JPH035545B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical field to which the invention pertains] The present invention relates to an oxygen sensor using a solid electrolyte having oxygen ion conductivity, such as a zirconia-based solid electrolyte, and in particular to measuring the amount of oxygen in a reducing atmosphere containing hydrocarbons in a metal heat treatment furnace. This invention relates to an oxygen sensor suitable for. [Prior art and its problems] Oxygen sensors using solid electrolytes as described above have a simple structure and are relatively inexpensive, and are widely used in instrumentation systems as part of process instrumentation in various industries. It has been incorporated. This type of sensor is required to have accurate detection output and good reproducibility even when the partial pressure of oxygen in the gas to be measured is low. It is required that the detection output is not affected as much as possible by impurities such as metal vapor, and that no deterioration occurs. In particular, in heat treatment furnaces for various metal materials, such as soaking furnaces, bright annealing furnaces, and carburizing furnaces, oxygen sensors come into direct contact with the high-temperature furnace air. Oxygen sensors are known. Figure 1 shows the main part of the detection part of the oxygen sensor. As shown in the figure, a cylindrical ZrO ₂ −
A pair of opposing inner electrodes 2 sandwiching a tube of an oxygen ion conductive solid electrolyte 1 such as Y ₂ O ₃ , ZrO ₂ -CaO, etc.
An outer electrode 3 is provided, and air (partial pressure = 0.21 atm) A, which is a gas with a known oxygen partial pressure, is flowed from a reference gas inlet pipe 4 to the inner electrode 2 side, and on the other hand, air (partial pressure = 0.21 atm) A, which is a gas with a known oxygen partial pressure, is supplied to the inner electrode 2 side. The outer electrode 3 exposed to the gas is used as a measurement electrode, and the electromotive force generated between these two electrodes 2 and 3 is measured by lead wires 5 and 6 connected to both electrodes, respectively.
From this output voltage and the temperature obtained by the thermocouple 7 provided close to the inner electrode 2, the unknown oxygen partial pressure is determined using the Nernst equation below. E=-0.0496×T×log(PO ₂ (2)/PO ₂ (1)) Where, E: Output voltage (mV), T: Sensor temperature (K), PO ₂ (2): Measured gas Oxygen partial pressure (atmospheric pressure), PO ₂ (1): Oxygen partial pressure of reference gas (atmospheric pressure) In the conventional oxygen sensor shown in Fig. 1, the inner electrode 2 and outer electrode 3 are coated with chemical plating, vacuum evaporation, Platinum (Pt) formed into a thin or thick film by sputtering or paste baking
is often used. However, the electrode formed by the above method had the following drawbacks. (a) The outer electrode 3, which comes into contact with the flowing high-temperature gas to be monitored, has poor adhesion to the solid electrolyte 1, and is likely to peel off due to the difference in thermal expansion coefficient between the outer electrode 3 and the solid electrolyte 1. (b) Dust such as silica (SiO ₂ ) and alumina (Al ₂ O ₃ ) or metal vapors such as zinc (Zn) and iron (Fe) coexisting in the furnace air react with the outer electrode 3 to form low-melting compounds. This causes the outer electrode 3 to become brittle or to be consumed due to evaporation or scattering. (c) Carbon precipitated from the reducing gas atmosphere diffuses into the outer electrode 3 or the low melting point compound, and the outer electrode 3 is likely to become brittle and wear out. Note that the dust contains a lot of the aforementioned silica and alumina, which are components of the refractories in the furnace walls, and if the cutting agent used when machining parts to be heat treated is insufficiently cleaned, this dust will be mixed with the parts in the furnace. The above-mentioned zinc and other vapors are likely to be generated in the furnace. FIG. 2 shows a schematic structure of an oxygen sensor having an improved electrode structure in consideration of the above drawbacks. In the figure, the same reference numerals as in FIG. 1 represent the same parts. In the oxygen sensor shown in FIG. 2, the outer electrode 8 is a thick platinum wire of 0.5 mm to 1 mm wound spirally and installed on the outer surface of the hemispherical closed end of the solid electrolyte tube 1. The lead wire 6 connected thereto is inserted from the right side of the figure into the inner hole of an alumina protection tube 10 equipped with a gas inlet 11 to be measured. The solid electrolyte tube 1 is pressed against the inner surface of the tip of the protection tube 10 by a spring (not shown) on the right side of the figure, so that the outer electrode 8 is pressed against the outer surface of the closed end of the solid electrolyte tube 1. The inner electrode 9 is a platinum wire mesh, and is inserted between the inner surface of the closed end of the solid electrolyte tube 1 and the tip of the reference gas introduction tube 4, and is pressed against the inner surface. Compared to the sensor with the structure shown in FIG. 1, the oxygen sensor with the structure shown in FIG. 2 is less susceptible to deterioration of the contact between the outer electrode 8 and the solid electrolyte 1 even when used in a harsh high-temperature atmosphere. , has been successfully put into practical use.
