Structural transformation,dielectric and multiferroic properties of (Gd1-xBax)(Fe1-xTix)O3 ceramics by tuning composition

The structural transformation, ferro-paraelectric and magnetic ordering induced dielectric phase transitions, multiferroic properties, relaxation and conduction mechanisms of (Gd_1-xBa_x)(Fe_1-xTi_x)O₃ ceramics with different compositions (x = 0.0–0.4) have systematically been investigated. The chemical route is adopted for material synthesis. Rietveld refinement reveals the structural transformation from orthorhombic to mixed phase (coexistence of orthorhombic and tetragonal phase) for x > 0.2. The analysis of the FESEM micrographs suggests a decrease in average grain size with the increase in BT in the composition. Dielectric anomalies are explained based on ferroelectric-paraelectric phase transition of BaTiO₃, polarization induced by a spin reorientation, and antiferromagnetic ordering of GdFeO₃. The multiferroic properties of all the samples are studied from P-E, M-H, and magneto-electric plots. The improved magnetic properties in compositions x = 0.1–0.3, make them suitable candidates for spintronic devices. The maximum magneto-electric coupling coefficient of 4.77 mV/cm?Oe in the composition x = 0.3, makes it very much suitable for application in magnetoelectric coupling devices.     

A near-infrared study of thermally induced structural changes in polyethylene crystal

Thermally induced structural changes of polyethylene (PE) have been studied by means of near-infrared (NIR) spectroscopy in the course of heating up to the melting temperature. NIR bands characteristic of the regular orthorhombic phase, the conformationally disordered hexagonal phase, and the amorphous phase have been successfully identified. It has been found that for the unoriented PE sample, the disordering process of orthorhombic lattice starts above room temperature and that it mostly occurs above 100 °C for the uniaxially oriented PE sample. In the latter case, the enhancement of crystallinity has clearly been detected just below T_m due to the reorganization of crystalline lattice. For the geometrically constrained ultradrawn PE sample, the phase transition from orthorhombic to hexagonal phase has been detected immediately below the melting point. The NIR bands characteristic of the hexagonal phase have been confirmed definitely. Usefulness of NIR spectroscopy has been demonstrated successfully in such a study of thermally induced phase transition behavior of PE samples with appreciable thickness, for which mid-IR spectroscopy is difficult to apply because of the intensity saturation of various key bands.     

Thermal stability and hardness of ReB2 type hexagonal OsB2 with the addition of W

This work attempts to understand the effect of W addition on microstructure, thermal stability, and hardness of ReB₂ type hexagonal osmium diboride (h-OsB₂). h-OsB₂ samples with W atomic concentration of (Os+W) from 0% to 30% were synthesized by mechanochemical method combines with pressure-less sintering. The XRD patterns of the as-synthesized powders indicate the formation of Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solution, which has a ReB₂-type hexagonal structure. After being high temperature sintered, part of the h-OsB₂ phase of the pure OsB₂ transformed to orthorhombic (o) phase, while the h-OsB₂ phase was maintained with the addition of W, which suggests that the thermal stability of the sample was remarkably improved. A macroscopically homogeneous structure with some pores can be found from all groups of the as-sintered Os_1-xW_xB₂ (x?=?0, 0.1, 0.2, 0.3) samples, with some B-rich areas distributed in the W doped samples. The lattice parameters of the Os_1-xW_xB₂ (x?=?0, 0.1, 0.2 and 0.3) solid solutions linearly decreased with the increase of the W concentration. The micro-hardness of the OsB₂ sintered samples is 25?±?2?GPa under an applied load of 0.49?N, which increased to 34?±?2?GPa, 38?±?2 and 37?±?2?GPa, respectively when the W concentration increased from 10, 20 and 30?at%. The increased hardness of the h-OsB₂ can be mainly attributed to the improvement of thermal stability with the addition of W.     

