JP4916357B2 - Semiconductor gas detector - Google Patents

Description

  The present invention relates to a semiconductor type gas detection element including a gas sensitive part.

  In general, a semiconductor type gas detection element includes a gas sensitive part mainly composed of a metal oxide semiconductor, and when a gas to be detected (hereinafter referred to as “detected gas”) contacts the gas sensitive part, The gas to be detected reacts with the metal oxide semiconductor, and exchange of electrons is performed. Since the resistance value of the metal oxide semiconductor changes due to the exchange of electrons, a gas sensor equipped with a semiconductor gas detection element detects the gas to be detected by taking out the change in the resistance value of the metal oxide semiconductor as a sensor output. Can do.

  A gas sensor including a semiconductor gas detection element has high durability and is used for a home gas leak alarm. In recent years, housing has become more airtight and organic gas generated from daily life has diversified and increased. Therefore, gas that is different from the gas to be detected (hereinafter referred to as "interference gas") is detected. And may give an alarm. As a technique for preventing such a false alarm, it is known to provide a catalyst layer for removing the interference gas on the surface of the gas sensitive part of the semiconductor type gas detection element to increase the selectivity to the gas to be detected (for example, Patent Document 1). As the catalyst layer, for example, there is one using a noble metal catalyst supported on a metal oxide, and in this type of semiconductor gas detection element, a relatively reactive interference gas such as hydrogen or ethanol is burned in the catalyst layer. It can be removed (see, for example, Patent Document 2).

JP-A 49-129596 JP 2002-139469 A

  However, in a semiconductor type gas detection element using a noble metal catalyst in the catalyst layer, when used for a long time at the operating temperature of the gas sensor, the noble metal fine particles aggregate, the oxidation activity of the noble metal catalyst is reduced, and the interference gas cannot be burned and removed. There was a problem. In this case, as the time passes, the interference gas easily passes through the catalyst layer and reaches the gas sensitive part, and the selectivity to the gas to be detected is lowered. Therefore, an alarm device using such a semiconductor gas detection element Etc., there was a risk of misreporting.

  This invention is made | formed in view of the said subject, and it aims at providing the semiconductor type gas detection element which has gas selectivity over a long period of time.

In order to achieve the above object, the first characteristic configuration of the semiconductor gas detection element according to the present invention is a semiconductor gas detection element comprising a gas sensitive part and a catalyst layer covering the gas sensitive part,
The catalyst layer includes a metal oxide semiconductor containing at least one metal oxide selected from the group consisting of tin oxide, indium oxide, titanium oxide, zinc oxide, iron oxide, and cerium oxide, cerium, tin, aluminum, The metal composite oxide contains at least one metal element selected from the group consisting of lanthanum, niobium, yttrium, zirconium, molybdenum, ruthenium, neodymium, gadolinium, vanadium, silicon, and magnesium.

  Tin oxide, indium oxide, titanium oxide, zinc oxide, iron oxide, and cerium oxide have high oxidation activity. In this configuration, the metal oxide semiconductor containing these metal oxides is selected from the group consisting of cerium, tin, aluminum, lanthanum, niobium, yttrium, zirconium, molybdenum, ruthenium, neodymium, gadolinium, vanadium, silicon, and magnesium. A metal composite oxide (solid solution) in which at least one metal element is solid-dissolved is used as a catalyst layer of a semiconductor gas detection element.

That is, the present inventors pay attention to the fact that the crystallite size is reduced by dissolving the metal element in the metal oxide semiconductor, and the metal element is dissolved in the metal oxide semiconductor having oxidation activity as the catalyst layer. By using the metal composite oxide, it is possible to suppress the growth of crystallites under high temperature while preventing the oxidation of the metal oxide semiconductor, and to prevent deterioration of the catalyst layer over time. I found.
Therefore, in the semiconductor gas detection element according to the present configuration, the oxidation activity of the catalyst layer is unlikely to change, so that gas selectivity can be maintained over a long period of time.

