JPH07209163A - Gas detection sensor - Google Patents

Abstract

PURPOSE:To obtain a sensor for detecting gas which is highly sensitive, can maintain sensitivity for a long time, and can detect reducing gas and noncombustible gas providing a heater etc. for heating a gas-adsorption film. CONSTITUTION:Electrodes 2a and 2b are provided on both surfaces of a crystal vibrator plate 1 and gas-adsorption films 3a and 3b are applied to the surface of the electrodes 2a and 2b. Further, flat-plate shaped ceramic heaters 8a and 8b are installed in parallel with the vibration plate 1 using a lead frame 9 for support which is connected to a lead pin 6c fixed to an insulation support stand 7. When gas is adsorbed to the adsorption films 3a and 3b, the resonance frequency of the vibration plate 1 changes according to the weight of gas, thus detecting gas. At this time, by heating with the heaters 8a and 8b, the adsorption/desorption of gas become reversible, thus preventing gas from being adsorbed and hence sensitivity from being reduced regardless of repeated measurement.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detecting sensor used for detecting a gas, and more particularly to a gas detecting sensor capable of detecting a flame-retardant gas.

[0002]

2. Description of the Related Art Conventionally, tin oxide (SnO ₂ ) or zinc oxide (Zn oxide) has been used to detect reducing gas in the atmosphere.
A semiconductor gas sensor using a sintered body of a metal oxide such as O) is known. This utilizes the phenomenon that the electrical resistance of a metal oxide decreases when it contacts a reducing gas.

Normally, oxygen in the atmosphere is adsorbed on the surface of a metal oxide with negative ions, and when a reducing gas is generated, an oxidation reaction occurs due to the adsorbed oxygen. At this time, the electrons captured by the adsorbed oxygen move to the metal oxide, so that the electron concentration of the metal oxide increases and the electric resistance decreases. Therefore, the semiconductor gas sensor is effective for detecting reducing gas in the atmosphere, but is not suitable for detecting flame-retardant gas or use in an oxygen-free atmosphere.

Sulfurous acid gas (SO ₂ ) which is a flame retardant gas,
A potentiostatic electrolysis gas sensor is known as a sensor for detecting hydrogen chloride (HCl), hydrogen fluoride (HF), and the like. This is a method in which the interface between the electrode and the electrolyte is kept at a specific potential and the electrolysis current generated by electrolyzing the gas is measured.

Although this type of sensor has high sensitivity to both reducing gas and flame-retardant gas, it has a drawback that its life is short because it uses an electrolytic solution. Also, the gas that can be detected is limited, and a gas such as SOF ₂ which is a decomposition gas of SF ₆ cannot be detected.

On the other hand, in recent years, in the field of gas sensors, a crystal oscillator type gas sensor has been attracting attention as a gas sensor having high sensitivity and capable of operating at room temperature. This sensor has a structure in which an organic material or the like is formed as a gas adsorption film on an electrode of a crystal oscillator plate.

Then, the principle of detecting the weight change Δw of the gas adsorbed on the gas adsorption film as the change Δf of the resonance frequency of the crystal resonator given by the principle equation such as the following equation (1) is used.

Δf = −2.3 × 10 ⁶ × f ² × Δw / A (1) where f is the resonance frequency of the crystal unit, and A is the electrode area of the crystal unit. Further, this quartz oscillator type gas sensor can detect not only the change in the resonance frequency but also the change in the impedance, the phase, and the inductance as the change in the electrical characteristics.

From this detection principle, the crystal oscillator type gas sensor has high sensitivity to both reducing gas and flame retardant gas, and ppb
The gas of the order can be detected. And JP-A-55-42
As disclosed in Japanese Patent Application Laid-Open No. 054 and Japanese Patent Application Laid-Open No. 1-229935, the operating temperature is generally room temperature, and development is underway for detecting odorous gas.

[0010]

The crystal oscillator type gas sensor as described above has a wide range of gases that can be detected. For example, the gas detection sensor described in JP-A-55-42054 can detect gases such as water vapor, hydrocarbons, carbon dioxide, and sulfur dioxide. However, SOF ₂ cannot be detected.

The gas detection sensor disclosed in Japanese Patent Laid-Open No. 1-22935 developed as a flame-retardant gas detection sensor can detect HF, which is one of the decomposed gases of SF ₆ , but not SOF _2. Cannot be detected.

