Mechanical waves, piezoelectric effect

For mechanical wave measurements, notice should be taken of the advances in technology. It is particularly notable that the major advances in materials description have not resulted so much from improved resolution in measurement of displacement and/or time, but in direct measurements of the derivative functions of acceleration, stress rate, and density rate as called for in the theory of structured wave propagation. Future developments, such as can be anticipated with piezoelectric polymers, in which direct measurements are made of rate-of-change of stress or particle velocity should lead to the observation of recognized mechanical effects in more detail, and perhaps the identification of new mechanical phenomena. 

A piezoelectric mass sensor is a device that measures the amount of material adsorbed on its surface by the effect of the adsorbed material on the propagation of acoustic waves. Piezoelectric devices work by converting electrical energy to mechanical energy. There are a number of different piezoelectric mass sensors. Thickness shear mode sensors measure the resonant frequency of a quartz crystal. Surface acoustic wave mode sensors measure the amplitude or time delay. Flexure mode devices measure the resonant frequency of a thin Si3N4 membrane. In shear horizontal acoustic plate mode sensors, the resonant frequency of a quartz crystal is measured. 

The simplest SAW sensor is a two-terminal transmission (delay) line in which the acoustic (mechanical) wave is piezoelectrically launched in one oscillator, called the transmitter. It travels along the surface of the substrate and is then transformed back into an electrical signal by the reverse piezoelectric effect at the receiving oscillator (Fig. 4.18). 

Any type of acoustic transducer, such as quartz crystal microbalance (QCM) or surface acoustic wave device (SAW), is fundamentally based on the piezoelectric effect. This was first described in 1880 by Jacques and Pierre Curie as a property of crystalline materials that do not have an inversion centre. When such a material is subjected to physical stress, a measurable voltage occurs on the crystal surfaces. Naturally, the opposite effect can also be observed, i.e. applying an electrical charge on a piezoelectric material leads to mechanical distortion, the so-called inverse piezo effect. These phenomena can be used to transfrom an electrical signal to a mechanical one and back, which actually happens in QCM and SAW. Different materials are ap-pHed for device fabrication, such as quartz, Hthium tantalate, lithium titanate... 

Mass-sensitive sensors involve piezoelectric effects and surface acoustic waves. The piezoelectric effect was discovered in 1880. Piezoelectricity is the ability of some materials, mostly crystals and ceramics, to generate an electric potential in response to mechanical stress. The piezoelectric effect was mainly utilized for immunosensors and nucleic acid sensors because antigen-antibody association and DNA hybridization cause relatively large changes in mass. Mass-sensitive... 

An acoustic wave biosensor utilizes acoustic or mechanical waves as a detection mechanism to obtain medical, biochemical, and biophysical information about the analyte of interest [1,2]. It detects changes in mass, elasticity, conductivity, and dielectric properties from mechanical or electrical variations. These devices also employ the piezoelectric effect to excite acoustic waves electrically at an input transducer and to receive the waves at the output transducer [2]. 

The piezoelectric phenomena have been used to generate ultrasonic waves up to microwave frequencies using thin polyfvinylidene fluoride) transducers. In the audio range a new type of loudspeaker has been introduced using the transverse piezolectric effect on a mechanically biased membrane. This development has been of considerable interest to telephone engineers and scientists. 

Ultrasound waves are mechanical vibrations (frequency 20 kHz-10 GHz) produced by a piezoelectric device. These waves can be established in a liquid sample and produce cavitation. Very high temperatures are associated with the locations where cavitation occurs, so the effect can be exploited to assist sample preparation [143]. 

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