Oxygen sensor and micromechanical electrical element including the same

Abstract

To provide a sensor that can measure oxygen density in detected gas.SOLUTION: An oxygen sensor comprises membrane electrode assemblies provided: respectively with solid electrolyte membranes; and respectively with first and a second membrane electrodes arranged on respective surfaces of the solid electrolyte membranes. An electrode facing member is arranged so as to face the second electrode. A reference oxygen concentration space is defined by a wall part provided to surround the second electrode, between the solid electrolyte membrane and the member. The member or the wall part is composed of an oxygen permeable material. The first electrode is arranged so that the first electrode faces a measurement objective atmosphere, and the second electrode is connected to a positive pole of a power source, and the first electrode is connected to a negative pole of the power source, and placed under a condition where oxygen density is increased in a reference oxygen density space, and the connection of the first electrode and second electrode to the power source is released and further the oxygen density in the measurement objective atmosphere which the first electrode faces, is measured by measuring electromotive force generated between the first electrode and the second electrode.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an oxygen sensor and a micro-mechanical electrical element including the same. [Background technology]

[0002] Various oxygen sensors using oxide ion-permeable solid electrolytes are known. These types of oxygen sensors are broadly divided into electromotive force types and limiting current types. For example, Patent Document 1 describes an electromotive force type oxygen sensor consisting of an electrochemical oxygen pump unit, a sealed space, and an electrochemical sensor unit. In this oxygen sensor, an external voltage is applied between the two electrodes of the electrochemical oxygen pump unit, and oxygen gas present in the sealed space is forcibly discharged to the outside in accordance with the principle of an electrochemical oxygen pump, thereby controlling the reference gas chamber to a predetermined low oxygen partial pressure.

[0003] Patent Document 2 describes an oxygen sensor in which a current is applied between an outer electrode and an inner electrode to introduce oxygen in a detection gas into the vicinity of the inner electrode, and the introduced oxygen is used as a reference gas. In this oxygen sensor, in order to introduce oxygen in a detection gas into the vicinity of the inner electrode, a buffer layer having a porous structure containing zirconia and aluminum is provided on the outer surface of the inner electrode, allowing oxygen to enter the buffer layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 05-240833 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-112918 Summary of the Invention

[0005] The oxygen sensor in Patent Document 1 attempts to accurately measure the oxygen concentration in the test gas by controlling the oxygen partial pressure in the reference gas chamber to a low reference value. However, because the oxygen partial pressure in the reference gas chamber is controlled to a low value, even a slight change in the oxygen partial pressure has a large effect on the electromotive force, making it difficult to improve detection accuracy.

[0006] The oxygen sensor described in Patent Document 2 uses oxygen as a reference gas by increasing the partial pressure of oxygen introduced near the inner electrode above that of atmospheric oxygen, and forms a cylindrical element. This necessitates the need for a porous and strong relaxation layer. However, making the relaxation layer porous and strong is disadvantageous in terms of miniaturizing the oxygen sensor. It is not easy to apply the formation of such a porous layer and the manufacture of such a cylindrical element to, for example, microelectromechanical systems (MEMS).

[0007] SUMMARY OF THE INVENTION An object of the present invention is to provide an oxygen sensor that can overcome the various drawbacks of the prior art described above.

[0008] The present invention provides a first membrane electrode assembly including a first solid electrolyte membrane having oxide ion conductivity and a first electrode disposed on one surface of the solid electrolyte membrane; a second membrane electrode assembly including a second solid electrolyte membrane having oxide ion conductivity and a second electrode disposed on one surface of the solid electrolyte membrane; the first membrane electrode assembly and the second membrane electrode assembly are arranged such that the first solid electrolyte membrane in the first membrane electrode assembly and the second solid electrolyte membrane in the second membrane electrode assembly face each other with a gap between them; an intermediate electrode is disposed between the first solid electrolyte membrane of the first membrane electrode assembly and the second solid electrolyte membrane of the second membrane electrode assembly so as to be in contact with both solid electrolyte membranes, and a reference oxygen concentration space is defined by an oxygen-permeable wall portion provided so as to surround the intermediate electrode; The oxygen sensor is configured to measure the oxygen concentration in the atmosphere to be measured that the first electrode faces, by placing the first electrode so that it faces the atmosphere to be measured, connecting the intermediate electrode to the positive terminal of a power source and the second electrode to the negative terminal of the power source to create a state in which the oxygen concentration in the reference oxygen concentration space is increased, and further measuring the electromotive force generated between the first electrode and the intermediate electrode.

[0009] The present invention also provides a membrane electrode assembly including a solid electrolyte membrane having oxide ion conductivity and a first electrode and a second electrode disposed on either side of the solid electrolyte membrane, an electrode opposing member is disposed to face the second electrode; a reference oxygen concentration space is defined between the solid electrolyte membrane and the member by a wall portion provided so as to surround the second electrode, At least one of the member and the wall portion is made of an oxygen-permeable material, The oxygen sensor measures the oxygen concentration in the atmosphere to be measured that the first electrode faces by positioning the first electrode so that the first electrode faces the atmosphere to be measured, connecting the second electrode to the positive terminal of a power source and the first electrode to the negative terminal of the power source to create a state in which the oxygen concentration in the reference oxygen concentration space is increased, disconnecting the first electrode and the second electrode from the power source, and measuring the electromotive force generated between the first electrode and the second electrode.

