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Improving the poor electrical conductivity of hard materials is important, as it will benefit their application. High-hardness metallic Mo2B was synthesized by high-pressure and high-temperature methods. Temperature-dependent resistivity measurements suggested that Mo2B has excellent metallic conductivity properties and is a weakly coupled superconductor with a T c of 6.0 K. The Vickers hardness of the metal-rich molybdenum semiboride reaches 16.5 GPa, exceeding the hardness of MoB and MoB2. The results showed that a proper boron concentration can improve the mechanical properties, not necessarily a high boron concentration. First-principles calculations revealed that the pinning effect of light elements is related to hardness. The high hardness of boron-pinned layered Mo2B demonstrated that the design of high-hardness conductive materials should be based on the structure formed by light elements rather than high-concentration light elements.

The connection between magnetism and superconductivity has long been discussed since the discovery of Fe-based superconductors. Here, we report the discovery of a pressure-induced transition from a spin to a superconducting state in novel MnN2 based on ab initio calculations. The superconducting state can be obtained in two ways: the first is the pressure-induced transition from an AFM-P21/m to an NM-I4/mmm phase at 30 GPa, while the other is the pressure-induced transition from an FM-I4/mmm phase to magnetic vanishing at 14 GPa, which leads to a structural transition with the distortion of octahedrons to tetragonal pyramids. NM-I4/mmm-MnN2 is superconductive with Tc ≈ 17.6 K at 0 GPa. In the second way, electronic structure calculations indicate that the system transforms from a high-spin state to a low-spin state due to increasing crystal-field splitting, causing disappearance of magnetism; more electron occupancy around the Fermi level drives the emergence of superconductivity. Remarkably, I4/mmm-MnN2 can achieve mutual spin-to-superconducting state transformation by pressure. Moreover, the AFM-P21/m-MnN2 phase is extremely incompressible with the hardness above 20 GPa. Our results provide a reasonable and systematic interpretation for the connection between magnetism and superconductivity and give clues for achieving spin-to-superconducting switching materials with certain crystal features.

The indirect method of obtaining in situ particle velocity of sample, under dynamic high-pressure loading experiments, generates an error due to the influence of rarefaction wave from the sample-wi...

The cold metal transfer (CMT) process, one of wire and arc additive manufacturing, was utilized to fabricate 316L stainless steel. As-built CMT 316L consists of austenitic coarse columnar grains aligned with depositing direction mainly and tiny areas of scattered ferrite (less than 5%) in dendrite. For comparison purposes, both as-built CMT 316L and annealed wrought 316L were dynamically characterized by Split-Hopkinson Pressure Bar. CMT 316L owns a nearly isotropic characteristic in dynamic compressions. Compared to wrought material, dynamic yield strength and flow stress for the CMT 316L are higher at small strains (below 10%) due to the special cellar structure in matrix but drop lower when the strain was larger than 10% for effect of twinning induced plasticity. Last but not least microstructural characterizations from a series of dynamic compressions of CMT 316L show that average twin thickness reaches maximum at strain rates about 1000–1500 s−1 and followed decreases with the increase of strain rates.

The combination of various desired physical properties greatly extends the applicability of materials. Magnetic materials are generally mechanically soft, yet the combination of high mechanical hardness and ferromagnetic properties is highly sought after. Here, we report the synthesis and characterization of nanocrystalline manganese boride, CrB-type MnB, using the high-pressure and high-temperature method in a large volume press. CrB-type MnB shares the specificity of large numbers of unpaired electrons of manganese ions and strong covalent boron zigzag chains. Thus, manganese mono-boride exhibits "strong" ferromagnetic, magnetocaloric behavior, and possesses high Vickers hardness. We demonstrate that zigzag boron chains in this structure not only play a pivotal role in strengthening mechanical properties but also tuning the exchange correlations between manganese atoms. Nontoxic and Earth-abundant CrB-type MnB is much more incompressible and tougher than traditional ferromagnetic materials. The unique combination of high mechanical hardness, magnetism, and electrical conductivity properties makes it a particularly promising candidate for a wide range of applications.

