JP3373741B2 - Limit current type oxygen sensor device and sensor driving method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a limiting current type oxygen sensor device used for detecting and measuring oxygen concentration in the fields of medical oxygen concentrators, oxygen deficiency monitoring systems, etc., and is particularly composed of a solid electrolyte. The present invention relates to a limiting current type oxygen sensor device and a sensor driving method configured by using an ionic conductor.

[0002]

2. Description of the Related Art Conventionally, as a limiting current type oxygen sensor, one using stabilized zirconia as an ion conductor is known. This kind of limiting current type oxygen sensor is generally formed with electrodes made of a porous material for applying a monitoring voltage on both sides of an ionic conductor, and one of the electrodes is provided with a diffusion-controlled gas for supplying gas. It is covered with a cap that has gas diffusion holes. A heater is provided on the cap to set the ion conduction plate at a monitoring temperature of several hundred degrees.

FIG. 3 shows the characteristics of a limiting current type oxygen sensor of this type. When a sensor voltage is applied between the electrodes, an output current proportional to the voltage flows while the voltage is small. If the sensor voltage is further increased, the current will eventually be saturated. The current in this saturation region is called the limiting current, and the magnitude of the limiting current is as shown by the oxygen concentration in FIG. 3 as a parameter.
There is a certain correspondence with the oxygen concentration. Therefore, for example, in FIG.
The oxygen concentration can be detected from the limiting current value obtained by the monitoring voltage Vs.

The current flowing through the ionic conductor is based on the movement of oxygen ions, and its current value depends on the voltage and temperature. Therefore, the limiting current type oxygen sensor is 400 to 50
The monitoring temperature is set to 0 ° C. and voltage driving is performed. The setting of the monitoring temperature is usually performed by providing the heater on the sensor body and energizing the heater as described above. In many cases, the limiting current type oxygen sensor is driven by a method in which the heater is always energized and a monitoring voltage is constantly applied between the electrodes.

However, if the oxygen sensor is driven by constantly applying the monitoring temperature and the monitoring voltage, the characteristics of the ionic conductor deteriorate due to the deterioration of the ionic conductivity. This characteristic deterioration appears as the slope of the output current with respect to the sensor voltage becomes gentle, as indicated by the alternate long and short dash line in FIG. When such characteristic deterioration occurs, the limiting current region becomes narrower. In the example of FIG. 3, when the oxygen concentration is 21%, the monitoring voltage Vs deviates from the limiting current region, and a current value smaller than the limiting current value is measured. Will be done. Therefore, when the oxygen concentration range to be measured and the monitoring voltage are defined by the specifications, if the monitoring voltage deviates from the limiting current region within that range, the correct oxygen concentration cannot be measured, and the oxygen sensor reaches the end of its life.

[0006]

In recent years, there has been a strong demand for a long life of the limiting current type oxygen sensor in various application fields, and specifically, it is required to be able to detect oxygen concentration stably and accurately for three years or more. It has become. However, in the conventional driving method in which the heater is continuously energized and the monitoring voltage is continuously supplied, it is the actual situation that about 20 to 30% of the sensors reach the end of their lives within the usage period of 3 years. On the other hand, an intermittent driving method has been proposed in which a monitoring voltage is intermittently applied to a limiting current type oxygen sensor (see Japanese Patent Application No. 59-158158). In this method, since the measurement is not performed during the period when the monitoring voltage is not applied, it is not necessary to energize the heater. Therefore, it is possible to reduce the power consumption by simultaneously turning on and off the monitoring voltage application and the heater energization. it can. When this intermittent drive method is used, the progress of deterioration is suppressed as compared with the case of the continuous energization described above, but still about 5-10% of the sensor reaches the end of its service life within 3 years of use. Further, if such intermittent driving is performed, the entire sensor is subjected to a heat cycle, so that there is a problem in that element breakdown is likely to occur.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a limiting current type oxygen sensor device and a sensor driving method capable of stably and highly accurately measuring oxygen concentration for a long period of time. I am trying.

[0008]

SUMMARY OF THE INVENTION The present invention is directed to an ionic conductor made of a solid electrolyte, a porous electrode pair for applying an electric field to the ionic conductor, and one surface of the electrode pair. A method for driving a limiting current type oxygen sensor having a gas diffusing means for supplying a diffusion-controlled gas and a heater for heating the ionic conductor, in which the heater is always energized so that the ionic conductor Heating temperature to 40
It is characterized in that a monitoring voltage is intermittently applied between the electrode pairs while maintaining the temperature at 0 ° C. or higher . In the present invention, the ratio of the ON time and the OFF time for applying the monitoring voltage (hereinafter, simply referred to as ON / OFF ratio) is preferably set in the range of 1/12 to 1.

