JPS6398557A - Low temperature operation type oxygen sensor - Google Patents

Abstract

PURPOSE:To increase a response speed at low temp., by providing a perovskite type composite oxide electrode to the surface of an oxygen ion conductive solid electrolyte. CONSTITUTION:An oxygen ion conductive solid electrolyte element 1 wherein 10mol% of calcium oxide is added to cerium oxide is fixed to an alumina magnetic pipe 2 and the interface between of them is sealed with silver to enhance airtightness. Perovskite type composite oxide A1-xA'xBO5-delta (wherein A is La, A' is an alkaline earth metal and B is a transition metal) finely pulverized to about 1mum or less is formed into a paste using an org. solvent such as terpentine oil and a small amount of n-butyl acetate is added to the resulting paste to prepare a composition which is, in turn, applied to the inner and outer surfaces of the element 1 and baked at about 80 deg.C in air to form electrodes 3. This oxygen sensor is exposed to gas to be inspected and the electromotive force at this time is measured through a conductive metal lead terminal 4. This oxygen sensor certainly operates at 300-400 deg.C with a sufficiently high response speed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a low temperature operating type oxygen sensor suitable for combustion control such as exhaust gas treatment.

[Conventional technology]

Conventionally, this type of oxygen sensor uses an oxygen ion conductive solid electrolyte such as Zr pseudo-Y, 0. system or ZrQ! Some use stabilized zirconia made of -0aO system or oxide semiconductors made of oxygen-deficient non-stoichiometric composition, such as TiO and J.

Solid electrolyte oxygen sensors can be divided into those that utilize the electromotive force of a solid battery and those that utilize the oxygen ion current that flows through a solid electrolyte when a voltage is applied.

On the other hand, oxide semiconductor oxygen sensors utilize changes in electronic conductivity in response to changes in the stoichiometric composition of oxides at high temperatures.

In both of the above methods, platinum electrodes have conventionally been used to detect oxygen concentration using electrical signals. This platinum electrode is used for the purpose mentioned above, and serves as a site for electrochemical reactions, i.e., oxygen shedding in the concentration equilibrium between the oxygen partial pressure of the other gas atmosphere and the oxygen ions or oxygen vacancies in the solid. Therefore, it is an extremely important part of this type of oxygen sensor.

Ordinary platinum electrodes have appropriate porosity to improve the permeability of oxygen in the atmosphere, and the method for manufacturing them is to paste finely divided platinum black with an organic solvent such as turpentine oil. In addition to the method of coating the film after applying it, there are also methods of vapor deposition, sputtering, or chemical plating.

In this platinum electrode, the three-phase interface of metal-electrolyte-gas phase serves as a reaction point, so the interfacial conductivity is determined by the length of the three-phase interface line. This interfacial conductivity decreases significantly at low temperatures compared to the ionic conductivity of the solid electrolyte. This is why the platinum electrode is made porous, because by making it porous, the three-phase interface line can be lengthened, and the effective area of the electrode can be expanded. However, forming a porous platinum electrode has the problem that a large amount of platinum is required in order to obtain sufficient conductivity on the electrode surface.

All oxygen sensors with the structure described above are heated to 700°C.
Practical operation does not occur unless the temperature is relatively high.

Currently, various types of oxygen sensors have been proposed as a means to solve this problem. For example, in the case of an oxide semiconductor type oxygen sensor, a good example is an oxygen sensor with a thin film structure that takes full advantage of the characteristics of an oxide with low resistivity.

On the other hand, with respect to solid electrolyte oxygen sensors as well, attempts have been made to make the electrolyte thinner, but the technological process has not yet been established, and in the case of the solid battery type oxygen sensor mentioned above, the structural There are many difficult aspects.

A solid-state battery type oxygen sensor is the most basic and commonly used method using a solid electrolyte. The principle of this type of oxygen sensor is that it consists of an oxygen concentration cell, for example, and its electromotive force is used to determine the oxygen partial pressure in the sample gas, p o,
This is intended to measure II. The theoretical electromotive force E of this battery is expressed by P: known oxygen partial pressure, Pop': tested oxygen partial pressure, T: absolute temperature, R: gas constant, and F: Faraday constant.

As mentioned above, various types of oxygen sensors based on various principles have been proposed, but the operating temperature of all of them is 700°C.
The temperature is as high as 0.degree. C. or higher, and there is nothing that can be put to practical use at lower temperatures.

In addition, when oxygen sensors are generally used at high temperatures, ■ A heating source is required to raise the temperature of the sensor and its atmosphere. ■ Because the oxygen sensor is at a high temperature, it is easily affected by the external temperature and cannot be kept at a constant temperature. It becomes difficult to maintain.

As a result, a temperature gradient occurs in the oxygen sensor element, reducing measurement accuracy.

■ Large heat loss due to heat leaking to the outside ■ This causes problems such as accelerated deterioration of the electrolyte and electrodes, shortening their lifespan.

