Biomedical Engineering Reference
In-Depth Information
(a)
(b)
131.27
(nm)
79.40
(nm)
131.27
(nm)
79.40
(nm)
0.00
0.00
40.00
40.00
20.00
20.00
20.00
20.00
40.00
40.00
0.00
0.00
0.00
0.00
50.00 × 50.00 (µm) Z-Max 131.27 (nm)
50.00 × 50.00 (µm) Z-Max 79.40 (nm)
(c)
118.99
(nm)
118.99
(nm)
0.00
40.00
20.00
20.00
40.00
0.00
0.00
50.00 × 50.00 (µm) Z-Max 118.99 (nm)
FIGURE 9.42
Reversible surface trench morphology upon thermal cycling. (From Wu et al., 2005, with permission to use from
American Institute of Physics, USA.)
Frequency Response
Applications of microactuators require not only large recovery stress and large transfor-
mation deformation, but also high frequency and fast response (narrow transformation
hysteresis). One of the challenges for the successful application of TiNi films is effective
reduction of hysteresis to increase operating frequency. External heat is necessary for gen-
erating phase transformation and actuation, and the response speed of TiNi microactua-
tors is mainly limited by their cooling capacities. The binary TiNi alloy films have a large
temperature hysteresis of about 30°C. TiNi films with smaller hysteresis are preferred for
faster actuation. The hysteresis could be slightly reduced by decreasing the cyclic tempera-
ture amplitude and/or increasing working stress. R-phase transformation usually has a
much smaller temperature hysteresis, which is useful for MEMS applications (Tomozawa,
2006). However, the problem in R-phase transformation is that the strain and stress (or
force) generated are too small to be of many practical uses. Addition of Cu in TiNi films is
effective in reducing the hysteresis. Compared with TiNi binary alloy, TiNiCu alloys also
show less composition sensitivity in transformation temperatures, lower martensitic yield
