Ferroelectricity

Polarization, Capacitance, Dielectric Properties

V

qC Capacitance

L

AC 0

VacC

C

Capacitance of a parallel plate capacitor

Relative Dielectric Constant

Polarization, Capacitance, Dielectric Properties

A

q

V

pP

Electric Dipole Moment

Polarization

xqp

displacement of the e- cloud with respect

to the core due to external bias, occurs in

ALL dielectric materials

Displacement of ions with

respect to each other due to

external bias, only in ionic

materials

Materials already possesing

permanent dipoles, H2O, BaTiO3,

oils, waxes, amorphous

polymers,…

Dielectrica, Paraelectrica and Ferroelectrica

Induced dipoles Permanent dipoles Permanent dipoles

With spontaneous

polarization

Some Important Definitions

EPD S

D: electrical displacement

: dielectric constant

E: electrical field

Ec: coercive field

dijk: piezoelectric coefficient (third rank tensor)

p: pyroelectric coefficient

Qijkl: electrostrictive coefficient (fourth rank tensor)

total

0

0

1

1f b

E

Consider Gauss’s Law in the

presence of a dielectric:

:total Total charge density

:f Free charge density

:b Bound charge density

Pb

PE f 0 f PE0

f D

Then,

electric displacement D

Integral form:

enc

surface

fd Q D a

0 D E P

Properties Spontaneous polarization in the absence applied electrical field.

Extremely high dielectric constant (~500-15,000).

Strong non-linear dielectric response to an applied electrical field.

High strain response to applied electrical field piezoelectricity

Strong variation in polarization with temperature pyroelectricity

Perovskite Structure

Typical Perovskite

Ferroelectrics

•Pb(Zr,Ti)O3-PZT

•Ba(Sr,Ti)O3-BST

•KNbO3 and LiNbO3 •Pb(Ca,Ti)O3 -PCT

•Pb(Sr,Ti)O3 –PST

•Pb(Mg1/3Nb2/3)O3-PbTiO3

Dielectric Constant: Slope of the P vs. E curve

jiji EP 0

Field dependence of dielectric permittivity Tunability

Paraelectric Ferroelectric

-100 -75 -50 -25 0 25 50 75 1000

500

1000

1500

2000

2500

Die

lect

ric

con

sta

nt 3

3/

0

Electric Field, E3 [kV/cm]

-300 -200 -100 0 100 200 300200

400

600

800

1000

1200

Die

lect

ric

co

nst

an

t 3

3/

0

Electric Field, E3 [kV/cm]

Ferroelectrica:

Two polarized states of equal energy but

opposite direction

Spontaneous Polarization and the Hysteresis

Spontaneous Polarization and the Hysteresis

Free energy is that portion of energy that is available to

perform thermodynamic work; i.e., work mediated by

thermal energy.

Most useful to describe properties of solid materials at

constant pressure that can expand or shrink as a function

of temperature.

F = U -TS Where U is the internal energy stored in a certain volume

Natural variables:

Differential:

Ginzburg Landau Theory of Phase Transitions

Any crystal in thermodynamic equilibrium can be completely specified by the

values of a number of variables:

Temperature T, entropy S, electric Feld E, polarization P, stress s and strain s.

We are applying externally electric fields E and elastic stresses s, the crystal

responds with the polarization and strain.

Expand the free energy in powers of the dependent variables, with unknown

coefficients.

Simple example: a single component of the polarization, and ignore the strain field.

Finding the minima of F:

Paraelectric Materials:

If a; b; c are all positive, the free energy (for E = 0) has a minimum at the origin.

This provides a relationship between the polarizability and the field (in linear

response, for small electric field) which defines the dielectric susceptibility

Ferroelectric Materials:

If a < 0, while b; c > 0: The free energy will have a double minimum at a finite

polarization P.

The ground state has a spontaneous polarization and is a ferroelectric.

A phase transition happens if a changes continuously with temperature and changes

sign at a temperature To.

Assuming a linear variation of a(T) with temperature: a0 ~ (T - To).

A second-order, or continuous phase transition. The order parameter (here

spontaneous polarization) vanishes continuously at the transition temperature Tc = To.

If b < 0 (while c remains positive).

T > To the free energy may have a subsidiary minimum at non-zero P.

As a is reduced (temperature lowered), this minimum will drop in energy to below that

of the unpolarized state, and will be the thermodynamically favored configuration.

This happens at the Curie temperature Tc, which is higher than To.

At any temperature between Tc and To the unpolarized phase exists as a local

minimum of the free energy.

The order parameter jumps discontinuously to zero at Tc.

First-order or discontinuous transition.

Why Ferroelectrics?

Wireless

Communications

Filters Oscillators P

E

+PE

+PR

-PE

-PR

1

0

Lead

Oxygen

Titanium

Ferroelectric

Memories

EC

or

0 100 200 300 400 5000.3

0.4

0.5

0.6

PbTiO3

TC=490oC

Sponta

neous P

ola

riza

tion (

C/m

2)

Temperature [oC]

Temperature Dependence of Spontaneous

Polarization

TE

T

P

T

Dp S

E

Pyroelectricity

P

T

E>>0 E=0

Why Ferroelectrics?

Exceptional Pyroelectric Response

Tc

Temperature P

ola

riza

tio

n

Volts

TE

T

P

T

Dp S

E

Pyroelectricity

0 100 200 300 400 500 600

0.390

0.395

0.400

0.405

0.410

0.415

PbTiO3

TC=490oC

a0

c

a

Lattic

e P

ara

mete

r [n

m]

Temparature [oC]

Electrostriction: Coupling between Polarization

and Self-Strain

Piezoelectric effect: Strain due to an applied electric field

kkijij Edx

Strain due to combined Electrostrictive and Piezoelectric effect

lkijkljkikij

lkijklkkijij

PPQPd

PPQEdx

1

0

1

Under non-zero external stress

klijkllkijklkkijij XSPPQEdx

Polarization Switching by an Electric Field

E=0

E>0

E≈EC

Electrical (or 1800-domains) to minimize depolarization.

Polarization Switching by an Electric Field

Non-Volatile RAMs (memory)

Dynamic RAMs (capacitors)

Tunable Microwave Devices

Pyroelectric Detectors/Sensors

Optical Waveguides

Piezoelectric Sensors/Actuators, MEMS

Applications of Ferroelectrics

Non-Volatile RAMs (memory)

Non-Volatile RAMs (memory)

Smart cards use ferroelectric memories. They can hold relatively large amounts of information

and do not wear out from use, as magnetic strips do, because they use contactless radio

frequency input/output. These cards are the size and shape of credit cards but contain

ferroelectric memory that can carry substantial information, such as its bearer's medical history

for use by doctors, pharmacists and even paramedics in an emergency. Current smart cards carry

about 250 kilobytes of memory.

Tunable Microwave Devices / Optical Waveguides

Filters

Phase

shifters Delay

lines

Oscillators

-100 -75 -50 -25 0 25 50 75 100

0

500

1000

1500

2000

2500

Die

lect

ric

con

stan

t 3

3/

0

Electric Field, E3 [kV/cm]

(E=0)

0

E

tunability

Pyroelectric Detectors/Sensors

Piezoelectric Sensors/Actuators, MEMS