JP2005265614A - Gas sensor element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas sensor element hard to cause the occurrence of rich shift, and to provide its manufacturing method. <P>SOLUTION: In the gas sensor element 1, having a solid electrolyte body 11 and the reference electrode 112 and electrode 111 on the side of a gas to be measured provided on the surface of the solid electrolyte body 11 and constituted so that the surface on the contact side with the gas to be measured of the electrode 111 on the side of the gas to be measured is covered with a porous layer 22, the porous layer 22 contains a nitride. The nitride is added to the porous layer 22, by blowing a plasma jet containing a nitrogen atom against the porous layer 22. It is preferable to add the nitride centering the surface coming into contact with the gas to be measured in the porous layer 22. Further, the ratio of the number of nitrogen atoms is preferably 0.2-100%, with respect to the number of oxygen atoms contained in the porous layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gas sensor element used in various gas sensors such as a gas sensor installed in an exhaust pipe or the like for combustion control of an internal combustion engine for a vehicle, and a manufacturing method thereof.

A gas sensor such as an oxygen sensor or an air-fuel ratio (A / F) sensor is provided in the exhaust system of a vehicle internal combustion engine such as an automobile engine, and the air-fuel ratio (A / F) is detected from the oxygen concentration in the exhaust gas. May be used to control combustion (exhaust gas control feedback system).
In particular, in order to efficiently purify exhaust gas using a three-way catalyst, it is important to control the air-fuel ratio to a specific value in the combustion chamber of the vehicle internal combustion engine.
And as a gas sensor element incorporated and used in the said gas sensor, the gas sensor element which exists in the following patent documents 1 is generally used.
As a configuration of such a gas sensor element, there is one in which a tip portion is covered with a porous layer as disclosed in Patent Document 1.
The porous layer is used as a diffusion resistance layer, for example, in the case of a limiting current type gas sensor element, and as a protective layer for protecting the measured gas side electrode from poisonous substances, for example, in the case of an oxygen concentration electromotive force type gas sensor element. There are many cases.

JP-A-12-065782

By the way, the inventors have found a phenomenon in which an A / F sensor element with a built-in gas sensor left in the exhaust pipe of a vehicle causes a rich shift in output within about 10 seconds after the start of the internal combustion engine. .
This phenomenon does not always occur. For example, this phenomenon does not occur when the vehicle internal combustion engine is restarted while the engine is stopped for a very short time. Although the shift amount increases depending on the standing time, it is almost saturated when left for about 1-2 days.
This rich shift makes the combustion of the vehicle internal combustion engine extremely unstable.
That is, although the system that has received the rich signal works in the direction of controlling the air-fuel ratio of the vehicle internal combustion engine to the lean side, the actual exhaust gas state is leaner than the indicated value of the air-fuel ratio sensor, so in the worst case, There is a risk of misfire (engine stop). In addition, since the control point is largely deviated, the concentration of atmospheric pollutant gas such as NOx may be increased in the exhaust gas.

By the way, since the rich shift phenomenon occurs when the gas sensor is left in the exhaust pipe for a long time, it is considered to be an influence of some exhaust gas component adhering to the A / F sensor element.
In order to confirm this phenomenon, the gas sensor element and various test pieces that were left in the exhaust pipe for a long time were used, and the components and desorption amount of the desorbed gas generated by heating each were investigated. 8 and described in FIG.
8 and 9, the vertical axis represents the relative adhesion amount.

As is apparent from FIGS. 8 and 9, it has been found that most of the gas species adhering to the gas sensor element is H ₂ O, that is, water vapor (water).
And it became clear that most of the adhering moisture adhered to the diffusion resistance layer, that is, the porous layer.
Furthermore, when the gas sensor was left in a high humidity atmosphere, it was possible to confirm the reproduction of rich shift.

