High-pressure high-temperature synthesis and structure of α-tetragonal boron

Abstract

Microcrystals of α-tetragonal (α-t) boron with unit cell parameters a=9.05077(6) and c=5.13409(6) Å and measured density 2.16–2.22 g cm^?3 were obtained by pyrolysis of decaborane B₁₀H₁₄ at pressures of 8–9 GPa and temperatures of 1100–1600 ^○C. The crystal structure is in good agreement with the model proposed by Hoard et al (1958 J. Am. Chem. Soc. 80 4507). However, compared to the original model, we found small deformations of icosahedra and changes in the interatomic distances within the unit cell of the synthesized α-t boron.     

Synthesis of boron-carbon phases with the α-tetragonal boron structure at 8–9 GPa

Our results demonstrate that phases with the structure of α-tetragonal boron can be obtained in the B-C-H system at pressures of 8–9 GPa and temperatures only below 1600–1700°C. The formation of these phases takes place at carbon content up to 20% relative to the boron in the growth system. Increasing the carbon concentration in the growth system reduces the length of the tetragonal cell diagonal. No B₅₀C₂ has been obtained in this study, which can be accounted for by the specifics of the doping process at high pressures.     

Preparation and high-temperature properties of skin–core structure SiC ceramic fibers

Journal of Materials Science - A new type of skin–core structure SiC ceramic fibers was prepared from polycarbosilane by employing the curing process of alternating air and vacuum atmosphere...     

Morphology-controllable synthesis and gas-sensing properties of α-MoO3

Different morphologies of orthorhombic molybdenum trioxide (MoO₃) were successfully synthesized through a facile hydrothermal method followed by a subsequent heat treatment. We found that the surfactants played a critical role in the formation process and the possible formation mechanism was discussed. Especially when chromic chloride hexahydrate was added as a surfactant, the unique net-like structure was prepared. Furthermore, the gas-sensing properties of the sample were tested towards the reducing ethanol gas. The results indicated that the MoO₃ with a net-like structure shows the largest response than the other structures.     

High-pressure phase transitions and structure of Al–20 at % Si hypereutectic alloy

Phase transformations of an Al–20 at % Si high-silicon hypereutectic alloy have been studied by differential barothermal analysis at temperatures of up to 800°C in argon compressed to 100 MPa. High pressure has been shown to raise the melting point of the alloy by 5°C during heating and to lower the eutectic solidification temperature by 5°C during cooling in comparison with the canonical phase diagram of the Al–Si system. At a temperature of 553°C, heating and cooling lead to silicon dissolution and decomposition of the aluminum-based solid solution, respectively. After high-pressure solidification, the silicon particles in the alloy have a bimodal size distribution. Quantitative porosity characteristics in the alloy after a barothermal scanning cycle are similar to those in the as-prepared alloy. The lattice parameters of the silicon and aluminum remain unchanged. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.     

High-pressure/high-temperature synthesis, crystal structure, and electrical properties of CaCu3 ? xFexV4O12

A perovskite-like compound of composition CaCu_{3 − x} Fe _x V₄O₁₂ (x = 0.7−1.0) has been synthesized at high pressures (p = 5.0−8.0 GPa) and temperatures (t = 1000−1300°C). Its crystal structure has been determined by X-ray diffraction (sp. gr. Im[`3]Im\bar 3, Z = 2, a = 7.3051(5) angles have been evaluated. The electrical conductivity of the high-pressure phase CaCu₂FeV₄O₁₂ exhibits metallic behavior.     

Hydrothermal synthesis and magnetic properties of α-MnO2 nanowires

In the present study, one-dimensional (1D) α-MnO₂ nanowires with width of 50–60 nm, length about several micrometers have been successfully prepared under hydrothermal conditions in the presence of sodium carboxymethyl cellulose. The samples were characterized by X-ray diffraction, scanning electron microscope, superconducting quantum interference device and N₂ adsorption–desorption experiment. The magnetic measurement reveals that the α-MnO₂ nanowires exhibit a ferromagnetic behavior at 5 K and a paramagnetic behavior at 300 K. The N₂ adsorption–desorption experiment shows that surface area is 160.4 m² g^?1, which is even larger than those of mesoporous nanostructures. At the same time, the possible formation mechanism for the formation of α-MnO₂ nanowires has been proposed according to the experimental results.     

