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MECHANISM Crystal classes Of the thirty-two Crystal Class es, twenty-one are non-centrosymmetric (not having a centre of symmetry), and of these, twenty exhibit direct piezoelectricity the remaining one being the cubic class 432. Ten of these are polar (i.e. spontaneously polarise), having a Dipole in their Unit Cell , and exhibit Pyroelectricity . If this dipole can be reversed by the application of an electric field, the material is said to be Ferroelectric .
In a piezoelectric crystal, the positive and negative Electrical Charge s are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. Each of these sites forms an Electric Dipole and dipoles near each other tend to be aligned in regions called Weiss Domains . The domains are usually randomly oriented, but can be aligned during ''poling'' (not the same as Magnetic Poling ), a process by which a strong electric field is applied across the material, usually at elevated temperatures. When a mechanical stress is applied, this symmetry is disturbed, and the charge Asymmetry generates a Voltage across the material. For example, a 1 cm cube of Quartz with 500 Lbf (2 KN ) of correctly applied force upon it, can produce a voltage of 12,500 V. Piezoelectric materials also show the opposite effect, called converse piezoelectricity, where the application of an electrical field creates mechanical deformation in the crystal. MATHEMATICAL DESCRIPTION Piezoelectricity is the combined effect of the electrical behavior of the material: : where ''D'' is the Electric Displacement , is Permittivity and ''E'' is Electric Field Strength , and Hooke's Law : : where ''S'' is Strain , ''s'' is Compliance and ''T'' is Stress . These may be combined into so-called ''coupled equations'', of which the strain-charge form is: : : where the superscript ''E'' indicates a zero, or constant, electric field; the superscript ''T'' indicates a zero, or constant, stress field; and the subscript ''t'' stands for transposition of a Matrix . The strain-charge from may also be written as: : : The bending forces generated by converse piezoelectricity are extremely high, of the order of tens of millions of pounds (tens of meganewtons), and usually cannot be constrained. The only reason the force is usually not noticed is because it causes a displacement of the order of one billionth of an inch (a few Nanometre s). HISTORY A related property known as Pyroelectricity , the ability of certain Mineral crystals to generate electrical charge when heated, was known of as early as the 19th Century , and was named by David Brewster in 1824 . In 1880 , the brothers Pierre Curie and Jacques Curie predicted and demonstrated piezoelectricity using tinfoil, glue, wire, magnets, and a jeweler's saw. They showed that crystals of Tourmaline , Quartz , Topaz , Cane Sugar , and Rochelle Salt (sodium potassium tartrate tetrahydrate) generate electrical polarization from mechanical stress. Quartz and Rochelle salt exhibited the most piezoelectricity. Converse piezoelectricity was mathematically deduced from fundamental thermodynamic principles by Lippmann in 1881 . The Curies immediately confirmed the existence of the "converse effect," and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals. APPLICATION Buffalo rawhide ceremonial rattle filled with quartz crystals. The rattle produces flashes of light created by the piezoelectric effect of quartz crystals being subjected to mechanical stress when the rattle is shaken in darkness.]] The Uncompahgre Ute Indians from Central Colorado are one of the first documented groups of people in the world credited with the application of piezoelectricity and the Piezoelectric effect involving the use of quartz crystals to generate both light and electricity. The Ute employed an ingenious invention which allowed the creation of light by piezoelectricity thousands of years before the modern world learned of the concept. The Ute constructed special ceremonial rattles made from buffalo rawhide which they filled with clear quartz crystals collected from the mountains of Colorado and Utah. When the rattles were shaken at night during ceremonies, the friction and mechanical stress of the quartz crystals impacting together through the translucent buffalo hide produced flashes of electric light. These rattles were believed to call spirits into Ute Ceremonies, and were considered extremely powerful religious objects. The second practical application for piezoelectric devices was Sonar , first developed during World War I . In France in 1917 , Paul Langevin (whose development now bears his name) and his coworkers developed an Ultrasonic Submarine detector. The detector consisted of a Transducer , made of thin quartz crystals carefully glued between two steel plates, and a Hydrophone to detect the returned echo. By emitting a high-frequency Chirp from the transducer, and measuring the amount of time it takes to hear an echo from the sound waves bouncing off an object, one can calculate the distance to that object. The use of piezoelectricity in sonar, and the success of that project, created intense development interest in piezoelectric devices. Over the next few decades, new piezoelectric materials and new applications for those materials were explored