Further, when the gas sensor left in a high-humidity atmosphere was heated and examined for the generated water, it was found that the desorption of adhering water appears remarkably in two temperature regions centered around 150 ° C and 400 ° C. .
Adhered moisture is considered to be attached to the diffusion resistance layer by physical adsorption and chemical adsorption accompanied by crystallization. Especially in chemical adsorption, for example, when the porous layer is made of aluminum oxide,
Al ₂ O ₃ (porous layer, solid) + 3H ₂ O (gas) → 2Al (OH) ₃ (porous layer, solid)
It is considered that moisture is firmly attached to the surface of the porous layer by such a reaction.
Furthermore, the above reaction is an integrated reaction, and it is estimated that the progress is relatively slow from the viewpoint of conservation of momentum (this is consistent with the fact that a rich shift will not occur unless the gas sensor is left in the wet exhaust pipe for a long time). .)

From the above, it is considered that the rich shift is caused to disappear by the following process.
That is, the rich shift occurs after the gas sensor is left in a high-humidity atmosphere such as an exhaust pipe of a vehicle internal combustion engine. When the gas sensor is left in a high-humidity atmosphere, vaporized moisture is generated in the A / F sensor element installed in the gas sensor. (Water vapor) enters, and moisture is physically adsorbed and / or chemically adsorbed mainly on the porous layer of the A / F sensor element.
This moisture is desorbed and vaporized from the porous layer by heating the A / F sensor element.
The vaporized water, that is, water vapor, tends to be expelled to the outside through the porous layer while expanding in volume, but since the porous layer has a diffusion resistance more or less, it takes some time for the water vapor to pass through the porous layer. Cost.
Accordingly, the water vapor pressure rises inside the A / F sensor element (particularly in the vicinity of the measured gas side electrode), and the oxygen partial pressure falls relatively. As a result, a rich shift occurs in the output of the gas sensor.
The water vapor gradually escapes to the outside through the porous layer, and at the same time, exhaust gas around the element starts to be taken into the A / F sensor element. As a result, the rich shift is reduced with the passage of time, and a normal output is obtained.

Thus, it is thought that the rapid volume expansion of water vapor due to desorption of water → vaporization causes a large rich shift of about 1 to 2 A / F.
Of course, this problem does not occur if the atmosphere to be left is dry, but the exhaust pipe of the parked vehicle has a high humidity atmosphere due to moisture contained in the exhaust gas as a combustion product, and a rich shift occurs. It is considered an easy environment.

The rich shift due to the above mechanism occurs in an element using a limiting current type oxygen sensor element such as an A / F sensor element, but also in an element using an oxygen concentration electromotive force type oxygen sensor element. A rich shift phenomenon due to a similar mechanism has been confirmed. Even in the oxygen concentration electromotive force type oxygen sensor element, the gas side electrode to be measured is often covered with a porous layer, and the porous layer has an appropriate diffusion resistance even if the pores are rough. This is because a certain amount of time is required for water vapor to escape.
As described above, it is an object of the present invention to provide a gas sensor element in which a rich shift hardly occurs and a manufacturing method thereof.

The first invention has a solid electrolyte body, a reference electrode provided on the surface of the solid electrolyte body, and a measured gas side electrode, and the surface of the measured gas side electrode on the side in contact with the measured gas is porous. In a gas sensor element covered with a layer,
In the gas sensor element, the porous layer contains a nitride.

In the gas sensor element according to the first invention, the porous layer contains a nitride.
The rich shift phenomenon of the gas sensor element occurs due to moisture adsorption and subsequent moisture desorption by heating.
Here, since the porous layer contains nitride, the porous layer becomes hydrophobic and moisture adsorption is prevented.
Therefore, the rich shift phenomenon hardly occurs.

The second invention has a solid electrolyte body, a reference electrode provided on the surface of the solid electrolyte body, and a measured gas side electrode, and the surface of the measured gas side electrode on the side in contact with the measured gas is a nitride. In manufacturing a gas sensor element covered with a porous layer containing
A gas sensor element manufacturing method is characterized in that a nitride is formed on the porous layer by spraying a plasma jet containing atomic nitrogen on the porous layer.