Since these gas detection sensors are used at room temperature, the sensitivity greatly changes depending on the ambient temperature, and the gas adsorbed on the gas adsorption film remains adsorbed as it is depending on the material. descend.
Therefore, the detection accuracy is poor and the life of the sensor is short.

An object of the present invention is to solve the above problems and to provide a gas detection sensor which is highly sensitive and capable of maintaining sensitivity for a long period of time and which can detect both reducing gas and flame retardant gas. To do.

[0014]

In order to solve the above problems, the present invention provides a vibrator plate having a pair of electrodes, the resonance frequency of which changes according to the amount of adsorbed gas, and at least one of the vibrator plates. A gas adsorption film made of a metal oxide formed on an electrode, and a heater for heating the gas adsorption film.
There is provided a gas detection sensor having:

[0015]

When a gas is adsorbed on a gas adsorption film formed on the surface of an oscillator plate such as a quartz oscillator plate, the electrical characteristics of the oscillator represented by the resonance frequency change depending on the weight of the gas, and Can be detected. At this time, by heating the heater, the adsorption and desorption of gas becomes reversible, and the state in which gas is adsorbed does not remain. Therefore, the sensitivity does not decrease even if the measurement is repeated. Further, since the gas adsorption film used in the present invention is formed of a thermochemically stable metal oxide, it can withstand many times of measurements as compared with the conventional gas adsorption film.

[0016]

EXAMPLE An example of the present invention will be described below using a commercially available 9 MHz AT cut thickness quartz crystal oscillator plate as an oscillator plate. FIG. 1 is a schematic sectional view showing the structure of a gas detection sensor according to an embodiment of the present invention. Electrodes 2a and 2b are provided on both sides of the crystal diaphragm 1, and each electrode 2a,
Gas adsorption films 3a and 3b are applied on the upper surface of 2b, and lead wires 5a and 5b are connected to the electrodes 2a and 2b through conductive pastes 4a and 4b. The electrodes 2a, 2
In b, a gold (Au) layer is applied on the surface with a chromium (Cr) layer as a base. Further, the lead wires 5a and 5b are connected to the lead pins 6a and 6b to support the crystal diaphragm 1. The lead pins 6a and 6b are fixed to the insulating support base 7. Furthermore, flat ceramic heaters 8a and 8b are
Using the supporting lead frame 9 connected to the lead pin 6c fixed to the insulating support base 7, the crystal diaphragm 1 is used.
It is installed in parallel with. The inside is protected by a cap 11 to which a protective net 10 made of stainless steel is fixed.

Specifically, tin oxide and copper oxide were prepared as the gas adsorption films 3a and 3b. A pyrolysis method of an organic compound containing a metal element was used for the preparation, and as a raw material for tin oxide, 2
-A copper-containing resin was used as a raw material for tin ethylhexanoate and copper oxide. Each of these was adjusted to a concentration of about 30% by weight in n-butanol used as a solvent to prepare a raw material liquid. After immersing the entire surface of the crystal oscillator plate in the raw material liquid,
It was dried at about 120 ° C. for 30 minutes and baked at about 500 ° C. for 30 minutes. The steps of coating, drying and baking were repeated twice to form a thin film having a thickness of about 0.4 μm. After forming the gas adsorption film, it was mounted on the element structure shown in FIG. 1 to obtain a gas detection sensor 12.

More specifically, as shown in FIG. 2, a disk-shaped crystal vibrating plate 1 is used, and a gas adsorption film 3 is formed on a circular electrode 2a.
A is arranged. Further, a pad region was made to project in a part of the electrode 2a, and the conductive paste 4a was applied thereto. The back side has the same structure.

Next, the sensitivity characteristics were evaluated as follows. First, the gas detection sensor 12 is attached to the sealed terminal 14 of the pressure-resistant sealed container 13 having an internal volume of 2.5 l as shown in FIG. And the gas detection sensor 12 is a pressure-resistant sealed container 13
Driven by an oscillation circuit 15 outside the frequency measuring device 1
6, the resonance frequency can be monitored. The oscillation circuit 15 has a structure as shown in FIG. 4, and the gas detection sensor 12 and the oscillation circuit 15 are connected to the lead wires 6a and 6b.
Connected by. The inside of the pressure-resistant sealed container 13 is evacuated, SF ₆ which is an inert gas is filled from the gas cylinder 17 to 1 atm and left for a certain period of time. After that, the test gas was sampled with the syringe 18 and injected so that the pressure-resistant sealed container 13 had a predetermined concentration, and the sensitivity characteristics were evaluated. As the test gas, SOF ₂ which is one of the typical components of the decomposition gas of SF ₆ and further SO ₂ and HC
Four flame-retardant gases of 1 and HF were used.

