JP2008275588A - Combustible gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a combustible gas sensor capable of remarkably improving its measuring sensitivity and measuring accuracy by increasing the contact area between a measuring object gas and oxidation catalyst particles to increase the amount of heat generated by oxidation reaction and to improve the transfer property of the heat of oxidation reaction while reducing the whole size. <P>SOLUTION: A thermopile 4 is formed by joining dissimilar metal to the upper surface of an Si substrate 2. A porous catalyst layer 6 with oxidation catalyst particles supported on a porous material layer, or a chain-like catalyst layer 7 with a large number of oxidation catalyst particles connected and linked in a chain form is provided on a thermal contact part 4a of the thermopile 4. The porous catalyst layer 6 or the chain-like catalyst layer 7 is connected to the thermal contact part 4a of the thermopile 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  In order to prevent a disaster such as an explosion of a combustible gas such as CO, HC, formaldehyde, and hydrogen in a petrochemical factory, for example, the present invention measures the calorific value of the gas to be measured. The present invention relates to a combustible gas sensor represented by a hydrogen gas sensor used for measuring the concentration of a combustible gas contained in a gas, particularly hydrogen gas.

  This type of combustible gas sensor has a general structure in which an oxidation catalyst such as platinum is laminated on the surface of a temperature measuring element such as a thermistor, thermocouple, thermometer resistor, etc. via an insulating layer. However, since the general-purpose gas sensor with such a laminated structure has a large heat capacity, the amount of heat due to the oxidation reaction heat of the combustible gas is small. Therefore, as a change in voltage, current, or electrical resistance due to a change in the amount of heat. There is a problem that the output sensitivity is small and the measurement sensitivity of the low concentration combustible gas is very low.

  In addition, a film such as alumina containing an oxidation catalyst such as platinum or palladium is formed (deposited) on the hot contact portion of the thermopile through an insulating film, while the cold contact portion of the thermopile is exposed to flammable hydrogen gas, etc. The temperature of the hot junction is increased by combustion accompanying the contact of the gas with the coating containing a catalyst such as platinum, and a thermoelectromotive force is generated between the hot junction and the cold junction in a low temperature state. A catalytic combustion type gas sensor that detects the concentration of the combustible gas by detecting the thermoelectromotive force has also been used more frequently than before (for example, see Patent Document 1). Compensation circuit for ambient temperature is unnecessary compared with the general-purpose gas sensor with the above-mentioned laminated structure, and although the measurement sensitivity can be improved by that amount, the heat capacity is still large and lacks responsiveness, and low concentration combustible The measurement sensitivity of the gas there is a problem that satisfactory results can not be obtained.

  In order to solve the problems and problems of the general-purpose gas sensor and the catalytic combustion type gas sensor having the above-described laminated structure, the present applicants form a temperature measuring element such as a thermopile on the insulating film formed on the semiconductor substrate surface. In addition, an oxidation catalyst such as platinum or ruthenium is directly formed on the thermosensitive part of the temperature measuring element, or is formed and supported via an adhesive layer containing a highly heat conductive metal material such as Cr or Ti. There has already been proposed a combustible gas sensor provided with a heater for maintaining the oxidation catalyst in an active state (see, for example, Patent Document 2).

JP-A-5-10901 JP 2006-71362 A

  The flammable gas sensor disclosed in Patent Document 2 can reduce the heat capacity of the temperature measuring element by adopting a thin film technology such as the formation of an insulating film, a temperature measuring element, and an oxidation catalyst on the semiconductor substrate surface. In addition, the flammable gas in the gas to be measured comes into contact with the oxidation catalyst to generate heat of oxidation reaction, and by detecting the amount of heat, it is possible to measure a predetermined flammable gas concentration such as hydrogen, Although the measurement sensitivity and responsiveness can be improved compared to the general-purpose gas sensor and catalytic combustion type gas sensor described above, the heat of oxidation reaction generated on the surface of the oxidation catalyst and the heat transfer by which the reaction heat is transferred to the heat sensitive part of the temperature measuring element The performance (speed and efficiency) is not sufficient, and there is still room for improvement in terms of measurement sensitivity and measurement accuracy.

