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Understanding the direct transformation from graphite to diamond has been a long-standing challenge with great scientific and practical importance. Previously proposed transformation mechanisms1–3, based on traditional experimental observations that lacked atomistic resolution, cannot account for the complex nanostructures occurring at graphite−diamond interfaces during the transformation4,5. Here we report the identification of coherent graphite−diamond interfaces, which consist of four basic structural motifs, in partially transformed graphite samples recovered from static compression, using high-angle annular dark-field scanning transmission electron microscopy. These observations provide insight into possible pathways of the transformation. Theoretical calculations confirm that transformation through these coherent interfaces is energetically favoured compared with those through other paths previously proposed1–3. The graphite-to-diamond transformation is governed by the formation of nanoscale coherent interfaces (diamond nucleation), which, under static compression, advance to consume the remaining graphite (diamond growth). These results may also shed light on transformation mechanisms of other carbon materials and boron nitride under different synthetic conditions.

From the perspective of high-temperature applications, materials with excellent high-temperature mechanical properties are always desirable. The present work demonstrates that the binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm obtained by high-pressure and high-temperature sintering exhibits excellent mechanical properties at both room temperature and high temperature up to 1000 °C. Specifically, the binder-free nanopolycrystalline WC ceramic still maintains a considerably high Vicker hardness HV of 23.4 GPa at 1000 °C, which is only 22% lower than the room temperature HV. This outstanding thermo-mechanical stability is superior to that of typical technical ceramics, e.g. SiC, Si3N4, Al2O3, etc. Nanocrystalline grains with many dislocations, numerous low-energy, highly stable Σ2 grain boundaries, and a relatively low thermal expansion coefficient, are responsible for the observed outstanding high-temperature mechanical properties.

Traditional ceramics or metals cannot simultaneously achieve ultrahigh strength and high electrical conductivity. The elemental carbon can form a variety of allotropes with entirely different physical properties, providing versatility for tuning mechanical and electrical properties in a wide range. Here, by precisely controlling the extent of transformation of amorphous carbon into diamond within a narrow temperature–pressure range, we synthesize an in situ composite consisting of ultrafine nanodiamond homogeneously dispersed in disordered multilayer graphene with incoherent interfaces, which demonstrates a Knoop hardness of up to ~53 GPa, a compressive strength of up to ~54 GPa and an electrical conductivity of 670–1,240 S m–1 at room temperature. With atomically resolving interface structures and molecular dynamics simulations, we reveal that amorphous carbon transforms into diamond through a nucleation process via a local rearrangement of carbon atoms and diffusion-driven growth, different from the transformation of graphite into diamond. The complex bonding between the diamond-like and graphite-like components greatly improves the mechanical properties of the composite. This superhard, ultrastrong, conductive elemental carbon composite has comprehensive properties that are superior to those of the known conductive ceramics and C/C composites. The intermediate hybridization state at the interfaces also provides insights into the amorphous-to-crystalline phase transition of carbon.

Columbite-rich multiphase TiO2 nanoceramics with outstanding mechanical and dielectric properties were successfully prepared through high-pressure sintering by using anatase-type TiO2 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 TiO2 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 TiO2 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 TiO2 crystal. These results enrich the fundamental knowledge of columbite-rich TiO2, thereby enabling the exploitation of new applications.

Boron carbide (B4C) 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 B4C ceramics. Herein, by spark plasma sintering (SPS) homemade high-purity nanopowders, single phase B4C 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 B4C ceramics.

p-Type BiSbTe alloys possess the highest ZT near room temperature, but inferior mechanical properties due to the atomically layered structure with weak van der Waals interactions. Here we report the successful preparation of porous BiSbTe alloys by sequential high-pressure synthesis, ball milling, and spark plasma sintering. Porosity usually has a negative effect on the material's mechanical properties. However, both thermoelectric and mechanical properties of our porous bulks are greatly enhanced, which benefit from the unique microstructure featuring closely bonded equiaxed grains and uniformly distributed pores (with a porosity of ca. 0.15). The optimal Bi0.42Sb1.58Te3 shows a maximum ZT of 1.27 around 350 K and an average ZT of 1.15 in the measured temperature range of 298–463 K. Furthermore, excellent mechanical properties including hardness, bending and compressive strengths are achieved in these porous samples, which are competitive with (or even better than) those of dense samples. The current structural architecture strategy can promote the development of porous thermoelectric materials, especially materials with similar anisotropic crystal structures.

The design and synthesis of superhard carbon nitrides have always been a focus of attention. Recently, an experiment has revealed the existence of the previously predicted three-dimensional strongl...

