Online proceedings - EDA Publishing Association

80˚C. Additionally, we looked into military fatigues and their
fabric compositions as well as a cotton polyester fabric coated
with nickel and copper. The military fatigues come in a variety
of compositions which might convolute testing, but the
FlecTron® Nickel/Copper fabric boasts strong characteristics
such as a high surface conductivity of less than 0.1 Ohms/sq.,
and can withstand temperatures up to 200°C as shown in
Table 1. The fabric is also flexible, lightweight, breathable,
durable, washable, and tear resistant. Since the fabric is coated
with an electroless metal layer that is conductive it also serves
as the bottom electrode for the piezoelectric circuit as well as
the substrate/wearable garment.
11-13 May 2011, Aix-en-Provence, France
PVDF/TrFE (from Solef, see spec sheet in Appendix A) in
cyclopentanone was chosen for its flexibility and durability as
well as for its piezoelectric properties and required processing
temperatures. Other materials tested such as zinc oxide were
brittle and cracked, thus failing any fatigue testing. Others
were too rigid to deform in such a way that would make them
practical in this application. The cyclopentanone has a
relatively high boiling point compared with other solvents, as
well as a strong ability to dissolve PVDF.
III. FABRICATION
To prepare the PVDF solution enough powdered
PVDF/TrFE) is added to a cyclopentanone solvent to produce
a 15 wt% PVDF solution. The mix is then heated to 70˚C
while being magnetically stirred in a sealed container until the
PVDF is completely dissolved (~4 hrs). For testing purposes,
membranes were created by spin coating the PVDF solvent
onto a silicon wafer and allowing to cure at 70˚C for 4 hours,
which produces a ~6um membrane when the spin coater is set
to 500 rpm.
2
Table 1. FlecTron properties (from manufacturer, 2010).
There are four possible crystalline phases for PVDF: alpha,
beta, and gamma are the most common [6]. Of these, only the
beta phase is piezoelectric. The beta phase is formed when alltrans
polymer chains pack themselves into an orthorhombic
crystalline lattice. Because all the fluorine atoms are on the
same side of the backbone the lattice lacks a center of
symmetry causing net dipole moment. Upon crystallization
out of the melt the most common phase is the alpha phase. The
chains in the alpha phase are of the form trans-gauche-transgauche.
They also pack into an orthorhombic lattice because
the fluorine atoms alternate sides of the backbone therefore
exhibiting no net dipole, and thus not piezoelectricity.
Fig. 4. Gold coated PVDF membrane in flexing test jig.
To minimize any bubbles in the film, the just spin coated
wafer was placed in a vacuum desiccators for 2 minutes prior
to curing. Finally, 100 nm gold was sputter coated on the
membrane at room temperature to form the top electrode as
shown in Fig. 4. The bottom electrode can be sputter coated
onto the silicon wafer prior to the spin coat of PVDF if
desired, as it will adhere to the PVDF when the membrane is
separated from the silicon substrate. The electrodes were then
connected using copper wire bonded to the gold electrodes
using silver conductive epoxy (cross section shown in Fig. 5).
Fig. 3. Solef brand PVDF crystalline phase diagram.
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