Color cathode ray tube and method of manufacturing the same

Abstract: A color cathode ray tube is disclosed in which a coating (20) containing fine particles of tungsten oxide and/or bismuth oxide and a binder containing aluminum phosphate is formed on the surface of a shadow mask (7) on the side of an electron gun (6).

Description: The present invention relates to a color cathode ray tube and, more particularly, to an improvement in a shadow mask for use in a color cathode ray tube. The shadow mask of a color cathode ray tube has a large number of apertures. These apertures are so designed as to have a geometrical one-to-one correspondence with phosphor layers. Each aperture thus designed has a function of passing an electron beam emitted from an electron gun such that the electron beam impinges only on a phosphor layer which is in a geometrical one-to-one correspondence with that aperture. Therefore, the aperture is also called a color selecting electrode. Normally, in a color cathode ray tube in operation, about a 15% to 20% portion of an entire electron beam emitted from an electron gun reaches a phosphor screen through the apertures of a shadow mask, and the remaining 80% to 85% portion of the beam impinges on the surface of the shadow mask. As a result, the kinetic energy of the electron beam is converted into a thermal energy to heat the shadow mask to about 80°C. Generally, the base material used in the shadow mask is a cold-rolled iron plate 0.1 to 0.3 mm in thickness whose thermal expansion coefficient is 12 × 10 -6 /°C at 20°C to 100°C. Upon being heated as above, the shadow mask causes thermal expansion so-called doming. This thermal expansion brings about a geometric positional deviation between the apertures of the shadow mask and a phosphor layer. Consequently, a portion of an electron beam passing through the apertures impinges on a phosphor layer of another color, leading to a purity drift. To improve a color cathode ray tube which causes a significant purity drift due to the doming phenomenon, Jpn. Pat. Appln. KOKOKU Publication No. 42-25446, e.g., has proposed the use of an iron-nickel alloy, such as an invar alloy, whose thermal expansion coefficient is nearly 1/10 that of iron. Unfortunately, the invar alloy is expensive and has a high yield strength after annealing and a low yield in mask molding. Therefore, color cathode ray tubes using this invar alloy are very expensive compared to those using iron. For this reason, methods of forming a coating on the surface of a shadow mask and suppressing a purity drift caused by the doming by the function of this coating have been conventionally proposed. Representative methods of the prior art will be described below. In a first method, as proposed in Jpn. Pat. Appln. KOKAI Publication No. 60-54139, crystallized glass of lead borate is coated on the surface of a shadow mask and bonded by a high-temperature heat treatment, thereby suppressing the doming. In this method, however, lead as a harmful substance is contained in the glass layer. Therefore, care must be taken in handling the material to ensure a safe working environment and to prevent an environmental pollution. A second method uses a coating solution containing particles of a heavy metal substance whose atomic number exceeds 70, as proposed in Jpn. Pat. Appln. KOKOKU Publication No. 60-14459. In this method, the coating solution is spray-coated on the surface on the electron beam incident side of a shadow mask, forming a coating having an electron beam reflecting property. This KOKOKU Publication No. 60-14459 also describes that it is effective to spray-coat a water-soluble suspension containing fine particles of a heavy metal, such as bismuth oxide, on the electron beam incident surface of the shadow mask after the formation of the coating. However, the effect of preventing the purity drift resulting from the doming of the shadow mask depends only upon a single element such as bismuth oxide. Consequently, this effect is unsatisfactory as compared with the effect of a shadow mask having no electron beam reflecting coating. A third method suppresses the doming by increasing the thermal conductivity or thermal radiation efficiency of the shadow mask, in addition to imparting the electron beam reflecting property discussed above. As a method of this sort, Jpn. Pat. Appln. KOKAI Publication No. 4-48530 has proposed a method by which bismuth oxide particles, tungsten particles, and partially graphitized carbon particles are mixed with water glass, and the resultant solution is coated on a shadow mask to form a composite coating on the electron beam incident surface of the shadow mask. In this method, the purity drift preventing effect as the purpose of the method is relatively good. However, since the particle sizes of the raw materials are large, it is difficult to uniformly mill the materials even if the materials are milled and stirred to have an average particle size