JP7287776B2 - Gas detection method, gas sensor and gas detection device - Google Patents

Description

TECHNICAL FIELD The technical field relates to a gas detection method, a gas sensor, and a gas detection apparatus for detecting a gas to be detected using a semiconductor in an oxygen-free atmosphere.

BACKGROUND ART Conventionally, in technical fields such as fuel cells, desulfurization treatment has been performed to remove sulfur compounds contained in commercial gas or the like when obtaining high-purity methane gas using commercial gas or the like as a raw material. In order to monitor whether this desulfurization process is being performed properly, there is a demand for a gas sensor that detects sulfur compounds remaining in gas after desulfurization. For example, city gas, which is a type of commercial gas, contains sulfur compound gas for odorization to facilitate discovery of gas leaks, in addition to methane gas as fuel. Sulfur compound gases include, for example, t-butyl mercaptan (TBM), dimethyl sulfide (DMS) and ethyl mercaptan. In order to detect the sulfur component contained in a substantially oxygen-free gas stream such as city gas, for example, in Patent Document 1 (Japanese Patent No. 4201709), the integrated amount of hydrogen sulfide passing through is detected. A sulfur content detection sensor has been proposed.

Japanese Patent No. 4201709

By the way, as described in Patent Document 1, in a sulfur content detection sensor that integrates the passing amount of hydrogen sulfide, it is difficult to know the generation of sulfur compound gas unless a certain amount of sulfur compound gas passes. Therefore, even if there is a problem with desulfurization and the passage of sulfur compound gas increases, it is difficult to detect it, and a large amount of sulfur compound gas may flow downstream of the sensor installation location.

It is an object to provide a gas detection method, a gas sensor, and a gas detection device that exhibit good responsiveness to polar gases even in an oxygen-free atmosphere.

A plurality of aspects will be described below as means for solving the problem. These aspects can be arbitrarily combined as needed.
A gas detection method according to a first aspect comprises a gas sensor comprising a first electrode, a second electrode, and a metal oxide semiconductor connected in series between the first electrode and the second electrode in an atmosphere of a non-polar gas other than oxygen. and setting the temperature of the gas sensor to a predetermined temperature of less than 100 ° C. in a non-polar gas atmosphere that does not contain the polar gas to be detected, and the resistance value between the first electrode and the second electrode is measured to obtain a reference resistance value, the gas sensor is set to a predetermined temperature, and the resistance value between the first electrode and the second electrode in the atmosphere of the nonpolar gas is compared with the reference resistance value to obtain the nonpolar gas and detecting polar gases entrained in the atmosphere of the.
In the gas detection method according to the first aspect, it is detected that the polar gas to be detected is mixed in the atmosphere of the non-polar gas at a relatively low temperature of less than 100 ° C. in the oxygen-free non-polar gas. Since the resistance value of the oxide semiconductor is reduced, the polar gas can be detected with good responsiveness even in an oxygen-free atmosphere.

The gas detection method described above may be configured such that the polar gas is a sulfur compound gas and the non-polar gas is a hydrocarbon gas. A gas detection method thus constructed is capable of detecting sulfur compound gases, e.g. can be used as a method.
The gas detection method described above is configured such that the metal oxide semiconductor is porous, and in the step of detecting the polar gas, the resistance value between the first electrode and the second electrode is measured without heating the gas sensor. may be In the gas detection method configured in this way, since the gas sensor is not heated, the safety during measurement can be improved. A polar gas can be detected with high accuracy.
The gas sensing method described above may comprise cleaning the gas sensor at a temperature of 200° C. or higher prior to sensing the polar gas. In the gas detection method configured in this manner, detachment of the polar gas adhering to the metal oxide semiconductor can be promoted, so the polar gas can be repeatedly detected with good accuracy.

A gas sensor according to one aspect comprises a first electrode, a second electrode, and a metal oxide semiconductor connected in series between the first electrode and the second electrode, wherein the metal oxide semiconductor contains a substance other than oxygen. In a non-polar gas atmosphere and in a predetermined temperature range of less than 100 ° C., the resistance value between the first electrode and the second electrode is higher than the resistance value between the first electrode and the second electrode when the atmosphere does not contain the polar gas to be detected. is configured to reduce the resistance value between the first electrode and the second electrode when
A gas sensor according to a first aspect detects that a polar gas to be detected is mixed in an atmosphere of a non-polar gas at a relatively low temperature of less than 100° C. in an oxygen-free non-polar gas. Since the resistance value of the semiconductor is reduced, the polar gas can be detected with good responsiveness even in an oxygen-free atmosphere.
The gas sensor described above may be configured such that the metal oxide semiconductor contains tin oxide as a main component. In the gas sensor configured in this way, it is easy to realize a metal oxide semiconductor so as to exhibit good responsiveness in detecting polar gases even in an oxygen-free atmosphere.

The gas sensor described above may be configured to include a first catalyst that acts on at least part of the atmosphere surrounding the metal oxide semiconductor to reduce sensitivity to gases other than the polar gas to be detected. . In the gas sensor configured in this manner, noise caused by changes in the resistance value between the first electrode and the second electrode caused by gases other than the polar gas to be detected is reduced, so that when detecting the polar gas to be detected, the noise is reduced. Improves accuracy.
The gas sensor described above may comprise a second catalyst that acts on at least a portion of the atmosphere surrounding the metal oxide semiconductor to increase its sensitivity to the polar gas to be sensed. In the gas sensor configured in this way, the sensitivity to the polar gas to be detected is improved, making it easier to detect the polar gas to be detected.
The gas sensor described above may be one in which the second catalyst is antimony. In the gas sensor configured in this way, the sensitivity to the polar gas to be detected can be sufficiently improved.
The gas sensor described above may be configured to include a heater arranged at a position capable of heating the metal oxide semiconductor. A gas sensor configured in this way may repeatedly detect polar gases, and heating during measurement may make it easier to measure the resistance value. , the gas sensor can be heated by a heater at the time of measurement, thereby improving the convenience of using the gas sensor.
In the gas sensor described above, the polar gas is a sulfur compound gas and the non-polar gas is a hydrocarbon gas. The gas sensor configured in this manner is used as a gas sensor for detecting, for example, a sulfur compound gas added as an odorant to a commercial gas in an oxygen-free gas stream of a hydrocarbon gas such as a commercial gas. be able to.
The gas sensor described above may be one in which the metal oxide semiconductor is porous.