Room temperature synthesis and formation process of α-NaYF4 nanocubes

In this article, α-NaYF₄ nanocubes were synthesized with a simple and environmentally friendly method at room temperature. The structure and morphology of the products were characterized by X-ray diffraction and scanning electron microscopy. The influence of the F^?/Y³⁺ molar ratio on the morphology of the products was examined. The products changed from spherical nanocrystals to cubic nanocrystals when the F^?/Y³⁺ molar ratio gradually increased from 4:1 to 16:1, and excessive F^? ions were necessary for the formation of regular NaYF₄ nanocubes in our synthesis procedure. The formation process of these α-NaYF₄ nanocrystals was traced via time-dependent experiments. Several nanometer-sized NaYF₄ nanocrystals changed into 100 nm sized NaYF₄ nanocrystals with prolonging of the reaction time. Additionally, the photoluminescence properties of Eu³⁺-doped samples were investigated. The excitation spectrum consists of the characteristic absorption peaks of Eu³⁺ corresponding to the direct excitation from the europium f-electrons. The charge-transfer band of Eu³⁺–F^? is not present in our measured range. The ⁵D₀ → ⁷F₁ transition is much stronger than the ⁵D₀ → ⁷F₂ transition in our samples.     

Controlled synthesis of spherical α-NiS and urchin-like β-NiS microstructures

Spheres and self-assembled wires of α-NiS (the hexagonal phase) have been successfully synthesized by a facile mix solvothermal route (glycol and water). Varying the volume ratio between glycol and water, and leaving the other conditions unchanged, the urchin-like β-NiS (the rhombohedral phase) microstructures were also prepared. The as-obtained products were characterized by means of X-ray powder diffraction patterns (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The TEM results show that the wires of α-NiS are constructed by a number of spheres of α-NiS as primary building units. On the basis of the TEM results, a phenomenological elucidation of the growth mechanism of α-NiS architectures has been presented as the directional aggregating and the Ostwald ripening. Moreover, the optical properties of the α-NiS and β-NiS microstructures were investigated by the room-temperature ultraviolet-visible (UV-Vis) absorption spectroscopy and the photoluminescence (PL) spectroscopy.     

Size-controlled synthesis of Rod-like α-FeOOH nanostructure

One-dimensional goethite (α-FeOOH) nanorods were successfully fabricated by a hydrothermal route without any template. Experimental results reveal that concentrations of Fe^3 + and ethylenediaminetetraacetic disodium salt (Na₂EDTA) affect the phase composition and size of the as-synthesized products. The size of the rod-like α-FeOOH increased when the concentration of Na₂EDTA was increased, where Na₂EDTA acts as a nucleation inhibitor. α-Fe₂O₃ nanoparticles were produced when the concentration of Fe^3 + was increased from 0.02 to 0.08 and 0.40 M. A possible formation mechanism was proposed based on the results of the time dependent experiments. Different electrolytes and surfactants can affect the size and the aspect ratio of the as-prepared nanorod-like α-FeOOH. Na₂SO₄ induced the decreasing of the size of the as-prepared sample. KCl and PVP affected the aspect ratio of the nanorods.     

A force field approach of structure and formation energy of defects of boron nitride nanotubes with tetragon–octagon pairs

We investigate the structure and the formation energy of defects of boron nitride (BN) nanotubes with tetragon–octagon pairs (4884) through a force field approach. Four kinds of 4884 defects [type A and type B for (n, 0) and (n, n) nanotubes] are considered. The instability of 4884 defects leads to the strong structural distortion of BN nanotubes and the formation energy of 4884 defect increases with increasing radius. The formation energy of the defects of type A is smaller than that of type B, because the defects of type B strongly affect the structural distortion of BN nanotubes. The length deviation of BN nanotubes with 4884 defects is also investigated, which decreases with larger radius. This result shows that the 4884 defect of (n, n) nanotubes is structurally more favorable than (n, 0) nanotubes.     

Effects of Bi excess on the structure and electrical properties of high-temperature BiFeO3–BaTiO3 piezoelectric ceramics

BiFeO₃–BTiO₃(BF–BT) ceramics as a promising candidate for lead-free high-temperature piezoelectric ceramics were studied with a special emphasis on the compositional dependence of piezoelectric properties. Excess Bi was added to compensate for the evaporation of Bi³⁺ ions during sintering and this addition was found to be effective in improving the piezoelectric properties of BF–BT ceramics. The microstructure, dielectric and piezoelectric properties of excess Bi doped BF–BT ceramics were investigated. Maximum piezoelectric constant d ₃₃ = 142 pC/N and k _p = 0.302 were obtained with 0.04 Bi doping. At the same time, an enhanced Curie temperature T _c, 452 °C, was obtained. The combination of improved piezoelectric properties and increased T _c makes these ceramics suitable for elevated temperature piezoelectric devices.     

