Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 87

short distances characteristic of nanoparticle-formed grains. 165 Ceramic struc-
tures can be made more easily and much less expensively if such net shape-
forming processes are made more capable.
The subject of structural nanomaterials was given a comprehensive review at
a recent Army-sponsored NRC Workshop on June 20-21, 2001. 166 The workshop
dealt with the synthesis, assembly, processing, fabrication, and manufacturing of
a variety of nanomaterials along with structure characterization and the modeling,
simulation, and application of these materials. Methods for producing grain sizes
of less than 50 nanometers for ceramics, metals, polymers, and composites were
discussed. Most of the workshop participants, dealing with nanosize particles as
basic building blocks, concluded that while progress is being made, challenges
remain on virtually every front in developing processes to consolidate nanosize
powders into useful and stable forms at useful manufacturing scales. Also dis-
cussed at the workshop were severe plastic deformation processes that “work”
materials to produce ultrafine grain sizes and increased strength. The equal-channel
angular pressing type of severe plastic deformation is reported to be especially
promising for Air Force applications such as lightweight aerospace structures. 167
The mechanical strength of nanosize, superstrong fibers such as carbon
nanotubes and perhaps other materials such as boron nitride nanofibers, coupled
with the lower density of these materials, offers the potential for much lighter
composite structures that are stronger and tougher than conventional structural
materials. This is a key advantage sought in the aerospace world for spacecraft,
aircraft, and military systems. Theoretical studies suggest that the Young's modu-
lus and the breaking strength of single-wall carbon nanotubes, for example, should
be exceptionally high. Recent experimental measurement of breaking strength
and Young's modulus tends to support the theory. 168 However, the mechanical
properties of such nanowires, -tubes, or -ribbons must be not only probed and
characterized but also manipulated and optimized at the scale of individual tubes
and assemblies of tubes if these concepts are to be made practical.
The mechanical properties of assemblies of these wires or fibers must be
understood as must be their properties in conjunction with the matrix material
with which they are to be hybridized. Carbon fibers with a tensile strength up to
6 GPa are commercially available. Initial experimental measurements on 4-mi-
cron-long, single-wall carbon nanotube (SWNT) “ropes” consisting of tens to
hundreds of individual SWNTs bound in van der Waals contact have yielded
values up to 45 GPa. 169 The hope is that millimeter-long SWNTs can be formed
into longer fibers or dispersed into a composite matrix while still maintaining a
significant fraction of this observed order-of-magnitude improvement in strength
over conventional carbon fibers.
Very large yield strengths (a measure of how much the material can stretch
without breaking) of 5 to 10 percent have been observed for carbon nanotubes.
This perhaps excessive ability to stretch may make this type of material imprac-