A gas detection device according to one aspect comprises first and second electrodes and a metal oxide semiconductor connected in series between the first and second electrodes, wherein the metal oxide semiconductor contains a non-oxygen other than oxygen. In a polar gas atmosphere and within a predetermined temperature range of less than 100° C., the polar gas in the atmosphere is higher than the resistance value between the first electrode and the second electrode when the atmosphere does not contain the polar gas to be detected. a gas sensor configured to reduce the resistance between the first and second electrodes when included; and intermittently applying a voltage to the gas sensor between the first and second electrodes. and a resistance measuring device for measuring a resistance value between the first electrode and the second electrode from the voltage applied and the current flowing through the metal oxide semiconductor.
In the gas detection device according to the first aspect, since the voltage applied to the metal oxide semiconductor is intermittent in order to exhibit good responsiveness in detecting polar gases even in an oxygen-free atmosphere, the voltage is constantly applied. Compared to the case of continuing, deterioration of the metal oxide semiconductor can be suppressed, and power consumption can be reduced.
The gas detection device described above further includes a heater connected to the resistance measurement device and arranged at a position capable of heating the gas sensor. It may be configured to measure the resistance between the two electrodes. The gas detection device configured as described above can promote detachment of the polar gas adhering to the metal oxide semiconductor, so that the polar gas can be repeatedly detected with good accuracy.

The gas detection method, gas sensor, or gas detection device of the present disclosure has good responsiveness to polar gases such as sulfur compound gas even in an oxygen-free atmosphere.

FIG. 2 is a block diagram showing how the gas sensor is attached; FIG. 4 is a partially enlarged cross-sectional view showing a state where the gas sensor is attached; (a) a top view of the gas sensor, (b) a side view of the gas sensor viewed from one direction, (c) a side view of the gas sensor viewed from another direction, (d) a cross-sectional view taken along the line II of FIG. 3(c) . (a) A schematic cross-sectional view for explaining a decrease in carriers due to oxygen, and (b) a schematic cross-sectional view for explaining an increase in carriers when oxygen is deprived. (a) A schematic cross-sectional view for explaining electrical conductivity in the absence of polar gas, (b) A schematic cross-sectional view for explaining electrical conductivity in the presence of polar gas. FIG. 4 is a schematic cross-sectional view for explaining the effect of the first catalyst; The graph which shows the fall of the resistance value by non-desulfurization gas. FIG. 4 is a diagram for explaining the sensitivity of a metal oxide semiconductor; The graph for demonstrating the detachment|desorption by temperature rising of the sulfur compound from the tin oxide semiconductor to which the catalyst is not added. A graph for explaining desorption by temperature rise of a sulfur compound from a tin oxide semiconductor to which the 1st catalyst was added. 5 is a graph for explaining desorption of a sulfur compound from a tin oxide semiconductor to which a second catalyst is added due to temperature rise. (a) A graph for explaining changes in the resistance value of the gas sensor, (b) a graph enlarging a portion of FIG. 12(a). FIG. 2 is a schematic diagram for explaining a configuration related to a bead-type tin oxide semiconductor and lead wires; FIG. 2 is a partially broken perspective view for explaining the configuration of a bead-type gas sensor; (a) A schematic diagram for explaining the configuration of a plate-type tin oxide semiconductor and lead wires, (b) A schematic diagram for explaining the configuration of the rear surface of FIG. 15(a). FIG. 2 is a partially broken perspective view for explaining the configuration of a plate-type gas sensor; (a) A schematic diagram for explaining the configuration of a plate-type tin oxide semiconductor without a heater and a lead wire, (b) A schematic diagram for explaining the configuration of the back surface of FIG. 17(a). FIG. 2 is a circuit diagram for explaining a first example of the configuration of the gas detection device; FIG. 4 is a circuit diagram for explaining a second example of the configuration of the gas detection device; FIG. 4 is a circuit diagram for explaining a third example of the configuration of the gas detection device; FIG. 11 is a circuit diagram for explaining a fourth example of the configuration of the gas detection device; 4 is a flow chart showing the flow of gas detection; 4 is a timing chart showing pulse signals output by the drive circuit in gas detection without heating. 4 is a timing chart showing pulse signals output by the drive circuit upon detection of gas to be heated;

(1) Application of Gas Sensor FIGS. 1 and 2 show a gas sensor incorporated downstream of a desulfurizer in a fuel cell system as an application example of the gas sensor.
Fuel cell system 100 includes source 110 , desulfurizer 120 , reformer 130 and fuel cell 140 . The supply source 110 supplies fuel containing a predetermined sulfur compound as an odorant to the desulfurizer 120 . The supply source 110 is, for example, facilities and equipment for supplying city gas, or, for example, a gas container containing compressed gas. The desulfurizer 120 removes sulfur compounds contained in the fuel from the fuel. Desulfurizer 120 is connected to source 110 by a flow path such as gas pipe 101 . The reformer 130 produces a reformed fuel such as hydrogen by a known reforming reaction. The fuel cell 140 uses the fuel reformed by the reformer 130 to generate electricity. The fuel cell system 100 is supplied with methane gas, for example, commercial gas odorized with sulfur compound gas such as t-butyl mercaptan (TBM), dimethyl sulfide (DMS) and ethyl mercaptan. Examples of gases that are hydrocarbon gases and non-polar gases are methane gas, propane gas and butane gas, and examples of gases that are sulfur compound gases and are polar gases are t-butyl mercaptan, dimethyl sulfide and ethyl mercaptan. . Here, a polar gas is a gas composed of polar molecules, and a non-polar gas is a gas composed of non-polar molecules. In other words, a polar gas is a gas whose constituent molecules have an electric dipole moment, and a non-polar gas is a gas whose constituent molecules do not have an electric dipole moment. Here, a case where a commercial gas is used as the fuel in the fuel cell system 100 will be described, but the gas used as the fuel in the fuel cell system to which the gas sensor of the present disclosure is applied is not limited to the commercial gas. It may be gas.

