WO2012033054A1

WO2012033054A1 - Contact burning-type gas sensor, manufacturing method therefor, and control circuit therefor

Info

Publication number: WO2012033054A1
Application number: PCT/JP2011/070174
Authority: WO
Inventors: 文雄西野; 大三廣島; 慶一廣瀬; 純平吉國
Original assignee: 立山科学工業株式会社
Priority date: 2010-09-08
Filing date: 2011-09-05
Publication date: 2012-03-15
Also published as: JP4754652B1; JP2012058067A

Abstract

A contact burning-type gas sensor provided with a sensing element (1) and a reference element (2) which are thermally isolated from each other and form a pair, wherein in the sensing element (1), a platinum-based electrode wire (4) is embedded in a thermistor ceramic (3a), and a reactive film (1a) into which a reactive catalyst for gas to be sensed is mixed is provided on the surface of the thermistor ceramic (3a), in the reference element (2), a platinum-based electrode wire (4) is embedded in a thermistor ceramic (3a), and a nonreactive film (2a) which does not contain the reactive catalyst for the gas to be sensed and has the same thermal capacity as the reactive film (1a) is provided on the surface of the thermistor ceramic (3a), and the sensing element (1) and the reference element (2) are respectively provided with terminals (5) leading to the electrode wires (4) thereof.

Description

Contact combustion type gas sensor, manufacturing method thereof and control circuit thereof

The present invention relates to a catalytic combustion type gas sensor and a manufacturing method thereof, and more particularly to a catalytic combustion type gas sensor having a high output resolution at a low concentration and a structure that realizes a long life and a manufacturing method thereof.

As a contact combustion type gas sensor that can detect gas such as hydrogen, carbon monoxide, or ethanol, it consists of a detection electrode and a reference electrode, and the temperature rise due to the combustion reaction that occurs at the detection electrode when a predetermined gas is present The type of detecting gas by detecting the difference between the output and the reference electrode without a combustion reaction is widely known.

This type of gas sensor generally uses a metal coil as a temperature detection unit and heater, and a catalyst suitable for the detection gas is supported on a ceramic carrier, which is coated and fired to form a detection electrode. On the other hand, a catalyst having a heat capacity matched to the carrier of the detection electrode is prepared by removing the catalyst, and this is applied and baked as a reference electrode (see Patent Documents 1 to 3 below).

In addition, the realization of the catalytic combustion gas sensor with a thin film structure with a small heat capacity is also introduced. This sensor has a structure including a detection electrode and a reference electrode, and the detection principle is the same, but it is known that high response and high sensitivity can be obtained by using a membrane structure or a thermistor.

Japanese Patent Laid-Open No. 7-120425 JP 2003-85675 A JP 2005-321215 A

However, in the type using the coil made of the above metal, since the temperature change of the catalyst is detected by the change of the electric resistance of the coil, the coil resistance value is required to have very high accuracy. Requires very advanced processing techniques. As a result, it is difficult to match the characteristics of the detection electrode and the reference electrode, resulting in poor yield during mass production.
In addition, when the firing temperature of the catalyst material becomes high, it is necessary to use a noble metal wire such as platinum for the coil, and the sensor including the coil becomes more expensive.

In addition, many catalyst materials are supported on ceramics, but if the difference in thermal expansion coefficient between the ceramic carrier and the metal coil is large, mutual adhesion is lost due to repeated heating, and the reaction at the detection electrode There is a problem that the heat generation versus output characteristics deteriorate.
In addition, when the reaction material heat generation is very small, the change in the electrical resistance of the coil is small, and the resistance difference between the detection electrode and the reference electrode is small, resulting in poor resolution and low response to changes in concentration. There is a problem that it must be a sensor.

On the other hand, when the thin film structure is adopted, the heater capacity cannot be increased to a temperature that reaches the gas combustion temperature, and if the temperature is increased to the gas combustion temperature, the heater life is shortened. There is a problem.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a catalytic combustion type gas sensor that is relatively easy to manufacture, has high reliability, has high sensitivity, and has a long life, and a method for manufacturing the same.

The catalytic combustion type gas sensor according to the present invention, which has been made to solve the above problems, is a catalytic combustion type gas sensor provided with a pair of a sensing element and a reference element which are thermally isolated from each other.

