Boron Phases Research Articles

Hydration properties of boron modified active belite cement concrete

In this study, hydration heat and speed of Boron Modified Active Belite (BAB) cement concretes were determined using by semi-adiabatic method. After the development of the theory and analysis of the BAB cement concrete production, the current work includes analyzes of hydration heat in BAB cement concrete. The same experiments were repeated in 6 different cement types including BAB cement, and their hydration properties were determined. For samples hydration heat-time and hydration velocity-time graphs for all types of cement were prepared comparatively. Additionally in order to determine the strength properties of hydration relations, Schmidt hammer, unit weight, compressive strength tests are applied to samples and the test results are interpreted with the help of graphs. As a result, hydration properties of steam cured and non-steam cured boron modified active belite cement concrete were comparatively analyzed and heat of hydration of BAB cementitious mortar samples were found significantly lower than other types of cement. As the cause of this situation is the lack of Alite (C3S) phase and low C3A (Belit) content of boron modified active belite cement.

Enhanced fracture toughness of boron carbide from microalloying and nanotwinning

Fracture toughness is one of the most important mechanical properties of structural materials. Particularly, enhancing the fracture toughness of super-hard materials is essential for their applications. Here, we applied density functional theory to examine how the microalloying and nanotwinning affect the fracture toughness of superhard boron carbide (B4C). We find that replacing C-B-C chains with two-atom chains especially weakly coupled O atoms can significantly improve the fracture toughness of B4C. In addition, inserting nanotwins can significantly enhance the fracture toughness of B4C and boron phases. Our results provide useful information to design boron based superhard materials with enhanced fracture toughness.

Elucidation of the role of rotation speed and stirring direction on AA 7075-B4C surface composites formulated by friction stir processing

In the present research investigation, aluminum–boron carbide surface composites were fabricated using friction stir processing technique. Boron carbide powder particles were incorporated into AA 7075 substrate by the thermomechanical mixing generated through multiple passes of friction stir processing. A parametric investigation was conducted to encounter homogeneous boron carbide powder particles distribution in the substrate matrix by employing various parameter combination sets like tool rotational speed and alteration in tool travel direction. Microstructural characterizations were performed by means of optical microscopy, scanning electron microscopy and X-ray diffraction analysis to investigate on boron carbide powder particles distribution, phases present, and grain morphologies in the substrate matrix. Homogeneous distribution of boron carbide powder particles was observed for surface composites processed at lowest tool rotational speed. Uniform boron carbide powder particles distribution in the processed zone along with various strengthening mechanisms brought about two-fold increase in microhardness and wear resistance of the prepared composites.

Revealing and exploiting hierarchical material structure through complex atomic networks

One of the great challenges of modern science is to faithfully model, and understand, matter at a wide range of scales. Starting with atoms, the vastness of the space of possible configurations poses a formidable challenge to any simulation of complex atomic and molecular systems. We introduce a computational method to reduce the complexity of atomic configuration space by systematically recognising hierarchical levels of atomic structure, and identifying the individual components. Given a list of atomic coordinates, a network is generated based on the distances between the atoms. Using the technique of modularity optimisation, the network is decomposed into modules. This procedure can be performed at different resolution levels, leading to a decomposition of the system at different scales, from which hierarchical structure can be identified. By considering the amount of information required to represent a given modular decomposition we can furthermore find the most succinct descriptions of a given atomic ensemble. Our straightforward, automatic and general approach is applied to complex crystal structures. We show that modular decomposition of these structures considerably simplifies configuration space, which in turn can be used in discovery of novel crystal structures, and opens up a pathway towards accelerated molecular dynamics of complex atomic ensembles. The power of this approach is demonstrated by the identification of a possible allotrope of boron containing 56 atoms in the primitive unit cell, which we uncover using an accelerated structure search, based on a modular decomposition of a known dense phase of boron, γ-B28.

Three-dimensional buckled honeycomb boron lattice with vacancies as an intermediate phase on the transition pathway from \u03b1-B to \u03b3-B

In the phase diagram of elemental boron, an unknown high-pressure form was identified as γ-orthorhombic boron (γ-B28), provoking studies of the extraordinary properties of its main building blocks, B12 icosahedra and B2 dumbbells. Although two low-pressure phases, α- and β-rhombohedral boron (α-B12 and β-B106), are also composed of icosahedra, the detailed kinetics and mechanisms of the structural transition from α-B12 or β-B106 to γ-B28 remain poorly understood. We report on new metastable boron phases formed during the transition in high-pressure high-temperature conditions that were discovered using the crystal structure search method. The metastable phases are understood to be a three-dimensional buckled defective honeycomb lattice in which boron vacancies lead to a dynamically and mechanically stable structure with triangular motifs. We suggest that the metastable phases act as intermediate states on the transition pathway from α-B12 to γ-B28 owing to their structural flexibility and low enthalpies, in the framework of Ostwald’s step rule.

