Chapter 1 - Smart Materials
1.1 Introduction to Smart Materials
Many definitions have been given to describe ''smart materials.'' A first definition states that the key characteristic of a ''smart material'' is its ability to respond to an external stimulus in a ''technically controlled and useful way'' (Harvey, 2000). The use of the expression ''technically controlled and useful'' is important because all materials react to external stimuli in some sort or other (for example, all materials respond to temperature by changing their volume). However, to be classified as a ''smart material,'' the response must be one that is useful for a particular application. Consequently, any discussion of smart materials should include at least some applications of these materials. Also, according to this definition, animals and plants could potentially be considered as smart material structures; however, the focus is generally only on materials that are studied in engineering and material sciences.
Furthermore, the use of the term ''smart material'' often has a historical context, only being coined for relatively new materials. For example, let's consider a bimetallic strip. Bimetallic strips have been fabricated for centuries and are made of two strips of metals glued together so that the difference in the coefficient of thermal expansion causes the strip to bend in response to a change in temperature. This technique has commonly been used to open and close mechanical valves and electrical circuits (Lane, 2003). In this case, heat either comes from the natural environment or it is engineered into a system that the bimetallic strip is part of. However, bimetallic strips are generally not thought of as smart material because they have been around and used for a long time.
Smart materials are also often characterized by the fact that they transform energy from one type to another, for example, from mechanical energy to magnetic energy. Table 1 lists the various effects that relate the input-electric field, magnetic field, stress, heat and light-to the output- charge/current, magnetization, strain, temperature, and light.
Figure . Some Transducer Properties of Smart Material (Sabat, 2009)
Numerous materials that can inherently convert one form of energy into another have been discovered. As can be seen in Figure 1, transducers are materials that respond to a physical stimulus, such as a change in temperature, pressure, or illumination, and produce an output signal in order to monitor or operate a system. As indicated by the first shaded column in Figure 1, actuators use the converse piezoelectric effect, elasticity, magnetostriction, thermal expansion, and the photostriction phenomenon. A specific instance of the thermal expansion is called ''shape memory''; indeed, shape memory alloys are a kind of thermally expanding material.
On the other hand, actuators are materials that respond to a stimulus in the form of a mechanical property change such as a dimensional or a viscosity change. Furthermore, a "sensor" requires charge (or current) output in most cases. Thus, conducting, semiconducting, magnetoelectric, piezoelectric, pyroelectric, and photovoltaic materials are used for detecting electric fields, magnetic fields, stress, heat, and light, respectively (see the second shaded column in Figure 1). Note that ferroelectric materials exhibit most of these effects, except magnetic-related phenomena which are extremely weak. Hence, ferroelectric materials are very useful materials with multiple applications and they are widely studied. Also, note that in a more narrow sense, actuators are often referred to as materials or devices that generate mechanical strain (or stress) as the output.
Smart materials are also often incorporated into ''smart structures'' or ''smart material systems'', which are structures that act as the structural support for the material (Smith, 2005).
It is also important to stress the importance of the ''non-expected behavior'' of smart materials. For example, elastic materials that generate strain outputs from stress, which is what one would expect, are sometimes referred to as "trivial" materials (Harvey, 2000). On the other hand, piezoelectric and pyroelectric materials that generate an electric field from the input of heat or stress, which is an unexpected behavior, are coined as ''smart materials.'' Most transducer effects have a corresponding converse effect, such as the electrocaloric and converse piezoelectric effects. If this is the case, both sensing and actuating functions can be performed with the same material. In this sense, piezoelectric materials are most popularly used in smart structures and systems because the same material can be used for both sensing and actuating, in theory. Even though transducers, in general, are devices that convert input energy to a different energy type of output, the piezoelectric "transducer" is often used to identify a device that possesses both sensing and actuating functions, for example underwater sonar. As such, in this paper I will mainly treat piezoelectric transducers, sensors, and actuators in chapter 2, 3 and 4. However, chapter 1 will provide brief descriptions of some other smart materials.
