JPH0230661B2 - GASUKENCHISOSHI - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a gas detection element using a composite metal oxide semiconductor used to detect combustible gas. Conventional configuration and its problems In recent years, various research and development activities have become active regarding materials for sensing elements for flammable gases. This is strongly attributable to the fact that explosions and poisoning accidents caused by flammable gas occur frequently, mainly in households and in various factories, and have become a major social problem. Among these, in particular, those that detect propane gas or city gas have been developed and put into practical use with considerably high levels of sensitivity and reliability. These are widely applied, for example, to various gas leak alarms. On the other hand, CO detection is becoming a hot topic as another societal need for gas disaster prevention. This is largely due to the spread of various gas appliances and the airtightness of housing structures. In other words, the problem is poisoning due to incomplete combustion of gas appliances or CO generated from new building materials in the early stages of a fire. In particular, the latter is an extremely important social problem because it accounts for the majority of deaths caused by fire. However, at present, there is no inexpensive and simple gas sensor that can accurately detect CO, and the situation is such that the above-mentioned social needs are not fully met. The reason for this is that in the case of a sensor for general combustible gases, the concentration of combustible gas to be detected is at least a fraction of the lower explosive limit; In this case, extremely small amounts of CO must be detected. In other words, while the purpose of sensors for other combustible gases is to prevent gas explosions, the main purpose of sensors for CO is to prevent CO poisoning; This is because the detection target must be a value that is extremely significant compared to the limit. For this reason, CO sensors are not sensitive to various other gases even if they are present;
This means that we must be sensitive to the presence of extremely small amounts of CO. In other words, the CO sensor is required to have selectivity to CO and high reliability. In low-cost, highly reliable combustible gas sensors, oxide semiconductors that are kept at high temperatures are often used to detect changes in their resistance. Several types of oxide semiconductors have been found that are highly sensitive or selectively sensitive to CO, but unfortunately, sensors with sufficient reliability have not yet been obtained. be. Purpose of the Invention The present invention was made in view of the above situation, and is intended to realize a gas detection element that is highly sensitive to CO and highly reliable. By the way, it is well known that ferric oxide (Fe ₂ O ₃ ) has α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ as typical crystal phases. α-Fe ₂ O ₃ is γ-Fe ₂ O ₃ at high temperature (e.g. 500℃
above). This transition temperature varies depending on the preparation conditions and fine structure of γ-Fe ₂ O ₃ or material composition (impurities, etc.). Therefore, it can be said that α-Fe ₂ O ₃ is a much more thermally stable material than γ-Fe ₂ O ₃ . However, although both α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ are the same ferric oxide, they have different crystal structures, and have different physical and chemical properties, as well as gas sensitivity characteristics (to some extent). There is also a very large difference in the property that the electrical resistance value of the susceptor material decreases significantly when it comes into contact with a reducing gas at high temperatures. Specifically, γ-Fe ₂ O ₃ itself has very large gas-sensitive properties, but α-
Fe ₂ O ₃ itself is hardly sensitive to gas. γ-Fe ₂ O ₃ has already been put into practical use as a sensitive material for gas sensors, taking advantage of its large gas-sensitivity properties. However, the problem with using γ-Fe ₂ O ₃ is that, as mentioned above, at high temperatures above the transition temperature,
This results in the transition to α-Fe ₂ O ₃ , which is not originally sensitive to gas. Therefore, in reality α−
Efforts are being made to find ways to increase the transition temperature to Fe ₂ O ₃ and how to improve the activation energy for that transition. Although γ-Fe ₂ O ₃ has a very high gas sensitivity to general flammable gases,
This means that it has extremely low sensitivity to CO. On the other hand, α-Fe ₂ O ₃ by itself shows almost no sensitivity characteristics to general flammable gases or even CO. However, Ti, Zr or Sn
at least one of the following, and also Cd or Au
