Twisted-layer boron nitride ceramic with high deformability and strength.

Moiré superlattices formed by twisted stacking in van der Waals materials have emerged as a new platform for exploring the physics of strongly correlated materials and other emergent phenomena1-5. However, there remains a lack of research on the mechanical properties of twisted-layer van der Waals materials, owing to a lack of suitable strategies for making three-dimensional bulk materials. Here we report the successful synthesis of a polycrystalline boron nitride bulk ceramic with high room-temperature deformability and strength. This ceramic, synthesized from an onion-like boron nitride nanoprecursor with conventional spark plasma sintering and hot-pressing sintering, consists of interlocked laminated nanoplates in which parallel laminae are stacked with varying twist angles. The compressive strain of this bulk ceramic can reach 14% before fracture, about one order of magnitude higher compared with traditional ceramics (less than 1% in general), whereas the compressive strength is about six times that of ordinary hexagonal boron nitride layered ceramics. The exceptional mechanical properties are due to a combination of the elevated intrinsic deformability of the twisted layering in the nanoplates and the three-dimensional interlocked architecture that restricts deformation from propagating across individual nanoplates. The advent of this twisted-layer boron nitride bulk ceramic opens a gate to the fabrication of highly deformable bulk ceramics.

Structural transition and migration of incoherent twin boundary in diamond.

Grain boundaries (GBs), with their diversity in both structure and structural transitions, play an essential role in tailoring the properties of polycrystalline materials1-5. As a unique GB subset, {112} incoherent twin boundaries (ITBs) are ubiquitous in nanotwinned, face-centred cubic materials6-9. Although multiple ITB configurations and transitions have been reported7,10, their transition mechanisms and impacts on mechanical properties remain largely unexplored, especially in regard to covalent materials. Here we report atomic observations of six ITB configurations and structural transitions in diamond at room temperature, showing a dislocation-mediated mechanism different from metallic systems11,12. The dominant ITBs are asymmetric and less mobile, contributing strongly to continuous hardening in nanotwinned diamond13. The potential driving forces of ITB activities are discussed. Our findings shed new light on GB behaviour in diamond and covalent materials, pointing to a new strategy for development of high-performance, nanotwinned materials.

Coherent interfaces govern direct transformation from graphite to diamond.

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.

Ultrastrong conductive in situ composite composed of nanodiamond incoherently embedded in disordered multilayer graphene.

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.

Theoretical insights into the structural, mechanical, and electronic properties of bcc-C40 carbon.

Novel materials displaying multiple exceptional properties are the backbone of the advancement of various industries. In the field of carbon materials, the combination of different properties has been extensively developed to satisfy diverse application scenarios, for instance, conductivity paired with exceptional hardness, outstanding toughness coupled with super-hardness, or heat resistance combined with super-hardness. In this work, a new carbon allotrope, bcc-C40 carbon, was predicted and investigated using first-principles calculations based on density functional theory. The allotrope exhibits unique structural features, including a combination of sp3 hybridized diatomic carbon and four-fold carbon chains. The mechanical and dynamic stability of bcc-C40 carbon has been demonstrated by its elastic constants and phonon spectra. Additionally, bcc-C40 carbon exhibits remarkable mechanical properties, such as zero homogeneous Poisson's ratio, superhardness with a value of 58 GPa, and stress-adaptive toughening. The analysis of the electronic properties demonstrates that bcc-C40 carbon is a semiconductor with an indirect band gap of 3.255 eV within the HSE06 functional, which increases with the increase in pressure. At a pressure of 150 GPa, bcc-C40 carbon transforms into a direct band gap material. These findings suggest the prospective use of bcc-C40 carbon as a superhard material and a novel semiconductor.

Novel carbon allotropes in all-sp2 bonding networks: self-assembling design and first-principles calculations.

Compared with traditional structure prediction methods, the purposeful bottom-up approach is better able to obtain structures with specified performance. In this study, we established two novel carbon phases in purely sp2-bonded networks, termed H61-carbon and H62-carbon, using a self-assembling approach. These carbyne-connected carbon allotropes had helix chains joined by cumulative double-bond chains. We certified the new carbon allotropes to be dynamically and mechanically stable. Both of these carbon allotropes exhibited excellent mechanical properties, and they had metallic and superconductive characteristics featuring superconducting transition temperatures of 10 K (H61-carbon) and 7.4 K (H62-carbon), respectively. These results provide an important strategy for the design of novel carbon allotropes with specified properties.

Hardness and electronic properties of Si-C-N structures.

Three novel hexagonal Si-C-N structures, namely SiC3N3, SiC7N6, and SiC13N14, were constructed on the basis of the α-Si3N4 crystal structure. The stability of the three structures is demonstrated by analyzing their elastic constants and phonon dispersion spectra and by calculating their formation energies. The calculated band structures and partial densities of states suggest that the SiC3N3 and SiC7N6 structures possess hole conductivity. The electron orbital analyses indicate that the SiC3N3 and SiC7N6 crystals possess three-dimensional and one-dimensional conductivity, respectively. SiC13N14 is a semiconductor with a wide bandgap of 4.39 eV. Based on two different hardness models and indentation shear stress calculations, the Vickers hardness values of SiC3N3, SiC7N6, and SiC13N14 are estimated to be 28.04/28.45/16.18 GPa, 31.17/34.19/20.24 GPa, and 40.60/41.59/36.40 GPa. This result indicates that SiC3N3 and SiC7N6 are conductive hard materials while SiC13N14 is a quasi superhard material.

