Sensor for measurement of air/fuel ratio

Abstract: An air/fuel ratio (A/F) sensor for detecting the A/F over a wide range covering a lean region and a rich region which are partitioned by the stoichiometric A/F comprises a solid electrolyte (1) made of an oxygen ion conductive metal oxide, first and second electrodes (2a, 2b) sandwiching the solid electrolyte and being operable by a predetermined voltage applied thereacross to ionize oxygen near the solid electrolyte (1) and diffuse oxygen ions into the solid electrolyte (1), and a gas diffusion layer (3; 3A) in the form of a porous sintered layer covering the second electrode (2b) and made of electrically insulating metal oxide fine particles having a mean particulate size of 1 µm or less so that oxygen or molecules of other gas components in the exhaust gas can move through pores of the gas diffusion layer (3; 3A).

Description: BACKGROUND OF THE INVENTION This invention relates to sensors for measurement of air/fuel ratio and more particularly to an air/fuel ratio measuring sensor which can detect the air/fuel ratio over a wide range covering low air/fuel ratio (rich region) and high air/fuel ratio (lean region) and the stoichiometric air/fuel ratio in between. The sensor is suitable for use in a fuel controller of an automobile engine. In a fuel control system for an automobile using an air/fuel ratio sensor, the actual air/fuel ratio in the mixture gas is measured by detecting the concentration of oxygen and unburnt gases (CO, H 2 , HC) in the exhaust gas, and information indicative of the air/fuel ratio (hereinafter abbreviated as A/F) is fed back to a fuel flow control circuit which in turn controls the fuel flow such that the mixture gas maintains a target A/F. The A/F at which gasoline fuel in mixture gas is completely burnt is known as the stoichiometric A/F which is 14.7. Presently available sensors are stoichiometric sensors (so-called O 2 sensor) for detecting the stoichiometric A/F point or lean sensors for continuously measuring the A/F over a range covering the stoichiometric A/F and the lean region. Automobiles are operated under widely varying ambient and load conditions, and therefore the A/F must be controlled adaptively over a wide range in accordance with running condition and load. For example, the A/F has to be controlled to be lean for light load, to be rich for heavy load and low temperature and to have the stoichiometric valve if a 3-way catalyzer is activated. However, technical difficulties are encountered in manufacture of a sensor which can measure the A/F in the rich region because the structure of a gas diffusion layer of the sensor causes a bottleneck. The gas diffusion layer structure plays an important role in measurement of the rich A/F as will be described below with reference to Figs. 1 to 3. Fig. 3 shows the relation between A/F value and concentration of components in the exhaust gas. Excepting the five components shown in Fig. 3, nitrogen is predominent in the exhaust gas. In the lean region, nitrogen and oxygen are predominant as components in the exhaust gas and when compared with these components, carbon monoxide, hydrogen and hydrocarbon are contained by a very small amount in the exhaust gas. Contrarily, in the rich region, the amount of oxygen contained in the exhaust gas is small but carbon monoxide, hydrogen and hydrocarbon which stand for incompletely burnt components are predominant. Fig. 1 shows a fragmentary section of an A/F sensor. In Fig. 1, 1 designates a solid electrolyte made of, for example, zirconia through which oxygen ions can permeate, 2a and 2b porous reaction electrodes across which a predetermined voltage V is applied, and 3 a porous gas diffusion layer made of, for example, ceramic. In the case of the lean A/F, the oxygen concentration is higher on the exhaust gas side than on the reaction electrode 2b and consequently oxygen molecules move through the gas diffusion layer 3 and reach then reaction electrode 2b at which they are ionized. Oxygen ions O 2 - than pass through the solid electrolyte 1 to reach the electrode 2a. The amount of moving oxygen ions is detected to provide a pump current value Ip which is increased as the applied voltage increases. The gas diffusion layer 3 is so configured as to limit the diffusion speed of oxygen molecules passing through the layer 3. The diffusion speed depends on the size and number of pores present in the gas diffusion layer 3. When the diffusion speed of oxygen is properly limited, characteristics can be obtained wherein the current