ceramic materials
TRANSCRIPT
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Dr. Madhavrao K. Deore
M.Sc., Ph. D.
Department of Physics,
M.V.P.Samaja’s, Arts, Science and
Commerce College, Ozar(Mig),
Nashik, -422206, India
CERAMIC MATERIALS
CERAMIC MATERIALS Ceramic materials are inorganic, non-metallic materials made from
compounds of a metal and a non metal. Ceramic materials may
be crystalline or partly crystalline.
The word ceramic comes from the Greek word
keramiko of pottery"
or for pottery from keramos.
Ceramics materials are the phases containing a compounds of
metallic and non-metallic elements. In short ceramics are the
inorganic non metallic materials such as silicates, aluminates, oxides,
carbides, borides and hydroxides. Since there are many possible
combinations of metallic and non-metallic atoms and there are many
several structural arrangement of each combination.
Ceramics always composed of more than one element. Bonds are
partially or totally ionic, can have combination of ionic and covalent
bonding (electronegativity)
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SPECTRUM OF CERAMICS USES
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http://www.ts.mah.se/utbild/mt7150/051212%20ceramics.pdf
Comparison metals v ceramics
Ceramics Metals
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Properties of Ceramic materials: Ceramic materials are brittle, hard, strong in compression, durability,
Weak in shearing and tension.
Chemically inert -They withstand chemical erosion that occurs in an acidic or
caustic environment. In many cases withstanding erosion from the acid and
bases applied to it.
Non-conductors of heat: Ceramics generally can withstand very high
temperatures such as temperatures that range from 1,000 °C to 1,600 °C
(1,800 °F to 3,000 °F)
Non-conductors of electricity (insulator): porcelain is widely used to make
electrical insulators, but some ceramic compounds are superconductors.’
They are excellent dielectric.
Low cost of raw materials and fabrication for some ceramics.
Good appearance control through surface treatments, colorization, etc
But not all ceramics behave in this way. For example, graphite is a very soft
ceramic and conducts electricity well, whereas diamond is a very good
conductor of heat. Ceramics called ferrites are particularly good conductors of
electricity and superconductors have almost no electrical resistance at all.
Ceramic matrix composites, made by embedding fibers of a strengthening
material in what is known as a ceramic matrix, are not at all brittle.
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Classification of Ceramic Materials:
Although the ceramic materials are not as simple as metals, they may be classified
and understood in terms of their internal structure.
Generally the ceramic materials may be divided in to three classes according to
their common characteristics features
a)Clay products
Silicon & Aluminium as silicates
Potassium compounds
Magnesium compounds
Calcium compounds
b) Refractories
c) Glasses :
Three common types of glass:
Soda-lime glass - 95% of all glass, windows containers etc.
Lead glass - contains lead oxide to improve refractive index
Borosilicate - contains Boron oxide, known as Pyrex.
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CLASSIFICATION OF CERAMICS
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Ceramic materials are also classified in the following ways also i)Functional classification indicating to application
Group Example Applications
Abrasive Natural - garnet, diamond,
Synthetic abrasives -
silicon carbide,
diamond, fused alumina,
etc.
Sand paper, Cutting, Polishing grinding, lapping, or
pressure blasting of materials
Pure Oxide
ceramics
MgO, Al2O3.SiO2 Al2O3-Protection tubes for thermocouple, Furnace Parts,
laboratory ware e.g crucible, boat, dishes
Non oxide
ceramics
SiC, Si3N4, B4C,BN SiC-Slide rings, Bearings, sliding bushing
Si3N4-Metal forming tooles, rollers
Fixed clay
product
Bricks, Tiles, Procelein
Inorganic
glasses
Windo glass,Lead
glass
Optical, Composite –reinforce, House hold,
Containers
Cementing
Material
s
Portland cement, Lime Structural
Rocks Granites, Sandstones 8
White wares Pottery, tableware, sanitary
ware, wall tile, etc.
Pottery, tableware, sanitary ware, wall tile, etc.
Minerals Quartz, Calcite
Refractories Silica bricks,
Magnesite
Firebricks for furnaces and ovens. Have high Silicon
or Aluminium oxide content.
Brick products are used in the manufacturing plant for
iron and steel, non-ferrous metals, glass, cements,
ceramics, energy conversion, petroleum, and
chemical industries
Advance
ceramics
Advanced ceramic materials are now well established
in many areas of everyday use, from fridge magnets to
an increasing range or industries, including metals
production and processing, aerospace, electronics,
automotive and personnel protection.
