JP2847979B2 - Oxide semiconductor gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide semiconductor gas sensor, and more particularly, to a gas-sensitive layer made of a metal oxide semiconductor, for example, titania, whose resistance varies according to the oxygen partial pressure of an atmospheric gas. The present invention relates to an oxide semiconductor gas sensor provided above. The gas sensor of the present invention
Preferably, it has a plate-like or film-like structure in which a heater is built in the substrate, and can be advantageously used as an automobile oxygen concentration sensor for measuring the oxygen concentration in exhaust gas from automobiles.

[0002]

2. Description of the Related Art As is well known, various structures have been proposed as exhaust gas sensors for automobiles or, in particular, oxygen concentration sensors. U-tube type (bulk type), plate type and film type are typical types. Particularly recently, a plate type with a built-in heater to improve low-temperature activity and to simplify the structure is used. However, there is a tendency to develop a film-shaped sensor.

As a typical example of such an oxygen concentration sensor,
An oxygen concentration sensor described in JP-A-61-231448 can be mentioned. This sensor is a so-called chip type sensor with a heater, and, as shown in a cross section in FIG. 7, a platinum electrode 23 and 24, a gas-sensitive layer 25, and a It has a structure in which a trap layer 27 for trapping deposits (such as lead compounds) in exhaust gas is provided. The illustrated oxygen concentration sensor 20
Can be advantageously manufactured according to the thick film lamination technology, in which case, as a material of the gas-sensitive layer 25, platinum-containing titania that is resistant to a high-temperature reducing atmosphere can be generally used, and as the trap layer 27, titania or alumina can be used. Etc. can be used. In this oxygen concentration sensor, even when platinum particles of the gas-sensitive layer 25 grow and try to fall out of the layer when the ambient gas is at a high temperature, the trap layer 27 exists above the layer, so the trapped layer is trapped in this trap layer. It does not fall into the atmosphere gas. Therefore, this Japanese Patent Application Laid-Open No. 61-231448
According to the invention described in Japanese Patent Application Laid-Open Publication No. H11-107, there is an effect that it is possible to suppress a decrease in the ability to detect oxygen concentration during use in a high-temperature atmosphere gas.

[0004]

However, conventional oxide semiconductor gas sensors, including the above-described titania chip type sensor with heater, have problems that must be solved due to their unique structure. . That is, in the case of such a sensor, the deposits in the exhaust gas can be sufficiently captured by the outermost trap layer as described above, and there is no problem. However, unburned combustible gas (hydrocarbon HC , Hydrogen H ₂ , carbon monoxide CO, etc.)
Has a problem of burning by the action of a noble metal catalyst in the trap layer and / or the titania-sensitive gas layer. Such combustion of the combustible gas causes a partial increase in the temperature of the gas-sensitive layer due to the heat generated at that time, causing the grain growth of titania particles and, as a result, a change in titania resistance. As will be readily appreciated, any change in the measured titania resistance raises a significant problem in that the final measured oxygen concentration will be inaccurate.

The change in titania resistance can be easily understood from the graph of FIG. 8 showing the temperature dependence. FIG.
Is a graph plotting the results of a durability test performed to evaluate the temperature dependence of titania resistance in a titania chip type sensor with a heater as shown in FIG. 7. As time elapses after the predetermined time (curve B) from the curve A), it increases by about 0.3 to 0.5 digit. This increase in the titania resistance is almost independent of the change in the sensor element temperature.

Accordingly, an object of the present invention is to prevent the growth of semiconductor particles of the metal oxide semiconductor constituting the gas-sensitive layer due to the combustion of the unburned combustible gas contained in the high-temperature atmosphere gas. To provide a simple oxide semiconductor gas sensor.

