JPH11142356A - Semiconductor gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor gas sensor whose power consumption is reduced. SOLUTION: A gas sensing body 1 is formed of SnO2 so as to be a nearly spherical shape, and its maximum outside dimension is formed to be about 0.8 mm. A coil-shaped electrode 2 which is used also as a heater is composed of platinum, and it is buried inside the gas sensing body 1. In addition, a platinum electrode 4 is buried inside the gas sensing body 1 so as to be passed through the center of the electrode 2 which is used also as the heater. A first filter layer 5 is formed by adding any of W, Mo and V to Al2 O3 , and it is formed on the surface of the gas sensing body 1. A second filter layer 6 is formed by adding any of Pt and Pd to Al2 O3 , and it is formed on the surface of the first filter layer 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor gas sensor for detecting flammable gas, carbon monoxide, carbon dioxide, and odorous component gas with low power consumption in general household and industrial fields.

[0002]

2. Description of the Related Art Conventionally, as a household gas leak alarm for detecting a gas leak of a flammable gas such as a city gas or a propane gas, a gas-sensitive body mainly composed of tin oxide is used. There are a semiconductor gas sensor that detects a combustible gas from a change in resistance of a gas-sensitive body due to the adhesion of a combustible gas to a gas sensor, and a hot wire gas sensor. In these gas sensors, a large amount of electric power is required because the gas sensing element that detects the flammable gas is constantly heated to a relatively high temperature by a heater, and the battery is used for a long time. I couldn't do that. As shown in FIG. 11, a sensing element 10 of this semiconductor gas sensor includes a gas-sensitive body 1 containing tin oxide (SnO ₂ ) as a main component, and a gas-sensitive body 1.
A heater coil 2 embedded in the gas-sensitive body 1 for heating the gas-sensitive body 1 and an electrode 4 embedded in the gas-sensitive body 1. The gas to be detected is detected from the resistance change.

As a carbon monoxide alarm for detecting carbon monoxide generated by incomplete combustion, there is a similar semiconductor gas sensor (see JP-B-53-43320 and JP-B-2-20682). In a semiconductor gas sensor, two levels of power are alternately supplied at regular intervals to a heater embedded in a gas-sensitive body, and the temperature of the gas-sensitive body is alternately switched between a high-temperature state and a low-temperature state. Cleaning is performed to remove the adsorbed gas adsorbed on the surface of the gas-sensitive body at a high temperature, and then the temperature of the gas-sensitive body is switched to a low-temperature state. Detecting carbon oxide.

[0004]

In the case of detecting a combustible gas in the semiconductor gas sensor having the above structure, it is necessary to constantly heat the temperature of the gas-sensitive body 1 to about 400 ° C. Requires about 400 mW of power. Therefore, when, for example, two AA batteries are used as the power supply of the semiconductor gas sensor, the battery life is less than one day.

In order to reduce the power consumption of the semiconductor gas sensor, it is conceivable to operate the semiconductor gas sensor intermittently. However, since the semiconductor gas sensor is large and the heat capacity of the gas-sensitive body 1 is large, a heater is required. After power is supplied to the coil 2, the temperature of the gas-sensitive body 1 becomes high (about 4
It takes about 10 seconds to reach a constant temperature of (00 ° C.) (b in FIG. 8). Therefore, even if this semiconductor gas sensor is operated intermittently, at least 10
Power must be supplied for seconds. Here, when gas is detected at a rate of about once every 150 seconds, the heater coil 2
The time when power is not supplied to the gas sensing body 1 is about 140 seconds.
Although the power consumption of the semiconductor gas sensor is reduced to about one-fifteenth as compared with the case where is continuously heated, the semiconductor gas sensor cannot be used for a long time of about one year using a battery as a power source, for example.