A limiting current type oxygen sensor device according to the present invention comprises an ionic conductor made of a solid electrolyte, a pair of porous electrodes for applying an electric field to the ionic conductor, and one of the pair of electrodes. a gas diffusion means for supplying a surface to diffusion limited gas of, the ion conductor 400 ° C. or higher
It is characterized in that it is provided with a heater which is constantly energized so as to be held at a constant temperature, and a drive means which intermittently applies a monitoring voltage between the electrode pairs.

According to the present invention, when a limiting current type oxygen sensor whose characteristics are deteriorated due to aging is heated for a certain time by energizing the heater without applying a monitoring voltage, the characteristics are restored to almost the initial state. It is based on the knowledge of the inventors of the present invention. FIG. 4 shows a limiting current type oxygen sensor using stabilized zirconia as an ionic conductor at a temperature of 400
The graph shows the change over time in the characteristics when the temperature is set to ° C and the device is used continuously for 3 years. The curve A in the figure is the initial characteristic, and the limiting current is obtained between the monitoring voltage Vs = 0.8 to 2.0V. Curve B is the deterioration characteristic after three years of use, and the limiting current region is almost gone. With respect to the oxygen sensor whose characteristics have deteriorated, when the heater is kept in the ON state (the state in which the ionic conductor is about 400 ° C.) and the monitoring voltage is not applied for a certain period of time, a curve C in FIG. 4 is obtained. Thus, it was discovered that the characteristics recovered and that the limiting current region close to the initial characteristics was obtained. It has also been confirmed that a minute short-circuit current flows if the electrodes are short-circuited while the heater is energized.

The inventors' consideration of the above experimental results is as follows. An oxygen sensor having a structure in which stabilized zirconia, which is a dielectric ceramic, is used as an ionic conductor and sandwiched by electrodes is a kind of capacitor, and therefore charges are accumulated when an electric field is applied between both electrodes. The charge accumulation inside the ionic conductor affects the ionic conductivity, and the sensor characteristics deteriorate as the accumulated charge amount increases.

As shown in FIG. 5, the places where charges are easily accumulated are the crystal grain boundaries 52 of the sintered ion conductor 51 and the voids 54 formed at the interface between the electrode 53 and the ion conductor 51. The oxygen sensor having more voids 54 as described above is more likely to accumulate charges. Therefore, it is presumed that the difference in the characteristic deterioration between the sensor elements in the conventional driving method is caused by the difference in the existence ratio of the void portion 54, in other words, the difference in the degree of sintering of the electrode or the ion conductor in the manufacturing process. To be done.

In order to confirm the validity of the above consideration, the present inventors conducted various experiments. First, when the oxygen sensor is set to a constant monitoring temperature of 400 ° C. and a constant monitoring voltage of 1.5 V is applied for one year, the charge amount accumulated in the ionic conductor is 10 ⁻⁵ to 10 ⁻⁴ C. It was confirmed that As a result of conducting experiments with various use conditions changed,
The following things became clear. The amount of charge stored in the ionic conductor depends on the monitoring voltage, the monitoring temperature, and the usage time. Specifically, the higher the monitor voltage and the longer the usage time, the larger the accumulated charge amount. Regarding the monitored temperature, the higher the monitored charge, the smaller the accumulated charge amount. The degree of deterioration in sensor characteristics increases as the amount of accumulated charge increases. If the monitoring voltage is not applied with the heater turned on, the electric charge accumulated in the ionic conductor is gradually discharged regardless of whether or not the electrodes are short-circuited. The higher the holding temperature due to heating by the heater, the shorter the time required for charge discharge.

From the above examination results, it is understood that it is important to suppress an increase in the amount of accumulated charge in order to extend the life of the limiting current type oxygen sensor using an ionic conductor. In the conventional driving method that constantly supplies the monitoring voltage and the monitoring temperature,
An increase in the accumulated charge amount cannot be suppressed. In another driving method in which the monitoring voltage and the monitoring temperature are applied intermittently at the same time, the discharge temperature is zero during the period when the monitoring voltage is zero, but the monitoring temperature is also lowered during this period, so the discharge is insufficient. Therefore, the accumulated charge amount gradually increases with the use time.

