arXiv subjects

R. Ruoff

Publications and source records attributed to R. Ruoff.

5 recordsLinked to original sources

arXiv2016

Characterization of hexagonal boron nitride layers on nickel surfaces by low-energy electron microscopy

The thickness and interfacial geometry of hexagonal boron nitride (hBN) films grown by chemical vapor deposition on polycrystalline nickel foils is studied using low-energy electron microscopy (LEEM). The reflectivity of the electrons, measured over an energy range of 0 - 20 eV, reveals distinct minima and maxima. The measured data is compared with simulations based on a first-principles description of the electronic structure of the material. From this comparison, the number of hBN layers and the separation between the lowest hBN layer and the nickel surface is deduced. The coupling of interlayer states of the hBN to both image-potential and Shockley-type surface states of the nickel is discussed, and the dependence of the reflectivity spectra on the surface orientation of nickel grains is examined.

cond-mat.mes-hall↗

arXiv2005

Dynamic Quantized Fracture Mechanics

A new quantum action-based theory, Dynamic Quantized Fracture Mechanics (DQFM), is presented that modifies continuum-based dynamic fracture mechanics. The crack propagation is assumed as quantized in both space and time. The static limit case corresponds to Quantized Fracture Mechanics (QFM), that we have recently developed to predict the strength of nanostructures.

cond-mat.mtrl-sci↗

arXiv2005

Nanoscale Weibull Statistics

In this paper a modification of the classical Weibull Statistics is developed for nanoscale applications. It is called Nanoscale Weibull Statistics. A comparison between Nanoscale and classical Weibull Statistics applied to experimental results on fracture strength of carbon nanotubes clearly shows the effectiveness of the proposed modification. A Weibull's modulus around 3 is, for the first time, deduced for nanotubes. The approach can treat (also) a small number of structural defects, as required for nearly defect free structures (e.g., nanotubes) as well as a quantized crack propagation (e.g., as a consequence of the discrete nature of matter), allowing to remove the paradoxes caused by the presence of stress-intensifications.

cond-mat.mtrl-sci↗

arXiv2002

Effects of Defects on the Strength of Nanotubes: Experimental-Computational Comparisons

The failure stresses and strains of nanotubes given by theoretical or numerical predictions are much higher than observed in experiments. We show that defects can explain part of this discrepancy: for an n-atom defect with 2<=n<=8, the range of failure stresses for a molecular mechanics calculation is found to be 36GPa to 64GPa. This compares quite well with upper end of the experimental failure stresses, 11GPa to 63GPa. The computed failure strains are 4% to 8%, whereas the experimental values are 2% to 13%. The underprediction of failure strains can be explained by the slippage that occurred in the experiments. The failure processes of nanotubes are clearly brittle in both the experiments and our calculations.

physics.atm-clus↗

arXiv2002

Atomistic Simulations of Nanotube Fracture

The fracture of carbon nanotubes is studied by atomistic simulations. The fracture behavior is found to be almost independent of the separation energy and to depend primarily on the inflection point in the interatomic potential. The rangle of fracture strians compares well with experimental results, but predicted range of fracture stresses is marketly higher than observed. Various plausible small-scale defects do not suffice to bring the failure stresses into agreement with available experimental results. As in the experiments, the fracture of carbon nanotubes is predicted to be brittle. The results show moderate dependence of fracture strength on chirality.

physics.atm-clus↗