In the case of a semiconductor gas sensor for detecting carbon monoxide, the temperature of the gas-sensitive body 1 is increased to remove carbon monoxide adsorbed on the surface of the gas-sensitive body 1.
In order to alternately switch between a state in which the temperature of the gas-sensitive body 1 is lowered to detect carbon monoxide and the state in which the temperature is lowered, the power supplied to the heater coil 2 is switched between large and small stages.
The surface temperature of the gas-sensitive body 1 decreases from about 400 ° C. at a high temperature to about 100 ° C. at a low temperature and becomes approximately 1 ° C. until it becomes substantially constant.
It takes more than 0 seconds (b in FIG. 8). At this time, while the surface temperature of the gas-sensitive body 1 decreases, an equilibrium state is established between the surface of the gas-sensitive body 1 and carbon monoxide according to the surface temperature of the gas-sensitive body 1, and this equilibrium state is established. Since it takes a relatively long time to perform the process, after the surface temperature of the gas-sensitive body 1 becomes substantially constant at about 100 ° C., the surface temperature of the gas-sensitive body 1 is reduced to 100 ° C. The effect of the equilibrium formed with the carbon oxide remains. Therefore, after the surface temperature of the gas-sensitive body 1 starts to decrease from about 400 ° C., the surface temperature becomes about 10 ° C.
It takes about 90 seconds until the temperature reaches 0 ° C. and an equilibrium state is formed between the surface of the gas sensing element 1 and carbon monoxide (that is, until carbon monoxide can be detected). Thus, the period for detecting carbon monoxide is determined by the time required to heat the gas-sensitive body 1 to a high temperature state (about 60 seconds) and the temperature of the gas-sensitive body 1 from a high temperature state to a low temperature state to reduce the carbon monoxide. It becomes the sum (about 2 minutes and 30 seconds) of the time (90 seconds) until detection becomes possible,
Carbon monoxide could only be detected about once every 2 minutes 30 seconds. Therefore, a dead time of about 2 minutes and 30 seconds occurs before the next detection of carbon monoxide, which may delay the detection of carbon monoxide, and detects a highly dangerous gas such as carbon monoxide. In this case, the detection delay becomes a problem. In addition, since the gas sensing element 1 is large and its heat capacity is large, the power consumption of the heater coil 2 is large, and it has not been possible to use the semiconductor gas sensor using a battery as a power source for a long time.

[0007] Recently, an alarm which incorporates two semiconductor gas sensors for detecting flammable gas and two for detecting carbon monoxide has been used. However, a semiconductor gas sensor for detecting carbon monoxide has been used. It took about 2 minutes and 30 seconds to detect carbon, so the dead time was long, and the power consumption of both sensors was large, so that there was a problem that a battery could not be used as a power source. Further, since two gas sensors are provided in one alarm device, there is a problem that the manufacturing cost of the alarm device increases.

Further, as a semiconductor gas sensor for detecting a flammable gas or carbon monoxide, its characteristics are stable for a long time,
It is desirable that the sensitivity be as low as possible with respect to gases other than the detection target (generally, hydrogen is a typical gas type as these so-called miscellaneous gases). The present invention has been made in view of the above-described problems, and has as its object the purpose of being small in size, consuming less power, being able to use a battery as a power source for a long time by intermittently driving, and preventing gas other than the detection target. It is an object of the present invention to provide a semiconductor gas sensor with reduced sensitivity, and to provide a semiconductor gas sensor with improved aging characteristics.

[0009]

In order to achieve the above object, according to the first aspect of the present invention, there is provided a substantially spherical gas-sensitive body having an outer dimension of about 0.8 mm or less, whose electrical resistance changes by adsorbing a gas. And a heater electrode made of coil-shaped platinum embedded in the gas-sensitive body, a platinum electrode embedded in the gas-sensitive body so as to penetrate the center of the coil of the heater / electrode, and a surface of the gas-sensitive body And a filter layer for removing miscellaneous gas formed in the heater electrode, and the platinum electrode is provided so as to penetrate the center of the coil of the heater / electrode. The size of the gas-sensitive body can be reduced, the heat capacity of the gas-sensitive body can be reduced, and the effect of miscellaneous gases can be reduced by the filter layer.

According to a second aspect of the present invention, in the first aspect of the invention, the filter layer is formed by adding any of W, Mo or V to SiO _2. Since it is formed by adding any of W, Mo and V to Al ₂ O ₃ , the long-term aging characteristics are improved, and the hydrogen gas sensitivity of the gas-sensitive body can be reduced at a high temperature of the gas-sensitive body. it can.

According to a fourth aspect of the present invention, in the first aspect of the present invention, the filter layer is formed by adding either Pt or Pd to SiO _2, and in the fifth aspect of the present invention, the filter layer is formed of Al ₂ Since it is formed by adding either Pt or Pd to O ₃ , the sensitivity of the gas-sensitive body to hydrogen gas can be reduced at a low temperature of the gas-sensitive body. According to the invention of claim 6, in the invention of claims 2 to 5, the filter layer comprises a double filter layer of the filter layer of claim 2 or 3 and the filter layer of claim 4 or 5. The long-term aging characteristics can be improved, and the sensitivity of the gas-sensitive material to hydrogen gas can be reduced at high and low temperatures.

According to a seventh aspect of the present invention, in the first to sixth aspects of the present invention, two-level electric power is supplied to the heater / electrode at a predetermined cycle to heat the gas-sensitive body at two-level temperatures, Since the flammable gas is detected when the temperature of the gas body is high, flammable gas such as methane gas and propane gas can be detected with low power consumption. In the invention of claim 8,
In the invention according to any one of claims 1 to 6, electric power of two levels is supplied to the heater / electrode at a predetermined cycle so that the gas-sensitive body is raised and lowered.
Heat at the temperature of the stage, carbon monoxide when the gas sensitive body is low temperature,
Since carbon dioxide and odorous component gases are detected, by applying two levels of power to the heater / electrode, the gas to be detected can be detected with low power consumption.