However, when used in an atmosphere such as a carburizing furnace where the furnace air contains hydrocarbons and has a high carbon potential in the gas phase, which tends to cause solid phase carbon precipitation, carbon is concentrated near the outer electrode 8, which is rich in catalytic activity. Due to the autocatalytic action of the precipitated carbon, carbon is deposited and packed tightly between the closed end of the solid electrolyte tube 1 and the alumina protection tube 8, and the gas to be measured is However, the solid electrolyte 1 may not reach the outer electrode 8, or poor contact between the outer electrode 8 and the solid electrolyte 1 may occur, resulting in a decrease in sensor performance. This precipitated carbon may cause the oxygen sensor to
It can be removed by firing at 800℃~1000℃,
Although it is possible to restore the performance of the oxygen sensor, if such deterioration occurs during furnace operation, the operation must be temporarily stopped and the atmosphere inside the furnace changed to an oxidizing atmosphere to remove the precipitated carbon. The sensor performance must be restored by performing a burnout. Such a shutdown of the furnace must be avoided as much as possible since it reduces operational efficiency and increases product costs. Oxygen sensors with the electrode structure shown in Figure 2 sometimes had to be burnt out about once every 20 days, although this varied depending on the operating conditions. As a method to prevent the deterioration of oxygen sensor performance due to such precipitated carbon, it has been proposed in Japanese Patent Application Laid-Open No. 130191/1985 that a method of intermittently applying scavenging gas to the outer electrode is effective. The use of air with a high partial pressure may cause the solid electrolyte to be damaged due to thermal shock due to local combustion of carbon in the electrodes and high temperatures, or the scavenging gas may enter the furnace from the inlet of the gas to be measured. It spreads and disturbs the furnace air,
By changing the oxygen partial pressure, you can change the carburization level,
In extreme cases, it may react with hydrogen or carbon monoxide in the reactor air, causing combustion or explosion, so it is not very practical. On the other hand, if a gas with a low oxygen partial pressure is used as the scavenging gas, it will naturally take a long time to remove the precipitated carbon, so this is also not practical. The publication further states that gold or a conductive material containing an element whose electron shell's d-level is completely filled with 10 electrons and which does not have a catalytic effect on the decomposition of methane can be used for the electrode. However, it is unsuitable as an electrode material compared to platinum in terms of thermal and chemical stability and catalytic activity, and it deteriorates within a short period of time in the atmosphere during carburizing. There is a risk of it getting lost. In addition to the above, porous alumina is installed as a filter between the solid electrolyte tube and the outer electrode, and by returning the unbalanced composition of the measurement gas to equilibrium before the measurement gas reaches the outer electrode, the outer electrode There is a method to suppress the amount of carbon precipitation in the
There is a risk that carbon will precipitate within the pores of the porous alumina and the porous alumina will be destroyed by the stress of the precipitated carbon, and this method cannot essentially prevent carbon precipitation. [Objective of the Invention] In view of the above-mentioned drawbacks of the prior art, the object of the present invention is to provide a method that minimizes performance decline and deterioration even when used in a highly reducing atmosphere where carbon tends to precipitate from the gas to be measured. The goal is to obtain an oxygen sensor. [Summary of the Invention] According to the present invention, the above object is achieved by sintering a basic oxide or a basic oxide and a neutral oxide containing at least one of an alkali metal and an alkaline earth metal as a constituent element. It is a compact body,
