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Method of Making a Shadow Mask Verfahren Zur Herstellung Einer Schattenmaske Procédé De Fabrication D'un Masque D'ombre

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Method of Making a Shadow Mask Verfahren Zur Herstellung Einer Schattenmaske Procédé De Fabrication D'un Masque D'ombre Europäisches Patentamt (19) European Patent Office Office européen des brevets (11) EP 0 209 346 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publication and mention (51) Int. Cl.7: H01J 29/07 of the grant of the patent: 01.03.2000 Bulletin 2000/09 (21) Application number: 86305399.7 (22) Date of filing: 14.07.1986 (54) Method of making a shadow mask Verfahren zur Herstellung einer Schattenmaske Procédé de fabrication d'un masque d'ombre (84) Designated Contracting States: • Tokita, Kiyoshi DE FR GB 1-1, Shibaura 1-chome Minato-ku Tokyo (JP) • Kida, Kaneharu (30) Priority: 17.07.1985 JP 15598185 1-1, Shibaura 1-chome Minato-ku Tokyo (JP) (43) Date of publication of application: (74) Representative: 21.01.1987 Bulletin 1987/04 Kirk, Geoffrey Thomas et al BATCHELLOR, KIRK & CO. (73) Proprietor: 102-108 Clerkenwell Road KABUSHIKI KAISHA TOSHIBA London EC1M 5SA (GB) Kawasaki-shi, Kanagawa-ken 210-8572 (JP) (56) References cited: (72) Inventors: EP-A- 0 139 379 EP-A- 0 156 427 • Koike, Norio GB-A- 2 080 612 US-A- 3 760 214 1-1, Shibaura 1-chome Minato-ku Tokyo (JP) US-A- 3 878 428 US-A- 4 558 252 • Matsuda, Hidemi 1-1, Shibaura 1-chome Minato-ku Tokyo (JP) • The New Encyclopaedia Britannica, 15th edition, vol.7, page 218 ("lead") Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). EP 0 209 346 B1 Printed by Xerox (UK) Business Services 2.16.7/3.6 EP 0 209 346 B1 Description [0001] This invention relates to a method for making a shadow mask for colour cathode ray tubes. [0002] In general, a shadow mask type colour cathode ray tube comprises an electron gun in the tube emitting three 5 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 pre- cisely 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, 10 the rest of the electrons impinge on the shadow mask and their energy is converted into heat energy, raising the tem- perature 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 15 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 20 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 par- ticularly 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 25 regions occur on the shadow mask, causing local doming. [0003] 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 30 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 35 electrodes, impairing the withstand voltage characteristic, and so forth, so that the quality of the colour cathode ray tube is markedly reduced. [0004] 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 temper- 40 ature, 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 con- 45 trast and lowering colour impurity. [0005] 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. 50 [0006] 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 decreas- ing elastic reflection of the electron beams at the shadow mask surface and controlling expansion from shadow mask heat evolution produced by the electron beams. [0007] According to the present invention, a method of manufacturing a shadow mask for a colour cathode ray tube 55 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, zirco- nium 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 2 EP 0 209 346 B1 layer comprising amorphous metal oxide or hydroxide or a mixture thereof and the filler. [0008] Any desired alkoxide, such as methoxide M(OCH3)n (where N means a metal), ethoxide M(OC2H5)n, n-pro- poxide M(O.n-C3H7)n, or isopropoxide M(O.iso-C3H7)n, butoxide M(O.n-C4H9)n, or isobutoxide M(O.iso-C4H9)n may be used. Those which are readily soluble at ordinary temperature in water-soluble low alcohols, such as methanol, ethanol 5 or propanol, are easiest to handle industrially. [0009] 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 10 mask. [0010] 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; 15 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 character- 20 istics 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.
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