Investigadores en China y los Estados Unidos han encontrado una manera de producir una variedad de carbono que es muy duro y muy elástico por calentamiento a alta presión. Este «carbono vítreo comprimido» (CVC), también es muy liviano y potencialmente podría fabricarse en grandes cantidades. Esto significa que podría ser una buena opción para varios tipos de aplicaciones, desde chalecos antibalas a nuevos tipos de dispositivos electrónicos. La capacidad única de carbono de vincularse a través de enlaces sp 2 y sp 3 da lugar a una serie de excelentes propiedades físicas, tanto mecánicas como eléctricas. En este documento se muestra que sometiendo a presión y temperatura a carbono vítreo con enlaces sp 2 se producen redes de grafeno vinculadas entre si con enlaces sp 3 . Los CVC tienen resistencias a la compresión específicas extraordinarias, más de dos veces la de las cerámicas de uso común y al mismo tiempo una rápida recuperación elástica en respuesta a deformaciones locales.
High-performance materials that couple low weight and high strength with elasticity are demanded for a vast range of applications. Finding the optimum strength-to-weight ratio is not an easy task, and certain compromises must generally be made between different classes of materials. Common metals are ductile yet heavy and have maximum yield strengths limited to about 2 GPa (1). The elastic strain of metals is usually limited to <2% because of dislocation or twin formation when the applied stress reaches a critical value. High-tech ceramics have superior compressive strengths and hardness over metals (2, 3). For example, cemented tungsten carbide has an extreme compressive strength up to 9 GPa, but its heavy weight leads to excessive energy consumption. Light-element compounds, such as silicon carbide and boron carbide, are low-density and high-strength and are thus preferred materials for military armor and aerospace shields to protect certain vital equipment. Nevertheless, most ceramics can only withstand small elastic strains (<2%) and undergo brittle fracture immediately after deformation. By virtue of martensitic transformations, fine-scale ceramics can simultaneously increase elasticity and strength (4). However, so far, it remains a major challenge to make ceramics with the optimum combination of weight, strength, and elastic properties.
Carbon gives rise to remarkable classes of materials with combined properties, such as low weight, high strength, hardness, elasticity, and tunable electronic properties, because of the flexibility to form sp-, sp2-, and sp3-hybridized bonds. Diamond, entirely composed of sp3 bonds, is a three-dimensional (3D) superhard insulator, whereas fully sp2 graphene is a 2D Dirac semimetal with out-of-plane flexibility. Mixed sp2– and sp3-bonded carbon phases are expected to have advantages by integrating mechanical and electrical properties. By introducing local sp3 buckling between graphene sheets, hard and elastic semiconducting thin films of amorphous carbon have been produced by multiple deposition techniques (5). Unfortunately, these films have significant residual internal stresses, which limit their thickness and usefulness (6). As a result, it is desirable to synthesize uniform bulk forms of mixed sp2-sp3 carbons with manageable microstructures and versatile capabilities.
The most direct means to synthesize mixed sp2-sp3 forms of carbon is by the controlled compression of pure sp2 carbons. For example, both highly sp2-hybridized graphite-like and sp3-hybridized diamond-like amorphous carbons can be quenched from compressed fullerenes (7, 8), and some of them also show high hardness and elastic recovery but with a very low compressive strength of 0.2 to 0.3 GPa, probably due to the restructuring heterogeneity from the collapse of fullerenes (9). Glassy carbon (GC), as a typical disordered sp2 carbon, can be manufactured into various shapes with a great variety of unique material properties, including high strength, low density, high-temperature resistance in inert gas up to 3000°C, and extreme corrosion resistance. Type I GC, which is produced by firing polymeric precursors at temperatures below 2000°C, mainly consists of randomly distributed curved graphene layer fragments (10, 11). Type II GC, fabricated at higher temperatures above 2500°C, contains self-assembled fullerene-like spheroids of nanometer sizes, dispersed within and interconnected by a 3D disordered multilayer graphene matrix (10, 11). During cold compression of type I GC to 44.4 GPa, synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced sp2-to-sp3 bonding change (12). The transition appeared to be reversible upon releasing pressure so that the cold-compressed GC was not quenchable to ambient pressure (12). While overheating GC at pressures above 15 GPa, fully sp3 superhard nanocrystalline diamonds were produced (13, 14). Thus, there is a gap to synthesize recoverable sp2-sp3 carbons from GC conversion and to further explore the suitable synthetic conditions needed. Moreover, it is well known that the sp2 carbon precursors with different crystal structures would undergo distinct phase transitions under pressure due to kinetic factors, which makes the phase diagrams unexpectedly complex, for example, the transitions of typical graphite and fullerenes (7, 15). Therefore, the comprehensive search of metastable phase transitions of various carbon precursors with pressure is needed because this may reveal key insights for producing more new carbon allotropes with unprecedented properties.
Here, we report a series of lightweight, ultrastrong, hard, elastic, and conductive type of amorphous carbons in bulk form by compressing GC at previously unexplored moderate temperatures. Structure and bonding were studied by x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and Raman spectroscopy. Indentation hardness and elastic recovery were derived from the load/unload-displacement curves, whereas axial compressive stress-strain relations were established using a diamond anvil cell (DAC) technique.
Fuente: http://advances.sciencemag.org
