electrical and magnetic properties of materials

7/30/2019 Electrical and Magnetic Properties of Materials

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Electrical

Properties


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V - is the difference in volts between two locations (called the potential

difference) volts (J/C)

I- is the amount of current in amperes that is flowing between these two

points amperes (C/s)

R - is the resistance in ohms of the conductor between the two locations of

interest [ ohms (V/A)

Electricity

It is well known that one of the subatomic particles of an atom is the

electron. Atoms can and usually do have a number of electrons circling its

nucleus. The electrons carry a negative electrostatic charge and under

certain conditions can move from atom to atom. The direction of

movement between atoms is random unless a force causes the electrons to

move in one direction. This directional movement of electrons due to some

imbalance of force is what is known as electricity.

Amperage

The flow of electrons is measured in units called amperes or amps for

short. An amp is the amount of electrical current that exists when a numberof electrons, having one coulomb of charge, move past a given point in one

second. A coulomb is the charge carried by 6.25 x 1018

electrons or

6,250,000,000,000,000,000 electrons.

Electromotive Force

The force that causes the electrons to move in an electrical circuit is

called the electromotive force, or EMF. Sometimes it is convenient to think

of EMF as electrical pressure. In other words, it is the force that makeselectrons move in a certain direction within a conductor. There are many

sources of EMF, the most common being batteries and electrical

generators.

The Volt


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The unit of measure for EMF is the volt. One volt is defined as the

electrostatic difference between two points when one joule of energy is

used to move one coulomb of charge from one point to the other. A joule is

the amount of energy that is being consumed when one watt of power

works for one second. This is also known as a watt-second. For our

purposes, just accept the fact that one joule of energy is a very, very small

amount of energy. For example, a typical 60-watt light bulb consumes

about 60 joules of energy each second it is on.

Resistance

Resistance is the opposition of a body or substance to the flow of electrical

current through it, resulting in a change of electrical energy into heat, light,

or other forms of energy. The amount of resistance depends on the type of

material. Materials with low resistance are good conductors of electricity.

Materials with high resistance are good insulators.

Electrical Resistivity

Electrical resistivity is the reciprocal of conductivity. It is in the opposition

of a body or substance to the flow of electrical current through it, resulting

in a change of electrical energy into heat, light, or other forms of energy.

The amount of resistance depends on the type of material. Materials with

low resistivity are good conductors of electricity and materials with high

resistivity are good insulators

The value of R is influenced by specimen configuration, and for many

materials is independent of current.

The resistivity is independent of specimen geometry but related to R

through the expression


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l - is the distance between the two points at which the voltage is measured

A - is the cross-sectional area perpendicular to the direction of the current

ELECTRICAL CONDUCTIVITY

ELECTRICAL conductivity is the ability of a material to carry theflow of an electric current (a flow of electrons). Imagine that you

attach the two ends of a battery to a bar of iron and a

galvanometer. (A galvanometer is an instrument for measuring

the flow of electric current.) When this connection is made, the

galvanometer shows that electric current is flowing through the

iron bar. The iron bar can be said to be a conductor of electric

current. Replacing the iron bar in this system with other materials

produces different galvanometer readings. Other metals also

conduct an electric current, but to different extents. If a bar of

silver or aluminum is used, the galvanometer shows a greater flow

of electrical current than with the iron bar. Silver and aluminumare better conductors of electricity than is iron. If a lead bar is

inserted, the galvanometer shows a lower reading than with iron.

Lead is a poorer conductor of electricity than are silver, aluminum,

or iron.Many materials can be substituted for the original ironbar that will produce a zero reading on the galvanometer. These

materials do not permit the flow of electric current at all. They are

said to be nonconductors, or insulators. Wood, paper, and mostplastics are common examples of insulators Many materials can

be substituted for the original iron bar that will produce a zero

reading on the galvanometer. These materials do not permit the

flow of electric current at all. They are said to be nonconductors,


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or insulators. Wood, paper, and most plastics are common

examples of insulators

Electrical Resistance

Another way of describing the conductivity of a material is

through resistance. Resistance can be defined as the extent to

which a material prevents the flow of electricity. Silver, aluminum,iron and other metals have a low resistance (and a high

conductivity). Wood, paper, and most plastics have a high

resistance (and a low conductivity). The unit of measurement for

electrical resistance is called the ohm (abbreviation: ). The ohm

was named for German physicist Georg Simon Ohm (17891854),

who first expressed the mathematical laws of electrical

conductance and resistance in detail. Interestingly enough, the

unit of electrical conductance is called the mho (ohm written

backwards). This choice of units clearly illustrates the reciprocal

(opposite) relationship between electrical resistance and

conductivity.

