KR100414195B1 - SOLID ELECTROLYTE-TYPE SOx SENSOR USING OXIDE REFERENCE ELECTRODE - Google Patents

Abstract

The present invention relates to a SO _x gas sensor using a solid electrolyte, comprising a solid electrolyte, a sensing electrode using a sulfate, and a reference electrode using a Na-Ti-O-based oxide two-phase mixture. It is about. Conventional solid electrolyte type SO _x gas sensors generally have problems such as unstable electromotive force, unsuitable for continuous measurement, or inability to exhibit stable sensitivity at high temperature. However, the SO _x gas sensor according to the present invention is glassy. By including the protective film, the reaction is suppressed so that Na ₂ SO ₄ is not produced around the reference electrode, thereby providing the SO _x gas sensor with stabilized electromotive force. In addition, the sensor manufactured by the method of the present invention not only has excellent reproducibility, but also exhibits stable sensitivity even at high temperatures.

Description

SOLID ELECTROLYTE-TYPE SOSOR USING OXIDE REFERENCE ELECTRODE

The present invention relates to a SO _x gas sensor. More particularly having a reference electrode as SO _x gas sensor using a solid electrolyte, and using the sulphate sensing electrode, with the oxide two-phase mixture of type Na-Ti-O, relates to a gas sensor composed of a glassy protective film.

SO _2, SO ₃ SO _x, such as is a typical toxic gas generated by an automobile exhaust pipe, the boiler, as well as one cause of the air pollution, it can cause acid rain detection of SO _x gas is the most important element for the measurement of air pollution Known as one of the

Conventionally, methods for measuring SO _x include 1) collecting a test gas into an absorbent liquid and measuring chemical analysis, absorbance measurement and electrical conductivity of the collected liquid, and 2) directly quantifying the test gas with an infrared spectrometer. It was common. However, since method 1) uses a collection liquid, it is difficult to operate and handle, and continuous measurement is difficult. In addition, method 2) is not only expensive to use an infrared spectrometer, but also has a large amount of infrared absorption of CO ₂ . _{2 had} to be removed, and continuous measurement was impossible.

In contrast, the solid electrolyte SO _x gas sensor measures the amount of SO _x gas from the electromotive force generated by contacting the sensing electrode with SO _x . The simple principle of the sensing electrode-solid electrolyte-reference electrode is shown in FIG. As a result, it is possible not only to manufacture a small element type sensor, but also to increase gas selectivity by using a sensing electrode that selectively detects a specific gas, and to quantitatively measure gas concentration.

The use of such a solid electrolyte solves the problems of the conventional SO _x gas measurement method to some extent, but such a solid electrolyte sensor is generally unstable due to unstable electromotive force, or after a contact with the SO _x sensor. The response time was too long to make continuous measurement difficult or to exhibit stable sensitivity at high temperatures. The cause of unstable electromotive force, which is pointed out as the main problem, is generally recognized because pyrosulfate and the like are generated near the interface of the electrode.

As a way to overcome the disadvantages of the solid electrolyte type SO _x sensor as described above, efforts such as stabilizing electromotive force by preventing gas diffusion between the reference electrode and the sensing electrode using a test tube-type ZrO ₂ tube (Japanese Patent Laid-Open No. 6-174692) There was this. However, by solving the various problems of the SO _x sensor as described above, while having the stability of electromotive force, the reproducibility of the measured value, the stability at high temperature, the possibility of continuous measurement, etc., the structure is simple, and the SO _x gas is easy to manufacture and handle. Sensors have not yet been developed, and coupled with the environmental issues of interest, there is an increasing demand for the development of practical SO _x gas sensors.

The present inventors conducted a study to develop a new SO _x sensor that can solve the problems of the conventional solid electrolyte SO _x sensor as described above, as a result, a sulfate compound as a sensing electrode, Na-Ti-O as a reference electrode By using an oxide two-phase mixture of the system and forming a glassy protective film on the surface of the solid electrolyte around the reference electrode to stabilize the electromotive force, to develop a SO _x sensor excellent in the reproducibility of the SO _x measurement, to complete the present invention Reached.

1 is a schematic cross-sectional view showing the structure of a solid electrolyte type SO _x gas sensor using an oxide reference electrode according to the present invention.

Figure 2 is a graph comparing the response characteristics for SO _x of the sensor with a glass protective film and the sensor without a glass protective film around the oxide reference electrode according to the present invention.

3 is a graph showing the response characteristics of the sensor according to the concentration change of SO _x in the sensor with a glass protective film according to the present invention.

4 is at each temperature between 500~700 ℃, a graph showing the relationship between the output voltage of the SO _x gas concentration sensor according to the present invention, the SO _x.

