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Integrated Pyr oelectric Sensors 133

140
T. Furukawa in Adv.Coll.Interf.Sci.
12 Saturation polarization 71, 183 (1997)
10 Remanent polarization 120
8
100
6 Calculated
4 80
D (μC/cm 2 ) 2 0 P r (mC/m 2 ) 60
–2
–4
–6 Coercive field 40
–8
–10 20
–12
0
–250 –200 –150 –100 –50 0 50 100 150 200 250 0 20 40 60 80 100
E (MV/m) PTrFE PVDF
VDF content (mol %)
Furukawa
B. Stadlober, M. Zirkl
Naber, Nature Mat. 4, 243 (2005)
Neumann, Ferroelectrics 118, 319 (1991)
FIGURE 4.7 Left: Typical hysteresis loop of a fl exible P(VDF-TrFE) sensor element
(55:45) indicating the coercive fi eld, the saturation polarization, and the remnant
polarization. The different curves correspond to loop measurements with increasing
maximum electric fi eld. Right: Remnant polarization as a function of the VDF content
20
derived by Furukawa and other data from literature included as well as data from
the sol-gel processed PVDF copolymer fi lms. (Reprinted from Ref. 20. Copyright
1997, with permission from Elsevier.)

field strength, the field where all the dipoles in the material switch in
the opposite orientation, is reached at about 80 MV/m. The maxi-
mum achievable polarization is called the saturation polarization and
is about 200 MV/m.
In Fig. 4.7 at right, the theoretical and measured values of the
remnant polarization P vs. the PVDF content (as derived by Furu-
r
20
kawa ) are shown. It turns out that the highest polarizations are
reached between 50 and 80% PVDF content. At very low PVDF con-
centrations, the few polar molecules prevent a high polarization and
ferroelectricity is lost, while at high PVDF concentrations the creation
of the α-phase is preferred and, moreover, the crystallinity is reduced
to less than 50%. Between 80 and 50% there is a gradual decrease in
the remnant polarization that is attributed to a decrease in the aver-
age dipole moment, because the dipole moment of a TrFE unit is one-
half that of a VDF unit. The P values from the sol-gel processed
r
P(VDF-TrFE) copolymers nicely agree with the Furukawa values (the
highest reported so far), whereas other reported values are signifi-
cantly smaller, thus indicating less ferroelectric activity.
All ferroelectric materials have a Curie temperature. At this
temperature they undergo a transition from the ferroelectric to the
paraelectric phase. Even if the polymer chains stay in the ferroelectric
all-trans configuration, the macroscopic spontaneous polarization is
lost under this transition because the alignment of the polymer chains,
with the dipole moments pointing in a preferential direction, is lost.