JP3496869B2 - Method for preventing sensitivity drop of solid electrolyte type carbon dioxide gas sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a solid electrolyte type carbon dioxide sensor technology.

[0002]

2. Description of the Related Art FIG. 1 shows a model cross-sectional view of an example of a solid electrolyte type carbon dioxide gas sensor element. Reference numeral 1 in the figure denotes a solid electrolyte, which is usually NASICON (Na _{1 + x} Z
r ₂ P _{3− x} Si _x O ₁₂ , where x is 1 or 2) or the like is used. A detection electrode 2, a gold electrode covered with a metal carbonate such as lithium carbonate, and a reference electrode 3 made of platinum are arranged with the solid electrolyte 1 sandwiched therebetween, and a porous ceramic layer 4 is formed as a whole.
The structure is covered by the gas filter 5 via. The reference electrode 3 is in contact with a porous ceramic substrate 7 having a heater 6, and the heater 6 keeps the solid electrolyte 1 at a temperature suitable for its ion conduction (usually 350 ° C. or higher).

Here, an electromotive force is generated between the detection electrode 2 and the reference electrode 3 depending on the concentration of carbon dioxide gas in the atmosphere in which the sensor element is placed. That is, the carbon dioxide gas concentration in the atmosphere can be known by measuring this electromotive force. Generally, the metal carbonate is lithium carbonate having relatively good water resistance, or lithium carbonate as a main component, and barium carbonate, calcium carbonate, and strontium carbonate are added to further improve water resistance. This mixture is used by fixing this metal carbonate on the NASICON surface by fusion or the like and heating it to 350 to 500 ° C. by the heater 6.

The melting temperature of lithium carbonate is around 710 ° C., and the melting temperature of a mixture of lithium carbonate and barium carbonate, calcium carbonate, strontium carbonate, etc. is 650.
Although the temperature is around ℃, eutectic occurs between lithium carbonate and NASICON at the actual operating temperature, and as confirmed by instrumental analysis, it melts at 550 ℃ or less due to the diffusion of lithium into NASICON. Furthermore, it is further reduced by impurities other than these carbonates, and it is necessary that the temperature be 400 ° C. or lower for long-term use, and it is actually desired that the temperature be 350 ° C. or lower.

FIG. 2 shows the results of examining the time-dependent characteristics of a solid electrolyte type carbon dioxide gas sensor element at 350 ° C., 400 ° C. and 450 ° C. EMF in the figure
Represents the sensor electromotive force, and the initial electromotive force is zero.
The metal salt used in this sensor is an equimolar mixture of lithium carbonate and calcium carbonate. According to FIG. 2, in this solid electrolyte type carbon dioxide gas sensor element,
Use at 340 ° C to 400 ° C, preferably 340 ° C to 3
It turns out that it is desirable to use at around 50 ° C. If the temperature is lower than 340 ° C., the ionic conductivity due to NASICON becomes low, which is not practical. The solid electrolyte type carbon dioxide gas sensor element is a thermal concentration battery, and the sensor output follows the Nernst equation (equation (I)).

[0006]

[Number 1] EMF = Const + (RT / 2F ) lnP (CO 2) (I)

In the formula, Const is the activity of lithium carbonate, and P (CO ₂ ) is the partial pressure of carbon dioxide in the atmosphere to be measured. Here, a practical sensor output can be obtained even at 350 ° C, which can be said to be the practically lowest usable temperature of NASICON, but the lowest usable temperature varies depending on the sensor. The amount is decreased, and the initial performance cannot be maintained due to the influence of water and the like even at the reference electrode, and as a result, the reduction in output becomes noticeable as shown in FIG.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to improve the above-mentioned conventional problems, that is, to provide an effective method for preventing a decrease in sensitivity of a solid electrolyte type carbon dioxide gas sensor.

[0009]

In order to solve the above-mentioned problems, the method for preventing a decrease in the sensitivity of a solid electrolyte type carbon dioxide gas sensor according to the present invention solves the above problems.
The heat-up process is performed at a temperature of 40 ° C. to 400 ° C. and a sensor temperature of 450 ° C. or higher and 550 ° C. or lower for 2 minutes or longer and 3 minutes or shorter on a regular basis.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION By the heat-up treatment which is the constitution of the present invention, substances etc. generated at the interface of each electrode at the time of measurement are rapidly decomposed to restore the initial performance, and the state is maintained for a long period of time. Can be maintained.

