Civil Engineering Reference
In-Depth Information
proton-conducting membrane was synthesized by immersing dehydrated BC into 5%
solution of phosphoric acid, H
3
PW
12
O
24
.29H
2
O. A complete fuel cell fabricated using
dif erently functionalized BC was studied at 3 dif erent operating temperatures, 20, 30
and 40°C (Figure 17.22). h e maximum power density obtained from these sheet-type
fuel cells was found to be 12.1 mWcm
-2
at an operating temperature of 40°C, three
orders of magnitude higher than that based on Pd nanoparticles. h e biggest advantage
of such fuel cells is that since BC is thermally stable up to 275°C, the fuel cells can be
operated at elevated temperatures to increase the cell ei ciency.
17.6
Optically Transparent and Mechanically Flexible
Composites
Flexible optoelectronic devices which are of use for displays must have several physical
properties co-existing in a single material. h ey are: high optical transparency, >85%;
high electrical conductivity, comparable to metallic values; relatively low modulus with
large elasticity and low coei cient of thermal expansion. As can be clearly seen, it is
nearly impossible to i nd a single material having all these properties. Hence, materi-
als that are generally researched for l exible optoelectronic devices are composites or
dif erent materials present in the form of multilayers. In this context, BC of ers a good
choice to be used as a matrix for synthesizing l exible optoelectronic composites with
a refractive index of 1.618 along the i bers and 1.544 in the transverse direction, which
are very similar to those of many commercially used clear glasses [67]. h e coei cient
of thermal expansion (CTE) of pure BC is also extremely low, 0.1× 10
-6
°C
-1
[68] .
Various types of thermosetting resins such as epoxy, acrylic and phenol-formaldehyde
are ideal choices to be incorporated into BC to synthesize l exible transparent compos-
ites. Yano
et al.
[69] synthesized transparent composites by ini ltration using these three
resins with BC. h e typical BC content in these composites was in the range of 60-70
wt%, and in spite of such large BC content, the composites have >75% transparency to
visible light of wavelength 400-800 nm (i gure 17.23a). h e composites exhibit a tensile
strength of 325 MPa with a Young's modulus of 20 GPa, three times less than that of
glass and i ve times larger than that of engineered plastics. In addition, the coei cient of
thermal expansion (CTE) is found to decrease to 6×10
-6
°C
-1
compared to 1.2×10
-4
°C
-1
for the pure resin. To achieve nearly total transmission to visible light, it is proposed
that an optical refractive index matching between the dif erent constituents in a com-
posite is essential. In order to investigate this, Nogi
et al.
[67] synthesized composites
with nine dif erent types of resins whose refractive index varied between 1.492 and
1.544. It was found that the transmittance in the forward direction varied in the range
85 to 88%, indicating that perfect refractive index matching is not required to achieve
high transmittance (Figure 17.23b). h ey also found that although the refractive index
of the individual constituents of the composite decreased with increasing temperature
in the range 20-80°C, the transmittance does not vary signii cantly (Figure 17.23c).
h ese results clearly show that light transmission capability of a composite is a function
of the dif erence in the refractive indices of the constituents and does not depend on
absolute values of the refractive index, Another variable in these composites is the frac-
tion of BC. Changing the BC i ber content from 7.4 to 66% increased the thickness of
