RU2371841C2 - Acoustoelectric surface acoustic wave sensor - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to acoustoelectric devices on surface acoustic waves (SAW) and can be used for determining physical and chemical parametres of gaseous media (liquids), mainly for designing wireless remote analytical systems based on surface acoustic wave sensors and radio frequency identification systems. The acoustoelectric surface acoustic wave sensor has two monocrystalline piezoelectric-crystal plates separated by a vacuum gap, on the inner surface of one of the plates of which there are two identical, oppositely placed interdigital transducers, one of which is the input transmitting, which coverts the input electric signal into an electroacoustic wave and an output receiving, which converts the electroacoustic wave λ into an electric signal; the input port for inlet of gas or liquid into the vacuum gap; an output port for outlet of gas or liquid from the vacuum gap. Thickness of the piezoelectric-crystal plates H₁, H₂ is less than or comparable to wavelength λ, and width of the vacuum gap H is less than wavelength λ.

EFFECT: increased sensitivity and efficiency, which allows for using the invention in making high-efficiency acoustoelectric gas and liquid analysers.

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Description

The invention relates to devices for surface acoustic waves (SAW), and more particularly to SAW sensors using a SAW propagating in a system of two thin piezoelectric plates separated by a vacuum gap, and can be used to determine the physical and chemical parameters of gaseous media (liquids), using the correlation of the change in the speed of such an acoustic wave or frequency generated in the resonator of a SAW device to create wireless remote analytical systems based on SAW sensors and radio systems astotnoy identification.

Known SAW sensor [1, 2], containing a single piezoelectric plate, the thickness of which is comparable to the wavelength. As a working acoustic wave in the known SAW sensor, a Lamb wave is used, propagating in a single piezoelectric plate, the thickness of which is comparable to the wavelength λ. A disadvantage of the known solution is the low sensitivity.

Known SAW sensor [3], containing two piezoelectric plates separated by a vacuum gap (the width of the gap is comparable to or less than the wavelength). Moreover, the thickness of one piezoelectric plate is comparable to the wavelength λ, and the thickness of the other piezoelectric plate is much larger than the wavelength λ. In this device, a slit electro-acoustic wave (ЩЭАВ) propagates, the speed and frequency of which change with a change in the type and composition of the gas (liquid) [4].

The effectiveness of such sensors is relatively low, which does not allow their use in the design of high-performance acoustoelectronic pressure sensors, gas and liquid analyzers.

The closest to the claimed technical solution in terms of features is a SAW sensor [3], adopted as a prototype.

A SAW sensor [3] using a slotted electroacoustic wave includes: a piezoelectric medium having, in one part, a thin piezoelectric membrane and a semi-infinite piezoelectric medium in another part, separated by a vacuum gap, inside which a slotted acoustic wave propagates; input interdigital transducer (IDT), formed on the inner surface of the piezoelectric medium on one side to convert the electrical input signal into a slotted acoustic wave; the output IDT formed on the same inner surface of the piezoelectric medium on the other hand, opposite the input IDT, for receiving a propagating slotted acoustic wave and for converting the wave into an electrical signal; an inlet port for liquid to enter the vacuum gap in a piezoelectric medium; an output port for liquid to exit the vacuum gap in the piezoelectric medium, whereby the liquid is sensed by the device. When a fluid flows through a vacuum gap, the dielectric constant and viscosity of a fluid are determined by measuring the frequency and velocity of the crevice acoustic wave.

The disadvantage of the prototype is the low sensitivity and lack of efficiency of the sensor.

The objective of the invention is to provide an acoustoelectronic SAW sensor with increased sensitivity for use in determining the type and composition of gas (liquid).

The technical result of this invention is to increase the sensitivity and efficiency of the SAW sensor in determining the physical and chemical parameters of gaseous media (liquids), mainly in wireless remote analytical systems based on SAW sensors.

The technical result is achieved by the fact that an acoustoelectronic SAW sensor containing two single-crystal piezoelectric plates separated by a vacuum gap, on the inner surface of one of which two identical interdigital transducers are arranged opposite each other: an input transmitter for converting an input electrical signal into an electroacoustic wave and an output receiving, used to convert the electro-acoustic wave into an electrical signal; inlet port for gas (liquid) to enter the vacuum gap; the outlet port for the exit of gas (liquid) from the vacuum gap, characterized in that the thickness of the piezoelectric plates H ₁ , H _{2 is} less than or comparable with the wavelength λ, and the width of the vacuum gap H is less than the wavelength λ.