In modern medicine, advanced ceramics – often
referred to as bioceramics – play an increasingly
important role. Bioceramics such as alumina and
zirconia are hard, chemically inert materials that
can be polished to a high finish. They are used as
dental implants and asbone substitutes in
orthopaedic operations such as hip and knee
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ii) Structural classification indicating to structural
criteria
Group Example
Crystalline ceramics Single phase MgO(Magnesium Oxide) is used
as insulation material in heating elements and
cables.
Aluminium Oxide
Beryllium Oxides
Boron Carbide
Tungsten Carbide.
Non Crystalline Natural and synthetic inorganic glasses
Glass bounded
ceramics
Fixed clay product crystalline phases are held in
glass matrix
Cement Crystalline or noncrystalline phases
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Ceramic Crystals:
AX-type ceramic crystal structures:
Most common ceramics are made of equal number of cations and anions, and are
referred to as AX compounds (A-cation(+), and X-anion(-)).
These ceramics assume many different structures, named after a common material
that possesses the particular structure. The characteristic feature of any AX
compounds is tha the A atom are co-ordinated with only X atoms as immediate
neighbour and the X atom have only A atom as first neighbours. Thus A and X
atoms are highly ordered.
There are three principle ways in which AX compound can form cubic crystals so
that two types of atoms are are in equal numbers and possess the ordered
coordination. the prototypes are
•Cesium Chloride structure (CsCl)
•Sodium Choride(NaCl)
•Zinc Blende structure(ZnS)
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Cesium Chloride (CsCl) structure
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It has body centred cubic (bcc) arrangement, with Cs+ at the center and Cl- at the
corners.
Each Cs+ ion is surrounded by 8 Cl- ions and each Cl- ions is surrounded by 8
Cs+ ions. Therefore the crystal has 8:8 co-ordination.
The Coordination number: 8
Each atom A has eight X neighbour and in extension and vice -versa
The anions are located at each of the corners of a cube, whereas the cube
centre is a single cation.
Interchange of anions with cations produces the A unit cell for CsCl structure
same crystal structure.
The CsCl structure looks like BCC, but is a uniquely different crystal structure
because two different ions are involved. It is simply cubic array of atoms.
The location are in 8-fold interstitial site (i.e. 8f sites) within simple cubic
pattern, because the location occupied by A atoms are in the sites surrounded by
eight neighbours.
The atomic positions of CsCl structure is - Cl: 0, 0, 0 Cs: 0.5, 0.5, 0.5 (can
interchange if desired)
The lattice constant is a =
For Cesium Chloride Structure: rC = rCs = 0.170 nm, rA = rCl = 0.181 nm ⇒ rC/rA =
0.94
It has one CsCl per unit cell because,
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Sodium Choride(NaCl) structure
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The crystal lattice of NaCl reveals that the ions are arranged in a three dimensional
pattern, the positive ion alternating with a negative ion.
In NaCl crystal there are 6 Cl surrounding the Na, and 6 Na around each Cl.
The NaCl type structure has an FCC array of anions with positive ions located 6-fold
interstitial site(6-f site)
It has a face centred arrangement (or CCP).
Cl- ions occupy the corners and face centers, Na+ occupy body centre and edge
centers.
Each Na+ is surrounded by six Cl- and each Cl- is surrounded by 6 Na+. Therefore it
has 6:6 coordination
The distance between the immediate neighbour in NaCl is fixed
i.e
NaCl structure: rC = rNa = 0.102 nm, rA = rCl = 0.181 nm ⇒ rC/rA = 0.56
The Na's occupy the octahedral sites in the Cl sublattice, and the Cl's occupy the
octahedral sites in the Na sublattice.
The basis consist of one Na+ and one Cl- separated by one half the body diagonal of
a unit cube. Hence it may be considered as two face cantered its origin mid way a-way
along a cubic, sub-lattices, one Na+ having its origin at a point (000) and other of Cl- ion
having its origin mid-away along a cube at appoint (a/2,0,0). There are four molecules
of NaCl in each unit cube with ions in the position as given as
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Number of NaCl Units per unit cell.
i) Cl- ions
Total number of Cl- ions = 4
ii) Na+ ions
One at the body centre contributes fully
= 1 x 1 = 1
Total Number of Na+ = 3+1=4
Therefore total NaCl units in one unit cell = 4
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Zinc Blende (ZnS) structure:
Zinc sulphide (or zinc sulphide) is an inorganic compound with the chemical
formula of ZnS. This is the main form of zinc found in nature, where it mainly
occurs as the mineral zinc blende (sphalerite). Although this mineral is
usually black because of various impurities, the pure material is white, and it
is widely used as a pigment. In its dense synthetic form, zinc sulphide can
be transparent, and it is used as a window for visible
optics and infrared optics.
