FR2609550A1 - DETECTOR OF THE OXYGEN CONTENT - Google Patents

Abstract

THE INVENTION RELATES TO A DETECTOR OF THE OXYGEN CONTENT. IN THIS DETECTOR, INCLUDING A SOLID ELECTROLYTE 1, ON THE EXTERIOR SURFACES ON WHICH TWO EXTERIOR ELECTRODES 2A, 2B ARE ARRANGED, THE ELECTRODE 2B IS COVERED BY TWO COATINGS OF METALLIC OXIDES INSULATING FROM THE ELECTRICAL POINT OF VIEW, CONSISTING OF A DIFFUSING LAYER, THE FIRST LAYER 3A BEING FORMED BY MEANS OF THE PLASMA SPRAYING PROCESS, TO A THICKNESS OF 10-500 MM, WHILE THE OTHER LAYER 3B IS FORMED BY MEANS OF THE COATING PROCESS CARRIED OUT WITH A SOL-GEL, TO A THICKNESS OF 0.01-20 MM. APPLICATION ESPECIALLY IN DEVICES FOR ADJUSTING THE AIR-FUEL RATIO OF MOTOR VEHICLES.

Description

The present invention relates to a sensor

  to measure the oxygen content and in particular a detector

  extended range of measurement, which can be used for

  of an internal combustion engine, and this in a wide range from a low to a high content. In the stoichiometric detectors of the prior art (which detect the theoretical air-fuel ratio ≥ 1) or in detectors of low levels (detecting the air-fuel ratio only for low levels),

  the oxygen diffusion layer was formed on a thick

  between 50 and 450 μm by means of plasma spraying using a magnesia spinel powder. The

  oxygen diffusion layer had properties such as

  that the porosity rate was between about 5-

  10% and that the mean diameter of the fine pores was equal to

  20-50 nm in accordance with a measurement performed with a

  rosemeter using mercury. However, in order to increase

  the combustion efficiency in an automobile, it is neces-

  to control the air-fuel ratio over a wide range of

  due between the rich side, where the content is high,

  and the poor side, where it is weak.

  On the other hand, in order to measure the oxygen content

  not on the rich side, it is necessary that the resistance vis-à-vis

  the diffusion is greater than that of the layer

  the oxygen diffusion system, of the prior art, describing

te above.

  That is to say, a dense layer of diffusion,

  which allows to control the speed of diffusion of

  cules in unburned gases, is necessary to order

  the air-fuel ratio on the rich side.

  However, the broadcast layer should not

  ter no feature reducing the life of the detector

  because of a pore obstruction in the diffusion layer

  by impurities in the exhaust

  because of the densification or the delay of the moment of ar-

  riveted at the reaction electrode, which reduces the rate of response. Until then it was not possible to satisfy these conditions by modifying only the rate

  of porosity or the thickness of a single layer.

  From these considerations it has been proposed in JP-A-53-13980 and JP-A-53-116896 to form the diaper

  in the form of a two-layer structure pos-

sedant of different densities.

  In the first document above the first layer, which is close to the electrode, is made with a dense structure of alumina of a thickness of 30 μm, by means of the plasma spraying method, and the second layer, which is located on the outer side, is made with a low density structure, to a thickness of 80 pm, using the same method. In the second document, the first layer is made in the form of a sparse structure of a magnesia spinel, over a thickness of 300 m, and the second layer is made with a dense structure on

a thickness of 2 mm.

  In the techniques of the prior art, op

  does not take into account the relationship between thickness or density

  diffusion layer and thermal inertia,

  the conductivity or response characteristic of this cou-

  che. In the first case, a problem lies in the fact that cracks appear in the thick and sparse outer layer, under the effect of cooling and heating cycles. In the second case, it is difficult to form the dense and thick outer layer and furthermore this can not be used since the resistance is too great

  and that the response characteristic is bad.

  The object of the present invention is to provide a detector for measuring the oxygen content and which

  is provided with a layer for the diffusion of the gas,

  denoting the optimal function of gas diffusion.

  The purpose described above can be achieved by

  reading the diffusion layer in the form of a two-layer structure, one of which is formed, so as to be sparingly dense and relatively thick, by a magnesia spinel by means of the conventional plasma spraying method, while the other is an extremely dense, relatively thin layer formed by the coating process carried out

with a sol-gel.