value Ip* is saturated over a certain voltage range as shown in Fig. 2. The saturated current value is called a diffusion limiting current value. The limiting current value changes with the A/F value. Accordingly, the A/F can be determined by detecting the limiting current Ip while keeping the voltage V constant. In the case of the rich A/F, carbon monoxide, hydrogen and hydrocarbon which stand for incompletely burnt components in the exhaust gas diffuse through the gas diffusion layer 3. Oxygen in atmosphere, on the other hand, is ionized while passing through the electrode 2a, and the oxygen ions move through the solid electrolyte 1 and combine with the carbon monoxide, hydrogen and hydrocarbon. Since the size of the molecule of the incompletely burnt components is far smaller than that of an oxygen molecule, the gas diffusion layer having ability to properly limit the diffusion speed of oxygen molecule can not limit the diffusion speeds of the incompletely burnt components, with the result that excessive amounts of incompletely burnt components migrate to the electrode 2b. On the other hand, the diffusion speed of oxygen prevailing on the atmosphere side is limited by a flow path resistance between the outer cap housing the sensor and the solid electrolyte. Further, because of low temperatures near the electrode 2b, oxidation (combustion) reaction of incompletely burnt components with ionized oxygen (O 2 - ) at the electrode 2b is insufficient, thus preventing complete combustion. Accordingly, limiting current characteristics for the rich A/F can not be obtained and the rich A/F can not be detected. In order to realize detection of the rich A/F, the flow of excessive amounts of incompletely burnt components must be suppressed to decrease absolute value of the limiting current Ip*. To this end, a gas diffusion layer capable of limiting the diffusion speed of the incompletely burnt components is needed. As well known in the art, the diffusion limiting current Ip* is expressed by the following theoretical formula (1): where F: Faraday constant R : gas constant T: absolute temperature of gas S : equivalent sectional area of pores in the gas diffusion layer ℓ : thickness of the gas diffusion layer αi: conversion constant Di: diffusion coefficient of molecules Pi: partial pressure of gas The values of the limiting currents shown in Fig. 2 are determined by inserting values of constants and variables into equation (1). By unifying the constants, equation (1) can be reduced to equation (2), Ip* = K · S ℓ where K : constant. As is clear from equation (2), the limiting current Ip* is determined by the equivalent sectional area S of pores representative of the denseness of the gas diffusion layer and the thickness ℓ of the gas diffusion layer. As the thickness ℓ of the gas diffusion layer increases, the limiting current Ip* decreases. But an excessively large thickness affects response speed and durability and therefore there exists an upper limit of the thickness. Accordingly, the limiting current Ip* essentially depends on the equivalent sectional area S of pores of the gas diffusion layer. The smaller the equivalent sectional area S, that is, the denser the gas diffusion layer, the smaller the limiting current Ip* becomes to meet effective detection control in the rich region. A conventional stoichiometric sensor (O 2 sensor) is disclosed in, for example, JP-A-13980 published on February 8, 1978 which corresponds to a Japanese patent application by Suzuki et al filed July 23, 1976. The sensor in this literature has a gas diffusion layer prepared through plasma spraying process. The gas diffusion layer has a two-layer structure in which two layers have different thicknesses, with the outer layer close to exhaust gas having a thickness of 80 µm and larger pores and the inner layer close to solid electrolyte having a thickness of 30 µm and smaller pores. A lean sensor is disclosed in, for example, U.S. Patent No. 4,356,065 to Dietz issued October 26, 1982. The patented sensor has a gas diffusion layer of a two-layer structure prepared through plasma spraying process. The thickness of a first layer having larger pores is 300 µm and the thickness of a second inner layer having smaller pores is 2 nm. The gas diffusion layers of the two known sensors are prepared through plasma spraying process. The plasma spraying process is one of excellent methods for formation of porous films but is disadvantageous in that: (1) A prepared layer inevitably has a relatively large thickness and accordingly its response speed