JP7389420B2 - Porous solid electrolyte gas sensor - Google Patents

Description

The present invention relates to a gas sensor using a porous solid electrolyte membrane.

A typical solid electrolyte gas sensor that uses electromotive force as a response output uses a dense sintered solid electrolyte, which complicates the manufacturing process. Even in a mixed potential type detection mechanism in which the detection electrode and counter electrode are exposed to the same atmosphere and no reference atmosphere is required, a dense sintered body is still used. The inventors have proposed a CO sensor that can operate at room temperature using a NASICON sintered body of Na ⁺ ion conductor (Patent Documents 1 and 2, Non-Patent Document 1), but there are similar problems. For this reason, it has been difficult to miniaturize gas sensors, simplify the manufacturing process, and make them more intelligent by mounting them on MEMS hot plates.

JP6425,309B US9939,404B

Electrochimica Acta 155 (2015) 8-15

An object of the present invention is to provide a gas sensor with a new structure using a porous solid electrolyte membrane, thereby simplifying the manufacturing process of the gas sensor and facilitating miniaturization of the gas sensor.

The porous solid electrolyte gas sensor of this invention is
a support such as a substrate or a membrane equipped with a micro-hot plate heater;
a pair of electrodes both fixed on a support and having different compositions;
A porous solid electrolyte membrane fixed on a support so as to cover a pair of electrodes,
The electromotive force between the pair of electrodes is output.

Since the gas sensor of the present invention outputs electromotive force, only a very small amount of current needs to flow through the solid electrolyte. Therefore, it is sufficient that the solid electrolyte particles are in contact with each other at a neck or the like, and gas can be detected even with a porous solid electrolyte membrane. The solid electrolyte membrane is preferably a porous alkali metal ion conductor membrane that can detect gases such as CO and H2 at room temperature.

When at least one of a pair of electrodes contains a noble metal and a metal oxide, a difference in electromotive force can be generated between the electrodes due to the metal oxide. In particular, one electrode is mixed with a metal oxide so that the electromotive force increases when it comes into contact with CO, and the other electrode is mixed with a metal oxide of another type so that the electromotive force decreases when it comes into contact with CO. By containing , a large gas response can be obtained.

Preferably, a membrane provided with a heater of a MEMS micro hot plate is used as the supporting membrane. This membrane is spanned over a cavity provided in the substrate. Using a MEMS micro hot plate makes it easy to integrate other sensors and drive circuits. Further, a micro hot plate heater can be used for baking the electrode and solid electrolyte membrane.

Cross-sectional view of the gas sensor of the example Plan view of the gas sensor of the example Electron micrograph of the cross section of the alumina substrate and NASICON film in the gas sensor of the example Electron micrograph of a cross section of the NASICON membrane in the gas sensor of the example Electron micrograph showing the fractured surface of the NASICON membrane and the Pt (15Bi ₂ O ₃ ) electrode in the gas sensor of the example Electron micrograph showing the Pt (15Bi ₂ O ₃ ) electrode and its surroundings in the gas sensor of the example Diagram showing the diffusion of Bi element to the Pt counter electrode during heat treatment in a disk-shaped NASICON sintered body Diagram showing the response of a gas sensor using a NASICON film deposited at 800℃ to 300ppmCO Diagram showing the response of a gas sensor using NASICON film deposited at 1000℃ to 300ppmCO Diagram showing the response of a gas sensor using a NASICON film deposited at 900°C to 300ppmCO Characteristic diagram showing the response to 300 ppm CO at room temperature of a gas sensor that combines a Pt (15CeO ₂ ) electrode and a plain Pt electrode. Characteristic diagram showing the response of a gas sensor that combines a Pt (15Bi ₂ O ₃ ) electrode and a Pt (15CeO ₂ ) electrode to 300 ppm CO at room temperature. Characteristic diagram showing the response of the gas sensor in Figure 12 to 300 ppm CO at -10°C

The best embodiments for carrying out the present invention are shown below.