[0010] The present invention further provides a membrane electrode assembly including a solid electrolyte membrane having oxide ion conductivity, a first electrode and a second electrode disposed on one side of the solid electrolyte membrane, and an intermediate electrode disposed on the other side of the solid electrolyte membrane, an electrode opposing member is disposed so as to face the intermediate electrode, a reference oxygen concentration space is defined between the solid electrolyte membrane and the member by a wall portion provided so as to surround the intermediate electrode, At least one of the member and the wall portion is made of an oxygen-permeable material, The oxygen sensor measures the oxygen concentration in the measurement atmosphere facing the first electrode by arranging a first electrode and a second electrode so that the first electrode faces the measurement atmosphere and the second electrode faces the measurement atmosphere or the outside air, connecting the intermediate electrode to the positive terminal of a power source and the second electrode to the negative terminal of the power source, thereby increasing the oxygen concentration in the reference oxygen concentration space, and measuring the electromotive force generated between the first electrode and the intermediate electrode. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing the structure of an embodiment of an oxygen sensor of the present invention. [Figure 2] FIG. 2 is a graph showing how the relationship between the concentration of oxygen gas in the test gas and the electromotive force of the concentration cell depends on the concentration of oxygen gas contained in the reference gas. [Figure 3] FIG. 3 is a schematic diagram showing the structure of another embodiment of the oxygen sensor of the present invention. [Figure 4] FIG. 4 is a schematic diagram showing the structure of yet another embodiment of the oxygen sensor of the present invention. [Figure 5] FIG. 5 is a schematic diagram showing the structure of yet another embodiment of the oxygen sensor of the present invention. [Figure 6] FIG. 6 is a graph showing the change in voltage over time when the concentration of oxygen gas is measured using the oxygen sensor of the embodiment shown in FIG. [Figure 7] FIG. 7 is a schematic diagram showing the structure of yet another embodiment of the oxygen sensor of the present invention. [Figure 8] FIG. 8 is a schematic diagram showing the structure of yet another embodiment of the oxygen sensor of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will now be described based on preferred embodiments with reference to the drawings. Fig. 1 shows one embodiment of an oxygen sensor of the present invention. The oxygen sensor 1 shown in the figure includes a first membrane electrode assembly 10 and a second membrane electrode assembly 20.

[0013] The first membrane electrode assembly 10 includes a first solid electrolyte membrane 100 having oxide ion conductivity and a first electrode 101 disposed on one surface of the solid electrolyte membrane 100. On the other hand, the second membrane electrode assembly 20 includes a second solid electrolyte membrane 200 having oxide ion conductivity and a second electrode 102 disposed on one surface of the solid electrolyte membrane 200.

[0014] The first solid electrolyte membrane 100 and the second solid electrolyte membrane 200 may be made of the same material or different materials as long as they have oxide ion conductivity. The first electrode 101 and the second electrode 102 may be made of the same material or different materials as long as they are conductive.

[0015] In the oxygen sensor 1, the first membrane electrode assembly 10 and the second membrane electrode assembly 20 are arranged so that the other side of the first solid electrolyte membrane 100 of the first membrane electrode assembly 10 (i.e., the side on which the first electrode 101 is not disposed) and the other side of the second solid electrolyte membrane 200 of the second membrane electrode assembly 20 (i.e., the side on which the second electrode 102 is not disposed) face each other with a gap between them. FIG. 1 shows the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200, both of which are flat plates, arranged substantially parallel to each other and spaced a certain distance apart. The gap between the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200 is not critical in the present invention and may be set to an appropriate value depending on the size of the oxygen sensor 1, the application situation, and the like. Generally, setting the gap between the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200 to be 0.05 mm or more and 10 mm or less enables highly accurate measurement of the concentration of oxygen gas.

[0016] An intermediate electrode 120 is disposed between the other surface of the first solid electrolyte membrane 100 in the first membrane electrode assembly 10 and the other surface of the second solid electrolyte membrane 200 in the second membrane electrode assembly 20 so as to be in contact with both solid electrolyte membranes 100, 200. The intermediate electrode 120 may be made of the same material as the first electrode 101 and the second electrode 102 described above, or may be made of a different material, as long as it is conductive.

[0017] In the oxygen sensor 1, an oxygen-permeable annular wall portion 30 is provided between the first solid electrolyte membrane 100 of the first membrane electrode assembly 10 and the second solid electrolyte membrane 200 of the second membrane electrode assembly 20. The annular wall portion 30 has a structure that allows oxygen gas to flow between the inside and outside of the wall portion. The annular wall portion 30 is provided to surround the intermediate electrode 120. As a result, in the oxygen sensor 1, the annular wall portion 30 defines a reference oxygen concentration space S between the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200. In FIG. 1 , the reference oxygen concentration space S is defined by the annular wall portion 30, the first solid electrolyte membrane 100, the second solid electrolyte membrane 200, and the intermediate electrode 120. The reference oxygen concentration space S is in communication with the outside world through the annular wall portion 30, which is made of an oxygen-permeable material, such as a porous material.

[0018] The volume of the reference oxygen concentration space S is not critical in the present invention and may be set to an appropriate value depending on the size of the oxygen sensor 1, the situation in which it is used, etc. Generally, the volume of the reference oxygen concentration space S is set to 0.01 mm 3 More than 1000mm 3 If the following setting is made, the concentration of oxygen gas can be measured with high accuracy. If the intermediate electrode 120 is made of a porous material or the like and has a void, the void can also be used as the reference oxygen concentration space S. Therefore, it is not necessary to provide a complete space between the intermediate electrode 120 and the annular wall portion 30.

[0019] The cross-sectional shape of the annular wall portion 30 is not particularly limited as long as it is annular. For example, a cylindrical annular wall portion having a circular or rectangular cross section can be used. The shape of the annular wall portion 30 to be used can be appropriately selected depending on the shape of each solid electrolyte membrane 100, 200, the size of the oxygen sensor 1, etc.

[0020] In the oxygen sensor 1, the first solid electrolyte membrane 100, the first electrode 101 disposed on each side thereof, and the intermediate electrode 120 constitute a first unit cell. Similarly, the second solid electrolyte membrane 200, the second electrode 102 disposed on each side thereof, and the intermediate electrode 120 constitute a second unit cell. The intermediate electrode 120 serves as both an electrode of the first unit cell and an electrode of the second unit cell. The first unit cell functions as a concentration cell. Meanwhile, the second unit cell functions as an oxygen pump. More specifically, as shown in FIG. 1 , a voltmeter 60 is connected between the first electrode 101 and the intermediate electrode 120 of the first unit cell to measure the electromotive force generated between the electrodes of the concentration cell. In the second unit cell, a DC power supply 40 is connected between the second electrode 102 and the intermediate electrode 120 to apply a voltage between the electrodes. In this case, the intermediate electrode 120 is connected to the positive electrode of the DC power supply 40 , and the second electrode 102 is connected to the negative electrode of the DC power supply 40 .