It is conceivable to add powder containing nitride or the like during the production of the porous layer and sinter together with the porous layer, or spray the nitride onto the porous layer. There is a risk of becoming a thing.
Therefore, in the second invention, a plasma jet containing atomic nitrogen is blown onto the porous layer, nitrogen having high reactivity is sufficiently allowed to enter the porous layer, and nitriding is performed to the inside.
Thereby, a porous layer sufficiently containing nitride can be obtained, and a hydrophobic porous layer can be obtained. Accordingly, it is possible to obtain a gas sensor element in which moisture adsorption hardly occurs and a rich shift phenomenon hardly occurs.
As described above, according to the first and second aspects of the invention, it is possible to obtain a gas sensor element that is difficult to shift rich and a method for manufacturing the same.

The gas sensor element according to the first and second aspects of the present invention is based on an oxygen ion current flowing through an electrochemical cell composed of an electrode to be measured and a reference electrode provided on an oxygen ion conductive solid electrolyte body. A predetermined gas concentration is measured.
As such a gas sensor element, in addition to the oxygen sensor element for measuring the oxygen concentration in the gas to be measured, a specific gas is decomposed to generate oxygen ions, and the concentration of the specific gas is measured based on the oxygen ions. A gas sensor element having such a structure is known. Examples of the specific gas here include NOx, CO, and HC.
Further, as a gas sensor element according to the first and second inventions, the gas sensor element is installed in an exhaust system of a vehicle internal combustion engine or the like, the oxygen concentration in the exhaust gas is measured, and the engine room of the internal combustion engine based on the measured value, Some measure the A / F in the combustion chamber (A / F sensor element).

The gas sensor element according to the first and second inventions performs measurement based on the oxygen ion current generated from the gas concentration difference between the measured gas side electrode and the reference electrode. There are a current type element and an oxygen concentration electromotive force type element, and the present invention can be applied to both of them.
The measurement gas side electrode of these sensors is usually provided with a protective layer for preventing poisoning from the measurement gas. Further, in a limiting current type sensor such as an A / F sensor, a limiting current is obtained by a porous diffusion resistance layer.

In the first invention, it is preferable that the porous layer contains a nitride centering on the surface in contact with the gas to be measured.
Thereby, since the porous layer can be hydrophobized around the surface serving as the introduction inlet of the gas to be measured, moisture adsorption of the porous layer can be suppressed more reliably and rich shift can be prevented.

In the first invention, the ratio of the number of nitrogen atoms to the number of oxygen atoms contained in the porous layer is preferably 0.2% or more (claim 3).
By setting it to 0.2% or more, it is possible to secure a nitride that covers at least the surface of the porous layer, and the porous layer can be hydrophobized around the surface that is easily in contact with the gas to be measured. It is possible to reliably suppress moisture adsorption and prevent rich shift.
In order to prevent the moisture adsorption, a sufficient effect can be obtained if a layer of about several nanometers on the surface of the porous layer is nitrided.

When measuring the ratio of oxygen and nitrogen atoms, Electron
Examples of methods include Probe Microscope Analysis (EPMA), Auger Electron Spectroscopy (AES), and Secondary Ion Mass Spectrometry (SIMS). These methods can directly measure the amount of atoms.
In addition, there is a method for measuring the ratio of the number of oxygen and nitrogen atoms that can be used when the porous layer is made of aluminum oxide, such as X-ray probe spectroscopy (XPS). That is, the ratio of the number of atoms of oxygen and nitrogen can be measured by comparing the ratio of Al—N and Al—O.

According to the first invention, it is preferable that the porous layer is made of aluminum oxide and the nitride is made of aluminum nitride.
Since the porous layer is often arranged in contact with the gas side electrode to be measured, and therefore has an insulating property, and there may be a high temperature atmosphere as a usage environment of the gas sensor element, it is desirable to have a heat resistance. Aluminum oxide meets the above requirements.
Aluminum nitride is an inexpensive and sufficiently stable material for practical use, and can be easily obtained by processing aluminum oxide.
The porous layer may be composed of spinel (MgAl ₂ O ₄ ). In this case, magnesium nitride or aluminum nitride may be used as the nitride.

