JPH055689A - Oxygen gas sensor - Google Patents

Abstract

PURPOSE:To prevent the deterioration of a sensor element operable even at room temperature without any need of consideration of maintenance and environment resistance. CONSTITUTION:Reed screen-like electrodes (IDT) 2, 4 for excitation and receiving are provided on a piezoelectric board 1. Each reed screen-like electrode (IDT) 2, 4 is composed of the combination of comb tooth-like electrodes 2a, 2b, 4a, 4b. The input of an electrical signal is received by the use of the other hand electrode 2a of the reed screen-like electrode (IDT) 2 for excitation to produce a resilient surface wave on the piezoelectric board 1 and by the use of the other hand electrode 4a of the reed screen-like electrode (IDT) 4 for receiving the resilient surface wave is received to convert into an electric signal, which is output. Moreover, a lead wire 6 of the side of the reed screen-like electrode (IDT) 2 for excitation is connected to another lead wire 8 of the side of the reed screen-like electrode (IDT) 4 for receiving through a feedback amplification circuit 12 to compose an oscillation circuit. And an oxygen induction thin film 20 is provided in a resilient surface wave propagation route between both the reed screen-like electrodes (IDT) 2, 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen gas sensor used for measuring oxygen gas concentration.

[0002]

2. Description of the Related Art Conventionally, galvanic cell type oxygen gas sensors, ceramic solid electrolyte type oxygen gas sensors, etc. are known as oxygen gas sensors used for measuring oxygen gas concentration for disaster prevention, process control, environmental measurement and the like.

The galvanic cell type oxygen gas sensor measures the oxygen gas concentration by utilizing the current change due to the redox reaction between the electrodes constituting the galvanic cell and oxygen.
Further, the ceramic solid electrolyte type oxygen gas sensor constitutes a concentration battery using, for example, stabilized zirconia, and measures the oxygen gas concentration by using electromotive force generated according to oxygen partial pressure.

[0004]

However, the galvanic battery type battery described above uses an alkaline electrolytic solution such as potassium hydroxide or sodium hydroxide or an acidic electrolytic solution such as sulfuric acid or hydrochloric acid, and therefore leaks, Since there is a risk of corrosion, it is necessary to consider maintenance and environmental resistance, such as maintaining the sealing property against leakage, selecting a chemical-resistant container material, and supplementing the solution.

Also, the ceramic solid electrolyte type is
It is not necessary to consider maintenance such as sealing property against leakage and consideration of environment resistance, but since the operating temperature of the sensor is as high as 350 to 500 ° C., deterioration of the sensor element is likely to occur, which is also a cause of shortening the life. It was

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an oxygen gas sensor which can be operated even at room temperature without the need to consider maintenance and environmental resistance.

[0007]

In order to achieve such an object, the present invention has the following constitution as a means for solving the problem. That is, a comb-shaped electrode for exciting a surface acoustic wave and a comb-shaped electrode for receiving are provided on a piezoelectric substrate, and an oxygen sensitive thin film is provided in a propagation path of the surface acoustic wave between the two comb-shaped electrodes to form an oscillation circuit. This is the configuration of an oxygen gas sensor which is characterized in that a change in the amount of oxygen that reacts with the oxygen sensitive thin film is output as a frequency change amount.

[0008]

The oxygen gas sensor of the present invention having the above structure is
The surface acoustic wave generated from the interdigital transducer for excitation propagates through the interface between the piezoelectric substrate and the oxygen sensitive thin film, reaches the interdigital transducer for reception, is converted into an electric signal, and is output. When the oxygen gas concentration changes, the amount of oxygen that reacts with the oxygen sensitive thin film changes.

Due to this change in the amount of oxygen, the propagation velocity of the surface acoustic wave propagating through the interface between the piezoelectric substrate and the oxygen sensitive thin film changes, and the frequency of the output electric signal also changes. Therefore, the change in the oxygen gas concentration can be reflected in the change amount of the oscillation frequency.

