Electric field–temperature phase diagram of sodium bismuth titanate-based relaxor ferroelectrics

The electric field–temperature phase diagrams of three bismuth sodium titanate-based relaxor ferroelectrics are reported, namely 0.94(Na_1/2Bi_1/2TiO₃)–0.06(BaTiO₃), 0.80(Na_1/2Bi_1/2TiO₃)–0.20(K_1/2Bi_1/2TiO₃) and 0.75(Na_1/2Bi_1/2TiO₃)–0.25(SrTiO₃). Relaxor behavior is demonstrated by temperature-dependent dielectric permittivity measurements in the unpoled and poled states, as well as by the field-induced phase transition into a ferroelectric phase from the relaxor phase. From temperature-dependent thermometry measurements, we identified the threshold electric field to induce the ferroelectric phase and obtained the released latent heat of the phase transition. We determined the nonergodic and ergodic relaxor phase temperature range based on the absence or presence of reversibility of the relaxor to ferroelectric transition. For all three compositions, the electric field–temperature phase diagram was constructed and a critical point was identified. The constructed electric field–temperature phase diagrams are useful to find optimum operational ranges of ferroelectrics and relaxors for electromechanical and electrocaloric applications.     

Variational‐based modeling of micro‐electro‐elasticity with electric field‐driven and stress‐driven domain evolutions

Recently, increasing interest in so‐called functional or smart materials with electromechanical coupling has been shown such as ferroelectric piezoceramics. These materials are characterized by microstructural properties, which can be changed by external stress and electric field stimuli, and hence find use as the active components in sensors and actuators. The electromechanical coupling effects result from the existence and rearrangement of microstructural domains with uniformly oriented electric polarization. The understanding and efficient simulation of these highly nonlinear and dissipative mechanisms, which occur on the microscale of ferroelectric piezoceramics, are a key challenge of the current research. This paper does not offer a substantially new physical model of these phenomena but a new mathematical modeling approach based on a rigorous exploitation of rate‐type variational principles. This provides a new insight in the structure of the coupled problem, where the governing field equations appear as the Euler equations of a variational statement. We outline a variational‐based micro‐electro‐elastic model for the microstructural evolution of both electrically and mechanically driven electric domains in ferroelectric ceramics, which also incorporates the surrounding free space. To this end, we extend recently developed multifield incremental variational principles of electromechanics from local to gradient‐extended dissipative response and specialize it by a Ginzburg–Landau‐type phase field model, where the thickness of the domain walls enters the formulation as a length scale. This serves as a natural starting point for a canonical compact, symmetric finite element implementation, considering the mechanical displacement, the microscopic polarization, and the electric potential induced by the polarization as the primary fields. The latter is defined on both the solid domain and a surrounding free space. Numerical simulations treat domain wall motions for electric field‐driven and stress‐driven loading processes, including the expansion of the electric potential into the free space. Copyright © 2012 John Wiley & Sons, Ltd.     

The Electrocaloric Effect in BaTiO<Subscript>3</Subscript>–SrTiO<Subscript>3</Subscript> Solid Solution

We have studied the electrocaloric effect in 0.68BaTiO₃–0.32SrTiO₃ solid solution, as well as the dielectric, pyroelectric, and piezoelectric properties of this compound in the vicinity of the first-order phase transition in a bias electric field. The dielectric and electromechanical contributions to the pyroelectric and electrocaloric effects are discussed.     

Ginzburg–Landau Analysis of the Doll–Näbauer Experiment

The Doll–Näbaucr experiment is reconsidered with respect to its theoretical interpretation in terms of the Ginzburg–Landau theory. We find analytical results for the temperature and magnetic field dependence of the order parameter, the transition temperature in finite fields, and the magnetic flux trapped inside the cylinder. This work is intended to demonstrate once again the predictive power and the conceptual beauty of the Ginzburg–Landau theory and is devoted to Vitalij Ginzburg on the occasion of his 90th birthday.     