[0021] The method for measuring the oxygen concentration in a test gas using the oxygen sensor 1 having the above configuration is as follows. First, the first electrode 101 is positioned so that it faces the atmosphere to be measured, and the second electrode 102 faces the outside air (generally the atmosphere). In this state, the entire oxygen sensor 1 is heated to cause the first and second solid electrolyte membranes 100, 200 to exhibit oxide ion conductivity. The heating temperature varies depending on the materials used to form the first and second solid electrolyte membranes 100, 200. For example, when the first and second solid electrolyte membranes 100, 200 are made of the materials described below, practical oxide ion conductivity is exhibited even at temperatures below approximately 600°C. The heating temperature is preferably 200°C or higher, more preferably 300°C or higher, and even more preferably 400°C or higher. By heating the entire oxygen sensor 1 to cause the first and second solid electrolyte membranes 100, 200 to reach the above temperature or higher, sufficient measurement accuracy can be obtained. It should be noted that this temperature is not a set temperature but the actual temperature of the first and second solid electrolyte membranes 100 and 200.

[0022] Once the first and second solid electrolyte membranes 100, 200 exhibit oxide ion conductivity, the intermediate electrode 120 is connected to the positive electrode of the DC power supply 40, and the second electrode 102 is connected to the negative electrode of the DC power supply 40. This activates the oxygen pumping function of the second unit cell, reducing oxygen gas contained in the ambient air to oxide ions. The oxide ions migrate through the second solid electrolyte membrane 200 and reach the intermediate electrode 120. Upon reaching the intermediate electrode 120, the oxide ions release electrons and convert to oxygen gas. The oxygen gas thus generated accumulates in the reference oxygen concentration space S. However, because the reference oxygen concentration space S is partially defined by the annular wall 30 made of an oxygen-permeable material and is connected to the outside, the pressure in the reference oxygen concentration space S does not increase excessively. As a result, the reference oxygen concentration space S maintains a certain pressure while maintaining a high partial pressure of oxygen gas. The oxygen concentration in this state is referred to as the reference oxygen concentration. It is preferable to set the reference oxygen concentration to 100 vol% oxygen in order to enable highly accurate measurements. Whether the oxygen concentration in space S is 100 vol% can be determined by measuring the electromotive force between the electrodes in an atmosphere with a known test gas concentration, such as an oxygen concentration of 20.9% in normal air.

[0023] When the reference oxygen concentration is reached in space S, the electromotive force generated between first electrode 101 and intermediate electrode 120 in the first unit cell is measured by voltmeter 60. During the measurement, a DC voltage is continuously applied between second electrode 102 and intermediate electrode 120 in the second unit cell to maintain the oxygen concentration in reference oxygen concentration space S at a certain high level. Based on the measured electromotive force, the concentration of oxygen gas in the atmosphere to be measured is calculated using the Nernst equation shown below. E = (RT / 4F)ln(P O2 A / P O2 B ) In the formula, E represents the electromotive force (V) generated between the first electrode 101 and the intermediate electrode 120, R represents the gas constant, T represents the absolute temperature (K), F represents the Faraday constant, and P O2 A represents the concentration of oxygen gas in the atmosphere to be measured, and PO2 B represents the concentration of oxygen gas in the reference oxygen concentration space S. In the above formula, E, R, T, F and P O2 B Since is known, P O2 A That is, the concentration of oxygen gas in the atmosphere to be measured can be calculated.

[0024] The advantages of using the second unit cell as an oxygen pump in the oxygen sensor 1 to increase the oxygen concentration in the reference oxygen concentration space S will be explained with reference to FIG. 2 . This figure is a graph plotting the relationship between oxygen gas concentration and electromotive force according to the Nernst equation for the oxygen sensor 1 of this embodiment. In this figure, lines A and B represent simulation results when T = 600°C and the reference oxygen concentrations are set to 0.010 vol% and 0.020 vol%. As is clear from comparing lines A and B, even when the reference oxygen concentration differs only slightly, such as 0.010 vol%, the electromotive force differs by several tens of millivolts, resulting in significant differences in the calculated oxygen gas concentration. In contrast, lines C through E represent simulation results when T = 600°C and the reference oxygen concentrations are set to 80 vol%, 90 vol%, and 100 vol%. As is clear from comparing lines C through E, there is almost no difference in electromotive force even when the reference oxygen concentration differs by as much as 20 vol%. As a result, the calculated oxygen gas concentration is almost the same. Thus, according to the present invention, the measurement accuracy can be improved by measuring the oxygen gas concentration with the reference oxygen concentration set high.

[0025] In particular, as is clear from FIG. 2, with the oxygen sensor 1 of this embodiment, the absolute value of the electromotive force increases as the concentration of oxygen gas in the test gas decreases, which is a concentration range in which measurement errors are likely to occur, and therefore there is an advantage that measurement errors are less likely to occur even when the concentration of oxygen gas is low.

[0026] Furthermore, unlike conventional oxygen sensors, the oxygen sensor 1 of this embodiment does not use air as the reference gas for the concentration cell, making it less susceptible to the influence of impurities contained in the air. This also allows the concentration of oxygen gas to be measured with high accuracy.

[0027] To further enhance the above advantages, it is advantageous to measure the oxygen gas concentration in the reference oxygen concentration space S under conditions in which the oxygen gas concentration is set to preferably between 60 vol% and 100 vol%, more preferably between 80 vol% and 100 vol%, and even more preferably between 90 vol% and 100 vol%. The oxygen gas concentration in the reference oxygen concentration space S is measured by measuring the electromotive force between the electrodes in an atmosphere of a known test gas concentration, such as an oxygen concentration of 20.9% in normal air.

[0028] To set the oxygen gas concentration in the reference oxygen concentration space S within the above range, for example, the voltage of the DC power supply is preferably set to 0.05 V or more and 3 V or less, and the volume of the reference oxygen concentration space S is set to 0.01 mm 3 Over 800mm 3 or less, or the amount of oxygen permeating through the annular wall portion 30 may be reduced.

[0029] In order to ensure stable measurements, it is advantageous to maintain the pressure of the reference oxygen concentration space S within a specific range in addition to the concentration of oxygen gas in the reference oxygen concentration space S. From this perspective, it is advantageous to measure the concentration of oxygen gas under conditions in which the pressure (Pa) of the reference oxygen concentration space S is set relative to the atmospheric pressure (Pa) in the atmosphere to be measured (i.e., the value of pressure (Pa) in the reference oxygen concentration space S / atmospheric pressure (Pa) in the atmosphere to be measured) is preferably set to 1.0 or more and 3.0 or less, more preferably 1.0 or more and 2.0 or less, and even more preferably 1.0 or more and 1.5 or less.