According to a ninth aspect of the present invention, in the first to sixth aspects of the present invention, two-level power is supplied to the heater / electrode at a predetermined cycle to heat the gas-sensitive body at two-level temperature, Since a flammable gas is detected at a high temperature of a gas body, and carbon monoxide, carbon dioxide, and a malodorous component gas are detected at a low temperature of the gas-sensitive body, the flammable gas,
It is possible to detect carbon monoxide, carbon dioxide, carbon dioxide, and a malodorous component gas, and to further reduce the power consumption of the semiconductor gas sensor by switching the power supplied to the electrode serving also as a heater in two levels, high and low.

According to a tenth aspect of the present invention, in the seventh to ninth aspects of the present invention, the power supply is stopped when the gas sensitive body is at a low temperature because the power in the two levels is power when the power supply is turned on and off. By doing so, the power consumption of the semiconductor gas sensor can be further reduced. According to an eleventh aspect of the present invention, in the first to tenth aspects, a filter for removing alcohol vapor or silicon vapor from a gas in contact with the gas-sensitive body is provided in the gas passage from the outside to the gas-sensitive body. According to a twelfth aspect of the present invention, in the eleventh aspect, the filter is made of either activated carbon or silica gel. In a thirteenth aspect, the filter is made of activated carbon and silica gel. It is possible to prevent alcohol vapor as miscellaneous gas and silicon vapor as poisoning gas from adhering.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A semiconductor gas sensor according to this embodiment has a resin base 11 as shown in FIGS.
And an electrode pin 12 embedded and fixed in the base 11, a sensing element 10 electrically connected to the electrode pin 12 via the heater electrode 2 and the platinum electrode 4, and a sensor cover 13 mounted on the base 11. A stainless steel wire mesh 14 is provided on the ceiling surface of the sensor cover 13,
Gas is guided from outside to the sensing element 10 via the wire mesh 14.

As shown in FIG. 1, the sensing element 10 includes a substantially spherical gas-sensitive body 1 made of SnO ₂ having a total length and a total width of about 0.8 mm, and a coil-like body embedded in the gas-sensitive body 1. A heater / electrode 2 made of platinum, a platinum electrode 4 made of platinum embedded in the gas-sensitive body 1 so as to penetrate the coil portion of the heater / electrode 2, and a second electrode formed on the surface of the gas-sensitive body 1. The first filter layer 5 includes a first filter layer 5 and a second filter layer 6 formed on the surface of the first filter layer 5. The first filter layer 5 is made of, for example, alumina (Al ₂
O ₃ ) to which a second component such as tungsten (W), molybdenum (Mo) or vanadium (V) is added,
The second filter layer 6 is formed by adding a second component such as platinum (Pt) or palladium (Pd) to Al ₂ O ₃ . In the present embodiment, the first and second filter layers 5, 5
Although Al ₂ O ₃ is used as the base material for SiO ₆ , SiO ₂ may be used instead of Al ₂ O ₃ , and only one of the first and second filter layers 5 and 6 is formed. You may. Further, in the present embodiment, the outer dimension of the gas-sensitive body 1 is set to approximately 0.8 mm, but the outer dimension of the gas-sensitive body 1 is set to 0.1 mm.
The size is not limited to 8 mm, but may be any value as long as the outer diameter of the gas-sensitive body 1 is approximately 0.4 mm to 0.8 mm or less.

As described above, since the platinum electrode 4 is disposed so as to pass through the center of the coil of the heater / electrode 2, the heater / electrode 2 and the platinum electrode 4 can be disposed in a united manner. Compared to semiconductor gas sensors
The size of the gas sensing body 1 can be reduced. Therefore,
The heat capacity of the gas-sensitive body 1 is reduced, and
The power required to heat to 0 ° C. is about 120 mW, about one third that of a conventional semiconductor gas sensor.

When the conventional semiconductor gas sensor and the semiconductor gas sensor of the present embodiment are intermittently heated to detect flammable gas once every 150 seconds, the present sensor heats the gas-sensitive body 1 to a high temperature. Therefore, the time (heating time) for supplying electric power to the heater / electrode 2 is sufficiently about 0.8 seconds as shown in FIG. 8A (about 10 seconds in the conventional semiconductor gas sensor). Is about 12
0 mW (about 40 m in the case of a conventional semiconductor gas sensor).
(0 mW), the average power consumption per second is reduced to 0.64 mW, which is about 1/40 of that of the conventional semiconductor gas sensor. If this sensor is used intermittently, the battery can be used as a power source for a long time ( See Table 1).

