I. E. Kuznetsova

Liquid level sensor using ultrasonic Lamb waves

This paper describes a novel, noninvasive method for measurement of liquid level in closed metal tanks that are under high pressure. It is based on the use of ultrasonic Lamb waves propagating along the tank wall. Contact with liquid substantially changes the characteristics of these waves and this can be used as an indicator of liquid presence. Theoretical analysis shows that the symmetric and antisymmetric Lamb wave modes, both fundamental and higher order, are sensitive to presence of the liquid. The optimal wave frequency depends on the thickness of the tank wall and wall material. A prototype level sensor based on this principle has been developed. It uses two pairs of wedge transducers to generate and detect Lamb waves propagating along the circumference of the gas tank. An operating frequency of 100 kHz is found to be optimal for use with tanks having a wall thickness of 30-50 mm. Prototype sensors developed under this program have been used successfully in oil fields in the far northern region of Russia.

Investigation of quasi-shear-horizontal acoustic waves in thin plates of lithium niobate

It has been shown recently that an acoustic wave which is nearly polarized in the shear horizontal direction can exist in thin plates of arbitrary elastic materials if the ratio (h is the plate thickness, the acoustic wavelength) is less than unity. We refer to this as a QSH (quasi-shear-horizontal) acoustic wave. Preliminary results showing attractive properties of this wave have been published. This paper presents detailed theoretical and experimental investigation of QSH waves in thin plates of lithium niobate. Results confirm that the QSH wave provides a number of attractive properties for use in sensing and signal processing applications. These include the following: (i) a phase velocity nearly constant for all values of ; (ii) the ability to propagate in contact with a liquid medium and (iii) a very high value of electromechanical coupling coefficient. Experimental measurements of phase velocity and coupling coefficient are found to be in good agreement with theoretical calculations. The influence of a biasing electric field on the propagation of QSH waves has been calculated. It is shown that biasing voltages of less than 200 V can produce fractional time delay changes greater than 0.15% in QSH wave delay lines fabricated on thick lithium niobate plates.

Propagation of QSH (quasi shear horizontal) acoustic waves in piezoelectric plates

The characteristics of QSH (quasi shear horizontal) acoustic waves propagating in thin plates of Y-cut, X-propagation lithium niobate are investigated theoretically and experimentally. The fractional velocity change (/spl Delta//spl nu///spl nu/) produced by electrical shorting of the surface is calculated as a function of the normalized plate thickness h//spl lambda/ (h=plate thickness, /spl lambda/=acoustic wavelength). It was found that values of /spl Delta//spl nu///spl nu/ as high as 0.18 could be obtained. Experimental measurements show good agreement with theory. The properties of QSH waves propagating in the presence of a perfectly conducting electrode separated from the piezoelectric plate by a small air gap have been studied theoretically and experimentally. It was found that by varying the height of the gap, the phase shift through a 3.2-MHz QSH wave delay line can be varied by more than 230/spl deg/. We have also theoretically investigated the influence of a thin layer of arbitrary conductivity on the velocity and attenuation of the QSH wave. Calculations show that the variations in these parameters can be as high as 18% and 5 dB per wavelength for a change in layer surface conductance from 10/sup -7/ to 10/sup -5/ S. Results obtained in this paper confirm the attractive properties of QSH waves for a variety of sensing and signal processing applications.

New method of parasitic mode suppression in lateral-field-excited piezoelectric resonator

A new method of parasitic mode suppression in piezoelectric resonators with transverse (lateral) electric field is described, which employs the partial coverage of electrodes by a damping layer. The potential of the proposed method is demonstrated for a resonator based on an X-cut lithium niobate plate. It is experimentally confirmed that an acoustic wave with transverse electric polarization exhibits stiffening, the degree of which depends on both the electromechanical coupling coefficient and the wave aperture

Biological sensor based on a lateral electric field-excited resonator

This paper describes a biological sensor based on a lateral electric field-excited resonator using an X-cut lithium niobate plate. Its potential was shown through the example of biological interaction between bacterial cells and specific bacteriophages. The detection was based on the analysis of the measured real and imaginary parts of electrical impedance for a resonator loaded by the biological suspension under study. It has been shown that the sensor is sensitive to specific interactions between bacterial cells and specific bacteriophages in a pure state as well as in the presence of extraneous microflora. The degree of electrical impedance variation resulting from the biological interaction depends on the numbers of phage particles and bacteria cells. The sensor may be used not only for the qualitative analysis of bacteria but also for their quantitative detection.

