JPS6129661B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a flammable gas detection element, in particular, which has a spinel-type crystal structure and is itself flammable (reducible).
This invention relates to a combustible gas detection element that has increased sensitivity and greatly improved response return characteristics by adding additives to lithium ferrite (LiFe ₅ O ₈ ), which has the ability to detect gas. In recent years, various research and developments have been made on combustible gas detection elements. Typical examples include those using n-type metal oxides such as stannic oxide (SnO ₂ ) and zinc oxide (ZnO).
However, in order to use it as a practical device,
Noble metal catalysts such as platinum (Pt) and palladium (Pd) must be added as other sensitizers to these materials, and catalyst poisoning by various gases has become an important issue. Recently, among ferric oxides, gamma-type ferric oxide (γ-
It has been discovered that Fe ₂ O ₃ ) exhibits excellent gas sensitivity characteristics, and development of gas sensing elements using this as a sensitive material is progressing. The present invention was discovered through research conducted in parallel with various spinel materials and various additives that improve their properties. It has sufficient gas sensitivity and response recovery characteristics. Specific details will be described below based on examples. [Example 1] 3 g of lithium oxide (Li ₂ O), 108 g of ferric oxide (Fe ₂ O ₃ ), and 7 g of zinc oxide (ZnO) were weighed out, water was added to them, and a stainless steel plate was prepared. Mixed in a stainless steel pot for 5 hours. This mixture was heated at a temperature of 200 °C for 12
Let it dry for an hour and then use an organic binder.
The particles were sized to a size of 100 to 200μ. The powder obtained in this way is pressure-molded into a rectangular parallelepiped shape,
It was fired in air at a temperature of 1100°C for 2 hours. 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, thereby creating 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 element temperature was maintained at 350°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 Isobutane gas with a purity of 99% or more is flowed into the tank at a volume ratio of 10 ppm/sec, and its concentration is 0.05% by volume and 0.5% by volume.
Measurements were taken when each was reached. The gas concentrations to be measured were chosen to be 0.05% by volume and 0.5% by volume.
This is because it is practically necessary for a combustible gas detection element to detect concentrations in the range of several tenths to several tenths of the lower explosive limit (LEL) of isobutane gas, about 2%. For the device immediately after fabrication obtained as described above, the temperature of the device is increased by energizing the heater.
The gas sensitivity characteristics were measured while maintaining the temperature at 350°C.
As a result, Ra is 420KΩ, Rg (0.05%) is 98K
Ω, Rg (0.5%) was 18KΩ. That is,
The resistance change ratio in the gas concentration region of 0.05% by volume and 0.5% by volume is 5.44. This value is a large value that has not been seen in conventional semiconductor gas sensing elements.
Also, the gas sensitivity (ratio of Ra and Rg) is 4.3 and 4.3, respectively.
23, which was large enough for practical use. On the other hand, in a general gas sensing element, the response return characteristic is an important characteristic factor along with the resistance change ratio and gas sensitivity in the above-mentioned detection concentration range. This is because response recovery characteristics as quick as possible are desired from the viewpoint of detecting gas as quickly as possible and, when applied to a meter, etc., from the viewpoint of wanting to shorten the measurement interval as much as possible. Here, for convenience, response time T ₁ and recovery time T ₂ were defined and evaluated as follows. In other words, when the sensing element is quickly inserted into a measurement container in an atmosphere of 0.1% by volume isobutane gas, the resistance value Rg (0.05) in 0.05% by volume isobutane gas (98K in this example)
Ω) is defined as the response time T ₁ , and when returning to normal air from this state, Ra
The time taken to reach 90% of the resistance (420KΩ in this example) was defined as the recovery time _T2 . As a result of measurement based on this definition, in this example T ₁ = 2.6
seconds, T ₂ =12.4 seconds. This is the result of a similar experiment without adding ZnO, with T ₁ = 8.7 seconds and T ₂ =
It can be seen that the response return characteristic is much better than that of 37.3 seconds. This is extremely effective from an applied standpoint. By adding ZnO to LiFe ₅ O ₃ in this way, the sensitivity can be increased without any loss in the resistance change ratio (Rg (0.05%)/Rg (0.5%)), and the response recovery characteristics can be greatly improved. It can be seen that the results can be improved.Next, the effects of combinations and amounts of these additives will be specifically shown in the following example. [Example 2] 8 g of LiO ₂ and Fe ₂ O ₃ Weigh out 108g of
ZnO, stannic oxide SnO ₂ , titanium oxide TiO ₂ and tungsten oxide WO ₃ were weighed in various combinations and added in various amounts, and each was added to the above-mentioned Li ₂ O,
In addition to Fe ₂ O ₃ , water was added to each of these and mixed for 5 hours using a stainless steel bowl in a stainless steel pot. These mixtures were dried at a temperature of 200°C for 12 hours and then calcined at a temperature of 1100°C for 2 hours. Furthermore, after pulverizing this powder, it was rectified 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 5 mm each, and the thickness is 0.5 mm.
An alumina substrate was prepared, and a pair of comb-shaped electrodes were formed by printing comb-shaped gold paste on the surface at 0.5 mm intervals and baking it. Then, 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, apply the above paste to the surface of the board by approximately 70 μm.
After printing to a thickness of 600 mm and air drying at room temperature,
Baking was carried out in normal air at a temperature of .degree. C. for 1 hour. During this baking process the paste evaporates and
The resulting sintered film has sufficient mechanical strength for practical use. The thickness of this gas sensitive material was approximately 50μ. The gas detection characteristics of each of the detection elements obtained as described above were measured in the same manner as in Example 1. In Example 1, isobutane gas was used as the detection gas, but in this example, commercially available propane gas (purity of 98% or more) was used. Its characteristics are shown in Table 1. However, the element temperature during measurement was 350°C.

