JPS6157572B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a combustible gas sensing element which contains indium oxide as the main active ingredient and which not only has excellent basic element performance but also excellent humidity characteristics. Most of the combustible gas detection elements that have been put into practical use are made using n-type oxide semiconductors such as tin oxide (SnO ₂ ), zinc oxide (ZnO), or γ-ferric oxide (γ-Fe ₂ O ₃ ). It consisted of a sintered body as an active ingredient. This invention also provides n
The present invention aims to provide a novel and practical combustible gas detection element whose main active ingredient is indium oxide, which is a type oxide semiconductor. Indium oxide shows a change in resistance value sufficient to be used as an element when it comes into contact with flammable gas.
That is, it has sufficient gas sensitivity characteristics. but,
The sintered body has a very low element resistance value, so
If this is attempted to be used in a gas leak alarm, problems such as difficulty in circuit design will occur, which poses practical difficulties. Therefore, in order to increase the element resistance to a practical level without destroying the excellent properties of indium oxide, the inventors considered the use of suitable additives and conducted detailed studies on various additives. did. In the process, they discovered that iron oxide is an excellent additive. Furthermore, in general, oxide semiconductors for gas detection are
When the combustible gas concentration increases to a certain degree, the rate of resistance change with respect to gas concentration change does not increase in proportion to the concentration and tends to reach saturation.
Indium oxide is no exception to this; moreover, in the case of indium oxide, saturation is already reached when the combustible gas is at a relatively low concentration, and the relationship between the change in element resistance and the concentration in the practical concentration range is poor. There is also the problem that it is not linear, that is, the concentration dependence (concentration separability) of the element resistance is somewhat small, but it has been found that this problem can be immediately solved by adding iron oxide as a subcomponent. Ivy. By the way, in this type of gas detection element,
Considering the environmental conditions at the site of use, it is desirable to minimize the influence of humidity. Therefore, by using iron oxide as a subcomponent, it is not enough to be satisfied with merely improving the basic device performance of low resistance value and concentration dependence, but it is also desired to improve humidity characteristics. The present invention was completed based on the above findings, and includes a sintered body and a combustible material that detects the presence of combustible gas by detecting the electrical resistance of the sintered body. A gas detection element,
The main component of the active ingredients of the sintered body is indium oxide,
The subcomponent is iron oxide, and the proportion of iron oxide in the active ingredient (however, when calculating this, indium oxide is converted to In ₂ O ₃ , and iron oxide is converted to Fe ₂ O ₃ ). ) is 5 to 60% by weight. Next, this invention will be explained in more detail. As described above, in the combustible gas detection element according to the present invention, the component showing the ability to detect gas (gas detection ability), that is, the main component in the active component is composed of indium oxide, and the subcomponent is is now composed of iron oxide. That is, in this invention, iron oxide is used as the above-mentioned additive, which solves the problem that the element resistance value of indium oxide is too low and the problem that the concentration dependence is small. I am planning to do so. In the device according to the present invention, it can be considered that indium oxide normally exists in the oxidized form of In ₂ O ₃ in the device, but the presence of indium oxide in other oxidized forms is also within the scope of the present invention. It may be considered that iron oxide is present in the element in the oxidized form of α-Fe ₂ O ₃ in most cases, but of course cases where iron oxide exists in other oxidized forms also fall within the scope of the present invention. By the way, in order to be put to practical use as an element for configuring a gas leak alarm, the resistance value of the element must fall within a certain range, and the rate of change in resistance value must also be maintained above a certain level. Of course. Therefore, prior to completing this invention, the inventors investigated factors that affect the element resistance value and the rate of change in resistance value. As a result, it was found that the amount of subcomponents added was the most important factor. As a result of further experiments and research, the following findings were obtained. In other words, the amount of iron oxide added is 60% by weight in the active ingredients (however, when calculating this percentage, indium oxide is converted to In ₂ O ₃ , and iron oxide is converted to Fe ₂ O ₃ ). (hereinafter the same applies), there is a tendency for the performance of the device to deteriorate; on the other hand,
If the amount added is less than 5% by weight, the effect of increasing the low resistance value of the element cannot be expected to be very high, and the effect of addition is also small in terms of gas sensitivity, concentration separation, etc.
Furthermore, in gas sensing elements, there has been no improvement in humidity characteristics, which is an important performance next to basic element performances such as low element resistance, gas sensitivity, and concentration separation. For this reason, in this invention,
The appropriate proportion of iron oxide in the active ingredient is 5 to 60% by weight, and therefore indium oxide is said to account for the remainder, 40 to 95% by weight. Note that when producing a gas detection element, a component that functions as a binder or a component that functions simply as an extender may be added to the component that exhibits gas detection ability. Even in such a case, it falls within the scope of the present invention as long as the component exhibiting gas detection ability consists of indium oxide and iron oxide. In this specification, the combustible gas detection element according to the present invention has been stated that the main active ingredient is indium oxide and the sub-component is iron oxide.
Indeed, as mentioned above, this is the result of taking into consideration that components other than those exhibiting gas detection ability are often added when actually producing gas detection elements. However, even though I have said this,
Of course, the scope of the present invention also falls within the scope of the present invention if the combustible gas detection element is composed of only the above-mentioned active ingredients, and the present invention is not intended to exclude such cases. As the form of the combustible gas detection element according to the present invention, a form constituted by a sintered body is generally selected because good gas sensitivity can be easily obtained and stability over time is good. It is not limited to this, and may be formed into a thin film or a thick film, for example, and its form is free. In addition, the manufacturing raw materials, manufacturing methods, etc. are appropriately selected in consideration of the ease of obtaining the raw materials, cost, purpose of use, etc. The starting raw material for manufacturing can be of any type as long as it is indium oxide or iron oxide when it becomes an element (the desired oxide itself may be used), and intermediates added to the starting raw material as necessary can be used. It does not matter whether the processing is different or not. However, to give examples for reference, starting materials for indium oxide include In ₂ O ₃ , InCl ₃ .xH ₂ O, In(NO ₃ ) ₃ , etc., and starting materials for iron oxide include Fe ₃ O ₄ , γ-
Examples include Fe ₂ O ₃ and α-Fe ₂ O ₃ . Since this invention is configured as described above,
Provides a combustible gas detection element containing indium oxide as the main active ingredient, which has an appropriate low resistance value, sufficient gas sensitivity and concentration dependence (concentration separability), and excellent humidity characteristics. can do. Next, examples will be described together with comparative examples. The main component and raw material for indium oxide is
In ₂ O ₃ was used, and Fe ₃ O ₄ was used as a raw material for iron oxide, which is a subcomponent. These raw materials were blended in proportions such that the element composition was as shown in the table below, and mixed thoroughly using an Ishikawa-type crusher.
Weighed out a certain amount (20mg) of iron oxide powder, compressed it into a cylindrical element shape with a diameter of 2mmφ and a length of about 2mm, in which a platinum wire electrode was embedded.
A gas sensitive body (sintered body) was produced by firing at 600°C or 800°C for 3 hours in air. The gas sensitive body of the example is
As a result of analysis by line diffraction, it was confirmed that the raw material Fe ₃ O ₄ was converted to α-Fe ₂ O ₃ after firing. A coiled heater was attached around each of the gas sensitive bodies obtained above, and the cap was further covered with a stainless steel wire mesh cap to serve as a gas detection section. The results of investigating the gas sensitivity characteristics of each element are shown in the table below, and all of the examples were superior to the comparative examples. The gas-sensitive characteristics were measured by applying a constant voltage to the coiled heater to maintain the temperature of the element at a constant 450°C, and then contacting the gas-sensitive element with isobutane-containing air with an isobutane concentration of 0.1% by volume and 0.3% by volume. The investigation was conducted by measuring the electrical resistance value of the material and determining its change.

