JPS6152418B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical field to which the invention pertains) The present invention relates to improvements in gas detection elements. (Prior Art and its Problems) Many examples of gas-sensitive elements using oxide semiconductors as gas-sensitive members have been proposed. In most cases, N-type oxide semiconductors are used, and the gas is detected by utilizing the fact that its resistance decreases upon contact with a reducing gas. On the other hand, when a P-type oxide is used as the semiconductor, it goes without saying that its resistance value increases when it comes into contact with a reducing gas. In this case, in order to increase the sensitivity to gas, not only a semiconductor but also a catalyst is used, and a heater is provided to keep the element at a high temperature. For example, as shown in FIG. 1, an element is known in which a metal wire is embedded in a gas sensitive body 1 as a heater. In the case of this element, one of the pair of electrodes 2 and 3 also serves as a heater, making it difficult to equalize the temperature within the element and making assembly difficult. In addition, since the heating was not uniform, there was a risk of a lack of reliability over a long period of time.
On the other hand, as another example, as shown in FIG. 2, a structure in which an insulating material such as alumina is used as the substrate 4, a heater 7 is provided on one side, and a pair of electrodes 2, 3 and a gas sensitive body 1 are provided on the opposite side is considered. It will be done. In this case, as described above, the substrate 4 is generally made of a heat-resistant material such as alumina, magnesia, or mullite. In the case of the conventional method, for example, the gas sensitive layer 1 shown in FIG. 2 is usually provided directly on the substrate by a thick film printing method or the like. With such a configuration, the adhesive strength of the gas sensitive layer shown in FIG. 2 to the substrate was insufficient, and there was a risk of peeling off due to mechanical vibration or the like. Furthermore, even without mechanical vibration, the adhesive strength tended to decrease due to the impact caused by the interruption of the heater. One way to improve this problem is to raise the baking temperature of the thick film, lengthen the baking time, or both to ensure that the thick film of the gas sensitive layer is bonded to the substrate with sufficient strength. There is a way to do it. However, this method also promotes sintering of the gas sensitive material particles constituting the thick film, which greatly impairs the gas detection sensitivity, which is the original objective. Another possible method is to mix a material that melts at a relatively low temperature, such as glass, into the thick film printing paste and bake it at a relatively low temperature. However, in this case as well, the molten glass covers the surface of the particles of the gas-sensitive material or penetrates between the particles, increasing the element resistance and significantly impairing the gas-sensitive characteristics. (Object of the invention) In view of the above points, the present invention improves the shortcoming of the conventional example of low mechanical strength, and has sufficient mechanical strength.
The object of the present invention is to provide a sensing element with high gas sensitivity characteristics. (Summary of the invention) The present invention includes an insulating substrate provided with a pair of electrodes,
A gas sensitive body made of a metal oxide semiconductor provided between electrodes on the insulating substrate, and a thin film of a metal oxide provided between the gas sensitive body and the insulated substrate and obtained by thermal decomposition of an organometallic compound. This gas sensing element can be manufactured as follows. That is, a step of providing a pair of electrodes on an insulating substrate, a step of providing a gas sensitive body made of a metal oxide semiconductor via a coated dry film of an organometallic compound, and a step of sintering the gas sensitive body and simultaneously applying the organic metal compound. It can be manufactured from a step of thermally decomposing a coated and dried oxide film to form a metal oxide thin film. That is, the present invention is the conventional gas sensing element shown in FIG. 2 as described above, which is characterized in that a metal oxide layer obtained by thermal decomposition of an organometallic compound is provided between the gas sensitive body and the insulating substrate. and
It is possible to have sufficient adhesion strength between the insulating substrate and the gas-sensitive member and to obtain excellent gas-sensitive characteristics. More specifically, the organometallic compound used in the present application is generally represented by (Me) and n(OR)m.
Note that Me is a metal element, R is nitrogen, carbon, hydrogen,
In this application, as organometallic compounds, metal alcohol compounds,
Metal alkoxides, naphthenic acids, etc. can be used. Further, organometallic compounds are usually liquids with high viscosity, but they can also be solids. These are well soluble in alcohols such as propyl alcohol and organic solvents such as tetralin and terpineol, and can be freely diluted. Adjust the organometallic compound to an appropriate viscosity by adding a diluent if necessary. Next, after screen-printing the organometallic compound on the insulating substrate, a gas sensitive layer made of a paste-like metal oxide semiconductor is provided. When this is fired,
The thin film of the organometallic compound decomposes into extremely fine oxide particles, which form a structure in which the gas sensitive body and the insulating substrate are mechanically bonded. As mentioned above, such reactions occur at relatively low temperatures, and in the case of many organic metals, calcination at 500 to 600°C is sufficient. Furthermore, the particles are bonded by a chemical reaction and cannot be simply mechanically bonded, making them extremely strong. In addition, since the sintering temperature is relatively low, grain growth of the metal oxide semiconductor does not occur.
