View source for Dielectric heating

{{short description|Heating using radio waves}}
[[File:microwave oven flashon.jpg|thumb|300px|A [[microwave oven]] uses dielectric heating to cook food.]]
'''Dielectric heating''', also known as '''electronic heating''', '''radio frequency heating''', and '''high-frequency heating''', is the process in which a [[radio frequency]] (RF) alternating electric field, or [[radio wave]] or [[microwave]] [[electromagnetic radiation]] heats a [[dielectric]] material. At higher frequencies, this heating is caused by [[molecular dipole]] rotation within the dielectric.

== Mechanism ==
{{unreferenced section|date=June 2016}}

Molecular rotation occurs in materials containing [[polar molecule]]s having an [[electrical dipole moment]], with the consequence that they will align themselves in an [[electromagnetic field]]. If the field is oscillating, as it is in an electromagnetic wave or in a rapidly oscillating electric field, these [[molecule]]s rotate continuously by aligning with it. This is called dipole rotation, or dipolar polarisation. As the field alternates, the molecules reverse direction. Rotating molecules push, pull, and collide with other molecules (through electrical forces), distributing the energy to adjacent molecules and [[atom]]s in the material. The process of energy transfer from the source to the sample is a form of radiative heating.<!-- <ref name="homepages.ed.ac.uk">[http://homepages.ed.ac.uk/ah05/basicintro.html ] {{webarchive |url=https://web.archive.org/web/20100706231044/http://homepages.ed.ac.uk/ah05/basicintro.html |date=July 6, 2010 }}</ref> -->

[[Temperature]] is related to the average [[kinetic energy]] (energy of motion) of the atoms or molecules in a material, so agitating the molecules in this way increases the temperature of the material. Thus, dipole rotation is a mechanism by which energy in the form of electromagnetic radiation can raise the temperature of an object. There are also many other mechanisms by which this conversion occurs.<ref>{{cite book|last1=Shah|first1=Yadish|title=Thermal Energy: Sources, Recovery, and Applications|publisher=CRC Press|location=Baton Rouge, FL|url=https://books.google.com/books?id=QdArDwAAQBAJ&q=Temperature+is+related+to+the+average+kinetic+energy+(energy+of+motion)+of+the+atoms+or+molecules+in+a+material,+so+agitating+the+molecules+in+this+way+increases+the+temperature+of+the+material.+Thus,+dipole+rotation+is+a+mechanism+by+which+energy+in+the+form+of+electromagnetic+radiation+can+raise+the+temperature+of+an+object.+There+are+also+many+other+mechanisms+by+which+this+conversion+occurs.&pg=PT902|access-date=27 March 2018|isbn=9781315305936|date=2018-01-12}}</ref>

Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the [[microwave oven]] where it operates most effectively on liquid [[water]], and also, but much less so, on [[fat]]s and [[sugar]]s. This is because fats and sugar molecules are far less [[chemical polarity|polar]] than water molecules, and thus less affected by the forces generated by the alternating electromagnetic fields. Outside of cooking, the effect can be used generally to heat solids, liquids, or gases, provided they contain some electric dipoles.

Dielectric heating involves the heating of electrically insulating materials by [[dielectric loss]]. A changing electric field across the material causes energy to be dissipated as the molecules attempt to line up with the continuously changing electric field. This changing electric field may be caused by an electromagnetic wave propagating in free space (as in a microwave oven), or it may be caused by a rapidly alternating electric field inside a capacitor. In the latter case, there is no freely propagating electromagnetic wave, and the changing electric field may be seen as analogous to the electric component of an antenna [[near and far field|near field]]. In this case, although the heating is accomplished by changing the electric field inside the capacitive cavity at radio-frequency (RF) frequencies, no actual [[radio wave]]s are generated or absorbed. In this sense, the effect is the direct electrical analog of magnetic [[induction heating]], which is also near-field effect (thus not involving radio waves).{{citation needed|date=February 2013}}

Frequencies in the range of 10–100&nbsp;[[MHz]] are necessary to cause dielectric heating, although higher frequencies work equally well or better, and in some materials (especially liquids) lower frequencies also have significant heating effects, often due to more unusual mechanisms. For example, in conductive liquids such as salt water, ''ion-drag'' causes heating, as charged ions are "dragged" more slowly back and forth in the liquid under influence of the electric field, striking liquid molecules in the process and transferring kinetic energy to them, which is eventually translated into molecular vibrations and thus into thermal energy.{{citation needed|date=February 2013}}