  The second characteristic configuration of the semiconductor gas detection element according to the present invention is that the metal complex oxide is formed by dissolving 0.01 to 10 mol% of the metal element in the metal oxide semiconductor.

  If this is a metal composite oxide in which a metal element is solid-dissolved in an amount of 0.01 to 10 mol% with respect to the metal oxide semiconductor, the crystallite size becomes smaller, the specific surface area becomes larger, and the high temperature Even underneath, the growth of crystallites can be suppressed, so that the oxidation activity is hardly lowered.

  According to a third characteristic configuration of the semiconductor gas detection element according to the present invention, the metal oxide semiconductor is an n-type semiconductor, and the metal element is a metal ion having a lower valence than the metal element of the metal oxide. It is in point to include.

When a current flows through the catalyst layer, it functions as a gas sensitive material and may affect the sensor output.
In this configuration, when the metal oxide semiconductor is an n-type semiconductor, the oxidation activity is controlled by solid-dissolving at least a metal element that becomes a metal ion having a valence smaller than that of the metal element of the metal oxide. The resistance value of the metal composite oxide can be increased while maintaining for a long time. Thereby, since it becomes difficult to flow an electric current through a catalyst layer, it can prevent that a catalyst layer functions as a gas sensitive material, and can maintain a stable sensor output over a long period of time.

  According to a fourth characteristic configuration of the semiconductor gas detection element according to the present invention, the metal oxide semiconductor is a p-type semiconductor, and the metal element is a metal ion having a higher valence than the metal element of the metal oxide. It is in point to include.

  When the metal oxide semiconductor is a p-type semiconductor as in this configuration, at least by controlling the valence by dissolving a metal element that becomes a metal ion having a valence larger than the metal element of the metal oxide, The resistance value of the metal composite oxide can be increased while maintaining the oxidation activity for a long period of time. Thereby, since it becomes difficult to flow an electric current through a catalyst layer, it can prevent that a catalyst layer functions as a gas sensitive material, and can maintain a stable sensor output over a long period of time.

  A fifth characteristic configuration of the semiconductor gas detection element according to the present invention is that, in the third characteristic configuration, the metal oxide semiconductor is made of tin oxide, and the metal elements are cerium and aluminum.

  As in this configuration, when cerium and aluminum are dissolved in a metal oxide semiconductor made of tin oxide having oxidation activity, the resistance value of the metal composite oxide can be increased while maintaining the oxidation activity over a long period of time. it can.

  Hereinafter, an embodiment of a gas sensor using a semiconductor gas detection element according to the present invention will be described with reference to the drawings. Here, although a hot wire type semiconductor gas detection element is illustrated as a semiconductor type gas detection element, it is not restricted to this. Other semiconductor gas detection elements include substrate type semiconductor gas detection elements.

  As shown in FIG. 2, the gas sensor according to the present embodiment is configured by incorporating a hot-wire semiconductor gas detection element Rs and fixed resistors R0, R1, and R2 into a bridge circuit. The bridge circuit is energized constantly or intermittently by a power source E so that the hot-wire semiconductor gas detection element Rs has a temperature suitable for detection. The resistance value of the hot-wire semiconductor gas detection element Rs changes when the gas to be detected is adsorbed. For this reason, in the gas sensor according to the present embodiment, the change in the resistance value of the hot-wire semiconductor gas detection element Rs is extracted as a deviation voltage, and this is used as the sensor output V to measure the concentration of the gas to be detected in the air. can do.

  As shown in FIG. 1, the hot-wire semiconductor gas detection element Rs of the present embodiment is provided with a gas-sensitive part 2 and a catalyst layer 3 that covers the gas-sensitive part 2 on a coiled noble metal wire 1. . The noble metal wire 1 has the same material, wire diameter, coil diameter, number of coil turns, and the like as those used in a conventional hot-wire semiconductor gas detection element, and is not particularly limited. In this embodiment, the wire diameter is 20 μm. A platinum wire is used, the coil diameter is 200 μm, and the number of coil turns is 12 turns.

