GB2169714A - Gas sensors - Google Patents

Abstract

A gas-phase diffusion limited sensor comprises a sensing electrode and a capillary passageway (24) for conducting gas to be sensed from atmosphere to the sensing electrode. The capillary (24) is provided in a bushing (22) made of "ceramic" material, which may be an interference fit or soldered in the sensor cap. "Ceramic" is defined as any material that can be shaped and then hardened by means of heat. <IMAGE>

Description

SPECIFICATION Gas sensor The present invention relates to gas sensors e.g. oxygen sensors and especially to diffusion-limited gas-phase sensors.

It is known, for example from British Patent No. 1,571,282, to restrict the flow of gas to the sensing electrode of a gas sensor by interposing a capillary between the sensing electrode and the atmosphere. British Patent Specification 1,571,282 proposes the use of a hypodermic needle to define the capillary but now for ease of manufacture, the capillary is made by pulling a thin wire through a softened plug of solder mounted in the cap of the gas sensor. However, this has several disadvantages: (1) Because the solder is molten, it is difficult to ensure that all gas sensors have capillaries with identical lengths, and this is disdvantageous since the length of the capillary affects the output of the sensor.

(2) The very act of drawing a wire through a plug of solder is hard to control and consequently the diameter of the capillaries in different sensors can vary: the diameter of the capillary also affects the output of the sensor.

(3) The operation of forming a capillary in a solder plug must be done by hand and is time-consuming.

(4) Solder corrodes and the corrosion products can alter the diameter of the capillary thereby altering the output characteristics of the sensor.

The present invention aims to solve these problems in a relatively simple way.

According to the present invention, there is provided a gas sensor that includes a sensing electrode and a capillary gas passageway for conducting gas to be sensed to the sensing electrode, the capillary passageway being formed in a bushing made of a ceramic material.

By the term "ceramic", we mean all materials that can be shaped and then hardened by means of heat.

A capillary may easily be formed in the ceramic bushing in a number of ways, for example ceramic material in the green state is formed into a small plug, a hole of known, small diameter is drilled through the plug, and the plug is then fired in a furnace to form the bushing. Ceramics technology is nowadays well advanced and the drill diameter necessary to produce a capillary of predetermined size in the fired bushing can readily be calcuiated. Alternatively the bushing can be slip-cast in a mould having a central pin extending through it. The plug and the pin are removed while in the green state, the pin is then removed and the moulded bushing is fired in a furnace; in this way, a capillary of predetermined size is formed. A third method is to extrude a tube of ceramic material in the green or plastic state with a central hole of predetermined diameter.The tube can be fired and then cut into individual bushings or it can be cut into sections before firing. Using any of these three methods, bushings having capillaries of uniform length and diameter can be produced and the methods can be automated.

The ceramic bushing can be mounted in a gas sensor housing using any method that ensures a gas-tight fit between the bushings and the housing. For that reason, only three methods will be mentioned: Firstly, the ceramic bushing can be pushed or driven into a hole in a soft-metal housing part that is slightly smaller than the diameter of the bushing. The seal between the bushing and the housing will, in practice, be adequate over most temperature ranges that are used.

When the sensor is to be used over a large temperature range, however, the bushing material can be chosen to have a coefficient of thermal expansion matched to that of the surrounding housing material. Thus, for example, the bushing may be made of yttria-stabilized zirconia and the housing from mild steel since they have nearly identical thermal expansion coefficients. Secondly, the bushing may be soldered into a hole in a gas sensor housing.

This is a relatively straightforward operation if one end of the hole in the housing is countersunk so that when a bushing has been inserted into the hole in the housing, a small annular groove extends around the bushing. A pre-stamped solder ring may be inserted into the groove and the assembly heated until the solder melts. In order that a gas-tight bond is formed between the solder and the bushing, the bushing should be metallised, for example by electroless plating or vapour-deposition.

