JP2008008666A - Limiting current type oxygen sensor and concentration detecting and measuring method of oxygen using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a limiting current type oxygen sensor which detects and measures the concentration of oxygen with high precision even if the concentration of oxygen is low, is easy to manufacture and is reduced in cost. <P>SOLUTION: The limiting current type oxygen sensor 10 is equipped with: an ion conductor 11; electrodes 12a and 12b; a gas diffusion mechanism 16 for supplying a gas limited in its diffusion rate; and a heater 17 for heating the ion conductor 11. The gas diffusion mechanism 16 has: a gas diffusion hole 14 formed so as to pierce a cap 13; and the internal space 15 communicating with the gas diffusion hole 14. The gas diffusion mechanism 16 is constituted so that the concentration gradient of oxygen in the internal space 15 satisfies the condition of a predetermined formula. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a limiting current type oxygen sensor used for detecting and measuring oxygen concentration and an oxygen concentration detecting and measuring method using the same, and particularly to detect and measure oxygen concentration accurately even at a low oxygen concentration. The present invention relates to a limiting current type oxygen sensor that can perform and a method for detecting and measuring oxygen concentration using the same.

  Conventionally, as a limiting current type oxygen sensor, for example, a solid electrolyte stabilized zirconia (Ytria Stabilized Zirconia: YSZ) using, for example, yttrium oxide (YtO) as an additive is known as an ion conductor. FIG. 8 is a schematic diagram showing the structure of a conventional limiting current oxygen sensor. As shown in FIG. 8, a general conventional limiting current type oxygen sensor 100 includes an anode side electrode 102 and a cathode side electrode 102 made of a porous material for applying a monitoring voltage to both surfaces of an ion conductor 101 of a solid electrolyte. , 103, and a cap 106 having a gas diffusion mechanism composed of gas diffusion holes 104, 105 and an internal space 106a for supplying a diffusion-controlled gas to one electrode 103 side. ing.

  Further, a heater 107 for setting the ion conductor 101 to a monitoring temperature of, for example, several hundred degrees C. is provided outside the cap 106, and a lead wire 108 is connected to the heater 107. The gas diffusion hole 104 is formed in a state of penetrating the cap 106 toward the electrode 103, and the gas diffusion hole 105 is formed in a state of penetrating in the direction along the surface of the electrode 103. Here, as the limiting current type oxygen sensor, among these gas diffusion holes 104 and 105, for example, a type in which the gas diffusion hole 105 is not formed is often used.

  Such a limiting current type oxygen sensor 100 has a structure in which when a monitoring voltage is applied between the electrodes 102 and 103, an output current proportional to the voltage flows through the ion conductor 101 while the voltage is small. Become. Further, the limiting current type oxygen sensor 100 has a characteristic that the output current eventually saturates when the monitoring voltage is further increased. The output current in such a saturation region is called a limit current, and the magnitude of this limit current has a certain relationship with the oxygen concentration. Therefore, in such a limit current type oxygen sensor 100, it is possible to detect and measure the oxygen concentration from the limit current value obtained by the monitoring voltage.

  Further, the current flowing through the ion conductor 101 of the limiting current oxygen sensor 100 is based on the movement of oxygen ions, and the current value depends on the voltage and temperature. For this reason, the limiting current oxygen sensor 100 is driven at a voltage set to a monitoring temperature of about 400 ° C. to 500 ° C., for example. Normally, the monitoring temperature is set by providing a heater 107 on the cap 106, which is the main body of the limiting current oxygen sensor 100 as described above, and energizing it.

  In many cases, such a limiting current oxygen sensor 100 is driven by applying a monitoring voltage between the electrodes 102 and 103 while the heater 107 is always energized (for example, a patent). Reference 1). In the conventional limiting current type oxygen sensor 100 configured as described above, in order to increase the output current value when detecting and measuring a low concentration of oxygen, not only the gas diffusion hole 104 but also the gas diffusion hole is formed in the cap 106. A gas diffusion mechanism in which holes 105 are formed is provided.