传统陶瓷材料的刚性结构变形能力非常有限, 通常在很小的应变下就发生断裂. 具有层状原子结构的陶瓷, 原子层间存在较弱的相互作用, 因而具有大的变形潜力. 我们以氮化硼(BN)为例, 研究了该类陶瓷的室温压缩行为. 分别以洋葱结构BN纳米颗粒和石墨状六方BN纳米片为原料, 采用放电等离子烧结(SPS)技术分别制备了BN-I和BN-II陶瓷材料. 在BN-I中, 随机取向的BN纳米薄片构成三维互锁的结构, 而在BN-II中, BN纳米薄片表现出垂直于SPS压缩方向的择优取向. BN-I的压缩强度为 343 MPa, 断裂应变为4.2%. 相比之下, BN-II的强度和应变分别为 112 MPa 和2.2%. 不同的微观组织结构导致了BN-I和BN-II压缩性能的差异. 此外, 这两种陶瓷材料均表现出1.1%的塑性变形. 该研究表明, 对于具有层状原子结构的陶瓷, 其纳米片作为结构基元构筑成无(或低)择优取向的三维互锁结构, 有望同时提高其强度和变形能力.

Summary New carbon forms that exhibit extraordinary physicochemical properties can be generated from nanostructured precursors under extreme pressure. Nevertheless, synthesis of such fascinating materials is often not well understood. That is the case of the C60 precursor, with irreproducible results that impede further progress in the materials design. Here, the semiconducting amorphous carbon, having band gaps of 0.1–0.3 eV and the advantages of isotropic superhardness and superior toughness over single-crystal diamond and inorganic glasses, is produced from fullerene at high pressure and moderate temperatures. A systematic investigation of the structure and bonding evolution is carried out with complementary characterization methods, which helps to build a model of the transformation that can be used in further high-pressure/high-temperature (high p,T) synthesis of novel nano-carbon systems for advanced applications. The amorphous carbon materials produced have the potential of accomplishing the demanding optoelectronic applications that diamond and graphene cannot achieve.

Ternary boron-carbon-nitrogen (B-C-N) compounds are considered to possess hardness comparable to diamond and thermal stability comparable to c-BN. Explorations for desirable B-C-N phases have been continuous. However, the nonconductive properties of most B-C-N compounds narrow the applications of these compounds. Herein, we propose a sp2-sp3 hybridized phase of t-B2C3N2, which consists of diamond-like BC blocks connected with single N-N bonds. Elastic constants and phonon dispersion curves confirm that t-B2C3N2 is mechanically and dynamically stable. The structure processes 2D metallicity in a strong 3D network. Furthermore, hardness and electron-phonon calculations reveal that t-B2C3N2 is superhard and superconductive with a superconducting critical temperature reaching 2.3 K.

Cubic boron nitride (cBN) exhibits superior hardness and strength as compared with other ceramics that are commonly used as abrasives and cutting tools. The recently synthesized polycrystalline cBN bulk material with ultrafine nanotwin substructures possesses a remarkable combination of high hardness, fracture toughness, and thermal stability. The twin substructure has been demonstrated to exert dislocation-blocking effect similar to grain boundary, leading to strengthening of materials. So far, the synthesis of cBN nanoparticles with ultrafine nanotwin substructures has not yet been realized. Herein, we report on the synthesis of cBN nanoparticles from onion-like boron nitride (oBN) under high pressure and high temperature. Multiple characterization methods, including X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy, revealed that the as-prepared cBN nanoparticles contained high-density nanotwin substructures. The use of the highly wrinkled oBN precursor and well-designed synthetic method were the key to obtain these unique ultrafine nanotwinned cBN nanoparticles.

Sulfur (S) possesses the largest number of allotropes among all elements. Herein, the formation of fibrous S via two different synthesis paths (path 1: S-I → S-II → fibrous S; path 2: S-I → liquid S → a-S → S-II → fibrous S) was investigated through in-situ Raman spectroscopy and synchrotron X-ray diffraction (XRD) analyses in a diamond anvil cell (DAC). Raman spectroscopy revealed the kinetic transformations from S-I to S-II and liquid S to S-II under high temperature and high pressure, and an intermediate amorphous phase was observed during the phase transition of liquid S to S-II. By decompressing S-II to ∼0.9 GPa at room temperature, the fibrous S would be formed and recovered to ambient conditions regardless of the paths. However, the difference of synthesis paths would result in the distinct microstructures and mechanical properties of fibrous S in bulk polycrystalline forms synthesized by a large press. In particular, the bulk sample synthesized from path 2 was composed of numerous aligned ultrafin...

Boron carbide is an extremely hard material extensively used in the industry. In this study, carbon-coated B4C 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 B4C, and high-resolution transmission electron spectroscopy revealed that the carbon-coated B4C nanoparticles exhibited regular B4C-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 B4C/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 B4C ceramics.

