Electrochemical Gage for Measuring Partial Pressures of Oxygen

This invention relates to an electrochemical oxygen gage with a solid oxide electrolyte and a solid internal reference: this oxygen gage is constructed preferably in miniaturised form and is capable of functioning under any pressure, high pressures included.

It will be remembered that an oxygen gage with solid oxide electrolyte can be assimilated to a galvanic cell of the type:

In which:

M and M' are unoxidizable electronic conductors, for example consisting of one of the noble metals,

THE SOLID OXIDE ELECTROLYTE IS AN OXIDE CERAMIC OR POSSIBLY A VITREOUS PHASE WITH PURE IONIC CONDUCTION, FOR EXAMPLE A SOLID SOLUTION OF OXIDES DEFECTIVE IN OXIDE IONS AND WITH A FLUORITE-TYPE STRUCTURE, SUCH AS THAT DESCRIBED BY D. Yuan and F. A. Kroeger in J. Electrochem. Soc. 118, 841 (1971).

The "reference" compartment uses a chemical system consisting of either a gaseous mixture (pure oxygen, oxygen-gas mixture, CO--CO.sub.2, H.sub.2 O), or a metal-oxide mixture, for example Fe--Fe.sub.1-x`O, Ni-NiO, etc. or even a mixture of two oxides of the same metal, for example (CoO--Co.sub.3 O.sub.4.

The use, particularly for measuring partial pressures of oxygen, for example in specific atmospheres or in molten metallic phases, etc., of electrochemical oxygen gages in which the chemical system used to establish the reference pressure P.sub.ref consists of a gaseous mixture of the type described above, is very awkward. In order to obtain accurate measurements, it is particularly required that in every case circulation must be ensured of a gas whose oxygen partial pressure has been exactly predetermined in the reference compartment.

The use of gages in which the reference pressure is established by a mixture or couple of the form metal:oxide of the same metal, or oxide:oxide, or similar redox couple, therefore seems a priori much more satisfactory. This mixture which will be referred to from now on as "internal reference" enables the partial pressure of oxygen to be established with reference to a fixed value, when the temperature is constant and when heterogeneous equilibrium is reached. It is then possible, once the temperature is maintained constant and heterogeneous equilibrium established, to obtain accurate measurements, while dispensing with circulation of gas in the reference compartment and consequently with all the difficulties posed by keeping the partial pressure of oxygen in this gas constant.

There have already been described a great number of redox couples of the above-mentioned type which, when placed in a galvanic cell of the kind mentioned above, would be capable of engendering a partial pressure of oxygen from the moment when heterogeneous equilibrium is reached. A number of gages utilising this principle have also already been described. However, known gages have various drawbacks which considerably limit their possibilities of use.

It will in fact be noted that, in the redox couples whose use in gages has already been suggested, the partial pressures of oxygen in the internal reference which are obtained at equilibrium are very weak, and these gages can function only at very high temperatures, generally above 800.degree. - 1000.degree. C. Furthermore, bringing the internal reference and the gas to be studied to thermodynamic equilibrium at the chosen operational temperature takes an extremely long time, so that obtaining a measurement with these gages requires a prolonged period of time; moreover, that results are often distorted when the temperature is subject to fluctuations, since these cannot be compensated for each time by an immediate return to equilibrium of the thermodynamic system under consideration.

The extreme weakness of the partial pressures of oxygen in the reference compartment of known gages also means that the slightest deficiency in the permeability of the electrolyte, the smallest leak, renders the gage completely useless.

This invention therefore aims at remedying these defects, and in particular at producing an electrochemical oxygen gage with an internal reference, capable of being used in relatively low temperatures, of being rapidly brought to a state of equilibrium at the requisite operational temperature and, if this temperature should vary, of being rapidly corrected; and which, moreover, offers high mechanical resistance.

In accordance with this invention a palladium-palladium oxide redox couple is used as the internal reference of an electrochemical gage of the type in question.

