Acoustic matching

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). 

There are a number of factors which have to be considered when deciding which transducer to use for a particular application. The most important of these are the frequency, crystal diameter and acoustic matching. An ultrasonic transducer generates ultrasound over a range of frequencies which depends on its resonant frequency and the degree of damping of the crystal. The resonant frequency fr of a transducer is determined by its thickness and the... 

Parameter defining the acoustic matching of a crystal and film material... 

For efficient transfer of power from the generator to the medium, usually water, the two must be acoustically matched. The discontinuity can be smoothed by fixing a 2/4 thick layer of material having an acoustic impedance intermediate between that of the radiating surface material and water, and polymers having impedances of about 3.5 Mrayl are readily available. The velocity of sound in them is approximately 2500 ms-1 so that the thickness required at 50 kHz is about 12 mm. In practice the transducer is often bonded to an ultrasonic cleaning tank and then the tank and water become a complicating part of the transducer. 

To form the array a 70 /un thick poled sheet (5x0.6 mm) of PZT is bonded between two metallized polymer sheets. The PZT is sliced into the 64 elements by diamond-cutting, the cut just penetrating the bottom polymer layer so allowing the whole to be wrapped around the catheter tip. The outer polymer film, about 20 /nn thick, serves as a 2/4 acoustic matching layers and also carries the thin copper tracks to electrically address each element. 

The subscripts 1 and 2 refer to the material the wave travels in and the material that is reflected by or transmitted into, respectively. These equations show that the maximum transmission of ultrasound occurs when the impedances and Z2 of the two materials are identical. The materials are then said to be acoustically matched. If the materials have very different impedances, then most of the US is reflected. The reflection and transmission of ultrasound at boundaries has important implications on the design of ultrasonic experiments and the interpretation of their results. In addition, measurements of the reflection coefficient are often used to calculate the impedance of a material. 

The accuracy of the method depends on the correct determination of the characteristics of the ultrasonic field, and accurate measurement of the temperature rise of an absorber of closely controlled energy equivalent. It is also essential that the absorbing medium has a high absorption coefficient and that the acoustic matching within the system is good. Castor oil is among the best liquids which have been used to calibrate transducers. 

CMUTs have the potential to make ultrasound a far more versatile and important imaging modality. This technology has made advances in acoustic matching and the microminiaturization of electronics. It enables higher-frequency imaging, allows clinicians to view small features in the body, and is useful for ultrasound imaging especially in the area of volumetric in vivo imaging, intravascular, and intracardiac research. 

Acoustic impedance is the most important parameter for configuring the acoustic matching layer. Using acoustic velocity c and density p of the propagation material, acoustic impedance Z can be represented by the following (33.1). 

When the acoustic waves move into the boundary of the propagation media, acoustic impedance determines basic characteristics such as reflection or transmission. Referring to Figure 33.1, if Zp is the acoustic impedance of the piezoelectric element, is the acoustic impedance of air, Z is the acoustic impedance of the acoustic matching layer, and d is the thickness of the acoustic matching layer, acoustic impedance matching conditions can be represented by (33.2), with thickness being represented by (33.3) [3]. 

Here, X is the ultrasonic wavelength in an acoustic matching layer. If (33.2) and (33.3) are satisfied, the transmission rate of the plane wave will be maximum. If the acoustic matching layers are of a multilayered structure, it is necessary for (33.2) and (33.3) to be satisfied at any continuous three layers. 

In fact, solid materials with such low acoustic impedance are extremely rare. For the acoustic matching layer of an airborne ultrasonic transducer, non-woven fabrics made of polymer or polymer materials packed in small hollow glass spheres are generally used. 

Acoustic impedance of these materials is around 1,000-2,000x10 kg/(m s), more than ten times the value of ideal acoustic impedance. That is, the current performance of ultrasonic transducers with an acoustic matching layer is restricted by the limitations of the acoustic characteristics of its acoustic matching layer. Silica aerogel has an extremely low density as a solid. We focused on this fact and researched the potential regarding the acoustic matching layer of ultrasonic transducers, as we describe in more detail in the next section. 

From these basic experiments, it was found that silica aerogels had very low acoustic impedances compared to those of the current matching layers [7-9], and are therefore very promising as materials for the acoustic matching layers of high-sensitivity airborne ultrasonic transducers. 

The acoustic properties of ultrasonic transducers with an aerogel acoustic matching layer were studied using a one-dimensional Krimholtz-Leedom-Matthaei (KLM) equivalent circuit [10, 11] to simulate using computer. Figure 33.6 shows the structure of the... 

Figure 33.8. Simulation model of two-acoustic matching layer and material properties.

Figure 33.9. Simulation result of two-acoustic matching layer ultrasonic transducer.

The acoustic matching layer, next to the piezoelectric element, is called the first matching layer, and the other layer is called the second matching layer. Figure 33.9 shows the results of the computer simulations of the two-acoustic matching layer type of ultrasonic transducer. 

AC-ML has two acoustic matching layers. In the cross section of the structure in Figure 33.10, the lower matching layer consists of a porous ceramic fiUed with an aerogel, and the upper matching layer consists of aerogel only. AC-ML has the foUowing characteristics ... 

Figure 33.10. Schematic view of aerogel composite acoustic matching layer consisting of aerogel and a porous ceramic.

Red line and black line are the calculated results of aerogel ultrasonic transducer and ultrasonic transducer with conventional matching layer respectively. The properties of conventional acoustic matching layer are 0.54 x 10 kg/m density, 2,400 m/s acoustic velocity, and 1,296 X 10 kg/m s acoustic impedance. 

The thickness of each layer of AC-ML and conventional acoustic matching layer were optimized so that it could efficiently transmit and receive ultrasonic waves at a fi equency of 500 kHz. 

Figures 33.15 and 33.16 show the experimental results of the time-domain and frequency-domain responses. The red line and the black line show the aerogel ultrasonic transducer and conventional ultrasonic transducer. An acoustic matching layer of...

Acoustic matching layer Antireflective layer of acoustic wave... 

First ferroelectric polymer - polyvinilidene fluoride (PVDF or PVF2) - was discovered in 1969. Extensive research has been focused on this substance and their copolymers withtrilluoroethylene (TrFE) since that time. Due to its resistivity to the harmful chemical substances is this polymer used in stractural coatings to prevent damage. Another excellent functional property is a veiy low value of the acoustic impedance, which allows for the better acoustic matching to water environment. Due to this property P(VDF/TrFE) copolymer is being applied mostly in hydrophones (Nalwa 1995) and ultrasound imaging transducers. PVDF polymer and its blends with TrFE are commercially available in the market. 

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