Search query: superhard

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abstract: Abstract A novel carbon allotrope, denoted P2/m C54, is proposed in this work. The structural properties, mechanical properties, mechanical and dynamical stability, relative formation enthalpy, anisotropy in Young's modulus and electronic properties for P2/m C54 are investigated in this work. First, P2/m C54 is found to remain mechanically and dynamically stable below 100 GPa, with a relative formation enthalpy that is 0.729 eV/atom less than that of T carbon. The bulk modulus, shear modulus and Young's modulus of P2/m C54 are calculated to be 345 GPa, 332 GPa and 754 GPa, respectively; the hardness of P2/m C54 is calculated to be 54.1 GPa and 70.4 GPa using Chen's model and Lyakhov and Oganov's model, respectively. All these calculations suggest that P2/m C54 is a potential superhard material. Furthermore, the band gap of P2/m C54 is determined to be 4.95 eV; in other words, P2/m C54 is an indirect band gap semiconductor material with a wide band gap at ambient pressure.
author_names: Xiaozhen Li

Mengjiang Xing
title: Prediction of a novel carbon allotrope from first principle calculations: A potential superhard material in monoclinic symmetry
year: 2020

abstract: A metastable sp3 bonded carbon allotrope, Penta C20, consisting entirely of carbon pentagons linked through bridge like bonds, was proposed and studied in this work for the first time. Its structure, stability, and electronic and mechanical properties were investigated based on first principles calculations. Penta C20 is thermodynamically and mechanically stable, with equilibrium total energy of 0.718 and 0.184 eV/atom lower than those of the synthesized T carbon and supercubane, respectively. Penta C20 can also maintain dynamic stability under a high pressure of 100 GPa. Ab initio molecular dynamics (AIMD) simulations indicates that this new carbon allotrope can maintain thermal stability at 800 K. Its Young's modulus exhibits mechanical anisotropy. The calculated ideal tensile and shear strengths confirmed that Penta C20 is a superhard material with a promising application prospect. Furthermore, Penta C20 is a direct band gap carbon based semiconducting material with band gap of 2.89 eV.
author_names: Weibin Zhang

Changchun Chai

Qingyang Fan

YanXing Song

Yintang Yang
title: Penta C20: A Superhard Direct Band Gap Carbon Allotrope Composed of Carbon Pentagon
venue: Materials
year: 2020

abstract: Abstract We establish a novel carbon phase in purely sp3 bonded networks with a 14 atom orthorhombic unit cell, termed C14 carbon, by swarm structural searches. This novel carbon phase is dynamically and mechanically stable under ambient conditions. The C14 carbon exhibits a high bulk modulus of 407 GPa and Vickers hardness of 84.3 GPa, which are comparble to those of diamond. Simulated X ray diffraction pattern is in satisfactory agreement with the previously unresolved diffraction peaks found in detonation soot samples. Interestingly, electronic band structure calculations reveal that the C14 carbon is a semiconductor with a direct band gap of around 4.60 eV. These results establish a novel carbon phase with outstanding mechanical and electronic properties and offer insights for further exploring other carbon allotropes.
author_names: Xigui Yang

Chaofan Lv

Shijie Liu

Jinhao Zang

Jinxu Qin

Mingrun Du

Dongwen Yang

Xing Ru Li

Bing-bing Liu

Chongxin Shan
title: Orthorhombic C14 carbon: A novel superhard sp3 carbon allotrope
year: 2020

abstract: Abstract Single phase high entropy monoborides (HEMBs) of the CrB prototype structure have been synthesized for the first time. Reactive spark plasma sintering of ball milled mixtures of elemental precursor powders produced bulk (V0.2Cr0.2Nb0.2Mo0.2Ta0.2)B, (V0.2Cr0.2Nb0.2Mo0.2W0.2)B, and (V0.2Cr0.2Nb0.2Ta0.2W0.2)B HEMB specimens of ~98.3 99.5% relative densities. Vickers hardness was measured to be ~22 26 GPa at an indentation load of 9.8 N and ~32 37 GPa at 0.98 N. In particular, the load dependent hardness of (V0.2Cr0.2Nb0.2Ta0.2W0.2)B is higher than those of ternary (Ta0.5W0.5)B (already considered as superhard) and hardest reported high entropy metal diborides, and on a par with the classical superhard boride WB4.
author_names: Mingde Qin

Qizhang Yan

Haoren Wang

Chongze Hu

Kenneth S Vecchio

Jian Luo
title: High entropy monoborides: Towards superhard materials
year: 2020

abstract: Abstract Materials with superhard and conductive properties are valuable and have great potential applications in multifunctional devices under extreme conditions. Here we propose two carbon polymorphs with superior superhard and conductive properties via first principle calculations. These two carbon phases, called Orth C10 and Orth C10' contain 10 carbon atoms buckled through sp2 sp3 hybridized bonds in an orthorhombic unit cell with Pmmm and Pmm2 symmetry, which are energetically more stable than fullerene C60 at 0 GPa, and more favorable than graphite at the pressure above 34.4 and 45.4 GPa, respectively. More importantly, their Vickers hardness are as high as 62.2 and 59.9 GPa, respectively, close to the hardness of cubic boron nitride, the second hardest material. Specially, Orth C10' owns ultra high axial incompressibility even beyond that of diamond at a pressure above 40 GPa. Furthermore, both Orth C10 and Orth C10' are metallic, which is rare in superhard carbon polymorphs. These outstanding natures make Orth C10 and Orth C10' potential materials for electronic devices and mechanical tools.
author_names: Lingyu Liu