How conductance takes place

Electrical conductivity occurs because of the ease with whichelectrons can be removed from atoms. All substances consist of

atoms. In turn, all atoms consist of two main parts: a positively


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charged nucleus and one or more negatively charged electrons.

An atom of iron, for example, consists of a nucleus with 26

positive charges and 26 negatively charged electrons. The

electrons in an atom are not all held with equal strength.Electrons close to the nucleus are strongly attracted by the

positive charge of the nucleus and are removed from the atom

only with great difficulty. Electrons farthest from the nucleus are

held only loosely and are removed quite easily. A block of iron can

be thought of as a huge collection of iron atoms. Most of the

electrons in these atoms are held tightly by the iron nuclei. But a

few electrons are held looselyso loosely that they act as if theydon't even belong to atoms at all. Scientists sometimes refer to

this condition as a cloud of electrons. Normally these "free"

electrons have no place to go. They just spin around randomly

among the iron atoms. That situation changes, however, when a

battery (or other source of electric current) is attached to the iron

block. Electrons flow out of one end of the battery and into the

other. At the electron-rich end of the battery, electrons flow into

the piece of iron, pushing iron electrons ahead of them. Since all

electrons have the same negative charge, they repel each other.

Iron electrons are pushed away from the electron-rich end of the

battery towards the electron-poor end. In other words, an electric

current flows through the iron

ENERGY BAND STRUCTURES IN SOLIDS


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In isolated atom electrons occupy well defined energy states.

When atoms come together to form a solid, their valence

electronsinteract with each other and with nuclei due to

Coulomb forces. In addition, two specific quantum mechanicaleffects happen. First, by Heisenberg's uncertainty principle,

constraining the electrons to a small volume raises their energy,

this is calledpromotion. The second effect, due to the Pauli

Exclusion Principle, limits the number of electrons that can have

the same energy. As a result of these effects, the valence

electrons of atoms form wide electron energy bands when they

form a solid. The bands are separated by gaps, where electronscannot exist.

Energy Band Structures and Conductivity

The highest filled state at 0 K Fermi Energy (EF). The two

highest energy bands are:

Valence band the highest band where the electrons are

present at 0 K

Conduction band - a partially filled or empty energy band

where the electrons can increase their energies by going

to higher energy levels within the band when an electric

field is applied

Metals

In metals (conductors), highest occupied band is partially

filled or bands overlap.


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Conduction occurs by promoting electrons into conducting

states that starts right above the Fermi level. The

conducting states are separated from the valence band by

an infinitesimal amount. Energy provided by an electric

field is sufficient to excite many electrons into conducting

states.

Semiconductors and insulators

In semiconductors and insulators, the valence band is

filled, no more electrons can be added (Pauli's principle).Electrical conduction requires that electrons be able to

gain energy in an electric field. To become free, electrons

must be promoted (excited) across the band gap. The

excitation energy can be provided by heat or light.

Insulators: wide band gap (> 2 eV)

Semiconductors: narrow band gap (< 2 eV)

Energy Band Structures and Conductivity

(semiconductors and insulators)

In semiconductors and insulators, electrons have to jump across the

band gap into conduction band to find conducting states above Ef .The energy needed for the jump may come from heat, or from

irradiation at sufficiently small wavelength (photo excitation). The

difference between semiconductors and insulators is that in

semiconductors electrons can reach the conduction band at ordinary


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temperatures, where in insulators they cannot. The probability that

an electron reaches the conduction band is about exp(-Eg/2kT)

where Eg is the band gap. If this probability is < 10-24 one would not

find a single electron in the conduction band in a solid of 1 cm3. Thisrequires Eg/2kT > 55. At room temperature, 2kT =0.05 eV Eg > 2.8

eV corresponds to an insulator. An electron promoted into the

conduction band leaves a hole (positive charge) in the valence band,

that can also participate in conduction. Holes exist in metals as well,

but are more important in semiconductors and insulators.

Energy Band Structures and Bonding (metals, semiconductors,

insulators)

Relation to atomic bonding:

Insulatorsvalence electrons are tightly bound to (or shared with) the

individual atoms strongest ionic (partially covalent) bonding.

Semiconductors - mostly covalent bonding somewhat weaker bonding.

Metalsvalence electrons form an electron gas that are not bound

to any particular ion.

