DEVICE AND COMPOSITION FOR LIBERATING NITROGEN GAS DURING THE MANUFACTURE OF A CATHODE RAY TUBE

Abstract: There is described a composition able to liberate nitrogen gas during the manufacture of a cathode ray tube and in particular to control the release of this gas after its heating in the atmosphere. This composition comprises Fe₄N, Ni and Al in the form of powders preferably in certain weight ratios. There is further described the preferred particle size distributions of the three components. There is further described a preferred form of capsule, for the release of nitrogen gas, containing said composition.

Description: This invention relates to a device and composition of matter for the release of nitrogen during the manufacture of a cathode ray tube. The release of nitrogen during the manufacture of a cathode ray tube is well known in the art. See for example US-A- 3.973.816. It is often preferable to release the nitrogen in two stages. For instance US-A- 3.669.567 describes a getter device in which there is first a release of nitrogen before evaporation of barium based getter material and there is a second release of nitrogen during the latter part of barium evaporation. During the manufacture of a cathode ray tube it is customary to perform a sealing process in which a soft glass "frit" is used to join the face plane portion of tube to the conical portion. US-A-4.052.641 describes a heating schedule in performing the frit seal process as follows: 25° to 100°C at 3.5°C per minute hold 100°C for 30 minutes 100°C to 410°C at 8°C per minute hold 410°C for 15 minutes cool to 25°C at 4°C per minute It will be realized that different cathode ray tube manufactures use different frit sealing cycles but it will be realized from the aforesaid that temperatures of more than 400°C such as those up to 450°C may be involved for times of the order of 3 to 4 hours. Before the frit sealing process takes place there is very frequently positioned within the cathode ray tube a color selection mask or shadow mask. The shadow mask is often also used as a location for the getter device. However whatever the position of the getter device it will be appreciated that if the getter device is present during the frit sealing process it will also reach temperatures of more than about 400°C and be exposed to these temperatures for several hours. Unfortunately it has been found that with getter devices having a second source of nitrogen and subjected to the above conditions then, on subsequent flashing of the getter device the second source of nitrogen presents certain disadvantages. A first disadvantage is that the nitrogen is released at an undesirable period of time relative to the evaporation of the barium. Another disadvantage is that the nitrogen is released exceedingly quickly and a nitrogen pressure is created within the CRT outside the range of pressures desired to effectively control the distribution and subsequent sorption capabilities of the barium film. Yet another disadvantage is the fact that the extremely rapid production of nitrogen creates a miniature explosion of the nitrogen releasing material thus producing large quantities of loose particles. These loose particles may enter the gun region of the cathode ray tube causing short circuits of the electrodes or the particles may find their way to the shadow mask and block passage of the image producing electron beams thus degrading picture quality. It is therefore an object of the present invention to provide a means for the controlled release of nitrogen gas after having been subject to a frit sealing process conditions. It is another object of the present invention to provide a means for the controlled release of nitrogen without the creation of loose particles after having been subject to a frit sealing process. It is yet a further object of the present invention to provide a composition for the controlled release of nitrogen after having been subject to the conditions of a frit sealing process. It is another object of the present invention to provide a capsule containing a composition for the controlled release of nitrogen after having been subject to the conditions of a frit seal process. The invention provides a composition of matter as described in claim 1 and devices comprising such a composition succeeding in these objects. These and other objects and advantages of the present invention will become clear to those skilled in the art of reference to the following description and drawings wherein: Fig. 1 is a cross sectional view of a capsule containing nitrogen releasing material of the present invention; and Fig. 2 is a cross sectional view of a getter device utilizing a capsule containing nitrogen releasing material of the present invention. It has surprisingly been found that a mixture of particulate Fe₄N, particulate Ni and particulate Al are capable of providing the controlled release of nitrogen gas after having been