In the preferred oxygen gage according to this invention the internal reference compartment is bounded at least in part by the solid electrolyte, this compartment contains a mass of palladium, of palladium oxide, or of a mixture of palladium and palladium oxide, at least one electronic conductor (hereafter referred to as "interior electronic conductor") is in contact both with the electrolyte within this compartment and with the contents of the compartment, and this compartment is hermetically closed, preferably sealed at the point where the electronic conductor leaves this compartment, so as to prevent any direct contact between the contents and the outside atmosphere.

The use of such a gage naturally implies the presence of a second electronic conductor in electrical contact (hereafter called "exterior electronic conductor") with the solid electrolyte in the measuring compartment or its equivalent, especially when the gage is used for determining the partial pressure of oxygen in a specific atmosphere, of a bath, for example of molten metal, etc.

In one recommended method of construction of the gage according to the invention, the compartment containing the internal reference takes the form of a tube or sheath element, formed by the solid oxide electrolyte itself.

The use of a palladium-palladium oxide couple in electrochemical gages of this type enables all the defects mentioned above to be remedied.

It has in fact been discovered that the kinetics of the bringing to equilibrium of this couple in the conditions described above is very rapid; the fact that the partial pressures of the reference oxygen of the couple according to the invention are considerably more important than those of other couples used hitherto, and this at much lower temperatures, is also shown to be particularly favourable to the efficient functioning of the gages. Purely as an indication, these partial pressures can vary between values of the order of 10.sup.-3 and the order of the atmosphere depending on the operational temperature. Gages so constituted are both more accurate and, especially where very high partial pressures of the reference oxygen are concerned, less sensitive to minor permeability deficiencies of the electrolyte.

Gages so constituted function perfectly at temperatures below 800.degree. C., temperatures which are low in the technical field under consideration. The oxygen exchange reaction between the palladium and the palladium oxide of the internal reference is still sufficiently rapid and reversible at 400.degree. C.

Another advantage deriving from use of the Pd-PdO couple lies in the possible regeneration, under relatively low recharge potential, of the internal reference by electrolytic reduction of the oxide or electrolytic oxidation of the reference metal, after operation of the minigage has caused the transformation of the palladium-palladium oxide mixture into one or other of its pure constituents, or at least into proportions which are no longer compatible with correct functioning of the minigage.

Electrochemical gages according to the invention may easily be miniaturized. Among the essential advantages of miniaturization must be reckoned the homogeneity of temperature in the masses of the internal reference and the electrolyte sheath respectively, thus eliminating risks of error due to temperature gradients, which in larger gages can often be avoided only with great difficulty. This advantage becomes more appreciable in that in the neighbourhood of 500.degree. C., a rise in temperature of 20.degree. C. results in the effective doubling of the reference pressure.

Miniaturization is also accompanied by an increase in the mechanical resistance of these gages, so that they may be used for measuring the relative oxygen content in atmospheres maintained at high pressures. These minigages are also easily utilised in cold enclosures. They are then used in conjunction with small furnaces which, given that minigages according to the invention may be constructed in very small sizes, may be of inconsiderable bulk and consume hardly any electric power.

Other characteristics of the invention will emerge in the course of the description which follows of preferred methods of construction of electrochemical gages according to the invention and the results which may be obtained with them, especially with reference to the figures, in which:

FIG. 1 shows an axial section across a gage constructed according to one of the preferred methods for the invention,

FIG. 2 is the same gage seen from above,

FIG. 3 is a diagram of such a minigage in conjunction with a furnace which enables it to be brought to the desired temperature,

FIG. 4 is a diagram of a variant of the gage in FIG. 1 and a method of utilising this gage, particularly for determining the partial pressure of oxygen in a liquid bath, for example in a molten metal,

FIG. 5 shows the curves representing variations of the emf of such a gage as a function of the temperature, for various constant values of exterior partial pressures of oxygen,

FIG. 6 shows the curves representing variations in the emf of such a gage as functions of the exterior partial pressure of oxygen for various temperature values.

