JPS6156789B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a gas detection element using a metal oxide semiconductor used to detect combustible gas. Conventional configuration and its problems In recent years, various research and development activities have become active regarding materials for sensing elements for flammable gases. This is strongly attributable to the fact that explosion accidents caused by flammable gases and poisoning accidents caused by harmful gases occur frequently, mainly in households and in various factories, and have become a major social problem. In particular, propane gas has a lower explosive limit (LEL).
Because it has a lower specific gravity and a higher specific gravity than air, it tends to stagnate in rooms, causing many accidents and causing many casualties every year. In recent years, gas detection elements using metal oxides such as stannic oxide (SnO ₂ ) and gamma-type ferric oxide (γ-Fe ₂ O ₃ ) have been put into practical use, and are used in gas leak alarms, etc. It is applied. Even if an accident such as a gas leak occurs, the presence of propane gas can be quickly detected before the LEL is reached, making it possible to prevent an explosion. 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, the demand for gas detection elements that can sensitively detect methane gas, which is the main component of LNG, is increasing. Of course, gas detection elements sensitive to methane gas have already been developed, but most of them use noble metal catalysts as sensitizers in the sensitive material.
Problems include catalyst poisoning by various gases, low selectivity to methane gas, and large changes in characteristics over time. For example, since methane gas itself is a very stable gas, a sensing element with sufficient sensitivity must be extremely active. Efforts have been made to use a noble metal catalyst added to the susceptor material, or to operate the susceptor at a considerably high temperature, for example, 450° C. or higher. 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 with high sensitivity to methane even at relatively low operating temperatures without adding any noble metal catalyst. Structure of the Invention The present invention is directed to alpha-type ferric oxide (α-Fe ₂ O ₃ ).
This discovery was made while studying the effects of various anions contained in the gas sensing element on the gas sensitivity characteristics, as well as the effects of additives. That is, the gas sensing element of the present invention contains α-Fe ₂ O ₃ containing 0.005 to 10% by weight of sulfate ions,
The gas sensitizer contains at least one of Ge and Th as an additive, in a total amount of 0.1 to 50 mol in terms of GeO ₂ and ThO ₂ , respectively. By adding Ge or Th to the material α-Fe ₂ O ₃ containing sulfate ions, the gas sensitivity characteristics and its reliability are dramatically improved, and the resistance to methane gas mentioned above is sufficiently large for practical use. This was achieved by discovering that it was possible to achieve high sensitivity. Description of Examples Examples of the present invention will be described below. First, in Example 1, the amount of sulfate ions contained in α-Fe ₂ O ₃ was kept constant, and
The cases where the amount of Ge or Th added and the combination thereof are changed will be described. Example 1 Additive for incorporating sulfate ions into 200 g of commercially available ferric oxide (Fe ₂ O ₃ ) reagent (which was confirmed to be entirely α-Fe ₂ O ₃ phase by X-ray diffraction). 40g of ferrous sulfate ( _FeSO4-7H2O ) reagent _as
and mixed for 2 hours in a rice cooker. These mixtures were equally divided into several parts, and commercially available germanium oxide (GeO ₂ ) and thorium oxide (ThO ₂ ) 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 to form particles with a size of 100 to 200μ. Next, these powders were pressure-molded into a rectangular parallelepiped shape and heated in air at a temperature of 600°C.
Baked for an hour. Next, Au 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 a sensing element was fabricated. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element.
The element temperature 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 50 mm in which dry air was stirred slowly to the extent that no turbulence occurred, and the resistance value (Rg) in gas was determined by 99% purity inside
The above methane (CH ₄ ) and hydrogen (H ₂ ) gases were made to flow at a volume ratio of 10 ppm/sec, and each was measured when the concentration reached 0.2 volume %. The gas concentration to be measured was chosen to be 0.2% because the practically required detection concentration for a gas detection element is in the range of several tenths to several tenths of the lower explosive limit temperature (LEL) of the gas. Each of the above gases
This is because the LEL is approximately 2% to 5% by volume. Furthermore, the presence of sulfate ions (SO ₄ ^-- ) contained in the gas sensitive material was confirmed by infrared absorption spectrum, and the amount contained was identified from the TG-DTA curve and fluorescent X-ray analysis. As a result, the amount of sulfate ions contained in these sintered sensitizers ranged from 0.41 to
It was 0.57% by weight. Figures 1 and 2 show the dependence of the gas sensitivity characteristics on the amount added when each additive is added individually. Sensitivity characteristics are evaluated by (i) gas sensitivity (Ra/Rg), (ii) rate of change in resistance over time △R (rate of change in resistance value from the initial value when the sensitive body is held at a temperature of 400°C for 2000 hours) did. Table 1 also shows the gas sensitivity (Ra/
Rg) and resistance change rate over time (ΔR). Note that ΔR is written in parentheses in the table. As is clear from Figures 1 to 2 and Table 1, by adding Ge or Th alone or in combination, the gas sensitivity characteristics (gas sensitivity:
Ra/Rg) has greatly improved. Also noteworthy is the change in resistance value over time, and the addition of these additives significantly reduces the rate of change. It can thus be seen that the addition of Ge or Th can dramatically improve gas sensitivity characteristics and reliability. In the present invention, the total amount of additives is limited to 0.1 to 50 mol%.
As seen in the figure and Table 1, no effect on improving gas sensitivity characteristics and reliability was observed.
On the other hand, if it exceeds 50 mol%, the resistance value itself becomes high,
This is also because the characteristics lack stability. In the table 〓
The marked items correspond to these,
In Table 1, it is listed as a comparative example.

