JPS5957152A - Gas detecting element - Google Patents

Abstract

PURPOSE:To obtain a gas detecting element operated as a lower temp. as compared with a conventional one and having high sensitivity, especially, for CH4 without adding a noble metal, by using a gas sensor obtained by baking a composition prepared by adding one or more of Sn and Zr to TiO2 containing a specific amount of SO4<--> in a ratio within a specific range. CONSTITUTION:A compound of one of Sn and Zr or compounds of both of them are mixed in a mixture containing 0.005-10wt% of SO4<--> obtained by adding a titanium sulfate solution to a commercial TiO2 powder to form a composition containing 0.1-50mol% of Sn and/or Zr as a total addition amount on the basis of SnO2 and ZrO2 and the obtained composition is molded under pressure and baked to obtain a sensor as a sintered body or said composition is formed into a paste which is, in turn, applied to a substrate by printing and the printed substrate is baked to obtain the gas sensor as a sintered film. A comb shaped electrode is provided to the surface of the sensor while a heater to the back surface thereof to prepare a gas detecting element which is small in the change of a resistance value with the elapse of time, has high sensitivity to combustion gas, especially, for CH4, operated at a low temp. of about 400 deg.C without adding a noble metal to the sensor.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a gas detection element using a metal oxide semiconductor used to detect combustible gases.

The problems are similar to those of the conventional structure, and various research and development efforts have been made on materials for detecting flammable gases. This is strongly attributable to the fact that explosions caused by flammable gases and poisoning accidents caused by toxic gases occur frequently, mainly in households and in various factories, and these have become major social problems.

Propane gas, in particular, has a low explosive limit (If!: L) and a higher specific gravity than air, so it tends to stagnate in rooms, resulting in numerous accidents and injuries every year.

In recent years, gas detection elements using metal oxides such as stannic oxide (SnOz) and gamma-type ferric oxide (γ-Fezes) have been put into practical use and are being applied to gas leak alarms, etc. . And even if a situation such as a gas leak occurs, L
Before reaching the EL, the presence of propane gas can be detected as early as possible, making it possible to prevent explosions.

Incidentally, in Japan, liquefied natural gas (LNG), whose main component is methane gas, has come to be used for general household use and is gradually becoming popular. Therefore, this LN
The demand for gas detection elements that can detect methane gas, the main component of G, with high sensitivity is also increasing.

Of course, gas detection elements sensitive to methane gas have already been developed, but many of them use noble metal catalysts as sensitizers in the sensitive material, so there are problems with catalyst poisoning by various gases and problems with methane gas. They have problems such as low selectivity and large changes in characteristics over time.

For example, since methane gas itself is a very stable gas, a detection element with sufficient sensitivity must be extremely active. , a noble metal catalyst is added to the susceptor material, or the susceptor is heated at, for example, 450°C.
Efforts have been made to operate at considerably higher temperatures. However, the current situation is that the properties are still insufficient for practical use.

OBJECTS OF THE INVENTION The present invention has been made in view of the above circumstances, and is intended to realize a gas detection element that has high sensitivity to methane even at relatively low operating temperatures without adding any noble metal catalyst.

Structure of the Invention The present invention is a gas sensing element using titanium oxide (Tj, Oz) as a gas sensing element, and the effects of various anions contained therein on the gas sensitivity characteristics as well as the effects of additives have been studied. It was discovered in the middle of the day.

That is, in the gas detection element of the present invention, sulfate ions are 0.0
At least one of Sn and Zr is added as an additive to 1°io2 containing 05 to 10% by weight, and the total amount of additives is 0.1 in terms of 5n02 and ZrO2, respectively.
~50 mol% is used as a gas sensitive material, and by adding Sn or Zrf to TlO2 containing sulfate ions, which is the base material of the gas sensitive material, gas sensitive characteristics and its reliability are improved. This was achieved based on the discovery that the sensitivity was dramatically improved and that it was possible to achieve a sufficiently high sensitivity for practical use even to the aforementioned methane gas.

DESCRIPTION OF EMBODIMENTS The present invention will be described in detail below. In Example 1, the amount of sulfate ions contained in TlO2 was kept constant,
A case will be described in which the amount of Sn or Zr as an additive and the combination thereof are changed.

[Example 1] Commercially available titanium oxide ('I'102) reagent 200y-,
Titanium sulfate solutions f and f 2 Of were added as additives to completely contain sulfate ions, and mixed for 2 hours using a sieve machine. These mixtures were equally divided into several parts, and commercially available tin oxide (SnO2) and zirconium oxide (ZrO2) reagents were added to each of them, either singly or in combination. Then, each powder was further dry-mixed for 3 hours using a miller. Then, an organic binder was added to each of these, and the particles were sized into particles having a size of 100 to 200 μm. Next, these powders were pressure-molded into a rectangular parallelepiped shape and fired in air at a temperature of 600°C for 1 hour.