1.2 Shape Memory Alloys
Shape memory alloys (SMAs) are actuators that, upon proper thermal and mechanical treatment, have the ability to remember up to two shapes which they had previously occupied (Janot, et al., 2001). When subjected to a mechanical stress below a certain critical temperature, these special materials can be plastically deformed beyond their elastic limit, but then they are able regain their original shape if they are heated above that certain temperature. Nickel-titanium, also called Nitinol, is the most common SMA and has some of the best shape memory properties. In addition, it has a relatively low transformation temperature. Figure 2 depicts the phase transformation that occurs from the austenite phase to the martensite phase above the transformation temperature.
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Figure . Austenite and Martensite structure in Nitinol (Memry, 2010)
Although they cannot match the outstanding shape memory capabilities of Ni-Ti alloys, copper-based alloys are a less expensive alternative. The most common copper-based material is Cu-Zn-Al and Cu-Al-Ni, which can generally achieve a shape memory strain of 4 to 5%, compared to approximately 8% for Ni-Ti (Janot, et al., 2001). They also have a broader range of transformation temperatures depending on their exact composition. Other alloys, especially iron-based alloys (Fe-Mn-Si, Fe-Pt, and Fe-Ni), exhibit shape memory but cannot regain their shape to the same extent as Nitinol and even copper-based alloys. SMAs are also produced in other forms such as composites and ferromagnetic alloys. Some of the current applications of SMAs include use in spacecraft, aircraft, automobiles, electronics, medicine, process systems, robotics, and domestic appliances. See (Smith, 2005) for more applications of SMAs.
1.3 Electrochromic Materials
When an electric current is passed through an electrochromic material, its optical properties changes, and as a result the material gains a significantly different appearance. In fact, the absorption, reflection, and transmission properties can be altered enough to cause a change in its color, reflectivity or transparency. This phenomenon relies on the electrochemical reactions of oxidation and reduction, involving a transfer of electrons and ions between electrodes and the electrochromic material (Vinogradov, 2002). When an electric current is applied and electrons and ions are being exchanged from one electrode to the other, there is an amount of energy dissipated with wavelengths in the visible spectrum that can be absorbed, therefore causing the material to change color, and hence become less transmissive. The degree to which the color is changed depends on the extent of the reaction, which is basically controlled by the applied electric field. Oxides of the following metals exhibit electrochromic properties: tungsten, iridium, manganese, molybdenum, rhodium, titanium, vanadium, and niobium (Vinogradov, 2002). Lithium nickel oxide, LiNiO, which turns from clear to a dark gray, is the most studied electrochromic material. The reaction is relatively slow, but the coloration remains almost indefinitely after the electric current is removed. Another widely studied material is V2O5, which changes from the red end of the spectrum to the blue end. A very special material is polyaniline, which can change from being transparent to green, then blue, and then purple. Then, upon electrochemical reduction, the colors come back to their original state. Electrochromic materials can be used to create smart windows capable of reducing their transmittance by over 85% in only a few seconds when an electric field is applied, as shown in Figure 3.
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Figure . Smart glass. Left: Clear Configuration. Right: Dark Configuration (Inc, 2010)
Once the electric field is removed the effect remains, but it is reversible when the current is reversed. For more applications of electrochromic materials, see (Lane, 2003) and (Inc, 2010).
1.4 Electrorheological and Magnetorheological Fluids
Electrorheological (ER) and magnetorheological (MR) fluids experience a nearly instantaneous change in their rheological properties [1] upon the application of an electric or magnetic field (Harvey, 2000). This change is reversible and occurs also nearly instantaneously upon the removal of the applied field. The physical changes can be quite substantial, turning a low viscosity fluid into a much more viscous, almost-solid substance. These special fluides typically consist of polarizable particles suspended in a carrier fluid. For ER fluids, the dispersed particles are commonly metal oxides, aluminosilicates, silica, organics, or polymers (Harvey, 2000). MR fluids use ferromagnetic or paramagnetic solid particles. ER and MR fluids both use the same type of carrier fluid, which can be any high electrical resistivity substance with low-viscosity, such as silicone oil. Activators, surfactants, and other additives are also commonly included in both types of fluids to improve their properties. ER and MR fluids are most commonly used in damping applications. Some applications include exercise equipment, valves, braking and clutch systems, as well as shock absorbing systems. (Harvey, 2000) discusses other applications in great detail.