It has been found that by containing at least one of the following, it becomes extremely sensitive to CO. The present invention has been made based on these newly discovered facts. Note that when γ-Fe ₂ O ₃ is used, the sensitivity to CO is not increased even if these additives are used;
No effect of addition was found. As mentioned above, α-Fe ₂ O ₃ is a thermally very stable material, so it is very advantageous as a sensitive material for gas sensing elements that use constant heating of the sensitive body. Needless to say. Structure of the Invention The gas detection element of the present invention includes Ti, Zr, and Sn.
At least one of them is TiO ₂ ,
α-Fe ₂ O ₃ containing 0.1 to 50 mol% in total in terms of ZrO ₂ and SnO ₂ , 1.0 to 20 in terms of Cd and Au, 0.1 to
The material added with 10% by weight was used as the gas sensitive material, and this is the base material of the gas sensitive material.
Cd and α-Fe ₂ O ₃ containing Ti, Zr or Sn
By adding Au, the gas sensitivity characteristics and its reliability are dramatically improved.
This was achieved by discovering that it was possible to achieve a sensitivity that was sufficiently high for practical use even to CO. Description of Examples Examples of the present invention will be described below. First, in Example 1, the effect of adding CdO and Au will be described when an additive, ie, one of Ti, Zr, and Sn, is added to the base material. Example 1 30 g of commercially available ferric chloride (FeCl ₃ .6H ₂ O) and 60 g of ferrous sulfate (FeSO ₄ .7H ₂ O) were dissolved in 1 part of water and stirred while maintaining the temperature at 50°C. To this, commercially available titanium sulfate solution (Ti(SO ₄ ) ₂ ), zirconium oxychloride (ZrOCl ₂ .8H ₂ O) and tin chloride (SnCl ₄ .
5H ₂ O) in Tables 1 to 3 (Additives in base material)
It was added so that the composition was as shown in the figure. While maintaining the temperature of the solution at 50° C., an 8N ammonium hydroxide (NH ₄ OH) solution was added dropwise to the solution at a rate of 60 c.c./min until the hydrogen ion concentration of the solution reached 7. Immediately after the dropwise addition was completed, the coprecipitate was suction-filtered. The powder obtained in this way is
Dry at 110°C. Leave this dry material in the air
Heat treatment was performed at 400°C for 1 hour. Tables 1 to 3 (Additives)
A solution prepared by dissolving commercially available cadmium oxide (CdO) and chloroauric acid (HAuCl ₄ 4H ₂ O) in water and adjusting the concentration to 100 mg/ml as shown in Figure 3. Added in multiple combinations. The respective powders were then dry mixed for 3 hours using a mixer. Two platinum wires are embedded in this powder, and the diameter is 2
It was press-molded into a columnar shape with a height of 3 mm and a height of 3 mm, and was fired at 500° C. for 1 hour in air. The obtained porous sintered body was attached to a sensing element header, a coil-shaped heater was placed around the sintered body, and an explosion-proof stainless steel net was covered to obtain a sensing element. FIG. 1 shows the structure of a gas detection element. In the figure, 1 is a sintered body in which electrodes 3 and 4 made of two platinum wires are embedded. 2 is a heater for heating the sintered compact 1, and the heater pin 1
Electric power is supplied to the heater from heater frames 7 and 8 from heater frames 7 and 8. The resistance of the sintered body 1 is measured from the electrodes 3 and 4 through the frames 5 and 6 to the pins 9 and 10.
It is configured to be measured between Heater pins 11 and 12 and pins 9 and 10 are header 1
3, and a stainless steel wire mesh 14 is attached to the header. Regarding the sensing element obtained as above,
We investigated the gas sensitivity characteristics and the lifespan of the battery under normal use (350°C). To measure the gas sensitivity characteristics, a current is passed through the heater part of the sensing element in advance, and the temperature of the sensing element is set to 350°C.
℃, insert it into a measuring box with a known volume, and use a syringe to inject test gas (CO gas (mixture of CO 5.0% and N ₂ 95.0%), and _H2 gas (99% or more)) into the measurement box, and the CO or _H2 concentration is 0.01% by volume.
(100 ppm), the resistance value of the sintered sensitive body was measured. The gas concentration to be measured was chosen to be 100 ppm because the permissible concentration of CO for industrial hygiene is 100 ppm, so it is necessary to be sensitive to at least this concentration or lower. The gas sensitivity characteristics are: (i) gas sensitivity (resistance value in air (Ra)/resistance value in gas (Rg)), (ii) resistance change rate over time △R (sensor held at 350°C for 2000 hours) Tables 1 to 3 show additives (CdO or
Gas sensitivity (Ra/Rg) when adding Au) and
The rate of change in resistance over time (ΔR) is shown. Note that ΔR is written in parentheses in the table.