An element in which tin oxide and copper oxide were respectively formed as gas adsorption films on the surface of a crystal resonator plate was heated to a surface temperature of about 300 ° C. by heating with a heater, and the concentration of each test gas was about 20.
The frequency changes measured for ppm are shown in Table 1 below. As shown in Table 1, it can be seen that any element using the tin oxide film and the copper oxide film shows a frequency change of ten-odd Hz to several tens Hz with respect to four kinds of gases.

[0021]

[Table 1]

Further, FIG. 5 shows the results of measuring the amount of frequency change with respect to the number of measurements with respect to SOF _{2 of} 20 ppm. The measurement was performed at intervals of 1 to 2 days. Although the tin oxide film and the copper oxide film both show some transient fluctuations, they showed similar frequency changes even after about 100 measurements, and no decrease in sensitivity was observed. Also, similar characteristics were obtained when measured with other gases.

The film thickness of the gas adsorption film in this embodiment is 0.
Particularly good characteristics can be obtained in the case of about 01 to 1 μm.
The metal oxide used as the gas adsorption film is not limited to tin oxide and copper oxide described in the above examples, but similar effects can be obtained as long as it is a metal oxide such as tungsten oxide having gas adsorption ability and high temperature stability. To be

A crystal oscillator was used as the oscillator, but in addition to this, a tuning fork oscillator, a ceramic piezoelectric oscillator, a single crystal silicon or polycrystalline silicon oscillator, a surface wave (SAW) device, etc. A device that outputs a change as a change in the electrical characteristics of the vibrator plate can be used.

Further, the gas detecting sensor of the present invention can detect not only the change of the oscillation frequency but also the change of impedance, phase and inductance by using a method such as a network analyzer method. it can.

The operating temperature of the sensor due to the heater heating varies depending on the gas adsorption film and the type of gas, but the sensitivity can be detected at about 100 ° C to 400 ° C. In addition, the following method may be used in order to perform more accurate measurement using the gas detecting sensor of the present invention.

First, a reference sensor having the same characteristics as the present sensor is put in a container such as a bellows so that the temperature and the pressure become the same as the present sensor and gas is not adsorbed to the sensor. Then the reference sensor does not adsorb the gas, so
Highly accurate measurement can be performed by performing differential detection using the outputs of this sensor and the reference sensor. The other is a method of storing previously measured temperature and pressure characteristics in a memory and compensating for the characteristics based on temperature and pressure information.

[0028]

As described above, according to the present invention, it is possible to provide a gas detection sensor which has high sensitivity and can maintain the sensitivity for a long period of time, and which can detect both reducing gas and flame retardant gas.

[Brief description of drawings]

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

FIG. 2 is a top view of a crystal resonator plate according to an example of the present invention.

FIG. 3 is a schematic diagram of a sensitivity characteristic evaluation device for a flame-retardant gas used in an example of the present invention.

FIG. 4 is an oscillator circuit diagram used in an embodiment of the present invention.

FIG. 5: SOF ₂ 20 ppm in the example of the present invention
FIG.

[Explanation of symbols]

 1: Quartz diaphragm 2a, 2b: Electrode 3a, 3b: Gas adsorption film 4a, 4b: Conductive paste 5a, 5b: Lead wire 6a, 6b, 6c: Lead pin 7: Insulating support 8a , 8b: Ceramic heater 9: Supporting lead frame 10: Protective net 11: Cap 12: Gas detection sensor 13: Pressure-resistant sealed container 14: Sealed terminal 15: Oscillation circuit 16: Frequency measuring instrument 17: Gas cylinder 18: Syringe

Claims (1)

[Claims] 1. A vibrator plate having a pair of electrodes, the resonance frequency of which changes according to the amount of adsorbed gas, a gas adsorbing film made of a metal oxide formed on at least one of the electrodes, A gas detection sensor having a heater for heating the gas adsorption film.