JPS6045369B2 - Flammable gas detection element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a combustible gas detection element, and an object thereof is to provide an element that exhibits a large resistance change ratio in response to combustible gas, that is, a reducing gas, and has excellent long-term energized life characteristics. It is something to do.

In recent years, with the spread of gas appliances, accidents due to gas leaks have been occurring frequently, and various methods to prevent these accidents have been studied.

2. Description of the Related Art As one of the typical gas detection elements that have been used in the past, one using an n-type metal oxide semiconductor is known. However, in practice, this is not always fully satisfactory because the rate of change in electrical resistance with respect to a unit change in gas concentration is small, the detected concentration lacks quantification, or the long-term energized life characteristics are insufficient. It's not something.
On the other hand, recently, among ferric oxides, gamma-type ferric oxide, which has a spinel-type crystal structure, has been developed.
1 Mori, f, ■, -A-, and L-I-1 have been identified, and development of gas detection elements using them as sensitive bodies is progressing. By the way, ferric oxide has various crystal structures, each with completely different chemical and physical properties, so unless the crystal structure is specified, ferric oxide cannot talk about the physical properties of

Among them, the most well-known is alpha-type ferric oxide (α-Fe.O
. ). In addition, γ-Fe used in the present invention
. O. Furthermore, although the manufacturing method and crystal structure are not very clear, the existence of beta-type ferric oxide (β-Fe2O.), delta-type ferric oxide (δ-Fe.O.), etc. has been reported. However, among these ferric oxides with various crystal structures, only γ shows practical gas sensitivity characteristics.
-Fe. 03 only.

This γ-Fe.

O. The gas sensitivity characteristics when using
It is large when it is in the range of ~4000C, and among them,
Sensitivity (normally expressed as the ratio of the resistance value F Ra in air to the resistance value Rg in the gas concentration to be detected) becomes large at about 350°C. by the way,
What is practically important for a gas detection element, rather than the above-mentioned sensitivity, is how large the rate of change in resistance per unit gas concentration (hereinafter referred to as resistance change ratio) is in the concentration range to be detected. That is to say. In other words, in the gas concentration range to be detected, how can the concentration be quantitatively detected as a change in resistance value? In addition, so-called atmosphere sensors such as gas detection elements, in which the elements themselves are exposed to the outside air and are used under harsh conditions, must undergo an aging process more or less immediately after manufacture. This is a step that should be omitted if possible from the standpoint of device manufacturing, production, and cost. This requires that the element itself maintain stable characteristics over a long period of time immediately after manufacture. The device according to the invention basically consists of γ-Fe2O
It is composed of 3 and γ-Al2O3, and has a large resistance change ratio, so it has good quantitative gas concentration detection, and has extremely stable energized life characteristics over a long period of time immediately after manufacture. It is something that can be demonstrated.

Below, specific characteristics will be described using examples.

Example 1 Triiron tetroxide (Fe3O4) with an average particle size of 0.2 μm
) powder and 0.7 mol of γ-Al2O3 powder.
Three moles were weighed out, water was added, and wet mixed and ground using stainless steel balls in a stainless steel pot.

The mixture was dried in vacuo at a temperature of 80°C and the resulting powder was calcined in vacuo at a temperature of 700'C. Triethanolamine was added to the baked ingredients to form a paste. On the other hand, the substrate for the gas detection element is 5T each in the vertical and horizontal directions.
T0n1 thickness. A 0.5Tr$L alumina substrate was prepared, and gold paste was applied to the surface at intervals of 0.5TWL and baked to form comb-shaped electrodes.A commercially available ruthenium oxide glaze resistor was placed between the gold electrodes on the surface. was printed and baked to make a heater.

Next, the paste described above was printed on the surface of the substrate to a thickness of about J70 μm, air-dried at room temperature, and then baked at a temperature of 400° C. for 1 hour, usually in air. At this stage, the main component Fe3O4 was oxidized to γ-Fe2O3, and at the same time the solvent in the paste evaporated to form a sintered film with sufficient mechanical strength for practical use. The thickness of the gas sensitive body thus obtained was approximately 50P7TL. The operating temperature of the device was controlled by passing a current through the heater section and adjusting the current value.

