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{{Short description|Electrically insulating substance able to be polarised by an applied electric field}}
{{Distinguish|dielectric constant|dialectic}}
{{More citations needed|date=December 2022}}
[[File:Capacitor schematic with dielectric.svg|thumb|A polarised dielectric material]]
{{Electromagnetism}}

In [[electromagnetism]], a '''dielectric''' (or '''dielectric medium''') is an [[Insulator (electricity)|electrical insulator]] that can be [[Polarisability|polarised]] by an applied [[electric field]]. When a dielectric material is placed in an electric field, [[electric charge]]s 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 [[Polarisation density|dielectric polarisation]], positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly [[Chemical bond|bonded]] molecules, those molecules not only become polarised, but also reorient so that their [[Symmetry axis|symmetry axes]] align to the field.<ref name=britannica>{{cite encyclopedia|title=Dielectric|encyclopedia=[[Encyclopædia Britannica]]|publisher=[[Encyclopædia Britannica, Inc.]]|location=[[Chicago]], [[Illinois]]|url=https://global.britannica.com/science/dielectric|quote=Dielectric, insulating material or a very poor conductor of electric current. When dielectrics are placed in an electric field, practically no current flows in them.|access-date=20 November 2021|archive-date=27 April 2021|archive-url=https://web.archive.org/web/20210427153917/https://global.britannica.com/science/dielectric|url-status=dead}}</ref>

The study of dielectric properties concerns storage and dissipation of electric and [[magnetic energy]] in materials.<ref>[[Arthur R. von Hippel]], in his seminal work, [https://archive.org/details/dielectricmateri00vonh_0/page/n13/mode/2up  ''Dielectric Materials and Applications''], stated: "''Dielectrics''... are not a narrow class of so-called insulators, but the broad expanse of ''nonmetals'' considered from the standpoint of their interaction with electric, magnetic or electromagnetic fields. Thus we are concerned with gases as well as with liquids and solids and with the storage of electric and magnetic energy as well as its dissipation." (p. 1) (Technology Press of MIT and John Wiley, NY, 1954).</ref><ref>{{cite journal|last1=Thoms|first1=E.|last2=Sippel|first2=P.|last3=et.|first3=al.|title=Dielectric study on mixtures of ionic liquids|journal=Sci. Rep.|volume=7|issue=1|pages=7463|date=2017|doi=10.1038/s41598-017-07982-3|pmid=28785071|pmc=5547043|arxiv=1703.05625|bibcode=2017NatSR...7.7463T}}</ref><ref>{{cite journal|last1=Belkin|first1=A.|last2=Bezryadin|first2=A.|last3=Hendren|first3=L.|last4=Hubler|first4=A.|title=Recovery of Alumina Nanocapacitors after High and Low Voltage Breakdown|journal=Sci. Rep.|volume=7|issue=1|pages=932|date=2017|doi=10.1038/s41598-017-01007-9|pmid=28428625|bibcode=2017NatSR...7..932B|pmc=5430567}}</ref> Dielectrics are important for explaining various phenomena in [[electronics]], [[optics]], [[solid-state physics]] and [[cell biophysics]].<ref>{{cite journal|last=Hossain|first=Shadeeb|date=2020-12-27|title=Malignant cell characterisation via mathematical analysis of bio impedance and optical properties|url=https://doi.org/10.1080/15368378.2020.1850471|journal=Electromagnetic Biology and Medicine|volume=40|issue=1|pages=65–83|doi=10.1080/15368378.2020.1850471|issn=1536-8378|pmid=33356700|s2cid=229694503}}</ref><ref>{{cite journal|last=Hossain|first=Shadeeb|date=2020-04-02|title=Biodielectric phenomenon for actively differentiating malignant and normal cells: An overview|url=https://doi.org/10.1080/15368378.2020.1737804|journal=Electromagnetic Biology and Medicine|volume=39|issue=2|pages=89–96|doi=10.1080/15368378.2020.1737804|issn=1536-8378|pmid=32138569|s2cid=212565141}}</ref>

==Terminology==
Although the term ''[[Insulator (electricity)|insulator]]'' implies low [[Electrical resistivity and conductivity|electrical conduction]], ''dielectric'' typically means materials with a high [[polarisability]]. The latter is expressed by a number called the [[relative permittivity]]. ''Insulator'' is generally used to indicate electrical obstruction while ''dielectric'' is used to indicate the [[Energy storage|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]]. The polarisation of the dielectric by the applied electric field increases the capacitor's surface charge for the given electric field strength.<ref name=britannica/>

