JPS59220641A - Gas detecting element - Google Patents

Abstract

PURPOSE:To accelerate a response speed while attaining miniaturization and to attain to enhance reliability, by imparting a different kind of electrode materials to both surfaces of a sintered body used as an oxygen ion conductor having a special chemical formula while covering the sintered body and the electrodes with a porous protective layer. CONSTITUTION:A gas detecting element is prepared by such a method that a different kind of electrodes are baked to the upper and lower surfaces of a sintered disc body 3 by using a ceramic composition represented by chemical formula Y4(WxZr1-x)3O12 (wherein X is 0-0.5) to form electrodes 1, 2 and, after lead wires 4 are attached to both electrodes 1, 2, the sintered disc body and the electrodes are further covered with a porous protective layer 6. The oxide used as a stock material in fabricating the ceramic composition consists of yttrium oxide, tungsten oxide and zirconium oxide each of which has purity of 99.9% or more and a predetermined amount of said oxides are weighed, mixed, filtered, calcined and ground to obtain an oxide mixture which is, in turn, mixed with an about 5% polyvinyl alcohol solution as a binder to prepare a granulated powder. This powder is sintered while the sintered one is cut so as to obtain a thickness of 0.5mm. and, after a platinum electrode is baked to the single surface of each cut sintered electrode, a silver electrode is baked to the opposite surface thereof and the lead wire is connected. Further, alumina is plasma-sprayed to form a porous protective layer 6.

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,
Solid electrolyte oxygen sensors using zirconia (ZrO,) stabilized with calcia (Cab) or yttoria (Y10*) are well known.

As shown in Fig. 1, this has a structure in which electrodes 1 and ti2 made of porous platinum layers are provided inside and outside of a pipe 3 made of a solid electrolyte, and lead wires 4 are also provided. . To measure the oxygen concentration, a ZrO* 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 the electrodes 1 and 2 and the oxygen partial pressure is given by the Nernst relational expression (1).

F, -(RT/4F) ln(Po2/Po5)
(1) Po2 and Pop are the partial pressures of oxygen in the gas in which electrode 1 and electrode 2 are placed, respectively. According to this equation, 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.

Oxygen concentration meters using this type of oxygen concentration battery are used, for example, to determine the oxygen concentration in automobile exhaust gas or to control the amount of dissolved oxygen in molten steel.

However, the conventional solid electrolyte materials and gas sensing elements using the same as described above have the following drawbacks.

Already Calcia (Cab) or Ittria (Y20
The ZrO2 pipe stabilized by
Unless the temperature is 1400°C or higher, dense porcelain cannot be obtained and manufacturing is not easy. 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 zirconia pipe itself is also prone to cranking, resulting in characteristic deterioration.

Since it is necessary to supply a gas having a known oxygen degree, such as air or oxygen, as a reference gas to one electrode portion, the shape is not large and it is difficult to miniaturize. Since the temperature of the gas to be detected still needs to be around several hundred degrees, its use has naturally been limited.

In order to eliminate the above-mentioned drawbacks, an element having a structure as shown in FIG. 2 has been proposed. That is, the stabilized zirconia disk 30 has electrodes 1 and 2 formed by baking Pt on the front and back surfaces thereof, and furthermore, a catalyst layer 5 and an electrode lead wire 4 are formed on one of the electrodes. 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 the problem that 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.

Unexploded EII] The nocus detection element is 'f4(WxZrt-
x) is expressed by the chemical formula 11012, 0≦X≦05
As shown in FIG. 3, using a porcelain composition shown in the range of It is characterized by a structure in which a lead wire 4 is attached and the sintered body and electrode are further covered with a porous protective layer 6.

The present invention will be described in detail below based on examples. -Y4
The oxide used as a raw material to produce the ceramic composition (Wx Z rI-x ) so +s has a purity of 99
9% or more of 1,1 um oxide (Y2O2), tungsten oxide (Wow), zirconium mide (Z
r0,2) by the chemical formula Y4(WxZrt-x)sets, respectively fL predetermined position amount so that X=0.2,
The mixture was mixed with pure water in a ball mill for 46 hours, filtered and dried, calcined at 1150° C. for 2 hours, and pulverized in a Raikai machine, followed by mixing with 5% polyvinyl alcohol solution as a binder to produce granulated powder. This'h is the diameter] - Q ax
Press molded into a diameter of approximately 5 mm and heated at 1000℃ to 1600℃
Sintered with

It was further cut to a thickness of 0.511 t, and as shown in FIG. 3, a platinum electrode was baked on one side, a silver electrode was baked on the other side, and a lead wire was connected. Furthermore, alumina was sprayed as a porous protective layer 6.

