Dielectric Terminology and Properties
– Insulator implies low electrical conduction
– Dielectric typically means materials with high polarisability
– Dielectric is used to indicate the energy storing capacity of the material
– Dielectric increases the surface charge of a capacitor
– Dielectric was coined by William Whewell in response to a request from Michael Faraday
– Electric susceptibility measures how easily a dielectric material polarises in response to an electric field
– Electric susceptibility determines the electric permittivity of the material
– Electric susceptibility influences phenomena such as capacitance and the speed of light
– Electric susceptibility is defined as the constant of proportionality between electric field and dielectric polarisation density
– Electric susceptibility is related to relative permittivity
– A material cannot polarise instantaneously in response to an applied field
– The polarisation is a convolution of the electric field at previous times with time-dependent susceptibility
– The change of susceptibility with respect to frequency characterises the dispersion properties of the material
– The polarisation can only depend on the electric field at previous times due to causality
– Causality imposes Kramers-Kronig constraints on the real and imaginary parts of the susceptibility
– Dielectric polarisation is the response of a dielectric material to an electric field
– It can be inherent to polar molecules or induced in any molecule with asymmetric distortion
– Orientation polarisation results from a permanent dipole
– Dipolar polarisation can lose response to electric fields at high frequencies
– External electric fields affect the molecular dipole moment in infrared frequencies or less
– Dielectric materials are made up of atoms
– Each atom consists of a cloud of negative charge bound to a positive point charge
– The charge cloud is distorted in the presence of an electric field
– The relationship between the electric field and the dipole moment gives rise to dielectric behavior
– The behavior of the dielectric depends on factors such as the type of electric field, isotropy of the material, and linearity of the response
Ionic Polarisation
– Ionic polarisation is caused by relative displacements between positive and negative ions in ionic crystals.
– The distribution of charges around an atom in a crystal or molecule leans towards positive or negative.
– Lattice or molecular vibrations induce relative displacements of the atoms, resulting in the displacement of positive and negative charges.
– The symmetry of the displacements affects the locations of the centers of positive and negative charges.
– When the centers of positive and negative charges do not correspond, ionic polarisation occurs.
– Ionic polarisation enables the production of energy-rich compounds in cells and the establishment of the resting potential at the plasma membrane.
– It allows energetically unfavourable transport of ions and cell-to-cell communication.
– All cells in animal body tissues maintain a voltage difference across their plasma membrane, known as the membrane potential.
– The electrical polarisation in cells is a result of the interplay between ion transporters and ion channels.
– Neurons have different types of ion channels in different parts of the cell, giving them different electrical properties.
Dielectric Dispersion and Relaxation
– Dielectric dispersion is the dependence of the permittivity of a dielectric material on the frequency of an applied electric field.
– It is a frequency-dependent response of a medium for wave propagation.
– Different types of polarisation lose their response to the electric field at different frequency ranges.
– At higher frequencies, dipolar polarisation, ionic polarisation, and molecular distortion polarisation lose their response.
– In the frequency region above ultraviolet, permittivity approaches a constant value.
– Dielectric relaxation is the momentary delay in the dielectric constant of a material.
– It is caused by the delay in molecular polarisation with respect to a changing electric field.
– Dielectric relaxation is often described in terms of permittivity as a function of frequency.
– The relaxation process depends on the structure, composition, and surroundings of the sample.
– The Debye equation is commonly used to describe dielectric relaxation.
– Variants of the Debye Equation include the Cole-Cole equation, Cole-Davidson equation, Havriliak-Negami relaxation, Kohlrausch-Williams-Watts function, and Curie-von Schweidler law.
Applications of Dielectrics
– Capacitors: Dielectric materials in capacitors prevent direct electrical contact between conducting plates, increase capacitance, and allow for higher stored charge at a given voltage.
– Dielectric resonator: Used in electronic circuits for frequency reference, often in microwave applications. Dielectric resonators can also function as dielectric resonator antennas (DRA).
– BST thin films: Barium strontium titanate (BST) thin films studied for fabrication of microwave components, such as voltage-controlled oscillators, tunable filters, and phase shifters. Improved tunability and dielectric properties observed with magnesium-doped BST films.
– Practical dielectrics: Solid dielectrics like porcelain, glass, and plastics commonly used in electrical engineering. Gaseous dielectrics include air, nitrogen, and sulfur hexafluoride. Dielectric fluids like mineral oil used in transformers and high voltage capacitors.
– Electrets: Specially processed dielectrics that retain excess internal charge or polarization, similar to magnets. Electrets have various practical applications in homes and industries. Piezoelectric materials and ferroelectric materials are also useful dielectrics.
Capacitors
– Charge separation in capacitors causes an internal electric field.
– Commercial capacitors use solid dielectric materials with high permittivity.
– Dielectric materials prevent direct electrical contact between conducting plates.
– High permittivity allows for greater stored charge at a given voltage.
– Dielectric materials for capacitors are chosen to resist ionization and operate at higher voltages.
Note: The content has been organized into 5 comprehensive groups, combining identical concepts. Source: https://en.wikipedia.org/wiki/Dielectric
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In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor, because they have no loosely bound, or free, electrons that may drift through the material, but instead they shift, only slightly, from their average equilibrium positions, causing dielectric polarisation. Because of dielectric polarisation, positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field (for example, if the field is oriented parallel to the positive x axis, the negative charges will shift in the negative x direction). This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarised, but also reorient so that their symmetry axes align to the field.
The study of dielectric properties concerns storage and dissipation of electric and magnetic energy in materials. Dielectrics are important for explaining various phenomena in electronics, optics, solid-state physics and cell biophysics.
