Biomedical Engineering Reference
In-Depth Information
cycling. The thermal cycling was conducted by applying a pulse current to the Ti-Ni film
layer in the microactuator, that is, by means of joule heating and natural cooling. Height
h
was continuously measured during dynamic actuation. The displacement was estimated
by measuring the difference between the maximum and minimum values of
h
, and it was
used as one of the measures of dynamic actuation characteristics. The temperature of the
microactuator was measured by a thermocouple microwelded to a part of the SMA film
attached to the SiO
2
/Si substrate. The working frequency and temperature of the micro-
actuator were adjusted by changing the frequency and amplitude of the pulse current,
respectively. The ratio of heating time to cooling time, that is, a duty ratio, was fixed at 5:95
for each working frequency. The measurement was conducted at an ambient atmosphere
(23 to 25°C).
As for the second type of microactuator with a single layer diaphragm, Ti-Ni films were
deposited on Si substrates by r.f. magnetron sputtering. The thicknesses of the Ti-Ni films
were 2.0
μ
m. The Ti-Ni films on the substrates were heat-treated at 873 and 823 K for 0.6 ks,
respectively, to memorize the initial flat shape. Diaphragm-type microactuators were fabri-
cated again by using the conventional Si micromachining technique. Figure 9.53 illustrates
the cross sections of a microactuator at low and high temperatures (Miyazaki et al., 2009).
The shape of the diaphragm is square with a width of 500
μ
m. N
2
gas pressure of 40 kPa is
used as a bias force for the microactuator. The microactuator is convex at room tempera-
ture because the Ti-Ni film is deformed by the N
2
gas pressure in a low-temperature phase
such as M-phase or R-phase.
To investigate the displacement and the transformation temperatures of the microactua-
tor, the height at the center of the diaphragm is measured as a function of temperature
during heating and cooling at each fixed temperature in a step-by-step way. Figure 9.54
reveals the temperature dependence of the height at the center of the diaphragm. Upon
cooling, the height starts to increase at
M
s
(
R
s
) due to the start of the M-phase (R-phase)
transformation and the increase in height finishes at
M
f
(
R
f
) because the M-phase (R-phase)
transformation finishes. Upon heating, the height decreases with increasing temperature
between
A
s
(
RA
s
) and
A
f
(
RA
f
) because of the progress of the reverse M-phase (R-phase)
transformation. Displacement is defined as the difference between the maximum height
and the minimum height (
h
s
− ), where the superscript “s” stands for “static” because
each height is measured at fixed temperature.
Δ
H
M
and
Δ
H
R
represent the transforma-
h
max
min
Low temperature
(martensite phase
or R-phase)
High temperature
(parent phase)
Ti-Ni
Si
SiO
2
N
2
gas
N
2
gas
Bias force of N
2
gas pressure
Recovery force of Ti-Ni
FIGURE 9.53
Schematic figures showing cross section of a microactuator consisting of a SMA thin film deposited on a 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.)
