JEE Main & Advanced Chemistry The Solid State / ठोस प्रावस्था Properties Of Solids

Some of the properties of solids which are useful in electronic and magnetic devices such as, transistor, computers, and telephones etc., are summarised below,

(1) Electrical properties : Solids are classified into following classes depending on the extent of conducting nature.

(i) Conductors : The solids which allow the electric current to pass through them are called conductors. These are further of two types; Metallic conductors and electrolytic conductors. The electrical conductivity of these solids is high in the range \[{{10}^{4}}-{{10}^{6}}oh{{m}^{-1}}c{{m}^{-1}}\]. Their  conductance decrease with increase in temperature.

(ii) Insulators : The solids which do not allow the current to pass through them are called insulators. e.g., rubber, wood and plastic etc. the electrical conductivity of these solids is very low i.e., \[{{10}^{-12}}-{{10}^{-22}}oh{{m}^{-1}}c{{m}^{-1}}\].

(iii) Semiconductors : The solids whose electrical conductivity lies between those of conductors and insulators are called semiconductors. The conductivity of these solid is due to the presence of impurities. e.g. Silicon and Germanium. Their conductance increase with increase in temperature. The electrical conductivity of these solids is increased by adding impurity. This is called Doping. When silicon is doped with P (or As, group 5^th  elements), we get n-type semiconductor. This is because P has five valence electrons. It forms 4 covalent bonds with silicon and the fifth electron remains free and is loosely bound. This give rise to n-type semiconductor because current is carried by electrons when silicon is doped with Ga (or in In/Al, group 3^rd  elements) we get p-type semiconductors.

Superconductivity : When any material loses its resistance for electric current, then it is called superconductor, Kammerlingh Onnes (1913) observed this phenomenon at 4K in mercury. The materials offering no resistance to the flow of current at very low temperature (2-5K) are called superconducting materials and phenomenon is called superconductivity.

Examples, \[N{{b}_{3}}\]Ge alloy (Before 1986)

                            \[L{{a}_{1.25}}B{{a}_{0.15}}Cu{{O}_{4}}\] (1986)

                           \[YB{{a}_{2}}\]\[C{{u}_{3}}{{O}_{7}}\] (1987)

Following are the important applications of superconductivity,

(a) Electronics,                           (b) Building supermagnets,

(c) Aviation transportation,       (d) Power transmission

“The temperature at which a material enters the superconducting state is called the superconducting transition temperature, \[({{T}_{c}})\]”. Superconductivity was also observed in lead (Pb) at 7.2 K and in tin (Sn) at 3.7K. The phenomenon of superconductivity in other materials such as polymers and organic crystals.  Examples are

(SN)_x, polythiazyl, the subscript x indicates a large number of variable size.

(TMTSF)₂PF₆, where TMTSF is tetra methyl tetra selena fulvalene.

(2) Magnetic properties : Based on the behavior of substances when placed in the magnetic field, there are classified into five classes.

Magnetic properties of solids

 (3) Dielectric properties : A dielectric substance is that which does not allow the electricity to pass through them but on applying the electric field, induced charges are produced on its faces. In an insulator, the electrons are strongly held by the individual atoms. When an electric field is applied polarization takes place because nuclei are attracted to one side and the electron cloud to the other side. As a result, dipoles are created. Such type of crystals shows the following properties,

(i) Piezoelectricity : In some of the crystals, the dipoles may align themselves is an ordered way so as to give some net dipole moment. When mechanical stress is applied in such crystals so as to deform them, electricity is produced due to the displacement of ions. The electricity thus produced is called piezoelectricity and the crystals are called piezoelectric crystals. Examples, Quartz, Rochelle’s salt ( sod. pot. tartarate). Piezoelectric crystals act as mechanical-electric transducer. These crystals are used as pick-ups in record players where they produce electric signals by application of pressure.

(ii) Pyroelectricity : On heating, some polar crystals produce a small electric current. The electricity thus produced is called pyroelectircity.

(iii) Ferroelectricity : In some of the piezoelectric crystals, a permanent alignment of the dipoles is always there even in the absence of the electric field, however, on applying field the direction of polarization changes. This phenomenon is called ferroelectricity and the crystals as ferroelectric crystal. Example, Potassium hydrogen phosphate \[(K{{H}_{2}}P{{O}_{4}})\], Barium titanate \[(BaTi{{O}_{3}})\].

(iv) Antiferroelectricity : In some crystals, the dipoles in alternate polyhedra point up and down so that the crystals does not possess any net dipole moment. Such crystals are said to be antiferroelectric. Example, Lead zirconate \[(PbZr{{O}_{3}})\]. Ferroelectrics are used in the preparation of small sized capacitors of high capacitance. Pyroelectric infrared detectors are based on such substances. These can be used in transistors, telephone, computer etc.