  In addition, since the contact area of the gas to be measured with the oxidation catalyst is not so large, the amount of heat generated by the oxidation reaction is small, and there is a limit to improving the measurement sensitivity of flammable gases, especially at low concentrations. For this purpose, it is indispensable to provide a heater for constantly maintaining the oxidation catalyst in an active state, and there is a problem that the entire gas sensor is likely to be enlarged.

  The present invention has been made in view of the above-mentioned circumstances, and its purpose is to increase the contact area between the gas to be measured and the oxidation catalyst while increasing the amount of generation of oxidation reaction heat while reducing the overall size. An object of the present invention is to provide a combustible gas sensor that improves heat transfer to a temperature measuring element and can realize a significant improvement in measurement sensitivity and measurement accuracy even with a low concentration of combustible gas.

  In order to achieve the above object, a combustible gas sensor according to the present invention forms a temperature measuring element on an insulating film formed on the surface of a semiconductor substrate, and detects the calorific value of the measurement target gas with the temperature measuring element. A combustible gas sensor configured to measure the concentration of the combustible gas in the measurement target gas, wherein the porous material layer carries the oxidation catalyst particles on the heat sensitive part of the temperature measuring element. A porous catalyst layer or a chain catalyst layer formed by joining a large number of oxidation catalyst particles in a chain form is provided, and this catalyst layer is integrally coupled to the heat sensitive part.

According to the combustible gas sensor of the present invention having the above-described characteristic configuration, when the measurement target gas comes into contact with the porous catalyst layer or the chain catalyst layer, the combustible gas in the measurement target gas is supported on the porous catalyst layer. It is oxidized by the oxidation catalyst particles or the chain oxidation catalyst particles forming the chain catalyst layer to generate reaction heat. For example, when the combustible gas is hydrogen gas (H ₂ ),
2H ₂ + O ₂ → 2H ₂ O + Q (1)
As shown by the following reaction formula, hydrogen gas (H ₂ ) molecules react with oxygen gas (O ₂ ) molecules to generate water molecules (H ₂ O). At this time, reaction heat Q is generated. This reaction heat is efficiently transmitted from the surface of the oxidation catalyst particle through the porous layer or directly from the surface of the chain oxidation catalyst layer particle to the heat sensitive part of the temperature measuring element, and the heat sensitive part is heated. Here, the oxidation catalyst particles are supported on the porous layer, or the oxidation catalyst particles are connected in a chain, so that the contact area between the combustible gas and the oxidation catalyst particles in the measurement target gas is sufficiently large. Since it can be made large, the calorific value Q according to the above reaction formula (1) is very large, and the temperature rise degree of the heat sensitive part of the temperature measuring element can be increased. Therefore, by using a porous catalyst layer carrying oxidation catalyst particles or a chain catalyst layer in which oxidation catalyst particles are connected in a chain, the overall temperature of the gas sensor can be reduced while the temperature of the temperature sensing element is increased. Even if it is a low concentration flammable gas such as hydrogen in the measurement target gas, the efficiency can be improved and the concentration measurement sensitivity and measurement accuracy can be significantly improved.

  As the porous catalyst layer in the combustible gas sensor according to the present invention, platinum, palladium, rhodium, iridium, nickel, ruthenium is selected from porous materials selected from layered fibers, porous ceramics, fibrous or clustered carbon nanotubes. It may be formed by supporting oxidation catalyst particles selected from noble metals including (Claim 2). In particular, the porous catalyst layer is formed by bonding oxidation catalyst particles to clustered carbon nanotubes formed by growing on a plurality of nuclei aggregated by heat treatment of a nickel layer, an iron layer or a cobalt layer. When used (Claim 3), it is possible to secure a larger contact area with the gas to be measured, to increase the bonding strength between the porous catalyst layer and the thermosensitive part of the temperature measuring element, and to be strong and to have an oxidation reaction heat It also has excellent heat transfer properties (speed, efficiency) to the heat sensitive part, and can further improve measurement sensitivity.