[0030] To set the pressure of the reference oxygen concentration space S within the above range, for example, the voltage of the DC power supply is preferably set to 1.0 V or more and 3.0 V or less, and the volume of the reference oxygen concentration space S is set to 0.01 mm 3 More than 1000mm 3or below, or by adjusting the amount of oxygen permeating through the annular wall portion 30. These methods of adjusting the oxygen gas concentration and pressure in the reference oxygen concentration space S can also be applied to other embodiments of the oxygen sensor described later.

[0031] Figure 3 shows another embodiment of the oxygen sensor shown in Figure 1. The oxygen sensor 1 shown in Figure 3 differs from the oxygen sensor 1 shown in Figure 1 in that the intermediate electrode 120 provided in the oxygen sensor shown in Figure 1 is divided into a third electrode 103 that is in contact only with the first solid electrolyte membrane 100 and a fourth electrode 104 that is in contact only with the second solid electrolyte membrane 200, and these electrodes are independent of each other. By separating the electrodes, deterioration of the electrodes is less likely to occur, and an improvement in the sensor life can be expected.

[0032] 3 includes a first membrane electrode assembly 10 having a first solid electrolyte membrane 100 and a first electrode 101 and a third electrode 103 disposed on either side of the solid electrolyte membrane 100. On the other hand, the second membrane electrode assembly 20 includes a second solid electrolyte membrane 200 and a second electrode 102 and a fourth electrode 104 disposed on either side of the solid electrolyte membrane 200. The first electrode 101 to the fourth electrode 104 may be made of the same material or different materials as long as they are conductive.

[0033] In the oxygen sensor 1, the first membrane electrode assembly 10 and the second membrane electrode assembly 20 are arranged so that the third electrode 103 of the first membrane electrode assembly 10 and the fourth electrode 104 of the second membrane electrode assembly 20 face each other with a gap between them. FIG. 1 shows the third electrode 103 and the fourth electrode 104, both of which are flat plates, arranged substantially parallel to each other and spaced a certain distance apart. The gap between the third electrode 103 and the fourth electrode 104 is not critical in the present invention and may be set to an appropriate value depending on the size of the oxygen sensor 1, the application situation, and the like. Generally, setting the gap between the third electrode 103 and the fourth electrode 104 to be 0.05 mm or more and 10 mm or less enables the concentration of oxygen gas to be measured with high accuracy.

[0034] In the oxygen sensor 1 shown in Fig. 3, the annular wall portion 30 is provided so as to surround the third electrode 103 and the fourth electrode 104. As a result, in the oxygen sensor 1, the annular wall portion 30 defines a reference oxygen concentration space S between the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200. In Fig. 3, the reference oxygen concentration space S is defined by the annular wall portion 30, the first solid electrolyte membrane 100, the second solid electrolyte membrane 200, the third electrode 103, and the fourth electrode 104. The reference oxygen concentration space S is in communication with the outside world through the annular wall portion 30, which is made of a material that is permeable to oxygen gas.

[0035] In the oxygen sensor 1 shown in FIG. 3, the first membrane electrode assembly 10 constitutes a unit cell. Similarly, the second membrane electrode assembly 20 also constitutes a unit cell. The first membrane electrode assembly 10 functions as a concentration cell. Meanwhile, the second membrane electrode assembly 20 functions as an oxygen pump. In detail, as shown in FIG. 3, a voltmeter 60 is connected between the first electrode 101 and the third electrode 103 of the first membrane electrode assembly 10 so as to measure the electromotive force generated between the two electrodes of the concentration cell. In the second membrane electrode assembly 20, a DC power supply 40 is connected between the second electrode 102 and the fourth electrode 104 so that a voltage is applied between the two electrodes. In this case, the fourth electrode 104 is connected to the positive electrode of the DC power supply 40, and the second electrode 102 is connected to the negative electrode of the DC power supply 40.

[0036] The method for measuring the oxygen concentration in a test gas using the oxygen sensor 1 shown in FIG. 3 is as follows. First, the first electrode 101 is positioned so that it faces the atmosphere to be measured, and the second electrode 102 is positioned so that it faces the outside air (generally the atmosphere). Under this condition, the entire oxygen sensor 1 is heated to induce oxide ion conductivity in the first and second solid electrolyte membranes 100, 200. Next, the fourth electrode 104 is connected to the positive electrode of the DC power supply 40, and the second electrode 102 is connected to the negative electrode of the DC power supply 40. This activates the oxygen pumping function of the second membrane electrode assembly 20, reducing oxygen gas contained in the outside air to oxide ions. The oxide ions migrate through the second solid electrolyte membrane 200 and reach the fourth electrode 104, where they are converted into oxygen gas. The oxygen gas thus generated accumulates in the reference oxygen concentration space S. As a result, the reference oxygen concentration space S maintains a certain pressure while the partial pressure of oxygen gas is high.

[0037] When the reference oxygen concentration is reached in the space S, the electromotive force generated between the first electrode 101 and the third electrode 103 in the first membrane electrode assembly 10 is measured by the voltmeter 60, and the concentration of oxygen gas in the atmosphere to be measured is calculated according to the Nernst equation described above.

[0038] Figure 4 shows yet another embodiment of the oxygen sensor shown in Figure 1. The oxygen sensor 1 shown in Figure 4 includes a first membrane electrode assembly 10 and a second membrane electrode assembly 20, similar to the oxygen sensor shown in Figure 1. The first membrane electrode assembly 10 includes a first solid electrolyte membrane 100 and a first electrode 101 disposed on one surface of the solid electrolyte membrane 100. The second membrane electrode assembly 20 includes a second solid electrolyte membrane 200 and a second electrode 102 disposed on one surface of the solid electrolyte membrane 200.

