JP2010043905A - Gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas sensor enhanced in sensitivity while having a simple structure. <P>SOLUTION: The gas sensor 10 includes a positive electrode 1a of a platinum wire, a negative electrode 1b of the platinum wire, and a catalyst layer 3 for covering both of the positive electrode and the negative electrode and characterized in that the positive electrode and the negative electrode are connected through the catalyst layer 3. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a gas sensor, and more specifically to a gas sensor having a simple structure and improved detection sensitivity.

  The gas sensor is sensitive to a specific gas contained in the gas and emits an electric signal having a strength corresponding to the gas concentration. Various gas sensors have been developed and put into practical use since the semiconductor gas sensor first put into practical use in Japan in 1969, and have been widely used in combustible gas leak alarms and automobile exhaust gas purification systems. . Among these, a gas sensor using an oxide semiconductor utilizes the fact that an electric resistance value changes when gas adsorption occurs on the surface of the oxide semiconductor. Simple structure, high sensitivity, excellent responsiveness and stability. Usually, a heating means such as a ceramic heater is provided to detect a change in electric resistance due to the gas adsorption at a high temperature at which the sensitivity is increased.

In recent years, an engine with good fuel efficiency has been emphasized in order to satisfy the waste gas regulations, and development to increase the efficiency of the engine by combustion at a lean fuel concentration has been promoted. In this case, it is necessary to accurately detect NOx in the exhaust gas and feed it back to the mixture ratio of fuel and air, injection timing, injection time, and the like (Non-Patent Document 1). For this reason, research and development of many gas sensors are in progress. For example, the ceramic heater electrode and the sintered oxide semiconductor are arranged so as to sandwich the high thermal conductive insulating substrate, and the electrodes of the ceramic heater are comb-like to ensure temperature uniformity. A gas sensor has been proposed (Patent Document 1). In addition, gas sensors using an oxide semiconductor in the form of a thin film tend to be used, and a thin film electrode containing gold or platinum as a main component is used as an electrode (Patent Document 2). The thin film electrode mainly composed of gold or platinum is formed by screen printing a paste containing gold or platinum.
S. Nakagawa, T. Hori & M. Nagano, "A New Feedback Control of a Lean NOx Trap Catalyst", SAE Technical Paper Series, 2004-01-0527 JP 2001-281190 A JP 2003-232763 A

  According to the method disclosed in the above patent document, the function of the gas sensor can be obtained under a high temperature environment. However, the rate of change in electrical resistance due to gas adsorption is limited, and there is much room for improvement in detection sensitivity. In addition, since the structure of the entire gas sensor is complicated, it is desired to reduce the manufacturing cost in combination with a reduction in the amount of expensive materials used by making the structure simpler. An object of the present invention is to provide a gas sensor having a simple structure and improved sensitivity.

  The gas sensor of the present invention includes a metal positive electrode, a metal negative electrode, and a catalyst layer covering the positive electrode and the negative electrode, and the positive electrode and the negative electrode interpose the catalyst layer. It is characterized by being connected.

  With the above configuration, since the electrodes are positioned with the catalyst layer forming the coating layer interposed therebetween, it is possible to form a sensor having a small electrode interval and a large change rate of electrical characteristics. For this reason, it is possible to obtain a highly sensitive sensor with a simple structure. In addition, the metal positive side electrode and the negative side electrode may be hollow, and the catalyst coated body may have a catalyst coated on the outside of the hollow structure.

  The above plus side electrode and minus side electrode can be used as wires. This makes it possible to easily form a catalyst-coated wire using a wire that is easy to handle. For example, a gas sensor having a large gas adsorption capacity can be obtained by bundling a large number of positive wires and a large number of negative wires with a covering layer interposed therebetween. Also in this case, the wire may be a solid wire or a hollow wire.

