RU2137243C1 - Evaporated gas trap with high yield of barium - Google Patents

Abstract

FIELD: production of barium. SUBSTANCE: invention refers to " welded " evaporated gas trap that can be sealed by welding and has the capacity to liberate barium in the amount of not less than 300 mg for use in kinescopes of customary type or in flat displays. Gas trap includes container where mixture of powdery alloy BaAlBaAl₄ and nickel in the form of briquette is placed. Its upper surface has changeable number of radial recesses. Non- solid metal element is provided inside briquette. EFFECT: proposed gas trap gives possibility to control instantaneous evaporation of barium, prevents enlargement of volume of briquette from powders, ejection of particles from it and partial melting of container providing for normal operation of kinescope tube. 14 cl, 7 dwg, 1 tbl

Description

 The present invention relates to a “weldable” vaporizable getter device with a high barium yield. As is known, vaporized gas-trapping materials are mainly used to maintain vacuum in the internal space of picture tubes for televisions and computer monitors. The use of vaporized getter materials in the interior of flat panel displays, which is currently under development, is also under investigation.

The getter material, which is widely used in picture tubes, is a metal barium, which is deposited in the form of a thin film on the inner walls of the tube. To obtain this film, devices are used, known in the art as vaporizable getter devices, which are inserted into the tube during its manufacture. These devices contain an open metal container containing inside a compound of barium and aluminum, BaAl ₄ , and nickel powders, Ni, in a weight ratio of about 1: 1. Evaporation is caused by induction heating of the device by means of a coil outside this tube, in an activated process, also defined as "flash evaporation" when the temperature of the powders reaches a value of about 800 ^o C, the following reaction takes place:
BaAl ₄ + 4Ni ---> Ba + 4NiAl
This reaction is highly exothermic and raises the temperature of the powders to about 1200 ^o C, with the evaporation of barium; then barium vapor settles after sublimation on the walls of the tube with the formation of a metal film. Evaporable getter devices are well known in the art, for example, U.S. Pat. US Pat. No. 3,558,962 describes a gas trapping device in which a metal element, preferably a wire mesh, is immersed in a powder briquette in a characteristic position in order to obtain a more uniform temperature in the interior of the powder briquette.

The manufacturing process of picture tubes, both conventional and flat type, involves the step of welding together two glass parts with each other, which is carried out with the so-called "Sealing by welding" operation, causing the glass solder to melt or soften with a melting point of about 450 ^o C between two parts in the presence of air. In conventional picture tubes, a gas trapping device can be introduced after sealing by welding through the neck provided for the body of the electron gun, however, in this case, the size of the gas trapping device is determined by the diameter of the neck, and the precise positioning of the device inside the tube is difficult. On the other hand, in the case of flat displays, it is practically impossible to position the gas collecting device after the sealing step by welding. As a result, manufacturers of picture tubes show an increasing tendency to insert a gas trap before sealing by welding. During the sealing step by welding, the gas trap at a temperature of about 450 ^° C. is in contact with atmospheric gases and vapors released from the fusible glass solder. The main result is the surface oxidation of nickel. During the instant evaporation of barium, nickel oxide enters into a highly exothermic reaction with aluminum, which is difficult to control: this can lead to an increase in the volume of the briquette from the powders, to the ejection of particles from it, and to partial melting of the container, thus being a hindrance to the normal operation of the gas trapping device and tube as a whole. These problems can theoretically be overcome by using a device with less powder during the flash operation, this should lead to a more controlled evaporation of barium, but with a longer evaporation time, which may not be acceptable in the production of picture tubes.

 Evaporable getter devices that can withstand sealing by welding without modification or even without the occurrence of the defects described above are defined as “weldable”.

 Weldable getter devices are already manufactured and sold by the authors. These devices can be manufactured using traditional technologies until some critical parameter values are exceeded: in particular, it is impossible to exceed certain specified values of the thickness of the briquette from the powders, since at too large values of the thickness the amount of heat that is released in the volume of the briquette from the powders dissipates too slowly causing the problems described above. It was empirically found that the ratio between the amount of barium contained in the device, given in mg, and the diameter of the device, given in mm, should not be more than about 10. For reasons related to the production of picture tubes, the maximum possible diameter of gas trapping devices is about 20 mm, so that the maximum amount of barium that can be evaporated from the welded getter devices manufactured by traditional technologies is about 200 mg. However, large picture tubes require amounts of evaporated barium of at least 300 mg, and this requirement cannot be met with weldable devices known from the literature. Weldable gas capture devices capable of evaporating amounts of barium in excess of 200 mg will be defined in the following part of the description and in the claims as “high yield devices”.