  For optimal conditions, it is preferable

  to form the first layer, which is closer to the electricity

  trode, using a plasma spray, over a thick layer of

  10 to 500 m, and to form the second layer, which is located on the outer side, by means of the coating process performed with a solgel, to a thickness of 0.01 to Pm.

  A detector arranged in this way can

  sea satisfactorily the control function of

  the diffusion rate, based on the low porosity rate

  one of the layers, despite the small thickness of

  this layer, since it is formed by means of the

  coated with a sol-gel. Since this layer is thin, the total thickness of the resistive diffusion layer is kept low and the number of occurrences of thermal deformations due to the difference in the coefficients of thermal expansion of the electrolyte in the solid state and of the resistive diffusion layer is weak, which reduces

the appearance of cracks.

  Other features and advantages of the pre-

  invention will be apparent from the description given hereinafter

  reference is made to the accompanying drawings, in which: - Figure 1 is a cross-sectional view

  a detector to measure the oxygen content in accordance with

according to the present invention;

  FIG. 2 is a diagram representing the

  output characteristic obtained in accordance with the present invention;

  FIG. 3 is a diagram for explaining

  the relationship between the air-fuel ratio and

  gaseous substances in the exhaust gas;

  FIG. 4 is a diagram for explaining

  the principle of control, of the limiting current type, of the ratio of air-fuel

  FIG. 5 is a diagram for explaining

  the relationship between the limit current and the air-

  fuel; FIG. 6 is a cross-sectional view of a detection element having a resistive gas diffusion layer; FIG. 7 is a diagram serving to explain a resistive gas diffusion layer of the prior art;

  FIG. 8 is a diagram for explaining

  quer the resistive layer of diffusion of gases, according to the present invention; FIG. 9 represents an image, obtained by means of the scanning electron microscope, of the surface

  of a fusion injection layer of a magnes spinel

sie; and

  FIG. 10 represents a cross-sectional image

  Versale, taken under a scanning electron microscope,

  very dense SiO2 layer, formed by means of the

performed with a sol-gel.

  A combustion system for an automobile used

  reading an air-fuel ratio detector is a system that detects the state of combustion while measuring the content

  oxygen in the exhaust, and returns the information

  obtained in a circuit controlling the quantity of

  sent and the quantity of air so as to control the mixing ratio of gasoline and air (excess ratio

  A / F air). In the area where the air-fuel ratio is higher than

  than the theoretical value 1 = 1 (A / F = 14.7 / 1), ie on the lean side, almost all the exhaust gases are formed by 02 * and contain only very little CO, HC and H2, which are unburned gases (Figure 3). Here, the O 2 molecules pass through the diffusion layer and are ionized by the catalytic reaction occurring at the outer reaction electrode so that the oxygen ions move from the o side where the exhaust gases are present towards the reaction zone. atmosphere (Figure 4). So, since it is necessary to control the speed of

  the oxygen passing through the resistive diffusion layer, at least

  yen of this layer, this resistive diffusion layer must

  have some appropriate density. The oxygen atoms

  which occur at the reaction electrode, are ionized, as previously indicated, and have an electrical output signal, a limiting current characteristic as shown in FIG. 5, since the oxygen content in the exhaust gas varies, depending on

air-fuel ratio.

  The theoretical relationship (1) indicated below and giving the limit current 4F.Do E.S will be explained below.

I 2 *. (1)

  Ip R.T 2 '' o F: Faraday constant R: gas constant Do: diffusion constant of oxygen molecules O2 T: absolute temperature

  E: diffusion rate in the resistive layer of diffraction

fusion of gases (oxygen)

  : effective diffusion distance in the resistance layer

  diffusion of gases (oxygen) S: surface of the electrode

PO: partial pressure of oxygen.