  Further, as the chain catalyst layer in the combustible gas sensor according to the present invention, a large number of oxidation catalyst particles are dispersed by a dispersing agent and heat-treated, so that the oxidation catalyst particles are joined together in a chain to be porous. The formed one can be used (claim 4). In this case, the gas to be measured can be directly brought into contact with the chain-like oxidation catalyst particles, and the heat of oxidation reaction can be increased to further improve the predetermined measurement sensitivity and measurement accuracy.

  In addition, as the temperature measuring element in the combustible gas sensor according to the present invention, it is preferable to use a thermopile (Claim 5).

  Furthermore, in the combustible gas sensor according to the present invention, the oxidation catalyst particle-supporting porous catalyst layer or the porous catalyst layer may be formed on the back surface of the heat sensitive part of the temperature measuring element. In this case, even when there is a slight misalignment when the porous catalyst layer or the porous catalyst layer is formed on the thermosensitive element, the catalyst layer touches the cold junction part of the thermometer element due to the structure. There is no decrease in sensitivity due to an unnecessary temperature rise in the cold junction. As a result, it is possible to improve measurement sensitivity and measurement accuracy and to easily mass-produce combustible gas sensors with uniform characteristics.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a longitudinal sectional view of a combustible gas sensor according to a first embodiment of the present invention. This combustible gas sensor 1-1 has a cavity 3 formed by etching on the back surface of a central portion of a silicon (Si) substrate (an example of a semiconductor substrate) 2 and is measured on the upper surface of the Si substrate 2 corresponding to the cavity 3. As an example of the temperature element, for example, a thermopile 4 is formed which joins dissimilar metals 4A and 4B such as polysilicon and aluminum, and generates and outputs a thermoelectromotive force by the Seebeck effect according to the amount of heat received. Insulating films 5 a and 5 b such as SiO ₂ are formed on the surface of the thermopile 4 and the entire upper surface of the Si substrate 2 around the thermopile 4.

On the insulating film 5a portion made of SiO ₂ on the hot contact portion 4a which is the heat sensitive portion of the thermopile 4, as shown in the enlarged model diagram of FIG. As an example, a porous catalyst layer 7 in which platinum (Pt) particles 6b are scattered and supported is provided, and this porous catalyst layer 7 is chemically or physically attached to the insulating thin film 5a portion on the hot contact portion 4a of the thermopile 4. It is combined with.

  As the oxidation catalyst particles in the porous catalyst layer 7, in addition to the Pt particles 6b, noble metal particles such as palladium (Pd), rhodium (Rh), iridium (Ir), nickel (Ni), ruthenium (Ru) are used alone. Or you may use combining.

In the combustible gas sensor 1-1 of the first embodiment configured as described above, when a measurement target gas containing a combustible gas such as hydrogen contacts the porous catalyst layer 7 carrying the Pt particles 6b and enters. The flammable gas in the measurement target gas, for example, hydrogen gas (H ₂ ) molecules, comes into contact with the Pt particles 6b supported on the porous catalyst layer 7 and, as shown in the above equation (1), oxygen gas (O ₂ ) It reacts with molecules to produce water molecules (H ₂ O), at which time reaction heat Q is generated.

  Here, since the Pt particles 6b in the porous catalyst layer 7 are supported on the fibrous porous material 6a such as carbon fiber, the contact area between the combustible gas in the measurement target gas and the Pt particles 6b is large, and The reaction heat Q becomes very large, and the large reaction heat quickly passes from the surface of the Pt particles 6b to the hot contact portion 4a of the thermopile 4 through the fibrous porous material 6a such as carbon fiber having a high thermal conductivity, and The temperature of the hot contact portion 4a is rapidly and greatly increased as a result of efficient transmission, and a large thermoelectromotive force is generated between the hot contact portion 4a and the cold contact portion. By measuring the thermoelectromotive force and calculating the amount of heat per unit time, the concentration of combustible gas such as hydrogen gas can be measured with very high sensitivity.