 Even by revising solutions known from the literature that provide excellent results in the case of non-weldable gas-trapping devices, it is impossible to obtain welded gas-trapping devices with a high yield: in fact, making radial recesses on the surface of the briquette are made of powders, as described in US Pat. No. 5,118,988, operation Barium vapor after sealing by welding, however, causes the briquette to swell or to eject particles from it. In the devices of US Pat. No. 3,558,962, a metal element, preferably a wire mesh, is immersed in a powder briquette in a specific position. A case is described in which the mesh is in contact with the bottom of the container or is even welded to the bottom itself, or pressed against the free surface of a powder briquette. In this case, both of these grid positions also prevent the weldable getter devices from being produced, causing the problems described above.

 The production of weldable gas capture devices without size restrictions and, as a result, devices with high output, is the goal of various patents.

 U.S. Patent 4,127,361, owned by the author of this application, describes gas trapping devices that can be made weldable by a protective layer of organosilanes, despite its effectiveness, this coating process is too slow and therefore not acceptable for industrial production.

 US Pat. No. 4,342,662 and Japan Patent Hei 2-6185, both of which are owned by Toshiba, describe vaporizable getter devices that are weldable (hereinafter also simply referred to as weldable getter devices), which are obtained by coating the entire powder cake with a glassy boron oxide film containing up to 7% silicon oxide, or, accordingly, only nickel with a glassy film of only barium oxide. The manufacture of these devices, however, is difficult because the film must have a controlled and reproducible thickness.

 The aim of the present invention is to provide an evaporative getter device free from the disadvantages known from the literature.

According to the present invention, this goal is achieved by using a welded vaporized gas capture device with a high barium output, which includes: a metal container open from above; a mixture of BaAl ₄ and Ni powders in a container, formed as a briquette, on the upper surface of which radial depressions are created; a non-continuous metal element of a generally flat shape and generally parallel to the bottom of the container; moreover, the metal element is immersed in a briquette of powders in a position remote from the bottom of the container, and so as not to come into contact with the free surface of the briquette itself.

 The invention will be further explained with reference to the drawings, in which FIG. 1 represents some possible embodiments of solid metal elements; FIG. 2 to 6 are cross-sectional views of some possible embodiments of the gas collecting devices of the present invention.

 For the purposes of the present invention, it is necessary that the metal element be immersed in a powder briquette in such a position that it is removed from the bottom and does not merge with the surface. In fact, induction heating of the gas collecting device is carried out mainly by means of a container and a metal element immersed in the powder, which then transfer heat to the powder of the gas collecting material. It has been noticed that in the contact areas between the metal element and the bottom of the container, heat transfer to the powders is particularly effective, and local overheating occurs if these contact areas are too many or they are too long, the heat not dissipated causes an increase in the volume of the powder briquette and, in some cases, melting of some parts of the device. In contrast, if a metal element merges with the free surface of the bag, the surface itself is divided into regions that are poorly connected to each other and are subject to ejection into the internal space of the tube during instant evaporation.

 The metal element can be made of various metals, such as iron alloys, nickel alloys or aluminum alloys, it is preferable to use AISI 304 steel due to its simple processing in cold form.

 The metal element may have various shapes, provided that it is solid and substantially flat.

 The discontinuity condition is necessary because the element will not impede the release of barium vapor produced by a portion of the powders between the element itself and the bottom of the container. This condition can be obtained using a variety of geometric shapes. Some possible designs are presented in FIG. 1, for example, the metal element can be made in the form of a chamomile-shaped cut out metal plate, like element 10 in the drawing, showing a central hole to facilitate the exit of barium from the underlying powders, it can be a cut out plate having a plurality of holes distributed over the surface either randomly or in an ordered manner, as shown by element 12, or it can be a wire mesh, as described in the aforementioned US patent 3558962.

