JPH01257254A - Gas detecting element - Google Patents

Abstract

PURPOSE:To enlarge a specific surface area and to improve aging stability, by using ZnO powder having a needle crystal structure wherein a core part is provided and the needles are extending in the directions of different four axes from the core part as a main raw material for detecting element. CONSTITUTION:A piece of pure zinc wire whose purity is 99.99% undergoes flame coating in air by an arc discharge type flame coating method. The powder of the wire is recovered inputted into ion exchanged water and stirred in an automated mortar. The material is left alone for a specified time. Thereafter, the material is dried and water content is removed. The powder is put in a crucible made of alumina porcelain, and excessive heat treatment is performed. As a result, a specified ZnO having an apparent bulk specific gravity of 0.09 is formed. Aqueous solution of polyvinyl alcohol is added into the ZnO powder having a needle crystal structure as an organic binder. Thus, the powder is granulated and regulated in grain size. The granules are formed into a cylindrical shape by pressing. Platinum wires (electrodes) 2 and 2' are embedded. The element formed in this way is baked in an electric furnace. A sintered body 1 is placed in a coil shaped heater 3. The electrode part is bonded. The element is covered with an explosion proof net, and the detecting element is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a gas detection element used in a gas leak alarm or the like, and has high sensitivity to various gases.

Conventional technology Conventionally, gas detection elements have been known that utilize phenomena such as resistance changes in platinum wires and discoloration of palladium chloride aqueous solutions, but these all have low gas sensitivity.
It has drawbacks such as the fact that the detection method is not simple. Also,
Although gas chromatographs and chemical analysis methods have high accuracy, they have drawbacks such as lack of immediate analysis and large-scale and expensive equipment.

Problems to be Solved by the Invention On the other hand, n-type semiconductors such as stannic oxide (Snow), zinc oxide (ZnO), and cadmium oxide (CdO) have been used as relatively inexpensive and simple detection methods. Gas sensing elements are known. This type of gas detection element is capable of instantly and quantitatively detecting combustible gases, and has high gas sensitivity. In order to improve the responsiveness of gas adsorption and desorption, these are configured to be used while being constantly maintained at a high temperature. However, in this type of sensing element, changes in resistance value over time,
In other words, the issue is whether the lifespan is sufficient. This means that devices using the aforementioned metal oxide semi-dead bodies do not have sufficient sensitivity when used alone, so various noble metal catalysts are added to activate the device. Therefore, it is thought that it becomes unstable and has a large effect on the lifespan.

In addition, a sintered body whose main component phase is cubic spinel represented by the general formula M Zna-zxoa (where M is 3n
, Ti) has already been disclosed to obtain large gas sensitivities (particularly for methane gas) without the presence of a catalyst. For example, Japanese Patent Publication No. 61-29660.

However, even in the gas detection element using the sintered body described above, there are factors that cause instability in the lifetime performance.

It is well known that in order to obtain sufficient life stability, it is important to sinter at a higher temperature to form a sintered body that is airtight. However, on the other hand, in order to obtain sufficient gas sensitivity characteristics, it is necessary to increase the specific surface area to sufficiently increase the area in contact with the gas. These contradictory conditions pose a major challenge for practical application.

Currently, gas is becoming increasingly popular as an energy source for ordinary households. However, the dangers of gas are well known. In order to eliminate the dangers of gas and enable its safe use, there is an urgent need for household gas leak alarms to become widespread.

Means for Solving the Problems The present invention uses ZnO powder with a core and an acicular crystal structure extending from the core in four different axes directions as the main raw material of a gas sensing element. A stable sintered body can be obtained.

By using a working core and a ZnO needle-like powder having a needle-like crystal structure extending from the core in four different axial directions, a sensitive material with a large specific surface area, which is important for obtaining gas-sensitive characteristics, was obtained. Further, even if the firing temperature is increased, the overall sintering does not progress much, and sintering occurs from the contact portions of the acicular powder, so that a porous but structurally stable sensitive material can be obtained. Hereinafter, the gas sensing element according to the present invention will be specifically described based on Examples.

Example (Example 1) First, a ZnO powder having a core and an acicular crystal structure extending from the core in four different axes directions was created by thermal spraying a pure zinc wire with a purity of 99.99% using an arc discharge method. 1 kg of the powder (metallic zinc powder) is recovered, poured into 500 g of ion-exchanged water, and stirred in a mortar-type crusher for about 20 minutes. Next, it is left in water at a temperature of 26° C. for 72 hours. After standing in the water, the powder is dried at 150° C. for 30 minutes to remove water on the surface of the powder. Next, put this powder into an alumina porcelain crucible and
It is placed in a furnace maintained at 0° C. and heated for 1 hour.

As a result, nodular zinc oxide was produced in the lower layer of the crucible, and zinc oxide whiskers with an apparent cavity specific gravity of 0.09 were produced in the upper layer. The proportion of whiskers in the produced zinc oxide was 86-1%.