Structure–property relations in xCaTiO3–(1−x)SrMg1/3Nb2/3O3 based microwave dielectrics

Solid solutions of the system (x)CaTiO₃–(1−x)Sr(Mg_1/3Nb_2/3)O₃ (CTSMN) were prepared by the mixed oxide route. All compositions were found to be single phase by X-ray diffraction; pellets had densities >95% of the theoretical X-ray density. Scanning electron microscopy coupled with energy dispersive X-ray analysis revealed a homogeneous distribution of cations within instrument sensitivity in all samples except for pure SMN which showed evidence of some unreacted MgO. SMN is reported as a 1:2 ordered trigonal/hexagonal complex perovskite structure in the X-ray data files but electron diffraction revealed superlattice reflections characteristic of both in-phase and anti-phase rotations of octahedra arising from further structural phase transitions on cooling. These reflections were retained throughout the solid solution range but ±1/3{hkl} ordered superlattice reflections were absent in samples where x > 0.1. In general, the microwave quality factor (Q_u*f_o) decreased as x increased and the temperature coefficient of resonant frequency (TCf) was approximately linearly proportional to the permittivity; at x∼0.20: TCf∼0, ε_r ∼45 and Q_u*f_o ∼9000.     

Anisotropic field induced phase transitions and negative electrocaloric effect in rhombohedral Mn doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The electrocaloric effect (ECE) of Mn doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT:Mn) single crystals with particular emphasis on the impact of crystallographic orientations and phase transitions were investigated systematically. Orientation-dependent phase transitions have been demonstrated by the dielectric and strain behaviors. Intriguingly, the negative ECE of –0.02?°C and –0.002?°C were obtained firstly in [001]-oriented PIN-PMN-PT:Mn crystals near the rhombohedral→tetragonal phase transformation and in [011]-oriented crystals near the rhombohedral→orthorhombic phase transformation, respectively. However, only the positive ECE was found in [111]-oriented crystals near the tetragonal→rhombohedral phase transition. Additionally, the maximum ECE temperature changes calculated in [001]-, [011]- and [111]-oriented crystals were 0.33?°C, 0.46?°C and 0.38?°C, respectively. Our results suggest that the negative ECE is attributed to electric field-induced phase transitions, whose critical field decreases with the increase of temperature. The phase transition-mediated coexistence of positive and negative effects in the relaxor-ferroelectric single crystals is beneficial to enhance the efficiency of the solid-state cooling devices.     

New insights into synthesis of nanocrystalline hexagonal BN

Uncovering the mechanism behind nanocrystalline hexagonal boron nitride (h-BN) formation at relatively low temperatures is of great scientific and practical interest. Herein, the sequence of phase transformations occurring during the interaction of boric acid with ammonia in a temperature range of 25–1000 °C has been studied in detail by means of thermo-gravimetric analysis, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The results indicate that at room temperature boric acid reacts with ammonia to form an ammonium borate hydrate (NH₄)₂B₄O₇x4H₂O. Its interaction with ammonia upon further heating at 550 °C for 1 h leads to the formation of turbostratic BN. Nanocrystalline h-BN is obtained either during heating in ammonia at 550 °C for 24 h or at 1000 °C for 1 h. This result is important for the development of novel cost-effective and scalable syntheses of h-BN nanostructures, such as nanosheets, nanoparticles, nanofibers, and nanofilms, as well as for sintering h-BN ceramic materials.     

Time resolved WAXS/SAXS observations of crystallisation in oriented melts of ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been drawn in the melt state at 140, 145 and 150 °C at extension rates ∼1 s⁻¹ while simultaneously recording two dimensional SAXS and WAXS with a time resolution of 0.1 s. The first observable crystallisation is mainly in the orthorhombic form at a level of about ∼1 wt%. At higher draw ratios additional crystallisation is in the hexagonal form up to ∼10 wt%. The crystallisation is accompanied by strong SAXS equatorial scatter with maxima at ∼25 nm period; in some cases meridional maxima are also visible at ∼120 nm. Substantial crystallisation occurs on subsequent cooling to 130 °C, accompanied by strong meridional maxima of narrow lateral width. The observed crystal forms are consistent with a temperature-strain phase diagram, favouring hexagonal at higher strains. There are indications that the thermodynamic orthorhombic to hexagonal transition T_tr is above 150 °C so that all the observable hexagonal structures are metastable. The initial orthorhombic crystals are associated with the high molecular weight tail and provide the strain hardening to enable the formation of subsequent hexagonal crystals. The equatorial SAXS lobes are interpreted in terms of lateral density fluctuations that are associated with an arrangement of columns of oriented chains comprising both orthorhombic and hexagonal structures. The columns are embryonic shish structures that on cooling nucleate kebab overgrowths.     