The electrorheological effect occurs in an ER fluid when an electric field is applied, causing the uniformly dispersed solid particles to become polarized. Once polarized, they begin to interact with each other, and form chain-like structures parallel to the electric field direction (see Figure 4).
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Figure . Electrorheological Effect. Left: No Applied Electric Field. Right: With Applied Electric Field (Lane, 2003).
Upon further intensification of the electric field, the chains begin to form thicker columns. A dramatic change in the fluid rheological properties is associated with this change in the structure. The chain-like assemblies give the fluid a greater yield stress. Then, upon removing the electric field, the particles lose their polarization and return to their free state. The period of time over which these events occur is on the order of milliseconds. The magnetorheological effect is similar to the ER effect, but obviously, instead of an electric field, a magnetic field is applied to polarize the particles.
1.5 Magnetostrictive Materials
Magnetic materials have internal areas called domains, within which all magnetic dipoles are oriented in the same direction. Domains with different orientations are separated by domain walls or boundaries. With the application of an external magnetic field, the boundaries move and domains rotate and align themselves in the same direction, ultimately causing a slight shape change in the bulk material. The magnetostriction effect is illustrated in figure 5.
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Figure . Magnetic Domains. Left: No Applied Magnetic Field. Middle: Weak Magnetic Field. Right: Strong Magnetic Field.
Magnetostrictive materials used in actuators exhibit a change in shape when an external magnetic field is applied. The reverse effect, used in sensors, is called piezomagnetism, where a magnetic field is produced upon the application of mechanical strain. Although usually minimal, ferromagnetic materials, such as iron, nickel and cobalt, exhibit magnetostriction to some extent (Dapino, 2002). Furthermore, ferromagnetic elements are often alloyed with iron. Rare earth elements have also exhibited significantly higher magnetostriction but only at temperatures lower than room temperature. Other common magnetostrictive materials include nickel-based alloys and composites containing magnetostrictive particles. Magnetostrictive sensors and actuators are used for transducers, transformers, vibration and noise control, ultrasonics, adaptive optics, linear motors, speakers, and mechanical torque sensors (Dapino, 2002). Terfenol is used almost exclusively as the choice material for magnetostrictive applications because of its exceptional magnetostrictive properties. Terfenol, TbFe2, is an intermetallic compound, and has been widely studied by the Naval Surface Warfare Center in the United States of America (NSWF, 2010).
Chapter 2 - Piezoelectric Materials
2.1 Introduction to the Piezoelectric Effect
Certain materials produce electric charges on their surfaces as a consequence of applying mechanical stress. When the induced charge is proportional to the mechanical stress, it is called the direct piezoelectric effect and was discovered by Jacques and Pierre Curie in 1880 (Uchino, 1997). Materials that show this phenomenon also conversely have a geometric strain generated that is proportional to an applied electric field. This is the converse piezoelectric effect. The root of the word "piezo" is the Greek word for "pressure"; hence the original meaning of the word piezoelectricity implied "pressure electricity" (Sabat, 2005). Piezoelectric materials couple electrical and mechanical parameters. The material used earliest for its piezoelectric properties was single-crystal quartz. Quartz crystal resonators for frequency control appear today at the heart of clocks and are also used in TVs and computers. Ferroelectric polycrystalline ceramics, such as barium titanate and lead zirconate titanate, exhibit piezoelectricity when electrically poled (Uchino, 1997). Because these ceramics possess significant and stable piezoelectric effects, that is, high electromechanical coupling, they can produce large strains and hence are extensively used as transducers. Piezoelectric polymers, notably polyvinylidene difluoride, trifluoroethylene and other piezoelectric composites that combine a piezoelectric ceramic and a passive polymer have been developed and offer high potential (Harrison, 2002). More recently, thin films of piezoelectric materials have been researched due to their potential use in microdevices. Piezoelectricity is being extensively used in fabricating various devices such as transducers, sensors, actuators, surface acoustic wave devices, and frequency controls. In this chapter, the fundamentals of the piezoelectric effect will be described, then the crystalline model will be given, followed by an explanation of the pyroelectric effect, the electrostrictive materials, and finally the relaxor ferroelectric materials.
