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 Example Pyroelectrics Capacitors/dielectrics Electrostriction/actuators Medical ultrasound/high efficiency transducers Piezoelectrics Electrooptics Pb(Sc1/2 Ta1/2)O3 (Ba0.60 Sr0.40)TiO3 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 (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 Permittivity frequency dependence Permittivity behavior in Paraelectric range Remnant polarization (Pr) Scattering of light X-Ray diffraction Sharp 1st or 2nd order transition about Tc Weak frequency dependence Follow Curie-Weiss Relation above Tc Strong remnant polarization Strong anisotropy (birefringent) Line splitting (cubic to tetragonal) Broad-diffused phase transition about Tmax Strong frequency dependence Follow Curie-Weiss Square Relation above Tmax Weak remnant polarization Very weak anisotropy to light (pseudo-cubic) No line splitting (pseudo-cubic structure) Comparison of normal and relaxor ferroelectrics

Lead Magnesium Niobate (PMN) System Spontaneous polarization (Ps) 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 Diffused/broad dielectric behavior 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 Dielectric Hysteresis 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 x = 35 for MPB compositions  Large piezoelectric strain > 1%  High electromechanical coupling factor (k33 > 90%)  Relaxor-based piezoelectric crystals for next generation transducers 1-x PZN – x PT Single Crystals 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