In the fuel cell system 100, desulfurization is performed to remove sulfur compound gas from the supplied commercial gas in order to suppress deterioration of the system. FIG. 1 shows a gas pipe 101 through which hydrocarbon gas after desulfurization passes, and a gas sensor 10 installed in the gas pipe 101 . The gas sensor 10 is fitted into the opening 101a of the gas pipe 101 and fixed with a screw or the like. A gap formed between the gas pipe 101 and the gas sensor 10 is sealed with an O-ring 19 . The seal of this O-ring 19 prevents the hydrocarbon gas from leaking out of the gas pipe 101 and also prevents outside air from entering the gas pipe 101 . In this manner, the gas pipe 101 is filled with hydrocarbon gas and maintained in a substantially oxygen-free state.
The top surface of the gas sensor 10 is shown in FIG. 3(a), and the side surfaces of the gas sensor 10 viewed from two directions perpendicular to each other are shown in FIGS. 3(b) and 3(c). FIG. 3D shows a cross section of the gas sensor 10 taken along line -I. It comprises a substrate 13 covering the bottom of the casing 11 , lead wires 14 fixed to the substrate 13 , and an n-type metal oxide semiconductor 15 attached to the substrate 13 . The lead wire 14 is connected to a first electrode and a second electrode (not shown). The n-type metal oxide semiconductor 15 is connected in series between the first electrode and the second electrode.
The inside of the cap 12 is a cavity 12b. The gas flowing from the circulation portion 12a of the cap 12 contacts the n-type metal oxide semiconductor 15 exposed inside the cavity 12b. Therefore, if the flow portion 12 a faces the inside of the gas pipe 101 , the gas flowing through the gas pipe 101 contacts the n-type metal oxide semiconductor 15 .

(2) Change in resistance value of n-type metal oxide semiconductor 15 In the n-type metal oxide semiconductor 15 of the present disclosure, a change in resistance when polar molecules to be detected are mixed in an oxygen-free atmosphere will be described. The n-type metal oxide semiconductor 15 is mainly metal oxides selected from, for example, tin oxide, tungsten oxide, indium oxide, zinc oxide, titanium oxide, strontium titanate, barium titanate and barium stannate, or combinations thereof. Ingredients. In order to facilitate understanding of changes in the resistance value of the n-type metal oxide semiconductor 15 of the present disclosure, first, a mechanism of changing the resistance value of the metal oxide semiconductor in a conventional gas sensor will be described.

(2-1) Change in Resistance Value of Conventional Metal Oxide Semiconductor Using FIGS. 4(a) and 4(b), reasons why the resistance value of metal oxide semiconductor changes in an atmosphere with oxygen will be explained. Carriers 46 caused by lattice defects are present in the n-type tin oxide SnO _2-x (tin oxide semiconductor 45) shown in FIG. 4(a). When this tin oxide is heated to 300° C. or higher in the atmosphere, oxygen in the atmosphere is dissociated. This dissociated oxygen traps electrons and adsorbs negative charges to the tin oxide semiconductor 45 . Expressing this in a formula, 1/2O ₂ +2e ⁻ →O ²⁻ ad(SnO _2−x ). where ad indicates negative charge adsorption. As a result, when some of the carriers 46 (negative charge) in the tin oxide semiconductor 45 are captured by the adsorbed oxygen 47, the carriers 46 in the tin oxide semiconductor 45 decrease, and the tin oxide semiconductor 45 becomes electrically charged. less conductivity. In other words, the oxygen 47 adsorbed to the tin oxide semiconductor 45 captures the carriers 46, thereby increasing the resistance value of the tin oxide semiconductor 45. FIG.
When oxygen 47 adsorbed to the tin oxide semiconductor 45 exists as shown in FIG. 4(a), and carbon monoxide gas is mixed in the atmosphere as shown in FIG. 4(b), Oxygen adsorbed on the tin oxide semiconductor 45 reacts with carbon monoxide. Oxygen and carbon monoxide react to produce carbon dioxide. This reaction can be represented by a formula: CO+O ²⁻ ad(SnO _2−x )→CO ₂ +2e ⁻ . As a result, the negative charges captured by the adsorbed oxygen 47 are supplied to the tin oxide semiconductor 45, the carriers 46 in the tin oxide semiconductor 45 increase, and the electrical conductivity of the tin oxide semiconductor 45 increases. In other words, the oxygen 47 adsorbed to the tin oxide semiconductor 45 is taken away by carbon monoxide, so that the resistance value of the tin oxide semiconductor 45 decreases.

(2-2) Change in resistance value of metal oxide semiconductor of the present disclosure Using FIGS. 5A and 5B, change in resistance value of metal oxide semiconductor in an oxygen-free atmosphere Explain why. In the example described with reference to FIGS. 5(a) and 5(b), an atmosphere filled with methane gas is considered, but the same applies to the atmosphere composed of other non-polar gases such as ethane gas and propane gas. can be thought of as As shown in FIG. 5A, even if methane gas (CH ₄ ) exists around the tin oxide semiconductor 45, it is considered that the behavior of the carriers 46 of the tin oxide semiconductor 45 is hardly affected. . On the other hand, as shown in FIG. 5B, methyl mercaptan (CH 3 SH) exists around the tin oxide semiconductor 45, and methyl mercaptan (CH ₃ SH) is adsorbed on the tin oxide semiconductor 45 _. Then, it is considered that the carrier 46 of the tin oxide semiconductor 45 is affected by the influence of methyl mercaptan. As a result, the electrical conductivity of the tin oxide semiconductor 45 increases and the resistance value of the tin oxide semiconductor 45 decreases.

Since methyl mercaptan is simply adsorbed on the tin oxide semiconductor 45, if the atmosphere is changed to a state of only methane gas (CH ₄ ), the tin oxide semiconductor 45 can be gradually removed without heating the tin oxide semiconductor 45 as in the conventional case. Then, methyl mercaptan is desorbed from the tin oxide semiconductor 45, and the resistance value of the tin oxide semiconductor 45 increases. For example, in the change in the resistance value of a conventional metal oxide semiconductor, if oxygen dissociation does not occur at a temperature of 300 ° C. or higher, the metal oxide semiconductor like the change in the resistance value of the metal oxide semiconductor of the present disclosure No significant increase in resistance value is observed.
In addition, the big difference between the phenomenon when the resistance value of the conventional metal oxide semiconductor changes and the phenomenon when the resistance value of the metal oxide semiconductor of the present disclosure changes is that the metal oxide semiconductor It is a change in the gas to be detected that has passed through the surroundings. When a change in resistance of a conventional metal oxide semiconductor is observed, it undergoes a chemical change, for example carbon monoxide (CO) to carbon dioxide (CO ₂ ). On the other hand, when a change in the resistance value of the metal oxide semiconductor of the present disclosure is observed, no chemical change occurs, for example methyl mercaptan (CH ₃ SH) does not change before and after the measurement.
As described above, by changing from the state shown in FIG. 5(a) to the state shown in FIG. 5(b), the resistance value changes even if oxygen is not involved. Therefore, by measuring the change in the resistance value of the n-type metal oxide semiconductor of the present disclosure, t-butyl mercaptan (TBM), dimethyl sulfide (DMS) and ethyl mercaptan can be detected.

