Editing Magnetic core (section)

== Core materials ==
An electric current through a wire wound into a [[electromagnetic coil|coil]] creates a [[magnetic field]] through the center of the coil, due to [[Ampere's circuital law]].  Coils are widely used in electronic components such as [[electromagnet]]s, [[inductor]]s, [[transformer]]s, [[electric motor]]s and [[Electric generator|generator]]s.  A coil without a magnetic core is called an "air core" coil.  Adding a piece of [[ferromagnetic]] or [[ferrimagnetic]] material in the center of the coil can increase the magnetic field by hundreds or thousands of times; this is called a magnetic core.  The field of the wire penetrates the core material, [[magnetization|magnetizing]] it, so that the strong magnetic field of the core adds to the field created by the wire.  The amount that the magnetic field is increased by the core depends on the [[magnetic permeability]] of the core material. Because side effects such as [[eddy current]]s and [[hysteresis]] can cause frequency-dependent energy losses, different core materials are used for coils used at different [[frequency|frequencies]].

In some cases the losses are undesirable and with very strong fields saturation can be a problem, and an 'air core' is used. A former may still be used; a piece of material, such as plastic or a composite, that may not have any significant magnetic permeability but which simply holds the coils of wires in place.

===Solid metals===

==== Soft iron ====

"Soft" ([[annealing (metallurgy)|annealed]]) [[iron]] is used in magnetic assemblies, [[direct current]] (DC) electromagnets and in some electric motors; and it can create a concentrated field that is as much as 50,000 times more intense than an air core.<ref>{{cite web|url=http://www.physicsforums.com/archive/index.php/t-164613.html|title=Soft iron core}}</ref>

Iron is desirable to make magnetic cores, as it can withstand high levels of [[magnetic field]] without [[Saturation (magnetic)|saturating]] (up to 2.16 [[Tesla (unit)|tesla]]s at ambient temperature.<ref>Daniel Sadarnac, ''Les composants magnétiques de l'électronique de puissance'', cours de Supélec, mars 2001 [in french]</ref><ref>{{Cite journal|last1=Danan|first1=H.|last2=Herr|first2=A.|last3=Meyer|first3=A.J.P.|date=1968-02-01|title=New Determinations of the Saturation Magnetization of Nickel and Iron|journal=Journal of Applied Physics|volume=39|issue=2|pages=669–70|doi=10.1063/1.2163571|issn=0021-8979|bibcode=1968JAP....39..669D|doi-access=free}}</ref>)  Annealed iron is used because, unlike "hard" iron, it has low [[coercivity]] and so does not remain magnetised when the field is removed, which is often important in applications where the magnetic field is required to be repeatedly switched.

Due to the electrical conductivity of the metal, when a solid one-piece metal core is used in [[alternating current]] (AC) applications such as transformers and inductors, the changing magnetic field induces large [[eddy current]]s circulating within it, closed loops of electric current in planes perpendicular to the field.  The current flowing through the resistance of the metal heats it by [[Joule heating]], causing significant power losses.  Therefore, solid iron cores are not used in transformers or inductors, they are replaced by [[lamination|laminated]] or powdered iron cores, or nonconductive cores like [[ferrite (magnet)|ferrite]].

==== Laminated silicon steel ====
{{main article|Silicon steel}}
[[File:Laminated core eddy currents 2.svg|thumb|upright=1.4|''(left)'' Eddy currents ''(<span style="color:red;">I, red</span>)'' within a solid iron transformer core. ''(right)'' Making the core out of thin [[laminations]] parallel to the field ''(<span style="color:green;">B, green</span>)'' with insulation between them (''C'') limits the eddy currents to circulate within each individual lamination, reducing the total current.  In this diagram the field and currents are shown in one direction, but they actually reverse direction with the alternating current in the transformer winding.]]

In order to reduce the eddy current losses mentioned above, most low frequency power transformers and inductors use [[laminations|laminated]] cores, made of stacks of thin sheets of [[silicon steel]]:

===== Lamination =====
[[Image:EI Lam.jpg|thumb|right|Typical EI Lamination.]]