SO _x sensor using a solid electrolyte according to the present invention comprises a solid electrolyte made of a conductor, is attached at one side of the solid electrolyte is attached to the other surface of the sensing electrode, the solid electrolyte consisting of a sulfate compound, Na- And a glassy protective film coated on the surface of the solid electrolyte around the reference electrode.

The structure and operation principle of the SO _x detection sensor of the present invention will be described in detail.

A. Structure of Gas Sensor

First, a solid electrolyte sensor for detecting SO _x is manufactured with the following basic structure.

1st lead wire / sensing electrode / solid electrolyte / oxide reference electrode / 2nd lead wire

(However, the first and second lead wires: platinum paste or platinum, sensing electrode: sulfate compound, solid electrolyte: Na + ion conductor (NASICON), reference electrode: two-phase mixture of Na-Ti-O-based compound.)

Corresponding to the above basic structure, the solid electrolyte sensor according to the present invention is manufactured, for example, in the structure shown in Scheme 1 below.

Scheme 1

The right part of Scheme 1 is an oxide reference electrode in which only oxygen is involved in the reaction, and the left part is used as a sensing electrode for detecting sulfur oxides represented by SO _x .

In the present invention, as a solid electrolyte, an alkali metal sulfate, a complex compound of B ₂ O ₃ -Li ₂ SO ₄ -Al ₂ O ₃ system and Na ₂ OB ₂ O ₃ -Na ₂ SO ₄ -Al ₂ O ₃ system, Na-β-alumina, NASICON, etc. can be used, and using NASICON is preferable because it can lower the sintering temperature and is advantageous in terms of conductivity.

As the sulfate compound of the sensing electrode, K ₂ SO ₄ , Na ₂ SO ₄ , Li ₂ SO ₄ , etc. may be used according to the metal ions of the solid electrolyte and the oxide reference electrode. As a preferable combination, a NASICON electrolyte and a Na— In the case of employing a Ti-O-based oxide reference electrode, it is preferable to use Na ₂ SO ₄ .

In addition, as the oxide reference electrode, Na-Ti-O type such as Na ₂ Ti ₃ O ₇ -Na ₂ Ti ₆ O ₁₃ , Na ₂ Ti ₆ O ₁₃ -TiO _2, Na ₂ TiO ₃ -Na ₂ Ti ₃ O _7, and the like. Preference is given to using one of the oxide biphasic mixtures of Na ₂ Ti ₃ O ₇ -Na ₂ Ti ₆ O ₁₃ .

B. How it Works

The half cell reaction of the oxide reference electrode is a reaction in which oxygen relates, for example, as shown in Scheme 2 below.

Scheme 2

In the sensing electrode,

Scheme 3

As such, a reaction involving both SO ₃ and O ₂ occurs.

Therefore, the two half-cell reactions achieve the overall reaction as shown in Equation 4 below.

Scheme 4

By this reaction, electromotive force of constant magnitude is generated at the both ends of the sensor according to the concentration of SO _3. The magnitude of the output voltage (E) is determined according to the following Equation 5.

Scheme 5

Output voltage:

(E: gas sensor output voltage, E °: reference voltage (constant), R: gas constant, F: Faraday constant, T: temperature, Pso ₃ : SO ₃ partial pressure).

In the atmosphere, most of SO _x exists in the form of SO _{2. At} high temperature (about 500 ° C.), SO ₃ is generated by reaction equilibrium between gas components as shown in Equation 6 below.

Scheme 6

The ratio between the gas components in thermodynamic equilibrium is determined by the following equation:

Scheme 7

Here, K _T is a constant determined by temperature, and using this relation, the total concentration of SO _x obtained by adding SO ₂ and SO ₃ can be obtained.

Scheme 8

Substituting this into an equation representing the electromotive force of the sensor can be expressed as follows.

Scheme 9

Output voltage:

(E: gas sensor output voltage, Eo: reference voltage (constant), R: gas constant, F: Faraday constant, T: temperature, Po ₂ : O ₂ partial pressure, Pso ₃ : SO ₃ partial pressure, Pso ₂ , _total : SO ₂ partial pressure at room temperature).

According to this equation, if the oxygen partial pressure is known, the concentration of SO _x can be measured through the electromotive force formed across the sensor.

Next, the role of the glassy protective film used in the present invention will be described in detail. SO _x present in air may react with Na ₂ O in the electrolyte to generate Na ₂ SO ₄ , as shown in Scheme 10 below.

Scheme 10

(Where R is gas constant, T is temperature, ΔG ° is Gibbs free energy, p so ₃ : SO ₃ partial pressure, a Na ₂ O: Na ₂ O activity, a Na ₂ SO ₄ : Na ₂ SO ₄ Activity).

