JPS63137491A - Piezoelectric element type sensor - Google Patents

Abstract

PURPOSE:To enable measurement in a gas containing a large quantity of moisture and having low insulating properties, etc., by coating an external exposed surface in a filmy metallic electrode formed onto a piezoelectric element with an electrical insulating film. CONSTITUTION:A sensor element having a pair or filmy electrodes 3 on a piezoelectric element 2 and shaping a chemical composition sensitive film 4 on at least one electrode 3 is formed. A quartz oscillator or Rochelle salt is used as the element 2, and a metallic thin-film consisting of gold, silver, etc., is employed as the filmy electrode 3 and applied onto both surfaces of the element through evaporation or sputtering. A plasma polymerization organic acid containing an ammonium group, etc., is used as the sensitive film 4 on the electrode 3, or a glass film through an alkoxide solution method is employed as a glass thin-film on the electrode. The external exposed surfaces of the metallic electrodes 3 are coated with electrical insulating films 5, thus allowing measurement at the time of immersion into a gas or a liquid containing a large number of moisture and having low insulating properties.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to a piezoelectric element type sensor. More specifically, the present invention relates to a piezoelectric element type sensor that is useful as a sensor for various chemical components such as a moisture sensor, a humidity sensor, an ion sensor, and can also be applied as a sensor for other chemical components.

(b) Prior Art Piezoelectric elements, especially crystal oscillators, have been used as transmitting elements in electronic and communication equipment for a long time. This is because the crystal is physically and chemically very stable, the oscillation frequency is extremely stable, vibration loss is low, the cutting method,
This is because various frequencies can be obtained depending on the shape of the vibrator, temperature characteristics can be controlled by impurity concentration, and the production technology for artificial quartz crystal has been established as an alternative to natural quartz crystal.

In recent years, utilizing the characteristics of a quartz crystal resonator, a pair of film-like electrodes are formed on the crystal resonator, and on at least one of the film-like electrodes, Polymer electrolyte membrane containing polar groups and plasma polymerization 8! Humidity/moisture sensor configured as a moisture-sensitive membrane by forming a membrane, ■ Ion sensor configured as an ion-sensitive membrane by forming an organic or M-based polymer membrane containing an ion-sensitive substance, ■ Forming an enzyme-immobilized membrane. As a result, enzyme sensors configured as membranes sensitive to various substrates for enzymes have been put into practical use or proposed. All of these are configured to detect a chemical component using as an indicator a change in the oscillation frequency of a crystal oscillator based on a change in the sensitive lI weight when the target chemical component is temporarily captured in a sensitive film. It is.

(c) Problems to be Solved by the Invention The above-mentioned crystal resonator has various vibration modes depending on its cut form, but its electrical characteristics differ from the equivalent circuit shown in Figure 5 regardless of its cut form. It is indicated by. 1 in the diagram
1. The series resonant circuit of Ci and Rt represents the mechanical vibration of the vibrator, and in parallel there is a parallel capacitance JIC as a crystal dielectric.
oS exists. Among these, Ll and C1 are the elastic modulus of crystal, piezoelectric constant, vibration mode, cutting direction, size and shape,
Although it is determined by the electrode arrangement and is not affected by the environment,
R1 is greatly influenced by the environment of the vibrator. Therefore, it is necessary to stabilize R1 as much as possible, and from this point of view, the various sensors described above can be used to measure moisture and humidity in a gas that maintains insulation properties, or with only one side of the sensor immersed in a liquid. Its usage was limited. For example, when measuring moisture, ions, substrates, etc. in a liquid by immersing the whole body in the liquid, or when measuring moisture and humidity in a gas with low insulation properties (for example, high humidity gas, etc.) In this case, leakage occurred between the pair of electrodes and had a large effect on R1, so that it could not be used on the sole of the body. Furthermore, this phenomenon also occurs when dew condensation occurs between the electrodes of a moisture/humidity sensor in a gas, causing a large error in measured values.

