WO2004090924A2

WO2004090924A2 - Evaporable getter device for projection tubes

Info

Publication number: WO2004090924A2
Application number: PCT/IT2004/000130
Authority: WO
Inventors: Daniele Martelli; Corrado Carretti; Giorgio Longoni
Original assignee: Saes Getters S.P.A.
Priority date: 2003-04-07
Filing date: 2004-03-17
Publication date: 2004-10-21
Also published as: CN1739180A; ITMI20030679A1; WO2004090924A3

Abstract

There is disclosed an evaporable getter device particularly suitable for being used in tubes for the projection of images on wide screen; the device (10), being ring-shaped, comprises a container (11) of powders (12) of barium or calcium compounds, formed of a bottom wall (22) connected to an external wall (20) and an internal wall (21), both sloping or curved towards the outside in their upper portion, to obtain a distribution of barium or calcium vapours compatible with the application.

Description

ΕVAPORABLE GETTER DEVICE FOR PROJECTION TUBES"

The present invention refers to an evaporable getter device for projection tubes.

Projection tubes belong to the family of cathode ray tubes (generally known in the field as CRTs) and are used for reproducing images on big dimensions screens (generally greater than 50 inches), both for domestic use, for the so-called "home-cinema" use, and for public or private-projections, for example in the case of presentations during conferences. The reproduced image may be in black and white, but more commonly it is in color; in the latter case three tubes are employed, each capable of projecting an image in one of the three fundamental colors, the resultant of which is the final color image. Projection tubes as generally known with the abbreviation PRTs. Projection tubes are similar for geometrical construction and operational principles to the tubes used in traditional television sets (defined in the following of the "direct vision" type); figure 4 shows in an extremely schematic view the cross-section of a CRT tube. A CRT comprises a sealed and evacuated glass tube T, having a front portion S forming the screen and a rear portion N, connected by a conical or pyramidal portion F; in zone N there is an electron gun G kept in position by suitable centering devices C (only one shown in the figure). The electron beam produced by the gun is focused and directed through the "magnetic yoke", a magnetic deflector arranged outside of the tube in a slightly advanced position with respect to the gun (approximately in the area of connection between the portions N and F), in such a way to scan all the points of the screen; on the screen there is a deposit of materials, called phosphors, which, when stimulated by the incident electrons, re-emit the energy absorbed in the form of visible light, thus generating the image. The internal surface of the portions F and S of the tube is made equipotential through the deposit of a thin layer of graphite and/or aluminum, to avoid the generation of localized electric fields which could deflect the electron beam; during the operation of the tube, between this equipotential area and the gun area there is a potential difference of the order of tens of thousand volts whereby, to avoid short circuits, between the two areas there is interposed a layer of insulating material R, with which the centering devices C get in contact. In all these tubes (PRTs and direct vision tubes) it is fundamental that the internal space of the tube is kept under high vacuum (pressures of the order of about 10^"4 - 10^"5 Pa) to avoid the diffusion of the electron beam due to collisions with gas molecules, which would result in the impossibility to form a clear image on the screen; furthermore, the partial pressure of oxidizing species has to be kept at pressures lower than about 10^"7 Pa, to avoid contamination of the cathode.

As it is known, to keep the vacuum, on a portion of the internal surface of the tube a thin deposit of active metal is generally formed, which is capable of interacting with almost any gas (except the rare gases and the hydrocarbons) by chemically fixing them; as the active metal it is possible to use calcium or barium, but in the remaining description reference will be made mainly to the latter, which is the more commonly used metal. The barium deposit is formed by evaporation from a compound thereof, generally BaAl , when the tube has been evacuated and sealed. For this purpose it is known to use the so-called evaporable getter devices, which are generally formed of an open metallic container wherein it is present compressed powder of the barium compound, or a compressed mixture of powders of said compound and of a promoter of the barium evaporation, generally nickel. To avoid the deposit of barium onto undesirable areas inside the tube, it is also known to shape the walls of the metallic container, or to add suitable deflectors to the container, so as to direct the vapours of the metal in preferred directions; evaporable getter devices with shaped walls or deflectors are known from several patents, among which USA patents No. 2,869,014, 2,907,451, 3,195,716, 3,996,488 and 6,071,080 and the Korean patent No. 314,997.

The barium evaporation inside the CRTs may, however, cause several problems. A first problem is that of the so-called "arc currents"; the deposit of amounts, even small, of barium on the layer R may jeopardize its electric insulation properties, thus causing in the system even intense electric discharges which, flowing through the centering devices of the gun (C), may damage the control electronic components of this latter.

