Synthetic Crystals of Silver with Carbon: 3D Epitaxy of Carbon Nanostructures in the Silver Lattice

Abstract

Only minimum amounts of carbon can be incorporated into silver, gold, and copper in a thermodynamically stable form. Here, the structure of stable silver carbon alloys is described, which are produced by thermoelectrically charging molten silver with carbon ions. Transmission electron microscopy and Raman scattering are combined to establish that large amount of carbon is accommodated in the form of epitaxial graphene-like sheets. The carbon bonds covalently to the silver matrix as predicted from density functional theory (DFT) calculations with bond energies in the range 1.1-2.2 eV per atom or vacancy. Graphitic-like sheets embedded in the crystal lattice of silver form 3D epitaxial structures with the host metal with a strain of ≈13% compared to equilibrium graphene. The carbon nanostructures persist upon remelting and resolidifi cation. A DFT-based analysis of the phonon density of states confi rms the presence of intense vibration modes related to the Ag C bonds observed in the Raman spectra of the alloy. The solid silver-high carbon alloy, termed "Ag-covetic," displays room temperature electrical conductivity of 5.62 × 10 7 S m −1 even for carbon concentrations of up to ≈6 wt% (36 at%). This process of incorporation of carbon presents a new paradigm for electrocharging assisted bulk processing.

Sp2 carbon embedded in Al-6061 and Al-7075 alloys in the form of crystalline graphene nanoribbons

“…Similar to the case of Ag covetic considered in our previous publication [15], First-principles DFT based calculations indicate that bonding between C and Al at edges of graphene ribbons and at C vacancies ( Figure 13) has a covalent character which is similar to the Ag covetic [15]. An interesting feature is that each interfacial Al atom can form more than one bond (two or three) with under coordinated carbon atoms positioned at the ribbon edge, which is related to its three valence electrons.…”

“…These ribbons contain small graphitic crystals with a preferred orientation relative to the Al lattice and similar to Ag covetic [15] with (111) Al // (0001) graphite and <1…”

“…Applying this technique, different metal-covetics of Al, Cu, Au, Ag, Zn, Sn, Pb and Fe have been produced with nanostructured carbon. In the case of Ag with carbon, termed Ag covetic, 3D epitaxial carbon network was observed [15]. The stability of these carbons in the metal lattice has been verified by cycles of re-melting and solidification, indicating that atomic bonding exists between the carbon and the metal rather than simply forming a composite with trapped carbon inside the molten metal.…”

Simultaneously enhancing mechanical properties and electrical conductivity of aluminum by using graphene as the reinforcement

“…The results showed that the electrical conductivity of graphene (∽67MS/m) was significantly higher than that of CNTs (∽20MS/m). At the same time, studies focused on the electrical properties of graphene reinforced Al nanocomposite were limited [13,14]. Thus, in this study, we chose graphene as the reinforcement to enhance the mechanical properties and electrical conductivity of pure Al.…”

A Study to Define Critical Current Limit for Covetic Al Formation

“…Although the role of high current applied to the melt is not fully known, it is assumed that DC current is necessary to obtain covetics and contributes to covetic structure formation by transforming large carbon structures into activated carbon (Knych et al, 2014). Salamanca-Riba et al (2015) suggest that covetic structures begin to form for DC current values above a threshold value, called critical current. Hence, carbon atoms re-arranged to form alternating graphene layers within metal matrix while below that threshold value covetics are not obtainable.…”