ELECTRICAL RESISTIVITY OF METALS

The resistivity is defined by scattering events due to theimperfections and thermal vibrations. Total resistivity tot can bedescribed by the Matthiessen rule:

total=thermal+impurity+deformation


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Where thermal - from thermal vibrations, impurity - from

impurities, deformation - from deformation-induced defects

Conductivity / Resistivity of MetalsInfluence of temperature:

Resistivity rises linearly with temperature (increasing thermal

vibrations and density of vacancies)

T = o + aT

Influence of impurities: Impurities that form solid solution

i = Aci(1-ci)

ci is impurity concentration,

A composition independent constant

Two-phase alloy ( and phases) rule-of-mixtures:

i = V + V

Influence of plastic deformation:In general, presence of any imperfections (crystal defects)

increases resistivity

-- grain boundaries

-- dislocations

-- impurity atoms

-- vacancies


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Normally, the influence of plastic deformation on electricalresistivity is weaker than the influence of tempera ture and

impurities

Materials of Choice for Metal Conductors

One of the best material for electrical conduction (lowresistivity) is silver, but its use is restricted due to the high

cost. Most widely used conductor is copper: inexpensive,

abundant, high , but rather soft cannot be used inapplications where mechanical strength is important. Solid

solution alloying and cold working in prove strength but

decrease conductivity. Precipitation hardening is

preferred, e.g. Cu-Be alloy When weight is important one

uses aluminum, whichis half as good as Cu and more

resistant to corrosion. Heating elements require low (high R), and resistance to high temperature oxidation:

nickel-chromium alloy

Semiconductivity

Some materials cannot be classified as either conductors or

insulators. Semiconductors, for example, are materials thatconduct an electric current but do so very poorly.

Semiconductors were not well understood until the mid-

twentieth century, when a series of remarkable discoveries

revolutionized the field of electrical conductivity. These


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discoveries have made possible a virtually limitless variety of

electronic devices, ranging from miniature radios and handheld

calculators to massive solar power arrays and orbiting

telescopes

Superconductivity

Superconductivity is a property that appears only at very low

temperatures, usually close to absolute zero (273C). At such

temperatures, certain materials lose all resistance to electric

current; they become perfect conductors. Once an electriccurrent is initiated in such materials, it continues to flow

without diminishing and can go on essentially forever. The

discovery of superconductivity holds enormous potential for

the development of electric appliances. In such appliances, a

large fraction of the electrical energy supplied to the device is

lost in overcoming electrical resistance within the device. Thatlost energy shows up as waste heat. If the same appliance were

made of a superconducting material, no energy would be lost

because there would be no resistance to overcome. The

appliance would become, at least in principle, 100 percent

efficient

Extrinsic semiconductors

n-type extrinsic semiconductors


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The hole created in donor state is far from the valence band

and is immobile. Conduction occurs mainly by the donated

electrons (thus n-type). ~ n|e|e ~ ND |e|e (for extrinsic n-

type semiconductors)

P-type extrinsic semiconductors

Excess holes are produced by substitutional impurities that

have fewer valence electrons per atom than the matrix. A

bond with the neighbors is incomplete and can be viewed

as a hole weakly bound to the impurity atom. Elements incolumns III of the periodic table (B, Al, Ga) are donors for

semiconductors in the IV column, Si and Ge. Impurities of

this type are called acceptors, NA = NBoron ~p The energy

state that corresponds to the hole (acceptor state) is close

to the top of the valence band. An electron may easily hop

from the valence band to complete the bond leaving ahole behind. Conduction occurs mainly by the holes (thus

p-type). ~ p|e|p ~ NA |e|p

Carrier mobility

Ionic Materials


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In ionic materials, the band gap is large and only very few

electrons can be promoted to the valence band by thermal

fluctuations. Cation and anion diffusion can be directed by

the electric field and can contribute to the total

conductivity: total = electronic + ionic

High temperatures produce more Frenkel and Schottky defects

which result in higher ionic conductivity.

Polymers

Polymers are typically good insulators but can be made to

conduct by doping. A few polymers have very high electrical

conductivity - about one quarter that of copper, or about twice

that of copper per unit weight

Q = magnitude ofcharge stored on each plate.

V = voltage applied to the plates

Dielectric Materials
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The dielectric constant of vacuum is 1 and is close to 1 for air

and many other gases. But when a piece of a dielectric material

is placed between the two plates in capacitor the capacitance

can increase significantly.