Zinc sulfide (ZnS) is a unique compound that forms two types of crystalline
structures. These two polymorphs are wurtzite and zincblende (also known
as sphalerite). Wurtzite has a hexagonal structure, while zincblende is
cubic. The two types have these features in common: a 1:1 stoichiometry
of Zn:S. a coordination of 4 for each ion (4:4 coordination) tetrahedral
coordination.
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The S- - ions present at corners and at the face centers and
Zn++ ions are present in alternate tetrahedral voids. Each
Zn2+ ion is surrounded tetrahedrally by S-2 ions each and S-
2 ion is surrounded tetrahedrally by Zn2+ ions. Therefore this structure has 4:4 coordination.
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Zn2+ ions occupy every other tetrahedral hole in the fcc array of S2− ions. Each Zn2+ ion is
surrounded by four S2− ions in a tetrahedral arrangement.
The lattice constant is a =
Zinc Blende (ZnS) Structure = rZn / rS = 0.074/0.184 = 0.402
Since 0.225 < 0.402 < 0.414,
(if the cation:anion radius ratio is less than about 0.414 the structure is tetrahedral and
coordination number is 4).
For example, ZnS contains an FCC lattice of S2− ions, and the cation:anion radius ratio is only
about 0.40, so we predict that the cation would occupy either a tetrahedral hole or an octahedral
hole. In fact, the relatively small Zn2+ cations occupy the tetrahedral holes in the lattice.
The Zn2+ ions occupy every other tetrahedral hole, as shown in Figure "The Zinc Blende
Structure", giving a total of 4 Zn2+ and 4 S2− ions per unit cell and a formula of ZnS. The zinc
blende structure results in a coordination number of 4 for each Zn2+ ion and a tetrahedral
arrangement of the four S2− ions around each Zn2+ ion.
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Number of S2- ions per unit cell 8 at corners contributed
6 at the face centre contribute
Total number of S-2 per unit cell = 4.
Since Zn2+ ions occupy half the tetrahedral sites, the number of
Zn2+ ions per unit cell will be 4.
Total number of ZnS per unit cell = 4.
Site Zn S
Central 4 0
Face 0 6(1/2) = 3
Corner 0 8(1/8) = 1
Total 4 4 20
Mechanical Behaviour of Ceramics:
The mechanical properties of the ceramic phases are determined in many ways
depending upon how the force is applied : Tensile, compressive , shear, transverse,
horizontal or impact.
Tensile strength in ceramic is theoretically high but in practice it is quite low. failure are
often due to stress concentrated at the pores, grain corners of micro crack. Glass fibbers
posses very high vale of tensile strength.
Compression strength in ceramic is many times greater than tensile strength. Therefore
the ceramics like bricks, cement and glass are always used in compression and not in
tension. tempered glass is used in doors , car windows due to high compressive
strength.
Ceramics are generally high shearing strength and low fracture strength. therefore they
commonly failed in brittle manner or by fracture.
Transverse are difficult to ascertain in ceramic materials. Therefore they are not used in
places where transverse strength is an important criteria.
Tortional rarely considered as a critical property of ceramics.
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The mechanical properties of ceramic phase
(a)Hardness
The hardness of ceramic materials is a property which is of high
significance as it relates to the ability of the material to withstand
penetration of the surface through a combination of brittle fracture
and plastic flow.
Ceramic phases are hard because they generally cannot undergoes
plastic deformation.
Abrasive materials such as emery consist of Al2O3, Silicon Carbide
(SiC) and TiC. They are equally important for grinding and cutting
metals and similar manufacturing process. SiC has ZnS and TiC has
NaCl structure. 22
(b) Notch Sensitivity Definition: The extent to which the sensitivity of a material to fracture is increased
by the presence of a surface inhomogeneity such as a notch, a sudden change in
section, a crack, or a scratch.
Low notch sensitivity is usually associated with ductile materials, and high notch
sensitivity with brittle materials.
