Method of making a shadow mask

Abstract: The shadow mask of a colour cathode ray tube is arranged in the vicinity of a phosphor screen. The shadow mask is provided with through-holes that select the electron beams emitted from the electron guns. A layer comprising, as a filler, one or other of a metal, metal oxide, metal carbide, metal nitride, or a mixture of two or more of these, for example zirconium powder and, as a binder, one of an amorphous metal oxide compound, amorphous metal hydroxide compound or a mixture of these, for example silicon oxide, is formed on the surface of the shadow mask. This layer increases thermal radiation of the shadow mask, reduces electron scattering, and raises the residual emissivity.

Description: This invention relates to a method for making a shadow mask for colour cathode ray tubes. In general, a shadow mask type colour cathode ray tube comprises an electron gun in the tube emitting three electron beams, a shadow mask distributing these beams selectively by colour, and a phosphor screen emitting light in the three colours, red, green and blue, on excitation by these beams. The image formed on the screen is observed through an envelope panel. In the shadow mask there are provided a large number of apertures which correspond precisely with the phosphor pattern of the respective colour on the screen. As the effective electron beams passing through these apertures during colour cathode ray tube operation represent somewhat less than a third of the incoming beams, the rest of the electrons impinge on the shadow mask and their energy is converted into heat energy, raising the temperature of the shadow mask. In a normal operating television set, the shadow mask is thereby heated to a temperature of about 80°C. In the special colour cathode ray tubes used in the instrument panels in aircraft cockpits, the shadow mask temperature can rise to around 200°C. Most shadow masks consist of a lamina 0.1 to 0.3 mm thick, made by cold rolling, of which the main constituent is iron of thermal expansion coefficient 1.2 x 10 -5 /°C. The rigid L section mask frame that supports the shadow mask skirt is about 1mm thick, is likewise made by cold rolling, and is subjected to blackening treatment. Thermal expansion readily occurs when the shadow mask is heated. Since the shadow mask periphery is adjacent to the blackened mask frame, which has a large heat capacity, heat is transferred from the shadow mask periphery to this mask frame by radiation or conduction. This results in the temperature of the shadow mask periphery falling below the temperature at its center, producing a temperature difference between the center and periphery. This produces the "doming" phenomenon caused by relative thermal expansion taking place principally at the center. Consequently the distance between the shadow mask and phosphor screen alters, disturbing the accurate landing of the electron beams and thus impairing colour purity. This phenomenon of mislanding due to doming is particularly evident when the colour cathode ray tube has just been switched on. Also, if part of the picture is locally of high luminance and especially if such high luminance portions are stationary for some time, high electron flow density regions occur on the shadow mask, causing local doming. With regard to this doming phenomenon in colour cathode ray tubes, there have been a number of proposals aimed at promoting dispersal of heat from the center of the shadow mask. For instance, in U.S. Patent No. 2826538 ( Hunter et al.), it is proposed to facilitate shadow mask heat dispersal by providing a black layer of graphite on the shadow mask surface. Such a graphite layer in the colour cathode ray tube acts as an excellent radiator, lowering the shadow mask temperature. However, such a black graphite layer has the following drawbacks. The thermal cycle of the heating process involved in the manufacture of the colour cathode ray tube impairs the adhesion of the black layer so that, when the colour cathode ray tube is subjected to vibration, part of this layer separates and minute flakes fall off. When this happens, flakes adhering to the shadow mask cause blockage of the electron apertures, adversely affecting the characteristics of the image on the phosphor screen. Flakes adhering to the electron gun cause sparks between the electrodes, impairing the withstand voltage characteristic, and so forth, so that the quality of the colour cathode ray tube is markedly reduced. It has been proposed in EP-A-0139379 to control the doming phenomenon by thermally bonding a layer of lead borate glass to the shadow mask. The shadow mask is heated to a temperature which is at least as high as the normal operating temperature of the mask and the borate glass is applied to the mask. When the mask cools to room temperature, the layer prevents full thermal contraction and the mask retains residual tensile stress which reduces expansion of the mask