As mentioned above, piezoelectric materials can provide coupling between electrical and mechanical energy and thus have been extensively used in a variety of electromechanical devices. The direct piezoelectric effect is most obviously used to generate charge or high voltage in applications such as the spark ignition of gas in heaters, cooking stoves, and cigarette lighters (Sabat, 2005). Using the converse effect, small mechanical displacements and vibrations can be produced in actuators by applying an electric field. Acoustic and ultrasonic vibrations can be generated by an alternating field tuned at the mechanical resonant frequency of a piezoelectric device and can be detected by sensing the vibrations incident on the material, which is usually used for ultrasonic transducers. Other applications of piezoelectricity are piezoelectric-based sensors, accelerometers, automobile knock sensors, vibration sensors, strain gages, flow meters, etc.
2.2 Crystalline Model
All crystals can be classified into 32 point groups according to their crystallographic symmetry, as seen in Figure 6.
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Figure . Piezoelectric Crystalline Model. Adapted from PH513 Notes: Ferroelectric Transduction Materials: Properties and Applications (Sabat, 2010).
These point groups are divided into two classes; one has a center of symmetry, and the other lacks it. There are 21 noncentrosymmetric point groups. Crystals that belong to 20 of these point groups exhibit piezoelectricity. Although cubic class 432 lacks a center of symmetry, it is not piezoelectric. Of these 20 point groups, 10 polar crystal classes contain a unique axis, along which can exist an electric dipole moment. The pyroelectric effect appears in any material that possesses a polar symmetry axis. The materials in this category develop an electric charge on the surface due to to the change in the dipole moment as temperature changes. The pyroelectric crystals whose spontaneous polarization are reorientable with the application of an electric field of sufficient magnitude are called ferroelectrics (Sabat, 2010).
2.3 Piezoelectric Materials
Piezoelectrics are materials that exhibit an electrical polarization with an applied mechanical stress (direct effect), or a physical change with an applied electric field (converse effect). They are used for both sensing and actuating devices. Lead zirconate titanate (PbZrTiO3) is the most common piezoelectric material as it may be doped with different concentrations of different atoms in order to produce different materials with a range of dielectric constants to meet the requirements of specific applications. Other piezoelectric materials that are often used are barium titanate (BaTiO3), lead titanate (PbTiO3), lead metaniobate (PbNb2O6), and PVDF (Harrison, 2002). Polymers are generally favored for sensing applications while ceramics are favored for actuating applications. Figure 7 depicts the piezoelectric effect observed in lead zirconate titanate upon the application of a compressive force relative to the crystal structure.
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Figure . Piezoelectric Effect in PZT upon the Application of a Compressive Force (Tuller, 2002).
Piezoelectricity is a relatively ''mature'' technology with numerous applications throughout the military and the commercial sectors. Devices using piezoelectrics include: adaptive optics, hydrophones and sonobuoys, fuse devices, depth sounders, thickness gauging, flaw detection, level indicators, alarm systems, strain gauges, airplane beacon locators, fetal heart detectors, and tire pressure indicators among many others (Uchino, 2000).