[0039] The oxygen sensor of this embodiment differs from the embodiment shown in FIG. 1 in the arrangement of the first membrane electrode assembly 10 and the second membrane electrode assembly 20. Specifically, the first membrane electrode assembly 10 and the second membrane electrode assembly 20 are arranged so that one of the two surfaces of the first solid electrolyte membrane 100 on which the first electrode 101 is arranged and one of the two surfaces of the second solid electrolyte membrane 200 on which the second electrode 102 is arranged face each other with a gap between them. The intermediate electrode 120 is arranged so as to be in contact with both the surface of the first solid electrolyte membrane 100 on which the first electrode 101 is arranged and the surface of the second solid electrolyte membrane 200 on which the second electrode 102 is arranged. However, the first electrode 101 is not in contact with the second solid electrolyte membrane 200, and the second electrode 102 is not in contact with the first solid electrolyte membrane 100. Therefore, with respect to the first solid electrolyte membrane 100, the first electrode 101 and the intermediate electrode 120 are disposed on the same surface of the first solid electrolyte membrane 100, and with respect to the second solid electrolyte membrane 200, the second electrode 102 and the intermediate electrode 120 are disposed on the same surface of the second solid electrolyte membrane 200. In Fig. 4, the first electrode 101 and the second electrode 102 are not disposed so as to overlap when viewed in the thickness direction of the oxygen sensor 1, i.e., the vertical direction of the paper in the figure, but instead, the electrodes 101, 102 may be disposed so as to overlap.

[0040] In the oxygen sensor 1 shown in FIG. 4, the first solid electrolyte membrane 100, the first electrode 101, and the intermediate electrode 120 constitute a first unit cell. The second solid electrolyte membrane 200, the second electrode 102, and the intermediate electrode 120 constitute a second unit cell. The first unit cell functions as a concentration cell. Meanwhile, the second unit cell functions as an oxygen pump. Specifically, as shown in FIG. 4, a voltmeter 60 is connected between the first electrode 101 and the intermediate electrode 120 of the first unit cell to measure the electromotive force generated between the two electrodes of the concentration cell. In the second unit cell, a DC power supply 40 is connected between the second electrode 102 and the intermediate electrode 120 to apply a voltage between the two electrodes. In this case, the intermediate electrode 120 is connected to the positive electrode of the DC power supply 40, and the second electrode 102 is connected to the negative electrode of the DC power supply 40.

[0041] The method for measuring the oxygen concentration in a test gas using the oxygen sensor 1 shown in FIG. 4 is as follows. First, the first electrode 101 is positioned so that it faces the target atmosphere, and the second electrode 102 is positioned so that it faces the target atmosphere or the outside air (generally the atmosphere). In this state, the entire oxygen sensor 1 is heated to develop oxide ion conductivity in the first and second solid electrolyte membranes 100, 200. Next, the intermediate electrode 120 is connected to the positive electrode of the DC power supply 40, and the second electrode 102 is connected to the negative electrode of the DC power supply 40. This activates the oxygen pumping function of the second unit cell, and oxygen gas accumulates in the reference oxygen concentration space S. When the reference oxygen concentration is reached in the space S, the electromotive force generated between the first electrode 101 and the intermediate electrode 120 is measured with the voltmeter 60, and the concentration of oxygen gas in the target atmosphere is calculated according to the Nernst equation described above.

[0042] The oxygen sensor 1 of this embodiment also measures the concentration of oxygen gas in the atmosphere to be measured with an elevated reference oxygen concentration, which has the advantageous effect of improving measurement accuracy. Furthermore, the oxygen sensor 1 of this embodiment has the advantage that the manufacturing process of the oxygen sensor 1 can be simplified because the first electrode 101 and the intermediate electrode 120 are disposed on the same surface of the first solid electrolyte membrane 100, and the second electrode 102 and the intermediate electrode 120 are disposed on the same surface of the second solid electrolyte membrane 200.

[0043] 1, 3, and 4, any material having oxide ion conductivity can be used without any particular limitation for the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200. For example, it is preferable that at least one of the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200 is made of a compound containing A, M, and O (A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba; M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Si, Ge, Zn, Sn, W, and Mo). In particular, when the first solid electrolyte membrane 100 is made of a compound containing A, M, and O, it is preferable because the transport number of oxide ions can be increased and a stable electromotive force can be obtained.

[0044] In addition, at least one of the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200 is represented by the general formula: A 9.33+x [T 6.00-y M y ]O 26.0+z (A in the formula is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. T in the formula is an element containing Si or Ge, or both. M in the formula is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo), in which x in the formula is a number of -1.33 or more and 1.50 or less, y in the formula is a number of 0.00 or more and 3.00 or less, and z in the formula is a number of -5.00 or more and 5.20 or less, and the ratio of the number of moles of A to the number of moles of T is preferably 1.33 or more and 3.61 or less. In particular, the second solid electrolyte membrane 200 used in the second membrane electrode assembly 20 acting as an oxygen pump is represented by the general formula: 9.33+x [T 6.00-y M y ]O 26.0+zIn the case where the porous carbon material is made of a compound containing a composite oxide represented by the formula (I), the oxygen pumping action is more pronounced at a lower temperature, which is preferable.

[0045] The general formula: A 9.33+x [T 6.00-y M y ]O 26.0+z The compound represented by the formula (I) is an oriented apatite-type oxide ion conductor, and from the viewpoint of enhancing oxide ion conductivity, it is preferable that the degree of orientation measured by the Lotgering method is 0.6 or more.

[0046] The thickness of the first solid electrolyte membrane 100 and the second solid electrolyte membrane 200 is not critical in the present invention, and they only need to have a strength sufficient to withstand the use of the oxygen sensor 1. The thickness of these electrolyte membranes can generally be 0.1 μm or more and 1.0 mm or less. The thicknesses of these electrolyte membranes may be the same or different.

[0047] The first electrode 101 to the fourth electrode 104 and the intermediate electrode 120 may be made of a metal material, but it is preferable that at least one of them is made of a conductive oxide in order to improve the conductivity of oxide ions. In particular, it is preferable that the oxide has a perovskite structure represented by MNO3 (M is one or more elements selected from the group consisting of Ca, Sr, Ba, La, Pr, and Y; N is one or more elements selected from the group consisting of Ni, Ti, V, Zr, Cr, Mn, Fe, Co, Mo, Ru, Pd, and Re) in order to further improve the oxide ion conductivity between the solid electrolyte membrane and the electrodes. An intermediate layer such as Sm-doped CeO2 may be provided between the solid electrolyte membrane and the electrodes.

