386
11 Templated Systems
Conducting polymers. Electrodeposition of extremely narrow conducting polymer
fibres into DCTs provides a unique opportunity to measure the conductivity of the
polymer fibres along their axis [214]. These experiments revealed that the high degree
of polymer chain orientation leads to a much higher conductivity in the case of the
ultranarrow polymer fibres than that of the polymer films with a higher chain disorder.
11.2.10 Templates Obtained from Directionally Solidified
Fibrous Eutectic Metallic Systems
Eutectic systems exhibit a composition corresponding to the lowest temperature
where the eutectic system may exist as a melt. When a eutectic system is solidified,
the composition of the solid phases being crystallized is far in composition from
the eutectic melt. Solidification of eutectic melts hence results in a phase mixture of
defined composition. A classical example for eutectic systems is the Cu–Ag mixture
(~28 wt.% Cu) in which the solubility of the components in each other is less than 5
at.% at the eutectic temperature (779 °C, a temperature much lower than the melting
point of either of the components). Pseudobinary eutectics are systems in which one
of the components is an alloy of fixed composition and the other component is a
metallic element. A typical example for pseudobinary eutectics is NiAl–X where X
is Mo, Re or W. The mole fraction of X in the latter eutectic systems is less than 2
at.%.
When a eutectic melt is solidified at a low rate and without a significant temperature gradient, interpenetrating random fibrous texture can form. Instead, if the solidification is directionally oriented (DO) with a temperature gradient of a few tens
of K cm
–1 and with the motion of the solidification front at the rate in the range of
10–3000 mm h
–1 to the same direction as the temperature gradient (Bridgman-type
growth), a fibre-like morphology is formed in which the fibre axis direction coincides
with the normal of the solidification front. The diameter of the fibres formed varies
between 100 and 900 nm from one system to another, but the wire diameter distribution is narrow for each particular sample. The relative position of the nanowires is
random. In contrast to other template formation methods, the fibres are sometimes
not cylindrical because they are single crystalline and have a well-defined polygonal
cross section [222, 223].
If the chemical/electrochemical reactivity of the matrix-forming and fibre-forming
materials is different enough, selective dissolution can be applied for structuring
the solidified eutectic system. If the reactivity of the matrix is higher than that of
the embedded wire in a suitable etching medium, self-supporting nanowires can
be obtained, while in the opposite case, a porous template with nanoholes can be
prepared. The illustration of all these processes together with the images of the
resulting nanostructures can be seen in Fig. 11.11.
Regarding the NiAl–X systems, the passivation of the NiAl phase makes it possible
to dissolve the embedded metal wires. Regardless of the additional metal that makes
11 Templated Systems
Conducting polymers. Electrodeposition of extremely narrow conducting polymer
fibres into DCTs provides a unique opportunity to measure the conductivity of the
polymer fibres along their axis [214]. These experiments revealed that the high degree
of polymer chain orientation leads to a much higher conductivity in the case of the
ultranarrow polymer fibres than that of the polymer films with a higher chain disorder.
11.2.10 Templates Obtained from Directionally Solidified
Fibrous Eutectic Metallic Systems
Eutectic systems exhibit a composition corresponding to the lowest temperature
where the eutectic system may exist as a melt. When a eutectic system is solidified,
the composition of the solid phases being crystallized is far in composition from
the eutectic melt. Solidification of eutectic melts hence results in a phase mixture of
defined composition. A classical example for eutectic systems is the Cu–Ag mixture
(~28 wt.% Cu) in which the solubility of the components in each other is less than 5
at.% at the eutectic temperature (779 °C, a temperature much lower than the melting
point of either of the components). Pseudobinary eutectics are systems in which one
of the components is an alloy of fixed composition and the other component is a
metallic element. A typical example for pseudobinary eutectics is NiAl–X where X
is Mo, Re or W. The mole fraction of X in the latter eutectic systems is less than 2
at.%.
When a eutectic melt is solidified at a low rate and without a significant temperature gradient, interpenetrating random fibrous texture can form. Instead, if the solidification is directionally oriented (DO) with a temperature gradient of a few tens
of K cm
–1 and with the motion of the solidification front at the rate in the range of
10–3000 mm h
–1 to the same direction as the temperature gradient (Bridgman-type
growth), a fibre-like morphology is formed in which the fibre axis direction coincides
with the normal of the solidification front. The diameter of the fibres formed varies
between 100 and 900 nm from one system to another, but the wire diameter distribution is narrow for each particular sample. The relative position of the nanowires is
random. In contrast to other template formation methods, the fibres are sometimes
not cylindrical because they are single crystalline and have a well-defined polygonal
cross section [222, 223].
If the chemical/electrochemical reactivity of the matrix-forming and fibre-forming
materials is different enough, selective dissolution can be applied for structuring
the solidified eutectic system. If the reactivity of the matrix is higher than that of
the embedded wire in a suitable etching medium, self-supporting nanowires can
be obtained, while in the opposite case, a porous template with nanoholes can be
prepared. The illustration of all these processes together with the images of the
resulting nanostructures can be seen in Fig. 11.11.
Regarding the NiAl–X systems, the passivation of the NiAl phase makes it possible
to dissolve the embedded metal wires. Regardless of the additional metal that makes