electrical and magnetic properties of materials research

7/30/2019 Electrical and Magnetic Properties of Materials research

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ARMIDA

Electrical and Magnetic

Properties of Materials


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Electrical

Properties


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Electrical properties are the physical conditions that allow anelectrical charge to move from atom to atom in a specific material.These properties differ greatly between the three major types ofmaterials: solids, liquids and gases. Electric properties of solid

materials like metal are high, while electric charges do not move aseasily in water and have an even more difficult time with gases. Ineach element, there are exceptions: some solids are poorconductors, and some gases can become excellent conductors.

Solids and electricity often are a perfect combination forconductivity. The electrical properties of cooper, steel, and othermetals provide the optimum opportunity because of the physicalcloseness of atoms. When electrons can pass easily between atoms,this promotes electrical conductivity. Solids like silver, copper and

aluminum are popular with electrical work because very littleenergy is lost when electricity travels through these metals.


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Not all solids, however, possess the strong electricalproperties of metal. Items like glass, wood and

plastic are considered insulators because the tightlypacked electrons do not share electrical chargeseasily. When an electrical current is introduced tothese materials, nothing happens. These solids are

still valued in electrical work, but often to protecthumans against electrical charges


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OHMS LAW

The relationship between current, voltage, andresistance is given by Ohms law.

This law states that the amount of current passingthrough a conductor is directly proportional to the

voltage across the conductor and inverselyproportional to the resistance of the conductor


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OHMS LAW

Ohms law can be expressed as an equation:

V=IR

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 isflowing between these two points amperes (C/s)

R - is the resistance in ohms of the conductorbetween the two locations of interest [ ohms (V/A) ]


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OHMS LAW

V = I R in most cases V =DV = V2 V1


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OHMs LAW

Electricity

It is well known that one of the subatomic particles of an atom is the electron. Atoms canand usually do have a number of electrons circling its nucleus. The electrons carry anegative 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 electronsto 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 isthe amount of electrical current that exists when a number of electrons, having onecoulomb of charge, move past a given point in one second. Acoulomb is the chargecarried 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 theelectromotive force, or EMF. Sometimes it is convenient to think of EMF as electricalpressure. In other words, it is the force that makes electrons move in a certain directionwithin a conductor. There are many sources of EMF, the most common being batteriesand electrical generators.


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OHMs LAW

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

electrostatic difference between two points when one joule of energy isused to move one coulomb of charge from one point to the other. Ajoule is the amount of energy that is being consumed when one watt ofpower works for one second. This is also known as awatt-second. For

our purposes, just accept the fact that one joule of energy is a very, verysmall amount of energy. For example, a typical 60-watt light bulbconsumes about 60 joules of energy each second it is on.

ResistanceResistance 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 onthe type of material. Materials with low resistance are good conductorsof electricity. Materials with high resistance are good insulators.


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OHMS LAW

Electrical ResistivityElectrical resistivity is the reciprocal of conductivity.It is in the opposition of a body or substance to theflow of electrical current through it, resulting in achange of electrical energy into heat, light, or otherforms of energy.

The amount of resistance depends on the type of

material. Materials with low resistivity are goodconductors of electricity and materials with highresistivity are good insulators


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OHMS LAW

The value of R is influenced byspecimen configuration, and formany materials is independent ofcurrent.

The resistivity is independent ofspecimen geometry but related toR through the expression

l - is the distance between the twopoints at which the voltage ismeasured

A - is the cross-sectional areaperpendicular to the direction ofthe current


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OHMS LAW

Schematic representation of theapparatus used to measureelectrical resistivity

The units for (resistivity) areohm-meters ( -m)

I ammeter


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ELECTRICAL CONDUCTIVITY

ELECTRICAL conductivity is the ability of a material tocarry the flow of an electric current (a flow of electrons).

Imagine that you attach the two ends of a battery to abar of iron and a galvanometer. (A galvanometer is aninstrument for measuring the flow of electric current.)

When this connection is made, the galvanometer showsthat electric current is flowing through the iron bar. Theiron bar can be said to be a conductor of electric current.


