Page 246 - Mechanics of Microelectromechanical Systems

4. Microtransduction: actuation and sensing 233
constrained recovery, force actuation, flow control and actuation at microscale.
Micrometer-order thick titanium-nickel (Ti-Ni) films that were sputter-
deposited have demonstrated excellent actuation and reaction-time properties.
The shape memory effect (SME) was discovered in a gold-cadmium (Au-
Cd) alloy as early as 1951, whereas the same effect in Ti-Ni alloys was
reported in 1963. More details regarding the structure, properties and
applications of shape memory alloys can be found in Otsuka and Wayman
[10] who gave a synthetic view on the evolution lines in the shape memory
alloy research. The shape memory effect consists in a phase transformation of
an alloy under thermal variation. At lower temperatures, the martensite phase
of an SMA – with lower symmetry and therefore more easily deformable – is
stable, whereas at higher temperatures, the austenite phase (also called the
parent phase) – of cubic, higher symmetry, which renders the SMA less
compliant/deformable under mechanical action – is stable.

Figure 4.45 Thermo-mechanical cycle in a SMA with shape memory effect

It is thus possible to utilize the sequence of Fig. 4.45 in order to realize
the SME. A temperature decrease is first applied which initiates the
martensitic transformation from austenite to martensite. By subsequently
applying the mechanical load, the SMA component in its martensitic phase
(which is called twinned martensite, with at least two orientations of its
potential deformation) at low temperature can be altered into deformed