Notch or crack is stress raiser. The true stress (σ)
at the tip of the notch ( Fig ) exceeds the nominal
tensile stress (S) by a factor that includes √c where
c is the crack length from the surface or half of the
internal crack width .
Furthermore the stress concentration is greater
when the tip of the notch is sharp, and less when it
is rounded to a larger radius of curvature
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If σ exceeds, yields the strength in the ductile material the tip of the notch
will deform to a larger radius and stress concentration will be reduced.
If the notch is crack in nonductile material, the radius of curvature may be of
atomic dimension.
A crack that reaches only 0.1μm to 10 μm into the surface may multiply the
stress by a factor of 102 to 103.
Deeper crack are more sever. Even though the ceramics are strong in shear,
stress concentration σ is found to exceed the bound strength between the
atoms. Thus crack may propagates. This increases the crack length (c) and
gives more force to the stress concentration still further, until sudden failure
occurs.
The ceramic materials are generally weak in tension because of their
resistance to shear at the crack tip.
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Electromagnetic Behaviour of Ceramics: Electrical properties of Ceramics: (a) Dielectric
These are the materials or insulators which have the unique characteristic of being
able to store electric charge.
The electrons in these substances are localized in the process of bonding the atoms
together.
Obviously, covalent or ionic bonds, a mixture of both, or Vander Waals bonding
between closed shell atoms give rise to solids or gases which exhibit dielectric or
insulating properties.
Dielectric materials may be gases, liquids or solids with the exception of air which is
the insulating material between the bare conductors of the overhead electric grid
system.
Liquid dielectrics are used mainly as impregnates for high voltage paper insulated
cables and capacitors as filling and cooling media for transformers and circuit breakers.
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Most common properties of dielectric materials are:
(i) Dielectric constant
(ii) Dielectric strength
(iii) Insulation resistance
(iv) Surface resistivity
(v) loss factor
(vi) Tangent of loss factor in terms of a capacitor or phase difference
(vii)Polar and non-polar materials.
Materials, which are capable of separating electrical conductors circuit breakers, e.g. silicon, oils, liquid dielectrics have high dielectric constant, high resistance, high dielectric strength when moisture and impurities are removed from them. They have high temperature dissipation capacity and least dielectric losses.
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DIELECTRIC MATERIALS Mica: is the widely used insulating material in switch gears armature windings, electrical heating devices like iron, hot plates etc. It is also used in capacitors for high frequency application. Mica is an inorganic compound of silicates of aluminium, soda potash and magnesia. It is crystalline in nature and can be easily split into very thin flat sheets. The two important types of mica are: (i) muscovite and (ii) phlogopite. Mica has a good dielectric strength and mechanical strength. Its dielectric constant varies between 5 and 7.5, loss tangent between 0.0003 and 0.015 and dielectric strength between 700 and 1000 kV/mm.
Asbestos: is also used as an insulator in the form of paper, tape, cloth and board. Asbestos is widely used in panel boards, insulating tubes and cylinders in the construction of air cooled transformers. Asbestos is an inorganic material, which is used to designate a group of naturally occurring fibre material. Asbestos has good dielectric and mechanical properties.
Ceramics: are generally non-metallic inorganic compounds, e.g. silicates, aluminates, oxides, carbides, borides, nitrides and hydroxides. Ceramics used as dielectrics may be broadly described as alumina, porcelains, ceramics, titanates, etc. These have excellent dielectric and mechanical properties. The dielectric constant of most commonly used ceramics varies between 4 and 10. These are used in switches in plug holders, thermocouples, cathode heaters, vacuum type ceramic metal seals etc. Ceramic capacitors may be operated at high temperatures and can be moulded into any shape and size.
Electric grade ceramics are used for the manufacturing of insulators, terminal blocks, plates, frames, coils, etc. They must have low losses, good insulating properties and high strength.
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Glass: is an inorganic insulating material, which comprises of complex system of oxides. Silica (SiO2) is the most essential constituent of many commercially used glasses. It is fused with alkali (like potash, soda etc.) and some base (like lime, lead oxide etc.). The silica glass (having 100% SiO2) is the best insulating material. The dielectric constant of glass varies between 3.7 and 10 and loss tangent between 0.0003 and 0.01 and dielectric strength between 2.5 and 50 kV/mm. Glass is used in electric bulbs, X-ray tubes, mercury switches, electronic valves as insulating material. It is also used in capacitors as dielectric material.