Gas sensitivity is extremely high. Moreover, since a large amount of CO ₂ and H ₂ O are generated by the decomposition reaction of the organometallic compound mentioned above, pores through which gas can enter and exit are automatically secured in the gas sensitive body. Furthermore, in the present invention, when an organometallic compound containing the same metal element as the main component of the gas sensitive body is used, when the organometallic compound is decomposed in the firing process of the element, the metal element in the organometallic compound is Not only does it "wet" well with the main component particles of the gas sensitive material, improving adhesive strength, but even after firing, the metal acid compound thin film obtained by thermal decomposition of the organometallic compound and the gas sensitive material contain the same metal element. Since it contains oxides, the coefficient of thermal expansion is similar, resulting in excellent peeling resistance. In addition, by making the insulating substrate the same as the main component of the gas sensitive body (however, the resistance value of the insulating substrate is high due to electron valence control), the thermal expansion coefficients of the gas sensitive body and the insulating substrate are similar, and firing Heat after time
Peeling due to shock can be prevented. Furthermore, if the resistance value of the metal acid compound thin film obtained by thermal decomposition of an organometallic compound is made higher than the resistance value of the gas sensitive body, it means that a high resistance body is connected in parallel with the gas sensitive body, and the apparent Gas sensitivity can be increased. Next, a specific manufacturing example of the gas sensing element according to the present invention will be shown. First, an insulating substrate is cut according to the size of the element, and a pair of detection electrodes is provided on one surface of the insulating substrate. Further, a heater layer for heating the element is provided on the other surface. Next, on the detection electrode side of this insulating substrate, an organometallic compound diluted with an organic solvent or the like is applied as necessary to at least the portion where the gas sensitive body and the insulating substrate are in contact. Thereafter, this is dried at room temperature, and further dried in a dryer at, for example, 100° C. for about 1 hour. The organometallic film thus produced has now been completely dried and its volatile components have been sufficiently volatilized. A paste in which a gas sensitive material is dispersed in an organic solvent is printed on this organometallic compound film. After the printed gas-sensitive paste is sufficiently dried at room temperature, it is dried in a dryer at 100° C. for about 1 hour, for example, and fired in an electric furnace to attach each lead wire to obtain a gas-sensitive element. In this case, the firing temperature is about 300° C. to 800° C., and a gas sensing element that is sufficiently firmly adhered to the insulating substrate can be obtained. As the gas sensitive material, it is preferable to use ZnO + 0.5 mol % Al ₂ O ₃ system in addition to SnO ₂ system as described in detail in Examples below. Also in this case, 10 to 40 wt% zinc hexaate as an organometallic compound
It is preferable to use one diluted with 60 to 90 wt% propyl alcohol. In addition, the metal oxide thin film obtained by thermal decomposition of an organometallic compound in the present invention has a thickness of 3000 Å or more.
It is preferable to form one with a thickness of 20,000 Å. The structure of the gas detection element according to the present invention manufactured in this manner is shown in cross section in FIG. In the figure, 1 is a gas sensitive body, 2 and 3 are detection electrodes, 4 is an insulating substrate, 5 and 6 are detection lead wires, 7 is a heater,
8 and 9 are heater lead wires, and 10 is a metal oxide thin film produced by decomposition of an organometallic compound. (Examples of the Invention) The specific manufacturing method of the device of the present invention will be shown in Examples below, and the mechanical strength and characteristics of the device according to each Example are shown in Table 1 together with Reference Examples. Example 1 A thick film heater 7 is provided on one side of an alumina substrate 4 as an insulating substrate, and a thick film heater 7 is provided on the other side by thick film printing.
Au electrodes 2 and 3 are provided. A solution prepared by diluting aluminum hexaate as an organometallic compound with propyl alcohol is applied between the Au electrodes and dried at room temperature for 1 hour. After further drying for about 1 hour in a dryer set at 100°C, it is taken out and sufficiently cooled to room temperature. Next, on the surface of the organometallic compound film, a paste prepared by adding 0.5 wt% of PdO to SnO ₂ to SnO ₂ powder to which 2 wt% of Sb ₂ O ₃ was added using an ordinary screen printer and dispersing it in an organic solvent was applied to the surface of the organometallic compound film. Electrodes 2, 3
I printed it so that it spanned the space. Let this paste dry for 1 hour at room temperature, then
After drying at 100℃ for 1 hour, dry at 600℃ in an electric furnace.
Fired at 800℃. Attach the lead wires 5 and 6 to this element, turn on the current, and after the element is sufficiently stabilized, put it into the measurement tank and the i-C ₄ H ₁₀ , H ₂ , and CO will be 2000 PPm each.
We injected as much gas as possible and examined the resistance changes. In addition, the adhesive strength is determined by preparing adhesive tapes with different adhesion strengths and attaching them to the surface of the resulting gas-sensitive material.When one end of the tape is folded 180 degrees and peeled off in parallel, the gas-sensitive material adheres to the adhesive tape. Judgment was made based on whether or not it peeled off. The results are shown in Examples 1-1 to 1-3 in Table 1.
In addition, elements obtained in the same manner as in Example 1 except that no organometallic compound was used were used in Reference Examples 1-1 to 1-1.
Shown as 1-3. Example 2 Manufactured using the same substrate method as in Example 1, and using the same gas sensitive material, the organometallic compound used was tin hexaate to which aluminum hexaate was added so that the atomic ratio contained 3 mol% of Al. . The measurement method was the same as in Example 1, and the results are shown in Example 2 in Table 1.
Shown in 1 to 2-3. Example 3 SnO ₂ containing 3 mol% Al ₂ O ₃ was used as an insulating substrate, and a heater, electrodes, etc. were provided in the same manner as in Example 1, and aluminum hexaate, tin hexaate, and zirconium hexaate were used between the detection electrodes, respectively. A gas sensing element according to the present invention was obtained in the same manner as in Example 1 except that organometallic compounds such as tin naphthenate were diluted with propyl alcohol, and the measurement results were summarized in Examples 3-1 to 3-3. 5 in Table 1.