Further, the first invention can be applied to both the cup-type and laminated gas sensor elements.
An example of a laminated gas sensor element is described in Example 1, and an example of a cup-type gas sensor element is described in Example 5.

(Example 1)
The gas sensor element concerning this invention is demonstrated using FIG. 1, FIG.
As shown in FIG. 1, the gas sensor element 1 of this example includes a solid electrolyte body 11, a reference electrode 112 provided on the surface of the solid electrolyte body 11, and a measured gas side electrode 111, and the measured gas side electrode The surface of the side 111 in contact with the gas to be measured is covered with the porous layer 22.
The porous layer 22 contains a nitride.
The porous layer 22 is made of aluminum oxide, and as shown in FIG. 2, the surface in contact with the gas to be measured is made of nitride 222 in the entire area of the layer, and the porous layer 22 contains oxygen atoms contained therein. The ratio of the number of nitrogen atoms contained to the number is 0.2% or more, and the nitride 222 is made of aluminum nitride.

Details will be described below.
As shown in FIG. 1, the gas sensor element 1 according to this example is a laminated flat plate element, which is a limiting current type element. The heating element substrate 14 provided with the heating element 140 that generates heat when energized, the insulating spacer 12 provided with the reference gas chamber 120 for introducing the atmosphere serving as the reference gas, the reference electrode 112 and the measured gas side electrode on the surface The oxygen ion conductive solid electrolyte body 11 provided with 111 and the diffusion resistance layer 2 laminated so as to cover the surface side of the solid electrolyte body provided with the gas side electrode 111 to be measured. The diffusion resistance layer 2 includes a dense layer 21 and a porous layer 22. Since the dense layer 21 does not transmit the gas to be measured, the gas to be measured is introduced from the end surface 220 of the porous layer 22 facing the side surface 101 of the gas sensor element 1.
The porous layer 22 is made of an aluminum oxide ceramic and is nitrided by a method according to Example 2 described later, and the surface in contact with the gas to be measured is covered with aluminum nitride.

Here, FIG. 2 is a schematic diagram of the ceramic structure of the porous layer 22.
Since the porous layer 22 has an aluminum oxide ceramic structure, the porous layer 22 includes a large number of aluminum oxide grains 221 and gaps between grains formed between the grains 221.
The porous layer 22 is subjected to nitriding treatment by introducing atomic nitrogen from the end face 220 (measurement gas inlet) shown in FIG. 1 (see Example 2 for details of the nitriding treatment), and therefore On the surface, aluminum nitride (nitride 222) is formed by replacing oxygen atoms with aluminum oxide.
Atomic nitrogen in an active and high energy state during the nitriding treatment can reach the interior of the porous layer 22 sufficiently through the gaps between the grains. By performing the nitriding treatment for a predetermined time, the porous layer 22 It is also possible to nitride the whole.

As described above, in the gas sensor element 1 according to this example, the entire layer is formed with aluminum nitride, which is the nitride 222, by nitriding in the porous layer 22 from the end face 220 facing the outside of the element 1 with the porous layer 22. Contains.
The rich shift phenomenon of the gas sensor element 1 occurs due to moisture adsorption when left unattended and subsequent desorption of moisture due to heating at engine startup. Since the porous layer 22 contains the nitride 222, the porous layer 22 becomes hydrophobic and moisture adsorption is prevented.
Therefore, according to this example, it is possible to obtain a gas sensor element in which a rich shift phenomenon is unlikely to occur.

(Example 2)
In this example, a method for manufacturing a gas sensor element according to Example 1 will be described.
That is, a sheet for a heating element substrate, a molded body for a spacer, a sheet for a solid electrolyte body, a sheet for a porous layer, and a sheet for a dense layer are prepared using a predetermined ceramic material and a binder.
A printing section that serves as a heating element, a reference electrode, a measured gas side electrode, or the like is provided on the obtained heating element substrate sheet. Thereafter, these sheets are laminated, pressure-bonded and integrated so as to obtain the element according to FIG. 1 to obtain an unfired laminated body.
The unfired laminate was fired using a heating furnace heated at a predetermined temperature profile to obtain a fired body. Thereafter, the fired body was polished to a predetermined size, and finally the body center part was sealed with glass to obtain a gas sensor element.

