Preface
The authors of this book represent the microelectronics department of National Technical University of Ukraine (Kiev Polytechnic Institute), founded 120 years ago and named after Igor Sikorsky, who trained at this institute. Functional dielectrics have been studied, elaborated, and taught in the microelectronics department for many years, and the authors have published hundreds of articles and several books in this area.
The task of this book is to provide engineers specializing in materials science application and research in electronics and information technology with up-to-date and clear ideas about the nature of electrical polarization, dielectric nonlinearity, electrical charge transfer mechanisms, thermal properties, and the nature of thermal stable permittivity in low-loss microwave dielectrics and other functional dielectrics. The book describes in detail the intrinsic mechanisms of electrical polarization and energy transformations in noncentrosymmetric crystals, which are responsible for converting thermal, mechanical, optical, and other actions into electrical signals. In order to extend the use of functional dielectrics into the field of nanoelectronics, this book contains a versatile and comprehensive presentation of the main physical processes that provide the electrical, mechanoelectrical, thermoelectrical, and other conversion phenomena in polar crystals.
More detailed descriptions are given to electrical manifestations of polar sensitivity in crystals; interaction of polarization with conductivity is described, as well as anomalies in thermal expansion coefficient and the main peculiarities of heat transfer in polar-sensitive crystals. Some applications of polarity theory to pyroelectrics and piezoelectrics are described. Microwave dielectrics are also of great importance in modern electronics, because they are widely used in communication devices. In them, small dielectric losses can be achieved in single-phase compositions, based on “hard” paraelectric material that resists polar structure formation. The thermal stability of permittivity can be obtained when paraelectricity is suppressed by paramagnetism.
Nonlinear dielectrics provide an important opportunity for rapid control by permittivity, used in microwave phase shifters. Other frequency-agile devices are also described: tunable dielectric resonators and phase shifters consisting of microwave dielectrics coupled with control by a piezoelectric air gap that avoids inserted losses. The authors express their deep gratitude to their colleagues from “Igor Sikorsky Kiev Polytechnic Institute” for their interest in this work and for their valuable comments. The authors are also grateful to their students who pointed out hardto- understand concepts and asked us for clarification. The responsibility for any remaining errors or shortcomings is, of course, ours.
We want to give special thanks to Prof. Dr. Sci. Yuriy Prokopenko, Ass. Prof. Dr. Victor Kazmirenko, Ass. Prof. Dr. Dmitriy Tatarchuk, and Dr. Yuriy Didenko for their useful collaboration.
Finally, we are deeply grateful to the staff of Elsevier: to Kayla Dos Santos and Peter Adamson for their patience in eliminating the shortcomings of the manuscript.
Introduction
The dielectrics intended for use in electrical and electronic devices must have good electrical insulating properties, including very low electrical conductivity and high electrical strength. At the same time, in present-day electronics technology, other properties of dielectrics have gained special importance, namely those that allow for conversion of energy or information; these dielectrics may be considered functional materials.
The functional (or active, or adaptive, or controlled, or smart) dielectrics actively react to changes of temperature, pressure, mechanical stress, electrical and magnetic fields, light illumination, and even smell. Functional dielectrics can be classified as pyroelectrics, ferroelectrics, piezoelectrics, electrets, quantum-electronics materials, microwave dielectrics with tunable permittivity, etc.
Active dielectrics perform different functions. For example, the piezoelectrics convert mechanical energy into electrical energy (and vice versa) that is used in piezoelectric filters, ultrasound emitters, piezoelectric transformers, piezoelectric motors, etc. Pyroelectrics convert heat into electricity and are applied as sensitive detectors of radiation, thermal-vision devices, etc. The nonlinear properties of ferroelectrics and paraelectrics and the constant electrical field produced by electrets allow use of these functional dielectrics for modulation, detection, amplification, registering, storing, displaying, and other types of electrical conversion of signals carrying information. It is also necessary to mention the use of functional dielectrics in multifunctional electronic devices, along with the active search for new technological solutions to challenges regarding application of these dielectrics in the field of information technology.
The main property of any dielectric is its electrical polarization, i.e., the effect of separation of electrical charges that remain bonded in spite of their shifting. As a result, an electrical moment appears (as a product of charge magnitude and displacement); the volumetric density of this moment is polarization P. One unusual feature of functional dielectrics is that their electrical polarization can be induced not only by an electrical field, but also by other causes. A comparison of conventional (nonpolar) dielectrics and two categories of polar dielectrics are shown in Table I.1. The essential distinction between active (functional) dielectrics and ordinary dielectrics is obvious. In many applications of functional dielectrics (such as sensors, actuators, filters, transformers, motors, etc.), they are subjected to external thermal, electrical, mechanical, and other influences—scalar, vector, or tensor types. Table I.1 shows that a conventional dielectric electrically reacts only to electrical field action: E ) P, while polar piezoelectrics and pyroelectrics, besides the E ) P response, are capable of electrical response to other actions: mechanical X ) P and thermal dT) dP. At the same time, piezoelectrics respond to electrical action not by ordinary polarization only, of P ¼ ε0χE, but also produce an electromechanical response P0 ¼ (e2/c)E, while pyroelectrics, in addition, also produce an electrothermal response P00 ¼ (γ2Т) E/(ε0C).
In view of possible applications of dielectrics as functional (converting) elements in electronics, one should identify and describe their properties not only in terms of their exclusively electrical characteristics but also their capability to manifest various electrical, mechanical, and thermal properties. These materials are important also for miniaturization of microwave and telecommunications equipment. In this case, a decisive role is played by the value of permittivity (ε), since the planar dimensions of microwave devices are reduced exactly by the factor ε. In some functional dielectrics (paraelectrics), a large value of ε can be obtained together with low dielectric losses, which has important technical application in high-frequency devices. Some functional dielectrics allow electrical control by permittivity that can be used for electrically controlled microwave devices.
For all the reasons mentioned, interest in ferroelectrics, paraelectrics, piezoelectrics, and pyroelectrics has recently grown in the field of electronics materials science, precisely because of their new applications in instrumentation engineering and electronics, and also owing to significant progress in modern microelectronic and nanoelectronic technologies.