JP2811976B2 - 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
A type in which a thermistor element for temperature compensation is connected in series with the sensor element is adopted. INDUSTRIAL APPLICABILITY The gas sensor of the present invention can be advantageously used as an automobile oxygen concentration sensor for measuring the oxygen concentration in exhaust gas of an automobile.

[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 sensors. U-tube type (bulk type), plate type and film type are typical structures.
There is a tendency to develop a plate-like or film-like sensor that has a built-in heater to improve low-temperature activity and has a simplified structure.

A conventional thick film laminated titania oxygen sensor is:
For example, it can have a structure as shown in FIGS. 8A and 8B, and here, FIG.
It is sectional drawing which followed the line segment AA of (A). The sensor element 20 is composed of an alumina substrate 21 having a built-in heater 22 for controlling the temperature of the sensor element, and a sensitive portion located at the tip thereof has platinum electrodes 23 and 24 for measuring the resistance of titania. Further, a gas-sensitive layer 25 made of a paste obtained by mixing a titania powder and an organic binder and applying and baking is supported. In addition, platinum electrode 23 and
24 is connected to a platinum lead wire 28. Here, the heater 22 is built into the alumina substrate 21 because the resistance value of titania shows a strong dependence not only on the oxygen partial pressure but also on the temperature. Because there is.

When an oxygen concentration is measured using the titania oxygen sensor shown in FIG. 8, an operation circuit as shown in FIG. 9 is usually used. That is, a titania resistor and a reference resistor (fixed resistor) outside the sensor are connected in series, and rich-lean judgment is made based on a voltage change applied to both ends of the reference resistor. In the operation circuit shown in the figure, the voltage application is D
This is performed using the C power supply 29. However, when the oxygen concentration is measured in this manner, the temperature dependence of titania resistance is not
As shown in Figure 10, the operating temperature range is about 350-400 ° C due to its large (ie, the change in resistance with temperature).
However, it is difficult to operate the sensor over the entire exhaust gas temperature range (about 400 to 900 ° C.) of an automobile. Also,
Even if it is assumed that the sensor operates, the duty ratio (rich / lean output time ratio;%) is greatly shifted at both ends of the sensor operating temperature as shown in FIG.
For this reason, when the exhaust gas temperature is low, it is necessary to raise the element temperature by the built-in heater. However, on the high temperature side, it is impossible to control and the temperature becomes high.

[0005] In addition, if the built-in heater is used, there are additional problems such as that the manufacture of the sensor becomes complicated and that an increase in manufacturing cost cannot be avoided. Here, Esper et al. Announced a heater-free system at the 1979 SAE meeting (MJEsper et al .: SAE Meeting, D
etroit, Michigan, Feb.-Mar. 1979, Paper No.79014
0). This method is described in the book “Automotive Ceramics”.
According to the description on pages 125 to 125, a thermistor element for temperature compensation is connected in series with the oxygen sensor element of titania. More specifically, this method is characterized in that a titania thermistor element cut off from the exhaust gas atmosphere is provided separately from the sensor element in order to compensate for the temperature characteristics of the titania oxygen sensor. However, there are some problems with the heater-less method. For example, in order to shut off the titania thermistor element from the exhaust gas atmosphere, it is necessary to seal the titania element serving as the thermistor with glass or the like after sintering. (Rich-lean) causes a resistance change, and appears as noise in the sensor output. Further, even if the glass seal is successfully formed, there is a problem that the sensor is broken by thermal shock during use, or the resistance of the glass becomes noise.

[0006]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an oxide semiconductor gas sensor which does not have the problems of the prior art, and more specifically, does not require the use of a heater and has a high or low ambient gas temperature. It is an object of the present invention to provide an oxide semiconductor gas sensor having excellent durability that can stably operate a sensor in a wide temperature range regardless of the atmosphere and the rich and lean atmospheres and that can integrally fire a thermistor element.

[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. And a temperature compensation thermistor element connected in series to the sensor element, wherein the temperature compensation thermistor element is formed from a mixture of an n-type oxide semiconductor and a p-type oxide semiconductor. This can be achieved by an oxide semiconductor gas sensor.