Figure 1 shows the design of the proposed acoustoelectronic SAW sensor.

The design contains two single-crystal piezoelectric plates (1) thick

H ₁ , H ₂ , separated by a vacuum gap (2) of a width N.

On the inner surface of one of the piezoelectric plates, the transmitting (input) and receiving (output) IDTs are applied by photolithographic method (3).

The input IDT is used to convert an electrical input signal into an acoustic wave. The output IDT, formed on the same inner surface of the piezoelectric medium on the other hand, opposite the input IDT, is used to receive a propagating acoustic wave and convert the wave into an electrical signal.

In the system, a thin piezoelectric plate - a vacuum gap - a thin piezoelectric plate propagates an electro-acoustic wave, which we will hereinafter call the electro-acoustic wave in a system of two thin piezoelectric plates separated by a vacuum gap.

In the vacuum gap (2) through the inlet port (4) is introduced, and through the outlet port (5) the studied gas (liquid) is discharged. The receiving and transmitting IDTs are covered with a dielectric layer (6) to eliminate the IDT contact with the test gas (liquid). Piezo plates themselves are mounted on massive dielectric (metal) substrates (7). The width of the vacuum gap is determined by the thickness of the dielectric spacers (8).

The process of determining the type and composition of the test gas (liquid) is as follows:

the input IDT, formed on one side of the inner surface of the piezoelectric medium, converts the electric signal into an acoustic wave due to the piezoelectric effect;

the output IDT, formed on the other side of the same surface opposite the input IDT, is used to receive a propagating acoustic wave and convert the wave into an electrical signal;

the inlet port (4) serves to enter the gas (liquid) into the vacuum gap;

the output port (5) serves to exit the gas (liquid) from the vacuum gap;

the dielectric constant of the gas (liquid) passing through the vacuum gap is determined by determining the change in the speed or frequency of the acoustic wave in the presence of gas (liquid) in the vacuum gap (2).

When an acoustic wave propagates in a vacuum gap (2) between two piezoelectric plates, the phase velocity of the acoustic wave depends on the dielectric constant of the gas (liquid) in the vacuum gap (2). If we designate the speed of an acoustic wave propagating in a vacuum gap as v ₀ , and the speed of an acoustic wave propagating in a vacuum gap with a gas (liquid) as v ₁ , then these velocities will differ the more, the greater the dielectric constant of the gas (liquid) . The type of gas (liquid) is determined by its dielectric constant.

In order to determine the type of gas (liquid) (dielectric constant), the frequency f _{0 of the} acoustic wave propagating in the vacuum gap (2) is measured, and the phase velocity v _{0 is} calculated. In this case, the relationship between frequency and speed is established using the relation v ₀ = λ * f ₀ .

When a gas (liquid) passes in the vacuum gap (2) through the inlet port (4), the frequency f _{1 of the} acoustic wave is measured and the phase velocity v _{1 is} calculated. The frequency f _{1 of the} acoustic wave is determined after the stationary flow of gas (liquid) is established in the vacuum gap (2). In this case, the relationship between frequency and speed is established using the relation v ₁ = λ * f ₁ .

Thus, the dielectric constant of a gas (liquid) passing through the vacuum gap (2) can be determined by the difference in frequencies f ₀ and f ₁ or by the difference in speeds v ₀ and v ₁ .

For this, it is necessary to preliminarily tabulate the dependence of the change in the frequency or velocity of the acoustic wave on the dielectric constant for different types of liquid and form these dependencies in the form of a database.

The dielectric constant of a gas (liquid) can be determined by searching the corresponding nearest values in this database, so the SAW device can work as a gas (liquid) sensor.

The method for determining the type of gas (liquid) using a SAW sensor includes the following steps:

(a) measuring the frequency of the acoustic wave passing through the empty vacuum gap and calculating the phase velocity;

(b) measuring the frequency of the acoustic wave and calculating the phase velocity in the presence of gas (liquid) passing in the vacuum gap through the inlet port;

(c) determining the dielectric constant of the gas (liquid) passing through the vacuum gap by changing the frequency or velocity of the wave and thereby identifying the type of gas (liquid).