subject to a frit sealing process consisting of heating in air at a temperature of up to about 400°C for a period of up to about 4 hours. The particulate Fe₄N can have any particle size distribution which is capable of withstanding the frit process when in admixture with nickel powder and aluminium powder. Preferably however the particulate Fe₄N has a particle size distribution of: The particulate Ni has a particle size distribution of: The particulate Al may have any particle size distribution which is capable of withstanding a frit seal process and preferably has a particle size distribution of: The weight percentage of the components Fe₄N, Ni and Al can vary over a wide range. It is preferable that the particulate Fe₄N is present at a percentage of from 40 to 80%. At percentage compositions less than 40% there is insufficient Fe₄N to create a suitable pressure within CRT whereas if the percentage is greater than 80% there is an insufficient Ni and Al to cause a sufficiently exothermic reaction to aid the release of N₂. The Ni should be present in a weight percentage of from 20 to 40%. At a percentage weight less than 20% there is insufficient Ni for the exothermic reaction to aid release of N₂ whereas if the percentage Ni is higher than 40% there is a risk of too much heat being produced with an excessive speed in the release of N₂. The Al should be present in a weight percentage of from 5 to 20%. If the percentage weight of Al is less than 5% there is insufficient Al present to produce sufficient heat in the exothermic reaction to cause release of N₂ from the Fe₄N, whereas if more than 20% of Al is present then the exothermic reaction may take place too violently causing excessive speed in the release of N₂. Preferably the nitrogen releasing composition comprises 60% Fe₄N, 30% Ni and 10% Al. In the broadest aspects of the present invention the ratio of Ni to Al can vary widely but generally is chosen to produce the maximum amount of heat when the Ni is combined with the Al. The maximum amount of heat is generally produced within weight ratios of Ni to Al of 1 : 10 to 10 : 1. An ideal ratio of Ni to Al is about 3 : 1. In the broadest aspects of the present invention the ratio of Fe₄N to the combined weight of Ni and Al is chosen to release substantially all the nitrogen from the Fe₄N within a very short period of time generally less than five and preferably less than three seconds. The Fe₄N is generally present in the composition within a weight ratio of 1 : 5 to 5 : 1 and preferably 1 : 2 to 2 : 1 compared to the combined weight of Ni and Al. In use the composition is preferably adopted as a second source of nitrogen in a N₂ doped getter, and is preferably contained within a holder such as a capsule. The capsule may be of any suitable material for holding the mixture for instance ceramic. Preferably it is of metal. Even more preferably it is of nichrome or stainless steel. Any suitably shaped holder may be used but it is preferably in the form of a hollow cylinder closed at one end. Such a N₂ releasing capsule for use in a so called "delayed nitrogen doped getter" will now be described with reference to Fig. 1 and Fig. 2 wherein 102 is a nitrogen gas releasing capsule comprising a holder 104 in the form of a cylinder 106 integrally closed at one end by means 108. In this particular case the open end 110 is horizontally flared in the form of a flange 112. Flange 112 has present a series of pips for subsequent projection welding for support purposes 114, 114′, 114″. Fig. 2 illustrates the use of a nitrogen gas releasing capsule as a delayed nitrogen doped gas source or second gas source in a nitrogen doped getter. The evaporable getter device 202 comprises a ring shaped holder 204 supporting an evaporable getter metal vapour releasing material 206. Evaporable getter material generally comprises a barium aluminium alloy of composition approximately BaAl₄ in admixture with approximately an equal weight of Ni powder. In addition there is usually incorporated a small amount of Fe₄N as a first source of N₂. This mixture is any mixture known in the art, see for example US-A- 4.077.899 which is capable of withstanding the frit process. In this particular embodiment the evaporable getter device 202 further comprises a reflecting shield 208 which serves to hold a ceramic disk 210 to prevent a heated getter device from cracking the nearby glass wall of a cathode ray tube. A nitrogen gas releasing capsule 212 identical in all respects to capsule 102 shown in Fig. 1 is projection welded to the centre portion of reflecting shield 208. The term "composition of matter for the release of nitrogen gas" as used in the specification and claims herein is meant to include the composition prior to nitrogen gas release. Furthermore the term "getter metal vapor releasing material" as used in the specification and claims herein is meant to include both the material prior to and after getter metal