[0010]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is an explanatory diagram showing a schematic configuration of an oxygen gas sensor which is an embodiment of the present invention. By photolithography on the piezoelectric substrate 1 such as a crystal ST cut plate,
Exciting and receiving interdigital transducers
ucer, hereinafter referred to as IDT. ) 2 and 4 are provided at a predetermined interval.

Each of the excitation and reception IDTs 2 and 4 is composed of a combination of comb-shaped electrodes 2a, 2b, 4a and 4b. One electrode 2a of the exciting IDT 2 receives an electric signal input through the lead wire 6 and causes the piezoelectric substrate 1 to generate a surface acoustic wave by the piezoelectric action. One electrode 4a of the receiving IDT 4 receives the surface acoustic wave, converts the surface acoustic wave into an electric signal by a piezoelectric reaction, and outputs the electric signal via the lead wire 8. Each ID
The other electrodes 2b, 4b of T2, 4 are lead wires 10,
It is grounded via 11.

Further, a feedback amplifier circuit 12 is provided between the lead wire 6 on the excitation IDT 2 side and the lead wire 8 on the reception IDT 4.
Connected through. The electric signal output from the reception IDT 4 is amplified by the feedback amplification circuit 12 and is fed back to the excitation IDT 2 to form an oscillation circuit.

The above-mentioned comb-teeth-shaped electrodes 2a, 2b, 4a,
Al is used for the material of 4b, and each IDT 2 and 4 has a logarithm of 200, an electrode width of 8.2 μm, and a crossing length of 6.56.
mm, electrode thickness 3000 angstrom vacuum deposition film. The cross length is the length in which the teeth of the comb-shaped electrode are arranged, and the logarithm is the number of pairs of the comb teeth.
Further, in this embodiment, the distance between the IDTs 2 and 4 is 7 mm, and the surface acoustic wave device having such a structure has a fundamental frequency of about 100 MHz.

An oxygen sensitive thin film 20 is provided in the surface acoustic wave propagation path between the IDTs 2 and 4, and in the present embodiment, LaF ₃ (lanthanum fluoride) is used and the film thickness is 100.
It is configured as a 0 angstrom vacuum deposited film. In addition to the above-mentioned crystal, the piezoelectric substrate 1 has an L
LiNbO ₃ (lithium niobate), LiTaO ₃ (lithium tantalate), may also be used such as PZT. In addition, on non-piezoelectric substrates such as glass, quartz, silicon, Al ₂ O ₃ ,
ZnO, AlN, CdS, ZnS, BiPbO ₁₉ , Li
NbO may be used after the formation piezoelectric thin film ₃ and the like.

The electrodes 2a, 2b, 4 of the IDTs 2, 4 are also
In addition to Al described above, Pt, Au, and
The oxygen sensitive thin film 20 is made of ITO, etc.
In addition to F ₃ , SrCl ₂ , SnO ₂ , PbSnF ₄ , Eu
_{F 3, Nb 2 O 5,} TiO 2, CrO 3, Cr 2 O 3 , etc. or a similar effect by using these composite compounds are obtained.

Next, the operation of this embodiment will be described.
When an electric signal having a controlled frequency is input to the one electrode 2a of the excitation IDT 2 via the lead wire 6, the comb-shaped electrodes 2a and 2b are distorted in mutually opposite phases due to the piezoelectric action. A surface acoustic wave is generated from the excitation IDT 2.

The surface acoustic wave generated from the exciting IDT 2 propagates through the interface between the piezoelectric substrate 1 and the oxygen sensitive thin film 20, reaches the receiving IDT 4, and is converted into an electric signal. The frequency of the electric signal affects the oscillation frequency. And
When the concentration of the oxygen gas to be detected changes, the amount of oxygen that reacts with the oxygen-sensitive thin film 20 changes. Due to this change in the amount of oxygen, the propagation velocity of the surface acoustic wave propagating through the interface between the piezoelectric substrate 1 and the oxygen sensitive thin film 20 changes, and the frequency of the electric signal to be output also changes.

