JPH1031004A - Method of measuring oxygen concentration, and oxygen concentration sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To dispense with a reference gas and perform a highly precise measurement by equalizing oxygen concentration between the outside and inside of a case by an oxygen filter mechanism for transmitting only oxygen. SOLUTION: Porous electrodes 2, 3 are arranged on the inside and outside with an oxygen ion conductive zirconia sintered body 1 between to constitute an oxygen filter mechanism. Oxygen ion is moved within the sintered body to uniform the oxygen concentration between the outside and inside of a case. The outside of the electrode 3 is protected by a porous plate 4, a limit current type oxygen concentration measuring element 5 is provided within the case, and interrupted from the external atmosphere by a seal 6. When a voltage is applied between the electrodes 5c, 5d of the measuring element 5, the oxygen within the pores of the porous plate 5a is ionized by the electrode 5c, moved, and released as oxygen gas. Since oxygen of the quantity corresponding to the oxygen concentration in the atmosphere is supplied through the porous plate 4, the current between the electrodes 5c, 5d is detected, whereby the oxygen concentration in the atmosphere can be measured. Thus, a reference gas is dispensed with, and a highly precisely measurement can be preformed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring an oxygen concentration in an atmosphere and an oxygen concentration sensor.

[0002]

2. Description of the Related Art As a method for measuring the oxygen concentration in a gas, there are known magnetic methods, galvanic cell methods, zirconia methods and the like. Among them, the zirconia oxygen concentration measurement method is widely used because the apparatus is compact and has good responsiveness and accuracy.

[0003] The zirconia oxygen concentration measurement method utilizes the oxygen ion conductivity of a zirconia membrane, and two main methods are known.
One is a concentration cell method, in which a pair of gas-permeable electrodes is arranged with a stabilized zirconia film, which is an oxygen ion conductor, interposed between the electrodes caused by a difference in oxygen concentration on both sides of the zirconia film. The potential difference is measured and converted into a concentration difference. Note that this method requires a reference gas whose oxygen concentration is known.

The other is a limiting current method. A pair of gas-permeable electrodes is arranged with a stable zirconia membrane interposed therebetween. When a voltage is applied to this electrode, an oxygen pump action is used in which oxygen ions move from the anode side to the cathode side of the zirconia membrane,
In addition, a current value corresponding to the oxygen concentration is obtained by controlling the supply of oxygen gas to the sensor by a cover having a small hole or a porous body (hereinafter, both are referred to as “oxygen gas diffusion controlling body”). Is converted to an oxygen concentration.

Although the limiting current method does not require a reference gas having a known oxygen concentration unlike the concentration cell method, the pores of the oxygen gas diffusion-controlling body used to control the oxygen gas transporting process are limited by Since the supply of oxygen to the sensor is limited by molecules smaller than oxygen molecules, such as methane and hydrogen, the measurement in an atmosphere containing methane or hydrogen in the combustion exhaust gas shows a value lower than the actual oxygen concentration . In addition, in the limiting current method, it is necessary to maintain a high temperature of about 700 ° C. in order to impart sufficient oxygen ion conductivity to the zirconia film. Measurement errors are likely to occur because the gas is burned and the oxygen is consumed.

As an example of using an oxygen sensor in addition to combustion exhaust gas, there is a nitrogen reflow soldering device used in various semiconductor mounting processes. This device has a low oxygen concentration to prevent metal oxidation,
An oximeter is required, but if a limiting current sensor is applied to this field, an error will occur due to flammable gas such as hydrocarbons contained in the flux vapor.

Further, when the concentration in these atmospheres becomes high, for example, when the organic solvent such as isopropyl alcohol used for removing the flux is largely evaporated after the soldering, oxygen is consumed by burning on the surface of the zirconia sensor. Measurement becomes impossible. In addition, it is possible to prevent the above-mentioned obstacles by previously removing interference components from the gas to be measured by an adsorption filter using activated carbon or the like, but in this case, the capacity of the filter is likely to be saturated,
Therefore, much labor is required for maintenance such as replacement and cleaning.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art, that is, an oxygen concentration measurement method capable of accurately measuring the oxygen concentration even in the presence of a flammable gas without requiring maintenance or a reference gas. The aim is to provide a method.