of, e.g., approximately 2 µm by using a ball mill or the like. Consequently, it is difficult to obtain a sharp particle size distribution of the milled particles. In order to prevent deformation or clogging of the mask apertures, the thickness of the coating must be controlled to about 3 µm. However, since substances having no sharp particle size distributions and different specific gravities are mixed, it is impossible to obtain a homogeneous mixture as the coating solution. This inhomogeneous coating solution cannot be spray-coated, so it is difficult to perfectly coat a shadow mask with this coating solution. A fourth method is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 62-110240. In this method, an amorphous metal oxide material or the like is used as a binder to form a layer containing a metal with a small atomic number, thereby improving the thermal radiation efficiency. In addition, a purity drift is prevented by performing electrostatic correction for the electron beam path by electrification. As described above, a number of means of forming a coating on the surface of a shadow mask have been conventionally proposed to suppress the doming of the shadow mask. In these means, in coating a substance which does not melt at temperatures applied during the color cathode ray tube manufacturing process, water glass or a metal alkoxide is used as a binder to allow film formation. Since, however, water glass contains an alkali metal, a carbonate is readily formed. This carbonate produces carbonic acid gas in a heat treatment during the manufacture, and a portion of the gas is readily adsorbed in the tube. The carbon dioxide adsorbed is released to poison the cathode upon impingement of an electron beam when the cathode ray tube is in operation. This degrades the emission characteristics. On the other hand, a metal alkoxide does not form a perfect metal oxide by a heat treatment at about 500°C. Consequently, hydrogen gas is produced when the color cathode ray tube is operated. Ionized hydrogen impinges on the phosphor screen to cause ion burn on the phosphor screen. This results in a decreased luminance. The present invention has been made to solve the above conventional problems and has as its object to provide a color cathode ray tube in which a coating made from inorganic materials is formed on the surface of a shadow mask to reduce thermal expansion caused by heat generated by impingement of an electron beam, thereby reducing a purity drift resulting from the doming and a degradation in the emission life. In addition, in the manufacturing process of a color cathode ray tube, a shadow mask is exposed to spray washing with water, and heat during sealing and evacuation. Therefore, a coating to be formed on the surface of a shadow mask preferably has a water resistance and a heat resistance. Unfortunately, a heat treatment at 500°C or higher is required to form a coating with a water resistance and a heat resistance by the use of conventionally proposed binders. This increases the thermal economic burden. It is, therefore, another object of the present invention to provide a color cathode ray tube including a shadow mask having a coating with a water resistance and a heat resistance. The present invention includes the following two aspects. The first aspect of the present invention is a color cathode ray tube comprising: a phosphor screen; a shadow mask with a large number of apertures, arranged in the vicinity of the phosphor screen; and an electron gun generating an electron beam passing through the apertures of the shadow mask to excite the phosphor screen; wherein the shadow mask has a coating having a thickness of 2 to 15 µm which is formed on an electron gun side of the shadow mask and contains fine particles of tungsten oxide and/or bismuth oxide in an amount of 15 to 60 wt% and a binder containing aluminum phosphate. The second aspect of the present invention is a method for manufacturing a color cathode ray tube comprising: a phosphor screen; a shadow mask with a large number of apertures, arranged in the vicinity of the phosphor screen; and an electron gun generating an electron beam passing through the apertures of the shadow mask to excite the phosphor screen; wherein the method comprises the steps of preparing a suspension by dispersing fine particles of tungsten oxide and/or bismuth oxide in an amount of 15 to 60 wt% in a binder containing aluminum phosphate, coating the suspension on a surface, on an electron gun side, of the shadow mask to form a coating film, and calcining the coating film, thereby forming a coating having a thickness of 2 to 15 µm on the shadow mask. The resultant shadow mask may be arranged on the faceplate such that the coating opposes the electron gun. According to the present invention, a coating for improving a purity drift of a shadow mask type color cathode ray tube can be obtained at a relatively low temperature. Additionally, since the adhesion of the coating is increased to decrease the gas release amount, neither the emission characteristic nor the pressure resistance is degraded. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a sectional view showing the overall arrangement of a color cathode ray tube according to the present invention; FIG. 2 is a sectional view showing the main components of a shadow mask according to the present invention; and FIG. 3 is a schematic view showing the measurement conditions of a purity drift. The present invention is an improvement of a shadow mask for use in a color cathode ray tube. A shadow mask used in the present invention has a shadow mask substrate and a coating formed on the shadow mask substrate. This coating contains fine particles of tungsten oxide and/or bismuth oxide and a binder containing aluminum phosphate. The coating is formed by, e.g., preparing a suspension by dispersing the fine particles of tungsten oxide and/or bismuth oxide in the binder containing aluminum phosphate, coating the resultant suspension on at least one surface of a shadow mask, and calcining the resultant coating film. The effect of the coating can be obtained by arranging the shadow mask thus manufactured on a faceplate such that the coating opposes an electron gun. Tungsten and bismuth contained in this coating have large atomic numbers and consequently a high electron reflecting power. Assuming the thermal radiation efficiency of a perfect black body is 1, those of bismuth oxide, tungsten, and tungsten trioxide are 0.80 to 0.85, 0.95 to 0.98, and 0.91 to 0.95, respectively. That is, tungsten trioxide and bismuth oxide have high thermal radiation efficiencies. Therefore, a temperature rise in the shadow mask can be greatly decreased by a high electron beam reflecting power and a high thermal radiation efficiency. This makes it possible to reduce a purity drift caused by thermal expansion of the shadow mask. A preferred addition amount of tungsten oxide and/or bismuth oxide is 15 to 60 wt%. If the addition amount is smaller than 15 wt%, the effect of suppressing a purity drift tends to be unsatisfied. If the addition amount is larger than 60 wt%, the strength of the coating film tends to be weakened, thereby peeling of the film can be easily to cause. In addition, in the present invention aluminum phosphate is contained in the binder for forming the coating with the electron beam reflecting property and the thermal radiation property discussed above. This allows formation of a coating with a sufficient film strength. Furthermore, the use of aluminum phosphate prevents production of a gas when the color cathode ray tube is in operation. This protects the cathode from poisoning by the gas and prevents ion burn of the phosphor of the phosphor screen. That is, since aluminum phosphate is a water-soluble phosphate containing no alkali metal, this substance does not produce a carbonate and consequently does not produce a gas during operation. Moreover, although at room temperature aluminum phosphate is a liquid represented by, e.g., the chemical formula Al 2 O 3 ·nP 2 O 5 ·mH 2 O (n = 2 to 5, m = 5 to 7), upon being calcined it changes into a solid represented by, e.g., the chemical formula Al 2 O 3 ·nP 2 O 5 ·mH 2 O (n = 1 or 2, m = 1 or less). This makes formation of a strong coating feasible. In the present invention, Al 2 O 3 ·3P 2 O 5 ·6H 2 O is preferably used as a liquid. A temperature for calcining preferably ranges between 180 and 600°C. A period for calcining preferably ranges between 30 and 120 minutes. Note that tungsten trioxide and bismuth oxide are extremely stable substances over a temperature range from room temperature to 500°C which is used during the manufacturing process of a color cathode ray tube, and are almost insoluble in water or alcohols. Therefore, these particles hardly dissolve after the film formation. Each of tungsten trioxide and bismuth oxide preferably has a particle size of about 0.2 to 2 µm, within which range the dispersibility particularly in the suspension is increased. In addition, by adding boron oxide to the binder, a water-resistant film can be obtained by low-temperature calcination at about 200°C. The addition amount of this boron oxide is desirably 10 to 25 wt% of the amount of aluminum oxide to be contained in the aluminum phosphate. If the addition amount is smaller than 10 wt%, an objective water resistance cannot be obtained. If the addition amount is larger than 25 wt%, the film strength decreases. As discussed above, a relatively strong film can be formed by the use of aluminum phosphate as the binder. However, in some cases the binder may slightly reacts with the base material of the shadow mask since the binder itself is acidic. This slight reaction causes a decrease in the adhesion. Therefore, in case this reaction between the binder and the shadow mask material is a problem, it is desirable to add aluminum oxide or magnesium oxide powder as a filler in the