A second problem is that of the parasitic emission (known in the field with the definition "stray emission"), due to the barium which deposits on the grids of the electron gun; the potential difference acting in this area of the tube may cause the extraction of electrons by field effect from barium deposits; the so released electrons are then accelerated towards the screen, but since they are not emitted by the cathode are deflected by the yoke in an anomalous way and not as a part of the collimated beam, giving rise to phenomena of spurious luminosity on the screen.

In the case of direct vision tubes the incidence of these phenomena is more limited or easier to be solved. In these tubes the image is visualized directly on the screen, therefore it is sufficient that the electrons of the beam arrive thereon with an energy sufficiently high to excite the phosphors; as a consequence of this functional request, the potential differences between the electron gun and the screen are generally lower than about 25 kV; furthermore, the possible deposit of thin layers of barium on the screen area does not impair the quality of the image; the possibility that the barium also gets on the screen allows to arrange the evaporable getter device approximately in the middle of the F area and mainly directed towards the F and S areas ("antenna" position), thus minimizing the possibility of deposit both on R layer and on the gun grids.

On the contrary, in the case of PRTs, the image must be projected outside the tube, at distances even of meters; to keep the focusing of the image at these distances, the energy with which the electron beam strikes on the screen is necessarily higher than that required in direct vision screens; the functional consequences are that the potential differences between S area and gun are in this case higher, of the order of 30 kV, and that in this case it is necessary to minimize the deposit of barium on the screen. These requests oblige to arrange the evaporable getter device as far as possible from the screen; in practice, the device is generally arranged close to the electron gun, and normally concentric and anterior thereto. This arrangement is however critic due to the problems of arc currents and stray emission, made even more critic in this case by the higher potential differences.

Moreover, since a PRT is generally of lower size than a direct vision tube, the space available for the barium deposit is smaller, but it is in any case necessary to provide a suitable surface of deposit to ensure the gas sorption speed and capacity required by the application.

An evaporable getter device for PRTs has thus more stringent and partly contrasting requirements than one for direct vision screens, since it must ensure (more than it is required by the direct vision tubes) that barium does not deposit on undesirable areas, but also that the metal is present in the form of a sufficient diffused deposit. The known getter devices, developed for direct vision screens, are not capable of satisfying these needs, causing too much localized deposits and therefore of reduced sorption speed and capacity (as in the case of the devices of US patents No. 2,869,014 and 2,907,451) or because they are not reliable in avoiding the barium deposit in undesirable areas (as in the case of the devices of the other above mentioned patents).

The object of the present invention is to provide an evaporable getter device which solves the above mentioned problems, particularly ensuring the reduction of the phenomena of arc currents and of stray emission and simultaneously gas sorption speed and capacity suitable for the use in projection tubes.

This object is achieved according to the present invention by means of an evaporable getter device as defined in claim 1 and in the claims depending thereon.

The invention will be described in the following with reference to the drawings wherein:

- Fig. 1 shows the device of the invention in a perspective view;

- Fig. 2 shows the device of the invention in a partially cut-out view; and

- Fig. 3 shows in a cut-out view a detail of the device of the invention, pointing out some characteristic dimensions and angles of its geometry; - Fig. 4 shows schematically a sectional view of a generic CRT tube.

Referring to figure 1, it is shown that the device of the invention, 10, is formed of a ring-shaped (that is, such that the top view thereof is a ring annulus) container 11, generally made of metal, inside which there is a packet of powders, 12, comprising at least a material capable of releasing barium or calcium.

This device satisfies the above explained requirements thanks to its particular geometrical features, as shown in detail with reference to figures 2 and 3. The container of the device is defined by an external wall 20, an internal wall 21 and a bottom wall 22 connected to each other; the two walls 20 and 21 are formed of two lower portions, respectively 23 and 24, parallel to axis X-X' of the device, and two upper portions, respectively 25 and 26, bent or curved outwards; portions 23 and 24 have respectively radius Ri and R₂. The lower portion of the container, wherein the packet of powders 12 is present, has thereby a substantially rectangular cross-section.

The dimensions and orientation of portions 25 and 26 are of fundamental importance to achieve the objects of the invention. The portion 25 is essentially a frustoconical section, and is bent outwards in such a way that, in a diametral plane of the device, said portion forms with portion 23 an angle comprised between 20° and 50°, and preferably of about 45°; the outward extension of portion 25 is such that its radius R_e at its most external point, 27, is comprised between Ri and 1,1 times Rj. The portion 26, when seen in section, may be defined by a segment (thus resulting substantially a frustoconical section) or by a curved line; this portion is bent or curved outwards in such a way that, on a diametral plane of the device, the tangent to the external edge of said portion forms with wall 24 an angle β comprised between 5° and 50°, preferably comprised between 20° and 40° and more preferably of about 30°; the outward extension of portion 26 is such that its radius Ri at its most external point, 28, is comprised between R₂ and Ri, and preferably is such that is valid the relation: R₂ + (Ri - R₂) / 2 ≥R; > R₂.

