JP2012112701A - Semiconductor type gas sensing element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor type gas sensing element which has a gas sensitive part with reduced electric resistance and is highly sensitive to a detected gas.SOLUTION: A semiconductor type gas sensing element is constituted by providing a gas sensing part on a noble metal wire where 0.8 mol% or less of antimony and 0.8 mol% or less of cerium are solid-dissolved in tin oxide in the gas sensing part.

Description

  The present invention relates to a semiconductor type gas detecting element in which a gas sensitive part is provided on a noble metal wire.

  In general, a semiconductor type gas detection element includes a gas sensitive part whose main component is a metal oxide semiconductor. In the gas sensitive portion, oxygen is adsorbed on the particle surface of the metal oxide semiconductor in an atmosphere in which a gas to be detected (hereinafter referred to as “detected gas”) does not exist, and is generated by this adsorbed oxygen. Since the space charge layer spreads toward the inside of the particle, the conduction path of free electrons is narrowed and the electric resistance is high. On the other hand, in the atmosphere where the gas to be detected exists, the adsorbed oxygen adsorbed on the gas sensitive part is desorbed from the surface of the metal oxide semiconductor particles by the oxidation-reduction reaction with the gas to be detected. Becomes thinner and the electric resistance of the gas sensitive part becomes lower. The semiconductor gas detection element detects the gas to be detected by taking out this change in electrical resistance as a sensor output.

  In addition, since the semiconductor type gas detection element used as the prior art in this invention is a general technique, prior art documents, such as a patent document, are not shown.

  However, in such a semiconductor gas detection element, a noble metal wire is provided, and the noble metal wire serves as both an electrode and a heater. In the semiconductor gas detection element, the electric resistance of the semiconductor gas detection element is the electric resistance of the gas sensitive part. It becomes a combined electrical resistance of the resistance and the electrical resistance of the noble metal wire. For this reason, when detecting a gas to be detected having a lower concentration (small change in electric resistance), it is necessary to increase the contribution ratio of the change in the electric resistance of the gas sensitive part in the combined electric resistance of the semiconductor gas detection element. Therefore, it is required to reduce the electric resistance of the gas sensitive part itself.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a semiconductor type gas detection element that is highly sensitive to a gas to be detected by reducing the electric resistance of a gas sensitive part.

  The inventors pay attention to the fact that the electric resistance of the metal oxide semiconductor is reduced by dissolving a metal having a valence larger than that of the metal of the metal oxide semiconductor in the metal oxide semiconductor, As a result of intensive studies, especially when tin oxide is used as a metal oxide semiconductor, a predetermined amount of antimony having a valence higher than tin is solid-dissolved, and further a predetermined amount of cerium is solid-dissolved, so that the gas to be detected On the other hand, the inventors have found that a highly sensitive gas detection element can be obtained, and have reached the present invention.

  That is, the first characteristic configuration of the semiconductor type gas detection element according to the present invention is a semiconductor type gas detection element in which a gas sensitive part is provided on a noble metal wire, and the gas sensitive part has an antimony content of 0. It is the point which made it solid-solve in the range of 8 mol% or less and the range of 0.8 mol% or less of cerium.

Since the electric resistance of tin oxide can be reduced by dissolving antimony in tin oxide as in this configuration, the sensitivity to the gas to be detected can be improved.
On the other hand, when cerium is dissolved in tin oxide, grain growth during firing of tin oxide can be suppressed, and the primary particle diameter of tin oxide can be reduced. The change in the electrical resistance of tin oxide due to adsorption / desorption of adsorbed oxygen adsorbed on the surface of tin oxide particles becomes more pronounced when the particle diameter is smaller. For this reason, the sensitivity with respect to to-be-detected gas can be improved by making cerium dissolve in tin oxide like this structure.
Further, when cerium is dissolved in tin oxide, it is possible to suppress grain growth during high-temperature operation as a gas detection element, so that thermal stability is also improved.

  Therefore, according to this configuration, it is possible to provide a semiconductor gas detection element that is highly sensitive to the gas to be detected and has excellent thermal stability over a long period of time.

  The second characteristic configuration of the semiconductor gas detection element according to the present invention is that the ratio of cerium and antimony dissolved in tin oxide is Ce / Sb ≦ 1.