In the limiting current type oxygen sensor 100 having a gas diffusion mechanism having a structure in which holes such as the gas diffusion holes 104 and 105 are provided in the cap 106, the gas diffusion mechanism includes a Faraday constant (F), a diffusion coefficient (D), a hole, and the like. Pore area (S), gas total pressure (P), gas constant (R), temperature (T), hole length (l), oxygen partial pressure (P _O2 ), and output current value (IL) Therefore, the oxygen concentration is measured by being configured to satisfy the condition of the following formula (1).

Japanese Patent No. 3373741

  However, in the above-described conventional limiting current oxygen sensor 100, not only the gas diffusion hole 104 but also the gas diffusion hole 105 must be newly formed in the cap 106 in order to detect and measure a low concentration oxygen concentration. For this reason, the number of processes in the manufacturing process of the limiting current type oxygen sensor 100 increases, and the sensor structure itself becomes complicated, resulting in an increase in cost.

  Further, since the new gas diffusion hole 105 in the limiting current oxygen sensor 100 is formed so as to penetrate in the direction along the surface (electrode surface) of the electrode 103 of the ion conductor 101, the gas diffusion hole 105 is formed. There is a problem that variation in manufacturing accuracy is likely to increase depending on processing accuracy compared to a type that is not performed, resulting in variations in sensor characteristics and unstable measurement accuracy.

  Furthermore, in the case of a type in which the gas diffusion hole 105 is not provided in place of the gas diffusion hole 105 instead of the gas diffusion hole 105 as in the conventional limiting current oxygen sensor described in Patent Document 1 described above, for example, In the limiting current region in a state where the oxygen concentration is as low as 1% or less, the limiting current value does not satisfy the condition of the above formula (1), so that there is a problem that accurate oxygen concentration detection and measurement cannot be performed.

  The present invention has been made in view of these points, and can detect and measure the oxygen concentration with good accuracy even at a low oxygen concentration, and can be easily manufactured to prevent variation in characteristics. It is an object of the present invention to provide a limiting current type oxygen sensor capable of reducing cost and a method for detecting and measuring oxygen concentration using the same.

The limiting current type oxygen sensor according to the present invention includes an ion conductor made of a solid electrolyte, a porous electrode pair for applying an electric field provided on the ion conductor, and one electrode side of the electrode pair. A gas diffusion mechanism for supplying a diffusion-controlled gas; and a heater for heating the ion conductor, wherein the gas diffusion mechanism includes an internal space in contact with one electrode of the electrode pair, and the internal space A gas diffusion hole communicating with the outside, and an oxygen concentration gradient in the internal space includes a Faraday constant (F), a diffusion coefficient (D), an oxygen concentration (C _O2 ), and a hole area (S) of the gas diffusion hole , The hole length (l) in the penetration direction of the gas diffusion hole, the distance (l _in ) between the electrode in the internal space and the inner surface facing it, the effective sectional area (S _in ) of the internal space, and the output current value (I based on the relationship of _lim), 1 / _{I l m} = ₍₁ / 4FDC O2), characterized in that it is configured to satisfy the equation of _{{(l / S) + (} l in / S in)}.

An oxygen concentration detection and measurement method according to the present invention includes an ion conductor made of a solid electrolyte, a porous electrode pair for applying an electric field provided on the ion conductor, and one surface side of the electrode pair. A method for detecting and measuring oxygen concentration using a limiting current oxygen sensor comprising a gas diffusion mechanism for supplying a diffusion-controlled gas to a gas and a heater for heating the ion conductor, Forming an internal space that is in contact with one electrode of the electrode pair and a gas diffusion hole that communicates the internal space with the outside, and an oxygen concentration gradient in the internal space includes a Faraday constant (F), a diffusion coefficient (D), Oxygen concentration (C _O2 ), hole area (S) of the gas diffusion hole, hole length (l) in the penetration direction of the gas diffusion hole, distance between the electrode in the internal space and the inner surface facing it (l _in ), The internal space 1 / I _lim = (1/4 FDC _O2 ) {(l / S) + (l _in / S _in )} based on the relationship between the effective area (S _in ) and the output current value (I _lim ) The oxygen concentration is detected and measured by performing an operation so as to satisfy the above condition.

