GB2316178A - Solid electrolyte gas sensor - Google Patents

Abstract

The gas sensor includes a hollow solid electrolyte mass 12 enclosing a cavity 13. Reference electrode material is located in the cavity and connected via a gas tight seal to reference wire 24. A gas sensitive electrode 16 is provided on the outside of the solid electrolyte and preferably comprises a grid of gold wires coated with a layer 20 of a salt which reaches a dissociative equilibrium with the gas to be sensed. Carbon dioxide can be sensed using a sodium carbonate layer, and sulphur dioxide using a layer of sodium sulphate. The solid electrolyte may be an inorganic ceramic such as sodium * small Greek beta *-alumina or an organic polymer solid such as poly(ethylene oxide) complexed with lithium perchlorate.

Description

Gas Sensor This invention relates to a gas sensor which gives an electrical signal in the presence of a desired gas, and is particularly but not exclusively applicable to sensors for a non-combustible gas such as CO2 or SO2.

One such type of gas sensor is described in EP 0 182 921B (K.K. Advance) consisting of a disc of solid electrolyte such as ss-alumina, part of whose surface is covered with a layer of a metal salt capable of reaching a dissociative equilibrium with the gas, this layer being in contact with a measurement electrode on the surface of the electrolyte, and a reference electrode of silver is in contact with the opposite surface of the electrolyte, all other surfaces of the disc being coated with a gasimpermeable, gas-intercepting layer. This sensor acts as a cell, and has an emf depending on the concentration of the gas, for example CO2. Another design of gas sensor, specifically for measuring SO2 concentration is described in US 4 855 034 (Sugimoto et al/NGK Spark Plug Co). This too uses a ss-alumina solid electrolyte disc, each surface of which is laminated to a pellet of sodium sulphate, and each pellet is provided with a platinum electrode. A reference gas is passed over one surface and the gas to be sensed is passed over the other surface, so the sensor acts as a cell. However neither of these sensors is entirely satisfactory: the former has a response which deteriorates with long term use, while the latter requires a flow of a reference gas.

According to the present invention there is provided a gas sensor comprising a pellet of a solid electrolyte defining an enclosed cavity, a gas responsive electrode on a part of the outer surface of the pellet, a reference electrode material inside the cavity, a conductor extending through the electrolyte to provide electrical contact to the reference electrode material, and a gas-tight seal where the conductor passes through the electrolyte.

Preferably the gas-responsive electrode comprises a measurement electrode on the surface of the pellet and a layer of a metal salt capable of reaching a dissociative equilibrium with a gas to be measured and which covers the exposed surface of the measurement electrode and adjacent regions of the surface of the pellet. The layer of metal salt is desirably no more than 200 pm thick, preferably no more than 100 pin thick, and must be gas-permeable. The metal salt in the gas-responsive electrode must be a salt of a cation which may be conducted by the electrolyte. For example to measure carbon dioxide, a suitable salt would be lithium or sodium carbonate, and the electrolyte would conduct lithium or sodium ions respectively. To measure sulphur dioxide a suitable salt would be lithium or sodium sulphate.

The use of a pellet with a cavity minimizes the area over which the reference electrode material must be sealed from the surrounding atmosphere; this is only required where the conductor passes through the electrolyte, where there may be an access hole. The cavity may be of larger cross-section than the access hole, so giving a larger volume of reference electrode material.

Preferably the reference electrode material contains cations which are the same as those in the layer of metal salt. The reference electrode material may be a metal or an alloy which is at least partly molten during operation. For example it might be sodium, or it might be a mixture of aluminium chloride, sodium chloride and either iron, aluminium or nickel (these components being initially provided in the form of a mixture of powders). The reference electrode material provides a constant activity of the cation, for example of sodium. Molten materials are desirable as they ensure good interfacial contact with the electrolyte.