formation of the coating. This is so because the reaction between the shadow mask and base material is reduced by a reaction between stoichiometrically excess phosphoric acid contained in the binder and aluminum oxide or magnesium oxide, and this further reduces the release amount of a gas. The addition amount of the aluminum oxide powder can be nearly stoichiometrically equal to excess phosphoric acid contained in the binder made from aluminum phosphate. That is, aluminum phosphate contains excess phosphoric acid with respect to aluminum oxide, as represented by the chemical formula Al 2 O 3 ·3P 2 O 5 ·6H 2 O. The addition amount of the aluminum oxide powder is set as above in order to effectively use this excess phosphoric acid. The addition of the aluminum oxide powder increases the adhesion of the film because the amount of the excess phosphoric acid which changes into solid aluminum phosphate, Al 2 O 3 ·P 2 O 5 , increases. A preferable addition amount of aluminum oxide or magnesium oxide is 70 to 140 wt% of the amount of the excess phosphoric acid. If the addition amount is less than 70 wt%, increase in adhesion of the film tends to be little. An addition amount exceeding 140 wt% increases the particle size after the film formation, leading to problems such as clogging of the apertures of the shadow mask or removal of the particles. Note that a magnesium oxide powder, in place of an aluminum oxide powder, can also be added to the aluminum phosphate binder. In this case the binder hardens immediately after the addition of magnesium oxide. Therefore, it is desirable to use a two-part mixing type spray gun in spraying to mix and coat a solution of a concentration higher than that of the coating solution in the above embodiment and a magnesium oxide suspension. Note also that the thickness of the coating is preferably about 2 to 15 µm. If the thickness is smaller than 2 µm, the effect of a purity drift tends to be unsatisfied. If the thickness is larger than 15 µm, clogging of apertures tend to be occurred in abundance and make an orbit of the electron been intercepted so as to make the orbit narrow. Example 1 One example of the present invention will be described below with reference to the accompanying drawings. As illustrated in FIG. 1, a shadow mask type color cathode ray tube generally has an envelope consisting of a rectangular panel 1, a funnel 2, and a neck 3. Stripes of a phosphor layer 4 which luminesce in red, green, and blue respectively are formed on the inner surface of the panel 1. The neck 3 incorporates an in-line type electron gun 6 for impinging electron beams 5, corresponding to red, green, and blue emitting phosphor layers, arranged in line along the horizontal axis of the panel 1, respectively. A shadow mask 7 having a large number of fine apertures is fixed to a mask frame 8 at a position near the phosphor layer 4, at which the shadow mask 7 opposes the phosphor layer 4. The mask frame 8 is supported in the panel 1 by engagement with stud pins 10 embedded in the vertical inner walls of the inner surface of the panel 1 via a holder 9. This permits the spacing between the mask 7 and the phosphor layer 4 to fall within the range of a design value. A deflecting device 12 deflects and scans the electron beams 5, thereby reproducing images. Note that the components of the color cathode ray tube are not limited to the in-line electron gun and the stripe phosphor screen as discussed above as long as the tube includes a shadow mask. FIG. 2 is a partial sectional view of the shadow mask 7. Referring to FIG. 2, the shadow mask 7 has a number of apertures 7a. A coating 20 (to be described later) is formed at least on a non-aperture portion between the apertures 7a on the surface opposing the electron gun. The shadow mask 7 is manufactured by forming a flat mask by using photoetching and molding the mask into a predetermined curved shape. In the manufacture, to decrease the mechanical strength of the material, a flat mask having a predetermined aperture size is annealed in a hydrogen reducing atmosphere at 700°C to 800°C. The resultant flat mask is so press-molded as to have a desired curvature, and is degreased with an organic solvent or a high-temperature alkali solution to remove the molding oil. Thereafter, the resultant mask is passed through a high-temperature gas atmosphere at 550 to 650°C which contains carbon dioxide gas as the main constituent. Consequently, a corrosion-resistant black oxide film consisting primarily of Fe 3 O 4 is formed on the surface of the mask. Thereafter, the coating of the present invention is formed on the surface, on the side of the electron gun, of the blackened shadow mask. Note that the black oxide film described above has a corrosion resistance. Therefore, even if pinholes or the like are formed in the coating of the present invention which is constructed from an inorganic substance, this black oxide film suppresses gathering of red rust during the