US3856518A

US3856518A - Method of electrophotographically manufacturing a television screen using hygroscopic material

Info

Publication number: US3856518A
Application number: US00335942A
Authority: US
Inventors: F Strik; Soerland N Van
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1972-03-04
Filing date: 1973-02-26
Publication date: 1974-12-24
Anticipated expiration: 1991-12-24
Also published as: AR193568A1; DE2308178A1; FR2174936A1; JPS5138216B2; DE2308178B2; ES412252A1; NL7202907A; CA982388A; FR2174936B1; GB1358147A; AU5268873A; BE796240A; IT981034B; JPS48102973A

Abstract

A method of electrophotographically manufacturing a display screen of a colour television display tube. For enhancing the contrast, a light-absorbing layer is provided between the phosphor patterns. In order to prevent pollution of the previously provided phosphor patterns with the phosphor of phosphor patterns which are provided afterwards, the phosphor particles are previously provided with a layer of a hygroscopic material. The hygroscopic material is wetted and hence made conductive between the provision of two phosphor patterns. The hygroscopic material is dried and hence made non-conductive between the provision of the last phosphor pattern and the provision of the light-absorbing layer.

Description

Strik et a1.

U.S. Philips Corporation, New York, NY.

Filed: Feb. 26, 1973 Appl. No.: 335,942

Assignee:

Foreign Application Priority Data Mar. 4, 1972 Netherlands 7202907 U.S. Cl 96/1 R, 96/361, 117/335 CM. 117/335 CP Int. Cl G03g 13/22, C030 3/28 Field 01' Search 96/1 R, 1.2, 36.1; 117/33.5 CM, 33.5 CF

References Cited UNITED STATES PATENTS 6/1964 Hays et a1. 96/1 R X 1 Dec. 24, 1974 3,365,292 1/1968 Fiore et a], 96/361 3,475,169 10/1969 Lange 96/1 R 3,489,556 1/1970 Drozd 96/1 R 3,489,557 1/1970 Lange et al. 117/335 CM 3,615,459 10/1971 Kaplan 117/3315 CM 3,615,462 10/1971 Szegho et al 96/361 3,632,339 1/1972 Khan 96/361 3,745,398 7/1973 Oikawa 96/36.] X

Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or FirmFrank R. Trifari [57] ABSTRACT A method of electrophotographically manufacturing a display screen of a colour television display tube. For enhancing the contrast, a light-absorbing layer is provided between the phosphor patterns. In order to prevent pollution of the previously provided phosphor patterns with the phosphor of phosphor patterns which are provided afterwards, the phosphor particles are previously provided with a layer of a hygroscopic material. The hygroscopic material is wetted and hence made conductive between the provision of two phosphor patterns. The hygroscopic material is dried and hence made non-conductive between the provision of the last phosphor pattern and the provision of the light-absorbing layer.

1 Claim, 1 Drawing Figure METHOD OF ELECTROPI-IOTOGRAPHICALLY MANUFACTURING A TELEVISION SCREEN USING IIYGROSCOPIC MATERIAL The invention relates to a method of electrophotographically manufacturing a display screen of a colour television display tube comprising the steps of: providing a conductive layer which can be fired on a face panel of the tube and providing a photoconductive layer which can be fired on the conductive layer; providing a first pattern of phosphor regions by uniformly electrically charging the photoconductive layer, forming a latent charge image which comprises charge in the regions corresponding to the said first pattern by exposing the photoconductive layer to light thrdugh a mask, and depositing phosphor particles charged with opposite polarity from an insulating developing liquid on the said charge image; providing in an analogous manner at least one subsequent pattern of phosphor regions; providing a light absorbing layer between the phosphor regions by uniformly electrically charging the photoconductive layer with the phosphor regions, uniformly exposing to light the photoconductive layer with the phosphor regions, and depositing lightabsorbing particles charged with the same polarity from an insulating developing liquid between the charged phosphor regions; firing the face panel for removing the conductive layer and the photoconductive layer. The invention also relates to a colour television display tube manufactured by said method.

Such a method is known from the US. Pat. No. 3,475,l69. This patent describes a few variations of the electrophotographic method. In a first variation which is termed direct imaging, the charged particles are deposited on the charged regions of the charge image because the charged particles have the opposite polarity of the charge image. In a second variation which is termed reverse imaging, the charged particles are deposited between the charged regions of the charge image because the charged particles have the same polarity as the charge image. Moreover, two exposure methods are possible. In the first method, a positive light pattern is formed of the apertures in a colour selection electrode. in a second method a negative light pattern is formed of the apertures in a colour selection electrode, that is to say that each aperture in the colour selection electrode is reproduced by a shadow region on the photoconductive layer.

