Highly Oriented Crystalline PE Nanofibrils Produced by Electric‐Field‐Induced Stretching of Electrospun Wet Fibers

In order to enhance the molecular orientation of electrospun nanofibers, a novel collection technique is proposed and applied to the spinning of polyethylene from high temperature solution. The technique makes use of a parallel‐electrode collector that acts before solidification of the fiber occurs. The resulting multiple‐necking morphology is composed of fine nanofibrils with very small diameter and narrow size distribution. The crystalline orientation of the nanofibrils was analyzed by TED. The formation mechanism of the nanofibrils is discussed. The strong elongational effect of the electric‐field‐induced stretching force in the parallel‐electrode collector is demonstrated by the orientational analysis and by observation of the multiple‐necking morphology.

     

Magnetic Field‐Induced Ferroelectric Switching in Multiferroic Aurivillius Phase Thin Films at Room Temperature

Single‐phase multiferroic materials are of considerable interest for future memory and sensing applications. Thin films of Aurivillius phase Bi₇Ti₃Fe₃O₂₁ and Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ (possessing six and five perovskite units per half‐cell, respectively) have been prepared by chemical solution deposition on c‐plane sapphire. Superconducting quantum interference device magnetometry reveal Bi₇Ti₃Fe₃O₂₁ to be antiferromagnetic (T_N = 190 K) and weakly ferromagnetic below 35 K, however, Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ gives a distinct room‐temperature in‐plane ferromagnetic signature (M_s = 0.74 emu/g, μ₀H_c =7 mT). Microstructural analysis, coupled with the use of a statistical analysis of the data, allows us to conclude that ferromagnetism does not originate from second phase inclusions, with a confidence level of 99.5%. Piezoresponse force microscopy (PFM) demonstrates room‐temperature ferroelectricity in both films, whereas PFM observations on Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ show Aurivillius grains undergo ferroelectric domain polarization switching induced by an applied magnetic field. Here, we show for the first time that Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ thin films are both ferroelectric and ferromagnetic and, demonstrate magnetic field‐induced switching of ferroelectric polarization in individual Aurivillius phase grains at room temperature.     

A Comparison of Electric‐Field‐Driven and Pressure‐Driven Fiber Generation Methods for Drug Delivery

Polymeric fibers are prepared by using electric field driven fiber production technology—electrospinning and pressure driven fiber production technology—pressurized gyration. Fibers of four different polymers: polyvinylidene fluoride (PVDF), poly(methyl methacrylate (PMMA), poly(N‐isopropylacrylamide), and polyvinylpyridine (PVP), are spun by both techniques and differences are analyzed for their suitability as drug carriers. The diameters of electrospun fibers are larger in some cases (PVDF and PMMA), producing fibers with lower surface area. Pressurized gyration allows for a higher rate of fiber production. Additionally, drug‐loaded PVP fibers are prepared by using two poorly water‐soluble drugs (Amphotericin B and Itraconazole). In vitro dissolution studies show differences in release rate between the two types of fibers. Drug‐loaded gyrospun fibers release the drugs faster within 15 min compared to the drug‐loaded electrospun fibers. The findings suggest pressurized gyration is a promising and scalable approach to rapid fiber production for drug delivery when compared to electrospinning.     

(Pb,Sm)(Zr,Sn,Ti)O3 Multifunctional Ceramics with Large Electric‐Field‐Induced Strain and High‐Energy Storage Density

Recently, the progress of integrated electronics has led to a strong demand for materials and devices with multiple functions. In this study, we achieved Pb_0.985Sm_0.01 (Zr_0.64Sn_0.28Ti_0.08)O₃ (PSZST) multifunctional ceramics which showed simultaneously large electric‐field‐induced strain (0.63%) and high recoverable energy density (1.743 J/cm³) at room temperature. Moreover, the strain and recoverable energy density exhibited a slight frequency fluctuation in the frequency range of 1–10 Hz. Their variations were less than 8% and 1.3% and the values were all higher than 0.58% and 1.722 J/cm³, respectively. The large strain, high‐energy density, and their good frequency stability in a wide range indicate that the PSZST ceramic is quite promising for application in multifunctional devices.     

