Formation and stability of cellular carbon foam structures: an ab initio study

Structural variety and stability of carbon honeycomb cellular structures

A new synthesized carbon honeycomb allotrope reported previously, built from graphene nanoribbons connected by sp3-bonded carbon junction lines, forms a family of cellular structures with high porosity and sorption capacity. In this work we first propose a complete set of possible honeycomb structures of different wall chiralities both the armchair and zigzag types, including considered earlier only theoretically, for the structural analysis of such structures by means of the high-energy electron diffraction method. The “completeness” of the model set made it possible to obtain nearly perfect coincidence of the experimental and calculated diffraction intensities. The contribution of graphite fragments and random structures, also involved in the analysis, turned out to be zero. Only a limited number of honeycomb structures of different types almost ideally describes the experiment. Thus we conclude that polydomain structures corresponding to a set of basic models formed in this investigation rather than formations dominated by random structures. The samples under study have demonstrated the unique cellular stability since were stored in vacuum ∼4.5 months before the reported measurements. Along with the original results the history of the carbon honeycomb cellular structures is briefly presented.

Roughening for Strengthening and Toughening in Monolayer Carbon Based Composites

Three-dimensional (3D) aggregation of graphene is dramatically weak and brittle due primarily to the prevailing interlayer van der Waals interaction. In this report, motivated by the recent success in synthesis of monolayer amorphous carbon (MAC) sheets, we demonstrate that outstanding strength and large plastic-like strain can be achieved in layered 3D MAC composites. Both surface roughening and the ultracompliant nature of MACs count for the high strength and gradual failure in 3D MAC. Such properties are not seen when intact graphene or multiple stacked MACs are used as building blocks for 3D composites. This work demonstrates a counterintuitive mechanism that surface roughening due to initial defects and low rigidity may help to realize superb mechanical properties in 3D aggregation of monolayer carbon.

Absorption of atomic and molecular species in carbon cellular structures (Review article)

The paper presents a brief review of the recent developments in the field of absorption of atomic and molecular species in carbon cellular structures. Such absorbing objects can be distinctly recognized among a large family of carbon porous materials owing to potential and already observed in experiments very high capacity to soak and to keep inside different substances, which at usual conditions outside the porous matrices may often stay only in a gaseous form. High capacity filling is attained owing to single graphene-like walls separating different cells in the whole structures providing their lightweight. This property of cellular structures makes them very promising for numerous technological applications such as hydrogen storage in fuel cells and molecular sieving in membranes made from such structures or for their usage in microelectronics, photovoltaics and production of Li-ion batteries. Independently of the targeted applications gases are good candidates for probing tests of carbon matrices themselves.

Three dimensional porous SiC for lithium polysulfide trapping

A series of 3D porous SiC materials with active sp² hybridized Si atoms have been designed for lithium polysulfide retention in Li–S batteries. The shuttle effect can be effectively depressed by the strong Si⋯S interaction between Li₂S_n and the 3D porous SiC hosts.

Bottom-up Design of Three-Dimensional Carbon-Honeycomb with Superb Specific Strength and High Thermal Conductivity

Low-dimensional carbon allotropes, from fullerenes, carbon nanotubes, to graphene, have been broadly explored due to their outstanding and special properties. However, there exist significant challenges in retaining such properties of basic building blocks when scaling them up to three-dimensional materials and structures for many technological applications. Here we show theoretically the atomistic structure of a stable three-dimensional carbon honeycomb (C-honeycomb) structure with superb mechanical and thermal properties. A combination of sp² bonding in the wall and sp³ bonding in the triple junction of C-honeycomb is the key to retain the stability of C-honeycomb. The specific strength could be the best in structural carbon materials, and this strength remains at a high level but tunable with different cell sizes. C-honeycomb is also found to have a very high thermal conductivity, for example, >100 W/mK along the axis of the hexagonal cell with a density only ∼0.4 g/cm³. Because of the low density and high thermal conductivity, the specific thermal conductivity of C-honeycombs is larger than most engineering materials, including metals and high thermal conductivity semiconductors, as well as lightweight CNT arrays and graphene-based nanocomposites. Such high specific strength, high thermal conductivity, and anomalous Poisson's effect in C-honeycomb render it appealing for the use in various engineering practices.

Are the experimentally observed 3-dimensional carbon honeycombs all-sp2 structures? The dangling p-orbital instability

3-Dimensional all-sp² honeycomb carbon structures are unstable, due to dangling bonds, formed on the junction atom unhybridized p-orbitals.

