S.G. Joshi

Investigation of acoustic waves in thin plates of lithium niobate and lithium tantalate

The general properties of fundamental antisymmetric A/sub 0/, symmetric S/sub 0/, and shear horizontal SH/sub 0/ acoustic waves propagating in thin piezoelectric plates have been theoretically investigated on samples of lithium niobate (LiNbO/sub 3/) and lithium tantalate (LiTaO/sub 3/). The results obtained will be useful for a proper development of various physical, chemical, and biological sensors and devices for signal processing based on plate acoustic waves.

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.

Acoustic waves in piezoelectric plates bordered with viscous and conductive liquid

The influence of a viscous or conductive liquid on the characteristics of acoustic waves propagating in thin piezoelectric plates is investigated theoretically as well as experimentally. Experimental results are found to be in good agreement with the theoretical calculations. The data presented in this paper is essential for the proper design of various acoustic wave sensors operating in contact with viscous and/or conductive liquids.

A study of polymer/water interactions using surface acoustic waves☆

Application of surface acoustics waves to the study of fundamental properties of hygroscopic polymer/water systems is reported. The basic device used is a surface acoustic wave (SAW) delay-line oscillator, the propagation path of the delay line being coated with a film of the polymer to be studied. The frequency of the SAW oscillator is measured as a function of temperature at various relative humidities. Thermodynamic functions of free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) are then determined. The transient response of the SAW oscillator to step changes in humidity is also studied, and may be described by a generalized relaxation equation containing two additive terms. The utility of these data for analyzing the thermodynamics and kinetics of water-vapor sorption and desorption is discussed. The very high surface sensitivity and mass resolution (about 1 ng) afforded by the SAW oscillator measurements are advantageous relative to conventional sorption measurements. The approach reported here provides a relatively simple yet precise method for characterizing materials of interest for chemical-sensing applications.

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.

Propagation of a quasi-shear horizontal acoustic wave in Z-X lithium niobate plates [and conductivity sensor application]

It is found that an acoustic wave which is nearly polarized in the shear horizontal (SH) direction can propagate along the X axis of a Z-cut lithium niobate plate if the ratio h//spl lambda/, where h=plate thickness and /spl lambda/=acoustic wavelength, is less than about 0.5. Attractive properties of this quasi-SH wave include: (1) phase velocity nearly constant for all values of h//spl lambda/; (2) ability to propagate in contact with a liquid medium; and (3) electromechanical coupling coefficient as high as 0.15. These properties make the wave attractive for use in a variety of sensor and signal processing applications. An example of sensor applications is illustrated by using the wave to measure conductivity of liquids (aqueous KCl solution). The frequency of a 12-MHz quasi-SH mode oscillator fabricated on a 0.48 wavelength thick Z-X lithium niobate plate is found to vary by more than 80 kHz for variation in KCI concentration from 0 to 0.15%.

Ultrasonic instrument for measuring bolt stress

Abstract An ultrasonic instrument for the accurate measurement of bolt stress is described. The instrument, which is based on the pseudo-continuous-wave technique, uses carrier phase detection to track the frequency of the mechanical resonance of the bolt. The basic operation of the instrument and the experimental results obtained are discussed.

Flow sensors based on surface acoustic waves

Abstract The basic theory, experimental performance, and current status of flow sensors based on the use of surface acoustic waves (SAWs) are discussed. The sensor consists of a delay-line stabilized SAW oscillator heated to a suitable temperature above ambient, and placed in the path of the flowing fluid. Convective cooling caused by fluid flow changes the substrate temperature and thereby the oscillator frequency. Attractive properties of this sensor include high sensitivity, wide dynamic range, and direct digital output. An example of the performance that has been achieved is illustrated by a 73 MHz sensor that shows greater than 140 kHz change in frequency for variations in flow rate from 0 to 1000 ml/min. One of the limitations of the SAW flow sensor is its slow speed of response. This problem can be overcome by using a Lamb wave device fabricated on a thin membrane.

Surface-acoustic-wave (SAW) flow sensor

The use of a surface-acoustic-wave (SAW) device to measure the rate of gas flow is described. A SAW oscillator heated to a suitable temperature above ambient is placed in the path of a flowing gas. Convective cooling caused by the gas flow results in a change in the oscillator frequency. A 73-MHz oscillator fabricated on 128 degrees rotated Y-cut lithium niobate substrate and heated to 55 degrees C above ambient shows a frequency variation greater than 142 kHz for flow-rate variation from 0 to 1000 cm/sup 3//min. The output of the sensor can be calibrated to provide a measurement of volume flow rate, pressure differential across channel ports, or mass flow rate. High sensitivity, wide dynamic range, and direct digital output are among the attractive features of this sensor. Theoretical expressions for the sensitivity and response time of the sensor are derived. It is shown that by using ultrasonic Lamb waves, propagating in thin membranes, a flow sensor with faster response than a SAW sensor can be realized.<<ETX>>