Embodiments of the structure of the gas sensor are shown in FIGS. 1 to 13. 1 and 2 show the structure of a gas sensor 2, where 3 is a substrate made of Si or the like, 4 is a cavity provided in the substrate 3 by etching, and the upper part of the cavity 4 is covered with a diaphragm-shaped support film 6. The support membrane 6 may bridge the cavity 4 instead of covering the cavity 4 in the form of a diaphragm. The support film 6 is composed of a single layer or a multilayer insulating film, and in the embodiment, a film-like Pt heater 8 is provided between the multilayer insulating films. The Si substrate 3 to heater 8 constitute a MEMS micro hot plate.

A pair of electrodes 10 and 11 are formed on the support film 6, one is a sensing electrode and the other is a counter electrode, and have different compositions. The film thickness of the electrodes 10 and 11 was set to 10 μm in the example, but it is arbitrary. Further, a porous NASICON film 12 (hereinafter simply referred to as NASICON film 12) is formed on the support film 6 so as to cover the electrodes 10 and 11. Pads 15 and 16 are connected to the ends of the electrodes 10 and 11, pads 17 and 18 are connected to both ends of the heater 8, and lead wires (not shown) are bonded to the pads 15 to 18. In this way, the electromotive force between the electrodes 10/11 is used as the output of the gas sensor 2. Note that the pads 15 to 18, etc. may be connected to wiring on the bottom surface of the substrate 3 via conductive through holes or the like. Further, the heater 8 may also serve as one of the electrodes 10 and 11.

The NASICON membrane 12 is a porous thick film, for example, with a thickness of about 10 μm to 5 mm, and a film with a thickness of about 0.1 to 0.3 mm was used, but even a film with a thickness of 3 mm worked as a gas sensor. Alkali metal ion conductors other than NASICON may be used; for example, the Na ion conductor and the lithium ion conductor LISICON are similar alkali metal ion conductors and both can be used for CO detection at room temperature. Furthermore, a porous solid electrolyte membrane other than an alkali metal ion conductor may be used as long as it is an ion conductor that is conductive at room temperature.

By changing the electrode composition between the sensing electrode and the counter electrode, the reaction between the electrode and gases such as CO, H2, ammonia, hydrogen sulfide, alcohol, ketones, and hydrocarbons changes. As a result, a response to the gas occurs, for example as an electromotive force. For example, one electrode may be made of a noble metal such as Pt, Au, Rh, Pd, etc., and the other electrode may be made of a metal oxide such as perovskite.

Both the sensing electrode and the counter electrode are electrodes whose main component is a noble metal (e.g., 70 wt% or more of the noble metal), and one of them is mixed with a metal oxide (e.g., 30 wt% or less, 0.01 wt% or more, preferably 30 wt% or less). (0.1wt% or more), a difference in properties occurs between the electrodes, and the gas can be detected. Metal oxides that cause a positive electromotive force response to reducing gases such as CO include Bi ₂ O ₃ and Cr ₂ O ₃ , and metal oxides that cause a negative electromotive force response. Examples include CeO ₂ , V ₂ O ₅ , WO ₃ , Ta ₂ O ₃ , In ₂ O ₃ , etc. Note that this has already been published in Patent Document 2. Preferably, one electrode is mixed with at least one type of metal oxide that causes a positive electromotive force response, and the other electrode is mixed with at least one type of metal oxide that causes a negative electromotive force response.

The heater 8 is used to bake the electrodes 10, 11 and the NASICON film 12, and since the gas sensor 2 of the embodiment operates at room temperature, there is no need to operate the heater 8 when detecting gas. However, when operating at a low temperature such as -40°C, or when detecting hydrocarbons, VOCs, etc., the heater 8 may be operated. The reason why the MEMS micro hot plate is used is to bake the electrodes 10 and 11 and the NASICON film 12 on the gas sensor 2. This is also because it can be integrated with other sensors such as a temperature sensor and humidity sensor, and also with ancillary circuits of a gas sensor. In the example, data is shown when electrodes 10 and 11 and a NASICON film 12 are provided on an alumina substrate instead of a MEMS gas sensor.

