GB2200460A

GB2200460A - Solid electrolyte oxygen concentration detector

Info

Publication number: GB2200460A
Application number: GB08800305A
Authority: GB
Inventors: Katsuyoshi Terakado; Sadayasu Ueno; Norio Ichikawa
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1987-01-09
Filing date: 1988-01-07
Publication date: 1988-08-03
Also published as: FR2609550A1; KR880009273A; CN88100069A; GB8800305D0; DE3800370A1; JPS63171356A

Abstract

A limit current type solid electrolyte oxygen concentration detector includes an electrically insulating metal oxide multilayer (3a, 3b) gas diffusion resistive layer covering the outer surface of porous outer electrode (2). The layer (3a) e.g. of magnesia spinel and thickness 10-500 mu m, is loose and is formed by the plasma spray method. Layer (3b), e.g. of silica ceramic and thickness 0.01-20 mu m, on the other hand is very dense and is formed by the sol-gel method. Other materials for the layers are alumina, zirconia, titania, calcia and magnesia. The layers (3a) and (3b) may be interchanged. <IMAGE>

Description

OXYGEN CONCENTRATION DETECTOR BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a detector for measuring oxygen concentration and in particular to a wide-ranged detector used for the control of an internal combustion engine, which is usable for a wide range-from lean to rich.

DESCRIPTION OF THE PRIOR ART In prior art stoichiometric sensors (detecting the theoretical air-fuel ratio A=1) or lean sensors (detecting the air-fuel ratio only for low concentrations).

the oxygen diffusion layer was formed at a thickness between 50 and 450 pm by the plasma spray using magnesia spinel powder. The oxygen diffusion layer had properties that the porosity ratio is about 5 - 10% and that the average diameter of fine pores was about 20 - 50 nm according to a measurement using a porosimeter used mercury. However, in order to increase the combustion efficiency for an automobile, it is necessary to control the air-fuel ratio over a wide range from the rich side, where the concentration is high, to the lean side, where it is low.

On the other hand, in order to measure the oxygen concentration on the rich side, the resistance against diffusion should be greater than that of the prior art oxygen diffusion resistive layer described above.

That is, a dense diffusion layer, which can control the diffusion speed of particles in unburned gas, is necessary for controlling the air-fuel ratio on the rich side.

However the diffusion layer must have no factors of shortening the life of the detector due to stoppage of pores in the diffusion layer by impurities in exhaust gas because of the densification or the retardation of the arrival time to the reaction electrode, which lowers the response speed. Heretofore it was not possible to satisfy these conditions by varying only the porosity ratio or the thickness of a single layer.

In view of this point it has been proposed in JP-A-53-13980 and JP-A-53-116896 to form the diffusion layer in a two-layered structure having different densities.

In the former the first layer, which is close to the electrode, is made densely of alumina in a thickness of 30 pm by the plasma spray method and the second layer, which is on the outer side, is made loosely in a thickness of 80 pm by the same method. In the latter the first layer is made loosely of magnesia spinel in a thickness of 300 pm and the second layer is made densely in 2 mm.

In the prior art techniques no attention is paid to the relation between the thickness or the density of the diffusion layer and the heat resistivity, the productivity or the response characteristics thereof. The former has a problem that cracks are produced in the outer loose and thick layer by cooling and heating cycles. For the latter it is difficult to form the outer dense and thick layer and further it is not practical, because the resistance is too great and the response characteristics are bad.

SUMMARY OF THE INVENTION The object of this invention is to provide a detector for measuring the oxygen concentration provided with a gas diffusion layer having the optimum gas diffusion function.

The object described above can be achieved by forming the diffusion layer in a two-layered structure, one of the two layers being made loosely and relatively thickly of magnesia spinel by the conventional plasma spray method, the other being extremely dense and relatively thin, formed by the sol-gel coating method.

To the optimum it is preferable to form the first layer, which is closer to the electrode, by the plasma spray to a thickness of 10 to 500 pm and the second layer, which is on the outer side, by the sol-gel coating method to a thickness of 0.01 to 20 pm.

A detector constructed in such a way can perform, satisfactorily the function of controlling the diffusion speed owing to the small porosity ratio of one of the layers, inspite of the. small thickness thereof, because it is formed by the sol-gel coating method.