On the other hand, like the conventional semiconductor gas sensor,
This semiconductor gas sensor is intermittently heated to produce a gas-sensitive body 1.
In the case where carbon monoxide is detected at a low temperature of the sensor, the sensor also serves to heat the gas-sensitive body 1 to a high temperature.
The time (heating time) for supplying power to the sensor is 0.8 seconds (about 60 seconds for a conventional semiconductor gas sensor), and the average power consumption per second (0.64 mW) of the sensor is:
Since the average power consumption per second (160 mW) of the conventional semiconductor gas sensor is reduced to about 1/250 or less, the semiconductor gas sensor can be driven for a long time using a battery as a power source (see Table 2). .

[0020]

[Table 1]

[0021]

[Table 2]

In the conventional semiconductor gas sensor, carbon monoxide is detected about 90 seconds after the temperature of the gas-sensitive body 1 is lowered, and is detected once every about 150 seconds. On the other hand, in this semiconductor gas sensor, since the thermal time constant of the gas-sensitive body 1 is reduced, the voltage applied to the heater / electrode 2 is reduced as shown in FIG.
Since carbon monoxide can be detected about one second after the temperature of the gas sensing element 1 is switched to a low temperature state, the dead time is determined by the time required to heat the gas sensing element 1 to a high temperature (about 0.8 seconds) and the time required for gas sensing. Body 1
(Approximately 1.8 seconds) with the time (approximately 1 second) for lowering the temperature of the device to a low temperature. Therefore, a flammable gas of about 150
In the case of detecting once per second, carbon monoxide can be detected for 148.2 seconds in the detection cycle (about 150 seconds).

As described above, in the semiconductor gas sensor of the present embodiment, the temperature of the gas-sensitive body 1 is intermittently heated in two steps of high and low by alternately applying electric power of two steps in high and low to the heater / electrode 2. A flammable gas such as methane gas or propane gas can be detected when the gas-sensitive body 1 is at a high temperature, and carbon monoxide can be detected when the gas-sensitive body 1 is at a low temperature. And carbon monoxide can be detected. Also, since the power consumption of this semiconductor gas sensor is significantly reduced as compared with the conventional semiconductor gas sensor, even when a commercial power supply is used as a power supply, the power supply circuit can be downsized and the circuit cost can be reduced. In addition, the entire semiconductor gas sensor can be reduced in size.

By the way, as a semiconductor gas sensor for detecting flammable gas or carbon monoxide, the characteristics are stable for a long time, and gases other than the gas to be detected (generally, hydrogen is a typical gas type for these so-called miscellaneous gases). Is preferably as low as possible. Therefore, in the semiconductor gas sensor of the present embodiment, for example, W is added to the first filter layer 5 made of alumina formed on the surface of the gas-sensitive body 1, and the first filter layer stabilizes the sensor characteristics for a long time. In addition, the sensitivity of hydrogen is reduced when the temperature of the gas-sensitive body 1 is low (that is, at the time of detecting carbon monoxide). Further, for example, Pt is added to the second filter layer 6 made of alumina formed on the surface of the first filter layer 5, so that the temperature of the gas-sensitive body 1 is high (that is, flammable gas such as methane gas or propane gas). At the time of gas detection) to reduce the sensitivity of miscellaneous gas (hydrogen). Therefore, the first and second filter layers 5 and 6 can stabilize the sensor characteristics for a long period of time and reduce the sensitivity of miscellaneous gases in the high-temperature state and low-temperature state of the gas-sensitive body 1. The same effect can be obtained by adding Mo or V as the second component instead of W to the first filter layer 5, and the second filter layer 6 can be formed by adding Pd instead of Pt to the second component. The same effect can be obtained even if added as a. Also,
The same effect can be obtained even when the material of the first and second filter layers is SiO ₂ .

As shown in FIGS. 6 and 7, a bottomed cylindrical cap 15 having an external filter 16 made of activated carbon is mounted on the cover 13 so that the sensing element 10 can be externally connected to the sensing element 10. An external filter 16 is disposed in the path through which the gas flows, and alcohol vapor as a miscellaneous gas and silicon vapor as a poisoning gas are removed from the external filter 16.
To reduce the alcohol sensitivity of the sensing element 10 and protect the sensing element 10 from poisonous substances such as silicon, so that the sensing element 10 is hardly affected by alcohol, silicon gas, or the like. The material of the external filter 16 is not limited to activated carbon, and the material of the external filter 16 may be silica gel (SiO ₂ ) or a combination of activated carbon and silica gel.

FIG. 9 shows a circuit for driving the semiconductor gas sensor. In this circuit, the constant voltage circuit 50 converts the AC power supply voltage into a constant DC voltage V _CC ,
_CC is applied to the series circuit of the switching element Q and the heater electrode 2 of the sensing element 10, and is applied to the series circuit of the load resistor R and the sensing element 10. Further, the detection state of the sensing element 10 is monitored and the heater is also used. Electrode 2
Is applied to a signal processing unit 60 that performs switching control of the voltage applied to the power supply.