[Problem that the invention seeks to solve]

The present invention eliminates the drawbacks of the conventional oxygen concentration battery type oxygen sensor and uses a perovskite composite oxide instead of a platinum electrode. The present invention aims to provide an actuated oxygen sensor.

[Means for solving problems]

The present invention provides the oxygen ion conductive solid electrolyte surface with the formula A +-
1A'z BOs-δ (A is La, A' is an alkaline earth metal.

This is a low-temperature operating type oxygen sensor characterized by having a perovskite-type composite oxide electrode represented by B (transition metal).

Perovskite-type composite oxides include "at, xSr
z Mn0s-a (X-[11~115) e
La+-xSrz Coo@-δ(xmo, 1-xSr
xMnO3-δ (x=0.1-0.5).

r, +a+-1s1"z Fe06-δ(X-α1~Q
, 5) l and LA+-1csrzN10g-δ
(x-o, o1-xSrxMnO3-δ (x=0.1~
0.05) can be used.

The above range of X is the solid solution range of Sr, and it has been confirmed by X-ray diffraction that other phases are precipitated in addition to the perovskite phase when the composition exceeds this range.

In addition, as an oxygen ion conductive solid electrolyte, 0eo1-
CaO system, OeO, -GdtOg system e Zr01-
YtOs system.

Any one oxide of ZrO, -CaO series and B i,03 series can be used, especially 0eO1 to 1
An electrolyte containing 0 to 40 mol of CaO is preferable, and an electrolyte having this composition range has excellent pure oxygen ion conductivity.

[Effect]

Oxygen-deficient perovskite-type composite oxide is a chemically functional material with mixed conductivity that has high conductivity of oxygen ions providing oxygen vacancies and electron or hole conductivity, and can be used as a solid electrolyte with oxygen ion conductivity. By providing the above composite oxide as an electrode, a gas phase/oxide/electrolyte structure can be obtained. First, at the gas phase/oxide interface, by using an oxide electrode with high catalytic activity, adsorption and desorption of oxygen is performed reversibly even at low temperatures, and the oxide/electrolyte interface is also used to transfer oxygen ions into the solid electrolyte. It can be used as a migration path. As a result, there is no need to make the electrode porous as in the case of platinum electrodes, and the electrode exhibits excellent electrode performance as an oxygen sensor even at low temperatures of 700° C. or lower, which is normally impractical.

〔Example〕

An oxygen ion conductive solid electrolyte prepared by adding 10 moles of calcium oxide (C-O) to cerium oxide (060) was used as a sensor element having a structure as shown in FIG. This solid electrolyte element 1 was installed in an alumina magnetic tube 2, and its interface was covered with a silver shield (not shown) to improve airtightness. Both the inner and outer surfaces of the element 1 are coated with perovskite-type composite oxide As, which has been made into fine particles of less than 1 μm.
-1A'
Apply a small amount of butyl mixed with 800% in the air.
The electrode 3 was formed by baking at a temperature of .degree.

The particle size of the perovskite type composite oxide is set to 1 μm or less in consideration of the adhesion strength when baked onto an electrolyte.

The oxygen sensor as described above was exposed to a test gas, and the electromotive force at that time was measured from the conductive metal lead terminal 4 connected to the electrode.

Horse gas mixed with 10% oxygen gas at 1 chill was used as the test gas, and the total flow rate was 500 to 500 m4/min.

It was supplied under atmospheric pressure. Note that pure oxygen gas was supplied as a reference gas at a rate of 500 d/min under atmospheric pressure.

The configuration of this oxygen sensor is as follows.

1'o,' (o,), MOloeOl-oao/
Mo, pol'(o, -Hz)MO:Las
-zA'zBo s-δ type composite, oxide electrode (Example 1
) First, La 1-18rzMno consists of an oxygen-deficient non-stoichiometric composition in which 8r of 10-5061I/z is substituted at the La site of perovskite-type composite oxide LaMnO3.
An oxygen sensor operation test was conducted using i-δ (X-11~(L5)) as an electrode. Among them, the sensor electromotive force was The measurement results are shown in Figure 2 A. The solid line in Figure 2 shows the theoretical electromotive force at each oxygen partial pressure to be tested obtained from equation (1). These are the results of measuring the electromotive force of an oxygen sensor with platinum electrodes attached by sputtering.

As is clear from this, the electromotive force of a commonly used oxygen sensor equipped with a platinum electrode follows the theoretical electromotive force at 600°C in the case of the 0eO1 solid electrolyte of this example.
The temperature was above average.

In other words, if the temperature is below 600°C, no theoretical electromotive force can be obtained at all and it becomes unusable. On the other hand, in the oxygen sensor using the Laαssr (12MnOi-δ electrode) of this example, the sensor electromotive force followed the theoretical electromotive force up to approximately 400°C, as shown in Figure 2A. This means that the operating temperature can be lowered to approximately 400°C.

