GB2188431A - Solid electrolyte oxygen probe - Google Patents

Abstract

An oxygen measuring probe for high temperatures, comprises of a porous air supply pipe 2 located inside a solid-electrolyte pipe 1 which is short by comparison with the length of the probe, an alkali- resistant covering tube 12 with pores or gas passages, and a porous alkali-resistant protective casing 22. Electrodes are formed from wires of non-scaling metal 3,9 and grains of material of electronic or mixed conductivity, 7,16 while in order to separate the gas spaces loose charges of fine ceramic alkali- resistant insulating powder 14 are provided between consolidated layers 13,15 of such powders. The probe head thus constructed is attached to the carrier pie 18, in which electrical conductors and the air-feed pipe 25 are embedded in a mass of loose ceramic particles 26. The probe requires a reduced solid electrolyte and noble metal construction and is resistant to high temperature, glass, ceramics, and metal impurities which poison platinum. <IMAGE>

Description

SPECIFICATION Oxygen probe The invention relates to an oxygen measuring probe suitable for high temperatures and including a galvanic solid-electrolyte cell in a protective casing, an air-feed pipe in a carrier pipe, the probe useful for continuous oxygen measurement, preferably at temperatures of over 1000"C in the gas phase, for example in melting furnaces, in stoves for ceramics of all types and in metallurgical plant.

For oxygen measurement at high temperatures 40-60cm long pipes, closed at one end and comprising zircon dioxide are known. Stability against high temperature effects and against impurities, or components affecting the probes, technical gases are achieved at the expense of considerable consumption of noble metals and protective tubing around the measuring cell. For example, in order to form the air reference electrode and its electrical conductor, the entire internal surface of the solidelectrolyte tube is platinized, while externally, to form the measuring electrode, a noble metal coating of about 5 cm in length is applied up to a predetermined thickness to the solid electrolyte. This consumes some 8-10 g of platinum metals (not including the thermoelement) for each probe, which is beyond effective recovery after the useful probe life.Despite such consumption these probes seldom last longer than 8 months in meeting specific requirements.

If elements such as lead, arsenic or antimony are present by impurity or otherwise in the gas to be analysed, then platinum electrodes directly exposed to this gas are rapidly rendered ineffective, particularly under reducing conditions. Fusible particles in the gas under analysis may obstruct pores of the electrode coatings on impact, or those of the protective ceramic, thus rapidly rendering the sensor sluggish and ineffective. Finally, major interference is also caused by the vapours of inorganic compounds which, at high temperatures, come into contact with the analysis gas from normal types of glass and many ceramics.

They may react dirctly from the gas phase with constituents of the solid bodies of which the probe is made and thus destroy the solid assembly.

The invention seeks to provide an apparatus in which consumption of material may be reduced and preferably minimized and with which oxygen may be accurately and continuously measured for a considerable time at high temperatures even under the influence of impurities or other interfering components occurring in gas phases above gas melting vats and ceramics.

The invention is based on modifying the method of constructing probes with solid-electrolyte cells for oxygen measurement in gases, whereby reduced or minimum consumption of solid electrolyte and noble metal, and largely unimpeded potentiometric recording of oxygen concentration in a current of gas (which may contain gas mixture dust at low temperatures, liquid particles at a high temperature and increasing vapours of inorganic substances at increasing temperature) may be possible over a long period.

According to this invention there is provided an oxygen measuring probe suitable for high temperatures, comprising a galvanic cell in a protective casing and a porous air supply pipe axially mounted in a carrier pipe, a solid electrolyte pipe shorter than the length of the probe, an alkali-resistant covering tube with pores or other gas passages, and a porous alkali-resistant protective casing, in the intermediate spaces of which, for the formation of the electrodes, non-scaling metallic structures and grains of material of electronic or mixed conductivity are provided, while in order to separate the gas spaces loose charges of fine ceramic alkali-resistant insulating powder are provided between consolidated layers of such powders, the probe head thus constructed being attached to the carrier pipe, in which electrical conductors and the air-feed pipe are embedded in a mass of loose ceramic particles.