[0048] The first electrode 101 to the fourth electrode 104 and the intermediate electrode 120 may have various planar shapes, such as a circle or a polygon. If necessary, the electrodes may be processed to increase their surface area by a known method. For example, the surface area can be increased by providing irregularities on the electrode surface. This processing can further promote oxygen transfer.

[0049] Like the thickness of the solid electrolyte membrane, the thickness of the first electrode 101 to the fourth electrode 104 and the intermediate electrode 120 is not critical in the present invention, and it is sufficient that they have sufficient strength to withstand the use of the oxygen sensor 1. The thickness of these electrodes can generally be 0.01 μm or more and 100 μm or less. The thicknesses of these electrodes may be the same or different.

[0050] The oxygen-permeable annular wall portion 30 can be made of a material that is stable at the operating temperature of the oxygen sensor 1, such as a ceramic material. Examples of ceramic materials that can be used include alumina, zirconia, silica, and zeolite. To incorporate the annular wall portion 30 into the oxygen sensor 1, for example, an adhesive may be applied to the upper and lower end surfaces of the annular wall portion 30, and then the annular wall portion 30 may be joined to the first membrane electrode assembly 10 and the second membrane electrode assembly 20. For example, a zirconia-based adhesive may be used as the adhesive.

[0051] Next, other embodiments of the oxygen sensor of the present invention will be described with reference to Figures 5 to 8. In these embodiments, differences from the embodiment shown in Figures 1 to 4 will be described, and the description of the embodiment shown in Figures 1 to 4 will be applied as appropriate to points not specifically described. In Figures 5 to 8, the same members as those in Figures 1 to 4 are assigned the same reference numerals.

[0052] The oxygen sensor 1a shown in Fig. 5 includes a membrane electrode assembly 10a. The membrane electrode assembly 10a includes a solid electrolyte membrane 100a having oxide ion conductivity, and a first electrode 101a and a second electrode 102a disposed on each side of the solid electrolyte membrane 100a. The membrane electrode assembly 10a functions as both a concentration cell and an oxygen pump depending on the switching of a switch 70.

[0053] The solid electrolyte membrane 100a may be made of the same material as that of the first solid electrolyte membrane 100 or the second solid electrolyte membrane 200 in the oxygen sensor 1 of the embodiment shown in Figures 1 and 3. The first electrode 101a and the second electrode 102a may be made of the same material as that of the first electrode 101 to the fourth electrode 104 and the intermediate electrode 120 in the oxygen sensor 1 of the embodiment shown in Figures 1 and 3. The material that makes up the first electrode 101a and the material that makes up the second electrode 102a may be the same or different.

[0054] In the oxygen sensor 1a, an electrode opposing member 50 is disposed so as to face the second electrode 102a. In Fig. 5, the electrode opposing member 50 is depicted as a flat plate-shaped member. In Fig. 5, the electrode opposing member 50 and the second electrode 102a are shown in close contact with each other with no gap between them, but this is not limiting, and the electrode opposing member 50 and the second electrode 102a may be disposed with a gap therebetween.

[0055] An annular wall 31 is provided between the solid electrolyte membrane 100a and the electrode opposing member 50. The annular wall 31 is provided so as to surround the second electrode 102a. As a result, in the oxygen sensor 1a, the annular wall 31 defines a reference oxygen concentration space S between the solid electrolyte membrane 100a and the electrode opposing member 50. In FIG. 5, the reference oxygen concentration space S is defined by the annular wall 31, the solid electrolyte membrane 100a, the electrode opposing member 50, and the second electrode 102a.

[0056] At least one of the electrode opposing member 50 and the annular wall portion 31 is made of an oxygen-permeable material. Such a material may be, for example, a porous material. Therefore, the reference oxygen concentration space S communicates with the outside via at least one of the electrode opposing member 50 and the annular wall portion 31. From the viewpoint of ease of manufacturing when miniaturizing the oxygen sensor 1a, it is preferable that only the annular wall portion 31 be made of an oxygen-permeable material.

[0057] 1, 3, and 4, the annular wall portion 31 in the oxygen sensor 1a of this embodiment does not necessarily need to be oxygen permeable, as described above. The annular wall portion 31 may be made of any material that can withstand the operating temperature of the oxygen sensor 1a.

[0058] The method for measuring the oxygen concentration in a test gas using the oxygen sensor 1a having the above configuration is as follows. First, these components are arranged so that the first electrode 101a faces the atmosphere to be measured and the electrode facing member 50 faces the outside air (generally the atmosphere). In this state, the entire oxygen sensor 1a is heated so that the solid electrolyte membrane 100a exhibits oxide ion conductivity. The heating temperature can be the same as in the embodiments shown in FIGS. 1, 3, and 4.

[0059] Once the solid electrolyte membrane 100a exhibits oxide ion conductivity, the second electrode 102a is connected to the positive electrode of the DC power supply 40, and the first electrode 101a is connected to the negative electrode of the DC power supply 40. This activates the oxygen pumping function, reducing oxygen gas contained in the atmosphere to be measured to oxide ions. The oxide ions move through the solid electrolyte membrane 100a and reach the second electrode 102a. Upon reaching the second electrode 102a, the oxide ions release electrons and convert to oxygen gas. The oxygen gas thus generated accumulates in the reference oxygen concentration space S. However, because the reference oxygen concentration space S is partially defined by an oxygen-permeable material and is connected to the outside, the pressure in the reference oxygen concentration space S does not increase excessively. As a result, the reference oxygen concentration space S maintains a certain pressure while achieving a reference oxygen concentration, where the partial pressure of oxygen gas is high. Setting the reference oxygen concentration to 100 vol% oxygen is preferable for highly accurate measurements. Whether or not the oxygen concentration in the space S is 100 vol % is determined as described above.

[0060] When the oxygen concentration in the reference oxygen concentration space S increases and reaches the reference oxygen concentration in the space S, the first electrode 101a and the second electrode 102a are disconnected from the DC power supply 40 by the switch 70. At the same time, the electromotive force generated between the first electrode 101a and the second electrode 102a is measured by the voltmeter 60. Thereafter, the oxygen concentration in the measurement atmosphere facing the first electrode 101a is calculated according to the Nernst equation using the same procedure as in the embodiment shown in FIG.