Since this layer is thin1 the whole thickness of the diffusion resistive layer is kept to be small and the number of thermal strain productions due to the difference in the thermal expansion coefficient between the solid state electrolyte and the diffusion resistive layer is small, which reduces generations of cracks.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a detector for measuring the oxygen concentration according to this invention; Fig. 2 is a scheme representing output characteristics obtained according to this invention; Fig. 3 is a scheme for explaining the relation between the air-fuel ratio and gaseous components in the exhaust gas; Fig. 4 is a scheme for explaining the principle of the limit current type control of the air-fuel ratio; Fig. 5 is a scheme for explaining the relation between the limit current and the air-fuel ratio; Fig. 6 is a cross-sectional view of a detecting element having a gas diffusion resistive layer; Fig. 7 is a scheme for explaining a prior art gas diffusion resistive layer; Fig. 8 is a scheme for explaining the gas diffusion resistive layer according to this invention;; Fig. 9 is an SEM image of the surface of a magnesia spinel fusion injection layer; and Fig. 10 is a cross-sectional SEM image of an SiO2 high density layer formed by the sol-gel coating method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A combustion system for an automobile using an air-fuel ratio sensor is a system, which detects the combustion state, while measuring the oxygen concentration in the exhaust gas and feeds back information thus obtained to a circuit controlling the supplied amount of gasoline and that of air in order to control the mixing ratio of gasoline and air (air excess ratio A/F). In the region, where the air-fuel ratio is greater than the theoretical value B=1 (A/F = 14.7/lj, i.e. on the lean side, almost all the part of the exhaust gas is 2 and CO, HC and H2, which are unburned gases, are contained extremely slightly (Fig. 3).Here molecules of O2 pass through the diffusion layer and are ionized by the catalytic reaction at the outer reaction electrode so that oxygen ions move from the exhaust gas side to the atmospheric side (Fig. 4). At this time, since it is necessary to control the speed of oxygen passing through the diffusion resistive layer by means thereof, the diffusion resistive layer should have a certain suitable density. The oxygen atmos arriving at the reaction electrode are ionized, as stated previously, and show limit current characteristics as electric output, as indicated in Fig. 5, because the oxygen concentration in the exhaust gas varies, depending on the air-fuel ratio.

Now the following theoretical equation (1) indicating the limit current will be explained.

where F: Faraday constant R: gas constant D02: diffusion constant of oxygen molecules T: absolute temperature E: diffusion rate in the gas (oxygen) diffusion resistive layer effective diffusion distance in the gas (oxygen) diffusion resistive layer S: area of the electrode P02: partial pressure of oxygen This Eq.(l) is well known and the limit current indicated in Fig. 5 can be determined by using values of these variables. Putting some of the constants together, Eq. (1) can be transformed into the following Eq. (2): I ~ K - g ... (2) p That is, the limit current I is determined, p depending on Q representing the density of the gas diffusion resistive layer and the electrode area S.The limit current becomes smaller with decreasing electrode area S. However, since a too small electrode area has influences on the reaction speed and the precision, it should~be greater than a certain value. Therefore the limit current I is determined by the effective diffusion p distance Q and thus the greater Q is, i.e. the more dense the gas diffusion resistive layer is, the smaller I is.

p That is, such a detector can be efficiently utilized for the detection and the control at the rich side region.

Since on the rich side the 2 content is small and unburned gases CO, HC and H2 are abundant in the exhaust gas, these unburned gases pass through the diffusion layer.

On the contrary, oxygen ions pass through the solid state electrolyte from the atmospheric side in the direction, which is opposite to that observed on the lean side, and react with unburned gases on the outer electrode.

However, since particles constituting the unburned gas component are much smaller than 0, 2 molecules, it is not possible to control the amount thereof passing through the diffusion layer by means of a prior art diffusion layer and thus the control on the rich side becomes impossible. That is, in order to perform the control on the rich side, a dense diffusion layer is required, which can control the diffusion speed of unburned gas particles.

However no stoppage due to impurities in the exhaust gas must be produced in pores in the diffusion layer by the densification.

The basic construction of an embodiment of the detector according to this invention and the operation of the oxygen diffusion layer will be described below. The first layer on the outer electrode is a magnesia spinel layer formed by the plasma spray. It is important that this first layer is looser than the second diffusion layer formed by the sol-gel coating method. In particular the density of the first layer has a close relation with the catalytic reaction on the electrode and it must have a value suitable for obtaining good response characteristics as a detector. As a measure therefor, although it is not optimum, it is preferable that the porosity ratio is about 5 - 10% and that the average diameter of pores measured by using a porosimeter used mercury is 30 - 40 nm.Further a preferable thickness of the first layer is 10 - 500 pm. Since an inattentively thick layer is apt to give rise to cracks due to the difference in the thermal expansion coefficient from that of the solid state electrolyte, it is more preferably 20 - 200 pm thick.