  FIG. 3 is a longitudinal sectional view of the combustible gas sensor according to the second embodiment of the present invention. In the gas sensor 1-2 of the second embodiment, a porous layer 8 made of a material having a high thermal conductivity such as carbon fiber or porous ceramic is formed in a thin film on the insulating film 5a portion on the hot contact portion 4a of the thermopile 4. In addition, the porous catalyst layer 7 similar to that of the first embodiment is formed on the thin film-like porous layer 8, and the other components are the same as those of the first embodiment. Are given the same reference numerals and their description is omitted.

In the combustible gas sensor 1-2 of the second embodiment, the measurement target gas is brought into contact with and penetrates into the porous catalyst layer 7 in substantially the same manner as the combustible gas sensor 1-1 of the first embodiment. Combustible gas in the gas, for example, hydrogen gas (H ₂ ) molecule comes into contact with the Pt particles 6b supported on the porous catalyst layer 7 and, as shown in the above formula (1), oxygen gas (O ₂ ) molecule. To generate water molecules (H ₂ O). At this time, the generated reaction heat passes from the surface of the Pt particles 6 b through the fibrous porous material 6 a such as carbon fiber having a high thermal conductivity and the thin film porous layer 8. The hot contact portion 4a of the thermopile 4 is efficiently transmitted to the hot contact portion 4a so that the temperature of the hot contact portion 4a rises rapidly and greatly to generate a large thermoelectromotive force with the cold junction portion. By calculating the amount of heat per hour, hydrogen gas It can be a combustible gas concentration measuring very sensitively.

FIG. 4 is a longitudinal sectional view of a combustible gas sensor according to a third embodiment of the present invention. The combustible gas sensor 1-3 according to the third embodiment forms a double insulating film 5 made of SiO ₂ and SiN on the thermopile 4 and an insulating film portion 5a corresponding to the hot contact portion 4a of the thermopile 4. A chain catalyst layer 9 is formed thereon. As shown in the enlarged model diagram (a) of FIG. 5, the chain catalyst layer 9 is an example of an oxidation catalyst particle, and a large number of Pt particles 6b having a relatively large particle diameter are dispersed from the Pt particles 6b by a dispersant. 5 is dispersed in a material having a low melting point, for example, a fiber material 10 and then heat treated to melt and disappear the fiber material 10 having a low melting point, as shown in FIG. Only the Pt particles 6b are connected to each other in a chain and are formed into a porous shape. Since the other configuration is the same as that of the first embodiment described above, the same members and the same parts are denoted by the same reference numerals, and description thereof is omitted.

  In the combustible gas sensor 1-3 of the third embodiment, when the measurement target gas directly contacts the chain Pt particles 6b forming the chain catalyst layer 9, the reaction occurs as shown by the above-described equation (1). Large reaction heat is generated, and the reaction heat is efficiently transmitted directly from the surface of the Pt particles 6b to the hot junction 4a of the thermopile 4 to raise the temperature and generate a large thermoelectromotive force with the cold junction. Then, by measuring the thermoelectromotive force and calculating the amount of heat per unit time, the concentration of combustible gas such as hydrogen gas can be measured with very high sensitivity.

  FIG. 6 is a longitudinal sectional view of a combustible gas sensor according to a fourth embodiment of the present invention. In the combustible gas sensor 1-4 of the fourth embodiment, a metal film, in particular, a gold (Au) thin film 11 is formed on the insulating film 5a portion on the hot contact portion 4a of the thermopile 4, and this Au thin film On the flat surface of the thin film 11, a plurality of carbon nanotubes (Carbon Nano Tubes, hereinafter referred to as CNT) 12 as representative examples of clustered carbon in which Pt particles are previously supported are disposed. Is connected to the hot contact portion 4a of the thermopile 4 by forming a CNT-R-S-Au structure by bonding the sulfur atom of the thiol group attached to one end of the thiol group and the Au atom of the Au thin film 11. A catalyst layer 7 is formed. Since the other structure is the same as that of the said 3rd Example, the same code | symbol is attached | subjected to the same member and the same site | part, and those description is abbreviate | omitted.

