JPH0230663B2 - 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 both 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 with a sensor for general combustible gases, the concentration of combustible gas to be detected is a fraction of the lower explosive limit, whereas with a sensor for CO In some cases, extremely small amounts
This is due to the need to detect CO. That is,
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, with the amount reaching the lower explosive limit. This is due to the fact that the target must be the detection of extremely small values in comparison. In low-cost, highly reliable combustible gas sensors, oxide semiconductors that are maintained 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 microstructure of γ-Fe ₂ O ₃ or material composition (impurities, etc.).
Therefore, compared to γ-Fe ₂ O ₃ , α-Fe ₂ O ₃ is
It can be said that it is a much more stable material thermally. 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 out how 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, it has been found that by containing at least one of Ge or Th and at least one of Cd or Au, 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 very thermally 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 present invention relates to alpha-type ferric oxide (α-Fe ₂ O ₃ ).
This discovery was made while studying the effects of additives on gas sensing elements that use gas as a gas sensitive material. That is, in the gas sensing element of the present invention, at least one of Ge and Th is
In α-Fe ₂ O ₃ containing 0.1 to 50 mol% in total in terms of GeO ₂ and ThO ₂ , at least one of Cd and Au is 1.0 to 20 and 0.1 in terms of CdO and Au, respectively.
~10% by weight was added as the gas sensitive material, and this is the base material of the gas sensitive material.
By adding Cd and Au to α-Fe ₂ O ₃ containing Ge or Th, the gas sensitivity characteristics and its reliability are dramatically improved. This was achieved by discovering that it was possible to achieve high sensitivity. Description of Examples Examples of the present invention will be described below. First, in Example 1, when an additive such as Ge or Au is added to the base material,
We will discuss the effect of the amount of CdO and Au added. Example 1 30 g of commercially available ferric chloride (FeCl ₃ 6H ₂ O),
60 g of ferrous sulfate (FeSO ₄ .7H ₂ O) was dissolved in 1 part of water and stirred while maintaining the temperature at 50°C. While keeping the temperature of this solution at 50℃, add 8N ammonium hydroxide (NH ₄ OH) solution to this solution.
The solution was added dropwise 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 thus obtained was heated to 110°C in air.
It was dried. This dried material is placed in the air at 400℃.
Heat treatment was performed for 1 hour. Commercially available germanium oxide (GeO ₂ ) and thorium oxide (ThO ₂ ) were added to this heat-treated powder so as to have the composition shown in Tables 1 to 2 (additives in base material). Furthermore, Tables 1 to 2 (Additives)
As shown in Figure 2, a solution prepared by dissolving commercially available cadmium oxide (CdO) and chloroauric acid (HAuCl ₄ 4H ₂ O) in water to a concentration of 100 mg/ml,
They were added singly or in combination. 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, 4 through the frames 5, 6 to the pins 9, 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 inject the test gas ((CO gas (mixed gas of CO 5.0% and N ₂ 95.0%)) with a syringe. and H ₂ gas (99% or more))) is injected into the measurement box, and the concentration of CO or H ₂ 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 (resistance change rate over time of 2000 hours at 350°C). Evaluation was made based on the rate of change in resistance value from the initial value when the resistance value was maintained. Tables 1 and 2 also show the gas sensitivity (Ra/Rg) and the rate of change in resistance over time (ΔR) when an additive (CdO or Au) is added.
Note that ΔR is written in parentheses in the table.

【table】

[Table] * Comparative example

【table】

[Table] * Comparative Example From Tables 1 and 2, CO can be reduced by adding CdO or Au singly or in combination to α-Fe ₂ O ₃ containing 01 to 50 mol% of GeO ₂ and ThO ₂ . It shows extremely high activity against gas and is stable over time, which means 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 effects of the amounts of CdO and Au added will be described when the sensitive body is a sintered film and a plurality of additives, ie, Ge and Th, are added to the base material. Example 2 Starting material is commercially available ferric chloride (FeCl ₄ 6H ₂ O)
Using 30g and _60g of ferrous sulfate ( _FeSO4.7H2O ),
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 ( _HAuCl4 .
A solution prepared by dissolving 4H ₂ O) in water to a concentration of 100 mg/ml was added singly or in combination, and each powder was dry mixed for 3 hours using a sieve. 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. In the figure, 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 having a length and width of 5 mm each and a thickness of 0.5 mm as a substrate for a gas sensing element. 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 body 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 3 and 4 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 additive amounts 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 the case where the amount of CdO added to α-Fe ₂ O ₃ containing 1.0 mol% each of GeO ₂ and ThO ₂ was changed (sample Nos. C-1 to C-5).
When the amount of Au added to α-Fe ₂ O ₃ containing 5.0 mol % was changed (sample Nos. C-6 to C-10), Figure 5 shows α-Fe ₂ O ₃ containing 25.0 mol % respectively. to CdO
When the amount of added Au was changed (Sample No.C-11
-C-15) shows the dependence of the gas sensitivity characteristics on the amount of addition. As is clear from Tables 3 and 4, 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 3 to 4 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 is α
The reason why the amount added to the base material containing -Fe ₂ O ₃ as a main component is limited to 1.0 to 20% by weight and 0.1 to 10.0% by weight, respectively, is based on the above-mentioned reason. By the way, in Examples 1 and 2, ferrous sulfate and ferric chloride were used as starting materials for iron, a commercially available oxide reagent was used as an additive, and chloroauric acid was used for Au. The present invention does not limit the starting materials or manufacturing process in any way as long as the final composition of the sensitive material is within the above-mentioned range. Effects of the Invention As explained above, the gas sensing element of the present invention uses α-Fe ₂ O ₃ containing Ge or Th as an additive.
Furthermore, it is a sintered body or a sintered body to which Cd and Au are added, making it possible to achieve high sensitivity to CO gas, which has been considered difficult to detect in trace amounts. This precisely responds to the growing social needs of a CO sensor that can prevent this from happening, as poisoning accidents due to CO caused by incomplete combustion of gas appliances or during the early stages of a fire tend to occur frequently these days. , and its 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]

Figures 1 and 2a and 2b are diagrams showing an example of the structure of a gas detection element according to the present invention, and Figures 3 to 5 are diagrams showing the amount of additives and CO FIG. 3 is a characteristic diagram showing the relationship between the sensitivity to H ₂ (Ra/Rg) and the rate of change in resistance over time (ΔR). 1... Sintered body.

Claims (1)

[Claims] 1. Germanium (Ge) and thorium (Th)
At least one of them is GeO ₂ ,
At least one of cadmium (Cd) and gold (Au) is added to alpha-type ferric oxide (α-Fe ₂ O ₃ ) containing 0.1 to 50 mol% in total in terms of ThO ₂ .
A gas sensing element characterized in that CdO and Au are added in an amount of 1.0 to 20 and 0.1 to 10% by weight, respectively, as a gas sensitive material. 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.