Biomedical Engineering Reference
In-Depth Information
then removing unwanted regions using lithography and etching leaving behind nanostructures. The
latter has been used widely for making components for electronic devices at the nanoscale. Both
approaches have been used widely and the success of the approaches is highly dependent on the par-
ticular requirements of the application. The essence of nanotechnology is the ability to control and
manipulate the nanostructures to provide unique properties of materials.
2.2.1
Synthesis by Mechanical Attrition
The grinding or milling of large coarse-grained materials to produce smaller sized particles has long been
a major component of ceramic processing and powder metallurgy. Accordingly, the uses of similar attrition
techniques in mechanical devices have also been tried in producing nanoparticles. This is a “top-down”
manufacturing process and under certain conditions the resulting particulate powders can exhibit nano-
structural characteristics. Although used extensively to prepare small particles this procedure is energy con-
suming and has limited success in providing true nanostructured components for dental materials.
The fundamental principle of size reduction in mechanical attrition devices such as ball mills lies
in the energy imparted to the sample during impacts between the milling media. For brittle materials,
particle fracture is described by Griffith theory
[2]
. The stress (
σ
F
) at which crack propagation occurs
leading to catastrophic failure, and hence size reduction depends on the surface energy (
γ
), modulus
of elasticity (
E
), and length of the initiated crack (
c
).
K
is an empirical constant that is usually deter-
mined experimentally for each type of material.
½
(2.1)
σ
=
K E
/
(
γ
)
F
When stress at the crack tip equals the strength of cohesion between the atoms, the crack becomes
unstable and propagates, leading to fracture. However, as fragments decrease in size, the tendency
to aggregate increases and particle fineness approaches a limit as milling continues and maximum
energy is expended
[3]
. Furthermore, the grinding media (i.e., the balls and inner walls of the ball
mill) become coated with fine particles that cushion the micro-bed from impact. The foregoing factors
limit the size reduction and hence provision of particles of nanometer scale becomes more difficult.
Another limitation of the milling process is that it generally produces a normal size distribution. Hence,
a substantial portion of the particles are higher than the required 100 nm size. The classification of particles
to eliminate the larger sizes has been tried but is generally of limited success. Often, dental composites
contain the unclassified milled particles. However, by virtue of the milling process although they contain a
fraction of nano-sized particles they cannot be classified as true nanofill composites.
2.2.2
Synthesis Through Sol-Gel Process
Among various processes available for synthesis of nanomaterials, sol-gel synthesis is one of the
most versatile and cost-effective methods for production. A sol is defined as a stable colloidal suspen-
sion of solid particles within a liquid while a gel is defined as a porous three-dimensional semi-solid
network that expands in a stable fashion
[4]
. There are many definitions of sol-gel systems. For den-
tal fillers, it is useful to consider the definition by Segal
[5]
who defines the sol-gel process as the
production of inorganic oxides, either from colloidal dispersions or from metal oxides. The starting
point of a sol-gel preparative method generally involves the mixing of different precursor chemicals,
usually in a liquid form. The precursors generate a solution containing a stable colloidal dispersion of