This is achieved by providing a basic refractory material having at least one opening through which the gas to be measured flows, in contact with an electrode exposed to the gas to be measured (hereinafter referred to as a measurement electrode). The main cause of carbon precipitation from the above-mentioned highly reducing furnace air is the rapid thermal decomposition reaction of hydrocarbon gases, especially isobutane gas. This reaction rate increases as the temperature increases, and it is thought that carbon is precipitated by the following reactions during the hydrocarbon decomposition process. C ₄ H ₁₀ (butane) → C + C ₃ H ₈ + H ₂ C ₃ H ₈ (propane) → C + C ₂ H ₆ + H ₂ C ₂ H ₆ (ethane) → C + CH ₄ + H ₂ CH ₄ (methane) → C + 2 H ₂ The rate of reaction is further increased by the catalytic activity of the material used as the measuring electrode. The present invention is based on the discovery, through experiments described below, that even when platinum, which has a very high catalytic activity, is used as the measuring electrode, carbon deposition can be effectively prevented by the above-described means. As the basic refractory to be placed in contact with the measurement electrode, oxides of alkali metals and alkaline earth metals such as magnesia (MgO) and calcia (CaO) as mentioned above are suitable. Since it is often impossible to form such a metal oxide, it is sufficient to incorporate such a metal oxide into an ordinary refractory material such as alumina (Al ₂ O ₃ ) porcelain to obtain the required basicity. Since alkali metal oxides naturally have higher basicity than alkaline earth oxides, a sufficient effect can be obtained by incorporating, for example, a relatively small amount of Na ₂ O into the refractory. Furthermore, if the refractory is porous, carbon deposits may occur in the pores, leading to deterioration of the refractory, so it is advisable to construct the refractory from a dense material. In the oxygen sensor of the present invention, the solid electrolyte material, for example, the zirconia solid electrolyte, is conventionally formed into a cylindrical shape with a closed end, and the measurement electrode is often disposed at the closed end, so that the solid electrolyte tube is closed. It is advantageous to insert the basic refractory according to the invention in the form of a cap between the end and the protective tube that covers it. As mentioned in the description of the prior art, since the solid electrolyte tube is pressed against the inner surface of the closed end of the protection tube, the basic refractory cap is brought into close contact with the measuring electrode due to this pressing force, and this portion Prevent carbon deposition from occurring. In some cases, the protective tube itself may be made of a basic refractory material so that the measuring electrode is pressed against the inner surface of the closed end of the protective tube. When the oxygen sensor of the present invention is applied to exhaust gas control or combustion control for automobiles, the solid electrolyte is often formed into a plate shape in order to make the sensor compact and robust. The present invention can be easily applied even if the shape of the solid electrolyte changes as described above. [Embodiments of the Invention] Examples of the present invention will be described in detail below based on the drawings. FIG. 3 shows a cross-sectional view of the structure of an oxygen sensor according to the present invention, in which the same parts as those of the prior art shown in FIGS. 1 and 2 are designated by the same reference numerals.
The solid electrolyte tube 1 is made of, for example, zirconia (ZrO ₂ ) in which 8 mol% of ittria (Y ₂ O ₃ ) is dissolved, and is a cylindrical body with a length of, for example, about 50 cm, with the left end of the figure closed. It is formed. This solid electrolyte tube 1 is inserted into an inner hole of a protective tube 10 made of alumina or the like, which has an opening 11 for introducing a gas to be measured and whose left end is closed in the figure, and is inserted into an inner hole of a protective tube 10 made of alumina or the like, which has an opening 11 for introducing a gas to be measured.
