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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The recently discovered graphite–diamond hybrid materials (Gradia) with mixed sp2- and sp3-hybridizations have opened up a new direction in carbon allotropes research. Herein, we reported Gradia-HZ, constituted by interfaced graphite and hexagonal diamond parts in the unit cell, which demonstrates distinct electronic and mechanical properties. With the modulation of graphite width, Gradia-HZ exhibits unexpected topological nodal-line semimetal, semiconductor, and normal metal integrating with a distinctive Quasi-1D electronic transport capability based on first-principles calculations. More interestingly, pressure-induced graphite phase transformation might be an implementable and effective method to regulate the structure and physical properties of Gradia-HZ. The discovery of rich and peculiar physical properties in Gradia-HZ, e.g., high-conductivity metals, semiconductors with variable bandgap, and topological semimetals, will arouse great research interest to graphite–diamond hybrid materials, to promote their development and application in advanced devices. [ABSTRACT FROM AUTHOR]

Scientific evaluation of the sustainable development efficiency of China's large and medium‐sized cities has far‐reaching significance to how to improve the well‐being of residents and improve the level of sustainable development. This research uses the parallel two‐stage SBM dynamic (PT‐DSBM) model to evaluate the sustainable development efficiency of 33 cities in eastern, central, and western China. The innovation of this article is divided into two points. First of all, the PT‐DSBM model is used to subdivide the sustainable development system into social, economic, and ecological subsystems and closely integrates it with residents' QOL, which further reveals the inherent mechanism of the sustainable development system. Second, the spatiotemporal evolution can further reveal the spatiotemporal changes of sustainable development and residents' QOL within the scope of this investigation. The results show the following. (1) The overall efficiency of urban sustainable development in China is high in the east and west and low in the central. (2) The social subsystem efficiency in western cities exhibits outstanding performance, eastern cities perform better in economic and ecological sustainable subsystems, while the efficiency of residents' QOL remains at a high level in the sample cities as a whole. (3) In the study of spatiotemporal evolution, the concentration of urban sustainable development decreases with time evolution. Finally, this research gives suggestions on social security, stakeholder engagement, and environmental policies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The nanocrystalline (Hf0.25Zr0.25Ta0.25Ti0.25)C high‐entropy carbide ceramics were successfully prepared through high‐pressure sintering of the self‐synthesized nanopowders for the first time. The results showed that the relative density, grain size, hardness, and elastic modulus of all the as‐prepared samples increased gradually with the increase of sintering temperatures from 1200 to 1600°C. The as‐fabricated samples at 1600°C possessed a relative density of 97.1% and a mean grain size of 98 ± 35 nm and simultaneously exhibited good comprehensive mechanical properties with Vickers hardness of 25.7 ± 0.6 GPa and fracture toughness of 4.3 ± 0.4 MPa·m1/2 at room temperature due to the grain refinement strengthening mechanism and microcrack deflection and bridging toughening mechanism. In addition, they showed a linear decrease in high‐temperature Vickers hardness with the increase of the temperature from room temperature to 1000°C, which was determined by the combined effects of the intrinsic softening of HEC‐1 ceramics at elevated temperature and the presence of ZrO2 impurity phase at the grain boundaries. [ABSTRACT FROM AUTHOR]

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 sp 2 -bonded networks, termed H6 1 -carbon and H6 2 -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 (H6 1 -carbon) and 7.4 K (H6 2 -carbon), respectively. These results provide an important strategy for the design of novel carbon allotropes with specified properties.

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-SiC w ) nanocomposites with the simultaneous containing 1D SiC w and 2D hBN phases were successfully fabricated via the high-pressure sintering of a mixture of SiC w and cBN nanopowders. The hBN was generated in situ via the limited phase transition from cBN to hBN. Nanocomposites with 25 wt.% SiC w exhibited optimal comprehensive mechanical properties with Vickers hardness of 36.5 GPa, fracture toughness of 6.2 MPa·m 1/2 , and flexural strength of 687.4 MPa. Higher SiC w 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, SiC w pull-out, crack deflection, and crack bridging.