Next, Au'f was vapor-deposited on the surface of this sintered body to form a pair of comb-shaped electrodes, and a platinum heating element was attached to the back surface with an inorganic adhesive to serve as a heater and to produce a sensing element. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element. The temperature of the element body was maintained at 400°C, and its gas sensitivity characteristics were measured.

The resistance value (Ra) in air was measured in a measuring container with a volume of 50β in which dry air was slowly stirred in a buried place without turbulence, and the resistance value in gas (Hq
) contains methane (CH4) with a purity of over 99% in this container.
and hydrogen (H2) gas at a volume ratio of 1 oppm
/second, and each measurement was made when the concentration reached 0.2% by volume/weight. Gas concentration to be measured is 0.2%
The choice was made based on the detection concentration 1, which is desirable for practical use as a gas detection element, and the lower explosive limit concentration (LEL) of the gas.
This is because the LltL of each of the above gases ranges from 1/0 to several fractions, and is approximately 2% by volume to 6% by volume.

Further, the presence of one sulfate ion (S04) contained in the gas sensitive body was confirmed by infrared absorption spectrum, and the amount contained was identified from the TG-D'lf'A curve and fluorescent X-ray analysis. As a result, the amount of sulfate ions contained in these sintered sensitive bodies was Q, 12 to 0.18, and 96 by weight.

FIG. 1 and FIG. 2 show the dependence of the gas sensitivity characteristics on the amount added when each additive is added alone. The sensitivity characteristics are (1) gas feeling TEjl (Ra/Rq),
(ii) The resistance change rate over time ΔR (the change rate of the resistance value with respect to the initial value when the sensitive body is held at a temperature of 400'C for 2000 hours) was evaluated. Table 1 also shows the gas sensitivity (Ra/Rq) when additives are used in combination.
and the resistance change rate over time (ΔR).

Note that ΔR is indicated in parentheses in the table. As is clear from Figures 1, 2, and Table 1, S
By adding n or Zr alone or in combination, gas sensitivity characteristics (gas sensitivity: l(a/R
g) has been greatly improved. Also noteworthy is the change in resistance value over time, and the rate of change is significantly reduced by adding these additives. In this way Sn,
It can be seen that the addition of Zr or Ti can dramatically improve gas sensitivity characteristics and reliability.

In the present invention, the added contracture 1ii is limited to 0.1 to 50 mol% because if it is less than 0.1 mol%, as shown in Figures 1 and 2 and Table 1, the gas sensitivity characteristics and reliability will be improved. This is because no effect is seen in increasing the resistance, and on the contrary, if it exceeds 50 mol %, the resistance value itself becomes high and the stability of the characteristics is lacking. Those marked with an X in the table correspond to these, and are listed as comparative examples in Table 1.

(Left below) Table 1 x Comparative Example By the way, it is generally known from physical chemistry that metal oxides that are amorphous to the extent that they can be placed in a sensitizer are more adsorbed by flammable gases than those that are crystallized. It is said that the phenomenon is likely to become active. However, even the commercially available TiO2 reagent used in this example, which was almost completely crystallized, showed extremely high activity by containing sulfate ions and further adding Sn or Zr. As a result, it can be seen that very large gas sensitivity and high reliability can be achieved.

In this Example 1, the sensitive body is a sintered body, the amount of sulfate ions contained is constant, and the amount of 2 additives is 1.
The cases where the combinations are different have been described.

In Example 2 shown below, the sensitive body is a sintered film, and contrary to Example 1, the type and amount of additives are kept constant and the amount of sulfate ions contained is varied. That is, in Example 2, it is fully confirmed that the present invention is effective even when the sensitive body is entirely sintered, and the effect of the amount of sulfate ions contained on the gas sensitivity characteristics will be described.

[Example 2] To 100 pieces of a commercially available titanium oxide reagent, commercially available tin oxide (SnOz) and zirconium oxide (Zr02) reagents were weighed out in the proportions shown in Table 2, and each sample was completely removed. The mixture was mixed on a paddle machine for 2 hours. Next, each of the mixed powders was divided into eight equal parts, titanium sulfate solutions prepared in advance at various concentrations were added thereto, and then all of the powders were mixed in a paddle paddle for one hour. In this way, there are three types of oxide compositions as representative examples (Samples A and G),
Eight types of samples with different amounts of sulfate ions and 124 types of samples were obtained for each oxide composition.