[0009]

According to a first aspect of the present invention, there is provided an oxygen concentration measuring method for measuring an oxygen concentration inside a case in which the inside and outside oxygen concentrations are equalized by an oxygen filter mechanism that transmits only oxygen. Having. With such a configuration, the oxygen concentration can be measured without being affected by combustible gas or the like.

[0010]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, an oxygen filter mechanism that allows only oxygen to pass is applied to a compound having a crown ether or the like that is known to pass through a molecule of a specific size, or to a specific diameter. May be formed by mixing a zeolite crystal having continuous pores with a polymer membrane, but at least a part of the case is composed of an oxygen ion conductor, and an electrode pair is sandwiched between the oxygen ion conductors. It can also be configured by disposing.

[0011] An oxygen filter mechanism that allows only oxygen to pass through may be formed by arranging at least a part of the case with an oxygen ion conductor and arranging electrodes inside and outside the case with the oxygen ion conductor interposed therebetween. Oxygen ions move in the ion conductor in a direction to make the oxygen concentration inside and outside the case uniform, and finally the oxygen concentration inside and outside the case becomes equal.

Incidentally, stabilized zirconia is known as an oxygen ion conductor. Stabilized zirconia needs to be maintained at a temperature of about 250 ° C. or higher in order to function as an oxygen ion conductor, but particularly when a relatively high temperature gas such as an internal environmental gas of a nitrogen reflow soldering apparatus is a measurement target. It is desirable that the temperature be as low as possible without heating and in order to avoid consumption of oxygen gas on the surface of the oxygen filter mechanism. The temperature at which methane gas burns in the surface atmosphere of such an oxygen filter mechanism is about 400 ° C., and about 250 ° C. does not significantly affect the oxygen concentration in the filter mechanism surface atmosphere. When the use environment temperature is about 250 ° C. or less, it can be dealt with by providing a heating means such as a heater in the case.

When the electrodes are short-circuited as described above,
It takes some time to equalize the oxygen concentration inside and outside the case. Therefore, a positive voltage may be applied to the electrode pair to positively move the oxygen ions. In this case, it is necessary to perform the comparison while comparing the oxygen concentrations inside and outside the case. As means for this purpose, for example, a measuring means such as a concentration difference battery for measuring the oxygen concentration inside and outside the case may be separately provided. However, if the application of the voltage is interrupted, the battery becomes a concentration difference battery. By applying a voltage to the electrode and measuring the voltage between the electrodes when the voltage is not applied, the oxygen concentration difference inside the case is detected, and by controlling the applied voltage (current) appropriately, the oxygen inside and outside the case is controlled. The concentrations can be matched quickly.

In the present invention, the material of the case other than the oxygen filter mechanism is not limited as long as it is airtight. Therefore, the material can be selected according to the environment to be measured. The shape of the case is not limited, but the response is better if the internal space is as small as possible. When the dead space of the internal space is large, it is desirable to use a spacer such as alumina together.

As a means for measuring the oxygen concentration inside the case, a limiting current type oxygen concentration measuring element can be used because a reference gas is difficult to obtain, although a concentration cell type oxygen concentration measuring element can be applied. desirable. It can be very compact and has good responsiveness. In general, in the case of a limiting current type oxygen concentration measuring element, the stabilized zirconia, which is an oxygen ion conductor, needs to be maintained at a temperature of about 700 ° C. However, there is no flammable gas in the case, and Since there is no interference by methane gas or hydrogen gas in the gas diffusion controlling body, the measurement becomes accurate.

[0016]

Embodiments of the present invention will be described below. Embodiment 1 FIG. 1A shows a cross section of an example of an oxygen concentration sensor in which an oxygen concentration measuring element is installed in a case having an oxygen filter mechanism that transmits only oxygen.
In the figure, reference numeral 1 denotes a zirconia sintered body having oxygen ion conductivity in an environment of about 300 ° C. or higher. The inner diameter of this is 3 mm and the length is 50 mm. Inside and outside the zirconia sintered body 1, there are porous gas permeable electrodes 2 and 3, and outside the gas permeable electrode 3, a porous plate is provided for protection. The limiting current type oxygen concentration measuring element 5 is installed in the case configured as described above, and a seal 6 is formed by a glass paste on the case to shield the inside of the case from the outside atmosphere.