Also, among other perovskite-type oxides, La l -XF3rz C! is effective as a sensor electrode. OO
5-δ(X=!01~0.5), La+-zSr
zNioi-δ(x-0,01~0.05) etc.,
Among these, LacLsSra2COOB-δ.

The electromotive force measurement results for the LaayySrusNiOs-δ electrode are shown in FIGS. 2B and 2O, respectively. In these cases, low-temperature operation up to approximately 400° C. was also possible.

(Example 2) When each electrode shown in Example 1 was used, the response speed on the low temperature side was measured. The results are shown in FIG. This is the temperature at which sensors using conventional platinum electrodes do not operate practically.
This is the response characteristic when the oxygen partial pressure is suddenly changed from 1 atm to 0.1 atm at 00°C. The broken line in the figure represents the oxygen partial pressure of 0.1 atm.

The theoretical electromotive force value at a temperature of 500°C is shown. As a result, the perovskite composite oxide electrode and oxygen sensors A, B, and C used in Example 1 all quickly followed sudden changes in oxygen partial pressure and reached the theoretical electromotive force, but the conventional platinum It was no longer possible to reach the electrodes within the measurement time.

Note that the response time in this embodiment is proportional only to the replacement rate of the simple mixed gas. Therefore, by changing the shape of the device and its dead volume, the response speed can be improved.

(Example 3) The principle of the oxygen concentration battery type sensor adopted in the present invention is entirely based on thermodynamic equilibrium theory, and therefore its electrochemical properties do not exhibit reversible action, which has no practical value. do not have. Therefore, here, we will use the perovskite type composite oxide electrode La us S r ax MnO described in the previous example.
The sensor electromotive force was tracked when the oxygen partial pressure in the test gas was repeatedly varied using s-δ and LaaaSrB12000g-δ. FIG. 4 shows a part of the response waveform at that time. As is clear from the figure, even in a temperature range that is conventionally impractical, the waveform of the reversible potential quickly follows fluctuations in oxygen partial pressure. Therefore, it was found that electrochemical reversible equilibrium occurred quickly in the electrode system, and practical low-temperature operation was possible.

(Example 4) Laa4Sra4MnOi-δ in the same manner as in Example 1.

LILa4Sr aaooos-δ, Laa@Sr,
The electromotive force measurement results of sensors using 2Fe05-δ and LaaiSra4Fe03-δ as electrodes are shown in FIG. 5 as A', p, yv, and yf curves, respectively.

La (14Sr aa MnO5-δ, A′ electrode up to about 350°C, Iaaa4Sra4COOi-δBe
At the electrode, up to about 500℃, LaaaSralIPe05
-δ, E and LaoSra4FeOs-δ, r-1 electrodes, the sensor electromotive force follows the theoretical electromotive force up to about 400°C, and if these electrodes are used as in Example 1, the practical operating temperature can be increased to about 400°C. The temperature can be lowered, especially for LaaaBr
aaOOOOs-δ means that the temperature can be lowered to about 300°C. Further, FIG. 6 shows the response speed corresponding to FIG. 5.

〔Effect of the invention〕

By adopting the above-mentioned configuration, the oxygen cell/cell of the present invention operates reliably with a sufficiently fast response speed at low temperatures of saa to 400°C, i.e., when used with a perovskite-type composite oxide electrode. be able to. and,
By widening the operating temperature range to the lower temperature side, it has become possible to expand the range of applications. Furthermore, since there is no need to use expensive platinum electrodes, the price of the sensor can be significantly reduced.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a tube-type oxygen sensor with one end sealed using a solid electrolyte, and FIGS. 2 to 6 are graphs showing characteristics of the examples. Sub-agents 1) Meifuku agent Ryo Hagiwara - Sub-agent Atsushi Anzai Figure 1 Figure 2 Operating temperature (0C) Figure 3 Figure 4 Response time (min) 200 300 40K) 500 600 7QO
aIn operating temperature (°C) Figure 6 Response time (S)

Claims (3)

[Claims] (1) Formula A_1_- on the surface of oxygen ion conductive solid electrolyte
A low-temperature operating oxygen sensor characterized by having a perovskite-type composite oxide electrode represented by _xA'_xBO_3_-_δ (A is La, A' is an alkaline earth metal, and B is a transition metal). (2) La_1_-_xSr_xMnO_3_-_δ (x=0
．． 1~0.5), La_1_-_xSr_xCoO_3
＿−＿δ(x=0.1~0.5), La_1_−_xS
r_xFeO_3_-_δ(x=0.1~0.5), and La_1_-_xSr_xNiO_3_-_δ(x
2. The low-temperature operating type oxygen sensor according to claim 1, characterized in that the oxygen sensor uses any one oxide in the range of 0.01 to 0.05). (3) The low-temperature operating oxygen sensor according to claim 1 or 2, wherein CeO_2 with 10 to 40 mol% of CaO added is used as the oxygen ion conductive solid electrolyte.