The second layer is a ceramic layer formed by the sol-gel coating method, which is very dense. This layer is disposed in particular for the purpose of restricting the diffusion of fine particles of CO, HC and H2, which are unburned gases, and controlling the speed thereof, in order to effect the detection at the rich side region. The film thickness is 0.01 - 20 pm, preferably 0.5 - 5 pm, because the gas diffusion becomes difficult, if it is too great.

An example will be explained more in detail.

Fig. 6 is a cross-sectional view of a limit current type detector for measuring the oxygen concentration according to this invention, which is used for the control of an automobile. That is, the element itself 1 is solid state electrolyte made of zirconium oxide (hereinbelow abbreviated to ZrO2) partially stabilized by yttrium oxide (hereinbelow abbreviated to Y2O2).and platinum (hereinbelow abbreviated to Pt) is plated on the inner and outer surfaces of the element as reaction electrodes 2a and 2b.

Since the outer electrode 2b relates to the electrode area influencing the characteristics represented by the theoretical equation (1) stated previously, it is formed with a high precision by using a mask at the Pt plating.

Further a lead electrode 4 connected with the outer electrode is formed at the same time by the Pt plating with a mask and covered with a thin glass insulating layer for excluding perfectly the reaction with the exhaust gas. Concerning the gas diffusion resistive layers 3a, 3b, which are important for this invention, at first the first layer 3a is formed by injecting fused magnesia spinel by the plasma spray method.

Fig. 9 shows an SEM image of the surface thereof. It can be seen there that powder particles in the semi-fused state are attached thereto. What is important is that gas diffused not only through gaps between accumulated powder particles but also through fine cracks (narrower than 0.1 - 0.2 pom). This is a feature of the ceramic fusion injection. Using magnesia spinel (MgO A1203), whose average particle diameter is about 15 pm, for the fusoin injection powder, the fusion injection is effected while supplying an amount of about 10 g per minute. Since the powder is fine and because of the nature thereof, it is highly hygrometric and apt to be influenced by the humidity in the room. Therefore it is difficult to supply stably the powder. When the supplied amount varies, the growth speed of the film accumulated on the reaction electrode varies also, which influences strongly the density of the coat. Consequently the limit current characteristics vary and thus it becomes impossible to provide a stable oxygen concentration measuring detector. For this reason it is necessary to supply the powder for the fusion injection in an always constant dry state. In a production installation according to this embodiment, in order to resolve this problem, a preheating device is attached to the powder supplying equipment, which dries the powder at a temperature between 80 and 1000C. The spray is effected with a power of 800 A, 50 V while using a mixture gas of argon (Ar) and nitrogen (N2) as the plasma gas.In the spray state the element is rotated with a speed of about 600 rpm and the spray is effected by injecting semi-fused magnesia spinel powder under the conditions described above by means of a plasma spray gun with a speed of 1000 m/min relative to the rotating element so that the coat, which is the first layer, is formed to a thickness of about 80 pm. Concerning the density of this coat, the measured porosity ratio is 5 - 10% and the average diameter of pores measured by means of a porosimeter used mercury is about 30 nm. Fig. 9 shows an SEM image of the surface of the first layer.

Next the second layer, which is a high density layer, is a ceramic (hereinbelow abbreviated to SiO2) layer about 1 pm thick formed on the first layer by the sol-gel coating method. Silica sol is used for the coating by the sol-gel coating method. The element, on which the plasma spray has been effected, is dipped in the silica sol at the-room temperature and sintered during a period of time of 30 minutes at a temperature of 7000C. This process is repeated 2 times so as to obtain a thickness of about 1 pm. This second layer is very dense. Fig. 10 shows an SEM image of a crosssection obtained by using an electron microscope.

A detector element is accomplished in this way, which has a high density layer capable of controlling the gas diffusion as the outermost layer and the gas diffusion resistive layer disposed thereunder, through which gas can diffuse with a reasonable speed and which is useful for increasing the reaction speed with the Pt electrode.

Fig. 1 shows a detector for measuring the oxygen concentration fabricated by using this detection element 1. The detection element 1 is secured to a plug body 5.

It is provided with an external sheath 7 for protecting the detection element at the extremity of the plug body and in addition a heater 6 for heating the element to a temperature between 600 and 7000C so as to make the material of the element, i.e. zirconia, electrolyte, within the element. Further lead lines 8 are disposed for taking out electric signals from and applying voltage to the outer reaction electrode 2b, the inner electrode 2a and the heater 6.By mounting the detector for measuring the oxygen concentration thus fabricated to the exhaust pipe of an automoble, making electric current flow through the heater so as to heat the solid state electrolyte of the element body to about 7000C, and applying voltage to the element, it was verified that the air-fuel ratio can be detected in the form of an output, which is linear in particular on the rich side up to A=0.6, as indicated by a full line in Fig. 2 representing output characteristics of the detector for measuring the oxygen concentration. On the contrary, by using a prior art diffusion film, it was possible to detect it on the rich side only up to A=0.8 and there was another inconvenience that the output is saturated at the rich side region, where the concentration is still higher.