The term ''[[:wikt:dielectric|dielectric]]'' was coined by [[William Whewell]] (from ''[[wiktionary:dia-|dia]]'' + ''electric'') in response to a request from [[Michael Faraday]].<ref>{{cite book|author=Daintith, J.|title=Biographical Encyclopedia of Scientists|publisher=CRC Press|year=1994|isbn=978-0-7503-0287-6|page=943}}</ref><ref>James, Frank A.J.L., editor. The Correspondence of Michael Faraday, Volume 3, 1841–1848, {{cite web|url=http://hermital.org/book/holoprt5-1.htm#F5.8|title=Letter 1798, William Whewell to Faraday, p. 442.|access-date=2012-05-18|archive-url=https://web.archive.org/web/20161223121046/http://hermital.org/book/holoprt5-1.htm#F5.8|archive-date=2016-12-23|url-status=dead}} The Institution of Electrical Engineers, London, United Kingdom, 1996. {{ISBN|0-86341-250-5}}</ref> A ''perfect dielectric'' is a material with zero electrical conductivity ([[cf.]] [[perfect conductor]] infinite electrical conductivity),<ref>{{cite book|url={{Google books|plainurl=yes|id=ZecSEXlJE0YC|page=21}}|title=Microwave Engineering – R. S. Rao (Prof.)|access-date=2013-11-08}}</ref> thus exhibiting only a [[Displacement current#History and interpretation|displacement current]]; therefore it stores and returns electrical energy as if it were an ideal capacitor.

==Electric susceptibility==
{{Main article|Electric susceptibility|Permittivity}}

The [[electric susceptibility]] <math>\chi_e</math> of a dielectric material is a measure of how easily it [[polarisation density|polarises]] in response to an electric field. This, in turn, determines the electric [[permittivity]] of the material and thus influences many other phenomena in that medium, from the capacitance of [[capacitor]]s to the [[speed of light]].

It is defined as the constant of proportionality (which may be a [[tensor]]) relating an electric field <math>\mathbf{E}</math> to the induced dielectric polarisation density <math>\mathbf{P}</math> such that

<math display="block">\mathbf{P} = \varepsilon_0 \chi_e \mathbf{E},</math>

where <math>\varepsilon_0</math> is the [[Vacuum permittivity|electric permittivity of free space]].

The susceptibility of a medium is related to its relative permittivity <math>\varepsilon_r</math> by

<math display="block">\chi_e\ = \varepsilon_r - 1.</math>

So in the case of a [[Vacuum#Electromagnetism|classical vacuum]],

<math display="block">\chi_e\ = 0.</math>

The [[electric displacement]] <math>\mathbf{D}</math> is related to the polarisation density <math>\mathbf{P}</math> by

<math display="block">\mathbf{D} \ = \ 
  \varepsilon_0 \mathbf{E} + \mathbf{P} \ = \ 
  \varepsilon_0 \left(1 + \chi_e\right) \mathbf{E} \ = \ 
  \varepsilon_0 \varepsilon_r \mathbf{E}.
</math>

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

<math display="block">\mathbf{P}(t) = \varepsilon_0 \int_{-\infty}^t \chi_e\left(t - t'\right) \mathbf{E}(t')\, dt'.</math>

That is, the polarisation is a [[convolution]] of the electric field at previous times with time-dependent susceptibility given by <math>\chi_e (\Delta t)</math>. The upper limit of this integral can be extended to infinity as well if one defines <math>\chi_e (\Delta t) = 0</math> for <math>\Delta t < 0</math>. An instantaneous response corresponds to [[Dirac delta function]] susceptibility <math>\chi_e (\Delta t) = \chi_e \delta (\Delta t)</math> .

It is more convenient in a linear system to take the [[continuous Fourier transform|Fourier transform]] and write this relationship as a function of frequency. Due to the [[convolution theorem]], the integral becomes a simple product,
<math display="block">\mathbf{P}(\omega) = \varepsilon_0 \chi_e(\omega) \mathbf{E}(\omega).</math>

The susceptibility (or equivalently the permittivity) is frequency dependent. The change of susceptibility with respect to frequency characterises the [[dispersion (optics)|dispersion]] properties of the material.