Nanocrystalline Cubic Silicon Carbide: A Route to Superhardness.

Superhard materials other than diamond and cubic boron nitride have been actively pursued in the past two decades. Cubic silicon carbide, i.e., ß-SiC, is a well-known hard material with typical hardness <30 GPa. Although nanostructuring has been proven to be effective in enhancing materials' hardness by virtue of the Hall-Petch effect, it remains a significant challenge to improve hardness of ß-SiC beyond the superhard threshold of 40 GPa. Here, the fabrication of nanocrystalline ß-SiC bulks is reported by sintering nanoparticles under high pressure and high temperature. These ß-SiC bulks are densely sintered with average grain sizes down to 10 nm depending on the sintering conditions, and the Vickers hardness increases with decreasing grain size following the Hall-Petch relation. Particularly, the bulk sintered under 25 GPa and 1400 °C shows an average grain size of 10 nm and an asymptotic Vickers hardness of 41.5 GPa. Boosting the hardness of ß-SiC over the superhard threshold signifies an important progress in superhard materials research. A broader family of superhard materials is in sight through successful implementation of nanostructuring in other hard materials such as BP.

Superhard sp2-sp3 hybridized B2C3N2 with 2D metallicity.

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.

Bioactive prenylated coumarins as potential anti-inflammatory and anti-HIV agents from Clausena lenis.

The phytochemical study on the stems of Clausena lenis resulted in the isolation of three new prenylated coumarins, clauselenins A-C (1-3), together with nine known prenylated coumarins (4-12). The chemical structures of new prenylated coumarins (1-3) were elucidated by means of comprehensive spectral analyses and the known compounds (4-12) were determined by means of comparing their experimental spectral data with those described data in the literatures. All isolated prenylated coumarins were assessed for their anti-inflammatory effects together with anti-HIV activities in vitro. Prenylated coumarins 1-12 displayed remarkable inhibitory effects against nitric oxide (NO) production induced by lipopolysaccharide in mouse macrophage RAW 264.7 cells in vitro with the IC50 values which are comparable to hydrocortisone. Meanwhile, prenylated coumarins 1-12 exhibited considerable anti-HIV-1 reverse transcriptase (RT) activities possessing EC50 values in the range of 0.17-9.08 µM. These findings indicate that the isolation and identification of these prenylated coumarins with pronounced anti-inflammatory effects as well as anti-HIV activities separated from the stems of C. lenis could be of great significance to the development of new anti-inflammatory and anti-HIV agents and their potential applications in the pharmaceutical industry.

Synthesis of cBN-hBN-SiCw Nanocomposite with Superior Hardness, Strength, and Toughness.

Nanocomposites with one-dimensional (1D) and two-dimensional (2D) phases can demonstrate superior hardness, fracture toughness, and flexural strength. Cubic boron nitride-hexagonal boron nitride-silicon carbide whiskers (cBN-hBN-SiCw) nanocomposites with the simultaneous containing 1D SiCw and 2D hBN phases were successfully fabricated via the high-pressure sintering of a mixture of SiCw and cBN nanopowders. The hBN was generated in situ via the limited phase transition from cBN to hBN. Nanocomposites with 25 wt.% SiCw exhibited optimal comprehensive mechanical properties with Vickers hardness of 36.5 GPa, fracture toughness of 6.2 MPa·m1/2, and flexural strength of 687.4 MPa. Higher SiCw contents did not significantly affect the flexural strength but clearly decreased the hardness and toughness. The main toughening mechanism is believed to be a combination of hBN inter-layer sliding, SiCw pull-out, crack deflection, and crack bridging.

Climate Change Impacts on Agricultural Production and Crop Disaster Area in China.

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.

High-Pressure Synthesis of cBN Nanoparticles with High-Density Nanotwin Substructures.

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.

Investigation on the Stability of Derivative Melam from Melamine Pyrolysis under High Pressure.

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.

Low-energy 3D sp2 carbons with versatile properties beyond graphite and graphene.

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.

Surface plasmon resonance spectroscopy and quartz crystal microbalance study of MutS binding with single thymine-guanine mismatched DNA.