The resistance value (Ra) in air is measured in a measurement container with a volume of 50e in which dry air is stirred slowly to the extent that no turbulence occurs. Isobutane gas with a purity of 99% or more flows at a rate of 10 ppm/sec by volume, and its concentration is 0.05
% and 0.5%, respectively. The gas concentrations to be measured were set to 0.05% and 0.5% to detect concentrations ranging from 1 to a fraction of the lower explosive limit (LEL) of isobutane gas, which is approximately 2%. What you do is
This is because it is practically necessary as a combustible gas detection element. Gas sensitivity characteristics were measured by energizing the heater of the element obtained as described above immediately after manufacture to maintain the element temperature at 350°C. As a result, Ra was 780KΩ, Rg(0.
05%) was 220K7Ω, and Rg (0.5%) was the immersion area Ω. That is, the resistance change ratio between the gas concentration regions of 0.05% and 0.5% is 5.79. This value is a large value that has not been seen in conventional semiconductor gas sensing elements.
By the way, this sensing element detects the JO. of isobutane gas. O
In the concentration range of 5% to 0.5%, the resistance value Rg of the sensitive body is proportional to C-1, where C is the gas concentration.

Therefore, the above-mentioned resistance change ratio can be evaluated using a constant n (hereinafter referred to as concentration coefficient). Incidentally, the resistance change ratio of 5.79 corresponds to n=0.763. Next, the heater of this element was energized to maintain the element temperature at 350°C, and a DC voltage of 10 μm was applied between the electrodes on the element side, and the element was left for a nodding period.

After that, we measured the gas sensitivity characteristics using the same method as described above, and found that
Ra=772KΩ, Rg(0.05%)=218KΩ,
Rg (0.5%) = 37KΩ, resistance change ratio = 5.89 (
n=0.770). Furthermore, after leaving this element for 200Vf in the same manner, the gas sensitivity characteristics were measured. As a result, Ra = 786KΩ, Rg (0.05%) = 22
9KΩ, Rg (0.5%) = 40KΩ, resistance change ratio = 5
．． 73. n=0.758. As can be seen from the above experimental results, the element according to the present invention has an extremely large resistance change ratio in the practically detectable concentration range, and its quantitative properties are very good.

In addition, its gas sensitivity characteristics are stable over a long period of time immediately after production, and no aging process is required.
It also has significant manufacturing advantages. Example 2 n-Fe2O3 and γ-Al.

Elements were fabricated in the same manner as in Example 1 with various composition ratios with O3, and gas sensitivity characteristics were measured for each element. In addition, alpha-type alumina (α-A
12O3: Generally speaking, when referring to aluminum oxide without specifying the crystal structure, it means this alpha-type aluminum oxide), γ-Al. Experiments were also carried out using O3 in place of O3, and devices with various composition ratios were fabricated under the same conditions as in Example 1, and the gas sensitivity characteristics of each were measured. Figure 1 shows the concentration coefficient (Rgcc
(expressed by n in C-1), Fig. 2 shows the energized life characteristics 20 hours after the element fabrication, and Fig. 3 shows the energized life characteristics and the amounts of γ-Al2O3 and α-AI2O3 after 200 hours. Compare and show the relationship between. As is clear from FIG. 1, the concentration coefficient n increases as the composition ratio of γ-/Vl2O3 increases, and reaches a maximum at 10 to 30 mol%.

On the other hand, when α-Al2O3 was used, no effect was observed. Furthermore, from FIGS. 2 and 3, it can be seen that by using γ-Al2O3, stable charging life characteristics are exhibited over a long period of time immediately after the device is fabricated. On the other hand, when α-Al2O3 is used, the resistance value changes for about an additional time immediately after the device is fabricated, but after that, stable characteristics are exhibited. Here, FIGS. 2 and 3 show the rate of change of the resistance value Rg (0.5%) with respect to the initial value at an isobutane gas concentration of 0.5%. Example 3 Fe with average particle size of 0.5 prrL
Weighed out 0.8 mol of 3O4 powder and 0.2 mol of γ-Al2O3 powder, wet-mixed them in the same manner as in Example 1,
And crushed.