[Table] Here, the gas sensitivity is expressed by the formula R _air −R ₀ . ₁ /R _air ×100, the concentration dependence is expressed by the formula R ₀ . ₁ /R ₀ . ₃ , and the humidity effect is expressed by the formula (at 20°C and RH 65%) R _air / (4
(at 0°C and _RH 85%). In the formula, R _air : resistance value of _the element in air R _0.1 ; resistance value of _{the element in air containing isobutane with an isobutane concentration of 1000 ppm R 0.3 ;} resistance value of the element in air containing isobutane with an isobutane concentration of 3000 ppm Each represents the resistance value of the element. As is clear from the results shown in the table above, in the single-component indium oxide system, the element resistance value is low and is out of the practical range, but by adding iron oxide to indium oxide, the element resistance value can be reduced. The resistance value increases and concentration separation in the practical concentration range improves. Furthermore, when comparing the samples at the same firing temperature, the samples of the Examples are less affected by humidity than the samples of the Comparative Examples.

Claims (1)

[Claims] 1 A combustible gas detection element that detects the presence of a combustible gas by detecting a sintered body and the electrical resistance of this sintered body, the element comprising a The component is indium oxide and the subcomponent is iron oxide, and the proportion of iron oxide in the active ingredient (however, when calculating this, indium oxide is converted to In ₂ O ₃ , and iron oxide is converted to Fe ₂ O ₃ A combustible gas detection element characterized in that the amount of the combustible gas is 5 to 60% by weight.