[0007]

According to the present invention, there is provided a gas-sensitive layer comprising a metal oxide semiconductor having a resistance value which changes in accordance with an oxygen partial pressure of an atmosphere gas. The difference between the density at the time of molding and the density after heat treatment at a temperature of 1400 ° C. is within 20% on the gas contact side of the gas-sensitive layer, and 1
An oxide characterized in that a combustible gas reaction layer having a highly heat-resistant material having a porosity of 30% or more when heat-treated at a temperature of 400 ° C. or more in combination with a noble metal catalyst, and a trap layer are sequentially provided. This can be achieved by a semiconductor gas sensor.

[0008] In the gas sensor of the present invention, as described above, the sensitive portion comprises the substrate and the gas-sensitive layer provided thereon. The sensitive part may have various shapes commonly used in this technical field, but is preferably a sensitive part of a chip type sensor with a heater, and hereinafter, refer to such a sensitive part. The present invention will be described as follows. The substrate of the sensitive part is preferably made of a material such as alumina, and the gas-sensitive part is preferably made of titania that is resistant to a high-temperature reducing atmosphere. However, if necessary, the gas-sensitive layer may be composed of a metal oxide semiconductor such as tin oxide or zinc oxide.

In the gas sensor according to the present invention, a heat-resistant material having a difference from the molding density within 20% and a porosity at that time of 30% or more at a heat treatment at 1400 ° C. is provided on the gas contact side of the gas-sensitive layer. It is characterized in that a combustible gas reaction layer is provided. This combustible gas reaction layer contains unburned combustible gas (HC, HC) in exhaust gas as described in the section of the prior art.
H _2, if the CO, etc.) had their gas is combusted in a gas sensitive layer is effective to prevent the elicit a partial high temperature of the layer. The flammable gas reaction layer is preferably made of a material having extremely poor sintering properties. Suitable materials include, but are not limited to, alumina, magnesia, zirconia, spinel, and the like. Can be.

[0010] The gas sensor of the present invention is generally used for the purpose of capturing the deposits in the exhaust gas to improve the durability of the sensor and protecting the gas-sensitive layer. In addition, it is recommended to further provide a trap layer on the gas-sensitive layer. As the material of the trap layer, the same titania or alumina as in the gas-sensitive layer can be used.

[0011] As described above, the gas sensor of the present invention comprises:
Although it can be advantageously used to measure the oxygen concentration in the exhaust gas, it can be used to measure the oxygen concentration in other gases or the concentration of other components as required.

[0012]

In the oxide semiconductor gas sensor of the present invention, a high heat resistant unburned gas reaction layer is formed between the trap layer and the oxide semiconductor gas-sensitive layer, and the reaction layer removes most of the unburned gas. It is intended to burn. In the configuration of the conventional sensor as shown in FIG. 7, the upper portions (portions not involved in the resistance change) of the trap layer 27 and the titania layer 25 can serve as a reaction layer. Burning does not affect the resistance value, but changes the pore size of the upper layer, which affects sensor characteristics, particularly responsiveness. Trap layer 27
A similar problem occurs even if unburned gas is burned in the furnace.
That is, the pore size of the trap layer changes as a result of combustion, and if the trap layer is made of a material having poor sinterability, such as an alumina-based material, a decrease in trapping capability and a decrease in adhesion to the alumina substrate may occur. Since a problem arises, the problem of unburned gas cannot be solved by the conventional two-layer structure.