Carbon nano-onions (CNOs), as one of the allotropes of carbon, have attracted great attention because of their excellent performance in many fields, especially in capacitors. Developing soft-chemistry synthesis methods is critically of importance, while the forming mechanism in this area is not clear. In this paper, we present a critical review of CNOs regarding the structure, especially interlayer spacing, and synthesis processes, elaborating the recent progress on soft-chemistry methods. Hansen solubility parameter theory is applied to predict and regulate the solubility of CNOs. This article would be inspirational and give new insights into understanding the formation and properties of CNOs.

A comparative study to assess the abrasive wear performance of steels for concrete mixing drum, coiled at different temperatures, was conducted by MLS-225 wet sand/rubber wheel abrasion wear tester...

Magnetic materials attract great attention due to their fundamental importance and practical application. However, the relatively inferior mechanical properties of traditional magnetic materials limit their application in a harsh environment. In this work, we report an outstanding magnetic material that exhibits both fantastic mechanical and excellent magnetic properties, CuAl2-type Fe2B, synthesized by the high pressure and high temperature method. The magnetic saturation of Fe2B is 156.9 emu g-1 at room temperature and its Vickers hardness is 12.4 GPa which outclasses those of traditional magnetic materials. It exhibits good conductivity with a resistivity of 5.6 × 10-7 Ω m. Fe2B is a promising strong ferromagnetic material with high hardness, which makes it a good candidate for multifunction applications in a harsh environment. The high hardness of Fe2B originates from the Fe-B bond framework, and the strong ferromagnetism is mainly attributed to the large number of unpaired Fe 3d electrons. The competition of Fe 3d electrons to fall into Fe-B bonds or Fe-Fe bonds is the main factor for its magnetism and hardness. This work bridges the chasm between strong ferromagnetism and high hardness communities.

Dwell/interface defeat phenomenon plays a pivotal rule in enhancing the protective ability of ceramic armor. In this study, the transition from dwell to penetration was studied experimentally and numerically, focusing on the dwell time, penetration and damage evolution in ceramic. A series of depth of penetration experiments were conducted of silicon carbide impacted by tungsten–iron–nickel (93%W–Fe–Ni) conical long-rod projectile (LRP) between 900 and 1400 m/s. The transition from interface defeat to extended dwell to penetration of ceramics was observed experimentally. Data analysis suggested that instead of a sharp transition velocity above which dwell phenomenon stopped, dwells with a phase of gradual decreasing time over a range of velocities were observed in experimental work. This observation was then studied by using numerical simulation software AUTODYN with two material models, Johnson–Cook for LRP and witness target, while Johnson–Holmquist for ceramic. The results of numerical simulation agreed well with the corresponding experimental data. Significant influences of nose shape of LRP, projectile material, ratio of each layer of target structure and thickness of back plate on transition from dwell to penetration were investigated numerically. It is shown that dwell/penetration transition predominantly depends on the damage accumulation in ceramic and the performance of back plate. In addition, mass erosion of LRP, rather than the decrease of velocity, functions vitally in affecting the transition when different projectile materials are taken into consideration.

As a typical transition metal nitride and promising spintronic material, CrN has many controversial results because of crystallographic nitrogen defects. One is the bulk softening resulting from electrons transition from localized to itinerated. Hence, well crystalline CrN synthesized is extremely important to clear the structure behavior under pressure. Compressed experiments of well crystalline CrN synthesized were measured up to 30.2 GPa in DAC. Our results show a similar bulk modulus from B1 = 210.7 GPa of Fm-3m phase to B2 = 214.2 GPa of Pnma phase across the whole process of phase transition between 5.3 GPa–8.1 GPa. The X-ray photoemission spectra indicate no remarkable change of electron transferring between Cr and N and no difference of p-d hybridization across the phase transition. It is found that the softening of CrN does not take place during the phase transition despite of the change of the d spin correlation. However, compared to N vacancies that destabilize structure, the nitrogen atoms play a very important role in the magnetic material as magnetic interaction medium ligand atoms and in material elastic properties as well. This work is significant to further understand high pressure behavior of transition metal nitrides.

