Biomedical Engineering Reference
In-Depth Information
the discovery of BaTiO
3
, the hydrogen hypothesis of ferroelectricity was
discarded. h erefore, the discovery of BaTiO
3
was so important that it was
the i rst ferroelectric without hydrogen bonds [5]. At the end of the 1960s,
Cummins and Cross [6] could demonstrate highly anisotropic optical and
electrical properties of Bi
4
Ti
3
O
12
, one of the most popular Bi-layer struc-
tured ferroelectrics. At er that a large number of ferroelectrics were discov-
ered with interesting physical properties suitable for practical applications.
13.1.2.1.2 Properties of Ferroelectrics
13.1.2.1.2.1 Ferroelectric Domains and Hysteresis Loop
Ferroelectric crystals possess regions with uniform polarization called fer-
roelectric domains. Within a domain, all the electric dipoles are aligned in
the same direction. h ere may be many domains in a crystal separated by
interfaces called domain wall.
h us ferroelectrics are dielectrics which do not show a linear rela-
tionship between polarization and applied electric i eld. h e relationship
between polarization and applied electric i eld shows a hysteresis loop.
13.1.2.1.2.2 Phase Transition and Curie Point
All the ferroelectric materials have a transition temperature called the
Curie point (T
c
). At a temperature T > T
c
the crystal does not exhibit fer-
roelectricity, while for T < T
c
it is ferroelectric. At the Curie point, the fer-
roelectrics undergo a phase transition either of the i rst order or of second
order shown in Figure 13.2 (a) and (b).
13.1.2.1.3 Applications of Ferroelectrics
Due to interesting structural, mechanical, thermal, electrical and optical
properties, ferroelectric materials are suitable for various important appli-
cations, some of which are given below.
1st Order
2nd Order
2nd Order
1st
Order
1st
Order
Tc
Temperature
Tc
Temperature
Figure 13.2
(a) Order parameters (η) ~ Temperature for a i rst and second order
ferroelectric phase transition. (b) h e same but with dielectric constant (ε
r
) ~ Temperature.