is poor and also weak in thermal stress in the operation; (2) During spraying, the material is exposed to high temperatures and cracks due to thermal stress tend to occur in the layer; (3) Many sensors can not be manufactured at a time; and (4) The manufacture cost is high. Other sensors are known from GB-A-2 200 460, from US-A-4,402,820 and from US-A-4,021,326. US-A-4,021,326 discloses a Nernst type A/F-sensor comprising a solid electrolyte and first and second electrodes. The thickness of the electrode on the exhaust gas side is between 0.1 and 50µm (col.5,l.16-18). This electrode is covered by a porous layer of metal oxides having a thickness between 0.1 and 500µm (col.5,l.19-29), wherein layers between 0.1 and 10 µm are preferably applied by thin-layer technology and layers between 15 and 500 µm by sintering, flame spraying or plasma spraying (col.6,l.12-16). The purpose of this porous layer is to achieve a sharp potential jump (col.4,l.18) upon transition between reducing and oxidizing atmosphere (col. 3, l. 9). This porous layer of metal oxide is preferably coated with a metallic layer as a getter layer for catalyst poisons. Alternatively the metal oxide layer is impregnated with a colloidal solution of silicon dioxide with subsequent firing of the very fine silicon dioxide powder (col.6,l.44-46, 59-63). Another sensor is known which does not rely on plasma spraying but uses a sintered material of superfine particles. This sensor corresponds to, for example, a gas sensor using superfine tin oxide particles described in National Technical Report, Vol. 26 (1980), page 457. The sintered material takes advantage of the fact that the surfaces of superfine particles are very active and highly adsorptive for gases, and is used as a solid electrolyte. However, this literature never teaches that the sintered material can be utilized for limitation of the diffusion speed of gases in the gas diffusion layer of the A/F sensor, such limitation being the task of the present invention, and the operation of the sintered material essentially differs from that of the gas diffusion layer of the A/F sensor. In the past, superfine particles have also been used for a partition film for gas separation, as exemplified in "KOGYOZAIRYO", vol. 31, No. 7 (1983), page 50. The partition film for gas separation is adapted to extract a desired gas component from a mixture gas. This literature describes that gas separation takes advantage of the fact that when gases pass through a space whose size is smaller than a mean free path of the gases, the diffusion coefficient of the gases depends on the size of the space. Exchangeability does not exist between this partition film for gas separation and the gas diffusion layer of A/F sensor to which the present invention pertains and the two are quite different from each other. While the pore diameter of the gas-separation partition film is 100 Å or less, the pore diameter of the gas diffusion layer of A/F sensor to which the invention pertains measures 200 to 500 Å. The gas-separation partition film is used in clean environment at room temperature but the A/F sensor is used in polluted environment at high temperatures. The former is applicable to selection of gas but the latter is applied to control of the speed of gas flow. Especially, because of different pore diameters, the partition film and the gas diffusion layer operate quite differently as will be described below. When gases diffuse through pores, the gas flow takes the form of a molecular flow if a pore has a diameter smaller than a mean free path of the gases and the molecular flow has a diffusion coefficient D A indicated by, D A = 9.7 x 10 3 d (T/M A ) 0.5 where d : pore diameter T : temperature M A : molecular weight of A component molecule. On the other hand, if the pore diameter is larger than the mean free path of the gases, the gas flow takes the form of a viscous flow having its diffusion coefficient D AB which is given by, D AB = 10 -3 T 1.75 (1/M A + 1/M B ) P(V A 1/3 + V B 1/3 ) where P : gas pressure V A : molecular volume of A component molecule. By substituting equations (3) and (4) into the previous equation (1), the diffusion limiting current for the molecular flow is, Ip* = C 1 P A T 0.5 · S ℓ where C 1 : constant P A : partial pressure of A component molecule and the limiting current for the viscous flow is, Ip* = C 2 T 0.75 P A P · S ℓ where C 2 : constant. Thus, equations (5) and (6) demonstrate that dependency of the limiting current upon temperatures exhibits counter positive and negative correlations