Poor electrical conductivity severely limits the diverse applications of high hardness materials in situations where electrical conductivities are highly desired. A "covalent metal" TaB with metallic electrical conductivity and high hardness has been fabricated by a high pressure and high temperature method. The bulk modulus, 302.0(4.9) GPa, and Vickers hardness, 21.3 GPa, approaches and even exceeds that of traditional insulating hard materials. Meanwhile, temperature-dependent electrical resistivity measurements show that TaB possesses metallic conductivity that rivals some widely-used conductors, and it will transform into a superconductor at Tc =7.8 K. Contrary to common understanding, the hardness of TaB is higher than that of TaB2 , which indicates that low boron concentration borides could be mechanically better than the higher boron concentration counterparts. Compression behavior and first principles calculations denote that the high hardness is associated with the ultra-rigid covalent boron chain substructure. The hardness of TaB with different topologies of boron substructure shows that besides incorporating higher boron content, manipulating light element backbone configurations is also critical for higher hardness amongst transition metal borides with identical boron content.

The increasing decomposition temperature of solid ammonia borane under high pressure introduces a solid–solid phase transition preceding decomposition. An earlier X-ray diffraction experiment sugge...

The hexagonal-close-packed (hcp) chromium dinitride (CrN2) with high hardness of 46 GPa has been selected as a model system to study the debatable hardness mechanism of AB2 type transition metal nitrides. The stress-strain relationship of hcp-CrN2 under different tensile and shear directions has been extensively studied. Theoretical shear simulation results indicate that the ( 1 0 1 ¯ 0 ) 〈 1 ¯ 2 1 ¯ 0 〉 slip direction of shear mode dominates the plastic deformation of hcp-CrN2, the ideal shear strength is only 30 GPa when the structural instability occurs, demonstrating that the hcp-CrN2 compound is not a superhard material but is indeed a hard material. Furthermore, the electronic calculations show the properties of semiconductors for hcp-CrN2, the atomic deformation mode of hcp-CrN2 indicates that the gradual separation of N2 dimers in the lattice is the main reasons for the decrease of hardness. Besides, our calculation results provide a reference value to judge transition metal nitrides whether can be a superhard material.

The D7b-type structure Mn3B4 was fabricated by high-temperature and high-pressure (HPHT) methods. Hardness examination yielded an asymptotic Vickers hardness of 16.3 GPa, which is much higher than that of Mn2B and MnB2. First principle calculations and XPS results demonstrated that double zigzag boron chains form a strong covalent skeletons, which enhances this structure's integrity with high hardness. Considering that the hardensses of MnB and Mn3B4 are higher than those of Mn2B and MnB2, zigzag and double zigzag boron backbones are superior to isolated boron and graphite-like boron layer backbones for achieving higher hardness. This situation also states that a higher boron content is not the sole factor for the higher hardness in the low boron content transition metal borides. Futhermore, the co-presence of metallic manganese bilayers contribute to the high d-electron mobility and generate electrical conductivity and antiferromagnetism in Mn3B4 which provide us with a new structure prototype to design general-purpose high hardness materials.

Inserting light element atoms into a transition metal host matrix simultaneously produces high hardness and high superconductivity transition temperature. Here, we report the synthesis of a polycrystalline ‘covalent metal’ – Mo3C2 (with space group: P63/mmc) with the high pressure and high temperature (HPHT) method. Direct magnetic susceptibility and electrical resistivity measurements show that Mo3C2 is a typical weakly coupled type-II superconductor with a high critical magnetic field and a superconductive temperature of 8.2 K. Moreover, compared with traditional superconductive materials, the asymptotic Vickers hardness value of Mo3C2 shows impressive high hardness. Its high superconductive temperature originates from its high Debye temperature and high density of states at the Fermi energy level. The high Debye temperature is closely correlated with the hybridization between Mo-4d orbitals and C-2p orbitals, and the metallic feature leads to the high density of states at the Fermi energy level N(EF). The advent of this class of high hardness superconductive materials bridges the gap between high hardness and superconductivity communities.

