A colour cathode ray tube

Abstract: A colour cathode ray tube (1) comprises a vacuum envelope (2) including a panel (3) with an inner surface, a neck (4) and a funnel (5) connecting the neck to the panel, a phosphor screen (6) provided on the inner surface of the panel for emitting visible light, an electron gun (7) provided in the neck for emitting a plurality of electron beams (8) towards the phosphor screen with a predetermined distance from the phosphor screen, and a porous layer (15) which is formed on a surface of the shadow mask facing the electron gun. The porous layer is formed by using metal alkoxide solution which includes black pigment containing both cobalt oxide and nickel oxide.

Description: This invention relates to a colour cathode ray tube and more particularly relates to a shadow mask for such a tube. Generally, a shadow mask type colour cathode ray tube comprises an evacuated envelope having a front panel, a phosphor screen on the inner surface of the panel and comprising phosphor layers which emit red, green and blue light, resepctively, when bombarded by respective electron beams from an electron gun, and an apertured shadow mask accurately located within the envelope at a predetermined distance from the phosphor screen. In the cathode ray tube, the amount of effective electron beams which pass through the apertures is one third or less of the total amount of the electron beams emitted from the electron gun. The remainder of the electron beams are converted to thermal energy by bombarding the shadow mask. Thus, the shadow mask is heated up to the order of 80°C during the operation of a normal TV set. Moreover, in the special colour cathode ray tubes used as a display, such as in aircraft cockpits, the temperature of the shadow mask sometimes rises to about 200°C. Generally, such a shadow mask is formed from thin plate, with a thickness of 0.1 mm - 0.3 mm, of so-called cold rolled steel with a thermal expansion coefficient as large as 1.2 x 10⁻⁵/°C. The shadow mask also has a blackened film which is formed on the surface of the plate. The shadow mask is fixed to a mask frame in the envelope along a skirt portion of the shadow mask. The mask frame is formed from the same cold rolled steel with an L-shaped cross-section of about 1 mm and on which a blackened film is formed. When the shadow mask is heated by the electronic beams, the temperature of the peripheral portion of the shadow mask is lower than that of the central portion of the shadow mask since the peripheral portion is contacted with the mask frame which has a large thermal capacity, so that the heat at the peripheral portion can be easily transmitted due to radiation and conduction. As a result, a temperature difference occurs between the central portion and the peripheral portion of the shadow mask, and a so-called doming phenomenon consequently occurs. When the doming phenomenon occurs, the distance between the shadow mask and the phosphor screen changes, and the colour purity deteriorates due to the disturbance to the accurate landing of the electron beams. This type of mislanding due to the doming phenomenon is particularly marked during the initial stage of the operation of the tube. Also, when an image including a high brightness portion is reproduced and the portion stops for a while, a local doming phenomenon occurs. To reduce the doming phenomenon by promoting heat radiation from the central portion of the shadow mask many proposals have been made. For example, it has been proposed in US-2826538 that the shadow mask supports a black layer consisting of graphite. Since the black layer acts as a good radiator, a temperature differential across the shadow mask can be prevented to some extent. However, the adhesion of the black layer to the shadow mask is reduced due to temperature changes during the heat treatment in the tube manufacturing process, and small pieces of the layer peel off due to the external vibration of the tube. The small pieces of the layer can cause deterioration of the picture quality by blocking some of the apertures of the shadow mask. Further, the pieces of the layer can cause sparking between the electrodes of the electron gun if the pieces enter the gun. Consequently, the effectiveness of the colour cathode ray tube is reduced. Another proposal has been described in EP-A-0139379 in order to prevent the doming phenomenon by increasing the mechanical strength of the shadow mask. According to this publication, the shadow mask has a layer of lead borate glass which is bonded to the surface of the shadow mask and is formed by a high temperature heat treatment. The shadow mask can remarkably reduce the doming phenomenon due to the glass layer. However, since the glass layer contains lead with a large atomic number, it is difficult to reduce the elastic rebound of the elctron beams which bombard the shadow mask. Furthermore, it has been proposed in EP-A-0209346 that a shadow mask has a layer with a good heat dissipation property formed on the surface of the mask. The layer comprises a metal or metal oxide as a filler and an amorphous metal oxide as a binder. DE-A-24 21 767 discloses cathode ray tubes wherein the shadow mask includes a black porous layer of nickel thereon. An object of this invention is to provide a colour cathode ray tube with good colour characteristics. According to this invention there is provided a colour cathode ray tube comprising an evacuated envelope having a panel with a phosphor screen on the inner surface of the panel, an electron gun assembly in the envelope for emitting