2.4 Pyroelectric Materials
Pyroelectrics, as previously mentioned, are materials that become electrically polarized upon an applied temperature change. Materials used as pyroelectrics include barium strontium titanate (BaSrTiO3), lead zirconate titanate (PbZrTiO3), barium strontium niobate (BaSrNb2O6), triglycine sulfates (TGS), lithium tantalate (LiTaO3), and polyvinylidene fluoride (PVDF) (Uchino, 2000). Ceramics are widely used due to their lower cost, availability, ease of processing and good stability. However, their weakness comes from their relative brittleness. Polymers, such as PVDF, provide a non-brittle alternative, but do not obtain the level of performance of ceramics. Pyroelectrics are widely used for infrared detection in surveillance and targeting applications. A pyroelectric infrared detector generally employs a thin film of pyroelectric material oriented with the electrode surfaces normal to its polarization direction. When infrared radiation is absorbed by the detector, the temperature of the pyroelectric material rises. This change in temperature alters the material's polarization, causing a change in surface charges across the material which produces an electrical signal via the electrodes, thus providing a signal that is proportional to the incoming radiation (Tuller, 2002).
2.5 Electrostrictive Materials
Electrostrictive materials, like piezoelectrics, exhibit a dimensional change upon an applied electric field or an electric polarization upon an applied mechanical stress. Electrostrictive materials, however, exhibit a non-linear response as opposed to the linear response of piezoelectrics. The advantages of electrostrictive materials over piezoelectrics are that they have almost no hysteresis, a quick response time, and higher displacements. These properties are desired for micro-positioner and adaptive optic actuation applications (Uchino, 1997). A disadvantage of available electrostrictors is their higher cost compared with piezoelectrics. Ceramic materials that are used in electrostrictive devices are in a class called "relaxor ferroelectrics." The term "relaxor" refers to the significant decrease in dielectric coefficients with increasing frequency. Lead lanthanum zirconate titanate (PLZT) is one of the most common electrostrictive materials. Besides, research on polymeric electrostrictors has taken place, but none have been developed thus far for practical use (Smith, 2005).
2.6 Relaxor Ferroelectric Materials
Figure . Comparison of the Strain Induced by the Electric Field for PZT and PMN-PT (Kholkin, 2002).Relaxor ferroelectrics differ from normal ferroelectrics, because they have broad phase transitions from the paraelectric to the ferroelectric state, strong frequency dependence of the dielectric coefficients, and weak remnant polarization. Lead-based relaxor materials have complex disordered perovskite structures of the general form Pb(A,B)O3, where A can be Mg2+, Zn2+, Sc3+, and B can be Nb5+, Ta5+, W6+ (Haertling, 1991). The B-site cations are distributed randomly in the crystal. In addition, relaxor-type materials such as the lead magnesium niobate Pb(Mg,Nb)O3, and lead titanate PbTiO3 solid solution exhibit electrostrictive phenomena that are suitable for actuator applications. Figure 8 shows a comparison of strain induced by the electric field for a typical piezoelectric (PZT) and also a relaxor (PMN-PT).
Note that a strain of 10-3 cm, which corresponds to a strain of approximately 0.1% according to (Kholkin, 2002), can be induced by the electric field, and that hysteresis is negligibly small for this electrostriction. Similar results and even higher strains were found by (Bobnar et al., 1999) and (Sabat, 2009).
The converse electrostrictive effect, which can be used for sensor applications, means that the permittivity, which is the derivative of polarization with respect to the electric field, changes as a function of stress. In relaxor ferroelectrics, the piezoelectric effect can be induced under a bias field such that the electromechanical coupling factors vary as the applied dc bias field changes. As the dc bias field increases, the coupling increases and eventually saturates (Sabat, 2009). Thus, these materials could potentially be used for ultrasonic transducers that could be tuned by a bias field.
