The electrical engineering handbook
Feisel, L.D. “Section V – Electrical Effect s and Devices” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000 Ever since the discovery of superconductivity in 1911, researchers have sought to raise the temperature at which superconductivity occurs. With the advent of high temperature superconducting (HTS) materials in 1986, superconductors have begun to emerge from the laboratory and appear in practical applications. A pioneer in this explosively advancing technology is Superconducting Technologies, Inc., Santa Barbara, California. This company uses thallium, the highest temperature material for making high temperature superconductors. Thallium remains conductive at temperatures above 77 ° K and can be cooled to working temperature by a liquid nitrogen system instead of the more difficult and more expensive helium method. Shown above is a high temperature superconductor being produced by a laser ablation system. (Photo courtesy of National Aeronautics and Space Administration.) © 2000 by CRC Press LLC © 2000 by CRC Press LLC V Electrical Effects and Devices 46Electroacoustic Devices P. H. Rogers Transduction Mechanisms•Sensitivity and Source Level•Reciprocity•Canonical Equations and Electroacoustic Coupling•Radiation Impedance•Directivity 47Surface Acoustic Wave Filters D. C. Malocha SAW Material Properties•Basic Filter Specifications•SAW Transducer Modeling•Distortion and Second-Order Effects•Bidirectional Filter Response•Multiphase Unidirectional Transducers•Single-Phase Unidirectional Transducers•Dispersive Filters•Coded SAW Filters•Resonators 48Ultrasound G. W. Farnell Propagation in Solids•Piezoelectric Excitation•One-Dimensional Propagation•Transducers 49Ferroelectric and Piezoelectric Materials K. F. Etzold Mechanical Characteristics•Ferroelectric Materials•Ferroelectric and High Epsilon Thin Films 50Electrostriction V. Sundar and R. E. Newnham Defining Equations•PMN-PT—A Prototype Electrostrictive Material 51Piezoresistivity A. Amin Equation of State•Effect of Crystal Point Group on Õ ijkl •Geometric Corrections and Elastoresistance Tensor•Multivalley Semiconductors•Longitudinel Piezoresistivity P l and Maximum Sensitivity Direction•Semiconducting (PTCR) Perovskites•Thick Film Resistors• Design Considerations 52The Hall Effect A. C. Ehrlich Theoretical Background•Relation to the Electronic Structure—(i) w c t << 1•Relation to the Electronic Structure—(ii) w c t >> 1 53Superconductivity K. A. Delin, T. P. Orlando General Electromagnetic Properties•Superconducting Electronics•Types of Superconductors 54Pyroelectric Materials and Devices R. W. Whatmore Polar Dielectrics•The Pyroelectric Effect•Pyroelectric Materials and Their Selection 55Dielectrics and Insulators R. Bartnikas Dielectric Losses•Dielectric Breakdown•Insulation Aging•Dielectric Materials 56Sensors R. L. Smith Physical Sensors•Chemical Sensors•Biosensors•Microsensors 57Magnetooptics D. Young, Y. Pu Classification of Magnetooptic Effects•Applications of Magnetooptic Effects 58Smart Materials P. S. Neelakanta Smart/Intelligent Structures•Objective-Based Classification of Smart/Intelligent Materials•Material Properties Conducive for Smart Material Applications•State-of-the-Art Smart Materials•Smart Sensors•Examples of Smart/Intelligent Systems•High-Tech Application Potentials © 2000 by CRC Press LLC Lyle D. Feisel State University of New York, Binghamton very high school student who takes a course in physics or even general science is—or at least should be—familiar with the first-order, linear electrical effects such as resistance, inductance, capacitance, etc. The more esoteric effects, however, are often neglected, even in otherwise comprehensive undergraduate electrical engineering curricula. These effects, though, are not only fascinating in their manifestations but are also potentially—and in some cases, currently—exceedingly useful in application. This section will describe many of these higher-order electrical and magnetic effects and some of the devices that are based upon them. Readers are invited not only to study the current applications but to let their imaginations extrapolate to other uses as yet unproposed. A number of phenomena are related to the interaction of mechanical energy with electrical energy. The field of acoustics deals with those situations where that mechanical energy takes the form of sound waves. Acoustic applications have been particularly fruitful, especially during the last two decades. Surface acoustic wave (SAW) filters are among the more useful applications. These elegant devices are a marriage of sophisticated signal theory and piezoelectricity, consummated on the bed of thin-film technology. Unlike some elegant devices, they have been commercially successful as well. A special class of acoustoelectric devices deals with acoustic frequencies beyond the range of human hearing. The field of ultrasonics and its related devices and systems are finding broad application in the area of nonde- structive testing. Of course, one of the testing applications where the nondestructive property is especially important is in investigating the human body. Medical imaging has provided considerable impetus for advances in ultrasonics in the last few years. Most people know that if a sample of certain types of material (e.g., iron) is