in accordance with the relation in magnitude between the pore diameter and the mean free path of the gases. In particular, when the pore diameter nearly equals the gas mean free path, the gas flow takes the form of a medium flow between the molecular and viscous flows and the limiting current becomes substantially unchangeable with temperature. Because of difference in pore diameter, the gas flow in the gas-separation partition film takes the form of a molecular flow but the gas flow in the gas diffusion layer takes the form of a medium flow. Accordingly, the output of a sensor using the gas diffusion layer depends on temperatures very little and this sensor is suitable for use as an A/F sensor. However, in contrast, since the output of a sensor using the gas-separation partition film remarkably depends on temperatures and this sensor is unsuitable for use as an A/F sensor. SUMMARY OF THE INVENTION An object of this invention is to provide an A/F sensor capable of measuring the rich A/F. A sensor according to the invention is defined in claim 1. Advantageous embodiments are defined in dependent claims 2 to 11. For manufacturing the gas diffusion layer of an A/F sensor in accordance with the invention a plasma spraying process is not required instead, by sintering fine particles of 1 µm or less mean particle size a very thin diffusion layer having a small pore diameter which can properly limit the diffusion speed of incompletely burnt components to permit measurement of A/F in the rich region can be prepared. The present invention features an A/F sensor which is robust to thermal stress and highly durable. The invention also features products which can be manufactured at a low cost and at a high yield. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged fragmentary view of an A/F sensor useful in explaining the principle of the sensor; Fig. 2 is a graph showing diffusion limiting current characteristics for various A/F values in the A/F sensor; Fig. 3 is a graph showing the relation between the A/F of a mixture gas and the concentration of components in exhaust gas; Fig. 4 is an enlarged fragmentary view illustrating the construction of an A/F sensor useful in explaining the principle of the sensor; Fig. 5 is a similar view of an A/F sensor useful in explaining the principle of the sensor; Fig. 6 shows an electron microscopic photograph of a gas diffusion layer of the A/F sensor according to the invention; Fig. 7 is a graphical representation useful in explaining a two-layer structure of the gas diffusion layer according to the invention. Fig. 8 shows an A/F sensor, which is not in accordance with the present invention, but which is useful in explaining the embodiment of Fig. 9. Figs. 9 to 11 are enlarged fragmentary views illustrating the construction of A/F sensors according to further embodiments of the invention; Fig. 12 deleted Fig. 13 is an overall sectional view of an A/F sensor to which the invention is applied; and Fig. 14 is a graph showing an output characteristic of the A/F sensor according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 4 illustrates, in sectional form, the essential part of a gas diffusion layer of a sensor for measurement of A/F according to an embodiment of the invention and the sensor is used for feedback control of A/F in automobiles. Referring to Fig. 4, a solid electrolyte element (hereinafter simply referred to as a solid electrolyte) is made of an oxygen ion conductive metal oxide, especially in this example, zirconium oxide (zirconia) which is partly stabilized by adding to zirconia yttrium oxide (yttria) taking the form of a solid solution in the zirconia. Platinum is plated on the inner and outer surfaces of the solid electrolyte 1 to form porous thin films of reaction electrodes 2a and 2b. Since the outer reaction electrode 2b is related to the pore sectional area S in the previously-described theoretical formula (1) which affects the characteristic, this electrode is formed with high dimensional accuracy by covering the surface not to be plated with a mask when plating platinum. A gas diffusion layer 3 made of an electrically insulative metal oxide covers the outer reaction electrode 2b. Denoted by 4 is a lead electrode and 6 a heater adapted to heat the solid electrolyte 1. The lead electrode 4 connected to the outer reaction electrode 2b is formed by plating platinum while being masked simultaneously with the electrode 2b. Further, the lead electrode 4 is covered with a dense glass insulating layer 8 so as to be completely shielded from