Ultrasound piezoelectric transducers

The ultrasound fields are generated by a focused, MRI compatible piezoelectric transducer (19). The physical characteristics of the transducer determine the size of the focal spot and can be tailored for the experimental needs. For example, we have performed successful experiments with 100-mm diameter, 80-mm radius of curvature transducers with a frequency of 0.7 MHz (see Note 1). 

Ultrasound amplitude (power). This variable is directly related to the amount of energy applied and has similar effects with baths and probes. The former, however, are not powerful sources as most of them use modern piezoelectric transducers which provide a low-intensity power (in the region of 1-5 W/cm ). 

Homer and Patel [12,13] saturated the l4N signal (Larmor frequency of 6.42 MHz) of a solution of AyV-dimethylformamide with varying intensities of ultrasound at different frequencies that were generated by piezoelectric transducers suspended in the NMR sample. No effect was observed at any intensity when 1.115-MHz ultrasound was applied to the sample. At 6 MHz there was some evidence of signal suppression, the effect increasing with ultrasonic intensity. At 6.42 MHz the saturation of the l4N resonance was almost complete even with low acoustic intensity. When the frequency was raised to 10 MHz the signal suppression stopped and the l4N resonance was once again observed. Similar results were also obtained with AyV-dimethylacetamide. 

A curved piezoelectric transducer was used for the high frequency 1.58 MHz work since this created cavitation at a focal point with intensities of approximately 3.4 kW/cm2. Low-frequency (20 kHz) ultrasound was produced with a commercial sonicator equipped with an exponential microhom. At high focal intensities (>1.5 kW/cm2), a single (100 ms) pulse of ultrasound produced depassivation at low intensities continuous ultrasonic exposure was required. In all cases, the induced depassivation was followed by precipitation of a metal salt film upon the metal surface prior to the oxide film formation. 

In many applications it is difficult to apply surface vibration because of the large mass of the heat transfer apparatus. The alternative technique is then utilized, whereby vibrations are applied to the fluid and focused toward the heated surface. The generators that have been employed range from the flow interrupter to the piezoelectric transducer, thus covering the range of pulsations from 1 Hz to ultrasound of 106 Hz. The description of the interaction between fluid vibrations and heat transfer is even more complex than it is in the case of surface vibration. In particular, the vibrational variables are more difficult to define because of the remote placement of the generator. Under certain conditions, the flow fields may be similar for both fluid and surface vibration, and analytical results can be applied to both types of data. 

Ultrasound through-transmission ultrasound ( ITU) or pulse echo (PE) ultrasound is the primary method for the acceptance of composite laminate materials using an automated scanning system with water-coupled piezoelectric transducers and acceptance standards. Many variations are applicable depending on the material, configuration, thickness, and sensitivity requirements. 

It is possible to use air as the coupling for a noncontact inspection using piezoelectric transducers. The noncontact application of ultrasound is desirable to avoid the... 

Piezoelectric transducers are key components in medical ultrasound imaging and are used both as the acoustic source and the detector (pulse-echo teclmique). The uses for ultrasound are numerous and include examination of the fetus in the mother s womb as shown in Figure 31.22 and high-resolution imaging of intravascular structures. PZT is the ceramic of choice for this application mainly because it has a high k and is inexpensive compared to some of the other options such as polymer piezoelectrics. 

FIGURE 31.22 Medical ultrasound image using ceramic piezoelectric transducers. (Scale is in centimeters.)... 

Doppler Ultrasound. Doppler ultrasound has been widely used to provide quaUtative measurements of the average flow velocity in large to medium-size vessels if the vessel diameter is known. These include the extracranial circulation and peripheral limb vessels. It is also used in an assessment of mapped occlusive disease of the lower extremities. The frequency used for Doppler ultrasound is typically between 1 and 15 MHz. The basis of this method is the Doppler shift, which is the observed difference in ftequency between sound waves that are transmitted from simple piezoelectric transducers and those that are received back when both transmitter and receiver are in relative motion. The average frequency shift of the Doppler spectrum is proportional to the average particulate velocity over the cross-sectional area of the sample. When us to measure blood flow, the transducers are stationary and motion is imparted by the flowing blood cells. In this event, red cell velocity V is described by the relationship... 

Piezoelectric transducers are the most common devices employed for the generation of ultrasound and utilise ceramics containing piezoelectric materials such as barium titanate or lead metaniobate. The piezoceramic element commonly used in ultrasonic cleaners and for probe systems is produced in the form of a disk with a central hole. Ceramic transducers are potentially brittle and so it is normal practice to clamp them between metal blocks. This serves both to protect the delicate crystalline material and to prevent it from overheating by acting as a heat sink. Usually two elements are combined so that their overall mechanical motion is additive (Figure 10.4). Piezoelectric transducers are better than 95% electrically efficient and can operate over the whole ultrasonic range. 