One form of oxygen measuring probe is characterized by an arrangement comprising, in a solid-electrolyte tube shorter than the length of the probe, in the axial direction, a porous alkali-resistant air supply tube, with a base or noble non-scaling metallic structure applied thereto in the zone of the electrode, and between this tube and the inner wall of the solid-electrolyte tube, in the zone of the electrode, a loose charge of grains of an oxidic material of electronic or mixed conductivity, consolidated ceramic composition delimiting this loose charge, and also a loose charge of fine ceramic insulating powder, and at the open ends of the solid-electrolyte tube consolidated ceramic insulating compound closing the intermediate space is provided thereby forming a solid-electrolyte tube, the probe further provided with a second non-scaling baseor noble-metal structure and applied in the zone of the electrode embedded axially in a covering tube of alkali-resistant ceramic metal with pores or apertures for the passage of gas, by a loose charge of fine ceramic alkaliresistant insulating powder, delimited by layers of consolidated alkali-resistant ceramic composition, the covering tube thus filled is fitted to a gas-tight carrier pipe and between covering tube and carrier pipe, loose fine alkali-resistant ceramic powder is filled onto a ring of consolidated ceramic composition, while a protective casing of consolidated alkali-resistant ceramic composition affixed to the carrier pipe is provided around the casing tube, the airfeed pipe loosely mounted on the porous airsupply pipe being embedded in the carrier pipe together with electrical conductors of the sensor, in a mass of loose ceramic particles.

Various properties of the measuring electrode may be improved if the space between the solid-electrolyte pipe and the casing pipe is filled, in the zone of the measuring electrode, with a loose charge of grains of an oxidic material, preferably of electronic conductivity, this charge being sealed, within the intermediate space, by a consolidated porous layer of alkali-resistant ceramic material.

Apertures provided for the passage of gas in the casing tube of alkali-resistant ceramic can be filled, together with the greater part of the space between the solid-electrolyte tube and the casing tube, with fine, alkali-resistant ceramic powder.

The structure is simplified if the casing tube and the protective casing together form a porous alkali-resistant protective body affixed to the carrier pipe and then the greater part of the space between the solid-electrolyte tube, protective body and carrier pipe can be filled with the same loose charge of fine ceramic alkali-resistant insulating powder.

The solid-electrolyte pipes of the shortness required for probes of the kind described do not need to retain their exact dimensions and are thus relatively easy to manufacture. In the probes they may be mounted in a position in which they are protected against mechanical thermal and chemical action. The structure of the electrodes may include oxidic electron conductors as a means of long-term stabilization, i.e. the cell resistance does not increase as a result of the re-crystallization and evaporation of platinum. With the use of non-scaling base-metal wires, probes free of noble metal can be obtained for temperatures of up to about 1200"C, and these unlike those containing platinum; can also be employed in waste gases containing lead.When probes usable in other cases up to 1 5000C are made with platinum-rhodium alloys the protected position of the noble-metal wires has a favourable effect on their durability. Compared to known probes oniy about half as much noble metal is required, and the noble-metal wires can be very largely recovered after the probe has been used.

To ensure resistance to alkalis with negligible electrical conductivity, probes may be constructed from compounds such as Al2O3, MgO, CaO and ZrSiO4. Aluminium silicates, such as those occurring in ordinary filter tubes, are most preferably avoided. Sinter spi nel and sinter magnesia are better than pure sinter corundum, sinter spinel MgAl2O4 being more suitable, where its expansion coefficient is concerned, for zirconium dioxide solid electrolyte. The gas permeability of the layer of powder employed may be controlled by using coarser powder for parts which have the desired permeability and finer powder when they are of the desired density. The consolidation of certain parts can be effected by the addition, to powders of highly sintered ceramics, of chemically prepared unsintered or low-sintered material.The use of loosely heaped powder of high-sintered material as a sealing layer prevents the formation of gas-permeable cracks during its use. The drawn-back position of the measuring electrodes and the fact that on the back of the casing pores remain in the flow, prevent the obstruction of the sensor with little drops of glass.

In order that the invention may be illustrated, embodiments thereof are now described by way of example only with reference to the accompanying drawings and in which: Figure 1 shows the component consisting of a sensor, with the solid-electrode tube open at both ends; Figure 2 shows the head of the probe with a porous covering tube and with a covered measuring electrode; Figure 3 shows the head of the probe with a covering tube of dense ceramic and with an open measuring electrode; Figure 4 shows the component consisting of a sensor with a solid-electrolyte tube closed at one end, Figure 5 shows the head of the probe with a porous protective body.