The mixture was dried in vacuo at a temperature of 80°C. The powder thus obtained was press-molded into a rectangular parallelepiped shape and sintered at a temperature of 800°C in a nitrogen stream. Then, after cooling to room temperature, the temperature was gradually raised in normal air and maintained at a temperature of 400°C for 1 hour to produce γ-Fe2.
A sintered body containing O3 as a main component was obtained. Gold was deposited on the surface of the sintered body thus produced to form a pair of comb-shaped electrodes. A platinum heating element was attached to the back side with an inorganic adhesive to serve as a heater, and this gas detection element was made. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element. The gas sensitivity characteristics were measured in the same manner as in Example 1 while maintaining the element temperature at 350°C. The results are summarized in the table below. Even if the element is used as a sintered body, although the absolute value of the resistance value itself is different from that of the sintered film shown in Examples 1 and 2, it has excellent initial characteristics and energized life characteristics. I know what you're doing. In this way,
In the element according to the present invention, even if its shape is a sintered film,
Even if it is a sintered body, the effects of using γ-Al2O3 can be fully exhibited. As mentioned above, gamma type ferric oxide (γ-Fe2O3) is
mole%, and gamma aluminum oxide (γ-Al2O
Gas sensing elements containing 0.5 to 50 mol % of It has excellent charging life characteristics over a long period of time.

In the device according to the present invention, the amount of γ-Al2O3 is
The reason why it is set at 0.5 to 70% with respect to the total amount of e2O3 is that if it is less than 0.5 mol%, the effect of γ-AI2O3 will not be seen, and if it exceeds 70 mol%, the resistance of the element will decrease. The value becomes abnormally high, and either the sintered film or the sintered body may not have sufficient mechanical strength as a practical element, making it impossible to use it as a practical gas sensing element. I have saved up. Furthermore, in the examples, the starting material was described as triiron tetroxide (Fe3O4), but in the state of the final device, the main component was γ-Fe2O.
There are no particular restrictions on the raw materials, as long as they have a rating of 3. In addition, among the aluminum oxide (Al2O3) crystal phases, γ-Al2O3 with a spinel structure was selected because even if α-7/Vl2O3, which is the most representative of the crystal phases, was used, after aging of about 201 Terama, Although γ-Al2 has excellent long-term energized life characteristics, it has no effect on increasing the resistance change ratio, that is, the concentration coefficient.
This is because O3 can maintain excellent properties for a long period of time. As described above, the gas detection element according to the present invention realizes an element with a large resistance change ratio, that is, a concentration coefficient, which is an extremely important element in practical use as an element that detects gas concentration with good quantitative accuracy, and is capable of long-term use. This provides an element with excellent long-term stability that maintains its characteristics over time. Moreover, it is of course possible to add and contain other components in order to further improve the properties or to obtain more suitable properties depending on the purpose. In the embodiments, the gas to be detected is limited to isobutane gas, but it goes without saying that the effects of the present invention are also effective for general flammable gases such as ethane, propane, and hydrogen.

[Brief explanation of drawings]

FIG. 1 shows γ in the combustible gas detection element according to the present invention.
-The relationship between the amount of Al2O3 and the concentration coefficient is expressed as γ-Al2O3
Figures 2 and 3 are diagrams that compare the relationship when α-Al2O3 is used instead of , and Figures 2 and 3 are diagrams that also compare and compare the energized life characteristics.

Claims (1)

[Claims] 1 Gamma type ferric oxide (γ-Fe_2O_3) at 99
．． 5-30% mol, and gamma-type aluminum oxide (
A sintered film or a sintered body containing 0.5 to 70 mol% of γ-Al_2O_3) is used as a gas sensitive body, and a pair of electrodes is provided on this gas sensitive body to detect the presence of combustible gas. A combustible gas detection element characterized by detecting a decrease in the electrical resistance value of a gas sensitive body.