  2 This relation (1) is well known and the current

  The limit reproduced in Figure 5 can be determined using

  values of these variables. By bringing together some of the constants, we can rewrite the relation (1) in the form of the following relation (2): Ip K -... (2) p -t

  That is, the current limit I is determined

  P mined as a function of t which represents the density of the resistive layer of diffusion of gases and according to the surface

  S of the electrode. The current limit decreases when the surface

  this S of the electrode increases. However, since too small a surface of the electrode affects reaction rate and accuracy, it must be greater than one

  certain value. This is why the current limit I is determined

P

  undermined by the effective diffusion distance e and consequently

  that is larger, that is, the more the layer resists

  If the diffusion of the gases is dense, the more I is weak. That is,

  p that such a detector can be used effectively for detection and control in the area on the rich side. Since, on the rich side, the content

  in 02 is low and the unburned gases CO, HC and H2 are pre-

  in large quantities in the exhaust gas,

  these unburned gases pass through the diffusion layer.

  On the contrary, the oxygen ions cross the elec-

  solid state trolyte from the atmosphere in a direction opposite to that observed on the lean side, and react with the unburned gases on the outer electrode. However, since the particles constituting the unburned gases are significantly smaller than the O 2 molecules, it is not possible to control the amount of such particles passing through the diffusion layer by means of a diffusion layer. prior art and therefore the control on the rich side becomes impossible. That is to say that, to achieve control on the rich side, it is necessary to have a dense diffusion layer, which allows to control

  the diffusion rate of the unburned desgaz particles.

  However, the pores in the layer of dif-

  the merger should not be blocked, due to the

2, c9550

  because of impurities in the exhaust gases.

  ment. The following is a description of the basic arrangement of a

  embodiment of the detector according to the present invention.

  and the operation of the oxygen diffusion layer. The first layer on the outer electrode is a layer formed by a magnesia spinel formed

  by plasma spraying. It is important that the first

  first layer is less dense than the second layer of

  formed by the coating process performed with a sol-gel. In particular, the density of the first layer is in close connection with the catalytic reaction

  electrode and must have an appropriate value to

  both obtaining a good response characteristic as a detector. As a provision in this sense and although this is not optimal, it is preferable that the porosity level is about 5-10% and that the average pore diameter, measured using a porosimeter using

  mercury, ie 30-40 nm. In addition, a preferred thickness

  ble of the first layer is 10-500 um. Since in a too thick layer it is possible that

  cracks due to the difference between the coefficients of

  temperature of this layer and the electrolyte in the state

  the thickness of the layer is more preferably

equal to 20-200 um.

  The second layer is a ceramic layer

  by the coating process carried out with a sol-gel, and which is very dense. This layer is provided in particular so as to limit the diffusion of fine particles of Co, HC and H2, which are unburned gases, and to regulate the flow rate so as to carry out the detection in the zone corresponding to the rich side . The thickness of the film is between 0.01-20 μm and preferably between 0.5-5 μm, since the diffusion of the gas becomes difficult, if

Thickness is too big.

  An example will be explained in more detail below. Figure 6 is a cross-sectional view of a

  limit current type detector, used to measure the

  oxygen in accordance with the present invention and which is used to carry out the control of an automobile. That is, element 1 itself is a solid state electrolyte consisting of zirconium oxide (hereinafter referred to as

  abbreviated as ZrO2) partially stabilized by an oxy-

  of yttrium (hereinafter abbreviated Y202) and

  platinum (hereinafter abbreviated as Pt) and

  on the outer and inner surfaces of the element to form reaction electrodes 2a and 2b. Since the outer electrode 2b is associated with the surface of the electrode influencing the characteristic represented by the above-mentioned theoretical relationship (1), it is formed with high accuracy by the use of a

  mask placed on the plating of Pt.

  lead electrode 4 connected to the outer electrode,

  where the plating is deposited in Pt, using a

  mask, and cover it with a thin insulating layer

  re to completely eliminate any reaction with the exhaust. With regard to the resistive layers

  3a, 3b of diffusion of gases, which are important for the pre-

  In this invention, at least the first layer 3a is formed by injection of a molten magnesia spinel by means of

  plasma spraying process.

  FIG. 9 represents an image obtained half way

  scanning croscope, from the surface of this layer. It can be seen that the powder particles in the semi-molten state are attached to this layer. What is important is that the gas has diffused not only through spaces between the accumulated powder particles, but also through

  thin cracks (less than 0.1-0.2 μm in width).