Densification of amorphous boron under pressure

The densification mechanism of amorphous boron under pressure is investigated using a constant pressure ab initio technique and found to be associated with two consecutive amorphous-to-amorphous phase transformations. Amorphous boron gradually transforms into a high density amorphous phase, followed by a first order phase transformation into another high density amorphous state. The high density amorphous phases of boron are not quenchable to ambient pressure. Most quasimolecular B12 icosahedra in the model are found to be stable at the highest the theoretical pressure of 280GPa reached in the present work and thus the phase transformations are principally due to the re-structural arrangements in the parts of the model connecting B12 icosahedra.

Band Gap of Semiconducting High Pressure Phase of Boron

The partially ionic high pressure phase of boron γ-B28 was synthesized at 12 GPa 1500oC using a multi-anvil press. The crystal structure of γ-B28 was confirmed by x-ray diffraction. The γ-B28 phase is metastable at ambient condition. Upon grinding, the sample back transform to α-B12 phase. Optical absorption was conducted to investigate the γ-B28 sample. The optical absorption edge was observed at a wavelength of 725nm indicating a band gap of 1.7 eV for this semiconducting high pressure phase, in a good agreement with theoretical calculation (1.5-1.7 eV).

Prediction of Two-Dimensional Phase of Boron with Anisotropic Electric Conductivity.

Two-dimensional (2D) phases of boron are rare and unique. Here we report a new 2D all-boron phase (named the π phase) that can be grown on a W(110) surface. The π phase, composed of four-membered rings and six-membered rings filled with an additional B atom, is predicted to be the most stable on this support. It is characterized by an outstanding stability upon exfoliation off of the W surface, and unusual electronic properties. The chemical bonding analysis reveals the metallic nature of this material, which can be attributed to the multicentered π-bonds. Importantly, the calculated conductivity tensor is anisotropic, showing larger conductivity in the direction of the sheet that is in-line with the conjugated π-bonds, and diminished in the direction where the π-subsystems are connected by single σ-bonds. The π-phase can be viewed as an ultrastable web of aligned conducting boron wires, possibly of interest to applications in electronic devices.

Electronic properties and stability phase diagrams for cubic BN surfaces

This contribution investigates structural and electronic properties as well as stability phase diagrams of surfaces of the cubic boron nitride (c-BN). Our calculated parameters for bulk c-BN agree reasonably well with both experimental and computed values available in the literature. Based on the energies of the three experimentally recognised phases of bulk boron, i.e. α-B36, β-B105 and γ-B28, we estimate enthalpy of formation of c-BN to be −2.8 eV. The c-BN(1 0 0) surface offers separate B and N terminations (denoted as c-BN(1 0 0)_B and c-BN(1 0 0)_N), whereas c-BN(1 1 1) and c-BN(1 1 0) are truncated with combinations of boron and nitrogen atoms (denoted as c-BN(1 1 1)_BN and c-BN(1 1 0)_BN). Optimised geometries of surfaces display interlayer displacements up to the three topmost layers. Downward displacement of surface boron atoms signifies a common geometric feature of all surfaces. Bulk c-BN and its most stable surface c-BN(1 0 0)_N possess no metallic character, with band gaps of 5.46 and 2.7 eV, respectively. We find that, only c-BN(1 0 0)_B configuration exhibits a metallic character. c-BN(1 1 0)_BN and c-BN(1 1 1)_BN surfaces display corresponding band gaps of 2.5 and 3.9 eV and, hence, afford no metallic property.

Conversion of vertically-aligned boron nitride nanowalls to photoluminescent CN compound nanorods: Efficient composition and morphology control via plasma technique

Two-dimensional BN/CN nanomaterials of various composition and morphology were synthesized in N2H2 plasma by plasma-enhanced hot filament chemical vapor deposition, with B4C used as precursor. The results of field emission scanning electron microscopy, X-ray diffractometer, transmission electron microscopy, micro-Raman and X-ray photoelectron spectroscopy evidence that the hexagonal boron nitride nanowalls with carbon and oxide phases were synthesized under about 1:1 ratio of H2 to N2, while the CN nanorods with boron and oxide phases are formed under other ratios of H2 to N2, and the nanowires made of oxide phases are grown without the plasma. Efficient control over the nanostructure composition and morphology was demonstrated, thus proposing a cheap and convenient fabrication technique for the advanced composite boron nitride/graphene materials. The photoluminescence properties of the synthesized BN and CN nanomaterials were also studied using the 325 nm line of HeCd laser as the excitation source. The fabricated nanomaterials can generate the ultraviolet, blue and green multi-PL bands, which are associated to the defects formed in these nanomaterials and the carbon clusters, respectively. These outcomes can make a great contribution to the design of functional BN- and CN-nanomaterials of various morphologies and compositions for the applications in various electronic and carbon-based optoelectronic nanodevices.