【table】

[Table] * Comparative example

【table】

[Table] * Comparative example

[Table] *Comparative example From Tables 1 to 3, TiO ₂ , ZrO ₂ and SnO ₂
to α-Fe ₂ O ₃ containing 0.1 to 50 mol% of each alone.
By adding CdO or Au alone or in combination, it shows extremely high activity against CO.
Furthermore, since this is stable over time, it can be seen that extremely high gas sensitivity and reliability can be achieved as a result. In this Example 1, the sensitive body is a sintered body, and the additive in the base material is one type of CdO
and the amount of Au additive. In Example 2 shown below, the effect of the amounts of CdO and Au added will be described when the sensitive body is a sintered film and a plurality of additives, namely Ti, Zr and Sn are added to the base material. Example 2 Starting material is commercially available ferric chloride (FeCl ₃ 6H ₂ O)
30 g and 60 g of ferrous sulfate (FeSO ₄ .7H ₂ O), titanium sulfate solution (Ti(SO ₄ ) ₂ ), zirconium oxychloride (ZrOCl ₂ .8H ₂ O) and tin chloride (SnCl ₄・Two or more types of 5H ₂ O) were added so as to have the composition shown in Tables 4 to 5 (additives in the base material), and a coprecipitate was obtained in the same manner as in Example 1. . This was dried and heat treated, and then commercially available cadmium oxide (CdO) and chloroauric acid (HAuCl _{4.4H 2} O) were added to it to give the composition shown in Tables 4 and 5 (additives). When dissolved in water, this concentration is
Solutions adjusted to 100 mg/ml were added singly or in combination, and the respective powders were dry mixed for 3 hours using a miller. This powder was sized to a size of 50 to 100 microns, and triethanolamine was added to form a paste. The structure of a sintered film type gas sensing element made using this is shown in FIGS. 2a and 2b as a front view and a back view, respectively. In the figure, the substrate of the gas detection element is vertical,
A pair of comb-shaped gold electrodes 2 were formed at an interval of 0.5 mm on the surface of an alumina substrate 1 with a width of 5 mm and a thickness of 0.5 mm. A gold electrode 3 for a resistor was also formed on the back surface at the same time, and during this time a glaze resistor 4 was printed and baked to form a heater. Next, the above paste was printed on the surface of the substrate to a thickness of about 70μ, and after air drying at room temperature, it was heated to 500℃.
The mixture was gradually heated until the temperature reached , and maintained at this temperature for 1 hour. At this stage, the paste evaporated and became a sintered film 5. The thickness of this gas sensitive member was approximately 55μ. A gas sensing element was thus obtained.

