Biomedical Engineering Reference
In-Depth Information
displacement measuring transducers) and medical instruments (e.g., blood flow detectors
and ultrasonic echography) to military applications (e.g., hydrophones and IR detectors).
One of the many reasons why PVDF has proved to be so popular with researchers in
the field of tactile sensors is because it is extremely light and inexpensive, and has many
wide-ranging applications. Furthermore, it is skin-like, can be prepared between 6
mand
2 mm in thickness and, because of this flexibility, can be formed into complex surface
structures [11]. In addition, PVDF has a bandwidth ranging between 0 Hz and the MHz
range and it is primarily due to this that the authors of this topic have opted to utilize
this medium as the transducer (sensing element) in much of their research and designs.
Although several investigators have reported measurements of the piezoelectric and
pyroelectric properties of PVDF film [5, 12-18], in order to consolidate previous find-
ings, some in-depth theoretical and experimental knowledge about PVDF characteristics
is still required. Tests are still being carried out on the commercial film supplied by Good-
fellow [19] in order to serve two purposes: firstly, to confirm the authenticity of all other
PVDF test procedures against those values provided by the manufacturer and, secondly,
to provide a base measurement for any future sensor system using PVDF film.
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3.4 Mechanical Characterization of Piezoelectric Polyvinylidene
Fluoride Films: Uniaxial and Biaxial
Uniaxial and biaxial PVDF films have three and two principal directions, respectively.
A traditional tension test machine can be used to measure the mechanical properties
(Young's modulus) of the films. Since the uniaxial film is mechanically drawn in one
direction, its mechanical properties in that direction are expected to be different than that
of the transverse direction. For the biaxial film, however, similar mechanical properties
for in-plane directions are expected. The following is a brief description of the machine
and method used for these measurements.
A precise mechanical testing machine (Bose, 3200 Series) was used to obtain the
stress-strain characteristics of the uniaxial PVDF film on two axes. The peak force of
the system was
225 N in a 12.5 mm range of displacement. The apparatus is shown in
Figure 3.4. The extension of the film was measured using a calibrated linear variable dif-
ferential transformer (LVDT). Three specimens of 110
±
m films were cut into a dumbbell
shape. The gauge length of each specimen was 26 mm with a width of 8 mm.
Tensile stress-strain characteristics were obtained for specimens oriented in two direc-
tions (see below). These averaged results are shown in Figure 3.5. The load (displacement)
was applied in a ramp fashion (0.1 mm s
-1
), and the extension and load were measured
during the test. These measurements showed that the Young's modulus, parallel to the
drawn direction, was 2.3 GPa, while in the perpendicular to the drawn direction it was 1.85
GPa. The measured modulus values compared favorably with the modulus ranges given
by Goodfellow, which were, respectively, 1.8-2.7 GPa for the longitudinal and 1.7-2.7
GPa for the transverse directions. Similar experiments were conducted to measure the
mechanical properties of biaxial film (Youngs modulus), for which the result obtained,
1.95 GPa, compared favorably with the value of 2 GPa provided by the manufacturer.
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