JPH0380257B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a gas-sensitive element, and particularly to a gas-sensitive element excellent for use in ventilation. [Technical background of the invention and its problems] Various studies have been conducted on gas-sensitive elements using gas-sensitive materials, such as SnO ₂ -based oxide semiconductors, whose resistance changes when in contact with various gases. There is. In general, catalysts and gas sensitizers are selected to have a specific sensitivity to almost a single gas such as CO gas, which is a toxic gas. In recent years, home electronics, that is, electronic control of various devices in general households, has been progressing. As part of this effort, there is research into a system that uses gas-sensitive elements to automatically ventilate the room depending on the atmosphere. The characteristics required of gas-sensitive elements used to issue control signals for such ventilation include uniform sensitivity to various gases that are unpleasant or harmful. Excellent responsive desorption characteristics. Must have excellent moisture resistance. etc. Gas is detected based on changes in the resistance value of the gas-sensitive element, but if the sensitivity is excellent only for a specific gas, it is difficult to set a threshold for issuing a control signal. In addition, since it is necessary to issue a ventilation ON/OFF signal, a quick response characteristic is required, and when the gas concentration exceeds a certain level, it is necessary to immediately issue an OFF signal.
In other words, it is necessary to shorten the time it takes to return to a steady state. Furthermore, since it is exposed to a general atmosphere, it is important that it has excellent moisture resistance because it is greatly affected by humidity. In addition, when the gas-sensitive element is not driven all the time, it is necessary to quickly settle into a stable state when it starts operating, and to enter a measurable state. Specific examples of gases to be measured by such a gas-sensitive element for ventilation include CO gas, cigarette smoke, alcohol, and the like. Conventional gas-sensitive elements using noble metal catalysts such as Pt and Pd are not necessarily sufficient for such ventilation applications, as they are too sensitive to CO, alcohol, etc., and have poor moisture resistance. There is also an attempt to mix a catalyst in which W or the like is supported on TiO ₂ into a gas sensitive material (Japanese Patent Application Laid-open No. 118953/1982).
However, problems remain with nonuniform sensitivity, moisture resistance, and long-term stability. [Object of the invention] The present invention has been made in consideration of the above points, and
It is an object of the present invention to provide a gas-sensitive element that has uniformly excellent sensitivity to gases that require ventilation, and also has excellent response desorption characteristics and moisture resistance. [Summary of the Invention] The gas-sensitive element of the present invention includes a gas-sensitive body having a pair of electrodes on a substrate, and a catalyst layer formed by carrying copper and tungsten together on a carrier is formed on the surface of the gas-sensitive body. It is characterized by the fact that That is, the present inventors have discovered that the above object can be achieved by using a catalyst layer that supports both copper and tungsten. CO, a toxic gas, is a gas that requires ventilation.
Typical examples include gas and cigarette smoke, which is a gas that causes discomfort. Due to its danger, the detection standard for CO gas is around 200 ppm, and empirical results indicate that the detection standard for cigarette smoke is around 500 ppm. According to the present invention, the sensitivity ratio is
It is approximately equal to 0.9 to 1.1, and it is very convenient to detect both. The same applies to other gases to be measured, such as alcohol (approximately 50 ppm). Regarding humidity resistance, if conventional catalysts using Pd-based catalysts are left in high humidity (90% humidity, room temperature), the resistance value will drift, making gas detection difficult. In addition, regarding the response and desorption characteristics, in the present invention, the response time is within 1 minute and the desorption time is within 3 minutes, compared to the conventional Pd system, which has a response time of 3 minutes and desorption time of 10 minutes.
This is much better than the previous model. By using copper as a catalyst, especially by using copper sulfate, moisture resistance is dramatically improved and sensitivity is also improved to a certain extent. The response and desorption characteristics to gas are also improved. However, when considered for ventilation, the sensitivity to tobacco smoke is low. However, by supporting tungsten, the sensitivity to tobacco smoke is also improved, and each gas has a substantially uniform sensitivity. The amount of this catalyst supported is preferably in the range of 0.1 to 30% by weight based on the carrier in terms of copper (Cu) and tungsten (W). If the amount is less than 0.1% by weight, the catalytic effect will not be exhibited, and if it is more than 30% by weight, the catalytic ability will be reduced. Here, the thickness of the gas sensitive thin film is 1000Å to 1μ.