If the metal salt is a sodium salt the solid electrolyte might be sodium -alumina, sodium '' -alumina, or Nasicon (Na3Zr2Si2PO12), or other sodium-conducting solid electrolytes. Where the metal salt is a lithium salt the electrolyte might for example be lithium ss-alumina (which may be represented as Li2O. 11A1203), or LisAlO4, or may be an organic polymer solid electrolyte such as poly (ethylene oxide) (PEO) complexed with a lithium salt. For adequate ionic conductivity through the inorganic electrolytes the sensor is desirably operated at a temperature of over 2000C, preferably in the range 300 to 4000C. The sensor is desirably provided with an electric heater to raise it to this temperature. The heater may be attached to a surface of the pellet, or the pellet might define a second cavity to accommodate a heater. The pellet with one or more cavities may be made by a powder-pressing route similar to that described in JP 64-42503 (Kaneda), where the cavity shape is provided by a suitably-shaped spacer of ice or dry ice, the powders are pressed at low temperature, and then the spacer is melted or sublimed out, and finally the pressed product is sintered. The spacer may be formed of any other material which can be removed prior to or during sintering. Alternatively the cavity may be defined by one or more recesses in two mating electrolyte blocks, which are sealed together to enclose the reference material in the cavity. It has been found that more reliable sensors are provided if the thickness of solid electrolyte between the electrodes is more than 3 mm, preferably at least 4 mm.

The invention will now be further described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a sectional view of a gas sensor; Figure 2 shows graphically the variation of emf of the sensor of Figure 1 with the concentration of carbon dioxide gas; Figure 3 shows a sectional view of an alternative gas sensor; Figure 4 shows a sectional view of another alternative gas sensor; and Figure 5 shows a sectional view of another alternative gas sensor.

Referring to Figure 1, a gas sensor 10 comprises a hollow pellet 12 of sodium ss-alumina. The pellet 12 is cylindrical, of diameter 9 mm and of height 2.5 mm, and defines a cylindrical cavity 13 of diameter 5 mm and height 1 mm. A hole 14 of diameter 0.07 mm extends radially through the side of the pellet 12 to communicate with the cavity 13. The pellet 12 may be made by pressing powdered sodium ss-alumina (or a powdered precursor for it), with a suitable binder, around a suitably-shaped piece of dry ice; allowing the dry ice to sublime; and then sintering.

One flat surface of the pellet 12 is screen-printed with a grid of 10 pin thick gold wires 16 connected to a current collector electrode 18 (the sizes of the wires 16 being exaggerated in the drawing). That surface of the pellet 12 is then coated with sodium carbonate, by painting on an aqueous solution, and drying it rapidly with a hot air blower. This procedure gives a layer 20 of sodium carbonate consisting principally of discrete lumps or aggregates of diameter about 100 pin. The aqueous solution may instead be allowed to dry naturally, forming a more uniform layer of thickness about 20 pin. The cavity 13 is filled with a mixture 22 of fine powders of aluminium chloride, sodium chloride, and aluminium, the number of moles of sodium chloride being greater than that of aluminium chloride. An aluminium wire 24 extends through the hole 14 to contact the mixture 22, and the wire 24 is sealed to the pellet 12 at the hole 14 by a glass seal 26.

The glass must be chemically inert to the sodium -alumina and to the mixture 22 in the cavity 13, and must have an expansion coefficient matched to those of the pellet 12 and the wire 24. A heater element 28 is attached to the other flat surface of the pellet 12.

In use of the sensor 10 the heater 28 is energized to maintain the pellet 12 at an operating temperature which may for example be 3000C. This operating temperature is such that the material of which the pellet 12 is made has adequate ionic conductivity, and may be maintained thermostatically, the heater 28 also comprising a temperature sensor. A high impedance voltmeter is connected to the wires 18 and 24 to measure the emf of the cell constituted by the sensor 10. The mixture 22 in the cavity 13 is molten (forming sodium aluminium chloride) and provides a reference sodium ion activity. (Any unreacted powdered metal in the mixture 22 improves its electronic conductivity). The carbonate layer 20 provides a sodium ion activity which depends upon the concentration of carbon dioxide in the surrounding atmosphere, as a consequence of a dynamic equilibrium which may be: Na2CO3 = 2 Na + CO2 + % O2 + The emf has been found to give a consiste t response to carbon dioxide, and Figure 2 shows graphically the experimentally measured values of emf for a sensor 10 over a wide range of different concentrations of carbon dioxide in dry air (between 125 ppm and 50000 ppm). The sensor 10 has been found to respond to a change of gas concentration within a few minutes, and to give consistent results during weeks of operation. With the carbonate layer 20 described above the sensor 10 is however also affected by changes in humidity.