MutS is a DNA mismatch binding protein that recognizes heteroduplex DNA containing mispaired or unpaired bases. In this study, we employ a quartz crystal microbalance (QCM) and a surface plasmon resonance (SPR) device for the study of MutS binding with DNA containing a single Thymine-Guanine (T-G) mismatch at different sites. Multi-step surface binding reactions are involved in the study, including probe DNA immobilization on the sensor surface through biotin-streptavidin-biotin bridge chemistry, target DNA hybridization to form T-G heteroduplexes, and MutS recognition of the mutation sites. The QCM frequency (d f) and motional resistance (d R, an impedance parameter reflective of QCM damping), as well as the SPR angle shift (d q ) are recorded for the binding reactions. The combined SPR and QCM data collection and analysis allow for an assessment of not only the amount of bound biopolymer but provide also information on also the structural properties of the streptavidin, DNA and MutS/DNA complexes. The affinity of the MutS/T-G heteroduplex assembly is determined by both the QCM and SPR methods through titration of the surface bound DNA with increasing MutS concentration. It is found that if the T-G mismatch is in the center of the DNA fragment, the MutS/DNA complex is more stable than if the mismatch is located near the end of the fragment.

Comparison of surface plasmon resonance spectroscopy and quartz crystal microbalance techniques for studying DNA assembly and hybridization.

In this study we evaluate the strengths and weaknesses of surface plasmon resonance (SPR) spectroscopy and quartz crystal microbalance (QCM) technique for studying DNA assembly and hybridization reactions. Specifically, we apply in parallel an SPR instrument and a 5 MHz QCM device with dissipation monitoring (QCM-D) to monitor the assembly of biotinylated DNA (biotin-DNA) on a streptavidin-modified surface and the subsequent target DNA hybridization. Through the parallel measurements, we demonstrate that SPR is more suitable for quantitative analysis of DNA binding amount, which is essential for interfacial DNA probe density control and for the analysis of its effect on hybridization efficiency and kinetics. Although the QCM is not quantitative to the same extent as SPR (QCM measures the total mass of the bound DNA molecules together with the associated water), the dissipation factor of the QCM provides a qualitative measure of the viscoelastic properties of DNA films and the conformation of the bound DNA molecules. The complexity in mass measurement does not impair QCM's potential for a kinetic evaluation of the hybridization processes. For quantification of target DNA, the biotin-DNA modified SPR and QCM sensors are exposed to target DNA with increasing concentration. The plots of SPR/QCM signals versus target DNA concentration show that water entrapment between DNA strands make the QCM sensitivity for the hybridization assay well comparable with that of the SPR, although the intrinsic mass sensitivity of the 5 MHz QCM is approximately 20 times lower.

Surface plasmon resonance spectroscopy and quartz crystal microbalance study of streptavidin film structure effects on biotinylated DNA assembly and target DNA hybridization.

Surface plasmon resonance (SPR) spectroscopy is employed for the study of biotinylated DNA assembly on streptavidin modified gold surfaces for target DNA hybridization. Two immobilization strategies are involved for constructing streptavidin films, namely, (1) physical adsorption on biotin-containing thiol treated surfaces through biotin-streptavidin links and (2) covalent attachment to 11-mercaptoundecanoic acid (MUA) treated surfaces through amine coupling. To understand the structural properties of the streptavidin films, a quartz crystal microbalance with energy dissipation monitoring (QCM-D) is used to monitor the streptavidin immobilization procedures. The simultaneously measured frequency (Deltaf) and dissipation factor (DeltaD) changes, together with the SPR angle shifts (Deltatheta), suggest that the streptavidin film assembled on the biotin-containing surface is highly rigid with a well-ordered structure while the streptavidin film formed through amine coupling is highly dissipative and less structured. The subsequent biotinylated DNA (biotin-DNA) assembly and target hybridization results show that the streptavidin film structure has distinct effects on the biotin-DNA binding amount. On the streptavidin matrix, not only the probe DNA density but also the strand orientation mediated by the streptavidin films has distinct effects on hybridization efficiency. Particularly, the molecularly ordered streptavidin films formed on the biotin-containing surfaces ensure a well-ordered DNA assembly, which in turn allows for a higher efficiency in target DNA capture and for a higher sensitivity in the hybridization analysis when compared to the biotin-DNA assembled on the less structured streptavidin films formed through amine coupling.

Detection of point mutation and insertion mutations in DNA using a quartz crystal microbalance and MutS, a mismatch binding protein.

MutS protein is a mismatch binding protein that recognizes mispaired and unpaired base(s) in DNA. In this study, we incorporate the MutS protein-based mutation recognition into quartz crystal microbalance (QCM) measurements for DNA single-base substitution mutation and 1-4 base(s) insertion (or deletion) mutation detection. The method involves the immobilization of single-stranded probe DNA on a QCM surface, the hybridization of target DNA to form homoduplex or heteroduplex DNA, and finally the application of MutS protein for the mutation recognition. By measuring the MutS binding signal, DNA containing a T:G mismatch or unpaired base(s) is(are) discriminated against perfectly matched DNA at target concentrations ranging from 1nM to 5 microM. Furthermore, the QCM damping behavior upon MutS-DNA complex formation is studied using a Network Analyzer. The measured motional resistance changes per coupled MutS unit mass (deltaR/deltaf) are found to be indicative of the viscoelastic or structural properties of the bound protein, corresponding to different binding mechanisms. In addition, the deltaR/deltaf values vary remarkably when the MutS protein binds at different distances away from the QCM surface. Thus, these values can be used as a "fingerprint" for MutS mismatch recognition and also used to quantitatively locate the mutation site.