The gas sensitive part 2 can use, for example, a metal oxide semiconductor containing a metal oxide such as tin oxide, zinc oxide, indium oxide, tungsten oxide, etc., and can be arbitrarily selected depending on the type of gas to be detected. In this embodiment, hydrocarbons such as methane and isobutane are used as the gas to be detected, a metal oxide semiconductor (Sb—SnO ₂ ) in which 0.1 mol% of antimony is added to tin oxide is used, and the film thickness is 0.24 mm. It is.

  The catalyst layer 3 is made of a metal oxide semiconductor containing at least one metal oxide selected from the group consisting of tin oxide, indium oxide, titanium oxide, zinc oxide, iron oxide, and cerium oxide, cerium, tin, aluminum, It is formed of a metal composite oxide in which at least one metal element selected from the group consisting of lanthanum, niobium, yttrium, zirconium, molybdenum, ruthenium, neodymium, gadolinium, vanadium, silicon, and magnesium is dissolved. The crystallite size is reduced by dissolving the metal element in the metal oxide semiconductor. Therefore, by using a metal composite oxide in which a metal element is dissolved in a metal oxide semiconductor having oxidation activity as the catalyst layer 3, the crystallites at high temperature while having oxidation activity of the metal oxide semiconductor. The growth of the catalyst layer 3 can be prevented and the deterioration of the catalyst layer 3 over time can be prevented. Therefore, in the hot wire type semiconductor gas sensing element according to the present embodiment, the interference gas can be burned and removed in the catalyst layer 3 over a long period of time, so that the gas selectivity can be maintained.

  As the metal oxide semiconductor, one having an oxidation activity with respect to an interference gas can be arbitrarily selected. The metal element that is solid-solved in the metal oxide is as described above, and any of those that can be substituted with the metal atom of the metal oxide and those that can be intruded into the metal oxide can be used. The solid solution amount of the metal element to be dissolved in the metal oxide is not particularly limited and can be up to the solid solution limit amount. However, as shown in an example described later (FIG. 6), the crystallite size is affected. 0.01 mol% to 10 mol% is preferable, and 1 mol% to 10 mol%, from which the influence increases, is more preferable. Further, since the crystallite size hardly changes at 3 mol% or more, 3 mol% to 10 mol% is more preferable.

  When an electric current flows through the catalyst layer 3, it may function as a gas sensitive material and affect the sensor output. For this reason, it is preferable to raise the resistance value of the catalyst layer 3 by controlling the valence of the metal oxide semiconductor. Thereby, since it becomes difficult to flow an electric current through the catalyst layer 3, it can prevent that the reaction which occurs in the catalyst layer 3 influences a sensor output, and can maintain a stable sensor output over a long period of time.

  When the metal oxide semiconductor is an n-type semiconductor, the valence control can be performed by dissolving a metal element that becomes a metal ion having a valence smaller than that of the metal oxide of the metal oxide semiconductor. In the case where the metal oxide semiconductor is a p-type semiconductor, it can be performed by dissolving a metal element that becomes a metal ion having a higher valence than the metal element of the metal oxide of the metal oxide semiconductor.

  Therefore, in order not to lower at least the resistance value of the metal oxide semiconductor, when the metal oxide semiconductor is an n-type semiconductor as the metal element to be dissolved, the metal of the metal oxide constituting the metal oxide semiconductor It is preferable to use a metal ion that is less than the valence of the element. When the metal oxide semiconductor is a p-type semiconductor, a metal that is greater than the valence of the metal element of the metal oxide constituting the metal oxide semiconductor. It is preferable to use an ion.

  Tin oxide, indium oxide, titanium oxide, zinc oxide, iron oxide, and cerium oxide can be used as an n-type semiconductor. Therefore, these metal oxides can themselves be used as metal oxide semiconductors. Alternatively, a metal oxide semiconductor obtained by adding a metal element which becomes a metal ion having a valence smaller than that of the metal oxide of the metal oxide to the metal oxide can be used. In this case, it becomes a p-type semiconductor.