Nanocrystalline growth activation energy of alumina polymorphs synthesised by mechanochemical technique

Abstract

Al₂O₃ nanopowders were synthesised via mechanochemical method using AlCl₃ and CaO as raw materials. The effect of thermal treatment on the structural evolutions and morphological characteristics of the nanopowders was investigated using X-ray diffractometry, transmission electron microscopy, scanning electron microscopy, differential thermal analysis and Rietveld refinement. The results showed that the average crystallite size of Al₂O₃ was <100 nm up to ～1200°C. The activation energy for Al₂O₃ nanocrystallite growth during calcinations was calculated to be ～22?598 and 30?195 J mol^?1 for η- and κ-alumina respectively, while for α-Al₂O₃, it was 8373 and 34?131 J mol^?1 at temperatures up to 1200°C and >1200°C respectively. The mechanism of nanocrystalline growth of Al₂O₃ polymorphs during annealing is also discussed.     

Structural and Kinetic Analysis of BaCl2 Nanocrystals in Fluorochlorozirconate Glass‐Ceramics

The presence of BaCl₂ nanocrystals and the crystallographic phase that they adopt controls the optical behavior of fluorochlorozirconate glass‐ceramics. We have used in situ X‐ray diffraction heating experiments and ex situ transmission electron microscopy to follow the BaCl₂ nanocrystal nucleation and growth processes as a function of heating rate and isothermal hold temperature. The BaCl₂ nanocrystals nucleate with the hexagonal crystal structure and grow as spherical particles to a size of ~10 to 20 nm. They then undergo a structural transformation to the orthorhombic phase and their shape changes to rounded disks, with diameters ranging from 150 to 250 nm, and thicknesses ranging from 80 to 120 nm. The change in size results from Ostwald ripening of the hexagonal BaCl₂ nanocrystals to form the orthorhombic BaCl₂ nanocrystals.     

Chain packing and phase transition of N-hexacosylated polyethyleneimine comb-like polymer: A combined investigation by synchrotron X-ray scattering and FTIR spectroscopy

In this paper, the chain packing and phase transition of comb-like polymer has been deeply analyzed with N-hexacosylated polyethyleneimine (PEI26C) as a template, fabricated through the reaction between n-hexacosyl bromide and PEI in a homogenous solution. The effect of long alkyl groups on the chain packing and phase transition of PEI26C was systematically investigated by synchrotron X-ray scattering and variable-temperature FTIR spectroscopy. PEI26C comb-like polymer exhibited an interesting structure-evolution process, and phase transformation from orthorhombic (β_O), monoclinic (β_M), to hexagonal (α_H) phase, and finally to amorphous state was demonstrated, indicating that large alkyl domains induced the complicated structures. Size-depended phase transition behavior provides an insight into the formation of metastable structure during the early stage of polymer crystallization.     

Containerless processing of metastable multiferroic composite in Ln-(Mn,Fe)-O system (Ln: Lanthanide)

It has been known that the hexagonal structure with a space group of P6₃cm is stable in the LnMnO₃ system (Ln: Lanthanoid) having rare-earth elements with smaller ionic radii than that of Gd³⁺. The hexagonal LnMnO₃ (h-LnMnO₃) that shows multiferroic properties such as ferroelectricity and ferromagnetism in one phase has attracted great interest in the field of electronic industry. Nevertheless, since h-LnMnO₃ shows antiferromagnetism as well as low magnetic transition temperature below 100 K, no materials for practical applications have been developed. As one of the solutions to this problem, a composite material composed of nano-sized h-LnFeO₃ and ferromagnetic phases is expected. Based on this idea, we tried to synthesize multiferroic composites consisting of h-LuFeO₃ and Fe₃O₄ by containerless processing technique. However, the results showed that not only the composite structure of h-LuFeO₃ and Fe₃O₄ but also orthorhombic phase (o-LuFeO₃) and Fe₂O₃ were identified. These results suggest that it is necessary to improve the stability of h-LnFeO₃ and suppress the chemical reaction from Fe₃O₄ to Fe₂O₃. From this point, we tested samples in which one-third of Fe ions was substituted by Mn ions. As a result, a fine composite structure composed of LuMn_1/3Fe_2/3O₃ and MnFe₂O₄ was obtained.     