  The gas diffusion mechanism is formed so that the distance between the electrode in the internal space and the inner surface facing the electrode is larger than the diameter of the gas diffusion hole. The gas diffusion mechanism is configured such that the ratio (S / l) of the hole area (S) to the hole length (l) of the gas diffusion holes is 50 μm to 250 μm.

  According to the present invention, the gas diffusion mechanism has an internal space that is in contact with one electrode of the electrode pair and a gas diffusion hole that communicates the internal space with the outside, and the oxygen concentration gradient in the internal space is a predetermined formula. It is configured to satisfy the following conditions. For this reason, the processing of the gas diffusion hole is simplified as compared with the case where a plurality of gas diffusion holes are provided as in the prior art, and the oxygen concentration detection measurement is accurately performed based on the concentration gradient satisfying the condition of the predetermined formula. It becomes possible. Thereby, for example, even if the oxygen concentration is a low oxygen concentration of 1% or less, it can be detected and measured with high accuracy and can be easily manufactured, and variations in processing accuracy characteristics can be suppressed and manufacturing costs can be reduced. .

  Hereinafter, a limiting current type oxygen sensor according to an embodiment of the present invention and a method for detecting and measuring oxygen concentration using the same will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing the structure of a limiting current oxygen sensor according to an embodiment of the present invention. FIG. 2 is a perspective view of the limiting current oxygen sensor as viewed from the direction of arrow A in FIG. FIG. 3 is a perspective view of the limiting current type oxygen sensor as viewed from the direction of arrow B in FIG. As shown in FIGS. 1 to 3, a limiting current oxygen sensor 10 according to an embodiment of the present invention applies an ionic conductor 11 made of a solid electrolyte and an electric field provided on the ionic conductor 11. Electrode 12a, 12b which is an electrode pair made of a porous material, a cap 13 which covers one of the electrodes 12a, 12b, and a heater 17 which heats the ion conductor 11. Yes.

  The ion conductor 11 is an insulator that exhibits conductivity by movement of internal ions at a high temperature of several hundred degrees Celsius, and is made of, for example, stabilized zirconia. Each of the electrodes 12 a and 12 b is made of, for example, porous platinum (Pt) or silver (Ag), and is formed on both surfaces of the ion conductor 11. The limiting current oxygen sensor 10 of this example is configured such that the electrode 12a serves as a cathode electrode and the electrode 12b serves as an anode electrode.

  The cap 13 is made of, for example, ceramic having a bottomed cylindrical appearance, and the side attached to the ion conductor 11 is formed in a concave shape. Further, one gas diffusion hole 14 penetrating in the thickness direction is formed in the center of the bottom portion of the bottomed cylindrical shape of the cap 13. The cap 13 is attached to the surface of the ion conductor 11 on the side of the electrode 12a serving as the cathode electrode so that the gas is supplied from only the gas diffusion hole 14 to the electrode 12a. An internal space 15 is formed by the surface of the ion conductor 11 on the electrode 12a side. The internal space 15 and the gas diffusion hole 14 constitute a gas diffusion mechanism 16 that supplies a diffusion-controlled gas to the electrode 12a.

  Therefore, in the limiting current oxygen sensor 10, the electrode 12a is provided in contact with the internal space 15, and the electrode 12b is provided in contact with the external atmosphere. Further, a heater 17 for heating the ion conductor 11 to a monitoring temperature of, for example, about 400 ° C. to 500 ° C. is provided on the outer surface of the cap 13 opposite to the side on which the ion conductor 11 is attached.

  As shown in FIG. 1, the gas diffusion hole 14 of the gas diffusion mechanism 16 is formed in the cap 13 so as to have a predetermined hole diameter (Sl) and a hole length (l) that is a predetermined length in the penetration direction. ing. Further, the internal space 15 of the gas diffusion mechanism 16 has a predetermined thickness in the penetrating direction of the gas diffusion hole 14 (that is, specifically, the surface of the electrode 12a and the inner wall surface of the cap 13 disposed to face this surface. (Distance) between (lin). And the gas diffusion mechanism 16 is comprised so that the oxygen concentration gradient in the internal space 15 may satisfy | fill the conditions of the predetermined formula mentioned later. Moreover, although the gas diffusion mechanism 16 is formed so that the thickness (lin) of the internal space 15 is larger than the hole diameter (Sl) of the gas diffusion hole 14 here, it is not limited to this.

