Color display tube

The invention relates to a color display tube comprising a shadow mask (12) placed in front of the 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. The object of such a layer is to reflect electrons incident on it so as to reduce local or overall heating. By means of the glass layer it is achieved that it crumbles away less easily, which inter alia is in favor of the picture quality, and that the shadow mask is more rigid.

The invention relates to a colour display tube comprising in an evacuated 
envelope means to generate a number of electron beams, a display screen 
having regions luminescing in different colours and a colour selection 
electrode which is present near the display screen and has apertures for 
passing the electron beams and associating each electron beam with 
luminescent regions of one colour, said colour 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 colour display tube. 
Heavy metal is to be understood to mean hereinafter a metal or an alloy of 
a metal having an atomic number higher than 70. 
A colour display tube of the type mentioned in the opening paragraph is 
known, for example, from GB-A No. 2 080 612. 
This Patent Application describes that a colour display tube having a 
colour 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 as a 
result of which the colour purity of the colour display tube would 
deteriorate. 
As a matter of fact a large part of the electrons, on their way to the 
display screen, are intercepted by the colour selection electrode, 
sometimes termed shadow mask, and cause local or entire heating of the 
shadow mask and hence doming. A layer with heavy metal on the side of the 
colour 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 already 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 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 colour display tube is the following. The 
colour selection electrode owes its rigidity for a considerable part to 
its curvature. Increasing the radius of curvature gives a weakening of the 
mask. In the present-day developments of the colour display tube there is 
a tendency to make the screen less convex or even flat. It is then desired 
to make the colour selection electrode more rigid. 
Another object of the invention is to provide a more rigid colour selection 
electrode. 
According to the invention the colour display tube mentioned in the opening 
paragraph is therefore characterized in that the heavy metal-containing 
layer on the colour 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 colour selection electrode as 
a whole becomes more rigid. By means of a glass layer having the indicated 
forming temperature it is achieved that no additional thermal treatments 
for the manufacture of the colour 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 colour 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. Cobalt is included as cobalt monoxide (CoO) to improve the flow 
properties of the glass. 
The glass layer comprising the heavy metal is preferably provided on the 
colour selection electrode by spraying a lead oxide and boron 
oxide-containing suspension on the colour selection electrode while on the 
other side of the colour 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 colour selection electrode so 
that afterwards no increased reflection at the said walls occurs and the 
picture quality would deteriorate. 
In a subsequent thermal treatment at, for example, approximately 
440.degree. 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 colour display tube in 
which the display screen and the cone are sealed together.

The colour 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 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 colour. 
This is realized in known manner by means of a colour 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 shadow 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 colour selection electrode 13 as a whole, according to the 
invention the layer 14 on the colour 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 proved to be very suitable. A lead borate glass 14 
comprises, for example, 0.25 mg of Pb and 0.04 mg of B per cm.sup.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 
colour selection electrode 12. 
The said aqueous suspension is sprayed on the colour 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 
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.degree. 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 colour display 
tube are sealed to 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.