properties of mag mat

8/13/2019 Properties of Mag Mat

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3) Magnetic materials

Hard magnetic soft magnetic

Typical hysteresis loop of soft magnetic material:

HH

cc

MM

H H

Soft

magneticHard

magnetic


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Necessary conditions for a GOOD soft magnetic

material:

a) Curie temperature above RT 3d elements: Fe,Co,Nib) High saturation magnetization (at RT) Fe,Co

c) Low coercivity low anisotropy, low magnetostriction,

microstructure

d) Low hysteresis area losses microstructure

e) resistivity


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Industrial soft magnetic materials:

Pure Iron:

Magnetization curve of pure iron


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Industrial soft magnetic materials:

Magnetization curve of Fe-C


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Magnetic properties of various grades of iron:


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High permeability materials:

High permeability materials mainly determined by microstructure.

Material shall have a low anisotropy and a low magnetostriction.

Estimate of rotational permeability:

Describes initial permeability but usual value is too small

1 = 45.6 104 erg/cm3

Ni: K1

= -4.8 104 erg/cm3

S

SSr

MKMrot

321)(

20

1

20 ++=

Fe: K


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Nickel-iron alloys:

Known as permalloy, used over a wide range of

compositions, from 30 to 80wt%Ni.Properties vary over composition range - optimum

composition must be selected for a particular

application.

High Ni content alloys: high permeability;

Around 50wt%Ni: high saturation magnetisationLow Ni content: high electrical resistance.

Special grades of Ni-Fe alloys: zero magnetostriction

and zero magnetic anisotropy mumetal -produced by a careful heat treatment and minor

additions of Cu and Cr. These alloys have extremely

high permeable, up to 300000 and intrinsic coercivityas low as 0.4A/m.


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Temperature and concentration dependence of

permeability:


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Magnetic properties of high permeability materials


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Silicon Iron

Iron-Silicon Alloys

Used for transformer cores - known as electrical steels.

Power applications: ac-electrical voltage; low f: 50-60Hz -

causes eddy currents in the transformer core.

Alloying Fe with Si - marked effect on electric resistivity -

increase a factor 4 for 3wt%Si. Silicon also reduce the

magnetostriction and the magnetocrystalline anisotropy.

Material is used laminated - typically 0.3 to 0.7mm thick.

Addition of too much silicon - material becomes extremely

brittle and difficult to produce - practical limitation of 4wt% Si

that can be added.


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Silicon Iron

Figure shows anisotropy of Fe - two types of possible

textures - known as cube-on-edge and cube texture.

Note that the cube texture has two type directions in

the plane of the sheet and provides an advantage if E-

shaped laminations are to be cut from the sheet.


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Silicon-Iron

Non oriented Silicon Iron:

High purity iron highest MS and high permeability high price!

For transformer application important:

a) Low hysteresis losses

b) Low eddy current losses


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Silicon-Iron

Addition of 1 4% of Si:

Reduces magnetocrystalline anisotropy

Reduces coercivity

Increases permeability

Increases resistivity reduces eddy current losses

Further increase of Si decrease of ductility

Reduction of MS


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Silicon-Iron

Production:

Decarbonized with hydrogen at 820C (%C < 0.001%)

Usually 3.2% Si in Fe; optimized grain size about 0.15 mm.

Smaller grains too many domains

Larger grains eddy current losses increase

Silicon steel available as: 0.35 mm, 0.5 mm and 0.65 mm thick.

Width up to 1070 mm

Total losses (d = 0.35 mm, B = 1.5 T, f 0 50 Hz): 2.5 W/kg


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Silicon-Iron

0.91.4012288600

1.31.5315174401

1.51.6435147804

Core loss (60

Hz; B = 1T;

W/kg

B(T) (H =

2.4 kA/m)

HC(A/m)

.max(A/m)

%Si


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Requirements for effective

transformer core materials:

a) High saturation polarisation JS

b) High permeability

c) Low magnetocrystalline ansitropy

d) High electrical resistivity minimize eddy currents

e) Low magnetostriction

f) Magnetic homogeneity single phase (?)g) Ductility

h) Stable properties over ambient temperature range


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Phase diagram Fe-Si:


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Magnetostriction of of Fe-Si:

0 150 300 450 600 750 900 1050

-15

-10

-5

0

5

10

15

Effective magnetostriction constants of Fe100-xSix alloys

pure Fe x = 2.1 x = 7.3 (caculated after literature)

x = 12.1 x = 13.8 x = 20.0 (our experiments)

s

(10

-6

)

Temperature (K)


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Magnetostriction of of Fe-Si:

0 2 4 6 8 10 12 14 16 18

Si concentration [at%]

8

6

4

s

(10-6)

2

0

-2

-4

Effective magnetostriction constants as a function of Si

concentration determined by strain-gauge

measurements on single crystals FeSi.