[0061] This embodiment also has the advantageous effect of improving measurement accuracy because the concentration of oxygen gas in the measurement target atmosphere is measured with the reference oxygen concentration set high. Moreover, unlike the oxygen sensor 1a described in Patent Document 2, the oxygen sensor 1a of this embodiment only requires that high-concentration oxygen gas be present in the reference oxygen concentration space S, and therefore does not require the electrode opposing member 50 to have high strength.

[0062] 6 shows an example of the results of measuring the voltage between the first electrode 101a and the second electrode 102a using the oxygen sensor 1a. As shown in the figure, when the switch 70 is turned on and a voltage from the DC power supply 40 is applied between the first electrode 101a and the second electrode 102a, the voltage E1 from the DC power supply 40 is observed between the two electrodes. While the voltage E1 is being applied, oxygen gas accumulates in the reference oxygen concentration space S due to the oxygen pumping action, and the concentration of oxygen gas increases. During this time, the oxygen sensor 1a does not measure the concentration of oxygen gas in the atmosphere being measured.

[0063] When a sufficient amount of oxygen gas has accumulated in the reference oxygen concentration space S and the oxygen gas concentration has increased sufficiently, the switch 70 is turned off. From this point on, the oxygen sensor 1a begins measuring the oxygen gas concentration in the measurement target atmosphere. At this time, the voltage between the first electrode 101a and the second electrode 102a, i.e., the electromotive force generated by the concentration cell principle, decreases below E1 as shown in FIG. 6 and indicates a value corresponding to the oxygen gas concentration in the measurement target atmosphere. The electromotive force shown in FIG. 6 is an example in which the oxygen gas concentration in the measurement target atmosphere is constant. The duration of measurement of the oxygen gas concentration by the oxygen sensor 1a, i.e., the time during which the switch 70 is turned off, depends on the change in the oxygen gas concentration in the reference oxygen concentration space S. Specifically, as long as the oxygen gas concentration in the reference oxygen concentration space S is constant, highly accurate measurement can be continued. When the concentration of oxygen gas in the reference oxygen concentration space S decreases (the oxygen gas dissipates to the outside through at least one of the electrode opposing member 50 and the annular wall portion 31), the switch 70 is turned on again as shown in Fig. 6 to accumulate oxygen gas in the reference oxygen concentration space S. Thereafter, the concentration of oxygen gas in the atmosphere to be measured is measured using the procedure described above.

[0064] In this way, oxygen sensor 1a of this embodiment alternately supplies oxygen to reference oxygen concentration space S and measures electromotive force by applying a pulsed voltage between first electrode 101a and second electrode 102a. By switching switch 70 to suitably use the concentration cell and oxygen pump functions, it is possible to achieve power savings in the sensor, making oxygen sensor 1a of this embodiment suitable for application to portable devices.

[0065] Although it depends on the dimensions of the oxygen sensor 1a and the degree of porosity of the electrode opposing member 50, it is preferable that the time period for which the switch 70 is turned on is 0.5 seconds or more and 10 seconds or less. On the other hand, the time period for which the concentration of oxygen gas in the atmosphere to be measured can be measured with the switch 70 turned off is generally 3 seconds or more and 200 seconds or less.

[0066] The oxygen sensor 1b shown in Fig. 7 includes a membrane electrode assembly 10b. The membrane electrode assembly 10b includes a solid electrolyte membrane 100b having oxide ion conductivity, a first electrode 101b and a second electrode 102b disposed on one side of the solid electrolyte membrane 100b, and an intermediate electrode 120b disposed on the other side of the solid electrolyte membrane 100b. The membrane electrode assembly 10b includes a concentration cell and an oxygen pump within its structure.

[0067] The solid electrolyte membrane 100b may be made of the same material as that of the first solid electrolyte membrane 100 or the second solid electrolyte membrane 200 in the oxygen sensor 1 of the embodiment shown in Figures 1 and 3. The first electrode 101b, the second electrode 102b, and the intermediate electrode 120b may be made of the same material as that of the first electrode 101 to the fourth electrode 104 and the intermediate electrode 120 in the oxygen sensor 1 of the embodiment shown in Figures 1 and 3. The materials that make up the first electrode 101b, the second electrode 102b, and the intermediate electrode 120b may be the same or different.

[0068] In the oxygen sensor 1b, an electrode opposing member 50 is disposed to face the intermediate electrode 120b. In Fig. 7, the electrode opposing member 50 is depicted as a flat plate-shaped member. In Fig. 7, the electrode opposing member 50 and the intermediate electrode 120b are shown in close contact with each other with no gap between them, but this is not limiting, and the electrode opposing member 50 and the intermediate electrode 120b may be disposed with a gap therebetween.

[0069] An annular wall portion 31 is provided between the solid electrolyte membrane 100b and the electrode opposing member 50. The annular wall portion 31 is provided so as to surround the intermediate electrode 120b. As a result, in the oxygen sensor 1b, the annular wall portion 31 defines a reference oxygen concentration space S between the solid electrolyte membrane 100b and the electrode opposing member 50. In FIG. 7, the reference oxygen concentration space S is defined by the annular wall portion 31, the solid electrolyte membrane 100b, the electrode opposing member 50, and the intermediate electrode 120.

[0070] The method for measuring the oxygen concentration in a test gas using the oxygen sensor 1b having the above configuration is as follows. First, the first electrode 101b and the second electrode 102b are arranged so that they face the atmosphere to be measured, and the electrode opposing member 50 faces the outside air (generally the atmosphere). In this state, the entire oxygen sensor 1b is heated to cause the solid electrolyte membrane 100b to exhibit oxide ion conductivity. Alternatively, these members may be arranged so that the first electrode 101b faces the atmosphere to be measured, and the second electrode 102b and the electrode opposing member 50 face the outside air (generally the atmosphere).

[0071] Next, the intermediate electrode 120b is connected to the positive electrode of the DC power supply 40, and the second electrode 102b is connected to the negative electrode of the DC power supply 40. This causes an oxygen pumping action, and oxygen gas accumulates in the reference oxygen concentration space S. The reference oxygen concentration space S is maintained at a certain pressure, and the reference oxygen concentration is reached, where the partial pressure of oxygen gas is high. Under this condition, the electromotive force generated between the first electrode 101b and the intermediate electrode 120b is measured by the voltmeter 60. Thereafter, the oxygen concentration in the measurement atmosphere facing the first electrode 101b is calculated according to the Nernst equation using the same procedure as in the embodiment shown in FIG. 1.