3 is biased to the left in the figure by a compression spring 14 built in. A basic refractory 12 according to the present invention is interposed between the hemispherical closed end of the solid electrolyte tube 1 and the similarly hemispherical closed end of the protective tube 10. As shown in a perspective view in FIG. 4, this refractory 12 is a cap-shaped refractory made of magnesia, for example, and has an opening 12a at its tip for allowing gas to be measured to flow therethrough. The measurement electrode 8 as an outer electrode is formed by spirally winding a platinum wire with a diameter of 0.75 mm, for example, and is attached to the solid electrolyte tube 1 as shown in the figure by the biasing force from the compression spring 14 mentioned above.
It bends along the curved surface of the hemispherical closed end of the refractory, makes good contact with the outer surface of the closed end, and also closely contacts the inner surface of the cap-shaped basic refractory 12. On the other hand, the reference electrode 9 as an inner electrode is made of a thin platinum wire mesh of, for example, about 55 meshes, and is attached to the reference gas introduction tube 4 which is biased toward the left in the figure by a spring 16 in the end housing 13. The left end opening presses against the inner surface of the closed end of the solid electrolyte tube 1 to maintain good contact. Lead wires 6 and 5 from the measurement electrode 8 and the reference electrode 9 are led out as measurement terminals 6a and 5a to the outside of the end housing 13, respectively, as shown. A temperature sensor such as a thermocouple 7 for measuring the temperature of the sensing portion of the solid electrolyte 1 is similarly led out to the right as a terminal 7a. The end housing 13 is a cylindrical body whose left end is fixed to the protective tube 10, and a female thread 13a is cut on the right side of the inner hole in the figure, and an annular adjustment fitting 1 is formed.
The outer male thread of No. 5 is screwed into this. The aforementioned compression spring 14 is installed between the collar 1a fixed to the solid electrolyte and this adjustment fitting 15, and by screwing the adjustment fitting 15 toward the back of the screw 13a, the biasing force of the compression spring 14 can be reduced. It can be strengthened. To the right of the compression spring 14 is another tension spring 1.
6 is arranged, and both ends thereof are connected to the aforementioned adjustment fittings 1.
5 is connected to the collar 4a fixed to the reference gas introduction pipe 4, so by screwing the adjustment fitting 15 toward the back of the screw 13a as described above, the left end of the reference gas introduction pipe 4 in the figure can be adjusted. It is adapted to be pressed against the reference electrode 9. Note that 17 is a seal that isolates the reference gas compartment and the measured gas compartment within the sensor from each other. The oxygen sensor configured in this manner is inserted through the furnace wall 18 of a heat treatment furnace or the like so that its detection portion is exposed to the furnace air. Reference numeral 19 is, for example, an alumina-based sealing adhesive. The tip detection part of the oxygen sensor is maintained at approximately the furnace temperature, and the opening 1 of the protection tube 10 is located at the contact part with the measurement electrode 8 at the tip of the solid electrolyte tube 1.
1 and basic refractory 12 are gas inlet holes 1 to be measured.
The furnace air enters through 2a, and the oxygen gas partial pressure to be measured reaches this part. On the other hand, from the right end of the reference gas introduction pipe 4 in the figure, a reference gas with a known oxygen partial pressure, for example, air with an oxygen partial pressure of 0.21 atmospheres, is supplied at a rate of, for example, 50 c.c. per minute by a means not shown, and the introduced A voltage is introduced from the left end opening of the tube 4 through the wire mesh of the reference electrode 9 into the contact area between the reference electrode 9 and the inner surface of the tip of the solid electrolyte tube 1, and a measurement voltage is applied between the measurement terminals 5a and 6a. Occur. The structure of the oxygen sensor described above is shown for the measurement of oxygen partial pressure in a high-temperature ambient atmosphere, but when used in a low-temperature ambient atmosphere, the tip detection part of the solid electrolyte tube 1 must be heated to a temperature that is advantageous for measurement. A heater for this purpose is built into the oxygen sensor. The basic refractory 12 is, for example, magnesia (MgO).