C = r o A / L

with r = 81 for water, 20 for acetone,12 for silicon, 3 for ice,

etc. A dielectric material is an insulator in which electric dipoles

can be induced by the electric field (or permanent dipoles can

exist even without electric field), that is where positive andnegative charge are separated on an atomic or molecular level

In the capacitor surface charge density (also calleddielectric

displacement) is D = Q/A = r oE = oE + P

Polarization is responsible for the increase in charge density

above that for vacuum

Mechanisms of polarization

electronic (induced) polarization: Applied electric field

displaces negative electron clouds with respect to positive

nucleus. Occurs in all materials.

ionic (induced) polarization: In ionic materials, applied electricfield displaces cations and anions in opposite directions

molecular (orientation) polarization: Some materials


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possess permanent electric dipoles (e.g. H2O). In absence of

electric field, dipoles are randomly oriented. Applying electric

field aligns these dipoles, causing net (large) dipole moment.

Ptotal = Pe + Pi + Po

Very high electric fields (>108 V/m) can excite electrons to the

conduction band and accelerate them to such high energies

that they can, in turn, free other electrons, in an avalanche

process (or electrical discharge). The field necessary to start

the avalanche process is called dielectric strength orbreakdown strength.


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Magnetic

Properties


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The study of atoms, electrons, neutrons, and protons is so

complex that throughout history scientists have developed

several models of the atom. From the early Greek concept of

the atom, about 2400 years ago, to today's modern atomic

model, scientists have built on and modified existing models, as

new information was discovered. There are still concepts on

which scientists do not fully agree. In an attempt to simplify the

concept and describe how some materials become magnetized,

we are using a simplification of the Niels Bohr Model of the

atom. Niels Bohr was a Danish scientist and made his model in

1913. In his model Bohr depicted electrons spinning and

orbiting the nucleus of an atom. In our exercise, the electron

appears to orbit in the same path around the nucleus, but

electrons do not really orbit in the same path. They change

their path with each revolution and are commonly described as

existing in clouds that surround the nucleus of an atom.

Because electrons move so quickly, it is impossible to see

where they are at a specific moment in time.


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What is the origin of magnetism?

The origin of magnetism is a very complicated concept. In

fact, there are some details about magnetism on the

atomic scale that scientists still do not fully agree on. To

begin to understand where magnetism originates and why

some materials can be magnetized while others cannot,

requires a fair amount of quantum theory. Quantum

theory is the study of the jumps from one energy level to

another as it relates to the structure and behavior of

atoms. However, explaining quantum theory is well

beyond the scope of this material, so this subject will be

reserved for high school and college chemistry and physics

classes. Nevertheless, the basic scientific principles of

magnetism can be explained if a few generalizations and

simplifications are made.

What is a magnetic field and how is it created?

A magnetic field describes a volume of space where thereis a change in energy. Later, you will see a simple way to

detect a magnetic field with a compass. As Ampere

suggested, a magnetic field is produced whenever an

electrical charge is in motion. The spinning and orbiting of


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the nucleus of an atom produces a magnetic field as does

electrical current flowing through a wire. The direction of

the spin and orbit determine the direction of the magnetic

field. The strength of this field is called the magnetic

moment.The motion of an electric charge producing amagnetic field is an essential concept in understanding

magnetism. The magnetic moment of an atom can be the

result of the electron's spin, which is the electron orbital

motion and a change in the orbital motion of the electrons

caused by an applied magnetic field.

What are paired electrons?

All the electrons do produce a magnetic field as they spin

and orbit the nucleus; however, in some atoms, two

electrons spinning and orbiting in opposite directions pair

up and the net magnetic moment of the atom is zero.Remember that the direction of spin and orbit of the

electron determines the direction of the magnetic field.

Electron pairing occurs commonly in the atoms of most

materials. In the experiment you observed a helium atom

showing two electrons spinning and orbiting around the

protons and neutrons of the nucleus. The two electronsare paired, meaning that they spin and orbit in opposite

directions. Since the magnetic fields produced by the

motion of the electrons are in opposite directions, they


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add up to zero. The overall magnetic field strength of

atoms with all paired electrons is zero. In general,

materials that have all paired electrons in the atoms and

thus have no net magnetic moment are called

diamagnetic materials; yet, there are some exceptions.

When placed in the magnetic field of a magnet,

diamagnetic materials will produce a slight magnetic field

that opposes the main magnetic field. Both ends of a bar

magnet will repel a diamagnetic material. If a diamagnetic

material is placed in a strong external magnetic field, the

magnetic field strength inside the material will be less than

the magnetic field strength in the air surrounding the

material. The slight decrease in the field strength is the

result of realignment in the orbit motion of the electrons.

