Color cathode ray tube

Abstract: A color cathode ray tube includes an electron gun, a fluorescent screen which emits light when it is hit by electron beams emitted from the electron gun, and a shadow mask arranged between the electron gun and the fluorescent screen and having a plurality of through holes for the electron beams. The color cathode ray tube further includes an electron beam reflecting film formed on an electron beam irradiated surface of the shadow mask using tungsten powder to which at least a metal that is not more likely to be oxidized than tungsten is added or coated. Alternatively, the powder may be coated with an oxide forming material (14, 14'), which oxide prevents oxidation of the tungsten powder during process baking.

Description: The present invention relates to a color cathode ray tube and, more particularly, to a technique for preventing a doming effect in which a shadow mask deforms because a temperature of the shadow mask is increased by thermal energy generated by collision of an electron beam emitted from an electron gun, causing color dislocation in a picture. A conventional color cathode ray tube is disclosed in, for example Japanese Patent Laid Open Gazette No. 55-76553, in which an electron beam reflecting film comprising bismuth oxide (Bi₂O₃) as a main element and binder such as water glass mixed therein is formed on an electron beam irradiated surface of a shadow mask. According to the above shadow mask, when an electron beam with high energy is irradiated on the surface of the shadow mask, doming caused by an increase of a temperature of the shadow mask can be prevented by increasing the ratio of the number of electrons elastically or inelastically scattered backward to the number of irradiated electrons, that is, the backward scattering coefficient, and thus scattering almost 30 % of the irradiated electron beam energy backward. In addition, such doming preventing technique is reported in detail in Japan Society of Electronics, Information, and Communication Engineers, "Doming prevention of CRT shadow mask by electron beam reflecting film", published on 9th February 1989. According to the conventional color cathode ray tube, an anti-doming effect obtained by the electron reflecting film formed on the shadow mask is approximately 30%, which is not sufficient for a large sized color cathode ray tube. In addition, it has been conventionally proposed that a film comprising metal tungsten as a main element is formed on a surface of the shadow mask instead of the film comprising Bi₂O₃ as a main element. When the film comprising metal tungsten as a main element is used, the anti-doming effect can be 50% in theory. As a result of ardent study by the inventors of the present invention for a long time, it is found that performance of the film can be improved by 50% or more as compared with that of the film comprising Bi₂O₃ as a main element. However, in case of the film comprising metal tungsten as a main element, since manufacturing process in which the film is baked is usually used, oxidation occurs in tungsten powder. As a result, the substantial anti-doming effect by the tungsten film is approximately 35%, which is almost the same as that of the conventional film comprising Bi₂O₃ as a main element. Reference is made to U.S. Patent No. 4 442 376 which discloses a colour display tube having a shadow mask positioned in front of a display screen. The shadow mask is coated at least on the side remote from the display screen with a heavy metal layer of selectively varied thickness. The heavy metal has an atomic number exceeding 70 and has a high electron reflection coefficient, thus minimizing energy absorbed by the shadow mask from electrons impinging on the mask. Reference is also made to "Werkstoffkunde für die Elektrotechnik und Elektronik", Hahn / Munke, Berlin 1986, page 465 which teaches that reactive refractory metals such as tungsten and molybdenum react with oxygen at relatively low temperatures and serve to prevent oxidation when used in the open. Concerns of the present invention are to solve the above problems and to provide a color cathode ray tube in which oxidation of tungsten powder is prevented even when the film is baked in the air and an improvement in the anti-doming effect. According to the invention there is provided a method of preventing doming in a colour cathode ray tube shadow mask due to irradiation thereof during use, including the steps of:- applying a mixture of tungsten particles and a binder to the surface of a shadow mask preform; and driving off said binder by baking; characterised in that said particles are coated with a material that is more readily oxidisable than tungsten, and that is active to form an oxide that shall limit or prevent further oxidation of said tungsten particles during baking. Further according to the invention the material, likely to be more readily oxidizable than tungsten, is magnesium, aluminium, a 4A group element, silicon, vanadium, manganese or molybdenum or any of these in combination. Figure 1 is a partially broken side view showing a color cathode ray tube