[Table] *Comparative example By the way, in general, metal oxides that are amorphous to some extent are more likely to become active in physicochemical phenomena such as adsorption phenomena for flammable gases than those that are crystallized. It is said that However, even the commercially available reagent α-Fe ₂ O ₃ used in this example, which is almost completely crystallized, shows extremely high activity by containing sulfate ions and further adding Ge or Th. Furthermore, since this is stable over time, it can be seen that extremely high gas sensitivity and high reliability can be achieved as a result. In Example 1, the sensitive body is a sintered body, the amount of sulfate ions contained is constant, and the amounts and combinations of additives are varied. 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 was confirmed that the present invention is effective even when the sensitive body is a sintered film, and the effect of the amount of sulfate ions contained on the gas sensitivity characteristics is described. . Example 2 To 100 g of a commercially available ferric oxide (Fe ₂ O ₃ ) reagent, commercially available germanium oxide (GeO ₂ ) and thorium oxide (ThO ₂ ) reagents were collected in the proportions shown in Table 2. , and were mixed for 2 hours using a sieve machine. Next, each mixed powder was divided into 8 equal parts,
Ferrous sulfate (FeSO ₄ -7H ₂ O) solutions adjusted in advance to various concentrations were added to this, and then each powder was mixed for 1 hour using a sieve.
In this way, there are 3 types of oxide compositions (Samples A to C) as representative examples, and 8 types of oxide compositions with different amounts of sulfate ions, for a total of 24 samples.
Various samples were obtained.

[Table] 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 size of 50 to 100μ, and triethanolamine was added to form a paste. On the other hand, as a substrate for the gas detection element, the length and width are each 5 mm, and the thickness is
Prepare a 0.5mm alumina substrate, and place 0.5mm on this surface.
A pair of comb-shaped electrodes was formed by printing gold paste in a comb shape at intervals of . A commercially available ruthenium oxide glaze resistor was printed on the back side of the alumina substrate between the gold electrodes and baked to form a heater. Next, the above paste was printed on the surface of the board to a thickness of about 70μ, and after air drying at room temperature, it was heated to 400℃.
The mixture was gradually heated until the temperature reached , and maintained at this temperature for 1 hour. At this stage, the paste evaporated and became a sintered film of the respective oxide composition containing sulfate ions. The thickness of this gas sensitive body was approximately 55μ. 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 of the above pastes on an alumina substrate, the paste itself was slowly heated at a temperature of 400°C in the same manner as described above. TG this
DTA and fluorescent X-ray analysis were performed.
Further, the presence of sulfate ions was confirmed by analyzing the infrared absorption spectrum as in Example 1. Example 1 shows the gas sensitivity characteristics of each sensing element.
It was measured in the same way as in the case of . 3 to 5 show the relationship between the gas sensitivity (Ra/Rg) and the sulfate ions contained in samples A to C having different oxide compositions, respectively. Table 3 also shows that samples A to C contain 2 to 5 sulfate ions as representative examples of aging characteristics.
The rate of change in resistance value over time when evaluated using the same method as in Example 1 for those containing % by weight is shown. In Example 2, methane and propane gas 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. Further, as can be seen from FIGS. 3 to 5, if the amount of sulfate ions is less than 0.005% by weight, there is no effect of adding Ge or Th, and the effects of the present invention cannot be expected. On the other hand, if it exceeds 10.0% by weight, it becomes impractical in terms of stability of properties or mechanical strength. The reason why the amount of sulfate ions contained in the gas sensing element of the present invention is limited to 0.005 to 10.0% by weight is based on the above-mentioned points.

[Table] By the way, in Examples 1 and 2, commercially available oxide reagents were used as starting materials, but in the present invention, the final composition of the reactor may be within the above-mentioned range. There are no limitations on starting materials or manufacturing methods. Furthermore, in the examples, methane, hydrogen, or propane gas was used as the test gas, but 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 also effective against Effects of the Invention As explained above, the gas sensing element of the present invention uses a sintered body or a sintered film obtained by adding Ge or Th as an additive to α-Fe ₂ O ₃ containing sulfate ions as a sensitive body. As a result, gas sensitivity has been dramatically improved, and even at a relatively low temperature of 400°C, it has extremely high sensitivity for methane gas, which has been difficult to detect in trace amounts without the use of precious metal catalysts. It is possible to realize this. 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 are characteristic diagrams showing the relationship between the amount of additives, sensitivity to methane and hydrogen (Ra/Rg), and rate of change in resistance over time (△R) in one embodiment of the present invention, and Figure 3 ~Figure 5 is a characteristic diagram showing the relationship between the sulfate ion content and the sensitivity (Ra/Rg) to methane and propane gas for three representative oxide compositions in another example of the present invention. .

Claims (1)

[Claims] 1. Germanium (Ge) and thorium (Th) are added as additives to alpha-type ferric oxide (α-Fe ₂ O ₃ ) containing 0.005 to 10% by weight of sulfate ions.
A gas sensing element characterized in that at least one of the additives contains 0.1 to 50 mol % of the total amount of additives in terms of GeO ₂ and ThO ₂ , respectively, as a gas sensitive material. 2. The gas detection according to claim 1, wherein the gas sensitive body is a sintered body obtained by pressure molding and firing, or a sintered film obtained by printing and firing a paste. element.