Search

The study of superhard materials plays a critical role in modern industrial applications due to their widespread applications as cutting tools, abrasives, exploitation drills, and coatings. The search for new superhard materials with superior performance remains a hot topic and is mainly considered as two classes of materials: (i) the light-element compounds in the B–C–N–O(–Si) system with strong and short covalent bonds, and (ii) the transition-element light-element compounds with strong covalent bonds frameworks and high valence electron density. In this paper, we review the recent achievements in the prediction of superhard materials mostly using the advanced CALYPSO methodology. A number of novel, superhard crystals of light-element compounds and transition-metal borides, carbides, and nitrides have been theoretically identified and some of them account well for the experimentally mysterious phases. To design superhard materials via CALYPSO methodology is independent of any known structural and experimental data, resulting in many remarkable structures accelerating the development of new superhard materials. [ABSTRACT FROM AUTHOR]

Due to the unique structures and superior chemical and physical properties, onion carbon holds great appeal for various applications, such as lubrication, energy storage and catalysis. However, conventional synthesis methods for onion carbon are often complex, resulting in products of low purity and quantity. Here we report the one-step combustion synthesis of onion carbon with high purity and tunable particle size by employing naphthalene as the precursor, and investigated the effect of combustion temperatures on the formation mechanism of onion carbon, leading to the achievement of controlled preparation of onion carbon. The size of synthesized onion carbon particles ranges from 50 to 190 nm, which increases with the synthesis temperature and shows a wider distribution at higher temperatures. Owing to the abundant oxygen-containing functional groups, as-synthesized onion carbon exhibits prolonged stability and solubility in organic solvents such as ethanol and glycerol. Furthermore, we tested the friction performance of stainless steel coated with onion carbon films by ball-on-disk mode on the upper surface with different loads and sliding speeds. Deposited onion carbon film improved the friction performance of stainless steel at high load and sliding speed. [Display omitted] [ABSTRACT FROM AUTHOR]

As an important phase-change material, GeTe has many high-pressure phases as well, but its phase transitions under pressure are still lack of clarity. It is challenging to identify high-pressure GeTe crystal structures owing to the phase coexistence in a wide pressure range and the reversibility of phase transitions. Hence, first-principles calculations are required to provide further information in addition to limited experimental characterizations. In this work, a new orthorhombic Cmca GeTe high-pressure phase has been predicted via the CALYPSO method as the most energetically favorable phase in the pressure range between ∼30 and ∼38.5 GPa, which would update the GeTe high-pressure phase transition sequence. The crystal structure of the Cmca phase is composed of alternate stacking puckered layers of Ge six-membered rings and Te four-membered rings along the b direction. The high density of states near the Fermi level and delocalization of electrons from the two-dimensional electron localization function indicate a strong metallic property of the Cmca phase. Electron–phonon coupling calculations indicate that the Cmca phase is superconductive below ∼4.2 K at 35 GPa. The simulated x-ray diffraction pattern of the Cmca phase implies that this phase might coexist with the Pnma-boat phase under high pressure. These results offer further understanding on the high-pressure structural evolution and physical properties in GeTe and other IV–VI semiconductors. [ABSTRACT FROM AUTHOR]

We present first-principles calculations of the structural and electronic properties of possible graphitelike BC4N constructed from 3r-graphite structure. Our calculation results show that the hexagonal BC4N structure stacked with one B-N layer and two C-C layers has the lowest total energy among all constructed BC4N structures. The calculated negative formation energy indicates that it may be synthesized from 3r-graphite and 3r-BN. Its stability has been further confirmed by mechanical stability criteria. The electronic properties show that it is a narrow band gap semiconductor. [ABSTRACT FROM AUTHOR]

We report here the high‐pressure and high‐temperature (HPHT) synthesis of well‐sintered B6O–diamond composites from B6O and carbon black nanopowders. The carbon black was transformed into diamond nanograins at HPHT conditions, and simultaneously formed high‐strength B6O–diamond interfaces. The ultrafine B6O and diamond nanograins and the high‐strength B6O–diamond interfaces synergistically construct excellent mechanical properties for the synthesized composites. The B6O–diamond composites possess a hardness (avg. 52 GPa) comparable to that of polycrystalline diamond (40–60 GPa), whereas the fracture toughness (avg. 7.2 MPa m1/2) is increased several times compared to previously synthesized polycrystalline B6O ceramics (1.7–3.1 MPa m1/2) and B6O‐based composites (3–4 MPa m1/2). Fracture behavior analysis demonstrates that the main toughening mechanisms in this B6O–diamond composite are nanotwin toughening, crack deflection, and crack bridging. [ABSTRACT FROM AUTHOR]

• ZrB 2 crystal surface calculations show Zr-terminated surface are more stable. • The work function of Zr-terminated ZrB 2 surfaces varies from 2.56 eV to 4.52 eV. • The Zr-terminated (11 2 ¯ 1) surface of ZrB 2 exhibits low surface energy. • ZrB 2 is a potential emission cathode material. In this study, the surface convergence, surface energy, and work function on various crystal facets of ZrB 2 were investigated by using the density functional theory calculations. Zr-terminated surface exhibits is more stable than the B-terminated surface. The electronic structure calculation reveals that ZrB 2 exhibits metallic property, and the high density of states near the Fermi level is mainly composed of Zr- d and B- p orbitals. The work function values of the Zr-terminated (0001), (10 1 ¯ 0), (11 2 ¯ 0), and (11 2 ¯ 1) surfaces of ZrB 2 are 4.52, 3.83, 4.00, and 2.56 eV, respectively. We found that the surface energy and work function of ZrB 2 (11 2 ¯ 1) are the lowest compared to ZrB 2 (0001), (10 1 ¯ 0) and (11 2 ¯ 0) surfaces, exhibiting good surface energy stability and providing excellent electron emission performance. This work provides valuable theoretical guidance for the exploration of ZrB 2 as a novel cathode material for electron emission. [ABSTRACT FROM AUTHOR]