Electron and phonon properties and gas storage in carbon honeycombs

A new kind of three-dimensional carbon allotrope, termed carbon honeycomb (CHC), has recently been synthesized [PRL 116, 055501 (2016)]. Based on the experimental results, a family of graphene networks has been constructed, and their electronic and phonon properties are studied by various theoretical approaches. All networks are porous metals with two types of electron transport channels along the honeycomb axis and they are isolated from each other: one type of channel originates from the orbital interactions of the carbon zigzag chains and is topologically protected, while the other type of channel is from the straight lines of the carbon atoms that link the zigzag chains and is topologically trivial. The velocity of the electrons can reach ∼10(6) m s(-1). Phonon transport in these allotropes is strongly anisotropic, and the thermal conductivities can be very low when compared with graphite by at least a factor of 15. Our calculations further indicate that these porous carbon networks possess high storage capacity for gaseous atoms and molecules in agreement with the experiments.

A first-principles study on three-dimensional covalently-bonded hexagonal boron nitride nanoribbons

We studied three-dimensional honeycomb-structure boron nitride (BN) allotrope using first-principles calculations and the tight-binding method. Interconnected by sp(3)-bonding at the vertices, hexagonal BN nanoribbons construct highly-porous, covalently-bonded hexagonal BN nanoribbons (CBBNs). We investigated the structural and mechanical properties of CBBNs with various sizes, compared with those of carbon and other BN allotropes. The mechanical and thermal stabilities are also checked. Our calculations show that, despite the high porosity and low mass density, CBBNs are stable and mechanically hard materials as cubic BN. Moreover, our calculated results suggest that CBBNs can be regarded as a binary alloy of sp(2)- and sp(3)-bonded BNs following the Vegard's rule in average bond lengths and bulk moduli. Calculated band structures show that the band gap of CBBNs has similar variation upon increasing size as BN nanoribbons and is also limited by the second-neighbor interaction between the pz states of sp(2)-bonded atoms in adjacent nanoribbons.

Phase coexistence and metal-insulator transition in few-layer phosphorene: a computational study

Based on ab initio density functional calculations, we propose γ-P and δ-P as two additional stable structural phases of layered phosphorus besides the layered α-P (black) and β-P (blue) phosphorus allotropes. Monolayers of some of these allotropes have a wide band gap, whereas others, including γ-P, show a metal-insulator transition caused by in-layer strain or changing the number of layers. An unforeseen benefit is the possibility to connect different structural phases at no energy cost. This becomes particularly valuable in assembling heterostructures with well-defined metallic and semiconducting regions in one contiguous layer.

Topologically protected conduction state at carbon foam surfaces: an ab initio study

We report results of ab initio electronic structure and quantum conductance calculations indicating the emergence of conduction at the surface of semiconducting carbon foams. The occurrence of new conduction states is intimately linked to the topology of the surface and not limited to foams of elemental carbon. Our interpretation based on rehybridization theory indicates that conduction in the foam derives from first- and second-neighbor interactions between p∥ orbitals lying in the surface plane, which are related to p⊥ orbitals of graphene. The topologically protected conducting state occurs on bare and hydrogen-terminated foam surfaces and is thus unrelated to dangling bonds. Our results for carbon foam indicate that the conductance behavior may be further significantly modified by surface patterning.

Formation of Nanofoam carbon and re-emergence of Superconductivity in compressed CaC6

Pressure can tune material's electronic properties and control its quantum state, making some systems present disconnected superconducting region as observed in iron chalcogenides and heavy fermion CeCu2Si2. For CaC6 superconductor (Tc of 11.5 K), applying pressure first Tc increases and then suppresses and the superconductivity of this compound is eventually disappeared at about 18 GPa. Here, we report a theoretical finding of the re-emergence of superconductivity in heavily compressed CaC6. The predicted phase III (space group Pmmn) with formation of carbon nanofoam is found to be stable at wide pressure range with a Tc up to 14.7 K at 78 GPa. Diamond-like carbon structure is adhered to the phase IV (Cmcm) for compressed CaC6 after 126 GPa, which has bad metallic behavior, indicating again departure from superconductivity. Re-emerged superconductivity in compressed CaC6 paves a new way to design new-type superconductor by inserting metal into nanoporous host lattice.