Therefore, with the oxygen gas sensor of this embodiment configured as an oscillation circuit, the change in oxygen gas concentration can be measured as the amount of change in oscillation frequency by a frequency counter (not shown). An example of measurement of oxygen concentration using such an oxygen gas sensor is shown below. First, the experimental system will be briefly described.
It is called AW. ) The oxygen gas sensor is held in a chamber with constant humidity, which is 25 ± 1 ° C in a high temperature water bath.
Is kept at. The oxygen gas concentration and temperature in the chamber are measured by a galvanic oxygen sensor and CA thermocouple, respectively, and the output from the SAW oxygen gas sensor is measured by a frequency counter. These measurement data are input to a computer as a processing system and displayed and stored.

In the experimental system thus constructed, the oxygen concentration in the chamber was changed and the reaction of the SAW oxygen gas sensor with respect to the oxygen gas concentration was investigated. The results are shown in FIG. The oxygen gas concentration at the start of measurement was 22%, and after confirming that the frequency was stable for 20 minutes, 5N oxygen gas was introduced into the chamber at a flow rate of 100 ml / min, as shown in FIG. , The frequency started to drop.

Then, when the oxygen gas concentration was 30%, the inflow of oxygen gas was once stopped and kept constant for 10 minutes. After that, oxygen gas is again supplied, and when the oxygen gas concentration is 40%, the oxygen gas inflow is temporarily stopped and kept constant for 10 minutes. Similarly, when the oxygen gas concentration is 50%, the oxygen gas inflow is temporarily stopped. After keeping it constant for a minute, it was returned to the atmosphere.

As a result, as can be seen from FIG. 2, the SAW
The oxygen gas sensor reacts with oxygen gas, and the frequency decreases as the gas concentration increases. Based on this result, a plot of the relationship between frequency and oxygen gas concentration by the method of least squares is shown in FIG. The correlation coefficient between the frequency and the oxygen gas concentration is 0.9997, and it can be seen that the two have a good linear relationship.

The slope of the straight line graph in FIG. 3 was minus 14.34, and it was found that the sensitivity of the SAW oxygen gas sensor used in this experiment was about minus 14 [Hz / O ₂ %]. In the conventional galvanic cell type oxygen gas sensor, since the electrolyte is used, there is a risk of leakage and corrosion, but the oxygen gas sensor of the present embodiment has such maintenance,
It is unnecessary to consider environmental resistance. Further, unlike the ceramic solid electrolyte type oxygen gas sensor, the sensor element does not have a high operating temperature and operates at room temperature without any problem. Therefore, deterioration of the sensor element is unlikely to occur and the life can be extended.

As described above, the present invention is not limited to such embodiments, and can be carried out in various modes without departing from the scope of the present invention.

[0024]

As described in detail above, the oxygen gas sensor of the present invention does not use an electrolytic solution or the like, does not require consideration of maintenance and environmental resistance, and can operate at room temperature. This has the effect of preventing deterioration.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a schematic configuration of an oxygen gas sensor which is an embodiment of the present invention.

FIG. 2 is a graph showing the response of frequency to oxygen gas concentration when an experiment was performed using the oxygen gas sensor of this example.

FIG. 3 is a graph showing frequency change characteristics of the oxygen gas sensor of this embodiment with respect to changes in oxygen gas concentration.

[Explanation of symbols]

1 ... Piezoelectric substrate 2, 4 ... IDT (=
Interdigital electrodes 2a, 2b, 4a, 4b ... Electrode 20 ... Oxygen sensitive thin film

Claims (1)

Claims: 1. A comb-shaped electrode for exciting a surface acoustic wave and a comb-shaped electrode for reception are provided on a piezoelectric substrate, and a propagation path of the surface acoustic wave is provided between the comb-shaped electrodes. An oxygen gas sensor characterized in that an oxygen sensitive thin film is provided to form an oscillation circuit, and a change in the amount of oxygen that reacts with the oxygen sensitive thin film is output as a frequency change amount.