Transcript Lead Magnesium Niobate (PMN)

Lead Magnesium Niobate (PMN) System
Lead Magnesium Niobate (PMN) System
Important Perovskite End Members for Relaxors
Important Relaxors Based on MPB Compositions
Lead Magnesium Niobate (PMN) System
Relaxor-Based Compositions for MLC
Lead Magnesium Niobate (PMN) System
Areas of Applications for Relaxors Ferroelectrics and Solid Solutions
Application
Pyroelectrics
Capacitors/dielectrics
Electrostriction/actuators
Medical ultrasound/high
efficiency transducers
Piezoelectrics
Electrooptics
Example
Pb(Sc1/2 Ta1/2)O3
(Ba0.60 Sr0.40)TiO3
Pb(Mg1/3Nb2/3)O3
Pb(Mg1/3Nb2/3)O3
Pb(Zn1/3Nb2/3)O3
Pb[(Mg1/3Nb2/3)1-xTix]O3
Pb[(Zn1/3Nb2/3)1-xTix]O3
Pb[(Sc½Nb½)1-xTix]O3
Pb(Zr1-xTix)O3
Pb[(Zn1/3Nb2/3)1-xTix]O3
Pb[(Sc½Nb½)1-xTix]O3
(Pb1-xLa2x/3)(Zr1-yTiy)O3
Lead Magnesium Niobate (PMN) System
Relaxor Ferroelectrics

Pb(B1B2)O3
(B1 ~ lower valency cation : Mg2+, Zn2+, Ni2+, Fe3+)
(B2 ~ higher valency cation : Nb5+, Ta5+, W6+)

PMN  Pb(Mg1/3Nb2/3)O3

Important Relaxor Ferroelectric with Tc ~ -10 C
Broad diffused and dispersive phase transition on cooling below Tc
Very high room temperature dielectric constant
Strong frequency-dependent dielectric properties

Nano-scaled compositional inhomogeniety

Chemically order-disorder behavior observed by TEM study

B-site 1:2 order formula with 1:1 order arrangement in the structure
(Most have rhombohedral symmetry due to slight lattice distortion)
Lead Magnesium Niobate (PMN) System
Dielectric properties of Pb(Mg1/3Nb2/3)O3
showing diffused phase transition and relaxor characteristics
(Tmax ( at 1 kHz) ~ -10 C with er max ~ 20,000)
Lead Magnesium Niobate (PMN) System
Property
Normal Ferroelectrics
Relaxor Ferroelectrics
Permittivity temperature
dependence
Sharp 1st or 2nd order
transition about Tc
Broad-diffused phase transition
about Tmax
Permittivity frequency
dependence
Weak frequency dependence
Strong frequency dependence
Permittivity behavior in
Paraelectric range
Follow Curie-Weiss Relation
above Tc
Follow Curie-Weiss Square
Relation above Tmax
Remnant polarization (Pr)
Strong remnant polarization
Weak remnant polarization
Scattering of light
Strong anisotropy
(birefringent)
Very weak anisotropy to light
(pseudo-cubic)
X-Ray diffraction
Line splitting (cubic to
tetragonal)
No line splitting (pseudo-cubic
structure)
Comparison of normal and relaxor ferroelectrics
Lead Magnesium Niobate (PMN) System
First-Order Phase Transition
Second-Order Phase Transition
Spontaneous polarization (Ps)
A discontinuity in the first-order phase transition 
 A continuous change in the second-order phase transition 
Relaxor ferroelectric  Ps decays continuously with temperature,
but does not follow the parabolic temperature dependence
as in the second-order phase transition
Lead Magnesium Niobate (PMN) System
Normal Ferroelectrics  Dielectric Behavior 
Relaxor Ferroelectrics
Normal ferroelectrics  the onset of spontaneous polarization occurs
simultaneously with the maximum in the paraelectric to ferroelectric phase
transition. No Ps above the transition temperature with a valid Curie-Weiss Law
Relaxor ferroelectrics  Three regimes : Regime I Above dielectric maximum
temperature, Regime II Between Td (depolarization temperature) and Tmax
(dielectric transition temperature), and Regime III Below Td
Lead Magnesium Niobate (PMN) System
Regime I : Electrostrictive region with existence of chemically ordered region
with no macro-scale ferroelectric domian  little or no hysteresis
Regime II : Freezing-out of macro-domain region in which with decreasing
temperature the polar regions grow and cluster  hysteresis is observed and
becomes more pronounced with decreasing temperature
Regime III : Macro-domain region becomes more stable which results to a large
spontaneous polarization and piezoelectric effects with large remnant strain
Lead Magnesium Niobate (PMN) System
Ordered and Disordered Perovskite Structures
Lead Magnesium Niobate (PMN) System
Ordered and Disordered Perovskite Structures
Fully disorder of the cations in the B-sites occupation

“Normal” ferroelectric materials (such as PZT)
Nano-scale order of the cations in the B-sites occupation

“Relaxor” ferroelectric materials (such as PMN)
Lead Magnesium Niobate (PMN) System
5 nm
Nano-scale ordered region in disordered matrix
Pb(Mg1/3Nb2/3)O3

Nano-scale ordered region with Mg:Nb = 1:1 (like in NaCl structure)

