GB2288908A - Ceramic thermocouple - Google Patents

Abstract

A ceramic thermocouple (10) comprises two ceramics, one (11) of which comprises silicone carbide and the other (13) of which comprises molybdenum disilicide. One (13) of the ceramics is resiliently urged towards the other (11) by a spring (15) so as to form a junction (14). <IMAGE>

Description

CERAMIC THERMOCOUPLE The present invention relates to a thermocouple assembly and more particularly to a thermocouple for measuring the temperature of a high temperature and corrosive environment by measuring the electromotive force generated at the junction of two semi-conducting ceramics.

It is known that two different metals when combined produce an electromotive force by the Seebeck effect in relation to the temperature of the junction between the metals. At temperatures over 1000 C in an oxidising or a corrosive atmosphere these metal thermocouples have a limited life due to oxidation of the metal junction and metal legs.

Even platinum containing thermocouples (types R and S) which are designed to operate at temperatures up to l70O0C may last only a few weeks in an oxidising environment due to volatilisation of the platinum.

It is known that two different semi-conducting ceramics may also produce an electromotive force in relation to their temperature. Ceramics are normally electrical insulators at room temperature but their resistivity decreases significantly with increasing temperature. This is opposite to that of metals whose resistivity increases with increasing temperature. At temperatures in excess of 200 C these ceramics may be used up to their maximum designed temperature in a particular environment. A commercially available thermocouple comprising boron carbide and carbon has been developed for measurements in high temperature systems. Such thermocouples have been found to possess certain disadvantages. Ceramics used for these thermocouples include silicon carbide, titanium dioxide, boron carbide and carbon.

Thermocouples employing carbon are limited to measuring temperatures below 500 C in an oxidising environment due to oxidation of the carbon. Similarly thermocouples employing titanium dioxide are limited to measuring temperatures below 900"C in an oxidising environment owing to oxidation of the titanium dioxide. Similarly thermocouples employing boron carbide are restricted to a maximum temperature of 800"C in an oxidising environment. In these situations carbon, titanium dioxide or boron carbide must be protected by the application of an inert gas such as nitrogen.

A further problem with known ceramic thermocouples is the stability of the junction between the two ceramics. This normally comprises a machined fit or the use of an adhesive bond between the two ceramics. Different ceramics expand at different rates and the integrity of the interface at these junctions normally deteriorates with extended periods of use.

The present invention has been made from a consideration of these problems.

According to a first aspect of the present invention there is provided a thermocouple assembly comprising two different semi-conducting ceramics in contact with each other at a junction, characterised in that one of said ceramics comprises silicon carbide and the other of said ceramics comprises molybdenum disilicide.

Thermocouples comprising these two ceramics are capable of operation in oxidising environments such as air at temperatures up to 18000C.

The thermocouple assembly preferably comprises an outer silicon carbide sheath and an inner molybdenum disilicide leg.

Preferably the junction between the two ceramics is achieved by resiliently urging one ceramic towards the other by a resilient body. This spring biased arrangement provides a good contact between the two ceramics at the junction throughout the working life of the thermocouple.

The resilient body, such as a coil spring, would usually be made from conventional materials such as steel. Thus when the thermocouple is located in a high temperature environment the resilient body should be isolated from that environment in some way. For example, if the thermocouple is being used to measure the temperature inside a boiler the thermocouple might pass through a hole in the boiler wall, a part of the thermocouple extending outside the boiler. The spring could be located here at the low temperature end of the thermocouple.

The contact faces of the two ceramics are preferably machined to a diamond finish.

The electromotive force (E.M.F.) of the thermocouple may be measured on a voltmeter or similar device. The two semiconducting ceramics are preferably separated and insulated, for example by washers or sleeving. Electrical connections on the two semi-conducting ceramics are made at the low temperature end of the thermocouple assembly.

In order that the present invention may be more readily understood a specific embodiment thereof will now be described by way of example only with reference to the accompanying drawings in which: Fig.l shows one thermocouple assembly in accordance with the present invention; and Fig.2 shows a graph of E.M.F. produced by the assembly of Fig.l when electrically heated in air.

Referring to the drawings a thermocouple assembly 10 for measuring the temperature of a gaseous or liquid environment comprises two legs of semi-conducting ceramic material. The first leg is a silicon carbide sheath 11 defining a blind channel 12. The second leg is in the form of a molybdenum disilicide rod 13. The rod 13 is located in the channel and extends beyond the open end thereof. The end of the rod located within the channel is urged towards the blind end of the sheath by a coiled spring 15 located at the opposite end of the thermocouple assembly 10. The contact faces of the two ceramics are machined to a diamond finish.

A high resistive aluminium oxide sleeving 16 is provided around the part of the rod which extends between the contact force and the opposite end of the outer ceramic sheath. High resistance aluminium oxide washers 17 are provided around this sleeving in order to locate the rod in the centre of the outer ceramic sheath 11. A further insulating washer is provided at the open end of the ceramic sleeve in order to close all that end of the sleeve.

In use the thermocouple assembly is passed through a wall 18 in a boiler, furnace or the like. A ceramic plug 19 plugs the gap between the outside of the sheath and the wall 18.

The spring 15 is located outside the wall 18 in order to prevent accelerated corrosion of the spring.

Measurements of E.M.F. generated by the thermocouple are taken outside the wall 18 using a voltmeter or a similar device.

Fig.2 shows the relationship between E.M.F. and temperature for the assembly shown in Fig.l when the assembly is heated in air. The relationship between temperature and E.M.F. was calculated as 70 microvolts/oC.

It is to be understood that the embodiment of the invention described herein is b way of illustration only.

Many modifications and variations are possible.

Claims (5)

1. A thermocouple assembly comprising two different semiconducting ceramics in contact with each other at a junction, characterised in that one of said ceramics comprises silicon carbide and the other of said ceramics comprises molybdenum disilicide.

2. A thermocouple assembly as claimed in claim 1, wherein the assembly comprises an outer silicone carbide sheath and an inner molybdenum disilicide leg.

3. A thermocouple assembly as claimed in claim 1 or claim 2, wherein the junction between the two ceramics is achieved by resiliently urging one ceramic towards the other by a resilient body.

4. A thermocouple assembly as claimed in claim 3, wherein the resilient body comprises a coil spring.

5. A thermocouple assembly as claimed in any preceding claim, wherein other than at the junction between the two ceramics, the two ceramics are separated and insulated.