 The element should be basically flat so that it can be immersed in a powder briquette, which usually has a thickness of a few millimeters, without contacting the bottom of the container and not reaching the surface of the powders. The condition that the metal element should not be in contact with the bottom of the container can be fulfilled in various ways. Some designs are shown in figures 2-6, where various metal elements are presented in accordance with various methods used to hold them at a distance from the bottom, but each of the different types of elements (a plate cut in the shape of a chamomile, a plate with holes, a wire mesh or others) can be used in each of the configurations described below. A possible getter device of the present invention is presented in cross-section in figure 2; such a device 20 is obtained by loading the first part 22 of powders onto the bottom of the container 21, applying a flat metal element 23 to their upper surface and coating the last with the remaining part of the powders 24. Finally, the powders are pressed in the container using a punch of a given shape, so that on the upper surface 25 of the briquette radial recesses 26, 26 ', ... are formed. The weight ratio between the number of powders placed in the container before and after placing the metal element 23 determines the position level of the element itself in the indicated flat layer and is therefore selected so that the element does not extend to surface 25 even where the recesses 26, 26 'are located, ...; as a rule, good results are obtained when this ratio is between 1: 2 and 1: 3. In another possible embodiment, as shown in FIG. Behind, the metal element 33 can be locally deformed to obtain some "mounting protrusions" 34; as shown in FIG. 3b, which is a cross-sectional view of the getter device 30 of the present invention, when the element 33 is placed in the container 31 where the powders 32 are present, the protrusions 34 maintain a predetermined distance from the bottom of the container 35. In this case, radial recesses are also present on the upper surface of the powder briquette 36 37, 37 ', .... Again, as shown in FIG. 4, representing a cross section of another possible getter device 40 of the present invention, it is possible to obtain protrusions 44 on the side walls 45 of the container 41 in which the powders 42 are present and to impose a metal element 43 on the protrusions 44; recesses 47, 47 ', ... are formed on the upper surface 46 of the briquette from the powders. Finally, as shown in FIG. 5, which is a cross-sectional view of yet another possible getter device 50 of the present invention, protrusions 54 can be formed on the bottom 55 of the container 51 containing powders 52, thereby forming support protrusions on which the metal element can be arranged; also in this case, depressions 57, 57 ', ... are formed on the surface of the briquette from the powders. The latter possibility is preferred when a container is used having mechanical fastening elements for a powder briquette, as described in US Pat. No. 4,642,516 and shown in FIG. 6; in this case, the metal element 63 can actually only be flat and located on these elements 64 of mechanical fastening. In the cases that are presented in FIG. 2-6, the position and size of the protrusions of both the container and the metal element determine the position of the latter in such a way that it should not merge with the upper surface of the powder briquette even where the radial recesses are located.

 The container of the device of the present invention may be any of the containers known from the literature. As a rule, it is made of steel, preferably of type AISI 304 or 305, due to the ease of cold processing by stamping and good resistance to oxidation conditions during the sealing operation of the picture tube by welding. The shape of said container is essentially a short cylinder, closed at the bottom and open at the top, although various modifications to this basic shape are possible, such as, for example, protrusions at the bottom or on the side walls, as described previously.

The powder briquette consists of a mixture of powdered BaAl ₄ and powdered nickel. The particle size of BaAl ₄ powders is usually less than about 250 microns, the particle size of nickel powders is usually less than about 60 microns. The weight ratio between the two materials is usually between about 1.2: 1 and 1: 1.2; preferably, this weight ratio is about 1: 1. A powder briquette is locally formed by loading a mixture of loose powders into a container and pressing them using appropriate punches. On the upper surface of this briquette, the necessary depressions are formed in the radial direction in different quantities, from 2 to 8, as described in the aforementioned US patent 5118988.

The device of the present invention can also be obtained in a nitrogen-containing version: the use of gas trapping devices containing small amounts of nitrogen compounds such as iron nitride, Fe ₄ N, germanium nitride, Ge ₃ N ₄ , or intermediate iron and germanium nitrides is known in the art. The purpose of these components is to create small nitrogen pressures in the tube during the instantaneous evaporation of barium, thus making it possible to obtain a wider and more uniform deposition of barium.

 The invention will be further illustrated by the following examples. These non-limiting examples represent some designs intended to familiarize those skilled in the art with the practice of the present invention and to present the best possible mode of carrying out the invention.

 Example 1

A gas trap is made by using an AISI 304 steel container, it has a diameter of 20 mm and a height of 4 mm, and at the bottom there are protrusions 1 mm high, similar to those shown in FIG. 5. Inside the container, there is a mesh made of AISI 304 steel with cells 1.5 mm wide, located on the protrusions at the bottom. A homogeneous mixture is loaded into the container, which consists of 775 mg of powdered BaAl ₄ with a total barium content of 403 mg and 875 mg of powdered nickel. The powder mixture is then pressed inside the container using a punch of such a shape as to form 4 radial recesses on the surface of the briquette. The sample thus obtained was treated at 450 ^° C. for 1 hour in air to simulate welding sealing conditions. Then the sample is placed in a glass flask, which is connected to the evacuation system, the flask is evacuated and a barium evaporation test is carried out, following the method described in ASTM F 111-72, while heating the device using radio frequency electromagnetic waves for 35 seconds with such a power, so that evaporation begins 15 seconds after the start of heating. The test results are presented in table 1, which contains comments on the details of the evaporation and the ratio of the residue and the vaporized amount of barium.

 Example 2

The test of example 1 is repeated with a mixture comprising a nitrogen source substance formed from 785 mg of powdered nickel, 825 mg of powdered BaAl ₄ and 40 mg of Fe ₄ N. The test results are presented in table 1.

 (Comparative) example 3.

 The tests of Example 1 are repeated, but without adding a wire mesh to the powder briquette. The test results are presented in table 1.

 (Comparative) example 4.

 The tests of Example 1 are repeated, but using a flat punch for pressing the powders in the container, so that the briquette does not receive radial recesses. The test results are presented in table 1.

 (Comparative) example 5.

 Repeat the tests of example 1, but using a container with a flat bottom and placing a wire mesh at its bottom. The test results are presented in table 1.

 (Comparative) example 6.

 The tests of Example 1 are repeated, but using a sample in which the mesh merges with the surface of the powder briquette: this sample is obtained by loading a mixture of powders into a container, placing the mesh on top of the powders and pressing everything together using a flat punch. The test results are presented in table 1.

 As is clear from the results in the table, only the devices of the present invention (examples 1 and 2) are apparently weldable, since they do not show problems associated with the swelling or ejection of the briquette from the powders or with the melting of the container; in addition, these devices provide a barium yield of 300 mg or more. In contrast, with all other devices, there are problems of swelling or ejection of powders, full or partial, or even the entire device is melted.

Claims (15)

1. Weldable evaporated getter device with a high output of barium containing a metal container, open at the top, a mixture of powdered BaAl ₄ and nickel in a container in the form of a briquette, on the upper surface of which are made radial recesses, a non-continuous metal element, essentially flat in shape and parallel to the bottom container, characterized in that the metal element is immersed in a briquette of powders in a position remote from the bottom of the container so as not to merge with the free surface of the briquette itself.  2. The device according to claim 1, characterized in that the metal element immersed in the briquette of powders is made in the form of a cut-out metal plate having the shape of a chamomile.  3. The device according to p. 1, characterized in that the metal element immersed in the briquette of powders is made in the form of a cut out metal plate having many holes distributed over its surface.  4. The device according to claim 1, characterized in that the metal element immersed in the briquette of powders is made in the form of a wire mesh. 5. The device according to claim 1, characterized in that the first part of the briquette of powdered BaAl ₄ and nickel is placed on the bottom of the container, on which there is a flat metal element coated with the remainder of the briquette of powdered BaAl ₄ and nickel, on the upper surface of which there are radial recesses .  6. The device according to claim 1, characterized in that the metal element is provided with protrusions made in the form of installation protrusions and immersed in a briquette of powders, so that the installation protrusions are in contact with the bottom of the container, while there are radial recesses on the upper surface of the powder briquette .  7. The device according to claim 1, characterized in that the side walls of the open top container are provided with protrusions that support a flat metal element, while radial recesses are provided on the upper surface of the powder briquette.  8. The device according to claim 1, characterized in that on the bottom of the container open at the top there are protrusions that support a flat metal element, while radial recesses are provided on the upper surface of the powder briquette.  9. The device according to claim 8, characterized in that the protrusions at the bottom of the container are round in shape. 10. The device according to claim 1, characterized in that the particles of BaAl ₄ powders have a size smaller than 250 microns.  11. The device according to p. 1, characterized in that the particles of nickel powders have a size less than 60 microns. 12. The device according to claim 1, characterized in that the powders of BaAl ₄ and Nickel are presented in a weight ratio of between about 1.2: 1 and 1: 1.2. 13. The device according to p. 12, characterized in that the powders of BaAl ₄ and Nickel are presented in a weight ratio of about 1: 1.  14. The device according to claim 1, characterized in that the recesses on the surface of the briquette of powders are presented in an amount of from 2 to 8.  15. The device according to claim 1, characterized in that the powder mixture also includes a nitrogen source substance selected from iron nitride, germanium nitride or intermediate iron and germanium nitrides.

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