The "minigage" shown in FIG. 1, and generally designated as 1, comprises a sheath 2, closed at its lower end 4 and made of a material suitable for constituting the solid electrolyte of such a minigage. This sheath provides a compartment 6 containing a mixture of palladium and palladium oxide which acts as internal reference. A lower electric conductor is maintained in contact with both the solid electrolyte and with the internal reference mixture, this conductor communicating with the exterior by means of the limbs 10a and 10b of a thermocouple. An exterior conductor 12 is maintained in contact with the electrolyte, in particular in the grooves 14 formed in the exterior surfaces and the bottom of the sheath 2. A stopper 16, preferably of the same material as the sheath 2, and a seal 18, ensures the tight seal of the compartment containing the internal reference around the conductors 10a, 10b and 12 and, consequently, prevents any exterior contact between this internal reference and the atmosphere.

In the construction shown in FIG. 1, the sensitive part of the thermocouple combined with the minigage is lodged in the interior compartment 6 of the gage. It may equally well be mounted on the exterior of the electrolyte. In particular, its two opposite limbs may be mounted in the two lateral grooves formed in the exterior surface of the sheath formed by the electrolyte, these limbs being then soldered together immediately under the lower part 4 of the gage.

In the construction which has just been described, the solid electrolyte may be any conventional solid oxide electrolyte for constructing oxygen gages e.g. a solid function of zirconia and yttrium oxide, stabilized zirconia, a solid solution of thoriabased oxides, etc. It must be made sufficiently tight e.g. by sintering or by cutting a monocrystal and its electronic conduction must be sufficiently weak under operational conditions.

The seal 18 should be constructed of a material which will remain sufficiently resistant to the operational temperature and sufficiently impermeable to oxygen. An enamel, particularly of Pyrex glass, is recommended.

The palladium-palladium oxide mixture is introduced into or formed in the compartment 6 in various ways:

1. by direct introduction into the gage before sealing of a carefully-blended mixture of powdered palladium and palladium oxide;

2. by introduction of powdered palladium only, then, after sealing, by partial oxidation by continuous passage of current, the interior electrode 8 being taken as an anode and the exterior electrode 12 kept in air;

3. by introduction into the reference compartment of palladium oxide and, after sealing, by partial reduction of the oxide by electrolysis at a sufficiently high temperature (of the order of 1,000.degree. C.), the interior electrode 8 being taken as a cathode.

It will be noted that this last method of proceeding may possibly be utilized for regeneration of the reference electrode of the minigage if, after prolonged operation at high oxygen pressure, the palladium has been completely transformed into oxide by the oxygen penetrating the reference compartment through leaks in the seal, permeation of the electrolyte or accidental "short-circuit" between the leads to the electrodes.

The electronic conductors or leads may be made of metal or alloy, unoxidisable under operational conditions, for example platinum alloyed with rhodium, or silver.

In order to improve contact with the electrodes, the electrolyte may be covered with a layer of unoxidizable material, electronically conducting or having mixed conduction properties, such as a precious metal or a semi-conducting oxide stable under operating conditions (for example V.sub.2 O.sub.5, RuO.sub.2, NiPr.sub.2 O.sub.4, etc.).

In this way a minigage has been constructed in accordance with FIGS. 1 and 2, using an electrolyte sheath of zirconia 12mm long and with an exterior diameter of 2mm. The useful volume for the internal reference of such a gage is of the order of 10mm.sup.3. The limbs 10a and 10b of the thermocouple are respectively of platinum and of platinum alloyed with rhodium. The exterior conductor 12 is also of platinum. Even smaller gages may be constructed with, for example, an exterior diameter of the order of 1mm.

A minigage so constructed may be used directly inside a heated enclosure containing the gaseous mixture of which it is desired to measure the oxygen partial pressure.