A modified sensor 10 differs from that described above only in having a different carbonate layer 20, using instead a sodium carbonate/barium carbonate equimolar mixture melted and quenched to form a layer 20 about 0.1 mm thick. This sensor 10 operates in the same way as that described above although it is substantially unaffected by humidity.

It will be appreciated that a gas sensor may differ from that described above while remaining within the scope of the present invention. The pellet might differ in size and shape, and for example might define a narrow hole formed by drilling instead of the cavity 13 and hole 14.

Furthermore the sensor might have two gas-sensitive electrodes side by side at the same flat surface of the sensor, which might sense different gases. For example one such electrode might incorporate sodium carbonate as described above, and the other a layer of sodium sulphate, in order to sense sulphur dioxide.

Referring now to Figure 3 there is shown an alternative gas sensor 30 which has several similarities to the sensor 10 of Figure 1. The sensor 30 comprises a cylindrical pellet 32 of sodium 13-alumina, of diameter 4 mm and of height 8 mm. Near the lower end of the pellet 32 is a cavity 34 of diameter 1.5 mm and of length 4 mm, which lies on the longitudinal axis of the pellet 32 and is formed by drilling in the green state, before the pellet is fired. The cavity 34 is filled with the same aluminium chloride/sodium chloride/aluminium powder mixture 22 as used in the sensor 10; an aluminium wire 35 (or alternatively a nickel alloy Kovar wire (trade mark)) provides electrical contact to this mixture, and is sealed to the pellet 32 by a glass seal 36. The other end (the top end as shown) is provided with a grid of 10 pin thick gold wires 38, embedded in a layer 39 of sodium carbonate, as in the sensor 10. The sensor 30 may be placed in a heated cavity, or an electrical heating element (not shown) may be provided on the curved outer surface of the pellet 32.

The sensor 30 operates in the same manner as does the sensor 10 described above, but the considerably greater thickness of solid electrolyte between the cavity 34 and the grid wires 38, 4 mm as compared to 0.75 mm, may improve the long-term stability of the sensor 30. However, the sensor 30 has a longer response time to changes in carbon dioxide concentration.

Referring now to Figure 4 there is shown another alternative gas sensor 40. The gas sensor 40 is of cylindrical shape, of diameter 12 mm and of height 5 mm.

One end surface carries a gold wire grid 42 covered by a layer 43 of sodium carbonate (as described in relation to the sensor 10) while the other end surface has an electrical heater 46. The sensor 40 comprises two pellets of sodium 13-alumina: a lower pellet 48 which defines a frustro-conical recess 49 and whose entire upper surface is coated with a layer 50 of gold; and an upper pellet 52 whose lower surface defines a peripheral rim surrounding a frustro-conical projection 54. The central part of the upper pellet 52 is 4 mm thick, and locates in the recess 49 to leave a cylindrical cavity between the bottom of the projection 54 and the bottom of the recess 49. This cavity is filled with the powder mixture 22 as described previously. After assembly, the two pellets 48 and 52 are heated so that the gold 50 forms a gas-tight seal, diffusion bonding around the rim, fixing the pellets 48 and 52 together and also providing electrical connection to the mixture 22 in the cavity. Pins 55 and 56 provide contacts respectively to the gold grid 42 and to the gold layer 50, extending through holes drilled through the pellets 48 and 52 while in the green state. The hole 58 for the pin 55 is sufficiently wide to ensure the pin 55 does not touch the gold layer 50.

The sensor 40 operates in the same fashion as the gas sensors 10 and 30; the large thickness of electrolyte between the reference electrode (the mixture 22) and the gas-responsive electrode (the grid 42), which is 4mm in this case, gives long-term stability.