In step (c), the dielectric constant of a gas (liquid) is determined by comparing the obtained data with the values established previously for the dielectric constant of known gases (liquids).

The design of IDT, applied by photolithographic method, can be of three types.

On figa presents the design of a conventional two-phase IDT. The width of the metal electrodes and the gap between them are equal to λ / 4. Figure 2b shows the design of a two-phase IDT with double (split) electrodes, by which the level of internal wave reflections from the electrodes can be significantly reduced, since the frequency of electrical resonance is 2 times less than the frequency of the mechanical one. In this case, the width of the electrodes and the gap between them are equal to λ / 8.

Figure 3 shows the construction of a single-phase transducer, the electrodes (11) of which are located on both sides of the surface of the piezoelectric plate (12). In this case, the lower electrode is continuous, and the width of the upper electrodes and the gap between them are equal to λ / 2.

The following can be used as single-crystal piezoelectric plates: XY-section piezoelectric plate of lithium niobate (LiNbO ₃ , trigonal syngonium), or potassium niobate (KNbO ₃ , rhombic syngony), or lead potassium niobate (PKN, orthorhombic syngonium). In the case of using piezoelectric plates of lithium niobate XY-section, a quasi-transverse normal acoustic wave propagates. This is a quasi-horizontal-transverse shear high-speed acoustic wave that has virtually no components of mechanical displacement normal to the surface. In the case of using piezoelectric plates of potassium niobate or leaded potassium niobate of the XY section, a purely transverse normal acoustic wave (SH wave) propagates. This is a horizontal-transverse shear high-speed wave that does not have components of mechanical displacement normal to the surface.

Due to this, such waves propagate in a system of two thin piezoelectric plates in contact with a gas (liquid), without radiation losses, which are caused by the radiation of wave energy into a gas (liquid) medium. In addition, a feature of the horizontal-transverse shear acoustic wave is that for a certain thickness of the piezoelectric plates (approximately 0.1 wavelength (λ)), the electromechanical coupling coefficient K ² can be very high, which significantly increases the sensitivity of the sensor.

Figure 4 shows the calculated dependence of the value of K ² and the phase velocity of the wave V on the relative thickness of the vacuum gap (N / λ) for the LiNbO ₃ piezoplate XY-slice thickness (H1 / λ = H2 / λ = 0.1, 0.2, 0.5).

Figure 5 shows the calculated dependence of the phase velocity of wave V on the relative permittivity of the gas (eps) in a system of two piezoelectric plates LiNbO ₃ XY-slice with a relative thickness of H1 / λ = H2 / λ = 0.1, separated by a vacuum gap of thickness H / λ = 0.05. As can be seen from FIG. 5, the dependence of the wave velocity V on eps is linear.

Literature

[1] United States Patent, patent Number 5, 189, 914, March 2, 1993, White, et al. Plate-mode ultrasonic sensor.

[2] United States Patent, patent Number 5, 212, 988, May 25, 1993, White, et al. Plate-mode ultrasonic structure including a gel.

[3] United States Patent, patent Number US 2005/0156484 A1, Jul. 21, 2005, V. Cherednick, M. Dvoesherstov, Y. L. Chee.

[4] M.Yu. Dvoesherstov, V.I. Cherednik, S.G. Petrov, A.P. Chirimanov. Numerical analysis of slotted electro-acoustic waves // Acoustic Journal. 2004.V.50. Number 4. C.1-6.

[5] M.Yu. Dvoesherstov, V.I. Cherednik, S.G. Petrov, A.P. Chirimanov. Lamb Electroacoustic Waves in Piezocrystalline Plates // Acoustic Journal, 2004, T.50, No. 5, pp. 633-639.

Claims (1)

An acoustoelectronic SAW sensor containing two single-crystal piezoelectric plates separated by a vacuum gap, on the inner surface of one of which there are two identical opposing pin probes located opposite each other, one of which is an input transmitter, which serves to convert an input electrical signal into an electroacoustic wave, and an output receiving, used to convert the electro-acoustic wave into an electrical signal; inlet port for gas or liquid to enter the vacuum gap; an output port for the exit of gas or liquid from the vacuum gap, characterized in that the thickness of the piezoelectric plates H ₁ , H _{2 is} less than or comparable with the wavelength λ, and the width of the vacuum gap (H) is less than the wavelength λ.

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