Biomedical Engineering Reference
In-Depth Information
nanoscale dimensions. Nanoscale is usually defined as smaller than one-tenth of a micrometer in at
least one dimension, though this term is sometimes also used for materials smaller than 1
m. A
natural, incidental, or manufactured material containing particles, in an unbound state or as an
aggregate or as an agglomerate and where, for 50% or more of the particles in the number size dis-
tribution, one or more external dimensions is in the size range 1
µ
100 nm. In specific cases and
where warranted by concerns for the environment, health, safety, or competitiveness, the number
size distribution threshold of 50% may be replaced by a threshold between 1% and 50%
[7]
.
An important aspect of nanotechnology is the vastly increased ratio of surface area to volume pres-
ent in many nanoscale materials, which makes possible new quantum mechanical effects. One example
is the “quantum size effect” where the electronic properties of solids are altered with great reductions
in particle size. This effect does not come into play by going from macro- to microdimensions.
However, it becomes pronounced when the nanometer size range is reached. A certain number of phys-
ical properties also alter with the change from macroscopic systems. Novel mechanical properties of
nanobiomaterials are the subject of nanomechanics research. Catalytic activities also reveal new behav-
ior in the interaction with biomaterials
[8]
. The chemical processing and synthesis of high-performance
technological components for the private, industrial, and military sectors require the use of high-purity
ceramics, polymers, glass-ceramics, and material composites. In condensed bodies formed from fine
powders, the irregular sizes and shapes of nanoparticles in a typical powder often lead to nonuniform
packing morphologies that result in packing density variations in the powder compact
[9]
.
Uncontrolled agglomeration of powders due to attractive Vander Waals forces can also give rise
to microstructural inhomogeneity. Differential stresses that develop as a result of nonuniform dry-
ing shrinkage are directly related to the rate at which the solvent can be removed and thus highly
dependent upon the distribution of porosity. Such stresses have been associated with a plastic-to-
brittle transition in consolidated bodies and can yield to crack propagation in the unfired body if
not relieved
[10,11]
. In addition, any fluctuations in packing density in the compact as it is pre-
pared for the kiln are often amplified during the sintering process, yielding inhomogeneous densifi-
cation. Some pores and other structural defects associated with density variations have been shown
to play a detrimental role in the sintering process by growing and thus limiting end-point densities.
Differential stresses arising from inhomogeneous densification have also been shown to result in
the propagation of internal cracks, thus becoming the strength-controlling flaws
[12,13]
. It would
therefore appear desirable to process a material in such a way that it is physically uniform with
regard to the distribution of components and porosity, rather than using particle size distributions
which will maximize density. The containment of a uniformly dispersed assembly of strongly inter-
acting particles in suspension requires total control over particle
particle interactions. It should be
noted here that a number of dispersants such as ammonium citrate (aqueous) and imidazoline or
oleyl alcohol (nonaqueous) are promising solutions as possible additives for enhanced dispersion
and deagglomeration. Monodisperse nanoparticles and colloids provide this potential
[14]
.
Monodisperse powders of colloidal silica, for example, may therefore be stabilized sufficiently to
ensure a high degree of order in the colloidal crystal or polycrystalline colloidal solid which results
from aggregation. The degree of order appears to be limited by the time and space allowed for
longer-range correlations to be established. Such defective polycrystalline colloidal structures
would appear to be the basic elements of submicrometer colloidal materials science, and, therefore,
provide the first step in developing a more rigorous understanding of the mechanisms involved in
microstructural evolution in high-performance materials and components
[15]
.