  This is a characteristic of the fusion injection of a

  Ceramic. Using a magnesia spinel (MgO.A1203), the average particle diameter of which is about 15 m, for the melt injection powder, the melt injection is carried out by adding a quantity of about 10 g per minute. Since the powder is fine and because of its nature, it is extremely hygrometric and able to be influenced by the humidity prevailing in the room. This is why it is difficult to have a stable delivery of the powder. When the quantity delivered varies, the speed of

  growth of the accumulated film on the reaction electrode

  It also varies, which strongly influences the density of the coating. As a result, the limiting current characteristic varies and it becomes impossible to obtain a stable oxygen content measuring detector. For this reason, it is necessary to supply the powder for

  the injection by fusion, in a dry state always constant.

  In a manufacturing facility conforming to this

  In order to solve this problem, an embodiment

  The heating device is connected to the device delivering the powder, this heating device drying the powder to a

  temperature between 80 and 100 C. The projection is carried out

  cut with an energy of 800 A, 50 V, by using a gas formed of a mixture of argon (Ar) and nitrogen (N2) as a plasma gas. During spraying, the element is rotated at a speed of about 600 rpm and the sputtering is performed by injecting a magnesia spinel powder in the semi-molten state under the conditions

  indicated above, by means of a plasma projection gun

  ma at a speed of 1000 m / min relative to the rotating element, so that the coating, which constitutes the first layer,

  is formed to a thickness of about 80 μm. With regard to

  the density of this coating, the measured porosity rate is equal to 510%, the average pore diameter, measured

  using a porosimeter using mercury is equal to

  viron 30 nm. FIG. 9 represents an image obtained half way

  scanning electronic croscope, of the surface of the first

down again.

  Then the second layer, which is a very dense ceramic layer (hereinafter abbreviated as SiO 2) having a thickness of about 1 μm, is formed on the first layer by means of the coating process carried out with a sol-gel. A silica sol is used to effect the coating by the coating process performed with a solgel. The element, on which plasma spraying was performed, is immersed in the silica sol at room temperature and sintered for a period of minutes at a temperature of 700 ° C. This process is repeated twice. to obtain a thickness of about 1 μm. This second layer is very dense. FIG. 10 shows an image, obtained by means of the scanning electron microscope, of a

cross section.

  In this way, a detection element is

  which has a very dense layer capable of controlling the diffusion of gases as the outermost layer,

  and a resistive gas diffusion layer, disposed

  below the previous one and through which the gases can

  broadcast at a reasonable speed and which is useful for

  increase the reaction rate of the electrode in Pt.

  1 represents a detector for measuring the oxygen content.

  gene and manufactured using this element of detection.

  detection unit 1 is attached to a candle body 5. It comprises

  an outer sheath 7 serving to protect it at the end of the body of the candle, and further a device

  6 for heating the element to a temperature of

  between 600 and 700 C in order to reduce the

  elemental element, namely zirconia, an electrolyte, inside the element. In addition, conductor wires 8 are arranged to take electrical signals on the outer reaction electrode 2b, the inner electrode 2a and the heater 6, and apply a voltage to these elements. By mounting the detector used to measure the oxygen content thus produced, on the exhaust pipe of an automobile, by circulating a current

  in the heating device so as to heat

  electrolyte in the solid state of the body of the element

  700 C and by applying a voltage to this element, it has been found that the air-fuel ratio can be detected under

  the form of an output signal which has a linear variation

  in particular on the rich side up to A = 0.6, as shown in Figure 2 by a solid line representing the output characteristic of the detector for measuring the oxygen content. On the contrary, by using a diffusion film of the prior art, it was possible

  to perform detection on the rich side only until

  that at X = 0.8, and another disadvantage was that the output signal is saturated in the area on the side

  rich, where the content is even higher. This invention

  This remarkably overcomes these disadvantages. In trans-

  placing this expression by that of the control characteristic, for the usual circulation (40-60 km / h) in flat terrain, the control is carried out in the zone situated on the side

  poor and the characteristic leads to an economic duct.

