Seung-Eek Park

Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals

The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3, morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s)>2500 pC/N and subsequent strain levels up to >0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for 〈001〉 oriented rhombohedral crystals, although 〈111〉 is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to an E-field induced phase transformation. High electromechanical coupling (k33)>90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Characteristics of relaxor-based piezoelectric single crystals for ultrasonic transducers

For ultrasonic transducers, piezoelectric ceramics offer a range of dielectric constants (K/spl sim/1000-5000), large piezoelectric coefficients (d/sub ij//spl sim/200-700 pC/N), and high electromechanical coupling (k/sub t//spl sime/50%, k/sub 33//spl sime/75%). For several decades, the material of choice has been polycrystalline ceramics based on the solid solution Pb(Zr/sub 1-x/B/sub 2x/)O/sub 3/ (PZT), compositionally engineered near the morphotropic phase boundary (MPB). The search for alternative MPB systems has led researchers to revisit relaxor-based materials with the general formula, Pb(B/sub 1/,B/sub 2/)O/sub 3/ (B/sub 1/:Zn/sup 2+/, Mg/sup 2+/, Sc/sup 3+/, Ni/sup 2+/..., B/sub 2/:Nb/sup 5+/ Ta/sup 5+/...). There are some claims of superior dielectric and piezoelectric performance compared to that of PZT materials. However, when the properties are examined relative to transition temperature (T/sub 3/), these differences are not significant. In the single crystal form, however, Relaxor-PT materials, represented by Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PZN-PT), Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PMN-PT) have been found to exhibit longitudinal coupling coefficients (k/sub 33/)>90%, thickness coupling (k/sub t/)>83%, dielectric constants ranging from 1000 to 5000 with low dielectric loss <1%, and exceptional piezoelectric coefficients d/sub 33/>2000 pC/N, the later promising for high energy density actuators. For single crystal piezoelectrics to become the next generation material of ultrasonic transducers, further investigation in crystal growth, device fabrication and testing are required.

Origin of the High Piezoelectric Response in PbZr1-xTixO3

High resolution x-ray powder diffraction measurements on poled PbZr1-xTixO3 (PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the so-called morphotropic phase boundary) have shown that for both rhombohedral and tetragonal compositions the piezoelectric elongation of the unit cell does not occur along the polar directions but along those directions associated with the monoclinic distortion. This work provides the first direct evidence for the origin of the very high piezoelectricity in PZT.

New High Temperature Morphotropic Phase Boundary Piezoelectrics Based on Bi(Me)O3–PbTiO3 Ceramics

New morphotropic phase boundary (MPB) piezoelectrics, with ferroelectric phase transition (Tc) exceeding that of PbZrO3–PbTiO3 (PZT), were investigated. Based on a perovskite tolerance factor-Tc relationship, new high Tc MPB systems were projected in the Bi(Me)O3–PbTiO3 system, where Me is a relatively large B+3-site cation. For the (1-x)BiScO3–(x)PbTiO3 solid solution, a MPB was found at x-0.64 separating the rhombohedral and tetragonal phases, with correspondingly enhanced dielectric and piezoelectric properties. A transition temperature Tc of ~ 450°C was determined with evidence of Tcs on the order of ≥ 600°C in the BiInO3 and BiYbO3 analogues, though issues of perovskite stability remain for the smaller tolerance end-member systems.

Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: The structure of PbZr0.52Ti0.48O3

The perovskitelike ferroelectric system PbZr1-xTixO3 (PZT) has a nearly vertical morphotropic phase boundary (MPB) around x=0.45–0.50. Recent synchrotron x-ray powder diffraction measurements have revealed a monoclinic phase between the previously established tetragonal and rhombohedral regions. In the present work we describe a Rietveld analysis of the detailed structure of the tetragonal and monoclinic PZT phases on a sample with x=0.48 for which the lattice parameters are, respectively, at=4.044 A, ct=4.138 A, at 325 K, and am=5.721 A, bm=5.708 A, cm=4.138 A, β=90.496°, at 20 K. In the tetragonal phase the shifts of the atoms along the polar [001] direction are similar to those in PbTiO3 but the refinement indicates that there are, in addition, local disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis. The monoclinic structure can be viewed as a condensation along one of the directions of the local displacements present in the tetragonal phase. It equally well corresponds to a freezing-out of the local displacements along one of the directions recently reported for rhombohedral PZT. The monoclinic structure therefore provides a microscopic picture of the MPB region in which one of the ‘‘locally’’ monoclinic phases in the ‘‘average’’ rhombohedral or tetragonal structures freezes out, and thus represents a bridge between these two phases.

Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite

The perovskitelike ferroelectric system PbZr1-xTixO3 (PZT) has a nearly vertical morphotropic phase boundary (MPB) around x=0.45–0.50. Recent synchrotron x-ray powder diffraction measurements have revealed a monoclinic phase between the previously established tetragonal and rhombohedral regions. In the present work we describe a Rietveld analysis of the detailed structure of the tetragonal and monoclinic PZT phases on a sample with x=0.48 for which the lattice parameters are, respectively, at=4.044 A, ct=4.138 A, at 325 K, and am=5.721 A, bm=5.708 A, cm=4.138 A, β=90.496°, at 20 K. In the tetragonal phase the shifts of the atoms along the polar [001] direction are similar to those in PbTiO3 but the refinement indicates that there are, in addition, local disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis. The monoclinic structure can be viewed as a condensation along one of the directions of the local displacements present in the tetragonal phase. It equally well corresponds to a freezing-out of the local displacements along one of the directions recently reported for rhombohedral PZT. The monoclinic structure therefore provides a microscopic picture of the MPB region in which one of the ‘‘locally’’ monoclinic phases in the ‘‘average’’ rhombohedral or tetragonal structures freezes out, and thus represents a bridge between these two phases.

Preparation and Characterization of High Temperature Perovskite Ferroelectrics in the Solid-Solution (1-x)BiScO3–xPbTiO3

The dielectric and piezoelectric properties of the new perovskite solid solution system (1-x)BiScO3–xPbTiO3 were investigated. This system is representative of a new group of high temperature piezoelectrics that includes Bi(Me)O3–PbTiO3, where Me+3 is a relatively large cation, Sc, Y, Yb, In, etc., and combinations thereof. In the (1-x)BiScO3–xPbTiO3 series, perovskite stability was achieved for x>50 mol% PbTiO3 being ferroelectric rhombohedral and transforming to ferroelectric tetragonal in the region x=64 mol% PbTiO3, designated as the morphotropic phase boundary (MPB). Analogous to (1-x)PbZrO3–xPbTiO3 (PZT), the dielectric and piezoelectric properties were enhanced for compositions near the MPB. Piezoelectric coefficient d33 values reached 450 pC/N, comparable to soft PZTs with a transition temperature of 450°C, more than 100°C higher than commercial PZT. The combination of high TC and excellent piezoelectric activity make (1-x)BiScO3–xPbTiO3 materials candidates for high temperature, and temperature stable actuators and transducers.

A monoclinic ferroelectric phase in the Pb(Zr1−xTix)O3 solid solution

A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr{sub 0.52}Ti{sub 0.48}O{sub 3} by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (a{sub t} = 4.037 {angstrom}, c{sub t} = 4.138 {angstrom}), and transforms below {approx} 250 K into a phase which, unexpectedly, has monoclinic symmetry (a{sub m} = 5.717 {angstrom}, b{sub m} = 5.703 {angstrom}, c{sub m} = 4.143 {angstrom}, {beta}= 90.53{sup o}, at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic ac plane close to the pseudocubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov.A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr{sub 0.52}Ti{sub 0.48}O{sub 3} by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (a{sub t}=4.037{Angstrom}, c{sub t}=4.138{Angstrom}), and transforms below {approximately}250 K into a phase which, unexpectedly, has monoclinic symmetry (a{sub m}=5.717{Angstrom}, b{sub m}=5.703{Angstrom}, c{sub m}=4.143{Angstrom}, {beta}=90.53{degree}, at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic {ital ac} plane close to the pseudocubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov. {copyright} {ital 1999 American Institute of Physics.}

Polarization Rotation via a Monoclinic Phase in the Piezoelectric 92%PbZn1/3Nb2/3O3-8%PbTiO3

The origin of ultrahigh piezoelectricity in the relaxor ferroelectric PbZn(1/3)Nb(2/3)O3-PbTiO3 was studied with an electric field applied along the [001] direction. The zero-field rhombohedral R phase starts to follow the direct polarization path to tetragonal symmetry via an intermediate monoclinic M phase, but then jumps irreversibly to an alternate path involving a different type of monoclinic distortion. Details of the structure and domain configuration of this novel phase are described. This result suggests that there is a nearby R-M phase boundary as found in the Pb(Ti,Zr)O3 system.

Crystallographically engineered BaTiO3 single crystals for high-performance piezoelectrics

Dielectric and piezoelectric properties of BaTiO3 single crystals polarized along the 〈001〉 crystallographic axes were investigated as a function of temperature and dc bias. Electromechanical coupling (k33)∼85% and piezoelectric coefficients (d33)∼500 pC/N, better or comparable to those of lead-based Pb(Zr, Ti)O3 (PZT), were found from 〈001〉-oriented orthorhombic crystals at 0 °C, as a result of crystallographic engineering. A rhombohedral BaTiO3 crystal polarized along 〈001〉 also exhibited enhanced piezoelectric performance, i.e., k33∼79% and d33∼400 pC/N at −90 °C, superior to PZTs at the same temperature. It was found that the crystal structure determined the (in)stability of the engineered domain state in BaTiO3 single crystals. Rhombohedral (3m) crystals at −100 °C exhibited a stable domain configuration, whereas depoling occurred in crystals in the adjacent orthorhombic phase upon removal of the E field.