The coating layer of the catalyst can be formed by a molten salt plating method. As a result, the above electrode can be coated with the catalyst layer with high coating strength very efficiently. This molten salt plating method does not allow other methods to follow, particularly in the case of a wire rod, in that it has both high production efficiency and high quality. As a result, the manufacturing cost can be reduced. An article coated by the molten salt plating method has an interdiffusion layer between the substrate and the article manufactured by the high temperature molten salt, and can be easily distinguished from the article prepared on the other side. In the present invention, the manufacturing method of the gas sensor is not limited to the molten salt plating method, and the oxidation is performed by oxidizing the metal phase deposited by the vapor phase method by the thermal oxidation method or the oxalic acid anodizing method. A physical semiconductor may be formed. Alternatively, the WO ₃ film may be formed directly by a vapor phase method.

  The sensor can include a heater for heating. Accordingly, the gas sensor main body can be operated in a high temperature range where the sensitivity can be increased for the sensor. For example, a ceramic heater (PTC heater or the like) can be used as the heater.

ZnO, WO ₃ , TiO ₂ , SnO ₂ , Nb ₂ O ₅ , MoO ₃ , In ₂ O ₃ , Ga ₂ O ₃ , Fe ₂ O ₃ , CuO, Cr ₂ O ₃ , Co ₃ O ₄ , At least one of semiconductors such as Bi ₂ O ₃ and Al ₂ O ₃ can be used as a main material. In general, when the detection target gas is determined, the type of catalyst semiconductor is determined. For example, the catalyst WO ₃ or Ga ₂ O ₃ is selectively highly sensitive to NO and hardly sensitive to other gases. By the above catalyst, sensing can be performed with high sensitivity using an optimum catalyst according to the type of gas to be inspected. In the above case, the term “catalyst” refers to an oxide semiconductor, and the electrical characteristics are related to electrical resistance.

  The above metals are platinum (Pt), ruthenium (Ru), palladium (Pd), iridium (Ir), platinum group such as osmium (Os), iron (Fe), cobalt (Co), nickel (Ni), etc. It can be an iron group or a noble metal such as vanadium (V), silver (Ag), gold (Au), or an alloy containing at least one of them. Thereby, an electrode having high durability in a high temperature environment can be obtained.

On the above catalyst layer, metals such as gold (Au), copper (Cu), platinum (Pt), titanium (Ti), zinc (Zn), and CeO, CuO, Fe ₂ O ₃ , MoO ₃ , NiO, Sb One or two or more of oxides such as ₂ O ₅ , SnO ₂ , Ta ₂ O ₅ , TiO ₂ , WO ₃ and ZnO can be contained as a sensitizer. Thereby, the detection sensitivity of the detection gas can be increased and the detection gas can be detected with high accuracy.

  The positive electrode covered with the catalyst layer and the negative electrode covered with the catalyst layer can be knitted to form a hole. Thereby, the contact area between the surrounding gas and the catalyst layer can be increased, and the detection sensitivity can be improved.

Any one of the gas sensors described above is provided as a first gas sensor, and further includes a second gas sensor different from the first gas sensor, and the second gas sensor includes a second catalyst different from the catalyst in the first gas sensor. And the second catalyst has the same form as the first gas sensor and is interposed between the positive electrode and the negative electrode in the second gas sensor, and the first gas sensor and the second gas sensor Can be for detecting different gases. Accordingly, for example, both NO and NO ₂ are often mixed, and when it is necessary to detect both individually and accurately, the above gas sensor having a small and simple structure can be used. it can. When detecting both NO and NO ₂ contained in the exhaust gas of a vehicle engine with high accuracy and feeding back to control the operating condition of the engine, the above gas sensor may be arranged in a narrow place. Yes, it can demonstrate its power. Needless to say, more gas sensors may be provided in addition to the second gas sensor.

  According to the present invention, it is possible to obtain a gas sensor having a simple structure and improved sensitivity.

