JPH06288952A - Gas sensor - Google Patents

Abstract

(57) [Summary] [Objective] To obtain a gas sensor that can be easily miniaturized, stably obtains the best response speed and detection sensitivity, and has high reproducibility of detection values. [Structure] A gas detector 11 and a temperature sensor 12 are provided on the front surface of an insulating substrate 10, and a heater 16 for heating the gas detector is provided on the rear surface of the substrate. The temperature sensor is a chip thermistor and is mounted on the surface of the substrate in the vicinity of the gas detector, and detects the temperature of the gas detector. Temperature sensor 12
The controller 20 controls the voltage applied to the heater 16 so that the temperature of the gas detector 11 becomes constant based on the detection output of the above.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor suitable for detecting a predetermined gas among gases such as hydrogen, carbon monoxide, alcohols, hydrocarbons, oxygen, carbon dioxide and chlorine. More specifically, it relates to a gas sensor integrated with a temperature sensor.

[0002]

2. Description of the Related Art In a gas sensor, when the temperature of a gas detector is low, its detection sensitivity is low and its response speed to a test gas is slow. Therefore, a heater is incorporated in the substrate on which the gas detector is attached to heat the whole to a high temperature. Further, it is known that the temperature of the gas detection body at this time has an optimum temperature at which the detection sensitivity is increased depending on the types of the gas detection body and the test gas. Table 1 is an example. In Table 1, when the gas detector is a semiconductor gas sensor made of a metal oxide semiconductor thin film, the optimum operating temperature according to the type of the test gas is shown.

[0003]

[Table 1]

Therefore, in order to obtain a highly reliable concentration of the test gas by the measurement using the gas sensor, it is necessary to accurately control the heating temperature of the heater. This type of conventional temperature control is performed by keeping the electric power to the heater constant. However, although the gas sensor using a metal oxide semiconductor thin film as the gas detector has excellent gas detection sensitivity, the gas detector is easily affected by the temperature of the measurement atmosphere in addition to the heat of the heater, and accurate temperature control is required. Difficult to do.

In order to solve this point, a gas sensor integrated with a temperature sensor has been conventionally proposed (Japanese Patent Laid-Open No. 4-273050). In this gas sensor, two sets of counter electrodes covered with the same metal oxide semiconductor thin film are provided on the surface of the same substrate, and a gas barrier heat-resistant insulation is provided on the metal oxide semiconductor thin film covering one set of the counter electrodes. Film is laminated and covered, and a thin film heater is provided on the back side of the substrate. In the metal oxide semiconductor thin film of this gas sensor, its electrical conductivity changes not only with the change in the concentration of the test gas but also with the change in temperature due to the semiconductor properties of the thin film. On the other hand, in a gas barrier heat-resistant insulating thin film that prevents contact with oxygen in the atmosphere and the test gas, its electrical conductivity does not change with changes in the concentration of the test gas, but only with temperature changes. Function as a temperature sensor. for that reason,
When the temperature of the measurement atmosphere changes, the electrode output covered with the heat-resistant insulating thin film is used to correct the electrode output covered with the metal oxide semiconductor thin film, that is, the temperature is compensated. As a result, the detection value of the gas sensor is not affected by the temperature of the measurement atmosphere and changes only with the concentration of the test gas.

[0006]

However, the heater incorporated in the gas sensor is merely applied with a constant voltage so that the gas detector has an optimum temperature as shown in Table 1. Therefore, when the temperature of the gas detector itself changes due to the temperature of the measurement atmosphere or the like, the configuration of the gas sensor described above has a problem that optimum detection sensitivity and response speed cannot be obtained. Especially, when two kinds of test gases are mixed, if the temperature change is automatically compensated by the above two thin films,
It becomes unclear which gas of the two kinds of gas is detected by the gas sensor, and there is a problem that the gas selectivity is changed or lowered. Further, the above gas sensor has a drawback that it is necessary to provide two thin film sensors on the surface of the substrate, a large substrate area is required, and miniaturization is difficult.

The object of the present invention is to easily miniaturize, and by controlling the voltage applied to the heater by the temperature sensor to keep the temperature of the gas detector at a predetermined temperature, the best response speed and detection sensitivity are stabilized. Another object of the present invention is to provide a gas sensor obtained by the above method and having high reproducibility of detection values.

[0008]

The structure of the present invention for achieving the above object will be described with reference to FIG. The present invention is
The gas detector 11 and the temperature sensor 1 are provided on the surface of the insulating substrate 10.
2 is provided, and a heater 16 for heating the gas detection body 11 is provided on the back surface of the substrate 10 to improve the gas sensor. The characteristic configuration is that the temperature sensor 12 is mounted on the surface of the substrate 10 in the vicinity of the gas detector 11 and
It is a chip thermistor that detects the temperature of. Further, as shown in FIG. 2, the gas sensor of the present invention may include a controller 20 that controls the voltage applied to the heater 16 so that the temperature of the gas detection body 11 becomes constant based on the detection output of the temperature sensor 12. preferable.

[0009]

The chip thermistor as the temperature sensor 12 is mounted on the surface of the substrate 10 near the gas detector 11 by soldering or the like. Thereby, the temperature of the gas detector itself can be detected by the chip thermistor 12, and the gas sensor can be downsized. Based on the output of the thermistor 12, the controller 2 keeps the temperature of the gas detector 11 constant.
If 0 controls the voltage applied to the heater 16, the temperature of the gas detector 11 is maintained in the optimum state, and the response speed and detection sensitivity of the gas sensor are stabilized.