Finally, a last condition necessary to achieve the objects of the invention is that the distance di between the points 27 and 28, in a cross-section along a diametral plane of device 10, is equal to at least 0.8 times the difference Ri - R₂. This last condition may be obtained in different ways by operating on the lengths of portions 25 and 26 and on angles e β. In an alternative embodiment, to obtain tins condition it is possible to operate on the height of the lower portions 23 and 24; in particular, called hi and h respectively the heights of the portions 23 and 24, it is possible to act in such a way that h₂ >1,5 hi. Container 11 is generally made of steel, in particular of type AISI 304 or

AISI 305, with a thickness comprised between about 0.10 and 0.20 mm.

As mentioned in the introduction, with the devices of the invention it is possible to evaporate both barium and calcium. Compounds, mixtures and conditions for the evaporation of barium are widely known in the field, for example from the mentioned patents or from US patents No. 4,642,516, 4,961,040, 6,104,138 and 6,306,314.

As to the use of calcium, proposed only in the last years, as a precursor compound it is possible to use the compound CaAl₂, subject of US Pat. No. 6,583,559 Bl, or calcium-rich Ca-Ba-Al ternary alloys, subject of published PCT application WO 03/038139; these compounds have characteristics of evaporation that are different from that of barium compounds, and for the evaporation of calcium therefrom it is possible to use the two steps method disclosed in published PCT application WO 03/043047. In case of devices for the evaporation of calcium, as promoter of evaporation it is preferable to use titanium. Finally, it is possible to apply to the devices of the invention arrangements known in the field to improve the characteristics of porosity of the deposit (which mainly increase the gas sorption speed), such as for example adding to the packet of powders a nitrogenated compound, such as Fe N, in a percentage by weight comprised between about 2% and 4% of the total weight of the powders. The device of the invention is similar for geometrical construction to that described with reference to figure 1 in patent US 2,907,451. This prior patent shows however only one generic shape of the device, with no construction details; the device of the invention is different therefrom above all for the fact that the projection of the portion 26 "falls" on the packet of powders 12, and for the greater size of the outlet conduit for the metal vapours, defined by d_\. The device of the prior art, designed for use in direct vision screens, is unable of satisfying all the requirements of the PRTs, and particularly it can not provide a sufficient diffusion of barium, such to ensure high gas sorption performances.

Claims

1. An evaporable getter device (10) formed of a metallic ring-shaped container (11) upperly open wherein is present a packet of powders (12) comprising at least a material capable of releasing barium or calcium, said container being formed of an external wall (20), an internal wall (21) and a bottom wall (22) connected to each other, said external wall comprising a lower portion (23) parallel to axis (X-X') of the device and an upper portion (25) bent outwards, said internal wall comprising a lower portion (24) parallel to axis (X-X') of the device and an upper portion (26) bent or curved towards the outside of the device, characterized in that: said lower portion (23) of the external wall (20) has a radius Ri; said lower portion (24) of the internal wall (21) has a radius R₂ < Ri ; said upper portion (25) of the external wall (20) is such that it forms with said lower portion (23) of the same wall an angle a comprised between 20° and 50°, and its length is such that for its radius at the most external point (27), R_e, it is valid the relation: l.lRi >R_e > Ri; said upper portion (26) of the internal wall (21) is bent or curved towards the outside, in such a way that the tangent to the external edge of said upper portion forms with lower portion (24) of the same wall an angle β comprised between 5° and 50°, and its length is such that for its radius at the most external point (28), R$, it is valid the relation: R_t _≥ _\ > R₂; and the distance, di, measured on a diametral plane of the device between said most external points of said upper portions is such that di >0,8(Rι -

R₂).

2. A device according to claim 1 wherein said angle αis about 45°.

3. A device according to claim 1 wherein said angle β is comprised between about 20° and 45°.

4. A device according to claim 3 wherein said angle β is about 30°.

5. A device according to claim 1 wherein said radius Rj is such that is valid the relation: R₂ + (Ri - R₂) / 2 >R_; > R₂.

6. A device according to claim 1 wherein the height h of the lower portion of the internal wall is equal to at least 1.5 times the height hi of the lower portion of the external wall.

7. A device according to claim 1 wherein said barium releasing material is BaAl₄.

8. A device according to claim 7 wherein said packet of powders also comprises nickel.

9. A device according to claim 1 wherein said calcium releasing material is CaAl₂.

10. A device according to claim 9 wherein said packet of powders also comprises titanium.