JPS6273155A - Characteristic improving method of oxygen sensor - Google Patents

Abstract

PURPOSE:To improve response speed in a low temperature region, by exposing an oxygen sensor element to a gas stream, which has a partial vapor pressure of 0.005-1atm, at a temperature of 80-250 deg.C to perform steam treatment. CONSTITUTION:A reference electrode is provided at one surface of a solid state electrolyte layer comprising a fluoride ion conductor, and a detecting electrode is provided at the other surface thereof. Thus, an oxygen sensor element is formed. The oxygen sensor element is exposed to a gas stream, which has a partial pressure of steam at 0.005-1atm, at a temperature of 80-250 deg.C, and steam treatment is performed. For example, a mixture of Sn and SnF2 is fused and fixed to a solid state electrolyte layer comprising an LaF3 single crystal as the fluoride ion conductor, and a reference electrode is formed. Platinum is vapor deposited and a detecting electrode is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for improving the characteristics, particularly the response speed, of a low-temperature operating type oxygen sensor using a fluoride ion conductor as a solid electrolyte.

[Prior art]

Conventionally, as a solid electrolyte oxygen sensor, an oxygen sensor using stabilized turfium has already been put into practical use and is widely used for air/fuel efficiency control in automobiles, Zeilers, etc. However, oxygen sensors using stabilized norconia cannot be used as oxygen sensors in low-temperature ranges because the resistance of stabilized norconia becomes high at temperatures below 500° C., so the operating temperature is limited to 500° C. or higher. Currently, some sensors using liquid electrolyte η, such as Clark-type oxygen sensors, are used for low-temperature applications, but because they use a liquid electrolyte, they are disadvantageous in terms of maintenance.
It also has the disadvantage of being difficult to miniaturize.

For this reason, research into oxygen sensors using solid electrolytes has attracted attention in recent years, but an oxygen sensor that is sufficiently practical at low temperatures has not yet been developed.

The solid electrolyte for the oxygen sensor is SrCt2.

5b205'nH2O, LaF3. Pb5nF4. B
iF3. PbF2 and the like are being studied, and among these, fluoride ion conductors are seen as promising because they have relatively high conductivity even at low temperatures, along with 5b205nH20. However, oxygen sensors using fluoride ion conductors operate in relatively high temperature ranges of at least 100°C even at low temperatures.
It is necessary to set the operating temperature to a region exceeding 10
It has the disadvantage that the response speed is extremely slow at temperatures below 0°C or at room temperature. For example, a response time of 90 minutes or more is required for a change in oxygen partial pressure from 0.2 atm to 1.0 atm at room temperature.

[Problem that the invention seeks to solve]

In view of the current situation, the present invention provides an oxygen sensor using a fluoride ion conductor as a solid electrolyte in a low temperature range of 10 to 10%.
The present invention is intended to provide a method for improving response speed in the temperature range from below 0° C. to room temperature.

The oxygen sensor element of the present invention using the fluoride ion conductor in the solid solute layer can also be used as a sensor for dissolved U', but when used as a dissolved oxygen sensor element, the response speed is relatively low. There is no great need for immediate improvement.

[Means to solve all problems]

The great invention employs the following configurations to achieve the above intention. That is, the present invention is constructed by disposing a reference electrode on one side of a solid solute layer made of a fluoride ion conductor and a detection electrode on the other side. A method for improving the characteristics of an oxygen sensor element is characterized in that a non-sensor element is subjected to a water vapor treatment by exposing it to a gas stream having a water vapor partial pressure of 4 (0005 to 1 atm) at a temperature of 80 to 250°C.

The fluoride ion conductor is not particularly limited, but triLaF! is a typical example. ,,BiF6. Pb
F2 and PbSnF4. In the present invention, one or more single crystals, polycrystals, green compacts, sintered bodies thereof, or vaporized N films appropriately selected from these are used.

A reference electrode is arranged on one side of a solid direct solution layer made of a fluoride ion conductor, and a detection electrode is arranged on the other side, thereby constructing an oxygen sensor element. The reference electrode and the detection electrode are constructed by the same means as conventionally employed in the construction of gold oxygen sensors. That is, the reference electrode is usually made of a mixture of metal and metal fluoride, namely Sn and SnF 2 lAg (!
:AgF, Pb and PbF2. Bi and B I F s
A mixture of these is used as a material, and these are placed on one side of the solid electrolyte layer by pressure bonding, fusion bonding, vapor deposition, or other appropriate means, and contact with the outside air is avoided after the lead wire is taken out by an appropriate method. It is then sealed and used as a reference electrode. The sensing electrode is constructed by using a noble metal such as platinum or 2 latium as a powder material, and disposing it on the J11 surface in addition to the all-solid electrolyte layer by vapor deposition, pressure bonding, or other means.

