JPS6152933B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses a gas sensitive material with a small particle size and porous microstructure, that is, a very large specific surface area, to detect flammable gases, especially methane gas, which has traditionally been difficult to detect with high sensitivity. The present invention also relates to a combustible gas detection element having a sensitivity sufficient for practical use. In recent years, various research and developments regarding combustible gas sensing element materials have become active. 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. 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 detect methane gas, which is the main component of LNG, with high selectivity is increasing. Of course, gas detection elements that are sensitive to methane gas have already been developed, but because they use noble metal catalysts as sensitizers in the sensitive material, there are problems with catalyst poisoning by various gases, and the selectivity for methane gas is low. These devices suffer from problems such as high dependence on humidity and ambient humidity. Therefore, the current situation is that the properties are still insufficient for practical use. The present invention provides a gas detection element that has characteristics that can solve these problems and has a sensitivity that is sufficiently high for methane gas for practical use. Since methane gas itself is a very stable gas, a detection element that has sufficient sensitivity for methane gas needs to be extremely active. Therefore, in order to achieve high sensitivity to methane gas, conventional techniques have been used such as adding a noble metal catalyst to the sensitive material or operating the sensitive material at a considerably high temperature. However, the device according to the present invention realizes a device with high sensitivity to methane even at a relatively low operating temperature without adding any noble metal catalyst. This will be specifically described in the following examples. [Example 1] 100 g of commercially available ferric sulfate (FeSO ₄ 7H ₂ O) was
It was dissolved in pure water kept at a temperature of 2°C and stirred well. An 8N ammonium hydroxide (NH ₄ OH) solution was added dropwise to this solution at a rate of 60 c.c. per minute until the pH of the solution reached 7. After the completion of the dropwise addition, the temperature of the solution was maintained at 50° C. for 10 minutes, and then returned to room temperature. At this stage, a brownish black coprecipitate was obtained. This coprecipitate was suction filtered and dried at a temperature of 110°C for 12 hours. The obtained dry powder was divided into two parts, and one part was treated at a temperature of 400°C for 1 hour in a nitrogen stream containing 20% hydrogen (hereinafter referred to as group V). The other one was treated in air at a temperature of 400° C. for 1 hour (hereinafter referred to as group A). That is, the former uses triiron tetroxide (Fe ₃ O ₄ ) powder, and the latter uses gamma-type ferric oxide (γ-Fe ₂ O ₃ ) containing a portion of alpha-type ferric oxide (α-Fe 2 O 3 ). ₂ O ₃ ) powder. After pulverizing each of these powders for 2 hours using a crusher, they are crushed to 100 to 200 μm using an organic binder.
The particles were sized into particles with a size of m. These powders were each shaped into a rectangular parallelepiped of 2 x 1.5 x 3 mm ³ at a weight of 400 kg/cm ^2.
For group V, it was baked in a vacuum at a temperature of 750°C for 1 hour, and then
It was treated in air at a temperature of 400°C for 20 hours.
In addition, for Group A, 750
It was baked for 1 hour at a temperature of °C. At this stage, the V group is a γ-Fe ₂ O ₃ receptor, and the A group is an α-
It became a sensitizer for Fe ₂ O ₃ . Next, Au was deposited on the surface of these sintered bodies 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, thereby creating a sensing element. The gas-sensitive characteristics are achieved by passing a current through this heating element and adjusting the current value to maintain the operating temperature of the element at 400℃.
This was investigated by measuring the resistance value between a pair of gold electrodes. By the way, the fine structure of the sensitive body was investigated using a sintered body at a stage before forming the Au electrode in the above-mentioned process. As a result, the average particle size of the sintered fine particles was 0.20 μ for both γ-Fe ₂ O ₃ and α-Fe ₂ O ₃ .
m, and the porosity of γ-Fe ₂ O ₃ is 68
%, and that of α-Fe ₂ O ₃ was 63%. The resistance value in air (R _a ) 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 in gas (R _g ) was measured using this 99% purity in the container
The gas sensitivity characteristics were investigated by flowing the above test gas at a volume ratio of 10 ppm/sec and measuring the resistance value of the sensitive body when a certain gas concentration was reached. In the case of V group, that is, γ-Fe ₂ O ₃ , R _a is
780 kΩ, and R _a in the case of A group, that is, α-Fe ₂ O ₃ was 665 kΩ. Methane (CH ₄ ), ethane (C ₂ H ₆ ), propane (C ₃ H ₈ ), isobutane (i-
C ₄ H ₁₀ ), hydrogen (H ₂ ), and ethyl alcohol (C ₂ H ₅ OH), each using 0.05
Measurements were made at gas concentrations of %, 0.2%, and 1.0%. The dependence of R _g on the concentration of each gas was as shown in Table 1. As can be seen from this table, while each element has sufficient sensitivity for practical use against methane, ethane, propane, isobutane, and hydrogen gases, it has extremely low sensitivity against ethyl alcohol. I understand. α-Fe ₂ O ₃ originally has very poor gas-sensitivity characteristics, but the microstructure of the sensitive material controlled in this way is effective against methane gas, which has traditionally been difficult to detect in trace amounts. It also has sufficient sensitivity for practical use. In addition, γ-Fe ₂ O ₃ itself is famous for its high sensitivity to gases such as propane, isobutane, and hydrogen, but it is particularly difficult to detect trace amounts of methane gas, which is chemically stable. It has been said that However, those whose fine structure is controlled as in the present invention exhibit considerably high sensitivity. Furthermore, another noteworthy feature of the device according to the present invention is that it has little dependence on ambient humidity. By the way, when this sensing element is placed in an ambient temperature atmosphere of 40°C and the surrounding relative humidity is varied from 35% to 95%, R _g (0.2) at a concentration of 0.2% for each gas is measured.
was measured. The results are also shown in Table 1. In the table, β _H indicates the ratio of R _g (0.2) at 35% and 95%, and indicates the degree of dependence on ambient humidity. As is clear from the table,
It can be seen that although there are some differences depending on the type of gas being tested, the dependence on ambient humidity is extremely small. This shows that the effectiveness of the present invention is great, considering that β _H of the conventional semiconductor type using a noble metal catalyst is about 1.25 or more.