Moreover, the fact that the polarisation can only depend on the electric field at previous times (i.e., <math>\chi_e (\Delta t) = 0</math> for <math>\Delta t < 0</math>), a consequence of [[causality]], imposes [[Kramers–Kronig relation|Kramers–Kronig constraints]] on the real and imaginary parts of the susceptibility <math>\chi_e (\omega)</math>.

==Dielectric polarisation==

===Basic atomic model===
[[Image:dielectric model.svg|right|thumb|400px|Electric field interaction with an atom under the classical dielectric model]]

In the classical approach to the dielectric, the 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 center. 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 [[dipole]] using the [[superposition principle]]. A dipole is characterised by its [[electrical dipole moment|dipole moment]], 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 behaviour of the dielectric. (Note that the dipole moment points in the same direction as the electric field in the figure. This is not 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 called [[Relaxation (physics)|relaxation]] time; an exponential decay.

This is the essence of the model in physics. The behaviour of the dielectric now depends on the situation. The more complicated the situation, the richer the model must be to accurately describe the behaviour. 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 ([[isotropy]] of the material)?
*Is the response the same everywhere ([[Homogeneity (physics)|homogeneity]] of the material)?
*Do any boundaries or interfaces have to be taken into account?
*Is the response [[Linear system|linear]] with respect to the field, or are there [[Nonlinear system|nonlinearities]]?

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:
<math display="block">\mathbf{M} = \mathbf{F}(\mathbf{E}).</math>

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 modelled include:

*[[Refractive index]]
*[[Group velocity dispersion]]
*[[Birefringence]]
*[[Self-focusing]]
*[[Harmonic generation]]

===Dipolar polarisation===
Dipolar polarisation is a polarisation that is either inherent to [[polar molecule]]s (orientation polarisation), or can be induced in any molecule in which the asymmetric distortion of the nuclei is possible (distortion polarisation). Orientation polarisation 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 polarisation in the absence of an external electric field. The assembly of these dipoles forms a macroscopic polarisation.

When an external electric field is applied, the distance between charges within each permanent dipole, which is related to [[chemical bond]]ing, remains constant in orientation polarisation; however, the direction of polarisation 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 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 10<sup>11</sup> Hz (in the microwave region). The delay of the response to the change of the electric field causes [[friction]] and heat.

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 polarisation disappears above the infrared.

===Ionic polarisation===
Ionic polarisation is polarisation caused by relative displacements between positive and negative [[ion]]s in [[ionic crystal]]s (for example, [[Sodium chloride|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 do not correspond, polarisation arises in molecules or crystals. This polarisation is called '''ionic polarisation'''.

Ionic polarisation causes the [[ferroelectric effect]] as well as dipolar polarisation. 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 polarisations in crystals is called a '''displacive phase transition'''.

====In cells====
Ionic polarisation enables the production of energy-rich compounds in cells (the [[proton pump]] in [[mitochondrion|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]]).

All cells in animal body tissues are electrically polarised – in other words, they maintain a voltage difference across the cell's [[plasma membrane]], known as the [[membrane potential]]. This electrical polarisation results from a complex interplay between [[Ion transporter|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 [[dendrite]]s, [[axon]], and [[soma (biology)|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.

==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 polarisation and changes in the electric field, the permittivity of the dielectric is a complex function of the frequency of the electric field. Dielectric dispersion is very important for the applications of dielectric materials and the analysis of polarisation systems.

This is one instance of a general phenomenon known as [[material dispersion]]: a frequency-dependent response of a medium for wave propagation.

When the frequency becomes higher:
# The dipolar polarisation can no longer follow the oscillations of the electric field in the [[microwave]] region around 10<sup>10</sup> [[Hertz|Hz]],
# The ionic polarisation and molecular distortion polarisation can no longer track the electric field past the [[infrared]] or far-infrared region around 10<sup>13</sup> Hz,
# The electronic polarisation loses its response in the ultraviolet region around 10<sup>15</sup> Hz.

In the frequency region above ultraviolet, permittivity approaches the constant ''ε''<sub>0</sub> in every substance, where ''ε''<sub>0</sub> is the permittivity of the free space. Because permittivity indicates the strength of the relation between an electric field and polarisation, if a polarisation process loses its response, permittivity decreases.

==Dielectric relaxation==
'''Dielectric relaxation''' is the momentary delay (or lag) in the [[dielectric constant]] of a material. 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 [[Electrical conductor|conducting]] surfaces). Dielectric relaxation in changing electric fields could be considered analogous to [[hysteresis]] in changing [[magnetic field]]s (e.g., in [[inductor]] or [[transformer]] [[Magnetic_core#Core_loss|cores]]). 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 polarisation 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 polarisation shows behaviour of the [[resonance]] or [[oscillator]] 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 ''ε'' of a medium as a function of the field's [[angular frequency]] ''ω'':

<math display="block">\hat{\varepsilon}(\omega) = \varepsilon_{\infty} + \frac{\Delta\varepsilon}{1 + i\omega\tau},</math>

where ''ε<sub>∞</sub>'' is the permittivity at the high frequency limit, {{nowrap|Δ''ε'' {{=}} ''ε<sub>s</sub>'' − ''ε<sub>∞</sub>''}} where ''ε<sub>s</sub>'' is the static, low frequency permittivity, and ''τ'' is the characteristic [[relaxation time]] of the medium. Separating into the real part <math>\varepsilon'</math> and the imaginary part <math>\varepsilon''</math> of the complex dielectric permittivity yields:<ref>{{cite book|title=Dielectric Phenomena in Solids|last=Kao|first=Kwan Chi|publisher=Elsevier Academic Press|year=2004|isbn=978-0-12-396561-5|location=London|pages=92–93}}</ref>

<math display="block">\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}</math>

Note that the above equation for <math>\hat{\varepsilon}(\omega)</math>is sometimes written with <math>1 - i\omega\tau</math> in the denominator due to an ongoing sign convention ambiguity whereby many sources represent the time dependence of the complex electric field with <math>\exp(-i\omega t)</math> whereas others use <math>\exp(+i\omega t)</math>. In the former convention, the functions <math>\varepsilon'</math> and <math>\varepsilon''</math> representing real and imaginary parts are given by <math>\hat{\varepsilon}(\omega)=\varepsilon'+ i \varepsilon''</math> whereas in the latter convention <math>\hat{\varepsilon}(\omega)=\varepsilon'- i \varepsilon''</math>. The above equation uses the latter convention.<ref>{{cite book|title=Theory of Electric Polarisation|last=Böttcher|first=C.J.F.|publisher=Elsevier Publishing Companys|year=1952|location=London|pages=231-232,348-349}}</ref> 

The dielectric loss is also represented by the loss tangent:

<math display="block">\tan(\delta) = \frac{\varepsilon''}{\varepsilon'} = \frac{\left(\varepsilon_s - \varepsilon_\infty\right)\omega\tau}{\varepsilon_s + \varepsilon_\infty \omega^2 \tau^2}</math>

This relaxation model was introduced by and named after the physicist [[Peter Debye]] (1913).<ref>Debye, P. (1913), Ver. Deut. Phys. Gesell. 15, 777; reprinted 1954 in collected papers of Peter J.W. Debye. Interscience, New York</ref> It is characteristic for dynamic polarisation with only one relaxation time.

===Variants of the Debye equation===
;[[Cole–Cole equation]]: This equation is used when the dielectric loss peak shows symmetric broadening.
;[[Cole–Davidson equation]]: This equation is used when the dielectric loss peak shows asymmetric broadening.
;[[Havriliak–Negami relaxation]]: This equation considers both symmetric and asymmetric broadening.
;[[Kohlrausch–Williams–Watts function]]: Fourier transform of [[stretched exponential function]].
;[[Curie–von Schweidler law]]: 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.
;[[Djordjevic–Sarkar approximation]]: This is used when the dielectric loss is approximately constant for a wide range of frequencies.

==Paraelectricity==
{{See also|Ferroelectricity}}

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. Contrary to the analogy with a paramagnetic material, no permanent [[electric dipole]] needs to exist in a paraelectric material. Removal of the fields results in the dipolar polarisation returning to zero.<ref>{{cite book|last=Chiang|first=Y.|year=1997|title=Physical Ceramics|publisher=[[John Wiley & Sons]]|location=New York}}</ref> The mechanisms that causes '''paraelectric''' behaviour are distortion of individual [[ions]] (displacement of the electron cloud from the nucleus) and polarisation of molecules or combinations of ions or defects.