Multiband Calculations on the Magnetic Properties of Superconductor KFe2As2

Based on two-band Ginzburg–Landau theory, we study the temperature dependence of upper critical fields for superconducting crystal KFe₂As₂. The results reproduce the experimental data in a broad temperature range and directly underlie the multigap superconductivity in this crystal. Our calculations also indicate that the specific heat jump at the superconducting critical temperature is about 0.77, in accordance with the experimental data.     

Fluctuation Hall Conductivity Beyond Linear Response in Layered Superconductors Under a Magnetic Field

We calculate fluctuation Hall conductivity of a strongly type-II superconductor in strong electric fields by using the time dependent Ginzburg–Landau approach. Thermal fluctuations, represented by the Langevin white noise, are assumed to be strong enough to melt the Abrikosov vortex lattice created by the magnetic field into a moving vortex liquid. The layered structure of the superconductor is accounted for by means of the Lawrence–Doniach model. The nonlinear interaction term in dynamics is treated within self-consistent Gaussian approximation and we go beyond the often used lowest Landau level approximation to treat arbitrary magnetic fields. The results are compared to experimental data on high-T _c superconductor YBa₂Cu₃O_7?δ .     

Three-Dimensional Anisotropic Fluctuation Diamagnetism Around the Superconducting Transition of Ba1−x K x Fe2As2 Single Crystals in the Finite-Field (or Prange) Regime

The magnetization around the superconducting transition was recently measured in a high-quality Ba_1?x K _x Fe₂As₂ single crystal with magnetic fields applied along and transverse to the crystal Fe-layers [J. Mosqueira et al. in Phys. Rev. B 83:094519, 2011]. Here we extend this study to the finite-field (or Prange) regime, in which the magnetic susceptibility is expected to be strongly dependent on the applied magnetic field. These measurements are analyzed in the framework of the three-dimensional anisotropic Ginzburg–Landau (3D-aGL) approach generalized to the short-wavelength regime through the introduction of a total-energy cutoff in the fluctuation spectrum. The results further confirm the adequacy of GL approaches to describe the fluctuation effects close to the superconducting transition of these materials.     

Multiferroic oxide composites: synthesis,characterisation and applications

Abstract

The nature of mechanical strain mediated electromagnetic coupling in multiferroic composites has been studied extensively in recent years. This review is on composites with ferromagnetic or ferrimagnetic oxides and ferroelectrics. Systems studied so far include samples with spinel ferrites, hexagonal ferrites or lanthanum manganites for the ferromagnetic phase and barium titanate, lead zirconate titanate (PZT), lead magnesium niobate–lead titanate (PMN-PT) or lead zinc niobate–lead titanate (PZN-PT) for the ferroelectric phase. Bilayer and multilayer heterostructures, bulk composites, core shell nanoparticles and core shell nanotubes and nanowires were investigated for their response to magnetic fields, termed direct magnetoelectric effect (DME). Several systems show a giant low frequency DME and resonance enhancement at bending and electromechanical resonance. The response of the composites to an electric field, called converse ME effect, is found to be strong in several ferrite–ferroelectric composites. The potential for use of the composites for pico-Tesla magnetic sensors and high frequency electric field tunable ferrite signal processing devices are also addressed in this review.     

Abnormal electrocaloric effect of Na0.5Bi0.5TiO3–BaTiO3 lead-free ferroelectric ceramics above room temperature

We demonstrate the electrocaloric effect (ECE) of Na_0.5Bi_0.5TiO₃–BaTiO₃ (NBT–BT) lead-free ferroelectric ceramics, which were fabricated by the solid-state reaction method. Based on a Maxwell relation, the ECE was characterized via P–T curves under different electric fields. The polarization of NBT increases monotonically within the temperature range of 25–145 °C. It indicates that the NBT has an abnormal ECE with a negative temperature change (ΔT₁₄₀ = −0.33 K at E = 50 kV/cm) opposite to that of the normal ferroelectrics. The 0.92NBT–0.08BT composition near the morphotropic phase boundary has a normal ECE under low electric fields and an abnormal ECE under high electric fields. The abnormal ECE character originates from the relaxor characteristic between ferroelectric and antiferroelectric phases, while the common ECE is always related to the normal ferroelectric–paraelectric phase transition.     