The signal processing section 60 alternately sets the high-temperature state period and the low-temperature state period of the timer 62 and the sensing element 10 by the time output of the timer 62, and sets the transistor in the high-temperature state period and the low-temperature state period, respectively. A function of controlling the on-duty of Q through the drive circuit 63, and
A function of outputting an alarm signal to the outside through the alarm control output circuit 67 when the degree of contamination exceeds the alarm operation threshold from the voltage value of the sensing element 10 taken at a predetermined timing during the low temperature state period and a preset reference value. Provided with an arithmetic control circuit 64 and an A / D converter for A / D converting the voltage across the load resistor R.
/ D conversion circuit 61, D / A conversion circuit 65 for D / A converting the voltage value between both ends of the digitally converted load resistance and outputting the analog output again as an analog output, and a memory for storing the reference value and the alarm operation threshold value The signal processing unit 60 comprises, for example, a one-chip microcomputer.

Here, the alarm signal of the alarm control output circuit 67 is used for drive control of a light emitting diode (LED) 71 for displaying an alarm and an external buzzer 72, and for controlling external devices such as a ventilator. Contact output. A temperature compensation circuit 80 provided as an external circuit of the signal processing unit 60
Is for correcting the voltage across the load resistor R to be A / D converted in accordance with the temperature characteristic of the sensing element 10 to eliminate the influence of temperature.

By the way, during the high temperature state of the gas sensing element 1, the gas sensing element 1 can be heated at substantially the same temperature (about 400 ° C.) as when a voltage of about 0.9 V is continuously applied to the heater / electrode 2. As described above, the drive circuit 63 turns on / off the transistor Q with a pulse signal so that the average value of the voltage applied to the heater / cumulative electrode 2 is about 0.9 V, and the period is about 0.8 seconds.

Further, in the low temperature state period, the driving circuit is designed to heat the gas-sensitive body 1 at substantially the same temperature (about 60 ° C.) as when a voltage of about 0.2 V is continuously applied to the heater / electrode 2. 63 turns on / off the transistor Q in response to the pulse signal, and the average value of the voltage applied to the heater / cumulative electrode 2 is about 0.
2 V, and the period is set to about 149 seconds.

In the present circuit, the high-temperature state and the low-temperature state of the gas-sensitive body 1 are alternately switched. In the high-temperature state of the gas-sensitive body 1, at the time of switching from the high-temperature state to the low-temperature state (ie, after 0.8 seconds). The arithmetic and control circuit 64 takes in the voltage between both ends of the load resistor R which has been A / D converted, determines the degree of contamination, and outputs a warning signal when the measured value exceeds the warning operation threshold by comparing with the warning operation threshold. Detects volatile gases. In the low-temperature state of the gas sensing element 1, the voltage between both ends of the load resistor R, which is sequentially A / D-converted by the arithmetic and control circuit 64, from the time when about 1 second has elapsed after switching to the low-temperature state to the next high-temperature state. In addition to the determination of the contamination degree, the alarm signal is compared with the alarm operation threshold, and if the alarm operation threshold is exceeded, an alarm signal is output to detect carbon monoxide.

As described above, the gas to be detected (carbon monoxide)
By setting the low-temperature state (0.8 seconds) and the on-duty and cycle of the pulse signal for turning on and off the transistor Q, carbon monoxide can be reliably detected in a short time. . In the circuit of FIG. 9, the voltage applied to the heater / electrode 2 of the sensing element 10 is set by switching control. As shown in the circuit of FIG. 10, the voltage applied to the heater / electrode 2 is set by continuous control. You may. In this circuit, connects the series circuit of a heater combined electrode 2 and the transistor Q ₂ between the output terminal of the constant voltage circuit 50, the transistor voltage value of the non-inverting input terminal of the operational amplifier OP which determines the reference voltage of the transistor Q ₂ Q so that the switch the reference voltage of the transistor Q ₂ in two stages by switching at ₁ on / off. Transistor Q
The on / off control of ₁ is performed by the arithmetic control unit 64 and the drive circuit 63
Performed by a control signal output through the corresponds to the period and the off period and the high temperature period of each sense gas body 1 to the low-temperature state period to turn on the transistor Q _1,
The setting is performed based on the time output of the timer 62.

In this circuit, when the gas to be detected is carbon monoxide, as shown in FIG. 12, the high temperature state period is set to about 0.8 seconds, and the voltage applied to the heater / electrode 2 is set to about 0 seconds. .9V and the low temperature state period is about 1
The voltage applied to the heater / electrode 2 is set to about 0.
By setting the voltage to 2 V, carbon monoxide can be detected in a short time by discriminating from combustible gases such as methane gas and propane gas.

In the present embodiment, the voltage applied to the heater / electrode 2 is switched between high and low to change the temperature of the gas sensitive body 1 between the high temperature state and the low temperature state. Turn on / off the power supply, and
May be switched between a high temperature and a low temperature, and the power consumption of the semiconductor gas sensor can be further reduced by stopping the power supply to the heater electrode 2 when the gas sensitive body 1 is at a low temperature.

