Abstract:
A color display tube comprises a shadow mask (12) placed in front of a display screen (8). The shadow mask (12), at least on the side remote from the display screen, is coated with a glass layer (14) comprising a heavy metal.

Description:
This is a division of application Ser. No. 879,830, filed June 26, 1986, now abandoned, which is a continuation of application Ser. No. 636,535, filed 8/1/84, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a color display tube comprising, in an evacuated envelope, means for generating a number of electron beams, a display screen having regions luminescing in different colors when struck by the beams, and a color selection electrode, disposed between the beam generating means and the display screen, which has apertures for passing the electron beams and associating each electron beam with luminescent regions of one color, said color selection electrode being coated at least on the side remote from the display screen with a layer which comprises a heavy metal. The invention further relates to a method of manufacturing such a color display tube. 
     Heavy metal is understood to mean hereinafter a metal or an alloy of a metal having an atomic number higher than 70. 
     A color display tube of the type mentioned in the opening paragraph is known, for example, from GB-A No. 2 080 612. 
     This Great Britain Patent Application describes that a color display tube having a color selection electrode which on the side remote from the display screen is coated with a layer of a material comprising a heavy metal, during operation shows considerably less local or overall doming. Such doming would deteriorate the color purity of the color display tube. 
     In fact, a large number of the electrons, on their way to the display screen, is intercepted by the color selection electrode, sometimes termed shadow mask, and causes local or entire heating of the shadow mask and, hence, doming. A layer with heavy metal on the side of the color selection electrode where the electron beams are incident has for its effect that the electron beams are refracted more strongly beyond the apertures and give less rise to heating and doming. 
     It is also stated in the above-mentioned literature reference that, depending on the increase of the thickness of the heavy metal-containing layer, the possibility of the occurrence of loose particles in the tube increases. These loose particles may give rise inter alia in the electron gun to high voltage flashovers and on the display screen to black spots in the displayed picture. 
     One of the objects of the present invention is to prevent the occurrence of loose particles in the tube by the action of the electron beams on the heavy metal-containing layer. 
     Another problem occurring in the color display tube is the following. The color selection electrode owes its rigidity for a considerable part to its curvature. Increasing the radius of curvature causes a weakening of the mask. In the present-day developments of the color display tube, there is a tendency to make the screen less convex or even flat. It is then desired to make the color selection electrode more rigid. 
     Another object of the present invention is to provide a more rigid color selection electrode. 
     SUMMARY OF THE INVENTION 
     According to the present invention the color display tube mentioned in the opening paragraph is characterized in that the heavy metal-containing layer on the color selection electrode is a glass layer having a forming temperature which is at most equal to the temperature of the manufacture of the envelope from the display screen and a cone. 
     By means of a glass layer it is achieved that parts cannot easily be detached from the layer, as well as that the color selection electrode as a whole becomes more rigid. By means of a glass layer having the indicated forming temperature, it is also achieved that no additional thermal treatments for the manufacture of the color display tube according to the invention are necessary. 
     The heavy metal is preferably lead. By means of lead, good glass layers can be obtained on a color selection electrode. 
     In addition to the heavy metal, at least one glass forming component is present in the glass layer. Boron is preferably present as such. 
     Particularly good results are obtained when the glass layer is a lead borate glass. 
     The glass layer preferably consists of 76-80% by weight (52-56 mol.%) of lead oxide, 15-20% by weight (33-45 mol.%) of boron oxide, 0-6% by weight (0-11 mol.%) of zinc oxide and 0.5-2% by weight (1-4 mol.%) of cobalt oxide. A glass layer having such a composition is very suitable as regards coefficient of expansion, melting properties and adhesion to the substrate. 
     The glass layer comprising the heavy metal is preferably provided on the color selection electrode by spraying a lead oxide and boron oxide-containing suspension on the color selection electrode while, on the other side of the color selection electrode, a sub-ambient pressure is maintained. 
     By means of this method it is achieved that no or hardly any particles deposit on the wall of the apertures in the color selection electrode so that afterwards no increased reflection at said walls occurs which would deteriorate the picture quality. 
     In a subsequent thermal treatment at, for example, approximately 440° C., the glass layer is formed and the glass layer does not flow into the apertures. Preferably, the thermal treatment simply coincides with the stage of manufacture of the color display tube in which the display screen and the cone are sealed together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described in greater detail with reference to an example and the accompanying drawing, in which: 
     FIG. 1 shows diagrammatically a color display tube according to the invention, and 
     FIG. 2 is a sectional view of a part of the color selection electrode of the tube shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The color display tube shown diagrammatically in FIG. 1 comprises a glass envelope 1 in which three (diagrammatically shown) electron guns 2, 3 and 4 are present to generate three electron beams 5, 6 and 7. 
     A display screen 8 is built up from a repeating pattern of phosphor stripes 9, 10 and 11 which can luminesce in blue, green and red and are each associated with each of the electron beams 5, 6 and 7 in such manner that each electron beam impinges only on phosphor stripes of one color. 
     This is realized in known manner by means of a color selection electrode (shadow mask) 12 which is placed at a short distance before the display screen 8 and which comprises rows of apertures 13 which pass a part of the electron beams 5, 6 and 7. 
     Only approximately 20% of the electrons, on their way to the display screen 8, pass through the apertures 13. In order to avoid local or overall heating of the shawdow mask, an electron reflective layer 14 is provided which has a heavy metal (see FIG. 2). 
     In order to avoid crumbling away of the layer 14 and to increase the rigidity of the color selection electrode 13 as a whole, according to the invention, the layer 14 on the color selection electrode is a glass layer having a forming temperature which is at most equal to the temperature of manufacturing the envelope 1 from the display screen 8 and a cone 16. 
     The heavy metal advantageously is lead, and boron is present in the glass layer 14 as a glass-forming component. A glass layer 14 consisting of lead borate glass has proven very suitable. A lead borate glass 14 comprises, for example, 0.25 mg of Pb and 0.04 mg of B per cm 2 . The glass layer 14 is provided from an aqueous suspension of a mixture consisting of approximately 16% by weight of boron oxide, 4% by weight of zinc oxide, 79% by weight of lead oxide and 1% by weight of cobalt oxide. The coefficient of expansion readily corresponds to that of the iron color selection electrode 12. 
     Said aqueous suspension is sprayed on the color selection electrode. During spraying, an air flow is maintained in the mask apertures 13 by exhausting the air on the non-sprayed side of the mask 12 by means of an exhauster. By means of these measures it is achieved that at most only a little suspension lands on the edge 15 of the apertures 13 so that no undesired electron reflection (taper reflection) takes place on said walls during operation of the tube. 
     The glass layer is formed by a thermal treatment of the shadow mask at approximately 440° C., the melting properties of the glass used being such that substantially no material lands in the apertures 13. 
     The coefficient of reflection for electrons of the layer 14 is approximately 45%. This results in a lower temperature of the mask 12 than in the absence of the layer 14 and hence in a smaller overall and local doming of the shadow mask. The thermal treatment simply coincides with the step in which the display screen 8 and the cone 16 of the color display tube are sealed to manufacture the envelope 1. 
     The invention is not restricted to the example described but may be varied in many manners by those skilled in the art without departing from the scope of this invention.