  Lead wires 18a and 18b are connected to the electrodes 12a and 12b, respectively, and these lead wires 18a and 18b are connected to a power source 30 for taking out and supplying a monitoring voltage. Further, an ammeter 31 and a voltmeter 32 are connected to the power supply 30 in series and in parallel, respectively. Further, a lead wire 19 is connected to the heater 17, and the lead wire 19 is connected to a heater power source 33. For example, the heater 17 is always energized by the heater power supply 33 during the period of use, and is set to a monitoring temperature of about 400 ° C.

  In the limit current type oxygen sensor 10 configured as described above, the heater current is supplied from the heater power source 33 to the heater 17 to generate resistance heat, thereby heating the limit current type oxygen sensor 10 itself to the above monitoring temperature and the power source 30 to generate electrodes. A predetermined monitoring voltage (V) is applied between 12a and 12b. When the monitoring voltage is applied, oxygen molecules contained in the gas existing in the internal space 15 of the gas diffusion mechanism 16 surrounded by the ion conductor 11 and the cap 13 obtain electrons through the electrode 12a. It becomes oxygen ions and enters the ion conductor 11. And this oxygen ion moves the inside of the ion conductor 11 through the oxygen ion void | hole in the ion conductor 11, for example in the thickness direction upper direction in FIG. The moved oxygen ions reach the electrode 12b, emit electrons, and become oxygen molecules again and are released into the external atmosphere. Due to the movement of oxygen ions, a current (A) flows between the electrodes 12a and 12b.

  At this time, due to the movement of oxygen ions, the internal space 15 of the limiting current oxygen sensor 10 becomes negative pressure, and gas flows from the external atmosphere through the gas diffusion hole 14. In this case, since the gas inflow amount is limited by the gas diffusion hole 14, in the current (I) -voltage (V) characteristics of the limiting current type oxygen sensor 10, the monitoring voltage applied between the electrodes 12a and 12b. It is possible to detect a limit current value at which the current does not change even when the value is increased.

  The limit current value of the limit current type oxygen sensor 10 depends on the oxygen concentration in the atmosphere, assuming that the shape of the gas diffusion hole 14, the sensor temperature at the time of measurement, the pressure of the atmosphere, etc. are constant. . Therefore, the present inventors analyze an appropriate oxygen concentration gradient based on the relationship between the limit current value and the oxygen concentration and apply it to the gas diffusion mechanism 16 so that the limit current oxygen sensor 10 can be used at a low oxygen concentration. It was also possible to detect and measure the oxygen concentration well.

  4 and 5 are graphs showing the relationship between the area and length of the gas diffusion holes and the output current in the limiting current type oxygen sensor. First, in the limiting current type oxygen sensor 10, the present inventors formed only one gas diffusion hole 14 at the center of the bottom portion of the cap 13 as described above, and then the hole area (S) of the gas diffusion hole 14. And the limit current value which is the sensor output current (IL) was measured in an environment where the hole length (l) was varied. As a result, the ratio between the hole area (S) of the gas diffusion hole 14 and the hole length (l) (hereinafter referred to as “S / l”) and the limit current (IL) have a relationship as shown in FIG. It was confirmed that this was true, and the characteristics represented by the broken line 41 and the solid line 42 were obtained. The broken line 41 indicates the characteristic represented by the theoretical value calculated using the above-described conventional equation (1), and the solid line 42 indicates the characteristic represented by the actual measurement value.

  As shown in FIG. 4, the limit current (IL) based on the actual measurement value indicated by the solid line 42 is larger than the theoretical value calculated using the conventional equation (1) indicated by the broken line 41 as the value of S / l increases. It turned out to show a small value. For example, when the value of S / l is in the range of about 50 μm to about 250 μm, it has been found that S / l and the limit current (IL) show a linear relationship. Therefore, for example, at a low oxygen concentration of 1% or less, it is assumed that there is a concentration gradient not only in the gas diffusion hole 14 of the gas diffusion mechanism 16 but also in the internal space 15, and an approximate expression is analyzed to analyze the gas diffusion. The following equation (2) that satisfies the oxygen concentration gradient applied to the mechanism 16 was calculated. When the value of S / l is in the range of about 50 μm to about 250 μm, the number of gas diffusion holes 14 may not be one.