It is preferable that the film thickness is in the range of 1 μm.
If it exceeds m, the sensitivity to gas decreases, and
When it is smaller than 1000 Å, the sensitivity decreases and at the same time its dispersion increases. Further, the thickness of the thick film catalyst layer is preferably in the range of 10 to 50 μm, and if it is outside this range, the catalytic effects such as sensitivity and response desorption characteristics will be degraded. As mentioned above, ventilation sensors are required to have average sensitivity to various gases. Therefore, for example, it is desirable that the sensitivity ratio between typical CO gas and cigarette smoke is S _sn (cigarette smoke sensitivity/S _cp (co sensitivity) = 1. CO gas is generally more sensitive than cigarette smoke. Therefore, in practice, it is preferably around 0.8 to 1.If the ratio of copper to tungsten in the catalyst layer becomes less than 0.1 in terms of atomic ratio (W/Cu), that is, if the amount of tungsten decreases, the sensitivity to cigarette smoke will decrease. However, when S _sn /S _cp becomes smaller than 0.8, it becomes virtually insensitive to cigarette smoke.Also, when W/Cu exceeds 10, that is, when the amount of Cu decreases, The overall gas sensitivity decreases.Therefore, in practice,
It is preferable that W/Cu is in the range of 0.1 to 10. The gas-sensitive element in the present invention is manufactured, for example, by the method shown below. First, as a substrate, a heat-resistant and insulating substrate such as a ceramic substrate such as Al ₂ O ₃ is used, and as an electrode.
It is formed using Au, Pt, etc., by a screen printing method, sputtering method, vapor deposition method, etc. These electrodes are provided facing each other on the gas sensitive member, and may be provided either between the gas sensitive member and the substrate or between the gas sensitive member and the catalyst layer. In addition, a cylindrical substrate is used,
A gas sensitive body may be formed on the outer peripheral surface of the gas sensitive body, or it may be shaped like a flat plate. Also, as a gas sensitive material, it is commonly used
An oxide semiconductor whose resistance value changes when it comes into contact with the gas to be measured, such as SnO ₂ -based, In ₂ O ₃ -based, Fe ₂ O ₃ -based, or ZnO-based, is used. This SnO ₂ system, In ₂ O ₃ system, Fe ₂ O ₃ system,
ZnO-based oxide semiconductors include SnO ₂ , In ₂ O ₃ , and
The main components are Fe ₂ O ₃ and ZnO, and if necessary Al ³⁺ ,
Sub-components such as Nb ⁵⁺ , Sb ³⁺ , and Sb ⁵⁺ are added. This gas sensitive material can be produced by sputtering method,
It is formed by vapor deposition, coating sintering, thermal decomposition of organic compounds, etc. However, gas sensitivity, reproducibility,
From the viewpoint of durability and the like, it is preferable to construct the gas sensitive body with a thin film using a sputtering method, a thermal decomposition method, or the like. Further, from the viewpoint of gas sensitivity, etc., it is preferable to use SnO ₂ type. Gas-sensitive elements are generally heated with a heater, but thin films have a smaller heat capacity.