Table 2 Several mixed powders thus obtained were heat treated in air at a temperature of 400°C for 2 hours. Further, this powder was sized to a total size of 50 to 100 μm, and triethylamine was added to form a paste. On the other hand, as a substrate for a gas detection element,
Prepare an alumina substrate with 6 sides each and a thickness of 0.6m, and apply gold paste (gold paste) in a comb shape on this surface at intervals of 0.5cm.
A pair of comb-shaped electrodes were formed by printing and baking. Then, on the back side of the alumina substrate, a commercially available ruthenium oxide glaze resistor was completely printed between the gold electrodes and baked to form a heater.

Next, it was printed to a thickness of about 70 μm on the surface of two series of paster substrates, air-dried at room temperature, and then gradually heated in a trench at a temperature of 400° C. and held at this temperature for 1 hour. At this stage, the paste evaporated to form a sintered film of the respective oxide composition containing sulfate ions. The thickness of this gas sensitive member was approximately 50 μm. A gas sensing element was thus obtained.

In addition, to identify the amount of sulfate ions contained in the gas-sensitive membrane, rather than printing a portion of each paste on an alumina substrate, a portion of each paste was slowly heated at a temperature of 400°C in the same manner as described above on a Benist box. This was carried out by TG-DTA and fluorescence Xi analysis. Further, the presence of sulfate ions was confirmed by analyzing the infrared absorption spectrum as in Example 1.

The gas sensitivity characteristics of each sensing element were measured in the same manner as in Example Φ1. FIGS. 3 to 5 show the relationship between the gas sensitivity (Ra/Rq) and the sulfate ions contained in samples A to G having different oxide compositions, respectively. Table 3 also shows, as a representative example of the characteristics over time, the change in resistance value over time when samples A and C containing 2 to 6 weight 9 sulfate ions were evaluated using the same method as in Example 1. Show rate. In Example 2, methane and propane were used as the test gases.

As is clear from FIGS. 3 to 5, almost the same characteristics as those obtained in Example 1 are obtained even when the sensitive body is a sintered film. Furthermore, as is clear from Table 3, the rate of change in resistance value over time is also very small, as in Example 1.

As can be seen from Figures 3 to 5, if the amount of sulfate ions is less than 0.005% by weight, there is no effect of adding Sn or Zr, and the effects of the present invention cannot be expected. On the other hand, 1
If it exceeds 0.0% by weight, it becomes impractical in terms of stability of properties or mechanical strength. The amount of sulfate ions contained in the gas detection element of the present invention 'io, oos ~ 1
o. The reason why it is limited to 0% by weight is due to the reason mentioned above.

Table 3 Incidentally, in Examples 1 and 2, a commercially available oxide reagent was used as the starting material, but in the present invention, the final composition of the reactor may be within the above-mentioned range. However, the starting materials and manufacturing methods are not limited in any way.

In addition, although methane, hydrogen, or propane were used as the test gases in the examples, the effects of the present invention are by no means limited to these gases, and can be applied to flammable gases such as ethane, isobutane, and alcohol. Of course, it is effective.

As described in detail, the gas sensing element of the present invention contains Sn or Z as an additive to titanium oxide containing sulfate ions.
A sintered body or sintered film added with r is used as a sensitive body, which dramatically improves gas sensitivity.
Even at a relatively low temperature of 400° C., extremely high sensitivity can be achieved for methane gas, which has hitherto been considered difficult to detect in trace amounts without the use of a noble metal catalyst.

This precisely responds to social needs, which are becoming increasingly demanding as city gas is replaced with natural gas (main component: methane gas), and its effects are extremely significant. Another effect of the present invention is a significant reduction in the life characteristics, especially the change in resistance value over time due to energization. In other words, this makes an extremely large contribution to improving the reliability of the element, which is the most important element of any sensing element.

[Brief explanation of the drawing]

Figures 1 and 2 show the additive storage and the sensitivity (R&/Rq) to methane and hydrogen in one embodiment of the present invention.
and characteristic diagrams showing the relationship with the resistance change rate over time (ΔR), and Figures 3 to 5 show the sulfate ion content and sensitivity to methane and globan (Ra/Rq) in other examples of unexploded t9J. FIG. 3 is a characteristic diagram showing the relationship between three typical oxide compositions. Name of agent: Patent attorney Toshio Nakao and 1 other person
b×(・B fight

Claims (2)

[Claims] (1) Titanium oxide (Ti02) containing 0.005 to 10% by weight of sulfate ions is added with tin (Sn
) and zirconium (Zr) in a total additive content of 0.1 to 50 mol % in terms of SnO2 and ZrO2, respectively, as a gas sensing element. (2) Claim (1) characterized in that the gas sensitive body is a sintered body obtained by pressure molding and firing, or a sintered film obtained by cross-paste printing and firing. The gas detection element described.