JPH03248057A - Gas sensor - Google Patents

Abstract

PURPOSE:To selectively detect a specific gas by providing a comb-like exciting electrode and a comb-like receiving electrode on a piezoelectric substrate, and applying a thin film including a host which has the gas to be detected as a quest between the two electrodes. CONSTITUTION:On a piezoelectric substrate 1, there are formed a comb-shaped exciting electrode 2 which excites a surface acoustic wave 3 and, a comb-shaped receiving electrode 4 which receives and converts the surface wave 3 to an electric signal. Moreover, a thin film 5 of gamma-cyclodextrin including a host which has a gas to be detected as a quest is applied to the surface of the substrate 1 between the electrodes 2 and 4. When an impulse voltage is impressed through an input terminal of the electrode 2 in the atmosphere, the surface wave 3 is excited by the electrode 2, spreading on the surface of the substrate 1, and converted to the electric energy by the electrode 4 and taken out as a high frequency output from an output terminal. A feedback amplifier circuit 8 is connected between the electrodes 2 and 4. Therefore, a signal received by the electrode 4 is amplified by the circuit 8 and fed back to the electrode 2. Thus, an oscillating circuit is constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to a gas sensor, and more specifically, to a gas sensor that uses a guest, which is a chemical species having a specific three-dimensional structure, and a host that selectively interacts with the guest. This invention relates to a gas sensor that applies an inclusion phenomenon that forms a guest-host complex between two gases.

(B) Conventional Technology Conventionally, metal oxide sintered semiconductor gas sensors and catalytic combustion gas sensors have been used as gas sensors for use in flammable gas leak alarms and gas concentration needles. The detection principle of these sensors utilizes the property that in semiconductor gas sensors, physical properties such as electrical resistance and work function change due to the adsorption phenomenon of gas on the semiconductor surface.
Further, a catalytic combustion type gas sensor utilizes the fact that a temperature change due to catalytic combustion of gas on the surface of a substance having a gas detection function is detected as a change in electrical resistance.

(c) Problems to be solved by the invention However, since the conventional gas sensor described above uses gas adsorption phenomena and combustion phenomena, it reacts with various gases in the gas to be detected, resulting in It was difficult to selectively detect only

In addition, the operating temperature of these gas sensors is generally 200~200℃.
Since it requires a high temperature of 500° C., there is a problem in that the device is likely to deteriorate.

(d) Means for Solving the Problems The gas sensor according to the present invention has a comb-shaped excitation t on a piezoelectric substrate.
An L pole and a comb-shaped receiving electrode are provided, and a thin film containing a host with the gas to be detected as a guest is deposited between these two electrodes.

(E) Effect According to the present invention, a thin film containing a host with the gas to be detected as a guest between the excitation and reception electrodes interacts with the guest, increasing its mass, and propagation of surface acoustic waves between the two electrodes. Since the conditions change, a selective detection action is performed on the gas to be detected.

(F) Example Hereinafter, an example of the present invention will be described in detail with reference to the drawings. FIG. 1 illustrates a schematic diagram of a gas sensor of the present invention. (1) is a piezoelectric substrate that forms the main part of the gas sensor, for example, L with a length of 10m, a width of 2 fins, and a thickness of 0.1m.
It is composed of iNbO5. (2) is this board (]
) is provided on one side of the surface of the piezoelectric substrate (1), and a comb-shaped excitation electrode (4) for exciting the surface acoustic wave (3) is provided on the other side of the piezoelectric substrate (1), facing the ay excitation electrode (2). With the comb-shaped receiving electrode, sN:! It functions to receive surface acoustic waves (3) propagating from the excitation electrode (2) to the surface of the piezoelectric substrate (1) and convert them into electrical signals. These comb-shaped electrodes (2) (4
) vs! It consists of 50 pairs of electrodes, an electrode spacing of 5 μm, a crossing length of 1 μm, and an electrode thickness of 1000 aluminum evaporated films. (5) is a thin film containing a host with the gas to be detected as a guest, which coats the surface of the piezoelectric substrate (1) between both comb-shaped electrodes t4i (2) and (4), for example, a film of γ-cyclodextrin with a film thickness of 1100 nm. A thin film is used. This thin film (5) is formed by a vacuum evaporation method under conditions of a vacuum degree of 10-' Torr and a substrate temperature of 25°C. FIG. 2 shows the chemical structural formula of this γ-cyclodextrin.

Next, the operation of the gas sensor having such a configuration will be explained. l in the atmosphere! When an impulse voltage f'JJ is applied through the input terminal (6) connected to the Y-type excitation electrode (2), the A-type excitation 112) generates distortion in opposite phases between the adjacent electrodes due to the piezoelectric effect, resulting in a surface acoustic wave. (3) is excited. This surface acoustic wave (3) propagates on the surface of the substrate (]), reaches the comb-shaped receiving electrode (4), is converted into electrical energy, and is extracted as a high-frequency output from the output end f(7).

