Mechanical properties of superhard boron subnitride B<Subscript>13</Subscript>N<Subscript>2</Subscript>

Microstructure and mechanical properties of bulk polycrystalline rhombohedral boron sub-nitride B₁₃N₂ synthesized by crystallization from the B–BN melt at 7 GPa have been systematically studied by micro- and nanoindentation, atomic force microscopy and scanning electron microscopy. The obtained data on hardness, elastic properties and fracture toughness clearly indicate that B₁₃N₂ belongs to a family of superhard phases and can be considered as a promising superabrasive or binder for cubic boron nitride.     

Thermoelastic equation of state of boron subphosphide B<Subscript>12</Subscript>P<Subscript>2</Subscript>

Compressibility of boron subphosphide B₁₂P₂ has been studied under quasi-hydrostatic conditions up to 26 GPa and 2600 K using laser-heated diamond anvil cell and angle-dispersive synchrotron X-ray diffraction. 300-K data fit yields the values of bulk modulus B ₀ = 192(11) GPa and its first pressure derivative B ₀′ = 5.5(12). It has been found that at ambient pressure the thermal expansion is quasi-linear up to 1300 K with average volume expansion coefficient α = 17.4(1) × 10^?6 K^?1. The whole set of experimental p–V–T data is well described by the Anderson-Grüneisen model with δ_T = 6.     

Preparation of B<Subscript>4</Subscript>C-ZrB<Subscript>2</Subscript>-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Emission Activity under Impact Destruction of A<Subscript>2</Subscript>B<Subscript>6</Subscript> Ceramics

In this paper, we present results of a study of the generation of electromagnetic emission during impact loading of ZnS and ZnSe ceramics prepared by various techniques. The choice of the type of mechanical action is associated with the typical applications of these ceramics. Separate contributions to emission activity from the dislocations movement and the microcracks development depending on the crystallite size in the ceramics are shown. The electromagnetic emission is compared with parallel time series of pulses of mechanoluminescence and acoustic emission. In all three cases, emission activity was recorded with a time resolution of 10 ns.     

Metastable phases in the Ni<Subscript>75</Subscript>Nb<Subscript>12</Subscript>B<Subscript>13</Subscript> alloy prepared under nonequilibrium conditions

The structure of Ni₇₅Nb₁₂B₁₃ alloys prepared by liquid quenching (LQ) and mechanical alloying (MA) has been studied by x-ray diffraction. The alloy prepared by LQ at a cooling rate of ～10⁶ K/s is shown to be fully amorphous, while MA yields an amorphous-crystalline material in which the predominant phase is an fcc Ni〈Nb,B〉 solid solution. The thermal stability of the alloys and their structural transformations on heating have been studied by differential scanning calorimetry. The amorphous phase obtained by LQ is shown to crystallize at 490°C. After heating to 720°C, the alloy consists of two equilibrium phases: Ni₂₁Nb₂B₆ (τ) and Ni₅Nb₃B₂ (z). Heating the MA alloy to 720°C leads to the formation of a stable τ-phase, while the Ni-based fcc solid solution remains supersaturated and, hence, metastable. Increasing the milling time leads to the formation of nanocrystalline τ and Ni₃B phases, in addition to the Ni-based fcc solid solution, which corresponds to the equilibrium phase composition of the Ni₇₅Nb₁₂B₁₃ alloy in the Ni-Nb-B phase diagram. The effect of high-energy milling on the phase composition of the alloy is similar to that of heat treatment.     

Aqueous electrocatalytic N<Subscript>2</Subscript> reduction under ambient conditions

Recently, the electrochemical N₂ reduction reaction (NRR) in aqueous electrolytes at ambient temperature and pressure has demonstrated its unique advantages and potentials. The reactants are directly derived from gaseous N₂ and water, which are naturally abundant, and NH₃ production is important for fertilizers and other industrial applications. To improve the conversion yield and selectivity (mainly competing with water reduction), electrocatalysts must be rationally designed to optimize the mass transport, chemisorption, and transduction pathways of protons and electrons. In this review, we summarize recent progress in the electrochemical NRR. Studies of electrocatalyst designs are summarized for different categories, including metal-based catalysts, metal oxide-derived catalysts, and hybrid catalysts. Strategies for enhancing the NRR performance based on the facet orientation, metal oxide interface, crystallinity, and nitrogen vacancies are presented. Additional system designs, such as lithium-nitrogen batteries, and the solvent effect are introduced. Finally, existing challenges and prospects are discussed.

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Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiN composites produced from TiO<Subscript>2</Subscript>-Modified Si<Subscript>3</Subscript>N<Subscript>4</Subscript> powders

Si₃N₄/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO₂ content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si₃N₄ + TiO₂ → TiN + NO + N₂O + 3Si. The Si₃N₄/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si₃N₄/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.     

Phase diagram of the W<Subscript>2</Subscript>B<Subscript>5</Subscript>-ZrB<Subscript>2</Subscript> system

The T-x phase diagram of the W₂B₅-ZrB₂ join in the ternary system Zr-W-B is mapped out. W₂B₅ and ZrB₂ form a eutectic at 20 mol % ZrB₂ with a melting point of 2180°C.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 290–292.Original Russian Text Copyright © 2005 by Ordanyan, Boldin, Suvorov, Smirnov.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

X-Ray luminescence of polycrystalline TbO<Subscript>2</Subscript>-doped Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>

This paper examines the effect of doping level on the X-ray luminescence of TbO₂-doped polycrystalline lithium tetraborate. It is shown that, when interpreting such spectra, it is convenient to proceed from the terms of free activator and constituent ions. We demonstrate that the emission lines of Tb³⁺ in doped polycrystalline lithium tetraborate are effectively excited in the band between 350 and 650 nm, which is predominantly due to electron transitions from the ⁵ D ₃ and ⁵ D ₄ excited states to spin-orbital levels of the ⁷ F _J ground multiplet. The emission lines of lithium and boron in single-crystal and polycrystalline undoped lithium tetraborate are effectively excited in the band between 274 and 550 nm.     

Two types of ABi<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>9</Subscript> layered perovskite-like ferroelectrics

Statistical analysis of the literature data on the structural and ferroelectric properties of ABi₂B₂O₉ layered perovskite-like compounds shows that these ferroelectrics can be divided into two distinct groups, one constituted by tetragonal and nearly tetragonal compounds, and the other by compounds with a significant orthorhombic (and possibly monoclinic) distortion. Surprisingly, there are no layered ABi₂B₂O₉ compounds in a wide intermediate range of structural parameters.     

Fe<Subscript>78</Subscript>Si<Subscript>9</Subscript>B<Subscript>13</Subscript> amorphous powder core with improved magnetic properties

The effects of pre-annealing treatments on the soft magnetic properties of the corresponding Fe₇₈Si₉B₁₃ amorphous powder cores were investigated. The amorphous powder cores prepared from pre-annealed powder have better soft magnetic properties compared with unpretreated powder core in the as-cast state. The result shows that the powder after pre-annealing in a magnetic field presents a regular domain structure and the soft magnetic properties of the corresponding powder cores are greatly improved. As the result, the magnetic-field annealed powder core has the highest effective permeability (μ _e) of 37, which is 23 % higher than the as-cast one and 7 % than only vacuum-annealed one. The total core loss (P _cv) for the core annealed in magnetic field is only 141 W/kg (100 kHz, 50 mT) and as low as 36 % of the P _cv for the powder core in the as-cast state. The one annealed in magnetic field also exhibits the best DC bias properties of 92 %. This work provides a novel approach to realizing low P _cv and high μ _e for Fe₇₈Si₉B₁₃ powder cores and also validates the application prospect of powder cores in the work condition of different ripple currents, different loads and a wide frequency (f) range (10 kHz–10 MHz).     

Microstructure characterization of in situ synthesized porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript>–Si<Subscript>2</Subscript>N<Subscript>2</Subscript>O composites using feldspar additive

Porous Si₃N₄–Si₂N₂O bodies fabricated by multi-pass extrusion process were investigated depending on the feldspar addition content (4–8 wt% Si) in the raw silicon powder. The diameter of the continuous pores was about 250 μm. The polycrystalline Si₂N₂O fibers observed in the continuous pores as well as in the matrix regions of the nitrided bodies can increase the filtration efficiency. In the 4 wt% feldspar addition, the diameter of the Si₂N₂O fibers in the continuous pores of the nitrided bodies was about 90–150 nm. A few number of rope typed Si₂N₂O fibers (∼4 μm) was found in the case of 8 wt% feldspar addition. However, in the 8 wt% feldspar addition, the matrix showed highly porous structure composed of large number of the Si₂N₂O fibers (∼60 nm). The relative densities of the Si₃N₄–Si₂N₂O bodies with 4 wt% and 8 wt% feldspar additions were about 65% and 61%, respectively.     

Dissolution of UO<Subscript>2</Subscript> in the N<Subscript>2</Subscript>O<Subscript>4</Subscript>-H<Subscript>2</Subscript>O system

The kinetics of the UO₂ dissolution in the N₂O₄-H₂O system was studied. At 25°C, the process is kinetically controlled, whereas at 55°C the process occurs initially under kinetic control (3 min) and then under diffusion-kinetic control. At 80°C, the process occurs exclusively under diffusion-kinetic control. The apparent activation energy was estimated at ∼39 kJ mol⁻¹.     

In-situ construction of 2D direct Z-scheme g-C<Subscript>3</Subscript>N<Subscript>4</Subscript>/g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> homojunction with high photocatalytic activity

Constructing all-solid-state Z-scheme junction is a very effective strategy to design highly active photocatalysts for solar energy conversion and environmental purification. We herein firstly construct 2D g-C₃N₄/g-C₃N₄ Z-scheme homojunction by using a bottom-up approach, during which the supramolecular complex is initially formed, followed by a facile thermal polycondensation. Based on the active species trapping experiments, Mott–Schottky test and band edge position analysis, the prepared 2D nanosheet g-C₃N₄/g-C₃N₄ homojunctions are found to be Z-scheme type, different from those available reported ones with a type-II energy alignment. Benefiting from the specific 2D morphology with large exposed surface area and Z-scheme junction with efficient separation and high redox abilities of the photoinduced electrons and holes, the obtained 2D g-C₃N₄/g-C₃N₄ homojunctions are much more active than the conventional g-C₃N₄/g-C₃N₄ homojunction (CN-MT) and bulk g-C₃N₄ (CN-M) under visible light irradiation, validating by the high rhodamine degradation rate of 0.833 h^?1, which is about 3.9 and 15.4 times higher than that of CN-MT (0.214 h^?1) and CN-M (0.054 h^?1), respectively. The present work sheds light on design of novel Z-scheme photocatalysts with specific morphology and thus further application in the field of environment or energy.     

Crystallization kinetic studies of CaBi<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>7</Subscript> glasses by non-isothermal methods

Transparent glasses of CaBi₂B₂O₇ (CBBO) were fabricated via the conventional melt-quenching technique. The amorphous and the glassy nature of the as-quenched samples were, respectively, confirmed by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC). The glass transition (T _g) and the crystallization parameters (crystallization activation energy (E _cr) and Avrami exponent (n)) were evaluated under non-isothermal conditions using DSC. The heating rate dependent glass transition and the crystallization temperatures were rationalized by Lasocka equation for the as-quenched CBBO glasses. There was a close agreement between the activation energies for the crystallization process determined by Augis and Bennet and Kissinger methods. The variation of local activation energy (E _c(x)) that was determined by Ozawa method increased with the fraction of crystallization (x). The Avrami exponent (n(x)) decreased with the increase in fraction of crystallization (x), suggesting that there was a changeover in the crystallization process from the bulk to the surface.     

Phase relations in the LaB<Subscript>6</Subscript>-W<Subscript>2</Subscript>B<Subscript>5</Subscript> system

The LaB₆-W₂B₅ join in the ternary system La-B-W is shown to have a eutectic phase diagram with t _e= 2220°C and a eutectic composition of 30 mol % LaB₆ + 70 mol % W₂B₅. Data are presented on LaB₆-containing systems potentially attractive for designing mixed-phase ceramics.     

Preparation of boron microcrystals via high-pressure,high-temperature pyrolysis of decaborane,B<Subscript>10</Subscript>H<Subscript>14</Subscript>

Crystalline boron has been prepared via high-pressure, high-temperature pyrolysis of decaborane, B₁₀H₁₄. We obtained α-tetragonal boron crystals at a pressure of 8–9 GPa and temperatures in the range 1100–1600°C and β-rhombohedral boron intergrowths at 3 GPa and 1200°C.     

Electrical Conductivity of AlN-Mg<Subscript>3</Subscript>N<Subscript>2</Subscript> Solid Solutions

AlN-Mg₃N₂ solid solutions are synthesized, and their electrical conductivity and ionic transference number are measured. In the range 500–1150°C, the conductivity of the solid solutions increases with Mg₃N₂ concentration. The ionic transference number is close to unity, and the ionic transport is mainly due to aluminum ions.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 819–822.Original Russian Text Copyright © 2005 by Lesunova, Burmakin.     

Refractive index of Al<Subscript>3</Subscript>C<Subscript>2</Subscript>B<Subscript>48</Subscript> aluminum borocarbide crystals

Aluminum borocarbide single crystals have been grown from an Al-based solution melt. The crystal lattice parameters have been determined, the dispersion of the refractive index in a 0.55–1.3 μm wavelength interval has been studied, and the temperature coefficient of the refractive index in a 300–600 K range has been measured. The crystals are characterized by a high refractive index in the visible spectral range in combination with at a high hardness, which makes them of interest for jewelry, as well as for both traditional and X-ray optics.