Method of manufacturing a picture display tube having a gas-absorbing layer; picture display tube thus manufactured, and gettering device suitable for such a method

A gettering device installed in a picture display tube includes an annular, channel-shaped metal holder containing an evaporable gettering metal and a heat-activated gas releasing material. The gas releasing material is disposed in the holder against a surface which rapidly increases in temperature in response to inductive heating of the holder. This material is covered by the gettering metal to protect it against deterioration by exposure to moist, high temperature air during tube manufacturing processes occurring after installation of the device. During inductive heating, the temperature of the gas releasing material increases faster than that of the gettering metal and begins releasing gas before the gettering metal begins to evaporate.

BACKGROUND OF THE INVENTION 
The invention relates to a method of manufacturing a picture display tube 
in which a gettering device is mounted in the tube. The gettering device 
comprises a metal holder in which are accommodated a source of evaporable 
gettering metal and a gas source of a material which releases gas upon 
heating. After evacuating the tube, the gettering device is heated 
inductively to release the gas from the gas source and to evaporate the 
gettering metal from the source of gettering metal. 
The invention further relates to a picutre display tube thus manufactured, 
as well as to a gettering device suitable for use in the above-mentioned 
method. 
Such a method is disclosed in U.S. Pat. No. 3,388,955. In this method the 
metal holder of the gettering device comprises a gettering metal to be 
evaporated which is mixed with a gas source of gasreleasing material. 
During the inductive heating of the gettering device, first the gas is 
released from the gas source and the gettering metal is then evaporated. 
The gettering metal is evaporated in a gas atmosphere to obtain a uniform 
distribution of gettering metal on an inner surface of the display tube. 
The known gettering device comprises a gas source consisting of iron 
nitride powder (Fe.sub.4 N) which is mixed with the source of gettering 
metal in powder form. In such a gettering device the iron nitride is 
attacked by moist air of approximately 450.degree. C., which condition is 
met, for example, in the manufacture of a colour television display tube 
when the display window and the cone of the display tube are sealed 
together by means of a sealing glass. The known gettering device may not 
be installed in the tube before the display window and the cone are 
secured together. This is a serious restriction inter alia in the 
manufacture of a colour display tube having a resistive layer provided 
internally on a part of the tube wall, as described in British patent 
specification No. 1,226,728. The resistive layer is provided near the 
neck-cone transition of the tube, which makes it necessary for the 
gettering device to be disposed in the tube in a place remote from the 
neck-cone transition, to avoid forming an electric shortcircuit on the 
resistive layer by means of gettering metal evaporated from the gettering 
device. Because of its inaccessibility after the window is secured, it is 
desirable to install the gettering device in a place remote from the 
cone-neck transition before the cone is secured to the window of the tube. 
Such installation is an improvement over the typical insertion of the 
gettering device by means of a resilient metal strip attached to the gun 
system disposed in the neck of the tube thereby avoiding the resilient 
force exerted on the gun system by the metal strip. 
A gas source which can be exposed to moist air of at least 450.degree. C. 
without any adverse effects is described in British patent specification 
No. 1,405,045. In this Specification the gas source comprises a germanium 
nitride, in particular Ge.sub.3 N.sub.4, as a gas-releasing material. 
Germanium nitride is a stable compound having a decomposition temperature 
which is comparatively high with respect to iron nitride. This has for its 
result that upon heating the gettering device, the gas source accommodated 
therein gives off its nitrogen only during the evaporation of the 
gettering metal. In order to obtain a layer of gettering metal which is 
porous through-out its thickness and hence is readily absorbing on an 
internal surface of the tube, it is necessary that during the heating of 
the gettering device the gas released from the gas source has already 
built up a sufficient pressure of approximately 133.times.1O.sup.-3 to 
666.times.1O.sup.-2 Pa in the tube before the gettering metal begins to 
evaporate. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method of manufacturing a 
picture display tube in which a gettering device is used which can be 
exposed to moist air of 450.degree. C. without any adverse effects and 
which has a gas source of which the giving-off of gas in the tube has been 
completed for the greater part before the gettering metal begins to 
evaporate. 