Dielectric heating at low frequencies, as a near-field effect, requires a distance from electromagnetic radiator to absorber of less than {{sfrac|2{{pi}}}} ≈ {{sfrac|6}} of a wavelength. It is thus a contact process or near-contact process, since it usually sandwiches the material to be heated (usually a non-metal) between metal plates taking the place of the dielectric in what is effectively a very large [[capacitor]]. However, actual electrical contact is not necessary for heating a dielectric inside a capacitor, as the electric fields that form inside a capacitor subjected to a voltage do not require electrical contact of the capacitor plates with the (non-conducting) dielectric material between the plates. Because lower frequency electrical fields penetrate non-conductive materials far more deeply than do microwaves, heating pockets of water and organisms deep inside dry materials like wood, it can be used to rapidly heat and prepare many non-electrically conducting food and agricultural items, so long as they fit between the capacitor plates.{{citation needed|date=February 2013}}

At very high frequencies, the wavelength of the electromagnetic field becomes shorter than the distance between the metal walls of the heating cavity, or than the dimensions of the walls themselves. This is the case inside a [[microwave oven]]. In such cases, conventional far-field electromagnetic waves form (the cavity no longer acts as a pure capacitor, but rather as an antenna), and are absorbed to cause heating, but the dipole-rotation mechanism of heat deposition remains the same. However, microwaves are not efficient at causing the heating effects of low frequency fields that depend on slower molecular motion, such as those caused by ion-drag.{{citation needed|date=February 2013}}

== Power ==
Dielectric heating must be distinguished from [[Joule heating]] of conductive media, which is caused by induced electric currents in the media.<ref name="Pryor">{{cite web|last1=Pryor|first1=Roger|title=Modeling Dielectric Heating: A First Principles Approach|url=https://www.comsol.com/paper/download/259871/pryor_paper.pdf|publisher=Pryor Knowledge Systems, Inc.|access-date=27 March 2018}}</ref> For dielectric heating, the generated power density per volume is given by:<ref name="Pryor" /><ref>
{{cite journal |last1=Vollmer |first1=Michael |date=2004 |title=Physics of the microwave oven |url=https://iopscience.iop.org/article/10.1088/0031-9120/39/1/006/meta |publisher=IOP |journal=Physics Education |volume=39 |issue=74      |pages= 74–81|doi=10.1088/0031-9120/39/1/006 |bibcode=2004PhyEd..39...74V |s2cid=250796895 |access-date= }}
</ref>

:<math>Q = \omega \cdot \varepsilon_\mathrm{r}'' \cdot \varepsilon_0 \cdot E^2,</math>

where ''ω'' is the [[angular frequency]] of the exciting radiation, ''ε''<sub>r</sub>″ is the [[imaginary part]] of the complex relative [[permittivity]] of the absorbing material, ''ε''<sub>0</sub> is the permittivity of free space and ''E'' the [[electric field strength]].  The imaginary part of the (frequency-dependent) relative permittivity is a measure for the ability of a dielectric material to convert electromagnetic field energy into heat, also called [[dielectric loss]]. (The real part of the permittivity is the normal effect of capacitance and results in non-dissipative [[reactive power]].)

If the [[electrical conductivity|conductivity]] ''σ'' of the material is small, or the frequency is high, such that {{nowrap|''σ'' ≪ ''ωε''}} (with {{nowrap|''ε'' {{=}} ''ε''<sub>r</sub>″ · ''ε''<sub>0</sub>}}), then Joule heating is low, and dielectric heating is the dominant mechanism of loss of energy from the electromagnetic field into the medium.

