JP4076465B2 - Semiconductor gas detector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a substrate type semiconductor gas detection element in which a sensitive layer mainly composed of a metal oxide semiconductor is provided on an insulating substrate provided with a pair of detection electrodes so as to cover the detection electrodes.
[0002]
[Prior art]
Conventionally, as this type of substrate type semiconductor gas detection element, for example, a pair of gold electrodes is vapor-deposited on an alumina substrate, the sensitive layer is formed over the electrodes, and the sensitive layer is used for various gases. The gas can be detected by detecting the change in the apparent resistance of the sensitive layer as it reacts. Here, in order to selectively detect only a specific detected gas, the sensitive layer is often provided with various catalysts.
[0003]
When a catalyst is provided in the sensitive layer, a component having a catalytic function (catalyst component) is included in the sensitive layer, particularly on the outer surface side, or a catalyst layer is provided separately from the sensitive layer on the sensitive layer outer surface. Is provided. When manufacturing the sensitive layer, a catalyst component may be added to the entire sensitive layer for the convenience of the manufacturing process, but this makes it possible to select the detected gas contained in the detection target gas such as indoor air and exhaust gas more selectively. It is thought that the reaction of the detected gas can be detected more selectively in the sensitive layer by selectively removing the contaminated gas contained together with the detected gas. (See Patent Document 1).
[0004]
The gas to be detected that has reached the outer surface side of the sensitive layer first comes into contact with the catalyst component on the outer surface portion of the sensitive layer, and mainly causes a catalytic reaction on the outer surface portion of the sensitive layer. become. Therefore, in such a case, a concentration gradient of the gas to be detected occurs in the film thickness direction of the sensitive layer, and even less gas than the surface part reaches the inside, so even if a catalyst part is provided on the inner side of the sensitive layer In some cases, the detected gas is wasted without being involved in the reaction, or a side reaction is caused to the reaction product of the detected gas. Therefore, the catalyst component is usually provided on the outer surface side of the sensitive layer in order to suppress waste and side reactions as described above.
[0005]
[Patent Document 1]
JP-A-2000-55852 [0006]
[Problems to be solved by the invention]
However, when the catalyst layer is provided in the sensitive layer in this way, on the contrary, the change in the apparent resistance of the entire sensitive layer based on the reaction of the gas to be detected in the sensitive layer cannot be sufficiently detected. In some cases, a change in the apparent resistance of the sensitive layer based on the reaction is selectively detected. At this time, on the contrary, the selectivity for selectively detecting the gas to be detected is lowered, or the apparent resistance value change rate (hereinafter referred to as sensitivity) is not sufficient.
[0007]
Therefore, in view of the above circumstances, an object of the present invention is to provide a technique for improving the gas selectivity and sensitivity by effectively improving the reactivity of a substrate type semiconductor gas sensing element to a gas to be detected. It is in.
[0008]
[Means for Solving the Problems]
The present inventors, due to the fact that the gas to be detected that has reached the surface of the gas sensing element first reacts preferentially with the catalyst contained in the catalyst layer at the outer surface portion of the sensitive layer, The change in the apparent resistance of the sensitive layer was thought to be mainly due to the fact that it occurred on the outer surface of the sensitive layer, leading to the above-mentioned problems, and as a result of intensive studies, the present invention was conceived.
That is, if the gas to be detected reacts mainly on the outer surface of the sensitive layer, the change in the apparent resistance of the sensitive layer mainly occurs on the outer surface of the sensitive layer. On the other hand, the electrical conductivity of the sensitive layer is greatly influenced by the electrical conductivity of the region between the electrodes (hereinafter referred to as the inter-electrode conducting region), so that the resistance change due to the reaction only on the outer surface side of the sensitive layer becomes small. Show the trend. Therefore, it can be explained that the change in the apparent resistance of the sensitive layer accompanying the detection of the gas to be detected in the previous period is not captured, leading to a decrease in the gas to be detected and a decrease in sensitivity.
[0009]
Therefore, in order to achieve this object, the semiconductor gas detection element of the present invention is
A substrate type semiconductor gas detection element for detecting a volatile organic substance (VOC) in which a sensitive layer mainly composed of tungsten oxide is provided on an insulating substrate provided with a pair of detection electrodes so as to cover the detection electrode. Thus, the current-carrying region between the detection electrodes in the sensitive layer is provided with a catalyst part that contains at least one of platinum and palladium and catalyzes the reaction of the gas to be detected. Further, the catalyst portion is formed by providing more catalyst components in the current-carrying region between the electrodes as compared with the surface portion of the sensitive layer.
It is preferable that the catalyst portion is provided between the detection electrodes on the insulating substrate.