The greatest feature of the present invention is that the oxygen sensor element constructed as described above can be heated to O, OO at temperatures of 80 to 250°C.
The entire steam treatment is carried out by exposure to a gas stream having a total steam partial pressure of 5 to 1 atm. The temperature of the steam treatment must be within the range of 80 to 250°C to cool the fruit, and preferably within the range of 100 to 200°C. Below 80°C, there is not much improvement in the response speed, and above 80°C, a dramatic improvement is seen, especially above 100°C. Furthermore, if the temperature exceeds 250°C, there is an adverse effect on the reference electrode, sealing agent, etc., and as a result, there is a tendency for the response speed to decrease or become saturated.

Further, 0.005 to 1 atm, more preferably Uo, 01 to
It is necessary to expose it to a gas stream having a water vapor partial pressure of Q, l atm. The type of atmospheric gas is not particularly limited, and usually air or an inert gas such as nitrogen or argon is used.

When the water vapor partial pressure is less than 0.005 atm, the effect of improving the response speed is significantly reduced, and when it exceeds 1 atm'i, the improving effect tends to be almost saturated.

A gas flow atmosphere that satisfies all of the above conditions of temperature range and water vapor partial pressure can be created, for example, by using a Raspin device containing water in a thermostat, and by flowing a constant flow rate of gas through it, the water vapor pressure is maintained at a constant level. Further, this gas can be obtained by such means as flowing this gas into a glass tube in which an oxygen gas sensor element is placed and heating it to a specific temperature in a partially electric furnace.

Although the time period for performing the steam treatment in such a gas flow atmosphere is not generally determined, it is desirable to perform the steam treatment sufficiently in order to improve the response speed.

Whether sufficient water vapor treatment has been carried out can be easily determined from the improvement effect on response speed. In general, the response speed improvement effect of steam treatment tends to reach saturation as the steam treatment time elapses, so it can often be said that steam treatment is sufficient if it is carried out for 3 to 24 hours, usually for about 5 to 15 hours.

Historically, the most preferred embodiment of the present invention is an embodiment in which aging treatment is performed subsequent to steam treatment. Here, the A-Song treatment refers to a means for smoothing charge transfer at the interface between the solid electrolyte and the electrode. By performing all of these aging processes, the degree of improvement in response speed becomes even more remarkable.

Typical examples of specific aging treatment methods include the following: (1) The ratio of 1 oxygen partial pressure is 2
This is a mode in which exposure is alternately made to an atmosphere with a high oxygen partial pressure and an atmosphere with a low V atmosphere. It is necessary that the ratio of oxygen partial pressures is 2 or more, preferably 3 or more, and if the partial pressure difference is less than this, the effect of aging treatment will not be seen much, and the oxygen partial pressure in an atmosphere with high oxygen partial pressure is If it is too small, the aging process will take a long time, so it is preferable that the oxygen partial pressure in an atmosphere with a high oxygen partial pressure is 0.01 atm or more. Inert gases such as nitrogen, argon, helium, etc. are representative examples, although they can be selected as appropriate.In addition, in the aging treatment of this embodiment, in order to obtain a sufficient aging effect, It is desirable to alternately expose the atmosphere to an atmosphere with a high oxygen partial pressure and an atmosphere with a low oxygen partial pressure at least 5 times, preferably 10 times or more.This number of times needs to be increased as the oxygen partial pressure ratio of both atmospheres becomes smaller, but for example, if the oxygen partial pressure is If the partial pressure is 1 atm and 0.2 atm, that is, the partial pressure ratio is 5, the exposure should be alternated about 5 to 30 times, that is, the atmosphere with high oxygen partial pressure and the atmosphere with low oxygen partial pressure VC5~
It is preferable to alternately expose about 30 times. Note that the lower the treatment temperature during the steam treatment, the greater the number of times during the aging treatment. The time for one exposure in each atmosphere is usually 5 to 60 minutes, and it is generally preferable to expose for a relatively long time in the early stages of aging treatment. Further, the oxygen partial pressure in the 5-300-degree atmosphere, where the oxygen partial pressure is low, may be zero or a value close to zero. For example, one suitable embodiment is to use air as the surrounding atmosphere and a nitrogen atmosphere as the low atmosphere.

21"i, this is a mode in which specie is applied from the outside to the variable @r. As a variable voltage, when a constant pressure is applied intermittently, or when the high and low voltages of the two heels are applied alternately, the inkstone is continuously applied. Examples include cases where a varying voltage is applied.In any of these cases, the difference between the highest and lowest voltage applied is 1.
It is desired that the voltage is 0 mV, preferably 20 mV or more.

Also, does the application time of variable voltage vary? 1 It is also related to the fluctuation period of construction pressure, but it is generally 30 minutes or more, and 1 usually 2 minutes.
It is preferable to spend about an hour or more.