For the measurement, as shown in Fig. 4, the sample prepared in the above method was placed on a quartz pipe 7 around which a heater 5 was wrapped, and when a gas of known concentration was flowed through the quartz pipe 7 at a rate of about 100 cc for 1 minute, it was converted to 'It'. The voltage generated for 4 hours was measured. The results are shown in FIGS. 5 to 7.

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

Figure 6 shows ■(, (straight line 1) and inbutane (1-C4H,
. .

These are the measured values when changing the mixing ratio of straight line 2) to air. The value of the electromotive force increased linearly as the gas family increased. The value also varies depending on the type of gas, indicating that selectivity to the gas can be obtained.

Normally, the concentration required to detect flammable gases is said to be 1/4 to 1/100 of the lower explosive limit. In the case of inbutane, this value is approximately 1100pp to 5000ppWj.
It can be concluded that the device according to the present invention has an output voltage of about 5 omv or more for this level of isobutane concentration, and is sufficiently effective.

When the same electrode material is used for electrode 1 and electrode 2 shown in FIG. 3, the induced voltage is 1077! If it is less than V, it will not exhibit effective characteristics as a gas detection element. Furthermore, even when one electrode was made of silver and the other electrode was made of palladium, good characteristics were shown as in the case of platinum.

Figure 7 shows the electromotive force of the gas sensing element of the present invention in air with 3000 ppm of carbon monoxide CC0)a when the blending ratio of tungsten and zirconium lamino in the sintered body composition is changed. ing.

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 necessary. In the Y4(WX zrI-x) 4012 ceramic composition according to the present invention, it is desirable that the blending ratio of tungsten and zirconium be in the range 0≦X<0.5.

Table 1 below shows that a temperature cycle test was repeated 500 times in which the gas sensing element of the present invention was kept 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. shows the rate of change in output voltage after

The conventional confectionery shown in Table 1 was prepared using zirconia (ZrO,N) stabilized with tecalcia (CaO) having the structure shown in FIG.

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 become separated due to temperature cycling, or cracks occur, resulting in a large drop in output voltage. On the other hand, the device according to the present invention exhibits stable characteristics with a small decrease in output voltage. It was also found that the porous protective layer is effective in reducing the change in electromotive force over time and stabilizing the device.

As described above, it is clear that 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 a large output voltage. It is.

In this example, combinations of silver and platinum and silver and palladium were shown as electrode materials, but other combinations, such as a mixture of nickel and nickel oxide, may be used for one electrode and silver or platinum for the other. The following characteristics are obtained.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIGS. 1 and 2 are structural diagrams of a conventional gas detection element. FIG. 3 is a structural diagram showing one embodiment of the gas detection element of the present invention. FIG. 4 is a diagram of an experimental apparatus for the gas detection element of the present invention. Fifth
6 are characteristic diagrams 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 FIG. 1, 1 and 2 are platinum electrodes, 3 is a solid electrolyte pipe, and 4 is a lead wire. In Figure 2, 1.2 is a platinum electrode, 3 is a solid electrolyte sintered body, 4 is a lead wire, and 5 is a platinum electrode.
is the catalyst layer. Figure 3. In FIG. 4, 1.2 is an electrode, 3 is a sintered solid electrolyte, 4.4' is a lead wire, 5 is a heater, 6 is a porous protective layer, and 7 is a quartz pipe. Figure 1 Figure 2 Figure 30 Figure 4 Figure 5 Okina (minute) Figure 6 4 history (Kano 0) Figure 7 Y4 (WX Z? 1-x)'30/20, j:)
/, U×

Claims (1)

[Claims] In a gas detection element in which electrodes are provided on both sides of a sintered body having oxygen ion conductivity and lead wires are attached to the electrodes, Y4(wXZr+-z)s is used as the sintered body.
Using an oxygen ion conductor represented by O+s (0≦X≦o, 5), different types of electrode materials were applied to both sides of the sintered body, and the sintered body and electrodes were covered with a porous platinum layer. A gas detection element characterized by: