Appendix A: Future Directions
A.7.3 Example for the Interaction of Structural, Aerodynamic,
9.2 Smart Ceramics: Transducers, Sensors, and Actuators
9.2.4 Applications of Piezoelectricity
phenomenologically calculated for compositions around the morphotropic phase boundary of PZT [17]. Th e maximum lon- gitudinal piezoelectric constant d33 (four to fi ve times the enhancement) and the electromechanical coupling factor k33
(more than 90%) in the rhombohedral composition were found at angles of 57° and 51°, respectively, canted from the spontane- ous polarization direction [111], which correspond roughly to the perovskite [100] axis.
Figure 9.22 shows the principle of the enhancement in electro- mechanical couplings. Because the shear coupling d15 is the high- est in perovskite piezoelectric crystals, the applied fi eld should be canted from the spontaneous polarization direction to obtain the maximum strain. Epitaxially grown, [001] oriented thin/thick fi lms using a rhomboherial PZT composition reportedly enhance the eff ective piezoelectric constant by—four to fi ve times.
9.2.4.2 Ultrasonic Transducer
One of the most important applications of piezoelectric materials is based on ultrasonic echo fi eld [20,21]. Ultrasonic transducers convert electrical energy into a mechanical form when generating an acoustic pulse and convert mechanical energy into an elec- trical signal when detecting its echo. Nowadays, piezoelectric transducers are being used in medical ultrasound for clinical applications that range from diagnosis to therapy and surgery.
Th ey are also used for underwater detection, such as sonars and fi sh fi nders, and nondestructive testing.
Th e ultrasonic transducers oft en operate in a pulse-echo mode. Th e transducer converts electrical input into an acoustic wave output. Th e transmitted waves propagate into the body, and echoes are generated that travel back to be received by the same transducer. Th ese echoes vary in intensity according to the type of tissue or body structure, and thereby create images. An ultrasonic image represents the mechanical properties of the tissue, such as density and elasticity. We can recognize anatomical structures in an ultrasonic image because the organ boundaries and fl uid-to-tissue interfaces are easily discerned. Th e ultrasonic imaging can also be done in real time. Th is means that we can follow rapidly moving structures such as heart without motional distortion. In addition, ultrasound is one of the safest diagnostic imaging techniques. It does not use ionizing radiation like x-rays and thus is routinely used for fetal and obstetrical imaging.
Useful areas for ultrasonic imaging include cardiac structures, the vascular system, the fetus, and abdominal organs such as liver and kidney. In brief, it is possible to see inside the human body by using a beam of ultrasound without breaking the skin.
Th ere are various types of transducers used in ultrasonic imaging. Mechanical sector transducers consist of single, rela- tively large resonators that provide images by mechanical scanning such as wobbling. Multiple element array transducers permit the imaging systems to access discrete elements individually and
enable electronic focusing in the scanning plane at various adjustable penetration depths by using phase delays. Th e two basic types of array transducers are linear and phased (or sector).
Linear array transducers are used for radiological and obstetri- cal examinations, and phased array transducers are useful for cardiological applications where positioning between the ribs is necessary.
Figure 9.24 shows the geometry of the basic ultrasonic trans- ducer. Th e transducer is composed mainly of matching, piezoelec- tric material, and backing layers [22]. One or more matching layers are used to increase sound transmissions into tissues. Th e backing is attached to the transducer rear to damp the acoustic return wave and to reduce the pulse duration. Piezoelectric materials are used to generate and detect ultrasound. In general, broadband transducers should be used for medical ultrasonic imaging. Th e broad bandwidth response corresponds to a short pulse length that results in better axial resolution. Th ree factors are important in designing broad bandwidth transducers. Th e fi rst is acoustic impedance matching, that is, eff ectively coupling the acoustic energy to the body. Th e second is high electromechanical coupling coeffi cient of the transducer. Th e third is electrical impedance matching, that is, eff ectively coupling electrical energy from the driving electronics to the transducer across the frequency range of interest. Th e operator of pulse-echo transducers is based on the thickness mode resonance of the piezoelectric thin plate. Th e thickness mode coupling coeffi cient, kt, is related to the effi ciency of converting electric energy into acoustic and vice versa. Further, a low planar mode coupling coeffi cient, kp, is benefi cial for limit- ing energies from being expended in a nonproductive lateral mode. A large dielectric constant is necessary to enable a good electrical impedance match to the system, especially in tiny piezo- electric sizes.
