Dielectric
[1] The study of dielectric properties concerns storage and dissipation of electric and magnetic energy in materials.
[2][3][4] Dielectrics are important for explaining various phenomena in electronics, optics, solid-state physics and cell biophysics.
[5][6] Although the term insulator implies low electrical conduction, dielectric typically means materials with a high polarisability.
Insulator is generally used to indicate electrical obstruction while dielectric is used to indicate the energy storing capacity of the material (by means of polarisation).
A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor.
[1] The term dielectric was coined by William Whewell (from dia + electric) in response to a request from Michael Faraday.
This, in turn, determines the electric permittivity of the material and thus influences many other phenomena in that medium, from the capacitance of capacitors to the speed of light.
It is more convenient in a linear system to take the Fourier transform and write this relationship as a function of frequency.
In the presence of an electric field, the charge cloud is distorted, as shown in the top right of the figure.
Important questions are: The relationship between the electric field E and the dipole moment M gives rise to the behaviour of the dielectric, which, for a given material, can be characterised by the function F defined by the equation:
Because the rotation is not instantaneous, dipolar polarisations 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 1011 Hz (in the microwave region).
The molecular vibration frequency is roughly the inverse of the time it takes for the molecules to bend, and this distortion polarisation disappears above the infrared.
Ionic polarisation enables the production of energy-rich compounds in cells (the proton pump in mitochondria) and, at the plasma membrane, the establishment of the resting potential, energetically unfavourable transport of ions, and cell-to-cell communication (the Na+/K+-ATPase).
This is one instance of a general phenomenon known as material dispersion: a frequency-dependent response of a medium for wave propagation.
This is usually caused by the delay in molecular polarisation with respect to a changing electric field in a dielectric medium (e.g., inside capacitors or between two large conducting surfaces).
The time lag between electrical field and polarisation implies an irreversible degradation of Gibbs free energy.
On the other hand, the distortion related to ionic and electronic polarisation shows behaviour of the resonance or oscillator type.
It is usually expressed in the complex permittivity ε of a medium as a function of the field's angular frequency ω:
in the denominator due to an ongoing sign convention ambiguity whereby many sources represent the time dependence of the complex electric field with
Paraelectricity is the nominal behaviour of dielectrics when the dielectric permittivity tensor is proportional to the unit matrix, i.e., an applied electric field causes polarisation and/or alignment of dipoles only parallel to the applied electric field.
Tunable dielectrics are insulators whose ability to store electrical charge changes when a voltage is applied.
The two have mismatched crystal spacing that produces strain within the strontium titanate layer that makes it less stable and tunable.
Commercially manufactured capacitors typically use a solid dielectric material with high permittivity as the intervening medium between the stored positive and negative charges.
[18] 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.
This can be seen by treating the case of a linear dielectric with permittivity ε and thickness d between two conducting plates with uniform charge density σε.
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.
[19] 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.
[20] This work improved tunability of bulk barium strontium titanate, which is a thin film enabler for electronics components.
[22] Researchers "doped" BST thin films with magnesium, analyzing the "structure, microstructure, surface morphology and film/substrate compositional quality" of the result.