in accordance with an embodiment of the present invention; Figure 2 is an enlarged sectional view showing a main part; Figure 3 is a graph showing a result of X-ray diffraction of a film formed in accordance with an example 1; Figure 4 is a graph showing a result of X-ray diffraction of a film formed in accordance with an example 2; Figure 5 is a graph showing a result of X-ray diffraction of a film formed in accordance with an example 3; Figure 6 is a schematic view showing (for reference purposes only) the state where tungsten is coated with metal which is less likely to be oxidized than tungsten; and Figure 7 is a schematic view showing the state where tungsten is coated according to the invention with metal which is more likely to be oxidized than tungsten. An embodiment of the present invention will be described in detail with reference to the drawings. Figure 1 is a partially broken side view showing a color cathode ray tube in accordance with an embodiment of the present invention. In Figure 1, reference numeral 1 designates an outer frame for keeping its inside highly vacuous and reference numeral 2 designates an electron gun built in a neck part la of the outer frame, which emits an electron beam toward a translucent glass panel part 1b serving as a part of the outer frame 1. Reference numeral 3 designates a shadow mask formed of a thin iron plate, in which a number of through holes 3a for electron beams are formed as shown in Figure 2. Reference numeral 4 designates a fluorescent screen on which three kinds of stripes of fluorescent materials emitting red, green and blue light are applied to an inner surface of the glass panel lb. These stripes of the fluorescent materials are arranged so as to each correspond to each of the electron beam through holes 3a in the shadow mask 3 in an electronic optical manner. According to the color cathode ray tube with above construction, an electron beam reflecting film 5 comprising tungsten having large reflection coefficient to the electron beam, as a main component is formed on the side of the electron beam irradiated surface of the shadow mask 3 as shown in Figure 2. The film may be formed of tungsten powder which are coated with 50 to 10000 ppm of one or more kinds of metals selected from a group of magnesium, aluminum, 4A group element, silicon, vanadium, or molybdenum prior to a baking process. Three electron beams emitted from the electron gun 2 are deflected so as to scan the whole surface of the fluorescent screen 4 by a deflecting apparatus (not shown) and then reach the shadow mask 3. The three electron beams pass through the electron beam through holes 3a of the shadow mask 3 and hit the stripes of the fluorescent materials on the fluorescent screen 4 to make them emit light, with the result that a color picture is projected on the fluorescent screen 4. Most of the electron beams which hit the shadow mask 3 are scattered toward the electron gun 2, that is, backward by the electron beam reflecting film 5, whereby thermal energy to be applied to the shadow mask 3 by the electron beams is reduced and then it is possible to prevent color dislocation in the picture caused by thermal deformation of the shadow mask 3. Hereinafter, a description will be given of doming examples and their results performed by the inventors by using the electron beam reflecting film 5 comprising tungsten as a main element and a metal applied thereto. The kind of the above metal is different in each example. Example 1 (for references purposes ; not falling within the scope of the claims) Tungsten powder to which silver is coated through chemical treatment (silver concentration is 6000 ppm) was sprayed on the shadow mask until this thickness became 10 µm (microns) and then baked at a temperature of 450°C for 30 minutes in the air. The thus formed film was subjected to X-ray diffraction. As a result, as shown in Figure 3, a material (WO₃) produced by oxidation is confirmed by a diffraction peak around a diffraction angle of 23.28 degrees, a product material (WO 2.9 ) is confirmed by a diffraction peak around that of 33 degrees and that of 37 degrees. Here, a diffraction peak of metal tungsten (W) was around a diffraction angle of 40 degrees. As can be seen from the result shown in figure 3, since metal tungsten sufficiently remained in the above tungsten powder, it was found that an amount of oxide (WO₃) was less than that of tungsten (W). In addition, a result of X-ray diffraction of the film formed on pure tungsten powder which was baked in the same condition as above is shown in Figure 4 for comparison. When the film was formed of pure tungsten powder, it was clear that metal tungsten (W) almost disappeared and an amount of oxide (WO₃) was overwhelmingly great. Then, the shadow mask on which the film was formed of tungsten powder coated in silver as described above was built in a 64 cm (25-inch) color CRT and then an anti-doming effect was measured. As a result, it was found that the anti-doming effect was 50%. Example 2 (for reference ; not falling within the scope of the claims) A film with a thickness of 7 µm (microns) was