DD287810A5 - METHOD FOR PRODUCING A HEATING LAYER - Google Patents

Abstract

The invention relates to a method for producing a heating layer for semiconductor gas sensors. It is a Aufschlaemmung of graphite oxide applied to the Heizflaeche of the sensor. The excess slurry of graphite oxide is removed by a subsequent air flow and the graphite oxide adhered to the heating surface is dried at low temperature. By subsequent slow heating, the graphite oxide is decomposed, whereby a heating layer of a graphitaehnlichen conductive film is formed, which is thermally very stable {heating layer; heating surface; Airflow; Contacting, thermal decomposition; Semiconductor gas sensor; slurry; graphite}

Description

Characteristic of the known state of the art

In planar gas sensors, heating layers of metal oxides, e.g. from ruthenium oxide (Oyabu, T .; J. of Appl. Phys. 53, p.7125-7130) was applied to the substrate of the gas sensor by screen printing. The raw materials used for these heating layers are expensive and difficult to obtain. In addition, the application of the heating layers by screen printing is limited only on flat surfaces.

Gas sensors in which the sensitive layer is located on a tube are generally heated by the fact that inside the tube is a metallic resistance wire, usually formed as a helix (e.g.

DE-AS 2640868). The introduction of the usually consisting of a very thin resistance wire heating coil in the interior of the tube is difficult without special tools and a uniform distance of the heating coil of the inner wall of the tube is not always guaranteed. In addition, the heat transfer is not optimal even with direct contact between the heating wire and the tube.

Goal of Erflndi ... ^

The object of the invention is to develop a method which makes it possible to reproducibly produce a heating layer on flat or otherwise shaped surfaces or in the interior of thin tubes in simple technology and with cheap starting materials which is suitable for the heating of semiconductor gas sensors.

Explanation of the essence of the invention

The object of the invention is to produce a heating layer for semiconductor gas sensors, which has a good adhesion to the heating surface and thus ensures a good heat transfer, which is thermally stable and ensures reliable contact with the power supply wires.

According to the invention this is achieved in that a slurry of graphite oxide is applied to the heating surface, with a subsequent air flow, the excess slurry of graphite oxide removed and adhering to the heating surface graphite oxide is dried at a temperature below 40 ⁰ C and then at a slow increase in temperature to 1000 ° C is heated. In this case, the graphite oxide decomposes with elimination of carbon dioxide and carbon monoxide and the gobiertes graphite-like and conductive film has a good adhesion to the heating surface. The contact with the power supply can be done before or after.

embodiment

For manufacturing a heatable tubular semiconductor gas sensor, a tube of a ceramic material of 2 mm in outer diameter and 12 mm in length is used. The inner diameter of the tube is 1 mm. For the power supply platinum wires of 25mm length and 0.1mm diameter are used. The first platinum wire is inserted from one side 8 mm into the interior of the tube and fixed by means of a refractory cement to the inner wall of the tube. The second platinum wire is also inserted 8 mm into the tube from the other side and fixed with the refractory cement so that it comes to rest on the inner wall of the tube exactly opposite the first platinum wire. After a brief baking of the putty and cooling of the tube, a slurry of 15% graphite oxide freshly prepared by known procedures is drawn into the interior of the tube in water. The graphite padding remains in the tube for about 2 minutes, rotating it constantly to prevent segregation. By means of a slow stream of air, the excess graphite oxide slurry is then blown out of the tube. On the inner wall of the tube and attached to the platinum wires there remains a thin layer of graphite oxide, which is dried for 45 minutes at 25 ⁹ C with an increased air flow. The tube containing the graphite oxide is then heated slowly in a special heating furnace, the temperature being increased at intervals of 5 minutes by 10 ° C. The heating is stopped when a temperature of 1000 ^° C. is reached Examination of

Heating layer to the protruding from d6m tubes platinum wires applied a voltage of 12 volts. After 5 minutes, a temperature of 300 ° C is reached in the middle of the tube outside. By further increasing the voltage, the tube is heated in the middle to red hot (about 800 ⁰ C). The tested in this way tube is then further processed in a known manner to a Meta'loxid gas sensor. By means of the heating layer, the applied sensitive semiconductor layer, which in this example consists of tin dioxide with additives, is baked at a temperature of about 800 ^° C. Subsequently, the sensor is operated at a temperature of 300 ⁰ C.

 The heating layer worked during the life of the sensor (12 months) without interference.

Claims (1)

A process for the preparation of a heating layer for semiconductor gas sensors, characterized in that applying a slurry of graphite on a heating surface is removed with a subsequent air flow, the excess slurry of graphite and dried, the adhesive remaining on the heating surface graphite at a temperature below 4O ⁰ C and is then heated at a moderate temperature increase to 1000 ⁰ C until the decomposition of the graphite oxide and that the contact to the power supply before or after done. Field of application of the invention The invention is applicable to heating for all forms of semiconductor gas sensors used for the determination of gases and vapors.