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Ductility of Fe-Si:


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Magnetostriction of Fe-Si:


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Grain oriented Silicon Iron

Transformer applications: flux lies predominantly in the

length of the laminations - it is desirable to enhance the

permeability in this direction. Can be achieved by various

hot and cold rolling stages - textured sheets - grain-oriented

silicon-steel, with the [001] direction in the length of thelamination.

The type crystal directions are the easy directions of

magnetisation - largest permeability.


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Grain oriented Fe-Si commercial production:

1. Starting material with a composition Fe-3.2%Si-0.03%C-

(0.06-0.1%)Mn-0.02%S

2. Cast ingot is hot rolled at about 1300C to a thickness 1.5-

2.5mm

3. Cold rolled to a final thickness of 0.2-0.35mm in two steps,with an intermediate heating at 800-1000C

4. Decarbonisation at 800C In H and N growth of new strain

free grains

5. Coated with MgO

6. Annealed at 1100-1200C in dry hydrogen and N to form the(110)[001] texture by secondary crystallization.


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Applications of Fe-Si steel


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Rapidly solidified Fe-Si alloys

Fe-Si with 6wt% Si

Advantages:

High electrical resistivity

Vanishing magnetostriction

Low magnetic anisotropy

Problem brittleness

Overcomed by:

Rapid quenching from the melt

High temperature annealing (above 1000C) +

Rapid cooling large grains with texture


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As quenched

Annealed T= 800C

Annealed T= 1200C


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As quenched

Annealed T

= 1100C

D = 15 m D = 80 m

i i ifi Si


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magnetization

coercivity

R idl lidifi d F Si ll


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S = 0

P ti f F Si Al ll


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Properties of Fe-Si-Al alloy

Contains 10% Si + 5% Al

Initial permeability about 30.000

Maximum permeability about 130.000

Because there K1 and S simultaneously zero!!!

make powderv(10 m)

Press and heat treatment Sendust alloy

But: very brittle

M t t lli i t f F Si Al ll


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Magnetocrystalline anisotropy of Fe-Si-Al alloy

R idl lidifi d F Si ll


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Comparison of different soft magnetic materials


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Comparison of different soft magnetic materials

Soft magnetic Ferrites


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Ferrimagnetic materials with spinel structure of the formula

MFe2O4

M...divalent metal ion

M = Mn, Fe, Co, Ni, or Cu, Zn, Mg, Cd

Properties of most important ferrimagnetic spinels:



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World market of soft magnetic materials


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World market of soft magnetic materials

World market of magnetic materials


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World market of magnetic materials

Preparation of ferrites


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Preparation of ferrites

Prepared from powdered carbonates or oxides

Milled to very fine powder

Compacted into required shape

Sintered at about 1000C

Advantages of ferrites:

High electrical resistivity

Large saturation magnetization is desired!

High permeability, low coercivity lowest value ofmagnetocrystalline anisotropy + low magnetostriction

Structure of spinels


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Magnetic structure of most important ferrites


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Magnetic structure of most important ferrites

Magnetic properties of most important ferrites


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ag et c p ope t es o ost po ta t e tes

Magnetic properties of most important ferrites


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g p p p

Note: K1 of Mn-Zn ferrite exceptional low Mn-Zn ferrites are

high permeability materials.

Most important commercial soft ferrites:

Mn-Zn and Ni-Zn ferrites.

Range of properties depend very much on composition,

preparation conditions, grain size, poriosity etc.

Magnetic properties of Mn-Zn and Ni-Zn ferrites.


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g p p

Mn-Zn ferrites for applications f < 500 kHz

Ni-Zn ferrites for applications 500 kHz until 50 MHz

Typical hysteresis loop of a Mn-Zn ferrite.


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yp y p

Composition and temperature dependence of


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p p p

magnetic properties of ferrites.

Frequency dependence of permeability of Ni-Zn


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ferrite at RT.

Frequency dependence of losses of Mn-Zn and Ni-Zn


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ferrites at RT.

Applications of soft magnetic ferrites.


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properties of mag mat

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