That is, the present inventors have Faraday constant (F), diffusion coefficient (D), oxygen concentration (C _O2 ), hole area (S) of the gas diffusion hole, hole length (l) in the penetration direction of the gas diffusion hole, The relationship among the thickness (l _in ) of the internal space, the effective cross-sectional area (S _in ) of the internal space, and the output current value (I _lim )

  5 is applied to the limiting current oxygen sensor 10, the hole length (l) and the hole area (S) of the gas diffusion hole 14 having the characteristics represented by the solid line 51 as shown in FIG. ) (Hereinafter referred to as “l / S”) and the reciprocal of the limit current (1 / IL). As a result, for example, when the above-mentioned S / l is 50 μm or less, the oxygen concentration can be detected and measured by applying the conventional equation (1) to the gas diffusion mechanism 16, but the value of the limit current (IL) Is not practical because of its small size.

The output current value (I _lim ) is equivalent to the limit current (IL). The effective cross-sectional area (S _in ) of the internal space 15 is based on the assumption that the flow of oxygen molecules in the internal space 15 spreads in a trapezoidal shape from the gas diffusion hole 14 toward the electrode 12a. It is set by taking about a half of the cross-sectional area of 14 and the cross-sectional area of the internal space 15 as a guide.

  FIG. 6 is a graph showing voltage (V) -current (I) characteristics of the limiting current type oxygen sensor. FIG. 7 is a graph showing the gas concentration characteristics of the limiting current type oxygen sensor. In the limiting current type oxygen sensor 10, for example, when the S / l of the gas diffusion hole 14 of the gas diffusion mechanism 16 is 150 μm and the monitoring voltage (Vs) -output current (Is) characteristics are evaluated in 1000 ppm oxygen gas, The result represented by the solid lines 61 and 62 as shown in FIG. Here, the solid line 61 represents the case where the conventional mathematical formula (1) is applied to the gas diffusion mechanism 16.

  As described above, when the oxygen concentration gradient defined by the above equation (2) is applied to the gas diffusion mechanism 16, the output current value indicated by the solid line 62 is lower than the output current value indicated by the solid line 61. It becomes. However, when the oxygen concentration gradient defined by Equation (2) is applied, the flat region 63 indicating the limit current can be clearly obtained. Further, as shown in FIG. 7, when the oxygen concentration gradient of the above formula (2) is applied in the internal space 15 of the gas diffusion mechanism 16 of the limiting current type oxygen sensor 10, the linear relationship shown by the solid line 71 is obtained. The characteristics of gas concentration and output current can be obtained.

  For this reason, for example, even with a low oxygen concentration of 1% or less, according to the limiting current type oxygen sensor 10 of the present invention, it is possible to detect and measure the oxygen concentration with good accuracy. Further, according to the limiting current type oxygen sensor 10 of the present invention, since the gas diffusion hole 14 of the gas diffusion mechanism 16 is one, it can be easily manufactured in the manufacturing process, and the variation in processing accuracy characteristics is suppressed. Manufacturing costs can be reduced.

  As described above, according to the present invention, the gas diffusion mechanism 16 of the limiting current oxygen sensor 10 is connected to the gas diffusion hole 14 formed in the cap 13 and penetrating toward the electrode 12a, and the gas diffusion hole 14. And the oxygen concentration gradient in the internal space 15 satisfies the above formula (2) as a predetermined condition. For example, the thickness (lin) of the internal space 15 is formed to be larger than the hole diameter (Sl) of the gas diffusion hole 14. For this reason, the processing of the gas diffusion hole 14 becomes simple, and it becomes possible to accurately detect and measure the oxygen concentration based on the oxygen concentration gradient that satisfies a predetermined condition. Thereby, for example, even if the oxygen concentration is a low oxygen concentration of 1% or less, it can be detected and measured with high accuracy and can be easily manufactured, and variations in processing accuracy characteristics can be suppressed and manufacturing costs can be reduced. .