Moreover, it is preferable that the said catalyst part is provided on the said insulated substrate by the at least 1 type of film forming means chosen from CVD, PVD, sputtering, and screen printing.
[0010]
[Function and effect]
That is, when a sensitive layer composed mainly of a metal oxide semiconductor is provided on an insulating substrate provided with a pair of sensing electrodes so as to cover the sensing electrode, the apparent resistance of the sensitive layer is brought into contact with the gas to be sensed. Can be taken out as an electrical signal from the electrode.
Here, providing the catalyst unit in the current region between the detecting electrodes in the sensitive layer, the gas to be detected to undergo catalytic reaction, reaches the catalyst portion is transmitted through the sensitive layer by diffusion, primarily It will rise to a chemical reaction by the catalyst in the catalytic section of the energizing area between the detection electrodes. Then, the inside of the sensitive layer, in particular in the energizing area between the detection electrode serving as a flow path of the current between the detection electrodes, the since the change in the apparent resistance of the sensitive layer is the larger, the apparent resistance of the sensitive layer It is possible to capture changes in the area with certainty and achieve high sensitivity.
If the catalyst part contains at least one component of platinum and palladium, it works as a catalyst that oxidizes the gas to be detected, and exhibits a difference in reactivity with various gases in the gas to be detected. It works effectively to produce and have selectivity for the gas to be detected.
It has been found that if the sensitive layer is based on tungsten oxide, the sensitive layer has gas selectivity for VOC (volatile organic substances). In general, hydrogen gas is known as a contaminant gas that interferes when VOC is detected. While the VOC reaction in the sensitive layer is an oxidation reaction, hydrogen gas also undergoes an oxidation reaction, and both reactions are catalyzed by platinum.
Here, when the platinum catalyst component in the base type semiconductor gas sensing element is present on the outer surface side of the sensitive layer, the hydrogen gas sensitivity is improved, whereas the VOC sensitivity is decreased. . On the other hand, if the platinum catalyst component is provided in the energization region between the detection electrodes, the hydrogen gas sensitivity does not increase so much, whereas the VOC sensitivity greatly increases. It was proved by experiments and found to be useful as a VOC gas sensing element.
[0011]
In addition, when a highly active contaminant gas undergoes a reaction in the sensitive layer, it is considered that the reaction occurs on the outer surface side of the sensitive layer, and therefore it is difficult to adversely affect the detection of the detected gas. Further, in this way, when the chemical reaction of the gas to be detected is captured, even if the less active gas of the gas to be detected in the gas to be detected reaches the catalyst portion in the sensitive layer, the catalytic reaction Therefore, only the gas to be detected is selectively detected. As a result, the substrate-type semiconductor gas sensing element can exhibit high gas selectivity.
[0012]
Moreover, it is preferable that the catalyst part is provided on the insulating substrate by at least one film forming means selected from CVD, PVD, sputtering, and screen printing. A detection electrode can be formed. Further, since the detection electrode can be formed by the same film forming means, and a thin film electrode can be formed, the current flowing from the electrode has a very narrow range of the inter-electrode energization region affected by the catalyst portion. A portion that gives a change in the apparent resistance can be effectively formed in the flow-sensitive layer.
Although the detection electrode itself may be made of platinum, the electrode itself exists outside the current-carrying region, so it functions as a catalyst part that catalyzes the reaction of the gas to be detected. Small effect.
[0013]
Since the rate of change in electrical resistance of the sensitive layer is governed by the change in electrical resistance of the sensitive layer in the sensing electrode conducting region provided on the insulating substrate, the interelectrode conducting region is very close to the insulating substrate in the sensitive layer. It will be formed in the region. Therefore, by attaching the catalyst part between the electrodes on the insulating substrate, the catalyst part can be reliably arranged in the inter-electrode energization region. Further, when the sensitive layer is formed on the insulating substrate, the catalyst portion can be arranged and formed in advance, so that the gas detection element can be easily manufactured.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
As shown in FIG. 1, a substrate type semiconductor gas detection element 10 of the present invention is provided by depositing gold comb electrodes 2 and 3 on the surface of an alumina substrate 1, and an interelectrode conduction region between the electrodes 2 and 3. A catalyst portion 4 made of a platinum thin film is formed on A by sputtering, a metal oxide semiconductor paste mainly composed of tungsten oxide is applied thereon, and fired at 600 ° C. for 2 hours to provide a sensitive layer 5. is there. A platinum thin film heater 6 is provided on the back surface of the alumina substrate 1 and is used to maintain the operating temperature of the gas detection element.
