JP3926949B2 - Oxygen sensor and electromotive force compensation method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an oxygen sensor capable of measuring an extremely low oxygen concentration on the order of ppb with high accuracy, and an electromotive force compensation method used for the oxygen sensor.
[0002]
[Prior art]
High-purity gas is used in a wide variety of fields such as semiconductor manufacturing processes, heat treatment of steel and metals (non-oxidation furnace), special metal welding, and food packaging. Among such high-purity gases, the production process of high-purity gas that requires oxygen concentration control or product purity management (oxygen concentration management) is performed in the ppb order, and yellow phosphorescence is used as an oxygen concentration measuring device. A formula or special galvanic type is used.
[0003]
[Problems to be solved by the invention]
However, yellow phosphorescent or galvanic oxygen sensors have various problems such as high cost and short life, and the need for fine maintenance. Therefore, there is a demand for an oxygen sensor that is small in size, easy to operate and maintain, low in cost, and has a long lifetime.
[0004]
One candidate for an oxygen sensor that can meet such demands is an oxygen sensor using zirconia (ZrO ₂ ) that exhibits good oxygen ion conductivity at high temperatures. However, in the ZrO ₂ oxygen sensor, gas adsorption at the metal electrode, oxygen ion movement due to the leakage current of the heater etc. flowing into the reference electrode, and slight oxygen intrusion from the porous material such as the electrode lead and electrode protective layer As a result, there is a problem that the sensor electromotive force (measured electromotive force) deviates from the theoretical electromotive force given by the Nernst equation in the ppb order trace oxygen concentration region. As a result, the oxygen concentration range in which high-precision measurement using a ZrO ₂ oxygen sensor is possible is limited to a range of 1 ppm or more. That is, it has been difficult to apply the conventional ZrO ₂ oxygen sensor to the detection of a trace oxygen concentration of the ppb order.
[0005]
[Means for Solving the Problems]
The present invention has been made in view of the above-mentioned problems of the prior art, and the object of the present invention is to enable detection of a trace oxygen concentration on the order of ppb with a ZrO ₂ oxygen sensor, which is small and excellent in maintainability. In addition, it is to provide an inexpensive oxygen sensor.
That is, according to the present invention, an electromotive force compensation method for an oxygen sensor including an oxygen ion conductive solid electrolyte and a metal electrode, wherein the sensor electromotive force is obtained by a Nernst equation in an oxygen concentration range of 0.1 ppb to 1 ppm. When there is a deviation from the theoretical electromotive force to the negative side larger than -0.5%, a predetermined DC constant current of 30 μA / cm ² or less per effective electrode area is passed from the reference electrode to the detection electrode to thereby generate the sensor electromotive force. A method for compensating an electromotive force of an oxygen sensor is provided.
[0006]
In this oxygen sensor electromotive force compensation method, it is preferable to keep the oxygen sensor at a predetermined temperature and determine the magnitude of the DC constant current so that the sensor electromotive force is within the range of the theoretical electromotive force ± 0.5%. . As the solid electrolyte, those containing zirconia as a main component are preferably used.
[0007]
Moreover, according to the present invention, as an oxygen sensor using the above-described electromotive force compensation method, an oxygen sensor including an oxygen ion conductive solid electrolyte and a metal electrode, in an oxygen concentration range of 0.1 ppb to 1 ppm, When the sensor electromotive force deviates from the theoretical electromotive force obtained by the Nernst equation to the negative side larger than -0.5%, a predetermined DC constant current of 30 μA / cm ² or less per effective electrode area from the reference electrode to the detection electrode There is provided an oxygen sensor characterized by having means for compensating the sensor electromotive force by flowing.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is an explanatory view showing a schematic configuration of an oxygen sensor 11 according to the present invention, and both (a) and (b) are cross-sectional views, and (b) passes through a broken line AA in (a) and is perpendicular to the paper surface. It is sectional drawing which showed the cross section. Reference electrode 13 is provided bottomed tubular zirconia porcelain to the inner peripheral surface of the (ZrO ₂ porcelain) 12, sensing electrode to face the reference electrode 13 across the ZrO ₂ porcelain 12 (the outer peripheral side of the ZrO ₂ porcelain 12) 14 is provided, and this portion becomes the sensor unit 21. The detection electrode 14 is embedded in a porous protective layer 15. A heater 16 is embedded in the ZrO ₂ porcelain 12 to raise the temperature of the sensor unit 21 to a predetermined temperature and maintain the temperature at a constant temperature.
[0009]
A DC constant current power supply 31 is provided so that a constant current can flow from the reference electrode 13 to the detection electrode 14. The voltage between the reference electrode 13 and the detection electrode 14, that is, the start of the oxygen sensor 11 is provided. A voltmeter 32 for measuring electric power is provided.