Acoustic emission is the burst of ultrasound emitted by a material under stress, detectable in practice with the aid of a piezoelectric transducer suitably attached to the article under test. Acoustic emissions from some composites, including fiber-reinforced plastics, are sometimes measured to detect the onset of yielding or permanent damage that may occur on repeated stressing. 

Piezoelectric transducers convert electric energy into ultrasound by applying an osciUating electric field to a piezoelectric crystal (such as quartz). These transducers, which work in liquids or air, can generate frequencies in the megahertz region with considerable power. In addition to natural crystals, ceramic piezoelectric materials, which can be febri-cated into any desired shape, have been developed. 

First system of piezoelectric ceramics (BaTiOs) has been developed at the end of World War II independently by the researchers in Japan, Soviet Union and USA. Ceramic material has been utilized for the applications in ultrasound (electroacoustic) transducers. Later in 1950 s the most important ceramic system ever - i.e. 

Figure 5. Sketches of several methodsusedtoputaliquidunda-mechanlcalteiision. (a) Acoustic method. A hemispherical piezoelectric transducer emits focused ultrasound bursts (arrows) into a bulk liquid [43]. (b) Metastable vapor-liquid equilibrium. A nanoporous membrane or gel mediates the equilibrium of a bulk volume of liquid and its subsaturated vapor [15]. (c) Bertbelot tube. A rigid container partially filled with a liquid in equilibrium with its vapor is heated until the liquid expands to fill the entire volume. Upon cooling, the liquid follows an isochore and its pressure decreases [44,45[. (d) Centrifugal method. A tube formed with two symmetrical bends at each end (a z-tube) is spun around its mid-point such that the pressure in the liquid drops due to the centripetal acceleration acting on the column of liquid [46[.

In the brewing industry, there is no ideal foam-control method currently in use, although ultrasonics have been tested to suppress foaming in fermenting vessels. The ultrasound generated by ceramic piezoelectric transducers has the unusual property that when an electric voltage is applied across their crystalline ceramic structure, deformation occurs, so that an alternating current thus produces a sinusoidal vibration. The resonance frequency depends on the physical dimensions of the... 

Nestleroth et al. [15], Segal et al. [16] considered some established novel signal processing schemes to assist in adhesive bond inspection. Sinclair et al. [17] and Filimonov [18] employed acoustic resonance methods for dynamic elastic modulus measurements in adhesively bonded structures. Yost and Cantrell [19], Achenbach and Parikh [20] and Nagy et al. [21] considered a nonlinear response of bonded structures to estimate material characteristics. In Achenbach and Parikh [20], failure was preceded by nonlinear behavior of thin boundary layers at the interfaces. Billson and Hutchins [22] considered lasers and EMATS in bond investigations. It was shown that this non-contact technique was reasonable when compared to that obtained by conventional piezoelectric transducers. Ince et al. [23] also characterized bonds with laser-generated ultrasound and through-transmission measurements. 

Although less work has been done on the acoustic reduction of Tj in solids than in liquids, it has been established that, as for liquids, the natural values of Tj in solids can be reduced by the application of ultrasound. By coupling 20 kHz ultrasound to a sample of trisodium phosphate dodecahydrate in an open mesh nylon sack immersed in a liquid, the normal value of the Tj was reduced from 7.1 s (obtained from MAS NMR measurements) to 2.1 s. Subsequently, similar reductions (by a factor of about two) have been observed for the values of for in diamonds to which high frequency piezoelectric transducers were bonded directly. 

In Ultrasound computed tomography, the interaction between ultrasound wave and the body is used to determine the distribution of sound velocity c and the tissue density p. Ultrasound signals are stimulated and received by piezoelectric transducers. Received data is also processed by above mentioned algorithms, i.e by applying Fourier transform algorithm in order to reconstruct internal stmctures of the body as indicated in the following diagram ... 

Piezoelectric transducers must be fitted with electrodes, which supply tte necessary alternating electrical field. These electrodes must not impede the irradiation of the ultrasonic waves therefore thin gold or silver films are usually depoated on the transducer surface by vacuum evaporation. A crystal prepared in this way can be used directly for the generation of ultrasound in a nonconducting liquid, e.g., oil. A typical experimental set-up of this type used for the irradiation of liquids h riiown in Fig. 1. The main disadvantage of this anangement is the difficulty in measuring... 

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