FIG. 1B is an enlarged view (cross-sectional view) of the limiting current type oxygen concentration measuring element 5. Reference numeral 5a denotes a perforated plate which is an oxygen gas diffusion controlling body. On one surface of the porous plate 5a, a porous gas-permeable electrode 5c, a zirconia thin film 5b as an oxygen ion conductor,
The porous gas-permeable electrodes 5b are laminated and formed in this order. A heater 5e made of platinum is provided on the other surface of the perforated plate 5a to keep the zirconia thin film 5b at a temperature (700 ° C.) suitable for its oxygen ion conduction. The end of the electrode 5c,
A lead wire 5f is connected to each end of the electrode 5d and both ends of the heater 5e.

In the limiting current type oxygen concentration measuring element 5, when a voltage is applied between the electrodes 5c and 5d, oxygen in the pores on the electrode 5c side of the porous plate 5a is ionized by the electrode 5c, and the zirconia thin film 5b It passes through the inside and moves toward the electrode 5d, where it is released as oxygen gas. At this time, the supply of oxygen to the electrode 5c of the porous plate 4 is diffusion-controlled by the pores of the porous plate 5c. At this time, since an amount of oxygen corresponding to the oxygen concentration in the atmosphere is supplied to the electrode 5c, the oxygen concentration in the atmosphere can be detected by detecting the current flowing between the electrode 5c and the electrode 5d. . The oxygen concentration sensor shown in FIG. 1 was placed in a test furnace (filled with nitrogen gas) at 300 ° C., and gases 1 to 7 having the compositions shown in Table 1 were switched and introduced into the test furnace every minute.

FIG. 2 shows a change (Example 1) of a value obtained by converting the sensor output at that time into an oxygen concentration in a nitrogen-oxygen mixed gas (hereinafter referred to as "output converted concentration"). Note that, as Comparative Example 1, the result of the limiting current type oxygen concentration sensor (the same as the limiting current type oxygen concentration measuring element 5 installed in the case of the oxygen concentration sensor of Example 1) simultaneously placed in this test furnace was also used. Both are shown.

[0020]

[Table 1]

FIG. 2 shows that the oxygen concentration sensor of the first embodiment can obtain an accurate oxygen concentration measurement result without being affected by flammable gas such as methane, propane, and isopropyl alcohol.

[Embodiment 2] In the oxygen concentration sensor of the embodiment 1, it took 20 seconds or more from the time of gas switching until the measured value became stable. Here, when actually used for atmosphere control, the time until the measured value stabilizes may cause a delay such as an action or an obstacle such as an overaction. Here, in order to shorten the time until the measured value stabilizes, an oxygen concentration sensor was prepared in which the void portion in the case of the oxygen concentration sensor of Example 1 was filled with alumina. FIG. 3 shows a cross-sectional view thereof. The one denoted by reference numeral 7 in the figure is the alumina powder.

When this sensor was used for evaluation in the same manner as in the oxygen concentration sensor of Example 1, it was confirmed that the response was improved in all the test gases. (See FIG. 4. In FIG. 4, Comparative Example 2 shows the limiting current type oxygen concentration sensor installed in the experimental furnace at the same time as the evaluation, as in Comparative Example 1 in Example 1.) Results).

[Embodiment 3] The oxygen concentration sensors of Embodiments 1 and 2 provide good responsiveness when the oxygen concentration is low, but frequently switch between a low oxygen concentration and a high oxygen concentration. Could not be accommodated. In such a case, it is possible to cope with using a sensor (Example 3) as shown in FIG. In the oxygen concentration sensors of Examples 1 and 2, the electrodes 2 and 3 provided in contact with the zirconia sintered body constituting the case were short-circuited. In the sensor shown in FIG. The electrode 3 is connected to the determination circuit 8.

The determination circuit detects a potential difference between the electrode 2 and the electrode 3 which occurs when there is a difference in oxygen concentration between the inside and outside of the case, and activates an oxygen pump circuit as necessary to apply a voltage to these electrodes 2 and 3. By applying the voltage, the difference in oxygen concentration between the inside and outside of the case is quickly eliminated. In addition, as described above, the electrode 2 and the electrode 3 can be used by alternately switching and using the electrode at the time of detecting the oxygen concentration difference and the electrode in the oxygen pump mechanism. In this case, one electrode pair is used. The production is easy because it is sufficient.