subjected to a magnetic field, it will exhibit a retained magnetic behavior. Few, however, realize that some materials exhibit a similar retention effect when an electric field is applied. Ferroelectricity is the phenomenon in which certain crystalline or polycrystalline materials retain electric polarization after an external electric field has been applied and removed. Since the direction of the polarization depends upon the direction of the applied field and since the polarization is quite persistent, memory devices can be based on this effect. Other applications have also been suggested. For decades, the frequencies of radio transmitters have been stabilized with “crystals.” In recent years, the effect called piezoelectricity —in which a mechanical strain induces an electric field and vice versa—has found many other applications. Like ferroelectrics, piezoelectric materials can be either crystalline or polycrystalline and can be fabricated in a variety of shapes. If an electric charge is moved with a velocity at some angle to a magnetic field, the charge will experience a force at right angles to both the charge velocity and the magnetic field. If the charge is inside a solid material, a charge inhomogeneity is created and an electric field results. This is the well-known Hall effect , which finds practical application in such devices as magnetic field meters and in more basic uses as measuring and understanding the properties of semiconductors. Probably the second electrical phenomenon observed by humans (lightning was probably the first), ferro- magnetism deals with the interaction of molecular magnetic dipoles with external and internal magnetic fields. Ferromagnetic materials retain some polarization after an external field is removed—a desirable property if the application is a permanent magnet or a recording device—but one which causes losses in a transformer. These materials have improved as the demands of magnetic recording have increased. If certain materials get cold enough, their resistivity goes to zero—not to some very small value but, as nearly as we can tell, zero. Superconductivity has been known as an interesting phenomenon for many years, but applications have been limited because the phenomenon only occurred at temperatures within a few degrees of absolute zero. Recent advances, however, have produced materials which exhibit superconductive behavior at substantially higher temperatures, and there is renewed interest in developing applications. This is certainly an area to watch in the next few years. Some very elegant devices have been developed to exploit the interactions between electric fields and photons or optical waves. Electrooptics is the key to many of the recent and, indeed, future advances in optical commu- nication. The phenomena are generally higher-order, nonintuitive, and exceedingly interesting, and the devices are generally quite elegant but simple. E © 2000 by CRC Press LLC We have come a long way since the first Atlantic Cable was fabricated using gutta-percha, tarred hemp, and pitch for insulation. Dielectrics and insulators are now better understood and controlled for a wide variety of applications. At one time the only property of real interest was dielectric strength, the insulator’s ability to stand up to high voltage. Today, many other properties, as well as ease and economy of fabrication, are at least as important. The word application appears many times in the preceding paragraphs. What are these applications? Many of the practical uses of the phenomena described in this section are in measuring the variables that define the phenomena. Thus, sensors constitute a primary application. For instance, the Hall effect can be used to measure magnetic fields, and mechanical strain can be measured using the phenomenon of piezoelectricity. Just as photons will interact with electric fields, so, too, will they affect and be affected by magnetic fields. Magnetooptics is the study and application of these interactions. As with electrooptics, the increased activity in optical communications has provided renewed interest in this field. The use of smart materials may solve a variety of engineering problems. In general, these are materials which change their properties to adapt to their environments, thereby doing their jobs better. This promises to be an area of increased activity in the future. Again, the reader is admonished not only to understand the applications presented in the following chapters but to understand, at least at the phenomenological level, the phenomena upon which the applications are based. Such understanding is likely to lead to even broader applications in the future. Nomenclature Symbol Quantity Unit a attenuation constant Np/m c o magnetic susceptibility of free space D diffraction constant E transducer efficiency e dielectric constant ⑀ complex permittivity F/m G T thermal conductance W/K h viscosity Poise h emissivity k quantum mechanical wave m –1 factor k 2 SAW coupling factor K thermal conductivity of W/m 2 /K pyroelectric Symbol Quantity Unit m molar mass kg R Hall coefficient m 3 /C S strain s conductivity S/m T stress N/m 2 t T thermal time constant of s element q f Faraday rotation coefficient V phase velocity m/s V Verdet constant W electromagnetic energy W/m 2 density Z R radiation impedance W