Biomedical Engineering Reference
In-Depth Information
(a)
(b)
SE
×25 2 mm
SE
×25 2 mm
FIGURE 9.51
TiNi microvalve fabricated with TiNi electrode on silicon membrane structure (a) top view of membrane and
TiNi electrode; (b) bottom view.
Ti-Ni-Cu, and Ti-Ni-Pd films, whereas the single layer diaphragm microactuators were
operated by the R-phase transformation induced in the Ti-Ni films. Figure 9.52 illustrates
the cross sections of the microactuator at room temperature and high temperature, respec-
tively (Miyazaki et al., 2009). Because the diaphragm consists of two layers with differ-
ent thermal expansion coefficients, an internal stress is generated in the diaphragm after
heat treatment, that is, compression in the SiO
2
layer and tension in the SMA film layer.
At room temperature, the Ti-Ni film layer is of martensite and can be easily deformed.
The crystal lattice of the martensite (low temperature phase) is an orthorhombic structure,
whereas that of the parent phase (high temperature phase) is a B2 structure (Miyazaki,
1990). Therefore, the diaphragm becomes convex as shown in Figure 9.52a to relax the
internal stress. By heating to a temperature above the reverse transformation temperature
of the SMA film layer, the diaphragm reverts to the initially memorized flat shape due to
the SME. By cooling to a temperature below the martensitic transformation temperature,
the diaphragm shape becomes convex again (Miyazaki et al., 2009).
As shown in Figure 9.52, the microactuator operates because of temperature variation.
Therefore, the temperature dependence of the height at the center of the diaphragm is
measured to investigate actuation process (Miyazaki et al., 2009). The height at the center
of the diaphragm is abbreviated to
h
. The parameter
h
is measured at each fixed tem-
perature during cooling and heating in a step-by-step way to characterize a quasi-static
actuation. Dynamic actuation is investigated by using a three-dimensional shape analyzer
equipped with a laser scanner. The microactuator is dynamically operated by thermal
500 µm
(a)
(b)
SMA (1.5-2.5 µm)
SiO
2
(1.0 µm)
Si
Internal stress
(bias force)
Shape recovery
force of SMA film
FIGURE 9.52
Schematic figures showing the cross section of a microactuator utilizing a SMA thin film deposited on a SiO
2
/
Si substrate at room temperature and a high temperature. (From Miyazaki, S., in Miyazaki et al., eds.,
Thin Film
Shape Memory Alloys: Fundamentals and Device Applications
, Cambridge University Press, UK, 2009, reproduced
with permission from Cambridge University Press.)