Table 9.5 compares the properties of ultrasonic transducer materials [7,23] Ferroelectric ceramics, such as PZT and modifi ed PT, are almost universally used as ultrasonic transducers. Th e success of ceramics is due to their very high electromechanical FIGURE 9.23 Cylindrical gyroscope commercialized by NEC-Tokin
(Japan). (Modifi ed from Encyclopedia of Smart Materials.) Vibrator
Holder
Support Lead
Ultrasonic beam
Input pulse
Backing
Piezoelectric element
Matching layer
FIGURE 9.24 Geometry of the fundamental transducer for acoustic imaging. (Modifi ed from Encyclopedia of Smart Materials.)
coupling coeffi cients. In particular, soft PZT ceramics such as PZT-5A and 5H type compositions are most widely used because of their exceedingly high coupling properties and because they can be relatively easily tailored, for instance, in the wide dielectric constant range. On the other hand, modifi ed PTs such as samarium -doped materials have high piezoelectric anisotropy: the planar coupling factor kp is much less than the thickness coupling factor kt. Because the absence of lateral coupling leads to reduced inter- ference from spurious lateral resonances in longitudinal oscilla- tors, this is very useful in high-frequency array transducer applications. One disadvantage to PZT and other lead-based ceramics is their large acoustic impedance (approximately 30 kg m−2 s−1 (Mrayls) compared to body tissue (1.5 Mrayls). Single or multiple matching layers of intermediate impedances need to be used in PZT to improve acoustic matching.
On the other hand, piezoelectric polymers, such as PVDF- trifl uoroethylene, have much lower acoustic impedance (4–5 Mrayls) than ceramics and thus match soft tissues better.
However, piezopolymers are less sensitive than the ceramics and they have relatively low dielectric constants that require large drive voltage and giving poor noise performance due to mis- matching of electrical impedance.
Piezoelectric ceramic/polymer composites are alternatives to ceramics and polymers. Piezocomposites that have 2-2 or 1-3 connectivity are commonly used in ultrasonic medical applica- tions. Th ey combine the low acoustic impedance advantage of polymers with the high sensitivity and low electrical impedance advantages of ceramics.
Th e design frequency of a transducer depends on the penetra- tion depth required by the application. Resolution is improved as frequency increases. Although a high-frequency transducer can produce a high-resolution image, higher frequency acoustic energy is more readily attenuated by the body. A lower frequency transducer is used as a compromise when imaging deeper struc- tures. Most of medical ultrasound imaging systems operate in the frequency range from 2 to 10 MHz and can resolve objects approximately 0.2–1 mm in size. At 3.5 MHz, imaging to a depth of 10–20 cm is possible, and at 50 MHz, increased losses limit the depth to less than 1 cm. Higher-frequency transducers (10–50 MHz) are used for endoscopic imaging and for catheter- based intravascular imaging. Ultrasonic microscopy is being done at frequencies higher than 100 MHz. Th e operating fre- quency of the transducer is directly related to the thickness and
velocity of sound in piezoelectric materials employed. As fre- quency increases, resonator thickness decreases. For a 3.5 MHz transducer, PZT ceramic thickness must be roughly 0.4 mm.
Conventional ceramic transducers, such as PZT, are limited to frequencies below 80 MHz because of the diffi culty of fabricating thinner devices [24]. Piezoelectric thin-fi lm transducers such as ZnO have to be used for microscopic applications (at frequencies higher than 100 MHz, corresponding to a thickness of less than 20 μm) [25].
9.2.4.3 Resonator and Filter
When a piezoelectric body vibrates at its resonant frequency, it absorbs considerably more energy than at other frequencies, result- ing in a fall of the impedance. Th is phenomenon enables using piezoelectric materials as wave fi lters. A fi lter is required to pass a certain selected frequency band or to stop a given band. Th e band width of a fi lter fabricated from a piezoelectric material is deter- mined by the square of the coupling coeffi cient k. Quartz crystals that have very low k value of about 0.1 can pass very narrow frequency bands of approximately 1% of the center resonance frequency. On the other hand, PZT ceramics whose planar coupling coeffi cient of about 0.5 can easily pass a band of 10% of the center resonance frequency. Th e sharpness of the passband depends on the mechanical quality factor Qm of the materials. Quartz also has a very high Qm of about 106, which results in a sharp cutoff of the passband and well-defi ned frequency of the oscillator.
A simple resonator is a thin disk electroded on its plane faces and vibrating radially for applications in fi lters whose center frequency ranges from 200 kHz to 1 MHz and whose bandwidth is several percent of the center frequency. Th e disk diameter must be about 5.6 mm for a frequency of 455 kHz. However, if the required frequency is higher than 10 MHz, other modes of vibration such as the thickness extensional mode are exploited, because of its smaller size disk. Trapped-energy type fi lters made from PZT ceramics have been widely used in the intermediate frequency (IF) range, for example, 10.7 MHz for FM radio receivers and transmitters. By employing the trapped-energy phenome- non, the overtone frequencies are suppressed. Th e plate is partly covered with electrodes of a specifi c area and thickness. Th e fundamental frequency of the thickness mode beneath the electrode is less than that of the unelectroded portion because of the extra inertia of the electrode mass. Th e longer wave charac- teristic of the electrode region cannot propagate in the unelec- troded region. Th e higher-frequency overtones can propagate into the unelectroded region. Th is is called the trapped-energy principle. Figure 9.25 shows a schematic drawing of a trapped- energy fi lter. In this structure, the top electrode is split so that coupling between the two parts is effi cient only at resonance.
More stable fi lters suitable for telecommunication systems have been made from single crystals such as quartz or LiTaO3. 9.2.4.4 Piezoelectric Transformer
Th e transfer of vibration energy from one set of electrodes to another on a piezoelectric ceramic body can be used to transform voltage. Th e device is called a piezoelectric transformer. Recently, TABLE 9.5 Comparison of the Properties of Ultrasonic
Transducer Materials
PZT Ceramic PVDF Polymer
PZT–Polymer
Composite ZnO Film
kt 0.45–0.55 0.20–0.30 0.60–0.75 0.20–0.30
Z (Mrayls) 20–30 1.5–4 4–20 35
e33T/e0 200–5000 10 50–2500 10
tan d (%) <1 1.5–5 <1 <1
Qm 10–1000 5–10 2–50 10
r (g/cm3) 5.5–8 1–2 2–5 3–6
offi ce automation equipment that has a liquid crystal display such as notebook-type personal computers and car navigation systems has been successfully commercialized. Th is equipment that uses a liquid crystal display requires a very thin transformer without electromagnetic noise to start the glow of a fl uorescent back- lamp. Th is application has recently accelerated the development of the piezoelectric transformers. Figure 9.26 shows the basic structure, where two diff erently poled parts coexist in one piezo- electric plate. Th e plate has electrodes on half of its major faces and on an edge. Th e plate is then poled in its thickness direction at one end and parallel to the long axis over most of its length. A low-voltage AC supply is applied to the large-area electrodes at a frequency that excites a length extensional mode resonance.
Th en, a high-voltage output can be taken from the small electrode and from one of the larger electrodes. Following the proposal by Rosen mentioned before, piezoelectric transformers of several diff erent structures have been reported [26]. A multilayer type transformers are proposed to increase the voltage step-up ratio [27]. Th e input part has a multilayer structure and has internal electrodes, and the output electrodes are formed at the side sur- face of the half of the rectangular plate. Th is transformer uses the piezoelectric longitudinal mode for the input and output parts.
9.2.4.5 Saw Device
SAW, also called a Rayleigh wave, is composed of a coupling between longitudinal and shear waves in which the SAW energy is confi ned near the surface. An associated electrostatic wave exists for a SAW on a piezoelectric substrate that allows electroacoustic coupling via a transducer. Th e advantages of SAW technology are that a wave can be electroacoustically accessed and trapped at the substrate surface and its velocity is approximately 104 times slower than an electromagnetic wave. Th e SAW wavelength is of the same order of magnitude as line dimensions that can be
photolithographically produced, and the lengths for both short and long delays are achievable on reasonable size substrates [28,29].
Th ere is a very broad range of commercial system applica- tions, including front-end and IF fi lters, community antenna television (CATV), and VCR components, synthesizers, analyz- ers, and navigators. In SAW transducers, fi nger electrodes pro- vide the ability to sample or tap the wave, and the electrode gap gives the relative delay. A SAW fi lter is composed of a minimum of two transducers. A schematic of a simple SAW bidirectional fi lter is shown in Figure 9.27. A bidirectional transducer radiates energy equally from each side of the transducer. Energy not received is absorbed to eliminate spurious refl ection.