[[Lamination|Laminated]] magnetic cores are made of stacks of thin iron sheets coated with an insulating layer, lying as much as possible parallel with the lines of flux.  The layers of insulation serve as a barrier to eddy currents, so eddy currents can only flow in narrow loops within the thickness of each single lamination.  Since the current in an eddy current loop is proportional to the area of the loop, this prevents most of the current from flowing, reducing eddy currents to a very small level.   Since power dissipated is proportional to the square of the current, breaking a large core into narrow laminations reduces the power losses drastically.  From this, it can be seen that the thinner the laminations, the lower the eddy current losses.

===== Silicon alloying =====
A small addition of [[silicon]] to iron (around 3%) results in a dramatic increase of the [[resistivity]] of the metal, up to four times higher.{{citation needed|date=August 2011}}  The higher resistivity reduces the eddy currents, so [[silicon steel]] is used in transformer cores. Further increase in silicon concentration impairs the steel's mechanical properties, causing difficulties for rolling due to brittleness.

Among the two types of [[silicon steel]], grain-oriented (GO) and grain non-oriented (GNO), GO is most desirable for magnetic cores. It is [[anisotropic]], offering better magnetic properties than GNO in one direction. As the magnetic field in inductor and transformer cores is always along the same direction, it is an advantage to use grain oriented steel in the preferred orientation. Rotating machines, where the direction of the magnetic field can change, gain no benefit from grain-oriented steel.

==== Special alloys ====
A family of specialized alloys exists for magnetic core applications. Examples are [[mu-metal]], [[permalloy]], and [[supermalloy]]. They can be manufactured as stampings or as long ribbons for tape wound cores. Some alloys, e.g. [[Sendust]], are manufactured as powder and [[sintering|sintered]] to shape.

Many materials require careful [[heat treatment]] to reach their magnetic properties, and lose them when subjected to mechanical or thermal abuse. For example, the permeability of mu-metal increases about 40 times after [[annealing (metallurgy)|annealing]] in hydrogen atmosphere in a magnetic field; subsequent sharper bends disrupt its grain alignment, leading to localized loss of permeability; this can be regained by repeating the annealing step.

==== Vitreous metal ====
[[Amorphous metal]] is a variety of alloys (e.g. [[Metglas]]) that are non-crystalline or glassy. These are being used to create high-efficiency transformers.  The materials can be highly responsive to magnetic fields for low hysteresis losses, and they can also have lower conductivity to reduce eddy current losses. Power utilities are currently making widespread use of these transformers for new installations.<ref>{{cite web |title=Metglas® Amorphous Metal Materials – Distribution Transformers |url=https://www.hitachimetals.com/infrastructure-energy/clean-energy-technology/metglas-amorphous-metal-materials-distribution-transformers.php |access-date=25 September 2020}}</ref> High mechanical strength and corrosion resistance are also common properties of metallic glasses which are positive for this application.<ref>{{Cite journal|last1=Inoue|first1=A.|last2=Kong|first2=F. L.|last3=Han|first3=Y.|last4=Zhu|first4=S. L.|last5=Churyumov|first5=A.|last6=Shalaan|first6=E.|last7=Al-Marzouki|first7=F.|date=2018-01-15|title=Development and application of Fe-based soft magnetic bulk metallic glassy inductors|url=http://www.sciencedirect.com/science/article/pii/S092583881732978X|journal=Journal of Alloys and Compounds|language=en|volume=731|pages=1303–1309|doi=10.1016/j.jallcom.2017.08.240|issn=0925-8388|url-access=subscription}}</ref>

===Powdered metals===
Powder cores consist of metal grains mixed with a suitable organic or inorganic binder, and pressed to desired density. Higher density is achieved with higher pressure and lower amount of binder. Higher density cores have higher permeability, but lower resistance and therefore higher losses due to eddy currents. Finer particles allow operation at higher frequencies, as the eddy currents are mostly restricted to within the individual grains. Coating of the particles with an insulating layer, or their separation with a thin layer of a binder, lowers the eddy current losses. Presence of larger particles can degrade high-frequency performance. Permeability is influenced by the spacing between the grains, which form distributed air gap; the less gap, the higher permeability and the less-soft saturation. Due to large difference of densities, even a small amount of binder, weight-wise, can significantly increase the volume and therefore intergrain spacing.