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.

Most dielectric materials are paraelectrics. A specific example of a paraelectric material of high dielectric constant is [[strontium titanate]].

The [[lithium niobate|LiNbO<SUB>3</SUB>]] crystal is [[ferroelectric]] below 1430 [[Kelvin|K]], and above this temperature it transforms into a disordered paraelectric phase. Similarly, other [[perovskite]]s also exhibit paraelectricity at high temperatures.

Paraelectricity has been explored as a possible refrigeration mechanism; polarising a paraelectric by applying an electric field under [[adiabatic|adiabatic process]] conditions raises the temperature, while removing the field lowers the temperature.<ref>{{cite journal|last1=Kuhn|first1=U.|last2=Lüty|first2=F.|doi=10.1016/0038-1098(65)90060-8|title=Paraelectric heating and cooling with OH—dipoles in alkali halides|journal=Solid State Communications|volume=3|issue=2|page=31|year=1965|bibcode=1965SSCom...3...31K}}</ref> A [[heat pump]] that operates by polarising 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 depolarising it, would result in refrigeration.

==Tunability==
''Tunable dielectrics'' are insulators whose ability to store electrical charge changes when a voltage is applied.<ref name=k>{{cite journal|last1=Lee|first1=Che-Hui|last2=Orloff|first2=Nathan D.|last3=Birol|first3=Turan|last4=Zhu|first4=Ye|last5=Goian|first5=Veronica|last6=Rocas|first6=Eduard|last7=Haislmaier|first7=Ryan|last8=Vlahos|first8=Eftihia|last9=Mundy|first9=Julia A.|last10=Kourkoutis|first10=Lena F.|last11=Nie|first11=Yuefeng|last12=Biegalski|first12=Michael D.|last13=Zhang|first13=Jingshu|last14=Bernhagen|first14=Margitta|last15=Benedek|first15=Nicole A.|last16=Kim|first16=Yongsam|last17=Brock|first17=Joel D.|last18=Uecker|first18=Reinhard|last19=Xi|first19=X. X.|last20=Gopalan|first20=Venkatraman|last21=Nuzhnyy|first21=Dmitry|last22=Kamba|first22=Stanislav|last23=Muller|first23=David A.|last24=Takeuchi|first24=Ichiro|last25=Booth|first25=James C.|last26=Fennie|first26=Craig J.|last27=Schlom|first27=Darrell G.|title=Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics|journal=Nature|year=2013|volume=502|issue=7472|pages=532–536|doi=10.1038/nature12582|pmid=24132232|bibcode=2013Natur.502..532L|hdl=2117/21213|s2cid=4457286}}</ref>

Generally, [[strontium titanate]] ({{chem|Sr|Ti|O|3}}) is used for devices operating at low temperatures, while [[barium strontium titanate]] ({{chem|Ba|1−x|Sr|x|Ti|O|3}}) substitutes for room temperature devices. Other potential materials include microwave dielectrics and carbon nanotube (CNT) composites.<ref name=k/><ref>{{cite journal|last1=Kong|first1=L. B.|last2=Li|first2=S.|last3=Zhang |first3=T. S.|last4=Zhai|first4=J. W.|last5=Boey|first5=F. Y. C.|last6=Ma|first6=J.|title=Electrically tunable dielectric materials and strategies to improve their performances|journal=Progress in Materials Science|date=2010-11-30|volume=55|issue=8|pages=840–893|doi=10.1016/j.pmatsci.2010.04.004|hdl=10356/93905|hdl-access=free}}</ref><ref>{{cite book|last1=Giere|first1=A.|last2=Zheng|first2=Y.|last3=Maune|first3=H.|last4=Sazegar|first4=M.|last5=Paul|first5=F.|last6=Zhou|first6=X.|last7=Binder|first7=J. R.|last8=Muller|first8=S.|last9=Jakoby|first9=R.|chapter=Tunable dielectrics for microwave applications|title=2008 17th IEEE International Symposium on the Applications of Ferroelectrics|year=2008|pages=1|doi=10.1109/ISAF.2008.4693753|s2cid=15835472|isbn=978-1-4244-2744-4}}</ref>

In 2013, multi-sheet layers of strontium titanate interleaved with single layers of [[strontium oxide]] produced a dielectric capable of operating at up to 125 GHz. 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.<ref name=k/>

Systems such as {{chem|Ba|1−x|Sr|x|Ti|O|3}} have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability. Films suffer significant losses arising from defects.