This invention was made in view of the above circumstances, and particularly provides a piezoelectric element that enables measurement in a gas with low insulation properties that contains a large amount of water, etc., or in a state where the entire body is immersed in a liquid. The present invention aims to provide a type sensor.

(d) Means for Solving the Problems Thus, according to the present invention, the sensor element comprises a pair of film electrodes on a piezoelectric element, and a chemical component sensitive film is formed on at least one of the film electrodes. There is provided a piezoelectric element type sensor characterized in that the externally exposed surface of each membrane electrode is covered with an electrically insulating thin film.

Examples of the piezoelectric element include a quartz crystal oscillator, tourmaline, Rotschl's salt, barium titanate crystal, zinc oxide crystal, etc. Among these, it is preferable to use a quartz crystal oscillator. Of the rough crystal oscillators, it is preferable to use a flat plate-shaped crystal oscillator with an A cut because it is easy to obtain a high oscillation frequency.

Further, as the film electrode, it is suitable to use a thin film of a metal such as gold or silver, and these are usually formed in pairs on both sides of the piezoelectric element by vapor deposition or sputtering.

As mentioned above, various organic and inorganic films can be used as the chemical component sensitive film. When forming a moisture-sensitive film, it is preferable to use a plasma-polymerized organic film containing ammonium groups or sulfonate groups on the surface layer, for example, as described in Japanese Patent Application Laid-Open No.
It can be formed on the above electrode according to the description in No. 24234 and Japanese Unexamined Patent Publication No. 61-66160. Furthermore, when configuring the membrane as a substrate-sensitive membrane for an enzyme, a glass thin film is first formed on the electrode by a solution method using an alkoxide, and the intended enzyme is immobilized thereon by chemical bonding by a silane coupling method or the like. It is preferable to form it by. These chemical component sensitive films are usually formed at least on the membrane electrode, but are preferably formed within the surface area of the membrane electrode, and more preferably in the center thereof.

An electrically insulating thin film is formed to cover the externally exposed surface of the film-like electrode of the piezoelectric element on which the sensitive film is formed. This coating may be applied to the entire surface of the piezoelectric element except for the sensitive film, as long as at least the electrode exposed surface is electrically isolated from the outside. The insulating thin film to be removed is preferably one that is chemically and physically stable and can be easily formed into a thin film, such as a plasma polymerized organic film or a glass film. In this case, a plasma-polymerized membrane having substantially no polar groups is suitable, and a polymer membrane obtained by subjecting an aromatic vinyl compound such as styrene to plasma polymerization as a polymer is preferable. . On the other hand, as the glass film, a glass film obtained through gelation using a metal alkoxide solution method is suitable.In particular, a glass film obtained by applying a solution of a lower alkoxysilane such as ethyl silicate or methyl silicate, drying, and heating is suitable. It is preferable to apply a glass film that is Note that the thickness of these electrically insulating thin films is usually suitably about 0.1 to 10). If the thickness is too thin, the insulation properties will deteriorate, and if it is too thick, the piezoelectric properties of the piezoelectric element will deteriorate, which is not preferable.

(E) Function The presence of the electrically insulating film prevents leakage between the pair of film-like electrodes, and keeps the inter-electrode resistance R1 of the piezoelectric element constant.

(f) Example Directly deposited on the center of both sides of an AT-cut crystal resonator (0.1 x 13 mmφ) by vapor deposition. A gold electrode (membrane electrode: approximately 1 ㎡ thick) was formed. This element (oscillation frequency 9.02
25' H2) was placed in a plasma polymerization chamber and masked under the following conditions: Monomer m: Styrene monomer pressure: 1°C orr Chamber temperature: 25°C Discharge frequency: 1KHz Discharge electrode: Approximately 20 at 3.5W Second plasma polymerization was performed to form a polystyrene film with a thickness of about 3000 mm at the center of the electrodes on both sides.