Lower permeability materials are better suited for higher frequencies, due to balancing of core and winding losses.

The surface of the particles is often oxidized and coated with a phosphate layer, to provide them with mutual electrical insulation.

==== Iron ====
Powdered iron is the cheapest material. It has higher core loss than the more advanced alloys, but this can be compensated for by making the core bigger; it is advantageous where cost is more important than mass and size. Saturation flux of about 1 to 1.5 tesla. Relatively high hysteresis and eddy current loss, operation limited to lower frequencies (approx. below 100&nbsp;kHz). Used in energy storage inductors, DC output chokes, differential mode chokes, triac regulator chokes, chokes for [[power factor]] correction, resonant inductors, and pulse and flyback transformers.<ref name="coilws"/>

The binder used is usually epoxy or other organic resin, susceptible to thermal aging. At higher temperatures, typically above 125&nbsp;°C, the binder degrades and the core magnetic properties may change. With more heat-resistant binders the cores can be used up to 200&nbsp;°C.<ref name="chalmers">{{cite web|author=Johan Kindmark, Fredrik Rosén |url=http://publications.lib.chalmers.se/records/fulltext/183920/183920.pdf |title=Powder Material for Inductor Cores, Evaluation of MPP, Sendust and High flux core characteristics |publisher=Department of Energy and Environment, Division of Electric Power Engineering, Chalmers University of Technology |location=Göteborg, Sweden |date=2013 |access-date=2017-06-05}}</ref>

Iron powder cores are most commonly available as toroids. Sometimes as E, EI, and rods or blocks, used primarily in high-power and high-current parts.

Carbonyl iron is significantly more expensive than hydrogen-reduced iron.

==== Carbonyl iron ====
{{main article|carbonyl iron}}

Powdered cores made of [[carbonyl iron]], a highly pure iron, have high stability of parameters across a wide range of [[temperature]]s and [[magnetic flux]] levels, with excellent [[Q factor]]s between 50&nbsp;kHz and 200&nbsp;MHz. Carbonyl iron powders are basically constituted of micrometer-size [[sphere]]s of iron coated in a thin layer of [[electrical insulation]]. This is equivalent to a microscopic laminated magnetic circuit (see silicon steel, above), hence reducing the [[eddy currents]], particularly at very high frequencies. Carbonyl iron has lower losses than hydrogen-reduced iron, but also lower permeability.

A popular application of carbonyl iron-based magnetic cores is in high-frequency and broadband [[inductor]]s and [[transformer]]s, especially higher power ones.

Carbonyl iron cores are often called "RF cores".

The as-prepared particles, "E-type"and have onion-like skin, with concentric shells separated with a gap. They contain significant amount of carbon. They behave as much smaller than what their outer size would suggest. The "C-type" particles can be prepared by heating the E-type ones in hydrogen atmosphere at 400&nbsp;°C for prolonged time, resulting in carbon-free powders.<ref name="handbkferr"/>

==== Hydrogen-reduced iron ====
Powdered cores made of [[hydrogen reduced iron]] have higher permeability but lower Q than carbonyl iron. They are used mostly for [[electromagnetic interference]] [[electronic filter|filters]] and low-frequency chokes, mainly in [[switched-mode power supply|switched-mode power supplies]].

Hydrogen-reduced iron cores are often called "power cores".