In addition to being efficient, it has significantly better response and desorption characteristics than thick film sintering. Furthermore, according to the pyrolysis method, a uniform thin film can be easily obtained regardless of the shape of the substrate, and this thin film can be created, for example, as follows. As an example, the case of SnO ₂ system is shown. First, a tin metal soap (for example, tin 2-ethylhexanoate) or a resin salt containing Sn,
Organic compounds containing Sn, such as tin alkoxides (ROSn; where R is an alkyl group), and organometallic compounds of tin (RSn; where R is an alkyl group or an aryl group), or organic compounds containing Nb or Sb. A sample solution of Sn having a predetermined concentration is prepared by dissolving a mixture to which a predetermined amount of the organic compound contained is added using an appropriate solvent such as toluene, benzene, or n-butyl alcohol. The Sn concentration is preferably in the range of 1.0 to 20% by weight. Next, this sample solution is applied to a substrate with a pair of electrodes, left in the air for a predetermined period of time (usually 30 minutes to 1 hour), and then heated to an appropriate temperature (usually about 120°C). vaporize it. After that, the whole body was heated in the air for 30 minutes to 1 hour at 400 to 700
When fired at a temperature of °C, the Sn-containing organic compound is thermally decomposed, and the Sn is oxidized to form a SnO ₂ thin film. Sn of the sample solution used
Although it varies depending on the concentration and cannot be unambiguously determined,
This coating-firing process is repeated about 1 to 4 times to form a SnO ₂ thin film of a predetermined thickness. When a thin film containing Nb and Sb as impurities is created, both Nb and Sb function as donors. The amount of Nb and Sb is preferably within the range of 0.005 to 0.05 in terms of atomic ratio (Nb/Sn or Sb/Sn) to Sn. Next, the catalyst layer will be described. For example, a catalyst layer using aluminum oxide (Al ₂ O ₃ ) as a carrier and supporting copper sulfate (CuSO ₄ ) and tungsten oxide (WOx) is manufactured as follows. First, Al ₂ O ₃ is immersed in an aqueous solution containing crystalline copper sulfate (CuSO ₄ .5H ₂ O) and ammonium tungstate at a predetermined ratio. After thorough stirring and mixing, 1~
Dry under reduced pressure for 2 hours, and then heat dry at 100°C.
Thereafter, it is ground into powder in a mortar or other method, and then placed in a quartz crucible and fired at a temperature of 400 to 600°C. The catalyst prepared in this way is made into a slurry using an aqueous solution of aluminum hydroxychloride as a binder, and this slurry is coated on a gas sensitive member to a predetermined thickness and dried, and then calcined at 400 to 500°C to form a catalyst. form a layer. The decomposition temperature of _CuSO4 is about 650
℃, it is preferable to carry out the reaction at a temperature below this temperature, for example, below 600°C. It is thought that this firing turns tungsten into oxide (WOx). In the present invention, by using a catalyst layer in which both Cu and W are supported, the sensitivity to various gases is increased, uniform values are obtained, and moisture resistance is greatly improved. In particular, moisture resistance is particularly effective when Cu is supported as copper sulfate. This is because, in addition to its catalytic effect, CuSO ₄ absorbs and releases moisture from the air to form a pentahydrate salt, and when heated, it becomes an anhydrous salt, which can suppress the effect of humidity on the gas-sensitive element. Conceivable. Also, as a support layer
It is also possible to use other carriers such as SiO ₂ , ZrO ₂ , SiO ₂ , SiO ₂ −Al ₂ O ₃ , but from the viewpoint of moisture resistance
_Al2O3 is _the best. It is also possible to use copper chloride (CuAl ₂ ) and copper nitrate (CuNO ₃ ) as raw materials for copper;
In terms of moisture resistance, the use of CuSO ₄ shows the most stable characteristics. In addition, gas-sensitive elements are generally equipped with a heater to control the operating temperature in order to improve gas response and gas selectivity, but for example, if a cylindrical substrate is used, a heater may be placed inside it. However, in the case of a flat plate shape, it can also be placed on the back side of the surface on which the gas sensitive body is formed, or under the gas sensitive body via the insulator surface. [Effects of the Invention] As explained above, according to the present invention, sensitivity to CO gas, tobacco smoke, etc. is uniformly excellent. Excellent moisture resistance. Excellent response and desorption properties. It is possible to obtain a gas-sensitive element that exhibits the following effects,
It is particularly effective as a ventilation sensor. [Embodiments of the Invention] Examples of the present invention will be described below. FIG. 1 is a cross-sectional view showing a gas-sensitive element of the present invention, and FIG. 2 is a perspective view of the gas-sensitive element of the present invention. (1) Formation of gas sensitive material Tin 2-ethylhexanoate with a Sn content
A sample solution was prepared by dissolving it in n-butanol to a concentration of 10% by weight. Furthermore, regarding Nb and Sb used as impurities, Nb-resinate and antimony n-butyrate ((n-
OC ₄ H ₉ ) ₃ Sb) were used to adjust the solution so that the atomic ratio to Sn was 0.1. This was applied to the outer surface of the tube of the substrate 1 on which a pair of electrodes 2 had been previously installed, as shown in Figure 1, and left in the air for 1 hour, then heated to 120°C to remove n-butanol. It was vaporized. The whole was then fired in air at 400°C for 1 hour. This application-
The firing process was repeated three times to create a product with a thickness of approximately 0.3 μm.