on subsequent rise of temperature. A pigment such as manganese dioxide or cobalt oxide may be added to the glass. However, since this glass layer, which is bonded to the surface of the shadow mask, contains a great deal of lead (which has a very high atomic number), it is difficult to reduce the elastic reflection of the electrons impinging on the shadow mask. Electron scattering causes emission of light from undesired parts of the screen, spoiling image contrast and lowering colour impurity. It has been proposed in US 3878428 for a colour cathode ray tube to have a shadow mask with an array of apertures therein. At least one of the major surfaces of the shadow mask and the inner surface of the phosphor screen have surface means for effecting faster radiative heat transfer from the central portion of the mask than from peripheral portions of the mask. In one embodiment the screen side of the mask carries a peripheral coating of aluminium. It is an object of the present invention to provide a method of making a shadow mask for a colour cathode ray tube which mask when produced by this method will improve picture contrast and purity drift characteristics by decreasing elastic reflection of the electron beams at the shadow mask surface and controlling expansion from shadow mask heat evolution produced by the electron beams. According to the present invention, a method of manufacturing a shadow mask for a colour cathode ray tube includes the step of spraying a suspension of metal alkoxide compound and a filler on to an apertured metal plate, said metal of the metal alkoxide compound being selected from the group consisting of silicon, titanium, aluminium, zirconium and mixture thereof, and said filler being selected from the group consisting of metal, metal oxide, metal carbide, metal nitride and mixture thereof, and subjecting the suspension on the metal plate to heat treatment to convert it to a layer comprising amorphous metal oxide or hydroxide or a mixture thereof and the filler. Any desired alkoxide, such as methoxide M(OCH 3 ) n (where N means a metal), ethoxide M(OC 2 H 5 ) n , n-propoxide M(O.n-C 3 H 7 ) n , or isopropoxide M(O.iso-C 3 H 7 ) n , butoxide M(O.n-C 4 H 9 ) n , or isobutoxide M(O.iso-C 4 H 9 ) n may be used. Those which are readily soluble at ordinary temperature in water-soluble low alcohols, such as methanol, ethanol or propanol, are easiest to handle industrially. In shadow masks produced according to this invention, the rise in temperature of the shadow mask is limited since the thermal radiation coefficient of this layer is high, so heat can easily escape. Furthermore, electron scattering is reduced because the atomic number of the metal contained in the layer is low. Additionally, this layer increases the residual emission either by gas adsorption or by suppresssing gas generation, since it is finely formed on the shadow mask. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is an axial cross-sectional view of a colour cathode ray tube; Figure 2 is an enlarged perspective view showing part of the shadow mask of the colour cathode ray tube of Figure 1; Figures 3 and 4 are schematic illustrations of the reproduced picture pattern, given in explanation of the purity drift characteristics of this embodiment of the invention; Figure 5 is a schematic illustration of the reproduced picture pattern, given in explanation of the contrast characteristics of this embodiment of the invention; Figure 6 is a partial cross-sectional view showing a shadow mask produced according another embodiment on this invention; Figure 7 is a characteristic graph showing the relationship between layer thickness and amount of beam movement for a product used for comparison, for the case of the pattern shown in Fig. 4. As shown in Fig. 1, the shadow mask type colour cathode ray tube of this embodiment is provided with an evacuated envelope consisting of an essentially rectangular panel 1, a funnel 2 and a neck 3. The inside of panel 1 is coated with a phosphor screen 4 formed by a phosphor layer in the form of stripes that emit respectively red, green and blue light. In-line electron guns 6 that emit three electron beams corresponding to red, green and blue are arranged in neck 3 in line along the horizontal axis of panel 1. A shadow mask 7, wherein a large number of slot-shaped apertures are arranged in the vertical direction and a large number of vertical rows thereby are provided in the horizontal direction, is fixedly supported by a mask frame 8 at a position adjacent to and opposite phosphor screen 4. Mask frame 8 is supported within the panel by means of stud pins 10 embedded in the inside wall of the vertical edge of panel 1 by means of resilient members 9. The three in-line electron beams 5 are deflected by a deflecting device 12 provided outside funnel 2 so that they are scanned over a rectangular area corresponding to rectangular panel 1. The colour picture is reproduced by colour-selecting