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Replacing the iron bar in this system with other materialsproduces different galvanometer readings. Other metalsalso conduct an electric current, but to different extents.If a bar of silver or aluminium is used, the galvanometer

shows a greater flow of electrical current than with theiron bar.

Silver and aluminum are better conductors of electricitythan is iron. If a lead bar is inserted, the galvanometershows a lower reading than with iron. Lead is a poorerconductor of electricity than are silver, aluminum, oriron.


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Many materials can be substituted for the originaliron bar that will produce a zero reading on thegalvanometer.

These materials do not permit the flow of electriccurrent at all. They are said to be nonconductors, orinsulators. Wood, paper, and most plastics are

common examples of insulators


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Many materials can be substituted for the originaliron bar that will produce a zero reading on thegalvanometer. These materials do not permit theflow of electric current at all. They are said to benonconductors, or insulators. Wood, paper, andmost plastics are common examples of insulators


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Electrical Resistance

Another way of describing the conductivity of a material isthrough 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 ahigh conductivity). Wood, paper, and most plastics have a

high resistance (and a low conductivity).

The unit of measurement for electrical resistance is called theohm (abbreviation: ). The ohm was named for Germanphysicist Georg Simon Ohm (17891854), who first expressedthe mathematical laws of electrical conductance andresistance in detail. Interestingly enough, the unit of electricalconductance is called the mho (ohm written backwards). Thischoice of units clearly illustrates the reciprocal (opposite)relationship between electrical resistance and conductivity.


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How conductance takes place

Electrical conductivity occurs because of the ease withwhich electrons can be removed from atoms. Allsubstances consist of atoms. In turn, all atoms consist oftwo main parts: a positively charged nucleus and one ormore negatively charged electrons. An atom of iron, for

example, consists of a nucleus with 26 positive chargesand 26 negatively charged electrons. The electrons in an atom are not all held with equal

strength. Electrons close to the nucleus are stronglyattracted 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 looselyand are removed quite easily.


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How conductance takes place

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 thiscondition as a cloud of electrons.

Normally these "free" electrons have no place to go. They just spinaround randomly among the iron atoms. That situation changes,however, when a battery (or other source of electric current) is attachedto 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 samenegative charge, they repel each other. Iron electrons are pushed awayfrom the electron-rich end of the battery towards the electron-poor end.In other words, an electric current flows through the iron.


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ENERGY BAND STRUCTURES IN SOLIDS

In an isolated atom electrons occupy well defined energystates

When atoms come together to form a solid, their valenceelectronsinteract with each other and with nuclei due toCoulomb forces. In addition, two specific quantum

mechanical effects happen. First, by Heisenberg's uncertaintyprinciple, constraining the electrons to a small volume raisestheir energy, this is calledpromotion. The second effect, dueto the Pauli exclusion principle, limits the number of electronsthat can have the same energy.

As a result of these effects, the valence electrons of atomsform wide electron energy bandswhen they form a solid.The bands are separated bygaps,where electrons cannotexist.


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ENERGY BAND STRUCTURES IN SOLIDS

Schematic plot ofelectron energy versusinter atomic separationfor an aggregate of 12

atoms upon closeapproach, each of the 1sand 2s atomic statessplits to form an electronenergy band consisting of12 states.


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Energy Band Structures and Conductivity

The highest filled state at 0KFermi Energy (EF) The two highest energy

bands are: Valence band the

highest band where theelectrons are present at 0K

Conduction band - apartially filled or emptyenergy band where the

electrons can increase theirenergies by going to higherenergy levels within the

band when an electric fieldis applied


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Metals In metals (conductors),

highest occupied band ispartially filled or bandsoverlap.

Conduction occurs bypromoting electrons intoconducting states, that startsright above the Fermi level. Theconducting states are separatedfrom the valence band by aninfinitesimal amount.

Energy provided by an electric

field is sufficient to excitemany electrons into conductingstates.

