Editing Magnetic core (section)

===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>