Environmental Engineering Reference
In-Depth Information
These alloys are already applied in different applications in various sectors
where the recovery (shape-memory effect) and superelasticity can be used. These
are, for example, different types of linear and rotary actuators applied in the
automotive, aerospace and robotics industries; in medical applications, where due to
its biocompatibility Ni
Ti is commonly used as orthopaedic implants, stents and
dental braces; as a heat engine; and in various different engineering (fasteners, seals,
connectors, clamps, valves, temperature controls, etc.) and practical applications
(glasses frames, underwire bras, cellphone retractable antenna, etc.) [
93
]. The heat
engine was one of the
-
Ti alloys as early as the 1970s. It
converts heat energy into mechanical and electrical energy by exploiting the
recovery force of the shape-memory material. It consists of a shape-memory
material (usually in the form of a wire or coil or better many coils), which alter-
natively moves (rotates) between high- and low-temperature media. The wires
entering the low-temperature medium become soft and are forced to bend. In the
next step, they enter the high-temperature medium, where they transform back to
the austenite phase, tend to straighten back to the original shape, and due to the
recovery force they generate the mechanical energy (movement).
The
rst applications of Ni
-
rst continuously operated shape-memory-alloy heat engine was developed
by R.M. Banks from the Lawrence Berkeley Laboratory of the University of
California in 1973. In later years, various different design concepts were developed
and tested. However, in order to be practical, these engines would typically be
required to produce hundreds of kilowatts. This was proven to be much more
dif
cult than expected, as for all of these devices the scale-up stopped at about one
kilowatt. In other words, the inventions at that time failed for both engineering and
economic reasons. However, the shape-memory-based heat engine has various
advantages, from its simple design, potential passive operation (only requires the
appropriate high- and low-temperatures media) and applicability to different oper-
ating conditions, so there might still be a promising and intriguing future for this
technology. More details on shape-memory heat engines can be found in, e.g.,
[
94
96
].
On the other hand, as explained in [
97
], polymeric materials are also capable of
shape-memory and superelastic effects, although the mechanisms responsible differ
dramatically from those of metal alloys. In contrast to shape-memory alloys,
polymers do not achieve any shape-memory and superelastic behaviour through the
martensitic transformation, but rather through a variety of physical means; the
underlying, very large extensibility being derived from the intrinsic elasticity of
polymeric networks. This is related to the entropy change caused by the second-
order phase transition between the soft and hard phases due to the polymeric chain
orientation change. A typical shape-memory polymer is natural rubber. In general,
they exhibit much larger transformation strains (up to 200 % or more), lower
densities and thermal conductivities as well as lower costs compared to the shape-
memory alloys and are therefore more suitable for various applications. However,
their potential for application in an elastocaloric cooling (or heat pumping) device is
somewhat lower (due to the above-mentioned properties and also the lower EsCE),
and thus further focus will be put on the shape-memory alloys.
-
Search WWH ::
Custom Search