Next, the pressure inside the sealed container was reduced to 0.1 tOrr, and N, N
,N',N'-Tetramethylhexanediamine vapor was introduced to 1 torr, and 6 torr was added under the same conditions.
Plasma polymerization was performed for 0 seconds to form a polymer film containing about 5,000 amine groups in close contact with the polystyrene thin film.

Next, this element was left in a methyl chloride vapor tank for 12 hours to alkylate the amino groups in the amine group-containing polymer film (surface II) and convert them into ammonium salt-type hydrophilic groups.

The device thus obtained is shown in FIG.

In the figure, 2 is a crystal resonator, 3 is a gold electrode, and 4 is a moisture-sensitive film made of a plasma-polymerized polymer containing an ammonium salt type hydrophilic group in its surface layer.

The surfaces of the moisture-sensitive films on both sides of the above device were masked, and this was placed in the plasma polymerization chamber again under the following conditions: Monomer: Styrene monomer pressure: 1 torr Chamber temperature: 25° C. Discharge frequency: 1 KHz Discharge power = 10 W (distance between electrodes about 50 mm) for about 3 minutes, the surface other than the moisture sensitive membrane was coated with a plasma polymerized film, and the exposed surface of the gold electrode was covered with a polystyrene thin film about 1 layer thick. A humidity sensor 1 according to an embodiment of the present invention as shown in FIG. 1 was obtained. In the figure, 5 indicates a polystyrene thin film produced by plasma polymerization.

This humidity sensor 1 is connected to an oscillation circuit board 9 as shown in FIG.
The electrode was attached with an insulating support 8.8 and the whole was immersed in a sodium chloride aqueous solution 6 of a predetermined concentration to evaluate electrical leakage between both electrodes. In the figure, 10 indicates a power supply (DC 15V), and 11 indicates a frequency counter.

The results are shown in FIG. In the figure, A shows the results for the above humidity sensor, and B shows the results for the sensor before forming polystyrene 115 (comparative example).

As described above, in the sensor of the example, the frequency does not change even when the sodium chloride concentration increases, and it can be seen that the insulation between the two electrodes is maintained extremely well.

The above sensor can be used as a moisture/humidity sensor in the gas phase even in a high humidity environment.

(G) Effects of the Invention According to the piezoelectric element force sensor of the present invention, good electrical insulation between both electrodes can be maintained even in a gas phase containing a large amount of water and ionic substances, as well as in a liquid phase. , it is possible to carry out the intended measurements even in such a system. Therefore, the application of piezoelectric elements to sensors is significantly expanded.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional structural explanatory diagram showing a humidity sensor according to an embodiment of the present invention, FIG. 2 is a planar structural explanatory diagram showing a piezoelectric element in the manufacturing process of the sensor according to the embodiment, and FIG. A configuration explanatory diagram of a device for testing electrical leakage of the sensor of the example, FIG. 4 is a graph diagram showing the results of the electrical leakage test together with a comparative example, and the fifth factor is an equivalent circuit diagram of a crystal oscillator. . 1...Humidity sensor, 2...Crystal resonator, 3...Gold electrode, 4...Moisture sensitive membrane, 5...Polystyrene Thin film. Figure 1 Figure 3

Claims (1)

[Claims] 1. Consisting of a sensor element comprising a pair of film electrodes on a piezoelectric element and a chemical component sensitive film formed on at least one of the film electrodes, the externally exposed surface of each film electrode is A piezoelectric element type sensor characterized by being coated with an electrically insulating thin film. 2. The sensor according to claim 1, wherein the electrically insulating thin film is a plasma polymerized organic film or a glass film. 3. The sensor according to claim 2, wherein the plasma polymerized organic film is a polymerized film of an aromatic vinyl compound. 4. The sensor according to claim 2, wherein the glass film is a glass film formed by a metal alkoxide solution method.