[Effect]

Although it is not clear how the water vapor treatment and aging treatment of the present invention improve the response speed of the oxygen sensor element, the present inventor speculates as follows. That is, it is expected that oxy7 fluoride th hydroxyfluoride is generated on the surface of the fluoride ion conductor due to the water vapor treatment, and this causes the oxygen in the gas phase and the fluoride in the fluoride ion conductor to be generated. It is speculated that the response speed is improved because the electrical connection between the compound ions is good, and the aging treatment smoothes charge transfer at the interface between the electrolyte and the electrode.

[Effect] By implementing the present invention, the response speed of the oxygen sensor element is dramatically improved. The degree of improvement is determined by the fluoride ion sj
T, 'iβ body, reference electrode and sensing electrode materials, oxygen sensor element j;, ν formation method, steam treatment and aging treatment modes, and operating temperature, etc.
Generally speaking, if the water vapor treatment is applied by about 3 to 6 times, and if the entire aging treatment is performed, then the effect of improving the response speed by about 0 to 40 times and shortening the hitting response time can be seen. - To give an example, a mixed exchange of Sn and SnF 2 was deposited on a solid electrolyte layer made of LaF 3 single crystal gold as a fluoride ion conductor to form a reference electrode, and platinum was fully vapor-deposited to form a sensing electrode. The oxygen sensor element has a temperature of 0,
The response time for a change in acid partial pressure from 2 atm to 10 atm requires 9() minutes, but by carrying out the steam treatment and aging treatment of the present invention, the response time can be reduced to within 5 minutes. It has improved until now. Hereinafter, specific effects of the present invention will be illustrated by giving examples and comparison examples of the present invention.

Prison Case 1 and Comparative Example 1 A sensor element using LaFs as a solid electrolyte was prepared by melting a mixture of Sn and SnF2 as a reference electrode on one side of an 11-sided LaFs single crystal (1× thick). After the lead wires were removed, they were coated with epoxy resin to avoid contact with the outside air, platinum was deposited on the other side as a sensing electrode, and the lead wires were taken out. In addition, a sensor element using BIF3 as a solid electrolyte with a diameter of I
Using a CrIL tablet press, Bi metal powder, BiF
At the end of the third round, PT black powder was layered in order and pressure molded into a tablet Bi core (reference Δ), PT Tunxin (S Zhige).
After removing the lead wires from each, the old electrodes were coated with epoxy resin to prevent frequent contact with the outside air. Furthermore, a sensor element using PbF2 as a solid polar material was fabricated on an alumina substrate with PB (reference electrode), Pb
It was constructed by sequentially stacking F2 (solid electrolyte) and Pt (sensing electrode) by vapor deposition.

Each of the three sensor elements obtained by the above method was fixed in a reaction tube installed in an electric furnace for steam treatment. On the other hand, place a glass bottle filled with water in a thermostatic chamber and add 100 to 1011 Ll/ml as atmospheric gas.
Dry air was passed through the dry air at a flow rate of n, and was passed through a glass tube wrapped around a suction heater to prevent dew condensation, and then flowed into the reaction tube in the electric furnace. Further, the temperature of the constant temperature bath was adjusted to adjust the water vapor partial pressure to 0.05 atm IcA.

The temperature of the steam treatment was measured using a thermoelectric lamp placed together with a sensor element in the reaction tube, and the steam treatment was carried out at 80° C. for 12 hours. In a similar manner, 60,100
, 150, 200. Three new elements at each temperature of 250 and 300C? After steam treatment, 7 elements of class 341 were obtained.

Next, one of the above sensor elements was fixed in a glass tube installed in an electric furnace, the two lead wires were taken out of the electric furnace, and the electromotive force between the two poles was measured using an electrometer and recorded. After adjusting the temperature, first, air was flowed through the glass tube at room temperature with a flow rate of 100 m, 137 m, and 1 nO, and when the electromotive force became stable, the test gas was switched to pure acid. The change in electromotive force was recorded, and the measurement was stopped when the change in electromotive force reached almost saturation. Find the value of the difference between the electromotive force 1-α from when the electromotive force starts to change until it stops changing, and the value becomes 90% of the value of the difference in electromotive force from when the electromotive force starts to change. The time required for each test was measured sequentially with the time taken as 90% response time.

However, devices using PbF2 could not operate stably at room temperature.

Next, similar measurements were carried out using the same device at an operating temperature of 100°C.

Similar measurements were also performed on elements that were not subjected to water vapor treatment.

These results are shown in Table 1. Note that A1.7 and 8 are comparative examples.

Example 2 The same procedure as in Example 1 was carried out except that the treatment temperature during steam treatment was set at 100,150°C, and the steam partial pressure was set at 0.005, 0.01, 0.05, 0.5, 1, and Qatm. The 90-chi response times of three types of sensor elements were determined to be 6111.