==Applications==

===Capacitors===
{{Main|Capacitor}}
[[Image:Capacitor schematic with dielectric.svg|thumb|upright|Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric (orange) reduces the field and increases the capacitance.]]

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''.<ref>Müssig, Hans-Joachim. ''Semiconductor capacitor with praseodymium oxide as dielectric'', {{US Patent|7113388}} published 2003-11-06, issued 2004-10-18, assigned to IHP GmbH- Innovations for High Performance Microelectronics/Institute Fur Innovative Mikroelektronik</ref>

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 ''ε'' and thickness ''d'' between two conducting plates with uniform charge density ''σ<sub>ε</sub>''. In this case the charge density is given by

<math display="block">\sigma_{\varepsilon}=\varepsilon\frac{V}{d}</math>

and the [[capacitance]] per unit area by

<math display="block">c=\frac{\sigma_{\varepsilon}}{V}=\frac{\varepsilon}{d}</math>

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

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

===Dielectric resonator===
{{Main|Dielectric resonator}}

A ''dielectric resonator oscillator'' (DRO) is an electronic component that exhibits [[resonance]] of the polarisation 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|dielectric resonator antenna]] (DRA).

===BST thin films===
From 2002 to 2004, the United States [[Army Research Laboratory]] (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.<ref name=Cole>{{cite journal|title=Novel tunable acceptor doped BST thin films for high quality tunable microwave devices|journal=Revista Mexicana de Fisica|volume=50|bibcode=2004RMxF...50..232C|last1=Cole|first1=M. W.|last2=Geyer|first2=R. G.|year=2004|issue=3|page=232}}</ref>

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.<ref>{{cite book|url=https://books.google.com/books?id=XzZLtRlUPNoC&q=tunable+microwave+devices+army+research+laboratory&pg=PA57|title=Developments in Dielectric Materials and Electronic Devices: Proceedings of the 106th Annual Meeting of The American Ceramic Society, Indianapolis, Indiana, USA 2004|last1=Nair|first1=K. M.|last2=Guo|first2=Ruyan|last3=Bhalla|first3=Amar S.|last4=Hirano|first4=S.-I.|last5=Suvorov|first5=D.|date=2012-04-11|publisher=John Wiley & Sons|isbn=9781118408193|language=en}}</ref> This work improved tunability of bulk barium strontium titanate, which is a thin film enabler for electronics components.<ref>{{cite book|url=https://books.google.com/books?id=gt_4CiNliKEC&q=tunable+microwave+devices+army+research+laboratory&pg=PA229|title=Ceramic Materials and Multilayer Electronic Devices|last1=Nair|first1=K. M.|last2=Bhalla|first2=Amar S.|last3=Hirano|first3=S.-I.|last4=Suvorov|first4=D.|last5=Schwartz|first5=Robert W.|last6=Zhu|first6=Wei|date=2012-04-11|publisher=John Wiley & Sons|isbn=9781118406762|language=en}}</ref>

In a 2004 research paper, U.S. ARL researchers explored how small concentrations of acceptor dopants can dramatically modify the properties of ferroelectric materials such as BST.<ref>{{cite journal|last1=Cole|first1=M. W.|last2=Hubbard|first2=C.|last3=Ngo|first3=E.|last4=Ervin|first4=M.|last5=Wood|first5=M.|last6=Geyer|first6=R. G.|date=July 2002|title=Structure–property relationships in pure and acceptor-doped Ba1−xSrxTiO3 thin films for tunable microwave device applications|journal=Journal of Applied Physics|language=en|volume=92|issue=1|pages=475–483|doi=10.1063/1.1484231|issn=0021-8979|bibcode=2002JAP....92..475C}}</ref>