heat treatment step. In addition, the black oxide film has fine projections and recesses compared to the surface of the shadow mask. This improves the adhesion of the coating to make the coating difficult to peel. The coating 20 of the present invention will be described in detail below. First, water was added to liquid aluminum phosphate represented by the chemical formula Al 2 O 3 ·3P 2 O 5 ·6H 2 O to adjust its viscosity to an appropriate value. Thereafter, tungsten oxide particles (average particle size 0.5 µm) containing tungsten trioxide as the main constituent were added to the material to prepare a suspension. In the preparation the ratio of the tungsten oxide and the aluminum phosphate binder in the coating solution was changed, as shown in Table 1. Each resultant suspension was spray-coated on the surface, on the side of an electron gun, of a shadow mask, which was molded as discussed above and on which the above-mentioned black oxide film was formed, by using an air spray gun or an air-less spray gun, thereby forming a coating film with a predetermined thickness. Since the coating solution had a viscosity coefficient larger than that of ethanol or water, scattering by the spray was little, and there was almost no sagging of the solution adhered to the shadow mask. Note that the appropriate film thickness is 2 to 15 µm. If the film thickness is less than 2 µm, the doming suppressing effect is decreased. If the film thickness is more than 15 µm, clogging of the apertures of the shadow mask frequently occurs. After the suspension was coated, the resultant shadow mask was placed in an oven, dried, and calcined. As an example, the calcination condition is that the shadow mask is heated from room temperature to 100°C over 10 minutes, kept at that temperature for one hour, again heated to 200°C over 20 minutes, kept at that temperature for 30 minutes, and then cooled to room temperature at a rate of 10°C/min. The coating film thus annealed has excellent characteristics at medium temperatures around 200°C and at high temperatures of 500°C or higher because of its strong bonding force. Therefore, the film is not adversely affected by heat applied during the manufacturing process. The coating film also has a water resistance and hence does not peel off by washing during the manufacture. The shadow mask on which the coating is formed in this manner is transported, with the coating faced to the electron gun, to the next stage, i.e., the assembly step of a color cathode ray tube. If aluminum phosphate used in the binder reacts with iron as the base material of the shadow mask to produce hydrogen, a pressure is applied to the coating from the inside. Consequently, cracks may form in the coating or the adhesion of the coating may deteriorate. Therefore, in case the base material and the binder readily react with each other, it is desirable to use an aluminum phosphate complex as the binder. Examples of an effective reagent for forming a complex with the aluminum of aluminum phosphate are alcohol amines such as ethanolamine, amino acids such as glycine, sarcosine and alanine, and ethylenediamine. Note that the complex is preferably a substance which does not remain in the film after the film formation. The complex is more desirably ethanolamine which is a low-molecular-weight, water-soluble substance which readily evaporates or decomposes and dissolves in a binder. The movement of an electron beam caused by the doming in each of the 25-inch color cathode ray tubes manufactured as discussed above was measured and compared with that of a conventional color cathode ray tube. The measurement was done as shown in FIG. 3. That is, 88-mm wide band-like white patterns were displayed at positions each separated by 160 mm from the center of the screen along the horizontal axis with an anode voltage of 26 kV and a cathode current of 1330 µA. A maximum movement of an electron beam which moved with time by thermal expansion of the shadow mask after turning on of the power switch was measured at each measurement point A. The measurement result is given in Table 1. Note that prior to the manufacture of the color cathode ray tubes, an adhesive tape peel test and a measurement of the water resistance were performed for the coating film of each shadow mask. Table 1 also shows these results. The adhesive tape peel test was conducted as follows. A cellophane adhesive tape, which size was 18 mm × 50 mm, was attached to the surface of the coating film. A rubber eraser was rubbed against the surface of the cellophane adhesive tape so as to make the cellophane adhesive tape completely adhere to the surface of the coating film. Immediately after adhesion, the cellophane adhesive tape was peeled in an instant with keeping a direction of peeling in a vertical to the surface of the coating film, then a sticking matter on the adhesive surface of the cellophane adhesive tape was observed. The evaluation was done as follows. o No