In the above, the heat-up treatment time depends on the heat-up temperature, but it is desirable that the heat-up treatment time is 10 minutes or less. Usually, the heat-up treatment is performed for 2 minutes to obtain a sufficient effect. Here, the heat-up treatment is regularly performed, so that the range of sensitivity decrease can be kept within a certain value. Become. Furthermore, it is preferable that the heat-up treatment time is 2 minutes or more and 3 minutes or less and the temperature is 450 ° C. or more and 550 ° C. or less because the sensor is less deteriorated and a sufficient effect can be obtained. Further, if the interval between the heat-up treatment and the next heat-up treatment is 1 day or longer and 1 week or shorter, it is possible to maintain the performance for an extremely long period of time. That is, if the frequency of heat-up is high, it will lead to melting of the carbonate, and if it is low, it will be difficult to maintain the performance.

The heating temperature can basically be determined very easily from the calculation of static resistance and dynamic resistance of the heater. In addition, when the heat-up is not performed, the sensor temperature can be used at the lowest usable temperature of 340 to 350 ° C. As a result, it is possible to stop the progress of lithium ion depletion, and to achieve a long sensor life. You can FIG. 3 shows a model of sensor output (output value to the atmosphere in this case) when heating is performed periodically. This is an example of intentionally narrowing the heat-up interval. The sensor output becomes high during the heat-up, and it takes several minutes for the output to stabilize after the end of the heat-up. However, since the frequency of the heat-up process is low in practice, there is no problem.

[0013]

EXAMPLES The method for preventing a decrease in sensitivity of the solid electrolyte type carbon dioxide gas sensor of the present invention will be specifically described below.
A solid electrolyte type carbon dioxide gas sensor element having a solid electrolyte composed of a sensing electrode and NASICON covered with an equimolar mixed metal carbonate of lithium carbonate and calcium carbonate and having a structure shown in FIG. went.

The acceleration experiment was conducted to examine the optimization of the sensor temperature under normal conditions, that is, when the heat-up process is not performed. 20 when the sensitivity to ordinary air (containing 360 ppm of carbon dioxide gas) is 0 mV
While detecting ΔEMF which is the sensitivity to air having carbon dioxide gas of 00 pmm, heat-up treatment (520 ° C.) for 2 minutes every 10 minutes and normal temperature of 340
The measurements were performed on the sensors at temperatures of 360 ° C., 380 ° C. At this time, the influence of the number of heat-up treatments on the output when the initial ΔEMF is set to 0 mV is shown in FIG. According to FIG. 4, the normal temperature of the sensor is set to 340 ° C., and heat-up treatment is performed at 520 ° C. for 2 minutes.
It can be seen that ΔEMF maintains the initial performance even after the 0-time treatment. In addition, this heat-up treatment 2000 times means
It is considered that the heat-up treatment is performed once a day, which corresponds to 2000 days (more than 5 years). This satisfies the demand for the life of the carbon dioxide gas sensor which is generally required.

As described above, the sensor (symbol: ●) subjected to the heat-up treatment (520 ° C.) once a day for 2 minutes at the normal temperature of 340 ° C. and the heat-up treatment performed at the temperature of 340 ° C. FIG. 5 shows the changes with time of ΔEMF of each sensor (symbol: ◯). It can be seen from FIG. 5 that ΔEMF can be stabilized for a long period of time by the effect of the periodic heat-up process for a short time.

The solid electrolyte type carbon dioxide gas sensor element used in the present invention uses a metal carbonate such as lithium carbonate. Therefore, if the sensor element temperature is kept at room temperature in a normal environment without using it for a long time, these metal Carbonate absorbs moisture, and as a result, the output value is not stable after the start of use, and the desired set level (normal atmosphere (carbon dioxide 300 to 400 ppm
It may take a long time (from several hours to several days) to reach the output value for (containing). Even after the use was started, the same was true when the use was suspended for a while and then resumed. This means that it takes a very long time until the operation can be confirmed after the sensor is started, and the reduction of the time is excessive. Here, in order to quickly reach the set level of the atmospheric EMF value, immediately after the start of use or the restart of use (in the present invention, including both of them, “immediately after start of use”)
It is desirable to perform a heat-up process (herein, the heat-up process is referred to as an “initial heat-up process” to distinguish it from the above-mentioned heat-up process).

