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Carbon is one of the most fascinating elements due to its structurally diverse allotropic forms stemming from its bonding varieties (sp , sp 2 and sp 3). Exploring new forms of carbon has been the eternal theme of scientific research. Herein, we report on amorphous (AM) carbon materials with a high fraction of sp 3 bonding recovered from compression of fullerene C60 under high pressure and high temperature, previously unexplored. Analysis of photoluminescence and absorption spectra demonstrates that they are semiconducting with a bandgap range of 1.5–2.2 eV, comparable to that of widely used AM silicon. Comprehensive mechanical tests demonstrate that synthesized AM-III carbon is the hardest and strongest AM material known to date, and can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance. [ABSTRACT FROM AUTHOR]

The phase transition of fullerene C60 under high pressure and high temperature has been widely studied, but the research on the spark‐plasma sintering of C60 is limited, and the mechanical properties of synthesized materials are still unknown. In this study, a series of amorphous carbon materials were synthesized by spark‐plasma sintering fullerene C60 at different temperatures. The structural characterizations showed that they were composed of multi‐graphene fragments with different sizes, curvatures, and ordering degrees. The densities of the synthesized amorphous carbons were 1.3‐1.4 g/cm3, which were lower than the values for graphite, but the mechanical properties were excellent. The highest compressive strength, indentation hardness, and elastic recovery of the amorphous carbons synthesized at different temperatures could reach up to ~1.25 GPa, ~3.8 GPa, and ~85.5%, respectively, which are far better than the values for commercial isotropic graphite materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nanocrystalline Bi2Te3 bulks were synthesized via a nanostructuring method that included steps of ball milling, cold pressing, and sintering. In the temperature range of 325–525 K, nanocrystalline Bi2Te3 bulks exhibit good thermoelectric properties with a peak ZT of 0.94. The experimental results suggest that the partially coherent boundaries allow the maintenance of low grain growth rate and low resistivity. The enhancement of ZT comes from a large reduction in the phonon thermal conductivity and a strong suppression of the bipolar effect at high temperature. [ABSTRACT FROM AUTHOR]

Although diamond is the hardest material for cutting tools, poor thermal stability has limited its applications, especially at high temperatures. Simultaneous improvement of the hardness and thermal stability of diamond has long been desirable. According to the Hall−Petch effect, the hardness of diamond can be enhanced by nanostructuring (by means of nanograined and nanotwinned microstructures), as shown in previous studies. However, for well-sintered nanograined diamonds, the grain sizes are technically limited to 10−30 nm (ref. 3), with degraded thermal stability compared with that of natural diamond. Recent success in synthesizing nanotwinned cubic boron nitride (nt-cBN) with a twin thickness down to ∼3.8 nm makes it feasible to simultaneously achieve smaller nanosize, ultrahardness and superior thermal stability. At present, nanotwinned diamond (nt-diamond) has not been fabricated successfully through direct conversions of various carbon precursors (such as graphite, amorphous carbon, glassy carbon and C60). Here we report the direct synthesis of nt-diamond with an average twin thickness of ∼5 nm, using a precursor of onion carbon nanoparticles at high pressure and high temperature, and the observation of a new monoclinic crystalline form of diamond coexisting with nt-diamond. The pure synthetic bulk nt-diamond material shows unprecedented hardness and thermal stability, with Vickers hardness up to ∼200 GPa and an in-air oxidization temperature more than 200 °C higher than that of natural diamond. The creation of nanotwinned microstructures offers a general pathway for manufacturing new advanced carbon-based materials with exceptional thermal stability and mechanical properties. [ABSTRACT FROM AUTHOR]

Superconducting properties have been investigated in the Ca-doped MgB2 (Mg1-xCaxB2) superconductors, which have been synthesized under high temperature and pressure. With increasing doping level x, a linear drop of the critical temperature Tc is observed, and the gap Δσ keeps nearly fixed and the gap Δπ deceases, giving rise to the persistent two-gap feature in the Mg1-xCaxB2 samples for x up to 0.07. Both the Sommerfeld constant γn and the relative weight γπ/γn are found to decrease with raising x, indicative of the decrease of Nπ(0). [ABSTRACT FROM AUTHOR]

The phase transition of fullerene C60 under high pressure and high temperature has been widely studied, but the research on the spark‐plasma sintering of C60 is limited, and the mechanical properties of synthesized materials are still unknown. In this study, a series of amorphous carbon materials were synthesized by spark‐plasma sintering fullerene C60 at different temperatures. The structural characterizations showed that they were composed of multi‐graphene fragments with different sizes, curvatures, and ordering degrees. The densities of the synthesized amorphous carbons were 1.3‐1.4 g/cm3, which were lower than the values for graphite, but the mechanical properties were excellent. The highest compressive strength, indentation hardness, and elastic recovery of the amorphous carbons synthesized at different temperatures could reach up to ~1.25 GPa, ~3.8 GPa, and ~85.5%, respectively, which are far better than the values for commercial isotropic graphite materials. [ABSTRACT FROM AUTHOR]

The thermal damage induced by microwave treatment in rocks has not been well explored, especially when using the dielectric-loaded converging waveguide antennas (DLCWAs). In this paper, the effects of microwave power level, heating time and working distance on the temperature increase, ultrasonic wave velocity reduction, and more importantly, the fracturing characteristics and the damage zone, are comprehensively investigated using a 6 kW microwave source and a customized DLCWA. A novel method is also proposed to adjust the P-wave velocity (V p) reduction across the thickness of the specimens and to determine the equivalent damage zone. The results show that the temperature and high temperature area, crack number, length and density, and the V p reduction rate as well as the damage zone all increase as either the power level or the heating time increases and decrease as the working distance increases. The damage zone can be divided into the crushed zone, fracture zone and discrete fragment zone based on the crack characteristics and the crack generation mechanisms. The method proposed is demonstrated to be reliable in predicting the damage zone induced by microwave heating. • Used a dielectric loaded converging waveguide antenna to fracture a biotite diorite. • Investigated the effects of microwave power level, heating time and working distance on thermal fracturing. • Proposed a novel method to estimate the P-wave velocity distribution across the total thickness. • Determined the equivalent thermal damage zone based on the spatial P-wave velocity reduction. [ABSTRACT FROM AUTHOR]