7.1 Mechanical ProPerties
scale and Properties
Of polycrystalline, nanocrystalline, and amorphous structures,
most materials are polycrystalline, made up of ordered crystals that
meet at boundaries where, inevitably, there is disorder. Figure 7.1a
suggests what they look like. Such materials have two structural
length scales: that of the crystals and that of the atoms that make
them up. The crystal size, typically, is between 0.1 mm and 1 mm.
This means that the disordered material occupies only a tiny fraction of the volume—less than one part in a million. Nanocrystalline materials, made by the processes described in Chapter 8, have
crystals that are much smaller, in the range of 10–100 nm. The
smaller the crystals, the greater the fraction of disordered material,
which now becomes large enough to influence mechanical and
other properties.
Suppose now that the crystal size shrinks further until it becomes
of atomic dimensions. The material is now completely disordered,
as in Figure 7.1b. Such materials are called amorphous, meaning
without structure. They have only one characteristic length scale,
that of the atoms or molecules that make them up. Polymers, too,
can adopt both crystalline and amorphous forms. The long molecular chains in some can fold neatly like a pleated blind to make
crystallites separated by confused tangles of disorder, as in Figure
7.2a. The molecular shape in others makes ordering difficult and
the structure, once more, is amorphous (Figure 7.2b), with no welldefined length scale beyond that of the molecule itself. The amorphous state, then, is a limit—that of the ultimate nanostructured
199
Nanomaterials: Properties
c haP ter 7
Nanomaterials, Nanotechnologies and Design
Copyright 2009 Elsevier Ltd. All rights of reproduction in any form reserved.
scale and Properties
Of polycrystalline, nanocrystalline, and amorphous structures,
most materials are polycrystalline, made up of ordered crystals that
meet at boundaries where, inevitably, there is disorder. Figure 7.1a
suggests what they look like. Such materials have two structural
length scales: that of the crystals and that of the atoms that make
them up. The crystal size, typically, is between 0.1 mm and 1 mm.
This means that the disordered material occupies only a tiny fraction of the volume—less than one part in a million. Nanocrystalline materials, made by the processes described in Chapter 8, have
crystals that are much smaller, in the range of 10–100 nm. The
smaller the crystals, the greater the fraction of disordered material,
which now becomes large enough to influence mechanical and
other properties.
Suppose now that the crystal size shrinks further until it becomes
of atomic dimensions. The material is now completely disordered,
as in Figure 7.1b. Such materials are called amorphous, meaning
without structure. They have only one characteristic length scale,
that of the atoms or molecules that make them up. Polymers, too,
can adopt both crystalline and amorphous forms. The long molecular chains in some can fold neatly like a pleated blind to make
crystallites separated by confused tangles of disorder, as in Figure
7.2a. The molecular shape in others makes ordering difficult and
the structure, once more, is amorphous (Figure 7.2b), with no welldefined length scale beyond that of the molecule itself. The amorphous state, then, is a limit—that of the ultimate nanostructured
199
Nanomaterials: Properties
c haP ter 7
Nanomaterials, Nanotechnologies and Design
Copyright 2009 Elsevier Ltd. All rights of reproduction in any form reserved.