Diamagnetic materials include zinc, gold, mercury, and

bismuth. Another key concept in magnetism is that

diamagnetic materials will oppose an applied magnetic

field. Both ends of a magnet will repel diamagnetic

materials.

Are all materials that have unpaired electrons magnetic?

Most materials with one or more unpaired electrons are atleast slightly magnetic. Materials with a small attraction to

a magnet are called paramagnetic materials, and those

with a strong attraction are called ferromagnetic


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materials. Aluminum, platinum, and manganese are some

paramagnetic materials. Iron, cobalt, and nickel are

examples of ferromagnetic materials.

MAGNETIC DOMAIN

A magnetic domain is region in which the magnetic fields

of atoms are grouped together and aligned. In the

experiment below, the magnetic domains are indicated by

the arrows in the metal material. You can think of

magnetic domains as miniature magnets within a material.In an unmagnetized object, like the initial piece of metal in

our experiment, all the magnetic domains are pointing in

different directions. But, when the metal became

magnetized, which is what happens when it is rubbed with

a strong magnet, all like magnetic poles lined up and

pointed in the same direction.The metal became amagnet. It would quickly become unmagnetized when itsmagnetic domains returned to a random order. The metal

in our experiment is a soft ferromagnetic material, which

means that it is easily magnetized but may not retain its

magnetism very long

DIAMAGNETISM AND PARAMAGNETISM

Diamagnetism is a very weak form of magnetism that isnonpermanent and persists only while an external field is

being applied. The magnitude of the induced magnetic


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moment is extremely small, and in a direction opposite to

that of the applied field Paramagnetism is a form of

magnetism whereby the paramagnetic material is only

attracted when in the presence of an externally applied

magnetic field. In a paramagnet, the magnetic moments

tend to be randomly orientated due to thermal

fluctuations when there is no magnetic field. In an applied

magnetic field these moments start to align parallel to the

field such that the magnetisation of the material is

proportional to the applied field.

Ferromagnetism

A ferromagnetic substance is one that, like iron, retains a

magnetic moment even when the external magnetic field

is reduced to zero.This effect is a result of a strong

interaction between the magnetic moments of theindividual atoms or electrons in the magnetic substance

that causes them to line up parallel to one another. In

ordinary circumstances these ferromagnetic materials are

divided into regions called domains; in each domain, the

atomic moments are aligned parallel to one another. The

most important class of magnetic materials is theferromagnetism: iron, nickel, cobalt and manganese, or

their compounds (and a few more exotic ones as well).


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Antoferromagnetism and Ferrimagnetism

Some ceramics also exhibit a permanent magnetization,termed ferrimagnetism. Ferrimagnetic substances have atleast two different kinds of atomic magnetic moments,

which are oriented antiparallel to one another (e.g. This

phenomenon of magnetic moment coupling between

adjacent atoms or ions occurs in materials other than

those that are ferromagnetic. In one such group, this

coupling results in an antiparallel alignment; the alignmentof the spin moments of neighboring atoms or ions in

exactly opposite directions is termed antiferromagnetism.

Manganese oxide (MnO) is one material that displays this

behavior.Fe3O4).

THE INFLUENCE OF TEMPERATURE ON MAGNETIC BEHAVIOR

With increasing temperature, the saturation magnetization

diminishes gradually and then abruptly drops to zero at Curie

Temperature

Hysteresis

Hysteresis is what allows us to make permanent

magnets.To make permanent magnets, we take our

material, create whatever shape we want, and then place

the material inside of a very strong magnetic field. The

domains inside the material align with the magnetic field,


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and when we remove the field, the domains stay aligned,

and we now have a new magnet. While these are magnets

are not truly permanent, some magnets domains will

not return to their original state for much longer than a

single lifetime.

Magnetic Anisotropy

The magnetic hysteresis curves will have different shapes

depending on various factors:

(1) whether the specimen is a single crystal or polycrystalline

(2) if polycrystalline, any preferred orientation of the grains

(3) the presence of pores or second-phase particles

(4) other factors such as temperature and, if a mechanical

stress is applied, the stress state.