Combustion synthesis of rod-like α-SiAlON seed crystals

Rod-like single-phase crystals of Y-α-SiAlON were synthesized by combustion of Si, Al, α-Si₃N₄, SiO₂ and Y₂O₃ powders. SEM observation of crystals, separated by chemical etching (HNO₃/HF=2:1) followed by ultrasonic agitation in ethanol showed rod-like crystals, assigned to almost pure and well crystallized α-SiAlON.     

Electron emission measurements and the defect structure of α-Al2O3

In this article, electron emission is used to study the defect structure of alumina. The need of a direct measurement of the position of the Fermi level (or the electron concentration in the conduction band) is shown by discussing the actual electrical data on alumina. The emission has been measured over a large temperature range (1400 to 2400 K) and the emission of a technical polycrystalline alumina is reported up to the melting temperature under a controlled oxygen partial pressure. Additional results are reported for titanium- and iron-doped polycrystalline aluminas. The results are discussed from two points of view. First the quantitative data concerning the work function are taken into account and the contribution of the surface layer is discussed. Secondly, the dependency of the electron emission on the oxygen partial pressure is explained by the defect chemistry of the oxide. The absence of variation of the electron concentration in a certain range of is due to a self compensation between donor and acceptor impurities.     

Hydrothermal synthesis and catalytic properties of α- and β-MnO2 nanorods

One-dimensional α-MnO₂ and β-MnO₂ single-crystalline nanostructures were prepared by hydrothermal process. The products were characterized in detail by multiform techniques: X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Experimental results indicate that the temperature plays important roles in determining produce α-MnO₂ and β-MnO₂ nanorods. In addition, the as-prepared α-MnO₂ and β-MnO₂ nanorods showed excellent catalytic performance in the Fenton-like reaction.     

Simple and rapid synthesis of α-Fe2O3 nanowires under ambient conditions

We propose a simple method for the efficient and rapid synthesis of one-dimensional hematite (α-Fe₂O₃) nanostructures based on electrical resistive heating of iron wire under ambient conditions. Typically, 1–5 μm long α-Fe₂O₃ nanowires were synthesized on a time scale of seconds at temperatures of around 700 ° ⊂. The morphology, structure, and mechanism of formation of the nanowires were studied by scanning and transmission electron microscopies, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman techniques. A nanowire growth mechanism based on diffusion of iron ions to the surface through grain boundaries and to the growing wire tip through stacking fault defects and due to surface diffusion is proposed. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

One-step synthesis,electromagnetic and microwave absorbing properties of α-FeOOH/polypyrrole nanocomposites

One-step synthesis of α-FeOOH/polypyrrole (PPy) nanocomposites is reported for the first time via a facile one-step chemical method in the presence of OH⁻, Fe²⁺, Fe³⁺ and pyrrole monomer. α-FeOOH nanorods are in situ formed in PPy matrix and the content of α-FeOOH nanorods increases with decreasing the molar ratio of pyrrole to Fe²⁺ ([Py]/[Fe²⁺] ratio). The electromagnetic and microwave absorbing properties of the nanocomposites are investigated as a function of the [Py]/[Fe²⁺] ratio. The results show that the PPy nanocomposites exhibit good conductivity (up to 16.10 S/cm) and antiferromagnetic behavior. The reflection loss evaluation based on the absorbing wall theory at the thickness of 2 mm shows that the nanocomposite at [Py]/[Fe²⁺] = 1.0 exhibits the best microwave absorbing property in the 2–18 GHz. And the corresponding reflection frequency range under −10 dB and −5 dB is 4.2 GHz and 5.8 GHz, respectively.     

Formation of three-dimensional urchin-like α-Fe₂O₃ structure and its field-emission application

A three-dimensional urchin-like α-Fe(2)O(3) microstructure is formed via a simple, template-free, and one-step thermal oxidation of Fe spheres in an air atmosphere at temperatures in the range of 300-450 °C. The urchin-like α-Fe(2)O(3) microstructure consists of crystalline α-Fe(2)O(3) nanoflakes grown perpendicularly on the surface of the sphere, a shell layer of α-Fe(2)O(3)/Fe(3)O(4), and an Fe core. During the oxidation process, the nanoflakes germinate and grow from cracks in the oxidation layer on the surface. The length of the nanoflakes increases with oxidation time. The tip diameters of the nanoflakes are in ranges of 10-20 nm at 300 °C, 20-30 nm at 350 °C, and 40-60 nm at 400 °C; the length can reach up to a few micrometers. The field-emission characteristics of the samples are experimentally studied and simulated. The results show that the urchin-like emitter has a low turn-on field of 2.8 V/μm, high field-enhancement factor of 4313, excellent emission uniformity of over 4 cm(2), and good emission stability during a 24 h test.