Functional Dielectrics for Electronics

Fundamentals of Conversion Properties

Yuriy Poplavko, Yuriy Yakymenko

• Publisher: Woodhead Publishing
• Year: 2020
• ISBN:
  9780128188361
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Functional Dielectrics for Electronics: Fundamentals of Conversion Properties presents an overview of the nature of electrical polarization, dielectric nonlinearity, electrical charge transfer mechanisms, thermal properties, the nature of high permittivity, low-loss thermostability and other functional dielectrics. The book describes the intrinsic mechanisms of electrical polarization and the energy transformations in non-centrosymmetric crystals that are responsible for converting thermal, mechanical, optical and other impacts into electrical signals. In addition, the book reviews the main physical processes that provide electrical, mechanoelectrical, thermoelectrical and other conversion phenomena in polar crystals.

Detailed descriptions are given to electrical manifestations of polar-sensitivity in the crystals, the interaction of polarization with conductivity, the anomalies in thermal expansion coefficient and main peculiarities of heat transfer in polar-sensitive crystals.

• Provides readers with a fundamental understanding of polar dielectric materials and their physical processes
• Includes different models of polar sensitivity and experimental confirmation of these models
• Discusses thermal expansion, heat transfer, dielectric nonlinearity and other important aspects for electronics applications

1

Physical nature of crystal internal polarity

Abstract

Particular properties of polar crystals (pyro-, piezo-, and ferroelectrics) are explained by the existence in their structure of peculiar polar-sensitive interatomic bonds, which gives new meaning to the traditional concept of spontaneous polarization. The hybridized ionic-covalent polar bonds originate mainly because of a distinction in the electronegativity of ions forming the polar crystal. Polar bonds cause electrical polarization as a response to nonelectrical homogeneous action (temperature or pressure). If a polar crystal is classified as a ferroelectric, polar-sensitive bonds cause a nonlinear response to an electrical field with a reversal of the orientation.

Polar-sensitive structure manifests itself in crystals as the ability to provide electrical (vector type) response to nonelectrical scalar or more complicated tensor types of actions. Hybridized ionic-covalent bonding causes a reduction in crystal symmetry, so polar crystals always belong to the noncentrosymmetric classes. Moreover, it is exactly the presence of such bonds that determines the noncentrosymmetric structure of some crystals. Hybridized ionic-covalent bonding is a main cause of pyroelectric and piezoelectric properties. The model used is based on the asymmetry in the electronic density distribution along the atomic bonds, which ensures the ability of a crystal to demonstrate a polar (electrical) response to nonelectrical action; this model is free from any assumption as to the presence of an internal electrical field in the polar crystal.

In some polar crystals, the temperature steadiness of polar-sensitive bonds can be described by the critical law M(T) ~ (θ − T)ⁿ, so a phase transition temperature θ exists, at which tensor M_ijk vanishes with temperature rise. The critical parameter is n = 1 if polar-sensitive bonds are arranged in a plane (2D case). In the event of a spatial (3D) arrangement of polar-sensitive bonds, the critical exponent is n = 2. These two cases differ from 1D dipole-type polar-sensitive bonds in ferroelectrics, which correspond to the well-known critical law M_i(T) with n = 0.5 (Landau's critical index). The thermodynamics of processes of electrical polarization is also briefly described.

Keywords

Dielectric polarization; Atomic bonding; Pyroelectric; Piezoelectric; Ferroelectric; Phase transitions

Particular properties of polar crystals (pyro-, piezo-, and ferroelectrics) are explained by the existence in their structure of peculiar polar-sensitive interatomic bonds, which gives new meaning to the traditional concept of spontaneous polarization. Hybridized ionic-covalent polar bonds originate mainly due to a distinction in electronegativity of ions forming polar crystals. Polar bonds cause electrical polarization as a response to a nonelectrical homogeneous action (temperature or pressure). If a polar crystal is a ferroelectric, the polar-sensitive bonds cause a nonlinear response to an electrical field with a reversal of orientation.

The polar-sensitive structure manifests itself in crystals as an ability to provide an electrical (vector-type) response to nonelectrical scalar or more complicated tensor types of actions. Hybridized ionic-covalent bonding causes a reduction in crystal symmetry, so polar crystals always belong to the noncentrosymmetric classes. Moreover, it is exactly the presence of such bonds that determines the noncentrosymmetric structure of some crystals. Hybridized ionic-covalent bonding is a main cause of pyroelectric and piezoelectric properties. The model used is based on the asymmetry in electronic density distribution along atomic bonds, which ensures the ability of a crystal to manifest a polar (electrical) response to nonelectrical action; the model is free from any assumption as to the presence of an internal electrical field in the polar crystal.

1.1 Basic elementary mechanisms of polarization

A substantially different approach to understanding polar crystal behavior is taken here. As a rule, the unique properties of polar crystals, in particular pyroelectricity, are widely conceptualized as being due to change in spontaneous polarization [1, 2]. A somewhat different implication of this concept is proposed here, and in this regard another physical mechanism is considered: the peculiar capability of a polar crystal to induce electrical polarization under uniform thermal or mechanical action, i.e., to generate an electrical response to nonelectrical influences [3].

Before discussing the nature of the peculiar polarization existing in polar crystals, the general conception of polarization in dielectrics needs to be clarified, since for this defining question it is necessary to take all details into account. The point is that any dielectric can be polarized by an applied electrical field, but only certain dielectrics can be polarized in a nonelectrical manner.

If electrical field E is applied to a dielectric, then an electrical polarization arises: P = ɛ₀χE, where ɛ₀ is an electrical constant and χ is dielectric susceptibility. Electrical polarization means that a separation of electrical charges occurs: for example, on the opposite surfaces of a plane-parallel dielectric sample, electrical charges of different signs appear; note that these charges are not free but closely bound to the dielectric. Traditionally, the polarization process is described by the electrical induction

$D={\varepsilon }_0\mathit{\varepsilon E}={\varepsilon }_{0}E+P,$

which includes induction of a vacuum ɛ₀E and polarization of the dielectric P = ɛ₀χE; dielectric permittivity ɛ = 1 + χ takes both processes into account. At the moment of the electrical voltage switching on, through the dielectric included in the electrical circuit, a reactive current of electrical charge displacement flows; then it terminates, if the remaining voltage is unchanged and the conductivity of the dielectric is insignificant. The voltage switching off is also accompanied by a jump of electrical depolarization current, which has an opposite sign to the charging current; in this way the electrical polarization reacts only to a change in electrical voltage. It is appropriate to note here that, if polarization is induced in a nonelectrical manner (me...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Introduction
• 1: Physical nature of crystal internal polarity
• 2: Manifestations of polar sensitivity in crystals
• 3: Thermal physics of polar crystals
• 4: Pyroelectricity
• 5: Piezoelectricity
• 6: High permittivity microwave dielectrics
• Index