[0017]
At this time, the alumina substrate has a plan view of 1 mm × 1.5 mm and a thickness of 0.4 mm, whereas the sensitive layer has the same plan view of 1 mm × 1 mm and has a thickness of 10 to 50 μm. The thickness of each electrode was 0.8 μm.
[0018]
As shown in FIG. 2, the gas detection device configures the gas detection device so that the above-described VOC detection element 10 is incorporated in the gas detection circuit 20 and a gas detection output is obtained. The gas detection output obtained from the VOC detection element 10 is input to the control unit 30 to determine whether or not the VOC concentration has reached a level requiring an alarm. Here, when it is determined that an alarm is required, the control unit 30 outputs an alarm signal to the alarm unit 40, and the alarm unit 40 sounds an alarm buzzer, an alarm sound, and the like.
[0019]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
(1) Non-catalytic substrate type semiconductor gas detector (non-catalytic)
A substrate-type semiconductor gas detection element was prepared in the same manner as in the previous embodiment except that the catalyst portion was not provided in place of the substrate-type semiconductor gas detection element described above.
When the toluene (an example of VOC) detection characteristic (0.1 to 3 ppm) and the hydrogen gas detection characteristic (50 to 300 ppm) of this substrate type semiconductor gas detection element were examined, FIG. 3 (1), FIG. 4 (1) It became like this.
The sensitivity is the ratio of the resistance value (Rair) in air and the resistance value (Rg) in the gas to be detected including the gas to be detected when the substrate type semiconductor gas sensing element is operated at 400 ° C. Rair / Rg).
[0020]
(2) Substrate-type semiconductor gas sensing element having a catalyst portion formed from the outer surface of the sensitive layer (entire sensitive layer) The sensitive layer 5 of the substrate-type semiconductor gas sensing element according to (1) above is coated with chloroplatinic acid. After impregnating the aqueous solution, it is calcined at 550 ° C. for 0.5 hour to carry platinum as a catalyst component. The amount of platinum impregnated was prepared by dropping a predetermined amount of a lead nitrate aqueous solution into the sensitive layer by using a capillary and impregnating the platinum with 0.005 mol% of tungsten oxide.
When the detection characteristics of this substrate type semiconductor gas detection element with respect to toluene and hydrogen gas were examined, they were as shown in FIGS. 3 (2) and 4 (2).
[0021]
(3) Substrate-type semiconductor gas sensing element having a catalyst portion in the interelectrode energization region (interelectrode energization region)
When the detection characteristics with respect to toluene and hydrogen gas of the substrate type semiconductor gas detection element of the previous embodiment were examined, the results were as shown in FIGS. 3 (3) and 4 (3).
[0022]
According to FIG. 3, the substrate-type semiconductor gas detection element has a lower toluene sensitivity by providing the platinum component on the outer surface side of the sensitive part (2), and has a higher toluene sensitivity by providing the platinum component in the inter-electrode conducting region. It can be seen that it is rising (3).
On the other hand, according to FIG. 4, the substrate type semiconductor gas detection element has a hydrogen sensitivity greatly increased by providing the platinum component in the entire sensitive part (2), and the hydrogen sensitivity is improved by providing the platinum component in the inter-electrode conducting region. It can be seen that the increase width is suppressed (3).
[0023]
That is, regarding the reactivity to toluene, the non-catalytic substrate-type semiconductor gas detection element of (1) had a certain sensitivity to toluene because toluene reacted while diffusing in the sensitive layer. In the substrate type semiconductor gas detection element in which the catalyst portion is formed from the outer surface of the sensitive layer, since the catalytic activity of platinum against toluene is extremely high, the platinum component is used as a catalyst, and toluene is the outer surface portion of the sensitive layer. It was considered that the resistance change based on the reaction occurred only on the outer surface portion of the sensitive layer, and was not reflected so much as the change in the synthetic resistance of the entire sensitive layer because the reaction could be sufficiently performed. On the other hand, in the substrate-type semiconductor gas detection element having a catalyst portion in the inter-electrode current-carrying region (3), toluene that diffuses and penetrates into the sensitive layer can react after reaching the inter-electrode current-carrying region. It is considered that the resistance change based on this occurs mainly in the inter-electrode energization region, greatly contributing to the change in the combined resistance of the entire sensitive layer, and a high toluene sensitivity can be obtained.
