Material science & Engineering : Electrical Properties of Metals, Se & Dielectrics
Electrical Properties of engineering materials:-
Electrical properties are the ability to transfer electrical current. Various electrical properties are resistivity, temperature coefficient of resistance, Electrical conductivity, dielectric strength, and thermoelectricity.
Some of the electrical properties of engineering materials are below
Electrical Resistivity
It is a property of the material that resists the flow of electric current through the material. It is a give-and-take of conductivity. Resistivity values are reported in micro ohm centimeters units.
Electrical Conductivity
It is a property of the material that allows the flow of electric current through the material. It is a parameter which indicates that how easily electric current can flow through the material. The conductivity of the material is give and take of resistivity. Electrical conductivity measures how well material accommodates the movement of an electric charge. It is the ratio of current density to electric field strength.
Electrical conductivity is a very useful property since values are affected by such things. Therefore, electrical conductivity information can be used for measuring the purity of water, checking for proper heat treatment of metals, and inspecting for heat damage in some materials.
Dielectric Strength
It is a property of the material that indicates the ability of it to withstand high voltages. Usually, it is specified for insulating material to represent their operating voltage.
Temperature Coefficient of Resistance
Temperature coefficient of resistance of material indicates the change in resistance of material with a change in temperature. Resistance of conductor changes with the change of temperature. As noted above, electrical conductivity values are reported at 20 degrees centigrade. This is done because the conductivity and resistivity of the material is depending on temperature. Thus conductivity of materials decreases as temperature increases.
Thermoelectricity
If a link formed by joining two metals is heated, a small voltage of millivolt is produced. This effect is called thermoelectricity or thermoelectric effect. This effect forms the basis of the operation of thermocouples and some temperature-based transducers. This can be used to generate electricity, measure temperature, and measure change in the temperature of objects.
Magnetic Properties of Material
Origin of magnetism lies in the orbital and spin motions of electrons and how electrons interact with one another. Magnetic properties of the material are those which determine the ability of material for a particular magnetic application. Read Engineering Materials – Deformation of Solid Materials
Permeability
It is the property of magnetic material which indicates that how easily magnetic flux is build up in the material. It is determined by the ratio of magnetic flux density to magnetizing force producing this magnetic flux density.
Hysteresis
Magnetic Hysteresis is an important material by which it first becomes magnetized and then de-magnetization process. The lack of retrace ability of magnetization curve is called hysteresis and is related to the existence of magnetic domains in material. Magnetic hysteresis is the rising temperature at which a given material ceases to be ferromagnetic or falling temperature at which it becomes magnetic.
Coercive Force
This force defined as the magnetizing force which is essential to neutralize completely magnetism in an electromagnet after the value of magnetizing force becomes zero. Also, read Engineering Materials -Comparison between Crystal Slip and Twinning.
Metals
The electrical property of a metal is determined by the ease of movement of electrons past the atoms under the influence of an electric field. This movement is particularly easy in copper, silver, gold, and aluminum—all of which are well-known conductors of electricity. The conductivity of a given metal is decreased by phenomena that deflect, or scatter, the moving electrons. These can be anything that destroys the local perfection of the atomic arrangement—for example, impurity atoms, grain boundaries, or the random oscillation of atoms induced by thermal energy. This last example explains why the conductivity of a metal increases substantially with falling temperature: in a pure metal at room temperature, most resistance to the motion of free electrons comes from the thermal vibration of the atoms; if the temperature is reduced to almost absolute zero, where thermal motion essentially stops, conductivity can increase several thousandfolds.
Semiconductors
Semiconductors possess specific electrical properties. A substance that conducts electricity is called a conductor, and a substance that does not conduct electricity is called an insulator. Semiconductors are substances with properties somewhere between them. Electrical properties can be indicated by resistivity. Conductors such as gold, silver, and copper have low resistance and conduct electricity easily. Insulators such as rubber, glass, and ceramics have high resistance and are difficult for electricity to pass through. Semiconductors have properties somewhere between these two. Their resistivity might change according to the temperature, for example, At a low temperature, almost no electricity passes through them. But when the temperature rises, electricity passes through them easily.
Semiconductors containing almost no impurities conduct almost no electricity. But when some elements are added to the semiconductors, electricity passes through them easily. Semiconductors comprising a single element are called elemental Semiconductors, including the famous semiconductor material Silicon. On the other hand, semiconductors made up of two or more compounds are called compound semiconductors and are used in semiconductors lasers, LED, etc.
Dielectric
It is the combination of two words- ‘Dia’ and ‘electric’. The electrical conductivity of a perfect dielectric is zero. A dielectric stores and dissipate the electrical energy similar to an ideal capacitor. Some of the main properties of a Dielectric material are Electric Susceptibility, Dielectric polarization, Dielectric dispersion, Dielectric relaxation, Tunability, etc…
Electric Susceptibility
The ease of a dielectric material that can be polarized when subjected to an electric field is measured by the electric susceptibility. This quantity also determines the electric permeability of the material.
Dielectric Polarization
An electric dipole moment is a measure of separation of negative and positive charge in the system. The relationship between the dipole moment (M) and the electric field (E) gives rise to the properties of dielectric. When the applied electric field is removed the atom return to its original state. The time taken by the atom to reach its original state is known as Relaxation time.
Total Polarization
There are two factors that decide the polarization of dielectric. They are the formation of dipole moment and their orientation relative to the electric field. Based on the elementary dipole type there can be either electronic polarization or ionic polarization. Electronic polarization Pe occurs when the dielectric molecules forming the dipole moment are composed of neutral particles.
Ionic polarization Pi and electronic polarization both are independent of temperature. Permanent dipole moments are produced in the molecules when there is an asymmetrical distribution of charge between different atoms. In such cases, orientational polarization Po is observed. If a free charge is present in the dielectric material it would lead to the Space charge polarization Ps. The total polarization of dielectric involves all these mechanisms. Thus the total polarization of the dielectric material is
PTotal = Pi + Pe + Po + Ps
Dielectric Dispersion
When P is the maximum polarization attained by the dielectric, tr is the relaxation time for a particular polarization process, the dielectric polarization process can be expressed as
P(t) = P[1-exp(-t/tr )]
The relaxation time varies for different polarization processes. Electronic polarization is very rapid followed by ionic polarization. Orientation polarization is slower than ionic polarization. Space charge polarization is very slow.
Dielectric Breakdown
When higher electric fields are applied, the insulator starts conducting and behaves as a conductor. In such conditions, dielectric materials lose their dielectric properties. This phenomenon is known as Dielectric Breakdown. It is an irreversible process. This leads to the failure of dielectric materials.
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