Effect of powder metallurgy Cu-B4C electrodes on workpiece surface characteristics and machining performance of electric discharge machining

The main aim of this study is to produce new powder metallurgy (PM) Cu-B4C composite electrode (PM/(Cu-B4C)) capable of alloying the recast workpiece surface layer during electric discharge machining process with boron and other hard intermetallic phases, which eventually yield high hardness and abrasive wear resistance. The surface characteristics of the workpiece machined with a PM/(Cu-B4C) electrode consisted of 20 wt% B4C powders were compared with those of solid electrolytic copper (E/Cu) and powder metallurgy pure copper (PM/Cu) electrodes. The workpiece surface hardness, surface abrasive wear resistance, depth of the alloyed surface layer and composition of alloyed layers were used as key parameters in the comparison. The workpiece materials, which were machined with PM/(Cu-B4C) electrodes, exhibited significantly higher hardness and abrasive wear resistance than those of machined with the E/Cu and PM/Cu. The main reason was the presence of hard intermetallic phases, such as FeB, B4C (formed due to the boron in the electrode) and Fe3C in the surface layer. The improvement of the surface hardness achieved for steel workpiece when using PM/(Cu-B4C) electrodes was significantly higher than that reported in the literature. Moreover, the machining performance outputs (workpiece material removal rate, electrode wear rate and workpiece average surface roughness (Ra)) of the electrodes were also considered in this study.

Separation of primary Si and impurity boron removal from Al-30%Si-10%Sn melt under a traveling magnetic field

Separation of primary Si phase and removal of boron in the primary Si phase during the solidification process of the Al-30%Si-10%Sn melt under a traveling magnetic field (TMF) were investigated. The results showed that the agglomeration layer of the primary Si can be formed in the periphery of the ingot while the inner microstructures mainly consist of the eutectic α-Al+Si and β-Sn phases. The intense melt flow carries the bulk liquid with higher Si content to promote the growth of the primary Si phase which is first precipitated close to the inner wall of crucible with a relatively lower temperature, resulting in the remarkable segregation of the primary Si phase. The content of impurity B in the primary Si phase can be removed effectively with an increase in magnetic flux intensity. The results of electron probe microanalysis (EPMA) clearly indicated that the average intensity of the B Ka line in the α-Al phase region of Al-Si-Sn alloy is higher in the case of solidification under TMF than that of normal solidification condition, suggesting that the electromagnetic stirring can promote the B removal from the primary Si phase.

A new orthorhombic boron phase B51.5–52 obtained by dehydrogenation of “α-tetragonal boron”

Abstract

Unveiling the atomic structure and electronic properties of atomically thin boron sheets on an Ag(111) surface.

Two-dimensional (2D) boron sheets (i.e., borophene) have a huge potential as a basic building block in nanoelectronics and optoelectronics; such a situation is greatly promoted by recent experiments on fabrication of borophene on silver substrates. However, the fundamental atomic structure of borophene on the Ag substrate is still under debate, which greatly impedes further exploration of its properties. Herein, the atomic structure and electronic properties of borophene on an Ag(111) surface have been studied using first-principles calculations and ab initio molecular dynamics simulations. Our results reveal that there exist three energetically favorable borophene structures (β5, χ1, and χ2) on the Ag(111) surface and their simulated STM images are in good agreement with experimental results, suggesting the coexistence of boron phases during the growth. All these stable borophene structures have a planar structure with slight surface buckling (∼0.15 Å) and relatively high hexagonal vacancy density (1/6 and 1/5) and exhibit typical metallic conductivity. These findings not only can be applied to solve the experimental controversies about the atomic structure of borophene on the Ag substrate but also provide a theoretical basis for exploring the fundamental properties and applications of 2D boron sheets.

Effects of B4C particle size on pore structures of porous TiB2–TiC by reaction synthesis

Porous TiB2–TiC were prepared by SHS using Ti and B4C powder mixtures. The pores, phase constituents, microstructures and composition distribution of TiB2–TiC were analyzed. The influences of B4C particle size and pressure in green compacts on pore sizes and structures of final products were also discussed. Via combustion front quenching, the reaction process between Ti and B4C particles was achieved. The results showed that the pore size decreased and combination between TiB2 and TiC became closer with the decrease of B4C particle size and increase of pressure. During the reaction, B4C particles decomposed layer by layer, and reacted with the melted Ti to form poor boron phases of TiB and Ti3B4 with needle-like shapes, which existed in quenched samples. With further reacting, TiB and Ti3B4 phases gradually changed to TiB2 with rectangle and hexagonal-plate shapes. The regular TiB2 particles were bound together by TiC, which constructed the skeleton of pores.