• Ultrafine-grained B 4 C- c BN composites were firstly fabricated by high pressure sintering. • The phase transition from c BN to h BN is avoided during the sintering process. • The fracture toughness of B 4 C-50 wt.% c BN composite is increased by over two times, compared with either commercial B 4 C. • The Vickers hardness of B 4 C-50 wt.% c BN composite reached up to 36.2 GPa, equivalent to B 4 C ceramics (35–40 GPa). Tough and hard ultrafine-grained B 4 C- c BN composites were firstly fabricated by high-pressure sintering mixed B 4 C and c BN nanopowders at 6 GPa and 1700 °C. The phase transition from c BN to h BN is avoided by high pressure during the sintering process. The effects of the c BN content on the densification and mechanical properties of B 4 C- c BN composites were evaluated. The results indicated that the hardness of the as-fabricated composites increased gradually with the increase of c BN content. The composite composed of 50 wt.% c BN exhibited excellent comprehensive mechanical properties with relative density of 98.6 %, density of 2.9 g/cm3, Vickers hardness of 36.2 GPa and fracture toughness of 6.7 MPa·m1/2. The introduction of superhard c BN maintained the lightweight and high hardness while enhancing the fracture toughness of the B 4 C. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed c BN grains. [ABSTRACT FROM AUTHOR]

Carbon is one of the most fascinating elements due to its structurally diverse allotropic forms stemming from its bonding varieties (sp , sp 2 and sp 3). Exploring new forms of carbon has been the eternal theme of scientific research. Herein, we report on amorphous (AM) carbon materials with a high fraction of sp 3 bonding recovered from compression of fullerene C60 under high pressure and high temperature, previously unexplored. Analysis of photoluminescence and absorption spectra demonstrates that they are semiconducting with a bandgap range of 1.5–2.2 eV, comparable to that of widely used AM silicon. Comprehensive mechanical tests demonstrate that synthesized AM-III carbon is the hardest and strongest AM material known to date, and can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance. [ABSTRACT FROM AUTHOR]

The recent successful synthesis of Pnnm -CN structure inspired researchers to explore new superhard 3D C–N compounds via a similar method, that is, high-pressure and high-temperature experiments involving carbon and nitrogen elements. Naturally, in some past high-pressure experiments involving C–N precursors, whether there are undiscovered C–N compounds in the product is worthy of being re-examined. This work proposed four novel carbon-rich C–N compounds, namely, r -C 17 N 4 , sc -C 4 N, sc -C 17 N 4 –I, and sc -C 17 N 4 -II, which possibly coexist with the unidentified simple cubic carbon phase in the shock compression products of the carbon black and tetracyanoethylene mixture. Most of the X-ray diffraction peaks from the experiment could be understood by the proposed C–N compounds. Furthermore, the calculations of formation enthalpies using experimental materials as precursors show that these four C–N compounds are more easily synthesized than the carbon structures previously used to explain the unknown phase. These phenomena indicate that C–N compounds might be synthesized in this experiment, not just the new carbon phase. The current study may provide a new idea for the experimental research of C–N compounds and inspire further research to reevaluate previous experimental findings. [Display omitted] [ABSTRACT FROM AUTHOR]

• A series of columbite-rich multiphase TiO 2 nanocrystalline ceramics were fabricated through high-pressure and high-temperature method. • The TiO 2 ceramic containing 85.25 % columbite phase has a considerable high Vickers hardness of 12.76 GPa. • The columbite-rich TiO 2 ceramic shows a colossal permittivity (∼8000) and low loss (∼0.2) at 1 kHz and room temperature. Columbite-rich multiphase TiO 2 nanoceramics with outstanding mechanical and dielectric properties were successfully prepared through high-pressure sintering by using anatase-type TiO 2 as precursor. High-pressure sintering combined with phase transformation assisted consolidation can effectively refine the grain size of the recovered samples. This process is conducive to obtain nanocrystalline columbite-rich TiO 2 ceramics with excellent performance. The highest hardness is approximately 12.76 GPa, which is 2.5 times higher than that of ordinary coarse-grained ceramics. The effect of columbite phase on the hardness of multiphase ceramics is discussed. The columbite-rich TiO 2 ceramic shows a colossal permittivity (∼8 × 103) and low loss (∼0.2) at 1 kHz and room temperature, which are superior to that of undoped rutile polycrystalline ceramics. This ceramic shows a steadier frequency-dependent dielectric permittivity and loss than rutile TiO 2 crystal. These results enrich the fundamental knowledge of columbite-rich TiO 2 , thereby enabling the exploitation of new applications. [ABSTRACT FROM AUTHOR]

Boron carbide (B 4 C) is an important structural ceramic material, however, the use of this material is limited by its poor sinterability and low fracture toughness. Nano-sized powders have small particle size, high chemical activity and great sintering ability, which can promote the fabrication of the dense B 4 C ceramics. Herein, by spark plasma sintering (SPS) homemade high-purity nanopowders, single phase B 4 C ceramics with density close to the theoretical value were prepared. Results showed that the best SPS condition was holding at temperature of 1850 °C for 4 min. Under this condition the sample exhibited enhanced mechanical properties including Vickers hardness of 33.9 GPa, fracture toughness of 3.81 MPa m1/2 and flexure strength of 554.7 MPa. Transmission electron microscopy (TEM) analysis shows that non-coherent twins exist at the grain boundary instead of amorphous phase or second phase, which further confirms the high purity and high densification of the obtained B 4 C ceramics. [ABSTRACT FROM AUTHOR]

Recent experiments reported a substantial toughening of nanotwinned diamond (nt-diamond) composite with coherently interfaced diamond polytypes (different stacking sequences). This discovery emphasizes the diversity of graphite-like structures in carbon onion precursors for synthesizing nt-diamond composite. We designed five new graphitic polytypes to investigate structural diversity in graphitic systems. Under ambient pressure, the energies of all the proposed polytypes are between those of graphite (AB structure) and a previously predicted AA structure. Dynamic and elastic stability results showed that four structures (AB'C', AB'D', AAB and ABCB) can be stable under ambient pressure. These four structures all have a large interlayer distance comparable to those in carbon onion and turbostratic graphite. Chiral graphitic structures (AB'D' and AB'C') with reversible electrical properties under slight stress might be responsible for the controversy about the bandgap of graphitic samples. Therefore, these four structures are most likely to be a part of the structures existing in carbon onion and turbostratic graphite or are structural defects in graphitic samples. [ABSTRACT FROM AUTHOR]

