Polymer Transducers

In 1969, Kawai69 discovered that polyvinylidene difluoride (PVDF, PVF2), which is an organic ferroelectric polymer, displayed strong piezoelectric properties. In the last decade, an organic copolymer known as polyvinylidene difluoride trifluoroethylene P(VDF-TrFE) with a higher coupling coefficient has become available. These polymers are used in hydrophones, audio microphones, and robotic tactile transducers. In addition, they find application in wave-propagation transducers for NDT, medical imaging, hi-fi stereophones, and tweeters. Table shows the piezoelectric and related properties of these copolymers.

A hydrophone is a very sensitive pressure transducer that is used to map the temporal and spatial acoustic pressure field of another transducer that is propagating acoustic energy through a fluid.

Hydrophones have to satisfy many requirements: low acoustic impedance to optimally match the fluid impedance and cause the least disturbance to the field being measured, small sensing spot size to obtain good spatial resolution, large bandwidth and flat frequency response to respond to harmonics of the measured signal, and good linearity to handle the wide dynamic range of pressures.

Two types of hydrophones most commonly used are the membrane hydrophone and the Lewin-type needle hydrophone.

In the membrane hydrophone, a metallized 250- to 500-μm-diameter circular dot is vacuum-deposited on either side of a poled copolymer film using a simple mask. This results in a single-element dot transducer. Suitable connections are made to the dots by vacuum-deposited metallic traces. The copolymer is then stretched and held in position over a circular hoop. The pressure of the acoustic field is measured as the electrical output of the dot transducer.

The needle hydrophone consists of a poled, 0.5- to 1.0-mm-diameter, PVDF or copolymer film transducer which is bonded to the flattened end of a hypodermic needle but electrically insulated from it. The electrical output of the transducer gives a measure of the acoustic field pressure.

Doppler Effect Transducers

When a sound wave at a given frequency is reflected from a moving target, the frequency of the reflected or backscattered sound is different. The shift in frequency is caused by the Doppler effect. The frequency is up-shifted if the target is moving toward the observer and down-shifted if it’s moving away. The Doppler shift in frequency is proportional to the velocity of the moving target and is given by 

Two techniques are used in the measurement of fluid flow velocity by the Doppler technique. The continuous-wave (CW) method, as used to determine the flow velocity of slurries in a pipe, is illustrated by Fig. 5.25a.52 The transmitted CW

Figure 5.25 Measurement of velocity by Doppler shift. (a) Continuous-wave (CW) method. (b) Pulse-wave (PW) method gives the peak velocity.

signal is partly reflected by the suspended particles or gas bubbles in the slurry. This backscattered signal is received by a second transducer and its output is compared with the transmitted signal. The Doppler shifted signal fD is given by Eq. 5.21. Knowing _ and f0, the velocity V can be obtained.

The second method, as used in a medical diagnostic application, is illustrated by Fig. 5.25b. A pulsed-wave (PW) signal is used to measure the blood flow velocity in a small blood sample volume or range cell localized in the bloodstream of a coronary artery. The device is constructed from a 0.45-mm-diameter, flexible and steerable guide wire with a 12-MHz transducer integrated into its tip.

The transducer transmits a sequence of 0.83-μs-duration pulses at a pulse repetition frequency of 40 kHz into the bloodstream. The range cell is located by time (range) gating the Doppler shifted backscattered signal generated by the red blood cells and received by the same transducer. This signal is compared with the transmitted signal, where as before the velocity V can be calculated using Eq. 5.21. In this case, cos ө=1.

Angle-Beam Transducers

Angle-beam transducers are used in nondestructive evaluation of castings and riveted steel connections and in the inspection of welded structural elements by the pulse-echo technique. This technique requires the ultrasonic beam to travel at a small angle to the surface of the structure Angle-beam transducers are based on the principle that a longitudinal wave incident on a solid 1-solid 2 interface is mode converted into a refracted shear wave and a refracted longitudinal wave, propagating in solid 2 as shown in Fig. 5.24. 

Fig 5.24 Angle-beam transducer.

The directions of the refracted waves are dictated by Snell’s law.These waves are used to selectively investigate welded joints,cracks, and other structural faults. According to Snell’s law, as _L1 is increased, _L2 and _S2 also increase. Corresponding to _L1 (crit.), _L2 becomes 90° and VL2 ceases to exist, and only VS2 propagates in solid 2. If _L1 is increased much beyond _L1 (crit.), VS2 also disappears. In this situation a SAW propagates on the surface of solid 2. SAWs are used in NDT to detect surface cracks.

