Page 71 - Nanotechnology an introduction

polymer and the hydrophilic radicals towards the air, whence they attract moisture, whose conductivity allows static charge accumulation to be
dissipated.
Polymers made conductive by the addition of electroactive materials are of growing importance in the automotive industry. If used for body panels,
for example, electrostatic spray painting is enabled, thereby reducing the need for primers, and which in turn improves painting reliability, reduces
costs and lessens the impact on the environment by using less volatile organic solvents. Conductive plastics have also been used to make fuel
pipes for automobiles, eliminating the build-up of static electricity and the danger of inadvertent ignition from a spark.

Much effort is currently being devoted to creating photovoltaic solar cells from semiconductor nanoparticles embedded in a matrix (the dielectric
constant of which must be less than that of the particles if the electrons are to be properly routed). The main advantage is that if the nanoparticles
are small enough to show size-dependent optical absorption (see Section 2.5), particles of different sizes can be blended to optimally cover the
solar spectrum. In addition, the fabrication of a nanoparticle–polymer composite should be significantly cheaper than the physical vapor deposition
of a thin semiconductor film on a substrate.

6.5.2. Metal–Matrix Composites

Conventional metal–matrix composites use two types of reinforcement: microparticles—usually silicon carbide—dispersed in an aluminium or
magnesium matrix to about 10–15 vol%; or continuous fibers—the most studied system is titanium with SiC reinforcement. The existing materials
show enhancements of a few % in stiffness, about 20% in strength and in superior wear resistance and fatigue initiation behavior.
Pure magnesium reinforced with 30 nm alumina particles at up to 1 vol% gives Young's modulus improvements of about 20%, yield strength
increases of 45% and some ductility improvement as well. The wear and creep performance is also improved significantly in both aluminium and
magnesium alloys. Silicon carbide nanoparticles used to reinforce magnesium result in a 100% increase in microhardness for 5 vol% SiC
nanoparticles. SiC nanowire reinforcement of ceramics improves toughness and strength by up to 100% for a few vol%.
Although extensive empirical work has been conducted, the parameter space is so vast (especially if one considers that distributions of sizes and
shapes, such as mixing particles and rods of different lengths, may offer better toughness and ductility) that our current knowledge must be
considered as being very limited. One general principle that has emerged is that a much smaller volume fraction of nanoparticles is required
compared with microparticles of the same chemical substance to achieve equivalent improvements in strength and stiffness. Thus the degradation
of toughness and ductility will not be as extreme and these nanomaterials may be more damage tolerant.
It is important that the particles are well dispersed and well bonded to the matrix in order to create good reinforcement. Ultrasonic waves are used
for dispersion of nanoparticles in the melt. There are interactive influences of the intrinsic properties of reinforcement and matrix and the size,
shape, orientation, volume fraction and distribution of the reinforcement. Usually the reinforcement causes diminution of the ductility and toughness
compared with the matrix alone. In the case of high-strength magnesium and aluminum alloys ductility is already limited; consequently attempts
have been made to use ductile pure metals as the matrix.
Principle process routes are:
• Stir casting: the nano-objects are distributed and suspended in the molten metal, e.g., via high-energy mixing. Melt sizes as large as 7 tons
are practicable. The slurry is then cast as billets or rolling bloom. Products with a volume fraction of reinforcement ranging from 10 to 40% are
available. The microstructure of the stir cast has to be finely controlled to develop uniform distributions of the particles. Spray casting followed by
hot extrusion is also used.
• Liquid metal infiltration: requires a preform consisting of a mat or other assembly of the ceramic fibers and particles to be infiltrated. A
volume fraction of between 40 and 70% is required to provide sufficient mechanical stability to withstand the infiltration forces without being
crushed. Liquid metal infiltration is extensively used by the automotive industry, and is now a preferred process for the thermal management
industry as it has the ability to produce near net shape components. Because of the relatively high volume fraction MMCs produced by this
process, applications tend to be those requiring wear resistance and high stiffness, rather than good toughness and ductility. The majority of
applications use aluminum as the matrix material.
• Powder metallurgical routes: these are used to produce continuous and discontinuous MMC in aluminum and titanium matrices. Sintered
preforms can be hot extruded to produce rods suitable for testing.
Current applications in the ground transportation industry account for 62% of the MMC market by mass. Aerospace applications are 5% by mass,
and general industrial markets comprise about 6% by mass (including cemented carbide and ceramic–metal composite (cermet) materials for
general tooling).

6.5.3. Self-Repairing Composites

The basic principle is the incorporation of hollow fibers and microspheres into conventional polymer matrix composites. These are presently
typically microsized (20–50 μm in diameter) rather than nanosized. Fibers and microspheres contain fluorescent dyes and polymer adhesives. If
the composite is damaged by fiber or resin cracking, the cracks intersect the hollow fibers and microspheres, which liberates the dyes and
adhesives, providing more visible indications of damage than the cracks themselves, and self-repairs the matrix cracks. Catalysts for the cure of
the adhesive can be placed in the resin separated from the fibers and capsule contents until the moment of release by damage. The concept has
also been applied to self-repair concrete structures.

Manufacture of such materials is relatively easily accomplished on a laboratory scale. Hollow glass fibers of 20–50 μm diameter can be easily
made, and hollow carbon fibers may also be made. Microcapsules containing adhesive can be created using in situ polymerization, followed by
incorporation into a conventional structural epoxy matrix material (Section 6.5.1). The required infiltration of the repair adhesive into the hollow
fibers is often a difficult manufacturing step.
The self-repair action repairs only cracks in the polymer matrix; there are no suggested routes to the repair of continuous fibers. It is important that
the repair material has sufficient time to flow from the containment vessels into the damaged region and has the time, which is very possibly long, of