operating range of the CRT. If desired, about 20% of collector electrode mernbers 62 can be formed from metallic powder whereas the remainder can be conductive phosphor. Alternatively, the array of dots can be made of the admixture of phosphors 64 and 66 surrounded by a layer of conductive phosphor.
Brighter write-through informa~ion is displayed by the present target because collectors 62 also emit light at write-through voltages above -8KV. Also, by killing the surface of the red phosphor particles with a phosphate layer, this raises the operating level of the storage tar~et thereby increasing the luminance of the green phosphor particles which also compensates ;5 for the loss in storage luminance of green phosphor particles due to ~he admix of the red phosphor particles to form the phosphor layer. The killed surfaces of the conductive phosphor collector electrode members vill collect secondary elec-trons during storage operation and they will not luminesce except above a threshold voltage level.

A dual electron gun structure can also be used in place of the switched voltage sources to a single cathode if desired.
From the above it should be obvious to one of ordinary skill in the art that various changes may be made in the above-described preferred embodiment without departing from the scope of the invention as defined by the following claims.

Claims (5)

Claims:

1. A storage target for a cathode-ray tube, comprising:
a substrate of electrically insulative material, means supported by said substrate for collecting secondary electrons, and a storage body of phosphor material on one side of said substrate, said phosphor material comprising a substantially uniform admixture of phosphor particles, including particles of a first phosphor capable of bistable storage of charge images and particles of a second, rare earth phosphor having nonluminescent material chemically bonded to the surfaces thereof in an amount sufficient to increase the operating level of said storage body and thereby increase the bright-ness of stored light images.

2. The storage target of claim 1, wherein said rare earth phosphor is selected from the group consist-ing of rare earth oxide phosphors, rare earth oxy-sulfide phosphors, rare earth vanadate phosphors, and mixtures thereof.

3. The storage target of claim 1, wherein said nonluminescent layer is a phosphate material.

4. The storage target of claim 2, wherein said rare earth phosphor consists essentially of a rare earth oxysulfide phosphor and said nonluminescent layer is an oxysulfate material.

5. The storage target of claim 1, wherein said second phosphor as a color emission different from that of said first phosphor.