Phase transitions, mechanical properties and electronic structures of novel boron phases under high-pressure: a first-principles study.

We have explored the mechanical properties, electronic structures and phase transition behaviors of three designed new phases for element boron from ambient condition to high-pressure of 120 GPa including (1) a C2/c symmetric structure (m-B16); (2) a symmetric structure (c-B56) and (3) a Pmna symmetric structure (o-B24). The calculation of the elastic constants and phonon dispersions shows that the phases are of mechanical and dynamic stability. The m-B16 phase is found to transform into another new phase (the o-B16 phase) when pressure exceeds 68 GPa. This might offer a new synthesis strategy for o-B16 from the metastable m-B16 at low temperature under high pressure, bypassing the thermodynamically stable γ-B28. The enthalpies of the c-B56 and o-B24 phases are observed to increase with pressure. The hardness of m-B16 and o-B16 is calculated to be about 56 GPa and 61 GPa, approaching to the highest value of 61 GPa recorded for α-Ga-B among all available Boron phases. The electronic structures and bonding characters are analyzed according to the difference charge-density and crystal orbital Hamilton population (COHP), revealing the metallic nature of the three phases.

Deformation Induced Solid–Solid Phase Transitions in Gamma Boron

We predict three new polymorphs of boron by applying density functional theory (PBE flavor) to large shear deformations starting from the recently discovered γ-B_(28) boron phase (stable above 9 GPa and 1000 K). We find that continuous deformation along the (100)/⟨001⟩ slip system leads to two new phases, named here as γ-B_(12)–(B_(2))_(6) and γ-B_(12)–(B···B)_(6). We show that these γ-B_(12)–(B_(2))_(6) and γ-B_(12)–(B···B)_(6) phases can also be obtained from uniaxial tensile and compressive deformations of the γ-B28 phase along the ⟨101⟩ direction, respectively. However, the reverse compressive loading on the newly formed γ-B_(12)–(B_(2))_(6) phase transforms it to itself, not the γ-B28 phase, because of the transferability of the three-center two-electron bond under deformation. This makes the new phase γ-B_(12)–(B_(2))_(6) a special type of superelastic material. In addition, application of reverse tensile deformation on the newly formed γ-B_(12)–(B···B)_(6) phase, transforms it to a third new phase, named α-B_(12)–BB, that is metallic, suggesting increased ductility that might make α-B_(12)–BB important for applications in electronic devices. We compared the structural character, mechanical properties, and electronic properties of these new phases to each other and to other phases of boron. We show that the three new phases are dynamically stable at zero pressure. These results show how modifying the connections between boron icosahedra using one to two atom chains can lead to dramatically different mechanical and electronic properties.

ChemInform Abstract: High‐Pressure Close‐Packed Structure of Boron.

AbstractBased on ab initio calculations, a high‐pressure close‐packed phase of boron (B10) with hexagonal P63/mcm symmetry is predicted to be stable over α‐Ga phase above 375 GPa to at least 500 GPa.

High-pressure close-packed structure of boron

Based on ab initio evolutionary algorithms, a high-pressure close-packed phase of boron with hexagonal P63/mcm symmetry is predicted, named as B10, which is stable over α-Ga phase above 375 GPa to at least 500 GPa. High pressure makes the typical B12 icosahedron collapse to form an incompressible linear atomic chain arrangement together with an isosceles triangle arrangement. The electron localization function calculations confirm that the B10 has strong covalency in this special atomic arranged structure. The vibration of the three atom's isosceles triangle in the framework of linear atomic chains induces an unusual superconductivity in B10. Electron–phonon calculations indicate that electron–phonon coupling parameter λ is 0.82 and the superconducting critical temperature is 44 K at 400 GPa.

High pressure synthesis and investigation of single crystals of metastable boron phases

In the present work we have confirmed the existence of the tetragonal boron phase T-50 that was disputed for more than a half of a century. This phase was reproducibly synthesized at high pressures and high temperatures (HPHT) in the form of single crystals and its structure was refined based on single-crystal synchrotron X-ray diffraction data. We also present here a previously unknown rhombohedral boron phase synthesized in a limited HPHT field. Its structure was solved based on in house single-crystal X-ray diffraction data and found to be similar to that of boron carbide. After hitherto known stable α-B, β-B, and γ-B polymorphs, T-50 and the newly obtained rhombohedral boron phases were designated in the present work as δ-B and ϵ-B, correspondingly. From the experimental evidences, both the phases are considered as metastable.