No abstract

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 ultrafine S nanofibers. The presence of intermediate amorphous phase and the microstructure difference of fibrous S might account for various contradictions among previously reported results.

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.

The reinforcements represented by graphene nanoplatelets, graphite, and carbon nanotubes have demonstrated the great potential of carbon materials as reinforcements to enhance the mechanical properties of TiO2. However, it is difficult to successfully prepare TiO2-diamond composites because diamond is highly susceptible to oxidation or graphitization at relatively high sintering temperatures. In this work, the TiO2-diamond composites were successfully prepared using high-pressure sintering. The effect of diamond on the phase composition, microstructure, mechanical properties, and tribological properties was systemically investigated. Diamond can improve fracture toughness by the crack deflection mechanism. Furthermore, the addition of diamond can also significantly reduce the friction coefficient. The composite composed of 10 wt.% diamond exhibits optimum mechanical and tribological properties, with a hardness of 14.5 GPa, bending strength of 205.2 MPa, fracture toughness of 3.5 MPa∙m1/2, and a friction coefficient of 0.3. These results enlarge the family of titania-based composites and provide a feasible approach for the preparation of TiO2-diamond composites.