FR2649827A1 - Device for heating an alumina tube and discharge tube fitted with such a device - Google Patents

Abstract

Device 5 for heating an alumina tube 10, employing a heating element 20 coiled around the said tube. According to the invention, the said device consists, in order starting from the surface of the tube 10, of a sanding 30 on the wall of the tube, a first layer 40 of alumina on which the heating element 20 is coiled, and a second layer 50 of alumina covering the said heating element. Application to discharge tubes comprising an alumina tube placed in a vacuum enclosure.

Description

"Heating device for an alumina tube and discharge tube provided with such a device".

Description:
The present invention relates to a device for heating an alumina tube using a heating element wound around said tube. It also relates to a discharge tube comprising an alumina tube placed in a vacuum enclosure and containing an alkali metal.

 The invention finds an advantageous application in the general field of heating an alumina tube and, more particularly, in that of preheating the internal tube fitted to a discharge tube.

 It is often necessary, in various applications, to provide heating for an alumina tube. In the case of a discharge tube for example, the heating of the alumina tube placed in a vacuum enclosure must allow, firstly, to bring the tube to a temperature close to 600 to 700 "C and, in a second step, to withstand a temperature of 1200 to 15000C outside operation.

 For this purpose, it is usual to use heating elements wound on the tube. However, this known solution is difficult to implement due to the poor quality of the thermal contracts with the alumina tube, the insufficient resistance to temperatures of 1200 to 1500 "C, as well as the difficulty of obtaining good -mechanical stability.

 Also, the technical problem to be solved by the object of the present invention is to provide a device for heating an alumina tube conforming to the preamble which would make it possible to obtain good thermal contact between the tube and the heating element and satisfactory mechanical and thermal resistance.

 The solution to the technical problem posed consists, according to the present invention, in that said heating device consists, in order from the surface of the tube, of a sanding of the wall of the tube, of a first layer alumina on which the heating element is wound, and a second layer of alumina covering said heating element.

 The heating device thus produced is mechanically very solid and compact, it withstands shock and vibration well. It also has excellent temperature resistance and ensures perfect thermal contact between the tube and the heating element.

 Certain discharge tubes consist of a vacuum enclosure in which an alumina tube containing an alkali metal is placed, the discharge occurring through the vapor of said alkali metal. When such a discharge tube is put into operation, it is necessary to vaporize the alkali metal which is initially in the solid state. In order to favor priming of the tube, the alumina tube must therefore be preheated. The use for this purpose of a heating device according to the invention is particularly recommended not only because of the advantages mentioned above, but also because it only gives rise to very low degassing.

 The description which follows with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be implemented.

 Figure 1 is a sectional view of a device for heating an alumina tube according to the invention.

 FIG. 2 is a sectional view of a discharge tube comprising heating devices similar to that of FIG. 1.

 FIG. 1 shows, in section, a device 5 for heating an alumina tube 10, using a heating element 20 wound around the tube 10. In the example illustrated in FIG. 2, the heating element 20 is in the form of a tungsten filament, or any other metal or alloy.

 FIG. 2 shows that the heating device 5 consists, firstly from the surface of the alumina tube 10, of a sandblasting 30 of the wall of the tube, intended to promote the attachment of the alumina in fusion. Then, a first layer 40 of alumina is deposited, by projection of alumina powder on the cold wall of the tube by means of a plasma torch at approximately 15,000 ° C. over a typical thickness of a few hundred microns. The heating filament 20 is then wound on this first layer 40 of alumina by means of suitable tools. Finally, a second layer 50 of alumina, which may have the same thickness as the first layer 40, covers said heating filament 20 and thus provides electrical insulation, mechanical strength and thermal diffusion towards the tube 10. Heating by the heating element 20 makes it possible to obtain a temperature of 7000C of the alumina tube. Excluding the heating operation and the discharge tube operation, this temperature can reach 1400cl.

The layers 40 and 50 are shown in FIG. 1 on an exaggerated scale with respect to the section of the tube 10.

 FIG. 2 shows, in section, a discharge tube comprising an alumina tube 10 placed in an enclosure 60 under vacuum and containing an alkali metal 70, such as cesium. The tube 10 also includes electrodes 80 intended to cause an electrical discharge in the cesium vapor after the latter has been vaporized. It is for the purpose of preheating the alumina tube 10 in order to vaporize the cesium that said tube is provided with heating devices 5 of the type described in relation to FIG. 1. The heating elements 20 are connected to a current source not shown, while the electrodes 80 are connected to a high voltage source, also not shown.

Claims (2)

CLAIMS: 1. Device (5) for heating an alumina tube (10), using a heating element (20) wound around said tube, characterized in that it is formed, in order from the surface of the tube (10), of a sandblasting (30) of the wall of the tube, of a first layer (40) of alumina on which the heating element (20) is wound, and of a second layer (50 ) alumina covering said heating element. 2. Discharge tube comprising a tube (10) of alumina placed in an enclosure (60) under vacuum and containing an alkali metal1 characterized in that said tube ('0) of alumina is preheated using at least one heating device according to claim 1.