reaction with the exhaust gas. Superfine particles are sintered to form the gas diffusion layer 3 which covers the outer surface of the insulating layer 8. Preferably, used as a material of the superfine particles is zirconia partly stabilized by yttria or magnesia spinel having the almost same thermal expansion coefficient as that of the material of the solid electrolyte 1. Fig. 5 illustrates, in sectional form, the essential part of a gas diffusion layer of A/F measuring sensor according to another embodiment of the invention. In Fig. 5, elements like those of Fig. 4 are designated by like reference characters and will not be described herein. In comparison with the Fig. 4 embodiment, a gas diffusion layer 3A of the Fig. 5 embodiment is configured differently from the layer 3. The gas diffusion layer is not always required to cover the entire outer surface of the solid electrolyte 1 and the gas diffusion layer 3A covering at least the entire surface of the outer reaction electrode 2b in accordance with the present embodiment is well adapted for the object. The sintered coating of superfine particles used in the embodiments of Figs. 4 and 5 will now be described specifically. It is recommended that the mean particulate size of superfine particles used in the present invention be 1 µm or less. Firstly, particles of zirconia partly stabilized by yttria (or magnesia spinel), preferably having a mean particulate size of 0.3 to 0.5 µm are mixed with water containing suitable peptizator and binder and dispersed for more than 18 hours by using a ball mill. A semi-finished device is dipped in the thus prepared dispersion liquid, dried with air and then sintered at around 1,300 ∼ - 1,500°C for one hour. This sintering temperature is relatively lower than the temperature for production of general ceramics which ranges from 1,800°C to 1,900°C. Through the above process a film having a thickness of about 50 µm can be prepared. For coating of the dispersion liquid, other methods than dipping may be employed including brushing, spraying, spin coating and blading. In this manner, a gas diffusion layer having diffusion speed limiting ability necessary for detection of the rich A/F can be obtained. Fig. 6 shows an electron microscopic photograph (4,000 magnifications) of the surface of the sintered layer of zirconia partly stabilized by yttria. The photograph demonstrates that crystal particles become slightly larger than the material particles so as to have a particulate size of a little less than 1 µm, thereby proving that sintering has proceeded. Exhaust gas components can diffuse through pores observed as black points in Fig. 6. In the embodiments of Figs. 4 and 5, the gas diffusion layer is depicted as a single layer structure but according to the invention the size of pores of the layer is changed in the direction of thickness of the layer. Fig. 7 shows distributions of two kinds of superfine particles over the distance ranging from the interior adjacent the electrode to the outer surface. It will be appreciated that the distributions of the two kinds of superfine particles change continuously in the direction of thickness of the gas diffusion layer and accordingly denseness of the layer changes also continuously. Specifically, a first gas diffusion layer of superfine particle I corresponds to the sintered superfine-particle coating of zirconia partly stabilized by yttria. In particular, a layer of particles having relatively large particle size is formed on the outer surface of the electrode 2b. Subsequently, a semifinished device is dipped in or coated with a dispersion liquid containing silica or zirconia particles having a mean particulate size of 0.1 µm or less (for example, 0.02 µm) to permit the superfine particles having the very small particle size to permeate into pores of the layer of large particle size, is then dried and thereafter sintered at around 700°C ∼ 900°C for 30 minutes. Through the above process, the superfine particles of an outer layer permeate into pores of an inner layer and denseness of the overall gas diffusion layer can be changed continuously in the direction of the overall thickness. In order to explain the embodiment of Fig. 9, reference is now made to Fig. 8, which does not constitute an embodiment of the invention. Fig. 8 illustrates, in sectional form, the essential part of a composite structure of the gas diffusion layer. In Fig. 8, elements like those of Fig. 4 are designated by like reference characters. Referring to Fig. 8, a first gas diffusion layer 3a is formed on