(2-3) Suppression of miscellaneous gas in metal oxide semiconductor When detecting a polar gas to be detected in the absence of oxygen, the first catalyst 91 (see FIG. 6) is placed in the atmosphere around the n-type metal oxide semiconductor. By acting on the miscellaneous gas inside, the influence of the miscellaneous gas on the resistance value of the n-type metal oxide semiconductor can be suppressed by the first catalyst 91 . In other words, the first catalyst 91 acts on miscellaneous gases around the n-type metal oxide semiconductor to reduce the sensitivity of the gas sensor 10 to the miscellaneous gases. To apply the first catalyst 91 to the n-type metal oxide semiconductor 15, the n-type metal oxide semiconductor 15 is made to support the first catalyst 91, for example.
FIG. 6 schematically shows the function of the first catalyst 91, and the first catalyst 91 shown in FIG. 6 is, for example, a noble metal catalyst. Noble metal catalysts include, for example, palladium (Pd), tungsten (W), platinum (Pt), rhodium (Rh), cerium (Ce), molybdenum (Mo) and vanadium (V). In FIG. 6, the tin oxide semiconductor 45 and the first catalyst 91 are illustrated separately, but this description is for the sake of explanation, and FIG. 6 shows the actual structure. not. The miscellaneous gas on which the first catalyst 91 acts includes, for example, a polar gas that is not to be detected.
As shown in FIG. 6, for example, methyl mercaptan (CH ₃ SH) is a polar gas to be detected, and methyl alcohol (CH ₃ OH) is a miscellaneous polar gas. In this case, by using palladium (Pd), which is a noble metal catalyst, as the first catalyst 91, the amount of methyl mercaptan is also reduced, but the amount of methanol is significantly reduced. As a result, when detecting methyl mercaptan, which is the target gas, the noise caused by methyl alcohol can be reduced, thereby improving the sensitivity to methyl mercaptan.

(2-4) Improving the sensitivity of the metal oxide semiconductor to the target gas When detecting the polar gas to be detected in the absence of oxygen, the second catalyst 92 (see FIG. 6) is placed around the n-type metal oxide semiconductor. By acting on the polar gas to be detected in the atmosphere, the second catalyst 92 can increase the influence of the polar gas to be detected on the resistance value of the n-type metal oxide semiconductor. In other words, the second catalyst 92 has the function of acting on the polar gas to be detected around the n-type metal oxide semiconductor to increase the sensitivity of the gas sensor 10 to the polar gas to be detected. That is. To apply the second catalyst 92 to the n-type metal oxide semiconductor 15, the n-type metal oxide semiconductor 15 is made to support the second catalyst 92, for example.
FIG. 7 shows changes in resistance value of an n-type tin oxide semiconductor carrying palladium and changes in resistance value of a tin oxide semiconductor carrying palladium and antimony.
The resistance values shown in FIG. It is a measurement result when measuring. The resistance value on the vertical axis of FIG. 7 is shown as a ratio to the resistance value of the gas sensor 10 in the desulfurized city gas 13A immediately before the undesulfurized city gas 13A starts to flow. In other words, the value obtained by dividing the resistance value Rs of the gas sensor 10 by the value Rs0 at the time of 0 second on the horizontal axis becomes the value on the vertical axis.

In FIG. 7, the period Pe1 before 0 seconds is the period during which the desulfurized city gas 13A flows through the gas pipe 101, and the period Pe2 after 0 seconds is the period during which the undesulfurized city gas 13A flows through the gas pipe 101. It is a period of time. During this period Pe1, the gas sensor 10 is in an atmosphere of non-polar hydrocarbon gases including methane gas, ethane gas and propane gas. and ethyl mercaptan are not working. On the other hand, during the period Pe2, the gas sensor 10 is in a non-polar hydrocarbon gas atmosphere (in an oxygen-free atmosphere) including methane gas, ethane gas, and propane gas, and the gas sensor 10 detects The polar gases t-butyl mercaptan, dimethyl sulfide and ethyl mercaptan are at work.
A graph Gh1 indicated by a solid line shows changes in the resistance value of the n-type tin oxide semiconductor supporting palladium when measured after being heated in the atmosphere before the measurement. A graph Gh2 indicated by a dashed-dotted line shows changes in the resistance value of the n-type tin oxide semiconductor supporting palladium and antimony when measured after being heated in the atmosphere before the measurement. A graph Gh3 indicated by a chain double-dashed line shows changes in the resistance value of the n-type tin oxide semiconductor supporting palladium and antimony when measured without heating in the atmosphere before the measurement.

As can be seen by comparing the graph Gh1 with graphs Gh2 and Gh3 in FIG. The sensitivity (ratio of change in resistance) to the polar gas to be detected is improved compared to the gas sensor 10 composed of a tin oxide semiconductor that is not coated.
FIG. 8 shows the sensitivity of the n-type tin oxide semiconductor supporting palladium and the n-type tin oxide semiconductor supporting palladium and antimony when the heating conditions of the heater are changed. Regarding the tin oxide semiconductor supporting palladium and antimony, high sensitivity was observed for dimethyl sulfide and the like to be detected, regardless of the presence or absence of heating, even in the absence of oxygen. However, when the tin oxide semiconductor supporting palladium is not heated, sufficient sensitivity to dimethyl sulfide, which is a detection target, cannot be obtained.

(2-5) Desorption of polar gas to be detected from metal oxide semiconductor FIGS. relationship is shown. The desorption amounts of the detected substances shown in FIGS. 9 to 11 were obtained as follows. Methyl mercaptan was allowed to adsorb to the tin oxide semiconductor heat-treated in dry air at room temperature for 2 hours and 30 minutes, replaced with helium (He) gas, and then the above-mentioned tin oxide semiconductor placed in the reaction tube was heated in an electric furnace. After heating, the amount of desorbed material was detected by mass spectrometry.
FIG. 9 shows the case of tin oxide only, FIG. 10 shows the case of tin oxide with palladium added, and FIG. 11 shows the case of tin oxide with palladium and antimony added. 9 to 11, the higher the peak intensity on the vertical axis, the greater the amount of desorption. 9 to 11, graph D1 is methyl mercaptan (CH ₃ SH), graph D2 is dimethyl sulfide (CH ₃ SCH ₃ ), graph D3 is sulfur dioxide (SO ₂ ), and graph D4 is dimethyl sulfide-derived substances. It is estimated that
From FIGS. 9 to 11, desorption of sulfur compound gas was confirmed at around 300.degree. From the comparison of these graphs, it can be seen that the adsorption amount of methyl mercaptan increases due to the effect of the palladium catalyst, and the adsorption amount further increases due to the effect of the antimony catalyst. At 400° C. or higher, substantially the same amount of desorption of sulfur compounds was observed in the case of only tin oxide, the case of adding palladium to tin oxide, and the case of adding palladium and antimony to tin oxide. Although not shown in the graph, a desorption peak was also observed at a low temperature of 200°C or less, but the desorption amount at a low temperature of 200°C or less is extremely small compared to the desorption amount around 300°C.
From the above, it can be seen that heating to a predetermined temperature of 200° C. or higher is effective for cleaning the gas sensor 10 by heating it so that the gas sensor 10 can be used repeatedly. Further, in order to shorten the cleaning time, it is preferable to heat to a predetermined temperature of 300° C. or higher.

(2-6) Behavior of the gas sensor 10 against methyl mercaptan Using FIGS. 12(a) and 12(b), the resistance of the gas sensor 10 when the gas sensor 10 is exposed to meryl mercaptan and then the methyl mercaptan is exhausted. Explain the value change. FIG. 12(b) shows an enlarged graph from -4 hours to 6 hours in FIG. 12(a). During the period shown in FIG. 12(a), the gas sensor 10 is kept at room temperature and is not heated. Moreover, the graph of four gas sensors 10 is shown by Fig.12 (a).
In the period Pe11 shown in FIG. 12(b), the gas sensor 10 is installed in the test chamber and the test chamber is sealed. During this period Pe11, the gas sensor 10 was exposed to the atmosphere. At period Pe12, the air in the test chamber was replaced with nitrogen (N ₂ ). In the period Pe13, the gas sensor 10 is left in the nitrogen atmosphere for one hour. In the next period, Pe14, methyl mercaptan was injected into the test chamber to expose the gas sensor 10 to 80 ppm methyl mercaptan.
Nitrogen gas was then passed through the test chamber for 10 minutes and methyl mercaptan was evacuated from the test chamber (period Pe15). Furthermore, the gas sensor 10 was left in the test chamber filled with nitrogen for 74 hours (period Pe16).
As can be seen from FIGS. 12(a) and 12(b), the injection of methyl mercaptan significantly decreased the resistance value of the gas sensor 10 (period Pe14). From the recovery of the resistance value of the gas sensor 10 in periods Pe15 and Pe16, it is considered that methyl mercaptan is desorbed. From such a phenomenon, it is conceivable that the resistance value is lowered not by chemical adsorption to the metal oxide semiconductor but by physical adsorption to the metal oxide semiconductor.
From the above results, it was found that it is effective to desorb the polar gas from the gas sensor 10 by heating, etc., in order to use the gas sensor 10 repeatedly.

(3) Structure of gas sensor An example of a gas sensor 10 having a structure different from that of the gas sensor 10 shown in FIGS. do.
(3-1) Bead-Type Gas Sensor FIG. 13 shows the tin oxide semiconductor 45 portion of the bead-type gas sensor 10 . As shown in FIG. 13 , the gas sensor 10 includes a coiled second platinum lead wire 24 in a tin oxide semiconductor bead 45 and a straight line passing through the coiled second platinum lead wire 24 . and a first platinum lead wire 23 having a shape.
An assembly including the bead-shaped tin oxide semiconductor 45, the first platinum lead wire 23 and the second platinum lead wire 24 shown in FIG. 13 is manufactured as follows. A first catalyst 91 and a second catalyst 92 are added to the calcined tin oxide, calcined, and pulverized. After that, the pulverized tin oxide, the first catalyst 91 and the second catalyst 92 are made into a paste, attached between the first platinum lead wire 23 and the second platinum lead wire 24, and formed into beads. The paste-like tin oxide, the first catalyst 91 and the second catalyst 92 are fired to form the tin oxide semiconductor 45, the first platinum lead wire 23 and the second platinum lead wire 24 in the state shown in FIG. be. At that time, the first platinum lead wire 23 and the second platinum lead wire 24 are connected to the first electrode 21 and the second electrode 22 shown in FIG. The first electrode 21 and the second electrode 22 are fixed and supported by a resin base 28 . After firing, the assembly of the gas sensor 10 is completed by covering with the cap 29 . The opening 29a of the cap 29 is covered with a highly breathable mesh 29b.
The tin oxide semiconductor 45 formed as described above is porous. As a result, the contact area with the polar gas to be detected increases, so that the influence of the polar gas on the carrier 46 of the tin oxide semiconductor 45 increases, thereby improving the detection sensitivity.

(3-2) Plate-Type Gas Sensor FIGS. 15A and 15B show the tin oxide semiconductor 45 portion of the plate-type gas sensor 10. FIG. The tin oxide semiconductor 45 shown in FIG. 15(a) is formed using thick film printing technology. This tin oxide semiconductor 45 is also preferably formed porous. In this gas sensor 10, for example, conductive layers 36a and 36b made of gold (Au) are provided on one main surface of an alumina substrate 35 having a side length of 2 mm, and gold (Au) is provided on the other main surface of the alumina substrate 35, for example. Au) conductive layers 36a, 36b, 36c and 36d are provided. The conductive layers 36a provided on one and the other principal surfaces are electrically connected to each other, and the conductive layers 36b provided on the one and the other principal surfaces are electrically connected to each other. A lead wire 37a is connected to the conductive layer 36a, a lead wire 37b is connected to the conductive layer 36b, a lead wire 37c is connected to the conductive layer 36c, and a lead wire 37d is connected to the conductive layer 36d. The lead wires 37a-37d are made of, for example, a noble metal alloy. The lead wires 37a-37d are connected to the conductive layers 36a-36d, respectively, by spot welding, for example.
A heater 38 is provided on the other main surface of the alumina substrate 35. The heater 38 is formed by printing a thick film of ruthenium oxide or platinum.