The phase transition of fullerene C60 under high pressure and high temperature has been widely studied, but the research on the spark‐plasma sintering of C60 is limited, and the mechanical properties of synthesized materials are still unknown. In this study, a series of amorphous carbon materials were synthesized by spark‐plasma sintering fullerene C60 at different temperatures. The structural characterizations showed that they were composed of multi‐graphene fragments with different sizes, curvatures, and ordering degrees. The densities of the synthesized amorphous carbons were 1.3‐1.4 g/cm3, which were lower than the values for graphite, but the mechanical properties were excellent. The highest compressive strength, indentation hardness, and elastic recovery of the amorphous carbons synthesized at different temperatures could reach up to ~1.25 GPa, ~3.8 GPa, and ~85.5%, respectively, which are far better than the values for commercial isotropic graphite materials. [ABSTRACT FROM AUTHOR]

Herein the ultrafine‐grained (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 high‐entropy diboride ceramics were successfully fabricated by high‐pressure sintering technology for the first time. The results showed that the grain size, relative density, and Vickers hardness of the as‐fabricated samples all increased gradually with increasing sintering temperatures from 1373 K to 1973 K. The relative density and mean grain size of the as‐sintered samples at 1973 K were 97.2% and 684 nm, respectively, and simultaneously they exhibited excellent comprehensive mechanical properties, combining a Vickers hardness of 26.2 GPa and a fracture toughness of 5.3 MPa·m1/2, which were primary attributed to the fine grain strengthening mechanism and microcrack deflection toughening mechanism. [ABSTRACT FROM AUTHOR]

Abstract: Diamond-like B3C10N3 model was built by stacking monolayer B3C10N3 which has the lowest energy. The first-principle calculations were performed by CASTEP code based on density functional theory (DFT) and basal physical properties of the model were investigated. The diamond-like B3C10N3 is semiconducting with direct energy gap of 1.65eV. The results of elastic constants (C ij ) satisfy the Born criteria, showing the mechanical stability. Bulk modulus calculated is about 398GPa, with hardness about 60.6GPa calculated by hardness formula of the covalent crystal, which might be one of superhard materials. The transformation pressure from layered B3C10N3 to diamond-like was proposed at about 25.3GPa. [Copyright &y& Elsevier]

Nanodiamonds represent a burgeoning class of carbon nanomaterials that possess favorable physical and chemical properties useful in abrasives, chemocatalysis, biomedicine, etc. Nowadays, the research on nanodiamonds has developed rapidly, with impressive control over particle size and morphology. However, the synthesis of nanodiamonds with ubiquitous nanotwinned substructure has yet to be realized. Here, we report the synthesis of nanotwin-structured nanodiamond particles from onion carbon precursors (with potassium chloride working as the isolation layer) under high pressure and high temperature conditions. The structural characterizations indicate that the synthetic nanodiamonds contain a nanotwinned substructure within individual particles, with an average twin thickness of ∼5 nm. The current work demonstrates an effective approach to nanodiamond particles with a nanotwinned substructure, which may promote applications in related areas. [ABSTRACT FROM AUTHOR]

Recently, heteroatom co-doped carbon material catalysts have shown extraordinary oxygen reduction reaction (ORR) catalytic activity, but a lack of understanding of the specific mechanism of the ORR has hindered their further development. In this study, we studied the geometric structure, stability, electronic properties, catalytic sites and detailed ORR pathways of P, N co-doped graphene (PN-Gra) by density functional theory (DFT) calculations, trying to reveal the effect of N and P co-doped graphene on the catalytic activity of the ORR. Our calculations indicate that the PN-Gra structure remains stable at high temperature (1000 K) and P, N atoms with their adjacent four carbon atoms constitute the active region to adsorb O2 more efficiently than single phosphorus or nitrogen doped catalysts. Combining energy barrier and free energy diagram analysis, the most favorable pathway is O2* → O* + O* → OH* + O* → 2OH* → OH* + H2O → 2H2O, which is thermodynamically favorable and the associated energy barrier is low, with an overpotential of 0.96 V. After comprehensive analysis and comparison, P, N co-doping does improve the catalytic ability due to the synergistic effect compared to single doping. We believe that this study provides researchers a way to understand the reaction mechanism and ways to improve the catalytic properties of carbon-based materials. [ABSTRACT FROM AUTHOR]

A fully tetrahedrally bonded boron nitride (BN) allotrope with an orthorhombic structure (O-BN) was investigated through first-principles calculations. O-BN has a bulk modulus of 371.8 GPa and a hardness of 66.4 GPa, thereby making it a superhard material with potential technological and industrial applications. O-BN becomes thermodynamically more stable than layered hexagonal BN (h-BN) at pressure above 1.5 GPa and is more favorable than the recently reported Pct-BN at any pressure. The phase transformations from h-BN and BN nanotubes to O-BN were respectively simulated, indicating the feasible synthesis of this superhard phase. [ABSTRACT FROM AUTHOR]

Through particle swarm optimization algorithm and first-principles structural optimizations, we have predicted two novel low-energy structures of BC7: graphite-like Amm2 structure and diamond-like P-4m2 structure. Structural stability of the proposed BC7 polymorphs was confirmed by calculating the elastic constants and phonon frequencies. Phase transition pressure from Amm2 to P-4m2 was determined to be at 2.2 GPa. Calculations for the electronic band structures demonstrated hole-type conductivity of the two novel phases. Ideal tensile strength along the <001> direction for the diamond-like BC7 was 155.2 GPa, which was approximately 52% higher than that of the recently predicted diamond-like BC5. Theoretical Vickers hardness of the diamond-like BC7 was 78.0 GPa, indicating that it is a superhard material. Electron-phonon coupling calculations revealed that diamond-like BC7 was superconducting with a critical temperature of ∼11.4 K. [ABSTRACT FROM AUTHOR]

Nanocrystalline Bi2Te3 bulks were synthesized via a nanostructuring method that included steps of ball milling, cold pressing, and sintering. In the temperature range of 325–525 K, nanocrystalline Bi2Te3 bulks exhibit good thermoelectric properties with a peak ZT of 0.94. The experimental results suggest that the partially coherent boundaries allow the maintenance of low grain growth rate and low resistivity. The enhancement of ZT comes from a large reduction in the phonon thermal conductivity and a strong suppression of the bipolar effect at high temperature. [ABSTRACT FROM AUTHOR]