Mechanical Imaging Transducers

Mechanical imaging transducers are used to visualize and display the location of impedance discontinuities in a medium through which a short ultrasonic pulse is made to propagate. This method of echo location is known as the pulse-echo technique.

Fig 5.22 The fundamental basis of the pulse-echo technique used in NDT and medical imaging.

 

Figure 5.22 illustrates the fundamental basis for the pulse-echo technique.

An ultrasonic transmit/receive (T/R) transducer is excited by an electrical pulse, and a corresponding stress pulse propagates into the medium. This stress pulse travels with a velocity V, and it encounters an acoustic impedance discontinuity at a distance d. At this discontinuity, some of the pulse energy is reflected back, and the remaining pulse energy propagates farther into the medium.

The first reflected pulse travels back to the same transducer, and since the T/R transducer is bidirectional, it registers the arrival of that pulse by generating an electrical signal at its output. This represents the transduction mechanism in the pulseecho mode. The total distance traveled by the initial pulse is 2d. Assuming that the velocity V is constant, the total round-trip time t is 2d/V. Now, t can be determined from an oscilloscope display, and consequently the distance d of the discontinuity from the transducer face is Vt/2. Other discontinuities can also be located in this manner.

Figure 5.23 Intravascular imaging transducer and processing circuits used to image the inside wall of an artery.

Figure 5.23 shows the cross section of a blood vessel,50 which could very well represent a cylindrical structure like a cladded metal pipe. In the center is a small single-element transducer which rotates when driven by a flexible shaft connected to a motor. In medical practice, the flexible shaft and transducer reside inside a catheter, which is a polyethylene tubing.

The catheter is not shown, for clarity. The space around the transducer is filled with an acoustic coupling fluid. The catheter(Sonicath*) is inserted into the blood vessel by * Sonicath is a trademark of Boston Scientific Corporation. making a small incision and is then advanced to the desired location.

When the transducer is excited, the ultrasonic stress pulse propagates through the fluid, catheter wall, and surrounding blood to reach the linings of the artery wall including the intima, media, and adventitia.47

For a given direction, the transducer sends an ultrasonic pulse toward the artery walls and remains in that position long enough to receive all the echoes from the impedance discontinuities. The transducer then rotates, points in another direction, and receives all the echoes. The process is repeated for a 360° circular scan. All the echoes are stored in memory. The image processing circuitry then uses these signals to modulate the intensity of a CRT which displays the cross-sectional image of the blood vessel as shown in Fig. 5.23.

Single-Element Transducers (Longitudinal mode transducers)

Single-element transducers. The transducers that we consider here are those connected with NDT of structural materials and medical ultrasound. In many ways the transducers used in these fields are quite similar, and there has been an explosion in their variety and availability in the last decade.

Fig 5.20 A single-element longitudinal mode transducer.

Fig 5.21 The on-axis acoustic pressure field of a single-element transducer

The frequencies employed in NDT and medical ultrasound cover the range from 100 kHz to 50 MHz. A single-element transducer is shown in Fig. 5.20. The active element, a piezoelectric ceramic, is poled in the thickness direction. The backing has an impedance ZB, and absorbs some or all of the energy from one face. The wear plate sometimes includes a quarter-wave matching layer to optimize the energy transfer of the propagating wave into the adjoining medium. Figure 5.21 shows the on-axis (dashed line) and the transverse ultrasonic fields of a circular piston transducer. The on-axis acoustic pressure field is divided into two regions, the near field, or Fresnel region and the far field, or Fraunhofer region. The extent of the Fresnel region is indicated by N. In this region the field goes through a series of maxima and minima and ends with a last maximum, which is considered as the effective focus of the transducer,44

The distance N is given by

In the transverse direction, the acoustic beam spreads as one moves away from the transducer face and its intensity drops. Focusing decreases the beam diameter and increases the intensity of the beam. Focusing can be achieved by bonding an acoustic lens to the PZT transducer or by using a spherically shaped PZT transducer. This increase augments the sensitivity of the transducer to locate small targets. In the pulse-echo mode, the –6-dB beam diameter at the effective focus is

where F is the effective focal length in the medium.

Single-element rectangular transducers can be assembled as a linear phased array transducer. In this configuration, the acoustic beam can be deflected in a sector scan format and focused by using delayed excitation signals.

Piezoelectric Wave-Propagation Transducers

Piezoelectricity derives its name from the Greek word “piezein,” to press. When a piezoelectric crystal is strained by an applied stress, an electric polarization is produced within the material which is proportional to the magnitude and sign of the strain—this is the direct piezoelectric effect. The converse effect takes place when a polarizing electric field produces an elastic strain in the same material.

