Size Dependent Properties

SIZE DEPENDENT PROPERTIES

Nearly all the properties as shown in figure 2.5, like hardness, strength, ductility, melting point and density, change for nanomaterials. These behaviors vary so significantly by a mere reduction in grain size. Nanomaterials are composed of grains and grain boundaries. Nanometre sized grains contains only a few thousands of atoms with in each grain.

A large number of atoms reside at the grain boundaries. As the grain size decreases, there is a significant increase in the volume fraction of grain boundaries (or) interfaces.

The properties of the materials are bound to be governed to a large extent by defect configurations. Hence the mechanical and chemical properties of nanomaterials are significantly altered due to defect dynamics. The elastic property of nanomaterials are different from that of bulk alloys due to the presence of increased fraction of defects.

Examples

1. Nanocrystalline ceramics are tougher and stronger than those with coarse grains.

2. Nano‒sized metals exhibit significant decrease in toughness and yield strength increases.

Important Properties of Nanomaterials

1. Electrical Properties

(i) Electrical conductivity decreases with a reduced dimension due to increased surface scattering. However, it can be increased, due to better ordering in micro‒structure.

Example: Polymeric fibres.

(ii) Nanocrystalline materials are used as very good separator plates in batteries, because they can hold more energy than the bulk materials.

Example: Nickel‒metal hydride batteries made of nanocrystalline nickel and metal hydride, require far less frequent recharging and last much longer.

2. Optical Properties

Reduction of material dimensions has pronounced effects on the optical properties. Optical properties of nano‒materials are different from bulk forms.

Reason for change in optical properties

The change in optical properties is caused by two factors

(i) The quantum confinement of electrons within the nano‒particles increases the energy level spacing.

Example: The optical absorption peak of a semiconductor nano‒particles shifts to a short wavelength, due to an increased band gap.

(ii) Surface plasma resonance, which is due to smaller size of nano‒particles than the wavelength of incident radiation.

Example: The colour of metallic nano‒particles may change with their sizes due to surface plasma resonance.

3. Mechanical properties

The nano‒materials have less defects compared to bulk materials, which increases the mechanical strength.

(i) Mechanical properties of polymeric materials can be increased by the addition of nano‒fillers.

(ii) As nano‒materials are stronger, harder and more wear resistant and corrosion resistant, they are us they are used in spark plugs.

Example: Nano‒crystalline carbides are much stronger, harder and wear resistant and are used in micro drills.

4. Magnetic properties

Magnetic properties of nano materials are different from that of bulk materials. Ferro‒magnetic behaviour of bulk materials disappear, when the particle size is reduced and transfers to super‒paramagnetics. This is due to the huge surface area.