  When antimony is dissolved in tin oxide, the electric resistance of tin oxide decreases according to the amount of the solid solution. On the other hand, when cerium is further dissolved in tin oxide in which antimony is dissolved, the rate of decrease in the electrical resistance of tin oxide is reduced according to the amount of the solid solution. For this reason, as in this configuration, cerium is dissolved by dissolving each so that the solid solution amount of cerium (mol%) / the solid solution amount of antimony (mol%) is 1 or less. The gas sensitivity can be improved by suppressing the influence on the electrical resistance.

  A third characteristic configuration of the semiconductor type gas detection element according to the present invention further includes a catalyst layer that covers the gas sensitive part, and the catalyst layer is formed by dissolving cerium in tin oxide in a range of 0.8 mol% or less. It is in the point.

  Since the primary particle diameter of tin oxide can be reduced by dissolving cerium in tin oxide as in this configuration, the specific surface area of the catalyst layer can be increased. For this reason, the selectivity with respect to to-be-detected gas can be improved.

  A fourth characteristic configuration of the semiconductor gas detection element according to the present invention is that the catalyst layer carries at least one kind of noble metal catalyst of platinum, palladium, gold, rhodium, ruthenium, and iridium.

  According to this configuration, since the chemical activity in the catalyst layer is increased, the selectivity for the gas to be detected can be further improved.

It is the schematic of the semiconductor type gas detection element which concerns on this embodiment. (A) is a graph which shows the change of the electrical resistance of tin oxide when changing the solid solution amount of antimony in tin oxide, and (b) is a graph for each concentration with respect to methane gas when the solid solution amount of antimony is changed. It is a graph which shows gas sensitivity. (A) is a graph which shows the change of the lattice constant of tin oxide when changing the solid solution amount of antimony in tin oxide, and (b) when long-term high-temperature operation is performed by changing the solid solution amount of antimony. It is a graph which shows a time-dependent change of the sensor output of the base. (A) is a graph which shows the change of the particle size of a tin oxide when changing the solid solution amount of cerium to a tin oxide, (b) is a graph which shows the change of the gas sensitivity with respect to methane gas by the solid solution presence or absence of cerium. It is. It is a graph which shows the time-dependent change of the sensor output of a base by the solid solution presence or absence of the cerium to tin oxide at the time of long-term high temperature operation. It is a graph which shows the change of the lattice constant of a tin oxide when changing the solid solution amount of the cerium to a tin oxide. It is the schematic of the semiconductor type gas detection element which concerns on another embodiment. (A) is a graph which shows the change of the gas sensitivity with respect to each gas kind when not providing a catalyst layer, (b) is a graph which shows the change of the gas sensitivity with respect to each gas kind at the time of using tin oxide as a catalyst layer. (C) is a graph showing changes in gas sensitivity with respect to each gas type when a catalyst layer in which cerium is dissolved in tin oxide is used as a catalyst layer, and (d) is a graph in which cerium is dissolved as a catalyst layer. It is a graph which shows the change of the gas sensitivity with respect to each gas kind at the time of using what carried | supported platinum on the made tin oxide.

  Hereinafter, an embodiment of a semiconductor gas detection element according to the present invention will be described with reference to the drawings. However, the present invention is not limited to this.

  As shown in FIG. 1, the semiconductor type gas detection element of the present embodiment has a coil-like noble metal wire 1 provided with a gas sensitive part 2. The noble metal wire 1 is not particularly limited in terms of material, wire diameter, coil diameter, number of coil turns, and the like, and a commonly used semiconductor gas detection element can be applied.

  The gas sensitive part 2 is a solid solution of tin oxide in a range of 0.8 mol% or less and cerium in a range of 0.8 mol% or less.

  When antimony is dissolved in tin oxide, the electric resistance of tin oxide can be reduced, and the gas sensitivity to the gas to be detected can be improved. On the other hand, in the addition of antimony to tin oxide, if antimony is not completely dissolved in the tin oxide immediately after the production of the semiconductor gas sensing element, it is solidified during operation at a high temperature as a semiconductor gas sensing element. Melting progresses and sensor output becomes unstable. For this reason, the solid solution amount of antimony in tin oxide is set to 0.8 mol% or less, which completely dissolves, as shown in Examples described later. The amount of antimony dissolved in tin oxide is preferably 0.8 mol% or less because the electric resistance of tin oxide is small, specifically, 0.1 mol% to 0.8 mol% is preferable. 0.2 mol% to 0.8 mol% is more preferable, 0.4 mol% to 0.8 mol% is more preferable, and 0.6 mol% to 0.8 mol% is still more preferable.