[Table] As mentioned above, Zn, Sn, Ti, and W are added to lithium ferrite (LiFe ₅ O ₈ ) as additives.
A combustible gas detection element using as a gas sensitive material at least one of which contains 0.5 to 50 mol% of the total amount of additives in terms of ZnO, SuO ₂ , TiO ₂ and WO ₃ in terms of oxides, respectively. It has a practically sufficient gas sensitivity, a large amount of change in resistance per unit concentration, and exhibits extremely excellent response recovery characteristics. In the present invention, the total amount of additives is limited from 0.5 mol% to 50 mol% because, as shown in Table 1, if it is less than 0.5%, it has no effect, and if it exceeds 50 mol%, the sensitivity and unit concentration This is because the amount of resistance change per hit decreases, and the response recovery characteristics are not improved. In addition, in the examples, oxides were used as starting materials in all cases, but any material containing lithium ferrite and a predetermined amount of any one of Zn, Sn, Ti, and W may be used, especially starting materials. It is not limited to. Further, although isobutane gas and propane gas were used as the detection gas, it goes without saying that the present invention is also effective for general flammable gases such as ethane, butane, and hydrogen. Furthermore, in order to further improve the characteristics of this element,
Alternatively, it is of course possible to add and contain other components in order to obtain more suitable characteristics depending on the purpose. As described above, the present invention improves gas sensitivity and resistance change per unit concentration by using lithium ferrite (LiFe ₅ O ₈ ) as a sensitive material and containing at least one of Zn, Sn, Ti, and W as an additive. Furthermore, it is possible to provide an element with extremely excellent response recovery characteristics.

Claims (1)

[Claims] 1. The receptor is lithium ferrite (LiFe ₅ O ₈ )
and at least one of Zn, Sn, Ti and W as additives, respectively ZnO, SnO ₂ , TiO ₂ , WO ₃
A combustible gas detection element characterized in that it contains 0.5 to 50 mol% of additives in terms of total amount. 2. Claim 1, characterized in that the sensitive body is a sintered film or a sintered body, a pair of electrodes are attached to the sensitive body, and combustible gas is detected by a change in resistance value between the electrodes. The combustible gas detection element described in .