adhesion to the tape. Δ A very slight adhesion to the surface layer. x Adhesion to some extent to the surface layer. The water resistance test was done in accordance with JIS K 5400. First, the substrate on which the coating was formed was dipped in water for two hours. Thereafter, whether the coating peeled, swelled, or softened was checked. The evaluation was done as follows. o None of creeps, expansion, cracks, peel, and color change occurred. Δ An extremely small amount of a removed substance was found in the water resistance test bath. x A removed substance was found to some extent in the water resistance test bath. As shown in Table 1, the doming suppressing effect of the color cathode ray tubes of this example was improved by 11 to 35% as compared with the conventional color cathode ray tube that was not treated. In addition, deterioration in the emission life characteristic of the cathode after the use of long periods of time remained unchanged from that in the non-treatment case in which no coating was formed. Also, no ion burn of the phosphor was brought about by hydrogen gas inside the tube. Note that in this example, tungsten oxide was used as the filler in the coating. However, a similar effect can be obtained for the electron beam moving amount even by use of bismuth oxide. In addition, the presence/absence of the aluminum phosphate complex as the binder has no effect on the moving amount of an electron beam. Tungsten oxide addition amount (%) Adhesive tape peel test Water resistance Electron beam moving amount (%) 10 ▵ ▵ 89 20 ▵ ▵ 75 30 o o 65 40 o o 68 50 o o 66 60 ▵ o 68 70 ▵ ▵ 71 80 ▵ ▵ 73 No coating 100 Example 2 In this example, each suspension used in Example 1 was added with boron oxide B 2 O 3 at a ratio of 15 wt% of the amount of aluminum oxide contained in aluminum phosphate. The resultant suspensions were used to form coatings under the same coating conditions and calcination conditions as in Example 1. The characteristics of the color cathode ray tubes according to this example were measured following the same procedures as in Example 1. The results are summarized in Table 2 below. Tungsten oxide addition amount (%) Adhesive tape peel test Water resistance Electron beam moving amount (%) 10 ▵ ▵ 88 20 ▵ o 73 30 o o 65 40 o o 69 50 o o 67 60 o o 70 70 ▵ ▵ 70 80 ▵ ▵ 71 No coating 100 The purity drift suppressing effect of the shadow masks of this example was improved by 12 to 35% compared to that of the non-treated mask. In addition, deterioration in the emission life characteristic of the cathode after the use of long periods of time remained unchanged from that of the cathode ray tube manufactured using the non-treated shadow mask. Also, no ion burn of the phosphor was brought about by hydrogen gas inside the tube. Furthermore, it was possible to form a coating with a sufficient water resistance by low-temperature calcination at about 200°C. Note that in this example, a similar effect was obtained for the electron beam moving amount even by use of bismuth oxide. Example 3 In this example, boron oxide B 2 O 3 was added to aluminum phosphate (Al 2 O 3 ·3P 2 O 5 ·6H 2 O) at a ratio of 20% of the amount of aluminum oxide contained in the aluminum phosphate. Water was added to the resultant material to obtain a proper viscosity. Thereafter, suspensions were prepared by changing tungsten oxide particles following the same procedures as in Example 1. By using these suspensions, coatings were formed under the same coating conditions and calcination conditions as in Example 1. The characteristics of the color cathode ray tubes according to this example were measured following the same procedures as in Example 1. The results are listed in Table 3 below. Tungsten oxide addition amount (%) Adhesive tape peel test Water resistance Electron beam moving amount (%) 10 ▵ o 90 20 o o 74 30 o o 65 40 o o 67 50 o o 66 60 o o 71 70 ▵ o 70 80 ▵ ▵ 70 No coating 100 The purity drift suppressing effect of the shadow masks of this example was improved by 10 to 35% compared to that of the non-treated mask. In addition, deterioration in the emission life characteristic of the cathode after the use of long periods of time remained unchanged from that of the cathode ray tube manufactured using the non-treated shadow mask. Also, no ion burn of the phosphor was brought about by hydrogen gas inside the tube. Note that in this example, a similar effect was obtained for the electron beam moving amount even by use of bismuth oxide. Each coating film added with a proper amount of aluminum oxide and annealed as discussed above did not peel off even in the above described peel test. In addition, the coating film had superior medium- and high-temperature characteristics derived from its strong bonding force. Therefore, the film was not adversely affected by heat applied during the manufacturing process. The coating film also had a water resistance and hence did not peel off by washing during the manufacture.