On the other hand, in the present invention, the monitoring voltage is intermittently applied while the heater is always energized.
Ri, ionic conductor monitoring voltage during the zero is performed sufficiently charge discharging is maintained at a high temperature, long-term charge storage amount used is kept small. Further, since the heater is continuously energized during use, an unnecessary heat cycle is not required, which also suppresses element deterioration. Therefore, according to the present invention, it is possible to obtain a limiting current type oxygen sensor device capable of stably and highly accurately measuring oxygen concentration for a long period of time.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a limiting current type oxygen sensor 10 and its drive circuit 20 according to an embodiment of the present invention. The oxygen sensor 10 comprises a pair of porous electrodes 12 made of Pt or the like on both surfaces of an ion conductor 11 made of stabilized zirconia.
a and 12b are provided, and the cap 13 is attached to the side of the electrode 12a serving as the anode. A gas diffusion hole 14 for supplying a diffusion-controlled gas to the electrode 12a is provided in the center of the cap 13, and a heater 15 for heating the ion conductor 11 is provided on the upper surface of the cap 13. There is.

The leads of the electrode pairs 12a and 12b are taken out to the outside and connected to a drive circuit 10 for applying a monitoring voltage. In the case of this embodiment, the drive circuit 10 includes a DC power source E for applying a monitoring voltage between the electrode pairs 12a and 12b.
1, a switch SW1 inserted between the DC power source E1 and the electrode 12a, and a timer 21 for intermittently turning on / off the switch SW1 during a use period. However, as shown by the broken line in the drawing, another switch SW2 is provided between the electrode pair 12a and 12b, and this is used by the timer 2
The output of No. 1 is inverted by the inverter 22 to be turned on and off, and the electrode pair 12a, 12
You may make it short-circuit between b. The heater 15 is constantly energized by a DC heater power source E2 during the period of use.

FIG. 2 shows an example of ON / OFF operation of the switches SW1 and SW2. The drive circuit as described above is provided, and the heater 15 is always energized during the period of use to provide the monitoring temperature (holding temperature), and the monitoring voltage is intermittently applied to prevent characteristic deterioration. The higher the holding temperature, the higher the ionic conductivity and the improved sensor function. However, if the holding temperature is set too high, the power consumption becomes large and the thermal stress inside the sensor element becomes large, so that the sensor element easily breaks. Therefore, in practice, the sensor function is fully exerted,
The temperature is selected within the range in which the characteristic deterioration is sufficiently suppressed. As will be made clear later by data, when the stabilized zirconia is used as the ionic conductor, the preferable holding temperature is 400 ° C. or higher.

Regarding the ON / OFF cycle of the monitoring voltage,
The length of time off is important. The on-time is the time for detecting the oxygen concentration, and during this time, the ionic conductor accumulates the charge and discharges the accumulated charge during the off-time, so in order to suppress the charge accumulation during long-term use, the on-time is preferably on. The / off ratio is 1 or less. It is preferable that the off time is as long as possible in order to suppress the charge accumulation, but since the off period is a period in which the oxygen concentration is not detected, when it is set to exceed 60 minutes, the oxygen concentration is detected at a fixed time. Since the number of times to do is very small, the application field is limited. Further, if the oxygen sensor is kept at a high temperature and left in an oxidizing atmosphere for a long time, the amount of oxygen deficiency in the stabilized zirconia may be reduced. The reaction of converting oxygen gas to oxygen ions at the platinum / zirconia interface is represented by the following chemical formula 1 (NLRobertson & JNMicael).
s; Electrochemical Engineering Applications, Vol.83,
No.254, p56-p63).

[0020]

## STR1 ## ^{_{O 2 (gass) + 4e -}} + 2Vo → 2O - O 2 (gass): oxygen gas e in the atmosphere ^-: electrons in the electrode Vo: oxygen vacancy O in Zirconia ^-: oxygen ions in the zirconia

Therefore, maintaining the sensor element for a long time in an atmosphere and temperature condition in which the sensor element is simply oxidized without applying a monitoring voltage to the electrode pair 12a, 12b only promotes oxygen supply to oxygen deficiency. , Not preferable. on the other hand,
Although data will be shown later, it has been confirmed that a life of three years or more can be stably obtained when the off time of the monitoring voltage is set to 60 minutes and the on time is set to 5 minutes. Therefore, the preferable on / off ratio of the monitoring voltage application is 1/12 ≦ on / off ratio ≦ 1
Becomes When the monitoring voltage off period exceeds 60 minutes, it is preferable to turn off the entire sensor, that is, turn off the heater. This is also effective for reducing power consumption.

[Embodiment 1] The oxygen sensor 10 shown in FIG. 1 was used in which the ion conductor 11 was ZrO ₂ -8 mol% Y ₂ O ₃ and the electrode 12 was used.
Platinum (Pt) for a and 12b, ZrO _{2-3 for} cap 13
It was prepared using mol% Y ₂ O ₃ , respectively. 10 of these
Zero oxygen sensors were used for 3 years by intermittently applying a monitoring voltage while maintaining the temperature at 450 ° C. in the atmosphere. The monitoring voltage Vs was 1.3 V, and the on / off ratio was 5 minutes / 5 minutes. After 3 years of use, it was confirmed that all 100 oxygen sensors showed a limiting current at a monitoring voltage of 1.3 V and had not reached the end of their lives.