Based on the detailed analysis of chemical bonds, we present a Vickers hardness expression for the covalency-dominant crystals such as transition-metal carbides and nitrides. Hardness is dependent not only on bond length, bond density, and ionicity of bond [F. M. Gao et al., Phys. Rev. Lett. 91, 015502 (2003)] but also on the metallicity of bond and orbital form in the crystal structure of a compound, and all of these parameters can be determined by first-principles calculations. The calculated hardness using our expression has a good agreement with the experimental values for known monocarbides, mononitrides of transition metals, and cubic Zr3N4 with Th3P4 structure. In addition, we have predicted the Vickers hardness of the recently predicted tetragonal BC3 and tetragonal B2CN, and the recently synthesized pyrite PtN2 and marcasite OsN2. Our method offers one useful technique to search for superhard materials in transition-metal carbides and nitrides. [ABSTRACT FROM AUTHOR]

We report two possible metastable high density BC6N phases originating from the diamond structure. The ground-state properties of the BC6N phases are calculated using first-principles density-functional methods and the Vickers hardness is estimated using a semiempirical microscopic model. The calculated results show that two BC6N phases are semiconductors with Vickers hardness about 79–80 GPa, indicating that the zinc-blende-like BC6N compounds are superhard material and have larger hardness than cubic BC2N and cubic boron nitride. [ABSTRACT FROM AUTHOR]

[Display omitted] Stable hard and conductive ceramics under ambient conditions are vital for many industrial applications. However, maintaining high hardness while achieving semiconducting-metallic transition in Si 3 N 4 materials is greatly challenging both experimentally and theoretically. Herein, two hexagonal Si–C–N structures, denoted SiC 6 N 7 -1 and SiC 6 N 7 -2, were constructed using α -Si 3 N 4 as a matrix model. Calculations of elastic constants and phonon spectra showed stable structures under ambient pressure. Both SiC 6 N 7 -1 and SiC 6 N 7 -2 can possibly be synthesized at the pressures attainable by large-volume press technology. Compared to a -Si 3 N 4 , both Si–C–N ternary compounds achieved a semiconductive-metallic transition owing to the elemental substitution. Property calculations showed conductive Si–C–N compounds with high hardness values of 52 GPa for SiC 6 N 7 -1 and 31 GPa for SiC 6 N 7 -2. Therefore, the introduction of C atoms tuned the intrinsic properties of both structures, achieving high hardness along with good electrical conductivity. Overall, these findings look promising for future design and synthesis paths of novel conductive ceramics with prospective applications. [ABSTRACT FROM AUTHOR]

A series of porous 3D C x (BN) y crystal structures with full sp 2 hybridization are obtained by polymerizing edge-functionalized graphene nanoribbons (GNRs) and boron nitride nanoribbons(BNNRs). Herein, the crystal structure and properties of C x (BN) y are systematically and comprehensively investigated by first-principles calculations, deriving that it is feasible to achieve performance tuning by controlling the nanoribbon width. Thermodynamic stability, these structures have energy advantages with respect to the previously proposed B-C-N structures, and the thermodynamic stability of the structures deteriorates with the increase of the carbon content. Electrical properties, the bandgap value is tunable between 0.1 and 1.1 eV, and the bandgap value increases with the width of BNNRs at the same width of GNRs. Mechanical properties, these structures are mechanically stable, and exhibit unexpected superstretching properties, with tensile lengths up to 2.4 times the original length, bulk modulus tunable between 137-167GPa, shear modulus tunable between 81-115GPa, and the incompressibility of the structure decreases as the nanoribbon width increases. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract By a first-principles approach combined with an evolutionary method, four novel cubic AlN phases (c P16, c F40, c I16, and c I24 AlN) are proposed as promising phases after detailed stability analysis. These four cubic AlN phases exhibit a hardness range of 7.8–17.0 GPa. The anisotropy of their mechanical properties, including Young's modulus, shear modulus, and Poisson's ratio, is systematically studied. Moreover, they are all semiconductors with band gaps smaller than those of experimental phases, and consist of polar covalent Al–N bonds with sp 3 hybridization. Their thermodynamic functions, including enthalpy, Gibbs free energy, and vibrational entropy, are calculated in the temperature range from 0 to 2000 K. Their thermodynamic properties, including Debye temperature and constant-volume specific heat, are investigated. The anisotropy of their minimum thermal conductivities is discussed on the basis of Clarke's model. We hope that this study of the mechanical, electronic, and thermodynamic properties of novel cubic AlN phases will provide guidance for the industrial applications of these four cubic AlN phases. Graphical abstract With thermodynamically, dynamically and mechanically stable, four cubic AlN phases (c P16-, c F40-, c I16-, and c I24-AlN) are proposed as the potential phases. They are all semiconductors with band gaps smaller than that of experimental phases as wz and rs. c I16-AlN is an indirect-gap semiconductor, the others are all direct-gap semiconductors. All the four cubic AlN are consisting of polar covalent Al-N bonds with sp 3 hybridized. They all possess good mechanical properties and low anisotropy, especially c P16-AlN. The systematic study of the mechanical, electronic and thermodynamic properties will provide guidance for the industrial applications of these four cubic AlN phases. Image 1 Highlights • Four cubic AlN phases (c P16, c F40, c I16, and c I24 AlN) are proposed as potential phases via a first-principles approach. • They all possess good mechanical properties (Young's modulus, hardness, etc.) and low anisotropy, especially c P16 AlN. • They are all semiconductors with small band gaps, and consisting of polar covalent Al–N bonds with sp 3 hybridization. [ABSTRACT FROM AUTHOR]