  In this embodiment, alcohol such as hydrogen or ethanol is used as an interference gas, and as shown in Examples (FIGS. 3 to 5) to be described later, tin oxide having oxidation activity with respect to these interference gases is used as a metal oxide semiconductor. The catalyst layer 3 is prepared by dissolving 3 mol% of cerium and further dissolving 1 mol% of aluminum which can be a metal ion having a valence (3) smaller than the valence (4) of tin in tin oxide.

Such a catalyst layer 3 can be formed by a coprecipitation method or a kneading method. In the catalyst layer 3 of the present embodiment, a mixed aqueous solution of tin chloride (SnCl ₄ ), ammonium cerium nitrate (Ce (NH ₄ ) ₂ (NO ₃ ) ₆ ), and aluminum chloride (AlCl ₃ ) is prepared. An aqueous ammonia (NH ₃ ) solution is added dropwise (coprecipitation method). Washed with water thereby resulting precipitate dried was ground, produced by firing at 700 to 800 ° C., the metal composite oxides of cerium and aluminum are dissolved in the tin oxide (Al _x Ce _y SnO _z) can do.

The metal composite oxide (Al _x Ce _y SnO _z), the crushed precipitate was added dropwise to ammonia (NH ₃₎ solution to mixed aqueous solution of tin chloride (SnCl ₄₎ and aluminum chloride (AlCl ₃₎ In this case, it can also be produced by kneading as an aqueous solution of cerium ammonium nitrate (Ce (NH ₄ ) ₂ (NO ₃ ) ₆ ) and firing (kneading method).

The metal composite oxide (Al _x Ce _y SnO _z ) produced in this way is attached in a paste form so as to cover the gas sensitive part 2 and fired by self-heating of the noble metal wire 1 to form the catalyst layer 3. Let it form. In the present embodiment, the thickness of the catalyst layer 3 is set to 0.1 mm.

  In addition, about the other structure of a gas sensor provided with the hot wire type | mold semiconductor type gas detection element Rs, a function, etc., it is the same as that of a conventionally well-known gas sensor.

(Measurement method of sensor output over time)
The hot-wire semiconductor gas detection element Rs according to the present embodiment as shown in FIG. 1 is used by being incorporated in the bridge circuit shown in FIG. 2, a voltage of 2.0 V is applied, and the operating temperature is about 450 ° C. Changes in sensor output over time with respect to air, methane (2000 ppm), isobutane (2000 ppm), hydrogen (5000 ppm), and ethanol (1000 ppm) were examined.

(Sensor characteristics of metal oxide)
In the hot-wire semiconductor gas detection element Rs according to the present embodiment, when the catalyst layer 3 is not provided, when the catalyst layer 3 is made of alumina (Al ₂ O ₃ ) having a particle size of about 30 nm, the particle size is applied to the catalyst layer 3. With respect to the case where tin oxide (SnO ₂ ) of about 49.5 nm was used, the change over time of the sensor output with respect to the above gas was examined. As a result, when the catalyst layer 3 is not provided (FIG. 3), the sensor output of hydrogen and ethanol increased with the passage of days, whereas when Al ₂ O ₃ was used for the catalyst layer 3 (FIG. 4). However, although there was almost no change in the sensor output with time, it was found that there was no selectivity between methane, isobutane and hydrogen, and there was no oxidation activity for the gas. Further, in the case of using SnO ₂ in the catalyst layer 3 (FIG. 5), but the sensor output of the hydrogen and ethanol early changes are small, was found to increase with the passage of days. That is, it was found that SnO ₂ has oxidation activity against hydrogen and ethanol, but the oxidation activity decreases with time.
Therefore, in the following using a SnO _2, were examined their oxidation activity.

(Solubility of cerium)
Changes in lattice constant (a-axis) and crystallite size when tin oxide was doped with different amounts of cerium (Ce) by the coprecipitation method were examined by Rietveld method. As a result, as shown in FIG. 6, it was found that the lattice constant (a-axis) was large until the Ce doping amount was at least 10 mol%, and that Ce was dissolved in SnO ₂ . The crystallite size changed when Ce was doped with at least 0.01 mol%, and rapidly decreased when doped with 1 mol%. Although it became smaller at 3 mol%, it turned out that it changes little more than that.
Therefore, the solid solution amount of Ce in SnO ₂ is preferably 0.01 mol% to 10 mol%, more preferably 1 mol% to 10 mol%, and even more preferably 3 mol% to 10 mol%.