Typical materials used in transducers are crystalline quartz, lithium niobate, several compositions of ferroelectric ceramics, ferroelectric polymers, and evaporated or sputtered films of cadmium sulfide and zinc oxide. The ferroelectric ceramics commonly used are PZT4, PZT5A, PZT5H, and PZT7. The elastic, dielectric, and piezoelectric properties of these materials can be found in Refs.

In addition, using these PZT materials, new and more efficient piezocomposites have been developed. Ferroelectric ceramics are not piezoelectric when they are manufactured and therefore have to be poled. Poling is done by applying a high dc voltage to the electrode faces normal to the thickness direction, while maintaining the ceramicm.

In a high-temperature environment.38 Ceramic transducers are generally rectangular, circular with parallel faces, or have a spherical curvature.

The conversion efficiency of a piezoelectric transducer is defined by the electromechanical coupling factor K.

The unit of acoustic impedance is kg/m2 • s, and it is expressed in Rayls. A mega Rayl (MRayl) is equal to 106 Rayls. Typical values of Z are  MRayl for quartz, 40.6 MRayl for brass, 1.5 MRayl for water, and 0.00043 MRayl for air.

Figure 5.18 Longitudinal and shear waves.

The piezoelectric transducers that are most commonly used generate longitudinal and shear waves which propagate with velocities VL and Vs, respectively. Figure 5.18 shows characteristics of these waves. The longitudinal mode is a compression wave, and its particle motion is in the direction of wave propagation. On the other hand, the particle motion in a shear wave is normal to the direction of wave propagation.

In some chemical, environmental, and physical transducers, surface acoustic waves (SAWs) are used. These waves travel on the surface, where mowhich they propagate is piezoelectric, then the electric field associated with the SAW can be used to dest of their energy is confined. SAWs propagate with a velocity VR that is less than VL and Vs. If the medium on tect the wave with interdigital transducers (IDTs).41

In wave-propagation transducers, maximum energy is transfered to another medium, when its impedance ZM is equal to the impedance Z0 of the transducer.

For example, the energy could be efficiently transferred from a PZT5A longitudinal mode transducer into a brass rod, whereas the transfer into water would be quite inefficient.  

To optimize that transfer, a quarter-wave matching layer is commonly used. The impedance ZML of the matching layer should be approximately and its thickness should be a quarter wavelength, evaluated using VL for that material.

Figure 5.19 shows a piezoelectric transducer with two electroded faces normal

Chemfet Transducers

The operation of the chemical field-effect transistor (ChemFET) transducer is similar to the operation of a standard metal-oxide-semiconductor field-effect transistor (MOSFET). ChemFETs are miniature transducers that are used for the measurement of the concentration of certain gases and for the determination of hydrogen ion concentrations (pH).

Fig 5.17 A cross section of a conventional MOSFET device.

Above the channel modulates ID as a function of the gate voltage VG. The VD vs. ID characteristics of a MOSFET as a function of VG display a linear region, and for small values of VD and ID, the channel behaves as a resistor. The transduction mechanism in a MOSFET is basically to sense the charge on the gate electrode and then use that charge to modulate the flow of charges in the channel between the source and the drain.

The Chem FET uses a modification of the MOSFET transduction mechanism.37 The palladium (Pd) gate MOSFET for hydrogen gas sensing and the ion-sensitive FET (ISFET) are two important examples of ChemFET transducers. In the Pd gate FET transducer the aluminum gate electrode is replaced by a Pd gate. Molecular hydrogen (measurand), in the air or by itself, is absorbed at the Pd surface, where it undergoes a catalytic dissociation into atomic hydrogen (Ha).

The atomic hydrogen then diffuses through the bulk of the Pd electrode and forms a dipole layer at the Pd-SiO2 interface.The polarization caused by the dipoles modulates the channel current ID in direct proportion to the hydrogen ion concentration.

In another ChemFET, a 10-nm-thick platinum film evaporated on top of the Pd gate electrode enables the measurement of ammonia (NH3) concentraton. If, instead, the gate electrode is a perforated film of platinum, the ChemFET will measure carbon monoxide.

The ISFET does not have a gate electrode and the SiO2 gate dielectric is exposed directly to the aqueous solution or the analyte whose pH is to be determined. For proper operation, an electrode is placed in the analyte and referenced to the bulk semiconductor. The transduction mechanism here is the formation of a charge at the analyte-oxide interface which is proportional to the pH of the analyte. This charge then modulates the channel current ID, and the electrical output of the ISFET is proportional to the pH of the analyte.