Enhanced Field‐Induced Strain in the Textured Lead‐Free Ceramic

Reactive template grain growth method was applied to prepare <001> grain oriented 0.94(Na_0.52K_0.48)NbO₃–0.06LiNbO₃ ceramics using plate‐like NaNbO₃ particles as template. A two‐step sintering procedure was put forward to obtain textured ceramics with electromechanical coupling factor k_p = 64%, piezoelectric constant d₃₃ = 220 pC/N, and in practical application the value of available (converse piezoelectric constant) is about 516 pm/V, which are much higher than these of random form with same composition. Based on the analysis for the behavior of piezoelectricity against the measuring frequency and the intensity of external field, a possible mechanism considering the interaction between defect dipoles induced by the doping of Cu²⁺ and spontaneous polarization dipoles was proposed to elucidate the field‐induced giant strain in textured ceramics. This study does not only provide an insight to the origin and coupling effect of two kinds of dipole but also renders a general approach of defect engineering to take advantage of point defect in ceramics realizing certain function enhancement.     

Investigation on the Electric‐Induced Light Scattering of PLZT Transparent Ceramics

Transparent ceramics of Pb_1?xLa_x(Zr_1?yTi_y)_1?x/4O₃ (PLZT) were fabricated by the two‐stage sintering method. The electric‐induced light scattering phenomenon for two compositions of PLZT ceramics was investigated. It was found that the composition with relatively bigger ferroelectric domains exhibits a relatively smaller light scattering driving field, but simultaneously a lower transparency in the original state. The dielectric properties, ferroelectric hysteresis loops, and ferroelectric domain structures of the two compositions were studied which are consistent with the electric‐induced light scattering results.     

Electric Field‐Induced Light Scattering in Eu‐Doped PMN‐PT Transparent Ceramics

Highly transparent Eu‐doped Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃(PMN‐PT) ferroelectric ceramics were obtained by a two‐stage sintering method. Eu doping play a significant role in the domain structures of PMN‐PT ceramics and resulted in different light scattering responses under electric field. The dielectric behaviors, ferroelectric properties, and domain configurations in the ceramics with increasing Eu doping concentration were studied, which were consistent with the electric field‐induced light scattering responses.     

Anisotropic Electrical Transport Properties of Aligned Carbon Nanotube/PMMA Films Obtained by Electric‐Field‐Assisted Thermal Annealing

This paper demonstrates how the electric‐field‐assisted thermal annealing of octadecylamine‐functionalized SWNT/PMMA films induces an increase in the composite transversal conductivity of several orders of magnitude and a decrease in the lateral conductivity. This difference has been rationalized in terms of the nanotube alignment into the polymer matrix along the electric field direction. This result provides an initial understanding of how electric fields can be used to control the bulk physical properties of such nanocomposites.

     

Molten Salt Synthesis of Bismuth Ferrite Nano‐ and Microcrystals and their Structural Characterization

Bismuth ferrite nano‐ and microcrystals were prepared by a facile molten salt technique in two kinds of molten‐salt‐based systems (NaCl–KCl and NaCl–Na₂SO₄). In the NaCl–KCl salt system, a systematic study indicating the effects of process parameters (e.g., calcination temperature, holding time as well as the molten salt ratios) on the bismuth ferrite formation mechanism and structural characteristics is reported. The results show that almost pure phase BiFeO₃ powders with minimum impurity phase of Bi₂Fe₄O₉ were synthesized at temperatures of 700°C–800°C, whereas high calcination temperature (e.g., 900°C) resulted in the formation of almost pure phase Bi₂Fe₄O₉ powders. The prolonged holding time increased the particle size via the Ostwald ripening mechanism; however, there was little effect on the particle morphology. Similar phenomenon occurred as increasing the molten salt ratios. In the NaCl–Na₂SO₄ salt systems, it is found that low NP‐9 (nonylphenyl ether, NP‐9) surfactant content (0–5 mL) led to the formation of almost pure phase BiFeO₃ powders, whereas high NP‐9 surfactant content (e.g., 20 mL) resulted in pure phase Bi₂Fe₄O₉ powders. The average particle size of the BiFeO₃ powders was decreased as increasing the NP‐9 surfactant content, whereas their morphologies did not change significantly. Because of the simplicity and versatility of the approach used, it is expected that this methodology can be generalized to the large‐scale preparation of other important transitional metal oxides with controllable sizes and shapes.     

Light‐Induced Bulk Architecturation of PDMS Membranes

One major challenge of biomaterial engineering is to mimic the mechanical properties of anisotropic, multifunctional natural soft tissues. Existing solutions toward controlled anisotropy include the use of oriented reinforcing fillers, with complicated interface issues, or UV‐curing processing through patterned masks, that makes use of harmful photosensitive molecules. Here, a versatile process to manufacture biocompatible silicone elastomer membranes by light degradation of the platinum catalyst prior to thermal cross‐linking is presented. The spatial control of network density is demonstrated by experimental and theoretical characterizations of the mechanical responses of patterned cross‐linked membranes, with a view to mimic advanced implantable materials.

