JPS59120951A - Gas sensitive element - Google Patents

Abstract

PURPOSE:To obtain a gas sensitive element which is small in size, fast in response speed and has high reliability by baking respectively different kinds of elelctrodes on the top and bottom surfaces of the disc of a sintered body using a ceramic compsn. expressed by a specific chemical formula and further coating the sintered body and electrodes with a porous protective layer. CONSTITUTION:The oxide used as a raw material for forming the ceramic compsn. (Y1-xLax)2WO6 is produced by using Y2O3, La2O3 and WO3 having >=99.8% purity, weigh ing respectively prescribed amts. of these materials so as to attain X=0.6 in the chemical formula (Y1-xLax)2WO6, mixing the weighed materials with pure water, filtering and drying the mixture, calcining the dried mixture, grinding the mixture with an automated mortar, mixing about 5% PVA soln. as a binder therewith and forming granulated powder. The granulated powder is press molded to about 10mm. diameter phi and about 5mm. length and the molding is sintered. The sintered molding is further cut to 0.5mm. thickness and a platinum electrode is baked on one side and thereafter a silver electrode is baked on the surface on the opposite side. Lead wires are connected to these electrodes. A porous protective layer 6 is additionally formed by spraying of alumina.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides the following methods:
The present invention relates to a gas detection element using a solid electrolyte material that detects gas by generating an electromotive force depending on the gas concentration.

As a gas sensing element using conventional solid electrolyte materials,
Calcia (CaO) or Ittria (Y, O,
) A solid electrolyte oxygen sensor using 2 luconia (ZrO□) stabilized with etc. is well known. As shown in Fig. 1, an electrode 1 and an electrode 2 made of a porous platinum layer are placed inside and outside a pipe 3 made of a solid electrolyte.
It has a structure in which a lead wire 4 is provided. To measure the oxygen concentration, a ZrO2 pipe is placed in the gas to be measured, and the inside of the pipe is filled with a gas having a known oxygen partial pressure, such as air or pure oxygen gas. At this time, the relationship between the electromotive force generated between electrode 1 and electrode 2 and oxygen partial pressure is +11
It is given by the Nernst relation of Eq.

E= (RT/4F)Jn(Po, /Po,, (I
I)ノ (1) (1) Here, P O2 and Po2 () are respectively the electrodes l and (
+) and the oxygen partial pressure in the gas in which the electrode 2 is placed.

According to this formula, the oxygen concentration in the gas to be measured can be determined from the value of the electromotive force generated based on the difference in oxygen partial pressure inside and outside the solid electrolyte pipe.

This type of oxygen concentration meter uses an oxygen concentration battery.

For example, measuring oxygen concentration in automobile exhaust gas.

It is used for applications such as controlling the amount of dissolved oxygen in molten steel.

However, the above-mentioned conventional solid electrolyte materials and gas sensing elements VC using the same have various runner-up points such as second order.

That is, calcia (CaO) or ittoria (Y,
03) etc., the ZrO2 pipe stabilized by
If it is heated to a high temperature of 1,400°C or higher, it will not be possible to obtain a dense porcelain, which is not easy to manufacture. Furthermore, the bonding surfaces between the electrodes 1 and 2 shown in FIG. 1 and the zirconia pipe 3 are susceptible to thermal shock, and the luconia pipe itself has the drawbacks of being susceptible to cracks and deterioration of Qf properties.

Also, use a gas with a known oxygen concentration, such as air or the like.

Since it is necessary to supply oxygen or the like as a reference gas to one of the vl and rod portions, the shape is large and there is also the drawback that it is difficult to miniaturize. Furthermore, since the temperature of the gas to be detected must be several hundred degrees, its use is naturally limited.

In order to eliminate the above-mentioned drawbacks, an element having a structure as shown in FIG. 2 has been proposed. That is, a Pt baked electrode 1 and an electrode 2 are provided on the front and back surfaces of a stabilized zirconia disc 30, and a catalyst layer 5 and an electrode lead #i are further provided on one electrode.
! 4. It is true that such a structure makes it easier to miniaturize the device. However, the time required for the output voltage to reach a certain value after gas introduction, that is, the response speed, is extremely slow, requiring 5 minutes or more. There was also a problem of ℃5 when the output voltage decreased due to deterioration of the catalyst layer.

An object of the present invention is to eliminate these drawbacks and provide a gas detection element that is small in size, has a fast response speed, and is highly reliable.

The gas detection element of the present invention is '(Y l, L a x
) 2 Represented by the chemical formula WOa, 02≦X≦1
Using a ceramic composition shown in the range of 0, different types of electrodes are baked on the upper and lower surfaces of a sintered disk 3 having the above composition as shown in FIG. 3 to form electrodes 1 and 2. , a lead wire 4 is attached, and the sintered body and electrode are further covered with a porous protective layer 6.

The following will be described in detail based on examples.

The oxide used as a raw material to create the porcelain composition (YX□xLaX)2WO6 has a purity of 99. s, 1 or more of yttrium oxide (Y, 03), lanthanum oxide (La20.), and tungsten oxide (WO3), each using the chemical formula (Y s −x Lax ) 2 WOs so that X = 0.6. Weigh the specified amount, mix it with pure water in a ball mill for 46 hours, dry it for 90 minutes, calcinate it at 1150℃ for 2 hours, and crush it in a Raikai machine, then mix it with 5% polyvinyl fluorocarbon solution as a binder and granulate it. produced powder. This was formed by IC press molding with a diameter of 10wnφ and a length of about 5m, and heated to 1000℃~1600℃.
Sintered at °C. It was further cut to a thickness of 0.5 mm, a platinum electrode was baked on one side as shown in FIG. 3, and a lead wire was connected to the other side by baking 1M and @. Furthermore, alumina was sprayed as a porous protective layer 6.