The sensing element includes a reaction electrode in which a platinum-based electrode wire is embedded in the thermistor / ceramics and a reaction gas reaction catalyst is mixed on the surface of the thermistor / ceramics. The reference element embeds the platinum-based electrode wire in the thermistor / ceramics. In addition, the surface of the thermistor / ceramics is provided with a non-reactive film that does not contain a detection gas reaction catalyst and has the same heat capacity as the reaction film, and each of the electrode wires has a detection element or a reference electrode. It is characterized by having a terminal to be connected.

In order to solve the above-mentioned problems, a method for manufacturing a catalytic combustion type gas sensor according to the present invention includes a thermistor slurry that covers a pair of platinum electrode wires arranged in parallel at a distance from each other, dried and fired to form a thermistor element. An element forming step, a reaction electrode forming slurry in which a detection electrode coating slurry mixed with a reaction catalyst of a detection gas is deposited on the surface of the thermistor element, dried and fired to form a reaction film of the detection element, and the thermistor element A non-reactive film forming step of forming a non-reactive film of a reference element by depositing a reference electrode coating slurry that does not contain a detection gas reaction catalyst on the surface of the electrode and drying and firing the reference electrode coating slurry having the same heat capacity as the detection electrode coating slurry; An assembly process is performed in which the detection element and the reference element are thermally isolated from each other and fixed to the housing.

The catalytic combustion type gas sensor uses a thermistor ceramic.
The thermistor ceramic is a ceramic having a property that a resistance value follows a temperature change and has a self-heating property by energization. Thermistor ceramics have a resistance temperature dependency that is greater than that of metals, so by using this for temperature detection, high sensitivity and high resolution can be realized. There is a feature that it is easy to manufacture a highly accurate resistor as compared with metal.

Moreover, the catalytic combustion type gas sensor has a structure capable of fulfilling the role of a heater by increasing the temperature by self-heating of the thermistor / ceramics by energizing the platinum-based electrode wire.
That is, on the surface of the temperature detection unit / heater, the sensing element is coated with a reaction film carrying a reaction catalyst, while the reference element is coated with a non-reactive film that is an inactive film having the same heat capacity as the sensing element. This is a sensor that outputs an electric signal (reaction signal) generated by detecting a temperature rise caused by a combustion reaction generated in the detection element when a target gas is present, and an electric signal (reference signal) generated from the reference element.

The gas sensor can be used as an index for determining the presence or absence of gas by integrally attaching a detection circuit that uses the difference between the reaction signal and the reference signal as a sensor output, or by attaching the detection circuit as an external circuit.

According to the catalytic combustion type gas sensor, there is no need for a structurally highly accurate metal coil, and it is only necessary to deposit a reaction film or a non-reaction film mixed with a reaction catalyst on the surface of the thermistor / ceramics. A highly reliable sensor can be provided without requiring advanced processing techniques.
In addition, by using thermistor / ceramics as a heater, self-heating can be adjusted so that the temperature at which the gas is likely to burn is detected at the detection electrode, and heat generation can be compensated between the detection electrode and the reference electrode. Therefore, an accurate temperature difference can be detected between the detection electrode and the reference electrode, and a highly accurate sensor can be provided.
In addition, the platinum electrode wire embedded in the thermistor / ceramics, unlike the thin film structure, not only provides sufficient durability when used as a heater, but also the expansion coefficient of the platinum electrode wire. Therefore, even if heating is repeated, stress that causes cracking or peeling at the interface with the thermistor / ceramics or reaction film does not occur.

In addition, the reaction catalyst is supported on a ceramic with the same thermal expansion coefficient as that of the thermistor / ceramic, and it is used as a reaction film, which ensures high accuracy over a long period of time without causing deterioration of characteristics even if heating is repeated. Can be used.
From these, it is possible to provide a contact combustion type gas sensor having high heat resistance, impact resistance, durability, and stability.

It is sectional drawing seen from the front side which shows an example of the contact combustion type gas sensor by this invention. (A), (B), and (C) are process drawings showing an example of a method for manufacturing a catalytic combustion gas sensor according to the present invention. It is a circuit diagram which shows the embodiment example of the contact combustion type gas sensor by this invention. 6 is a graph showing an example of gas (carbon monoxide) concentration-output characteristics of a catalytic combustion type gas sensor (thermistor type) according to the present invention and a conventional gas sensor (coil type).

Embodiments of a catalytic combustion type gas sensor (hereinafter referred to as a gas sensor) according to the present invention will be described below in detail with reference to the drawings.
The gas sensor shown in FIG. 1 is a gas sensor provided with a pair of a thermally isolated detection electrode SE and a reference electrode RE.