Referring firstly to Figure 1, the sensor with a solid-electrolyte tube 1 open at both ends is prefabricated as a component, in that a porous air-supply pipe 2, open at both ends, with a metallic conductor 3 wound up in the zone of the electrode, is inserted, and a layer 4 which consolidates itself is introduced, after which the fine ceramic powder 5 is filled in from one side and covered over with the selfconsolidating layer 6, while from the other side the grains 7 of an oxidic electron conductor are shaken in and covered over with the self-consolidating composition 8. A winding 8 of the second metallic conductor is placed on the outside, in the zone of the electrode, and taken over the insulating pieces 10.

The component 11 shown in Figure 1 is introduced, in a central position, into the probe heads shown in Figures 2 and 3.

According to Figure 2 the component 11 is situated in a casing tube 12 of porous spinel MgAl2O4. From one side fine powder 14 of highly sintered spinel MgAl2O4 is filled onto a self-consolidating layer 13 of spinel mortar and covered over with a self-consolidating layer 15, while from the other side grains 16 of an oxidic electron conductor are shaken in and covered over with a self-consolidating layer 17 or spinel mortar.

This provides a second-stage component which is inserted in the carrier pipe 18 in such a way that first of all a self-consolidating ring 19 of ceramic material is introduced, after which loose fine ceramic powder 20 is shaken onto it. The final stage of the construction of the probe head consists of the operation of encasing it in a high-temperature concrete which, inside a mould, in the lower part 21, with fine powder of high-sintered spinel and a high proportion of very finely ground siliconfree high-temperature cement, is rendered relatively gas-tight, while in the upper part 22, with coarse powder of high-sintered spinel and a low proportion of very finely ground silicon-free high-temperature cement, is rendered relatively porous.

According to Figure 3 the component 11 is situated in a protective tube 23 of gas-tight sinter corundum with gas apertures 24. Fine powder 14 of high-sintered spinel MgAl2O4 is filled onto a selfconsolidating layer 13 of spinel mortar, after the apertures 24 have been provided with a provisional outer covering of combustible material, and is then covered over with a self-consolidating layer 15.

This again provides a second-stage component which, as described in Figure 2, or in the form to be seen in Figure 3, can be built onto the carrier pipe 18.

The construction of the probe is completed by an operation in which in the interior a ceramic air-feed pipe 25 is loosely mounted on that end of the air-supply pipe 2 which protects from the solid-electrolyte pipe 1, the intermediate space between the pipes 18 and 25 being filled with corundum powder 26 to provide a bed for the metallic conductor wires. The probes are sealed in a simple practical manner on the side which is not shown in the drawings and which remains at ambient temperature and are connected up in the known manner by their electrical conductors to signal processing apparatus.

A thermo-element which will also resist the operating temperatures can be built into the probes at various points; it is also possible for one of the conductor wires to be used as a branch of a thermoelement and welded to a suitable second branch.

As an oxidic means of electronic or mixed conductivity for the cores 7 and 16 use can be made, for example, of a lanthanum-strontium-chromite or a mixed oxide of cerium- and praseodymium- or lanthanumoxide. Examples free of noble metal are produced with wires of nickel, chromium-nickel or iron-aluminium alloys, for example.

Figure 4 shows the construction of a sensor with a unilaterally connected solid-electrolyte pipe 27 as a component. The parts correspond to those indicated for the sensor according to Figure 1. Figure 4, however, also shows that the porous air-supply pipe 2 may already end before emerging from the solidelectrolyte pipe.

The component 28 shows in Figure 4 can be installed in place of the component 11 in the systems described by reference to Figures 2 and 3.

Figure 5 illustrates a probe head in which, in place of the protective tube and the casing, use is made of a porous protective body 29, mainly of spinel or magnesium oxide. Inside this casing structure, cast on by the aid of a mould, with the use of a pasty mortar, or attached as a prefabricated body, the component 11 or 28 can be acommodated with a covered or open measuring electrode. Figure 5 shows an embodiment in which the component 28 is mounted in a long layer of loose fine spinel powder 30, shaken into the unit, inside a prefabricated porous spinel body 29.

The body 29 is affixed, with an alkali-resistant high-temperature mortar 31, to the carrier pipe 18. The air-feed pipe 25 is inserted into the solidelectrolyte pipe 27 and, in the interior, takes the conductor 3 to the internal electrode of the sensor.

