Abstract: Electrochemically splitting water to produce clean hydrogen fuel requires high-performance electrocatalysts. Here, we report that ternary transition metal boride Co–Ni–B is an active candidate catalyst for water splitting. The Co–Ni–B nanoparticles were deposited on Ni foam (NF) by a facile electroless plating method. After being properly calcined under an Ar atmosphere, the Co–Ni–B@NF catalyst exhibits a low overpotential along with high stability in alkaline solutions for the oxygen evolution reaction, which is among the best reported levels for non-precious metal catalysts. Furthermore, the properly annealed Co–Ni–B@NF catalyst also shows good activity towards the hydrogen evolution reaction. The potential of Co–Ni–B@NF as a bifunctional catalyst for the overall water splitting was demonstrated, with 10 mA cm−2 achieved at 1.72 V. The favorable combination of high catalytic activity, facile preparation and low cost makes Co–Ni–B@NF a very promising bifunctional catalyst for electrochemical water splitting.

Abstract: Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mechanical properties as well as hydrogen storage capacity. Here, we report the experimental realization of 2D hydrogen boride sheets with an empirical formula of H1B1, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB2) with an average yield of 42.3% at room temperature. The sheets feature an sp2-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray atomic pair dist...

Abstract: Using first‐principles calculations, the structural, elastic, and electronic properties of MoAlB have been investigated. The optimized lattice constants exhibit fair agreement with the experimental results. The computed elastic constants satisfy the mechanical stability conditions for MoAlB. The Mo‐based boride MoAlB is elastically anisotropic and classified as brittle material. This boride is expected to be thermally conductive due to its high Debye temperature of 693 K. The metallic conductivity of this compound is predicted by means of electronic structure calculations. The chemical bonding in MoAlB is basically covalent that is assured with the results of DOS, Mulliken population, and charge density distribution. The hardness value of 11.6 GPa for MoAlB suggests that it is relatively soft compared to many others borides. The Fermi surface is formed due to low dispersive Mo 4d‐like bands, which makes the compound a conductive one.

Abstract: FeB and Fe2B hard ceramic phases were produced in Armco iron using gas-boriding in N2–H2–BCl3 atmosphere. This process was carried out at 920 °C (1193 K) for 3 h and caused acceleration in the diffusion of boron into the surface of a base material in comparison with other acceptable methods of diffusion boriding. FeB and Fe2B layers were characterized by a strong zonation, obtaining the average thickness of 32 μm and 125 μm, respectively. Young's moduli and hardness of iron borides were measured using the nanoindenter with a Berkovich diamond tip under load of 50 mN. The higher average indentation hardness (HIT = 20.95 ± 0.93 GPa) and Young's modulus (EIT = 308.86 ± 26.44 GPa) were characteristic of FeB phase. Fe2B boride was characterized by HIT = 17.42 ± 0.80 GPa, and EIT = 252.96 ± 15.57 GPa. The fracture toughness of iron borides was measured by microindentation technique using a Vickers diamond indenter under load of 100 gf (about 0.981 N). The average fracture toughness (Kc), measured in FeB zone, was equal to 1.79 ± 0.70 MPa·m1/2. Fe2B phase was characterized by higher fracture toughness, obtaining Kc = 2.42 ± 0.66 MPa·m1/2. However, at FeB/Fe2B interface the increase in brittleness was detected. Such a situation was caused by the differences in coefficients of thermal expansion of both iron borides and their mechanical properties. It could provide the preliminary cracks at this interface after cooling. During indentation, the value of shear stress probably exceeded the value of normal compressive stress. It could cause the failure at this interface, facilitating cracks' propagation.

Abstract: Non-noble metal nanomaterials (molybdenum sulfides, phosphides, carbides, and nitrides) have recently emerged as highly active electrocatalysts for the hydrogen evolution reaction (HER). Molybdenum borides in contrast have not been studied for their HER activity at the nanoscale, however, they were recently shown to be already efficient HER catalysts in the bulk (microscale). In this study, we report on the first nanocrystalline molybdenum boride (MoB2) synthesized by a simple, one-step, relatively low temperature (650 °C) and environmentally benign redox-assisted solid state metathesis (SSM) reaction. The obtained MoB2 nanospheres exhibit a low onset overpotential of 154 mV at 10 mA cm−2, a Tafel slope of 49 mV dec−1 and high stability. Furthermore, density functional theory (DFT) calculations show that several surfaces are active and that the optimum evolution of H2 occurs at a hydrogen coverage between 75% and 100% on the B-terminated {001} surface. These experimental and theoretical results open new avenues to design new architectures of inexpensive and highly efficient boron-based HER catalysts, such as boride nanospheres (with maximum active sites) or materials with B-terminated surfaces (e.g. {001} nanosheets of AlB2-type borides or even the recently discovered borophene and related 2D compounds).

