Biomedical Engineering Reference
In-Depth Information
3.3
Transmittance and absorption
In nanocomposites, particular attention should be paid to the intensity loss
of transmitting light by scattering, which varies with particle size.
Transparency is determined by reflection and absorption, as described by 28 :
I
I 0 ¼ð
2
e g h
1
R
Þ
½
3
:
1
where I is the intensity of the transmitted light and I 0 that of the incident
light, R is the reflection,
is the extinction coefficient (including scattering
and absorption components) and h is the optical path length. For
transparency, the conductive filler particles themselves must be transparent
and the scattering at the interface between the filler particles and the matrix
must be as small as possible. Scattering can be minimized either by matching
the refractive indices of the transparent conductive filler particles and the
polymer matrix or by reducing one dimension of the conductive filler
particles which thereby becomes much smaller than the wavelength of visible
light. Since matching of the refractive indices of a polymer and transparent
ceramic conductors may not be possible, nanosized conductive particles can
be used, where the concentration of the conductive particles is kept as low as
possible. Transparent conductive coatings have been fabricated by combin-
ing nanosized antimony-doped tin oxide particles and a polymer (photo-
graphic-grade gelatin or poly(n-butyl methacrylate) and poly(n-butyl
acrylate) lattices).
Sun et al. 28 fabricated transparent, conductive composite coatings from
suspensions of poly(vinyl acetate-acrylic) (PVAc-co-acrylic) copolymer
lattices (50-600 nm) and nanosized antimony-doped tin oxide particles
(
γ
15 nm). The suspensions were deposited as coatings onto poly(ethylene
terephthalate) substrates and dried at 50
~
C. They investigated the optical
transmittance and scattering behavior of the coatings and compared them
with PVAc-co-acrylic coating. They found that composite coatings had lower
transparency because of the Rayleigh scattering. The transparency of the
composite coatings was improved by a reduction in the coating thickness.
The best transparency for the coatings with a direct-current conductivity of
approximately 10 2 S/cm was around 85% at a wavelength of 600 nm.
Luo et al. 29 prepared and studied the optical properties of novel
transparent Al-doped zinc oxide (AZO)/epoxy nanocomposites. AZO
nanoparticles were prepared by calcinations of the precursor (precursor
synthesized via the homogeneous precipitation method) at different
temperatures. They studied the optical properties, namely visible-light
transmittance, UV and IR opaqueness, of the AZO/epoxy nanocomposites
as a function of the AZO content by using a UV
8
￿ ￿ ￿ ￿ ￿ ￿
VIS spectrophotometer
and a UV
VIS
near-infrared (NIR) spectrophotometer (Figs 3.4 and 3.5).
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