According to the invention, a method of manufacturing a picture display 
tube in which a gettering device is mounted in the tube, which gettering 
device comprises a metal holder in which are accommodated a source of 
evaporable gettering metal and a gas source of a material releasing gas 
upon heating, which gettering device, after evacuating the tube, is heated 
inductively to release the gas from the gas source and to evaporate the 
gettering metal from the source of gettering metal, is characterized in 
that a gettering device is used in which the gasreleasing material is 
concentrated in a layer which internally adjoins a wall portion of the 
metal holder. 
During inductive heating, the gettering device first will become warm at 
the area where the induction currents generated by the induction field in 
the gettering device are maximum. With a high frequency induction field, 
the gettering device will become warm first at the bottom and on the 
outside, that is to say that the metal holder of the gettering device 
leads in temperature relative to the filling of the holder. By 
concentrating the gas-relasing material in a layer which internally 
adjoins a wall portion of the metal holder of the gettering device it is 
achieved that during the inductive heating of the gettering device the 
temperature of the gas-releasing material leads with respect to the other 
contents of the metal holder, that is to say, the source of evaporable 
gettering metal. This has for its result that the gasreleasing material, 
even when it has a comparatively high decomposition temperature, gives off 
its gas before the gettering metal begins to evaporate from the metal 
holder. 
As compared with a gettering device in which the gas source in powder form 
is mixed with the source of gettering metal in powder form, the invention 
has the further advantage that the material of the gas source is better 
sealed from the surrounding atmosphere. The invention therefore permits 
the use of chemically less resistant gas sources in an atmosphere of warm 
moist air, for example, iron nitride. 
The invention also permits the use of gas sources which, if mixed with the 
source of gettering metal, during the gettering would have a negative 
influence on the yield of gettering metal. Due to the separated positions 
of the gas source and the source of gettering metal, this latter problem 
does not present itself in a gettering device according to the invention. 
The metal holder of the gettering device has a shape which is suitable for 
inductive heating and it usually consists of an annular channel or a 
circular tray. Both the source of gettering metal and the gas source 
consists of material in powder form which is pressed in the metal holder. 
However, the quantity by weight of gasreleasing material is small with 
respect to the quantity by weight of the material of which the source of 
gettering metal consists. The source of gettering metal usually is a 
mixture of nickel powder and a powdered alloy of aluminum and the 
gettering metal, in which mixture the content of nickel powder is 
approximately 40-60% by weight. This source of gettering metal determines 
substantially the total filling weight of the metal holder of the 
gettering device. The quantity by weight of gas-releasing material usually 
is one to a few per cent, of the total filling weight of the metal holder. 
In order to obtain a laminated structure with respect to the gas-releasing 
material and the material of the source of gettering metal, it is possible 
when filling the metal holder to dose the gasreleasing material 
separately. As a result of the small quantity of gas-releasing material 
required per gettering device, high requirements are imposed in this 
method on the dosing accuracy of the filling apparatus used for filling 
the metal holder. The filling apparatus often operates reliably only when 
a given minimum quantity of powder is dosed. If this minimum quantity is 
larger than the quantity of gas-releasing material required per gettering 
device, this problem can be solved by dosing the gasreleasing material 
while mixed with another powdered material. This means that the quantity 
of gas-releasing material required for a gettering device is supplemented 
with the other powdered material up to at least the quantity of powder 
required minimally for an accurate dosing. The layer which internally 
adjoins a wall portion of the metal holder of the gettering device 
according to the invention consists of a mixture of gas-releasing material 
in powder form and another material in powder form. This other material in 
powder form may be any suitable material, but preferably it is a material 
of which the source of gettering metal is also composed or of at least one 
of the components thereof. 
According to an embodiment of the invention, the layer in which the 
gas-releasing material is concentrated adjoins the bottom wall of the 
metal holder. The filling of the metal holder then comprises a first phase 
in which the metal holder is partly filled with the gas-releasing 
material, mixed or not mixed with another material in powder form. In a 
second phase the metal holder is then supplied with the desired quantity 
of material which forms the source of the gettering metal, and the 
powdered filling is then compressed in the holder. It is also possible to 
use pre-compressed filling bodies and post-compress these filling bodies 
in the metal holder. This method is particularly suitable and may be used 
either for the material of the source of the gettering metal or for the 
gas-releasing material mixed with another material in powder form, or for 
both sources. 