the outer reaction electrode 2b by coating superfine particles of zirconia partly stabilized by yttria. It is important that the first gas diffusion layer 3a be relatively rough. The denseness of this layer, which has close relation to catalytic reaction on the electrode, must be determined properly to permit the sensor to have good response speed. For example, by estimation, the porosity as measured by a mercury porosimeter is about 5 to 10% or the mean pore size is 300 to 400 Å. The thickness of the first gas diffusion layer 3a is 200 µm or less and with an unnecessarily large thickness, cracks due to difference in thermal expansion coefficient between the solid electrolyte 1 and the layer 3a tend to occur, leading to degradation of response speed. Accordingly, the thickness of the first gas diffusion layer 3a is preferably 100 µm or less. A second gas diffusion layer 3b is formed on the first gas diffusion layer 3a by coating sintered superfine particles. Particularly, in detecting low A/F values in the rich region, the second layer conveniently functions to limit the diffusion speed of fine gas molecules such as CO, H 2 and HC which stand for incompletely burnt gas components. With an excessively large thickness of the second gas diffusion layer, the gas diffusion is difficult to take place and therefore the thickness is 0.01 to 20 µm, preferably, 0.01 to 5 µm. To prepare the second gas diffusion layer 3b, an organic solvent containing silica particles or zirconia particles having a mean particulate size of 0.1 µm or less (for example, 0.02 µm) is coated on the first gas diffusion layer 3a after sintering thereof, then dried and finally sintered at 700°C for 30 minutes. This process repeats itself twice to provide a film thickness of about 1 µm. Through the above process, a composite gas diffusion layer can be completed which has the highly dense outer layer capable of limiting the diffusion speed of incompletely burnt gas components and the inner layer capable of permitting suitable gas diffusion and efficiently promoting the speed of reaction with the platinum electrode. Since the composite gas diffusion layer of Fig. 8 has a lamination structure of two distinctively separated layers, it does not constitute an embodiment of the invention. Figs. 9 to 11 illustrate, in sectional form, the essential part of composite structures of the gas diffusion layer according to embodiments of the invention. Firstly, referring to Fig. 9, this embodiment additionally has a transition gas diffusion layer 30. Especially where the porosity of the first gas diffusion layer 3a is high and the wettability of a solvent of the second gas diffusion layer 3b is high for a material of the first gas diffusion layer 3a, the intermediate transition gas diffusion layer 3c can be provided in which the material of the second gas diffusion layer 3b are impregnated in pores of the first gas diffusion layer 3a. Advantageously, in the Fig. 9 embodiment, even when the thermal expansion coefficient of first gas diffusion layer 3a greatly differs from that of second gas diffusion layer 3b, concentration of thermal stress can be mitigated by the transition gas diffusion layer 3c and a highly durable composite gas diffusion layer can be obtained. The Fig. 9 embodiment may be modified to extremities to provide an embodiment of Fig. 10 based on permeance to advantage. In the Fig. 10 embodiment, a material for formation of the second gas diffusion layer is perfectly impregnated in the first gas diffusion layer 3a to form a lamination structure of only first gas diffusion layer 3a and transition gas diffusion layer 3c. If in the Fig. 10 embodiment the thickness of the first gas diffusion layer is sufficiently small, the material for formation of the second gas diffusion layer is impregnated over the entirety of the first gas diffusion layer. In this case the gas diffusion layer includes the transition gas diffusion layer 3c alone as shown in Fig. 11 and the resulting structure may be interpreted in a broad sense as a lamination structure in which components continuously change in the direction of depth. This lamination structure is of the highest durability. In the foregoing embodiments, the gas diffusion layer is prepared by sintering and coating superfine particles, so that the cost can be reduced and the film thickness can also be reduced to improve the durability against thermal stress and the response speed. Also, variation in characteristics can be minimized to improve the yield. Further, the rough layer of the gas diffusion layer