Acoustic transducer surfaces

Preformed polymers, for example in the form of nano- or micrometer-sized particles, can be interfaced with the transducer in different ways. It has been suggested to entrap MIP-particles into gels [11] or behind a membrane [33,41] for use with electrochemical transducers. Others have spin-coated, a suspension of MIP particles in a solution of an inert, soluble polymer (polyvinyl chloride), which served as glue, onto an acoustic transducer surface [28]. Among the potential... 

Mercury binding leads to an increase of mass of the gold layer which can be detected by electro-acoustic transducers based on quartz microbalance (QMB the abbreviation QCM = quartz crystal microbalance is also widely used), surface acoustic waves (SAW)—devices [20] or microcantilevers [21,22], Adsorption of mercury vapour increases resonance frequency of shear vibrations of piezoelectric quartz crystals (Fig. 12.2). This process can be described by Sauerbrey equation [23]. For typical AT-cut quartz, this equation is... 

Acoustic transducers operating at 10 and (or) 20 kHz are capable of defoaming liquids provided the acoustic source is placed above the liquid surface upon which the foam is being generated. 

Acoustic transducers have also been used in immunosensors for water analysis (Guilbault, 1992). The resonant frequency of an oscillating piezoelectric crystal can be affected by a change in mass at the crystal surface. Piezoelectric immunosensors are able to measure a small change in mass. Recent publications have been based on immunosensors using a quartz crystal microbalance (QCM) for the detection of trace amounts of chemical compounds, such as dioxins (Kurosawa, 2003). 

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... 

Acoustical properties of soybeans can be used to help distinguish between healthy and diseased soybeans. Misra et al. (1990) measured acoustic properties of soybeans by transmitting sound waves through soybeans using acoustic transmission and by an impact force method. In the impact force method, a seed is dropped on an acoustic transducer creating an impulse wave. The acoustic transmission method was slow but was able to predict the mass of individual soybeans. The impact force method showed that diseased soybeans had a narrower bandwidth than healthy soybeans. Soybeans with wrinkled surfaces and diseased and damaged soybeans were detected from healthy soybeans based on wide variations at low frequencies. 

FIGURE 44.20 Spatial variation of the acoustic field across the transducer surface. The actual trapping positions are given by the near field pressure distribution as shown in the 3D-image. Particles wiU be trapped in clusters at the local pressure minima, indicated by the peaks in the figure. (Reproduced from Evander M., et al.. Analytical Chemistry, 79, 2984—2991, 2007. With permission.)... 

There are different possibilities to address the above set of equations which can be solved provided 2in > 3i, and provided the measured ToF information varies between measurement points. For the purpose of the present work we have taken two simplifying assumptions (a) one virtual source predominates at each measurement point, m and (b) each virtual source predominates at more than one measurement point. Note that assumption (b) ensures the condition 2m > 3i that is necessary to obtain solutions for Equations (2) and (3). These assumptions are justified by considering the defect surface as an acoustic secondary field source. At each measurement point the transducer predominantly receives signals from an... 

The use of air-bome ultrasound for the excitation and reception of surface or bulk waves introduces a number of problems. The acoustic impedance mismatch which exists at the transducer/air and the air/sample interfaces is the dominant factor to be overcome in this system. Typical values for these three media are about 35 MRayls for a piezo-ceramic (PZT) element and 45 MRayls for steel, compared with just 0.0004 MRayls for air. The transmission coefficient T for energy from a medium 1 into a medium 2 is given by... 

Fig. 2 shows the CFRP-sandwich specimen and the transducer mounted on the scanner. Fig. 23 presents a C-scan of the specimen as first interesting result. Only the defects visible from the outside are indicated. The distance between transducer and specimen was smaller than the focal length, so that the angle of incidence at the edge of the sound beam converts the longitudinal waves to Rayleigh-waves in the specimen. These waves provide a very sharp image of the surface. This method opens the possibility for a non-contact acoustic microscope. 

This report presents the results of investigations aimed at the creation of the surface wave transducer for the automated control. The basic attention is drawn to the analysis of the position of the front meniscus of the contact liquid when the surface waves excite through the slot gap and to the development of system for acoustic contact creation. 

Double-magnet systems are the most convenient ones for transducers on non-magnet materials. In this case magnetic moments are the normal ones to a surface of the pattern and the opposite ones to the each other. The magnetic field fills the whole zone of the acoustic contact in such positions of... 

The dependencies described are sufficient for designing the different types of ultrasonic transducers for testing by surface waves. The constant transmission of acoustic energy is provided. 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

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