(Sensor characteristics of solid solution)
In the hot-wire semiconductor gas detection element Rs according to the present embodiment, the above gas in the case of using 3 mol% of Ce dissolved in SnO ₂ as the catalyst layer 3 by the above coprecipitation method and firing at 700 ° C. The change of sensor output with respect to time was investigated. As a result, as shown in FIG. 7, it was found that the sensor outputs of hydrogen and ethanol have little change over time and the oxidation activity is difficult to decrease.
Therefore, it was found that the semiconductor gas detection element can maintain gas selectivity over a long period of time by using, as the catalyst layer 3, a metal composite oxide in which Ce is dissolved in SnO ₂ .

(Manufacturing method)
In the hot-wire semiconductor gas sensing element Rs according to the present embodiment, the catalyst layer 3 is prepared by dissolving 3 mol% of Ce in SnO ₂ by the above kneading method and firing at 700 ° C. The change in sensor output over time was examined. As a result, as shown in FIG. 8, the sensor outputs of hydrogen and ethanol increased with the passage of days, and it was found that the decrease in oxidation activity over time was larger than that in the coprecipitation method (FIG. 7). This is presumably because Ce was not sufficiently dissolved in the kneading method.

(Baking temperature)
In the hot-wire semiconductor gas sensing element Rs according to the present embodiment, 3 mol% of Ce was dissolved in SnO ₂ as the catalyst layer 3 by the above-described coprecipitation method and fired at 800 ° C. ((3 mol%) Ce—SnO ₂ ) The change over time of the sensor output for the above gas was investigated. As a result, as shown in FIG. 9, it was found that there was almost no change in the sensor output, and the change over time in the oxidation activity of the catalyst layer 3 could be kept small as compared with that calcined at 700 ° C. (FIG. 7).
Therefore, it was found that long-term stability of the semiconductor gas detection element is improved by using a solid solution fired at 800 ° C.

(Aluminum solid solution)
Changes in lattice constant (a-axis) and crystallite size when (3 mol%) Ce—SnO ₂ was further doped with different amounts of aluminum (Al) by the coprecipitation method were examined by Rietveld method. As a result, as shown in FIG. 10, it was found that the lattice constant (a axis) was small until the Al doping amount was at least 10 mol%, and Al was dissolved. It was also found that the crystallite size was small in change with respect to the Al doping amount.

(Resistance change)
SnO ₂ , (3 mol%) Ce—SnO ₂ , (3 mol%) Ce—SnO ₂ in which 1 mol% of Al is dissolved ((1 mol%) Al— (3 mol%) Ce—SnO ₂ ), (3 mol%) ) Ce-SnO ₂ in which 3 mol% of Al was dissolved ((3 mol%) Al— (3 mol%) Ce—SnO ₂ ) and Sb—SnO ₂ were applied on the comb-shaped electrode, and the resistance value was It was measured. As a result, as shown in FIG. 11, it was found that the solid solution of Al has a higher resistance value than SnO ₂ and (3 mol%) Ce—SnO ₂ . Further, the resistance value hardly changed between 1 mol% of Al and 3 mol% of Al.
Therefore, from the results of FIGS. 10 and 11, the solid solution amount of Al is preferably 1 mol% to 10 mol%, and more preferably 1 mol% to 3 mol%.

(Change in sensor output over time)
In the hot-wire semiconductor gas sensing element Rs according to the present embodiment, (1 mol%) Al- (3 mol%) Ce-SnO ₂ is used as the catalyst layer 3, and (3 mol%) Al- (3 mol%). With respect to the case of using Ce—SnO ₂ , the change with time of the sensor output with respect to the gas was examined. As a result, in the case of using a solid solution of 1 mol% of Al (FIG. 12) and the case of using a solid solution of 3 mol% of Al (FIG. 13), the case where Al is not dissolved (FIG. 9). There was no change in sensor output.
Therefore, it was found that the solid solution of Al does not affect the oxidation activity of (3 mol%) Ce—SnO ₂ .

