Environmental Engineering Reference
In-Depth Information
However, applying a high electric fi eld just below the Curie temperature lets the
domains which lie in the fi eld direction grow at expense of others. This operation
is called poling and causes a net dipole in the material. The matrix of piezoelectric
strain constants is of the form:
0 0 0 0
d
0
⎡
⎤
15
⎢
⎥
( 6 )
0
d
ddd
0
0
0 0
0 0 0
⎢
⎥
15
⎢
⎥
⎣
⎦
31
31
33
In the case of a patch, the three-direction denotes the out of plain and poling direction
and
d
33
is much larger then
d
31
. 4-6 denote shearing. Specifi c forms and applications
of PZT are discussed later.
3.1.2 PVDF
PVDF, or polyvenyldiphosphate, is one of the many polymers that can exhibit
piezoelectric effects. It is discussed here because it has the highest coupling coeffi -
cient and already applications exist. Still, the coupling is much lower than the PZT,
but a big advantage of PVDF is that it is very ductile. Making PVDF piezoelectric
requires two steps: obtaining the right crystal structure and secondly, obtaining the
net dipole. There are numerous ways to get to a piezoelectric form, but since most
available materials consist of so-called phase II form, the following procedure is
suggested [ 69 ].
The fi rst step, obtaining the right crystal structure, is performed by stretching
the material at elevated temperature after which it recrystallizes in phase I. This
form has a non-centrosymmetric crystal, and thus a dipole. However, the crystals
are still randomly orientated. Subsequent poling can be achieved by applying a
high electric fi eld from a corona at room temperature or a relative low fi eld at
elevated temperature. When poling, the PVDF's constitutive units are rotated
around their chain bonds and thus a net dipole is attained. Hundred percent align-
ment with the fi eld is not possible because some of the chains my have a vector
component in the poling direction.
For piezoelectric PVDF sheets the charge constant
d
which links a fi eld in the
out of plane poling direction to in-plane strain, is different for both in-plane direc-
tions:
d
31
d
42
, because of the uni-axial stretching. Therefore the
charge constant matrix has the following form:
≠
d
32
and
d
51
≠
0 0 0 0
d
0
⎡
⎤
15
⎢
⎥
( 7 )
0
d
ddd
0
0
0 0
0 0 0
⎢
⎥
24
⎢
⎥
⎣
⎦
31
32
33
The charge constants, the coupling coeffi cient and the compliance of PVFD are
much lower than those of PZT, but it is much easier to handle and less brittle. In
addition, PVDF with PZT granules dissolved in them have been proposed already
years back [70]. This boosts the piezoelectric coupling, while maintaining the duc-
tility of PVDF. However, Furukawa shows that the piezoelectric functionality of
the composite can mainly be attributed to the PZT part of the composite.
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