The magnetizing field is applied in [100], [110], and [111]

crystallographic directions. This dependence of magnetic

behavior on crystallographic orientation is termed

magnetic anisotropy.For each of these materials there is

one crystallographic direction in which magnetization is

easiest is termed a direction ofeasy magnetization a hardcrystallographic direction is that direction for which

saturation magnetization is most difficult

SOFT MAGNETIC MATERIALS


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Soft magnetic materials are those materials that are easily

magnetised and demagnetised. Soft magnetic materials

are: 1.small coercivities 2.used for electric motors

3.example: commercial iron 99.95 Fe

HARD MAGNETIC MATERIALS

Hard magnetic materials are utilized in permanent

magnets, which must have a high resistance to

demagnetization. In terms of hysteresis behavior, a hard

magnetic material has a high remanence, coercivity, and

saturation flux density, as well as a low initial permeability,

and high hysteresis energy losses. Hard magnetic

materials: large coercivities used for permanent magnets

add particles/voids to inhibit domain wall motion example:

tungsten steel - Hc = 5900 amp-turn/m)

High-Energy Hard Magnetic Materials

High-Energy Hard Magnetic Materials

Permanent magnetic materials having energy products in

excess of about 80 kJ/m3 (10 MGOe) are considered to be

of the high-energy type.

SamariumCobalt Magnets


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SmCo5- is a member of a group of alloys that are combinations

of cobalt or iron anda light rare earth element

-a number of these alloys exhibit high-energy, hard magnetic

behavior

- Energy products of these SmCo5 materials are considerably

higher than the conventional hard magnetic materials ; in

addition, they have relatively large coercivities

NeodymiumIronBoron Magnets:

Coercivities and energy products of these materials rival those of

the samariumco

Very high strength

Relatively low cost balt alloys

Two different processing techniques are available for the

fabrication of Nd2Fe14B magnets: powder metallurgy (sintering)

and rapid solidification (melt spinning). The powder metallurgical

approach is similar to that used for the SmCo5 materials.

For rapid solidification, the alloy, in molten form, is quenched very

rapidly such that either an amorphous or very fine grained and

thin solid ribbon is produced.

MAGNETIC STORAGE


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Transference to and retrieval from the tape or disk isaccomplished by means of an inductive readwrite head,

which consists basically of a wire coil wound around a

magnetic material core into which a gap is cut write or

record data by applying a magnetic field that aligns

domains in small regions of the recording medium read

or retrieve data from medium by sensing changes in

magnetization There are two principal types of magnetic

mediaparticulate and thin film.The thin-film storage

technology is relatively new and provides higher storage

capacities at lower costs. It is employed mainly on rigid

disk drives and consists of a multilayered structure. A

magnetic thin-film layer is the actual storage

component.This film is normally either a CoPtCr or CoCrTa

alloy, with a thickness of between 10 and 50 nm. A

substrate layer below and upon which the thin film resides

is pure chromium or a chromium alloy. The storage density

of thin films is greater than for particulate media because

the packing efficiency of thin-film domains is greater than

for the acicular particles; particles will always be separated

with void space in between.

SUPERCONDUCTIVITY

Superconductivity is basically an electrical phenomenon.Materials for which the resistivity, at a very low


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temperature, abruptly plunges from a finite value to one

that is virtually zero and remains there upon further

cooling. Materials thatdisplay this latter behavior are

called superconductors, and the temperature at which they

attain superconductivity is called the critical temperature

TC.Temperature dependence of the electrical resistivityfor normally conducting and superconducting materials in

the vicinity of 0 K. Superconducting materials may be

divided into two classifications designated as type I and

type II: Type I materials, while in the superconducting

state, are completely diamagnetic; that is, all of an applied

magnetic field will be excluded from the body of material,

a phenomenon known as the Meissner effect. Several

metallic elements including aluminum, lead, tin, and

mercury belong to the type I group. Type II

superconductors are completely diamagnetic at low

applied fields, and field exclusion is total. However, the

transition from the superconducting state to the normal

state is gradual and occurs between lower critical and

upper critical fields, designated HC1 and HC 2

TC= critical temperature - if T> TCnot superconducting

JC= critical current density - ifJ >JCnot superconducting

HC= critical magnetic field - if H > HCnot superconducting

vances in Superconductivity


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Research in superconductive materials was stagnant formany years.

-Everyone assumedTC,max

was about 23 K

-Many theories said it was impossible to increase TC

beyond this value

1987- new materials were discovered with TC> 30 K-ceramics of form Ba1-x Kx BiO3-y

-Started enormous race

+Y Ba2Cu3O7-x TC= 90 K

+Tl2Ba2Ca2Cu3Ox TC= 122 K

+difficult to make since oxidation state is very important

The major problem is that these ceramic materials areinherently brittle.


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electrical and magnetic properties of materials

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