JPS6160376B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a combustible gas detection element, and particularly to a semiconductor type combustible gas detection element that has extremely stable characteristics against long-term high-temperature operation, and a method for manufacturing the same. 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.
Since a semiconductor gas sensing element is normally required to have a fast response speed, the gas sensing element is used while being maintained at a high temperature in the atmosphere. Therefore, an oxide that is stable against an oxidizing atmosphere is selected as the gas sensitive material. Up until now, various oxides have been used as gas sensitive materials, but recently alpha-type ferric oxide (ferric oxide), which has a corundum-type crystal structure that was previously thought not to be sensitive to gases, has been introduced. α―
It has been discovered that Fe ₂ O ₃ ) exhibits excellent gas-sensitive properties, and studies are underway to develop gas sensing elements using this as a sensitive material. A gas sensor using this α-Fe ₂ O ₃ is
The gas sensitivity characteristics are remarkable when the temperature of the element is in the range of 350 to 450℃, and the sensitivity (resistance value in normal air)
Ra and the resistance value Rg in the gas concentration to be detected), and the rate of change in the resistance value per unit gas concentration in the concentration range to be detected is large, so the gas concentration to be detected can be determined with a quantitative degree. It has the excellent feature that it can be easily detected as a change in resistance value. However, like gas detection elements, the elements themselves are directly exposed to the outside air and used under harsh conditions, so they tend to become unstable, especially over long energized lifespans, making it difficult to always accurately detect gases. It was difficult. The reason for this is that the gas sensitive material is either a porous sintered material or a film-like sintered material formed on a substrate, but the activity changes due to being constantly held at a high temperature. Possible causes include grain growth and a decrease in porosity due to condensation.
All of these are thought to be caused by thermal history, and as a result of various studies on additives to prevent sintering, we found that La ₂ O ₃ , Ce ₂ O ₃ , Pr ₂ O ₃ , and Nd ₂ O ₃ It turned out to be very effective. The present invention is based on the results of this study. By the way, in general, in gas sensors, it is necessary to efficiently heat the sensitive body with as little electric power as possible, so the sensitive body naturally becomes small. The same applies to the ceramic semiconductor type. Therefore, if the various additives added for the purpose of improving characteristics etc. are not uniformly contained in the sensitive body, it will cause variations in characteristics between elements. For this reason, a solution method (co-precipitation method) is an effective manufacturing method because it provides a completely uniform composition. According to this method, in addition to excellent dispersive mixing, fine particle powder can be obtained, which leads to an increase in specific surface area, that is, high activation, and it is possible to obtain a material that can detect even stable gases such as methane. There are advantages. In addition, when focusing on fine particle powder, iron ions are co-precipitated, and after drying, La ₂ O ₃ ,
A method of mixing at least one of Ce ₂ O ₃ , Pr ₂ O ₃ , and Nd ₂ O ₃ is also considered. The present invention has been made in view of the above-mentioned matters,
Examples will be described below in comparison with comparative examples. [Comparative 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 each
of water and stirred while maintaining the temperature at 80°C. Further, while maintaining the temperature at 80° C., an 8N ammonium hydroxide (NH ₄ OH) solution was added dropwise to this solution at a rate of 60 c.c./min until the hydrogen ion concentration of the solution reached 7. After the dropwise addition was completed, the temperature of the solution was maintained at 60° C. for 10 minutes, and the coprecipitate was filtered off by suction. The powder thus obtained was placed in a vacuum container and vacuum dried. After pulverizing the obtained dried material for 2 hours using a grinder, it is pulverized to 100 to 200 μm using an organic binder.
The particles were sized to . Two platinum wires were embedded in this powder, which was then pressure-molded into a cylindrical shape with a diameter of 2 mm and a height of 3 mm, and fired at 550° C. for 2 hours 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 the gas detection element. In the figure, 1 is a sintered body in which electrodes 3 and 4 made of two platinum wires are embedded. Reference numeral 2 denotes a heater for heating the sintered body 1, and power is supplied to the heater from heater pins 11 and 12 through 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 1.