== Penetration ==
Microwave frequencies penetrate conductive materials, including semi-solid substances like [[meat]] and living tissue. The penetration essentially stops where all the penetrating microwave energy has been converted to heat in the tissue. Microwave ovens used to heat food are not set to the frequency for optimal absorption by water. If they were, then the piece of food or liquid in question would absorb all microwave radiation in its outer layer, leading to a cool, unheated centre and a superheated surface. <ref name="Quora answer: Why aren't microwaves tuned to the resonant frequency of water? What would happen if they were?">{{cite web |url=https://qr.ae/pGDVOd |title=Why aren't microwaves tuned to the resonant frequency of water? What would happen if they were? |last=Slepkov |first=Aaron |year=2018}}</ref> Instead, the frequency selected allows energy to penetrate deeper into the heated food. The frequency of a household microwave oven is 2.45&nbsp;GHz, while the frequency for optimal absorbency by water is around 10&nbsp;GHz.
<ref name="homepages.ed.ac.uk">{{cite web |url=http://homepages.ed.ac.uk/ah05/basicintro.html |archive-url=https://web.archive.org/web/20100706231044/http://homepages.ed.ac.uk/ah05/basicintro.html |archive-date=July 6, 2010 |title=A Basic Introduction to Microwave Chemistry |last=Whittaker |first=Gavin |year=1997}}</ref>

== Radio-frequency heating ==

The use of high-frequency electric fields for heating dielectric materials had been proposed in the 1930s. For example, {{US patent|2147689}} (application by Bell Telephone Laboratories, dated 1937) states:<blockquote>"''This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field.''"</blockquote>This patent proposed radio frequency (RF) heating at 10 to 20 [[megahertz]] (wavelength 15 to 30 meters).<ref>{{US patent|2147689}}. [http://ip.com/pat/US2147689 Method and apparatus for heating dielectric materials - J.G. Chafee]</ref> Such wavelengths were far longer than the cavity used, and thus made use of near-field effects and not electromagnetic waves. (Commercial microwave ovens use wavelengths only 1% as long.)

In agriculture, RF dielectric heating has been widely tested and is increasingly used as a way to kill pests in certain food crops after harvest, such as walnuts still in the shell. Because RF heating can heat foods more uniformly than is the case with microwave heating, RF heating holds promise as a way to process foods quickly.<ref name="Piyasena-2003-pmid-14669879">{{citation |author=Piyasena P |title=Radio frequency heating of foods: principles, applications and related properties—a review |journal=Crit Rev Food Sci Nutr |year=2003 |volume=43 |issue=6 |pages=587–606 |pmid=14669879 |doi=10.1080/10408690390251129|s2cid=24407944 |display-authors=etal}}</ref>

In medicine, the RF heating of body tissues, called [[diathermy]], is used for muscle therapy<ref>"Diathermy", [[Collins English Dictionary]] - Complete & Unabridged 10th Edition. Retrieved August 29, 2013, from [http://dictionary.reference.com/browse/diathermy Dictionary.com website]</ref> Heating to higher temperatures, called [[hyperthermia therapy]], is used to kill cancer and tumor tissue.

RF heating is used in the wood industry to cure glues used in plywood manufacturing, fingerjointing, and furniture construction.  RF heating can also be used to speed up drying lumber.

==Microwave heating==
[[Image:Microwave tunnel closeup.jpg|thumb|In addition to heating food, microwaves are widely used for heating in many industrial processes.  An industrial microwave tunnel oven for heating plastic parts prior to extrusion.]]

Microwave heating, as distinct from RF heating, is a sub-category of dielectric heating at frequencies above 100&nbsp;MHz, where an electromagnetic wave can be launched from a small dimension emitter and guided through space to the target. Modern [[microwave oven]]s make use of electromagnetic waves with electric fields of much higher frequency and shorter wavelength than RF heaters. Typical domestic microwave ovens operate at [[ISM band|2.45&nbsp;GHz]], but [[ISM band|915&nbsp;MHz]] ovens also exist. This means that the wavelengths employed in microwave heating are 0.1&nbsp;cm to 10&nbsp;cm.<ref>{{cite web|title=The Electromagnetic Spectrum|url=http://imagine.gsfc.nasa.gov/science/toolbox/emspectrum2.html|website=NASA Goddard Space Flight Center, Astronaut's Toolbox|access-date=November 30, 2016}}</ref> This provides for highly efficient, but less penetrative, dielectric heating.{{citation needed|date=February 2013}}

Although a capacitor-like set of plates can be used at microwave frequencies, they are not necessary, since the microwaves are already present as [[near and far field|far field]] type [[electromagnetism|EM]] radiation, and their absorption does not require the same proximity to a small antenna as does RF heating. The material to be heated (a non-metal) can therefore simply be placed in the path of the waves, and heating takes place in a non-contact process which does not require capacitative conductive plates.{{citation needed|date=February 2013}}

===Microwave volumetric heating===

[[Microwave volumetric heating]] is a commercially available method of heating liquids, suspensions, or solids in a continuous flow on an industrial scale. Microwave volumetric heating has a greater penetration depth, of up to {{convert|42|mm}}, which is an even penetration through the entire volume of the flowing product. This is advantageous in commercial applications where increased shelf-life can be achieved, with increased microbial kill at temperatures {{cvt|10|-|15 |C-change}} lower than when using conventional heating systems.