In this embodiment, carbon monoxide has been described as an example of a gas to be detected when the temperature of the gas-sensitive body 1 is low. However, the gas to be detected is not limited to carbon monoxide. It is also possible to detect gases that emit odors, such as carbon, ammonia, and mercaptans.

[0036]

Next, embodiments of the present invention will be described in more detail. (Embodiment 1) In this embodiment, the material of the sensing element 10 is SnO ₂ , but a method of adjusting SnO ₂ will be described below. First, an aqueous solution of SnCl ₄ is hydrolyzed with NH ₄ to obtain a stannic acid sol. The stannic acid sol is air-dried and calcined in air at, for example, 550 ° C. to 700 ° C. for about 1 hour to obtain SnO ₂ . This SnO ₂ is impregnated with an aqueous solution of Pd, and calcined at 500 ° C. for 1 hour in air to add Pd to SnO ₂ . Where S
Pd added to nO ₂ has a role of improving the sensitivity of the sensing element 10 to various gases and improving the response speed to various gases, and may not be added depending on the application. Also, instead of Pd, Pt, Rh, A
A noble metal such as u may be added.

To improve the concentration gradient of the sensing element 10, a second component is added to each of the gas-sensitive body 1 made of SnO ₂ and the filter layer made of alumina formed on the surface of the gas-sensitive body 1. Tables 3 and 4 show the measurement results of the sensor characteristics. Here, the addition of the second component to the gas-sensitive body 1 is performed by adding an aqueous solution of a known concentration of the second component to the gas-sensitive body in a predetermined amount (for SnO ₂ , the amount of the second component is 0.1 or 1) (0.0 mol%), followed by baking at about 500 ° C. The addition of the second component to the filter layer was performed by dipping an aqueous solution of a known concentration of the second component into alumina in a predetermined amount (the amount of the second component was 0.1 or 1 mol% with respect to alumina). Later
The paste was baked at about 500 ° C., mixed with a binder to form a paste, applied to the gas-sensitive body 1, and baked at about 500 ° C.

As shown in Tables 3 and 4, gas-sensitive body 1 (SnO ₂ )
In some cases, the sensitivity of the miscellaneous gas (hydrogen) was reduced when the second component was added, but the resistance value of the sensing element 10 became extremely large or the temporal characteristics deteriorated.
Nothing useful was found. On the other hand, the gas sensing body 1
Of the filter layers made of alumina formed on the surface of alumina and added with the second component, those obtained by adding tungsten (W), molybdenum (Mo), and vanadium (V) to alumina respectively (Nos. 5, 6, 9, and 10) , 13,1
4) The effect of improving the aging characteristics of the sensing element 10 is obtained. Further, in the case of adding platinum (Pt) or palladium (Pd) to alumina, respectively (Nos. 17, 18,
21 and 22), the effect of reducing the sensitivity of miscellaneous gas (hydrogen) can be seen when the temperature of the gas sensing element 1 is high (that is, when detecting a combustible gas such as methane gas or propane gas). Also,
Even when W, Mo, and V are added to alumina (No.
5, 6, 9, 10, 13, 14), the effect of reducing the sensitivity of hydrogen is obtained when the temperature of the gas-sensitive body 1 is low (that is, at the time of detecting carbon monoxide). The material of the filter layer is SiO
_In the case of ₂ , the same effect can be obtained.

[0039] Here, the hot CH ₄ resistance values in Table 3 and in Table 4, a CH ₄ is sensitive resistance value of the gas body 1 of high-temperature state when 1000 ppm (kW), the hot CH ₄ Sensitivity ,
The resistance value (k) of the gas-sensitive body 1 in a high-temperature state when placed in air
The Omega) is a value obtained by dividing the resistance value (kW) at the time of CH ₄ is 1000 ppm, the hot CH ₄ aging characteristics, CH ₄ is 1
Sensitive resistance value of the gas body 1 when allowed to stand for 100 days in a state of 000ppm is a value obtained by dividing the (kW) in the initial resistance value (kW), and the high temperature H ₂ sensitivity, high temperature when H ₂ is 1000ppm The resistance value (kΩ) of the gas-sensitive body 1 in the state is 1 for CH _4.
It is a value divided by the resistance value (kΩ) at 000 ppm.
Further, the low-temperature CO resistance, a CO ₂ is sensitive resistance value of the gas body 1 of the low-temperature state when the 100ppm (kW), in the low-temperature CO aging characteristics, CO ₂ is 100ppm state 10
Resistance (k) of the gas-sensitive body 1 in a low temperature state when left for 0 days
Ω) divided by the initial resistance value (kΩ).
₂ Sensitivity is a value obtained by dividing the resistance value (kΩ) of the gas sensing element 1 in a low temperature state when H ₂ is 1000 ppm by the resistance value (kΩ) when CO ₂ is 100 ppm.