In a given type of colour television display tube the phosphor regions on the display screen are smaller or narrower than the electron spots. This is of great importance since in that case a much smaller part of the surface of the display screen is occupied by phosphor regions and the intermediate space can be filled with a light-absorbing material. This has for its result that the colour television display tube can provide a much greater contrast because ambient light reflected by the screen is minimized. The tolerance which is necessasry between the dimensions of the phosphor regions and the dimensions of the electron spots is produced in that the electron spots overlap the phosphor regions.

A colour television display tube of the shadow mask type in which the phosphor regions are smaller than the electron spots and are separated by regions having a light-absorbing material is known from the US. Pat. No. 3,l46,368. This patent also describes a few methods of manufacturing such a tube. These methods come down to the fact that by carefully dosing the exposure to light, phosphor regions are manufactured on the screen which are smaller than the apertures in the colour selection electrode. This method is very difficult to perform in practice, while the achieved reduction farther is also not sufficient. Another known method starts from a colour selection electrode having small provisional apertures. After using the colour selection electrode for the exposure, said apertures are made larger by after-etching or by removing a layer which was provided thereon temporarily. Of course this method is very cumbersome, which is a decisive drawback in practice.

In the US. Pat. No. 3,6 I 5,459 a method is described with which phosphor regions which are smaller than the apertures in the colour selection electrode can be manufactured on the display screen in a simple and noncritical manner. In this method, each aperture in the colour selection electrode is represented during the exposure by a shadow region on the photoconductive layer. This exposure method is very suitable for use in combination with the electrophotographic provision of the phosphor regions and the light-absorbing layer. This is actually due to the free choice between the direct and reverse imaging which one has in a electrophotographic method. The exposure method described in the US. Pat. No. 3,615,459 combined with the electrophotographic method described in the US. Pat. No. 3,475,169, however, shows the drawback that previously provided patterns of phosphor regions are slightly polluted by the phosphor of patterns provided afterwards.

It is an object of the invention to prevent pollution of previously provided patterns by the phosphor and/or the light-absorbing material of patterns provided afterwards. Another object of the invention is to provide a method of manufacturing a colour television display tube which is capable of displaying strongly saturated colours.

According to the invention, a method of the type described in the first paragraph is characterized in that, prior to mixing the phospor phosphor with the developing liquid, the phosphor particles for at least one of the patterns are provided with a layer of a hygroscopic material which is electrically conductive in the moist condition, that between the provision of two patterns of phosphor regions the hygroscopic material on the face panel is brought into a moist condition and that between the provision of the last pattern of phosphor regions and the provision of the light-absorbing layer the hygroscopic material on the face panel is brought into a dry condition.

The invention is based on investigations which have proved that the phosphor regions of already provided patterns have insufficient electrical conductivity to be discharged during the subsequent exposure which is necessary to provide a subsequent pattern. For that purpose it would be necessary for the phosphor parti cles to be made slightly conductive. The provision of a conductive layer on the phosphor particles as such, however, serves no purpose, for in that case the phosphor regions would also lose their charge during the last uniform exposure for providing the light-absorbing layer. So it must be possible for the conductive phosphor particles to be made non-conductive again. According to the invention, a hygroscopic material is used for that purpose which in itself is non-conductive but which is made conductive by the absorption of moisture and is afterwards made non-conductive by drying.

It is to be noted that the influence of possible conductivity of the phosphor regions is stated in column 12 of the already mentioned U.S. Pat. No. 3,475,169. Here, however, it is described that the patterns of phosphor regions are made conductive afterwards by means of an agent which also serves as a fixative. According to said patent this would have for its result that during the electric charge of the photoconductive layer the charge of the phosphor regions leaks away, as a result of which the light-absorbing layer could by provided by a method of direct imaging and without exposure. Apart from the fact that said method does not prove satisfactory in practice, no solution is given in said passage for the problem of the pollution of the phosphor patterns.

The invention will be described in greater detail with reference to the following example and by means of the accompanying drawing which shows a part of a display screen of a colour television display tube during the performance of the method according to the invention.

