Introduction
There are a large number of techniques available to synthesize different types
of nanomaterials in the form of colloids, clusters, powders, tubes, rods, wires,
thin films etc. Some of the already existing conventional techniques to synthesize
different types of materials are optimized to get novel nanomaterials and some new
techniques are developed. Nanotechnology is an interdisciplinary subject. There are
therefore various physical, chemical, biological and hybrid techniques available to
synthesize nanomaterials. It can be seen from Box 3.1 that, for each type, there is
a large number of possibilities. The list is not complete but gives some commonly
used techniques; some of them will be described in this and in the next two chapters.
The technique to be used depends upon the material of interest, type of nanomaterial
viz. zero dimensional 0-D, one dimensional 1-D or two dimensional 2-D, their
sizes and quantity.
In this chapter we shall discuss some physical as well as related methods to obtain
nanomaterials.
It is one of the simplest ways of making nanoparticles of some metals and alloys
in the form of powder. There are many types of mills such as planetary, vibratory,
rod, tumbler etc. Usually one or more containers are used at a time to make large
quantities of fine particles. Size of container, of course, depends upon the quantity of
interest. Hardened steel or tungsten carbide balls are put in containers
along with powder or flakes <50 m of a material of interest. Initial material can be of arbitrary size and shape. Container is closed with tight lids. Usually 2:1 mass
ratio of balls to material is advisable. If the container is more than half filled, the
efficiency of milling is reduced. Heavy milling balls increase the impact energy on
collision. Larger balls used for milling produce smaller grain size but larger defects
in the particles. The process, however, may add some impurities from balls. The
container may be filled with air or inert gas. However this can be an additional
source of impurity, if proper precaution to use high purity gases is not taken. A
temperature rise in the range of 100–1,100 ıC is expected to take place during the
collisions. Lower temperatures favour amorphous particle formation. The gases like
O2, N2 etc. can be the source of impurities as constantly new, active surfaces are
generated. Cryo-cooling is used sometimes to dissipate the heat generated. During
the milling, liquids also can be used. The containers are rotated at high speed a few
hundreds of rpm around their own axis. Additionally they may rotate around some
central axis and are therefore called as ‘planetary ball mill’.
