Solid State
9.0 Properties of Solids
9.0 Properties of Solids
$1.$ Electrical Properties:
Solids can be broadly classified into three types, on the basis of electrical conductivity.
$a)$ Metals (conductors)
$b)$ Insulators
$c)$ Semi-conductors
Electrical conductivity of metals is very high and is of the order of $106 – 108$ $ohm^{–1} cm{–1}$ whereas for insulators, it is of the order of $10^{–12}$ $ohm^{–1}cm^{–1}$.
Semi-conductors have intermediate conductivity in the range of $10^2 – 10^{–9} ohm^{–1} cm^{–1}$. Electrical conductivity of solids may arise through the motion of electrons and holes (positive) or through the motion of ions.
The conduction through electrons is called n-type conduction and through (positive) holes is called p-type conduction.
Pure ionic solids where conduction can take place only through movement of ions are insulators. The presence of defects in the crystal structure increases their conductivity.
The conductivity of semi-conductors and insulators is mainly due to the presence of interstitial electrons and positive holes in the solids due to imperfections. The conductivity of semi-conductors and insulators increases with increase in temperature while that of metals decrease.
$2.$ Magnetic Properties
- Diamagnetic Materials: Those materials which are weakly repelled by the magnetic field are called diamagnetic materials. e.g. $Cu^+$, $TiO_2$, $NaCl$ and benzene. They do not have unpaired electrons.
- Paramagnetic Materials: The materials which are weakly attracted by magnetic field are called paramagnetic materials. These materials have permanent magnetic dipoles due to presence of atoms, ions or molecules with unpaired electron. e.g. $O_2$, $Cu^{2+}$, $Fe^{2+}$ etc.
But these materials lose their magnetism in the absence of magnetic field.
- Ferromagnetic Materials: The materials which show permanent magnetism even in the absence of magnetic field are called ferromagnetic materials. These materials are strongly attracted by the magnetic field. e.g. $Fe$, $Co$, $Ni$ and $CrO_2$. Ferromagnetism arises due to spontaneous alignment of magnetic moments of ions or atoms in the same direction.
Alignment of magnetic moments in opposite directions in a compensatory manner and resulting in zero magnetic moment gives rise to anti-ferromagnetism.
Alignment of magnetic moments in opposite directions resulting in a net magnetic moment due to unequal number of parallel and anti-parallel magnetic dipoles give rise to ferri-magnetism e.g. $Fe_3O_4$.
Ferromangetic and ferrimagnetic substances change into paramagnetic substances at higher temperature due to randomisation of spins. $Fe_3O_4$, is ferrimagnetic at room temperature and becomes paramagnetic at $850\ K$.
$3.$ Dielectric Properties:
The electrons in insulators are closely bound to the individual atoms or ions and thus they do not generally migrate under the applied electric field. However, due to shift in charges, dipoles are created which results in polarisation. The alignment of these dipoles in different ways i.e. compensatory way (zero dipole) or non-compensatory way (net dipole) impart certain characteristic properties to solids.
If the dipoles align in such a way that there is net dipole moment in the crystals, these crystals are said to exhibit piezoelectricity or piezoelectric effect i.e. when such crystals are subjected to pressure or mechanical stress, electricity is produced. Conversely, if an electric field is applied to such a crystal, the crystal gets deformed due to generation of mechanical strain. This is called inverse piezoelectric effect.
Some crystals which on heating, acquire electric charges on opposite faces, are said to exhibit pyroelectric effect. The solids, in which dipoles are spontaneously aligned in a particular direction, even in the absence of electric field are called ferroelectric substances and the phenomenon is known as ferroelectricity. If the alternate dipoles are in opposite direction, then the net dipole moment will be zero and the crystal is called anti-ferroelectric.
Ferroelectric solids – Barium titanate $(BaTiO_3)$, sodium potassium tartrate (Rochelle salt) and potassium hydrogen phosphate $(KH_2PO_4)$. Ant-?ferroelectric – Lead Zirconate $(PbZrO_3)$.
$4.$ Super Conducting Materials:
The material which offer no resistance to the passage of electricity is called superconductor or super conducting material. In this state, the materials become diamagnetic and are repelled by the magnets. Most of the metals become super conducting at low temperatures $(2 – 5K)$. Highest temperature at which super conductivity is known is $23\ K$ in alloys of niobium (e.g. $Nb_3Ge$). Many complex metal oxides have been found to possess super-conductivity at somewhat higher temperatures.
Material | Temperature |
$Nb_3Ge$ | $23K$ |
$Bi_2Ca_2Sr_2Cu_3O_{10}$ | $105K$ |
$Ti_2Ca_2Ba_2Cu_3O_{10}$ | $125K$ |