[0013] The gas sensor of the present invention, as shown in FIG.
The titania gas-sensitive layer 5 is the same as that of the related art, and has a three-layer structure in which a reaction layer 6 is formed from a material having a very poor sintering property, and a trap layer 7 is thinly provided so as to surround the reaction layer. Has become. Here, when the thickness of the trap layer 7 is increased, a change in the grain size of the trap layer occurs. Therefore, the thickness of the trap layer needs to be about 10 to 30 μm. The unburned gas hardly burns on the outermost surface of the sensor element 10, but penetrates into the interior for about several tens of microns and burns, and radiates heat to the surroundings. There is no problem if you pay attention to it. In addition, the material constituting the reaction layer 6 is determined in consideration of unburned gas burning in the reaction layer.
It must be highly heat resistant. Accordingly, as a material usable for the reaction layer, there are alumina, magnesia, zirconia, spinel and the like as described above. However, even among these materials, there are various characteristics of the powder, and a powder having good sinterability cannot be used. Here, when the unburned gas is burned in the reaction layer, it can be estimated that the temperature will be about 1400 ° C. microscopically, so it is necessary to select a powder having poor sinterability up to 1400 ° C. That is, the combustible gas reaction layer of the present invention is 1400
It is necessary to be made of a high heat-resistant material having a difference from the molding density within 20% in the heat treatment at ° C and a porosity at that time of 30% or more.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the oxygen concentration sensor shown in FIG. The oxygen concentration sensor in FIG. 1 is referred to as a sensor element 10 as a whole, as shown in FIG.
It consists of an alumina substrate 1 with a built-in heater 2. Electrodes 3 and 4 forming a pair in the sensitive part of this sensor element 10
Are each made of platinum. The sensitive part has a three-layer structure of a titania-sensitive gas layer 5, a combustible gas reaction layer 6, and a trap layer 7.

The sensor shown in FIG. 1 can be manufactured, for example, by using the sensor body after the electrodes are formed as shown in FIGS. That is, first, a sensitive part is formed by opening a window for exposing the platinum electrodes 3 and 4 in a part of the alumina substrate 1 having a shape as shown in FIG. As shown in FIG. 4, which is a cross-section taken along line IV-IV in FIG.
A built-in heater 2 is contained in an alumina substrate 1. At the end of the alumina substrate 1, a lead wire 8 connected to the electrodes 3 and 4 is provided.
Is attached.

The gas-sensitive layer 5 is formed as shown in FIG. That is, a paste obtained by further mixing a mixture of high-purity titania powder and a platinum-rhodium catalyst with an organic binder is applied to the window frame portion of the sensitive portion of the alumina substrate 1, dried, and fired at a high temperature. After the formation of the gas-sensitive layer 5 in this manner, the formation of the combustible gas reaction layer 6 is performed as shown in FIG. A paste obtained by further mixing a mixture of alumina powder and a platinum-rhodium catalyst with an organic binder is applied to the upper surface of the gas-sensitive layer 5 and dried. Next, although not shown, a trap layer is formed. The trap layer is formed by obtaining a paste from the alumina powder in the same manner as the above-described reaction layer (but not carrying a catalyst), applying the paste to the upper surface of the reaction layer, and drying. Subsequently, the dried reaction layer and the trap layer are simultaneously fired at a high temperature. The sensor according to the invention, shown in cross section in FIG. 1, is thus obtained. Example 1 A mixture of high-purity titania powder and a platinum-rhodium catalyst mixed with an organic binder in a window frame portion of an alumina substrate containing a heater having a shape as shown in FIGS. Coated at 200 μm thickness, dried and fired at 1200 ° C. for 2 hours. A titania-sensitive gas layer was formed.

Next, a reaction layer was formed. In order to form this reaction layer, four types of alumina powders each exhibiting sinterability were prepared (FIGS. 6 and 7), and a platinum-rhodium catalyst was loaded thereon as in the case of the above-described titania-sensitive layer. Then, it was made into a paste, applied on the gas-sensitive layer with a thickness of 200 μm, and dried. Next, in order to form a trap layer, an alumina powder having an average particle size of 0.5 μm was formed into a paste in the same manner as described above without carrying a catalyst, applied in a thickness of 30 μm, and dried. This layer was fired with the reaction layer at 1100 ° C. for 2 hours. The sensor shown in cross section in FIG. 1 was obtained.