When the ZnO whiskers obtained above were observed using an electron microscope, a core and needle-like powder with a crystalline structure extending in four different axial directions from the core were confirmed. In this powder, a crystal body consisting of a core and needle-shaped crystal parts extending from the core in four different axial directions can be clearly recognized. The above needle-like crystal part is
The diameter of the root part of the eye is 1-10 μm, and the length is 10
~200 μm. Acicular crystals with 3 or 2 axes are also observed, but some of these have 4 axes with folded Y.
It seems that he was a member. In addition, plate-like crystals were also observed. In any case, according to the above method, about 80% of the crystals have an acicular crystal structure extending in four different axial directions from the tread and the core.

As a result of XNIA analysis, all peaks were indicative of zinc oxide, indicating single crystallinity with few dislocations and lattice defects.

An aqueous solution containing 51% polyvinyl alcohol (PVA) as an organic binder is added to the ZnO powder having a core and an acicular crystal structure extending from the core in four different axes directions, and the powder is granulated and sized. Then, press molding was performed into a cylindrical shape with a length of 2 cranes and a diameter of 211 mm. The molding pressure at this time was 70
At 0 kg/cra, two platinum wires with a thickness of 100 μm were embedded in parallel during molding. This becomes the electrode of the element.

The element thus produced by common ceramic means was fired in an electric furnace at a temperature of 1350° C. for 1 hour in air.

This firing temperature is the temperature at which conventional ZnO powder sufficiently progresses sintering and becomes densified, but the ZnO powder of the present invention has a core and an acicular crystal structure extending from the core in four different axes directions.
In the case of powder, an element with a porosity of about 50% and a large specific surface area was obtained based on the density of the sintered body. On the other hand, the tensile strength of the buried platinum wire is approximately 200 to 350 g/wood, and the sinterability is considered to be sufficient.

Next, a coiled heater for heating this sintered body was prepared, and as shown in FIG. 1, the sintered body was placed inside the coiled heater and the electrode portions were bonded. In Fig. 1, 1 is the above sintered body, 2.2° is a platinum electrode wire for taking out the output, 3 is a coiled heater made of nichrome wire, 4 is cement for insulating the heater 3, 5.6.7 .8 is an electrical terminal for the heater 3 and the sintered body 1. The above structural pancage is covered with an explosion-proof net like the conventional element,
It was used as a gas detection element.

In the allll method, an ohmmeter was connected between the terminals 7 and 8, and in order to heat the heater 3, electricity was applied to the terminals 5 and 6 so that the temperature of the sintered body f was approximately 450°C. The heater power at this time was 1.15W. After confirming that the atmosphere to be measured was sufficiently clean air, the resistance value RA of the sintered body at that time was measured.

Next, add 3000pp of isobutane (i C=H+o)
The resistance value RM in air containing m was measured after being inserted into a gas atmosphere and left for 30 seconds. As a result, in the device using the above-mentioned tetrapod-shaped ZnO, the RA was reduced to 15Ω.
, RM is 2.5Ω, and sensitivity RA/RM is 6, which is sufficient for practical use.

On the other hand, with conventional granular ZnO, almost no sensitivity could be obtained under the above conditions because no noble metal catalyst was used and the result was a dense sintered body.

(Example 2) Various amounts of titanium oxide powder and stannic oxide were added to ZnO powder having the same core as the ZnO used in Example 1 and an acicular crystal structure extending from the core in four different axial directions. was added, more water was added and mixed well, and then it was dried. A device was prepared from the mixed powder using the same ceramic method as in Example 1, and fired in an electric furnace at a temperature of 1300° C. for 1 hour in air.

Table 1 shows the performance of the device obtained as described above.

In Table 1, both Ti01 and Sn0w amount, X=
1. It can be seen that the maximum sensitivity is near OO.

As a result, sufficient gas sensitivity characteristics can be obtained without using a catalyst as in the conventional case. Moreover, the compound Mx Z n 4-! by Tie, SnO, It can be seen that a gas sensing element that is stable over time can also be obtained by using XO4.

Effects of the Invention The present invention uses Zno whiskers with a core and an acicular crystal structure extending in four different axes directions from the core as a starting material for ZnO, thereby producing a gas that is stable over time and has rich gas-sensitive characteristics. A sensing element is obtained. That is,
ZnO powder with an acicular crystal structure extending from the core in four different axes directions has a small apparent cavity specific gravity, so sintering only progresses at the contact area, which is sufficient in terms of strength and has a small specific surface area. A large sintered body is obtained, which is suitable for use as a gas-sensitive body. In addition, by using the core and ZnO powder with an acicular crystal structure extending from the core in four different axes directions, it is possible to use Sufficient gas sensitivity can be obtained without the need for

[Brief explanation of the drawing]

FIG. 1 is a structural diagram showing an example of the structure of a gas sensing element according to the present invention. l...Sintered body, 2.2°...Platinum electrode wire, 3...Heater, 4...Cement,
5.6.7.8...Terminal.

Claims (2)

[Claims] (1) A gas sensing element characterized in that it uses a core and a sintered body mainly composed of zinc oxide with an acicular crystal structure extending from the core in four different axial directions as a gas sensitive body. (2) General formula MZn_4_-_2_xO_4 (however,
M is at least one selected from Ti and Sn.
75≦x≦1.25) A sintered body whose main component phase is a cubic spinel phase expressed by A gas detection element characterized by using needle-like powder.