JPS6356940B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a gas sensing element that exhibits a large resistance change with respect to carbon monoxide gas and a small change in resistance value over time. Although various materials have been considered for gas detection elements to date, gas detection elements using metal oxide semiconductors have been attracting attention from various quarters in recent years. Representative examples include elements using zinc oxide (ZnO), tin oxide (SnO ₂ ), spinel-type ferrite oxide, δ iron monoxide, titanium oxide (TiO ₂ ), and the like. All gas sensing elements using these materials utilize changes in the resistance value of an oxide semiconductor that occurs when gas molecules are adsorbed and desorbed onto the element surface. There are various types of these structures, but they can be broadly classified into sintered types, thin film types, and thick film types. Sintered type elements are made by making a metal oxide semiconductor into a paste with an appropriate solvent, attaching it in a bead shape to a thin coiled heater wire, embedding a pair of separate electrode wires, and then sintering it. It's something you can get. With this method, it is difficult to precisely control the size of the deposited metal oxide semiconductor bead, the electrode spacing, etc., so the element resistance value and the rate of change in resistance against gas vary widely, which affects the reproducibility of the element. There were also drawbacks such as poor performance and large changes in characteristics over time. A thin film type element has a structure in which a thin film of a metal oxide semiconductor is formed on a suitable substrate such as alumina by sputtering or vapor deposition, and electrodes for resistance measurement and a heater are provided on the thin film. This device has the same drawbacks as sintered devices, such as compositional deviation of the metal oxide semiconductor that occurs during sputtering or vapor deposition, poor uniformity of thin films such as variations in film thickness, and poor reproducibility of characteristics, and high manufacturing costs. There were flaws. On the other hand, the thick film type element has a structure in which, as shown in FIG. 1, a gas detection semiconductor 3 is printed on a substrate 1 on which an electrode 2 and a heating heater 4 have been provided in advance. Therefore, it has the advantage of being easy to manufacture, easy to control characteristic values, and low cost. However, conventional elements using metal oxide semiconductors have the drawback of exhibiting similar resistance changes with respect to various gases, and lack of selectivity with respect to gases. Furthermore, the detection sensitivity for carbon monoxide gas was poor, and almost no sensitivity was obtained at concentrations below 1000 ppm. Usually carbon monoxide concentration
If it is around 100ppm, it will last for several hours, and if it is around 200ppm, it will last for 2 hours.
After ~3 hours, abnormalities such as a headache occur.
At around 1000ppm, a headache occurs in about 40 to 50 minutes.1
It is said that if the damage lasts only a short period of time, it will be a serious situation that could be life-threatening. Therefore, a gas detection element that is effective even at carbon monoxide concentrations of 1000 ppm or less is desired. The reliability of the gas detection element is also an important factor. That is, it is also practically important to reduce changes in the resistance value of the gas detection element over time and changes in gas sensitivity over time. Typically, gas detection elements are used with the temperature of the element raised to 200 to 400°C and a DC or AC voltage applied. When used in this way, it takes a long time for the initial stabilization of the resistance, and in extreme cases it may take several days.
Furthermore, if the device is left in use, its resistance value will gradually change, and the gas sensitivity of the device will also change. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a carbon monoxide gas detection element that overcomes the above-mentioned conventional drawbacks. According to the invention, aluminum oxide (Al ₂ O ₃ )
Crystallized borosilicate glass containing 0.05 mol% to 2.0 mol% of stannic oxide (SnO ₂ ) and zirconium oxide (ZrO ₂ ) in a dissolved amount of 0.1 to 5.0 wt%, each in a weight ratio of 90 wt% to 90 wt%. 40wt% and 10wt
% to 60 wt%, and firing at a crystallization temperature of the borosilicate glass of 800° C. to 950° C. is obtained. According to the present invention, for carbon monoxide gas,
It is possible to realize a carbon monoxide gas detection element that has excellent high sensitivity and stable characteristics with extremely small changes over time. The present invention will be described in detail below based on one example. Example As a raw material, oxidized No. 2 with a purity of 99.8% and an average particle size of 2μ
Aluminum oxide (Al ₂ O ₃ ) to tin (SnO ₂ ) powder
Weighed to a ratio of 0.05 mol% to 2.0 mol%, mixed with pure water using a ball mill for 46 hours, and then over-dried.
It was calcined at 1200°C for 2 hours. Add the crystallized borosilicate glass frit shown in Table 1 to this calcined powder.
A mixed raw material containing 10 wt% to 60 wt% and an organic vehicle containing 10 parts by weight of terpionelle to 1 part by weight of ethyl cellulose were kneaded at a weight ratio of 70:30 to form a paste. This paste was applied by screen printing to an alumina substrate 1 on which a heater 3 and a resistance detection electrode 2, which had been printed and fired with an alloy electrode paste having a weight ratio of 30 platinum and 70 palladium, as shown in Figure 1, were formed. It was printed on top and fired for 30 minutes at the glass frit crystallization temperature of 800°C to 950°C to produce a gas sensing element. FIG. 2 shows the ratio R _G /R _O of the resistance R _G in the gas to the resistance R _O in the air of the gas detection element obtained in this way, with respect to the gas concentration. As is clear from the figure, the element according to the present invention exhibits excellent selectivity with respect to carbon monoxide gas (CO), especially with a large resistance change. FIG. 3 shows the relationship between firing temperature and R _G /R _O. From now on, especially when the firing temperature is 850℃~950℃
R _G /R _O is large and performance is excellent. The broken line in FIG. 3 is an example in which platinum and palladium are used as a resistance detection electrode, which means that the resistance change is small and the electrode is not effective as a carbon monoxide gas detection element. That is, it is desirable to select a material for the resistance detection electrode. The reason why the device of the present invention has such excellent performance is estimated as follows. That is, when the crystallized borosilicate glass is fired in the crystallization temperature range, the glass component that fixes the stannic oxide crystal particles to each other crystallizes in this temperature range, causing volumetric contraction and the generation of pores. This is due to an increase in the contact area between the ditin particles and the outside air. A particularly large resistance change with respect to CO gas strongly depends on the amount of zirconium oxide (ZrO ₂ ) in the glass and the amount of aluminum oxide (Al ₂ O ₃ ) dissolved in the stannic oxide (SnO ₂ ). Figure 4 is the first
The results of R _G /R _O for devices fabricated using glass frits with different weight ratios of ZrO ₂ shown in the table are shown. As is clear from Figure 4, the value of R _G /R _O for CO strongly depends on the amount of aluminum oxide added and the amount of zirconium oxide in the glass, and it is clear that the selectivity for carbon monoxide gas can be increased. It is. That is, in the present invention, a sintered body made of SnO ₂ containing borosilicate glass is used, Al ₂ O ₃ is added to SnO ₂ , and ZrO ₂ is added to borosilicate glass at the same time. By adjusting the amount of each type of additive to a predetermined amount, the value of R _G /R _O can be increased.