As shown in FIG. 16, the assembly including tin oxide semiconductor 45, alumina substrate 35, conductive layers 36a-36d, lead wire 37a and heater 38 includes first electrode 31, second electrode 32 and third electrode. 33 and the fourth electrode 34 are fixed to a resin base 39 which is fixedly supported. The lead wires 37a-37d are connected to the first electrode 31, the second electrode 32, the third electrode 33 and the fourth electrode 34 shown in FIG. After that, the assembly of the gas sensor 10 is completed by covering with the cap 40 . The opening 40a of the cap 40 is covered with a highly breathable mesh 40b.
The gas sensor 10 can also be configured without the heater 38, as shown in FIGS. 17(a) and 17(b). When the heater 38 is not provided, the material for forming the heater 38, the material for forming the conductive layers 36c and 36d, and the material for forming the lead wires 37c and 37d are omitted, and the manufacturing cost of the gas sensor 10 is reduced. can be reduced.

(4) Gas detector (4-1)
FIG. 18 shows a gas detection device 200 using the bead-type gas sensor 10 shown in FIG. A gas detection device 200 includes a gas sensor 10 , a constant voltage circuit 210 , a switching element 220 , a load resistor 230 and a controller 250 .
The constant voltage circuit 210 is connected to a commercial AC power supply 290 . The constant voltage circuit 210 rectifies the AC voltage supplied from the AC power supply 290 to generate a DC voltage Vc between the + side output terminal 211 and the - side output terminal 212 . For example, pnp transistors can be used for the switching elements 221 and 222 . The emitters of the switching elements 221 and 222 are connected to the + side output terminal 211 of the constant voltage circuit 210 . The switching element 221 has a collector connected to one of the two second electrodes 22 of the gas sensor 10 and a base connected to the drive circuit 254 via the resistor 241 . A collector of switching element 222 is connected to one end of load resistor 230 . The base of switching element 222 is connected to drive circuit 254 via resistor 242 . The other of the second electrodes 22 of the gas sensor 10 is connected to the negative output terminal 212 of the constant voltage circuit 210 . The first electrode 21 of the gas sensor 10 is connected to the other end of the load resistor 230 and the A/D conversion circuit 255 .
Therefore, when the switching element 221 is turned on, a current flows from one side of the second electrode 22 of the gas sensor 10 to the other side, and heat is generated in the gas sensor 10 . Further, when the switching element 222 is turned on, the voltage Vc is applied to the load resistor 230 and the gas sensor 10, and the tin oxide semiconductor 45 of the gas sensor 10 (see FIGS. 13 and 15(a)) is applied from the first electrode 21 of the gas sensor 10 (see FIGS. 13 and 15(a)). current flows to the other second electrode 22 through .

The controller 250 includes a central processing circuit 251 , a memory 252 , an output circuit 253 , a drive circuit 254 and an A/D conversion circuit 255 . A CPU, for example, can be used for the central processing circuit 251 . A driving circuit 254 of the controller 250 receives commands from the central processing circuit 251 and outputs pulse signals to the switching elements 221 and 222 to switch the switching elements 221 and 222 between the ON state and the OFF state, respectively. . The A/D conversion circuit 255 converts the voltage value generated across the load resistor 230 into a digital signal and transmits the digital signal to the central processing circuit 251 . The memory 252 stores data indicating the relationship between the operations of the switching elements 221 and 222 and the voltage across the load resistor 230 and the presence or absence of the polar gas to be detected. The output circuit 253 outputs a signal indicating that the polar gas to be detected has been detected to an external output device (not shown) such as an alarm lamp, an alarm buzzer, or a display device. The on-resistance of the switching element 222 is set to be sufficiently small with respect to the load resistance 230, so that the on-resistance of the switching element 222 can be ignored when detecting the polar gas to be detected.

(4-2)
When the gas sensor 10 has the first electrode 31 to the fourth electrode 34 as shown in FIG. 16, a gas detection device 200 as shown in FIG. 19, for example, can be constructed. The gas detection device 200 shown in FIG. 18 and the gas detection device 200 shown in FIG. 19 differ from the gas sensor 10 in the type of switching element, and only in the connection relationship.
Therefore, here, the connection relationship between the gas sensor 10, the switching elements 223 and 224, and the resistors 243 and 244 of the gas detection device 200 of FIG. 19 will be described. For the switching elements 223 and 224, for example, npn transistors can be used. The emitters of the switching elements 223 and 224 are connected to the negative output terminal 212 of the constant voltage circuit 210 . The switching element 223 has a collector connected to the third electrode 33 of the gas sensor 10 and a base connected to the drive circuit 254 via the resistor 243 . A collector of switching element 224 is connected to one end of load resistor 230 . The base of switching element 224 is connected to drive circuit 254 through resistor 242 . The first electrode 31 of the gas sensor 10 is connected to the other end of the load resistor 230 and the A/D conversion circuit 255 . The second electrode 32 and the fourth electrode 34 of the gas sensor 10 are connected to the + side output terminal 211 of the constant voltage circuit 210 .

(4-3)
In (4-1) and (4-2) above, the gas detection device 200 having a configuration for heating the gas sensor 10 was described, but the configuration for heating the gas sensor 10 may be omitted. For example, the gas sensor 10 may be replaced with a new gas sensor 10 once it detects a polar gas to be detected. For example, as shown in FIGS. 20 and 21, the heater function of the gas sensor 10 may be disabled. omit the explanation.

(5) Gas Detection Method (5-1) Case without Heating Here, a gas detection method using the gas detection device 200 shown in FIG. 20 will be described as an example. FIG. 22 shows an example flow of a gas detection method using the gas detection device 200. As shown in FIG.
First, the gas sensor 10 is placed in an atmosphere of a non-polar gas other than oxygen (step ST1). For example, the gas sensor 10 is installed in the gas pipe 101 shown in FIG. Non-polar gases include, for example, hydrocarbon-based gases such as city gas, propane gas and natural gas. When city gas is circulated in the gas pipe 101, the gas sensor 10 is installed in an atmosphere of non-polar gas mainly composed of methane gas. When the tin oxide semiconductor 45 is used as the gas sensor 10, the resistance value of the gas sensor 10 is also changed by moisture, so it is preferable to provide a filter for removing moisture. However, when the atmosphere contains almost no moisture, such as city gas, means for removing moisture can be omitted in gas detection by the gas sensor 10 .