Resins: are organic polymers and may be natural or synthetic. The synthetic resins are produced artificially. The commonly used synthetic resins are polyethylene, polystyrene, polyvinyl chloride, acrylic resins, teflon, nylon, etc. These have good dielectric and mechanical properties. The dielectric constant of resins varies between 2 and 4.5, the loss tangent between 0.0002 to 0.04 and dielectric strength is quite high. These are used in transformers, high frequency capacitors. These are also used as a dielectric material in d.c. capacitors.
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Uses of dielectrics
We have seen that dielectric materials are electrically insulative, yet
susceptible to polarization in the presence of an electric field. This
polarization phenomenon accounts for the ability of the dielectrics
to increase the charge storing capability of capacitors.
Now, we can summarize the main uses of dielectrics as follows:
(i) Piezoelectric and electro-optic devices
(ii) In capacitors, resistors and strain gauges
(iii) Thermionic valves, radiation detectors, electric devices,
dielectric amplifier
(iv) Dielectrics are usually used as ordinary insulators in power
cables, signal cables, electric motors, etc.
(v) Dielectrics are used in transformers and various form of
switchgear and generators where the dissipation problem of
heat is active, and a common way of getting rid of it is to
insulate with a transformer oil, i.e. mineral oil. 29
Feroelectricity The group of dielectric materials called ferroelectrics exhibit spontaneous polarization i.e., polarization in the absence of an electric field.
In a sense, ferroelectrics are the electric analog of the ferromagnets, which may display permanent magnetic behaviour.
In ferroelectrics, the polarization can be changed and even reversed by an external electric field.
The reversibility of the spontaneous polarization is due to the fact that the structure of a ferroelectric crystal can be derived from a non-polarized structure by small displacement of ions.
In most ferroelectric crystals, this non polarized structure becomes stable if the crystal is heated above a critical temperature, the ferroelectric Curie temperature (Tc); i.e. the crystal undergoes a phase transition from the polarized phase (ferroelectric phase) into an unpolarized phase (Paraelectric phase).
The change of the spontaneous polarization at Tc can be continuous or discontinuous.
The Tc of different types of ferroelectric crystals range from a few degrees absolute to a few hundred degrees absolute.
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In some ferroelectrics, spontaneous polarization can occur along
several axes that are equivalent in the paraelectric phase,
e.g. BaTiO3-type (or perovskite type) ferroelectrics.
One of the most common ferroelectrics is the Barium Titanate
(BaTiO3).
The spontaneous polarization in this crystal is a consequence of the
positioning of the Ba2+, Ti4+, and O2– ions within the unit cell (Fig.).
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At temperatures below the Curie point, the crystal distorts to tetragonal symmetry.
The titanium ion moves away from the centre of the unit cell (chosen as downward in the graphic), thus giving the unit cell a net dipole moment—with the positive end downward.
The Ba2+, ions are located at the corners of the unit cell, which is of tetragonal symmetry (a cube that has been elongated slightly in one direction). The dipole moment results from the relative displacements of the O2– and Ti4+ ions from their symmetrical positions as shown in the side view of the unit cell. We can see that O2– ions are located near, but slightly below, the centers of each of six faces, whereas the Ti4+ ion is displaced upward from the unit cell corner. Obviously, a permanent ionic dipole moment is associated with each unit cell.
However, when BaTiO3 is heated above its ferroelectric Tc (= 120°C), the unit cell becomes cubic, and all ions assume symmetric positions within the cubic unit cell; the material now has a perovskite crystal structure, and ferroelectric behaviour ceases.
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The piezoelectric effect was discovered by Jacques and Pierre Curie in
1880. They found that if certain crystal is mechanically strained, or when
the crystal is deformed by the application of an external stress, electric
charges appear on certain of the crystal surfaces; and when the direction
of the strain reverses, the polarity of the electric charge is reversed. This
is called the direct piezoelectric effect, and the crystals that exhibit it are
classed as piezoelectric crystals.
Conversely, when a piezoelectric crystal is placed in an electric field, or
when charges are applied by external means to its faces, the crystal
exhibits strain, i.e. the dimensions of the crystal change. When the
direction of the applied electric field is reversed, the direction of the
resulting strain is reversed. This is called the converse piezoelectric
effect.