Various materials are currently being used for SAW devices. Th e most popular single-crystal SAW materials are lithium niobate and lithium tantalate. Th e materials have diff erent properties depending on their cuts and the direction of propagation. Th e funda- mental parameters are the SAW velocity, the temperature coeffi - cients of delay (TCD), the electromechanical coupling factor, and the propagation loss. SAWs can be generated and detected by spa- tially periodic, interdigital electrodes on the plane surface of a piezoelectric plate. A periodic electric fi eld is produced when an RF source is connected to the electrode, thus permitting piezoelectric coupling to a traveling surface wave. If an RF source of a frequency f is applied to an electrode whose periodicity is p, energy conversion from an electrical to mechanical form will be maximum when
= 0=Vs, f f
p (9.20)
where
Vs is the SAW velocity
f0 is the center frequency of the device FIGURE 9.25 Trapped-energy fi lter. (Modifi ed from Encyclopedia of
Smart Materials.)
Bottom Top
Electrode
Ceramic plate
FIGURE 9.26 Rosen-type piezoelectric transformer. (Modifi ed from Encyclopedia of Smart Materials.)
High voltage output Low voltage input
FIGURE 9.27 Typical SAW bidirectional fi lter that consists of two interdigital transducers. (Modifi ed from Encyclopedia of Smart Materials.)
Input SAW Output
Interdigital electrode Piezoelectric substrate
SAW velocity is an important parameter that determines the center frequency. Another important parameter for many applications is the temperature sensitivity. For example, the temperature stability of the center frequency of SAW bandpass fi lters is a direct function of temperature coeffi cient for the velocity and delay time of the material used. Th e fi rst-order TCD time is given by
⎛ ⎞ ⎛ ⎞ ⎛ ⎞⎛ ⎞
⎛ ⎞⎜ ⎟ =⎛ ⎞⎝ ⎠⎜ ⎟−⎜ ⎟⎜ ⎟
⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠⎝ ⎠
s s
1 d 1 d 1 d
d d d ,
t L V
t T L T V T (9.21)
where
t = L/Vs is the delay time L is the SAW propagation length
Th e surface wave coupling factor, ks2, is defi ned in terms of the change in SAW velocity that occurs when the wave passes across a surface coated by a thin massless conductor, so that the piezo- electric fi eld associated with the wave is eff ectively shorted- circuited. Th e coupling factor, ks2, is expressed by
= −
2 f m
s
f
( ) ,
2 V V
k V (9.22)
where
Vf is the free surface wave velocity
Vm is the velocity on the metallized surface
In SAW applications, the value of ks2 relates to the maximum band- width obtainable and the amount of signal loss between input and output that determines the fractional bandwidth versus minimum insertion loss for a given material and a fi lter. Propagation loss, one of the major factors that determines the insertion loss of a device, is caused by wave scattering by crystalline defects and surface irregu- larities. Materials that have high electromechanical coupling factors combined with small TCD time are likely to be required. Th e free surface velocity, V0, of the material is a function of cut angle and propagative direction. Th e TCD is an indication of the frequency shift expected from a transducer due to a temperature change and is also a function of the cut angle and the propagation direction. Th e substrate is chosen on the basis of the device’s design specifi cations for operating temperature, fractional bandwidth, and insertion loss.
Table 9.6 shows some important material parameters of represen- tative SAW materials. Piezoelectric single crystals such as 128°Y–X
(128°-rotated-Y-cut and X-propagation)—LiNbO3 and X-112°Y (X-cut and 112°-rotated-Y-propagation)—LiTaO3 have been exten- sively employed as SAW substrates for VIF fi lters. ZnO thin fi lms, c-axis oriented and deposited on a fused quartz, glass, or sapphire substrate, have also been commercialized for SAW devices.
9.2.4.6 Actuators
Currently another important application of piezoelectric materials exists in the actuator fi eld [30]. Using the converse piezoelectric eff ect, a small displacement can be produced by applying an elec- tric fi eld to a piezoelectric material. Vibrations can be generated by applying an alternating electric fi eld. Th ere is a demand in advanced precision engineering for a variety of types of actuators that can adjust position precisely (micropositioning devices), suppress noise vibrations (dampers), and drive objects dynamically (USMs). Th ese devices are used in areas, including optics, astronomy, fl uid con- trol, and precision machinery. Piezoelectric strain and electrostric- tion induced by an electric fi eld are used for actuator applications.
Figure 9.28 shows the design classifi cation of ceramic actuators.