these beams landing on the phosphor stripe layer through the apertures of shadow mask 7. In some cases, the electron beams may not land accurately on the phosphor stripes for which they are intended, due to the effect of external magnetic fields such as the earth's magnetic field. This spoils the colour purity of the picture. To prevent this, a magnetic shield 11 of high permeability, made of high permeability metal sheet, is fastened to the inside of the funnel 2 by means of frame 8. The material of the shadow mask is for example low carbon steel sheet of thickness 0.1mm to 0.3mm whose main constituent is iron. A photo-resist film is obtained on both sides of this shadow mask by applying and then drying a photo-sensitive liquid consisting of for example alkali milk caseinate and ammonium bichromate. Next, a negative mask provided with the prescribed hole pattern is tightly stuck onto this photo-resist film and developed by exposure, so as to expose those parts of the metal surface where the through-holes are to be formed. Then through-holes having the prescribed aperture shape are formed by spraying etching liquid comprising ferric chloride onto the exposed metal surface. This shadow mask blank, in the form of a flat sheet formed with through-holes, is mounted in a prescribed outer frame. Its edges are clamped by a blank holder and die, and its main area, that is provided with the through-holes, is formed to the prescribed curved surface by a punch above and a knockout below. Its peripheral region is then bent over for example in the axial direction to provide a skirt for supporting and holding the main area of the mask. The skirt of the thus-formed shadow mask is supported and fixed in a rigid frame of for example L-shaped cross-section. Next, a film of thickness about 15 µm is applied to one side of the main area of the shadow mask, where the through-holes are provided, by spraying a suspension of for example, as in the following Example, an alkoxide of silicon and zirconia, e.g. Si(OC 2 H 5 ) + Zr(OC 4 H 9 ) 4 , containing silicon zirconate (ZrSiO 4 ) as a filler, onto the main area of the mask, which is concave towards the electron gun when it is arranged adjacent the screen. The filler is desired to be of a material containing metal component with smaller atomic number than that of lead. Example zircon (powder, mean particle diameter 0.7 micron) 500 gr alkoxide of silicon and zirconia 100 gr isopropyl alcohol 400 gr Various methods may be used to apply this suspension. The requirements which such methods must satisfy are that the suspension must be applied uniformly and the through-holes must not get blocked. Painting the suspension on using a brush, for example, is undesirable due to the risk of producing a non-uniform coating and blocking the holes. In this respect, with the spraying method, if the suspension is applied with a spraying pressure of about 3kg/cm 2 from a distance of 20cm to 30cm, a film of thickness about 15 µm as in the above Example can be formed in about 10 seconds. This is the preferred method for mass production since if there should be any foreign bodies stuck in the through-holes, they will be removed by the high pressure suspension liquid hitting the back of the mask. Thus a layer 13 as shown in Fig. 2 can be obtained by heating, in an atmosphere at 70°C or above, a shadow mask coated, on the surface facing the electron guns, with a suspension of an alkoxide compound of silicon and zirconia, containing zircon as a filler. The alkoxide compound of silicon and zirconia applied to shadow mask 7 undergoes hydrolysis due to the moisture in the air etc. in an atmosphere at 70°C or over, resulting in the formation of a film by a polycondensation reaction between the alkoxides, forming a zircon-containing mixed layer of amorphous silicon and zirconia metal oxides and metal hydroxides. Although in the above example, the suspension was heated after application, to shorten the manufacturing time, if the suspension is applied while heating to 70°C or more, the subsequent heat treatment step can be dispensed with. Also, since the alkoxide compound of silicon and zirconia has a good radiation absorption characteristic in the infra-red region, it has been found that satisfactory film formation can be achieved even at ordinary temperatures, without using an atmosphere of over 70°C, by irradiating the surface of the shadow mask with for example infra-red radiation whilst the suspension containing the alkoxide compound of silicon and zirconia is being applied. It is also possible to irradiate with infra-red radiation after applying the suspension. Once thus-completed shadow mask 7 has been assembled with the panel, the screen forming step is carried out. First of all, an azide photo-resist film is formed on the inside face of the panel, and exposed through through-holes 7a of shadow mask 7 using an ultra-high pressure mercury lamp. After developing the resist film, the