Constitutive law for ferroelectric and ferroelastic single crystals: a micromechanical approach

This paper examines the switching process occuring in ferroelectric and ferroelastic single crystals under electro-mechanical loadings. Ferroelectrics undergoing a cubic to tetragonal phase transition are considered. The single crystal energy has three origins: elastic, electric and the incompatibilities of the spontaneous strain and electric displacement fieds between domains. The stress and electric fields fluctuate and present jumps at the domain walls. As a consequence, they induce electro-elastic interaction energy. Thus, it involves dissipation that the present work aims to capture through a micromechanical approach.     

Dielectric properties,thermal expansion and heat capacity of (1 − x)Na0.5Bi0.5TiO3–xBaTiO3 single crystals (x = 0, 0.02, 0.025, 0.0325 and 0.05)

High-quality and large-size lead-free (1 − x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ single crystals (x = 0, 0.025, 0.0325 and 0.05) were grown using Czochralski method. The obtained samples were of pure perovskite structure with rhombohedral symmetry at room temperature. Thermal expansion, heat capacity, ferroelectric and dielectric properties were measured in a wide temperature range. The broad anomalies observed in thermal expansion and heat capacity were corresponded to structural, ferroelectric and dielectric anomalies, related to temperature features of polar regions and formation of a long-range order ferroelectric phase. The Burns temperature was found to increase with increasing BaTiO₃ content. At low-frequency (100 Hz–100 kHz) the samples showed diffuse phase transitions. The obtained results were discussed in terms of local electric and strain fields caused by a difference in ionic radii between (Na,Bi) and Ba ions.     

Ferroelectric-to-non-ergodic relaxor phase transition of (Bi0.5Na0.3K0.2) TiO3– (Ba0.8Ca0.2) TiO3 lead-free ceramics by SrTiO3 effect

In this work, a series compositions of [(0.95?x)(Bi_0.5Na_0.3K_0.2)TiO₃–xSrTiO₃–0.05(Ba_0.8Ca_0.2) TiO₃] Pb-free ceramics (0.0?≤?x?≤?0.2) abbreviation (BKNT–ST–BCT) were synthesized by conventional sintering method. Effect of ST addition on the crystal structure, domain structure, dielectric and ferroelectric properties were investigated. The crystal structure was monitored by XRD and the patterns shown phase crossover from tetragonal to pseudo-cubic caused by lower crystal symmetry of lattice effect at high content of ST. Morphology of sintered ceramics were characterized by SEM, while the domain structure of ST?=?0.0 at different temperatures was examined by In situ TEM. Diffused phase transition corresponding to ferroelectric to ergodic relaxor at lower T has been observed at depolarization temperature (T_d) at (ST < 0.15), while the permittivity peak which detected at ST?=?0.2 in whole range of temperature denote non-ergodic relaxor to paraelectric phase transition. All compositions show normal ferroelectric (P–E) loop at room temperature even ST?=?0.2 with low coercive field (E_c?~?15 kV/cm), while slim relaxor (P–E) loop was observed at (T?=?200 °C). Present normal ferroelectric properties of ST?=?0.2 are attributed to the domain growth and domain wall displacement above the domain switching electric field. All samples belonging to (x?≤?0.1) shown current peak corresponding to ferroelectric phase transition at domain switching field. Samples belonging to (x?≥?0.15) shown two nominal peaks where the second peak is corresponding to ferroelectric-to-another ferroelectric phase transition with different crystal symmetry.