It is a schematic diagram which shows the structure of the limiting current type oxygen sensor which concerns on one Embodiment of this invention. It is the perspective view which looked at the same limiting current type oxygen sensor from the arrow A direction in FIG. It is the perspective view which looked at the same limiting current type oxygen sensor from the arrow B direction in FIG. It is a graph showing the relationship between the hole area and hole length of a gas diffusion hole and output current in the limiting current type oxygen sensor. It is a graph showing the relationship between the hole area and hole length of a gas diffusion hole and output current in the limiting current type oxygen sensor. It is a graph which shows the voltage (V) -current (I) characteristic of the limiting current type oxygen sensor. It is a graph which shows the gas concentration characteristic of the same limiting current type oxygen sensor. It is a schematic diagram which shows the structure of the conventional limiting current type oxygen sensor.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Limit current type oxygen sensor, 11 ... Ion conductor, 12a, 12b ... Electrode, 13 ... Cap, 14 ... Gas diffusion hole, 15 ... Internal space, 16 ... Gas diffusion mechanism, 17 ... Heater, 18a, 18b, 19 …Lead.

Claims (5)

An ionic conductor made of a solid electrolyte;
A porous electrode pair for applying an electric field provided on the ion conductor;
A gas diffusion mechanism for supplying a diffusion-controlled gas to one surface side of the electrode pair;
A heater for heating the ion conductor,
The gas diffusion mechanism has an internal space that is in contact with one electrode of the electrode pair, and a gas diffusion hole that communicates the internal space with the outside, and an oxygen concentration gradient in the internal space has a Faraday constant (F). , Diffusion coefficient (D), oxygen concentration (C _O2 ), hole area (S) of the gas diffusion hole, hole length (l) in the penetration direction of the gas diffusion hole, the electrode in the internal space and the inner surface facing it Based on the relationship between the distance (l _in ), the effective cross-sectional area (S _in ) of the internal space, and the output current value (I _lim ),
1 / I _lim = (1/4 FDC _O2 ) {(l / S) + (l _in / S _in )}
A limiting current type oxygen sensor configured to satisfy the formula:   2. The limit according to claim 1, wherein the gas diffusion mechanism is formed such that a distance between the electrode of the internal space and an inner surface facing the electrode is larger than a diameter of the gas diffusion hole. Current-type oxygen sensor.   The gas diffusion mechanism is configured such that a ratio (S / l) between the hole area (S) and the hole length (l) of the gas diffusion hole is 50 μm to 250 μm. Item 3. The limiting current type oxygen sensor according to item 1 or 2. An ion conductor made of a solid electrolyte, a porous electrode pair for applying an electric field provided on the ion conductor, and a gas for supplying a diffusion-controlled gas to one side of the electrode pair A method for detecting and measuring oxygen concentration using a limiting current oxygen sensor comprising a diffusion mechanism and a heater for heating the ion conductor,
In the gas diffusion mechanism, an internal space that is in contact with one electrode of the electrode pair and a gas diffusion hole that communicates the internal space with the outside are formed.
The oxygen concentration gradient in the internal space includes a Faraday constant (F), a diffusion coefficient (D), an oxygen concentration (C _O2 ), a hole area (S) of the gas diffusion hole, and a hole length in the penetration direction of the gas diffusion hole ( l), based on the relationship between the distance between the electrode of the internal space and the inner surface facing the electrode (l _in ), the effective cross-sectional area of the internal space (S _in ), and the output current value (I _lim ),
1 / I _lim = (1/4 FDC _O2 ) {(l / S) + (l _in / S _in )}
A method for detecting and measuring oxygen concentration, wherein the oxygen concentration is detected and measured by performing an operation so as to satisfy the condition of the equation (1).   5. The oxygen concentration according to claim 4, wherein the internal space of the gas diffusion mechanism is formed such that a distance between the electrode and an inner surface facing the electrode is larger than a diameter of the gas diffusion hole. Detection measurement method.

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