MASK TUBE FOR DISPLAY, NOTABLY FOR COLOR TELEVISION

TUBE A MASQUE POUR ECRAN D'AFFICHAGE ET SURTOUT POUR TELEVISION COULEUR

English Abstract

Disclosed is a mask tube for display or color
television wherein the face of the iron-based mask
opposite the screen is coated with a heavy metal or a
heavy metal compound with an atomic number greater than
70.. The heavy metal or heavy metal compound is mixed
with an alkaline silicate, for example potassium
silicate. The ratio by weight between the heavy metal
and the silicate is between 7 and 17, and is preferably
equal to 12.

Claims

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WHAT IS CLAIMED IS:
1. A cathode ray tube including a phosphor
screen and an iron-based shadow mask having
oppositely disposed surfaces adjacent to the
screen, the surface of the iron-based mask facing
away from the screen being coated with a mixture
of a heavy metal or a heavy metal compound, with
an atomic number greater than 70, and an alkaline
silicate, the ratio, by weight, between the heavy
metal or the heavy metal compound, and the
silicate being between 7 and 17.
2. A tube according to claim 1, wherein
the ratio, by weight, between the heavy metal or
the heavy metal compound, and the silicate is 12.
3. A cathode ray tube according to claim
1, wherein the mixture of heavy metal, or of heavy
metal compound, with alkaline silicate comprises
grains with dimensions of the order of 0.5 to 0.8
µm.
4. A cathode ray tube according to claim
1, 2, or 3, wherein the heavy metal is bismuth.
5. A cathode ray tube according to claim
1, 2, or 3, wherein the alkaline silicate is
chosen from among at least one of the following
silicates: potassium silicate, lithium silicate,
sodium silicate.

Description

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MASK TUBE FOR DISPLAY, NOTABLY FOR COLOR TELEVISION
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a mask tube for display,
S notably for color television.
2. Description of the Prior Art
A mask type color television tube is formed by a
glass bulb that ends in a screen on which luminophors
are deposited. When these luminophors are struck by
electrons, they produce a light radiation that is
generally red, green or blue. When these luminophors
are struck by radiation, they produce a light radiation
which is generally red, green or blue. These
luminophors are arranged on the screen in triads. A
triad comprises a green luminophor, a red luminophor
and a blue luminophor. Each luminophor is excited by a
corresponding electron beam.
Before the tube, there is placed a mask having
holes of dimensions and arrangements such that, in each
triad, the luminophor of a determined color is excited
only by the electron beam assigned to this color.
A great part of the electrons of the three beams
is stopped by the mask. The impact of the electrons
heats the mask, and this rise in temperature causes a
deformation which may impair the colors of the image of
the screen.

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To reduce the heating of the mask, it has already
been proposed to coat the face of the steel mask,
pointed towards the electron guns (opposite the
screen), with a heavy metal, or a compound of a heavy
metal, with an atomic number greater than 70. This
heavy metal, bismuth for example, has a reflecting
effect for the electrons: this reduces the absorption
and, hence, the heating.
It is noted that the coating of heavy metal or of
a compound of heavy metal, on the mask has defects of
homogeneity, and that these defects impair the
reflecting capacity of the coating.
The invention overcomes this drawback.
SUMMARY OF THE IN~IENTION
According to the invention, the heavy metal or
heavy metal. compound is mixed with an alkaline
silicate, the ratio between the weight of heavy metal
and the weight of silicate being between 7 and 17, and
being preferably equal to 12.
The alkaline silicate is, for example, chosen from
among one of the following silicates: potassium
silicate, lithium silicate, sodium silicate.
The addition of an alkaline silicate, for example
potassium silicate, enables proper adhesion to the
metallic mask as well as proper homogeneity of the
mixture of the heavy metal, or of the heavy metal
compound, with this silicate. The quantity of heavy

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metal deposited per unit of area is greater than with
the known technique. The chosen proportion of silicate
is small enough for it not to appreciably disturb the
heating reduction effect given by the heavy metal.
Furthermore, this ratio between the weight of heavy
metal, or heavy metal compound, and the weight of
silicate provides fox optimum adhesion to the metallic
mask and enables the coating to be done with sputtering
or spraying equipment. It has been further noted that
if the proportion of heavy metal or heavy metal
compound is below the above-indicated limit of 7, no
proper adhesion of the coating to the mask would be
obtained. Moreover, should the heavy metal compound be
bismuth oxide, and should the alkaline silicate be
potassium silicate, no bismuth silicate would be
formed, notably with the ratio 12, since there is a
deficiency of silica with respect to bismuth.
The alkaline silicate is stable at high
temperature, namely a temperature of more than 400pC,
enabling it to withstand the heat treatment undergone
by the mask during the fabrication of the tube.
To obtain better results, it is preferable for the
mixture of heavy metal, or of heavy metal compound,
with alkaline silicate to be formed by grains with
dimensions of the order of 0.5 to 0.8 fun.
After the mixture of heavy metal, or heavy metal
compoent, with alkaline silicate has been used, an

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appreciable reduction in the temperature of the
iron-based mask is observed. This entails a reduction
of the order of 35$ to 45~ in the deformation of the
mask at the start of the operation of the tube.
Figure 1 is a view, seen under an electron
microscope, of the surface of a color television tube
mask coated with bismuth by means of a standard
technique, and figure 2 is a similar view, but relates
to the use of the invention, namely, in this example, a
surface view of a mask coated with a mixture of
potassium silicate and bismuth.
In these two figures, the clearer the mask, the
greater is the density of bismuth. It is seen that, on
the whole, the blank zones 10 in figure 2 have a
greater area than the blank zones 10 in figure 1. Thus,
at the surface, the density of bismuth is greater with
the invention. It is thus seen that the invention
enables the increase of the density of bismuth in
volume.
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Representative Drawing