The results are shown in Table 2.

In addition, water vapor partial pressure 1. When using Qatm, boiling steam was sent instead of air. Comparative Example 2 The same procedure as Example 2 was carried out except that when the treatment temperature during steam treatment was 100°C, the steam partial pressure was reduced to almost zero and dry air was flowed. The 90-chi response time of the device was measured.

In addition, in the case of a treatment temperature of 150°C during steam treatment,
Measurement was carried out in the same manner as in Example 2, except that the device was treated in an autoclave at 1500° and 2 atm for 12 hours.

The results are shown in Table 3.

Example 3 Regarding a sensor element using LaF s as a solid electrolyte, the 90-chi response time was measured in the same manner as in Example 1, except that the water vapor treatment time was changed to 3.6 hours and 24 hours.

The results are shown in Table 4.

Comparative Example 3 The 90% response time was measured in the same manner as in Comparative Example 1, except that the water vapor treatment time was changed to 3, 6, and 24 hours for a sensor element using L a Fs as the solid crude solution.

The results are shown in Table 5.

Example 4 The 90% response time of a sensor element using L a F 5 as the solid solute was measured in the same manner as in Example 2, except that the steam treatment time was 3.6 and 24 hours.

The results are shown in Table 6.

Comparative Example 4 The 90-hour response time of a sensor element using LaFs as the solid electrolyte was measured in the same manner as Comparative Example 2, except that the steam treatment time was 3, 6, and 24 hours.

The results are shown in Table 7.

Example 5 Example 1 except that the water vapor treatment temperature was 150° C. for the sensor element using LaF s as the solid electrolyte.
For the elements treated with water vapor under the same conditions as above, as an aging treatment, air and pure oxygen were alternately flowed 10 times each until the change in electromotive force was saturated. Regarding the sensor element after this aging treatment, the response time was measured in the same manner as in Example 1, with the operating temperature being room temperature and 100°C. The results show that the 90% response time is 2 at room temperature.
The temperature was 1 minute and 20 seconds at 100°C.

Example 6 Regarding a sensor element using LaFs as a solid electrolyte, the response time was measured in the same manner as in Example 5 except that pure nitrogen was used instead of pure oxygen as the gas during the aging treatment. Ta. The results showed that the 90% response time was 2 minutes and 30 seconds at room temperature and 1 minute and 15 seconds at IOO°C.

Example 7 A sensor element using LaFs as a solid electrolyte was subjected to aging treatment at an amplitude of 40 mV and a period of 2.
The response time was measured in the same manner as in Example 5 except that a 0 minute square wave was applied to both poles of the sensor element for 10 hours.The results showed that the 90% response time was 3 minutes at room temperature and 100 degrees Celsius.
It took 1 minute and 40 seconds.

Claims (7)

[Claims] (1) An oxygen sensor element composed of a reference electrode on one side of a solid electrolyte layer made of a fluoride ion conductor and a detection electrode on the other side was heated at a temperature of 80 to 250°C for 0.
A method for improving the characteristics of an oxygen sensor element, characterized by performing water vapor treatment by exposing it to a gas stream having a water vapor partial pressure of 0.005 to 1 atm. (2) The fluoride ion conductor is LaF_3, BiF_
3. The method for improving the characteristics of an oxygen sensor element according to claim 1, which is one or more single crystals, polycrystals, green compacts, or sintered bodies or vapor deposited films thereof selected from PbF_2 and PbSnF_4. (3) An oxygen sensor element composed of a reference electrode on one side of a solid electrolyte layer made of a fluoride ion conductor and a detection electrode on the other side was heated at a temperature of 80 to 250°C at 0.
A method for improving the characteristics of an oxygen sensor element, comprising performing a water vapor treatment by exposing it to a gas stream having a water vapor partial pressure of 0.005 to 1 atm, and then performing an aging treatment. (4) A method for improving the characteristics of an oxygen sensor element according to claim 3, wherein the aging treatment is performed by alternately exposing the oxygen sensor element to an atmosphere with a high oxygen partial pressure and an atmosphere with a low oxygen partial pressure in which the ratio of oxygen partial pressures is 2 or more. (5) A method for improving the characteristics of an oxygen sensor element according to claim 3, wherein the aging treatment is performed by exposing the element to a pure nitrogen atmosphere and to air alternately. (6) A method for improving the characteristics of an oxygen sensor element according to claim 3, wherein a varying voltage is applied externally as the aging treatment. (7) The fluoride ion conductor is LaF_3, BiF_
3. The method for improving the characteristics of an oxygen sensor element according to claim 1, which is one or more single crystals, polycrystals, green compacts, or sintered bodies or vapor deposited films thereof selected from PbF_2 and PbSnF_4.