JP5241744B2 - Oxygen sensor - Google Patents

Description

  The present invention relates to an oxygen sensor suitably used for detecting an oxygen concentration in, for example, automobile exhaust gas.

  In general, in an engine such as an automobile, a mixture of air and fuel is burned in a combustion chamber to obtain a desired output. Therefore, an air-fuel ratio (the air mass in the mixture is divided by the fuel mass). When (value) becomes small, incomplete combustion occurs, and conversely, when the air-fuel ratio becomes large, the output decreases. For this purpose, an oxygen sensor is attached to an exhaust pipe or the like in an automobile to detect the oxygen concentration in the exhaust gas, thereby feedback-controlling the air-fuel ratio.

  Here, as shown in FIGS. 1 and 2, the oxygen sensor has an oxygen sensor element in which an outer electrode 12 is formed on the outer surface of a cylindrical solid electrolyte 11 whose one end is closed and an inner electrode 13 is formed on the inner surface. 1, a gas to be detected is brought into contact with the outer electrode 12 and a reference gas is brought into contact with the inner electrode 13 to generate an electromotive force between the outer electrode 12 and the inner electrode 13.

Specifically, a cylindrical protector 21 that protects the oxygen sensor element 1 and is closed at the tip end except for the gas inlet port 211 to be detected, and a metal cylinder 22 disposed on the proximal end side of the protector 21 The oxygen sensor element 1 is accommodated in the casing 2 containing the oxygen sensor element 1 so that the space touching the outer electrode 12 and the space touching the inner electrode 13 are blocked. The outer electrode 12 is electrically connected to the metal cylinder 22. Are connected to each other and grounded, and the inner electrode 13 is electrically connected to the lead wire 3 for outputting the electromotive force as a detection signal to the outside of the casing 2, and the oxygen sensor element 1 and the protector. A metal cylinder 22 is attached to the exhaust pipe so that 21 protrudes into the exhaust pipe (not shown) of the automobile engine (see, for example, Patent Document 1).

Japanese Patent Laid-Open No. 10-332627

In the oxygen sensor described in Patent Document 1, it is described that zirconium oxide is used as a solid electrolyte. Generally, yttria (Y ₂ O ₃ ) is appropriately used as a stabilizer for a zirconia (ZrO ₂ ) raw material. A high-strength, high-toughness yttria-stabilized zirconia (YSZ), which is obtained by adding a small amount and calcining, and whose crystal phase mainly consists of a tetragonal phase or a mixed phase of a tetragonal phase and a cubic phase. Are known.

  However, when a member made of yttria-stabilized zirconia is used over a long period of time at high temperature, in a steam atmosphere or with contact with water, water and heat cause the tetragonal phase to become a monoclinic phase. The transformation is promoted, and the deterioration of strength and toughness becomes remarkable.

Here, when the oxygen sensor is attached to the exhaust pipe of the automobile engine, the oxygen sensor is disposed at a high temperature, and external rainwater enters the detected gas inlet 211 of the protector 21 through the exhaust pipe and reaches the oxygen sensor element 1. As a result, the tetragonal phase of yttria-stabilized zirconia tends to undergo a phase transformation into a monoclinic phase, and the zirconia tube may expand rapidly during this phase transformation, causing cracks. In addition to the change in the output characteristics of the detection signal, there is a problem that the output of the detection signal itself may be impossible.

  The present invention has been made in view of the above circumstances, has little transformation from a tetragonal phase to a monoclinic phase due to long-term use, and suppresses destruction of the solid electrolyte due to long-term use, with long-term reliability. An object is to provide an excellent oxygen sensor.

  The present invention comprises an oxygen sensor element in which an outer electrode is formed on the outer surface of a cylindrical solid electrolyte whose front end is closed, and an inner electrode is formed on the inner surface. An oxygen sensor in which a reference gas is brought into contact with an inner electrode to generate an electromotive force between the outer electrode and the inner electrode, wherein the solid electrolyte contains 1 to 4 mol% of disprussia and 3 to 7 It is characterized by comprising disprusia-ceria stabilized zirconia containing a total of 7 to 11 mol% of mol% ceria.

  Further, the present invention provides the above oxygen sensor, wherein the oxygen sensor element is protected and a cylindrical protector whose front end side is closed except for the detected gas inlet, and a metal cylinder connected to the base end side of the protector. The oxygen sensor element is disposed in a casing including a body so that a space where the front end side of the solid electrolyte is located in the protector and which touches the outer electrode and a space which touches the inner electrode are blocked. The metal cylinder is electrically connected to the outer electrode, and a lead wire for outputting the electromotive force as a detection signal to the outside of the casing is electrically connected to the inner electrode. It is characterized by that.

  According to the oxygen sensor of the present invention, the disprusia-ceria stabilized zirconia in which the solid electrolyte constituting the oxygen sensor element contains 1 to 4 mol% of disprusia and 3 to 7 mol% of ceria in a total of 7 to 11 mol%. It is possible to realize an oxygen sensor with excellent long-term reliability, in which the transformation from the tetragonal phase to the monoclinic phase due to long-term use is small, and the solid electrolyte is prevented from being destroyed by long-term use. it can.

  In addition, the oxygen sensor includes a cylindrical protector that protects the oxygen sensor element and is closed at the tip end except for the detected gas inlet, and a metal cylinder connected to the base end side of the protector. An oxygen sensor element is housed inside the casing so that the solid electrolyte tip side is located in the protector and the space that touches the outer electrode and the space that touches the inner electrode are blocked. An oxygen sensor for measuring the exhaust gas of an automobile by electrically connecting a cylindrical body and electrically connecting an inner electrode with a lead wire for outputting an electromotive force as a detection signal to the outside of the casing. Even if it is attached to the exhaust pipe of an automobile engine and used for a long time, there is little transformation from the tetragonal phase to the monoclinic phase due to long-term use. Quality is inhibited from breaking, it is possible to realize the oxygen sensor is excellent in long-term reliability.

It is a schematic sectional drawing which shows an example of embodiment of the oxygen sensor of this invention. It is an expanded sectional view of the oxygen sensor element shown in FIG.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic cross-sectional view showing an example of an embodiment of the oxygen sensor of the present invention, and FIG. 2 is an enlarged cross-sectional view of the oxygen sensor element shown in FIG.

The oxygen sensor shown in FIGS. 1 and 2 includes an oxygen sensor element 1 in which an outer electrode 12 is formed on the outer surface of a cylindrical solid electrolyte 11 whose front end is closed, and an inner electrode 13 is formed on the inner surface. A gas to be detected is brought into contact with the electrode 12 and a reference gas is brought into contact with the inner electrode 13 to generate an electromotive force between the outer electrode 12 and the inner electrode 13.

  The oxygen sensor element 1 is formed in a cylindrical shape whose front end is closed. As will be described later, a space that touches the outer surface of the solid electrolyte 11 and a space that touches the inner surface are blocked, and an outer electrode 12 formed on the outer surface. It is accommodated in the casing 2 so as to generate an electromotive force between the inner electrode 13 formed on the inner surface thereof.

  The outer electrode 12 and the inner electrode 13 are made of a noble metal such as platinum (Pt), gold (Au), silver (Ag), rhodium (Rh), palladium (Pd). The inner electrode 13 is made of gold.

  The casing 2 includes a protector 21 for protecting the oxygen sensor element 1 and a metal cylinder 22 connected to the base end side of the protector 21. Specifically, the opening of the protector 21 and the opening of the metal cylinder 22 are combined so that the internal space is connected.

The protector 21 is formed in a cylindrical shape whose front end side is closed except for the detected gas introduction port 211, and its base end side is fixed to the front end of the metal cylinder 22.

  A step is formed on the inner wall of the metal cylinder 22 so that the distal end side is narrow and the proximal end side is widened, and the outer wall of the oxygen sensor element 1 is formed so that the proximal end side protrudes outward. The oxygen sensor element 1 is fixed by an insulating cylinder 4 disposed in contact with the base end side of the oxygen sensor element 1 by using a seal member, if necessary. The protruding portion is accommodated in the casing 2 so as to be positioned on the protector 21, and the protruding portion is protected by the protector 21.

  At this time, the oxygen sensor element 1 is accommodated in the casing 2 so that the space touching the outer electrode 12 constituting the oxygen sensor element 1 and the space touching the inner electrode 13 constituting the oxygen sensor element 1 are blocked. . In addition, the metal cylinder 22 is electrically connected to the outer electrode 12 of the oxygen sensor element 1.

A male screw portion 221 is formed on the outer periphery on the front end side of the metal cylinder 22 so that the oxygen sensor element 1 and the protector 21 protrude into an exhaust pipe (not shown) of an automobile engine. 221 is screwed to the exhaust pipe, and the oxygen sensor is attached to the exhaust pipe. in this way,
The metal cylinder 22 is normally connected to the ground, for example, by being screwed to the exhaust pipe. Thereby, the outer electrode 12 of the oxygen sensor element 1 is also grounded.

  On the other hand, the lead wire 3 is electrically connected to the inner electrode 13 of the oxygen sensor element 1 via the conductive cylinder 51 and the connection conductor 52.

  The lead wire 3 is composed of a core wire made of a plurality of metal wires and an insulating covering tube made of a fluororesin such as polytetrafluoroethylene, and is provided on the base end side of the metal cylinder 22. It passes through a lead wire insertion hole (not shown) of the seal member 6 and is pulled out of the casing 2 and connected to a control unit (not shown) for engine control.

Here, the detected gas brought into contact with the outer electrode 12 of the oxygen sensor element 1 is an automobile exhaust gas, and the reference gas brought into contact with the inner electrode 13 is the atmosphere. The solid electrolyte 11 generates an electromotive force between the outer electrode 12 and the inner electrode 13 based on the difference in oxygen concentration when there is a difference in oxygen concentration between the outer exhaust gas and the inner atmosphere. The electromotive force is output to the outside of the casing 2 through the lead wire 3 as a detection signal.

  In addition, in order to heat the solid electrolyte 11 of the oxygen sensor element 1 and activate it at an early stage, a heater 7 may be provided inside the oxygen sensor element 1 as shown in FIG. The heater 7 is formed of a ceramic material into a small-diameter rod shape, and has a heating resistor embedded therein, and generates heat when power is supplied from the outside. Further, in order to supply power to the heater 7, lead wires 31 and 32 having the same configuration as the lead wire 3 electrically connected to the inner electrode 13 of the oxygen sensor element 1 are provided, and the base end of the metal cylinder 22 is provided. It passes through a lead wire insertion hole (not shown) of the seal member 6 provided on the side, is drawn out of the casing 2, and is connected to a control unit (not shown) for engine control.

  Thus, the control unit corrects the fuel injection amount based on the oxygen concentration detection signal output from the oxygen sensor, and optimally feedback-controls the air-fuel ratio. In addition, the control unit supplies power to the heater 7 through the lead wires 31 and 32 when the solid electrolyte 11 is at a low temperature, for example, when the engine is started, thereby heating the solid electrolyte 11 early. Activate.

  The solid electrolyte 11 is important to be a sintered body made of disprusia-ceria stabilized zirconia, which is an oxygen ion conductive material. Ceria as a stabilizer for zirconia has been recognized as a substance having good oxygen ion conductivity, but has a problem that it is weak in strength and cannot be used effectively as a structural material, but has good oxygen ion conductivity. By adding disprusia as a stabilizer in addition to ceria, the oxygen ion conductivity is good and the strength is strong, and further, deterioration is suppressed even in a high temperature and high humidity environment.

Specifically, the disprusia-ceria stabilized zirconia forming the solid electrolyte 11 is mainly composed of zirconia (ZrO ₂ ), 1 to 4 mol% of disprusia (Dy ₂ O ₃ ) and 3 to 7 mol%. It is important to be composed of disprusia-ceria stabilized zirconia containing 7 to 11 mol% of ceria (CeO ₂ ) in total.

  Here, the amount of the stabilizer is within the above range because if the disprusia is less than 1 mol%, the monoclinic phase is generated by 50% by mass or more at the time of firing, which is a manufacturing process, and cracks are likely to occur. On the contrary, when it exceeds 4 mol%, the strength (bending strength) is remarkably lowered.

  In addition, if the ceria is less than 3 mol%, the monoclinic phase is 50 mass% or more and cracks are generated during sintering. Conversely, if the ceria is more than 7 mol%, the strength (bending strength) decreases. .

  Furthermore, if the total amount of display Russia and ceria is less than 7 mol%, the amount of monoclinic crystals increases to 50% by mass or more under high temperature conditions, and the total content of display Russia and ceria is particularly preferable. Is preferably in the range of 7-11 mol%.

  As described above, the oxygen sensor of the present invention is characterized in that the solid electrolyte constituting the oxygen sensor element contains both disprusia and ceria as stabilizers in the above-described proportions, and thereby oxygen ion conductivity. In addition, the strength is high and the deterioration can be extremely reduced even in a high temperature and high humidity environment.

In addition, on the condition that 1 to 4 mol% of disprusia and 3 to 7 mol% of ceria are included in total in a range of 7 to 11 mol%, as a component other than these, for example, 100 masses of stabilizer composed of disprusia and ceria Up to 30% by mass may be substituted with a rare earth element oxide which is another stabilizer, such as calcia (CaO), magnesia (MgO) or the like. Furthermore, alumina (Al ₂ O ₃ ), silica (SiO ₂ ), iron oxide (Fe ₂ O ₃ ), titania (TiO ₂ ), etc. are sintered together as impurities in the raw materials and components mixed during the manufacturing process. You may contain to 3 mass% of the whole body. The zirconia raw material may contain hafnia (HfO ₂ ) which is difficult to separate.

  Such disprusia-ceria stabilized zirconia is mainly composed of a tetragonal phase or a mixed phase of a tetragonal phase and a cubic phase, and preferably has a tetragonal phase of 80% by mass or more from the viewpoint of strength and toughness. To do. In addition, such display Russia-ceria stabilized zirconia is such that the monoclinic phase ratio does not exceed 50 mass% even after long-term use.

The average crystal grain size of disprusia-ceria stabilized zirconia forming the solid electrolyte 11 is 1.0 μm or less from the point that precipitation of a monoclinic phase can be suppressed and a high-strength sintered body can be obtained. In particular, 0.4 μm or less is preferable.

  The solid electrolyte 11 as described above may be prepared as a raw material powder obtained by mixing powders of various components so as to have the above composition, but preferably a zirconia containing a predetermined amount of disprusia-ceria in advance. Prepare the raw material powder obtained by coprecipitation. And after forming this raw material powder into a predetermined shape by means of press molding, extrusion molding, injection molding or the like, it may be fired at 1300-1500 ° C. In addition, as a method of forming electrodes on the outer surface and inner surface of the solid electrolyte 11, a metal component for forming an electrode may be applied to each surface and then fired.

  In the above-described embodiment, the heater 7 is provided as the oxygen sensor. However, the present invention is not limited to this, and the heater 7 may be omitted.

  Since it is difficult to measure the bending strength of the solid electrolyte used as an oxygen sensor element, the width is 4 mm, the thickness is 3 mm, and the length is stipulated in JIS-R-1601. A test piece of a prism having a long side cross section of 36 mm in length was used.

  The test sample contains stabilizers (disprusia and ceria) in the proportions shown in Table 1, with the remainder being zirconia disprusia-ceria stabilized zirconia, and yttria stable containing 5 mol% yttria as a comparative example. A test sample made of zirconia was prepared.

  Then, after measuring the bending strength (Pa) and the amount ratio (mass%) of the monoclinic phase, a thermal degradation test was performed to measure the change in bending strength before and after the test and the amount ratio of the monoclinic phase. . The thermal deterioration test is a hot water treatment test (such a test is generally called a Pressure Cooker Test, hereinafter abbreviated as PCT), and is heated in boiling water at a temperature of 121 ° C. and 2 atm. The solid electrolyte is left for 100 hours.

  The bending strength was obtained from the three-point bending test method of JIS-R-1601 and was an average value of ten pieces.

  The amount of monoclinic phase was determined by the X-ray diffraction method described in RC Carvie and P.S.Nicholsona, J.Am.Ceram.Soc.Vol.55, No.6, p.303-305 (1972). Based on the above, the quantity ratio (mass%) of the monoclinic phase was determined by Equation 1.

  The results are shown in Table 1.

According to Table 1, sample No. used as an oxygen sensor element within the scope of the present invention. 3 to 13 and 15 show a large bending strength before PCT, and the amount ratio of monoclinic phase is 50 even after PCT.
It can be seen that a high strength value of bending strength of 3.9228 × 10 ⁸ Pa or more is obtained while being suppressed to mass% or less. In particular, sample no. In 3, 6, 7, and 9, even after PCT, the amount ratio of the monoclinic phase is 23% by mass or less and a bending strength of 5.88 × 10 ⁸ Pa or more is obtained.

On the other hand, sample no. In Nos. 12, 14, 16, and 17, the monoclinic phase after PCT exceeded 50% by mass, cracks were generated, or only a bending strength of less than 4.020 × 10 ⁸ Pa was obtained.

  In addition, Sample No. as a comparative example. 18 (yttria-stabilized zirconia containing 5 mol% yttria) had a high bending strength before PCT and a small amount of monoclinic phase, but the bending strength was severely deteriorated after PCT. The amount ratio of the oblique phase was remarkably large.

  From the above results, the solid electrolyte used as the oxygen sensor (oxygen sensor element) of the present invention includes a disprusia containing 7 to 11 mol% in total of 1 to 4 mol% of disprusia and 3 to 7 mol% of ceria. It can be seen that the use of ceria-stabilized zirconia provides high bending strength and further suppresses deterioration even in a high-temperature and high-humidity environment.

1: Oxygen sensor element
11: Solid electrolyte
12: Outer electrode
13: Inner electrode 2: Casing
21: Protector
211: Detected gas inlet
22: Metal cylinder
221: Male thread portion 3, 31, 32: Lead wire 4: Insulating cylinder
51: Conductive cylinder
52: Connection conductor 6: Seal member 7: Heater

Claims (2)

  An oxygen sensor element having an outer electrode formed on the outer surface of a cylindrical solid electrolyte whose front end is closed and an inner electrode formed on the inner surface is brought into contact with a gas to be detected and a reference to the inner electrode. An oxygen sensor in which a gas is brought into contact to generate an electromotive force between the outer electrode and the inner electrode, wherein the solid electrolyte is 1 to 4 mol% of disprussia and 3 to 7 mol% of ceria. An oxygen sensor comprising disprusia-ceria stabilized zirconia containing 7 to 11 mol% in total.   Inside the casing, which protects the oxygen sensor element and includes a cylindrical protector whose front end side is closed except for the detected gas inlet, and a metal cylinder connected to the proximal end side of the protector, the oxygen sensor The sensor element is accommodated such that the front end side of the solid electrolyte is located in the protector, and the space that touches the outer electrode and the space that touches the inner electrode are blocked. 2. The cylindrical body is electrically connected, and a lead wire for outputting the electromotive force as a detection signal to the outside of the casing is electrically connected to the inner electrode. The oxygen sensor described.

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