The mixture of the n-type oxide semiconductor and the p-type oxide semiconductor used as the material of the thermistor element in the present invention has the opposite characteristics to the atmospheric gas. Preferably, the element is retrofitted to the sensor element. The n-type oxide semiconductor and the p-type oxide semiconductor useful in the practice of the present invention are as listed below, respectively. n-type oxide semiconductors: MgFe ₂ O ₄ , TiO ₂ , ZnFe ₂ O ₄ , NiFe ₂ O ₄
Such. P-type oxide semiconductor: MgCr ₂ O ₄ , ZnCr ₂ O ₄ , NiCr ₂ O _{4 and the} like.

In the gas sensor according to the present invention, the sensitive part comprises the substrate and the gas-sensitive layer provided thereon as described above. The sensitive part can have various shapes commonly used in this technical field, provided that the substrate of the sensitive part is preferably made of a material such as alumina, and the gas sensitive layer is Preferably, it is composed of titania which is strong in a high-temperature reducing atmosphere. However, if necessary, for example, the gas-sensitive layer may be made of the above-described n-type oxide semiconductor or p-type oxide semiconductor. Above the gas-sensitive layer, a conventional method is generally used for the purpose of improving the durability of the sensor by capturing deposits (lead compounds and the like) in the exhaust gas and protecting the gas-sensitive layer. As has been
It is recommended to further provide a trap layer. As the material of the trap layer, the same titania or alumina as in the gas-sensitive layer can be used.

Further, if necessary, an unburned combustible gas (HC,
A reaction layer for burning H ₂ , CO, etc.) may be interposed. This is because if unburned combustible gases are present in the exhaust gas, these gases are prevented from burning in the gas-sensitive layer and causing a partial increase in temperature in the layer and thus a change in titania resistance. It is effective for This reaction layer is preferably made of a material having extremely poor sintering properties, and suitable materials include, but are not limited to, alumina, magnesia, zirconia, spinel, and the like. .

[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]

When measuring the oxygen concentration using the titania oxygen sensor of the present invention, an operation circuit as shown in FIG. 2 is used. That is, a titania resistor and a reference resistor (thermistor) inside the sensor are connected in series, and a rich-lean determination is made based on a voltage change applied to both ends of the reference resistor. In the operation circuit shown in the figure, the voltage is applied to the DC power supply 9.
This is performed using As will be understood, in the case of this operation circuit, unlike the conventional operation circuit of FIG. 9, the reference resistance is varied as a thermistor element.

As described above, the present invention is characterized in that a mixture of an n-type oxide semiconductor and a p-type oxide semiconductor is used as the thermistor material of the thermistor element. An n-type oxide semiconductor exhibits high resistance when lean (when oxygen is excessive) and low resistance when rich (when oxygen is insufficient). Conversely, a p-type oxide semiconductor shows low resistance when lean and high resistance when rich. When these two kinds of oxide semiconductors are mixed, a thermistor material having a stable resistance can be obtained without being affected by the atmosphere in a rich atmosphere or a lean atmosphere. Therefore, if this material is used as a thermistor, it is not necessary to use a glass seal or the like that causes the above-described inconvenience, and a durable reference resistance thermistor can be easily incorporated into the sensor element.

[0014]

FIG. 1 is a sectional view of a preferred example of an oxide semiconductor gas sensor according to the present invention, taking a titania oxygen sensor as an example. The sensor element 10 is composed of an alumina substrate 1 having no built-in heater, and its sensitive part has a titania thick film 5 which can act as a gas-sensitive layer, as shown in the figure.
In addition, the thermistor thick film 6 that can function as a thermistor element is embedded and is wrapped by the trap layer 7. The operation circuit of this titania oxygen sensor is shown in FIG.
With reference to the above.

The titania oxygen sensor of FIG. 1 can be manufactured, for example, as follows. First, as shown in FIG. 3, platinum electrodes 3 and 4 are formed in the illustrated electrode pattern on a 300 μm-thick alumina green sheet 11 formed by a doctor blade method using a screen printing method. Next, as shown in FIG. 4, a window frame sheet 13 and a cover sheet 14 each having a sensing element cavity 15 and a reference resistance thermistor cavity 16 punched into a window frame shape are formed on the alumina green sheet 11 after the formation of the platinum electrode. Laminate and crimp in order. At this time, the tip of the platinum lead wire 8 is
And 4. Next, the obtained laminate was
℃ to degrease the binder in the green sheet,
Further, firing is performed at 1500 ° C. An alumina substrate showing the configuration of the tip portion in FIG. 5 is obtained.