and developed. Development of piezoelectric devices and materials in the United States was kept within the companies doing the development, mostly due to the wartime beginnings of the field, and in the interests of securing profitable patents. New materials were the first to be developed — quartz crystals were the first commercially exploited piezoelectric material, but scientists searched for higher-performance materials. Piezoelectric devices found homes in many fields. Ceramic Phonograph cartridges simplified player design, were cheap and accurate, and made record players cheaper to maintain and easier to build. Ceramic Electret Microphones could be made small and sensitive. The development of the ultrasonic transducer allowed for easy measurement of viscosity and elasticity in fluids and solids, resulting in huge advances in materials research. Ultrasonic Time-domain Reflectometer s (which send an ultrasonic pulse through a material and measure reflections from discontinuities) could find flaws inside cast metal and stone objects, improving structural safety. However, despite the advances in materials and the maturation of manufacturing processes, the United States market had not grown as quickly. Without many new applications, the growth of the United States' piezoelectric industry suffered. In contrast, Japanese manufacturers shared their information, quickly overcoming technical and manufacturing challenges and creating new markets. Japanese efforts in materials research created piezoceramic materials competitive to the U.S. materials, but free of expensive patent restrictions. Major Japanese piezoelectric developments include new designs of piezoceramic filters, used in radios and televisions, piezo buzzers and audio transducers that could be connected directly into electronic circuits, and the piezoelectric igniter which generates sparks for small engine ignition systems (and gas-grill lighters) by compressing a ceramic disc. Ultrasonic transducers that could transmit sound waves through air had existed for quite some time, but first saw major commercial use in early television remote controls. These transducers now are mounted on several Car models as an Echolocation device, helping the driver determine the distance from the rear of the car to any objects that may be in its path. MATERIALS In addition to the materials listed above, many other materials exhibit the effect, including quartz analogue crystals like Berlinite (AlPO4) and Gallium Orthophosphate (GaPO4), Ceramic s with Perovskite or Tungsten - Bronze structures ( BaTiO3 , SrTiO3 , Pb(ZrTi)O3 , KNbO3, LiNbO3 , LiTaO3 , BiFeO3, NaxWO3 , Ba2NaNb5O5, Pb2KNb5O15). Polymer materials like Rubber , Wool , Hair , Wood fiber, and Silk often behave as electrets. Although this phenomenon is often confused with piezoelectricity, the two phenomena are distinct. the orientation of polarization in a piezoelectric is limited by the symmetry, whereas the polarization direction in an electret is not. The polymer polyvinylidene fluoride, PVDF , exhibits piezoelectricity several times larger than quartz. Bone exhibits some piezoelectric properties, due to the Apatite crystals: it has been hypothesized that this is part of the mechanism of bone remodelling in response to stress, as the electric fields on the apatite crystals stimulate further bone growth. APPLICATIONS Piezoelectric crystals are used in numerous ways: High-voltage sources Direct piezoelectricity of some substances like quartz, as mentioned above, can generate Potential Differences of thousands of Volt s.
Sensor s
Actuator s As very high voltages correspond to only tiny changes in the width of the crystal, this width can be changed with better-than- Micrometre precision, making piezo crystals the most important tool for positioning objects with extreme accuracy.
Frequency Standard s
Piezoelectric motors Types of piezoelectric motor include the well-known travelling-wave motor used for Auto-focus in Reflex Cameras , Inchworm Motor s for linear motion, and rectangular four-quadrant motors with high power density (2.5 Watt /cm³) and speed ranging from 10 nm/s to 800 mm/s. All these motors work on the same principle. Driven by dual orthogonal vibration modes with a Phase shift of 90°, the contact point between two surfaces vibrates in an Elliptical path, producing a Friction al force between the surfaces. Usually, one surface is fixed causing the other to move. In most piezoelectric motors the piezoelectric crystal is excited by a Sine Wave signal at the Resonant Frequency of the motor. Using the resonance effect, a much lower voltage can be used to produce a high vibration amplitude. Ultrasonic transducers Piezoelectric materials are used as Ultrasonic transducers for imaging applications (eg medical imaging, industrial Nondestructive Testing , or NDT) and high power applications (eg medical treatment, Sonochemistry and industrial processing). For imaging applications, the transducer can act as both a sensor and an actuator. Ultrasonic transducers can inject ultrasound waves into the body, receive the returned wave, and convert it to an electrical signal (a voltage). Most medical ultrasound transducers are piezoelectric. SEE ALSO
International Standards
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