Introduction to Nanotechnology

6.1. SOLID DISORDERED NANOSTRUCTURES 137
Melt
Metal Droplets (3 Q” QQ
Figure 6.5. Illustration of apparatus for making droplets of metal nanoparticles by gas atomiza-
tion. (With permission from I. Chang, in Handbook of Nanosfrucfured Materials and Nanofech-
nology, H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 1, Chapter 11, p. 501.)
materials, called gas atomization, a high-velocity inert-gas beam impacts a molten
metal. The apparatus is illustrated in Fig 6.5. A fine dispersion of metal droplets is
formed when the metal is impacted by the gas, which transfers kinetic energy to the
molten metal. This method can be used to produce large quantities of nanostructured
powders, which are then subjected to hot consolidation to form bulk samples.
Nanostructured materials can be made by electrodeposition. For example, a sheet
of nanostructured Cu can be fabricated by putting two electrodes in an electrolyte of
CuS04 and applying a voltage between the two electrodes. A layer of nanostructured
Cu will be deposited on the negative titanium electrode. A sheet of Cu 2 mm thick
can be made by this process, having an average grain size of 27 nm, and an enhanced
yield strength of 119 MPa.
6.1.2. Failure Mechanisms of Conventional Grain-Sized Materials
In order to understand how nanosized grains effect the bulk structure of materials, it
is necessary to discuss how conventional grain-sized materials fail mechanically.
A brittle material fractures before it undergoes an irreversible elongation. Fracture
occurs because of the existence of cracks in the material. Figure 6.6 shows an
example of a crack in a two-dimensional lattice. A “crack” is essentially a region of
a material where there is no bonding between adjacent atoms of the lattice. If such a
material is subjected to tension, the crack interrupts the flow of stress. The stress
accumulates at the bond at the end of the crack, making the stress at that bond very
high, perhaps exceeding the bond strength. This results in a breaking of the bond at
the end of the crack, and a lengthening of the crack. Then the stress builds up on the
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