【table】

[Table] * Comparative example

[Table] * Comparative example The gas sensitivity characteristics of each sensing element are shown in Example 1.
It was measured in the same way as in the case of . Tables 4 and 5 show the gas sensitivity (Ra/Rg) and resistance change rate over time (△R) when CdO and Au are added when there are multiple additives in the base material and the amounts added are different. Indicated.
Moreover, FIGS. 3 to 5 show the dependence of the gas sensitivity characteristics on the amount of addition when CdO and Au are added singly or in combination when the additives in the base material are constant. Figure 3 shows that when the amount of CdO added to α-Fe ₂ O ₃ containing 1.0 mol% each of TiO ₂ , ZrO ₂ and SnO ₂ was changed (sample Nos. D-4 to D-8), the fourth The figure shows the case where the amount of Au added to α-Fe ₂ O ₃ containing 5.0 mol% was changed (sample Nos. D-9 to D-13),
Figure 5 shows α-Fe ₂ O ₃ containing 15.0 mol% each.
When the amount of CdO and Au added was changed (Sample No.
The dependence of the gas sensitivity characteristics of D-14 to D-18) on the amount of addition is shown. As is clear from Tables 4 and 5, almost the same characteristics as those obtained in Example 1 are obtained even when the sensitive body is a sintered film. Further, the rate of change in resistance value over time is also very small, as in Example 1. Also, from Tables 4 to 5 and Figures 3 to 5
If the amounts of CdO and Au added are less than 1.0 and 0.1% by weight, respectively, there is no effect, and the effects of the present invention cannot be expected. On the other hand, the amount of CdO and Au added is 20.0 and 20.0, respectively.
If it exceeds 10.0% by weight, it becomes impractical in terms of reduced gas sensitivity or stability of properties. The amount of CdO and Au contained in the gas sensing element of the present invention added to the base material is 1.0-20.0 and 0.1-20.0, respectively.
The reason why it was limited to 10.0% by weight is based on the above-mentioned reason. By the way, in Examples 1 and 2, ferrous sulfate and ferric chloride were used as iron salts, and sulfates and chloride salts were used as additive salts as starting materials for the base materials.
Furthermore, for CdO, a commercially available oxide was used, and for Au, chloroauric acid was used, but in the present invention, the final composition of the reactor only needs to be within the above-mentioned range, and no starting materials are used. It does not limit the manufacturing process. Effects of the Invention As explained above, the gas sensing element of the present invention contains α-
A sintered body or sintered film made by adding Cd and Au to Fe ₂ O ₃ is used as a sensitive body, and this achieves high sensitivity for CO gas, which has been considered difficult to detect in trace amounts. It is possible.
This is an accurate response to the growing social needs of CO sensors that can prevent CO poisoning from incomplete combustion of gas appliances or from the early stages of a fire, as poisoning accidents tend to occur frequently these days. Yes, and the effects are extremely large. Another effect of the present invention is a significant reduction in the life characteristics, especially the change in resistance value over time due to energization. In other words, this makes an extremely large contribution to improving the reliability of the element, which is the most important element of any sensing element.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of the structure of a gas detection element according to the present invention, FIGS. 2a and 2b are front and back views of the same element, and FIGS. FIG. 2 is a characteristic diagram showing the relationship between the amount of additives, the sensitivity to CO and H ₂ (Ra/Rg), and the rate of change in resistance over time (ΔR). 1... Sintered body, 2... Heater, 3, 4... Electrode, 5, 6... Frame, 7, 8... Frame for heater, 9, 10... Pin, 11, 12... Pin for heater , 13... Header, 14... Stainless steel wire mesh.

Claims (1)

[Claims] 1. At least one of titanium (Ti), zirconium (Zr), and tin (Sn) is
Alpha-type ferric oxide (α-Fe ₂ O ₃ ) containing 0.1 to 50 mol% in total in terms of TiO _{2 ,} ZrO _{2 ,} and SnO 2
, at least one of cadmium (Cd) and gold (Au) is 1.0 in terms of CdO and Au, respectively.
~20, a gas sensing element characterized in that it is used as a gas sensitive material to which 0.1 to 10% by weight is added. 2. The gas detection according to claim 1, wherein the gas sensitive body is a sintered body obtained by pressure molding and firing, or a sintered film obtained by printing and firing a paste. element.