[0072] This embodiment also has the advantageous effect of improving measurement accuracy by measuring the concentration of oxygen gas in the measurement target atmosphere with a high reference oxygen concentration. Moreover, the oxygen sensor 1b of this embodiment has the advantages of improving measurement accuracy and shortening the oxygen pumping time.

[0073] FIG. 8 shows another embodiment of the oxygen sensor shown in FIG. 7. The oxygen sensor 1b shown in FIG. 8 differs from the oxygen sensor 1 shown in FIG. 7 in that the intermediate electrode 120b provided in the oxygen sensor shown in FIG. 7 is divided into a third electrode 103b and a fourth electrode 104b, which are independent of each other. The materials constituting the third electrode 103b and the fourth electrode 104b may be the same or different. The third electrode 103b and the fourth electrode 104b are surrounded by an annular wall portion 31. As a result, in the oxygen sensor 1b, as shown in FIG. 8, a reference oxygen concentration space S is defined by the annular wall portion 31, the solid electrolyte membrane 100b, the electrode opposing member 50, the third electrode 103b, and the fourth electrode 104b.

[0074] The method for measuring the oxygen concentration in a test gas using the oxygen sensor 1b having the above configuration is as follows. First, the first electrode 101b and the second electrode 102b are arranged so that they face the atmosphere to be measured and the electrode facing member 50 faces the outside air. In this state, the entire oxygen sensor 1b is heated to cause the solid electrolyte membrane 100b to exhibit oxide ion conductivity.

[0075] Next, the fourth electrode 104b is connected to the positive electrode of the DC power supply 40, and the second electrode 102b is connected to the negative electrode of the DC power supply 40. This causes an oxygen pumping action, and oxygen gas accumulates in the reference oxygen concentration space S. Under this condition, the electromotive force generated between the first electrode 101b and the third electrode 103b is measured by the voltmeter 60. Thereafter, the oxygen concentration in the measurement target atmosphere facing the first electrode 101b is calculated according to the Nernst equation using the same procedure as in the embodiment shown in FIG. 1. This embodiment also has the advantageous effect of improving measurement accuracy, since the concentration of oxygen gas in the measurement target atmosphere is measured with the reference oxygen concentration increased.

[0076] The oxygen sensors 1, 1a, and 1b of the above-described embodiments can be made smaller and more compact due to their structure, allowing for a significant reduction in power consumption. Therefore, the oxygen sensors 1, 1a, and 1b can be incorporated into a micro-electromechanical system (MEMS). This allows the oxygen sensors to be installed in mobile devices such as personal computers and mobile terminals, enabling oxygen gas concentrations to be measured in small spaces. [Industrial Applicability]

[0077] According to the present invention, an oxygen sensor capable of measuring the oxygen concentration in a gas to be detected with high accuracy is provided, and also according to the present invention, an oxygen sensor that can be made compact is provided.

Claims

1. a membrane electrode assembly including a solid electrolyte membrane having oxide ion conductivity and a first electrode and a second electrode disposed on each side of the solid electrolyte membrane; an electrode opposing member is disposed to face the second electrode; a reference oxygen concentration space is defined between the solid electrolyte membrane and the member by a wall portion provided so as to surround the second electrode, Of the member and the wall portion, only the wall portion is made of a material that is permeable to oxygen, An oxygen sensor that measures the oxygen concentration in the measurement atmosphere facing the first electrode by positioning the first electrode so that it faces the measurement atmosphere, connecting the second electrode to the positive terminal of a power source and connecting the first electrode to the negative terminal of the power source to create a state in which the oxygen concentration in the reference oxygen concentration space is increased, disconnecting the first electrode and the second electrode from the power source, and measuring the electromotive force generated between the first electrode and the second electrode.

2. 2. The oxygen sensor according to claim 1, wherein the solid electrolyte membrane is made of a compound containing A, M, and O (A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba; and M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Si, Ge, Zn, Sn, W, and Mo).

3. The solid electrolyte membrane is a membrane having a general formula: A ９．３３＋ｘ [T ６．００－ｙ M ｙ ]O ２６．０＋ｚ (In the formula, A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. T in the formula is an element containing Si or Ge, or both. M in the formula is Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, and wherein x is a number equal to or greater than -1.33 and equal to or less than 1.50, y is a number equal to or greater than 0.00 and equal to or less than 3.00, and z is a number equal to or greater than -5.00 and equal to or less than 5.20, and wherein the ratio of the number of moles of A to the number of moles of T is 1.33 or greater and equal to or less than 3.

61.

4. 4. The oxygen sensor according to claim 1, wherein at least one of the first electrode and the second electrode is made of an oxide.

5. The general formula: A ９．３３＋ｘ [T ６．００－ｙ M ｙ ]O ２６．０＋ｚ 4. The oxygen sensor according to claim 3, wherein the compound represented by the formula (I) is an oriented apatite-type oxide ion conductor, and has a degree of orientation of 0.6 or more as measured by the Lotgering method.

6. The oxide is MNO ３ 5. The oxygen sensor according to claim 4, having a perovskite structure represented by the formula: (M is one or more elements selected from the group consisting of Ca, Sr, Ba, La, Pr, and Y; and N is one or more elements selected from the group consisting of Ni, Ti, Zr, V, Cr, Mn, Fe, Cu, Co, Mo, Ta, Nb, Ru, Pd, and Re).

7. 7. The oxygen sensor according to claim 1, wherein the pressure (Pa) of the reference oxygen concentration space is set to be 1.0 or more and 3.0 or less relative to the atmospheric pressure (Pa) of the atmosphere to be measured, and the oxygen concentration is measured under this condition.

8. 8. The oxygen sensor according to claim 1, wherein the oxygen concentration in the reference oxygen concentration space is measured under a condition in which the oxygen concentration is set to 60% or more and 100% or less.

9. A micromechanical electrical element comprising an oxygen sensor according to any one of claims 1 to 8.

10. A method for measuring an oxygen concentration using the oxygen sensor according to any one of claims 1 to 8.

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