After molding the fine powder into a predetermined shape using a slurry casting method or a press molding method, the sample gas is heated and sintered at a high temperature of 1800° C. or higher to make it denser, after the gas introduction hole 12a to be measured is previously opened. In order to verify the effects of the present invention, the oxygen sensor A according to the present invention shown in FIG. 3 and the conventional oxygen sensor B shown in FIG. A performance comparison test was conducted in a continuous carburizing furnace under the operating conditions shown in Table 1. As the basic refractory, one made of magnesia was used.

[Table] Figure 5 shows the changes over time in the output values of both oxygen sensors when the furnace was continuously operated under the above operating conditions.Curve A represents the output value of oxygen sensor A, and curve B represents the output value of oxygen sensor B. It is expressed as a ratio to the calculation output. As can be seen from Figure 5, oxygen sensor B
It is understood that the output of oxygen sensor A decreased to less than 99% of the calculated output, but the output of oxygen sensor A did not change at all from the initial state for about 100 days. A 1% error in the output of the oxygen sensor corresponds to approximately 0.1wt% carbon content in terms of carburization.
If an error of 1% or more occurs, it is necessary to restore the performance of the sensor by burnout, since it greatly affects the quality of the carburized material. As a result of repeated performance tests on sensor A under the same conditions, it was found that at least an error of 1% occurred.
It was 100 days later. It was also confirmed that both Sensor A and Sensor B recover their normal output due to burnout after their performance deteriorates. As a result of the above, it was found that the oxygen sensor incorporating the basic refractory according to the present invention can take a longer time to recover its performance due to burnout than the conventional oxygen sensor. Long-term field tests have revealed that the lifespan of the sensor is longer than that of conventional oxygen sensors. The lifespan of this type of oxygen sensor is related to the degree of wear of the platinum wire used for the measurement electrode 8 in the operating atmosphere, and it is not always clear why the wear of the platinum wire of oxygen sensor A is less. However, it is clear that one of the causes of sensor deterioration is that carbon atoms gradually diffuse into the platinum wire in a high temperature and highly reducing atmosphere, accelerating the brittleness and wear and tear of the platinum wire. In sensor A, since the amount of carbon deposited on the outer electrode 3 is originally small, the amount of carbon diffused into the platinum wire is also small, and it is presumed that the life span will be improved compared to conventional oxygen sensors. Further, based on the result that the basic refractory member 12 is effective in suppressing carbon deposition, the inventors conducted the following experiment to investigate the cause. Dense refractory members 12 made of SiO ₂ −10wt% Al ₂ O ₃ , Al ₂ O ₃ , MgO, CaO, and Al ₂ O ₃ −3wt% Na ₂ O were each made by a known ceramic manufacturing process, and It was assembled into an oxygen sensor having the structure shown in Fig. 2 and set in a carburizing furnace, and the relationship between the material of the refractory member and the amount of carbon deposited on the measuring electrode was investigated. FIG. 6 shows the relationship between the amount of carbon deposited on the platinum wire of the outer electrode 3 of the sensor and the material of the refractory member after a 6-day field test. In the figure, the amount of precipitated carbon on the vertical axis of the graph is shown with the weight of carbon precipitated in a sensor that uses alumina as a refractory material as the standard, and the performance of a sensor that uses alumina as a refractory member is shown in Figure 2. It is almost equivalent to the conventional sensor shown. On the other hand, when the above-mentioned materials used as fire-resistant members are sorted based on the acid-basicity of the solid, the results are as shown in Table 2.