(Characteristics of metal composite oxide)
For each of SnO ₂ , (3 mol%) Ce—SnO ₂ , (1 mol%) Al— (3 mol%) Ce—SnO ₂ , the particle size, crystallite size, and specific surface area were examined. The particle size was calculated using X-ray small angle scattering (SAXS) and TEM observation, the crystallite size was calculated using the Rietveld method, and the specific surface area was calculated using the BET method. As a result, as shown in Table 1, it was confirmed that by dissolving Ce and Al in SnO ₂ , the particle size and crystallite size were reduced and the specific surface area was increased.

  Since the semiconductor type gas detection element according to the present invention can maintain gas selectivity over a long period of time, it can be applied to various gas alarms such as a home gas leak alarm.

Schematic of a hot-wire semiconductor gas detector according to this embodiment Configuration diagram of gas sensor according to this embodiment A graph showing the change over time of the sensor output of a hot-wire semiconductor gas detector without a catalyst layer Graph showing the time change of the sensor output of the hot wire type semiconductor type gas sensing element using Al ₂ O ₃ The graph which shows the time-dependent change of the sensor output of the hot wire type semiconductor gas detection element using SnO ₂ Graph showing change in lattice constant (a axis) and crystallite size with respect to Ce doping amount _{(3mol%) Ce-SnO 2} ( coprecipitation, 700 ° C.) graph showing the time change of the sensor output of the hot wire type semiconductor type gas sensing element using _{(3mol%) Ce-SnO 2} ( kneading method, 700 ° C.) graph showing the time change of the sensor output of the hot wire type semiconductor type gas sensing element using Graph showing the time change of the sensor output of the hot wire type semiconductor type gas sensing element using the _{(3mol%) Ce-SnO 2} (800 ℃) Graph showing changes in lattice constant (a-axis) and crystallite size with respect to Al doping amount Graph showing change in resistance value at operating temperature of metal composite oxide (1 mol%) Al- graph showing the time change of the sensor output hot wire type semiconductor type gas sensing element using the (3mol%) Ce-SnO ₂ (3 mol%) Al- graph showing the time change of the sensor output hot wire type semiconductor type gas sensing element using the (3mol%) Ce-SnO ₂

Explanation of symbols

Rs Hot wire type semiconductor gas detector (semiconductor gas detector)
1 Precious metal wire 2 Gas sensitive part 3 Catalyst layer

Claims (5)

A semiconductor type gas detection element comprising a gas sensitive part and a catalyst layer covering the gas sensitive part,
The catalyst layer includes a metal oxide semiconductor containing at least one metal oxide selected from the group consisting of tin oxide, indium oxide, titanium oxide, zinc oxide, iron oxide, and cerium oxide, cerium, tin, aluminum, Semiconductor gas detection containing a metal composite oxide in which at least one metal element selected from the group consisting of lanthanum, niobium, yttrium, zirconium, molybdenum, ruthenium, neodymium, gadolinium, vanadium, silicon, and magnesium is dissolved. element.   2. The semiconductor type gas detection element according to claim 1, wherein the metal complex oxide is a solid solution of 0.01 to 10 mol% of the metal element with respect to the metal oxide semiconductor.   3. The semiconductor type gas detection element according to claim 1, wherein the metal oxide semiconductor is an n-type semiconductor, and the metal element includes a metal ion having a valence lower than that of the metal element of the metal oxide.   3. The semiconductor gas detection element according to claim 1, wherein the metal oxide semiconductor is a p-type semiconductor, and the metal element includes a metal ion having a higher valence than the metal element of the metal oxide.   The semiconductor-type gas detection element according to claim 3, wherein the metal oxide semiconductor is made of tin oxide, and the metal elements are cerium and aluminum.

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