Chapter 3 - Research on Piezoelectric Materials
3.1 Past Research on Piezoelectric Materials
As already stated, Pierre and Jacques Curie discovered piezoelectricity in quartz in 1880. The discovery of ferroelectricity accelerated the creation of useful piezoelectric materials. Rochelle salt was the first ferroelectric discovered in 1921. Until 1940 only two types of ferroelectrics were known, Rochelle salt and potassium dihydrogen phosphate. In 1940 to 1943, unusual dielectric properties such as an abnormally high dielectric constant were found in barium titanate BaTiO3 independently by Wainer, Ogawa, and Golman (Kholkin, 2002). After the discovery, compositional modifications of BaTiO3 led to improvement in temperature stability and high voltage output. Piezoelectric transducers based on BaTiO3 ceramics became well established in a number of devices. In the 1950s, (Jaffe et al.) established that the lead zirconate titanate (PZT) creates strong piezoelectric effects. The maximum piezoelectric response was found for PZT compositions near the morphotropic phase boundary between the rhombohedral and tetragonal phases. Since then, PZT containing various additives has become the dominant piezoelectric ceramic for a variety of applications. The development of PZT-based solid solutions was a major success of the piezoelectric industry for these applications. In 1969, (Kawai et al.) discovered that certain polymers, notably polyvinylidene difluoride (PVDF), are piezoelectric when stretched during the fabrication process. Such piezoelectric polymers are also useful for some transducer applications. In 1978, (Newnham et al.) improved composite piezoelectric materials by combining a piezoelectric ceramic and a passive polymer whose properties could be tailored to the requirements of various piezoelectric devices. Another class of ceramic material which was discussed previously has recently become important: relaxor-type electrostrictors, such as lead magnesium niobate (PMN), typically doped with 10-15% lead titanate, which have potential applications in the piezoelectric actuator field. More recently, a breakthrough in the growth of high-quality, large, single-crystal relaxor piezoelectric compositions has sparked interest in these materials for applications ranging from high strain actuators to high frequency transducers for medical ultrasound devices due to their superior electromechanical characteristics (Smith, 2005). More recently, thin films of piezoelectric materials such as zinc oxide (ZnO) and PZT have been widely studied and developed for use in microelectromechanical (MEMS) devices.
3.2 Present Research on Piezoelectric Materials
This section summarizes the current status of piezoelectric materials: single-crystals, ceramics, barium titanate, lead zirconate titanate, polymers, composites, and thin films.
3.2.1 Single Crystals
Piezoelectric ceramics are widely used at present for a large number of applications. However, single-crystal materials retain their utility because they are essential for applications such as frequency stabilized oscillators and surface acoustic devices (Lane, 2003). The most popular single crystal piezoelectric materials are quartz, lithium niobate (LiNbO3), and lithium tantalate (LiTaO3). The single crystals are anisotropic in general, and have different properties depending on the cut of the materials and the direction of the bulk or surface wave propagation. Quartz is a well-known piezoelectric material that has been known for centuries. It belongs to the triclinic crystal system with point group 32 and has a Curie temperature of 537â-¦C (Tuller, 2002). Quartz oscillators using the thickness shear mode are extensively used as clock sources in computers and as frequency stabilized oscillators in televisions. Furthermore, another distinguishing characteristic of quartz is its extremely high mechanical quality factor Qm > 105 (Tuller, 2002).
Lithium niobate and lithium tantalate also belong to an isomorphous crystal system. The Curie temperatures of LiNbO3 and LiTaO3 are 1210â-¦C and 660â-¦C respectively (Tuller, 2002), which is thus higher than single crystal quartz. The crystal symmetry of the ferroelectric phase of these single crystals is 3m, and the polarization direction is along the c axis. [2] These materials have high electromechanical coupling coefficients for surface acoustic waves (SAW). In addition, large single crystals can relatively easily be obtained and produced. Thus, both materials are very important in SAW device applications.