graphite is applied and dried, developed using a decomposing agent, and narrow light-absorbing strips formed at prescribed positions on the inside face of the panel. Next, phosphor particles, in the form for example of a slurry to which phosphor particles for blue have been added, are applied on the inside face of the panel, onto a photoresist film consisting of ammonium dichromate and polyvinyl alcohol. Exposure and developing are then performed as above to form blue-emitting phosphor strips. Green-emitting and red-emitting phosphor strips are then successively formed in the same way to obtain the screen. When the panel has been completed by the above steps, it is bonded to the funnel using frit glass and, after exhausting and sealing, the prescribed steps are performed to obtain the colour cathode ray tube. The purity drift characteristics obtained by the inventors for 21 inch colour cathode ray tubes manufactured as above were as follows. The sample screen picture patterns used for these experiments are shown in Fig. 3 and Fig. 4. The pattern of Fig. 3 is one in which the whole screen is white, while the pattern of Fig. 4 is one in which part of the screen is white. In the Fig. 4 pattern, there are two white bands 51 of horizontal width 75mm disposed on the left and right respectively with their centers 140mm from the center of the screen, the rest of the screen being black i.e. not emitting light. The symbol x indicates the measurement points. The results of measurement of the amount by which the beams are displaced are shown in Table 1. The measurement conditions were Eb = 26.5 kV. Ik in the case of pattern (A) is 1,500 µA, and in the case of pattern (B) is 1,100 µA. Pattern (A)) Pattern (B) Comparative Example 100 100 This Invention 90 95 The comparative examples in the above Table were provided by 21 inch colour cathode ray tubes wherein, by heating at high temperature, lead borate glass was sealed and bonded in about 20 µm thickness to the surface, facing the electron guns, of shadow masks constructed as proposed in EP-A-0139379. The inventors found that the purity characteristic of colour cathode ray tubes according to this invention was better than that of the prior art colour cathode ray tubes. This was because the thermal emissivity (about 0.9) of the zircon-containing layer formed on the shadow mask is much greater than that of the prior art shadow mask, so radiation of heat from the shadow mask is promoted thereby limiting the rise in temperature of the shadow mask. Figure 7 shows the improvement of the beam displacement characteristic, in comparison with the prior art, for the pattern of Figure 4, obtained by varying the thickness of the applied layer. As can be seen from this graph, the preferred range of thickness is 1 µm to 30 µm. In this embodiment, zircon was used as the filler. However, the essence of this invention is not restricted to this, and a similar improvement in thermal emissivity and purity drift characteristic can be obtained by using dark pigments comprising other metal oxides, such as cobalt oxide, chromium oxide, iron oxide, or manganese oxide. Also carbides, such as silicon carbide, boron carbide, tungsten carbide etc. can be used as fillers with the same effect. No doubt this is because the thermal conductivity of these carbides is greater than that of the mild steel sheet, facilitating removal of heat generated in the shadow mask. Specifically, the thermal conductivity of the mild steel sheet is 0.11 cal/cm.sec°C, while that of silicon carbide is 1.0 cal/cm.sec°C, that of boron carbide is 0.65 cal/cm.sec°C, and that of tungsten carbide is 0.7 cal/cm.sec °C. Also nitrides, such as silicon nitride, boron nitride, or aluminium nitride etc. can be used as fillers with the same effect. It is believed this is because the volume resistivity of these nitrides is large (10 12 ohm-m to 10 14 ohm-m), so that when a large current flows, this layer acts as an electron-absorbing layer, becoming negatively charged. As a result, it is able to exert an electrostatic correction effect on the electron beams, which improves the purity drift characteristic. A similar effect is obtained by use of tungsten, lead, or bismuth etc. In the above embodiment the use of a compound of silicon and zirconia for the metal alkoxide compound is described. However, as in the case of the filler, the essence of the invention is not restricted to this, and alkoxide compounds of for example silicon, silicon and titanium, silicon and aluminium, titanium and zirconium etc. can be used. Next, for purposes of comparison, the contrast characteristic of a colour cathode ray tube manufactured with a shadow mask according to Japanese Patent Application No. 58-148843 referred to above but otherwise similarly to the colour cathode ray tube of this invention described above was obtained. For the purposes of the test, the picture pattern shown in Fig. 5 was reproduced. A white portion 31 of dimensions 300mm