Composites that consist of ceramic and C materials have attracted considerable attention in the microwave absorption field. In this work, we applied a facile and scalable synthesis strategy to prepare B 4 C nanoparticles with core–shell structures. XRD and Raman results confirmed the existence of B 4 C and amorphous C in the core–shell B 4 C nanoparticles. Morphological analysis revealed that the B 4 C nanoparticles exhibited regular morphologies with average diameter of approximately 100 nm and were well encapsulated by amorphous C shells. The C-coated B 4 C nanoparticles fabricated with a B/C ratio of 1:1.25 and with thicknesses of 1.5 mm provided the lowest reflection loss value of −60.8 dB at 15.5 GHz. The C coated B 4 C nanoparticles have considerable potential uses as microwave absorbers given their facile preparation and excellent microwave absorption performance. B 4 C nanoparticles with core‒shell structures were prepared through a facile and scalable synthesis strategy. Morphological analysis indicated that the B 4 C nanoparticles displayed regular morphology with average diameter of approximately 100 nm and were well encapsulated by an amorphous C shell. C-coated B 4 C nanoparticles with appropriate submicron amorphous carbon layer and thickness of 1.5 mm provided the lowest RL value of −60.9 dB. In addition, the C-coated B 4 C nanoparticles have good thermal stability and can be used at high temperature. The C-coated B 4 C nanoparticles prepared in this study have considerable potential applications as microwave absorbers given their facile synthesis and excellent microwave absorption performances. Image 1 [ABSTRACT FROM AUTHOR]

Abstract Boron carbide is an extremely hard material extensively used in the industry. In this study, carbon-coated B 4 C nanoparticles were synthesized through a one-step synthesis method using high-temperature firing cheap and readily available raw materials, namely, boric acid and sucrose. X -ray diffraction and Raman spectra analysis indicated the formation of B 4 C, and high-resolution transmission electron spectroscopy revealed that the carbon-coated B 4 C nanoparticles exhibited regular B 4 C-C core-hell structure with an average particle diameter of around 100 nm. This method is simple and time-saving, and can be adopted for the large-scale production of the core-shell nanoparticle. Directly using these core-shell structural nanoparticles as precursor, high-performance B 4 C/C ceramics with hardness reaching 34 GPa, fracture toughness reaching 3.3 MPa m1/2 were synthesized. This study has important implications in developing high-performance B 4 C ceramics. Highlights • A new method to prepare B 4 C-C core-hell nanopowders is proposed. • Grain growth of the nanopowders will not occur with the increase of temperature. • We synthesized high-performance B 4 C/C ceramics by using these nanopowders. [ABSTRACT FROM AUTHOR]

Utilizing the crystal structure prediction method (CALYPSO), a tetragonal B C O compound ( t I12 - B 6 C 4 O 2 with I 4 ¯ m2 symmetric structure) was predicted. Computed formation enthalpies, elastic constants and phonon dispersion spectra certify that t I12-B 6 C 4 O 2 is thermodynamically, dynamically and mechanically stable. Our results indicate that t I12 - B 6 C 4 O 2 has large mechanical moduli and high hardness (21.9 GPa). The directional dependences of the Young’s modulus, shear modulus and Poisson’s ratio have been visualized to analysis the mechanical anisotropy. The calculated band structure and partial density of state revealed that t I12 - B 6 C 4 O 2 is a typical for conductor with sp 3 hybrid B C and B O covalent bonds. [ABSTRACT FROM AUTHOR]

Three novel sp 2 - sp 3 hybridized BC 6 N phases with a sandwich structure, including a type of orthorhombic BC 6 N ( o -BC 6 N) and two types of tetragonal BC 6 N ( t -BC 6 N-1 and t -BC 6 N-2), are investigated through first principles calculations. The structural stabilities are confirmed by the calculated elastic constants and phonon dispersions. Calculated electronic band structures, density of states (DOS), and partial DOS show that the o -BC 6 N, t -BC 6 N-1, and t -BC 6 N-2 crystals may possess the metallicity with the conducting electrons from the p orbits of sp 2 -hybridized C atoms. Calculations of electron orbits indicate that the electrons in o- BC 6 N and t -BC 6 N-1 structures can conduct through the π bonds along the orientation parallel to the [1 0 0] and [0 1 0] directions in different layers. Moreover, the electrons in t -BC 6 N-2 structure can conduct along the orientation parallel to the [1 1 0] and [ 1 ¯  1 0] directions in different layers. The behavior of the linear electron conductivity in the layer and vertical direction of conduction between the adjacent layers imply that the three kinds of crystals have potential applications in the field-effect devices. Calculation results using the semi-empirical microscopic model show that o -BC 6 N, t -BC 6 N-1, and t -BC 6 N-2 are potential superhard materials with Vickers hardness of 52.4, 45.3, and 40.1 GPa, respectively. [ABSTRACT FROM AUTHOR]

Carbon materials with full sp2-hybridized bonding, e.g. zero-dimensional (0D) fullerenes, 1D carbon nanotubes, and 2D graphene, possess outstanding and unparalleled properties, and have unique scientific and technological importance. The theoretical design and experimental exploration of other types of novel sp2 carbon allotropes, especially with 3D architectures, is always a compelling scientific theme. Here we proposed a class of low-energy 3D sp2 carbons with exceptional properties, not only possessing excellent mechanical properties such as high 3D strength, rubber-like ultra-stretchability, and negative Poisson's ratio, but also including the electronic properties of graphite-like metallicity and graphene-like Dirac cone, which are the desirable properties across a broad range of potential applications. Furthermore, a design route was suggested to access these 3D sp2 carbons by the polymerization of edge-functionalized graphene nanoribbon arrays. [ABSTRACT FROM AUTHOR]

A new sp- sp -hybridized tetragonal carbon allotrope, namely Tetra-carbon, is predicted through the evolutionary particle swarm structural search. Tetra-carbon has a 3D framework composed of sp carbon helixes connected by linear sp carbon chains, similar to the interconnected network of propadienyl groups, which forms the well-proportioned microporous structure. Tetra-carbon is thermodynamically more stable than known graphdiyne and carbyne carbon and also shows mechanical and dynamic stabilities at ambient pressure. Tetra-carbon is a semiconductor with an indirect band gap of 3.27 eV and has anisotropic tensile strengths with an unexpected large tensile strain of 0.64 along the [001] direction. Base on the analysis of Poisson's ratios as well as the tensile strains, it is significantly revealed that Tetra-carbon is a mechanically ductile microporous carbon allotrope in contrast with the known brittle carbons such as diamond, potentially applied in the fields where the ductile metals are available. [ABSTRACT FROM AUTHOR]

Although various kinds of carbon nitride precursors have been proposed, s-triazine-based structures are hardly reported because of their unfavorable energy, higher than that of heptazine-based ones. In this study, we investigate the thermal stability of s-triazine-based melam processed at a high pressure of 5 GPa and a temperature of 400-700 °C and complete the analyses of the composition and structure of the treated samples through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and elemental analyses (EA). Results show that melam can stably exist up to 600 °C at 5 GPa. XRD and FTIR analyses reveal that residual melamine can be pyrolyzed into melam as temperature increases from 400 °C to 600 °C at a high pressure, suggesting that melam may be purified through high-pressure pyrolysis. Further melam polymerization at a higher pressure is a promising strategy for the preparation of s-triazine-based carbon nitride precursors used for bulk carbon nitride synthesis. [ABSTRACT FROM AUTHOR]

Carbon can be metal, semiconductor or insulator depending on its structures, and its conductivity can be one-dimensional (1D), 2D, and 3D correspondingly. Boron Nitride (BN) is isoelectronic to carbon, but one prominent difference between the two systems is their electronic properties. For many years, the synthesized and predicted BN allotropes were always insulators, irrespective of their structures. In this paper, with the help of structures searching based on first-principles calculations, a sp 2 / sp 3 -hybridized metallic monoclinic 3D BN ( M -BN) structure is proposed. M -BN can be viewed as puckered hexagonal-BN (h-BN) layers bucked by partial sp 3 B N bonds. The electronic property analysis revealed that the metallicity of M -BN is attributed to the delocalized 2 p electrons of the sp 2 B and N atoms. The metallic B atoms and N atoms are aligns in two arrays along a 1D axial direction, resulting in the unusual 1D dual-threaded conduction in 3D M -BN structure. The enthalpy calculation have revealed that M -BN is the most energetically favorable structure among the predicted metallic BN structures so far, and it might be obtained via compressing layered h-BN precursor theoretically. Due to the 3D strong framework, M -BN has an estimated Vickers Hardness of 33.7 GPa, indicating it is a potential hard material with novel 1D conduction. [ABSTRACT FROM AUTHOR]

Carbon allotropes with distinct sp , sp 2 , and sp 3 hybridization possess various different properties. Here, a novel all- sp 3 hybridized tetragonal carbon, namely the P carbon, was predicted by the evolutionary particle swarm structural search. It demonstrated a low density among all- sp 3 carbons, due to the corresponding distinctive microporous structure. P carbon is thermodynamically stable than the known C 60 and could be formed through the single-walled carbon nanotubes (SWCNTs) compression. P carbon is a direct bandgap semiconductor displaying a strong and superhard nature. The unique combination of electrical and mechanical properties constitutes P carbon a potential superhard material for semiconductor industrial fields. [ABSTRACT FROM AUTHOR]

The present study examined the effects of pressure (range: 1-6 GPa) on microstructure and mechanical properties of pure Zr. Pressure significantly affected refining of Zr microstructure. When 5 GPa pressure was applied, ω-phase was observed in processed specimen, and volume fraction sharply increased to 57.4% for specimen pressurized-quenched at 6 GPa. Benefitting from refinement of acicular-shaped α (α′) plates and the formation of equiaxial ω-phase, the yield strength of the sample quenched from 6 GPa reached ~616 MPa, which is almost twice as that of coarse-grained Zr. [ABSTRACT FROM AUTHOR]

The ω-zirconium (ω-Zr) is generally considered as the favorable strengthening phase in the application of Zr and its alloys. However, the intrinsic mechanism is still unclear. In this study, the elastic constants and anisotropic ideal shear strengths of ω-Zr and the common α-Zr phase are systematically studied by the first-principle calculations. Results reveal that the shear modulus of ω-Zr is 1.37 times that of α-Zr. The lowest shear strength of ω-Zr is 3.25 GPa along ( 1 0 1 ¯ 0 ) [ 0 0 0 1 ] , which is about 1.6 times the weakest shear strength (2.01 GPa) of α-Zr along ( 1 0 1 ¯ 0 ) [ 1 ¯ 2 1 ¯ 0 ] . In addition to the high strength, the critical strain of ω-Zr is 0.173, much larger than that (0.072) of α-Zr. The electron localization function (ELF) result shows that there are more electrons highly localized in ω-Zr than in α-Zr along their weakest shear directions, indicating the stronger metallic bonds of ω-Zr as the origin of the high shear strength and strain. In addition, based on the von Mises criteria, the dominating deformation mode for the polycrystalline material of ω-Zr is proposed along the { 1 0 1 ¯ 0 } 〈 0 0 0 1 〉 / { 1 0 1 ¯ 1 ¯ } 〈 1 ¯ 2 1 ¯ 0 〉 or { 1 1 2 ¯ 0 } 〈 0 0 0 1 〉 / { 1 0 1 ¯ 1 ¯ } 〈 1 ¯ 2 1 ¯ 0 slips, which is helpful to the understanding of deformation behavior in polycrystalline ω-Zr. [ABSTRACT FROM AUTHOR]

Carbon possesses various allotropes with diverse electronic properties from insulation to conduction due to the sp , sp 2 , and sp 3 bonding types. Here, a new sp 2 - sp 3 carbon allotrope, called C 48 , is proposed. C 48 presents a lilac-like structure with a cage-like center and four petal-like tubes around, and can be formed through compressing single-walled carbon nanotubes. C 48 is not only strong and hard in 3D but also has unexpected 1D conduction along eight divided linear pathways in the unit cell. This multithreaded 1D conduction in 3D framework is peculiar in carbon materials, and would be helpful to design singular electronic nanodevices. [ABSTRACT FROM AUTHOR]

A lonsdaleite-like orthorhombic structure B 2 CO ( o P8-B 2 CO) with strong sp 3 covalent B‐C and B‐O bonds has been predicted theoretically by using the crystal structure prediction package CALYPSO. The mechanical, dynamical and thermodynamic stabilities of o P8-B 2 CO are verified by independent elastic constants, phonon dispersion spectrum and formation enthalpy, respectively. Pressure may promote synthesis of o P8-B 2 CO, which is demonstrated by calculating the relationship of formation enthalpies for o P8-B 2 CO via pressure. Based on bond resistance model for hardness, o P8-B 2 CO has a high hardness value of 47.70 GPa. Band structures calculation illustrates that all B 2 CO phases are semiconducting with indirect band gaps and o P8-B 2 CO has the widest band gap (3.540 eV) relative to t P4-B 2 CO (1.658 eV) and t I16-B 2 CO (2.988 eV). The superhardness and tunable band gap open an extensive industrial application and scientific research for B 2 CO compounds. [ABSTRACT FROM AUTHOR]