  In addition, as a connection means of the said CNT12, as above-mentioned, in addition to the connection by the coupling | bonding of the sulfur atom of the thiol group attached to one end, and the Au atom of the Au thin film 11, insulation on the hot junction part 4a of the thermopile 4 A diamond thin film is formed on the film 5a by, for example, a CVD method, and a part of the diamond thin film is terminated with a functional group such as hydrogen, a hydroxyl group, a carboxyl group, and an amino group, and a part of the diamond thin film is formed on one end of the CNT 12 The carbon bonds of the CNTs 12 may be terminated by functional groups such as carboxyl groups and amino groups, and the ends of both functional groups may be connected by dehydration polymerization and chemical bonding, and one end of the CNT 12 may be connected to the silane coupling agent. May be terminated by direct chemical bonding to the insulating film 5a portion on the hot contact portion 4a of the thermopile 4.

  Further, as a combustible gas sensor according to a fifth embodiment of the present invention, as shown in FIG. 7, it is grown on a plurality of nuclei 14 aggregated by heat treatment of a thin nickel layer, iron layer or cobalt layer 13 by a CVD method or the like. Then, the clustered CNT 12 is formed, and the porous catalyst layer 7 formed by attaching the Pt particles 6b to the CNT 12 is chemically or physically bonded to the insulating film 5a portion on the hot contact portion 4a of the thermopile 4. It may be what you did. Since the other configuration is the same as that of the third embodiment, the entire configuration of the gas sensor is omitted.

  In the case of the combustible gas sensors of the fourth and fifth embodiments, the entire gas sensor is compact and compact by using the CNT 12 that has a very high thermal conductivity and a large contact area with the measurement target gas. It is possible to generate a large thermoelectromotive force rapidly while achieving the reduction in the concentration, and the concentration of combustible gas such as hydrogen gas can be measured with high sensitivity and accuracy.

  FIG. 8 is a longitudinal sectional view of a combustible gas sensor according to a sixth embodiment of the present invention. In the combustible gas sensor 1-6 of the sixth embodiment, a Pt particle-supported porous catalyst layer 7 is formed in a portion corresponding to the hot contact portion 4a on the back surface of the thermopile 4, and the surface of the thermopile 4 is A protective film 15 is formed. Since the other structure is the same as that of the said 1st Example, the same code | symbol is attached | subjected to the same member and the same site | part, and those description is abbreviate | omitted.

  In the combustible gas sensor 1-6 of the sixth embodiment configured as described above, the concentration of the combustible gas such as hydrogen gas is highly sensitive and accurate as in the first to fifth embodiments. In particular, when the porous catalyst layer 7 is formed on the back side of the thermopile 4 so as to correspond to the hot contact portion 4a of the thermopile 4, the formation thereof is possible. Even if there is a slight misalignment, the porous catalyst layer 7 does not touch the cold junction part of the thermopile 4 due to the structure, so that there is no sensitivity reduction due to an unnecessary increase in temperature of the cold junction part. As a result, the measurement sensitivity and measurement accuracy can be improved, and mass production of combustible gas sensors with uniform characteristics is easy.

  In the sixth embodiment, the catalyst layer formed on the back surface of the thermopile 4 corresponding to the hot contact portion 4a is not limited to the porous catalyst layer as shown in FIG. 2, but in FIG. Even the chain catalyst layer 9 composed of Pt particles 6b linked in a chain as shown in FIG. 6 is a porous catalyst layer 7 using CNTs 12 carrying Pt particles, as shown in FIGS. Also good.

  Moreover, as a forming material of the porous catalyst layer 7 in each of the above embodiments, porous ceramics may be used in addition to the layered fibers such as carbon fibers.