[0040]

[Table 3]

[0041]

[Table 4]

(Embodiment 2) In the first embodiment, only one filter layer is formed on the surface of the gas-sensitive body 1, but in this embodiment, the filter layer is formed on the surface of the gas-sensitive body 1 twice. Table 5 shows the sensor characteristics in the case of performing the above. The surface of the gas-sensitive body 1 is coated with alumina (Al ₂ O ₃ ) in which tungsten (W) is added at 1 mol% to form a first filter layer 5, and platinum (Pt) is added to alumina at 1 mol% The addition is applied to the surface of the first filter layer 5 to form the second filter layer 6.

According to Table 5, the first and second filter layers 5 and 5
6, the characteristics obtained by superimposing the characteristics obtained in step 6, ie, the sensitivity of hydrogen is low regardless of whether the temperature of the gas-sensitive body 1 is low or high, and the aging characteristics of methane gas (CH ₄ ) at high temperatures and carbon monoxide at low temperatures It can be seen that the temporal characteristics of (CO) are also good.

[0044]

[Table 5]

[0045]

As described above, according to the first aspect of the present invention, there is provided a substantially spherical gas-sensitive body having an outer dimension of about 0.8 mm or less whose electric resistance changes by adsorbing a gas, and a gas-sensitive body. A heater / electrode made of buried coil-shaped platinum;
A platinum electrode buried in the gas-sensitive body so as to penetrate the center of the coil of the heater / electrode, and a filter layer for removing various gases formed on the surface of the gas-sensitive body are provided. Since a platinum electrode is provided so as to penetrate the center of the coil, the heater electrode and the platinum electrode can be arranged in a unitary manner to reduce the size of the gas-sensitive body, thereby reducing the heat capacity of the gas-sensitive body. Therefore, the power consumption required to heat the gas-sensitive body can be reduced, and the semiconductor gas sensor can be operated for a long time using the battery as a power supply. In addition, the effect of the extraneous gas can be reduced by the filter layer, and the gas to be detected can be accurately detected.

According to a second aspect of the present invention, the filter layer is made of Si.
Formed by adding any of W, Mo or V to O ₂ ,
According to the third aspect of the present invention, since the filter layer is formed by adding any of W, Mo and V to Al ₂ O ₃ , the sensor characteristics are stabilized for a long time, and the high temperature Sometimes, the sensitivity of the gaseous substance to hydrogen gas can be reduced, and the effect of other gases other than the flammable gas to be detected can be reduced.

According to a fourth aspect of the present invention, the filter layer is made of Si.
The filter layer is formed by adding either Pt or Pd to O _2, and the filter layer is formed by adding Pt or Pd to Al ₂ O ₃ .
Or Pd is added, so that the sensitivity of the gas-sensitive body to hydrogen gas can be reduced at a low temperature of the gas-sensitive body, and the effect of miscellaneous gases other than the detection target gas can be reduced. There is.

According to a sixth aspect of the present invention, since the filter layer comprises a double filter layer of the second or third filter layer and the fourth or fifth filter layer, the sensor characteristics can be improved over a long period of time. There is an effect that hydrogen gas sensitivity of the gas-sensitive body can be reduced at the time of stabilization and at the time of high temperature and low temperature of the gas-sensitive body, and the influence of miscellaneous gases other than the detection target gas can be reduced.

According to a seventh aspect of the present invention, two-level power is supplied to the heater / electrode at a predetermined cycle so that the gas sensitive body is raised and lowered in two steps.
Since the flammable gas is detected when the temperature of the gas-sensitive body is high, the power supplied to the electrode serving as a heater is switched between high and low levels to reduce flammable gas such as methane gas and propane gas with low power consumption. There is an effect that a volatile gas can be detected.

According to an eighth aspect of the present invention, in the first to sixth aspects of the present invention, two-level power is supplied to the heater electrode at a predetermined cycle to heat the gas-sensitive body at two levels of temperature. Since carbon monoxide, carbon dioxide, and odorous component gases are detected when the temperature of the gas body is low, the gas to be detected can be detected with low power consumption by switching the power supplied to the electrode serving as a heater in two stages, high and low. This has the effect.

According to a ninth aspect of the present invention, in the first to sixth aspects of the present invention, two-level electric power is supplied to the heater electrode at a predetermined cycle to heat the gas-sensitive body at the two-level temperature. Since a flammable gas is detected at a high temperature of a gas body, and carbon monoxide, carbon dioxide, and a malodorous component gas are detected at a low temperature of the gas-sensitive body, the flammable gas,
Carbon monoxide, carbon dioxide, and odorous component gases can be detected, and the production cost of the semiconductor gas sensor can be reduced. In addition, since the power supplied to the heater / electrode is switched between high and low, the power consumption of the semiconductor gas sensor can be further reduced.