The obtained four types of sensors are exposed to a gas atmosphere at a gas temperature of 400 ° C. with a propane gas burner, and rich-lean switching is performed in a cycle of 2 seconds, and the sensor temperature is set to 800 ° C. with a built-in heater. Then, the durability test was performed continuously for 200 hours. Further, as a comparative sample, a sample having no reaction layer was prepared, and two types of the sample having the thickness of the trap layer of 30 μm and 100 μm were prepared and subjected to the same durability test. As a result, the four types of sensors provided with the reaction layers showed a very good resistance change after endurance as shown in FIG. Note that curves A and B in the figure respectively indicate the initial stage and after a predetermined time, as in the case of FIG. 8 described above. Note that neither the rich resistance nor the lean resistance has risen at all.

However, in the example of using alumina powder and FIG. 6, as the firing of the reaction layer itself progressed, the porosity remarkably decreased, and the responsiveness of the sensor deteriorated. In the comparative sample having a trap layer thickness of 30 μm, the flammable gas burned inside the titania layer, and the titania resistance greatly changed after a predetermined time as shown in FIG. Trap layer thickness is 10
In the case of 0 μm, the change in titania resistance was slightly smaller than that in the case of the thickness of 30 μm, but the sintering of the trap layer still proceeded, and the response was deteriorated.

From the above results, the characteristics required for the reaction layer are that it is desirable that the difference from the molding density in the heat treatment at 1400 ° C. is within 20% and the porosity at that time is 30% or more. Understood. In the present embodiment, alumina was used as the reaction layer material. However, as described above, magnesia, zirconia, spinel, etc., other than alumina, can also be used as the reaction layer material as long as the above conditions are satisfied.

[0021]

According to the present invention, the heat resistance of the sensor is considerably improved as compared with the conventional sensor, and as a result, the sensor control temperature can be increased by 100 to 150 ° C. as compared with the conventional sensor.
It can be controlled up to 850 ℃. Such a high temperature is comparable to the maximum exhaust gas temperature of an automobile. Further, the resistance change type sensor has a very large temperature dependence of the resistance value as shown in FIG. 8, and the characteristics change depending on the sensor temperature. Therefore, at present, the sensor temperature is controlled around 700 ° C by the built-in heater. However, if the exhaust gas temperature is lower than the control temperature, there is no problem. However, if the exhaust gas temperature is higher than the control temperature, the control cannot be performed. However, in the sensor of the present invention, the sensor temperature can be controlled near the maximum exhaust gas temperature, so that the sensor temperature can always be kept constant. Therefore, the temperature characteristics of the sensor can be made extremely small.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a structure of an oxygen concentration sensor according to the present invention.

FIG. 2 is a graph showing the temperature dependence of titania resistance in the sensor of the present invention.

FIG. 3 is a perspective view showing a sensor main body after electrodes are formed.

FIG. 4 is a cross-sectional view taken along line IV-IV of the sensor main body of FIG. 3;

FIG. 5 is a sectional view showing formation of a gas-sensitive portion of the sensor of the present invention in order.

FIG. 6 is a graph showing a change in porosity according to a firing temperature in the sensor of the present invention.

FIG. 7 is a cross-sectional view showing a structure of a conventional oxygen concentration sensor.

FIG. 8 is a graph showing the temperature dependence of titania resistance in a conventional sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Alumina substrate 2 ... Built-in heater 3 ... Platinum electrode 4 ... Platinum electrode 5 ... Titania gas-sensitive layer 6 ... Reaction layer 7 ... Trap layer 10 ... Sensor element

Claims (1)

(57) [Claims] 1. An oxide semiconductor gas sensor having a gas-sensitive layer made of a metal oxide semiconductor whose resistance value changes according to an oxygen partial pressure of an atmospheric gas on a substrate, wherein the gas of said gas-sensitive layer is On the contact side, the density at molding and the temperature of 1400 ° C
A high heat-resistant material having a difference from the density after heat treatment at a temperature of 20% or less and a porosity of 30% or more when heat-treated at a temperature of 1400 ° C. in combination with a noble metal catalyst. An oxide semiconductor gas sensor having a combustible gas reaction layer and a trap layer sequentially provided.