Next, a reference resistance value is obtained in a non-polar gas atmosphere that does not contain the polar gas to be detected (step ST2). At this time, the temperature of the gas sensor 10 is the same as the temperature of the atmosphere of the non-polar gas. Since the temperature of the atmosphere of the non-polar gas is within the predetermined temperature range of less than 100.degree. As shown in FIG. 12(b), the change in the resistance value of the gas sensor 10 when detecting the polar gas to be detected changes greatly as the digit of the resistance value differs. Therefore, if the polar gas to be detected is detected by taking the ratio of the resistance values before and after the change, false detection can be reduced compared to detecting by comparing the resistance value after the change and the threshold value.
For example, it is assumed that the gas pipe 101 in FIG. 2 is at an outside temperature (an arbitrary temperature within the outside temperature range normally observed in Japan, eg, -40° C. to 50° C., eg, 20° C.). In this case, if the gas pipe 101 is used in Japan, -40°C to 50°C is an example of a predetermined temperature range below 100°C. In gas detection device 200, drive circuit 254 outputs a pulse signal as shown in FIG. The time during which the pulse signal is high (the time during which the voltage is Vc) is, for example, 5 ms, and the time during which the pulse signal is low (the time during which the voltage is 0 V) is, for example, 1 hour. is. Measurement of the resistance value of the gas sensor 10 is performed when the pulse signal is high and the switching element 222 is turned on. The voltage value generated in the load resistor 230 is converted into a digital signal and transmitted from the A/D conversion circuit 255 to the central processing circuit 251 . In the central processing circuit 251 , the resistance value Rs of the gas sensor 10 is calculated from the voltage value V _RL generated in the load resistor 230 . For example, the resistance value Rs can be obtained by calculating Rs=(Vc/V _RL −1)×RL. The measured resistance value Rs is compared to a standard reference resistance value Rs00 stored in memory 252, for example. If the measured resistance value Rs is suitable compared with the standard reference resistance value Rs00, it is stored in the memory 252 as the reference resistance value Rs0 and used in the next detection.

Next, a polar gas mixed in the non-polar gas atmosphere is detected by comparing the resistance value between the first electrode and the second electrode and the reference resistance value in the non-polar gas atmosphere in a predetermined temperature range. (step ST3). For example, the gas detection device 200 compares the resistance value Rs measured in the next measurement in which the reference resistance value Rs0 is stored with the reference resistance value Rs0 stored in the memory 252 . The measurement of the resistance value Rs performed by the gas detection device 200 is performed in the same manner as in step ST2. As described with reference to FIGS. 7 and 12(a), if the polar gas to be detected is mixed, the resistance value is greatly reduced. The gas detection device 200 determines that a polar gas is mixed when (measured resistance value Rs)÷(reference resistance value Rs) calculated by the central processing circuit 251 becomes one-fifth or less.

(5-2) Heating Using the gas detection device 200 shown in FIG. 18 or 19, the gas sensor 10 can be heated using the heater 38 or the like. 18 or 19, when measuring the resistance value between the first electrodes 21, 31 and the second electrodes 22, 32, the solid-line pulse signal shown in FIG. 222 and 224 are driven, and the switching elements 221 and 223 are driven by the pulse signal of the dashed line.
By driving the switching elements 221 and 223, the second platinum lead wire 24 in FIG. 18 generates heat, and the heater 38 in FIG. 19 generates heat. The tin oxide semiconductor 45 is heated by the heat generated by the second platinum lead wire 24 in FIG. 18 or the heat generated by the heater 38 in FIG. . Gas detection can be stabilized by keeping the temperature for gas detection below 100° C. at which polar gases can be easily detected. In such a case, the gas detection device 200 may incorporate temperature-based output adjustment so that the appropriate power is applied to the second platinum lead 24 or the heater 38 depending on the temperature.
In addition, when the sensitivity of gas detection can be improved at 100° C. or more and less than 200° C., the heat generation of the second platinum lead wire 24 in FIG. 18 or the heat generation of the heater 38 in FIG. may be used.

Also, the heat generated by the second platinum lead wire 24 in FIG. 18 or the heat generated by the heater 38 in FIG. 19 may be used to clean the tin oxide semiconductor 45 . In this case, the pulse width of the pulse signal indicated by the dashed-dotted line may be increased so that the oxide semiconductor can be kept at a temperature suitable for cleaning.

(6) Features (6-1)
In anoxic conditions filled with non-polar gases such as methane gas, ethane gas and propane gas, at a relatively low temperature of less than 100 ° C., the polar gas to be detected, t-butyl mercaptan ( TBM), dimethyl sulfide (DMS), ethyl mercaptan, and other sulfur compound gases are mixed in the non-polar gas atmosphere, and the resistance value of the tin oxide semiconductor 45, which is an n-type metal oxide semiconductor, has decreased. can be detected by As a result, it is possible to exhibit good responsiveness in detecting polar gases even in an oxygen-free atmosphere.
The non-polar gas may be, for example, nitrogen gas, carbon dioxide, inert gas other than hydrocarbons. The hydrocarbon gas may be aromatic, for example. Also, the polar gas may be a gas other than the sulfur compound gas, such as carbon monoxide or alcohol.
(6-2)
When the polar gas is a sulfur compound gas and the non-polar gas is a hydrocarbon gas, in the absence of oxygen in a gas stream of a commercial gas, such as city gas, for example, an odorant is added to the commercial gas. It can be used to detect sulfur compound gases that have been added as Safety is improved by performing gas detection at a low temperature of less than 100°C.