A gas sensitive body 3 made of a SnO ₂ thin film was created. (2) Formation of catalyst layer CuSO ₄ , 5H ₂ O, and NH ₄ (W ₇ O ₂₄ )·4H ₂ O were dissolved in water, and Al ₂ O ₃ fine powder with a surface area of about 100 m ² /g was immersed and thoroughly stirred. . Separate Al ₂ O ₃ fine powder 1.5
After drying under reduced pressure for an hour to remove moisture, it was evaporated to dryness. Then, it was ground in a mortar, and the resulting powder was placed in a quartz crucible and fired at 400°C. This catalyst powder was put into an aqueous aluminum hydroxychloride solution (Al ₂ O ₃ 1%) to form a slurry. This slurry was applied onto the SnO ₂ thin film, dried, and the whole was fired at 400°C. Thickness
A catalyst layer 4 of CuSO ₄ -WOx supported Al ₂ O ₃ having a thickness of 20 μm was formed. As shown in FIG. 2, the gas-sensitive element constructed in this way is mounted and held on a pin 6 erected on an insulating plate 5 so as not to come into contact with anything else, thereby forming a gas detection device. In the figure, 7 represents a lead wire for the electrode, and 8 represents a heater. The heater 8 is used to adjust the surface temperature (operating temperature) of the element. In this example, SnO ₂ was used as the gas sensitive material, but ZnO, In ₂ O ₃ , and Fe ₂ O ₃ systems may also be used. (3) Measurement of sensitivity characteristics Using the gas-sensitive element of the present invention manufactured as described above, sensitivity was measured as Rair/Rgas to 200 ppm CO and 500 ppm cigarette smoke.
Here, Rair is the resistance value exhibited by the element in air that does not contain the measurement gas, and Rgas is the resistance value exhibited by the element in air containing the above-mentioned gases at respective concentrations. The larger Rair/Rgas means higher sensitivity. Note that the element temperature was approximately 350°C. Here, CO gas is measured by injecting a certain amount into a measuring tank, and cigarette smoke is collected using various brands from Japan Monopoly Corporation, for example, with a syringe at a rate that a normal person can absorb. A mixture of smoke and air) was injected into the measurement tank. The results are shown in Table 1.

[Table] As is clear from Table 1, the gas-sensitive element equipped with a catalyst layer supporting both copper and tungsten can absorb CO gas (200 ppm) and cigarette smoke (500 ppm).
Similar output can be obtained for . In the case of only copper (Comparative Example 1), the sensitivity to tobacco smoke is low, and in the case of only tungsten (Comparative Example 2), the sensitivity to CO decreases. Further, the response and desorption characteristics of Example 4 were measured, and the results are shown in FIG. 3a. The time taken to reach 90% of the saturated output in the measurement gas is defined as the response time, and similarly the time taken to reach 10% of the saturated output is defined as the desorption time. For comparison, Al ₂ O ₃ was added to the SnO ₂ sintered body.
Similar measurements were carried out when a supported Pd catalyst was used and the structure was similar to that of this example (Comparative Example 3).