Here, the effect of the initial heat-up process will be described with reference to FIG. When a new sensor element is used, its output (EMF) does not reach the set level even after 40 minutes due to the time required for stability as shown by the chain line. Immediately after the start of use, the initial heat-up treatment (550 ° C., 2 minutes) was repeated 4 times at 3 minute intervals (FIG. 6).
(Corresponding to the portion where the EMF value is significantly increased in the chart with the initial heat-up treatment indicated by the solid line).
As a result, the sensor element output could reach the set level in less than 20 minutes.

Since this initial heat-up process is performed only immediately after the start of use, the number of times is small and the influence on the sensor life is small, and the temperature of the initial heat-up process is large with respect to the time required to reach the set level. Considering that it has an effect and that the sensor element output should reach the set level as soon as possible after the start of use, it should be performed at a higher temperature (usually 50 ° C. or higher) than the above normal heat-up treatment. Is preferred.
Also, the initial heat-up process as shown in FIG.
It can be performed using a microprocessor, and can also be easily implemented by combining a timer and a comparator. As described above, after the atmospheric EMF of the sensor reaches the set level, the regular heat-up process is periodically performed, so that stable measurement can be performed.

[0019]

According to the method for preventing a decrease in sensitivity of the solid electrolyte type carbon dioxide gas sensor of the present invention, the sensor-based EMF is used.
Is a method for preventing the deterioration of the sensitivity of the solid electrolyte type carbon dioxide gas sensor, which is capable of slowing down the deterioration of the sensor and extremely extending the life of the sensor. In addition, it is possible to quickly measure a sensor that is left unused for a long period of time. Further, by regularly performing heat-up, it is possible to keep the decrease in sensitivity within a certain range, but at this time, a microprocessor or the like is not always necessary, and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a model cross-sectional view of an example of a solid electrolyte type carbon dioxide gas sensor element.

FIG. 2 is a diagram showing the results of examining the characteristics over time at 350 ° C., 400 ° C. and 450 ° C. in a certain solid electrolyte type carbon dioxide gas sensor element.

FIG. 3 is a diagram showing a heat-up time chart together with a heat-up time chart when two short-time heat-up processes are performed while detecting 2000 pmmΔEMF when the sensitivity to normal air is 0 mV.

FIG. 4 shows a change in atmospheric EMF of a solid electrolyte type carbon dioxide gas sensor when heat-up is periodically performed.

FIG. 5 is a diagram showing a quick baseline return effect by a heat-up process in a solid electrolyte type carbon dioxide gas sensor left unused for a long period of time.

FIG. 6 is a diagram showing an effect of an initial heat-up process.

[Explanation of symbols]

1 Solid electrolyte 2 detection poles 3 reference pole 4 Porous ceramic 5 gas filters 6 heater 7 substrate

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-96852 (JP, A) JP-A-9-145666 (JP, A) JP-A-9-329559 (JP, A) JP-A-8- 189915 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/416 G01N 27/406

Claims (3)

(57) [Claims] 1. The temperature at the time of use is 340 ° C. to 400 ° C.
In addition, the sensor temperature should be 450 ° C or higher and 550 ° C or lower.
A method for preventing a decrease in sensitivity of a solid electrolyte type carbon dioxide gas sensor , which comprises periodically performing a heat-up treatment for 2 minutes or more and 3 minutes or less . 2. The method for preventing sensitivity deterioration of a solid electrolyte type carbon dioxide gas sensor according to claim 1, wherein the interval between the heat-up treatment and the next heat-up treatment is 1 day or longer and 1 week or shorter. 3. The method according to claim 1, wherein the initial heat-up process is performed immediately after starting the use of the sensor, the measurement is started after the sensor output reaches a set level, and then the heat-up process is performed for a short time. Item 3. A method for preventing a decrease in sensitivity of a solid electrolyte type carbon dioxide gas sensor according to Item 2.