It is configured to be measured between 0 and 0. Heater pins 11, 12 and pins 9, 10 are fixed to a header 13, 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, the energized life at normal operating temperature (400°C), and the overload energized life at a temperature much higher than the normal operating temperature (600°C). To measure the gas sensitivity characteristics, a current is applied to the heater part of the sensing element in advance, and the temperature of the sensing element is set to 400°C.
℃, insert it into a measurement box with a known volume, and then inject test gas into the measurement box with a pillar gun to measure the resistance of the sintered sensitizer. did. Normal energization life is determined by constantly passing current through the heater part of the sensing element to maintain the temperature of the sensing element at 400°C, and measuring the gas sensitivity characteristics using the method described above over time. CH ₄ gas and B gas (H ₂ 65% and i-
C ₄ H ₁₀ (mixed gas with 35%), the rate of change in resistance over time, that is, {initial resistance Rg (5000ppm) - resistance Rg after t hours of energization (5000ppm)}/initial resistance Rg
The value ΔR/R (%) of (5000 ppm) was determined. Regarding the overload energization life, the temperature of the sensitive body was maintained at 600° C., and the temperature was measured with the above-mentioned method over time, and the rate of change in resistance over time was determined using the same method as for the normal energization life. The initial gas sensitivity characteristics are shown in Table 5 below (sample
No. A), Figure 2 shows the change in resistance change rate for CH ₄ during normal energized life, Figure 3 shows that for B gas, and overloaded energized life.
The change in resistance change rate for CH ₄ is shown in FIG. 6, and that for B gas is shown in FIG. 7.
The sample number in each figure is A. What can be seen from Table 5 and Figures 2, 3, 6, and 7 is that during normal energization life and overload energization life, the resistance to methane gas is on the positive side, that is, it tends to deteriorate; For B gas, there is a significant change in the negative side, that is, a sensitization tendency, so it is impossible to use it by attaching it to an actual alarm device as it is.
It can be seen that this is not desirable because it directly leads to a change in the detected concentration. [Example 1] The starting materials were commercially available ferric chloride and ferrous sulfate with a mixing ratio of 1:2, as in Comparative Example 1, and after sintering and oxidation, the materials shown in Tables 1 to 3 (Sample No. .B, C, E,
F, G, I, J, K, M, N, O, Q, R, S,
Lanthanum nitrate (La(NO ₃ ) ₃・6H ² O) and ceric nitrate (Ce
(NO ₃ ) ₃・6H ₂ O), praseodymium nitrate (Pr
(NO ₃ ) ₃・6E ₂ O) and neodymium nitrate (Nd
_{Comparative Example 1}
A sensing element was prepared in the same manner as above, and its characteristics were evaluated in the same manner. The initial gas sensitivity characteristics are shown in Tables 5 and 6 (sample Nos. C, G, K, O, R, S, T, U, V) below.
The transition of the resistance change rate with respect to CH ₄ during the normal energized life is shown in Figure 2 (Sample No. C, G, K, O) and Figure 4 (Sample No. R, S, T, U, V). The values for B gas are shown in Fig. 3 (Sample No. C, G, K, O) and Fig. 5 (Sample No. R, S, T, U, V).
Furthermore, Fig. 6 shows the change in resistance change rate with respect to CH ₄ during the life of overload application (sample Nos. C, G, K, O).
In addition, it is shown in Fig. 7 (Sample No.
C, G, K, O) respectively. Also, La(NO ₃ ) ₃・6H ₂ O, Ce(NO ₃ ) ₃・
6H ₂ O, Pr(NO ₃ ) ₃・6H ₂ O and Nd(NO ₃ ) ₃・
Detection elements created by varying the amount of 6H ₂ O co-precipitation
The rate of change in resistance after 5000 hours of charging (normal charging life) is shown in Figures 10 to 13. From the above results, after sintering La ₂ O ₃ , Ce ₂ O ₃ ,
By containing 0.5 to 20 mol% of at least one selected from Pr ₂ O _{3 and Nd 2 O 3 in the form of Pr 2 O 3 and} Nd ₂ O 3, the rate of change in resistance is extremely high in the temperature range higher than the normal usage temperature. It can be seen that it is stable. As can be seen from the figure, those containing less than 0.5 mol _{% of} La ₂ O ₃ , Ce ₂ O ₃ , Pr ₂ O ₃ and Nd _{2 O 3} have _a large rate of change in resistance; , Pr ₂ O ₃ and
If Nd ₂ O ₃ exceeds 20 mol %, the sensitivity to methane gas decreases, making it impractical due to the circuit configuration of the alarm, and there is no advantage in increasing the amount added any further, so neither is practical. From this, it can be seen that the effective amounts of La ₂ O ₃ , Ce ₂ O ₃ , Pr ₂ O ₃ and Nd ₂ O ₃ in the sintered sensitive body are 0.5 to 20 mol %.

