Biomedical Engineering Reference
In-Depth Information
electric i eld; antiferroelectric materials possess ordered
dipole moments that cancel each other completely within
each crystallographic unit cell.
•
Ferromagnetic
materials
possess a spontaneous magnetiza-
tion that is stable and can be switched hysteretically by an
applied magnetic i eld; antiferromagnetic materials possess
ordered magnetic moments that cancel each other com-
pletely within each magnetic unit cell.
•
Ferroelastic
materials
display a spontaneous deformation
that is stable and can be switched hysteretically by an applied
mechanical stress.
•
Ferrotoroidic
materials
possess a stable and spontaneous
order parameter that is taken to be the curl of magnetiza-
tion or polarization. By analogy with the above examples, it
is anticipated that this order parameter may be switchable.
13.1.2.1 Some Important Features of Ferroelectrics
h e ferroelectric materials show the following features:
• h ey exhibit spontaneous polarization.
• h ey show reversible polarization under the inl uence of
external electric i eld.
• h ey exhibit hysteresis loop below the Curie-point.
• Below Curie temperature they show ferroelectric behavior
and above Curie temperature they show paraelectric behavior.
• h ey show piezoelectric and pyroelectric properties and
belong to the polar class of materials.
• h ey have high resistivity and show insulator to semicon-
ductor behavior with the increase of temperature.
13.1.2.1.1 A Brief History of Ferroelectrics
h e birth of ferroelectrics started with the discovery of Valasek in 1921 [1]
that the polarization could be reversed by the application of an external
electric i eld in Rochelle salt. In 1935, Bush and Scherrer [2] discovered
the ferroelectric properties in KH
2
PO
4
, potassium dihydrogen phosphate
(KDP) and in (NH
4
)H
2
PO
4
, ammonium dihydrogen phosphate (ADP).
Like Rochelle salt, KDP and ADP exhibit good piezoelectric properties
above the Curie temperature T
c
. In 1943, Wainer and Soloman [3] dis-
covered the anomalous dielectric properties of BaTiO
3
. h e ferroelectric
behavior of BaTiO
3
was discovered by von Hippel
et al.
in 1946 [4]. With