The results are shown in Figure 3b. Note that FIG. 3 shows that gas was injected at t ₁ and gas was exhausted at t ₂ . As is clear from the figure, in the present invention, the response time is within 1 minute and the desorption time is within 3 minutes, which is extremely superior to Comparative Example 3, which has a response time of 3 minutes and a desorption time of 10 minutes or more. I know that there is. In addition, the time required for initial stabilization, that is, the time required to generate a steady state output after energizing the heater, is within 3 minutes in Example 4, but more than 1 hour in Comparative Example 3. Resulting in.
When considered as a ventilation sensor, it is necessary to be in a measurable state immediately after energization, and the present invention is also excellent in this respect. This is considered to be because the coexistence of Cu and W provides excellent thermal stability. Further, W/Cu in the catalyst layer was set to 2, and changes in sensitivity to 200 ppm of CO gas and 500 ppm of tobacco smoke with respect to changes in the amount of W were measured and are shown in FIG. The solid line is CO and the broken line is cigarette smoke. As is clear from Figure 4, when the amount of W is less than 0.1%, the overall sensitivity decreases, and when it is more than 20%, the difference in sensitivity between CO and tobacco becomes large. It can be seen that good sensitivity is shown in the range _{of 0.1 to 30% for 2O3 .} Furthermore, the change in sensitivity depending on the W/Cu atomic ratio in the catalyst layer is shown in FIG. 5 for the case where W is 3.0% by weight.
As is clear from Figure 5a, CO200ppm sensitivity S _cp
(solid line) and cigarette smoke 500 ppm sensitivity S _sn (broken line) both exhibit high sensitivity in the W/Cu range of 0.1 to 10. In addition, as is clear from figure b, the S _sn /S _cp ratio also increases with W/Cu.
With a ratio of 0.8 to 1.1 in the range of 0.1 to 10, very good results are obtained. Next, moisture resistance was measured. Figure 6 shows the fluctuations from the initial output after being left at room temperature and 90% humidity. In Example 4 of the present invention, the moisture resistance is within 5% as shown in Figure A, while in Comparative Example 3, it fluctuates by more than 20% as shown in Figure B, and the moisture resistance is extremely poor. . As described above, in the present invention, sensitivity to various gases is uniformly excellent. Excellent moisture resistance. Excellent response and desorption properties. Excellent initial stabilization properties. It is possible to obtain the following effects, making it an extremely excellent gas-sensitive element for ventilation.

[Brief explanation of drawings]

Fig. 1 is a sectional view of the device of the present invention, Fig. 2 is an assembly diagram of the gas detection device, Fig. 3 is a response desorption characteristic curve,
FIG. 4, FIG. 5, and FIG. 6 are sensitivity characteristic curve diagrams. 1... Substrate, 2... Electrode, 3... Gas sensitive body,
4...Catalyst layer.

Claims (1)

[Scope of Claims] 1. A substrate, a gas sensitive body made of an oxide semiconductor which is provided on the substrate and whose resistance value changes when it comes into contact with a gas to be measured, and a gas sensitive body provided on the surface of the gas sensitive body and having copper as a carrier. and a catalyst layer in which tungsten is also supported. 2. The gas-sensitive element according to claim 1, wherein the copper in the catalyst layer is copper sulfate. 3. The gas-sensitive element according to claim 1, wherein the ratio of copper and tungsten in the catalyst layer, expressed as an atomic ratio, is 0.1≦W/Cu≦10. 4. The gas-sensitive element according to claim 1, wherein the carrier in the catalyst layer is aluminum oxide. 5 When the ratio of copper and tungsten to the carrier in the catalyst layer is converted to Cu and W,
2. The gas-sensitive element according to claim 1, wherein the amount is 0.1 to 30 wt% by weight. 6. The gas-sensitive element according to claim 1, wherein the gas-sensitive member is an oxide semiconductor thin film formed by thermally decomposing an organometallic compound.