Title: Types of Ferroelectric Materials
1Types of Ferroelectric Materials
- Ferroelectric Materials can be structurally
categorized into 4 groups - Corner Sharing Octahedra
- 1.1 Perovskite-Type Compounds
- (such as BaTiO3, PT, PZT, PMN, and PLZT)
- 1.2 Tungsten-Bronze-Type Compounds
- (such as PbNb2O6)
- 1.3 Bismuth Oxide Layer Structured Compounds
- (such as Bi4Ti3O12 and PbBi2Nb2O9)
- 1.4 Lithium Niobate and Tantalate
- (such as LiNbO3 and LiTaO3)
- Compounds Containing Hydrogen Bonded Radicals
- (such as KDP, TGS, and Rochelle Salt)
- Organic Polymers (such as PVDF and co-polymers)
- Ceramic Polymer Composites (such as PZT-PE)
2Corner Sharing Octahedra
Mixed Oxide Ferroelectrics with Corner Sharing
Octahedra of O2- Ions
Inside each Octahedron ? Cation Bb (3 lt b lt
6) Space between the Octahedra ? Aa Ions (1 lta
lt 3)
3Corner Sharing Octahedra
In prototypic forms, Aa, Bb, and O2- ions
geometrically coincide ? Non-Polar Lattice ?
Phase Transitions ?Changes in Lattice Structure
? Aaand Bb ions displaced w.r.t. O2- ions ?
Polarized Lattice ?
4Perovskite-Type Compounds
Perovskite ? Mineral Name of Calcium Titanate
(CaTiO3)
General Chemical Formula ?ABO3 A ? Cation with
Larger Ionic Radii B ?Cation with Smaller Ionic
Radii O ? Oxygen
5Perovskite-Type Compounds
Perovskite ? Three-Dimensional Network of BO6-
Octahedra Perovskite ? Cubic-Close-Packed of A
and O ions with B in interstitial positions
Most Ferroelectric Perovskites A2B4O3 or
A1B5O3 Non-Ferroelectric Perovskites A3B3O3
6Perovskite-Type Compounds
Structural Classifications of A2B4O3 compounds
by A2 and B4 ionic radii
7Perovskite-Type Compounds
Barium Titanate (BaTiO3)
Ti ? 6 coordinated to Oxygen (Octahedron) Ba ? 12
coordinated to O (Cubic-Close-Packed) O ? 4
coordinated to Ba AND 2 coordinated to Ti
(Distorted Octahedron)
8Perovskite-Type Compounds
Barium Titanate (BaTiO3)
Cubic-Close-Packed (CCP) OR Face-Centered-Cubic
(FCC) (abc-abc-abc arrangement)
9(No Transcript)
10Barium Titanate (BaTiO3)
Ti ? 6 coordinated to Oxygen (Octahedron) Ba ? 12
coordinated to O (Cubic-Close-Packed) O ? 4
coordinated to Ba and 2 coordinated to Ti
(Distorted Octahedron)
11Crystal Chemistry of BaTiO3
12Phase Equilibria of BaTiO3 (BaO-TiO2)System
Very First Phase Equilibria
- Effects of BaO/TiO2 Ratio
- Very little solubility of excesses BaO or TiO2
- Excess TiO2 results in Ba6Ti17O40 separated
phase (melt at 1320 C) ? - liquid phase sintering below 1350 C ? wide grain
sizes (5 50 mm) - Excess BaO results in Ba2TiO4 separated phase
(melt at 1563 C) ? - solid insoluble phase acts as grain growth
inhibitor below 1450 C ? - smaller grain sizes (1 5 mm)
13Phase Transitions in BaTiO3
Cubic (m3m) ? Tetragonal (4mm) ? Orthorhombic
(mm2) ? Rhombohedral (3m) 120
C 0 C
-90 C
Paraelectric Phase
Ferroelectric Phase
14Phase Transitions in BaTiO3
Lattice Parameters Variation with Temperature
during the Phase Transitions
Through X-Ray and Neutron Diffractions, during
the Cubic-to-Tetragonal Phase (Structural)
Transition, Ba2, Ti4, and O2- (w.r.t. center
O2-) displaced along the c-axis 0.06 Å, 0.12 Å,
and 0.03 Å, respectively
15Phase Transitions in BaTiO3
Spontaneous Polarization (Ps) versus Temperature
I. No Spontaneous Polarization (Ps 0) II. Ps
along 001 directions of the original cubic III.
Ps along 110 directions of the original
cubic IV. Ps along 111 directions of the
original cubic (Ps 26 mC/cm2 at room
temperature)
16Phase Transitions in BaTiO3
Relative Permittivity of Single Crystal BaTiO3
Measured in the a and c Directions versus
Temperature
17BaTiO3 Ceramics and Modifications
- BaTiO3 ceramic was the first piezoelectric
transducer developed, BUT now use mainly for
high-dielectric constant capacitors because of
TWO main reasons - Relatively low Tc (120 C) limits its use as
high-power transducers - Low piezoelectric activities as compared to PZT
- BaTiO3 for capacitor applications require special
modifications to suppress its ferroelectric/piezoe
lectric properties, and simultaneously to obtain
better dielectric features. This is done through
additives and compositional modifications, which
can produce the following effects - Shift of Curie Point and other transition
temperatures - Restrict domain wall motions
- Introduce second phases and compositional
heterogeneity - Control crystallite size
- Control oxygen content and the valency of the Ti
ion
18Effects of A and B Sites Substitutions in BaTiO3
Curie Point and Phase Transitions Shifters This
would enable the peak permittivity to be used in
the temperature range of interest. For example,
Sr2 in the A site would reduce the Curie Point
towards room temperature, while Pb2 would raise
the Curie Point. This leads to tailoring
dielectric properties with A and B sites
substitutions.
19Modified BaTiO3 Ceramics (Tc Suppressors)
- Ba(Ti1-x Zrx )O3 Solid-Solution
- Low level addition the dielectric peak rises
sharply - ? Higher level addition results in peak
broadening (probably causes by macroscopic
heterogeneity in the composition
- Controlling the Permittivity
- Control of K in fine grained BT
- ? Control of dirty grain boundary impedance to
suppress the Curie Peak at Tc (as compared to
Curie point adjusted compositions above)
20Effects of Grain Sizes
At Curie Point large grain ? multiple domains ?
more domain wall motions ? higher K small grain
? single domain ? less domain wall motions due to
grain boundary ? lower K
At Room Temp large grain ? larger domains ? less
internal stress ? lower K small grain ? smaller
domains ? less internal stress relieved ? larger
internal stress ? higher K