The detection element 1 of the detection electrode SE includes a reaction film 1a in which a platinum-based electrode wire 4 is embedded in a thermistor / ceramic 3a and a reaction catalyst of a detection gas is mixed on the surface of the thermistor / ceramic 3a.
The reference element 2 of the reference electrode RE is a non-reactive film in which a platinum-based electrode wire 4 is embedded in the thermistor / ceramics 3a and the surface of the thermistor / ceramics 3a does not include a detection gas reaction catalyst and has the same heat capacity as the reaction film 1a 2a.
The detection electrode SE and the reference electrode RE are each provided with a terminal 5 connected to the platinum-based electrode wire 4 of the detection element 1 and the reference element 2.

The thermistor ceramics 3a in this example is a general thermistor composition.
For example, in the case of a known MnCoNiFe system or CaYMnCr system, a practical resistance value, resistance temperature coefficient, and long-term durability can be obtained even when operated at a temperature from room temperature to over 200 ° C.
A slurry having the thermistor composition (hereinafter referred to as the thermistor slurry) is applied to a jig in which a pair of platinum-based electrode wires 4 are stretched in parallel with a gap therebetween, dried, and fired. An inserted unit of bead-type thermistor element 3 is obtained (element forming step).
In this gas sensor, two units of the same thermistor element 3 are used.

The detection electrode coating slurry mixed with the detection gas reaction catalyst is deposited on the surface of one thermistor element 3, and dried and fired to form the reaction film 1a of the detection element 1 (reaction film forming step).

The detection electrode coating slurry includes a reaction catalyst and ceramics as a carrier for supporting the reaction catalyst, and a sintering aid is added as necessary.
The reaction catalyst varies depending on the gas to be detected. For example, platinum may be selected for hydrogen, palladium or cobalt oxide may be selected for carbon monoxide, and the like.
The ceramic is adjusted to the thermal expansion coefficient of the thermistor ceramics 3a (for example, 9 × 10 −6 / K) and matched with the thermal expansion coefficient of the thermistor ceramics 3a.
For example, an oxide such as Y203, Cr203, or Al203 may be selected.
As the sintering aid, glass frit or the like may be selected.

A reference electrode coating slurry obtained by removing the detection gas reaction catalyst from the detection electrode coating slurry material is deposited on the surface of the other thermistor element 3 in the same area as the reaction film 1a, dried and fired, and then the reference element 2 The non-reactive film 2a is formed (non-reactive film forming step).
At this time, the film thickness is adjusted so that the heat amounts of the reaction film 1a and the non-reaction film 2a are equal.
If the film thickness is about 5 μm to about 10 μm, which is several times the particle size of the sensing electrode coating slurry, an effective difference between the reaction signal and the reference signal can be obtained.

The detection element 1 and the reference element 2 obtained in this way are fixed to a casing that has been heat-insulated so that mutual heat does not interfere with each other. ).

Examples of the gas sensor will be described below.
The example is a gas sensor using a thermistor as a temperature detection and heater.

The thermistor slurry in this example is made through the following steps.
That is,
(1) Manganese oxide, cobalt oxide, nickel oxide, and iron oxide are weighed so as to be 40 mol%, 40 mol%, 10 mol%, and 10 mol%, respectively, and wet mixed in a ball mill for 12 hours.
(2) The obtained mixture is put into an alumina square bowl and calcined at 900 ° C. for 2 hours to synthesize.
(3) The resulting composite is wet-ground for 24 hours in a ball mill to obtain a thermistor powder having a particle size of about 1 μm.
(4) A thermistor slurry is obtained by weighing and mixing 70% by weight of the obtained thermistor powder and 30% by weight of 5% by weight of ethyl cellulose 10 cp as an organic vehicle.

The obtained thermistor slurry is applied to a jig in which linear platinum-based electrode wires 4 having a diameter of 60 μm are stretched in parallel at intervals of 400 μm and dried, followed by firing at 1100 ° C. for 2 hours to obtain a bead type thermistor element 3. .

The sensing electrode coating slurry in this example is made through the following steps.
In addition, the reaction catalyst in the said example is palladium which has a catalytic reaction with respect to carbon monoxide.
(1) The palladium powder, the alumina powder, and the sintering aid, whose particle diameters are adjusted to about 1 μm, are weighed to 7 wt%, 92 wt%, and 1 wt%, respectively, and mixed. By doing so, a sensing electrode coating material is obtained.
(2) Weigh and mix 70 wt% of the obtained sensing electrode coating material and 30 wt% of 5 wt% dissolved product of ethyl cellulose 10 cp as an organic vehicle to obtain a sensing electrode coating slurry.