Piezoelectricity and Effect of electric field and pressure
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The application of a mechanical stress produces in certain
dielectric (electrically non-conducting) crystals an electric
polarization (electric dipole moment per cubic meter) which
is proportional to this stress.
If the crystal is isolated, this polarization manifests itself
as a voltage across the crystal, and if the crystal is short-
circuited, a flow of charge can be observed during loading.
Conversely, application of a voltage between certain faces
of the crystal produces a mechanical distortion of the
material. This reciprocal relationship is referred to as the
piezoelectric effect 34
Piezoelectricity occurs only in insulating materials. Only few ceramic materials exhibit this property.
Piezoelectric materials are used extensively in transducers for converting a mechanical strain into an electrical signal.
Such devices include microphones, phonograph pickups, vibration-sensing elements, and the like.
The converse effect, in which a mechanical output is derived from an electrical signal input, is also widely used in such devices as sonic and ultrasonic transducers, headphones, loudspeakers, and cutting heads for disk recording. Both the direct and converse effects are employed in devices in which the mechanical resonance frequency of the crystal is of importance. Such devices include electric wave filters and frequency control elements in electronic oscillator circuits.
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The necessary condition for the piezoelectric effect is the absence of symmetry in the crystal structure. Of the 32 crystal classes, 21 lack a centre of symmetry, and with the exception of one class, all of these are piezoelectric.
Piezoelectric materials include titanates of barium and lead, zirconate (PbZrO3), ammonium dihydrogen phosphate (NH4H2PO4), natural quartz.
As stated earlier, this property is a characteristic of materials having complicated crystal structure with a low degree of symmetry. One may improve the piezoelectric behaviour of a polycrystalline specimen by heating above its Curie temperature and then cooling to room temperature in strong electric field.
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Piezoelectrics are required to have high piezoelectric
modulus and low losses.
The structure of piezoelectric ceramics is a solid
solution of barium titanate, barium niobate, lead niobate
or lead titanate.
Piezoelectric materials have the following important
applications:
(i) frequency resonators (ii) gramophone pickups
(iii) filters (iv) ultrasonic flaw detectors
(v) underwater sonar transducers and
(vi) air transducers (ear-phones, microphones,
hearing aids, etc.).
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Magnetic Properties of Ceramics:
Magnetic Ceramics:
Magnetic ceramics are important materials for a variety of
applications such data storage, tunnel junctions, spin valves, high
frequency applications etc.
These materials possess extra-ordinary properties such as
strong magnetic coupling, low loss characteristics and high
electrical resistivity which is often related to their structure and
composition.
Depending upon the type of application, based on the knowledge
of materials, one can choose appropriate material.
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Naturally magnetic materials have varying degrees
of magnetism that are characterized by two things.
First is the magnitude of the material’s magnetic
moment, which is a vector with a certain magnitude and
direction that determines the torque seen by the magnet
from an applied external magnetic field.
The second characterization is the sign of the material’s
susceptibility(χ)that describes how responsive a material
is to the applied magnetic field.
These characterizations separate magnetic materials
into the five types of magnetism- diamagnetism,
paramagnetism, ferromagnetism, antiferromagnetism and
ferrimagnetism. 39
Ferromagnetics have been known for thousands of years
and is the most common form of magnetism. These
materials led to the development of the compass,
electromagnets and generators without which we literally
would not be where we are today.
Ferromagnets have a positive susceptibility and magnetic
moments that align parallel with an applied external field,
therefore enhancing the total magnetism and maintain that
magnetism even when the applied field is removed.
Above a critical temperature a ferromagnet becomes
paramagnetic, where the magnetic moments still align in
the general direction of the applied field but the alignment
is much weaker than below the critical temperature and the
material no longer retains its magnetism when the applied
field is removed.
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A ferrimagnetic material is one that has populations of atoms with
opposing magnetic moments, as in antiferromagnetism; however,
in ferrimagnetic materials, the opposing moments are unequal and
a spontaneous magnetization remains.
This happens when the populations consist of different materials
or ions (such as Fe2+ and Fe3+).
Ferrimagnetism is exhibited by ferrites and magnetic garnets.
The oldest known magnetic substance, magnetite (iron(II,III)
oxide; Fe3O4), is a ferrimagnet; it was originally classified as a
ferromagnet before Néel's discovery of ferrimagnetism and
antiferromagnetism in 1948.
Ferrimagnetic materials are like ferromagnets in that they hold a
spontaneous magnetization below the Curie temperature, and show
no magnetic order (are paramagnetic) above this temperature.
