Alkali metal vapour dispenser

Abstract: Cesium dispenser with a metal carrier in which silicon-­ alkali metal compounds or germanium-alkali metal compounds are present along the grain boundaries of the carrier material (for example, Al or Ag).

Description: The invention relates to a device for releasing metal vapour of an alkali metal by heating. Such devices (dispensers) are used, for example in tubes comprising photocathodes (brightness intensifiers, X-ray image intensifiers) and photomultiplier tubes in which, for example a thin layer of cesium is provided so as to decrease the work function of the electrons. This type of dispenser may also be used in display tubes comprising semiconductor cathodes. A device of the type mentioned in the opening paragraph is described in GB-A-1,265,197 in which the powder comprises an alkali chromate. When heating the powder, which comprises cesium chromate in the example shown, this chromate is decomposed so that pure cesium is released. One of the drawbacks of such a device is that the dimensions of the pulverulent grains of the chromate are so small that when filling the holders these grains are irregularly supplied (poor flowing characteristics). This makes it difficult to manufacture the dispensers in a reproducible manner. A second drawback is the emission of unwanted gases during the supply of the alkali metal. It is true that such dispensers often comprise silicon and zirconium-aluminium in addition to the chromate for binding oxygen which is being released during the decomposition reaction, but notably zirconium-aluminium emits hydrogen and hydrocarbon gases at the decomposition temperature of the various alkali chromates (700-800°C), while also the envelope, which usually consists of nickel-chromium steel, emits these gases, notably carbon-containing gases; particularly the latter gases have a detrimental influence on the operation of photocathodes and semiconductor cathodes. Moreover, the alkali metal is supplied from the decomposition temperature; the supply of the alkali metal is therefore difficult to control or is not controllable at all. It is an object of the invention, inter alia to provide a device of the type described in the opening paragraph which can be manufactured in a more reproducible manner. It is a further object of the invention to provide a device, in which the release of the alkali metal vapour is controllable. Moreover, it is an object of the invention to reduce the emission of the unwanted gases as much as possible. To this end a device according to the invention is characterized in that the device comprises a metal carrier and in that within the metal carrier material a silicon alkali metal compound or germanium alkali metal compound for releasing said metal vapour is present. The use of a silicon alkali metal compound is known per se in devices for releasing alkali metal vapours from US-A-3.096.211. A silicate (which may be a silicate of the metal which is to be released) is shown which binds (in the example shown) a cesium chromate and acts at the same time as a moderator or decelerator, but does not release alkali metal vapour. An embodiment of a device according to the invention is characterized in that the silicon alkali compound or the germanium alkali compound is present along grain boundaries of the carrier metal. The invention is based on the recognition that this can be achieved by releasing the alkali metal by means of diffusion instead of by a decomposition reaction. Furthermore, the invention is based on the recognition that such a release method can be realised by refraining from pulverulent mixtures. Preferably, the carrier is wire-shaped. In the case of a silicon-cesium compound (presumably CsSi₄) cesium is released at approximately 530°C and diffuses to the exterior. The diffusion rate and hence the cesium supply can be controlled by means of the heating temperature. Since the filling process is now replaced by a diffusion process of alkali metal in the carrier material, such a dispenser can be manufactured in a reproducible manner. Moreover, the emission of unwanted gases is considerably lower due to the lower operating temperature. Suitable metals for the carrier are Al, Ag, Cu, Fe, Pt, Ti, V and W, while preferably Cs, Na, K and Ru are chosen for the alkali metals. Sodium, potassium and rubidium are very suitable for use in, for example brightness intensifiers and X-ray image intensifiers comprising photocathodes, while cesium is notably used in photomultiplier tubes and (display) tubes based on semiconductor cathodes. To obtain a directed release of vapour, the carrier may at least partly be provided with a layer of, for example chromium which is impenetrable to the alkali metal. This is notably advantageous in a device as proposed in Netherlands Patent Application NL-A-8700486 (PHN 12.047) in the name of the Applicant, in which device a plurality of semiconductor cathodes is mounted side by side so that a slit-shaped aperture may be used in a cesium dispenser facing the row of cathodes. Moreover, the extra layer protects the wire from melting. A directed release of the alkali metal can also be obtained by accommodating the carrier in a holder which has an aperture of the desired shape (for example, a slit or directed apertures). The invention will now be described in greater detail with reference to an example and the accompanying drawing in which Figure 1 shows diagrammatically a first embodiment and Figure 2 shows diagrammatically a second embodiment. A device according to the invention is obtained by starting from a preferably wire-shaped holder of silver or aluminium containing up to 10% by weight of silicon. Since the carrier material does not react with or hardly reacts with the silicon, the latter is predominantly present along the grain boundaries of the carrier material. This silicon may form a silicon-cesium compound with cesium (for example CsSi₄) in a manner which is analogous to that in EP-A-0 360 316 (PHN 12.657) filed simultaneously, in which the holder is coated with cesium metallic under suitable circumstances (10⁵ Pa (1 atm) argon pressure, T = 28°C) and is subsequently heated. During heating cesium diffuses along the grain boundaries and is bound to the silicon (to, for example CsSi₄). The duration of the diffusion is dependent on the temperature used (several hours at 550° C). The compounds thus formed are shielded from their ambient so that the device is substantially chemically stable. Such a wire-shaped cesium source (dispenser) (Figure 1) can be mounted in a vacuum tube (not shown), for example opposite a row of semiconductor cathodes as described in Netherlands Patent Application NL-A-8700486 (PHN 12.047). During heating cesium is released from the CsSi₄ from approximately 530°C; this cesium diffuses to the exterior via the grain boundaries. The diffusion rate and hence the cesium supply is controllable by adjusting the temperature. The wire (carrier) may be simultaneously carrier and heating wire. A directed supply of the cesium can be obtained by chromium-plating the larger part of the surface area of the wire 1 so that a narrow slit 3 remains in a protective layer 2 for the supply of the cesium. Moreover, the wire is now better protected against melting because the melting point is about 600° C. To prevent unwanted gas supply during the supply of cesium, the slit may be provided with a thin layer of gold or galvanic nickel. These materials pass cesium, but are barriers to oxidizing gases. Such a supply can also be achieved by accommodating the wire 1 for the greater part or entirely in a nickel-chromium envelope 4 which has one or more apertures (for example a slit 5) in the way desired (Figure 2). The invention is of course not limited to the example described. Other metals which are electrically conducting and do not react with cesium up to approximately 600°C can also be chosen for the carrier. These requirements are satisfied by, for example nickel, copper, iron, platinum, titanium, vanadium and tungsten. Instead of cesium a different alkali metal may alternatively be chosen, for example sodium or potassium, while silicon may be replaced by germanium. The manufacturing conditions then of course change, while also the metals suitable as a carrier need not be the same. In addition to the said example of the thin vacuum tube, alkali metal sources according to the invention can also be used very satisfactorily in other electron tubes (High Brightness Gun) and also in devices comprising photocathodes (brightness intensifiers, X-ray image intensifiers) and in photomultiplier tubes. Also suitable metal wires or other metal parts present in a vacuum tube may be used as carriers such as bonding flaps or interconnection wires of for instance semiconductor cathodes.