Referring now to Figure 5 there is shown another alternative gas sensor 60. In this case there is a reference electrode 62 which comprises a mixture of a powdered lithium intercalation compound such as lithium cobalt oxide, LixCoO2, powdered carbon, and an electrolyte of poly(ethylene oxide) (PEO) complexed with lithium perchlorate. The LixCoO2 may be made in the way described in EP 0 017 400, while the polymer/salt complex may be prepared from a solution of PEO and Lilo, in acetonitrile with an [EO]:[Li] ratio of 12. Embedded in the electrode mixture is a fine nickel wire mesh (not shown) connected to a contact wire 64, and the electrode 62 is 0.05 mm thick and 10 mm square in plan. The electrode 62 is sandwiched between two 15 mm square sheets 66 of PEO compiexed with lithium perchlorate (this electrolyte being the same as the electrolyte component of the electrode 62), each 0.05 mm thick, which are bonded to each other around the edges and bonded to the electrode 62 by heat and pressure treatment, by heating the assembly to 1250C and holding it together for 30 min. On the outer surface of one of the PEO sheets 66 is a nickel grid 67 and a coating 68 of lithium carbonate about 25 pin thick, and the grid 67 is connected to a contact wire 69.

In use the sensor 60 is heated to 1200C by an electrical heater (not shown). The reference electrode 62 provides a constant lithium ion activity, while the lithium ion activity adjacent to the grid 67 depends upon the carbon dioxide concentration in the surrounding atmosphere.

Hence the emf of the cell i.e. the voltage measured between the contact wires 64 and 69 enables the carbon dioxide concentration to be determined.

It will be appreciated that the reference source of lithium ions may be a different intercalation material instead of lithium cobalt oxide, for example lithium inserted into TiS2 or V6O13, or lithium/graphite LiC6, or may even be lithium metal. The lithium carbonate may alternatively be provided in the form of a mixture of lithium carbonate, powdered carbon, and PEO, cast as a sheet similarly to the reference layer 62, bonded to the outer surface of the PEO sheet 66, and provided with a nickel grid 67 for electrical contact. It will also be appreciated that the sensor 60 might utilise a different solid electrolye material instead of PEO, for example a film comprising a copolymer of vinylidene fluoride with 8 to 25% by weight of hexafluoropropylene, the film also containing a lithium salt such as lithium perchlorate dispersed in a solvent such as ethylene carbonate or propylene carbonate, the solvent providing 20 to 70% by weight of the film. Such an electrolyte is described in US 5 296 318 (Gozdz et al.), and can provide satisfactory conductivity at room temperature so that no heater would be required in the sensor 60.

Claims (11)

Claims

1. A gas sensor comprising a pellet of a solid electrolyte defining an enclosed cavity, a gas-responsive electrode on a part of the outer surface of the pellet, a reference electrode material inside the cavity, a conductor extending through the electrolyte to provide electrical contact to the reference electrode material, and a gastight seal where the conductor passes through the electrolyte.

2. A gas sensor as claimed in Claim 1 wherein the gasresponsive electrode comprises a measurement electrode on the surface of the pellet and a layer of a metal salt capable of reaching a dissociative equilibrium with a gas to be measured and which covers the exposed surface of the measurement electrode and adjacent regions of the surface of the pellet.

3. A gas sensor as claimed in Claim 2 wherein the layer of metal salt is no more than 200 pin thick, preferably no more than 100 pin thick.

4. A gas sensor as claimed in any one of the preceding Claims also comprising an electric heater to raise the temperature of the sensor to an operating temperature above ambient temperature.

5. A gas sensor as claimed in any one of the preceding Claims wherein the reference electrode material is at least partly molten at the operating temperature of the sensor.

6. A gas sensor as claimed in any one of the preceding Claims wherein the electrolyte is an inorganic ceramic electrolyte.

7. A gas sensor as claimed in any one of Claims 1 to 4 wherein the electrolyte is an organic polymer solid electrolyte.

8. A gas sensor as claimed in Claim 7 wherein the reference electrode material comprises both an intercalation compound and organic polymer electrolyte material.

9. A gas sensor as claimed in any one of the preceding Claims wherein the pellet of solid electrolyte comprises two sheets of solid electrolyte bonded together around their edges and defining the cavity for the reference material between them.

10. A gas sensor as claimed in any one of the preceding Claims wherein the thickness of solid electrolyte between the reference electrode material and the gas responsive electrode is at least 3 mm.

11. A gas sensor substantially as hereinbefore described with reference to, and as shown in, the aceampanying drawings.