  By dissolving cerium in tin oxide, the growth of tin oxide particles during firing can be suppressed, and the primary particle diameter of tin oxide can be reduced. For this reason, the change in the electrical resistance of tin oxide due to adsorption / desorption of adsorbed oxygen adsorbed on the surface of tin oxide particles can be increased, and the gas sensitivity to the gas to be detected can be improved. Moreover, since it is possible to suppress the growth of tin oxide grains when the semiconductor gas sensing element is operated at a high temperature, the thermal stability over a long period of time can be improved. In the case of cerium, as in the case of antimony, if the amount added to tin oxide increases, cerium does not completely dissolve in tin oxide, and the sensor output becomes unstable. For this reason, as shown in the Example mentioned later, the solid solution amount of the sensor to tin oxide is 0.8 mol% or less which completely dissolves. The amount of cerium dissolved in tin oxide is preferably larger at 0.8 mol% or less, specifically, 0.1 mol% to 0.8 mol% is preferable, and 0.2 mol% to 0.8 mol% is preferable. More preferably, 0.4 mol% to 0.8 mol% is more preferable, and 0.6 mol% to 0.8 mol% is still more preferable.

  In addition, when antimony is dissolved in tin oxide, the electric resistance of tin oxide decreases according to the amount of the solid solution. On the other hand, when cerium is further dissolved in tin oxide in which antimony is dissolved, the rate of decrease in the electrical resistance of tin oxide is reduced according to the amount of the solid solution. For this reason, it is preferable that antimony and cerium to be dissolved in tin oxide are dissolved in such a manner that the ratio thereof is Ce / Sb ≦ 1. Thereby, since the contribution ratio of the electrical resistance with respect to tin oxide of antimony becomes larger than the contribution ratio of the electrical resistance with respect to tin oxide of cerium, the electrical resistance of tin oxide can be reduced while suppressing grain growth of tin oxide. From such a viewpoint, it is more preferable to make a solid solution so that Ce / Sb ≦ 0.4 (mol%) / 0.6 (mol%).

  The gas sensitive part 2 can be produced by solid-dissolving antimony and cerium in tin oxide by a conventionally known method such as a coprecipitation method, followed by firing (calcination). The firing temperature is usually 600 ° C. to 900 ° C., but in particular, when the amount of antimony dissolved is increased, a higher one is preferable, for example, 700 ° C. to 900 ° C. is preferable. On the other hand, when the firing temperature is high, grain growth is promoted and the primary particle diameter of tin oxide tends to be large. Also in this regard, when antimony is solid-solved with cerium with respect to tin oxide as in the present invention, even if the firing temperature is increased so that a large amount of antimony is solid-dissolved, it is also dissolved at the same time. Grain growth can be suppressed by cerium.

  In the semiconductor gas detection element according to the present invention, the gas to be detected is not particularly limited, but it is known that the gas sensitivity is improved particularly with respect to methane, as shown in Examples described later.

Hereinafter, examples using the present invention will be shown to describe the present invention in more detail. However, the present invention is not limited to these examples.
(Example 1-1)
In the semiconductor gas detection element shown in FIG. 1, a platinum coil (wire diameter: 20 μm, coil diameter: 0.28 μm, number of coil turns: 12 turns) is used as the noble metal wire 1, and antimony (SnO ₂ ) is added to the antimony (SnO ₂ ). Changes in the resistance value of the gas sensitive part 2 when the solid solution amount of antimony is changed using a solid solution of Sb), and gas sensitivity (R _air / R _gas ) with respect to 10 ppm to 5000 ppm of methane gas (CH ₄ ) ) Was examined. In addition, the gas sensitive part 2 was produced by baking at 600-900 degreeC, after dissolving antimony in tin oxide by the coprecipitation method.

As a result, as shown in FIG. 2 (a), it was confirmed that the resistance value decreases as the solid solution amount increases by dissolving antimony in tin oxide. In particular, with respect to the solid solution amount of antimony, it was found that the resistance value suddenly decreased in the range of 0 to 0.8 mol%, and thereafter the resistance value changed gradually.
As for gas sensitivity to methane gas, as shown in FIG. 2 (b), it was confirmed that the gas sensitivity increased as the amount of antimony dissolved increased. In addition, it was found that as the amount of antimony dissolved increases, the difference in gas sensitivity due to the difference in methane gas concentration increases and the accuracy with respect to the gas concentration also improves.
Therefore, from the viewpoint of gas sensitivity, the amount of antimony dissolved in tin oxide is preferably 0.1 mol% or more, more preferably 0.2 mol% or more, further preferably 0.4 mol% or more, and 0.6 mol%. It has been found that the above is even more preferable.

(Example 1-2)
When antimony was dissolved in tin oxide, the change in the lattice constant of tin oxide was investigated when the amount of antimony dissolved was changed. Moreover, when using the same semiconductor gas sensing element as in Example 1-1 and not dissolving antimony in tin oxide, 0.2 mol%, 0.8 mol%, and 1.0 mol% of antimony were dissolved in the tin oxide, respectively. The change over time of the sensor output of the base during long-term high-temperature (350 to 600 ° C.) operation was examined.

As a result, as shown in FIG. 3A, it is found that the linearity cannot be maintained with respect to the rate of increase of the lattice constant with respect to the antimony solid solution amount after the antimony solid solution amount exceeds 0.8 mol%. It was. That is, it has been found that the Vegard rule that the magnitude of the lattice constant varies linearly depending on the amount of dopant added is not satisfied.
Further, as shown in FIG. 3 (b), when the solid solution amount of antimony is up to 0.8 mol%, the base output does not change with time even when operated at a high temperature, whereas the solid solution amount of antimony is 1.0 mol. % Showed that the base output increased with time. This is considered to be because the solid solution was not completed at the time when the gas sensitive part 2 was produced (fired), and the solid solution further progressed during the subsequent high temperature operation of the gas detection element.
From the above, it was found that when the solid solution amount of antimony exceeded 0.8 mol%, it was not completely dissolved in tin oxide.

  From Examples 1-1 and 1-2, in order to use as a gas detection element, the amount of antimony dissolved in tin oxide needs to be 0.8 mol% or less. 1 mol% to 0.8 mol% is preferable, 0.2 mol% to 0.8 mol% is more preferable, 0.4 mol% to 0.8 mol% is more preferable, and 0.6 mol% to 0.8 mol% is still more preferable I understood.

(Example 1-3)
In the same semiconductor gas detection element as in Example 1-1, the gas sensitive part 2 is obtained by changing the metal dissolved in tin oxide from antimony to cerium (Ce) as the gas sensitive part 2 and changing the solid solution amount of cerium. The change in the particle size of tin oxide constituting 2 was examined. Further, as the gas sensing unit 2, and (0.4mol%) Ce- (0.6mol% ) Sb-SnO 2, the electric resistance was prepared to be the same and (0.4mol%) Sb-SnO ₂ The change in gas sensitivity when the concentration of methane gas was changed and the change over time of the sensor output of the base during long-term high-temperature operation were investigated.

As a result, as shown in FIG. 4 (a), it was confirmed that the primary particle diameter of tin oxide became smaller as the amount of cerium solid solution increased.
As shown in FIG. 4B, when the electric resistance of the gas sensitive portion 2 is the same, the sensitivity to methane gas is better when the primary particle diameter of tin oxide is made smaller by dissolving cerium. I found it getting bigger.
Furthermore, as shown in FIG. 5, when cerium is dissolved ((0.4 mol%) Ce- (0.6 mol%) Sb-SnO ₂ ), when cerium is not dissolved ((0 .4 mol%) Sb—SnO ₂ ) was found to have a small change in sensor output with time. This is considered to be because grain growth during high temperature operation could be suppressed by dissolving cerium in solid solution.
Therefore, it was found that by dissolving cerium in tin oxide, it is possible to suppress the growth of tin oxide grains in the gas sensitive part 2 and to improve the thermal stability of the gas detection element.

(Example 1-4)
In the case where cerium was dissolved in tin oxide, the change in the lattice constant of tin oxide was investigated when the amount of cerium solid solution was changed.

  As a result, as shown in FIG. 6, Vegard's law was not established when the solid solution amount of cerium exceeded 0.8 mol%, and tin oxide exceeded 0.8 mol% when the solid solution amount of cerium exceeded 0.8 mol%. It was found that they were not completely dissolved.