vapor release. This term embraces both the material in the form sold with the getter device and in the form in which it is found in an operating tube wherein the bulk of the getter metal has been evaporated from the material and is in the form of a film on the inside surfaces of the tube. The compositions of the present invention have the property of not releasing particles even after being heated in air at temperatures of 100 to 450°C for a period of two hours. EXAMPLE 1 This example is designed to show the behaviour of a prior art composition of matter for the release of N₂ gas when held in a hollow cylinder of stainless steel after having been subjected to a frit sealing process in air. A powder mixture is prepared comprising 60% by weight of Fe₄N having a preferred particle size distribution as described above. The mixture further comprises 30% of powdered titanium having a particle size distribution of and 10% of particulate Al having a particle size distribution as described above. About 43 mg of the mixture is placed in a hollow cylinder and is subjected to 450°C for 2 hours in air. The cylinders are then placed in an evacuated system and heated to produce a release of N₂. A violent reaction occurs in which particles of the composition are ejected into the system. EXAMPLE 2 This example was carried out under the same conditions as Example 1 with the sole exception that in the composition for release of nitrogen gas the titanium was substituted with nickel having the particle size distribution described in US-A- 4.077.899 where its use is disclosed to avoid excessive exothermic reactions. This distribution was as follow: Also in this case a violent reaction takes place in which particles of the composition are ejected into the system. EXAMPLE 3 This Example is designed to show the behaviour of a composition of matter for the release of nitrogen gas of the present invention held in a capsule after having been subjected to a frit sealing process. A powder mixture is prepared having exactly the same weight composition and particle size ranges except that the Ni is replaced by Ni having a particle size distribution according to the Ni distribution as described above. Getter capsules are exposed to air at exactly the same temperature and for exactly the same time as in Example 1. When the capsules are caused to release N₂ in an evacuated system the N₂ is found to be released in a controlled manner without the expulsion of any loose particles from the capsules. Although the invention has been described in considerable detail with reference to certain preferred embodiments and applications it is intended that variations and modifications can be made within the spirit and scope of the claims.

Claims: A composition of matter for the release of nitrogen gas comprising: A. particulate Fe₄N, B. particulate Ni, having a particle size distribution of: C. particulate Al. The composition of claim 1, wherein the ratio of Ni to Al is that which releases the maximum amount of heat when they are combined; and wherein the Ni and Al are present in an amount sufficient to produce heat which releases substantially all the nitrogen from the Fe₄N. A composition of matter of claim 1, wherein the weight ratio of Ni to Al, namely B : C is 1 : 10 to 10 : 1; and wherein the weight ratio of Fe₄N to the combined weight of Ni and Al namely A : (B + C) is 1 : 5 to 5 : 1. A composition of matter of claim 1 for the controlled release of nitrogen gas after being subjected to heating in air at a temperature of 100 to 450°C for a period of about 2 hours comprising an intimate mixture of: A. particulate Fe₄N having a particle size distribution of: B. particulate Ni having a particle size distribution according to claim 1, C. particulate Al having a particle size distribution of: A nitrogen gas releasing capsule comprising: I. a holder; and II. a composition of matter according to claim 1 in which the particulate Al is supported by said holder. A nitrogen gas releasing capsule of claim 5 in which the weight percentages of A, B and C are in the ranges: A. from 40 to 80% B. from 20 to 40% C. from 5 to 20%. A nitrogen gas releasing capsule for use in a delayed nitrogen doped getter for the controlled release of a second amount of nitrogen gas after being subjected to heating in air at a temperature of 100 to 450°C for a period of 2 hours comprising: I. a hollow cylinder of stainless steel closed at one end; and II. a composition of matter according to claim 4, compressed within said cylinder, wherein the weight ratio of B : C is 1 : 10 to 10 : 1; and the weight ratio of A : (B+C) is 1 : 2 to 2 : 1 An evaporable getter device incorporating a composition of matter of claim 1. An evaporable getter device incorporating a capsule comprising the composition of matter of claim 4.

Inventors: MARTELLI, DANIELE
Applicants: SAES GETTERS S.P.A.
Application year: 1987
Main classification: H01J29
IPC codes: C01B21, H01J7, H01J29