a plurality of electron beams towards the phosphor screen, an apertured shadow mask located in the envelope in spaced apart relation from the screen, said shadow mask having a porous layer thereon, characterised in that said porous layer includes black pigment containing cobalt oxide and nickel oxide. The invention may furthermore provide a colour cathode ray tube comprising an evacuated envelope having a panel with a phosphor screen on the inner surface of the panel, an electron gun assembly in the envelope for emitting a plurality of electron beams towards the phosphor screen, an apertured shadow mask located in the envelope in spaced apart relation from the screen, said shadow mask having a porous layer thereon, characterised in that said porous layer is formed by using a metal alkoxide solution which includes black pigment containing cobalt oxide and nickel oxide. Since the porous layer covering the surface of the shadow mask is formed by the solution which includes black colour group pigments containing both cobalt oxide and nickel oxide, the porous layer has a greater pore diameter distribution and a smaller cumulative pore capacity, according to the invention. Therefore, the oxidizing gases, such as H₂O, CO₂ and CO, which are adsorbed to the porous layer and will deteriorate the emission characteristic of the oxide cathode of the electron gun, can be easily exhausted during the tube manufacturing process. Consequently, according to the invention, the porous layer on the surface of the shadow mask can improve the heat dissipation characteristic of the shadow mask due to the black colour group pigments and thus can suppress the doming phenomenon effectively. Also, the porous layer can improve the emission characteristic of the cathode of the electron gun. The film shown on EP-A-0209346 consists of a porous film containing hydroxyl groups (-OH), since the film is formed by a suspension containing amorphous metal oxide contained in metal alcoholate as a binder. Accordingly, the film absorbs the gases emitted during heating and drying when forming the film itself and the gases contained in the atmospheric air to which the film is exposed during the manufacturing process of the tube. It, however, is not easy to exhaust these gases from the film. The porous layer of this invention may be composed of a film formed from metal alcoholate in the same way as the film mentioned in EP-A-0209346. However, since the pigments used in the invention contain both cobalt oxide and nickel oxide, the pH of the solution which hydrolyses the alcoholate is large (this means that the solution is basic solution) during film formation, and thus the pore diameter distribution is larger and the cumulative pore capacity is smaller compared with that of the film described in EP-A-0209346. As a result, since the gases, such as the oxidizing gases, which have been adsorbed can easily be exhausted, the emission characteristic of the electron gun is improved. The cumulative pore capacity means the volume of the space portion per unit weight of the porous layer formed on the shadow mask, that is to say, it indicates the total volume of the pores in the layer. This means that the smaller the cumulative pore capacity, the smaller the volume of the pores. Therefore, it is desirable for the porous layer that the absorption of the undesirable gas is small, due to less total volume of the pores, and the gases can be easily exhausted due to large diameter of the pore. In other words, the film, which has a large pore diameter distribution and a small cumulative pore capacity, is desirable for the porous layer of the invention. According to the invention, it is preferable that both ranges of the pore diameter distribution (r) and the cumulative capacity (V) simultaneously satisfy the following equations, respectively:- 3000Å ≦ r ≦ 7000Å 200mm³/g ≦ V ≦ 500mm³/g Since it is hard to exhaust the oxidizing gases absorbed in the porous layer, it is preferable for practical use of the porous layer if r is greater than3000Å. Since tightness of the porous layer is deteriorated, it is preferable for practical use of the porous layer if r is smaller than 7000Å. Also, since the amount of the gases absorbed in the porous layer increases, it is preferable for the porous layer, practically, if V is smaller than 500mm³/g. It is preferable for the porous layer, practically, if V is greater than 200mm³/g, since the amount of hydrogen gas emitted from the porous layer decreases due to decreased amount of residual hydroxyl groups (-OH). For the contents of the oxides of Co and Ni in the black colour pigments, it is desirable for the pigments to contain 0.5 wt% to 15 wt% of cobalt oxide and nickel oxide more preferably 0.9 wt% to 15 wt% thereof. If the amounts of cobalt oxide and nickel oxide exceed 15 wt%, it is undesirable since the amount of Fe⁺⁺⁺, Mn⁺⁺⁺, Mn⁺⁺⁺, which realize the sharpness of absorption characteristic of the pigments, reduces. If the amount is less than 0.9 wt%, the basicity of the solution used to form the porous layer may not be exhibited. Also, it is desirable to adjust the ratio of CoO to NiO to mole ratio of 1:1, since the Ni ion acts as Ni⁺⁺⁺ when forming the porous layer. Furthermore, since the porous layer of this invention may include hydroxyl groups (-OH), hydrogen gas (H₂), which is a reductive gas, could be emitted from the layer when the layer is bombarded by the electron beams. As described in the following equation, since BaO in the oxide cathode is reduced by the hydrogen gas, the emission characteristic of the electron gun is