Ssemiconductors and insulators In semiconductors and insulators,

the valence band is filled, no moreelectrons can be added (Pauli'sprinciple). Electrical conductionrequires that electrons be able to gainenergy in an electric field. To become

free, electrons must be promoted(excited) across the band gap. Theexcitation energy can be provided byheat or light.

Insulators:

wide band gap (> 2 eV) Semiconductors:

narrow band gap (< 2 eV)


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METAL SEMI CONDUTORS ANDINSULATORS



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Energy Band Structures and

Conductivity(semiconductors andinsulators)

In semiconductors and insulators, electronshave to jump across the band gap intoconduction band to find conducting statesabove Ef

The energy needed for the jump may come

from heat, or from irradiation at sufficientlysmall wavelength (photo excitation).

The difference between semiconductors andinsulators is that in semiconductors electronscan reach the conduction band at ordinarytemperatures, where in insulators theycannot.

The probability that an electron reaches theconduction band is about exp(-Eg/2kT)where Eg is the band gap. If this probabilityis < 10-24 one would not find a singleelectron in the conduction band in a solid of 1cm3. This requires Eg/2kT > 55. At roomtemperature, 2kT =0.05 eV Eg > 2.8 eVcorresponds 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 aremore important in semiconductors andinsulators.

Energy Band Structures andBonding (metals,semiconductors, insulators)

Relation to atomic bonding:

Insulators valence electrons aretightly bound to (or shared with) theindividual atoms strongest ionic(partially covalent) bonding.

Semiconductors - mostly covalentbonding somewhat weaker bonding.

Metals valence electrons form anelectron gas that are not bound to anyparticular ion.


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ELECTRICAL RESISTIVITY OF METALS

The resistivity is defined byscattering events due to the

imperfections and thermalvibrations. Total resistivitytot

can be described by theMatthiessen rule:

total=thermal+impurity+deformation

where thermal - fromthermal vibrations,

impurity - from impurities,

deformation - fromdeformation-induced defects


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Electrical Resistivity of Metals

Conductivity / Resistivity ofMetals

Influence of temperature:

Resistivity rises linearly withtemperature (increasing thermalvibrations 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 plasticdeformation:In general, presence of any

imperfections (crystaldefects) increases resistivity

-- grain boundaries

-- dislocations-- impurity atoms-- vacancies Normally, the influence of

plastic deformation on

electrical resistivity isweaker than the influence oftempera ture and impurities


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Materials of Choice for Metal Conductors

One of the best material forelectrical conduction (lowresistivity) is silver,but its use isrestricted due to the high cost

Most widely used conductor iscopper: inexpensive, abundant,high , but rather soft cannot beused in applications wheremechanical strength is important.

Solid solution alloying and coldworking in prove strength butdecrease conductivity.Precipitation hardening ispreferred, e.g. Cu-Be alloy

When weight is important one usesaluminum,whichis half as goodas Cu and more resistant tocorrosion.

Heating elements require low (high R), and resistance to hightemperature oxidation: nickel-chromium alloy


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Semiconductivity

Some materials cannot be classified as either conductorsor insulators. Semiconductors, for example, are materialsthat conduct an electric current but do so very poorly.Semiconductors were not well understood until the mid-

twentieth century, when a series of remarkablediscoveries revolutionized the field of electricalconductivity. These discoveries have made possible a

virtually limitless variety of electronic devices, ranging

from miniature radios and handheld calculators tomassive solar power arrays and orbiting telescopes.

.


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Superconductivity

Superconductivity is a property that appears only at very lowtemperatures, usually close to absolute zero (273C). At suchtemperatures, certain materials lose all resistance to electriccurrent; 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 deviceis lost in overcoming electrical resistance within the device.That lost energy shows up as waste heat. If the same appliance

were made of a superconducting material, no energy would belost because there would be no resistance to overcome. Theappliance would become, at least in principle, 100 percentefficient


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n-type extrinsicsemiconductors

p-type extrinsicsemiconductors

The hole created in donor state is farfrom the valence band and is immobile.Conduction occurs mainly by thedonated electrons (thus n-type). ~n|e|e ~ ND |e|e (for extrinsic n-type semiconductors)

Excess holes are produced by substitutionalimpurities that have fewer valence electronsper atom than the matrix.

