JPH04223260A - Oxygen sensor element - Google Patents

Abstract

PURPOSE:To provide a limiting current type oxygen sensor element using a solid electrolyte of zirconia, of which a heater characteristic is not made unstable even when a heater voltage is made large relatively and which can conduct precise measurement of the concentration of oxygen. CONSTITUTION:A heater characteristic is made stable by forming at least a surface part of a portion wherein a heater is formed, out of a material constituted of at least one kind of ceramic constituent selected from a group comprising Al2O3, MgO, SiO2, BeO, CaO, ThO2, HfO2, TiO2, Cr2O3, Cr2C3, Be2C, TiC, ZrC, VC, NbC, HfC, TaC, SiC, TaC, SiC, B4C, Mo2C, WC, TiN, Si3N4, ZrN, HfN and AlN, or of this ceramic constituent of 50mol% or above and zirconia of 50mol% or below.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a limiting current type oxygen sensor element using a zirconia solid electrolyte, and in particular, the heater characteristics do not become unstable even when the heater voltage is relatively high, and the oxygen concentration can be accurately measured. The present invention relates to an oxygen sensor element that can be used in a wide range of applications, from general household room monitors to industrial oxygen deficiency monitors, oxygen concentration detection devices for oxygen concentration control, and the like.

0002

2. Description of the Related Art Various types of sensors have been proposed and used to measure the concentration of various gases in the air. In particular, oxygen concentration needs to be detected in a heated room, in underground equipment, in a manhole, in a ship's hold, in a silo, etc., and various types of oxygen sensors are used.

A typical oxygen sensor is a limiting current type oxygen sensor, which has a structure shown in FIG. 1, for example. That is, electrodes 3a and 3b are provided on both sides of an oxygen ion conductive substrate 1 made of a zirconia solid electrolyte, and a spacer 6 and a sealing plate 4 are provided to form an internal chamber 8 on the cathode side (electrode 3a side). It has a structure having at least one micro diffusion hole 2 communicating with the internal chamber 8. Although the oxygen sensor element shown in FIG. 1 has the micro diffusion holes 2 provided in the solid electrolyte substrate 1, the micro diffusion holes may be provided in the sealing plate instead of the solid electrolyte substrate. An example of this type of device is the oxygen sensor device shown in FIG. In the oxygen sensor element shown in FIG. 2, minute diffusion holes 2 are formed in a sealing plate 4 that forms an internal chamber 8. In addition,
Each part of the oxygen sensor element shown in FIG. 2 is numbered to correspond to each part of the oxygen sensor element shown in FIG. 1, respectively.

In the limiting current type oxygen sensor element as shown in FIGS. 1 and 2, oxygen in the internal chamber 8 is ionized into O2- at the cathode 3a, which is then ionized into O2- by the zirconia (mainly) solid electrolyte substrate 1. It moves inside towards the anode 3b, and at the anode O2- releases electrons and becomes oxygen gas.
Since the emitted electrons return to the power source through the anode, the oxygen concentration can be detected by detecting the current between the electrodes. Oxygen is sucked into the device through the minute diffusion holes 2 and exhausted at the anode 3b of the solid electrolyte substrate, but the amount of oxygen sucked in at this time is rate-limited depending on the size of the minute diffusion holes, etc., depending on the oxygen concentration. , the current value saturates at a certain value. This saturation current is called a limiting current, and since its value is approximately proportional to the oxygen concentration in the external atmosphere, the oxygen concentration can be measured by reading the limiting current.

By the way, the oxygen ion conductivity of zirconia increases as the temperature increases, so oxygen sensors usually have a
It is used after being heated to 50-450°C. Therefore, a heater is attached to the oxygen sensor element, and generally a heater 5 is formed on the sealing plate 4 as shown in FIGS. 1 and 2. This heater is formed on the surface of the sealing plate or the like by a thick film method or a thin film method. In the oxygen sensor element shown in FIGS. 1 and 2, the heater 5 meanders over the sealing plate 4, so it is shown in a separated state in the cross-sectional views of FIGS. 1 and 2.

[0006]

[Problems to be Solved by the Invention] As described above, since the oxygen sensor element uses a heater to raise the operating temperature to about 400°C, in order to maintain the airtightness of the internal chamber ( In order to prevent oxygen from entering and leaving the solid electrolyte substrate), it is common to form the sealing plate etc. from a material having a coefficient of thermal expansion equivalent to that of the solid electrolyte substrate. That is, in the conventional oxygen sensor element, a zirconia-based material is used as the sealing plate, and a heater is formed thereon.