formed on a shadow mask surface using tungsten powder alloyed by adding palladium (Pd) to tungsten by 500 ppm and then it was baked in the air. Then, this shadow mask was built in a 74 cm (29-inch) color CRT and then an anti-doming effect was measured. As a result, it was found that the anti-doming effect was 49%. Example 3 (for reference ; not falling within the scope of the claims) Figure 5 shows a result of X-ray diffraction of a film with a thickness of 10 µm (microns) which was formed using tungsten powder alloyed by adding molybdenum (Mo) to tungsten by 100 ppm and it was baked in the same condition as in the Example 1. As can be seen from Figure 5, it was found that metal tungsten also sufficiently remained in this Example 3 and the ratio of it was almost equal to that of produced oxide (WO₃). More specifically, it was found that molybdenum (Mo) had a practically enough anti-oxidation effect even if concentration of molybdenum was not so high. The shadow mask in accordance with the Example 3 was built in a 64 cm (25-inch) color CRT and then the anti-doming effect was measured. As a result, it was found that the anti-doming effect was 48%. Example 4 A film with a thickness of 15 µm (microns) was formed on a shadow mask surface using tungsten powder coated by aluminum (A1) in concentration of 10000 ppm and then it was baked in the air. Then, this was built in the 74 cm (29-inch) color CRT and then the anti-doming effect was measured. As a result, it was found that the anti-doming effect was 48%. Figure 6 is a schematic view showing an example by way of reference in which the tungsten in the above described Examples 1 and 2 is coated with metal which is less likely to be oxidized than tungsten (not falling within the scope of the claims). Figure 6(a) shows the state where fine-grain silver 11 is coated on the tungsten 10, Figure 6(c) shows the state where uniform coating layer is formed on the surface of the tungsten 10 using fine-grain silver 11' whose grains are smaller than those of silver 11, and Figures 6(b) and 6(d) show the state of tungsten of Figures 6(a) and 6(c) after baking, respectively. In Figure 6(b), because the silver grains are a little spaced from each other at the time of baking, the surface of the tungsten 10 is slightly oxidized and becomes tungsten oxide layer 12. Adhesion between silver grains 13 is seen on the coating layer of silver 11. Furthermore, in Figure 6(d), the surface of tungsten 10 is slightly oxidized. Figure 7 is a schematic view showing the state where tungsten in the above described Examples 3 and 4 is coated according to the invention with metal which is more likely to be oxidized than tungsten. Figures 7(a) and 7(b) show the state where tungsten 10 is coated with aluminum either in the form of particles or uniformly (respectively 14, 14′) in the same way as above described embodiment, and Figure 7(c) shows the state of tungsten of Figure 7(a) and 7(b) after baking, in both cases the surface of tungsten 10 is a little oxidized and aluminum 14 and 14′ become alumina, Al₂O₃ 15. As described above, for reference oxidation of tungsten can be prevented by adding a small amount only of metal, which is not likely to be oxidized as compared with tungsten (not falling within the scope of the claims), to tungsten which is likely to be oxidized, even when normal manufacturing process in which a film is baked in the air is used. As a result, the anti-doming effect can be fairly improved, while manufacturing costs are almost the same as when a film is formed of Bi₂O₃ in a conventional manner. In the present invention, a stable oxide layer can be promptly formed on a surface of tungsten powder by coating according to the invention with a material which is more likely to be oxidized as compared with tungsten so that thermal oxidation of tungsten powder itself can be prevented. As a result, the anti-doming effect of the film comprising tungsten as a main element can be fairly improved.

Claims: A method of preventing doming in a colour cathode ray tube shadow mask due to irradiation thereof during use, including the steps of:- applying a mixture of tungsten particles and a binder to the surface of a shadow mask preform; and driving off said binder by baking; characterised in that said particles are coated with a material that is more readily oxidisable than tungsten, and that is active to form an oxide that shall limit or prevent further oxidation of said tungsten particles during baking. A method as claimed in claim 1, characterised in that said material, likely to be more readily oxidizable than tungsten, is magnesium, aluminium, a 4A group element, silicon, vanadium, manganese or molybdenum or any of these in combination.

Inventors: HANADA TOHRU, KIMURA HIROSHI, KOITABASHI MASAYASU, WATANABE TETSUYA, YAMAMOTO MORIO, HANADA, TOHRU, C/O SUNTEC KABUSHIKI KAISHA, KIMURA, HIROSHI, C/O MITSUBISHI DENKI K. K., KOITABASHI, MASAYASU, C/O MITSUBISHI DENKI K. K., WATANABE, TETSUYA, C/O MITSUBISHI DENKI ENG. K.K., YAMAMOTO, MORIO, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01J29
IPC codes: H01J29, A45D7, H01J9, A45D8