Claims: A device for releasing metal vapour of an alkali metal by heating, characterized in that the device comprises a metal carrier and in that within the metal carrier material a silicon alkali metal compound or germanium alkali metal compound for releasing said metal vapour is present. A device as claimed in Claim 1, characterized in that the silicon alkali compound or the germanium alkali compound is present along grain boundaries of the carrier metal. A device as claimed in Claim 1 or 2, characterized in that the carrier is wire-shaped. A device as claimed in Claim 1, 2 or 3, characterized in that the carrier comprises a metal of the group of aluminium, silver, copper, iron, platinum, titanium, vanadium and tungsten. A device as claimed in any one of the preceding Claims, characterized in that the alkali metal belongs to the group of sodium, potassium, rubidium and cesium. A device as claimed in any one of the preceding claims, characterized in that the carrier is at least partly coated with a layer which is impenetrable to the alkali metal. A device as claimed in Claim 6, characterized in that the layer which is impenetrable to the alkali metal comprises apertures at the area at which the carrier is provided with a material penetrable by the alkali metal. A device as claimed in Claims 6 or 7, characterized in that the impenetrable layer comprises chromium. A device as claimed in Claim 7 or 8, characterized in that the material penetrable by the alkali metal comprises gold or tin. A device as claimed in any one of Claims 1 to 8, characterized in that the carrier is present in a holder having at least one aperture. A device as claimed in Claim 10, characterized in that the aperture is slit-shaped. An electron tube, characterized in that it comprises a device as claimed in any one of Claims 1 to 11. An electron tube, characterized in that it comprises a semiconductor cathode or a photocathode and a device as claimed in any one of Claims 1 to 11.

Inventors: SMOORENBURG ARNOLDUS DIRK JOHA, VAN DER MAREL CORNELIS, SMOORENBURG, ARNOLDUS DIRK JOHANNUS, VAN DER MAREL, CORNELIS
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1989
Main classification: H01J9
IPC codes: H01J9