By utilizing an evolutionary methodology on crystal structure search, we propose three new metastable phases for aluminum arsenide (AlAs) as follows: (1) a P 6 4 22 symmetric structure ( h P6-AlAs), (2) a C 222 symmetric structure ( o C12-AlAs), and (3) a I 4 ¯ 3 d symmetric structure ( c I24-AlAs). By controlling the unloading pressure rate, o C12-, h P6-, and c I24-AlAs may be acquired through quenching NiAs-AlAs. The elastic constants and phonon dispersion spectra are calculated to certify the mechanical and dynamic stabilities of three newly discovered phases. On the basis of first-principle study, we explore phase transformations under pressure for several AlAs polymorphs. The calculation of mechanical properties illustrates that o C12-, and h P6-AlAs possess similar hardness levels, which are higher than that of c I24-AlAs. Meanwhile, o C12-, and h P6-AlAs hold similar shear anisotropic factors, which are smaller than that of c I24-AlAs. Electronic band structure calculation reveals that at zero pressure, o C12-, and h P6-AlAs possess indirect band gaps of 0.468 eV and 1.356 eV, respectively. c I24-AlAs is a direct semiconductor with a gap value 1.761 eV. [ABSTRACT FROM AUTHOR]

Recently, the structural fragments of the 2H, 4H, 9R, and 15R diamond polytypes were found in the synthesized diamond crystals by high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM). This work mainly uses onion carbon as a raw material, a complex layered structure, thus attracting us to focus on the influence of precursor structure on the formation of diamond polytypes. Herein, we selected nine graphitic structures with different stacking sequences from previous work to study their phase transition under different compression methods using DFT (density functional theory) calculations. Most of them appeared as interlayer slips during compression and finally produced 3C, 2H, and 9R diamond polytypes. The hexagonal character unit (h) was derived only from the initial AA stacking layer in the graphite precursor after analyzing the phase transition process. Meanwhile, the design of a suitable preparation scheme that maximizes the retention of stacking faults from the precursor into the product is the key to the synthesis of non-3C(cubic) diamonds. Thus, this study provides insights into the high-pressure transformation of graphite systems with diverse structures and illustrates the reasons why non-3C diamonds are rare. [Display omitted] • Nine graphitic structures were selected to study their phase transition. • The AAB graphitic structure can transform into a 9R polytype or 3C diamond. • AA stacking and ultra-fast pressurization will contribute to synthetize polytypes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Detailed structural stability and properties of 4^3T175 were performed. • The Vickers hardness of 4^3T175 is 73 GPa. • 4^3T175 may be one of the products in the detonation soot. We present a systemical study on the stability, electronic and mechanical properties of a sp 3-bonded carbon allotrope with 4^3T175 topology. This carbon allotrope is composed of a C-centered orthorhombic unit cell with 20 atoms and is mechanically and dynamically stable over a pressure range from ambient conditions to 50 GPa. According to the calculations, 4^3T175 is an indirect band gap semiconductor with band gap of 2.84 eV and also a potential superhard material with Vickers hardness of 73 GPa. The simulated X-ray diffraction pattern can be matched to some of the diffraction peaks of those unidentified carbon phases in detonation soot samples, indicating its possible presence in these specimens. These findings yield a deeper understanding of 4^3T175 and provide a guidance to elucidate the unidentified carbon phase in the high-pressure synthesis products. [ABSTRACT FROM AUTHOR]

Carbon clathrate is a kind of typical carbon allotrope that has an open framework composed of various types of cages, displaying intriguing electronic and mechanical properties. This paper proposes a superhard superstrong carbon clathrate via first-principle calculations. This carbon clathrate, called C 60 clathrate, contains 60 carbon atoms buckled through sp 3 -hybridized bonds in a cubic unit cell with symmetry of Im-3m , and possesses the network topology of commonly called binodal net. C 60 clathrate is energetically more stable than fullerene C 60 at ambient pressure, and it is more favorable than graphite at a pressure above 50.1 GPa C 60 clathrate can be constructed by small C 24 cage and flat C 18 drum. It shows a high density of 3.34 g/cm 3 , which is the densest carbon clathrate to date. The estimated Vickers hardness, tensile strength, and shear strength of C 60 clathrate reach remarkably high values of 82.8, 90.7, and 76.4 GPa, respectively, indicating its superhard and superstrong characteristic. Band structure calculations indicate that C 60 clathrate is semiconductive with a direct band gap of 2.26 eV. C 60 clathrate can be potentially used in photovoltaic devices. [ABSTRACT FROM AUTHOR]

Four novel AlN phases with low density and high hardness are predicted by using a evolutionary methodology on structural search. All four phases are thermodynamically more favourable than the rock salt structure of AlN at ambient pressure and will be transformed to rock salt AlN at certain pressure. All these phases are mechanically and dynamically stable by checking the independent elastic constants and phonon dispersion spectra. The calculated mechanical properties reveal that the proposed AlN phases are hard, and the Vickers hardness of the AlN phases ranges from 13.2 GPa to 15.2 GPa. The calculated band structures of the novel AlN phases reveal that these AlN phases possess semiconductive properties with wide direct band gaps at G points, and the gaps range from 3.627 eV to 3.927 eV. [ABSTRACT FROM AUTHOR]

Searching for novel functional carbon materials is an enduring topic of scientific investigations, due to its diversity of bonds, including sp-, sp2-, and sp3-hybridized bonds. Here we predict a new carbon allotrope, bct-C12 with the body-centered tetragonal I4/mcm symmetry, from the compression of carbon nanotubes. In particular, this structure behaviors as the Dirac fermions in the kz direction and the classic fermions in the kx and ky directions. This anisotropy originates from the interaction among zigzag chains, which is inherited from (n, n)-naotubes. [ABSTRACT FROM AUTHOR]

