Dielectric

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A dielectric (or dielectric material) is an that can be polarized by an applied. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in an but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric, positive charges are displaced in the direction of the field and negative charges shift in the opposite 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 polarized, but also reorient so that their 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, , , and.

Terminology
Although the term  implies low, dielectric typically means materials with a high. The latter is expressed by a number called the. The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the storing capacity of the material (by means of polarization). A common example of a dielectric is the electrically insulating material between the metallic plates of a. The polarization of the dielectric by the applied electric field increases the capacitor's surface charge for the given electric field strength.

The term  was coined by (from - + electric) in response to a request from. A perfect dielectric is a material with zero electrical conductivity ( infinite electrical conductivity), thus exhibiting only a ; therefore it stores and returns electrical energy as if it were an ideal capacitor.

Electric susceptibility
The ?e of a dielectric material is a measure of how easily it  in response to an electric field. This, in turn, determines the electric of the material and thus influences many other phenomena in that medium, from the capacitance of s to the.

It is defined as the constant of proportionality (which may be a ) relating an electric field E to the induced dielectric P such that


 * $$\mathbf{P} = \varepsilon_0 \chi_e \mathbf{E},$$

where e0 is the.

The susceptibility of a medium is related to its relative permittivity er by


 * $$\chi_e\ = \varepsilon_r - 1.$$

So in the case of a vacuum,


 * $$\chi_e\ = 0.$$

The D is related to the polarization density P by


 * $$\mathbf{D} \ = \

\varepsilon_0 \mathbf{E} + \mathbf{P} \ = \ \varepsilon_0 \left(1 + \chi_e\right) \mathbf{E} \ = \ \varepsilon_0 \varepsilon_r \mathbf{E}. $$

Dispersion and causality
In general, a material cannot polarize instantaneously in response to an applied field. The more general formulation as a function of time is


 * $$\mathbf{P}(t) = \varepsilon_0 \int_{-\infty}^t \chi_e\left(t - t'\right) \mathbf{E}\left(t'\right)\, dt'.$$

That is, the polarization is a of the electric field at previous times with time-dependent susceptibility given by ?e(?t). The upper limit of this integral can be extended to infinity as well if one defines ?e(?t) = 0 for ?t < 0. An instantaneous response corresponds to susceptibility ?e(?t) = ?ed(?t).

It is more convenient in a linear system to take the and write this relationship as a function of frequency. Due to the, the integral becomes a simple product,
 * $$\mathbf{P}(\omega) = \varepsilon_0 \chi_e(\omega) \mathbf{E}(\omega).$$

The susceptibility (or equivalently the permittivity) is frequency dependent. The change of susceptibility with respect to frequency characterizes the properties of the material.

Moreover, the fact that the polarization can only depend on the electric field at previous times (i.e., ?e(?t) = 0 for ?t < 0), a consequence of, imposes on the real and imaginary parts of the susceptibility ?e(?).

Basic atomic model
In the classical approach to the dielectric model, a material is made up of atoms. Each atom consists of a cloud of negative charge (electrons) bound to and surrounding a positive point charge at its centre. In the presence of an electric field the charge cloud is distorted, as shown in the top right of the figure.

This can be reduced to a simple using the. A dipole is characterized by its, a vector quantity shown in the figure as the blue arrow labeled M. It is the relationship between the electric field and the dipole moment that gives rise to the behavior of the dielectric. (Note that the dipole moment points in the same direction as the electric field in the figure. This isn't always the case, and is a major simplification, but is true for many materials.)

When the electric field is removed the atom returns to its original state. The time required to do so is the so-called time; an exponential decay.

This is the essence of the model in physics. The behavior of the dielectric now depends on the situation. The more complicated the situation, the richer the model must be to accurately describe the behavior. Important questions are:
 * Is the electric field constant or does it vary with time? At what rate?
 * Does the response depend on the direction of the applied field ( of the material)?
 * Is the response the same everywhere ( of the material)?
 * Do any boundaries or interfaces have to be taken into account?
 * Is the response with respect to the field, or are there ?

The relationship between the electric field E and the dipole moment M gives rise to the behavior of the dielectric, which, for a given material, can be characterized by the function F defined by the equation:
 * $$\mathbf{M} = \mathbf{F}(\mathbf{E})$$.