Where K is the equilibrium constant of the above reaction and is a value dependent only on temperature. That is, when the above reaction time to achieve the balance, Na ₂ O and SO ₃ activity of the will have a product of a constant value (in the case of SO ₃ is the same as the concentration in the atmosphere), out of it has to achieve a balance The reaction proceeds in either direction.

The direction of the reaction there is thermodynamically determined by a degree of Na ₂ O and SO ₃ activity, the Na + reported in the literature - when calculated using the Na ₂ O activity in the electrolyte (NASICON), the concentration in air of SO ₃ It was found that the reaction equilibrium proceeds to the right even in very small cases (<1 ppm). This means that even in the presence of trace amounts of SO _x (SO ₂ , SO ₃ ) in the atmosphere, Na ₂ SO ₄ is spontaneously produced on the surface of the electrolyte.

The structure of the sensor for sensing SO _x according to the present invention has a solid electrolyte interposed therebetween, and one side is coated with a sulfate compound used as a sensing electrode, and the other side is used as a reference electrode. It consists of having a two-phase mixture of Na-Ti-O-based compound capable of fixing the activity of Na ₂ O. The two electrodes must be separated from each other by a solid electrolyte. If Na ₂ SO ₄ is spontaneously produced on the surface of the solid electrolyte, the two electrodes are electrically short-circuited.

The case of the upper part of the graph of FIG. 2 corresponds to this, and it can be seen that the signal of the sensor continues to converge to 0 even if the concentration of SO _{x in} the air is changed. In other words, the function as a sensor is lost.

In order to maintain the sensor without losing the characteristics of the sensor, it is essential to suppress the reaction so that Na ₂ SO ₄ is not formed around the oxide used as the reference electrode. In the present invention, the solid electrolyte around the reference electrode of the configured sensor By coating glass as a protective film on the surface, it was possible to effectively prevent the reaction that the surrounding SO _x and the electrolyte reacts to form Na ₂ SO ₄ .

In the present invention, the protective film is not directly related, so long as to inhibit contact of the electrolyte with the SO _x that can effectively prevent the generation of Na ₂ SO _4, the type of the protective film is limited and the reaction is able to detect the SO _x There is no Therefore, in order to form a protective film, a method of coating a glass of various compositions on the surface of the electrolyte through a heat treatment at a high temperature, or sputtering a substance (Al ₂ O _3, etc.) that does not react with each component of the sensor A well-known method, such as a method of coating using a technique, can be used.

Platinum paste and platinum wire were used as lead wires for reading out the sensor signal.

The form of the SO _x sensor of the present invention is not particularly limited, but is preferably in the form of a disk or an element.

Hereinafter, the present invention will be described in detail with reference to the following examples. However, in the SO _x sensor and its manufacturing method according to the present invention, the following embodiments are merely exemplary, and various modifications, changes, additions, etc. will be possible to those skilled in the art within the spirit and scope of the present invention. Etc. shall be regarded as belonging to the following claims.

EXAMPLE

Based on the accompanying drawings, the structure of the solid electrolyte type SO _x sensor using the reference electrode according to the present invention, a manufacturing method thereof and the sensor's response characteristics are as follows.

SO _x Structure of Gas Sensor

1 is a schematic cross-sectional view of a solid electrolyte type SO _x gas sensor including a sensing electrode made of sulfate, NASICON as a solid electrolyte, and a Na-Ti-O based two-phase mixed oxide reference electrode.

Among the three components of the sensor (solid electrolyte, sensing electrode, reference electrode), the sensing electrode is a mixture of platinum paste mixed with Na ₂ SO ₄ , and as the reference electrode, the platinum paste is fixed on a Na-Ti-O-based two-phase mixture. A mixture mixed in proportion was employed, and was attached to each opposite surface of NASICON used as a solid electrolyte. That is, as shown in Figure 1, the solid electrolyte type SO _x gas sensor according to the present invention is attached to the disk-shaped thin film sensing electrode (2) made of sulfate on one side of the disk-shaped solid electrolyte (3) made of NASICON On the other side, a disk-shaped reference electrode 5 made of Na-Ti-O-based two-phase mixed oxide is attached, and a glass-like film (4) is formed on the surface of the solid electrolyte around the reference electrode 5 of the configured sensor. It has a structure coated with glass.

Manufacturing method of gas sensor

1) Preparation of Reference Electrode

Na ₂ Ti ₃ O ₇ -Na ₂ Ti ₆ O ₁₃ and platinum paste as a two-phase mixture of Na-Ti-O compounds on one side of a disk-shaped electrolyte (NASICON) 6: 4 (volume ratio) After mixing well and thinly applying the ratio of), the reference electrode was attached by heat treatment at about 1000 ° C. for about 2 hours.

