Electrical conductivity measured in atomic carbon chains

The first electrical conductivity measurements of monoatomic carbon chains are reported in this study. The chains were obtained by unraveling carbon atoms from graphene ribbons while an electrical current flowed through the ribbon and, successively, through the chain. The formation of the chains was accompanied by a characteristic drop in the electrical conductivity. The conductivity of carbon chains was much lower than previously predicted for ideal chains. First-principles calculations using both density functional and many-body perturbation theory show that strain in the chains determines the conductivity in a decisive way. Indeed, carbon chains are always under varying non-zero strain that transforms its atomic structure from cumulene to polyyne configuration, thus inducing a tunable band gap. The modified electronic structure and the characteristics of the contact to the graphitic periphery explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure

Non-hexagonal-ring defects and structures induced by strain in graphene and in functionalized graphene

We perform {\textit ab initio} calculations for the strain-induced formation of non-hexagonal-ring defects in graphene, graphane (planar CH), and graphenol (planar COH). We find that the simplest of such topological defects, the Stone-Wales defect, acts as a seed for strain-induced dissociation and multiplication of topological defects. Through the application of inhomogeneous deformations to graphene, graphane and graphenol with initially small concentrations of pentagonal and heptagonal rings, we obtain several novel stable structures that possess, at the same time, large concentrations of non-hexagonal rings (from fourfold to elevenfold) and small formation energies

Chains of carbon atoms: A vision or a new nanomaterial?:

Linear strings of sp(1)-hybridized carbon atoms are considered as a possible phase of carbon since decades. Whereas the debate about the stability of the corresponding bulk phase carbyne continues until today, the existence of isolated chains of carbon atoms has meanwhile been corroborated experimentally. Since graphene, as the two-dimensional sp(2)-bonded allotrope of carbon, has become a vast field, the question about the importance of one-dimensional carbon became of renewed interest. The present article gives an overview of the work that has been carried out on chains of carbon atoms in the past one or two decades. The review concentrates on isolated chains of carbon atoms and summarizes the experimental observations to date. While the experimental information is still very limited, many calculations of the physical and chemical properties have been published in the past years. Some of the most important theoretical studies and their importance in the present experimental situation are reviewed

Synchrotron-based radioscopy employing spatio-temporal micro-resolution for studying fast phenomena in liquid metal foams

High-speed synchrotron-based radioscopy is applied to study a coalescence event (which lasts ∼2 ms) in situ in a liquid metal foam

Stability of various particle stabilised aluminium alloys foams made by gas injection

Aluminium alloy foams are created by injecting air into liquid alloys containing non metallic particles. In addition to an alloy containing the usual SiC particles, other types of metal particle composites are studied, which are created by in situ reactions in the melts two fluoride salts react and form TiB2 particles, and Ca addition or addition of CuO and SiO2 gives rise to the formation of various oxides and spinel particles. Injecting air into the molten composites through two different steel cannulas leads to the formation of first bubbles and then foam. The entire process is monitored in situ by X ray radioscopy. The goal is not only to understand how and what kind of particles stabilise gas injected foams better, but also to reduce the fraction of added particles, which could improve mechanical properties, solve recycling issues and reduce production costs. All the observed composites are shown to have the potential to be processed to metallic foam. Melts containing TiB2 particles are found to perform as well as those containing SiC even at lower volume fractions. Oxidation of alloy melts promoted by Ca addition gives rise to melts that exhibit good foamability. Melts oxidised by CuO and SiO2 addition show partial stability. Mg is found to be a required alloying element to create stable foams. Smaller bubbles can be produced using smaller injector needle openings. By reducing bubble size and using new variants of in situ generated particles, more stable foams can be achieved with a lower number density of stabilising particle

An experimental study of columnar crystals using monodisperse microbubbles

We investigate the ordered arrangements of monodisperse microbub bles con ned within narrow cylinders. These foams were imaged using X ray tomography, allowing the 3D positions of the bubbles of the foam to be accurately determined. The structure of these foams closely re semble the minimum energy con guration of hard spheres in cylindrical con nement as found in simulations. For larger ratios, , of cylinder to bubble diameter two and three layered crystals were formed. Each layer of these structures is found to be ordered, with each internal layer resem bling structures found at lower values. The average number of contacts per bubble is seen to increase wit

The rupture of a single liquid aluminium alloy film

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The present study is based on the idea of understanding the rupture of films in metal foams by studying free standing metallic films as a model system. Liquid dynamics, the velocity of the rupturing material as well as the behaviour of ceramic particles inside the melt were analysed optically ex situ and by synchrotron X-ray radiography in situ. It was found that the resistance of films to rupture is mainly based on the interaction between solid particles and an immobile oxide skin, the formation of which depends on the oxygen content of the surrounding atmosphere and the presence of magnesium

Ab initio study of transport properties in defected carbon nanotubes: an O(N) approach

A combination of ab initio simulations and linear-scaling Green's functions techniques is used to analyze the transport properties of long (up to one micron) carbon nanotubes with realistic disorder. The energetics and the influence of single defects (mono- and di-vacancies) on the electronic and transport properties of single-walled armchair carbon nanotubes are analyzed as a function of the tube diameter by means of the local orbital first-principles Fireball code. Efficient O(N) Green's functions techniques framed within the Landauer-Buttiker formalism allow a statistical study of the nanotube conductance averaged over a large sample of defected tubes and thus extraction of the nanotubes localization length. Both the cases of zero and room temperature are addressed.Comment: 15 pages, 12 figures (submitted to J. Phys: Condens. Matter

Influence of Gas Pressure and Blowing Agent Content on the Formation of Aluminum Alloy Foam

Apart from density and alloy composition, the structure of solid metallic foams determines their mechanical performance. Herein, the effect of ambient pressure on the foaming behavior of AlSi6Cu4 precursors is studied. In situ X ray radioscopy is applied while foaming in a furnace chamber pressurized up to 40 amp; 8201;bar. The content of the blowing agent TiH2 is varied for each of the pressures chosen. The foam density and morphology are analyzed quantitatively using time resolved X ray radioscopy sequences and postsolidification X ray tomography data. The optimal content of blowing agent as well as the pore sizes and distributions is found to depend strongly on the ambient pressure. At high pressures very small, round, and uniformly distributed pores are formed and crack formation is avoided during the gas nucleation stage, which helps to prevent structural defects. Adjusting the ambient pressure allows for better control of the foam structure and density, which is relevant for commercial production and applicatio