[0010]
First, the material which comprises the sensor part 21 of the oxygen sensor 11 is demonstrated. In the oxygen sensor 11, the ZrO ₂ porcelain 12 serves as a solid electrolyte and a partition that separates the measurement gas and the reference gas. The oxygen ion conductivity of the solid electrolyte is preferably high, and in ZrO ₂ , the ion conductivity can be changed depending on the type and amount of the element to be dissolved, so that a composition suitable for the purpose should be used appropriately. Is preferred.
[0011]
Specifically, as the ZrO ₂ porcelain 12, a stabilization made by dissolving various stabilizing materials such as yttria (Y ₂ O ₃ ), magnesia (MgO), calcia (CaO), and ceria (CeO ₂ ). ZrO ₂ or partially stabilized ZrO ₂ is preferably used. If the thickness between the reference electrode 13 and the detection electrode 14 is reduced, the resistance between the electrodes can be reduced.
[0012]
An important characteristic of the reference electrode 13 and the detection electrode 14 is that they have high oxygen ion catalytic properties. The oxygen ion catalytic property at the detection electrode 14 refers to the property of ionizing oxygen molecules in the measurement gas and taking them into the solid electrolyte, while the oxygen ion catalytic property at the reference electrode is conversely a solid that has moved from the detection electrode. It refers to the property of taking electrons from oxygen ions in the electrolyte and releasing them as oxygen molecules into the gas phase (standard gas).
[0013]
A platinum (Pt) electrode is suitably used as an electrode material having excellent characteristics. In addition, it is preferable that these electrodes have a porous property, and a large number of triple points (three-phase interfaces) where the gas phase, the electrode, and the three phases of the solid electrolyte are in contact with each other are formed. Therefore, it is also preferable to use a cermet electrode made of Pt and ZrO ₂ as the reference electrode 13 and the detection electrode 14.
[0014]
Next, the electromotive force compensation method of the present invention applied to the oxygen sensor 11 will be described in detail below including the operating principle.
Reference gas, such as air, oxygen concentration is known the inner space 18 of the measurement ZrO ₂ porcelain 12 (an oxygen partial pressure of the reference gas to P _1.) Flowing outside the measurement gas ZrO ₂ porcelain 12 (measurement gas When the oxygen partial pressure in contact with the.) to P _2, a difference in oxygen concentration partial pressure P ₁ · P ₂ between the inside and outside in which the ZrO ₂ porcelain 12 and the partition wall occurs, concentration cell is formed . At this time, the Nernst equation (E ₀ = (RT / 4F) · ln (P ₁ / P ₂ )) is provided between the reference electrode 13 and the detection electrode 14, where R: gas constant, F: A theoretical electromotive force E ₀ based on the Faraday constant, T: absolute temperature) is generated.
[0015]
Accordingly, since the oxygen partial pressure P ₁ is also constant if the temperature T is constant and the type and total pressure of the reference gas are fixed, the relationship between the oxygen partial pressure ln (1 / P ₂ ) and the theoretical electromotive force E ₀ is It is expressed by a linear function. Further, when the pressure ratio between the reference gas and the measurement gas is constant, the ratio of the oxygen partial pressures P ₁ and P ₂ can be replaced with the ratio of oxygen concentration. Therefore, as shown in FIG. 2, the relationship between the theoretical electromotive force E ₀ and the oxygen concentration can be represented by a straight line B when the horizontal axis represents the oxygen concentration of the measurement gas and the vertical axis represents the electromotive force.
[0016]
On the other hand, the actual sensor electromotive force when the conventional ZrO ₂ oxygen sensor is used (referred to as measured electromotive force when there is no compensation of electromotive force described later) E ₁ changes in FIG. Indicated by curve C in the middle. That is, when the oxygen concentration of the measurement gas is about 1 ppm or more, the sensor electromotive force E ₁ coincides with the theoretical electromotive force E ₀ almost according to the Nernst equation, so that the oxygen concentration can be accurately measured. . However, when the oxygen concentration is 1 ppm or less, the sensor electromotive force E ₁ deviates from the theoretical electromotive force E ₀ and decreases, and the change in value also decreases. As a result, the measurement accuracy in such a trace oxygen concentration region is drastically lowered.
[0017]
As described above, one of the reasons why the sensor electromotive force E ₁ does not increase in the trace oxygen concentration range is considered to be oxygen adsorption inside the detection electrode and / or oxygen concentration difference due to oxygen diffusion change in the thickness direction of the detection electrode. . Therefore, it is considered that by releasing the sensor unit from such a state, the sensor electromotive force E ₁ is brought close to the theoretical electromotive force E ₀ and a trace oxygen concentration of 1 ppm or less can be measured.