(6-3)
The resistance value between the first electrodes 21 and 31 and the second electrodes 22 and 32 can be measured without heating the gas sensor 10 by using the gas detection device 200 without a heater as shown in FIGS. are measuring. Since the gas sensor 10 is not heated, the safety during measurement can be improved, and since the tin oxide semiconductor 45 is porous, the sensitivity is improved compared to the case where the tin oxide semiconductor 45 is not porous, and the polar gas to be detected can be accurately detected. can be detected. Note that the n-type metal oxide semiconductor 15 other than the tin oxide semiconductor 45 is also preferably made porous.
(6-4)
If the gas sensor 10 is cleaned at a temperature of 200° C. or higher before detecting the polar gas to be detected, detachment of the polar gas adhering to the n-type metal oxide semiconductor 15 can be facilitated, and thus good repeatability can be achieved. , it becomes possible to detect polar gases.
(6-5)
Like the gas sensor 10 described above, a first catalyst 91 such as palladium that acts on at least part of the atmosphere surrounding the n-type metal oxide semiconductor to reduce sensitivity to gases other than the polar gas to be detected. (see FIG. 6) reduces noise caused by changes in resistance between the first electrodes 21 and 31 and the second electrodes 22 and 32 due to gases other than the polar gas to be detected. For example, in the example illustrated in FIG. 6, methyl mercaptan is the polar gas to be detected, and methyl alcohol is the miscellaneous gas other than the polar gas to be detected. As a result, in the gas sensor 10, the accuracy of detecting the polar gas to be detected is improved.

(6-6)
Like the gas sensor 10 described above, a second catalyst 92 such as antimony (FIG. 6) acts on at least a portion of the atmosphere surrounding the n-type metal oxide semiconductor to increase its sensitivity to the polar gas to be sensed. ), the sensitivity to the polar gas to be detected is improved, making it easier to detect the polar gas to be detected. In the example illustrated in FIG. 6, methyl mercaptan is the polar gas to be detected. A second catalyst 92 such as antimony is also effective for polar gases other than methyl mercaptan. Other gases, such as sulfur compound gases and polar gases, are also effective. Such gases include, for example, dimethyl sulfide.
(7) Modification (7-1) Modification A
In the above embodiment, the case where the atmosphere is directly exposed to the n-type metal oxide semiconductor 15 or the tin oxide semiconductor 45 has been described, but it may be configured so as to be exposed to the atmosphere that has passed through a filter. In this case, it is preferable to remove moisture and miscellaneous gases with a filter.
(7-2) Modification B
In the above embodiment, the case of using the n-type metal oxide semiconductor 15 has been described, but instead of the n-type metal oxide semiconductor 15, a p-type metal oxide semiconductor may be used.
(7-3) Modification C
In the above embodiment, the polar gas to be detected at a relatively low temperature of less than 100 ° C. in anoxic conditions filled with hydrocarbon gases such as methane gas, ethane gas, and propane gas, which are non-polar gases. The case of detecting that sulfur compound gases such as TBM, DMS and ethyl mercaptan are mixed in a non-polar gas atmosphere has been described. However, non-polar gases are not limited to hydrocarbon gases. Also, the non-polar gas is not limited to the sulfur compound gas. For example, when the hydrogen gas supplied by the reformer 130 shown in FIG. A gas sensor connected in series between one electrode and a second electrode can be used. In this case, the gas sensor 10 is attached to the gas pipe 102 downstream of the reformer 130, for example.

10 gas sensor 15 n-type metal oxide semiconductor 21, 31 first electrode 22, 32 second electrode 45 tin oxide semiconductor 200 gas detector

Claims (9)

placing a gas sensor comprising a first electrode, a second electrode, and an n-type metal oxide semiconductor connected between the first electrode and the second electrode in an atmosphere of a non-polar gas other than oxygen; ,
In an atmosphere of the non-polar gas that does not contain the polar gas to be detected, the temperature of the gas sensor is set to a predetermined temperature of less than 100° C., and the resistance value between the first electrode and the second electrode is measured. obtaining a reference resistance value;
The gas sensor is set to the predetermined temperature, and the resistance value between the first electrode and the second electrode in the atmosphere of the non-polar gas containing the polar gas to be detected is compared with the reference resistance value. detecting the polar gas entrained in the atmosphere of the non-polar gas;
The n-type metal oxide semiconductor is porous,
The polar gas is a sulfur compound gas,
the non-polar gas is a hydrocarbon gas,
In the step of detecting the polar gas, the resistance value between the first electrode and the second electrode is measured without heating the gas sensor.
Gas detection method. cleaning the gas sensor at a temperature of 200° C. or higher prior to detecting the polar gas;
The gas detection method according to claim 1. a first electrode and a second electrode;
an n-type metal oxide semiconductor connected between the first electrode and the second electrode;
with
The n-type metal oxide semiconductor is in an atmosphere of a non-polar gas other than oxygen and within a predetermined temperature range of less than 100° C., the first electrode and the configured such that the resistance value between the first electrode and the second electrode when the atmosphere contains the polar gas is smaller than the resistance value between the second electrodes,
The polar gas is a sulfur compound gas, the non-polar gas is a hydrocarbon gas,
A gas sensor, wherein the n-type metal oxide semiconductor is porous. wherein the n-type metal oxide semiconductor contains tin oxide as a main component,
The gas sensor according to claim 3. A first catalyst that acts on at least part of the atmosphere around the n-type metal oxide semiconductor to reduce sensitivity to gases other than the polar gas to be detected;
wherein the first catalyst is a noble metal catalyst;
The gas sensor according to claim 3 or 4. a second catalyst that acts on at least part of the atmosphere surrounding the n-type metal oxide semiconductor to increase sensitivity to the polar gas to be detected;
wherein the second catalyst is antimony;
The gas sensor according to any one of claims 3 to 5. A heater arranged at a position capable of heating the n-type metal oxide semiconductor,
The gas sensor according to any one of claims 3 to 6. A first electrode, a second electrode, and an n-type metal oxide semiconductor connected between the first electrode and the second electrode, wherein the n-type metal oxide semiconductor is in an atmosphere of a non-polar gas other than oxygen. and in a predetermined temperature range of less than 100 ° C., the polar gas in the atmosphere is higher than the resistance value between the first electrode and the second electrode when the atmosphere does not contain the polar gas to be detected a gas sensor configured to reduce the resistance value between the first electrode and the second electrode when
A voltage is intermittently applied to the gas sensor to generate a voltage between the first electrode and the second electrode from a voltage applied between the first electrode and the second electrode and a current flowing through the n-type metal oxide semiconductor. a resistance measuring device that measures the resistance of
with
The gas detection device, wherein the polar gas is a sulfur compound gas, and the non-polar gas is a hydrocarbon gas. further comprising a heater connected to the resistance measuring device and arranged at a position capable of heating the gas sensor;
After the resistance measuring device is heated by the heater and cleaned,
measuring the resistance between one electrode and the second electrode;
The gas detection device according to claim 8.

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