Claims: A color cathode ray tube comprising: a phosphor screen; a shadow mask (7) with a large number of apertures, arranged in the vicinity of said phosphor screen; and an electron gun generating an electrcn beam passing through said apertures of said shadow mask (7) to excite said phosphor screen; wherein; said shadow mask (7) has a coating (20) having a thickness of 2 to 15 µm, which is formed on an electron gun side of said shadow mask (7) and contains fine particles of tungsten oxide and/or bismuth oxide in an amount of 15 to 60 wt% measured in a suspension for coating and a binder containing aluminum phosphate. A color cathode ray tube according to claim 1, characterized in that the diameter of said fine particles of tungsten oxide and/or bismuth oxide is 0.2 to 2 µm. A color cathode ray tube according to claim 1 or 2, characterized in that said binder containing aluminum phosphate further contains boron oxide. A color cathode ray tube according to claim 3, characterized in that a content of the boron oxide is 10 to 25 wt% of an amount of aluminum oxide contained in the aluminum phosphate. A color cathode ray tube according to claim 1 or 2, characterized in that said binder containing aluminum phosphate further contains aluminum oxide and/or magnesium oxide in an amount corresponding to 70 to 140 wt% of a stoichiometric excess amount of phosphoric acid with respect to aluminum oxide contained in the aluminum phosphate. A method for manufacturing a color cathode ray tube comprising: a phosphor screen; a shadow mask (7) with a large number of apertures, arranged in the vicinity of said phosphor screen; and an electron gun generating an electron beam passing through said apertures of said shadow mask (7) to excite said phosphor screen; wherein said method comprises the steps of preparing a suspension by dispersing fine particles of tungsten oxide and/or bismuth oxide in an amount of 15 to 60 wt% in a binder containing aluminum phosphate, coating said suspension on a surface, on an electron gun side, of said shadow mask (7) to form a coating film, and calcining said coating film, thereby forming a coating (20) having a thickness of 2 to 15 µm on said shadow mask (7). A method according to claim 6, characterized in that a diameter of said fine particles of tungsten oxide and/or bismuth oxide is 0.2 to 2 µm. A method according to claim 6 or 7, characterized in that said binder containing aluminum phosphate further contains boron oxide. A method according to claim 8, characterized in that a content of the boron oxide is 10 to 25 wt% of an amount of aluminum oxide contained in the aluminum phosphate. A method according to claim 6 or 7, characterized in that said suspension is further added with aluminum oxide and/or magnesium oxide in an amount corresponding to 70 to 140 wt% of a stoichiometric excess amount of phosphoric acid with respect to aluminum oxide contained in the aluminum phosphate. A method according to anyone of claims 6 to 10, characterized in that a calcination temperature in the calcining step is 180 to 600°C. A method according to anyone of claims 6 to 10, characterized in that a calcination time in the calcining step is 30 to 120 minutes.

Inventors: MURAMATSU SACHIKO, OHTAKE YASUHISA, MURAMATSU, SACHIKO, OHTAKE, YASUHISA, MURAMATSU, SACHIKO, C/O INT.PROP.DIV. K.K. TOSHIBA, OHTAKE, YASUHISA, C/O INT. PROP. DIV. K.K. TOSHIBA
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1995
Main classification: H01J9
IPC codes: H01J29, H01J9