A feedback amplifier circuit (8) is connected between the comb-shaped excitation electrode (2) and the @-shaped receiving electrode (4) of the sensor having such a configuration, as shown in FIG.
) is connected, and the signal received by the comb-shaped receiving electrode (4) is amplified by this feedback amplifier circuit (8) and then sent back to the comb-shaped excitation electrode (2).
An oscillation circuit is constructed by feeding back the

When the sensor operating in this manner is fixed in a gas cell (10) as shown in Fig. 4 and carbon tetrachloride vapor is introduced into the gas cell (10), the sensor and the feedback amplifier circuit (8) are connected. The oscillation frequency starts to decrease, and the decrease I
l! The elephant stabilized at its reduced frequency after 5 minutes.

This phenomenon occurs because carbon tetrachloride molecules fill the cavities inside the molecules of the γ-cyclodextrin thin film (5), forming a host-guest complex, and the mass of the γ-cyclodextrin thin film (5) increases. By increasing the comb-shaped excitation type t
4i (2) (piezoelectric substrate opening) surface 18! It is believed that the cause is a change in the propagation conditions of the surface acoustic wave (3) that propagates to the receiving electrode (4). By introducing nitrogen gas, carbon tetrachloride molecules are expelled from the cavities inside the molecules of the γ-cyclodextrin thin film (5), and the host-guest complex collapses, so the oscillation frequency of the oscillation circuit is restored to the original chain. did.

When chloroform vapor was used instead of carbon tetrachloride under similar conditions, the oscillation frequency changed only slightly. The reason for this difference between carbon tetrachloride, a halogenated hydrocarbon with a similar structure, and chloroform is that the diameter of the cavity inside the molecule of the γ-cyclodextrin thin film (5) is larger than that of the carbon tetrachloride molecule. This carbon tetrachloride molecule forms a host-guest complex with r-cyclodextrin because it is almost equal to the carbon tetrachloride molecule, whereas the chloroform molecule is smaller than the carbon tetrachloride molecule, so the molecule forms a thin film of γ-cyclodextrin (5). This is probably because even if it fills the cavity inside the molecule, it immediately separates, making it impossible to form a host-guest complex.

Here, the operation of the gas sensor of the present invention will be explained using specific numerical values. When the oscillation circuit shown in Fig. 3 is oscillating at 90 MHz in the atmosphere, when the oscillation circuit is placed in the gas cell (10) shown in Fig. 4 and tetrachloride coal industry gas (9) is introduced, The oscillation frequency decreased by 500 Hz after 5 minutes. Further, as a comparative example, in the case of chloroform, the frequency change was 10 Hz.

(G) Effects of the Invention As is clear from the above description, the present invention provides a method in which a comb-shaped excitation electrode and a comb-shaped reception electrode are provided on a piezoelectric substrate, and a gas to be detected is placed as a guest at the same position of these picture electrodes. Since a thin film containing a host is deposited, changes in the weight of the thin film caused by selective host-guest complex formation with respect to a specific gas in this thin film are detected as changes in frequency. Therefore, it is possible to selectively detect specific gases that cannot be detected using conventional sensors such as catalytic combustion type or semiconductor type gas sensors.At the same time, since the detection operation can be performed in a room temperature atmosphere, there is less deterioration of elemental gas and the lifespan is extended. can be achieved.

[Brief explanation of drawings]

Fig. 1 is a perspective view of the element of the gas sensor of the present invention, Fig. 2 is the chemical structural formula of γ-cyclodextrin used in the invention, Fig. 3 is a perspective view showing the configuration of the gas sensor, and Fig. 4 is a perspective view of the gas sensor element of the present invention. It is a sectional view showing a detection state. (+)...Piezoelectric substrate, (2)...s1! :! Excitation electrode, (3)...Surface acoustic wave, (4)...ls type receiving electrode, (5)...γ-cyclodextrin thin film, (8)...
・Feedback amplifier circuit, (9)...Gas, (10)...Gas cell.

Claims (3)

[Claims] (1) A comb-shaped excitation electrode that excites surface acoustic waves on a piezoelectric substrate that propagates surface acoustic waves, and a comb-shaped receiving electrode that receives surface acoustic waves propagating from the electrode on the surface of the piezoelectric substrate. and a thin film containing a host with a gas to be detected as a guest is deposited on at least a part of the surface of the piezoelectric substrate between the two electrodes. (2) The gas sensor according to claim (1), wherein the thin film is made of γ-cyclodextrin and detects carbon tetrachloride gas. (3) Claim (1) wherein an oscillation circuit is constructed by connecting a feedback amplifier circuit between the comb-shaped excitation electrode and the comb-shaped reception electrode.
) or the gas sensor described in (2).