  Moreover, in each said Example, you may comprise so that a measurement object gas may be heated by incorporating a heater etc. around the thermopile 4. In this case, by adjusting the heating temperature of the gas to be measured by the heater, it is possible to measure combustible gas other than hydrogen contained in the gas to be measured, and to have selectivity for the gas to be measured. it can.

  In addition, when the measurement target gas contains a plurality of combustible gases, the combustible gas species are separated using a sampling device and a column, and oxygen is supplied to the combustible gas after the separation, and the thermopile 4 is separated. It is possible to measure the density | concentration of the combustible gas for every isolate | separated kind by making it react by.

  Further, in each of the above embodiments, the thermopile is used as the temperature measuring element. However, in addition to the above, even if the thermistor bolometer is used, the entire sensor can be downsized as described above. In this way, the measurement sensitivity and measurement accuracy of the combustible gas can be improved.

It is a longitudinal cross-sectional view of the combustible gas sensor of 1st Example which concerns on this invention. It is an expansion model figure of the porous catalyst layer in the combustible gas sensor of 1st Example. It is a longitudinal cross-sectional view of the combustible gas sensor of 2nd Example which concerns on this invention. It is a longitudinal cross-sectional view of the combustible gas sensor of 3rd Example based on this invention. (A), (b) is an expansion model figure of the porous catalyst layer in the combustible gas sensor of 3rd Example. It is a longitudinal cross-sectional view of the combustible gas sensor of 4th Example based on this invention. It is an expanded longitudinal cross-sectional view of the porous catalyst layer in the combustible gas sensor of 5th Example based on this invention. It is a longitudinal cross-sectional view of the combustible gas sensor of 6th Example based on this invention.

Explanation of symbols

1-1, 1-2, ..., 16 Flammable gas sensor 2 Si substrate (an example of a semiconductor substrate)
4 Thermopile (an example of a temperature sensor)
4a Hot junction (an example of a heat sensitive part)
5 Insulating film 6 Porous catalyst layer 6a Porous material 6b Pt fine particles (an example of oxidation catalyst fine particles)
7 Chain catalyst layer 8 Porous layer 12 CNT (an example of cluster carbon)
13 Nickel layer, iron layer or cobalt layer

Claims (6)

A temperature measuring element is formed on the insulating film formed on the semiconductor substrate surface, and the calorific value of the measurement target gas is detected by this temperature measurement element so as to measure the concentration of the combustible gas in the measurement target gas. A combustible gas sensor configured as follows:
A porous catalyst layer in which oxidation catalyst particles are supported on a porous material layer or a chain catalyst layer in which a large number of oxidation catalyst particles are connected in a chain form is provided on the thermosensitive part of the temperature measuring element, and this catalyst layer Is integrally coupled to the heat-sensitive part.   The porous catalyst layer is selected from precious metals including platinum, palladium, rhodium, iridium, nickel, and ruthenium as a porous material selected from layered fibers, porous ceramics, fibrous or clustered carbon nanotubes. The combustible gas sensor according to claim 1, wherein the combustible gas sensor is formed by supporting oxidation catalyst particles.   The porous catalyst layer is formed by bonding oxidation catalyst particles to cluster-like carbon nanotubes formed by growing on a plurality of nuclei aggregated by heat treatment of a nickel layer, an iron layer or a cobalt layer. Item 10. The combustible gas sensor according to Item 1.   2. The chain catalyst layer is formed in a porous shape by connecting a number of oxidation catalyst particles in a chain form by dispersing a large number of oxidation catalyst particles with a dispersant and performing a heat treatment. Flammable gas sensor.   The combustible gas sensor according to any one of claims 1 to 4, wherein a thermopile is used as the temperature measuring element. The combustible gas sensor according to any one of claims 1 to 5, wherein the oxidation catalyst particle-supporting porous catalyst layer or the chain catalyst layer is formed on a back surface of a heat sensitive part of a temperature measuring element.

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