(Embodiment 1)
FIG. 1 is a diagram showing a gas sensor 10 according to Embodiment 1 of the present invention. In this gas sensor 10, a constant voltage power supply 31 and an ammeter 33 are connected to the outside of the two electrodes 11 and 12. A plurality of plus side electrodes 1 a are connected to the plus side current collecting terminal 11. A plurality of negative side electrodes 1 b are connected to the negative side current collecting terminal 12. Both the plus side electrode 1a and the minus side electrode 1b are formed of platinum wires. A gas detector S is formed between the positive electrode 1a and the negative electrode 1b. The gas detection unit S is formed of a catalyst 3 and platinum wires 1a and 1b covered with the catalyst 3. The thickness of the catalyst layer 3 is preferably 0.01 μm to 1 μm.

A gas species to be detected is adsorbed to the gas detection unit S, and a change in electric resistance is generated, whereby the amount of adsorbed gas can be detected. The gas detection unit S (1) must have selectivity for the gas to be detected. That is, it is necessary to have a function of adsorbing only the gas species to be detected and not adsorbing other gas species. For example, it is necessary that NO ₂ adsorbs but NO does not adsorb. (2) In addition to the above selectivity, it is necessary to increase or decrease the amount of gas adsorbed in proportion to the concentration of gas to be detected in the environment where the gas sensor is placed. The condition (2) is normally satisfied in the case of a dilute concentration.

  FIG. 2 is a longitudinal sectional view of one minus side electrode 1b or plus side electrode 1a. It consists of platinum wires 1a and 1b and a catalyst layer 3 covering the platinum wires. The platinum wires 1a and 1b and the catalyst layer 3 covering the platinum wires 1a and 1b form a catalyst covering C. That is, the one in which the platinum wires 1a and 1b are coated with the catalyst 3 is referred to as a catalyst coating C. The gas detection unit S is formed by a set of catalyst coverings C. 3 is a cross-sectional view taken along line III-III in FIG. The plus side catalyst cover C and the minus side catalyst cover C are alternately inserted to form a set of catalyst covers C. The platinum wires 1a and 1b are conductors, but the catalyst 3 is an oxide semiconductor and forms an electrical resistance R between plus / minus. The gas to be detected is selectively adsorbed on the catalyst, causing a change ΔR in electrical resistance R.

  The electric resistance R and its change ΔR are generated between the positive current collector terminal 11 and the negative current collector terminal 12. Therefore, the positive side catalyst cover C and the negative side catalyst cover C are arranged so as to be in contact with each other as much as possible in order to detect a change in electrical resistance, that is, to ensure high sensitivity. Above, desirable. Under the above arrangement, the change ΔR in electrical resistance occurs in proportion to the amount of gas adsorption. The distance between the positive electrode platinum wire 1 a and the negative electrode platinum wire 1 b is about twice the thickness of the catalyst layer 3. Therefore, the distance between the electrodes is 0.02 μm to 2 μm, which is very small. Since the electrical resistance R is proportional to the distance between the electrodes, the small distance between the electrodes results in a small electrical resistance R. As a result, when the electric resistance change ΔR due to gas adsorption occurs, the electric resistance change rate (ΔR / R) becomes a large value. This means that the sensitivity of the gas sensor having the electrode arrangement using the platinum wire is high. Further, if gas adsorption occurs at any one place where the plus side catalyst cover C and the minus side catalyst cover C are in contact with each other and the electric resistance is lowered, it is immediately indicated on the ammeter 33.

  The catalyst 3 alone has gas selectivity, but it is usual to add a sensitizer in order to further improve the gas selectivity. FIG. 4 is a view showing a cross section of the catalyst cover C in a state where the sensitizer 5 is added to the catalyst 3. The addition amount of the sensitizer is usually several atomic percent or less, but may be added in an amount of about 5% by weight with respect to the catalyst. Table 1 shows examples of the detection target gas, a catalyst (sensitive layer) having high selectivity for the detection target gas, and a sensitizer corresponding to the catalyst. The above is an example, and in addition, there may be a catalyst and a sensitive agent.