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.<ref name=Cole/>

==Some practical dielectrics==
Dielectric materials can be solids, liquids, or gases. (A high [[vacuum]] can also be a useful,<ref>{{cite journal|last1=Lyon|first1=David|title=Gap size dependence of the dielectric strength in nano vacuum gaps|journal=[[IEEE Transactions on Dielectrics and Electrical Insulation]]|date=2013|volume=20|issue=4|pages=1467–1471|doi=10.1109/TDEI.2013.6571470|s2cid=709782}}</ref> 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. Some examples include [[porcelain]], [[glass]], and most [[plastic]]s. Air, [[nitrogen]] and [[sulfur hexafluoride]] are the three most commonly used [[gaseous dielectric]]s.
*[[Industrial coating]]s such as [[Parylene]] provide a dielectric barrier between the substrate and its environment.
*[[Mineral oil]] is used extensively inside electrical [[transformer]]s as a fluid dielectric and to assist in cooling. Dielectric fluids with higher dielectric constants, such as electrical grade [[castor oil]], are often used in [[high voltage]] capacitors to help prevent [[corona discharge]] 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 [[triboelectric effect]]). This can be useful, as in a [[Van de Graaff generator]] or [[electrophorus]], or it can be potentially destructive as in the case of [[electrostatic discharge]].
*Specially processed dielectrics, called [[electret]]s (which should not be confused with [[ferroelectric]]s), may retain excess internal charge or "frozen in" polarisation. Electrets have a semi-permanent 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 [[Stress (physics)|stress]], or (equivalently) change physical shape if an external voltage is applied across the material. This property is called [[piezoelectricity]]. Piezoelectric materials are another class of very useful dielectrics.
*Some ionic [[crystal]]s and [[polymer]] dielectrics exhibit a spontaneous dipole moment, which can be reversed by an externally applied electric field. This behaviour is called the [[Ferroelectricity|ferroelectric effect]]. These materials are analogous to the way [[ferromagnetic materials]] behave within an externally applied magnetic field. Ferroelectric materials often have very high dielectric constants, making them quite useful for capacitors.

==See also==
{{div col|colwidth=22em}}
*[[Permittivity#Classification of materials|Classification of materials based on permittivity]]
*[[Paramagnetism]]
*[[Clausius-Mossotti relation]]
*[[Dielectric absorption]]
*[[Dielectric losses]]
*[[Dielectric strength]]
*[[Dielectric spectroscopy]]
*[[EIA Class 1 dielectric]]
*[[EIA Class 2 dielectric]]
*[[High-κ dielectric]]
*[[Low-κ dielectric]]
*[[Leakage (electronics)|Leakage]]
*[[Linear response function]]
*[[Metamaterial]]
*[[RC delay]]
*[[Rotational Brownian motion]]
*[[Paschen's law]] – variation of dielectric strength of gas related to pressure
*[[Separator (electricity)]]
{{div col end}}

==References==
{{Reflist|30em}}

==Further reading==
*{{cite book |last=Jackson|first=John David|author-link=John David Jackson (physicist)|date=10 August 1998|orig-year=1962|title=Classical Electrodynamics|publisher=[[John Wiley & Sons]]|edition=3rd|url={{google books|plainurl=yes|id=FOBBEAAAQBAJ}}|oclc=535998|isbn=978-0-471-30932-1}}

*{{cite book |last=Scaife|first=Brendan K. P.|author-link=Brendan Scaife|title=Principles of Dielectrics|series=Monographs on the Physics & Chemistry of Materials|publisher=[[Oxford University Press]]|edition=2nd|date=3 September 1998|url={{google books|plainurl=yes|id=9GBP9uBwpScC}}|isbn=978-0-198-56557-4}}

==External links==
*[https://feynmanlectures.caltech.edu/II_10.html Feynman's lecture on dielectrics]
*[https://web.archive.org/web/20070116093447/http://wiki.4hv.org/index.php/Dielectric_Sphere_in_Electric_Field Dielectric Sphere in an Electric Field]
*[https://www.doitpoms.ac.uk/tlplib/dielectrics/index.php Dissemination of IT for the Promotion of Materials Science (DoITPoMS) Teaching and Learning Package "Dielectric Materials"] from the [[University of Cambridge]]
*{{Wikisource-inline|list=
**{{Cite Americana|short=1|wstitle=Dielectric|noicon=x}}
**{{Cite EB1911|short=1|wstitle=Dielectric|noicon=x}}
}}

{{Polarization states}}
{{Authority control}}

[[Category:Dielectrics| ]]
[[Category:Electric and magnetic fields in matter]]