The reference electrode coating slurry in this example is made through the following steps.
(1) Alumina powder and sintering aid whose particle size is adjusted to about 1 μm are weighed to be 98% by weight and 2% by weight, respectively, and mixed to obtain a reference electrode coating material. .
(2) A reference electrode coating slurry is obtained by weighing and mixing 70% by weight of the obtained reference electrode coating material and 30% by weight of a 5% by weight dissolved product of 10 cp of ethyl cellulose as an organic vehicle.

The sensing electrode coating slurry and the reference electrode coating slurry obtained as described above are applied so as to have a heat capacity equal to the surface of each thermistor element 3 serving as the sensing element 1 or the reference element 2, dried, and then 700 ° C. The sensing element 1 and the reference element 2 provided with the reaction film 1a or the non-reaction film 2a are obtained by baking for 1 minute.

According to the composition, the thermal expansion coefficients of the thermistor ceramics 3a of the thermistor element 3 and the reactive film 1a and the non-reactive film 2a are substantially equal.
The reaction catalyst is supported on a ceramic with the same thermal expansion coefficient as that of the thermistor ceramics 3a, and it is used as the reaction film 1a, so that it can be used with high accuracy for a long time without causing deterioration of characteristics even if heating is repeated. can do.

The detection element 1 and the reference element 2 are fixed to the pedestal 6 of the detection electrode SE and the pedestal 7 of the reference electrode RE to complete the detection electrode SE and the reference electrode RE. At that time, the

elements

1 and 2 are supported by the dust adhesion preventing mesh 8, and the platinum electrode wires 4 of the

elements

1 and 2 are connected to a pair of

terminals

5 and 5 fixed to the

bases

6 and 7.

Finally, the detection electrode SE and the reference electrode RE are fixed to the connection base (housing) 9 and a thermal partition 10 is installed between them to complete the main body of the gas sensor. At that time, the terminal 5 of each pole in the example is drawn out through the coupling base 9 and connected to the control circuit 15 through the terminal 5.
The

pedestals

6 and 7, the connecting pedestals 9, and the thermal partition 10 may be formed of a material such as ceramic having poor thermal conductivity.

The gas sensor is connected to the control circuit 15 shown in FIG.
That is, a detection voltage generation circuit 11 in which a voltage dividing resistor Rs and a detection element 1 are connected in series, a reference voltage generation circuit 12 in which a temperature setting resistor Rc and a reference element 2 are connected in series, and two power supply resistors Rx , Ry connected in series to each other, and the detection voltage generation circuit 11 and the reference voltage generation circuit 12 form a bridge circuit. A virtual short circuit 14 for forcibly matching the point voltage VR and the voltage dividing point voltage VQ of the reference voltage generating circuit 12 at both voltage dividing points R and Q without adding or removing current is added.

With the above circuit configuration, power (voltage) determined by the power supply voltage generation circuit 13 is supplied to the detection element 1 and the reference element 2, and the detection element 1 and the reference element 2 are directly heated.
Due to the characteristics of the thermistor whose resistance value decreases as the temperature rises, the currents of the detection voltage generation circuit 11 and the reference voltage generation circuit 12 increase under the equal power supply voltage, but the voltage at the voltage dividing point Q of the reference voltage generation circuit 12 Due to the drop, the generated voltage of the power supply voltage generating circuit 13 decreases, and the current of the detection voltage generating circuit 11 and the reference voltage generating circuit 12 that receives the power supply voltage is reduced.

If the output of the control circuit 15 is extracted from the voltage dividing point P of the detection voltage generating circuit 11 and the voltage dividing point Q of the reference voltage generating circuit 12, the thermistor ceramics 3a coated with the carrier and the heat capacity equal to that of the carrier are obtained. The concentration of the gas can be obtained by the difference between the voltage dividing points VP and VQ of the detection voltage generation circuit 11 and the reference voltage generation circuit 12 due to the temperature difference generated between the thermistor ceramics 3a coated with the inert substance. it can.

By incorporating the detection element 1 and the reference element 2 into the detection voltage generation circuit 11 and the reference voltage generation circuit 12 as described above, the detection element 1 and the reference element 2 can be directly heated. Further, by performing feedback control for adjusting the power supplied to the detection element 1 and the reference element 2 as described above, even if the temperature exceeds 200 ° C., the thermistor / ceramics 3a is stably heated at a constant temperature, and the atmosphere A gas sensor having no temperature dependence can be realized.