In the above production method, a plasma jet containing nitrogen atoms was sprayed on the end face of the porous layer at the stage of the fired body to perform a process of containing nitride in the porous layer.
A method for carrying out this plasma jet will be described in detail below.
First, the nitriding apparatus 3 that applies the plasma jet 35 to the fired body will be described.
As shown in FIG. 3, the nitriding apparatus 3 includes a plasma nozzle 34 that blows a plasma jet 35 onto the fired body 19 and a holder 36 that fixes the fired body 19 in a sealable processing chamber 30.
A processing gas 321 is introduced from a gas tank 32 to the plasma nozzle 34 through a pipe 322.
The plasma nozzle 34 is provided with a circulation device 33 for circulating the cooling water 331. Reference numeral 332 is a circulation pipe for the cooling water 331.
In addition, a power supply 31 and a lead wire 311 are provided for applying a voltage to the plasma nozzle 34 to turn the processing gas into plasma.

The process of spraying the plasma jet 35 will be described.
First, the fired body 19 is fixed to the holder 36 so that the end surface of the porous layer is directly below the plasma nozzle 34.
Next, the inside of the processing chamber 30 is evacuated using a vacuum pump (not shown).
Next, the processing gas 321 is supplied to the plasma nozzle 34, the power supply 31 is turned on, and a voltage is applied to the plasma nozzle 34.
Here, N ₂ was used as the processing gas 321, but hydrogen-nitrogen gas, ammonia-nitrogen gas such as H ₂ (˜5%) / N ₂ , NH ₃ (−10%) / N _2, etc. Etc. can be used.
Moreover, the output voltage can be adjusted in the range of 10-1000W. The power supplied from the power source 31 can be either direct current or alternating current. In the case of alternating current, a high frequency of 13.56 MHz, which is used industrially, is convenient.
The flow rate of the processing gas 321 was 1 to 10 liters / minute, and the irradiation time for one end face of the plasma jet 35 was 5 to 30 seconds.
In order to perform the nitriding treatment with the plasma jet 35 on the inside of the porous layer of the device, the plasma treatment was performed by jetting the plasma jet onto the end face 220 (both left and right) in FIG. Specifically, first, jetting is performed from the left side, then the element is inverted, and jetting is performed from the right side, thereby ensuring the nitriding treatment in the porous layer.
By the above, the gas sensor element provided with the porous layer containing nitride can be obtained.
Note that the step of spraying the plasma jet can also be performed after polishing the fired body.

(Example 3)
In this example, the performance of the gas sensor element provided with the nitride-containing porous layer according to Example 1 and the element not so was evaluated by the following method.
That is, the gas sensor element of Example 1 according to the present invention (manufactured by the manufacturing method according to Example 2) was left for 15 minutes or 1 day in a state where it was installed in the exhaust pipe of an automobile engine. Although the exhaust pipe contained water vapor exceeding the amount of water vapor at 100% at room temperature in a high temperature atmosphere of 200 ° C. or more immediately after the engine was stopped, it reached room temperature level within 2 hours and within 20 hours. It has been confirmed that the humidity is equivalent to that of outside air.
Further, as a comparative sample, a gas sensor element similar to that in Example 1 but not subjected to nitriding treatment by the method according to Example 2 was prepared, and this was applied to the exhaust pipe of an automobile engine under the same conditions as described above. Left alone.
Then, the automobile engine was started, and the output of the gas sensor element for 20 seconds after the start was obtained, which is shown in the diagrams according to FIGS.

  As is apparent from FIG. 4, in the measurement after being left for 15 minutes, the sensor output of the present invention and the comparative sample were almost normal, but after being left for one day, a large rich shift appeared in the comparative sample. However, it has been found that the sensor output from the element according to the present invention does not exhibit a clear rich shift and is sufficiently effective for air-fuel ratio control.