Wide bandgap semiconductors are crucially significant for optoelectronic and thermoelectric device applications. Metal nitride is a class of semiconductor material with great potential. Under high pressure, the bandgap of magnesium nitride was predicted to grow. Raman spectra, ultra-violet-visible (UV-Vis) absorption spectra, and first-principles calculations were employed in this study to analyze the bandgap evolution of Mg3N2. The widening of the bandgap has been first detected experimentally, with the gap increasing from 2.05 eV at 3 GPa to 2.88 eV at 47 GPa. According to the calculation results, the enhanced covalent component is responsible for the bandgap widening.

Complex boron substructures lead to diversity properties for transition metal borides (TMBs), that provides them many application possibilities in numerous fields. To clarify the actual effect of boron substructures on mechanical, magnetic and electrical properties, we prepared polycrystalline β-FeB samples with zigzag boron chains by high pressure and high temperature. β-FeB exhibits high saturation magnetization (79.54 emu/g), good antioxidant capacity (>800 K), high hardness (15.62 GPa) and low resistivity (3.4×10-6 Ω∙m); thus, it is a promising magnetic material for extreme environmental applications. Subsequently, we performed first-principle calculations combined with X-ray photoelectron spectroscopy analysis and found that the free electrons transferred from Fe atoms stabilize the zigzag boron chains. Spin selection occurs during electron transfer and bonding, with majority spin state electrons as the main participants. The zigzag boron chain substructure provides excellent mechanical properties, at the expense of electrical and magnetic properties. Therefore, we speculate that the spin-selective electrons transfer between the metal and boron substructure can effectively modulate the electrical, mechanical, and magnetic properties of TMBs. This study introduces an effective route for the design, preparation, and applications of high-hardness multifunctional TMBs.

Recent reports exposed an astonishing factor of high hardness that the connection between transition-metal (TM) atoms could enhance hardness, which is in contrast to the usual understanding that TM-TM will weaken hardness as the source of metallicity. It is surprising that there are two opposite mechanical characteristics in the one TM-TM bond. To uncover the intrinsic reason, we studied two appropriate mononitrides, CrN and WN, with the same light-element (LE) content and valence electron concentration. The two high-quality compounds were synthesized by a new metathesis under high pressure, and the Vickers hardness is 13.0 GPa for CrN and 20.0 GPa for WN. Combined with theoretical calculations, we found that the strong correlation of d electrons in TM-TM could seriously affect hardness. Thus, we make the complementary suggestions of the previous hardness factors that the antibonding d-electron state in TM-TM near the Fermi level should be avoided and a strong d covalent coupling in TM-TM is very beneficial for high hardness. Our results are very important for the further design of high-hardness and multifunctional TM and LE compounds.

Hard superconducting materials are of particular interest for electromechanical applications under high strain and stress. In this work, Mo2C was synthesized using high temperature and high pressure (HPHT) methods. In view of characterizations, Mo2C transforms into a superconductivity state below the temperature of 7.5 K and the asymptotic Vickers hardness is about 13.4 GPa. Its hardness is compatible with that of commonly used hard alloys, which is a potential hard superconducting material. Based on the results of our first-principle calculations, superconductivity and hardness come from the high density of states at the Fermi energy level and high hardness originates from the strong hybridization between Mo-4d orbitals and C-2p orbitals. By introducing the light element C atoms into the transition metal Mo to form a ‘covalent metal’ structure, the hardness and mechanical properties of the superconducting material can be improved which gives a way to design hard superconducting materials.