     

Near-field acoustic and piezoresponse microscopy of domain structures in ferroelectric material

Three kinds of near-field microscopy imaging mode including SEAM (Scanning electron acoustic microscopy), PFM (Piezoresponse force microscopy) and SPAM (Scanning probe acoustic microscopy) have been developed to investigate domain structures of ferroelectric ceramics, crystals and thin films in our studies. The domain imaging mechanisms are presented individually in three imaging modes. Sub-surface micro-domain configuration of ferroelectric BaTiO₃ ceramics and single crystal and their dynamic behavior under external fields were clearly visualized by SEAM. Ferroelectric domain structures of ferroelectric PZT thin film and PMN-PT single crystal were characterized by PFM. Nanoscale switching behavior and local field-induced nanoscale displacement behavior of domain structures in ferroelectric thin film were studied by PFM. Antiparallel domain patterns in ferroelectric transparent PLZT ceramics were also characterized by SPAM. The combination of SEAM, PFM and SPAM in application to imaging domain structures undoubtedly enrich our understanding of the nature of piezoelectricity and ferroelectricity at submicro-, even nano-meter scale.     

铅基弛豫铁电单晶体的生长技术

铅基弛豫铁电单晶体由于其优异的压电性能在机电换能领域具有广泛的应用前景.介绍了铅基弛豫铁电单晶体生长技术的研究进展,比较了不同生长技术的优缺点,指出了目前晶体生长中出现的问题,并展望了其研究方向.     

Dielectric and ferroelectric properties of phosphoric acid doped triglycine sulfate single crystals

Single crystals of triglycine sulfate (TGS) grown with 25–50 mol% phosphoric acid dopant in solution were investigated for their dielectric, ferroelectric and mechanical hardness properties. A 25 mol% of H₃PO₄ in solution yielded crystals with a large area in the ac plane without any seed crystal. The room temperature (300 K) dielectric constant (ε′) and tan δ in the frequency range 0.1–100 kHz were found to increase with increasing phosphoric acid concentration. A maximum shift of 2.5 K in the phase transition temperature (T_c) is observed for crystals grown with 50 mol% of H₃PO₄. Systematic lowering of Curie–Weiss constant (C_f) in ferroelectric phase and an increase in the paraelectric phase (C_p) indicates domain wall clamping effects in the doped samples. The changes in the spontaneous polarization (2.6–3.2 μC cm⁻²) with phosphoric acid doping are insignificant, but the high coercive fields (E_c=5 kV cm⁻¹), and distorted loops indicate the presence of stresses and inhomogeneities. The activation energy (ΔE) from temperature dependent d.c. conductivity measurements is lower in both the ferroelectric and the paraelectric phase for doped crystals indicating a strong influence of dopants on the conduction process. The microhardness of the doped crystals is lower in comparison to undoped TGS.     

Optical properties of (1−<Emphasis Type="Italic">x</Emphasis>)PbZn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>−<Emphasis Type="Italic">x</Emphasis>PbTiO<Subscript>3</Subscript> single crystals

The temperature dependence of the optical transmission and small-angle light scattering with and without applied constant electric field was studied in relaxor single crystals of 0.91PbZn_1/3Nb_2/3O₃-0.09PbTiO₃ (PZN-PT 91/9) and 0.93PbZn_1/3Nb_2/3O₃-0.07PbTiO₃ (PZN-PT 93/7) solid solutions in the region of two phase transitions: (i) from cubic paraelectric to tetragonal ferroelectric phase at T=T _c and (ii) from tetragonal ferroelectric to rhombohedral ferroelectric phase at T=T _rt. In the absence of external electric field, only the phase transition at T _c proceeds in both PZN-PT 91/9 and PZN-PT 93/7 crystals according to a percolation mechanism and is accompanied by the appearance of a sharp maximum in the small-angle light scattering intensity curve. In PZN-PT 93/7 crystals, the application of a relatively weak electric field induces an additional percolation type phase transition at T _rt.     