[Example 2]

In Example 1, the firing conditions were 750°C for 1 hour in both vacuum and air, but here the firing temperature was varied in the range of 550 to 1150°C in 100°C increments. A device was fabricated using the same method as above, and the dependence of the gas sensitivity characteristics and the microstructure of the sensor on the firing temperature was investigated. In addition, the firing time is 1
It was time. FIG. 1 shows the relationship between firing temperature, average grain size, and porosity. Figure 2 also shows the firing temperature and gas sensitivity characteristics (R
The relationship between _a and R _g (0.2% methane gas) is shown. As is clear from the figure, when the firing temperature is 950°C or higher, grain growth is promoted and the porosity is reduced. Additionally, the gas sensitivity characteristics have also changed accordingly. These results show that the gas-sensitive characteristics are largely related to the fine structure of the sensitive material, even if the composition of the sensitive material remains unchanged. That is, when the average particle size becomes 0.5 μm or more, the effective specific surface area decreases, the activity decreases, and the gas sensitivity also decreases. The same thing can also be said when the porosity is 35% or less. Conversely, the porosity is 85%
If it is more than that, the mechanical strength will be so low that it cannot be used as a practical device. As described above, the gas sensing element of the present invention is
Even for methane gas, which has been said to be chemically stable and difficult to detect with high sensitivity, it does not use a precious metal catalyst and even at a relatively low operating temperature of 400℃.
It has sufficient sensitivity for practical use. Furthermore, it also has the characteristics of low sensitivity to alcohol and low dependence on ambient humidity, which can cause false alarms in practice. Further, by adding other components, even higher sensitivity detection can be achieved. The following will be described in detail based on examples. [Example 3] 160 g of commercially available ferric sulfate (FeSO ₄ -7H ₂ O) was
It was dissolved in pure water kept at a temperature of 2°C and stirred well. In addition, commercially available zinc sulfate (ZnSO ₄
7H ₂ O) was dissolved in 1 pure water and stirred well. This solution was then poured into the Fe ion-containing solution from earlier and stirred thoroughly again. Next, an 8N ammonium hydroxide (NH ₄ OH) solution was added dropwise to this solution at a rate of 60 c.c. per minute until the pH of the solution reached 7. The temperature of the solution was kept at 50℃ for 10 minutes after the completion of dropping.
and then allowed to return to room temperature. At this stage, a brownish black coprecipitate was obtained.
This coprecipitate was taken out by suction and heated to 110°C.
dried for 12 hours at a temperature of The obtained dry powder was divided into two parts, and in the same manner as in Example 1, one part was subjected to reduction treatment (V group) and the other part was subjected to oxidation treatment (A group). Hereafter, Example 1
Two groups of devices were fabricated using exactly the same method as above, their gas sensitivity characteristics were measured, and their microstructures were investigated. The results are shown in Table 2. As seen in the table, by adding Zn compared to Example 1, although there is no major change in the microstructure, the sensitivity to methane, ethane, propane, and isobutane gases increases. , and the degree of gas concentration separation (the amount of change in resistance value per unit gas concentration) is increasing. On the other hand, for ethyl alcohol, there is not much difference observed compared to when no Zn is added. Also,
There is also almost no difference in the dependence on ambient humidity due to the addition of Zn. By combining components such as Zn in this manner, only gas sensitivity and gas concentration separation can be increased without impairing other properties. This effect is effective for both the V group and the A group, and its degree is almost the same.