[Table] Next, Table 2 shows the resistance R _O of the element obtained according to the present invention and the change over time in the resistance ratio R _G /R _O at a carbon monoxide concentration of 1000 ppm with respect to air. The results shown in Table 2 were measured again under an applied voltage of 20 V.DC to the device, with the device temperature kept at 300° C., and after being left in CO 1000 ppm for 500 hours, it was left in air for 2 hours. Second
From the table, crystallized glass is used.

[Table] Conventional elements that are not used have carbon monoxide gas concentration
If left in 1000ppm for a long time, the resistance remains reduced. Furthermore, the change in resistance against carbon monoxide gas also becomes small. On the other hand, the element of the present invention is stable with small changes in characteristics. As described above, the carbon monoxide gas detection element according to the present invention exhibits excellent selectivity to CO, exhibits little change in characteristics over time, and is extremely stable.

[Brief explanation of drawings]

Figure 1 shows the structure of a thick film type device. a...Cross-sectional view, b...Surface view, c...Back view, 1...Substrate, 2...Resistance detection electrode, 3...Semiconductor part, 4...Heater. FIG. 2: A diagram showing resistance change with respect to gas concentration in air. The broken line in the figure shows an example in which palladium and platinum are used as resistance detection electrodes. FIG. 3 is a diagram showing the relationship between element firing temperature and resistance change rate. The broken line in the figure is an example in which palladium is used for the resistance detection electrode. FIG. 4 is a diagram showing the relationship between the amount of zirconium oxide in the glass frit and the change in resistance in relation to the content of aluminum oxide.

Claims (1)

[Claims] 1 Aluminum oxide (Al ₂ O ₃ ) 0.05 mol% ~ 2.0
Crystallized borosilicate glass containing mol % of stannic oxide (SnO ₂ ) and zirconium oxide (ZrO ₂ ) in a dissolved amount of 0.1 to 5.0 wt %, respectively, by weight.
90 wt% to 40 wt% and 10 wt% to 60 wt%, and firing at a crystallization temperature of the borosilicate glass of 800°C to 950°C.