[0024]
On the other hand, regarding the reactivity to hydrogen gas, in the non-catalytic substrate type semiconductor gas sensing element of (1), the sensitive layer had almost no reactivity to hydrogen. On the other hand, in the substrate type semiconductor gas detection element in which the catalyst part is formed in the entire sensitive layer of (2), hydrogen gas reacts due to the catalytic effect of the platinum component, but the reaction site throughout the sensitive part. As a result, a resistance change due to a hydrogen gas reaction appears, which is considered to be reflected as a change in the combined resistance of the entire sensitive layer. However, in the substrate type semiconductor gas detection element having the catalyst portion in the inter-electrode energization region of (3), since the catalyst portion is arranged in the inter-electrode energization region inside the sensitive layer, there is a site where hydrogen gas reacts. It is thought that the sensitivity is reduced and the sensitivity cannot be obtained.
[0025]
Therefore, by arranging the catalyst portion that catalyzes the reaction of the detected gas in the inter-electrode energization region, the sensitivity to the detected gas is improved and the contamination gas that becomes the interference gas of the detected gas is reduced. It is considered that the increase in sensitivity could be suppressed and the gas selectivity of the substrate type semiconductor gas detection element could be greatly improved.
[0026]
[Another embodiment]
In the previous embodiment, the catalyst portion was formed on the insulating substrate and provided in the inter-electrode energization region. However, as long as the catalyst portion is formed in the inter-electrode energization region, It does not matter if it is provided. For example, a metal oxide layer is provided between the electrodes on the insulating substrate, a catalyst portion is formed by impregnating and supporting the metal oxide layer with a catalyst, and the metal oxide is covered with the electrode and the catalyst portion. A sensitive layer composed mainly of a semiconductor may be formed.
[0027]
Further, the catalyst is not limited to platinum, and may be another catalyst, which may be selected according to the gas type to be detected.
[0028]
Further, for example, the catalyst part can be provided between the electrodes as follows.
[0029]
A comb-shaped gold electrode was placed on the substrate, and palladium was deposited on the entire substrate surface (including the electrode) on which the electrode was placed to disperse the palladium fine particles, thus forming a catalyst part (controlling the deposition time of palladium) Thus, the electrode is not short-circuited). After that, a gas detection element having a sensitive layer provided by coating a WO3 film covering the entire catalyst portion was produced. In FIG. 5, the sensitivity characteristic with respect to toluene of the said gas detection element is shown. FIG. 5 shows that the sensitivity of this gas detection element is greatly improved over the WO3 sensitive layer when there is no catalyst. This is because the gas to be detected causes a catalytic reaction at the interface between the fine palladium particles dispersed in the electrode and the electrode energization region and the bottom surface of the sensitive layer, and the electrical resistance of the sensitive layer in the vicinity of the electrode surface between the electrode and the substrate changes greatly. by.
[0030]
Various detection electrodes such as gold, platinum, platinum-palladium alloy and the like can be used. The electrode does not function as a catalyst.
[0031]
Further, the metal oxide semiconductor constituting the sensitive layer is not limited to the one having tungsten oxide as the main material, and the one having various metal oxides such as tin oxide and zinc oxide as the main material is applied. Can do.
[Brief description of the drawings]
FIG. 1 is a partially broken perspective view of a substrate type semiconductor gas detection element of the present invention. FIG. 2 is a schematic diagram of a gas detection apparatus using the substrate type semiconductor gas detection element of the present invention. Fig. 4 is a graph showing the hydrogen detection characteristics of a substrate type semiconductor gas detection element. Fig. 5 is a graph showing the toluene detection characteristics of a substrate type semiconductor gas detection element (palladium catalyst). Graph [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Alumina substrate 2, 3 Gold comb-shaped electrode 4 Catalyst part 5 Sensitive layer 6 Platinum thin film heater 10 Substrate type semiconductor gas detection element 20 Gas detection circuit 30 Control part 40 Alarm part A Inter-electrode energization region

Claims (4)

A semiconductor-type gas sensing element comprising a sensing layer comprising tungsten oxide as a main component on an insulating substrate provided with a pair of sensing electrodes so as to cover the sensing electrode, wherein the sensing electrode in the sensitive layer A substrate-type semiconductor gas detection element for detecting a volatile organic substance, in which a catalyst portion including at least one of platinum and palladium and catalyzing a reaction of a gas to be detected is provided in a current-carrying region. 2. The substrate type semiconductor gas for detecting volatile organic substances according to claim 1, wherein a catalyst portion is formed by providing more catalyst components in a current-carrying region between the electrodes as compared with the surface portion of the sensitive layer. Sensing element. The substrate type semiconductor gas detection element for detecting volatile organic substances according to claim 1 or 2, wherein the catalyst part is provided between the detection electrodes on the insulating substrate. The volatile organic substance detection device according to any one of claims 1 to 3, wherein the catalyst portion is provided on the insulating substrate by at least one film forming means selected from CVD, PVD, sputtering, and screen printing . Substrate type semiconductor gas detector.

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