After forming the alumina substrate, a sensing element is formed (not shown). A platinum-rhodium catalyst is supported on 99.9% pure titania powder and mixed with an organic solvent to form a paste. This paste is applied in a thickness of 200 μm to the sensing element cavity of the alumina substrate. Subsequently, a thermistor element is formed. MgFe ₂ O ₄ powder as an n-type oxide semiconductor and p-type oxide semiconductor
MgCr ₂ O ₄ powder is mixed in an equimolar amount, and an organic solvent is mixed with the mixed powder to form a paste. This paste-like material is applied in a thickness of 200 μm to the reference resistance thermistor cavity of the alumina substrate. Then, after firing the substrate at 1200 ° C. for 2 hours, an alumina paste supporting a platinum-rhodium catalyst was applied at a thickness of 100 μm to form a trap layer for trapping deposits in the exhaust gas. Bake at 2 ° C. for 2 hours. The cross section of the sensor element thus formed is as shown in FIG.

In the titania oxygen sensor manufactured as described above, the value of the reference resistance is not fixed to the constant value of the titania as shown in FIG. So that it is in the middle of the rich resistance
The length of L ₁ and L ₂ shown in FIG. 5 may be adjusted. FIG. 7 shows the duty ratio characteristics of this sensor. As understood from the graph of FIG. 7, according to the present invention, extremely stable characteristics can be obtained in a wide temperature range as compared with the characteristics of the conventional sensor (FIG. 11).

[0018]

The temperature dependence of the titania resistance is extremely remarkable, so that the conventional gas sensor uses a heater in combination. However, in the gas sensor of the present invention, the temperature dependence of the titania resistance is so small as to be negligible. It is not necessary to use a heater together. Therefore, according to the present invention, not only is a circuit for controlling the heater unnecessary, but also the sensor element can be downsized, and the manufacturing cost can be greatly reduced. Furthermore, according to the present invention, even if the thermistor is exposed to the atmospheric gas, the characteristics of the two are offset, so that the sensor element does not need to be affected by the atmospheric gas, and the high-precision measurement has a sealing property. This can be achieved without consideration (without using a glass seal or the like).

[Brief description of the drawings]

FIG. 1 is a sectional view showing the structure of a titania oxygen sensor of the present invention.

FIG. 2 is a circuit diagram showing the operation of the titania oxygen sensor of the present invention.

FIG. 3 is a plan view showing formation of a platinum electrode for a sensor.

FIG. 4 is a perspective view showing the formation of an alumina substrate for a sensor.

FIG. 5 is a plan view showing a tip portion of the alumina substrate of FIG.

FIG. 6 is a graph showing temperature dependence of titania resistance in the sensor of the present invention.

FIG. 7 is a graph showing the temperature dependence of the duty ratio in the sensor of the present invention.

FIG. 8 is a plan view (A) and a cross-sectional view (B) showing the structure of a conventional titania oxygen sensor.

FIG. 9 is a circuit diagram showing the operation of a conventional titania oxygen sensor.

FIG. 10 is a graph showing the temperature dependence of titania resistance in a conventional sensor.

FIG. 11 is a graph showing temperature dependency of a duty ratio in a conventional sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Alumina substrate 5 ... Titania thick film 6 ... Thermistor thick film 7 ... Trap layer 9 ... DC power supply 10 ... Sensor element

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-56-84541 (JP, A) JP-A-63-45551 (JP, A) JP-A-55-129742 (JP, A) JP-A-55-129742 124059 (JP, A) JP-A-61-155946 (JP, A) JP-A-63-85344 (JP, A) JP-A-2-264854 (JP, A) JP-A-62-114355 (JP, U) (58) Field surveyed (Int.Cl. ⁶ , DB name) G01N 27/12

Claims (1)

(57) [Claims] 1. A gas-sensitive layer made of a metal oxide semiconductor whose resistance value changes according to the oxygen partial pressure of an atmospheric gas is provided on a substrate, and a temperature compensating thermistor element is connected in series with the sensor element. An oxide semiconductor gas sensor according to claim 1, wherein said temperature-compensating thermistor element is formed from a mixture of an n-type oxide semiconductor and a p-type oxide semiconductor.