The more recent development of single-crystal piezoelectrics started in 1981, when (Kuwata et al.) first reported an enormously large electromechanical coupling factor and a piezoelectric constant d33 of 1500 pC/N in single crystals solutions between the relaxor and normal ferroelectric materials for Pb(ZnNb)O3 and PbTiO3 (Haertling, 1991). Ten years after, (Yamashita et al.) from the Toshiba corporation and (Shrout et al.) from the Penn State University independently reconfirmed these values and further improved data that had been obtained in these few years, aimed at improving medical acoustic applications.
3.2.2 Ceramics: Perovskite Structure
Most of the piezoelectric ceramics have the perovskite structure ABO3, as shown Figure 9.
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Figure . Perovskite Structure. Adapted from (Sabat, 2009)
This specific structure consists of a simple cubic unit cell that has large cations A at the corners, a smaller cation B in the body center, and oxygens O in the centers of the faces. The structure is a network of linked oxygen octahedra surrounding the B cation. The piezoelectric properties of perovskite-structured materials can be easily tailored for applications by incorporating various cations in the perovskite structure (Sabat, 2009).
3.2.3 Barium Titanate.
Barium titanate (BaTiO3) is one of the most thoroughly studied and most widely used piezoelectric materials. Figure 10 shows the temperature dependence of dielectric coefficients in BaTiO3, that demonstrate the phase transitions in BaTiO3 crystal structure.
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Figure . Temperature Dependence of the Dielectric Coefficients in BaTiO3 (Harrison, 2002)
Three anomalies can be observed. The discontinuity at the Curie temperature which is around 130â-¦C is due to a transition from a ferroelectric to a paraelectric phase. The other two discontinuities are accompanied by transitions from one ferroelectric phase to another. Above the Curie point, the crystal structure is cubic and has no spontaneous dipole moments. At the Curie temperature, the crystal becomes polar, and the structure changes from a cubic to a tetragonal phase. The dipole moment and the spontaneous polarization are parallel to the tetragonal axis as seen in Figure 10. Just below the Curie temperature, in its tetragonal phase, the vector of the spontaneous polarization points in the [001] direction; below 5â-¦C it reorients in the [011] direction for the orthorhombic phase, and below −90â-¦C in the [111] direction for the rhombohedral phase (Harrison, 2002). The dielectric and piezoelectric properties of the ferroelectric ceramic BaTiO3 can be affected by its composition, microstructure, and by dopants in the A or B site of the crystalline structure. Modified ceramics of BaTiO3 that contain dopants such as Pb or Ca ions have been used as commercial piezoelectric materials (Smith, 2005).
3.2.4 Lead Zirconate Titanate.
Lead Zirconate Titanate Pb(Ti,Zr)O3, or (PZT), ceramics are widely used because of their superior piezoelectric properties. The phase diagram of PZT is shown in Figure 11.
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Figure . Phase Diagram for PZT (Harrison, 2002).
Below the Curie temperature, the crystalline symmetry of this solid solution is determined by the Zr and Ti content. Lead titanate has a tetragonal ferroelectric phase of the perovskite structure. As the Zr content increases, the tetragonal distortion decreases, and when Zr > 0.52, the structure changes from the tetragonal 4mm phase to another ferroelectric phase of rhombohedral 3m symmetry (Sabat, 2005) & (Sabat, 2009). The dielectric coefficients have their highest values near the morphotropic phase boundary. This enhancement in the piezoelectric effect is attributed to the increased ease of reorientation of the polarization in an applied electric field. Also, note that doping the PZT material with donor or receptor ions changes the dielectric properties dramatically. Doping with donor ions such as Nb5+ or Ta5+ provides soft PZTs, like PZT-5, because of the facility of domain motion due to the charge compensation of the Pb vacancy. On the other hand, acceptor ions such as Fe3+ or Sc3+ lead to hard PZTs, such as PZT-8, because oxygen vacancies ''pin'' the domain wall motion (Harrison, 2002).
3.2.5 Polymers
A piezoelectric polymer is a material where structured crystallites [3] appear in a completely random configuration. The relative proportion of crystallites strongly affects the piezoelectric behavior of the material (Vinogradov, 2002).