2) Formation of protective film

As described above, the glass was coated around the reference electrode to form a protective film. In order to form a protective film, borosilicate glass powder was applied to an organic solvent and evenly applied onto the surface of the solid electrolyte around the reference electrode material. It was. At this time, care should be taken not to expose the electrolyte between the glass protective film and the reference electrode material.

3) Preparation of sensing electrode

Na ₂ SO ₄ was used as a material to detect SO _x . As with the reference electrode, platinum paste and Na ₂ SO ₄ powder were mixed well at a ratio of 4: 6 (volume ratio), and then It was applied to the opposite side of the electrode and heat treated at about 850 ° C. for 30 minutes. In order to improve the sensitivity of the sensor by smoothly entering and exiting the gas, heat treatment was performed at the melting point of Na ₂ SO ₄ or lower to form a sensing electrode having a porous structure. do.

Response characteristics of gas sensor

FIG. 2 is a graph comparing the response characteristics to SO _x of a sensor having a glassy protective film around a reference electrode and a sensor without a glassy protective film.

First, in the case of the sensor cut off using glass to prevent the electrolyte surface from freely contacting the gas, it can be seen that a rapid signal change occurs each time the concentration of SO ₂ is changed and reaches a steady state. On the other hand, in the case of a sensor designed to freely contact the surrounding gas and electrolyte without using glass, the value changes at the moment of changing the concentration of SO ₂ , but gradually approaches zero. The phenomenon of convergence by value can be observed.

This means that when both ends of the electrolyte of the sensor are exposed to the surrounding gas, the sensor loses its characteristics, namely, sensitivity over time. It can be seen that the protective film made of glass effectively prevents this phenomenon.

3 is a graph showing the response characteristics of the sensor according to the change in the concentration of SO _x for the sensor fabricated by wearing a glass protective film. Measurement method other then given to maintain a certain amount of time at each concentration was measured in the electrical signal representative of the sensor through a digital multimeter (DMM), wherein the concentration of SO _x is from 1.25ppm to 10ppm according to change the concentration of SO _x Measured while changing to concentration.

When the concentration of SO _x is kept constant, the signal of the sensor is also kept constant. After changing the concentration of SO _x , the signal of the sensor changes rapidly and reaches a new equilibrium value. In addition, since the same sensor signal appears for the same gas concentration, the reproducibility is very excellent. 3 is a result of measuring the concentration of SO _x at 504 ° C., showing a similar pattern at other temperatures.

4 is a result measured at each temperature between 500 and 700 ° C. and shows a relationship between the concentration of SO _x and the output voltage of the sensor. As shown in FIG. 4, as the concentration of SO _x decreased, the voltage across the sensor increased, and as shown in the equation representing the output voltage, it can be seen that the linearity of the logarithm of the SO _x concentration was excellent. .

The solid electrolyte type SO _x gas sensor using the oxide reference electrode according to the present invention has a glassy protective film, and thus has a stable electromotive force, excellent sensitivity, and excellent reproducibility and thermal stability compared to the solid electrolyte type SO _x sensor without a protective film. This can provide a good sensor.

Claims (6)

Solid electrolyte consisting of a conductor, A sensing electrode attached to one side of the solid electrolyte and composed of a sulfate compound; A reference electrode attached to the other side of the solid electrolyte and comprising a two-phase mixture of Na-Ti-O-based compounds, Solid Oxide SO _x sensor, characterized in that a protective film made, including of the vitreous coating on the solid electrolyte surface around the reference electrode. The compound as claimed in claim 1, wherein the solid electrolyte includes an alkali metal sulfate, a B ₂ O ₃ -Li ₂ SO ₄ -Al ₂ O ₃ system and a Na ₂ OB ₂ O ₃ -Na ₂ SO ₄ -Al ₂ O ₃ system , Na-β-alumina and NASICON any one of a solid electrolyte type SO _x sensor characterized in that. The method of claim 1, wherein the oxide reference electrode is Na ₂ Ti ₃ O ₇ -Na ₂ Ti ₆ O ₁₃ , Na ₂ Ti ₆ O ₁₃ -TiO _2, and Na ₂ TiO ₃ -Na ₂ Ti ₃ O ₇ Solid electrolyte SO _x sensor, characterized in that consisting of any one. The solid electrolyte type SO _x sensor according to claim 1, wherein the protective film is coated by a high temperature heat treatment. The solid electrolyte type SO _x sensor according to claim 1, wherein the protective layer is coated using a thin film deposition technique. According to claim 1, wherein the SO _x sensor is a solid electrolyte SO _x sensor, characterized in that the disk or element shape,