The drawing shows a face plate 1 of a colour television display tube of the shadow mask type. An organic conductive layer 2 consisting of polyvinyl piperidinium chloride is provided on the face plate 1. An organic photoconductive layer 3 consisting of polyvinyl carbazol is provided on the conductive layer 2. A colour selection electrode 4 having apertures 5 is arranged immediately in front of the face plate I at exactly the same distance as afterwards in the operating tube. A light source 6 of comparatively large dimensions throws light through the apertures 5. ln practice, a correction lens for accurate matching of the place of the light source to the deflection point of the electron beam in the operating tube is arranged between the light source 6 and the shadow mask 4. The light spots 7 on the conductive layer 3 show a half-shadow 8. As a result of this, the light spots are considerably larger than the apertures 5. Prior to the exposure to light, the photoconductive layer 3 is provided with a negative surface charge. This is carried out by exposing the layer 3 to a corona discharge of an electrode which is at a high potential relative to the conductive layer 2. This is a known method which is also described in the U.S. Pat. No. 3,475,169. In the places illuminated by the light source the photoconductive layer 3 becomes conductive as a result of which the negative surface charge leaks away to the conductive layer 2. The light source 6 is first arranged in the deflection point of the electron beam which in the operating tube is to impinge upon the discrete regions with green phosphor. The light source is then arranged in the deflection point for blue. After this, only parts of the photoconductive layer which have not been exposed to light still contain negative electric charge. After removing the shadow mask 4, a suspension which comprises positively charged red luminescing phosphor particles in an insulating liquid is sprayed against the photoconductive layer. The said liquid consists of branched hydrocarbons, for example. a mixture of octane and nonane. Before the phosphor particles were suspended with the insulating liquid, they have been covered with a thin layer of a hygroscopic material for which, for example, a mixture of polyvinyl alcohol, choline chloride and glycerin is very suitable. The red phosphor adheres to the oppositely charged unexposed places which form a pattern of dots having diameters smaller than the diameters of the apertures 5.

It is to be noted that in direct imaging, a single exposure from the deflection point for the colour to be provided will also be sufficient when a special form of the light source is used as is described in the U.S. Pat. No. 3,152,900. With such an annular light source it is also possible to perform the exposure in such manner that the unexposed parts form a pattern of dots having diameters smaller than the diameters of the apertures 5.

After drying the provided layer of red phosphor dots, the display screen is exposed to moist air for some time. As a result of this the hygroscopic material with which the phosphor grains are covered absorbs some water and the phosphor dots become conductive.

The photoconductive layer 3 with the conductive phosphor dots is then again provided with a negative surface charge and exposed to light for providing the next pattern of phosphor dots. Since the red phosphor dots are conductive, the surface charge thereof can leak away via the exposed parts of the photoconductive layer 3. The next patterns of phosphor dots are provided in an entirely analogous manner.

The display screen provided with all the patterns of phosphor dots, usually red, blue and green, is then heated in dry air as a result of which the water is expelled from the hygroscopic material around the phosphor grains. The photoconductive layer 3 is then provided with a positive surface charge. After uniform exposure of the photoconductive layer 3, said charge leaks away everywhere with the exception of the phosphor dots which are now non-conductive. The space between the phosphor dots is then filled with a lightabsorbing material. For this purpose, a suspension is sprayed against the photoconductive layer 3 which consists of positively charged particles of a black pigmentation agent, for example graphite or a black metal oxide, in an insulating liquid. The light-absorbing material is repelled by the charge of the same sign of the phosphor dots and adheres to the remaining surface between the phosphor patterns.

The

organic layers

2 and 3 are then removed by firing after which the phosphors and the light-absorbing substance immediately adhere to the glass of the display screen 1.

It will be obvious that the invention is not only suitable for manufacturing a colour television display tube of the shadow mask type but that it may also be used in manufacturing other types of display tubes. The problem of pollution of previously provided patterns with the phosphorof patterns provided afterwards actually always occurs in those cases where the direct imaging as stated in the U.S. Pat. No. 3,475,169 is used for providing the phosphor patterns and/or subsequently a light-absorbing layer between the phosphor patterns is provided with the reverse imaging.