  From the above, in order to use as a gas detection element, the amount of cerium dissolved in tin oxide needs to be 0.8 mol% or less. On the other hand, in order to suppress the grain growth of tin oxide, improve gas sensitivity, and improve thermal stability, it is preferable that the amount of cerium solid solution is large. For this reason, the solid solution amount of cerium in tin oxide is preferably 0.1 mol% to 0.8 mol%, more preferably 0.2 mol% to 0.8 mol%, and further 0.4 mol% to 0.8 mol%. Preferably, 0.6 mol% to 0.8 mol% is even more preferable.

[Another embodiment]
In the semiconductor type gas detection element according to the above embodiment, as shown in FIG. 7, a catalyst layer 3 that covers the gas sensitive part 2 may be further provided. As a material constituting the catalyst layer 3, a material obtained by dissolving cerium in tin oxide in a range of 0.8 mol% or less is preferable. As described above, cerium can be dissolved in tin oxide to suppress the growth of tin oxide grains during firing and to reduce the primary particle diameter of tin oxide. For this reason, the specific surface area of a catalyst layer can be improved and the selectivity with respect to to-be-detected gas can be improved. As described above, the solid solution amount of cerium in tin oxide is set to 0.8 mol% or less which completely dissolves. The amount of cerium dissolved in tin oxide is preferably larger at 0.8 mol% or less, specifically, 0.1 mol% to 0.8 mol% is preferable, and 0.2 mol% to 0.8 mol% is preferable. More preferably, 0.4 mol% to 0.8 mol% is more preferable, and 0.6 mol% to 0.8 mol% is still more preferable.
In addition, a noble metal catalyst of at least one of platinum, palladium, gold, rhodium, ruthenium and iridium can be supported on the surface of the material constituting the catalyst layer.

(Example 2-1)
In the semiconductor type gas detection element shown in FIG. 7, the same platinum coil as that of Example 1-1 is used as the noble metal wire 1, and (0.4 mol%) Ce− (0.6 mol%) Sb—SnO is used as the gas sensitive part 2. _When (a) the catalyst layer 3 is not used, (b) SnO ₂ is used as the catalyst layer 3 (c) Ce— in which cerium is dissolved in tin oxide (0.7 mol%) When SnO ₂ is used, (d) (0.7 mol%) Ce-SnO ₂ loaded with (1 mol%) Pt is used, and methane (CH ₄ ), isobutane (i-C ₄ H ₈ ) Gas sensitivity to ethanol (EtOH) and hydrogen (H ₂ ) was examined.

As a result, compared with the case shown in FIG. 8A in which the catalyst layer 3 is not provided, in FIGS. 8B to 8D in which the catalyst layer 3 is provided, gas sensitivity to H ₂ and EtOH is reduced, and carbonization is performed. It was found that the hydrogen selectivity was improved.
Among those in which a catalyst layer 3, compared with the case of FIG. 8 using the SnO ₂ (b), in the case of FIG. 8 using the _{(0.7mol%) Ce-SnO 2} (c) is a hydrocarbon selected In the case of FIG. 8D using (0.7 mol%) Ce—SnO ₂ with (1 mol%) Pt supported, it was found that the methane selectivity was improved. It was.
As described above, in order to improve gas selectivity, it is preferable to provide the catalyst layer 3 in the gas sensitive part 2, and the catalyst layer 3 is preferably a solution in which cerium is dissolved in tin oxide, and cerium is added to tin oxide. It was found that a solution in which a noble metal catalyst is supported on a solid solution of is more preferable.

  Since the semiconductor gas detection element according to the present invention is highly sensitive to the gas to be detected, it can be applied to various gas sensors, various gas alarms, and the like.

1 Precious metal wire 2 Gas sensitive layer 3 Catalyst layer

Claims (4)

A semiconductor type gas detection element having a gas sensitive part on a noble metal wire,
The gas sensitive part is a semiconductor gas detection element in which antimony is dissolved in tin oxide in a range of 0.8 mol% or less and cerium in a range of 0.8 mol% or less.   2. The semiconductor type gas detection element according to claim 1, wherein a ratio of cerium and antimony to be dissolved in tin oxide is Ce / Sb ≦ 1.   The semiconductor gas detection element according to claim 1, further comprising a catalyst layer that covers the gas sensitive part, wherein the catalyst layer is formed by dissolving cerium in tin oxide in a range of 0.8 mol% or less.   4. The semiconductor type gas detection element according to claim 3, wherein the catalyst layer carries a noble metal catalyst of at least one of platinum, palladium, gold, rhodium, ruthenium and iridium.

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