When the thermistor is used as a contact combustion type gas sensor, it is not necessary to heat the thermistor with side heat from a heater or the like to cause a catalytic reaction. It can be simplified.

In addition, by using the voltage dividing resistor Rs and performing feedback control for adjusting the power supplied to the sensing element 1 by the voltage dividing point voltage VP between the sensing element 1 and the voltage dividing resistor Rs, the sensing element 1 Current limitation is also applied to the temperature-resistance characteristics, and the problem of thermal runaway due to overcurrent and the problem that the output signal changes depending on the usage environment temperature are solved at the same time.

FIG. 4 compares the carbon monoxide concentration-output characteristics of the above embodiment (thermistor type) and the conventional gas sensor (coil type) with current flowing so that both sensors are heated to the same temperature. is there.
Here, in the example used as a conventional gas sensor, a pair of 30 μm nickel wires is shaped into a 21-turn coil shape, one of which is coated with the same sensing electrode coating slurry as the above embodiment, and the other is coated with the other. After the reference electrode coating slurry is applied, the detection electrode SE and the reference electrode RE are formed by baking at the same temperature as in the above embodiment.

As a result of comparison, each example has an output characteristic proportional to the gas concentration. However, the above embodiment generates an output about 8.3 times that of the conventional gas sensor at the same carbon monoxide concentration. It was confirmed that extremely high sensitivity was obtained.

1 sensing element, 1a reactive film, 2 reference element, 2a non-reactive film,
3 thermistor element, 3a thermistor ceramics,
4 platinum electrode wires, 5 terminals,
6 pedestal (detection element), 7 pedestal (reference element), 8 mesh, 9 connecting pedestal,
10 thermal partition,
11 detection voltage generation circuit, 12 reference voltage generation circuit, 13 power supply voltage generation circuit,
14 virtual short circuit, 15 control circuit,
P voltage dividing point (detection voltage generation circuit), VP voltage dividing point voltage,
Q voltage dividing point (reference voltage generation circuit), VQ voltage dividing point voltage,
R voltage dividing point (power supply voltage generation circuit), VR voltage dividing point voltage,
Rs voltage dividing resistor, Rc temperature setting resistor, Rx power supply resistor, Ry power supply resistor,
SE detection electrode, RE reference electrode

Claims

In a contact combustion type gas sensor comprising a pair of a sensing element and a reference element which are thermally isolated from each other,
The sensing element includes a reaction film in which a platinum-based electrode wire is embedded in the thermistor / ceramics, and a reaction gas reaction catalyst is mixed on the surface of the thermistor / ceramics.
The reference element includes a platinum-based electrode wire embedded in the thermistor ceramic, and a non-reactive film that does not include a detection gas reaction catalyst on the surface of the thermistor ceramic and has the same heat capacity as the reaction film,
A catalytic combustion type gas sensor comprising a detection element and a reference element each having a terminal connected to each electrode wire.
Including the catalytic combustion type gas sensor according to claim 1;
A detection voltage generation circuit in which a voltage dividing resistor and a detection element are connected in series;
A reference voltage generation circuit in which a temperature setting resistor and a reference element are connected in series;
Constructed by connecting in parallel a power supply voltage generating circuit formed by connecting two power resistors in series,
A bridge circuit is configured by the detection voltage generation circuit and the reference voltage generation circuit,
A virtual short circuit for forcibly matching the voltage dividing point voltage of the power supply voltage generating circuit and the voltage dividing point voltage of the reference voltage generating circuit at both voltage dividing points without causing current to flow in and out;
A control circuit for a catalytic combustion type gas sensor in which a voltage dividing point of the detection voltage generating circuit and a voltage dividing point of the reference voltage generating circuit are output detection points corresponding to the concentration of the detected gas.
An element forming step of forming a thermistor element by covering a pair of platinum-based electrode wires arranged in parallel with a gap with a thermistor slurry, drying and firing;
A reaction film forming step in which a detection electrode coating slurry mixed with a detection gas reaction catalyst is deposited on the surface of the thermistor element, dried and fired to form a reaction film of the detection element;
A non-reactive film that does not contain a detection gas reaction catalyst on the surface of the thermistor element and that is coated with a reference electrode coating slurry having the same heat capacity as the detection electrode coating slurry, and is dried and fired to form a non-reactive film of the reference element Forming process;
A method for manufacturing a catalytic combustion type gas sensor, wherein an assembly process is performed in which the detection element and the reference element are thermally isolated from each other and fixed to a casing.