Large strain and relaxation behavior in CeO2 doped Bi0.487Na0.427K0.06Ba0.026TiO3 piezoceramics

xCeO₂-doped Bi_0.487Na_0.427K_0.06Ba_0.026TiO₃ lead-free piezoelectric ceramics (BNTC1000x, x=0, 0.3, 0.6, 0.8, 1.0, 1.2, 1.4 wt%), were synthesized by the solid-state reaction method. XRD patterns showed that all BNTC1000x ceramics exhibit pure single perovskite phase. At the critical composition BNTC12, a large electric-field-induced strain of 0.39% with normalized strain (S_max/E_max) of 561 pm/V was obtained under an electric field of 65 kV/cm. The ferroelectric phase was fully poled with electric field, and depoled once the applied electric field was removed. During that cycle, the non-180°-domains repeated switching and back-switching and the large strain was induced. The relaxation behavior was involved in BNTC1000x ceramics and induced by oxygen vacancy migration. Besides, this behavior was more predominant in BNTC12 than in BNTC0.     

A Numerical Investigation of the Effect of Induced Porosity on the Electromechanical Switching of Ferroelectric Ceramics

Ferroelectric ceramics have many applications ranging from microelectromechanical systems (MEMS) to explosively driven power supplies. In addition to chemical compositions and processing methods, porosity is an important material parameter that can affect both the electrical and mechanical responses of a ferroelectric. The main objective of the current study is to gain preliminary insight on the possible effect of porosity on the switching behavior of ferroelectrics.

Numerical simulation was used to address the research objective. The numerical code used is an arbitrary Lagrangian-Eulerian, multi-material, multi-physics finite element code developed by Sandia National Laboratories. To accomplish the research objective, a phenomenological electromechanical model developed by Landis was first implemented in the code. The effects of void density, which ranges from 0 to 11%, and shape, which includes sphere and cylinder, were then investigated through a parametric study.

The study indicates that the remanent polarization decreases with increased porosity density. For a given density, the porous solid that contains the cylindrical voids whose longitudinal axis is perpendicular to the applied electric field possesses the largest amount of the remanent polarization. The solid that contains the cylindrical voids whose longitudinal axis is parallel to the applied electric field has the least while the one containing spherical voids is intermediate. It is conjectured that the void shape effect is mainly due to the fact that the void perturbs the distribution of electric field and polarization with respect to from the applied electric field direction and different shapes of void result in different degrees of perturbation.

The limitation of the current numerical simulation and possible future work are also discussed.     

Phase evolution and origin of the high piezoelectric properties in lead-free BiFeO3–BaTiO3 ceramics

La-substitution effects for Bi³⁺-site in 0.7Bi_1.03(1-x)La_xFeO₃-0.3BaTiO₃ (abbreviated as BF30BT-100xLF with x = 0.00, 0.01, 0.035, 0.06, 0.07 and 0.10) ceramics were investigated systematically. All ceramics were synthesized by a conventional solid-state reaction method and quenched in water from its sintering temperature. The crystal structure Rietveld refinement shows that undoped BF30BT ceramic exhibited dominant rhombohedral (R) symmetry and gradually changed to the tetragonal (T) phase with La-doping. However, for x ≥ 0.07 compositions the lattice distortions (c_T/a_T and 90°−α_R) significantly decrease as a result crystal structures become close to the cubic-like (CL) phase. Hence, two different morphotropic phase boundaries (MPBs) were reported for BF30BT-100xLa ceramic system; one MPB-I between the R and T phases and the other MPB-II between T and CL phases. The largest direct piezoelectric coefficient (d₃₃ = 274 pC/N) with a high Curie temperature (T_C = 532 °C) for BF30BT-1LF composition was obtained due to the typical MPB-I between R and T phases. However, a maximum electric field-induced strain (S_max = 0.27%) with a high converse piezoelectric coefficient (d₃₃* = 500 pm/V) for BF30BT-7LF ceramic was mainly attributed to the MPB-II of T + CL phases and soft ferroelectric switching properties.     