  When an oxide semiconductor is used as a catalyst, there are many materials whose sensitivity is maximized within a range of 300 ° C to 450 ° C. For this reason, heating with a heater is required. FIG. 5 is a view showing an example in which the heater 21 is covered with the insulating layer 29 between the above-described sets of the catalyst coverings C. As shown in FIG. All the catalyst coverings C are in contact with the heaters 21 and 29 with the heater arrangement of the degree shown in FIG. For this reason, since the gas sensor 10 does not use a thermally unique material or structure, the temperature uniformity can be ensured by the arrangement of the heaters 21 and 29 that are sparser than those shown in FIG.

Next, a method for manufacturing the gas sensor 10 shown in FIGS. 1 to 3 will be described. First, the platinum wire to be the electrodes 1 a and 1 b is covered with the catalyst 3. The catalyst 3 is, for example, WO ₃ . In this case, there are a method in which a catalyst is coated by a molten salt plating method and a method in which a gas phase method and an oxidation method are combined.
(Molten salt plating method + oxidation method): 67 mol% KF, 26 mol% B ₂ O ₃ and 7 mol% WO ₃ were heated to 850 ° C. in a carbon crucible to obtain a molten salt Form. A predetermined portion of a platinum wire is immersed in the molten salt as a cathode and a tungsten rod is immersed as a counter electrode, and electrolysis is performed at a current density of 30 mA / cm ² for 4 minutes. Then, a platinum wire plated with tungsten (W) can be obtained by taking out the platinum wire from the molten salt and washing the adhered salt with water. Since the molten salt plating method is an atmospheric pressure process, continuous processing is easy and efficient, and a plating layer with high adhesive strength can be formed on the platinum wire because it has an interdiffusion layer between the substrate and the substrate. . After plating tungsten (W) on the platinum wire in this way, WO ₃ is formed by a thermal oxidation method. Due to the above features, it can be specified by the product of the gas sensor.

(Gas phase method + oxidation method): A metal thin film made of tungsten (W) is formed on the surface of the platinum wire by sputtering or CVD (Chemical Vapor Deposition). (A1) Then, by heat treatment in an oxidizing atmosphere, is oxidized to the WO _3. Or (B1) by a tungsten film nitric anodized, oxidized and WO _3. According to the gas phase method + oxidation treatment, a catalyst layer having many pores can be formed by appropriately selecting vapor deposition conditions and oxidation conditions. By increasing the number of pores, the contact area with the gas to be inspected can be increased, and the change ΔR in electrical resistance can be increased.

  The platinum wires 1a and 1b coated with the catalyst layer 3 are divided into the minus side and the plus side, and the catalyst cover C on one pole side contacts the catalyst cover C on the other pole side as alternately as possible. Arrange them and bind them together. The ends where the platinum wires are bound and exposed are connected to the respective current collecting terminals 11 and 12. The bundling method in which one electrode-side catalyst cover C is bound to the other electrode-side catalyst cover C while forming the gas detection portion S may be by sintering the catalyst serving as the coating layer. Alternatively, an adhesive that is solidified by fastening or sintering with another wire may be used.

  According to said method, the gas detection part S can be formed with the simple structure of the alternating insertion set of the catalyst sheath bodies C of a platinum wire. In addition, since the thickness of the catalyst sheath is very thin, the rate of change in electrical resistance due to gas adsorption can be increased, and high sensitivity can be ensured.

(Embodiment 2)
FIG. 6 is a diagram showing the gas sensor 10 according to the second embodiment of the present invention. The gas sensor 10 is characterized in that a plus-side catalyst cover C and a minus-side catalyst cover C are knitted to form a hole (stitch) 8. For this reason, all the connection points 9 on the plus side and the minus side can make sufficient contact with the atmosphere. The change in electrical resistance is captured most sensitively at the connection point where the plus side catalyst cover C and the minus side catalyst cover C intersect. In particular, in the grid-like net shown in FIG. 6, when the electrical resistance decreases due to gas adsorption, if the electrical resistance decreases at one connection point (lattice point) 9, the decrease in electrical resistance immediately causes a change in current. The ammeter can capture the change sensitively. For this reason, when the change in the composition of the atmosphere is severe, the change in the composition of the atmosphere can be captured sharply. The size of the stitch 8 can be easily adjusted by changing the density of the catalyst cover C constituting the lattice. Other structures have the same configuration and effects as those of the first embodiment.