When both the type of electric field and the type of material have been defined, one then chooses the simplest function F that correctly predicts the phenomena of interest. Examples of phenomena that can be so modeled include:



Dipolar polarization
Dipolar polarization is a polarization that is either inherent to s (orientation polarization), or can be induced in any molecule in which the asymmetric distortion of the nuclei is possible (distortion polarization). Orientation polarization results from a permanent dipole, e.g., that arising from the 104.45° angle between the asymmetric bonds between oxygen and hydrogen atoms in the water molecule, which retains polarization in the absence of an external electric field. 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 ing, remains constant in orientation polarization; however, the direction of polarization itself rotates. This rotation occurs on a timescale that depends on the and surrounding local  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 1011 Hz (in the microwave region). The delay of the response to the change of the electric field causes and heat.

When an external electric field is applied at 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
Ionic polarization is polarization caused by relative displacements between positive and negative s in s (for example, ).

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.

Ionic polarization causes the as well as. 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.

In cells
Ionic polarization enables the production of energy-rich compounds in cells (the in ) and, at the, the establishment of the , energetically unfavourable transport of ions, and cell-to-cell communication (the ).

All cells in animal body tissues are electrically polarized – in other words, they maintain a voltage difference across the cell's, known as the. This electrical polarization results from a complex interplay between and.

In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the s,, and 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.

Dielectric dispersion
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 : a frequency-dependent response of a medium for wave propagation.

When the frequency becomes higher:
 * 1) dipolar polarization can no longer follow the oscillations of the electric field in the  region around 1010 ;
 * 2) ionic polarization and molecular distortion polarization can no longer track the electric field past the  or far-infrared region around 1013 Hz, ;
 * 3) electronic polarization loses its response in the ultraviolet region around 1015 Hz.

In the frequency region above ultraviolet, permittivity approaches the constant e0 in every substance, where e0 is the permittivity of the free space. 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
Dielectric relaxation is the momentary delay (or lag) in the 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.g., inside capacitors or between two large surfaces). Dielectric relaxation in changing electric fields could be considered analogous to in changing s (e.g., in  or  ). Relaxation in general is a delay or lag in the response of a, 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.

In, 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, 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 or  type. The character of the distortion process depends on the structure, composition, and surroundings of the sample.

Debye relaxation
Debye relaxation is the dielectric relaxation response of an ideal, noninteracting population of dipoles to an alternating external electric field. It is usually expressed in the complex permittivity e of a medium as a function of the field's ?:


 * $$\hat{\varepsilon}(\omega) = \varepsilon_{\infty} + \frac{\Delta\varepsilon}{1 + i\omega\tau},$$

where e8 is the permittivity at the high frequency limit, ?e = es - e8 where es is the static, low frequency permittivity, and t is the characteristic of the medium. Separating the real and imaginary parts of the complex dielectric permittivity yields:


 * $$\begin{align}

\varepsilon' &= \varepsilon_\infty + \frac{\varepsilon_s - \varepsilon_\infty}{1 + \omega^2\tau^2} \\[3pt] \varepsilon'' &= \frac{(\varepsilon_s - \varepsilon_\infty)\omega\tau}{1+\omega^2\tau^2} \end{align}$$

The dielectric loss is also represented by:


 * $$\tan(\delta) = \frac{\varepsilon''}{\varepsilon'} = \frac{\left(\varepsilon_s - \varepsilon_\infty\right)\omega\tau}{\varepsilon_s + \varepsilon_\infty \omega^2 \tau^2}$$

This relaxation model was introduced by and named after the physicist (1913). It is characteristic for dynamic polarization with only one relaxation time.

Variants of the Debye equation

 * This equation is used when the dielectric loss peak shows symmetric broadening.
 * This equation is used when the dielectric loss peak shows asymmetric broadening.
 * This equation considers both symmetric and asymmetric broadening.
 * Fourier transform of.
 * This shows the response of dielectrics to an applied DC field to behave according to a power law, which can be expressed as an integral over weighted exponential functions..

Paraelectricity
Paraelectricity is the ability of many materials (specifically s) to become polarized under an applied. Unlike, this can happen even if there is no permanent that exists in the material, and removal of the fields results in the  in the material returning to zero. The mechanisms that cause paraelectric behaviour are the distortion of individual (displacement of the electron cloud from the nucleus) and polarization of molecules or combinations of ions or defects.