According to a tenth aspect of the present invention, in the seventh to ninth aspects of the present invention, since the power in the two levels is the power when the power supply is turned on and off, the power supply is stopped when the temperature of the gas sensitive body is low. By doing so, there is an effect that the power consumption of the semiconductor gas sensor can be further reduced. The invention according to claim 11 is provided with a filter for removing alcohol vapor or silicon vapor from a gas in contact with the gas-sensitive body in a gas flow path from the outside to the gas-sensitive body. The filter is made of either activated carbon or silica gel. According to the invention of claim 13, since the filter is made of activated carbon and silica gel, it is possible to prevent alcohol vapor or silicon vapor from adhering to the gas-sensitive material, This has the effect of reducing the alcohol sensitivity and protecting the gas-sensitive material from poisonous substances such as silicon.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a sensing element of a semiconductor gas sensor according to an embodiment.

2A and 2B show the semiconductor gas sensor according to the first embodiment, in which FIG. 2A is a partially broken front view, FIG. 2B is a partially broken top view, and FIG.
Is a bottom view.

3A and 3B show a state where a cover of the semiconductor gas sensor is removed, and FIG. 3A is a front view and FIG. 3B is a top view.

FIG. 4 shows a cover of the semiconductor gas sensor according to the first embodiment;
(A) is a front view which can be partially broken, and (b) is a top view.

FIG. 5 is a diagram showing an operation of the semiconductor gas sensor according to the first embodiment.

FIGS. 6A and 6B show a state in which a cap is mounted on the semiconductor gas sensor of the above, wherein FIG. 6A is a sectional view and FIG. 6B is a bottom view.

FIG. 7 shows a cap of the semiconductor gas sensor according to the first embodiment;
(A) is a front view, (b) is a bottom view.

FIG. 8 is a diagram showing a relationship between time and temperature of a gas-sensitive body of the semiconductor gas sensor according to the first embodiment.

FIG. 9 is a circuit diagram showing a driving circuit of the above.

FIG. 10 is a circuit diagram showing another driving circuit of the above.

FIG. 11 is a perspective view showing a conventional semiconductor gas sensor.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 gas-sensitive body 2 heater / electrode 4 electrode 5 first filter layer 6 first filter layer

Claims (13)

[Claims] 1. A heater / electrode comprising a substantially spherical gas-sensitive body having an outer dimension of about 0.8 mm or less whose electric resistance changes by adsorbing a gas, and a coil-shaped platinum buried in the gas-sensitive body. And a platinum electrode buried in the gas-sensitive body so as to penetrate the center of the coil of the heater / electrode, and a filter layer for removing miscellaneous gases formed on the surface of the gas-sensitive body. Semiconductor gas sensor. 2. The semiconductor gas sensor according to claim 1, wherein said filter layer is formed by adding one of W, Mo and V to SiO ₂ . 3. The semiconductor gas sensor according to claim 1, wherein said filter layer is formed by adding any of W, Mo and V to Al ₂ O ₃ . 4. The method according to claim 1, wherein the filter layer is made of Pt or Pd on SiO _2.
The semiconductor gas sensor according to claim 1, wherein the semiconductor gas sensor is formed by adding any one of the following. 5. The method according to claim 1, wherein said filter layer is made of Pt or Pt on Al ₂ O _3.
2. The semiconductor gas sensor according to claim 1, wherein said semiconductor gas sensor is formed by adding any of d. 6. A semiconductor gas sensor according to claim 2, wherein said filter layer comprises a double filter layer of the filter layer of claim 2 or 3 and the filter layer of claim 4 or 5. 7. A two-step power supply to a heater-and-electrode at a predetermined cycle to heat a gas-sensitive body at a two-step temperature and to detect a flammable gas at a high temperature of the gas-sensitive body. The semiconductor gas sensor according to claim 1, wherein 8. A gas-sensitive body is heated at a two-step temperature by supplying electric power in two steps to a heater / electrode at a predetermined cycle, and carbon monoxide, carbon dioxide, and odor are emitted when the gas-sensitive substance is at a low temperature. 7. The semiconductor gas sensor according to claim 1, wherein a component gas is detected. 9. A two-step electric power is supplied to a heater-and-electrode at a predetermined cycle to heat a gas-sensitive body at a two-step high and low temperature. 7. The semiconductor gas sensor according to claim 1, wherein carbon monoxide, carbon dioxide, and malodorous component gas are detected at a low temperature. 10. The semiconductor gas sensor according to claim 7, wherein the two levels of power are power when power is turned on and off. 11. A filter according to claim 1, wherein a filter for removing alcohol vapor or silicon vapor from a gas in contact with the gas-sensitive body is provided in a gas flow path from the outside to the gas-sensitive body. Semiconductor gas sensor. 12. The semiconductor gas sensor according to claim 11, wherein said filter is made of either activated carbon or silica gel. 13. The semiconductor gas sensor according to claim 11, wherein said filter is made of activated carbon and silica gel.