Non-stoichiometric short range chemical heterogeneity

Different ferroelectric transition temperature regions

Diffused/broad dielectric behavior
Lead Magnesium Niobate (PMN) System
PbSc1/2Ta1/2O3
Harmer and Bhalla
PbMg1/3Nb2/3O3
Randall et al.
Dark field TEM images showing nano-scale ordered
region in disordered matrix
Lead Magnesium Niobate (PMN) System
Polarization
Hysteresis
Dielectric
Birefringence
Features for Ordered and Disordered Ferroelectrics
Lead Magnesium Niobate (PMN) System
Structural Transition
Ferroelectric properties decay with increasing T 
Features for Ordered and Disordered Ferroelectrics
Lead Magnesium Niobate (PMN) System
Relaxor Ferroelectrics

PMN  Pb(Mg1/3Nb2/3)O3

Strong frequency-dependent dielectric properties
(Tmax shifts to higher temperature with increasing frequency)
(Dielectric losses are at the highest just below Tmax)

Dynamical thermal re-orientation of polar regions with frequency
(As frequency increases, the polar regions cannot keep up  er  and loss )

Dielectric relaxation similar to glass (follows a Vogel-Fulcher model)

However, no certain explanation for relaxor ferroelectrics

Freezing of micro-region and chemical fluctuation 
 Ordered-disordered region 
 Spin-glass model 
Lead Magnesium Niobate (PMN) System
One of the difficulties in processing PMN ceramics

Pyrochlore
(General formula RNb2O6 where R is a mixture of divalent ions)

Pb1.83Nb1.71Mg0.29O6.39  formed at 700-850 C

Paraelectric with room temperature er of 130

Strong reduction in er if present as inter-granular region in high er PMN region
(Not very significant if only discrete particles disperse in PMN matrix)

Pure Phase PMN with “Columbite Precursor Method”
(MgO + Nb2O5  MgNb2O6  MgNb2O6 + PbO  PMN)
Example of Pyrochlore Phase
Lead Magnesium Niobate-Lead Titanate (PMN-PT) System
Most widely studied relaxor materials  PMN-PT Solid Solutions

High-strain (0.1%) electrostrictive actuators
High dielectric constant (er > 25,000) capacitors
Lead Magnesium Niobate-Lead Titanate (PMN-PT) System
0.65 PMN - 0.35 PT  MPB Compositions with normal ferroelectric properties
High dielectric constant capacitors  0.90 PMN - 0.1 PT  Relaxor
(with Tmax near room temperature with large dielectric constant)
(large “electrostrictive” strain)
Lead Magnesium Niobate-Lead Titanate (PMN-PT) System
Dielectric Behavior of 0.9PMN-0.1PT Relaxor Ferroelectrics
Lead Magnesium Niobate-Lead Titanate (PMN-PT) System
Strain-Field Relation of 0.9PMN-0.1PT Relaxor Ferroelectrics
Lead Magnesium Niobate-Lead Titanate (PMN-PT) System
Electrostriction in Ferroelectric Materials

Basis of electromechanical coupling in all insulators
x = ME2 and x = QP2
(As compared to x ~ E for piezoelectric effects)

Large in ferroelectrics just above Tc
due to electrical unstabability of ferroelectrics (PMN, PZN, and PLZT)
(because of their diffused transition and possible field-activated coalescence of
micropolar region to macrodomain of the parent ferroelectric )

“Electrostrictive Mode”
“Field-Biased Piezoelectric Mode”

DC Bias Field  Induced Ferroelectric Polarization  Normal Piezoelectric
d33 = 2Q11P3e33
d31 = 2Q12P3e33
Lead Magnesium Niobate-Lead Titanate (PMN-PT) System
Advantages of Electrostriction

Minimal or negligible strain-field dependence hysteresis
(in selected temperature range)
More stale realizable deformation than observed in piezo-ceramics
No poling is required

Longitudinal strain
0.1% in PMN
0.3% in PLZT (La/Zr/Ti = 9/65/35)
Disadvantages of Electrostriction

Limited usable temperature range 
(due to a strong temperature dependence)
 Small deformation at low electric field 
(as a result of a quadratic nature of electrostriction)
PMN-PT and PZN-PT Single Crystals
1-x PMN – x PT Single Crystals
1-x PZN – x PT Single Crystals
x = 35 for MPB compositions
 Large piezoelectric strain > 1%
 High electromechanical coupling
factor (k33 > 90%)

Relaxor-based
piezoelectric
crystals
for
next
generation
transducers
x = 9 for MPB compositions
 Large piezoelectric strain ~ 1.7%
 High electromechanical coupling
factor (k33 = 92%)

Relaxor-based
piezoelectric
crystals for high performance
atuators
PMN-PT and PZN-PT Single Crystals
Comparison of field-induced strain for various
ceramics and single crystals
PMN-PT and PZN-PT Single Crystals
PMN-PT and PZN-PT Single Crystals
Engineered Domain States
Initially the domains are aligned as close as possible to the field
direction
 Increased polarization in rhombohedral structure
 As the field is increased to certain values, the domains collapse to
the <001> direction, as a result of rhombohedral-to-tetragonal phase
transition
 Large increase in polarization, hence piezoelectric properties