[0010]

Embodiments of the present invention will now be described in detail with reference to the drawings. As shown in FIG. 1, a gas detector 11 and a temperature sensor 12 are provided on the surface of a substrate 10 such as alumina or aluminum nitride, which has excellent heat resistance and insulation properties. In this example, the size of the substrate 10 is about 3 mm in length and about 3 mm in width,
The thickness is about 0.5 mm. The gas detector 11 is a metal oxide semiconductor thin film, which is a SnO ₂ thin film having a thickness of about 2 μm. This thin film is vacuum deposited, sputtered,
It is formed on the substrate surface by the ion plating method or the like. Reference numerals 13 and 14 denote electrodes of the gas detector, which are formed by printing gold paste in a predetermined shape, for example. The temperature sensor 12 is a chip thermistor. In this example, the size of the chip thermistor is about 2 mm in length and about 1.25 mm in width,
The thickness is about 1.25 mm. The chip thermistor is Mn,
Terminal pastes 12b, 12 are prepared by applying Ag paste on both end faces of a chip-shaped ceramic body 12a made of oxide powder of a metal such as Fe, Co, Ni, Cu, Al and baking it.
It is made by providing c. A circuit pattern (not shown) is formed by printing a conductive paste on the substrate surface, and the terminal electrode 1 of the temperature sensor 12 is formed on the circuit pattern.
2b and 12c are fixed by soldering 15. In this example, the terminal electrode 12b is soldered to the circuit pattern near the electrode 14 of the gas detector. A thin film heater 16 is provided on the back surface of the substrate 10. The heater 16 has a film thickness of about 2 μm by printing a paste such as gold or platinum.
formed in m. Reference numerals 17 and 18 denote heater electrodes. The heater 16 is provided so as to extend to the back surface of the substrate where the temperature sensor 12 is located.

As shown in FIG. 2, the detection output of the temperature sensor 12 is connected to the controller 20. Controller 2
The control output of 0 is connected to the heater 16. The controller 20 has a memory (not shown) built therein, and the memory stores a value corresponding to the temperature at which the gas (test gas) to be detected by the gas detector 11 can be optimally detected. Figure 2
1, the same reference numerals as those in FIG. 1 denote the same components. Further, 21 is a load, 22 is an input terminal of the gas sensor, 2
3 is its output terminal. In this measuring circuit, when the heater voltage is V _H , the circuit input voltage is V _C , the load resistance is R _L , and the output voltage is V _RL , the resistance R _S of the gas detector is expressed by the following equation. _{R S = {(V C /} V RL) -1} × R L ... (1) The temperature of the gas detection element itself T _S, the heat received from the heater through the substrate T _H, the temperature of the atmosphere T _F Then, T _S
Is expressed by the following equation. T _S = T _H −T _F (2) Here, assuming that the temperature of the temperature sensor itself is T _T , since the heater 16 extends to the back side of the temperature sensor 12, T _T
Is expressed by the following equation. T _T = T _H −T _F (3) From the relationship of the above equations (2) and (3), the temperature sensor 12
The temperature change of can be regarded as the temperature change of the gas detector 11.

When the gas sensor detects a predetermined gas, the controller 20 applies a voltage to the heater 16 so that the gas detector 11 has a temperature at which the gas can be detected optimally. When the temperature of the gas detector itself changes due to a change in the measurement atmosphere or the like, a difference occurs between the detection value of the temperature sensor 12 and the reference value stored in the memory. The controller 20 changes the voltage applied to the heater 16 so as to eliminate this difference. Thereby, the gas detector is always maintained at a predetermined temperature, and the response speed and detection sensitivity of the gas sensor are stabilized.

In the above example, Sn is used as the gas detector.
Although the O ₂ thin film is shown, the present invention is not limited to the semiconductor gas sensor including this metal oxide semiconductor thin film, and may include the other metal oxide semiconductor thin films shown in Table 1 above. Also,
The size of each component shown in the above example is an example, and the present invention is not limited to the above numerical values.

[0014]

As described above, according to the present invention, the temperature sensor is a chip thermistor having a very small mounting area, and it suffices to slightly increase the area of the insulating substrate. Can be downsized. Moreover, since the chip thermistor can be fixed by soldering, it can be easily mounted on the substrate. According to the gas sensor including the controller of the present invention, the temperature of the gas detector can be set and maintained at a desired temperature according to the output of the temperature sensor without applying a constant voltage to the heater as in the conventional case. The optimum conditions for detecting the test gas can be easily and reliably ensured.
As a result, the best response speed and detection sensitivity can be stably obtained without variation, and an excellent gas sensor with high reproducibility of detection values can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a gas sensor according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of the measurement circuit.

[Explanation of symbols]

 10 Insulating Substrate 11 Gas Detector 12 Temperature Sensor 16 Heater 20 Controller

Claims (4)

[Claims] 1. A gas detector (11) on the surface of an insulating substrate (10).
And a temperature sensor (12) are provided, and a heater (16) for heating the gas detection body (11) is provided on the back surface of the substrate (10), the temperature sensor (12) is a gas detection body ( 11) near the substrate
(10) A gas sensor, which is a chip thermistor mounted on the surface for detecting the temperature of the gas detector (11). 2. The controller (20) according to claim 1, further comprising a controller (20) for controlling the voltage applied to the heater (16) so that the temperature of the gas detection body becomes constant based on the detection output of the temperature sensor (12). Gas sensor. 3. The gas sensor according to claim 1, wherein the gas detector (11) is a metal oxide semiconductor thin film. 4. The gas sensor according to claim 1, wherein the heater (16) is provided so as to extend to the back surface of the substrate (10) on which the temperature sensor (12) is located.