Simple devices composed of a disk or a multilayer type use the strain
TABLE 9.6 Material Parameters for Representative SAW Materials
Material
Cut–Propagation
Direction k2 (%)
TCD
(ppm/C) V0 (m/s) er
Single crystal Quartz ST—X 0.16 0 3158 4.5
LiNbO3 128°Y—X 5.5 −74 3960 35
LiTaO3 X112°—Y 0.75 −18 3290 42
Li2B4O7 (110) —<001> 0.8 0 3467 9.5
Ceramic PZT-In(Li3/5W2/5)O3 1.0 10 2270 690
(Pb,Nd)(Ti, Mn, In)O3 2.6 <1 2554 225
Th in fi lm ZnO/glass 0.64 −15 3150 8.5
ZnO/Sapphire 1.0 −30 5000 8.5
FIGURE 9.28 Structures of ceramic actuators. (Modifi ed from Encyclopedia of Smart Materials.)
Multilayer
Bimorph
Moonie
v z
v z z v z
induced in a ceramic by the applied electric fi eld directly. Complex devices do not use the induced strain directly but use the amplifi ed displacement through a special magnifi cation mechanism such as unimorph, bimorph, and moonie. Th e most popularly used multi- layer and bimorph types have the following characteristics: Th e mul- tilayer type does not have a large displacement (10 μm), but has advantages in generation force (1 kN), response speed (10 μs), life- time (1011 cycles), and the electromechanical coupling factor k33
(0.70). Th e bimorph type has a large displacement (300 μm), but has disadvantages in generation force (1 N), response speed (1 ms), life- time (108 cycles), and the electromechanical coupling factor keff (0.10).
For instance, in a 0.65 PMN–0.35 PT multilayer actuator with 99 layers of 100 μm thick sheets (2 × 3 × 10 mm3), a 8.7 μm displacement is generated by a 100 V voltage, accompanied by a slight hysteresis.
Th e transmitted response of the induced displacement aft er the application of a rectangular voltage is as quick as 10 μs. Th e multi- layer has a fi eld-induced strain of 0.1% along the length [30].
Unimorph and bimorph devices are defi ned by the number of piezoelectric ceramic plates: only one ceramic plate is bonded onto an elastic shim, or two ceramic plates are bonded together.
Th e bimorph causes bending deformation because each piezo- electric plate bonded together produces extension or contraction in an electric fi eld. In general, there are two types of piezoelectric bimorphs: the antiparallel polarization type and the parallel polarization type, as shown in Figure 9.29. Two poled piezoelectric
plates t/2 thick and L long are bonded so that their polarization directions opposite or parallel to each other. In the cantilever bimorph confi guration where one end is clamped, the tip dis- placement dz under an applied voltage V is
= (3/2) 31( / ) (antiparallel type)2 2
z d L t V
d (9.23)
=3 31 ( / ) (parallel type).2 2
z d L t V
d (9.24)
Th e resonance frequency fr for both types is given by
= 2 E −1/2 r 0.16 / ( 11 ) ,
f t L rs (9.25)
where
r is the density
s11E is the elastic compliance
A metallic sheet (called a shim) is occasionally sandwiched between the two piezoelectric plates to increase the reliability; the structure can be maintained even if the ceramics fracture. Using the bimorph structure, a large magnifi cation of the displacement is easily obtainable. However, the disadvantages include a low response speed (1 kHz) and low generative force [30].
A composite actuator structure called “moonie” has been developed to amplify the small displacements induced in piezo- electric ceramics [31]. Th e moonie consists of a thin single or multilayer element and two metal plates that have narrow moon- shaped cavities bonded together. Th is device has characteristics intermediate between the conventional multilayer and bimorph actuators; it has an order of magnitude larger displacement (100 μm) than the multilayer, and much larger generative force (100 N) and quicker response (100 μs) than the bimorph.
Some examples of applications of piezoelectric and elec- trostrictive actuators are described here. Th e piezoelectric impact dot-matrix printer is the fi rst mass-produced device that uses multilayer ceramic actuators (Figure 9.30) [32]. Th e advantage of a piezoelectric printer head compared to conventional magnetic type are low energy consumption, low heat generation, and fast printing speed. Longitudinal multilayer actuators do not have a
FIGURE 9.30 Impact dot matrix printer head commercialized by NEC (Japan). (Modifi ed from Encyclopedia of Smart Materials.) Head
element
Platen Paper
Ink ribbon Guide
Piezoelectric actuator
Stroke amplifier Wire
Wire
Wire guide
(a) (b)
FIGURE 9.29 Two types of piezoelectric bimorphs: (a) antiparallel polarization type and (b) parallel polarization type. (Modifi ed from Encyclopedia of Smart Materials.)
V
V (a)
(b)