Polyvinylidene difluoride (PVDF) is one of the most common piezoelectric polymers. In 1969, the strong piezoelectricity of PVDF was observed by (Kawai et al.) The piezoelectric coefficients of the polymer were reported to be approximately 6-7 pCN-1, which is ten times larger than any other polymer. Unlike other popular piezoelectric materials, such as PZT, PVDF has a negative d33 value. Physically, this means that PVDF will compress instead of expand, or vice versa, when exposed to an electric field.
This piezoelectric material is stretched during its fabrication process; in other words, thin sheets of PVDF are drawn and stretched in the direction parallel to the plane of the sheet in order to convert the material into its polar phase. Large sheets can be manufactured and thermally formed into complex shapes. Such piezoelectric polymers are used for directional microphones and ultrasonic hydrophones (Vinogradov, 2002).
3.2.6 Composites
Piezoelectric composites made from piezoelectric ceramics and polymers are promising materials because their properties can be tailored exactly for a specific application. PZT rod composites are the most promising candidate, because they have high coupling factors, low acoustic impedance, mechanical flexibility, a low mechanical quality factor, a good match to water and human tissue, a broad operational bandwidth, and the possibility of making arrays by joining the electrodes appropriately (Ramamurthy, 1992). Piezoelectric composite materials are especially useful for underwater sonar and medical diagnostic ultrasonic transducers. For more applications of piezoelectric composites, especially for military use, refer to (Ramamurthy, 1992).
3.2.7 Thin Films
Aluminum nitride (AlN) and zinc oxide (ZnO) are both simple compounds that have the Wurtzite crystalline structures, as depicted in Figure 12.
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Figure . Wurtzite Crystalline Structure. Gray represents a metal. Yellow represents a nitride or an oxide (Berger, 1997).
These materials can be sputtered or deposited as thin films in the c-axis orientation on a variety of substrates. ZnO has sufficient piezoelectric coupling factors, and its thin films are widely used in bulk acoustic and SAW devices. Indeed, the fabrication of ZnO thin films has been widely studied and developed, as reported in (Harrison, 2002) & (Harvey, 2000). Yet, the performance of ZnO devices is limited due to their relatively small piezoelectric coefficients. Nevertheless, PZT thin films are expected to exhibit higher piezoelectric properties. In the past few years, the growth of PZT thin film has been carried out for use in microtransducers and microactuators and has shown some successful results [4] .
Chapter 4 - Conclusion: Future Research in Piezoelectricity
In this paper, important sensor and actuator technologies as well as current state-of-the-art piezoelectric materials for those technologies have been presented. The use of sensor and actuator materials is continuing to grow in an increasing effort to optimize operating systems. Some significant examples are:
Pyroelectric material sensors offer infrared detection capabilities used for night vision and motion detectors;
Piezoelectric materials offer both sensing and actuating applications for pressure detection and micropositioning applications respectively;
Electrostrictive and magnetostrictive materials offer non-linear micropositioning capabilities;
Shape memory alloys offer temperature sensing and a high strain response, although with a relatively slow response time.
Smart structures built with these materials will allow for more flexibility and greater functionality over conventional materials systems in various applications. There are numerous unexplored avenues for the application of sensing and actuating materials, but their development is still far from complete. Continued research on the materials themselves will generate new unknown applications, and will also provide improved performance in existing applications. Although these materials will not simply replace conventional hydraulic, pneumatic and electric motor actuators completely, the new actuators may eventually be implemented more widely than their long established counterparts, as they have a high energy density, a large operating bandwidth, and comparatively simple production. Traditional actuators can be large and complex systems, adding weight and various additional costs to the application. Sensor and actuator materials can decrease manufacturing, operating and maintenance costs, reduce weight and improve system performance. Therefore, when designing new systems, these materials should be given proper consideration as they present a very legitimate alternative to conventional systems.
