Electrical and Magnetic Properties

Electrical Properties

Electrical conductivity of solids may arise through the motion of electrons and positive holes (electronic conductivity) or through the motions of ions (ionic conductivity).

The conduction through electrons is called n-type conduction and through positive holes is called p – types conduction.

Electrical conductivity of metal is due to motion of electrons and it increases with the number of electrons available to participate in the conduction process.

Pure ionic solids where conduction can take place only through motion of ions are insulators. However, the presence of defects in the crystal structure increases their conductivity.

On the basis of electrical conductivity the solids can be classified into three types –

· Conductors

· Insulators

· Semi conductors

Metal (Conductors):

They allow the maximum portion of the applied electric field to flow through them and have conductivities in order of 10⁶ – 10⁸ ohm^-1.

Insulators:

They have low conductivities i.e. they do not practically allow the electric circuit to flow through them. The electrical conductivity is in order 10^-10 – 10^-20 ohm^-1 m^-1

Semi conductors:

The solids with intermediate conductivities at the room temperature. Semi conductors allow a portion of electric current to flow through them.

Actually semi conductors are those solids which are perfect insulators at absolute zero, but conduct electric current at room temperature.

Conduction of Electricity in Metals

A conductor may conduct electricity through movement of electrons or ions. Metallic conductors belong to the former category and electrolytes to the latter.

Metals conduct electricity in solid as well as molten state. The conductivity of metals depend upon the number of valence electrons available per atom. The atomic orbitals of metal atoms form molecular orbitals which are so close in energy to each other as to form a band. If this band is partially filled or it overlaps with a higher energy unoccupied conduction band, then electrons can flow easily under an applied electric field and the metal shows conductivity (below figure a).

If the gap between filled valence band and the next higher unoccupied band (conduction band) is large, electrons cannot jump to it and such a substance has very small conductivity and it behaves as an insulator (below figure b).

Distinction among (a) metals (b) insulators and (c) semiconductors. In each case, an unshaded area represents a conduction band.

Conduction of Electricity in Semiconductors

In case of semiconductors, the gap between the valence band and conduction band is small (above figure c). Therefore, some electrons may jump to conduction band and show some conductivity. Electrical conductivity of semiconductors increases with rise in temperature, since more electrons can jump to the conduction band. Substances like silicon and germanium show this type of behaviour and are called intrinsic semiconductors.

The conductivity of these intrinsic semiconductors is too low to be of practical use. Their conductivity is increased by adding an appropriate amount of suitable impurity. This process is called doping. Doping can be done with an impurity which is electron rich or electron deficient as compared to the intrinsic semiconductor silicon or germanium. Such impurities introduce electronic defects in them.

(i) n-type semiconductors

Silicon doped with 15 group elements like phosphorus is called ntype semiconductor. The conductivity is due to the presence of negative charge (electrons),

(ii) p-type semiconductors

Silicon doped with 13 group element like gallium is called p-type semiconductor. The conductivity is due to the presence of positive holes.

· Some typical 13-15 compounds are InSb, AlP and GaAs and SOme typical 12-16 compounds are ZnS, CdS. CdSe and HgTe.

· These exhibit electrical and optical properties of great use in electronic industry. Magnetic Properties of Solids

Magnetic Properties

Ø The magnetic properties of different materials are studies in terms of their magnetic moments which arise due to the orbital motion and spinning motion of the electron.

Ø As electron is charged particle, the circular motion of the electric charge causes the electron to act as a tiny electro magnet.

Ø The magnetic moment of the magnetic field generated due to orbital motion of the electron is along the axis of rotation.

Ø The electron also possesses magnetic moment due to the spin which is directed along the spin axis.

Ø Thus, magnetic moment of the electron is due to travelling in closed path (orbital motion) about the nucleus and spinning on its axis.

Ø For each electron spin magnetic moment is ±μB where μB, Bohr Magneton is the fundamental unit of magnetic moment and is equal to 9.27 × 10^-24 em².

Ø The magnetic moment due to orbital motion is equal to Mlμ B where Ml is the magnetic quantum number of the electron.

Demonstration of the magnetic moment associated with (a) an orbiting electron and (b) a spinning electron

On the basis of their magnetic properties, substances can be classified into five categories:

i. Paramagnetic

ii. Diamagnetic

iii. Ferromagnetic

iv. Antiferromagnetic and

v. Ferrimagnetic

Paramagnetic Substances

These are attracted by the magnetic field and have unpaired electrons. These lose magnetism in the absence of magnetic field, e.g., O², Cu²⁺, Fe³⁺, etc.

Diamagnetic Substances

These are weakly repelled by the magnetic field and do not have any unpaired electron, e.g., TiO₂, V₂O₅, C₆H₆, NaCI, etc.

Ferromagnetic Substances

These are attracted by the magnetic field and show permanent magnetism even ill the absence of magnetic field e.g., Fe, Co and Ni.

Anti-ferromagnetic Substances

These substances have net magnetic moment zero due to compensatory alignment of magnetic moments, e.g., MnO, MnO₂, FeO, etc.

Ferrimagnetic Substances

These substances have a net dipole moment due to unequal parallel and anti-parallel alignment of magnetic moments, e.g., Fe₃O₄, ferrites, etc.

Schematic alignment of magnetic moments in (a) ferromagnetic (b) antiferromagnetic and (c) ferromagnetic

Properties	Description	Alignment of Magnetic Dipoles	Examples	Applications
Diamagnetic	Feebly repelled by the magnetic fields. Non-metallic elements (excepts O2, S) inert gases and species with paired electrons are diamagnetic	All paired electrons	TiO2, V2O5, NaCl, C6H6 (benzene)	Insulator
Paramagnetic	Attracted by the magnetic field due to the presence of permanent magnetic dipoles (unpaired electrons). In magnetic field, these tend to orient themselves parallel to the direction of the field and thus, produce magnetism in the substances.	At least one unpaired electron	O2,Cu2+,Fe3+,TiO, Ti2O3,VO,VO2 , CuO	Electronic appliances
Ferromagnetic	Permanent magnetism even in the absence of magnetic field, Above a temperature called Curie temperature, there is no ferromagnetism.	Dipoles are aligned in the same direction	Fe, Ni, Co, CrO2	CrO2 is used in audio and video tapes
Antiferromagnetic	This arises when the dipole alignment is zero due to equal and opposite alignment.		MnO, MnO2, Mn2O, FeO, Fe2O3; NiO, Cr2O3, CoO, Co3O4,
Ferrimagnetic	This arises when there is net dipole moment		Fe3O4, ferrites