The very small size of such a minigage allows it to be used in small isothermic enclosures.

It is also possible to use it inside a large non-isothermic enclosure, if this includes a region where the temperature gradient is not too high. In particular, this makes it possible to measure the oxygen partial pressure in an enclosure where the maximum temperature is higher than the limit of temperature at which the minigage will operate.

Such a minigage is also capable of operating in cold enclosures. It must then be used in conjunction with a small furnace which, in view of the small size of the minigage, is not cumbersome and consumes very little electrical power. Such a furnace 20 is shown diagrammatically in FIG. 3, the gage 1 being then placed in the central canal 22 of this furnace. The conductors leading from the gage are then protected by an isolating sheath 24. The same minigage may also be used to measure the partial pressure of oxygen in a liquid bath, in particular a mass of molten metal 25 (FIG. 4). It is then equally necessary to protect the conductors leading from the gage. For this particular application, it will be useful to employ an apparatus of the kind shown in FIG. 4 or similar, the gage being then held by its upper part in a tube of isolating material 26 which forms an integral piece with a part 27 and in particular with the cover of the tank containing the bath: this tube thus protects the leads and thermocouple conductors from contact with the bath 25 and any corrosive vapours emanating from it.

For such an application, it is advisable to construct the exterior conductor 12 from materials which do not react with the liquid metallic phase and which do not dissolve in it. The external conductor 12 may moreover be eliminated and replaced with a separate lead 28 immersed in the bath.

Such minigages are capable of quite remarkable performances, as attested by the following trial results.

Various minigages have been constructed on the principles of the diagrams in the figures. They operate in a temperature range of 400.degree. - 1,000.degree. C. One of them, allowed to function continuously in air at 450.degree. C. (potential 183 mV), is still functioning perfectly after a period of six months. During the whole of that period its potential has remained constant within fluctuations of plus or minus 1 mV approximately, due to variations in the barometric pressure which cause corresponding variations in the partial pressure of oxygen in the air.

A minigage of this type mounted in a small furnace was tested (up to 100 bars) in a high-pressure shell with helium-oxygen and argon-oxygen mixtures. The results show that the minigage resists pressures of a least 100 bars at temperatures between 0.degree. and 700.degree. C. and that its potential at a given temperature is independent of the partial pressure of helium or argon and depends solely on the partial pressure of oxygen.

Conversely, a minigage according to the invention was submitted to a pressure of 300 bars of argon at 400.degree. C. for a period of an hour. After decompression, this gage continued to function normally.

A minigage according to the invention was brought several times from ordinary temperature to 500.degree. C. in about a minute. No deterioration due to thermal shock could be observed. Equilibrium tension is reached, when the gas under analysis is air, five seconds after stabilization of the temperature.

The lines in FIG. 5 represent variations of the emf (V) -- expressed in millivolts on the ordinate axis -- as a function of the temperature -- expressed in .degree.C. on the abscissa axis -- for various values, shown on each of the lines at the top of FIG. 5, of the partial pressure P.sub.m of oxygen in a test gas in which the minigage being tested was immersed. These variations are appreciably linear in the 400.degree. - 800.degree. C. range.

The lines in FIG. 6 give the value of the pressure P.sub.m -- expressed in atmospheres on the ordinate axis (logarithmic units) -- as a function of the emf -- expressed in millivolts on the abscissa axis -- at various operational temperatures between 500.degree. and 1,000.degree. C. It follows naturally that gages according to this invention may equally well be used at very high temperatures, providing that the solid electrolyte and the electronic conductors are made of materials resistant to such conditions, such as are well known to specialists.

Consequently minigages may be constructed which are exceptionally sensitive, accurate, solid and characterised by a particularly low cost price.

As will be obvious and indeed has already been demonstrated, this invention is in no way limited to the construction methods especially considered; on the contrary, it is open to any number of variations.