Paraelectricity can occur in phases where electric dipoles are unaligned and thus have the potential to align in an external  and weaken it.

An example of a paraelectric material of high dielectric constant is.

The crystal is  below 1430, and above this temperature it transforms into a disordered paraelectric phase. Similarly, other s also exhibit paraelectricity at high temperatures.

Paraelectricity has been explored as a possible refrigeration mechanism; polarizing a paraelectric by applying an electric field under conditions raises the temperature, while removing the field lowers the temperature. A heat pump that operates by polarizing the paraelectric, allowing it to return to ambient temperature (by dissipating the extra heat), bringing it into contact with the object to be cooled, and finally depolarizing it, would result in refrigeration.

Tunability
Tunable dielectrics are insulators whose ability to store electrical charge changes when a voltage is applied.

Generally,  is used for devices operating at low temperatures, while   substitutes for room temperature devices. Other potential materials include microwave dielectrics and carbon nanotube (CNT) composites.

In 2013 multi-sheet layers of strontium titanate interleaved with single layers of produced a dielectric capable of operating at up to 125 GHz. The material was created via. The two have mismatched crystal spacing that produces strain within the strontium titanate layer that makes it less stable and tunable.

Systems such as have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability. Such films suffer significant losses arising from defects.

Capacitors
Commercially manufactured capacitors typically use a 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. This can be seen by treating the case of a linear dielectric with permittivity e and thickness d between two conducting plates with uniform charge density se. In this case the charge density is given by


 * $$\sigma_{\varepsilon}=\varepsilon\frac{V}{d}$$

and the per unit area by


 * $$c=\frac{\sigma_{\varepsilon}}{V}=\frac{\varepsilon}{d}$$

From this, it can easily be seen that a larger e leads to greater charge stored and thus greater capacitance.

Dielectric materials used for capacitors are also chosen such that they are resistant to. This allows the capacitor to operate at higher voltages before the insulating dielectric ionizes and begins to allow undesirable current.

Dielectric resonator
A dielectric resonator oscillator (DRO) is an electronic component that exhibits 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. Such resonators are often used to provide a frequency reference in an oscillator circuit. An unshielded dielectric resonator can be used as a (DRA).

BST thin films
From 2002 to 2004, the (ARL) conducted research on thin film technology. 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. This work improved tunability of bulk barium strontium titanate, which is a thin film enabler for electronics components.

In a 2004 research paper, ARL researchers explored how small concentrations of acceptor dopants can dramatically modify the properties of ferroelectric materials such as BST.

Researchers "doped" BST thin films with magnesium, analyzing the "structure, microstructure, surface morphology and film/substrate compositional quality" of the result. The Mg doped BST films showed "improved dielectric properties, low leakage current, and good tunability", meriting potential for use in microwave tunable devices.

Some practical dielectrics
Dielectric materials can be solids, liquids, or gases. In addition, a high can also be a useful, nearly lossless dielectric even though its relative  is only unity.

Solid dielectrics are perhaps the most commonly used dielectrics in electrical engineering, and many solids are very good insulators. Some examples include, , and most s. Air, and  are the three most commonly used s.
 * s such as provide a dielectric barrier between the substrate and its environment.
 * is used extensively inside electrical s as a fluid dielectric and to assist in cooling. Dielectric fluids with higher dielectric constants, such as electrical grade, are often used in capacitors to help prevent  and increase capacitance.
 * Because dielectrics resist the flow of electricity, the surface of a dielectric may retain stranded excess electrical charges. This may occur accidentally when the dielectric is rubbed (the ). This can be useful, as in a or, or it can be potentially destructive as in the case of.
 * Specially processed dielectrics, called s (which should not be confused with s), may retain excess internal charge or "frozen in" polarization. Electrets have a semipermanent electric field, and are the electrostatic equivalent to magnets. Electrets have numerous practical applications in the home and industry.
 * Some dielectrics can generate a potential difference when subjected to mechanical, or (equivalently) change physical shape if an external voltage is applied across the material. This property is called . Piezoelectric materials are another class of very useful dielectrics.
 * Some ionic s and dielectrics exhibit a spontaneous dipole moment, which can be reversed by an externally applied electric field. This behavior is called the . These materials are analogous to the way  behave within an externally applied magnetic field. Ferroelectric materials often have very high dielectric constants, making them quite useful for capacitors.