〔Effect of the invention〕

As explained above, according to the present invention, in an oxygen sensor in which a measurement electrode and a reference electrode are provided facing each other on the surface of a sensor body made of a solid electrolyte, at least one of an alkali metal and an alkaline earth metal is provided in contact with the measurement electrode. A basic refractory comprising a basic oxide or a sintered dense body of a basic oxide and a neutral oxide, the basic refractory having at least one opening through which the gas to be measured flows. This prevents carbon from forming near the measuring electrode, even when used for long periods in a high-temperature, reducing atmosphere containing carbon compounds, or even when platinum, which has an extremely high carbon deposition catalytic effect, is used as the measuring electrode. The risk of precipitation and troubles caused by it can be significantly reduced compared to conventional oxygen sensors. Thereby, the oxygen sensor according to the present invention
In addition to almost completely eliminating the shortcomings of conventional sensors, which required frequent work to restore performance through burnout treatment, the oxygen sensor's lifespan was also reduced due to brittleness and wear of the measurement electrode within a relatively short period of time. It is possible to eliminate the disadvantages that occur.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views of main parts of sensors showing different examples of oxygen sensors according to the prior art, and FIGS. 3 and 4 show embodiments of oxygen sensors according to the present invention, of which FIG. FIG. 4 is a cross-sectional view of an oxygen sensor according to the present invention, FIG. 4 is a perspective view showing an example of a cap-shaped basic refractory used in the sensor, and FIG. 5 is an oxygen sensor according to the present invention and an oxygen sensor according to the prior art. Figure 6 is a graph showing a comparison of changes in measured output during long-term use, and Figure 6 is a line showing a comparison of the amount of carbon deposited between basic refractories and neutral or acidic refractories used in the present invention. It is a diagram. In the figure, 1: solid electrolyte tube as the sensor body, 8: measurement electrode, 9: reference electrode,
12: Basic refractory.

Claims (1)

[Scope of Claims] 1. A sensor body made of a solid electrolyte having oxygen ion conductivity; a reference electrode that contacts one surface of the sensor body and is placed in an atmosphere of a reference gas containing a predetermined amount of oxygen; a measurement electrode that is placed in contact with the other surface of the sensor body and placed in the atmosphere of the gas to be measured whose oxygen content is to be measured; In an oxygen sensor that measures the amount of oxygen that is present in contact with the measurement electrode, the oxygen sensor contains at least one of an alkali metal and an alkaline earth metal as a constituent element, and a basic oxide or a mixture of a basic oxide and a neutral oxide. An oxygen sensor using a solid electrolyte, characterized in that a basic refractory is provided which is a sintered dense body and has at least one opening through which the gas to be measured flows. 2. The oxygen sensor according to claim 1, which uses a solid electrolyte, wherein the basic refractory is made of a metal oxide. 3. The oxygen sensor according to claim 2, which uses a solid electrolyte, wherein the basic refractory is a metal oxide whose main component is magnesia (MgO). 4. The oxygen sensor according to claim 2, which uses a solid electrolyte, wherein the basic refractory is a metal oxide whose main component is calcia (CaO). 5. The oxygen sensor according to claim 2, which uses a solid electrolyte, wherein the basic refractory is a metal oxide containing Na ₂ O. 6. An oxygen sensor using a solid electrolyte according to claim 1, wherein the sensor body is made of a solid electrolyte formed in a plate shape. 7. The oxygen sensor according to claim 1, which uses a solid electrolyte, characterized in that the measurement electrode is made of platinum. 8. The oxygen sensor according to claim 1, which uses a solid electrolyte, characterized in that the measurement electrode is made of a platinum wire. 9. An oxygen sensor using a solid electrolyte according to claim 1, wherein the sensor body is formed of a solid electrolyte formed in a cylindrical shape with one end closed. 10. The oxygen sensor according to claim 9, which uses a solid electrolyte, characterized in that the basic refractory is a protective tube with one end closed, which is placed over a cylindrical solid electrolyte. 11. The oxygen sensor according to claim 9, wherein the basic refractory is a measuring electrode holding member interposed between the closed end of the solid electrolyte tube and the protective cylinder. Oxygen sensor used.