Applications of microwave volumetric heating include:
* [[Pasteurization]]
* [[Flash pasteurization]]
* [[Microwave chemistry]]
* [[sterilization (microbiology)|Sterilization]]
* [[Food preservation]]
* [[Biofuel]] production

==Food application==
In drying of foods, dielectric heating is usually combined with conventional heating. It may be used to preheat the feed to a hot-air drier. By raising the temperature of the feed quickly and causing moisture to move to the surface, it can decrease the overall drying time. Dielectric heating may be applied part-way through the drying cycle, when the food enters the falling rate period. This can boost the rate of drying. If dielectric heating is applied near the end of hot-air drying it can also shorten the drying time significantly and hence increase the throughput of the drier. It is more usual to use dielectric heating in the later stages of drying. One of the major applications of RF heating is in the postbaking of biscuits. The objectives in baking biscuits are to produce a product of the right size, shape, color, and moisture content. In a conventional oven, reducing the moisture content to the desired level can take up a large part of the total baking time. The application of RF heating can shorten the baking time. The oven is set to produce biscuits of the right size, shape, and color, but the RF heating is used to remove the remaining moisture, without excessive heating of the already dry sections of the biscuit.<ref>{{Cite book|title=Food Processing Technology: Principles and Practice|last=Fellows|first=P.J.|publisher=Woodhead Publishing|year=2017|isbn=978-0-08-101907-8|location=United Kingdom|pages=826–827}}</ref> The capacity of an oven can be increased by more than 50% by the use of RF heating. Postbaking by RF heating has also been applied to breakfast cereals and cereal-based baby foods.<ref>{{cite journal |last1=Brennan |first1=J.G. |title= DRYING {{!}} Dielectric and Osmotic Drying |journal= Encyclopedia of Food Sciences and Nutrition (Second Edition) |date=2003 |pages=1938–1942 |doi=10.1016/B0-12-227055-X/00372-2 |isbn=9780122270550 }}</ref>

Food quality is maximized and better retained using electromagnetic energy than conventional heating. Conventional heating results in large disparity in temperature and longer processing times which can cause overprocessing on the food surface and impairment of the overall quality of the product.<ref>{{Cite journal|last1=Datta|first1=Ashim K.|last2=Davidson|first2=P. Michael|date=2000-11-01|title=Microwave and Radio Frequency Processing|journal=Journal of Food Science|language=en|volume=65|pages=32–41|doi=10.1111/j.1750-3841.2000.tb00616.x|issn=1750-3841}}</ref> Electromagnetic energy can achieve higher processing temperatures in shorter times, therefore, more nutritional and sensory properties are conserved.<ref>{{Cite book|title=Food processing technology|last=Fellows|first=Peter|publisher=Woodheat publishing|year=2017|isbn=978-0-08-101907-8|pages=813–840}}</ref>

== See also ==
* [[Dielectric welding]]
* [[Electrosurgery]], which requires direct joule heating of tissue, and thus directly transmitted high frequency currents
* [[Specific absorption rate]]

== References ==
{{reflist|30em}}

== External links ==
{{refbegin}}
* {{cite book |last=Metaxas |first=A.C. |title=Foundations of Electroheat, A Unified Approach |year=1996 |publisher=John Wiley & Sons |isbn=0-471-95644-9}}
* {{cite book |last=Metaxas, A.C. |first=Meredith, R.J. |title=Industrial Microwave Heating (IEE Power Engineering Series) |year=1983 |publisher=Institution of Engineering and Technology |isbn=0-906048-89-3}}
* {{US patent|2147689}} – ''Method and apparatus for heating dielectric materials''
{{refend}}

{{DEFAULTSORT:Dielectric Heating}}
[[Category:Electric and magnetic fields in matter]]
[[Category:Heating]]
[[Category:Medical equipment]]
[[Category:Electromagnetism]]
[[Category:Food processing]]