Types of Ferroelectric Materials Ferroelectric Materials can be structurally categorized into 4 groups: 1. Corner Sharing Octahedra: 1.1 Perovskite-Type Compounds (such as BaTiO 3 , PT, PZT, PMN, and PLZT) 1.2 Tungsten-Bronze-Type Compounds (such as PbNb 2 O 6 ) 1.3 Bismuth Oxide Layer Structured Compounds (such as Bi 4 Ti 3 O 12 and PbBi 2 Nb 2 O 9 ) 1.4 Lithium Niobate and Tantalate (such as LiNbO 3 and LiTaO 3 ) 2. Compounds Containing Hydrogen Bonded Radicals (such as KDP, TGS, and Rochelle Salt) 3. Organic Polymers (such as PVDF and co-
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Types of Ferroelectric Materials Ferroelectric Materials can be structurally categorized into 4 groups: 1.Corner Sharing Octahedra: 1.1 Perovskite-Type.
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Types of Ferroelectric MaterialsFerroelectric Materials can be structurally categorized into 4 groups:
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)
Ti
Ba
O
Crystal Chemistry of BaTiO3
Phase 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 m) • 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 m)
Phase Transitions in BaTiO3
Cubic (m3m) Tetragonal (4mm) Orthorhombic (mm2) Rhombohedral (3m) 120 C 0 C -90 C
Paraelectric Phase Ferroelectric Phase
Phase 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
Phase Transitions in BaTiO3
Spontaneous Polarization (Ps) versus Temperature
I. No Spontaneous Polarization (Ps = 0)II. Ps along [001] directions of the original cubicIII. Ps along [110] directions of the original cubicIV. Ps along [111] directions of the original cubic
(Ps ~ 26 C/cm2 at room temperature)
Phase Transitions in BaTiO3
Relative Permittivity of Single Crystal BaTiO3 Measured in the a and c Directions versus
Temperature
BaTiO3 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/piezoelectric 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
Effects of A and B Sites Substitutions in BaTiO3
Curie Point and Phase Transitions ShiftersThis 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.
Modified 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)
Effects of Grain Sizes
At Curie Pointlarge 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 Templarge grain larger domains less
internal stress lower K
small grain smaller domains less internal stress relieved larger