However, in an oxygen sensor element having such a configuration, if the heater voltage is set to a relatively high voltage of, for example, 2.5 volts or more, the heater characteristics become unstable, and the accuracy of the oxygen sensor element also decreases. However, accurate measurements could not be made. This is because when a high voltage is applied to a heater installed on a zirconia-based substrate (sealing plate), the zirconia in the area in contact with the heater decomposes and metal zirconium precipitates on the surface of the substrate, which causes the heater characteristics to deteriorate. This seems to be because it becomes unstable.

[0008] Therefore, it is possible to obtain the required amount of heat by keeping the heater voltage low and increasing the current, but this method requires a power supply with a large current capacity, and the problem is that the power consumption of the sensor drive circuit also increases. There is.

[0009] Accordingly, an object of the present invention is to provide an oxygen sensor element which can eliminate the above-mentioned disadvantages, operate at high voltage and low current, and obtain stable and accurate values.

0010

[Means for Solving the Problems] As a result of intensive research in view of the above object, the present inventors have determined that at least the surface portion of the portion where the heater of the oxygen sensor element is formed is formed on a solid electrolyte substrate in the operating temperature range of the oxygen sensor element. If it is made of a ceramic material that has a coefficient of thermal expansion equivalent to The present inventors have discovered that the influence of changes in heater characteristics due to oxidation can be eliminated, thereby making it possible to provide an oxygen sensor element that can perform accurate measurements with low power consumption, and has conceived the present invention.

That is, the limiting current type oxygen sensor element of the present invention includes an oxygen ion conductive substrate made of a zirconia-based solid electrolyte, an anode and a cathode provided on both surfaces of the substrate, and an anode and a cathode provided on the cathode side. It has an internal chamber and a minute diffusion hole that communicates with the internal chamber, and at least the surface portion of the heater provided to heat and maintain the sensor element at an operating temperature is made of Al2O3, MgO.
, SiO 2 , BeO , CaO , ThO 2 , H
fO2, TiO2, Cr2O3, Cr2
C3, Be2C, TiC, ZrC, VC
, NbC, HfC, TaC, SiC, B4
C, Mo2C, WC, TiN, Si3N4
, ZrN, HfN, and AlN, or this ceramic component is 50 mol% or more, and zirconia is 50 mol% or more.
It is characterized by being made of a material containing mol % or less.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view showing an oxygen sensor element according to an embodiment of the present invention. This oxygen sensor element includes a zirconia-based solid electrolyte substrate 1, electrodes 3a and 3b formed on both sides thereof, a spacer 6 arranged to form an internal chamber 8 on the side where the electrode 3a is formed, and a seal. It has a stop plate 4. Further, a heater 5 for heating the oxygen sensor element to an operating temperature is formed on one surface of the sealing plate 4 (the surface opposite to the internal chamber). As described above, the oxygen sensor element of the present invention is similar to the conventional oxygen sensor element in its assembly structure, but as will be described later, the material used in the heater forming part for heating and maintaining the oxygen sensor element to the operating temperature is different. It is characterized by being limited to specific things.

As the solid electrolyte substrate 1, it is preferable to use zirconia in which at least one stabilizer such as yttria, calcia, ceria, etc. is dissolved.

In the oxygen sensor element of this embodiment, minute diffusion holes 2 are formed in a solid electrolyte substrate 1. Oxygen gas diffuses into the minute diffusion holes 2, and the diffusion rate is generally proportional to the hole diameter. Therefore, the pore diameter is determined so that the rate of diffusion within the pores becomes rate-determining. The ratio of the cross-sectional area S to the length L of the hole S/
L is preferably 0.2 to 0.9 μm.

Electrodes 3a and 3b formed on both sides of the substrate 1
is made of porous material. Since the electrodes 3a and 3b function as catalyst activation electrodes, Pt, Pd, Ag, Rh, In
It is preferable to use metal materials such as, alloy materials thereof, or a mixture of at least one of these metal materials and a difficult-to-sinter material such as zirconia or boron nitride to prevent sintering. . In particular, it is preferable to use Pt or a mixture of Pt and zirconia. Note that the electrode 3a formed within the internal chamber 8 serves as a negative electrode.