【table】

[Table] The results in Table 1 will be explained below. The composition of the gas sensitive body is the same for all reference examples and examples, and the gas sensitivity is expressed as R ₀ /Rgas, where R ₀ is the element resistance in air and Rgas is the resistance in each gas. Ta. Adhesive strength β was determined from the presence or absence of peeling when adhesive tapes of various adhesive strengths were attached to the gas sensitive member and one end of the tape was folded back and peeled off parallel to the adhesive surface. According to the results in Table 1, as shown in Reference Examples 1-1 to 1-3, the adhesive strength is extremely weak when using only the gas sensitive material. Also, reference examples 2-1 to 2- in which silica sol and alumina sol were used as a solidifying agent in this gas sensitive material.
3 and 3-1 to 3-3, a slight improvement in adhesion strength was observed, but in most cases cracks were generated within the plane, and when the firing temperature was raised to improve adhesion strength, the sensitivity to each gas suddenly increased. decreases to On the other hand, in the examples of the present invention, no cracking occurs at all, and when the firing temperature is increased, the adhesion strength is improved and the gas sensitivity remains sufficient. Further, it is desirable that the organometallic compound used has an electrical resistance higher than the element resistance after firing, but no practical problem occurred even when used alone. Also, Reference Examples 4-1 to 4-3 and Examples 3-1 to 3
-5, the main component of the substrate is the same as the main component of the gas sensitive material, and a comparison is made between cases where an organometallic compound is used and cases where an organometallic compound is not used. As a result, the method of the present invention showed sufficient strength even when fired at 600° C. by using a substrate with the same main components as the gas sensitive member, but the conventional method did not have much effect. Note that the gas sensitivity in Example 3 is for the case of firing at 700°C. Furthermore, this element was subjected to a drop test and the results are shown in Table 2. The test method is to weigh the element at 200g.
It is attached to an iron cylindrical object from a height of 1m to about 3cm.
It was dropped onto a cedar board placed on cm-thick concrete.

【table】

[Table] This result also shows that the method of the present invention did not cause peeling in all cases despite firing at a low temperature. Although the above embodiments have been described using SnO ₂ as the main component, similar effects can be obtained with I _o2 O ₃ , F _e2 O ₃ , Z _o O, T _i O ₂ , V ₂ O _f , etc. It was also obtained when other materials were used as the main ingredients. (Effects of the Invention) As is clear from the above results, the gas sensing element according to the present invention has excellent mechanical strength against peeling, dropping, etc. and excellent gas-sensitive characteristics, and can be said to be extremely effective in practice. .

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an example of the structure of a conventional gas detection element. FIG. 2 is a sectional view showing another example of the structure of a conventional gas detection element. FIG. 3 is a sectional view showing an example of the structure of the gas detection element according to the present invention. 1... Gas sensitive body, 2, 3... Electrode, 4... Insulating substrate, 10... Thin film of metal oxide obtained by thermal decomposition of an organometallic compound.

Claims (1)

[Claims] 1. An insulating substrate provided with a pair of electrodes, a gas sensitive body made of a metal oxide semiconductor provided between the electrodes on the insulating substrate, and between the gas sensitive body and the insulating substrate. 1. A gas sensing element comprising a thin film of a metal oxide obtained by thermal decomposition of an organometallic compound. 2. The gas sensing element according to claim 1, wherein the organometallic compound contains the same metal element as the main component of the gas sensitive body. 3. The gas sensing element according to claim 1, wherein the resistance value of the metal oxide thin film obtained by thermal decomposition of an organometallic compound is higher than the resistance value of the gas sensitive body. 4. The gas sensing element according to claim 1, wherein the insulating substrate is a sintered body having the same main component as the gas sensitive body and having a higher resistance than the gas sensitive body.