Abstract: MoAlB is the first and, so far, the only transition-metal boride that forms alumina when heated in air and is thus potentially useful for high-temperature applications. Herein, the thermal stability in argon and vacuum atmospheres and the thermodynamic parameters of bulk polycrystalline MoAlB were investigated experimentally. At temperatures above 1708 K, in vacuum and inert atmospheres, this compound incongruently melts into the binary MoB and liquid aluminum metal as confirmed by differential thermal analysis, quenching experiments, x-ray diffraction, and scanning electron microscopy. Making use of that information together with heat-capacity measurements in the 4–1000-K temperature range—successfully modeled as the sum of lattice, electronic, and dilation contributions—the standard enthalpy, entropy, and free energy of formation are computed and reported for the full temperature range. The standard enthalpy of formation of MoAlB at 298 K was found to be −132 ± 3.2 kJ/mol. Lastly, the thermal conductivity values are computed and modeled using a variation of the Slack model in the 300–1600-K temperature range.

Abstract: ConspectusBoron’s unique chemical properties and its reactions with metals have yielded the large class of metal borides with compositions ranging from the most boron-rich YB66 (used as monochromator for synchrotron radiation) up to the most metal-rich Nd2Fe14B (the best permanent magnet to date). The excellent magnetic properties of the latter compound originate from its unique crystal structure to which the presence of boron is essential. In general, knowing the crystal structure of any given extended solid is the prerequisite to understanding its physical properties and eventually predicting new synthetic targets with desirable properties. The ability of boron to form strong chemical bonds with itself and with metallic elements has enabled us to construct new structures with exciting properties. In recent years, we have discovered new boride structures containing some unprecedented boron fragments (trigonal planar B4 units, planar B6 rings) and low-dimensional substructures of magnetically active eleme...

Abstract: Ultrathin ceramic coatings are of high interest as protective coatings from aviation to biomedical applications. Here, a generic approach of making scalable ultrathin transition metal-carbide/boride/nitride using immiscibility of two metals is demonstrated. Ultrathin tantalum carbide, nitride, and boride are grown using chemical vapor deposition by heating a tantalum-copper bilayer with corresponding precursor (C2 H2 , B powder, and NH3 ). The ultrathin crystals are found on the copper surface (opposite of the metal-metal junction). A detailed microscopy analysis followed by density functional theory based calculation demonstrates the migration mechanism, where Ta atoms prefer to stay in clusters in the Cu matrix. These ultrathin materials have good interface attachment with Cu, improving the scratch resistance and oxidation resistance of Cu. This metal-metal immiscibility system can be extended to other metals to synthesize metal carbide, boride, and nitride coatings.

Abstract: Like many FeCrAl-based alloys, and some MAX phases, the atomically laminated boride, MoAlB, forms slow-growing, adherent Al2O3 scales when heated in air to 1350°C. Herein the oxidation of MoAlB ceramics in air was studied in the 1100–1400°C temperature range for up to 200 h. At 1400°C, the oxide scale was heavily cracked and spalled. At 1100°C, and up to 20 h, mass loss was recorded. At 1300°C and 1350°C, subparabolic, approximately cubic kinetics were observed, as a result of growth and coarsening of the Al2O3 grains in the oxide scale. At 1200°C, the weight gain kinetics were nearly linear, while the oxide thickening kinetics were approximately cubic likely due to cubic growth of Al2O3 and concurrent volatility of constituents in the oxide scale. The cyclic oxidation resistance was also good for up to 125, 1-hour, cycles at 1200°C. Analysis of grain coarsening and scale thickening kinetics suggest that oxygen grain boundary diffusivity is the rate controlling mechanism for the growth of Al2O3 scales at 1300°C and 1350°C. Dimensional changes at samples' corners after long oxidation at T > 1200°C may limit the maximum operational temperature of MoAlB.

Abstract: Boron nitride was recently identified as a highly selective catalyst for the oxidative dehydrogenation of propane (ODHP). In this communication we report that other boron-containing materials such as boron carbide, titanium boride, nickel boride, cobalt boride, hafnium boride and tungsten boride, as well as elemental boron itself show the same exceptional behavior as boron nitride. X-ray photoelectron and infrared spectroscopy suggest oxyfunctionalization of the surface. This observation disproves previous mechanistic hypotheses that edge sites on the boron nitride would be the active sites and rather suggests the formation of an analogous surface-stabilized BOx active site for all tested boride catalysts.

Abstract: The effects of micro-additions of boron and zirconium on grain-boundary (GB) structure and strength in polycrystalline γ(f.c.c.) plus γ'(L12) strengthened Co-9.5Al-7.5W-X at. % alloys (X = 0-Temary, 0.05B, 0.01B, 0.05Zr, and 0.005B-0.05Zr at. %) are studied. Creep tests performed at 850 °C demonstrate that GB strength and cohesion limit the creep resistance and ductility of the ternary B- and Zr-free alloy due to intergranular fracture. Alloys with 0.05B and 0.005B-0.05Zr both exhibit improved creep strength due to enhanced GB cohesion, compared to the baseline ternary Co-9.5Al-7.5W alloy, but alloys containing 0.01B or 0.05Zr additions displayed no benefit. Atom-probe tomography is utilized to measure GB segregation, where B and Zr are demonstrated to segregate at GBs. A Gibbsian interfacial excess of 5.57 ± 1.04 atoms nm-2 was found for B at a GB in the 0.01B alloy and 2.88 ± 0.81 and 2.40 ± 0.84 atoms nm-2 for B and Zr, respectively, for the 0.005B-0.05Zr alloy. The GBs in the highest B-containing (0.05B) alloy exhibit micrometer-sized boride precipitates with adjacent precipitate denuded-zones (PDZs), whereas secondary precipitation at the GBs is not present in the other four alloys. The 0.05B alloy has the smallest room temperature yield strength, by 6 %, which is attributed to the PDZs, but it exhibits the largest increase in creep strength (with an ~2.5 order of magnitude decrease in the minimum strain rate for a given stress at 850 °C) over the baseline Co-9.5Al-7.5W alloy.

Abstract: In order to improve the hardenability and toughness of high boron alloys, X Cu ( X = 0, 0.5, 1, 1.5, 2, 3, weight %) were added into high boron alloys. The Cu precipitation behavior and its effect on hardenability, microstructure, crystallography and mechanical properties of as-cast Fe-B-C-Cu alloys have been investigated systematically by means of SEM, XRD, TEM, EPMA and EBSD. The results indicate that Cu improves the hardenability of Fe-B-C-Cu alloys, while excessive Cu atoms segregating at the matrix attaching to boride promote the appearing of nano-Cu precipitation with body-centred cubic (bcc)structure, which can act as heterogeneous nuclei of the pearlite, leading to the reduction of hardenability. Therefore, the Cu concentration in Fe-B-C-Cu alloys should be controlled below 2 wt%. Observation of TEM reveals that the orientation relationship(OR) between martensite and retained austenite are [001] α // [011] γ and [110] α //[ 1 ( − ) 1 ( − ) 1] γ. As well, the OR between ferrite and M 3 C are (100) M // (010) M3C and [011] M //[001] M3C . EBSD analysis shows that the misorientation angles of the α-Fe and M 2 B phases are mainly low-angle with less than 5°. The α-Fe phase has the preferred crystal plane (111) // ND-type grain, while the M 2 B phase displays a preferred growth orientation [002] on the M 2 B crystal plane (001). A possible OR between borides and α-Fe in Fe-B-C-Cu alloys is (001) α- Fe //(001) M2B . In addition, the bulk hardness and impact toughness increase obviously with 1–2 wt% copper concentration owing to the increasing of volume fraction of martensite( V m ), which maybe a guiding copper addition in Fe-B-C-Cu alloys.

Abstract: We synthesized orthorhombic FeB-type MnB (space group: Pnma) with high pressure and high temperature method. MnB is a promising soft magnetic material, which is ferromagnetic with Curie temperature as high as 546.3 K, and high magnetization value up to 155.5 emu/g, and comparatively low coercive field. The strong room temperature ferromagnetic properties stem from the positive exchange-correlation between manganese atoms and the large number of unpaired Mn 3d electrons. The asymptotic Vickers hardness (AVH) is 15.7 GPa which is far higher than that of traditional ferromagnetic materials. The high hardness is ascribed to the zigzag boron chains running through manganese lattice, as unraveled by X-ray photoelectron spectroscopy result and first principle calculations. This exploration opens a new class of materials with the integration of superior mechanical properties, lower cost, electrical conductivity, and fantastic soft magnetic properties which will be significant for scientific research and industrial application as advanced structural and functional materials.

Abstract: The macrokinetic features of combustion in the Ta-Zr-B system were studied. Combustion is characterized by spin mode, suggesting the limiting role of gas-phase mass transfer of reagents. The mechanism of chemical reactions and phase formation in combustion wave was discussed. Primary layers of tantalum and zirconium borides were detected in the preheating zone at temperatures below the melting point of the reagents. After zirconium and boron melt, the temperature in the combustion zone reaches its maximum and zirconium diboride precipitates out of the oversaturated solution. Powders with a grain size of 1–3 μm were fabricated and hot-pressed into dense ultra-high-temperature ceramics (UHTCs). Boride ceramics with the record-setting hardness of 70 GPa, Young's modulus of 594 GPa, and elastic recovery of 96% were obtained. The measured heat conductivity of the solid solution (Zr,Ta)B2 was equal to 35–42 W/m K. Plasma torch tests demonstrated high oxidation resistance of the obtained ceramics at 2900–3000 °C.

Abstract: Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr2B2, AlMn2B2, AlFe2B2, AlCo2B2, and AlNi2B2. We find that the first three borides of this series are stable phases, while AlCo2B2 and AlNi2B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2B2, AlMn2B2, and AlFe2B2 are more brittle, while AlCo2B2 and AlNi2B2 are more ductile. We propose that the elasticity of AlFe2B2 can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono- and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.

Abstract: Inconel 625, a nickel-based superalloy, is used in a wide range of applications including the marine and petroleum industries under where it is subjected to harsh conditions such as high temperatures and highly corrosive environments. However, its wear resistance is limited and can be often considered unsatisfactory in some applications. If this alloy were to be used under abrasive wear conditions, its surface would have to be protected by a wear resistant coating. In this study, a two-step thermo-chemical borotitanizing treatment (including an initial boriding step followed by titanium diffusion) is proposed. Microstructural characterization (optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction) and mechanical properties (including micro-hardness and micro-abrasion wear) of the coated samples were conducted. Microstructural studies revealed a compact, homogenous, silicide-free coating, consisting of four distinct regions: a TiB 2 layer, a multi-phase boride layer, a diffusion zone and the substrate. Hardness values were significantly higher than those obtained by standard boriding treatments. Due to the nano-sized boriding agents used, the coatings formed on the surface were thicker than coatings obtained by methods such as nitriding, paste boriding and pack-boriding, and comparable to that of laser boriding. The wear resistance was improved by up to ten times in comparison with untreated Inconel 625. Grooving was the effective wear mechanism in untreated Inconel 625. However the increase in surface hardness achieved by the borotitanizing treatment changed the wear mechanism in the coated samples from grooving to rolling.

Abstract: Metal borides have mostly been studied as bulk materials. The nanoscale provides new opportunities to investigate the properties of these materials, e.g., nanoscale hardening and surface reactivity. Metal borides are often considered stable solids because of their covalent character, but little is known on their behavior under a reactive atmosphere, especially reductive gases. We use molten salt synthesis at 750 °C to provide cobalt monoboride (CoB) nanocrystals embedded in an amorphous layer of cobalt(II) and partially oxidized boron as a model platform to study morphological, chemical, and structural evolutions of the boride and the superficial layer exposed to argon, dihydrogen (H2), and a mixture of H2 and carbon dioxide (CO2) through a multiscale in situ approach: environmental transmission electron microscopy, synchrotron-based near-ambient-pressure X-ray photoelectron spectroscopy, and near-edge X-ray absorption spectroscopy. Although the material is stable under argon, H2 triggers at 400 °C decomp...

Abstract: New results about the evolution of the FeB-Fe2B layers during a diffusion annealing process (DAP) are presented in this work. First, the growth of the boride layers over the surface of an AISI 1045 steel was developed by means of the powder-pack boriding process (PPBP) at temperatures of 1173–1273 K with different exposure times for each temperature. The boron diffusion coefficients in the FeB and Fe2B were estimated according to the mass balance equations on the growth interphases, and expressed as a function of the boriding temperatures by the Arrhenius equation. Moreover, the DAP was conducted on borided samples obtained at 1273 K with 4–8 h of exposure using a SiC atmosphere, and considering the theoretical values of annealing times proposed by the extended model. The evolution of the boride layer microstructure was represented by the interphase velocities of the FeB/Fe2B and Fe2B/substrate, and the relationships between the growth of the Fe2B at the expense of the FeB layer for the applied range of annealing times.