Four three-dimensional graphdiyne polymers, namely Tri-C 18 , Hex-C 36 , Tri-C 54 , and Orth-C 72 , are predicted with first principles calculations. These carbon allotropes are constituted with interlinked sp 3 -hybridized carbon pillars and sp 2 -hybridized graphene-like nanoribbons. Graphdiyne polymers are energetically more stable than C 60 and graphdiyne, and show lower phase transition energy barriers from AA-graphdiyne than that of graphite–diamond transition. Except for the semimetallic Tri-C 18 , these polymers show narrow direct (or quasi-direct) band gap of 1.55–1.74 eV. The porous structure and the high in-plane Young’s modulus of graphdiyne sheet are inherited by these polymers. Meanwhile, a greatly enhanced Young’s modulus as high as ca . 1 TPa along the direction perpendicular to graphdiyne sheet and high hardness are developed in these systems. [ABSTRACT FROM AUTHOR]

Exploration of novel hard metastable phases of silicon nitride was performed using a recently developed particle-swarm optimization method within the CALYPSO software package. Three potential hard metastable phases of t -Si 3 N 4 , m -Si 3 N 4 , and o -Si 3 N 4 were predicted. These phases are mechanically and dynamically stable at ambient pressure based on their elastic constants and phonon dispersions. t -Si 3 N 4 and m -Si 3 N 4 exhibit lower energies than γ -Si 3 N 4 at pressures below 2.5 GPa and 2.9 GPa, respectively, which promise that the formers could be obtained by quenching from γ -Si 3 N 4 . o -Si 3 N 4 is a better high-pressure metastable phase than CaTi 2 O 4 -type Si 3 N 4 proposed by Tatsumi et al. and it can come from the transition of γ -Si 3 N 4 under 198 GPa. The theoretical band gaps of t -Si 3 N 4 , m -Si 3 N 4 , and o -Si 3 N 4 at ambient pressure were 3.15 eV, 3.90 eV, and 3.36 eV, respectively. At ambient pressure, the Vickers hardness values of t -Si 3 N 4 (32.6 GPa), m -Si 3 N 4 (31.5 GPa), and o -Si 3 N 4 (36.1 GPa) are comparable to β -Si 3 N 4 and γ -Si 3 N 4 . With the pressure increasing, t -Si 3 N 4 , m -Si 3 N 4 , and o -Si 3 N 4 will change from the brittle to ductile state at about 15.7 GPa, 7.3 GPa and 28.9 GPa, respectively. [ABSTRACT FROM AUTHOR]

By immersion of macroporous polystyrene beads covalently bound with negatively charged sulfonate groups in tungstic acid and subsequent carbonization in presence of sulfur vapor, the polystyrene-derived porous carbon (PDPC) with uniformly embedded WS 2 nanoparticles of ∼20 wt% (denoted as PDPC/WS 2 ) was fabricated as an anode material for Li-ion batteries. Compared to the pure PDPC, the uniform chemical vapor synthesized embedment of WS 2 nanoparticles lead to the highly improved electrochemical performance. At the high current rate of 1 A/g, it delivers the continuous increase in capacity with cycling and ∼100% Coulumbic efficiency over more than 1000 cycles, and reversible capacity of 282 mAh/g is retained after the 1220 cycles. By tuning again up to 2 A/g, the PDPC/WS 2 anode can maintain the impressive capacity retentions of 81% and 70% after 1000 and 2000 more cycles, respectively. The cycling test of the PDPC/WS 2 anode at the high rate of 4 A/g demonstrate the cycling stability over more than 7000 cycles. [ABSTRACT FROM AUTHOR]

Six novel superhard BC 4 N- x (x = a , b , c , d , e , f) structures with low energies are determined by ab-initio evolutionary methodology using crystal structure analysis by particle swarm optimization (CALYPSO) code. These BC 4 N polymorphs employ a sp 3-hybrid network with BN and C layers connected alternatively, and with low energies close to those of the previously proposed lowest energy c -BC 4 N. The possible precursors and phase transition paths from hBC 4 N- x to novel BC 4 N- x (x = a , b , c , d , e , f) are also determined, and the energies of the hBC 4 N- x are close to each other under ambient pressure. Moreover, simulated XRD patterns of these novel BC 4 N structures show agreement with experimental data. The Raman main peak positions and intensities of these structures are also consistent. All indications point out the experimentally synthesized samples likely contain various BC 4 N structures. [Display omitted] • Six novel BC 4 N structures with superhard semiconductors are proposed via first principle calculation. • The simulated XRD patterns of novel BC 4 N structures show agreement with experimental data. • The possible precursors and phase transition paths from hBC 4 N- x to novel BC 4 N- x (x = a , b , c , d , e , f) are also determined. [ABSTRACT FROM AUTHOR]

Abstract: Nanocrystalline TiC–ZrC solid solutions (Ti x Zr1−x C1−δ ) with densification of ~100% have been successfully prepared by spark plasma sintering. Due to the difference in ionic radii of Ti and Zr, the required sintering temperature for single-phase solid solutions exhibits an asymmetric dependence on x, being highest at x=0.7. At a temperature lower than the consolute one, the two-phase solid solution is formed, exhibiting the ability of grain size refinements to nanoscale. Due to the alternate reinforcements and the grain size refinements in TiC–ZrC solid solutions, the mechanical properties of Vickers hardness and fracture toughness are significantly increased when compared to TiC and ZrC. [Copyright &y& Elsevier]

Abstract: Recently, a new family of two-dimensional (2D) MXene materials was prepared by exfoliating the MAX phases (ACS Nano 2012, 6, 1322). Among all possible MXene phases, theoretically 2D Sc2C possesses the highest surface area per weight and thus is expected to have the highest gravimetric hydrogen storage capacities. In this work, using first-principles total energy pseudopotential calculations, we systematically investigated the hydrogen storage properties of 2D Sc2C phase. Depending on different adsorption sites, the hydrogens are bound by three modes: chemisorption, physisorption and Kubas-type interactions with the binding energies of 4.703, 0.087 and 0.164 eV respectively. The maximum hydrogen storage capacity was calculated to be 9.0 wt.%, which meets the gravimetric storage capacity target (5.5 wt.% by 2015) set by the U.S. DOE. Ab-initio molecular dynamic simulations confirmed that 3.6 wt.% hydrogen molecules storaged by Kubas-type interactions can be adsorbed and released reversibly at ambient conditions. [Copyright &y& Elsevier]