Manufacturing example of gas sensor and structure of NASICON membrane
NASICON (Na ₃ Zr ₂ Si ₂ PO ₁₂ ) powder was prepared and made into a paste with glycerin. As the electrode material paste, a mixture of noble metal paste and metal oxide powder, or a plain noble metal paste was used. Pastes for the sensing electrode and counter electrode were applied onto an alumina substrate, and NASICON paste was applied from above and dried. After drying, the substrate was fired, for example, in air, and the electrodes were fired and the porous NASICON film was formed (fired) at the same time. The firing of the electrode and the formation of the porous NASICON film may be performed separately. The firing atmosphere and the like are arbitrary, as long as a porous NASICON film can be formed.

The electrode composition was expressed as the weight ratio of Pt paste and metal oxide, and the Pt concentration in the Pt paste was 80 to 90 wt%. For example, Pt (15Bi ₂ O ₃ ) indicates that Pt paste is 85wt% and Bi ₂ O ₃ is 15wt%, and the electrode composition after firing is 82 to 84 wt% of Pt and 18 to 16 wt% of Bi2O3. Become.

Figure 3 shows the NASICON film on the alumina substrate visible at the bottom, and Figure 4 shows an enlarged view of the NASICON film. The firing conditions were 900°C x 30 minutes.

Figure 5 shows the fractured surface of the Pt (15Bi ₂ O ₃ ) electrode and NASICON membrane, and the alumina substrate can be seen at the bottom of Figure 5. FIG. 6 shows an enlarged view of the Pt (15Bi ₂ O ₃ ) electrode and its surroundings. The firing conditions were 900°C x 30 minutes.

Migration of metal oxides in electrodes A conventional gas sensor consists of a disk-shaped NASICON sintered body (sintered in air at 1100℃ for 4 hours), a sensing electrode (Pt (15Bi ₂ O ₃ )) and a counter electrode (Pt). The paste was applied and heat treated in air at temperatures of 700°C, 800°C, and 900°C for 30 minutes each. The composition of the sensing electrode and counter electrode after heat treatment was analyzed using XPS (X-ray
analyzed by photoelectron spectroscopy). Figure 7 shows the peak of Bi at the opposite electrode. The Bi concentration of the sensing electrode decreased with heat treatment, and when treated at 800°C or 900°C, Bi was observed on the counter electrode, and at 900°C, a high concentration of Bi was observed on the counter electrode. Among the metal oxides added to the electrode, Bi ₂ O ₃ , V ₂ O ₅ , and WO ₃ are materials that have a relatively low melting point or are easily sublimed. And FIG. 7 shows that the metal oxide mixed in the electrode can be moved by firing.

Effect of firing temperature The sensing electrode was Pt (15Bi2O3) and the counter electrode was Pt, and the firing conditions for the NASICON film were 800℃ x 30 minutes in air (Figure 8), 1000℃ x 30 minutes in air (Figure 9), and air. The temperature was changed to 900°C for 30 minutes (Figure 10). The response to 300 ppm CO in a dry atmosphere at room temperature is shown. In the Examples, gas responses in a dry atmosphere are shown unless otherwise specified. Temperature indicates ambient temperature, and the gas sensor was operated at ambient temperature without heating. In embodiments, the response to the gas is a change in electromotive force between the electrodes. At a firing temperature of 800°C (Fig. 8), the electromotive force was unstable, and at a firing temperature of 1000°C (Fig. 9), only a small CO response was obtained, but at a firing temperature of 900°C (Fig. 10), a large CO response was obtained. was gotten. These facts indicate that the firing temperature of the NASICON film is preferably around 900°C. In the following, the firing conditions were unified to 900°C in air for 30 minutes.

Gas Response Figures 10-12 show the response of the gas sensor to 300 ppm CO. Figure 10 above shows the results when a Pt (15Bi ₂ O ₃ ) pole and a plain Pt pole are combined, and Figure 11 shows the results when a Pt (15CeO ₂ ) pole and a plain Pt pole are combined. , FIG. 12 shows the results when Pt(15Bi ₂ O ₃ ) and Pt(15CeO ₂ ) poles are combined. The measurement temperature was room temperature, and the background was dry air. An electromotive force response to CO was obtained, and the response was increased by combining the Pt(15Bi ₂ O ₃ ) and Pt(15CeO ₂ ) poles (FIG. 12). Note that CO responses can be obtained even with humidified air as a background.