Ternary boron-carbon-nitrogen (B-C-N) compounds are considered to possess hardness comparable to diamond and thermal stability comparable to c-BN. Explorations for desirable B-C-N phases have been continuous. However, the nonconductive properties of most B-C-N compounds narrow the applications of these compounds. Herein, we propose a sp2-sp3 hybridized phase of t-B2C3N2, which consists of diamond-like BC blocks connected with single N-N bonds. Elastic constants and phonon dispersion curves confirm that t-B2C3N2 is mechanically and dynamically stable. The structure processes 2D metallicity in a strong 3D network. Furthermore, hardness and electron-phonon calculations reveal that t-B2C3N2 is superhard and superconductive with a superconducting critical temperature reaching 2.3 K.

As one of the largest agricultural countries in the world, China has always paid close attention to the sustainable development of agricultural production efficiency. However, with global climate change, extreme weather has become an exogenous factor that cannot be ignored, as it affects agricultural production. Most of the existing studies only consider the domestic natural resources and economic factors, without fully considering the external climate factors. This paper uses the super undesirable dynamic Slacks-Based Measures (SBM) under an exogenous variable model to simulate the external environmental factors by adding extreme weather days. The Dagum Gini coefficient and kernel density estimation are used to explore the regional differences in agricultural production in China. The results show that the agricultural production efficiency is higher in the eastern region, and the difference in agricultural production efficiency among the provinces in the middle and western regions is large, showing a trend of polarization. The difference in the Gini coefficient between the middle and western regions is more significant. The main contribution factor of the Dagum Gini coefficient is the inter-regional difference. The regional concentration degree of agriculture in China is decreasing, the regional distribution of agricultural water resources is more balanced, and the national regional difference gradually decreases. Finally, some suggestions are put forward, such as extreme weather control, agricultural water supply, and water-saving measures.

Few experimental and theoretical superconductivity works have focused on Li–B systems under the Li-rich regime. Here, we utilized the recently developed particle-swarm optimization method for structural search to propose three novel Li2B phases. These phases include the layered sandwich structure hP6–Li2B (P-3m1, 164) and the two chain structures mS12–Li2B (C2/m, 12) and oC12–Li2B (Cmcm, 63). The structural stability of the three novel Li2B phases was confirmed on the basis of elastic constant and phonon dispersion calculations. Under ambient pressure, the three Li2B structures are notably more thermodynamically stable than the previously proposed Cmcm-Li2B structure and oC12–Li2B may be experimentally synthesized by α-LiB and Li. Electronic band structure and partial density of states analyses showed that our three proposed phases were metallic. The quasi-2D character in the band structure of hP6–Li2B was consistent with that of MgB2 and indicated that hP6–Li2B has potential superconductive properties. Our electron–phonon coupling calculations illustrated that mS12– and oC12–Li2B exhibited superconductivity at 0.03 K under 10 Gpa and 0.14 K under 0 Gpa, respectively. Moreover, we found that hP6–Li2B has low superconductivity with a predicted Tc of 0.09 K at 20 GPa, and its λ and Tc values increased within the pressure range of 20–50 GPa and then decreased under pressures exceeding 60 GPa. The maximum λ and Tc values under 60 GPa were 0.33 and 0.65 K, respectively.

Both boron nitride (BN) and carbon (C) have sp, sp 2 and sp 3 hybridization modes, thus resulting in a variety of BN and C polymorphs with similar structures, such as hexagonal BN (hBN) and graphite, cubic BN (cBN) and diamond. Here, five types of BN polymorph structures are proposed theoretically, inspired by the graphite-diamond hybrid structures discovered in a recent experiment. These BN polymorphs with graphite-diamond hybrid structures possess excellent mechanical properties with combined high hardness and high ductility, and also exhibit various electronic properties such as semi-conductivity, semi-metallicity, and even one- and two-dimensional conductivity, differing from known insulators hBN and cBN. The simulated diffraction patterns of these BN hybrid structures could account for the unsolved diffraction patterns of intermediate products composed of so-called “compressed hBN” and diamond-like BN, caused by phase transitions in previous experiments. Thus, this work provides a theoretical basis for the presence of these types of hybrid materials during phase transitions between graphite-like and diamond-like BN polymorphs.

Three novel sp2-sp3 hybridized BC6N phases with a sandwich structure, including a type of orthorhombic BC6N (o-BC6N) and two types of tetragonal BC6N (t-BC6N-1 and t-BC6N-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-BC6N, t-BC6N-1, and t-BC6N-2 crystals may possess the metallicity with the conducting electrons from the p orbits of sp2-hybridized C atoms. Calculations of electron orbits indicate that the electrons in o-BC6N and t-BC6N-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-BC6N-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-BC6N, t-BC6N-1, and t-BC6N-2 are potential superhard materials with Vickers hardness of 52.4, 45.3, and 40.1 GPa, respectively.

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.