Relaxor-PT single crystals: observations and developments

Relaxor-PT based ferroelectric single crystals Pb(Zn?/?)Nb(?/?)O?-PbTiO? (PZNT) and Pb(Mg?/?)Nb(?/?)O?-PbTiO? (PMNT) offer high performance with ultra-high electromechanical coupling factors k?? > 0.9 and piezoelectric coefficients d??s > 1500 pC/N. However, the usage temperature range of these perovskite single crystals is limited by T(RT)-the rhombohedral to tetragonal phase transition temperature, which occurs at significantly lower temperatures than the Curie temperature T(C), a consequence of curved morphotropic phase boundaries (MPBs). Furthermore, these <001>-oriented crystals exhibit low mechanical quality Q and coercive fields, restricting their usage in high-power applications. In this survey, recent developments on binary and ternary perovskite relaxor-PT crystal systems are reviewed with respect to their temperature usage range. General trends of dielectric and piezoelectric properties of relaxor-PT crystal systems are discussed in relation to their respective T(C)/T(RT). In addition, two approaches have been implemented to improve mechanical Q, including acceptor dopants, analogous to hard polycrystalline ceramics, and anisotropic domain engineering, enabling low-loss crystals with high coupling for high-power applications.     

Multiaxial behavior of ferroelectric single and polycrystals with pressure dependent boundary effects

The external electric and mechanical fields applied at angles to the initial poled direction of the ferroelectric ceramics produce a significantly different nonlinear behavior to that of external fields applied parallel to the poling direction. This angle dependent response of ferroelectric single and polycrystals are predicted by the model proposed based on irreversible thermodynamics and physics of domain switching. The dissipation associated with boundary constraints in thin ferroelectric single crystals are incorporated in the model. As well, the pressure dependent constraints imposed by the surrounding grains on the grain of interest at its boundary during domain switching is correlated with the resistance experienced by a ferroelectric single crystal on its boundary during domain switching. Taking all the domain switching possibilities, the volume fractions of each of the variants in a grain are tracked and homogenized for macroscopic behavior. Numerical simulations were carried out for the multiaxial behavior of ferroelectric single and polycrystals under electrical, mechanical and electromechanical loading conditions.     

Spectroscopic Studies of Nb- and Hf-Doped Barium Titanate Crystals

The optical absorption spectra of Nb- and Hf-doped BaTiO₃crystals were found to exhibit marked changes not only at the ferroelectric transition (120°C) but also in the paraelectric phase (150–170°C). The intensity of the absorption band peaked at 700 nm notably increases on heating to 150–170°C, suggesting an increase in the concentration of F-centers.     

Lead-free piezoelectric ceramics with composition of (0.97−x)Na1/2Bi1/2TiO3-0.03NaNbO3-xBaTiO3

Compositions in (Na_1/2Bi_1/2)TiO₃ based ternary system, (0.97 – x) (Na_1/2Bi_1/2)TiO₃-0.03NaNbO₃-xBaTiO₃ (x = 0, 0.01, 0.02, 0.04, 0.05, 0.06, 0.08) are synthesized using conventional solid state reaction method. Influence of BaTiO₃ on crystal structure, dielectric and piezoelectric properties are investigated. All compositions can form single perovskite phase. Powder x-ray diffraction patterns can be indexed assuming a pseudo-cubic structure. Lattice constant increases with the increase of BaTiO₃ concentration. Rhombohedral distortion is observed in poled samples with BaTiO₃ concentration up to 6 mol%. Temperature dependence of dielectric constant and dissipation factor measurement reveals that all compositions experience two phase transitions: from ferroelectric to antiferroelectric and from antiferroelectric to paraelectric. Both transition temperatures, T _c and T _f, are lowered due to introduction of BaTiO₃. Ferroelectric to antiferroelectric phase transition has relaxor characteristics. Piezoelectric properties have relatively higher value around 1 mol% to 4 mol% BaTiO₃. In ceramics with x = 0.02, thickness electromechanical coupling factor (k _t) of 0.51 and piezoelectric charge constant (d ₃₃) of 110 × 10^–12 C/N are obtained. Addition of small amount of BaTiO₃ (x = 0.01, 0.02) improves piezoelectric properties compared to NBT-NN binary system, while T _f remains above 140°C, higher than that of NBT-BT binary system composition with similar piezoelectric properties. This is in favor of the possible application of them as lead-free piezoelectric ceramics.