Abnormal piezoelectric properties of acceptor doped 0.75BF-0.25BT lead-free ceramics for application in atomizer

Ecologically sustainable hard type piezoelectric ceramics are highly demanded for high power transducer applications in replacing lead-based piezoelectric ceramics. In this study, MnO was proposed as acceptor dopant to modify the electric properties of 0.75BiFeO₃-0.25BaTiO₃ piezoelectric ceramics. Results showed that MnO doping reduced the lattice distortion of rhombohedral phase and facilitated the formation of pseudo cubic phase of 0.75BiFeO₃-0.25BaTiO₃ system. Moreover, it caused the diversity of domain morphology and reduced the domain size as well. XPS analysis indicated that Mn²⁺ ions suppressed the transition from Fe³⁺ ions to Fe²⁺ ions and gave rise to the increase of oxygen vacancies. Accordingly, the introduction of MnO synergistically increases the d₃₃, K_p, and Q_m and lowers the tanδ of 0.75BiFeO₃-0.25BaTiO₃ ceramics. The abnormal enhancement in hard type properties is ascribed mainly to the increase of extrinsic contribution rather than the pinning effect arising from oxygen vacancies. Finally, based on an optimal 0.75Bi(Fe_0.985Mn_0.015)O₃-0.25BaTiO₃ system, an atomizer prototype was fabricated. The remarkable atomization effect suggests a competitive potential of 0.75BF-0.25BT ceramic for transducer application.     

Enhanced multiferroic properties in Pr-doped BiFe0.97Mn0.03O3 films

Bi_1−xPr_xFe_0.97Mn_0.03O₃ (x=0.00, 0.05, 0.10, 0.15, 0.20) thin films were deposited on FTO/glass substrate using chemical solution deposition. The influences of Pr doping on the crystalline structure and multiferroic properties were investigated. In the X-ray diffraction and Raman spectra results, the crystal structures of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films revealed a gradual transformation from the trigonal structure to the tetragonal structure. The leakage current densities of Bi_1−xPr_xFe_0.97Mn_0.03O₃ films are one order of magnitude lower than that of BiFeO₃. Compared with unsaturated polarization-electric field hysteresis loop of BiFeO₃ film, the Pr and Mn co-doped BFO films have significantly improved ferroelectric properties. The improved remnant polarization (Pr=91.3 µC/cm²) and the positive switching current (I=0.028 A) have been observed in Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film. The improved electrical properties are attributed to the structure transformation, increasing grain boundaries, low oxygen vacancies ratio and increasing Fe³⁺ concentration. In addition, the saturation magnetization of Bi_0.85Pr_0.15Fe_0.97Mn_0.03O₃ film is 1.81 emu/cm³, which is approximately three times higher than pure BiFeO₃ (M_s=0.67 emu/cm³).     

The study of domain rearrangement under external electric fields by scanning electron acoustic microscope

Compared with other methods, scanning electron acoustic microscope (SEAM) is a convenient, fast and nondestructive technique revealing ferroelectric domain without any special pre-treatment to the sample. It is also a promising technique for the dynamic study of domain structure of ferroelectric materials. In the initial research, detailed images of rearrangement of domains in BaTiO₃ ceramics induced by an external electric field were obtained. The results are discussed.     

Are the parameters of the empirical law dependent on the dielectric properties of the ferroelectric relaxors?

The empirical law, ? = ?_m/[1 + (T − T_m)^γ/2σ²] or ? = ?_m/{1+[(T − T_m)/Δ]^ξ}, are usually used to estimate the degree of the diffused phase transition of the ferroelectric relaxors. As the values of γ or σ (ξ or Δ) are larger, the diffused phase transition is more obvious. However, values of T_m and ?_m are different upon different compositions, dopants, synthesized methods, and others. Are the values of γ and σ (ξ and Δ) affected by T_m or ?_m which represents the characteristics of the relaxors? In this paper, we utilize the experimental data of the (1 − x)Pb(Fe_2/3W_1/3)O₃–xPbTiO₃ ceramic system and the mathematical derivation to conclude that the values of γ and σ (ξ and Δ) are not affected by ?_m and T_m. Therefore, they can be used to properly describe and compare the degree of the diffused phase transition between the different samples with different compositions, dopants and synthesized methods since the noises (T_m and ?_m) are filtered in these empirical laws. Furthermore, we can clearly clarify the physical meaning of theses parameters, γ and σ (ξ and Δ), as well as their effect on the composition for the further study.     

