Page 371 - Book Hosokawa Nanoparticle Technology Handbook

6.4 ELECTRIC PROPERTIES FUNDAMENTALS
nanometer scale by piezoresponse force microscope exceeds coercive field (E ), the polarization state
c
(PFM) which belongs to the family of scanning force changes drastically to the positive one. Even if the elec-
microscope (SPM) [5]. Here, the theory of tric field is turned off, the charged state is retained with
dielectrics/ferroelectrics and the methods for observing the state “0” and the crystals have spontaneous positive
fundamental physical properties cannot be described charge. In the crystal, Ti ions are displaced downward
due to limitations of space. The details have been pub- to the “1” state, while the upward displacement of the
lished in the great book [6–8] and the reviews [9,10]. Ti ions are seen in the “0” state. Namely, the ferro-
electrics can have positive or negative charge without
6.4.1.1 Crystal structure of perovskite-type applying an electric field. The polarization value with
ferroelectrics zero electric field is defined as remnant polarization
Fig. 6.4.1 shows the crystal structure of BaTiO with (P ). In the ferroelectric crystals, the sign of the P can
r
r
3
perovskite structure. In the crystal structure, divalent be controlled by electric field. The perovskite ferro-
2
4
Ba (Ba ) occupies the A site, and tetravalent Ti (Ti ) electrics such as BaTiO belong to the displacive-type
3
is positioned at the B site. Above the Curie tempera- ferroelectric in which ferroelectricity originates from
ture (T of 135 C for BaTiO ), ferroelectrics have a the displacement of the constituent ions from the cor-
3
C
center of symmetry and do not exhibit a ferroelectric responding positions in the parent cubic structure.
behavior. This state is categorized as paraelectric and Ferroelectrics exhibit piezoelectricity consequently
Ti ions are located at the center of the BO octahedra. from the crystallographic point of view. When stress
6
During cooling, ferroelectric phase transition occurs at is applied to piezoelectrics, the crystals generate elec-
the T and the crystals show a ferroelectric behavior. tric dipole proportional to the stress value, which is
C
The significant structural changes induced by the called piezoelectric effect. In the ignition devices,
phase transition are the elongation along the c axis and spark discharge by a high voltage of the piezoelectrics
shrinkage along the a axis of the unit cell and the off- caused by hitting the piezoelectrics is often used. In
center displacement of the Ti ions along the c axis. In contrast, applying a voltage enables us to obtain a
the ferroelectric crystals with low-symmetry tetrago- thickness change ( l) of the piezoelectrics which is
nal, the relative displacement between the centers of proportional to electric field (inverse piezoelectric
“ ” and “ ” charges generates dipole moment in the effect). The inverse piezoelectric effect has found a
unit cell, and the ferroelectric crystals possess electric wide variety of applications such as actuators etc.,
charge spontaneously. in which ferroelectric Pb(Zr,Ti)O is exclusively used.
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Recently, (K,Na)NbO [11,12] has attracted a great
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6.4.1.2 Ferroelectric polarization hysteresis and the deal of attention as a candidate ferroelectric for
applications of ferroelectrics Pd-free piezoelectric materials.
In the ferroelectric hysteresis loop drawn in Fig. 6.4.2,
let us consider the polarization change of the crystal 6.4.1.3 Dielectric permittivity and dielectric loss
–1
with the state “1” (negatively charged) when positive In insulating materials under electric field E (V m ),
electric field is applied to it. As soon as the field charged species respond to E and dipole moments are
2+
Ba
(A site)
2-
O

Ti 4+
(B site)

(a) Paraelectric Phase (b) Ferroelectric Phase
-
(Cubic, Pm3m) (Tetragonal, P4mm)
Figure 6.4.1
Crystal structures of barium titanate (BaTiO ) with perovskite structure.
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