Diamond-like B C compounds exhibit the favorable properties of high hardness and superconductivity. The explorations for novel B C phases are kept on going and it was desirable to explore whether the diamond-like structure can still exist stably or not when the content of boron increases. Herein we predicted a nondiamond-like structure t2-BC took place the lowest energy position of the diamond-like structure, which is completely different from the BCx (x ≥ 2) system. Based on elastic constants and phonon dispersion calculations, the mechanical and dynamic stability of t2-BC were confirmed. Furthermore, hardness and electron–phonon calculations reveal that t2-BC is superhard (Hv = 55.4 GPa) and superconductive with superconducting critical temperature reaching 6.4 K. These results have practical significance for the experimental synthesis of diamond and nondiamond like B C compounds.

The graphitic carbon nitride photocatalyst with a layered melon structure (Melon) has attracted widespread interest. However, this photocatalyst still bears marginal optical absorption in the visible-light region, a high recombination rate of photoexcited charge carriers, and low photocatalytic activity. Here, a pressure-induced nanocrystalline carbon nitride photocatalyst with a distinct and restacked crystal structure of melon (Re-Melon) with highly efficient photocatalytic activity in the long-wavelength region are synthesized. X-ray diffraction, high-resolution transmission electron microscopy, and electron paramagnetic resonance spectroscopy results revealed the crystal structure of nanocrystalline Re-Melon with nitrogen deficiency. Ultraviolet–visible diffuse reflectance spectra, a transient photocurrent response, and valence-band X-ray photoelectron spectroscopy spectra uncovered enhanced light absorption, efficient charge migration, and well-matched energy level. All these properties contribute to excellent photocatalytic hydrogen production performance in visible-light and long-wavelength regions.

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.

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.

We report first-principles calculations of the electronic and mechanical properties of three monoclinic BC8N structures (space group: Pm, No. 6), namely, BC8N-1, BC8N-2, and BC8N-3, by employing a newly developed ab initio particle swarm optimization methodology for crystal structure prediction. The mechanical stability and dynamical stability of these BC8N structures are confirmed based on the calculated results of elastic constants and phonon dispersions. Among the three proposed BC8N phases, BC8N-3 has a negative formation energy of −0.002 eV/atom, indicating that it may be synthesized from diamond and cubic boron nitride. Investigation of their electronic properties shows that all three BC8N phases are semiconductors with an indirect bandgap ranging from 2.52 eV to 4.61 eV. Using a semiempirical microscopic hardness theoretical model, we estimate that the three BC8N phases are potential superhard materials with the Vickers hardness of 75.72, 77.21, and 78.43 GPa. Based on the Pugh criterion, the B/G ratios of the proposed BC8N are 0.92, 0.92, and 0.90, which are all higher than that of diamond (0.83), implying their ductile nature. These multiple excellent properties enable BC8N materials to have wide potential applications as optical and electronic device absorbents, cutting tools, coatings, and so on.

The element carbon possesses various stable and metastable allotropes; some of them have been applied in diverse fields. The experimental evidences of both carbon chain and graphdiyne have been reported. Here, we reveal the mystery of an enchanting carbon allotrope with sp-, sp2-, and sp3-hybridized carbon atoms using a newly developed ab initio particle-swarm optimization algorithm for crystal structure prediction. This crystalline allotrope, namely m-C12, can be viewed as braided mesh architecture interwoven with multigraphene and carbon chains. The m-C12 meets the criteria for dynamic and mechanical stabilities and is energetically more stable than carbyne and graphdiyne. Analysis of the B/G and Poisson’s ratio indicates that this allotrope is ductile. Notably, m-C12 is a superconducting carbon with Tc of 1.13 K, which is rare in the family of carbon allotropes.

The magnetic properties and specific heat have been studied on α-NaCuPO4, an analog of ion-battery material LiFePO4. A ferromagnetic-like transition at 23 K was observed for the first time by the isofield magnetization measurement and confirmed by isothermal magnetization below 23 K. The compound obeys Curie-Weiss law with an antiferromagentic coupling feature. A canted antiferromagentism was suggested to account for the ferromagnetic-like transition. The contribution from the orbital magnetic moment is found to be non-ignorable. Furthermore, specific heat shows that the magnetic transition is broadened and the critical temperature is enhanced under magnetic field, revealing a low dimensionality feature.

An analytical model is developed to estimate the effect of creep deformation on the stress relaxation and distribution in the film/substrate bilayer structure during prolonged exposure to high temperature. In the model, either the film or the substrate subjecting to high-temperature creep is considered. Closed-form solutions for the residual stresses in the film and the substrate are derived. A relationship between the stress relaxation rate in the film/substrate system and the relaxation time is obtained. Case studies show that the ratios of the biaxial modulus and the thickness of the film to those of the substrate, the creep parameters, and the exposure temperature have significant influence on the residual stress relaxation rate.