A bond with the neighbors is incomplete andcan be viewed as a hole weakly bound to theimpurity atom.

Elements in columns III of the periodic table(B, Al, Ga) are donors for semiconductors inthe IV column, Si and Ge.

Impurities of this type are called acceptors,

NA = NBoron ~p The energy state thatcorresponds to the hole (acceptor state) isclose to the top of the valence band. Anelectron may easily hop from the valenceband to complete the bond leaving a holebehind. Conduction occurs mainly by theholes (thus p-type). ~ p|e|p ~ NA |e|p

Extrinsic semiconductors


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Carrier mobility

Ionic Materials 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 defectswhich 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 onequarter that of copper, or about twice that of copper per unit weight.


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CAPACITANCE

Capacitance is typified by a parallel platearrangement and is defined in terms of chargestorage:

where

Q = magnitude of charge

stored on each plate.V = voltage applied to theplates.


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Dielectric Materials

The dielectric constant of vacuumis 1 and is close to 1 for air andmany other gases. But when apiece of a dielectric material isplaced between the two plates incapacitor the capacitance canincrease significantly.

C = r o A / L

with r = 81 for water, 20 foracetone,12 for silicon, 3 for ice,etc.

A dielectric material is an

insulator in which electricdipoles can be induced by the

electric field (or permanent

dipoles can exist even withoutelectric field), that is wherepositive and negative chargeare separated on an atomic ormolecular level

In the capacitor surface

charge density (also calleddielectric displacement) is

D = Q/A = r oE = oE + P

Polarization is responsible for theincrease in charge density

above that for vacuum


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Dielectric Materials

Mechanisms of polarization electronic (induced) polarization:

Applied electric field displacesnegative electron clouds with respecttopositive nucleus. Occurs in allmaterials.

ionic (induced) polarization: In

ionic materials, applied electricfield displaces cations and anions inoppositedirections

molecular (orientation)polarization: Some materialspossess permanent electric dipoles (e.g.H2O). In absence of electric field,

dipoles are randomly oriented.Applying electric field aligns thesedipoles, causing net

(large) dipole moment.

Ptotal = Pe + Pi + Po


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Dielectric strength

Very high electric fields (>108 V/m) can exciteelectrons to the conduction band and acceleratethem to such high energies that they can, in turn,free other electrons, in an avalanche process (orelectrical discharge).

The field necessary to start the avalanche process iscalled dielectric strength or breakdown

strength.


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Piezoelectricity

In some ceramicmaterials, application ofexternal forces producesan electric (polarization)field and vice-versa

Applications ofpiezoelectricmaterials is based on

conversion of mechanical

strain into electricity(microphones, straingauges, sonar detectors)


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MagneticProperties


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The study of atoms, electrons, neutrons, and protons is socomplex that throughout history scientists have developedseveral models of the atom.

From the early Greek concept of the atom, about 2400 yearsago, to today's modern atomic model, scientists have built onand modified existing models, as new information wasdiscovered. There are still concepts on which scientists do notfully agree. In an attempt to simplify the concept and describehow some materials become magnetized, we are using a

simplification of the Niels Bohr Model of the atom.


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Niels Bohr was a Danish scientist and made his model in 1913.In his model Bohr depicted electrons spinning and orbitingthe nucleus of an atom. In our exercise, the electron appears to

orbit in the same path around the nucleus, but electrons donot really orbit in the same path. They change their path witheach revolution and are commonly described as existing inclouds that surround the nucleus of an atom. Becauseelectrons move so quickly, it is impossible to see where theyare 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. Infact, there are some details about magnetism on the atomicscale that scientists still do not fully agree on. To begin tounderstand where magnetism originates and why somematerials can be magnetized while others cannot, requires afair amount of quantum theory.