These inconveniences are remarkably improved by this invention. Changing this expression to that for driving characteristics, for the usual drive (40 - 60 km/h) in flatland the control is effected in the lean side region and the characteristics lead to an economical drive. On the contrary for the ascending drive on a road between mountains the control is effected in the rich side region, the output is increased and the driving characteristics are improved as a whole.Furthermore, in a current engine, in which a 3-dimensional control (control with respect to CO, HC and NO in the exhaust gas) is effected x by using an oxygen sensor (stoichiometric sensor), since it can happen that the air-fuel mixture is rich at a cold start, a strong acceleration, etc. so that X is increased up to about 0.6, the detector according to this invention can be utilized not only for the lean burn engine (engine for lean combustion control) but also for the wide range air-fuel ratio control in the current engine and thus this invention has effects for the descrease in the fuel cost and improvement in the driving characteristics and further it has a subsidiary effect that it is useful for the increase in the security, etc.

This invention relates to a detector for measuring the oxygen concentration used for the air-fuel ratio control and is characterized particularly in the gas diffusion resistive layer of the detector element The point of this invention consists in the two-layered structure consisting of a layer formed by the plasma spray and a high density layer formed by the sol-gel coating method. Although in the embodiment an example, in which magnesia spinel powder was used for the plasma spray, was shown, there is no special restriction concerning the kind of the powder. The effect of this invention can be exhibited, if the coat after the spray is so formed that e.g. the porosity ratio is 2 - 20% and the average diameter of pores measured by means of a porosimeter used mercury is 20 - 50 nm.That is, this invention is useful, even if the powder is a simple substance power of ceramics such as alumina, magnesia, silica, titania, zirconia, calcia, etc. or composite powder thereof, and any particle diameter of the powder may be used. In addition, concerning the high density layer, which is the second layer, although the coat made of silica by the sol-gel coating method was described in this embodiment, it is a matter of course that the same effect can be obtained by using alumina, zirconia, titania, calcia, magnesia, etc. as the material thereof.

Furthermore the same effect can be obtained, even if the high density layer described above is formed at first on the electrode by the sol-gel coating method and after that the loose layer stated above is formed thereon by the plasma spray method.

As explained above, according to this invenion, an oxygen concentration detector can be obtained, which is excellent in the heat resistivity owing to the small thickness of the layers and has good response characteristics.

Claims

Claims:

1. A limit current type oxygen concentration detector comprising: a solid state electrolyte element made of oxygen ion conductive metal oxides; porous thin film electrodes disposed on the front and rear surfaces of said element, a voltage being applied therebetween so that oxygen ions existing in the atmosphere, where said element is located, are diffused within said element for the purpose of measuring the oxygen concentration by obtaining the limit current corresponding to the concentration of the oxygen ions; and a gas diffusion resistive layer covering the front surface electrode of said element and made of porous electrically isolating metal oxides; wherein said gas diffusion resistive layer has a multilayered structure including at least two layers, one of them being loose, which is formed by the plasma spray method by using electrically insulating metal oxides having a suitable granulometry; the other being very dense, which is formed by the sol-gel coating method by using electrically insulating metal oxides.

2. An oxygen concentration detector according to Claim 1, wherein said loose layer formed by the plasma spray method is 10 - 500 -pm thick and said very dense layer formed by the sol-gel coating method is 0.01 - 20 pm thick.

3. An oxygen concentration detector according to one of Claims 1 and 2, wherein said loose layer is formed at first on the surface of the outer electrode by the plasma spray method and thereafter said very dense layer is formed on the outer surface thereof by the sol-gel coating method.

4. An oxygen concentration detector according to one of Claims 1 and 2, wherein it is so adjusted that characteristics representing variations of the air-fuel ratio X with respect to the voltage applied to said electrodes, in a state where said solid state electrolyte is heated to a temperature of 7000C, show a predetermined linear proportional property in a region above X = 0.6.

5. An oxygen concentration detector according to one of Claims 1 and 2, wherein said loose and dense layers are electrically insulating metal oxide layer made of at least one of alumina, magnesia, silica, titania, zirconia and calcia.

6. An oxygen concentration detector substantially as herein described with reference to, and as shown in, Figures 1-6 and 8-10 of the accompanying drawings.