On the form of high-frequency voltage-capacitance characteristics of metal-insulator-semiconductor structures with a ferroelectric insulating layer BaxSr1-хTiO3

A model of the high-frequency impedance of metal-ferroelectric-semiconductor structures has been composed and analyzed. An algorithm is formulated for recovering the impedance of the potential drop across the ferroelectric layer, band bending in a semiconductor, the total concentration of charged boundary states and minority charge carriers on the semiconductor surface from the experimental field characteristics as a function of the voltage across the structure. The results of high-frequency measurements of the capacitance and conductivity of Ni–Ba_0.8Sr_0.2TiO₃–Pt and Ni–Ba_0.8Sr_0.2TiO₃–Si structures in the paraelectric phase are presented. It is shown that Si does not evolve into a state of strong depletion in the entire range of external voltages, and the capacitances of the semiconductor and the metal-ferroelectric-silicon structures practically coincide. This is due to the almost complete screening of the ferroelectric layer polarization by the charges of electron traps at the Ba_0.8Sr_0.2TiO₃–Si contact. It is proposed to use dangling bond passivation methods realized in planar silicon technology to reduce the concentration of active trapping centers at the ferroelectric-semiconductor interface.     

High thermal stability of electric field-induced strain in (1−x)(Bi0.5Na0.5)TiO3-xBa0.85Ca0.15Ti0.9Zr0.1O3 lead-free ferroelectrics

In ferroelectric materials high electric field-induced strain (EFIS) with good thermal stability is of much interest from both fundamental research and potential applications. Here we propose a strategy to achieve high thermally stable EFIS based on electrostrictive effect and thermal stability of polarization. According to this strategy, we synthesized (1−x)(Bi_0.5Na_0.5)TiO₃-xBa_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BNT-xBCZT) ferroelectric ceramics in order to tailor the thermal stability of dielectric permittivity, polarization and EFIS. A dielectric platform with a wide temperature region is induced by increasing x from 0.24 to 0.36 gradually. From 30 °C to 150 °C, a variation of 20% polarization results in a change of 36% EFIS, suggesting a good thermal stability as expect. Temperature-insensitive electrostrictive coefficient Q₃₃ ranges from 0.0264 m⁴/C² to 0.0314 m⁴/C². These results not only prove the effectiveness of this strategy, but also suggest that this strategy can be applied to other ferroelectric materials to improve the thermal stability.     

Optimization of synthesizing leucine-binding nano-sized magnetite by a two-step transformation

The optimum conditions for synthesizing leucine (Leu)-binding nano-sized magnetite (NSM) particles by a two-step transformation (TST) process were studied. The formation and magnetization of as-synthesized NSM particles were investigated through variation of the acidity, the type of surface modifier, and the nature of the acid for pH adjustment. With increased acidity, the saturation magnetization of the NSM particles decreased, but the amount of Leu coated on the surface of NSM particles increased. After the influence of hydroxyl (OH⁻) groups on the surface of NSM particles was removed by using the dicarboxyl anion (C₂O₄²⁻) as a ligand in the first step, Leu was successfully bound with NSM particles in the second step. However, when polyethylene glycol (PEG) was used as a surface modifier, it was difficult to form the Leu-to-NSM particle complex. In terms of the acid used to modify pH, H₂SO₄ was slightly less effective than HCl in achieving saturation magnetization because the coordination for SO₄²⁻ anions is stronger than that of Cl⁻ anions. The preparation of other amino acid-binding NSM particles can be optimized in an analogous manner.