  FIG. 7 is a diagram showing a modification of FIG. 6 according to the embodiment of the present invention. In this modification, the plus-side or minus-side catalyst cover C is wound in a spiral shape, and a connection point 9 is formed at a location intersecting with the spiral-shaped catalyst cover C, while extending in the axial direction. The catalyst coating C to be disposed is disposed. Such a spiral arrangement and an extended arrangement along the axial direction on the inner surface thereof may be said to constitute “knitting”. The connection between the catalyst coverings C is preferably by sintering of the catalysts 3 forming the covering layer. As in the grid-like net shown in FIG. 6, even in the structure shown in FIG. Immediately causes a change in current, and the ammeter can capture the change sensitively.

  In the case of the modification shown in FIG. 7, it is preferable in terms of sensitivity to have a structure in which the atmosphere containing the detection target gas flows in the cylinder. Although not shown, when the linear catalyst cladding C extending in the axial direction is arranged outside the helical catalyst cladding C, the connection point 9 is located outside, so that the outside of the cylinder The atmosphere should flow.

(Embodiment 3)
FIG. 8 is a diagram illustrating the gas sensor 10 according to Embodiment 3 of the present invention. FIG. 9 is a cross-sectional view taken along line IX-IX in FIG. The gas sensor 10 can detect two types of detection gases at the same time. In many cases, the usefulness can be improved by detecting two kinds of gases at the same time. As such an example, NOx can be given. NOx is a term that refers to two types of gases, NO and NO ₂ . Since NO and NO ₂ are often converted into each other by chemical reaction or the like and exhibit relatively similar characteristics in many cases, it is more convenient to handle them together. Is more prevalent. In a gasoline engine, a diesel engine, or the like, it may be possible to perform the above feedback more appropriately by accurately knowing the ratio of NO to NO ₂ in the exhaust gas. In addition, it is good also as a gas sensor which distinguishes and detects not only the said 2 types of gas but 3 or more types of gas.

As shown in FIG. 8, in the gas sensor 10 detects the NO in the gas detection section _{S 1,} and also detect the NO ₂ in the gas detector _{S 2.} Between them, heaters 21 and 29 are arranged as shown in FIG. Gas detector _{S 1} of the electrode 1a, 1b and the gas detection section _{S 2} of the electrodes 51a, 51b may be both used a platinum wire. However, different catalysts and sensitizers are used. For example, in the gas detector S ₁ , Ga ₂ O ₃ may be used for the catalyst 3, and Ta ₂ O ₅ may be used for the sensitizer 5. In the gas detector S ₂ , it is preferable to use Cr ₂ O ₃ for the catalyst 53 and TiO ₂ for the sensitizer 55.

  As explained in the first embodiment, the catalyst structure in which the platinum wire is coated with the catalyst and the sensitizer is divided into the plus side and the minus side, and the structure of the aggregate inserted so as to be alternately contacted is as follows. Simple. The structure in which two simple catalyst-covered aggregates are formed and the heaters 21 and 29 are sandwiched between them is very simple as a gas sensor for detecting two kinds of gases at the same time. For this reason, since the volume can be reduced, for example, it can be easily attached to a narrow place such as an exhaust gas path of an internal combustion engine of an automobile. And all the members which comprise said gas sensor 10 are excellent in heat resistance. For this reason, the gas sensor 10 itself is also excellent in heat resistance, and can stably maintain the function of the gas sensor for a long period of time.

(Other embodiments)
In the above description of the embodiment, the exhaust gas of the internal combustion engine of the automobile is often used as a reference, but the gas sensor of the present invention is not limited to the exhaust gas of the internal combustion engine of the automobile. (1) Exhaust of various boilers, (2) Monitoring of atmospheres in public facilities and living spaces, (3) Monitoring of atmospheres of various factory facilities with controlled atmospheres, and the like. Further, it can be used not only for so-called gas but also for an odor (odor) sensor.

