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Friday, July 25, 2008

Dielectric and Magnetic properties of materials

Dielectric loss
When a dielectric material is subjected to an ac field, a part of the energy is lost each time the field changes its direction. This is due to the fact that each time the field is reversed, the direction of the dipoles has to change. In this process, a loss of energy due to friction occurs. This energy loss appears in the form of heat. This energy loss depends on the frequency of ac field.
Important applications of dielectric materials
1. As insulating materials
2. In capacitors
3. Dielectric ceramic materials are used in high voltage power lines
4. Liquid dielectrics such as petroleum oil are used in transformers for cooling, circuit breakers and in cables.
5. In dielectric heating devices
6. Non linear dielectrics (piezoelectrics and ferroelectrics where er depend on the intensity of the applied electric field) are used in many applications.
Ferro electricity (Qualitative)
Dielectric materials which possess electric polarization even in the absence of electric field (spontaneous polarization) are known as ferroelectric materials. They exhibit electrical hysteresis for the variation of polarization P with the applied electric field E analogous to the way ferromagnetic material exhibit magnetic hysteresis for variation of B with H.

Ferroelectric materials possess very high values of er of the order of 1000 to 10000. In ferroelectric materials the value of er changes with temperature according to the relation, er = C/T-TC above TC, where TC is called Curie temperature. This is known as Curie-Wiess law of ferroelectrics. Rochelle salt, BaTiO3 and PbTiO3 are examples of ferroelectric materials. These materials are widely used in capacitors, electronic circuits and in memory devices.
Piezoelectricity (Qualitative)
When one pair of opposite faces of certain asymmetric crystals such as quartz crystal is compressed, opposite electric charges appear on the other pair of opposite faces of the crystal. On application of tension, the polarities of the charges reversed. This phenomenon of the development of charges as a result of the mechanical deformation is known as piezoelectric effect. Crystals which exhibit this property are called piezoelectric crystals. Quartz, ammonium phosphate and lead zirconate titanate are examples of piezoelectric materials.
The converse of piezoelectric effect is also true. If an electric field is applied across one pair of faces of piezoelectric crystal, it gets deformed along the direction of the other opposite pair of faces. If an ac field is applied, crystal vibrates with the frequency of the field. This effect is also called inverse piezoelectric effect which is used for the production of ultrasonic waves. Piezoelectric materials are also used in gas igniters, vibrators, sensors, detectors and in printer heads.

Langevin’s theory of Diamagnetism (Qualitative treatment)
Diamagnetic materials are those which acquire feeble magnetism opposite to the applied magnetic field. i.e., they exhibit negative susceptibility. The order of susceptibility is around 10-6. Copper, Bismuth, Lead, Zinc, Inert gases etc. are examples.
Here all the electrons rotating in clockwise direction will have magnetic moment in one direction and the remaining electrons which rotate in the anticlockwise direction will have magnetic moment in opposite direction. Thus net magnetic moment in any direction must be zero.
When magnetic field is applied, the electrons rotating in clockwise direction will experience a Lorentz force directed radially outwards. Now to maintain equilibrium, electrons reduce its velocity which results in the reduction of magnetic moment in the direction of applied magnetic field.
Meanwhile the electrons which revolve in the anticlockwise direction will experience a force directing radially inwards. Now to maintain equilibrium, electrons increase its velocity which results in an increase of magnetic moment in a direction opposite to the magnetic field.

The entire ferromagnetic volume splits into a large number of small regions of spontaneous magnetization. These regions are called domains. In the absence of magnetic field, the relative orientation of the magnetic moments of various domains will be completely random, and thus the resultant magnetic moment of the material as a whole turns out to be zero.
Antiferromagnetism and ferrimagnetism
When the magnetic moments of sublattices in a crystal unit cell are equal in magnitude but opposite in direction, they cancel each other giving rise to antiferromagnetism. Their susceptibility is of the order of 10-3 to 10-5. MnO is an example for antiferromagnetic material. If the temperature of the antiferromagnetic material is raised above Neel temperature, material becomes paramagnetic.
When the magnetic moments of sublattices in a crystal unit cell are not exactly equal in magnitude and oriented in opposite in direction, the crystal possess a net resultant magnetic moment. This type of materials is called ferromagnetic materials whose saturation magnetization value is not as high as for ferromagnetic materials. Ferrites are examples for ferrimagnetic material.
Ferrites are mixed metal oxides with the general formula MFe2O4 or MO Fe2O3. By varying composition, we can appreciably vary the magnetic and other properties of ferrite materials. Metal ions can occupy tetrahedral A sites (surrounded by 4 oxygen ions) or octahedral B sites (surrounded by 6 oxygen ions). In ZnFe2O4 and CdFe2O4 F3+ ions occupy octahedral sites and Zn2+/Cd2+ ions occupy tetrahedral sites. They are called normal spinel ferrites. In NiFe2O4, CoFe2O4 and CuFe2O4, Fe3+ ions occupy both octahedral and tetrahedral sites while Ni2+, Co2+ and Cu2+ ions occupy octahedral sites. Hence they are called inverse spinel ferrites.
B-H graph in ferromagnetic materials
When a ferromagnetic material is taken through a cycle of magnetization, a curve as shown below is obtained. This curve is known as hysteresis curve (B-H curve).

When a magnetic field H is applied to a ferromagnetic material, the magnetic flux density B (or magnetisation M) will vary. As the magnetic field is increased, the flux density (or magnetisation) increases and reaches a saturation value Bs (or Ms). When the field intensity is reduced to zero, the flux density will not become zero, but will have a finite value which is called remanent flux density Br ( or remanent magnetisation Mr). This remanent flux density may be reduced to zero by applying a magnetic field in the opposite direction. The field Hc required to reduce the flux density to zero is called the coercive field.
Soft and hard magnetic materials
Magnetic materials which are easily magnetized and demagnetized are known as soft magnetic materials. They are characterized by thin hysteresis loop (loop area small) with low coercive field, low hysteresis loss and high initial permeability. All these properties are by virtue of the domain wall motion that occurs easily in soft magnetic materials. Permalloy, Silicon-Iron alloy and ferrites are examples.
Hard magnetic materials are those which have a high resistance to demagnetization. They are characterized by large hysteresis loop (loop are large) with high coercive field, high hysteresis loss and low initial permeability. In these materials, domain walls are highly immobile. Alnico alloy, Invar, Platinum-Cobalt alloy etc. are examples.

Applications of magnetic materials
High resistivity soft magnetic materials are used in the transformer cores since in ac conditions hysteresis loss factor and eddy current loss are important and for high resistivity soft magnetic materials these factors are low. Ferrites are used for high frequency applications. Hard magnetic materials are used in fabrication of permanent magnets. Magnetic materials are also used in memory devices, loud speakers, generators, switching devices, tape-recorders, telephones, electrical instruments, TV tubes etc.

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