Meng Hu

Zhisheng Zhao

Yilong Pan

Huafeng Dong
title: Superhard conductive orthorhombic carbon polymorphs
year: 2020

author_names: Heng Liu

Qingyang Fan

Fang Yang

Xinhai Yu

Wei Zhang

Sining Yun
title: tP40 carbon: A novel superhard carbon allotrope
year: 2020

abstract: The computational prediction of superhard materials would enable the in silico design of compounds that could be used in a wide variety of technological applications. Herein, good agreement was found between experimental Vickers hardnesses, Hv, of a wide range of materials and those calculated by three macroscopic hardness models that employ the shear and/or bulk moduli obtained from: (i) first principles via AFLOW AEL (AFLOW Automatic Elastic Library) and (ii) a machine learning (ML) model trained on materials within the AFLOW repository. Because $H_{\mathrm{v}\mathrm{ML}$HvML values can be quickly estimated, they can be used in conjunction with an evolutionary search to predict stable, superhard materials. This methodology is implemented in the XtalOpt evolutionary algorithm. Each crystal is minimized to the nearest local minimum, and its Vickers hardness is computed via a linear relationship with the shear modulus discovered by Teter. Both the energy/enthalpy and $H_{\mathrm{v,Teter}\mathrm{ML}$Hv,TeterML are employed to determine a structure's fitness. This implementation is applied towards the carbon system, and 43 new superhard phases are found. A topological analysis reveals that phases estimated to be slightly harder than diamond contain a substantial fraction of diamond and/or lonsdaleite.
author_names: Patrick Avery

Xiaoyu Wang

Corey Oses

Eric Gossett

Davide M Proserpio

Cormac Toher

Stefano Curtarolo

Eva Zurek
title: Predicting superhard materials via a machine learning informed evolutionary structure search
venue: npj Computational Materials
year: 2019

abstract: Two novel superhard 3D sp3 hybridized carbon allotropes (Cmmm C32 and P6/mmm C54) are proposed in this work through first principles calculations. Both Cmmm C32 and P6/mmm C54 have a honeycomb structure. P6/mmm C54 shows a wide direct bandgap (3.564 eV) while Cmmm C32 shows an ultrawide indirect bandgap (5.051 eV) as determined using the HSE06 hybrid functional. The hardnesses of Cmmm C32 and P6/mmm C54 are 83.72 and 54.01 GPa, respectively, and their calculated ideal strengths further confirm their superhardness characteristics. P6/mmm C54 is isotropic in the (001) plane. The minimum thermal conductivities of Cmmm C32 and P6/mmm C54 are larger than those of diamond C, illustrating that Cmmm C32 and P6/mmm C54 are potential semiconductor device materials.Two novel superhard 3D sp3 hybridized carbon allotropes (Cmmm C32 and P6/mmm C54) are proposed in this work through first principles calculations. Both Cmmm C32 and P6/mmm C54 have a honeycomb structure. P6/mmm C54 shows a wide direct bandgap (3.564 eV) while Cmmm C32 shows an ultrawide indirect bandgap (5.051 eV) as determined using the HSE06 hybrid functional. The hardnesses of Cmmm C32 and P6/mmm C54 are 83.72 and 54.01 GPa, respectively, and their calculated ideal strengths further confirm their superhardness characteristics. P6/mmm C54 is isotropic in the (001) plane. The minimum thermal conductivities of Cmmm C32 and P6/mmm C54 are larger than those of diamond C, illustrating that Cmmm C32 and P6/mmm C54 are potential semiconductor device materials.
author_names: Wei Zhang

Changchun Chai

Qingyang Fan

YanXing Song

Yintang Yang
title: Two novel superhard carbon allotropes with honeycomb structures
venue: Journal of Applied Physics
year: 2019

abstract: Superhard materials are of great interest in various practical applications, and an increasing number of research efforts are focused on their development. In this article, we demonstrate that machine learning can be successfully applied to searching for such materials. We construct a machine learning model using neural networks on graphs together with a recently developed physical model of hardness and fracture toughness. The model is trained using available elastic data from the Materials Project database and has good accuracy for predictions. We use this model to screen all crystal structures in the database and systematize all the promising hard or superhard materials, and find that diamond (and its polytypes) are the hardest materials in the database. Our results can be further used for the investigation of interesting materials using more accurate ab initio calculations and/or experiments.
author_names: Efim A Mazhnik

Artem R Oganov
title: Application of machine learning methods for predicting new superhard materials
year: 2020

abstract: Abstract A fabrication route based on aqueous colloidal processing plus transient liquid phase assisted spark plasma sintering (SPS) with Ti Al additives is described for the environmentally friendly obtention of superhard B4C composites reinforced with reduced graphene oxide (rGO) having orthotropic and isotropic microstructures. It is shown that the former, which have coarse rGO platelets preferentially aligned perpendicular to the SPS pressing direction, can be prepared from mixtures of B4C and Ti Al particles with a source of thick, large rGO nanoplatelets by imposing smooth co dispersion conditions to avoid platelet re exfoliation and fragmentation. The latter, which have fine rGO platelets randomly oriented, can be fabricated from mixtures of B4C and Ti Al particles with a source of thin, small rGO nanoplatelets by applying intensive sonication to promote platelet re exfoliation and fragmentation during co dispersion. Finally, it is shown that these orthotropic and isotropic B4C/rGO composites are equally superhard, and that, as expected, their microstructures interact differently with the cracks. Finally, this processing route is simple, and easily adaptable/extensible to make other ceramic/rGO composites with orthotropic and isotropic microstructures.
author_names: Cristina Ojalvo

Rodrigo Moreno

Fernando Guiberteau

Angel L Ortiz
title: Processing of orthotropic and isotropic superhard B4C composites reinforced with reduced graphene oxide
year: 2020