The electrode 3a, which is a negative electrode, is connected to the internal chamber 8 as described above.
This internal chamber 8 is defined by the solid electrolyte substrate 1, the spacer 6, and the sealing plate 4 described above. As this spacer 6, it is preferable to use an inorganic substance (sealing material such as glass) having a coefficient of thermal expansion comparable to that of the solid electrolyte substrate 1.

In the oxygen sensor element of this embodiment, the sealing plate 4
serves as a substrate for forming the heater 5, and at least the surface portion (the portion in contact with the heater) of the sealing plate 4, which is the forming portion of the heater, is mainly composed of a specific oxide ceramic, carbide ceramic, or nitride ceramic. It consists of a substance. Such oxide ceramics include Al
2O3, MgO, SiO2, BeO, C
aO, ThO2, HfO2, TiO2,
Examples include Cr2O3. In addition, carbide ceramics include Cr2C3, Be2C, Ti
C, ZrC, VC, NbC, HfC, TaC
, SiC, B4C, Mo2C, WC, and the like. Furthermore, as nitride ceramics, TiN
, Si3N4, ZrN, HfN, AlN
etc. Preferred ceramics include Al2O, which is industrially easily available and has relatively high strength.
3, MgO, SiO2, etc.

[0018] In the present invention, the above-mentioned ceramics or these are used as main components (50 mol% or more), and zirconia is
A heater is formed on ceramics containing 0 mol% or less. In the oxygen sensor element shown in FIG. First, on the heater forming side,
A configuration may be employed in which a thin ceramic layer made of the above-mentioned material (including ceramics containing 50 mol % or less of zirconia) is formed, and the heater is formed on the thin ceramic layer.

The above-mentioned ceramic, or a mixture or solid solution of it and zirconia, has a coefficient of thermal expansion that is the same as or slightly smaller than that of the zirconia-based solid electrolyte substrate at the operating temperature of the oxygen sensor element (approximately 350 to 400°C). , and does not cause distortion of the element due to heating. Therefore, the airtightness of the internal chamber is maintained well.

Furthermore, if each of the above-mentioned ceramics covers the surface of the substrate on which the heater is formed, the heater
Even when a high voltage of volts or higher is applied, the influence of changes in heater characteristics due to deterioration of zirconia is eliminated, the element temperature is kept constant, and good oxygen concentration measurement can be performed. When each of the above-mentioned ceramics containing zirconia is used for the heater forming portion, the zirconia content must be 50 mol % or less as described above. If the zirconia content exceeds 50 mol%, the zirconia will deteriorate and the heater characteristics will change when a high voltage of 2.5 volts or more is applied to the heater, which is not preferable.

[0021] The heater is one in which a heating element is formed in a planar or linear form on a substrate (or on a sealing plate as in this embodiment), and is formed by screen printing using platinum paste or photolithography. It can be formed by the following methods.

Note that platinum wire or the like can be used as the lead wire 7 connected to the heater 5 and the electrodes 3a, 3b.

FIG. 2 is a schematic cross-sectional view showing an oxygen sensor element according to another embodiment of the present invention. In this embodiment, the internal chamber 8 is formed relatively large, and the end portion of the sealing plate 4 is bent to have a structure that also serves as a spacer. Further, the micro diffusion holes 2 are formed not in the solid electrolyte substrate 1 but in the sealing plate 4. The rest of the structure is substantially the same as that of the oxygen sensor element shown in FIG.

In this embodiment, the sealing plate 4 is formed of the above-mentioned oxide ceramics, carbide ceramics, nitride ceramics, or a solid solution of these and zirconia.

The present invention will be explained in more detail by the following specific examples. Example 1 An oxygen sensor element having the structure shown in FIG. 1 was produced. As the solid electrolyte substrate 1, 8 mol% of yttria as a stabilizing material was dissolved in zirconia as a solid solution. The solid electrolyte substrate 1 was molded into a sufficiently dense plate with no open pores, and the plate thickness was 0.2 mm.

The micro diffusion holes 2 were formed approximately at the center of the solid electrolyte substrate 1 so as to have an axis in the thickness direction. The pore diameter was approximately 10 μm.

The electrodes 3a and 3b were formed by applying a conductive paste made by adding an organic binder and an organic solvent to platinum powder and firing the paste. The paste was applied to the solid electrolyte substrate 1 except for about 1 mm width at the outer periphery. Further, lead wire portions connected to the electrodes 3a and 3b were formed in the same manner as the electrodes 3a and 3b.

As the sealing plate 4, an alumina/zirconia plate with a thickness of 0.2 mm was used. This alumina/zirconia plate was produced by mixing alumina powder and zirconia powder at a molar ratio of 9:1, molding the mixture, and firing the mixture.