and the solid electrolyte element body are made of the same material, i.e., zirconia partly stabilized by yttria and as a result the difference in thermal expansion coefficient can be minimized to suppress, to advantage, the generation of thermal stress. The overall construction and output characteristic of a limiting current type A/F measuring sensor having such a gas diffusion layer as prepared in accordance with the foregoing embodiments will now be described with reference to Figs. 13 and 14. An embodiment of the sensor capable of exhibiting an output characteristic as shown in Fig. 14 is illustrated, in sectional form, in Fig. 13. Referring to Fig. 13, a solid electrolyte 1 is fixedly connected to a plug 5. The plug 5 has, at its tip, a cap 7 adapted to protect a gas diffusion layer 3 prepared in accordance with the foregoing embodiments from impurities in the exhaust gas. Disposed in the solid electrolyte 1 is a heater 6 for heating the solid electrolyte element to 600 to 700°C so that zirconia, the material of the element, may be activated to act as an electrolyte. For picking-up of electrical signals and application of voltage, lead wires 9a, 9b and 9c are connected to an inner reaction electrode 2a, an outer reaction electrode 2b and the heater 6, respectively. When the thus constructed limiting current type A/F measuring sensor is mounted to the exhaust pipe of an engine and a voltage is applied across the reaction electrodes 2a and 2b while heating the solid electrolyte 1 to about 700°C by current conduction to the heater 6, an output characteristic as represented at solid line in Fig. 14 can be obtained, indicating that the output voltage changes linearly with the A/F over the lean region, stoichiometric A/F equalling 14.7 and rich region. With the conventional gas diffusion layer, the output characteristic is abruptly decreased in the rich region where the fuel concentration is high, as represented at dotted line in Fig. 14, and the detection of the A/F in the rich region is limited by A/F = 12. This disadvantage can be eliminated by the present embodiment of the invention. Fuel control in an automobile engine is practiced using the above sensor, proving that normal running (40 to 60 Km/h) on the level ground is controlled in the lean region to achieve economical running, running on ascending slopes such as roads in the mountains is controlled in the rich region to provide an improved output characteristic and the running characteristic can be improved as a whole. In an automobile engine employed with A/F feedback control system including a conventional O 2 sensor and 3 way catalyst, the mixture sometimes takes rich air/fuel ratio at cold start or wide throttle opening. The present invention can apply to the conventional 3 way catalyst system to control the rich A/F region thereby to improve fuel economy and drivability. The material of zirconia particles partly stabilized by yttria used for the rough layer of the gas diffusion layer in the previous embodiments is not limitative. For example, a sintered film having a porosity of 2 to 20% measured by a mercury porosimeter or a mean particulate size of 200 to 500 Å can fulfil effects of the invention. Thus particles of ceramic such as alumina, magnesia, silica, titania or calcia or a mixture thereof may be used to advantage. The material of silica particles used for the dense layer of the gas diffusion layer in the embodiments previously described is not limitative and obviously alumina, magnesia, titania or calcia may be used to attain the same effects.

Claims: A sensor for measurement of air/fuel ratio (A/F) of the limiting current type comprising: a solid electrolyte (1) made of an oxygen ion conductive metal oxide; first and second electrodes (2a, 2b) in the form of porous thin films which are respectively provided on the atmosphere side surface and the exhaust gas side surface of said solid electrolyte (1), wherein, when said first and second electrodes (2a, 2b) are connected to a voltage source of a predetermined voltage oxygen near said solid electrolyte (1) is ionized and diffused into said solid electrolyte (1); and a gas diffusion layer (3; 3A), having a sintered layer (3a, 3c) contacting and covering said second electrode (2b), the gas diffusion layer (3; 3A) being made of electrically insulating metal oxide fine particles so that oxygen or molecules of other gas components in the exhaust gas can move through pores of said gas diffusion layer (3; 3A), characterized in that in the vicinity of said second electrode (2b) the metal oxide fine particles of the sintered layer (3a; 3c) have a mean particulate size of 1 µm or less; in the vicinity of the exhaust gas side surface of the gas diffusion layer (3; 3A) the metal oxide fine particles of the sintered layer (3b, 3c) have a smaller mean particulate size of 0.1 µm or less; the gas diffusion layer (3; 3A) has a sintered layer (3c) having a transition layer structure wherein the composition ratio of said metal oxide fine particles having mean particle sizes of 1 µm or less and of 0.1 µm or less continuously varies in the direction of thickness of the gas diffusion layer, and in said transition layer structure the mean particulate size, the porosity and mean diameter of pores gradually increase from the exhaust gas side surface of said gas diffusion layer toward said second electrode (2b). An A/F measuring sensor according to claim 1, characterized in that said gas diffusion layer (3; 3A) includes three sintered layers, the first sintered layer (3a) in which the metal oxide fine particles have a mean particulate size of 1 µm or less being contiguous to said second electrode (2b); the second sintered layer (3b) in which the metal oxide fine particles have a mean particulate size of 0.1 µm or less being contiguous to the exhaust gas side surface of the gas diffusion layer (3; 3A); and the third sintered layer (3c) forming said transition layer structure between said first and second sintered layers (3a, 3b). An A/F measuring sensor according to claim 1, characterized in that said gas diffusion layer (3; 3A) includes two sintered layers, the first sintered layer (3a) in which the metal oxide fine particles have a mean particulate size of 1 µm or less being contiguous to said second electrode (2b); and the second sintered layer (3c) forming said transition layer structure (Fig. 7) and having metal oxide fine particles of a mean particulate size of 0.1 µm or less in the vicinity of the exhaust gas side surface of the gas diffusion layer (3; 3A). An A/F measuring sensor according to claim 1, characterized in that said gas diffusion layer (3; 3A) includes a single sintered layer (3c) forming said transition layer structure (Fig. 7) and having the metal oxide fine particles having a mean particulate size of 1 µm or less in the vicinity of said second electrode (2b) and having a mean particulate size of 0.1 µm or less in the vicinity of the exhaust gas side surface. An A/F measuring sensor according to any one of claims 1 to 4, characterized in that in the vicinity of said second electrode (2b) the sintered layer (3a, 3c) has substantially the same thermal expansion coefficient as that of said solid electrolyte (1). An A/F measuring sensor according to any one of claims 1-5, characterized in that in the vicinity of said second electrode (2b) the sintered layer (3a, 3c) is made of fine particles mainly containing zirconium oxide, and in the vicinity of the exhaust gas side surface the sintered layer (3b, 3c) is made of fine particles containing one or both of silicon oxide and zirconium oxide. An A/F measuring sensor according to any one of claims 1 to 5, characterized in that in the vicinity of said second electrode (2b) the sintered layer (3a, 3c) is made of fine particles containing magnesia spinel, and in the vicinity of the exhaust gas side surface the sintered layer (3b, 3c) is made of fine particles containing one or both of silicon oxide and zirconium oxide. An A/F measuring sensor according to claim 2 or 3, wherein the thickness of said second sintered layer (3b, 3c) is smaller than that of said first sintered layer (3a). An A/F measuring sensor according to claim 8, wherein the thickness of said second sintered layer (3b, 3c) is preferably 0.01 µm to 5 µm and the thickness of said first sintered layer (3a) is preferably 100 µm or less. An A/F measuring sensor according to any one of claims 1 to 4, characterized in that in the vicinity of said second electrode (2b) the porosity of the sintered layer ranges from 5 % to 10 %. An A/F measuring sensor according to any one of claims 1 to 4, characterized in that in the vicinity of said second electrode (2b) the mean diameter of pores of the sintered layer is 30 to 40 nm when measured by a mercury porosimeter.

Inventors: HARADA TAKESHI, IBARAKI YOSHIRO, ICHIKAWA NORIO, KANAMARU MASATOSHI, TERAKADO KATSUYOSHI, UENO SADAYASU, HARADA, TAKESHI, IBARAKI, YOSHIRO, ICHIKAWA, NORIO, KANAMARU, MASATOSHI, TERAKADO, KATSUYOSHI, UENO, SADAYASU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: G01N27
IPC codes: G01N25, G01N27, B05D5