Transition metal (TM) interaction induces many fantastic properties in transition metal light element compounds (TMLEs). Here, we report the synthesis of polycrystalline Mn2B specimen with strong Mn–Mn interaction by high pressure and high temperature (HPHT) method. Magnetic characterization gives anti-ferromagnetic and spin-glass behavior with Neel temperature of 43 ​K. Micro-hardness measurement yields an asymptotic Vickers hardness of 14.1 ​GPa which is far higher than manganese metal. To interpret the mechanical and magnetic properties, the chemical bond and electronic structure were investigated by first principle calculation. Although the distance between manganese atoms has been enlarged by inserting boron atoms, anti-ferromagnetic behavior is attributed to the insufficient Mn–Mn distance enlargement. Based on the bonds character, these manganese atoms layers are pinned together by quasi-boron-chains which is contradictory with fully isolated boron atoms configuration. Quasi-covalent boron chains and partial covalent Mn–B bonds attribute to its high hardness. Tuning Mn–Mn interaction is demonstrated as a promising method to modulate d-electron related magnetic and hardness property.

The stable polymeric nitrogen and polynitrogen compounds have potential applications in high-energy-density materials. For beryllium nitrides, there is one known crystalline form, Be3N2, at ambient conditions. In the present study, the structural evolutionary behaviors of beryllium polynitrdes have been studied up to 100 GPa using first-principles calculations and unbiased structure searching method combined with density functional calculations. One stable structural stoichiometry of beyrllium polynitride has been theoretically predicted at high pressures. It may be experimentally synthesizable at high pressures less than 40 GPa. It is therefore possible to synthesize BeN4 by compressing solid Be3N2 and N2 gas under high pressure and BeN4 may be quenching recoverable to ambient conditions. The predicted high-pressure P21/c-BeN4 compound contains a novel variety of polynitrogen, extended polymeric 3D puckered N10 rings network. To the best of our knowledge, this is the first time that stable N10 rings netw...

In this work, the structures, phase sequence, and metallic properties of K2S have been systematically explored. We confirm that the P63/mmc phase is the best possible candidate for the stable structure of K2S at low pressure range. Although the phases of P63/mmc and Cmcm K2S are semiconductors, two new structures of P6/mmm and Pm1 emerge with metallic characters at high pressures. The analyses of electronic localization functions reveal that the conductivity mainly comes from the electrons surrounding S atom chains, which supplies a potential way to improve the conductivity of sulfur to enhance the electrode recharge ability and rate capability in alkali sulfide battery under pressure.

Alkali metal compounds exhibit novel characteristics under pressure, such as antimetallization in CLi4, high superconductivity at 80 K in highly compressed Li3S, and the existence of unexpected stable stoichiometries of sodium chlorides, etc., which have greatly prompted us to explore KxS compounds at pressure. We found several stable structures with a variety of stoichiometries and proposed a phase diagram on the K-rich side first. Chemical rules established at ambient pressure are frequently violated when high pressure is applied, Na3Cl and NaCl3 as unusual stoichiometries of sodium chloride have been reported in high-pressure conditions. However, KS, with its counterintuitive chemical formula, has been discovered theoretically even at ambient pressure, and possesses the same stability as K2S. The mechanism of superconductivity in Pmm K3S is deeply investigated, comparing with the reported Pmm Li3S. The weak electron–phonon coupling mainly contributes to the weak superconductivity in K3S, which is fully in contrast to the mechanism of interstitial charge localization which dominates the low Tc in the reported Li3S.

Aiming at finding new superconducting materials, we performed systematical simulations on phase diagrams, crystal structures, and electronic properties of vanadium hydrides under high pressures. The VH, VH2, VH3, and VH5 species were found to be stable under high pressures; among these, VH2 had previously been investigated. Moreover, all three novel stoichiometries showed a strong ionic character as a result of the charge transfer from V to H. The electron–phonon coupling calculations revealed the potentially superconductive nature of these vanadium hydrides, with estimated superconducting critical temperature (Tc) values of 6.5–10.7 K for Rm (VH), 8.0–1.6 K for Fmm (VH3), and 30.6–22.2 K for P6/mmm (VH5) within the pressure range from 150 GPa to 250 GPa.