  On the contrary, in the case of driving on a road

  aunt between mountains, control is carried out in the area on the rich side, the output signal is increased

  and the control characteristic is improved in its

  appears. In addition, in a conventional engine in which a

  three-dimensional control (control in conjunction with CO, HC and NO X

  in exhaust gases) is achieved by means of

  an oxygen sensor (stoichiometric detector, since it can happen that the air-fuel mixture is rich during a cold start, a strong acceleration,

  etc. so that k is increased to about 0.6, the detection

  according to the present invention can be used not only for the lean combustion engine (engine for

  control of a poor combustion), but also for

  the air-fuel ratio over a wide range in the conventional engine, and therefore the present invention

  reduce fuel costs and improve the characteris-

  In addition, it has the side effect of being useful for increasing safety and so on.

  The present invention relates to a sensor

  to measure the oxygen content and used for

  of air-fuel ratio and is characterized in particular by

  the resistive layer of diffusion of gases, which

  detection. The essential point of the present invention lies in the two-layer structure including a layer formed by the plasma spraying method, and a

  very dense layer formed by means of the coating process

  cut with a sol-gel. Although in this embodiment

  an example has been shown in which a

  dre a spinel of magnesia to carry out the spraying

  plasma, there is no particular restriction

  the type of the powder. The present invention can produce its effect if the coating obtained after spraying is formed such that, for example, the porosity ratio is 2-20% and the average pore diameter, as measured in FIG.

  average of a porosimeter using mercury, is equal to 20-

  nm. That is, the present invention is useful even though the powder is a single powder of a ceramic. such as alumina, magnesia, silica, titanium dioxide, zirconia, calcium oxide, etc. or a composite powder formed of such ceramics, and it is possible to use a powder whose particles have any diameter . In addition, with regard to the very dense layer, which constitutes the second

  layer, although it has been described in this form of

  the coating consisting of silica deposited at least once

  yen of the coating process performed with a sol-gel, it is

  obvious that the same effect can be achieved by using,

  material for this coating, alumina, zirconia, titanium dioxide, calcium oxide, magnesia, etc. Furthermore, the same effect can be obtained even if the very dense layer described above is first formed on the electrode by means of the coating process carried out with a sol-gel and the thin layer indicated above is formed on the first layer by the method

plasma spray.

  As explained above, in accordance with

  In accordance with the present invention, a detector can be

  oxygen content, which has excellent resis-

  because of the low thickness of the

  and has a good response characteristic.

Claims (5)

  A limit current type oxygen content detector comprising: a solid state electrolyte element (1) consisting of conductive metal oxides for the oxygen ions; thin porous films (2a, 2b) disposed on the front and rear surfaces   said element (1) and between which a voltage is applied   in such a way that the oxygen ions existing in the   mosphere o said element (1) is located, diffuse inside said element (1) in order to allow the measurement of the   oxygen content by determining the corresponding current limit   sputtering at the oxygen ion content; - a resistive gas diffusion layer (3) covering the electrode (2b) located on the front surface of said element (1) and consisting of electrically insulating porous metal oxides; characterized in that said resistive layer   (3) gas diffusion has a multilayer structure   tiples including at least two layers, one of which (3a) is sparse and is formed by the spraying process   through the use of isomeric metal oxides   electrically and suitably sized, and the other (3a) is very dense and formed   by means of the coating process carried out with a sol-gel,   the use of insulating metal oxides from the point of electric view.   2. Detector of the oxygen content according to the   claim 1, characterized in that said layer has a low   (3a) formed by the plasma spraying process   that has a thickness of 10-500 m and that said layer   very dense (3b) formed by means of the coating process   with a sol-gel has a thickness of 0.1-20 rm.   3. Oxygen oxygen detector   Claims 1 or 2, characterized in that said   thin layer (3a) is formed first on the surface of   the outer electrode (2b) by means of the spraying process   and said very dense layer (3b) is subsequently formed on the outer surface of the first layer at   medium of the coating process performed with a sol-gel.   4. Detector of the oxygen content according to one   Claims 1 or 2, characterized in that it is   so that the characteristic representing variations   X air-fuel ratio in relation to the applied voltage   said electrodes, in a state where said electrolyte (1) in the solid state is heated to a temperature of 600 C, is a predetermined linear proportional function in   an area above A = 0.6.   5. Detector of the oxygen content according to one   Claims 1 or 2, characterized in that said layer   Deu dense (3a) and said dense layer (3b) are formed by   insulating metal oxide layers from the point of view of   formed by at least one of the materials: alumina,   silica, titanium dioxide, zirconia and calcium oxide.