Quantum theoryis the study of the jumps from one energylevel to another as it relates to the structure and behavior ofatoms. However, explaining quantum theory is well beyondthe 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 aremade.

Wh t i ti fi ld d h i it


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What is a magnetic field and how is itcreated?

Amagnetic field describes a volume of space where there is a changein energy. Later, you will see a simple way to detect a magnetic fieldwith a compass. As Ampere suggested, a magnetic field is producedwhenever an electrical charge is in motion. The spinning and orbitingof the nucleus of an atom produces a magnetic field as does electrical

current flowing through a wire. The direction of the spin and orbitdetermine the direction of the magnetic field. The strength of this fieldis called the magnetic moment.

Wh t i ti fi ld d h i it


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What is a magnetic field and how is itcreated?

The motion of an electric charge producing amagnetic field is an essential concept inunderstanding magnetism. The magnetic momentof an atom can be the result of the electron's spin,which is the electron orbital motion and a change inthe orbital motion of the electrons caused by anapplied magnetic field.


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What are paired electrons?

All the electrons do produce a magnetic field as they spin andorbit the nucleus; however, in some atoms, two electronsspinning and orbiting in opposite directions pair up and thenet magnetic moment of the atom is zero. Remember that thedirection of spin and orbit of the electron determines thedirection of the magnetic field.

Electron pairing occurs commonly in the atoms of mostmaterials. In the experiment you observed a helium atomshowing two electrons spinning and orbiting around theprotons and neutrons of the nucleus.

The two electrons are paired, meaning that they spin and orbit

in opposite directions. Since the magnetic fields produced bythe motion of the electrons are in opposite directions, theyadd up to zero. The overall magnetic field strength of atoms

with all paired electrons is zero.


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What are paired electrons

In general, materials that have all paired electrons in theatoms and thus have no net magnetic moment are calleddiamagnetic materials;yet, there are someexceptions.

When placed in the magnetic field of a magnet,diamagnetic materials will produce a slight magneticfield that opposes the main magnetic field. Both ends of a

bar magnet will repel a diamagnetic material. If adiamagnetic material is placed in a strong external

magnetic field, the magnetic field strength inside thematerial will be less than the magnetic field strength inthe air surrounding the material.


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What are paired electrons

The slight decrease in the field strength is the resultof realignment in the orbit motion of the electrons.Diamagnetic materials include zinc, gold, mercury,and bismuth.

Another key concept in magnetism is thatdiamagnetic materials will oppose an appliedmagnetic field. Both ends of a magnet will repel

diamagnetic materials.

Are all materials that have unpaired


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Are all materials that have unpairedelectrons magnetic?

Most materials with one or more unpaired electronsare at least slightly magnetic. Materials with a smallattraction to a magnet are called paramagneticmaterials, and those with a strong attraction are

called ferromagnetic materials. Aluminum,platinum, and manganese are some paramagneticmaterials. Iron, cobalt, and nickel are examples offerromagnetic materials.


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MAGNETIC DOMAIN

Amagnetic domain is region in which the magneticfields of atoms are grouped together and aligned. In theexperiment below, the magnetic domains are indicated

by the arrows in the metal material. You can think ofmagnetic 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 pointingin different directions. But, when the metal becamemagnetized, which is what happens when it is rubbed

with a strong magnet, all like magnetic poles lined upand pointed in the same direction. .


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MAGNETIC DOMAIN

The metal became a magnet. It would quicklybecome unmagnetized when its magnetic domainsreturned to a random order.

The metal in our experiment is a soft ferromagneticmaterial, which means that it is easily magnetized

but may not retain its magnetism very long


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3. DIAMAGNETISM ANDPARAMAGNETISM

Diamagnetism is a veryweak form of magnetismthat is nonpermanent andpersists only while anexternal field is beingapplied

The magnitude of theinduced magnetic moment

is extremely small, and in adirection opposite to that ofthe applied field

The atomic dipoleconfiguration for a diamagnetic material

with and without a magnetic field. In theabsence of an external field, no dipolesexist; in the presence of a field, dipoles areinduced that are aligned opposite to thefield direction.


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3. Diamagnetism and Paramagnetism

Paramagnetism is a form ofmagnetism whereby theparamagnetic material is onlyattracted when in the presence ofan externally applied magneticfield

In a paramagnet, the magneticmoments tend to be randomlyorientated due to thermalfluctuations when there is nomagnetic field. In an appliedmagnetic field these momentsstart to align parallel to the fieldsuch that the magnetisation ofthe material is proportional tothe applied field.

Schematic showing themagnetic dipole momentsrandomly aligned in aparamagnetic sample


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4. Ferromagnetism

FERROMAGNETISM A ferromagnetic substance is one

that, like iron, retains a magneticmoment even when the externalmagnetic field is reduced to zero.

This effect is a result of a stronginteraction between the magneticmoments of the individual atoms or

electrons in the magnetic substancethat causes them to line up parallel toone another.

In ordinary circumstances theseferromagnetic materials are dividedinto regions called domains; in eachdomain, the atomic moments arealigned parallel to one another

The most important class of magneticmaterials is theferromagnetism: iron,nickel, cobalt and manganese, or theircompounds (and a few more exoticones as well).

Schematic showing the magnetic

dipole moments aligned parallel ina ferromagnetic material


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5. Antoferromagnetism and Ferrimagnetism

Some ceramics alsoexhibit a permanentmagnetization, termedferrimagnetism.

Ferrimagneticsubstances have at leasttwo different kinds ofatomic magneticmoments, which areoriented antiparallel toone another (e.g. Fe3O4 )

Schematic showingadjacent magneticmoments of differentmagnitudes aligned anti-

parallel.


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This phenomenon of magneticmoment coupling betweenadjacent atoms or ions occursin materials other than thosethat are ferromagnetic. In onesuch group, this coupling

results in an antiparallelalignment; the alignment of thespin moments of neighboringatoms or ions in exactlyopposite directions is termed

antiferromagnetism. Manganese oxide (MnO) is one

material that displays thisbehavior.

Schematic showing adjacent magnetic

dipole moments with equal magnitudealigned anti-parallel in anantiferromagnetic material. This is onlyone of many possible antiferromagnetic

arrangements of magnetic moments.

6 THE INFLUENCE OF TEMPERATURE


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6 THE INFLUENCE OF TEMPERATUREON MAGNETIC BEHAVIOR

With increasing temperature, the saturationmagnetization diminishes gradually and thenabruptly drops to zero at Curie Temperature, Tc.


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Hysteresis

Hysteresis is what allows us

to make permanentmagnets. To make permanent magnets,

we take our material, createwhatever shape we want, andthen place the material inside

of a very strong magneticfield. The domains inside thematerial align with themagnetic field, and when weremove the field, the domainsstay aligned, and we now havea new magnet. While theseare magnets are not trulypermanent, some magnetsdomains will not return totheir original state for muchlonger than a single lifetime.


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8. Magnetic Anisotropy

The magnetic hysteresiscurves will have differentshapes depending on

various factors:(1) whether the specimen is a

single crystal or

polycrystalline(2) if polycrystalline, any

preferred orientation of thegrains

(3) the presence of pores or

second-phase particles(4) other factors such astemperature and, if amechanical stress isapplied, the stress state.


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Magnetic Anisotropy

the magnetizing field is appliedin [100], [110], and [111]crystallographic directions

This dependence of magneticbehavior on crystallographicorientation is termed magnetic

anisotropy. For each of these materials there is

one crystallographic direction inwhich magnetization is easiest istermed a direction ofeasymagnetization

a hard crystallographic direction

is that direction for whichsaturation magnetization is mostdifficult


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9. SOFT MAGNETIC MATERIALS


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10. HARD MAGNETIC MATERIALS

Hard magnetic materials are

utilized in permanent magnets,which must have a high resistanceto demagnetization.

In terms of hysteresis behavior, ahard magnetic material has ahigh remanence, coercivity, and

saturation flux density, as well as alow initial permeability, and highhysteresis energy losses

Hard magnetic materials:

large coercivities

used for permanent magnets

add particles/voids to inhibit domainwall motion

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


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High-Energy Hard Magnetic Materials

High-Energy Hard MagneticMaterials

Permanent magnetic materials havingenergy products in excess of about 80kJ/m3 (10 MGOe) are considered to beof the high-energy type.

Samarium

Cobalt MagnetsSmCo5- is a member of a group of alloys

that are combinations of cobalt or ironanda light rare earth element

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

- Energy products of these SmCo5materials are considerably higher thanthe conventional hard magneticmaterials ; in addition, they haverelatively large coercivities

NeodymiumIronBoron

Magnets Coercivities and energy products of

these materials rival those of thesamariumco

Very high strength

Relatively low cost balt alloys

Two different processing techniquesare available for the fabrication ofNd2Fe14B magnets: powder metallurgy(sintering) and rapid solidification(melt spinning). The powdermetallurgical approach is similar tothat used for the SmCo5 materials.

For rapid solidification, the alloy, inmolten form, is quenched very rapidlysuch that either an amorphous or veryfine grained and thin solid ribbon isproduced.


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11 .MAGNETIC STORAGE

Transference to and retrievalfrom the tape or disk isaccomplished by means of aninductive readwrite head, whichconsists basically of a wire coil

wound around a magneticmaterial core into which a gap iscut

write or record data by applying amagnetic field that aligns

domainsin small regions of the recordingmedium

read or retrieve data frommedium by sensing changesin magnetization


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11 .MAGNETIC STORAGE

There are two principal types of

magnetic mediaparticulate andthin film.

The thin-film storage technologyis relatively new and provideshigher storage capacities at lowercosts. It is employed mainly onrigid disk drives and consists of amultilayered structure. Amagnetic thin-film layer is theactual storage component.Thisfilm is normally either a CoPtCror CoCrTa alloy, with a thicknessof between 10 and 50 nm. A

substrate layer below and uponwhich the thin film resides ispure chromium or a chromiumalloy.

The storage density of thin films

is greater than for particulatemedia because the packingefficiency of thin-film domains isgreater than for the acicularparticles; particles will always beseparated with void space in

between.

S CO C


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12 .SUPERCONDUCTIVITY

Superconductivity is basicallyan electrical phenomenon

Materials for which theresistivity, at a very lowtemperature, abruptlyplunges from a finite value toone that is virtually zero and

remains there upon furthercooling.

Materials thatdisplay thislatter behavior are calledsuperconductors, and thetemperature at which they

attain superconductivity iscalled the critical temperatureTC

Temperature dependence ofthe electrical resistivity fornormally conducting andsuperconducting materials inthe vicinity of 0 K.

Mercu

Copper

(normal)

SUPERCONDUCTIVITY


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12. SUPERCONDUCTIVITY

Superconducting materialsmay be divided into twoclassifications designated astype I and type II:

Type I materials, while in thesuperconducting state, arecompletely diamagnetic; that

is, all of an applied magneticfield will be excluded from the

body of material, aphenomenon known as the

Meissner effect. Severalmetallic elements including

aluminum, lead, tin, andmercury belong to the type Igroup

Type II superconductors arecompletely diamagnetic at lowapplied fields, and field exclusionis total. However, the transitionfrom the superconducting state tothe normal state is gradual and

occurs between lower critical andupper critical fields, designatedHC1 and HC 2

TC= critical temperature - if T> TCnot superconducting

JC= critical current density - ifJ>JCnot superconducting

HC= critical magnetic field - ifH>HCnot superconducting

SUPERCONDUCTIVITY


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SUPERCONDUCTIVITY


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SUPERCONDUCTIVITY


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SUPERCONDUCTIVITY


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Advances in Superconductivity Research in superconductive materials was stagnant for many

years.-Everyone assumed TC,maxwas about 23 K-Many theories said it was impossible to increase TCbeyond 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 are inherentlybrittle.

electrical and magnetic properties of materials research

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