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[Comparative example 2]

Commercially available α-Fe ₂ O ₃ and La ₂ O ₃ , Ce ₂ O ₃ , Pr ₂ O ₃ and Nd ₂ O ₃ were blended as shown in Table 4 (Sample Nos. X, Y, Z), A sensing element was produced by carrying out the steps from mixing and pulverizing to baking in the same manner as in Example 1. Characteristic evaluation was also performed in the same manner. As shown in Table 6 (Sample No. X), the initial gas sensitivity was so low that it could not be used practically. From this, it can be seen that when α-Fe ₂ O ₃ is used as a gas sensitive material, it is necessary to make it into fine particles by a coprecipitation method. In the embodiment, the sensitive body of the sensing element was explained by sintering the molded body, but it is also possible to obtain the sensitive body by making a paste of this sintered body raw material, applying it onto a substrate, and baking it. The raw material salts have been explained using ferric chloride, ferrous sulfate, lanthanum nitrate, ceric nitrate, praseodymium nitrate, and neodymium nitrate, but are not limited to these, but include water-soluble iron, lanthanum, cerium,
All praseodymium and neodymium salts are effective. Furthermore, although firing was performed at 550° C. in the examples, the firing temperature is not limited to this temperature, and any firing temperature that provides a strength of the sintered body that can withstand practical use may be used. As described in detail above, the element of the present invention has α-
Contains Fe ₂ O ₃ and at least one of La ₂ O ₃ , Ce ₂ O ₃ , Pr ₂ O ₃ and Nd ₂ O ₃ , so it maintains extremely stable characteristics even during long-term high temperature operation. It is something that can be done. As a result, variations in characteristics are significantly reduced, making it possible to realize a highly reliable combustible gas detector.

[Brief explanation of the drawing]

FIG. 1 is a front view showing an example of the structure of a combustible gas detection element according to the present invention, FIG. 2, FIG. 4, and FIG.
The figure shows 5000ppm of methane gas during normal energized life.
Figures 3, 5, and 9 are diagrams showing changes in the rate of change of the resistance value of a gas detection element according to an embodiment of the present invention in an atmosphere with respect to its initial value.
Figure 6 showing the rate of change in resistance value in a gas atmosphere
The figure shows the rate of change in the resistance value in a methane gas atmosphere during the life of overload energization, Figure 7 shows the rate of change of the resistance value in a B gas atmosphere during the life of overload energization, and Figure 10. FIGS. 11, 12, and 13 show the amount of Le ₂ O ₃ , amount of Ce ₂ O ₃ ,
FIG. 3 is a diagram showing the relationship between the amount of Pr ₂ O ₃ and the amount of Nd ₂ O ₃ and the rate of change in resistance. 1... Sintered body, 2... Heater, 3, 4... Electrode, 5,
6, 7, 8... Frame, 9, 10... Electrode pin,
11, 12... Heater pin, 13... Base, 14
...Stainless steel wire mesh.

Claims (1)

[Claims] 1. Contains 99.5 to 80 mol% of alpha-type ferric oxide (α-Fe ₂ O ₃ ) prepared by a coprecipitation method, and contains lanthanum (La), cerium (Ce), and praseodymium (Pr).
and at least one selected from four types of oxides of neodymium (Nd) in a total amount of 0.5 to 20 mol%. 2. In addition to iron ions, at least one of lanthanum, cerium, praseodymium, and neodymium ions is co-precipitated to produce a gas sensitizer raw material;
A method for producing a combustible gas detection element, the method comprising obtaining a detection element by firing the combustible gas detection element. 3 Co-precipitate iron ions and mix at least one of the oxides of lanthanum, cerium, praseodymium, and neodymium with the resulting α-Fe ₂ O ₃ to obtain a raw material for a gas sensitizer, and then sinter it. 1. A method for producing a combustible gas detection element, characterized in that the detection element is obtained by: