Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: May 2015

Methods used for Nanomaterial Synthesis.

The methods of synthesis of nanoparticles are well known for a long time as compared to the other nanomaterial. For the synthesis of nanoparticles, the processing conditions need to be controlled in such a manner that the resulting nanoparticles have the following characteristics:

(i) identical size of all particles,

(ii) identical shape,

(iii) identical chemical composition and crystal structure

(iv) Individually dispersed with no agglomeration.

Nanoparticles can be synthesized by both top-down or bottom-up approaches. Two well-known top-down approaches are milling (or attrition) and thermal cycling. Attrition produces nanoparticles of a wide range of diameter ranging from 20 nm to several hundred nanometers.

The shape of the particles varies as well. They may contain impurities from the milling medium. The nanoparticles made by this process are usually used in the fabrication of Nano composites and bulk materials having Nano grains where perfections in size and shape, and presence of impurities do not matter significantly.

Moreover, some of the defects can get annealed during the sintering process. A bulk material having very small thermal conductivity but a large coefficient of thermal expansion may be subjected to repeat thermal cycling to produce very fine particles. However, this technique is difficult to design and the control of particle size and shape is difficult.

The bottom-up methods are more popular than the top-down methods. There are several bottom-up methods such as homogeneous and heterogeneous nucleation processes, microemulsion based synthesis, aerosol synthesis, spray pyrolysis and template-based synthesis. In this lecture, some of these techniques will be discussed.

TTT diagram and its importance:

TTT diagram stands for “time-temperature-transformation” diagram. It is also called isothermal transformation diagram.

TTT diagrams give the kinetics of isothermal transformations.

TTT diagram gives

i) Nature of transformation-isothermal or athermal (time independent) or mixed

ii) Type of transformation-reconstructive, or displacive.

iii) Rate of transformation

iv) Stability of phases under isothermal transformation conditions

v) Temperature or time required to start or finish transformation

vi) Qualitative information about size scale of product

Hardness of transformed products