What is claimed is:

l. A method of electrophotographically manufacturing a display screen of a color television display tube comprising a shadow mask and a face panel, said method comprising the steps of:

a. providing said face panel comprising a fireable conductive layer disposed thereon and a fireable photoconductive layer disposed on said conductive layer;

b. providing first phosphor particles comprising an outer layer of a hygroscopic material which is electrically conductive in a moist condition;

face panel by uniformly electrostatically charging said photoconductive layer and selectively discharging said photoconductive layer by exposing f. rendering said hygroscopic material on said face panel in a dry condition;

g. following step (f) providing a light-absorbing layer between said phosphor regions by uniformly elecsaid photoconductive layer through a shadow 5 trostatically charging said photoconductive layer mask, thereby forming a latent charge image which with said phosphor regions, selectively discharging comprises charges in the regions corresponding to regions of said photoconductive layer and said dethe said first pattern and then depositing on the posited phosphor regions by uniformly exposing said charge image and from an insulating developsaid photoconductive layer with the said phosphor ing liquid, first phosphor particles charged with op- 10 regions. and depositing between the charged phosposite polarity to said latent charge image;

phor regions and from an insulating developing liqd. rendering said hygroscopic material of said first uid, light-absorbing particles charged with the phosphor particles in a moist condition; same polarity as said photoconductive layer; and e. then repeating step (c) above with second phosh. firing the face panel to remove said conductive phor particles to provide second phosphor regions [5 layer and said photoconductive layer at said face panel;

Claims

1. A METHOD OF ELECTROPHOTOGRAPHICALLY MANUFACTURING A DISPLAY SCREEN OF A COLOR TELEVISION DISPLAY TUBE COMPRISING A SHADOW MASK AND A FACE PANEL, SAID METHOD COMPRISING THE STEP OF: A. PROVIDING SAID FACE PANEL COMPRISING A FIREABLE CONDUCTIVE LAYER DISPOSED THEREON AND A FIREABLE PHOTOCONDUCTIVE LAYER DISPOSED ON SAID CONDUCTIVE LAYER; B. PROVIDING FIRST PHOSPHOR PARTICLES COMPRISING AN OUTER LAYER OF A HYGROSCOPIC MATERIAL WHICH IS ELECTRICALLY CONDUCTIE IN A MOIST CONDITION; C. PROVIDING A FIRST PATTERN OF PHOSPHOR REGIONS AT SAID FACE PANEL BY UNIFORMLY ELECTROSTATICALLY CHARGING SAID PHOTOCONDUCTIVE LAYER AND SELECTIVELY DISCHARGING SAID PHOTOCONDUCTIVE LAYER BY EXPOSING SAID PHOTOCONDUCTIVE LAYER THROUGH A SHODOW MASK, THEREBY FORMING A LATENT CHARGE IMAGE WHICH COMPRISES CHARGES IN THE REGIONS CORRESPONDING TO THE SAID FIRST PATTERN AND THEN DEPOSITING ON THE SAID CHARGE IMAGE AND FROM AN INSULATING DEVELOPING LIQUID, FIRST PHOSPHOR PARTICLES CHARGED WITH OPPOSITE SPONDING TO THE SAIDLATENT CHARGE IMAGE; D. RENDERING SAID HYGROSCOPIC MATERIAL OF SAID FIRST PHOSPHOR PARTICLES IN A MOIST CONDITION; E. THEN REPEATING STEP (C) ABOVE WITH SECOND PHOSPHOR PARTICLES TO PROVIDE SECOND PHOSPHOR REGIONS AT SAID FACE PANEL; F. RENDERING SAID HYGROSCOPIC MATERIAL ON SAID FACE PANEL IN A DRY CONDITION; G. FOLLOWING STEP (F) PROVIDING A LIGHT-ABSORBING LSAYER BETWEEN SAID PHOSPHOR REGIONS BY UNIFORMLY ELECTROSTATICALLY CHARGING SAID PHOTOCONDUCTIVE LAYER WITH SAID PHOSPHOR REGIONS, SELECTIVELY DISCHARGING REGIONS OF SAID PHOTOCONDUCTIVE LAYER AND SAID DEPOSITED PHOSPHOR REGIONS BY UNIFORMLY EXPOSING SAID PHOTOCONDUCTIVE LAYER WITH THE SAID PHOSPHOR REGIONS, AND DEPOSING BETWEEN THE CHARGED PHOSPHOR REGIONS AND FROM AN INSULATING DEVELOPING LIQUID, LIGHT-ABSORBING PARTICLES CHARGED WITH THE SAME POLARITY AS SAID PHOTOCONDUCTIVE LAYER, AND H. FIRING THE FACE PANEL TO REMOVE SAID CONDUCTIVE LAYER AND SAID PHOTOCONDUCTIVE LAYER.