Example 2 The same oxygen sensor as in Example 1 was tested under the same conditions as in Example 1 except that the on / off ratio was 5 minutes / 60 minutes. After three years of use, it was confirmed that all 100 oxygen sensors had not reached the end of their life.

Example 3 The same oxygen sensor as in Example 1 was tested under the same conditions as in Example 1 except that the holding temperature was 400 ° C. After three years of use, it was confirmed that all 100 oxygen sensors had not reached the end of their life. In addition,
As a result of performing the same test with the holding temperature at 350 ° C,
The life of 100 oxygen sensors was reached after 3000 hours of use. However, this shows that the preferable monitoring temperature of the ionic conductor 11 used in the examples is 400 ° C. or higher, and when the ionic conductor of another material is used,
It is fully conceivable that it will function well even at 350 ° C.

Comparative Example 1 With respect to the same oxygen sensor as in Example 1, the holding temperature was 400 ° C. and the monitoring voltage Vs = 1.
3V was continuously applied and used. Of 100 oxygen sensors,
6 defective after 1 year of use, 8 defective after 1 to 2 years, 2
~ 12 pieces became defective in 3 years. After 3 years of use, 74 out of 100 survived.

[Comparative Example 2] The same oxygen sensor as in Example 1 was used by intermittently driving it by turning on and off the heater voltage and the monitoring voltage at the same time. The holding temperature when the heater is turned on is 400 ° C., and the holding temperature when the heater is turned off is room temperature. The on / off ratio was 5 minutes / 5 minutes. Oxygen sensor 10
Out of 0 pieces, 2 pieces are defective after 1 to 2 years of use period, 4 after 2 to 3 years
The piece became defective. In addition, cracks were observed in the ion conductive plates of the two oxygen sensors after one year of use.
Therefore, after 3 years of use, 92 out of 100 survived.

[0027]

As described above, according to the present invention, the monitoring voltage is intermittently applied while the heater is always energized, thereby deteriorating the characteristics of the limiting current type oxygen sensor using the ion conductor. It is possible to suppress the oxygen concentration and stably and accurately measure the oxygen concentration for a long period of time.

[Brief description of drawings]

FIG. 1 shows a limiting current type oxygen sensor and its drive circuit according to an embodiment of the present invention.

FIG. 2 shows a switch control operation by the drive circuit of the embodiment.

FIG. 3 shows characteristics of a limiting current type oxygen sensor.

FIG. 4 shows changes with time in characteristics of a limiting current type oxygen sensor.

FIG. 5 shows how charges are accumulated inside the sensor element.

[Explanation of symbols]

10 ... Limiting current type oxygen sensor, 11 ... Ion conductor, 1
2a, 12b ... Electrode pair, 13 ... Cap, 14 ... Gas diffusion hole, 15 ... Heater, 20 ... Drive circuit, 21 ... Timer,
22 ... Inverter, SW1, SW2 ... Switch, E1,
E2 ... DC power supply.

Continuation of the front page (56) Reference JP-A-9-318593 (JP, A) JP-A-61-36016 (JP, A) JP-A-59-65755 (JP, A) JP-A-61-294352 (JP , A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/41

Claims (4)

(57) [Claims] 1. An ionic conductor made of a solid electrolyte, a porous electrode pair for applying an electric field provided in the ionic conductor, and a diffusion-controlled gas supplied to one surface of the electrode pair. A method for driving a limiting current type oxygen sensor having a gas diffusing means for controlling the ion conductor and a heater for heating the ion conductor, wherein the heater is constantly energized to heat the ion conductor.
A method of driving a limiting current type oxygen sensor, wherein a monitoring voltage is intermittently applied between the electrode pairs while maintaining the temperature at 400 ° C. or higher . 2. The method for driving a limiting current type oxygen sensor according to claim 1, wherein the ratio of the on time and the off time for applying the monitoring voltage is set in the range of 1/12 to 1. 3. An ion conductor made of a solid electrolyte, a porous electrode pair for applying an electric field provided in the ion conductor, and a diffusion-controlled gas is supplied to one surface of the electrode pair. Gas diffusing means, and constantly energizing to keep the ionic conductor at 400 ° C or higher
Heater and, limiting current type oxygen sensor device being characterized in that a driving means for applying intermittently monitored voltage between said electrode pairs being. 4. The limiting current type oxygen sensor device according to claim 3, wherein a ratio of an on-time and an off-time at which the monitoring voltage is applied by the driving means is set in a range of 1/12 to 1. .