The third possible method is to apply a layer of solder paste to the outside of a metallised bushing. The bushing is then pushed into a hole in the sensor housing that is the same size or only slightly larger than the bushing so that part of the solder paste is scraped off by the edge of the hole as the bushing is inserted into it. On heating, the solder paste sets to form a gas-tight seal.

In the second and third methods mentioned in the preceding paragraph, it is preferred that the two end faces of the bushing are left unmetallised so that the solder does not adhere to them; this can be achieved by making the bushings by extruding and firing a tube of ceramic material as discussed above and metallising the resulting tube before cutting it up into individual bushings.

As stated above, the bushing is made of a ceramic material and the preferred ceramic materials are tungsten carbide, silicon nitride, ferrite, alumina, and a stabilized zirconia, e.g.

yttria-, magnesia-, or calcium-stabilized zirconia.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a cap and a capillary bushing of a sensor according to the present invention, and Figure 2 is a cross-sectional view of an alternative form of a cap and capillary bushing of a sensor according to the present invention.

Referring initially to Fig. 1, the reference number 20 indicates a mild steel cap of an oxygen sensor. It has a central hole in which a ceramic bushing 22 is accommodated. A capillary 24 extends through the bushing 22 and permits a controlled flow of gas from the atmosphere to a sensing electrode (not shown) located below the capillary. Before the bushing is pushed into the cap, the central hole in the cap is slightly smaller than the diameter of the bushing. The act of pushing the bushing into the hole deforms the edge 26 of the hole. We have found that the resulting joint is gas-tight, particularly if the material from which the bushing and the cap are made have similar coefficients of thermal expansion over the operating temperature of the sensor.

For this reason the cap is preferably made of mild steel and the bushing of yttria-stabilized zirconia.

Fig. 2 illustrates an alternative way of securing a bushing 30 in a cap 32. As in the Fig. 1 arrangement, the cap 32 has a central hole but, unlike the cap of Fig. 1, it is of the same or slightly larger diameter than the bushing 30. The hole has a countersunk portion 34.

To secure the bushing 30 in the cap 32, the bushing is placed in the hole, a ring of solder is placed in the annular countersunk groove 34 around the bushing and the assembly is heated until the solder melts to form a solder pool 36 which, on hardening, seals the bushing in an air-tight manner in the cap. In order to obtain a good bond between the solder 36 and the bushing 30, the latter should be metallised, e.g. by electroless plating or vapour deposition. The bushing 30 includes a capillary channel 38.

Claims (10)

1. A gas-phase diffusion-limited sensor comprising a sensing electrode and a capillary gas passageway for conducting gas from the atmosphere to the sensing electrode, wherein the capillary passageway is formed in a bushing made of a ceramic material.

2. A sensor as claimed in claim 1, wherein the bushing is made from tungsten carbide, silicon nitride, ferrite, alumina or a stabilized zirconia.

3. A sensor as claimed in claim 1 or claim 2, wherein the sensor has a cap and wherein the bushing is located in an opening in the cap, the periphery of the cap around the opening being deformed to produce an interference fit with the bushing and thereby a gas-tight seal between the bushing and the cap.

4. A sensor as claimed in claim 3, wherein the coefficient of thermal expansion of the bushing is approximately equal to that of the cap material.

5. A sensor as claimed in claim 4, wherein the bushing is made of yttria-stabilized zirconia and the cap is made of mild steel.

6. A sensor as claimed in claim 1 or claim 2, wherein the sensor has a cap and the bushing is soldered into an opening in the cap.

7. A sensor as claimed in claim 6, wherein the cap has a groove around the top or bottom of the opening, said groove being filled with solder.

8. A sensor as claimed in claim 7, wherein the groove is a countersunk portion of the opening.

9. A sensor as claimed in claim 1, which is an oxygen sensor.

10. A sesor substantially as hereinbefore described in connection with and as illustrated in Figs. 1 and 2 of the accompanying drawings.