ReB2-type MnB2 has always been considered to be the ground-state structure of MnB2. However, subsequent theoretical study has revealed that this structure is easy to decompose into elemental Mn and B under ambient conditions, which motivated us to look for a stable MnB2 structure at high pressures. Using structure prediction algorithm USPEX and density functional theory calculations, we found a stable multi-layered MnB2 structure with space group Immm at high pressure. The calculated hardness of Immm-MnB2 is 22.5 GPa, which makes it a potential hard multifunctional material along with its conductive and magnetic properties. The hexagonal graphene-like boron networks of Immm-MnB2 contribute to its hardness and stability.

We systematically study the stability, crystal structure, electronic property, and superconductivity of protactinium hydride (PaH n ) (n = 1-9) at a pressure range of 1 atm to 300 GPa by using the first principle of density functional theory. PaH n compounds are very rich, featuring six stoichiometries, such as PaH, PaH3, PaH4, PaH5, PaH8 and PaH9. PaH8 possesses the highly symmetrical crystal structure Fm-3m with cubic H8 units, which is predicted to be thermodynamically stable above 32 GPa. This phase maintains a dynamically stable decompression at 10 GPa. Electron-phonon coupling (EPC) calculations show that Fm-3m-PaH8 exhibits high superconducting critical transition temperature (T c) value of 79 K at 10 GPa due to a strong EPC and large logarithmic average frequency. The T c values of Fm-3m-PaH8 decrease with increasing pressure. Interestingly, superconducting PaH8 appears at low pressure, prompting experimental research.

© 2019, Springer Science+Business Media, LLC, part of Springer Nature. Al/Ni composites are typical structural energetic materials, which have dual functions of structural and energetic characteristics. In order to investigate the influence of manufacturing methods on shock-induced chemical reaction (SICR) behaviour of Al/Ni composites, Al/Ni multi-layered composites with 3–5 cold-rolling passes and Al/Ni powder composites were obtained. Microstructural observation using scanning electron microscopy (SEM) and two-step impact initiation experiments were performed on the four Al/Ni composites. Furthermore, mesoscale simulations, through importing SEM images into the finite element analysis to reflect the real microstructures of the composites, were performed to analyse the particle deformation and temperature rise under shock compression conditions. The experimental results showed the distinct differences on the SICR characteristics among the four Al/Ni composites (i.e. by 3, 4 and 5 cold-rolling passes and powder compaction). The manufacturing methods provided the control of the particle sizes, particle distribution and the content of the interfacial intermetallics at scale of different microstructures, which ultimately affected the temperature distribution, as well as the contact between Al and Ni in Al/Ni composites under shock loading. As a result, the Al/Ni powder composites showed the highest energy release capacity among the four composites, while the energy release capability of Al/Ni multi-layered composites decreased with the growth of rolling passes.

The D7b-type structure Mn3B4 was fabricated by high-temperature and high-pressure (HPHT) methods. Hardness examination yielded an asymptotic Vickers hardness of 16.3 GPa, which is much higher than that of Mn2B and MnB2. First principle calculations and XPS results demonstrated that double zigzag boron chains form a strong covalent skeletons, which enhances this structure's integrity with high hardness. Considering that the hardensses of MnB and Mn3B4 are higher than those of Mn2B and MnB2, zigzag and double zigzag boron backbones are superior to isolated boron and graphite-like boron layer backbones for achieving higher hardness. This situation also states that a higher boron content is not the sole factor for the higher hardness in the low boron content transition metal borides. Futhermore, the co-presence of metallic manganese bilayers contribute to the high d-electron mobility and generate electrical conductivity and antiferromagnetism in Mn3B4 which provide us with a new structure prototype to design general-purpose high hardness materials.