x 100mm was disposed in the middle of the top of this screen 30, the remainder 32 being black. The measurement points, referred to as nf1 and nf2, are indicated by the symbol x and are located respectively 30mm and 60mm below the center of the screen. The luminance at these points nf1 and rf2 is shown in Table 2. The measurement conditions were that the anode voltage Eb of the colour cathode ray tube was 26.5 kV, the total cathode current Ik was 500 µA, and the colour of the white colour was 9,300K + 27MPCD. rf1 rf2 Comparative Example 200 100 Embodiment 75 70 It can be seen from Table 2 that the luminance of the dark portion is reduced in this embodiment of the invention. This means that the elastic spattering of electrons is decreased. This is dependent on the atomic numbers of the Si and Zr constituents of coating layer 13 (their atomic numbers are 24 and 40 respectively) being lower than the atomic numbers 82 and 56 of the Pb and Ba of the lead borate glass of the comparison product. The residual emission percentage after subjecting a colour cathode ray tube according to this embodiment to a 3,000 hours continuous operation test was then determined. It was found that the residual emission percentage was indeed improved, being 80% of the initial value. For the prior art product, a residual emission percentage of 70% is standard. Thus this represents an improvement of better than 10%. This is interred to be because of gas adsorption by the coating layer of this embodiment. The amorphous silicon oxide (SiO 2 ) that is used as a binder appears to be particularly effective in this respect. Also it is thought that generation of gas is suppressed by the formation of a fine coating layer on the shadow mask surface. It is therefore particularly effective to form the coating on the surface of the shadow mask facing the electron guns, since this surface reaches a very high temperature when the electron beams impinge on it and so tends to generate unstable gases. Of course, formation of such a coating increases the manufacturing process, but, as shown in Fig. 6, if the whole of the shadow mask surface is covered by coating 13 according to this invention, practically all generation of unstable gases by the shadow mask can of course be suppressed. In the description of this embodiment, the suspension containing an alkoxide compound of zircon and silicon and zirconia was applied to the shadow mask before forming the phosphor screen, and a mixed zircon-containing layer of silicon and zirconia amorphous metal oxides and metal hydroxides was formed. However, if the presence of this coating layer causes a slight adverse photochemical effect in the exposure step when forming the phosphor screen, the formation of this coating can be carried out after formation of the phosphor screen. If the coating of this invention is formed on the surface of the shadow mask facing electron guns, it is not necessary to form a conductive coating. By this means, a 5 to 10% improvement in the purity drift characteristic can be obtained compared with the case where a conductive coating is formed. As described above, using a shadow mask produced according to this invention, a colour cathode ray tube can be obtained with improved contrast and purity drift characteristics, a better emission life characteristic, and which is well adapted for mass production.

Claims: A method of manufacturing a shadow mask, comprising an apertured metal plate, for a colour cathode ray tube which includes the step of spraying a suspension of metal alkoxide compound and a filler on to the apertured metal plate, said metal of the metal alkoxide compound being selected from the group consisting of silicon, titanium, aluminium, zirconium and mixture thereof, and said filler being selected from the group consisting of metal, metal oxide, metal carbide, metal nitride and mixture thereof, and subjecting the suspension on the metal plate to heat treatment to convert it to a layer comprising amorphous metal oxide or hydroxide or a mixture thereof and the filler. A method as claimed in claim 1, characterised in that said filler is selected from the group consisting of silicon carbide, manganese oxide, chromium oxide, iron oxide, cobalt oxide, copper oxide, zircon, zirconium and mixture thereof. A method as claimed in claim 1 or 2, characterised in that the thickness of said layer is 1-30 µm. A method as claimed in any preceding claim, characterised in that the apertured metal plate is heated to a temperature of 70°C to convert the suspension to said layer. A method as claimed in any preceding claim, characterised in that the layer is formed on the surface of the apertured plate which, in use, faces the electron gun of the colour cathode ray tube.

Inventors: KIDA KANEHARU, KOIKE NORIO, MATSUDA HIDEMI, TOKITA KIYOSHI, KIDA, KANEHARU, KOIKE, NORIO, MATSUDA, HIDEMI, TOKITA, KIYOSHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01J29
IPC codes: H01J29, H01J9