improved:- BaO + H₂ → Ba + H₂O Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 shows a sectional view of a colour cathode ray tube in accordance with one embodiment of the invention; and Figure 2 shows a perspective view of the shadow mask shown in Figure 1. In Figure 1, a colour cathode ray tube 1 includes an evacuated envelope 2 which has a generally rectangular panel 3, a neck 4 and a funnel 5 connecting the neck 4 to the panel 3. On an inner surface of the panel 3, a phosphor screen 6, which includes a plurality of phosphor stripes for emitting red, green and blue light, respectively, is formed. In the neck 4 is provided an in-line type electron gun 7 which emits three electron beams 8 aligned in a direction along a horizontal axis of the panel 3 for bombarding the phosphor stripes, respectively. Also, a shadow mask 9, which has a plurality of apertures 9b arranged in lines in the vertical direction and in the horizontal direction, is supported near the phosphor screen 6 by a mask frame 10. The mask frame 10 is secured by stud pins 11 which are embedded in a skirt of the panel 3 through elastic members 12. A deflection yoke 13 is provided on the outside of the funnel 5 for deflecting the electron beams 8 so that the electron beams scan the phosphor screen 6. Furthermore, an inner shield 14, which is composed of a ferro-magnetic metal plate, is fixed to the mask frame 10 so as to surround a path of the electron beams 8. As shown in Figure 2, on a rear surface 9a of the shadow mask facing the electron gun is formed a porous layer 15, which is formed from a mixed layer of metal oxide of amorphous silicon and zirconium, metallic hydroxides and black colour pigments containing both cobalt oxide and nickel oxide. The porous layer 15 was formed in the following manner. At first, a solution, which had the constituents given below, was prepared. For the pigment, the inorganic pigments shown in Table 1 was used. The mean particle diameter of these pigments was adjusted to 0.7 µm. This solution was coated on the rear surface of the shadow mask to have a thickness of 15 µm by spray method. Finally, the porous layer was obtained by heating the shadow mask in an atmosphere at a temperature more than 70°C. During the heating, hydrolysis was caused on the alcoholate compounds of silicon and zirconia coated on the shadow mask by moisture contained in the atmosphere. Thus, by the condensation polymerization reaction between the alkoxides, a film was formed and, finally, the porous layer composed of a mixed layer of metal oxide amorphous silicon and zirconium, metallic hydroxides and black colour pigments. For comparative example, the porous layers were formed in the same manner as in the embodiment for the various types of the pigments in Table 1. The shadow masks with the porous layers were incorporated in colour cathode ray tubes with 21 inches-size screen, respectively. After the tubes were continuously operated for 3000 hours, residual emission coefficients were measured. The results are shown in Table 2. Furthermore, after the residual emission coefficient test, the tubes were dismantled and the porous layers were peeled off for measuring the pore diameter distributions and the cumulative pore capacities. The measurement was done for the pores with diameter between 40 Å and 75000 Å by the mercury porosimeter method. The result are shown in Table 3. As is clear from the results of these measurements, the residual emission characteristic of the tubes according to the embodiments are improved. The mean pore diameter of the porous layer according to the embodiments were greater than that of the comparative examples. Also, the cumulative pore capacity were smaller than that of the comparative examples. In the above embodiments, (Fe, Mn, Co, Ni)O black group pigment is used. Similar results can be obtained when (Fe, Mn, Co, Ni, Si)O black pigment is used.

Claims: A colour cathode ray tube comprising an evacuated envelope having a panel with a phosphor screen on the inner surface of the panel, an electron gun assembly in the envelope for emitting a plurality of electron beams towards the phosphor screen, an apertured shadow mask located in the envelope in spaced apart relation from the screen, said shadow mask having a porous layer thereon, characterised in that said porous layer includes black pigment containing cobalt oxide and nickel oxide. A colour cathode ray tube as claimed in claim 1 wherein the said porous layer is derived from an alcoholate suspension or solution which includes black pigment containing cobalt oxide and nickel oxide. A colour cathode ray tube according to claim 1 or 2, wherein the pigment additionally contains manganese dioxide and iron oxide. A colour cathode ray tube according to any preceding claim wherein the pigment contains 0.5 wt% to 15 wt% of cobalt oxide, preferably 0.9 wt% to 15 wt% thereof, and further contains 0,5 wt% to 15 % wt% of nickel oxide and preferably 0.9 wt% to 15% thereof. A colour cathode ray tube as claimed in claim 4 wherein the pigment contains 1.0 wt% to 15 wt% of cobalt oxide and 0.9 wt% to 15 wt% of nickel oxide. A colour cathode ray tube according to any preceding claim, wherein the porous layer is capable of emission of hydrogen gas when the porous layer is bombarded by the electron beams. A colour cathode ray tube according to any preceding claim, wherein the molar ratio of cobalt oxide to nickel oxide is about 1:1.

Inventors: KOIKE NORIO INTELLECTUAL PROPE, KOIKE, NORIO INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01J29
IPC codes: H01J9, H01J29