Self-consistent calculations of strain-induced band gap changes in semiconducting (n, 0) carbon nanotubes

First-principles density-functional calculations of the electronic structure, energy band gaps (Eg), and strain-induced band gap changes in moderate-gap single-walled (n,0) carbon nanotubes (SWNTs) are presented. It is confirmed that (n,0) SWNTs fall into two classes depending upon n mod 3=1 or 2. Eg is always lower for “mod 1” than for “mod 2” SWNTs of similar diameter. For n\u3c10, strong curvature effects dominate Eg; from n=10 to 17, the Eg oscillations, amplified due to σ−π mixing, decrease and can be explained very well with a tight-binding model which includes trigonal warping. Under strain, the two families of semiconducting SWNTs are distinguished by equal and opposite energy shifts for these gaps. For (10,0) and (20,0) tubes, the potential surface and band gap changes are explored up to approximately ±6% strain or compression. For each strain value, full internal geometry relaxation is allowed. The calculated band gap changes are ±(115±10) meV per 1% strain, positive for the mod 1 and negative for the mod 2 family, about 10% larger than the tight-binding result of ±97 meV and twice as large as the shift predicted from a tight-binding model that includes internal sublattice relaxation

Molecular modeling of the influence of crosslink distribution on epoxy polymers

Experimental studies on epoxies report that the microstructure consists of highlycrosslinked localized regions connected with a dispersed phase of low-crosslink density epoxy. Because epoxies play a major role in many structural applications, the influence of the crosslink distribution on the thermo-mechanical properties must be determined. But as experiments cannot reliably report the exact number or distribution of crosslinked covalent bonds present in the molecular network, molecular modeling is a valuable tool that can predict the influence of crosslink distribution on thermo-mechanical properties. In this study, molecular dynamics are used to establish well-equilibrated molecular models of an EPON 862-DETDA epoxy system with a range of crosslink densities and distributions. Crosslink distributions are varied by forming highly crosslinked clusters within the epoxy network and then forming additional crosslinks that connect between clusters. Results of simulations on these molecular models indicate that the thermal expansion coefficient decreases with overall crosslink density, both above and below the glass transition temperature. It is also found that within the range of crosslink distributions investigated, there is no discernible influence of crosslink distribution on the linear thermal expansion coefficient of the epoxy. © 2012 by Gregory M. Odegard

Predicting Mechanical Properties Using Continuum Mechanics-Based Approach: Micro-mechanics and Finite Element Analysis

The mechanical properties of nano-structured materials are important field of exploration in the fields of materials science and other engineering disciplines. Thorough understanding of underlying material structure and resulting properties require a variety of tools depending on the length scales of interest. This chapter reviews continuum mechanics-based techniques, with an emphasis on micro-scale modeling techniques: analytical and computational. In addition to micro-mechanics, different approaches to multiscale modeling are presented. With the appropriate choice of techniques, models can be bridged across multiple length scales leading to mechanistic understanding of the mechanics of materials. Some illustrative examples are also discussed that utilize the techniques presented here

Characterization of infiltration capacity of permeable pavements with porous asphalt surface using cantabrian fixed infiltrometer

Porous asphalt is used in Permeable Pavement Systems, but it is sensitive to surface clogging, which leads to a loss in its infiltration capacity. Test methods based on the use of permeable pavement models, which are manufactured in a laboratory and assessed under different clogging conditions, such as slope, rain, and runoff, have been widely applied to the study of permeable pavements with concrete blocks but not to the study of porous bituminous mixtures. The Cantabrian Fixed (CF) Infiltrometer has been used for the study of porous asphalt with void percentages between 20 and 33%. Three clogging scenarios were studied: 1) newly placed surface, 2) surface with an average maintenance level, and 3) clogged surface. Each clogging scenario was tested with five different slopes: 0, 2, 5, 8, and 10% and three repetitions. The direct rainfall simulation was produced by five lines of bubblers over the 0.25 - m 2 piece, and the runoff was simulated by one perforated pipe over a plastic ramp at the beginning of the surface. From the analysis of the results, it was concluded that a suitable design of a porous bituminous mixture, with a void percentage that increases with depth, along with surface brushing are enough to ensure and maintain a good infiltration capacity. Finally, an empirical, conservative model for estimating the porous asphalt infiltration capacity, based on the length, the clogging scenario, and the surface slope, is proposed