     

Magnetic‐Field‐Induced Dielectric Anomaly and Electric Polarization in Co4Ta2O9

We report magnetic and electric properties of Co₄Ta₂O₉, which is a collinear antiferromagnet with trigonal structure. At zero magnetic fields, no dielectric anomaly and electric polarization are observed in this compound. While, finite dielectric peaks and electric polarization develop under applied magnetic field, and increase with increasing field, showing magnetodielectric and magnetoelectric performance. We ascribe the magnetic‐field‐induced dielectric anomaly and electric polarization in Co₄Ta₂O₉ to spin fluctuation and domain effect.     

High Electric‐Induced Strain and Temperature‐Dependent Piezoelectric Properties of 0.75BF–0.25BZT Lead‐Free Ceramics

0.75BiFeO₃–0.25Ba(Zr_xTi_1?x) + 0.6 wt% MnO₂ (0.75BF–0.25BZT) ceramics with Mn addition were prepared by the solid‐state reaction method. The high‐field strain and high‐temperature piezoelectric properties of 0.75BF–0.25BZT ceramics were studied. Introduction of Zr in the solid solutions decreased the Curie temperature slightly, and improved the dielectric and piezoelectric properties obviously. The piezoelectric properties of 0.75BZT–0.25BT ceramics reached the maximum at Zr content of 10 mol%. The Curie temperature T_c, dielectric constant ε and loss tanδ (1 kHz), piezoelectric constant d₃₃, and planner electromechanical coupling factor k_p of 0.75BF–0.25BZT ceramics with 10 mol% Zr were 456°C, 650, 5%, 138 pC/N, and 0.30, respectively. The high‐field bipolar and unipolar strain under an electric field of 100 kV/cm reached up to 0.55% and 0.265%, respectively, which were comparable to those of BiScO₃–PbTiO₃ and “soft” PZT‐based ceramics. The typical “butterfly”‐shaped bipolar strain and frequency‐dependent peak‐to‐peak strain indicated that the large high‐field‐induced strain may be due to non‐180° domain switching. Rayleigh analysis reflected that the improved piezoelectric properties resulted from the enhanced extrinsic contribution by Zr doping. The unipolar strain of 0.75BF‐0.25BZT ceramics with 10 mol% Zr was almost linear from RT to 200°C. These results indicated that 0.75BF–0.25BZT ceramics were promising candidates for high‐temperature and lead‐free piezoelectric actuators.     

Electric‐Field‐Driven Phase Transition Process in (K,Na, Li)(Nb,Ta, Sb)O3 Lead‐Free Piezoceramics

The electric‐field‐driven phase transition in (K, Na, Li)(Nb, Ta, Sb)O₃ lead‐free piezoelectric ceramics was investigated by X‐ray diffraction, Raman spectra, and the temperature dependences of permittivity spectra. After poling under different electric fields, phase of the ceramics transformed gradually from orthorhombic–tetragonal coexisting phase to orthorhombic phase, indicating that the crystal structure of ceramics was strongly sensitive to electric field as an external stimulus. A secondary phase K₃Li₂Nb₅O₁₅ induced by electric field was detected in the ceramics with Li content of 7 mol%, which was close to the solubility limit of lithium. This field‐induced secondary phase resulted from the movement of Li ions and the structural deformation induced by electric field. Moreover, piezoelectric constant d₃₃ increased with the increasing poling field strength and the enhancement can be attributed to the field‐triggered domain switching. This study implied that in addition to temperature and composition, which has been reported in previous researches, electric field might be an effective way for inducing phase transition in lead‐free piezoelectric ceramics and improving the electrical performances simultaneously.     

Temperature and Frequency Dependence of Electric Field‐Induced Phase Transitions in PMN–0.32PT

Single crystal (1 ? x)Pb(Mg_1/3 Nb_2/3)O₃–xPbTiO₃ [PMN–xPT] (x = 0.32) is a relaxor‐ferroelectric material known to exhibit ‘giant’ piezoelectric behavior, with achievable strains in excess of 1% for samples of certain particular crystallographic orientations and chemical compositions close to the morphotropic phase boundary. In this study, we investigate the electric field‐induced structural phase transitions in single crystal PMN–0.32PT with time‐of‐flight neutron diffraction and macroscopic electrical polarization measurements, and show that both the frequency of the applied ac field and the temperature of the sample are critical factors in determining these phase transition fields.     