In the above description, when a positive voltage is applied to the inner electrode 2 and a negative voltage is applied to the outer electrode 3, oxygen gas is carried as oxygen ions in the zirconia sintered body 1 according to Faraday's law, and 1 equivalent at 96500 C Be transported. Here, 1A = 1C / sec, and the volume of one mole of oxygen molecule is 22.4 l in a standard state, and 0 ₂ + 4e ⁻ = 2O ² −. Therefore, the flow rate V of the oxygen gas carried by the current of 0.1 A is V ₁ is 5.8 × 10 ⁻³ ml / sec (standard state) according to Equation 1.

[0027]

V ₁ = 0.1 A × 60 sec / (4 × 96500) × 22.4 × 1000 [ml / min] = 0.348 [ml / min] = 5.8 × 10 ^-3 [ml / sec] ]

On the other hand, the diameter d inside the case is 3 mm and the length is 3 mm.
1 is 50 mm, and aluminum
If the porosity at the time of filling is 3%, the volume in the case
Quantity V _TwoIs 0.01 ml according to Equation 2.

[0029]

V ₂ = (πd ² 1/4) × a = 0.01 ml

Therefore, when there is a 10% difference in oxygen concentration between the inside and outside of the case from V ₁ and V ₂ , the time required to make the difference equal to or less than 200 msec, and the measurement-voltage application (50 msec) 400 m after the above concentration difference detection
After sec, the oxygen concentration inside and outside the case can be equalized, and the responsiveness is improved as compared with the sensors of the first and second embodiments. The determination circuit is a CPU, ROM
And a microprocessor unit or the like constituted by a RAM or the like, and the oxygen pump circuit can be constituted by a power supply circuit such as a constant current power supply circuit or a constant voltage power supply circuit.

[0031]

The method for measuring oxygen concentration according to the present invention is excellent in that it can measure oxygen concentration accurately even in the presence of flammable gas, without requiring maintenance or a reference gas.
In addition, the size of the measuring section can be made extremely small.

[Brief description of the drawings]

FIG. 1A is a diagram illustrating a cross section of an oxygen concentration sensor according to a first embodiment. (B) It is a figure which shows the cross section of the limiting current type oxygen concentration measuring element installed inside the case.

FIG. 2 is a diagram illustrating a situation when the oxygen concentration of a test gas is measured by the oxygen concentration sensor according to the first embodiment.

FIG. 3 is a diagram illustrating a cross section of an oxygen concentration sensor according to a second embodiment.

FIG. 4 is a diagram illustrating a situation when the oxygen concentration of a test gas is measured by the oxygen concentration sensor according to the second embodiment.

FIG. 5 is a diagram illustrating a cross section of an oxygen concentration sensor according to a third embodiment and a determination circuit and an oxygen pump circuit attached thereto.

[Description of Signs] 1 Sintered zirconia 2 Electrode 3 Electrode 4 Porous plate 5 Oxygen concentration measuring element 5a Porous plate 5b Zirconia thin film 5c, 5d Electrode 5e Heater 5f Lead wire 6 Seal 7 Alumina powder 8 Judgment circuit 9 Oxygen pump circuit

Claims (5)

[Claims] 1. An oxygen concentration measuring method, comprising: measuring an oxygen concentration inside a case in which the oxygen concentration inside and outside is equalized by an oxygen filter mechanism that transmits only oxygen. 2. The oxygen filter mechanism according to claim 1, further comprising an oxygen ion conductor constituting at least a part of the case, and an electrode pair disposed inside and outside the case with the conductor interposed therebetween. Oxygen concentration measurement method described in 1. 3. The oxygen concentration measuring method according to claim 2, wherein the oxygen concentration inside and outside the case is equalized by applying a voltage between the pair of electrodes. 4. The oxygen concentration measurement according to claim 3, wherein an oxygen concentration difference between the inside and outside of the case is detected by measuring an electromotive force between the pair of electrodes, and the applied voltage is controlled based on the result. Method. 5. An oxygen concentration sensor comprising: a case having an oxygen filter mechanism that allows only oxygen to pass therethrough; and an oxygen concentration detection unit that measures the oxygen concentration inside the case.