[0018]
Therefore, in the present invention, a small DC constant current I _M is passed from the reference electrode 13 to the detection electrode 14 and oxygen (oxygen ions) in the vicinity of the detection electrode 14 is discharged to the reference electrode 13 side. reducing the oxygen adsorption amount, also, by leveling the oxygen diffusion to the sensing electrode 14, to the relationship between the oxygen concentration and the sensor electromotive force E ₁ measurement gas is consistent with relationship Nernst equation.
[0019]
Specifically, in the oxygen concentration range of 0.1 ppb to ₁ ppm, the sensor electromotive force E ₁ deviates from the theoretical electromotive force E ₀ obtained by the Nernst equation to a negative side larger than −0.5%. Sometimes, the sensor electromotive force E ₁ is compensated by flowing a predetermined DC constant current I _M of 30 μA / cm ² or less per effective electrode area from the reference electrode 13 to the detection electrode 14. Incidentally, the sensor electromotive force after being compensated by such a method, hereinafter be expressed as compensating electromotive force E _2.
[0020]
In FIG. 3, a porous platinum electrode is used for the detection electrode and the reference electrode, a ZrO ₂ porcelain having an electrode effective area of 3.46 mm ² and a thickness between both electrodes of 220 μm is used, and the temperature of the sensor unit is 760 ° C. Each of the electromotive forces (compensated electromotive force E) when the electromotive force compensation is performed (I _M = 0.36 μA) and when the electromotive force compensation is not performed (I _M = 0 μA) is performed using the oxygen sensor held in a constant state. ₂ is a graph showing the relationship between the sensor electromotive force E ₁ ) and the oxygen concentration.
[0021]
Both the sensor electromotive force E ₁ and the compensated electromotive force E ₂ show the same measured value at an oxygen concentration of about 10 ppm or more. This is because the amount of oxygen ions moving by the weak DC constant current I _M from the triple point where the detection electrode, the measurement gas, and the ZrO ₂ porcelain are in contact to the reference electrode is small. That is, the change in oxygen concentration due to the DC constant current I _M is small in the high oxygen concentration region, and therefore, even if the DC constant current I _M for compensating electromotive force in the minute oxygen concentration region is supplied on the high oxygen concentration side, the measurement is performed. The accuracy is not adversely affected.
[0022]
Conversely, in a trace oxygen concentration range of 1 ppm or less, the amount of oxygen ions that are moved by the DC constant current I _M becomes so large that it cannot be ignored with respect to the oxygen concentration in the measurement gas. The compensation electromotive force E ₂ can be brought close to the theoretical electromotive force E ₀ .
[0023]
In such an electromotive force compensation method, in order to obtain high measurement accuracy, the magnitude of the DC constant current I _M is set so that the compensated electromotive force E ₂ falls within the range of the theoretical electromotive force E ₀ ± 0.5%. Is preferably determined. Here, when the temperature of the sensor unit changes, the oxygen adsorption amount and the gas diffusion coefficient at the detection electrode change, so the temperature of the sensor unit needs to be kept constant.
[0024]
In the oxygen sensor 11 of FIG. 1, the sensor unit 21 can naturally be maintained at a predetermined temperature by the heater 16. Therefore, the temperature management of the sensor unit can also be performed by keeping the heating power of the heater at a predetermined value. Therefore, in FIGS. 4A and 4B, the oxygen sensor that gives the result of FIG. 3 described above is used, and the heater heating power is 9 W (sensor part temperature: 811 ° C.) and 7 W (sensor part temperature: 695 ° C.), respectively. In this case, a graph showing how the magnitude of the DC constant current I _M affects the electromotive force of the oxygen sensor is shown.
[0025]
When the sensor temperature is high, that is, when the heater heating power is large, the graph of I _M = 0.36 μA indicating the theoretical electromotive force E ₀ in FIG. 4 shows that the sensor electromotive force is high and the slope with respect to the logarithmic oxygen concentration is steep. become. This is clear from the Nernst equation shown above. On the other hand, the deviation of the sensor electromotive force E ₁ (I _M = ₀ μA) from the theoretical electromotive force E ₀ increases as the heater power decreases. Further, when the compensation current value I _M is small, the electromotive force remains low, and when the compensation current value I _M is large, the electromotive force becomes too high. Therefore, the compensation current value I _M has an appropriate current value range. The width of the appropriate current value becomes narrower as the lower limit of the oxygen concentration region requiring linearity is lower. Therefore, when the compensation current I _M is applied, the voltage drop represented by the product of the sensor impedance Z _P and the compensation current I _M is uniformly superimposed on the sensor electromotive force E ₂ over the entire oxygen concentration. If the sensor temperature is low, the increase in electromotive force cannot be ignored and must be added when setting the compensation current.