At measuring points continuously subject to under-pressure the air reference electrode of sensor with a solid-electrolyte pipe open at both ends is supplied with live air by suction.

In the case of excess pressure or fluctuating pressures, on the other hand, as well as in the case of sensors with solid-electrolyte pipes closed at one end, a pump must be employed, for example, to ensure a continuous current of air through the air-feed pipe.

The invention inciudes use of probes as defined above in a method of oxygen determination.

Claims (8)

1. An oxygen measuring probe suitable for high temperatures, comprising a galvanic cell in a protective casing and a porous air supply pipe axially mounted in a carrier pipe, a solid electrolyte pipe shorter than the length of the probe, an alkali-resistant covering tube with pores or other gas passages, and a porous alkali-resistant protective casing, in the intermediate spaces of which, for the formation of the electrodes, non-scaling metallic structures and grains of material of electronic or mixed conductivity are provided, while in order to separate the gas spaces loose charges of fine ceramic alkali-resistant insulating powder are provided between consolidated layers of such powders, the probe head thus constructed being attached to the carrier pipe, in which electrical conductors and the air-feed pipe are embedded in a mass of loose ceramic particles.

2. Oxygen measuring probe suitable for high temperatures with a galvanic solid-electrolyte cell in a protective casing and with an air-feed pipe in a carrier pipe, characterized by the fact that, in a solid-electrolyte pipe shorter than the length of the probe, in the axial direction, a porous alkali-resistant air-supply tube with a base- or noble-metal non-scaling structure applied thereto in the zone of the electrode, and between this tube and the inner wall of the solid electrolyte tube, in the zone of the electrode, a loose charge of grains of an oxidic material of electronic or mixed conductivity, consolidated ceramic composition delimiting this loose charge, and also a loose charge of fine ceramic insulating powder and at the open ends of the solid-electrolyte tube consolidated ceramic insulating compoung closing the intermediate space, are inserted, that the solid-electrolyte tube thus prepared, and provided with a second non-scaling base- or noble-metal structure and applied in the zone of the electrode, is embedded axially in a covering tube of alkali-resistant ceramic metal with pores or other apertures for the passage of gas, by a loose charge of fine ceramic alkaliresistant insulating powder delimited by layers of consolidated alkali-resistant ceramic composition, that the covering tube thus filled is built onto the gas-tight carrier pipe, that between covering tube and carrier pipe, onto a ring of consolidated ceramic composition, loose fine alkaliresistant ceramic powder is filled, while a protective casing of consolidated alkali-resistant ceramic composition affixed to the carrier pipe, is placed around the casing tube, and that in the carrier pipe the air-feed pipe loosely mounted on the porous air-supply pipe is embedded, together with electrical conductors of the sensor, in a mass of loose ceramic particles of ceramic alkali-resistant composition is placed, and that in the carrier pipe the (air- feed pipe loosely mounted on the porous air supply pipe is embedded, together with electrical conductors of the sensor, in a mass of loose ceramic particles.

3. Probe in accordance with Claim 1 or 2, characterized by the fact that the space between the solid-electrolyte pipe and the casing pipe is filled, in the zone of the measuring electrode, with a loose charge of grains of an oxidic material, preferably of electronic conductivity, this charge being sealed, within the intermediate space, by a consolidated porous layer of alkali-resistant ceramic material.

4. Probe in accordance with any preceding claim characterized by the fact that the casing tube of alkali-resistant ceramic apertures are provided for the passage of gas and together with the greater part of the space between the soiid-electrolyte pipe and the casing tube are filled with fine alkaii-resistant ceramic powder.

5. Probe in accordance with any preceding claim characterized by the fact that the casing tube and the protective casing, in conjunction with each other, form a porous alkali-resistant protective body affixed to the carrier pipe, after which the greater part of the space between the solid-electrolyte pipe and the protective body and the carrier pipe is filled with the same loose charge of fine ceramic alkaliresistant insulating powder.

6. Oxygen measuring probe substantially as herein described with reference to any of the accompanying drawings.

7. Oxygen measuring probe substantially as herein illustrated in any of the accompanying drawings.

8. Use of an oxygen probe as claimed in any preceding claim in a method of oxygen determination.