Chapter 1. Mathematical Examination of Dielectrics 1 1. Introduction to dielectrics 1. See All Customer Reviews. Shop Textbooks. Read an excerpt of this book! Add to Wishlist.
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USD Sign in to Purchase Instantly. Temporarily Out of Stock Online Please check back later for updated availability. Overview An introduction to the physics of electrical insulation, this book presents the physical foundations of this discipline and the resulting applications. Contents 1. Read an Excerpt Click to read or download. Table of Contents Foreword ix Chapter 1.
Introduction to dielectrics 1 1. Perfect dielectrics 15 1. Forces exerted on polarized dielectrics 34 1. Dielectric losses 45 1. Residual charges 50 1. Electrets 51 1.
Characteristics of an insulator 52 1. Pyro and piezo-electricity 54 1. Currents in extended conductors 63 Chapter 2. Physical Examination of Dielectrics 81 2.
Orientation polarization results from a permanent dipole, e. The assembly of these dipoles forms a macroscopic polarization. When an external electric field is applied, the distance between charges within each permanent dipole, which is related to chemical bonding , remains constant in orientation polarization; however, the direction of polarization itself rotates. This rotation occurs on a timescale that depends on the torque and surrounding local viscosity of the molecules. Because the rotation is not instantaneous, dipolar polarizations lose the response to electric fields at the highest frequencies.
A molecule rotates about 1 radian per picosecond in a fluid, thus this loss occurs at about 10 11 Hz in the microwave region. The delay of the response to the change of the electric field causes friction and heat.
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When an external electric field is applied at infrared frequencies or less, the molecules are bent and stretched by the field and the molecular dipole moment changes. The molecular vibration frequency is roughly the inverse of the time it takes for the molecules to bend, and this distortion polarization disappears above the infrared.
Ionic polarization is polarization caused by relative displacements between positive and negative ions in ionic crystals for example, NaCl. If a crystal or molecule consists of atoms of more than one kind, the distribution of charges around an atom in the crystal or molecule leans to positive or negative. As a result, when lattice vibrations or molecular vibrations induce relative displacements of the atoms, the centers of positive and negative charges are also displaced.
The locations of these centers are affected by the symmetry of the displacements. When the centers don't correspond, polarization arises in molecules or crystals. This polarization is called ionic polarization.
What is Dielectric Constant?
Ionic polarization causes the ferroelectric effect as well as dipolar polarization. The ferroelectric transition, which is caused by the lining up of the orientations of permanent dipoles along a particular direction, is called an order-disorder phase transition. The transition caused by ionic polarizations in crystals is called a displacive phase transition.
All cells in animal body tissues are electrically polarized — in other words, they maintain a voltage difference across the cell's plasma membrane , known as the membrane potential. This electrical polarization results from a complex interplay between ion transporters and ion channels. In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the dendrites , axon , and cell body different electrical properties.
As a result, some parts of the membrane of a neuron may be excitable capable of generating action potentials , whereas others are not. In physics, dielectric dispersion is the dependence of the permittivity of a dielectric material on the frequency of an applied electric field. Because there is a lag between changes in polarization and changes in the electric field, the permittivity of the dielectric is a complicated function of frequency of the electric field.
Dielectric dispersion is very important for the applications of dielectric materials and for the analysis of polarization systems. This is one instance of a general phenomenon known as material dispersion : a frequency-dependent response of a medium for wave propagation. Because permittivity indicates the strength of the relation between an electric field and polarization, if a polarization process loses its response, permittivity decreases. Dielectric relaxation is the momentary delay or lag in the dielectric constant of a material. This is usually caused by the delay in molecular polarization with respect to a changing electric field in a dielectric medium e.
Dielectric relaxation in changing electric fields could be considered analogous to hysteresis in changing magnetic fields e. Relaxation in general is a delay or lag in the response of a linear system , and therefore dielectric relaxation is measured relative to the expected linear steady state equilibrium dielectric values. The time lag between electrical field and polarization implies an irreversible degradation of Gibbs free energy.
In physics , dielectric relaxation refers to the relaxation response of a dielectric medium to an external, oscillating electric field. This relaxation is often described in terms of permittivity as a function of frequency , which can, for ideal systems, be described by the Debye equation. On the other hand, the distortion related to ionic and electronic polarization shows behavior of the resonance or oscillator type. The character of the distortion process depends on the structure, composition, and surroundings of the sample.
Debye relaxation is the dielectric relaxation response of an ideal, noninteracting population of dipoles to an alternating external electric field. Separating the real and imaginary parts of the complex dielectric permittivity yields: . This relaxation model was introduced by and named after the physicist Peter Debye Paraelectricity is the ability of many materials specifically ceramics to become polarized under an applied electric field. Unlike ferroelectricity , this can happen even if there is no permanent electric dipole that exists in the material, and removal of the fields results in the polarization in the material returning to zero.
Paraelectricity can occur in crystal phases where electric dipoles are unaligned and thus have the potential to align in an external electric field and weaken it. An example of a paraelectric material of high dielectric constant is strontium titanate.
The LiNbO 3 crystal is ferroelectric below K , and above this temperature it transforms into a disordered paraelectric phase. Similarly, other perovskites also exhibit paraelectricity at high temperatures. Paraelectricity has been explored as a possible refrigeration mechanism; polarizing a paraelectric by applying an electric field under adiabatic process conditions raises the temperature, while removing the field lowers the temperature.
Tunable dielectrics are insulators whose ability to store electrical charge changes when a voltage is applied. Other potential materials include microwave dielectrics and carbon nanotube CNT composites. The material was created via molecular beam epitaxy. The two have mismatched crystal spacing that produces strain within the strontium titanate layer that makes it less stable and tunable. Such films suffer significant losses arising from defects.
Commercially manufactured capacitors typically use a solid dielectric material with high permittivity as the intervening medium between the stored positive and negative charges. This material is often referred to in technical contexts as the capacitor dielectric. The most obvious advantage to using such a dielectric material is that it prevents the conducting plates, on which the charges are stored, from coming into direct electrical contact. More significantly, however, a high permittivity allows a greater stored charge at a given voltage.
In this case the charge density is given by. Dielectric materials used for capacitors are also chosen such that they are resistant to ionization. This allows the capacitor to operate at higher voltages before the insulating dielectric ionizes and begins to allow undesirable current. A dielectric resonator oscillator DRO is an electronic component that exhibits resonance of the polarization response for a narrow range of frequencies, generally in the microwave band. It consists of a "puck" of ceramic that has a large dielectric constant and a low dissipation factor.
Such resonators are often used to provide a frequency reference in an oscillator circuit. An unshielded dielectric resonator can be used as a dielectric resonator antenna DRA. Barium strontium titanate BST , a ferroelectric thin film, was studied for the fabrication of radio frequency and microwave components, such as voltage-controlled oscillators, tunable filters, and phase shifters.
The research was part of an effort to provide the Army with highly-tunable, microwave-compatible materials for broadband electric-field tunable devices, which operate consistently in extreme temperatures. In a research paper, ARL researchers explored how small concentrations of acceptor dopants can dramatically modify the properties of ferroelectric materials such as BST. The Mg doped BST films showed "improved dielectric properties, low leakage current, and good tunability", meriting potential for use in microwave tunable devices.
Dielectric materials can be solids, liquids, or gases. In addition, a high vacuum can also be a useful,  nearly lossless dielectric even though its relative dielectric constant is only unity. Solid dielectrics are perhaps the most commonly used dielectrics in electrical engineering, and many solids are very good insulators.