Researchers at the University of Yanshan University’s State Key Laboratory of Metastable Material Science and Technology have developed a light weight, ultra-strong and elastic carbon (C) material known as 'compressed glassy carbon'.
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Due to the carbon's ability to have sp3, sp2 and sp hybridizations, it is capable of forming a variety of bonds that lead to various different structures. For example, the structure of diamond is joined entirely by sp3 hybridization, whereas the C atoms in the two-dimensional (2D) honey comb lattice of graphene shows sp2 hybridization. This ability of C atoms to form different bonding states is responsible for the variety of physical attributes of carbon materials, of which include its extraordinary electrical and mechanical properties.
High performance materials that are strong, light weight and elastic with tunable electrical properties are in great demand in a variety of diverse applications such as materials for military armor and aerospace.
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However, it has been rather difficult to find materials with an optimum strength to weight ratio that does not compromise certain aspects of the material. For example, common metals are strong, yet heavy and commonly exhibit poor elasticity and a ductile strength of about 2 GPa.
High-tech ceramics, such as cemented tungsten carbide, despite having a high compressive strength of 9 GPa, are not so useful as a result of its higher energy consumption that is a result of the weight of the material and its low elasticity.
Therefore, materials made up of carbon show great promise in finding such materials of interest that display strong, flexible and light-weight properties, which is a result of the ability of C atoms to form different bonding states.
While diamond, a sp3 hybridized form of carbon, is a super hard insulator, the sp2 hybridized graphene shows out of plane flexibility. Combining both the sp3 hybridized form and the sp2 hybridized form of carbon could potentially lead to a material with a superior strength and flexibility.
Controlled compression is a direct method to synthesize these hybrid sp2-sp3 forms of carbon, where the two dimensional sheets of pure sp2 hybridized layers of graphene-like materials are compressed under controlled conditions. Glassy carbons (GC) are disordered sp2 hybridized materials that are both strong and light-weight, as well as materials that are known to be highly resistant to corrosion and extreme temperatures measuring up to 3000º C.
While overheating at high pressures has led to the formation of super hard nano crystalline diamonds, cold compression allows for the formation of sp2-sp3 bridges, which are reversed upon the release of pressure. Therefore, a reliable and reproducible method and the appropriate conditions required to produce recoverable sp2-sp3 carbon form.
Zhisheng Zhao’s team from Yanshan University in China have used moderate temperatures and pressure conditions to synthesize the compressed glassy carbon (compressed GC) which is both flexible and super strong as a result of its mixed sp2-sp3 carbon structure.
The selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) images revealed that the compressed GCs showed long range disorder and short range order. While the C atoms in each layer is arranged with sp2 linkages, the interlinks formed at the curved surfaces between the carbon layers are found to be of sp3 bonds.
The raw GCs were found to have a hardness of only 5 GPa as a function of applied loads. When subjected to the same tests as the raw GCs, the compressed GCs recovered from a 25 GPa pressure and temperatures of 400 ºC and 600 ºC have a hardness of 15 GPa while the compressed GCs recovered from 800 ºC and 1000 ºC have a hardness of 26 GPa.
Non-indentation tests revealed that the compressed GCs demonstrated unparalleled hardness and elastic recovery in comparison with common ceramics, polymers and metals. Compressed GCs were also found to be two to four times stronger than generally used materials such as cemented diamond, carbon fibers, silicon carbide, when the densities of all the materials are normalized.
The discovery of compressed glassy carbon could potentially replace silicon materials due to its tunable electrical properties while also having the desired mechanical properties. This class of sp2-sp3 makes it possible to combine a variety of desired features such as great flexibility, extraordinary specific compressive strength, tunable electrical conductivity, high hardness in one material which makes it find its use in a wide variety of applications.
References:
- “Compressed glassy carbon: An ultrastrong and elastic interpenetrating graphene network” M. Hu, J. He, et al. Science Advances. (2017). DOI: 10.1126/sciadv.1603213.
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