Phase transition,microstructure, and dielectric properties of Li/Ta/Sb co-doped (K,Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−xTa_xSb_0.07)O₃ (abbreviated KNLNST_x) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 °C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNST_x ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST_0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d₃₃=313 pC/N, k_p=47%, and ε_r=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d₃₃=200 pC/N and ε_r=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.     

Preparation and characterization of V2O3 micro-crystals via a one-step hydrothermal process

Phase pure V₂O₃ micro-crystals with a hexagonal dipyramid morphology were fabricated for the first time via a facile one-step hydrothermal method. The crystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). A hexagonal dipyramid structure of V₂O₃ enclosed by well-grown {012} facets was obtained by hydrothermally reducing VO(acac)₂ precursor with N₂H₄·H₂O at 220 °C for 48 h. The results indicated that V₂O₃ can be well crystallized up to micron size with distinguished facets by only one step hydrothermal treatment. The formation mechanism and morphology evolution for V₂O₃ micro-crystals were discussed. Based on our experiments, the V₂O₃ nuclei formed and grew by a phase transformation through a dissolution–recrystallization process of VOOH, and the formation of the hexagonal dipyramids was ascribed to the specific adsorption of Hacac to the {012} facets restraining the growth in the directions normal to the {012} facets. The present work provides a facile method for preparing phase pure V₂O₃ micro-crystals with hexagonal dipyramid morphology, which can be used as a new powder material for ceramic fabrication.     

Effects of Re addition on phase stability and mechanical properties of hexagonal OsB2

ReB₂‐type hexagonal Osmium diboride (OsB₂) has been predicted to exhibit higher hardness than its orthorhombic phase, but hexagonal‐orthorhombic phase transformation occurs at temperature higher than 600°C, resulting in the decrease in its hardness. Therefore, ReB₂‐type hexagonal OsB₂ samples with Re addition were produced by a combination of mechanochemical method and pressureless sintering technique, and the effects of Rhenium (Re) addition on phase composition, thermal stability and mechanical properties of OsB₂ were investigated in this study. X‐ray diffraction (XRD) analysis of the as‐synthesized powders by high‐energy ball milling indicates the formation of hexagonal Os_1‐xRe_xB₂ solid solution with Re concentration of 5 and 10 at.% without forming a second phase. After being sintered at 1700°C, part of the hexagonal phase in OsB₂ transformed to orthorhombic structure, while Os_0.95Re_0.05B₂ and Os_0.9Re_0.1B₂ maintained their hexagonal structure. This suggests that the thermal stability of the hexagonal OsB₂ was significantly improved with the addition of Re. Scanning electron microscopy (SEM) photographs show that all of the as‐sintered samples exhibit a homogeneous microstructure with some pores and cracks formed throughout the samples with the relative density >90%. The measurements of micro‐hardness, nano‐hardness, and Young's modulus of the OsB₂ increased with Re addition, and these properties of the sample with 5 at.% addition of Re is higher than that with 10 at.% Re.     

Increasing fracture toughness of zirconia-based composites as a synergistic effect of the introducing different inclusions

The effect of multi-walled carbon nanotubes (MWCNTs) and hexagonal boron nitride (h-BN) inclusions on the fracture toughness of yttria-stabilized zirconia (YSZ) ceramics has been studied. It is shown that an increase in the MWCNTs and h-BN content has a positive effect on the K_1C of zirconia ceramics. The greatest increase in the fracture toughness of YSZ ceramics was observed with the introduction of hexagonal boron nitride particles. For YSZ ceramics, the K_1C value was ≈6.1 MPa m^1/2, for ceramics with a 5 wt % of h-BN K_1C ≈ 9.2 MPa m^1/2. It was shown that an increase of the YSZ ceramics fracture toughness with the introduction of MWCNTs and h-BN, both and separately was provided by the combined action of several mechanisms of increasing the work of crack propagation. In addition, in all composites obtained in this work, the transformation of tetragonal ZrO₂ into monoclinic was observed.     