  Although the embodiments of the present invention have been described above, the embodiments of the present invention disclosed above are merely examples, and the scope of the present invention is not limited to these embodiments. The scope of the present invention is indicated by the description of the scope of claims, and further includes meanings equivalent to the description of the scope of claims and all modifications within the scope.

  According to the present invention, gas detection with high sensitivity can be performed with a simple structure, and two types of simultaneous gas detection devices can be easily obtained by taking advantage of the simple structure.

It is a figure which shows the gas sensor in Embodiment 1 of this invention. It is a figure which shows one catalyst coating body in the gas sensor shown in FIG. It is sectional drawing which follows the III-III line in FIG. It is sectional drawing of the catalyst coating body which coat | covered the wire with the catalyst and the sensitizer. It is sectional drawing which shows the gas detection part which has arrange | positioned a heater. It is a figure which shows the gas sensor in Embodiment 2 of this invention. It is a figure which shows the modification of the gas sensor in Embodiment 2 of this invention. It is a figure which shows the gas sensor in Embodiment 3 of this invention. It is sectional drawing which follows the IX-IX line in FIG.

Explanation of symbols

1a, 1b, 51a, 51b Platinum wire (electrode), 3,53 catalyst (sensitive layer), 5,55 sensitizer, 9 connection point (lattice point), 10 gas sensor, 11, 12, 61, 62 current collecting terminal , 21 heater, 29 insulator, 31, 81 constant voltage power source, 33, 83 ammeter, C catalyst covering, S, S ₁ , S ₂ gas detector.

Claims (9)

A metal positive electrode,
A metal negative electrode,
A catalyst layer covering the plus side electrode and the minus side electrode,
The gas sensor, wherein the plus side electrode and the minus side electrode are connected via the catalyst layer.   The gas sensor according to claim 1, wherein the plus side electrode and the minus side electrode are wires.   The gas sensor according to claim 1, wherein the catalyst layer is formed by a molten salt plating method.   The gas sensor according to claim 1, further comprising a heater for heating. Said _{catalyst, ZnO, WO 3, TiO 2} , SnO 2, Nb 2 O 5, MoO 3, In 2 O 3, Ga 2 O 3, Fe 2 O 3, CuO, Cr 2 O 3, Co 3 O 4, _{Bi 2 O 3, Al 2 O} 3, a semiconductor etc., characterized by at least one kind of the main material of the gas sensor according to any one of claims 1 to 4.   The metal is a platinum group such as platinum (Pt), ruthenium (Ru), palladium (Pd), iridium (Ir), and osmium (Os), and iron such as iron (Fe), cobalt (Co), and nickel (Ni). It is any one of a group, or noble metals, such as vanadium (V), silver (Ag), gold (Au), or an alloy containing at least one of these. The gas sensor according to any one of 1 to 5. On the catalyst layer, a metal such as gold (Au), copper (Cu), platinum (Pt), titanium (Ti), zinc (Zn), and CeO, CuO, Fe ₂ O ₃ , MoO ₃ , NiO, Sb ₂ The oxide of O ₅ , SnO ₂ , Ta ₂ O ₅ , TiO ₂ , WO ₃ , ZnO or the like, containing one or more of them as a sensitizer, according to claim 1, The gas sensor as described in any one.   The positive electrode covered with the catalyst layer and the negative electrode covered with the catalyst layer are knitted to form a hole. Gas sensor described in 1.   A gas sensor according to any one of claims 1 to 8 is provided as a first gas sensor, a second gas sensor different from the first gas sensor is further provided, and the second gas sensor is a catalyst in the first gas sensor. A second catalyst different from the first catalyst, and the second catalyst has the same form as the first gas sensor and is interposed between the plus side electrode and the minus side electrode in the second gas sensor, A gas sensor, wherein the first gas sensor and the second gas sensor are for detecting different gases.

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