Obtaining phage mini-antibodies and using them for detection of microbial cells with an electroacoustic sensor

Phage mini-antibodies to bacterial cells of strain Azospirillum brasilense Sp245 were obtained, and the possibility of using them for detection of microbial cells with a lateral field excited piezoelectric resonator was studied. It has been found that the frequency dependences of the real and imaginary parts of electrical impedance of such a resonator loaded with a suspension of A. brasilense Sp245 cells with the mini-antibodies differ significantly from the dependences of the resonator with a control cell suspension without mini-anti-bodies. The limit of possible determination of the concentration of microbial cells is found to be 103 cells/mL upon interaction with mini-antibodies. It has been ascertained that detection of A. brasilense Sp245 cells with the aid of mini-antibodies is possible even in the presence of other cultures, for example, E. coli BL-Ril and A. brasilense Sp7. Therefore, it has been shown for the first time that detection of microbial cells with an electroacoustic sensor is feasible.

Reflection of plate acoustic waves produced by a periodic array of mechanical load strips or grooves

The reflection of fundamental acoustic waves propagating in a thin piezoelectric plate by a periodic array of conducting strips of finite thickness or grooves has been theoretically and experimentally investigated. The analysis has shown that electrical shorting and mass loading affect the relationship of neighboring region impedances in a contrary manner. In some cases, these effects are comparable, and there exists a certain strip thickness for each piezoactive fundamental plate mode at which the reflection coefficient can become zero. A high efficiency of grooved reflector for plate acoustic waves was theoretically revealed. Experimental results for mass loading and grooved reflections, which have been obtained for an SH/sub 0/ wave propagating in the Y-X lithium niobate plate, are in a good agreement with the theory. They show a high efficiency of such reflectors and confirm the validity of using a model based on an equivalent circuit for the analysis of their operation. Investigations indicate that nearly 100% reflection of the SH/sub 0/ wave in the lithium niobate plate can be obtained with the use of a mass loading reflector containing 10 silver strips of thickness d/h=0.08 or a grooved reflector containing eight grooves of depth d/h=0.25. Here h is the plate thickness and d is the reflector thickness or depth.

The study of piezoelectric lateral-electric-field-excited resonator

The piezoelectric lateral-electric-field-excited resonator based on an X-cut lithium niobate plate has been investigated. Two rectangular electrodes were applied on one side of the plate so that the lateral electric field components were parallel to the crystallographic Y-axis and excited the longitudinal wave in the gap between the electrodes. The region around the electrodes was covered with a special absorbing varnish to suppress the spurious oscillations. The effect of the absorbing coating width on the resonant frequency and Q-factor of the lateral field-excited resonator was studied in detail with the series and parallel resonances for different width of the gap between the electrodes. As a result, we found experimentally the parameter regions of pure resonances and the boundaries of value variation for resonance frequency, Q-factor, and effective electromechanical coupling coefficient.

New method of change in temperature coefficient delay of acoustic waves in thin piezoelectric plates

As is well-known, the development of highly effective and thermostable acoustic devices assumes using the acoustic waves with high coefficient of electromechanical coupling (K2) and low temperature coefficient of delay (TCD). At present, it also is well-known that fundamental shear horizontal (SH0) acoustic waves in thin piezoelectric plates possess significantly more electromechanical coupling compared to surface acoustic waves (SAW) in the same material. However, although the value of TCD of SH0 waves is insignificantly less than for SAW, this is not enough for development of thermostable devices. This paper suggests a new way of decreasing TCD of SH0 waves in piezoelectric plates at a high level of electromechanical coupling. This way assumes to use the structure containing the piezoelectric plate and liquid with the special dependence of permittivity on temperature. Theoretical and experimental investigation showed that, for SH0 wave in YX LiNbO3 plate at hf=700 m/s (h=plate thickness, f=wave frequency) the presence of butyl acetate can decrease the value of TCD by six times at K2=30%. In a whole the obtained results open the wide prospect of using SH0 wave in thin piezoelectric plate for development of highly effective and thermo-stable acoustic devices

Influence of conducting layer and conducting electrode on acoustic waves propagating in potassium niobate plates

The influence of a thin conducting layer and a conducting electrode on the characteristics of acoustic waves propagating in thin plates of potassium niobate is investigated theoretically. The variations in velocity and attenuation as high as 50% and 30 dB per wavelength, respectively, can be achieved for a change in conductance of the thin film layer from 10/sup -7/ to 10/sup -5/ S.