According to an embodiment of the invention, the gas source consists of a 
gas-releasing material which gives off its gas only at temperatures higher 
than approximately 700.degree. C. The advantage of such a gas source is 
that the gettering device can be pre-degassed to approximately 650.degree. 
C., so that it is thoroughly liberated from gases which are not absorbed 
as such by the layer of gettering metal provided in the tube, for example, 
argon. This is of importance because such gases can shorten the life of 
the tube in which the gettering device is used. 
A very suitable gas-releasing material is germanium nitride, in particular 
Ge.sub.3 N.sub.4. Germanium nitride is chemically a particularly resistent 
compound which in a vacuum begins to decompose at approximately 
825.degree. C. and decomposes very rapidly at approximately 900.degree. C. 
When such a gas source is used in combination with a chemically resistent 
source of gettering metal, a gettering device is obtained which has the 
advantage, as compared with the known gettering devices, that it can be 
assembled in its place inside the tube envelope before the window and the 
cone of the display tube are secured together in the manufacture of a 
display tube. A chemically resistent material is to be understood to mean 
herein a material which can withstand the attack by moist air of 
approximately 450.degree. C. for at least one hour. As already stated, 
this is of particular importance in the manufacture of display tubes 
having a resistive layer disposed on an internal wall portion of the tube. 
Another very suitable chemically resistant gas source consists of an 
iron-chromium-germanium nitride, in particular Fe.sub.60 Cr.sub.7 
Ge.sub.33 nitride. This nitride gives off its nitrogen at approximately 
650.degree. C. and is preferably provided in the getter holder in the form 
of a precompressed ring. 
The resistance of the gettering device against the action of the ambient 
atmosphere as such is a great advantage, since this enables storage of the 
gettering device for a long period of time without reducing the usefulness 
of the gettering device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The gettering device shown in FIG. 1 comprises a chromium-nickel channel 1 
having a bottom 5 and two side walls 6 and 7, in which channel a powdered 
filling material 2 is compressed consisting of a layer 4 adjoining the 
bottom 5 and a layer 3 provided on the layer 4. The layer 4 comprises 
approximately 8 mg of gas-releasing material in the form of germanium 
nitride powder (Ge.sub.3 N.sub.4) which is dosed while mixed with 
approximately 36 mg of barium aluminum powder and 36 mg of nickel powder. 
The weight of the powder mixture in the layer 4 thus is approximately 80 
mg, which quantity can more easily be dosed than the comparatively small 
quantity of 8 mg of germanium nitride. The layer 3 forms the source of 
gettering metal and consists of approximately 1,070 mg of a mixture of 
barium aluminum powder and nickel powder in the weight ratio 1:1. 
The source of gettering metal, in the present case the source of barium, 
can withstand attack by moist air of approximately 450.degree. C. for at 
least one hour due to a suitable choice of the grain sizes of the barium 
aluminum powder and the nickel powder. As described in U.S. patent 
specification No. 4,077,899, the contents of which are deemed to be 
incorporated in this Application by reference, the nickel powder in such a 
source of gettering metal has for that purpose an average grain size 
smaller than 80 microns and a specific area smaller than 0.15 m.sup.2 per 
gram, while the average grain size of the barium alumiminium powder is 
smaller than 125 microns. 
For the inductive heating, the gettering device is subjected to a 
high-frequency induction field, in which the field lines have the 
direction indicated in FIG. 1 by the double arrow 8. As a result of this 
induction field, induction currents are formed in the metal holder 1 and 
the filling material 2, as a result of which the temperature of the 
gettering device rises. The induction currents will be largest at the 
outer circumference 7 and at the bottom 5 of the gettering device, so that 
the gettering device will become warm there first. In the gettering device 
shown in FIG. 1, the germanium nitride will hence decompose and give off 
its nitrogen before the barium begins to evaporate from the source of 
gettering metal 3. 
The layer 4 may also be provided in the metal holder 1 as a precompressed 
ring. FIG. 2 shows an embodiment which is slightly varied in this respect. 