Response at low temperature Figure 13 shows the response at -10°C to 300 ppm CO. The electrodes were a combination of Pt (15Bi ₂ O ₃ ) and Pt (15CeO ₂ ) electrodes, and a response to CO was obtained even at -10°C.

Gas Detection Model The inventor estimates the gas detection model as follows. Because a porous NASICON membrane is used, gas diffuses to the interface between the NASICON membrane and the electrode, where a three-phase interface between NASICON, the electrode, and the gas occurs. At the three-phase interface, the equilibrium reaction of equation (1) occurs between oxygen and Na ions in the air.
2Na ⁺ +1/2O ₂ +2e ^- → Na ₂ O (1)

When CO is added to the atmosphere, the equilibrium reaction of equation (2) is added, and the electrode potential is determined at the point where the reactions of equation (1) and (2) are balanced. Therefore, the electromotive force depends on gas such as CO.
CO + Na ₂ O → 2Na ⁺ +2e ^- +CO ₂ (2)

When a metal oxide is mixed into the electrode, a non-stoichiometric redox reaction of the metal oxide is added, as shown in equation (3). In the formula, δ represents a minute amount, and M represents a metal element. The equilibrium in equation (3) shifts to the left side of the equation for substances such as CeO2, which are difficult to reduce, and shifts to the right side of the equation for substances that are easily reduced, such as Bi2O3 and Cr2O3. For this reason, a difference in electromotive force occurs between the electrode and a single Pt electrode, and it is thought that the difference in electromotive force depends on the gas in the atmosphere.
MO _x +2δNa ⁺ +2δe ^- → MO _x-δ +δNa ₂ O (3)

Effects The effects of the examples are shown.
1) Since a membrane-like solid electrolyte is used, the gas sensor can be easily miniaturized and integrated (Figures 1 and 2). This is possible because the output is electromotive force and the solid electrolyte has low electrical conductivity.
2) In particular, if the insulating film with the heater of the MEMS micro-hot plate is used as the supporting film, it will be easy to integrate it with other sensors and control circuits. Further, a hot plate heater can be used for forming the electrode and the porous solid electrolyte membrane.
3) Gas diffuses within the porous solid electrolyte membrane, and gas can be detected even if an electrode is provided between the solid electrolyte and the support membrane (Figures 10 to 12).
4) When an alkali ion conductor is used as the solid electrolyte membrane and the sensor is operated without heating, the alkali ion Gases are not oxidized in the conductor.
5) CO can be detected even at low temperatures without activating the heater (Figure 13).
6) The metal oxide mixed into the electrode moves through heat treatment (Figure 7). When the electrode is provided between the support (the substrate or the support membrane in the example) and the solid electrolyte membrane, the movement of metal oxides in the electrode can be reduced.
7) The porous solid electrolyte membrane traps poisonous components in the atmosphere, making the electrodes less susceptible to poisoning.

2 Gas sensor 3 Si substrate 4 Cavity 6 Support membrane 8 Heater 10, 11 Electrode 12 Porous NASICON membrane 15 to 18 Pad

Claims (4)

a support and
a pair of electrodes both fixed on a support and having different compositions;
a porous solid electrolyte membrane fixed on the support so as to cover the pair of electrodes,
configured to output an electromotive force between the pair of electrodes ,
The solid electrolyte membrane is a porous alkali metal ion conductor membrane,
A porous solid electrolyte gas sensor , wherein at least one of the pair of electrodes contains a noble metal and a metal oxide . The pair of electrodes both contain a noble metal and a metal oxide, and the types of metal oxides are different between the pair of electrodes,
2. The porous solid electrolyte gas sensor according to claim 1 , wherein one of the pair of electrodes has an increased electromotive force upon contact with CO, and the other of the pair of electrodes has an electromotive force that has decreased upon contact with CO. 3. The porous solid electrolyte gas sensor according to claim 1 , wherein the support is a support film that spans over a cavity provided in the substrate, and the support film is equipped with a heater. The porous solid electrolyte gas sensor according to any one of claims 1 to 3, wherein the solid electrolyte membrane and the pair of electrodes are both configured to allow gas movement between them and the atmosphere. .

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