JPS6129663B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a combustible gas detection element,
By adding various additives to cadmium ferrite (CdFe ₂ O ₄ ), which has a spinel-type crystal structure and does not have sufficient detection ability for combustible gases on its own, the sensitivity is increased and the response recovery characteristics are improved. The object of the present invention is to provide a combustible gas detection element with greatly improved characteristics. In recent years, various research and developments have been conducted on combustible gas sensing element materials. 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, platinum (Pt) must be used as a sensitizer in addition to these materials.
It is necessary to add noble metal catalysts such as or palladium (Pd), 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. In addition, in parallel with this, research on the gas sensitivity characteristics of a series of various spinel materials is progressing, and as a result, the general formula is Cd _1-x Fe _2+x O ₄ (0.20≦x≦0.85)
It is also known that ferrite whose main component is iron percadmium ferrite with a cubic spinel phase represented by , exhibits great gas sensitivity characteristics. However, cadmium ferrite (CdFe ₂ O ₄ ) having a stoichiometric composition has low gas sensitivity and is not satisfactory for use as a practical device. To be more specific, the ratio of the resistance value of the sensitive material in normal air to the resistance value in an atmosphere containing 0.2% isobutane gas (gas sensitivity) is at most 2.
This value could not necessarily be said to be sufficient gas sensitivity from a practical point of view. The present invention provides a combustible gas detection element that improves this point, and in which at least one of Zn, Sn, Ti, and W is added to CdFe _{2 O 4} as an additive. _2.By using a sensitizer containing 0.5 to 50 mol% of additives in terms of WO ₃ in total, it has sufficient sensitivity for practical use and is one of the extremely important characteristic factors of gas detection elements. This makes it possible to provide an element with significantly improved response recovery characteristics. Hereinafter, the combustible gas detection element of the present invention will be specifically explained based on Examples. [Example 1] Weigh out 13 g of cadmium oxide (CdO), 22 g of ferric oxide (Fe ₂ O ₃ ), and 5 g of zinc oxide (ZnO), add water to them, and place them in a stainless steel pot. Mixed in a stainless steel bowl for 5 hours. Next, this mixture was dried at a temperature of 200° C. for 12 hours, and then sized into particles with a size of 100 to 200 μ using an organic binder. The powder thus obtained was pressure-molded into a rectangular parallelepiped shape and 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. Change the body temperature to 350
The gas sensitivity characteristics were measured while maintaining the temperature at ℃. 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 isobutane gas as a volume ratio
It was flowed in at a rate of 10 ppm/sec, and measurements were taken when the concentrations reached 0.05% by volume and 0.5% by volume, respectively. The gas concentration to be measured is 0.05% by volume and 0.5
The reason for choosing the volume percentage is that 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, approximately 2%. Because there is. Regarding the device thus obtained immediately after fabrication, the heater was energized to maintain the device temperature at 350° C., and its gas sensitivity characteristics were measured. As a result, Ra is
320KΩ・Rg (0.05%) is 88KΩ, Rg (0.5%) is
It was 18KΩ. That is, the resistance change ratio in the gas concentration region of 0.05% by volume and 0.5% by volume is 4.89. This value is a large value that has not been seen in conventional semiconductor gas sensing elements. Also, gas sensitivity (Ra and
Rg) are 3.6 and 17.8, respectively, which are sufficient 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 (88K 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 (320KΩ in this example) was defined as the recovery time _T2 . As a result of measurement based on this definition, in this example T ₁ = 2.4
seconds, T ₂ =11.7 seconds. This is the result of a similar experiment without adding ZnO, with T ₁ = 9.2 seconds and T ₂ =
It can be seen that the response return characteristic is much better than that of 39.6 seconds. This is extremely effective from an applied perspective. In this way, by adding ZnO to CdFe ₂ O ₄ , the resistance change ratio (Rg (0.05%) / Rg (0.5%))
It can be seen that the sensitivity can be increased and the response return characteristic can be significantly improved. Next, in a second example, the effects of combinations and amounts of additives will be specifically shown. [Example 2] 26g of CdO and 44g of Fe ₂ O ₃ were weighed out, and
ZnO, stannic oxide SnO ₂ , titanium oxide TiO ₂ and tungsten oxide WO ₃ were weighed in various combinations and added in different amounts, and each of the above-mentioned CdO,
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 fired at a temperature of 1100°C for 2 hours. Furthermore, after pulverizing this powder, it was sized to a size of 50 to 100 microns, and triethanolamine was added to form a paste. On the other hand, an alumina substrate with a thickness of 5 mm and a thickness of 0.5 mm was prepared as a substrate for the gas detection element, and a pair of comb-shaped electrodes were formed by printing gold paste on the surface in a comb shape at 0.5 mm intervals and baking it. A commercially available ruthenium oxide glaze resistor was printed on the back side of the alumina substrate between the metal electrodes and baked to form a heater. Next, print the base plate described above to a thickness of about 70μ on the surface of the substrate, air dry it at room temperature, and then heat it to 600℃.
Baked in normal air at a temperature of 1 hour.
During this baking process, the paste evaporated, resulting in a sintered film with sufficient mechanical strength for practical use. The thickness of this gas sensitive body 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 the table below. However, the element temperature during measurement was 350°C.