However, there is sometimes a temperature below the Curie
temperature at which the two opposing moments are equal,
resulting in a net magnetic moment of zero; this is called the
magnetization compensation point.
41
Antiferromagnetic Antiferromagnetic are like ferromagnets but their the magnetic
moments of atoms or molecules, usually related to the spins of
electrons, align in a regular pattern with neighboring spins (on
different sublattices) pointing in opposite directions.
This alignment occurs spontaneously below a critical temperature
known as the Neel temperature.
The Neel temperature is named after Louis Neel who discovered
antiferromagnetics and was awarded the Nobel Prize in Physics for
his work in 1970.
the Neel temperature the material becomes paramagnetic.
Antiferromagnets are less common compared to the other types of
magnetic behaviors, and are mostly observed at low temperatures.
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The magnetic susceptibility of an antiferromagnetic material
typically shows a maximum at the Néel temperature. In contrast, at
the transition between thef erromagnetic .
Antiferromagnetic materials occur commonly among transition
metal compounds, especially oxides. Examples include hematite,
metals such as chromium, alloys such as iron manganese (FeMn),
and oxides such as nickel oxide (NiO), Mangenese II oxide(MnO) .
There are also numerous examples among high nuclearity metal
clusters. Organic molecules can also exhibit antiferromagnetic
coupling under rare circumstances, as seen in radicals such as 5-
dehydro-m-xylylene. 43
In a crystalline structure the ions in one plane have a parallel spin alignment
with each other and the ions in an adjacent plane have antiparallel spin
alignment, Figure. This creates two opposing magnetic moment sublattices
and thus a total magnetic moment of zero.
The antiparallel alignment is the result of superexchange of spin energy
within the material. An example of this antiferromagnetic phenomenon is
Mangenese II oxide. MnO is ionic with linear chains of Mn2+ and O2- ions. The
oxygen ion has a full set of valence electrons in the p orbitals directly
influencing the spin of the neighboring Mn2+ ions. The gray (spin down) and
black (spin up) circles represent the Mn ions. The oxygen ions, (open circles)
do not contribute to the antiferromag-netic behavior.
44
A schematic of the superexhange mechanism is shown in Figure.
It is energetically favorable for this ionic compound to possess a
degree of covalent bonding. Since both the Mn and O ions have full
electron shells, hybridization occurs by the donation of the O2-
electrons to the open orbitals of the Mn2+ ions. The Mn2+orbitals
containing up-spin electrons receives one down-spin electron from
the O2- p orbital leaving one up-spin O2- electron. The O2- ion is then
able to donate it’s up-spin to the next Mn2+ ion in the chain
completing the bonds. This donation, however, only occurs if the
nextMn2+ ion has it’s d elections in the down-spin orientation. Since
by Hund’s Rule all unpaired electrons must align with parallel spins
within an orbital, all of the Mn2+ electron spins must be flipped. This
is the superexchange. Thus the Mn2+ ions are aligned with opposing
spins within the crystalline structure.
superexchange of
MnO.
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FERRITE Yogoro Kato and Takeshi Takei of the Tokyo Institute of
Techn
A ferrite is a type of ceramic compound composed
of iron oxide (Fe2O3) combined chemically with one or
more additional metallic elements. They are
both electrically nonconductive and ferrimagnetic,
meaning they can be magnetized or attracted to a
magnet.ology synthesized the first ferrite c
Ferrites are usually non-
conductive ferrimagnetic ceramic compounds derived
from iron oxides such as hematite (Fe2O3)
or magnetite (Fe3O4) as well as oxides of other metals.
Ferrites are, like most of the other ceramics, hard
and brittleompounds.
Many ferrites are spinels with the formula AB2O4, where
A and B represent various metal cations,
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Ferrites can be divided into two families based on their
magnetic coercivity, their resistance to being
demagnetized.
Soft Ferrites:
Soft ferrites, are materials which are easy to magnetize or
demagnetize i.e. materials with low coercive field
strengths and thus so that they can reverse the direction
in alternating fields without dissipating much energy since
the area of B-H (or M-H) loop is small, while the material's
high resistivity prevents eddy currents in the core, another
source of energy loss.
Because of their comparatively low losses at high
frequencies, they are extensively used in the cores
of RF transformers and inductors in applications such
as switched-mode power supplies and loopstick
antennas used in AM radios.
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The most common soft ferrites are:
Manganese-zinc ferrite (MnZn, with the formula MnaZn(1-
a)Fe2O4). MnZn have higher permeability and saturation
induction than NiZn.
Nickel-zinc ferrite (NiZn, with the formula NiaZn(1-a)Fe2O4).
NiZn ferrites exhibit higher resistivity than MnZn, and are
therefore more suitable for frequencies above 1 MHz.
For applications below 5 MHz, MnZn ferrites are used;
above that, NiZn is the usual choice. The exception is
with common mode inductors, where the threshold of
choice is at 70 MHz.
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•Hard Ferrites:
In constrast, pernmanent ferrite magnets are made of
hard ferrites, which have a high corecivity and high
remanence after magnetization.
Iron oxide and barium or strontium carbonate are used in
manufacturing of hard ferrite magnets.
The high coercivity means the materials are very resistant
to becoming demagnetized, an essential characteristic for
a permanent magnet.
They also have high magnetic permeability. These so-
called ceramic magnets are cheap, and are widely used in
household products such as refrigerator magnets.
The maximum magnetic field B is about 0.35 tesla and the
magnetic field strength H is about 30 to 160 kiloampere
turns per meter (400 to 2000 oersteds).[8] The density of
ferrite magnets is about 5 g/cm3. 49
The most common hard ferrites are:
•Strontium Ferrite, SrFe12O19 (SrO·6Fe2O3):
Used in small electric motors, micro-wave devices,
recording media, magneto-optic media,
telecommunication and electronic industry.
•Barium Ferrite, BaFe12O19 (BaO·6Fe2O3):
A common material for permanent magnet applications.
Barium ferrites are robust ceramics that are generally
stable to moisture and corrosion-resistant.
They are used in e.g. loudspeaker magnets and as a
medium for magnetic recording, e.g. on magnetic stripe
cards.
Cobalt Ferrite, CoFe2O4 (CoO·Fe2O3):
Used in some media for magnetic recording.
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Applications of Magnetic Ceramics: In Electronic Inductors, Transformers and Electromagnets:
Soft ferrites like Mn-Zn and Ni-Zn ferrites are used as core materials in
these applications in the frequencies ranging from a 100 kHz to 100
MHz. Typically these ferrites have high electrical resistance which
results in very low eddy current losses. Most common radio magnets,
including those used in loudspeakers, are ferrite magnets. Ferrite
magnets have largely displaced Alnico magnets in these applications.
Ferrites are also used for power transformers which are used to
transmit either over a single frequency or within a range such as in
ultrasonic generators. For high frequency applications, upto about 5
MHz, Ni-Zn ferrites are useful while for frequencies upto 100 kHz, Mn-
Zn ferrites are preferred due to their higher permeabilities.
Equipment Shielding:
Here, due to their high impedance to high frequency currents, ferrite
components of Ni-Zn and Mn-Zn ferrites are able to prevent high
frequency electrical noise due to electromagnetic interference from
exiting or entering the equipment. 51
Data Storage ( Magnetic Recording Tapes and Hard Disks):
In the magnetic tapes, elongated 0.2-.5 μm long hard magnetic oxide particles
of γ -Fe2O4 are embedded in nonmagnetic binder. The particles have single
domains magnetized along their major axes which are aligned in the plane of
the tape. The coercive fields are typically between 50-100 kA.m-1. In magnetic
hard-disks, core element is produced by forming several layers of materials
(nonmagnetic underlayer, magnetic layer, overcoat, plus layer of lubricants
on a nonmagnetic disk substrate). Here, the read/write head is not in direct
contact with the hard disk (in contrast to floppy disk) due to an air bearing (˜
50 nm); air flow is caused by the relative velocity between disk and head.
These memories have high storage density of about 10 GB.in-2 and short
access time.
Early computer memories stored data in the residual magnetic fields of hard
ferrite cores,which were assembled into arrays of core memory. Ferrite
powders are used in the coatings of magnetic recording tapes. One such type
of material is iron (III) oxide.
Absorbing Materials:
In stealth aircrafts, ferrite particles are used as a component of radar-
absorbing materials or coatings and in the absorption tile lining in the rooms
used for electromagnetic compatibility measurements.
Microwave Applications in the Frequency Ranges of 1-300 GHz:
Materials like Mg-ferrites, Li-doped Ferrites and garnets are used for such
applications such as phase shifters, circulators and isolators.
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