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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
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elecur.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elevol.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elevol.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elecur.html#c27/30/2019 Electrical and Magnetic Properties of 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.
7/30/2019 Electrical and Magnetic Properties of Materials
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