[0026]
The compensation current is set by first measuring electromotive force at several points in a region where the oxygen concentration is high and the electromotive force linearity (correlation between logarithmic oxygen concentration and electromotive force) is ensured and the oxygen concentration is 10 ppm or more. Find a linear equation. Next, an electromotive force E ₀ of the set oxygen concentration is obtained by extrapolation from the obtained linear equation, a sensor is inserted into the set oxygen concentration (gas having a known concentration), and the sensor electromotive force is the calculated set electromotive force. Thus, a current is passed between the electrodes to obtain a compensation current I _M. When the sensor impedance is high and the voltage drop cannot be ignored, the compensation current I _M is obtained by the same method after adding the voltage drop to the calculated set electromotive force. A gas having a trace oxygen concentration of 0.1 ppb targeted by the present invention can be easily obtained by passing a cylinder gas through an oxygen scavenger or the like.
[0027]
When the constant DC current I _M is a constant value and the compensation electromotive force E ₂ deviates significantly from the theoretical electromotive force E ₀ in a part of the oxygen concentration range, the measurement range is divided into a plurality of ranges. It is also possible to set a predetermined DC constant current I _M for each measurement range and match the compensated electromotive force E ₂ with the theoretical electromotive force E ₀ . In this case, since the range of the theoretical electromotive force E ₀ to be detected in each measurement range is known, the measurement range is not appropriate unless the compensated electromotive force E ₂ matches the theoretical electromotive force E ₀ . Move to a measurement in another range.
[0028]
As described above, as it is the sensor unit of the conventional ZrO ₂ oxygen sensor, the processing method of the electromotive force signal from the sensor unit, by applying an electromotive force compensation method of the present invention, conventionally measured in ZrO ₂ oxygen sensor This makes it possible to measure the oxygen concentration of 0.1 ppb to 1 ppm with high accuracy.
[0029]
Further, the electromotive force compensation method of the oxygen sensor of the present invention can be used for calibration of oxygen concentration in other gas sensors. For example, in a carbon monoxide sensor having a structure equivalent to the ZrO ₂ oxygen sensor of the present invention, oxygen interferes with the detection of carbon monoxide, so the oxygen concentration is monitored and the detected carbon monoxide concentration is corrected. The method to do is taken. That is, the method for compensating the electromotive force of the oxygen sensor of the present invention is extremely effective as a means for correcting the oxygen concentration in a gas sensor other than oxygen.
[0030]
【The invention's effect】
The oxygen sensor and the electromotive force compensation method according to the present invention are simple in structure, low in price, easy to handle and excellent in maintainability, and using a ZrO ₂ oxygen sensor, which has been difficult in the past, measures oxygen concentration on the ppb order. Can be performed with high accuracy.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a schematic configuration of a zirconia oxygen sensor according to the present invention.
FIG. 2 is a graph showing the relationship between sensor electromotive force and theoretical electromotive force in a conventional zirconia oxygen sensor.
FIG. 3 is a graph showing the relationship between the compensated electromotive force and the oxygen concentration when the electromotive force compensation method for a zirconia oxygen sensor according to the present invention is used.
FIG. 4 is a graph showing the relationship between the value of the DC constant current used for electromotive force compensation of the zirconia oxygen sensor according to the present invention and the compensated electromotive force.
[Explanation of symbols]
11 ... oxygen sensor, 12 ... ZrO ₂ porcelain, 13 ... reference electrode, 14 ... detection electrode, 15 ... protective layer, 16 ... heater, 18 ... inner space, 21 ... sensor unit, 31 ... constant current source, 32 ... voltmeter .

Claims (5)

An electromotive force compensation method for an oxygen sensor comprising an oxygen ion conductive solid electrolyte and a metal electrode,
When the sensor electromotive force has a deviation from the theoretical electromotive force obtained by the Nernst equation to the negative side larger than -0.5% in the oxygen concentration range of 0.1 ppb to 1 ppm, per effective electrode area from the reference electrode to the detection electrode An electromotive force compensation method for an oxygen sensor, wherein the sensor electromotive force is compensated by flowing a predetermined DC constant current of 30 μA / cm ² or less. 2. The method of claim 1, wherein the magnitude of the DC constant current is determined so that the sensor electromotive force falls within the range of the theoretical electromotive force ± 0.5%. 3. The method for compensating an electromotive force of an oxygen sensor according to claim 1, wherein the oxygen sensor is maintained at a predetermined temperature and the magnitude of the DC constant current is determined. The method for compensating an electromotive force of an oxygen sensor according to any one of claims 1 to 3, wherein the solid electrolyte is mainly composed of zirconia. An oxygen sensor comprising an oxygen ion conductive solid electrolyte and a metal electrode,
When the sensor electromotive force has a deviation from the theoretical electromotive force obtained by the Nernst equation to the negative side larger than -0.5% in the oxygen concentration range of 0.1 ppb to 1 ppm, per effective electrode area from the reference electrode to the detection electrode An oxygen sensor comprising means for compensating the sensor electromotive force by flowing a predetermined DC constant current of 30 μA / cm ² or less.

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