[Example 4]

Commercially available ferric sulfate (FeSO ₄ 7H ₂ O),
An aqueous solution containing iron ions, tin ions, and zinc ions was prepared in the same manner as in Example 3 using stannic chloride (SnCl _{4 .5H 2} O) and zinc sulfate (ZnSO 4 .7H ₂ O). By changing the mixing ratio of these aqueous solutions, various mixed solutions with different concentrations were prepared, and coprecipitates were obtained using NH ₄ OH. and,
These were dried and pulverized to obtain a raw material powder for the sensitive material. The obtained fine powder raw material is mixed with an organic binder to
The particles were sized to a size of 200 μm. These various powders were each shaped into a rectangular parallelepiped (2 x 1.5 x 3 mm
³ ) Pressure molded at a pressure of 400Kg/cm ² and molded in air.
It was baked at a temperature of 800°C for 1 hour. Au was 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 form a heater, thereby producing a sensing element. A current was passed through this heating element, the current value was adjusted to maintain the operating temperature of the element at 400°C, and its gas sensitivity characteristics, average particle size, and porosity were measured. The results are shown in Table 3. As in Examples 1 and 3, the measurement gases used were methane, ethane, propane, isobutane, hydrogen, and ethyl alcohol. Regarding R _g , the value when the gas concentration is 0.2% is shown. Regarding β _H , the values are shown for isobutane gas, which produces the largest amount of β _H among these measurement gases.

[Table] As is clear from Table 3, when the total amount of Sn and Zn is less than 0.5 mol% in terms of SnO ₂ and ZnO, respectively, no effect is seen by adding these components. On the other hand, if it exceeds 70 mol%, although some products may exhibit some degree of gas sensitivity, the resistance value may become abnormally low, or the resistance value may vary greatly or change over time, making it impossible to put it into practical use. Become. For this reason,
The total amount of Sn and Zn components was limited to 0.5 to 70 mol% in terms of oxides. As described above, the element according to the present invention achieves high sensitivity detection even for methane gas, which is currently in strong social demand, by operating it at a relatively low temperature without using a noble metal catalyst. Moreover, it has low sensitivity to ethyl alcohol,
Furthermore, since the dependence on ambient humidity is extremely small, it has also become possible to realize a combustible gas detection element that produces fewer false alarms in practical use. In the examples, FeSO ₄ 7H ₂ O,
Although ZnSO ₄ 7H ₂ O and SnCl ₄ 5H ₂ O were used,
It is not particularly limited to these. In addition, we have described the use of sintered bodies as gas sensitive bodies, but of course this is not limited to sintered bodies, and it goes without saying that other shapes such as thick films are also acceptable. Nor. Furthermore, it is also possible to add other components for the purpose of improving properties.

[Brief explanation of the drawing]

FIG. 1 shows the relationship between the firing temperature of the element and the average particle size and porosity of the sensitive material in the present invention, and FIG. 2 shows the relationship between the firing temperature and gas sensitivity characteristics.

Claims (1)

[Scope of Claims] 1. A gas sensitizer obtained by heat-treating fine particles of an iron compound, the sensitizer being gamma-type ferric oxide (γ-Fe ₂ O ₃ ) or alpha-type ferric oxide. (α-Fe ₂ O ₃ ), and the fine structure of the sensitive material has an average particle size of 0.5 μm or less and a porosity of 35
A combustible gas detection element characterized by ~85%. 2. The sensitizer contains at least one of tin (Sn) and zinc (Zn) together with the iron oxide component in a total amount of 0.5 to 70 mol% in terms of stannic oxide (SnO ₂ ) or zinc oxide (ZnO), respectively. The combustible gas detection element according to claim 1, characterized in that it includes: