Alkali metal vapour dispenser

Abstract: Alkali metal source based on a powder of silicon or germanium grains with a shell from a compound of silicon or germanium and the metal.

Description: The invention relates to a device for releasing metal vapour of an alkali metal, comprising a holder which contains a powder of grains from which alkali metal is released upon heating. The invention also relates to a method of manufacturing such a device. 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 the 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 zirconiumaluminium 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. 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 powder comprise grains of silicon or germanium with a shell of a respective compound of silicon or germanium and the alkali metal, in which the diameter of the grains is at least 50 µm and at most 200 µm. It is found that when using such a powder, for example the combination of silicon-cesium, cesium diffuses from the powder already from 530°C. The extent of diffusion is temperature-dependent and can be satisfactorily controlled over a wide range. The said grains can be easily manufactured; the powder thus obtained has good flowing characteristics so that the holders can be filled in a reproducible manner. Moreover, since the diffusion takes place at a considerably lower temperature than the said decomposition reaction of cesium chromate, the emission of unwanted gases is also much smaller, all the more because additional mixtures such as zirconium-aluminium can now be dispensed with. The use of a silicon-alkali compound in an alkali-metal dispenser is known per se from US-A-4.195.891 but here, the compound is mixed with an additional mixture of zirconium-aluminium. In one of the examples using a silicon-potassium compound heating to 700°C is necessary to obtain a good yield, leading to the problems mentioned above of gas-emission. The powder is preferably introduced into a holder which is substantially tubular and has one or more apertures (for example a slit) for releasing the alkali metal. This provides the possibility of a directed metal vapour supply. In this connection tubular is understood to mean any regular or irregular cross-section (triangular, square, etc.), but preferably circular. Such holders are known per se from US-A-3.945.949, which shows a tubular holder for dispensing an alkali-metal vapour into an electric discharge tube. Preferably, sodium, potassium, rubidium or cesium are chosen as alkali metals. Sodium and potassium 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. A method of manufacturing such a device is characterized in that grains of silicon or germanium powder are mixed in an inert atmosphere with liquid alkali metal, and the mixture is heated, diffusing the alkali metal into the silicon or germanium grains, thereby forming a powder comprising grains of silicon or germanium with a shell of a respective compound of silicon or germanium and the alkali metal. Since the powder thus obtained is slightly hygroscopic and is usually not immediately stored in an evacuated space, the outer layer is preferably oxidized. The powder thus obtained is eminently protected from moisture absorption. The invention will now be described in greater detail with reference to some embodiments and the drawing in which Fig. 1 shows diagrammatically a device according to the invention. The device of Fig. 1 comprises a holder 2 which is substantially cylindrical in this example and which is made of, for example nickel-chromium. The holder 2 has metal caps 5 and electric terminals 3 at both of its ends for the passage of current. For a directed supply of the alkali metal vapour the holder 2 has a slit 4. The holder contains a powder from which an alkali metal, in this example cesium, is released upon heating. The heat treatment takes place because a current is passed through the walls of the cylindrical holder via the electric terminals 3. The relevant powder is obtained in this example from silicon powder having a grains size of between 50 and 200 µm which is mixed with cesium in an inert argon (or nitrogen) atmosphere. Pressure and temperature are such (for example, 1 atmosphere, 28°C) that the silicon powder is in close contact with the cesium. During a subsequent temperature increase to approximately 550°C the cesium diffuses into the silicon and forms a shell comprising a cesium-silicon compound (presumably CsSi₄). The rate of this diffusion process is dependent on the temperature and the thickness of the shell of the quantity of cesium. The powder thus obtained is very suitable for manufacturing processes in which reproducible dispenser properties are obtained. Notably the grain size is very favourable for a continuous filling of chutes from which the holders 2 are manufactured (good flowing characteristics). When used in a cesium dispenser it is found that the powder already releases cesium in vacuo from 530°C because the silicon-cesium compound decomposes and the cesium is released via diffusion. Since the cesium supply is determined in the first instance by this diffusion, an adjustable supply by means of temperature control is possible. However, the powder thus formed is slightly hygroscopic. This is no drawback if substantially all manufacturing steps ranging from manufacture to assembly in an electron tube or photocathode substantially entirely take place in vacuo or in an inert atmosphere. Since in practice the powder is often stored temporarily, it is heated in air for some time (for example 60 min. at 250° C) so as to inhibit the hygroscopicity. Instead of silicon powder germanium powder may also be used as a starting material, while also various other alkali metals can be chosen (sodium, potassium, rubidium). Dependent on the combinations chosen, the manufacturing conditions (pressure, temperature) must also be chosen differently so that notably the diffusion of the alkali metal into the silicon or germanium grains is ensured. The finished powder is introduced into a chute. Due to the good flowing characteristics of the powder, the chute is continuously filled with a substantially constant quantity of powder per unit of length. After filling, such a chute is folded up while leaving a narrow slit. The tube thus obtained is sawn whereafter the separate parts are provided with caps 5 and electric terminals 3. The dispensers may be used in photocathodes for brightness intensifiers and X-ray image intensifiers, in photomultiplier tubes and for providing material decreasing the work function (particularly cesium) on semiconductor cathodes for electron tubes.

Claims: A device for releasing metal vapour of an alkali metal, comprising a holder which contains a powder of grains from which the alkali metal is released upon heating, characterized in that the powder comprises grains of silicon or germanium with a shell of a respective compound of silicon or germanium and the alkali metal, in which the diameter of the grains is at least 50 µm and at most 200 µm. A device as claimed in Claim 1, characterized in that the holder is substantially tubular and has a slit for releasing the alkali metal. A device as claimed in Claim 2, characterized in that the holder is made of metal and that its ends are provided with terminals for the passage of current. 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 holder consists of nickel-chromium steel. A method of manufacturing a powder for use in a device for releasing metal vapour of an alkali metal, characterized in that grains of silicon or germanium powder are mixed in an inert atmosphere with liquid alkali metal, and the mixture is heated, diffusing the alkali metal into the silicon or germanium grains, thereby forming a powder comprising grains of silicon or germanium with a shell of a respective compound of silicon or germanium and the alkali metal. A method as claimed in Claim 6, characterized in that the outer layer of the grains is oxidized after the diffusion. An electron tube, characterized in that it comprises a and a device as claimed in any one of Claims 1 to 5. An electron tube, characterized in that it comprises a semiconductor cathode or a photocathode and a device as claimed in any one of the Claims 1 to 5.

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