The ring 9 shown in this Figure consists of a precompressed body of the 
same composition as the layer 4 in FIG. 1. The ring 9 internally adjoins 
the wall 7 of the holder 1 instead of the bottom 5. In this construction 
also the germanium nitride incorporated in the ring 9 gives off its 
nitrogen before the barium begins to evaporate from the source of 
gettering metal denoted by 3 in this Figure also. 
When gas sources having a comparatively low decomposition temperature are 
used, the position of the gas source in the metal holder is of minor 
importance. When, for example, an iron-chromium-germanium nitride, such as 
Fe.sub.60 Cr.sub.7 Ge.sub.33 nitride, is used, the gas source may also be 
provided in the form of a precompressed ring 9' or 9", as shown in FIG. 2. 
The same applies when a gas source is used consisting of iron nitride 
(decomposition temperature of approximately 500.degree. C.) 
Since a gettering device according to the invention gives a freedom of 
choice with respect to the moment in the manufacturing process of a 
display tube at which the gettering device is mounted inside the envelope 
of the display tube, the invention is extremely suitable for use in the 
manufacture of display tubes, said moment of mounting being in an early 
stage of the manufacturing process. This aspect of the invention will be 
described with reference to FIG. 3. The colour television display tube 
shown diagrammatically in this Figure has a neck 10, a cone 11 and a 
window 12 of glass. A layer 13 of areas luminescing in red, green and blue 
is provided on the inside of the window 12 and in known manner forms a 
pattern of lines or a pattern of dots. The tube further comprises a metal 
shadow mask 15 which, like a metal magnetic screening cap 17, is secured 
to a metal supporting frame 16. In an annular metal holder 20 of a 
gettering device characterized according to the invention are provided a 
source of gettering metal in the form of a mixture of barium-aluminum 
powder and nickel powder, as well as a source of nitrogen in the form as 
described with reference to FIG. 1 or 2. A metal strip 19 is welded to the 
holder 20 and is secured to the screening cap 17 at 22. It is also 
possible to secure the strip 19 to a high voltage contact 26 sealed in the 
tube wall. After providing this gettering device in place, the window 12 
is secured to the cone 11 in a vacuum-tight manner by means of a sealing 
glass 18. In this process which lasts approximately one hour and takes 
place in a furnace at a temperature of approximately 450.degree. C., water 
vapour is released from the sealing material. The gettering device 
characterized according to the invention can be exposed to these 
circumstances without any objection. After the sealing process, a system 
of guns 14 shown diagrammatically and with which three electron beams can 
be generated, is placed in the neck of the tube and the tube is evacuated. 
Finally, the gettering device 20 is brought to a temperature range by 
inductive heating, in which first nitrogen is introduced in the tube by 
thermal decomposition of the germanium nitride and an exothermally 
occurring reaction is then brought about between the barium-aluminium and 
the nickel. The barium evaporates, is scattered by the nitrogen, and is 
deposited as a thin layer of gettering metal on surfaces inside the space 
defined by the mask 15 and the screening cap 17. The place and the spatial 
orientation of the gettering device are such that the part of a resistive 
layer 25 on the internal surface of the tube between the line 24 and the 
system of guns 14 is not covered with barium. Such a resistive layer 
functions to minimize the detrimental results which a possible high 
voltage breakdown in the tube may have for certain components in the 
control circuit connected thereto. In a usual connection of the gettering 
device to a gun system, or to an element connected to a gun system, the 
resistive layer is short circuited by the deposited barium, which is 
prevented by the above-indicated placement of the gettering device. 
Although the invention has been described with reference to a gettering 
device which comprises as a source of gettering metal a mixture of 
barium-aluminum powder and nickel powder, it is not restricted thereto. 
The invention may also be performed while using other gettering metals, 
such as strontium, calcium and magnesium. In order to obtain a chemically 
resistant source of gettering metal, measures other than those described 
above may also be taken. For example, the nickel powder in this source may 
be replaced by a chemically resistant nickel-titanium compound or an 
iron-titanium compound. It is also possible to cover the surface of the 
source of gettering metal exposed to the atmosphere with a protective 
layer of, for example, aluminum or an organo-silicon compound.