For the measurement, as shown in Fig. 4, the sample tabulated in the above method was placed in a quartz pipe (eve 7) wrapped around a heater 5, and a gas of known concentration was flowed through the quartz pipe 7 at a rate of about 100 cc for one minute. The voltage induced between the electrodes was measured.The results are shown in the fifth section.
It is shown in FIGS.

FIG. 5 shows the change in electromotive force over time when air containing 3000 ppm of carbon monoxide (Co 2 ) was flowed into a quartz pipe heated with a heating heater so that the sample temperature reached 400'C. The electromotive force reaches 80% of its steady value within 5 seconds,
It showed a sufficiently fast response speed. The recovery performance is also good after switching the test gas to air only, returning to the steady value within about 1 minute.
It was found that the performance was sufficient for practical use.

Figure 6 shows the measured values when the mixing ratio of I(2 (line 1) and inbutane (line 2) to air was changed.The value of electromotive force increases almost linearly as the gas concentration increases. The value also differs depending on the type of gas, indicating that selectivity to the gas can be obtained.

Normally, the concentration required for detection of flammable gas is 1/4 to 17,100 or less of the lower explosive limit. In the case of isobutane, this value is about 1,100 ppm to 5,000 ppm, and it can be concluded that the device of the present invention has an output voltage of about 50 mV or more for this level of isobutane concentration, and is sufficiently effective.

Further, when the same electrode material is used for electrode 1 and electrode 2 shown in FIG. 3, the induced voltage is 10 mV or less, and the device does not exhibit effective characteristics as a gas sensing element. Furthermore, even when one electrode was made of steel and the other electrode was made of palladium, good characteristics were shown as in the case of platinum.

Figure 7 shows the behavior of the gas sensing element of the present invention in air with a carbon monoxide (co) concentration of 3000 ppm when the mixing ratio of yttrium oxide and lanthanum oxide in the sintered body composition was changed. Indicates the power (・ru.

In practice, a method for detecting an electromotive force of 10 mV or less is more difficult due to the circuit configuration. Therefore, it is desirable that the electromotive force be as large as possible, and 50 mV or more is required. According to the present invention, it is desirable that in the ceramic composition represented by Y I can do it.

Table 1 below shows the results after 500 repetitions of a temperature cycle test in which the gas sensing element of the present invention was held in an electric furnace kept at room temperature to 500°C for 5 minutes, then taken out to room temperature and left for 20 minutes. Indicates the rate of change in output voltage.

The conventional elements in Table 1 had the structure shown in FIG. 2 and were manufactured using zirconia (ZrO2) stabilized with calcia (CaO).

As a result of the tests in Table 1, it has been found that in conventional devices, the solid electrolyte and the electrode or the electrode and the catalyst layer separate due to temperature cycling, or cracks occur, resulting in a significant drop in output voltage. , - The device invented by Rikiki exhibits stable characteristics with a small decrease in output voltage. It has also been found that the porous protective layer reduces the change in electromotive force over time and is effective in stabilizing the device.

As described above, the gas sensing element of the present invention is small and easy to sinter, is resistant to temperature cycles, has sufficient stability, has a fast response speed, and exhibits useful performance in practice, such as an output voltage of 30°F. That is clear.

In addition, in this example, as the N electrode material, a combination of @ and platinum and silver and palladium is used as one electrode, and other combinations such as a mixture of nickel and nickel oxide σ) are used as one electrode,
Similar characteristics can be obtained by using chime or platinum as the other material.

Further, in the examples, the porous protective layer was formed by alumina spraying, but the features of the present invention will not be lost even if the porous protective layer is formed by a different method or method.

[Brief explanation of drawings]

FIGS. 1 and 2 are structural diagrams of conventional gas detection elements. FIG. 3 is a structural diagram of a gas sensing element according to the present invention. FIG. 4 is a diagram of an experimental apparatus for the gas detection element of the present invention. Figure 5. FIG. 6 is a characteristic diagram of the gas detection element of the present invention. FIG. 7 is a diagram showing the relationship between the composition ratio of the sintered body used in the gas sensing element of the present invention and the electromotive force. In Figure 1, 1 and 2 are platinum X electrodes, 3 is a solid electrolyte pipe, and 4 is a soil electrode. In Figure 2, 1 and 2 are platinum electrodes, 3 is a sintered body of solid electrolyte, 4 is the lead wire, 5
is the catalyst layer. In Figures 3 and 4, 1.2 is an electrode, and 3 is a sintered solid electrolyte. 4.4' is a lead wire, 5 is a Hilter, 6 is a porous protective layer, and 7 is a quartz vibrator. O 1 En O 2 Figure t 3 Mouth f 〃 Kasumi/ ,, -y

Claims (1)

[Claims] A gas sensing element comprising electrodes provided on both sides of a sintered body having oxygen ion conductivity and lead wires attached to the electrodes, wherein (Y,.L ax ) 2wo is used as the sintered body. Using an oxygen ion conductor represented by the chemical formula 6 (0,2≦X≦1.0), different types of electrode materials are applied to both sides of the sintered body, and the sintered body and electrodes are protected by porous A gas detection element characterized by being coated with a layer.