Example 4
In this example, the ratio of the number of nitrogen atoms to the number of oxygen atoms contained in the porous layer (= nitrogen / oxygen ratio, unit is atomic%) and the rich shift are caused by the gas sensor element manufactured by the method according to Example 2. The relationship with the abnormal output amount was measured and shown in the diagram of FIG.
In this example, elements having different nitrogen / oxygen ratios (nitrogen / oxygen ratio = 0, 0.05, 0.10, 0.15, 0.20, 0.30, and those having a nitrogen / oxygen ratio of 0 are nitrided It was produced by appropriately changing the time for which a comparative sample that does not contain an object in the porous layer was exposed to the plasma jet. In addition, five elements were produced for each ratio.
The abnormal output amount was calculated by using the peak value (A / F addition / subtraction value) of the shift amount, and the result is shown in the diagram according to FIG.
From the figure, it was found that when the nitrogen / oxygen ratio increases to some extent, the abnormal output approaches almost 0, so that it is preferable to contain nitride so that the nitrogen / oxygen ratio is 0.20 atomic%.

(Example 5)
In this example, the present invention is applied to a cup-type gas sensor element.
As shown in FIG. 7, a bottomed cup-type solid electrolyte body 11 having a reference gas chamber 40 for introducing air therein, a reference electrode 112 provided on the inner surface, and a measured gas side electrode 111 provided on the outer surface, The porous layer 22 is provided so as to cover the measured gas side electrode 111.
A rod-shaped ceramic heater 49 is inserted into the reference gas chamber 40.
The entire porous layer 22 contains nitride uniformly.
The other detailed configuration is the same as that of the first embodiment, and has the same effects as the first embodiment.

Sectional explanatory drawing of the gas sensor element concerning Example 1. FIG. BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure | tissue of a porous layer concerning Example 1. FIG. Explanatory drawing of the nitriding processing apparatus used when the porous layer of a gas sensor element contains nitride in Example 2. FIG. The diagram which shows the time change of the sensor output after leaving for 15 minutes in the gas sensor element concerning this invention concerning the example 3 and a comparative sample. The diagram which shows the time change of the sensor output after 1 day leaving in the gas sensor element concerning this invention concerning the example 3 and a comparative sample. The diagram which shows the relationship between the abnormal output amount and nitrogen / oxygen ratio concerning Example 4. FIG. Sectional explanatory drawing of the cup-type sensor element concerning Example 5. FIG. The diagram which shows the kind and relative adhesion amount of the gas which adhere to a gas sensor element. The diagram of the relative adhesion amount of H ₂ O adhering to the diffusion resistance layer of the gas sensor element and the measured gas side electrode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Gas sensor element 11 Solid electrolyte body 111 Gas side electrode to be measured 112 Reference electrode 22 Porous layer

Claims (6)

It has a solid electrolyte body, a reference electrode provided on the surface of the solid electrolyte body, and a measured gas side electrode, and the surface of the measured gas side electrode that is in contact with the measured gas is covered with a porous layer In the gas sensor element,
The gas sensor element, wherein the porous layer contains a nitride.   2. The gas sensor element according to claim 1, comprising a nitride centering on a surface of the porous layer in contact with the gas to be measured.   3. The gas sensor element according to claim 1, wherein the ratio of the number of nitrogen atoms to the number of oxygen atoms contained in the porous layer is 0.2% or more.   4. The gas sensor element according to claim 1, wherein the porous layer is made of aluminum oxide, and the nitride is made of aluminum nitride. 5.   5. The gas sensor element according to claim 1, wherein the gas sensor element is a cup type or a laminated type. A porous layer having a solid electrolyte body, a reference electrode provided on the surface of the solid electrolyte body, and a gas side electrode to be measured, the surface of the gas side electrode being in contact with the gas to be measured containing a nitride In manufacturing a gas sensor element covered with
A method for producing a gas sensor element, comprising forming a nitride in the porous layer by spraying a plasma jet containing atomic nitrogen on the porous layer.

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