==== MPP (molypermalloy) ====
An alloy of about 2% [[molybdenum]], 81% [[nickel]], and 17% iron. Very low core loss, low hysteresis and therefore low signal distortion. Very good temperature stability. High cost. Maximum saturation flux of about 0.8 tesla. Used in high-Q filters, resonant circuits, loading coils, transformers, chokes, etc.<ref name="coilws">{{cite web|url=http://www.coilws.com/index.php?main_page=page&id=41|title=How to choose Iron Powder, Sendust, Koolmu, High Flux and MPP Cores as output inductor and chokes : CWS Coil Winding Specialist, manufacturer of transformers, inductors, coils and chokes|first=The Zen Cart™ Team and|last=others|website=www.coilws.com}}</ref>

The material was first introduced in 1940, used in [[loading coil]]s to compensate capacitance in long telephone lines. It is usable up to about 200&nbsp;kHz to 1&nbsp;MHz, depending on vendor.<ref name="chalmers"/> It is still used in above-ground telephone lines, due to its temperature stability. Underground lines, where temperature is more stable, tend to use ferrite cores due to their lower cost.<ref name="handbkferr">{{cite book|url=https://books.google.com/books?id=StbgBwAAQBAJ&dq=powder+core+%22carbonyl+iron%22&pg=PA185|title=Handbook of Modern Ferromagnetic Materials|first=Alex|last=Goldman|date=6 December 2012|publisher=Springer Science & Business Media|via=Google Books|isbn=9781461549178}}</ref>

==== High-flux (Ni-Fe) ====
An alloy of about 50–50% of nickel and iron. High energy storage, saturation flux density of about 1.5 tesla. Residual flux density near zero. Used in applications with high DC current bias (line noise filters, or inductors in switching regulators) or where low residual flux density is needed (e.g. pulse and flyback transformers, the high saturation is suitable for unipolar drive), especially where space is constrained. The material is usable up to about 200&nbsp;kHz.<ref name="coilws"/>

==== Sendust, KoolMU ====
An alloy of 6% aluminium, 9% silicon, and 85% iron. Core losses higher than MPP. Very low [[magnetostriction]], makes low audio noise. Loses inductance with increasing temperature, unlike the other materials; can be exploited by combining with other materials as a composite core, for temperature compensation. Saturation flux of about 1 tesla. Good temperature stability. Used in switching power supplies, pulse and flyback transformers, in-line noise filters, swing chokes, and in filters in [[phase-fired controller]]s (e.g. dimmers) where low acoustic noise is important.<ref name="coilws"/>

Absence of nickel results in easier processing of the material and its lower cost than both high-flux and MPP.

The material was invented in Japan in 1936. It is usable up to about 500&nbsp;kHz to 1&nbsp;MHz, depending on vendor.<ref name="chalmers"/>

==== Nanocrystalline ====

A [[nanocrystalline]] alloy of a standard iron-boron-silicon alloy, with addition of smaller amounts of [[copper]] and [[niobium]]. The grain size of the powder reaches down to 10–100 nanometers. The material has very good performance at lower frequencies. It is used in chokes for inverters and in high power applications. It is available under names like e.g. Nanoperm, Vitroperm, Hitperm and Finemet.<ref name="chalmers"/>

===Ceramics===

==== Ferrite ====
{{main article|Ferrite core}}

[[Ferrite (magnet)|Ferrite ceramics]] are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters. As ceramics, they are essentially insulators, which prevents eddy currents, although losses such as hysteresis losses can still occur.

=== Air ===
<!-- arguably this is not a magnetic core, but the articles need a section to explain the downsides of magnetic cores, and why high permeable cores are not always used, and 'air core' is a well recognized term of art. --->
A coil not containing a magnetic core is called an ''air core''.  This includes coils wound on a plastic or ceramic form in addition to those made of stiff wire that are self-supporting and have air inside them. Air core coils generally have a much lower [[inductance]] than similarly sized ferromagnetic core coils, but are used in [[radio frequency]] circuits to prevent energy losses called [[core loss]]es that occur in magnetic cores.  The absence of normal core losses permits a higher [[Q factor]], so air core coils are used in high frequency [[resonant circuit]]s, such as up to a few megahertz. However, losses such as [[proximity effect (electromagnetism)|proximity effect]] and [[dielectric losses]] are still present. Air cores are also used when field strengths above around 2 Tesla are required as they are not subject to saturation.