Effect of consolidation pressure up to 1.8 GPa on the sintering of nanocrystalline nc-Y2O3

Sintering of nanocrystalline (nc) monoclinic yttrium oxide (Y₂O₃) was studied in the homologue temperature range of 0.4–0.7T_M. Samples were isostatically consolidated at super high pressure (SHP), up to 1.8 GPa. The combined effects of consolidation pressure and sintering temperature on the properties and microstructure are explored. The physical properties of the samples, mainly elastic modulus follow in general the density changes, or the interparticle contact area, but are also affected by sintering temperature. The effect of compaction pressure and sample density on the phase transition is studied and discussed.     

Symmetry-bridging phase as the mechanism for the large strains in relaxor-PbTiO3 single crystals

Relaxor-PbTiO₃ piezoelectric single crystals have been of a great interest, since the discovery of ultrahigh piezoresponse demonstrated in <001> -oriented crystals of the composition at the rhombohedral side of morphotropic phase boundary. It has been proposed that the exceptionally large piezoelectric properties should originate from an electric-field-induced polarization rotation that involves a reversible phase transformation between rhombohedral and tetragonal via monoclinic symmetry. However, this commonly accepted polarization rotation mechanism has its limit in explaining still the excellent piezoelectricity even at a small excitation field far below the coercive field. Here, we show by a comparative study using single crystals from two distinct processing techniques, the polarization rotation has, if ever, little influence on the strain properties of <001 > -oriented rhombohedral relaxor-PbTiO₃ single crystals. Instead, they may come from a reversible shear-mode piezoelectric contribution from electric-field-susceptible ‘symmetry-bridging’ unit-cell-level phases, the polarization direction of which spans monoclinic symmetry.     

Evolving antiferroelectric stability and phase transition behavior in NaNbO3-BaZrO3-CaZrO3 lead-free ceramics

The stability of antiferroelectricity in NaNbO₃ ceramics was found to evolve with co-doping x mol% CaZrO₃ and 6 mol% BaZrO₃, from a dominant ferroelectric (FE) orthorhombic Q phase (x = 0) to a gradually stabilized antiferroelectric (AFE) orthorhombic P phase owing to different ionic radii of Ba and Ca ions. Although a complete AFE P phase appears at x = 0.5, the field induced AFE-FE phase transformation is irreversible at first, and then becomes partially reversible at x = 1 and finally completely reversible at x = 3. The above-mentioned change process proves to be associated with the enhancing stability of antiferroelectricity with x, as evidenced by means of dielectric, polarization and strain properties as well as in/ex-situ synchrotron x-ray diffraction and Raman spectra. A composition-field phase diagram for the NN-based lead-free AFE ceramic was constructed on basis of the phase structural change, which would provide a clear understanding of how ion doping influences its antiferroelectricity.     

A new modification of silicon carbide with a rhombic lattice

Data of an x-ray structural study of single crystals of silicon carbide α-SiC(6H) and a new, previously unknown modification γ-SiC with a rhombic lattice are described. The rhombic-lattice parameters are related to the parametera of the hexagonal and cubic lattices of SiC in the following way:a _r=a _h,b _r=3a _h,c _r=3a _c. Translated from Steklo i Keramika, No. 3, pp. 19 – 22, March, 2000.     

Hydrothermal phase transformation of hematite to magnetite

Different phases of iron oxide were obtained by hydrothermal treatment of ferric solution at 200°C with the addition of either KOH, ethylenediamine (EDA), or KOH and EDA into the reaction system. As usually observed, the α-Fe₂O₃ hexagonal plates and hexagonal bipyramids were obtained for reaction with KOH and EDA, respectively. When both KOH and EDA were added into the reaction system, we observed an interesting phase transformation from α-Fe₂O₃ to Fe₃O₄ at low-temperature hydrothermal conditions. The phase transformation involves the formation of α-Fe₂O₃ hexagonal plates, the dissolution of the α-Fe₂O₃ hexagonal plates, the reduction of Fe³⁺ to Fe²⁺, and the nucleation and growth of new Fe₃O₄ polyhedral particles.