High‐Temperature Dielectric Relaxations in LiF Single Crystals

By means of dielectric permittivity, electric modulus and impedance, the dielectric properties of LiF single crystals were investigated in the temperature range of 30°C–800°C and frequency range of 50 Hz–10 MHz. Two thermally activated relaxations, R1 and R2, were observed. The relaxation R1 showing activation energy around 0.8 eV was found to be related to the Li‐ion diffusion in the crystal. The relaxation R2 contains three Arrhenius segments, the low‐, mid‐, and high‐T segments, separated by boundary temperatures of 325°C and 425°C. These segments in the order of ascending temperature were found to be associated with F₃, F₃⁺ centers, F₂ centers, and F centers, respectively.     

Electric‐oxidation kinetics of molybdenite concentrate in acidic NaCl solution

The electric‐oxidation kinetics of molybdenite concentrate in NaCl electrolyte was investigated in this study. The effects of liquid‐to‐solid ratio, stirring speed, and concentration of NaCl on the dissolution rate were determined. It was found that the dissolution rate increases with increase in liquid‐to‐solid ratio, stirring speed, and concentration of NaCl. A shrinking particle model is presented to describe the dissolution and to analyse the data. The apparent activation energy of this dissolution process was found to be 8.2 kJ/mol; it was established that the leaching process is mainly controlled by diffusion, and the kinetics formula of this research system can be expressed as: .     

Ferroelectric Transition and Low‐Temperature Dielectric Relaxations in Filled Tungsten Bronzes

A comprehensive review on the latest development of the ferroelectric transition and low‐temperature dielectric relaxations of filled tungsten bronze ceramics are presented together with some new issues. In the filled tungsten bronze ceramics M_6?pR_pTi_2+pNb_8?pO₃₀ (p = 1, 2; M = Ba or Sr; R = rare earth or Bi), a ferroelectric transition is generally indicated together with up to three low‐temperature dielectric relaxations. The ferroelectric transition is determined as 4/mmm → 4 mm, and the low‐temperature dielectric relaxations are deeply concerned with the structure modulations due to the order/disorder of ions in A1 and A2 sites, their random cross occupancy, and the order/disorder of B‐site ions. Both the ferroelectric transition and low‐temperature dielectric relaxations are dominated by the composition and radius difference between A1‐ and A2‐site ions, ?r. The normal ferroelectric transition might be expected if the ratio of the biggest ion and second big ion is 2:1, otherwise the diffuse or relaxor ferroelectric is expected. Meanwhile, the larger ?r generally results in the normal ferroelectric, and the smaller ?r will lead to the diffuse or relaxor ferroelectric. Moreover, the effects of A sites order/disorder and the random cross occupancy of A‐site ions are primary, and the effects of B‐site ordering/disordering are secondary. The right ratio of 2:1 for A2‐ and A1‐site ions and the large ?r should be the guidelines for designing the possible multiferroic tungsten bronzes.     

Ferroelastic Domain Switching in Tetragonal Zirconia Single Crystals—Microstructural Aspects

Ferroelastic domain switching is one of the possible toughening mechanisms in ceramic materials. Microstructural evidence of domain reorientation (switching) in polydomain tetragonal zirconia single crystals is observed upon the application of a unidirectional compressive stress at 1000° and 1400°C. Dark-field imaging of the three {112{ tetragonal twin variants in a [111] zone indicates that two sets of twin variants grow at the expense of the third set upon application of uniaxial compression. The diminishing variant is the one with its c axis parallel to the compression axis. Inducing cracks into polydomain tetragonal zirconia single crystals provides further evidence of domain reorientation near the crack surface. It is noted again that two sets of twin variants grow at the expense of the third set. A construction for the orientation relationship of domains and their twin boundaries is presented, and a relation between shear stress and reorientation is proposed.     

Low‐Temperature Relaxations Associated with Mixed‐Valent Structure in Sr2TiMnO6

The dielectric properties of Sr₂TiMnO₆ ceramic samples were investigated as functions of temperature (100 K ≤ T ≤ 320 K) and frequency (100 Hz ≤ f ≤ 10 MHz). Two thermally activated dielectric relaxations were observed. The sample was confirmed to possess multivalent states of Mn and Ti ions and the coexistence of electron holes and electrons. Our results revealed that both relaxations are bulk effect related to localized carriers hopping inside grains. It was suggested that the low‐temperature relaxation (LTR) can be related to dipolar effect due to electron holes, and the high‐temperature relaxation (HTR) was associated with the electrons hopping between Ti³⁺ and Ti⁴⁺ ions.