A heater 5 made of platinum whose resistance value was controlled to 35 ohms was formed on one side of the obtained sealing plate 4 by a thin film method. The heater was formed in a meandering manner on the sealing plate 4. A lead wire portion was also connected to this heater 5 using the above-mentioned conductive paste.

Using the oxygen sensor element thus prepared, oxygen gas was detected in the following manner. First, a DC voltage of 9 volts was applied to the heater 5 to maintain the element temperature at 400°C. The current value flowing through the heater at this time was approximately 125 mA. In this state, electrodes 3a and 3b
A DC voltage of 1.4 volts was applied between the two, and the oxygen concentration in the atmosphere was measured.

Even when the above measurements were carried out for a long time (200 hours), the operating temperature of this oxygen sensor element did not change, indicating good heater stability. Therefore, there was no change in the limiting amount of current flowing between the two electrodes, and it was possible to perform good oxygen gas concentration detection.

Examples 2 and 3 Oxygen sensor elements were produced in the same manner as in Example 1, except that the sealing plate 4 forming the heater 5 was made of the following material. Example 2: The molar ratio of alumina and zirconia in the sealing plate 4 was set to 7:3. Example 3:
The molar ratio of alumina and zirconia in the sealing plate 4 was set to 5:5.

When oxygen gas was detected using the above oxygen sensor element in the same manner as in Example 1, it was found that in the oxygen sensor element of Example 2, the heater stability was good and the element could be kept at a constant temperature. This enabled reliable oxygen gas detection. Further, in the oxygen sensor element of Example 3, although the stability of the heater was somewhat unstable, the element could be maintained at a substantially constant operating temperature.

Comparative Example 1 An oxygen sensor element was produced in the same manner as in Example 1, except that the sealing plate 4 forming the heater 5 was made of the following material. Comparative Example 1: The molar ratio of alumina and zirconia in the sealing plate 4 was 3:7.

When oxygen gas was detected using the above oxygen sensor element in the same manner as in Example 1, the heater was unstable and the element could not be kept at a constant temperature. Detection could not be performed.

[0036]

As explained above, in the oxygen sensor element of the present invention, the heater has good stability, the element operating temperature is constant, and oxygen can be detected with high accuracy.

Furthermore, a relatively high voltage of 2.5 volts or more can be applied to the heater of the oxygen sensor element of the present invention, thereby making it possible to keep the heater current low. In a conventional limiting current type oxygen sensor using a zirconia material for the heater substrate, the heater voltage is set to about 2.3 volts at most for safety reasons, and therefore the heater current is about 500 mA. On the other hand, in the oxygen sensor element of the present invention, this heater current is 125 mA.
It is possible to operate with a current value of about 1/4 or less.

According to the oxygen sensor element of the present invention, power consumption can be reduced and the life of the element itself can be extended. Such an oxygen sensor element can be used in a wide range of applications, from general household room monitors to industrial oxygen deficiency monitors, oxygen concentration detection devices for oxygen concentration control, and the like.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing an oxygen sensor element according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing an oxygen sensor element according to another embodiment of the present invention.

[Explanation of symbols]

1 Solid electrolyte substrate 2. Micro diffusion pores 3a, 3b electrode 4 Sealing plate 5 Heater 6 Spacer 7 Lead wire 8　　　　　　　Inner room

Claims (2)

[Claims] 1. An oxygen ion conductive substrate made of a zirconia-based solid electrolyte, an anode and a cathode provided on both sides of the substrate, an internal chamber provided on the cathode side, and a microelectrode that communicates with the internal chamber. In a limiting current type oxygen sensor element having a diffusion hole, at least the surface portion of a forming part of a heater provided to heat and maintain the sensor element at an operating temperature is made of Al2O3, MgO, SiO2,
BeO , CaO , ThO 2 , HfO 2 , T
iO 2 , Cr2 O 3 , Cr2 C 3 , B
e2C, TiC, ZrC, VC, NbC, H
fC, TaC, SiC, B4C, Mo2
C, Mo2C, WC, TiN, Si3N4
, ZrN, HfN, AlN, or 50 mol% or more of this ceramic component and 50 mol% of zirconia.
An oxygen sensor element characterized by being formed of a material consisting of the following: 2. The oxygen sensor element according to claim 1, wherein the heater is provided on a sealing plate that forms the internal chamber.