[Table] Items marked with * in the margin of the table are those in which the total amount of additives is less than the amount stated in the claims (No. 1, 2) or exceeds (No. 12 to
16), which is a comparative example for clarifying the effects of the present invention. In other words, No. 1 and 2 in the table have a total additive content of less than 0.5 mol%,
Sensitivity is low, and response recovery characteristics are also poor. However, when the total amount of additives is 0.5 mol % or more, the effect of addition becomes noticeable regardless of whether one or more additives are used, sensitivity increases, and response return characteristics become extremely good (Nos. 3 to 11). On the other hand, when the total amount of additives exceeds 50 mol%, the sensitivity decreases and the response return characteristics also deteriorate (Nos. 12 to 16). As mentioned above, Zn, Sn, Ti and W are added to cadmium ferrite (CdFe ₂ O ₄ ) as additives.
At least one of them is an oxide ZnO, SnO ₂ ,
From 0.5 to the total amount of additives in terms of TiO ₂ and WO ₃
The combustible gas detection element of the present invention using a gas sensitive material containing 50 mol% has practically sufficient gas sensitivity and exhibits extremely excellent response recovery characteristics. In addition, in the present invention, the total amount of additives is limited from 0.5 mol% to 50 mol%, as shown in the table, there is no effect of addition below 0.5 mol%, and when it exceeds 50 mol%, sensitivity decreases. This is because the response is degraded and the response return characteristic is not improved. In Example 1, the sensitive body is a sintered body, and in Example 2, the sensitive body is a sintered film, but as can be seen from the contents of both Examples, the sensitive body is a sintered body, The effects of the present invention are effective in any case of sintered films. In addition, in the examples, oxides were used as starting materials in all cases, but any material that ultimately contains cadmium ferrite and Zn, Sn, Ti, and W may be used, and the starting materials are particularly limited. It's not a thing. In addition, isobutane gas and propane gas were used as detection gases, but ethane, butane,
It goes without saying that the present invention is also effective for general combustible gases such as hydrogen. Furthermore, it is of course possible to add and contain other components in order to further improve the characteristics of this element or to obtain more suitable characteristics depending on the purpose. As is clear from the above description, the present invention uses cadmium ferrite (CdFe ₂ O ₄ ) as an additive.
By using a sensor containing Zn, Sn, Ti, and W, it is possible to provide a combustible gas detection element with extremely excellent gas sensitivity and response recovery characteristics.

Claims (1)

[Claims] 1. The sensitizer is cadmium ferrite (CdFe ₂ O ₄ )
and at least one of Zn, Sn, Ti and W as additives, respectively ZnO, SnO ₂ , TiO ₂ ,
A combustible gas detection element characterized by containing 0.5 to 50 mol% of additives in total in terms of _WO3 . 2. In the statement of claim 1, the sensitive body is a sintered film or a sintered body, and a pair of electrodes are attached to the sensitive body, and combustible gas is detected by a change in resistance value between the electrodes. A combustible gas detection element characterized by: