Atomistic simulations of the solubilization of single-walled carbon nanotubes in toluene

Solubilization of the armchair, metallic (10,10) single-walled carbon nanotubes (SWCNTs) in toluene is modeled using molecular dynamics simulations. Inter- and intra-molecular atomic interactions in the SWCNT + toluene system are represented using COMPASS (Condensed-phased Optimized Molecular Potential for Atomistic Simulation Studies), the first ab initio forcefield that enables an accurate and simultaneous prediction of various gas-phase and condensed-phase properties of organic and inorganic materials.The results obtained show that due to a significant drop in the configurational entropy of toluene, the solvation Gibbs free energy for these nanotubes in toluene is small but positive suggesting that a suspension of these nanotubes in toluene is not stable and that the nanotubes would fall out of the solution. This prediction is consistent with experimental observations.     

Reactor length-scale modeling of chemical vapor deposition of carbon nanotubes

Chemical Vapor Deposition (CVD) of carbon nanotubes from a gas mixture consisting of methane (carbon precursor) and hydrogen (a carrier gas) in the presence of cobalt, nickel or iron catalytic particles in a cylindrical reactor is modeled at the reactor length-scale by solving a continuum-based coupled boundary-layer laminar-flow hydrodynamics, heat-transfer, gas-phase chemistry and surface chemistry problem. The model allows determination of the gas-phase fields for temperature, velocity, and various species as well as the surface-species coverages and the carbon deposition rate. Various available experimental and theoretical assessments are used to construct the necessary database for gas-phase and surface chemistry and gas-phase transport parameters. A reasonably good agreement is found between the model predicted and the experimentally measured carbon nanotubes deposition rates over a relatively large range of processing conditions.     

Effect of martensitic transformation in Ti–15 at %V β-phase particles on lamellar boundary decohesion in γ-TiAl Part II Finite element analysis of crack-bridging phenomenon

The commercial finite element package ABAQUS has been used to analyse the crack bridging process by Ti-15 at%V -phase particles dispersed in -TiAl matrix in the presence of particle–matrix decohesion. Both the particle–matrix decohesion potential and the -phase materials constitutive relations are found to have a major effect on the ductility, fracture toughness and failure mode of the – two-phase material. The interface potential is found to primarily affect the distribution of the normal interface strength ahead of the advancing interfacial crack and the mode (gradual versus sudden) of decohesion. The -phase materials constitutive relations are found to influence the location of nucleation of the interfacial cracks and, in turn, the mode of decohesion. A metastable -phase that can plastically deform at low stress levels by undergoing a stress-assisted martensitic transformation, but experience a high rate of strain hardening is found to give rise to the largest levels of ductility and fracture toughness is the – two-phase material. © 1998 Kluwer Academic Publishers     

Effect of martensitic transformation in Ti–15 at % V β-phase particles on lamellar boundary decohesion in γ-TiAl Part I Derivation of interface decohesion potentials

Molecular statics atomistic simulations of the Ti–15 at% V body-centred cubic (bcc) -phase–-TiAl interface and – lamellar boundary decohesion processes have been carried out to determine the corresponding decohesion potentials. The potentials are subsequently related to the dislocation structure of the interface–boundary. Atomic interactions have been represented using the appropriate embedded atom method (EAM) interatomic potential functions. The results obtained show that the decohesion potential functions are quite complex because they have to account for the instabilities that occur under some modes of interface decohesion and for the periodic character of the interfacial shear. Lastly, the use of decohesion potentials to derive constitutive relations for continuum-type interfacial elements and their implementation in the finite element method are presented. © 1998 Kluwer Academic Publishers     

A “four-ferrocene” modified stem-loop structure as a probe for sensitive detection and single-base mismatch discrimination of DNA

We report the use of a four-ferrocene modified oligonucleotide as a probe for DNA detection with a gold electrode microsystem. This oligonucleotide is synthesized by automated solid-phase synthesis with four successive ferrocene moieties at the 5′-end and a C6-thiol modifier group at the 3′-end. The grafting of this 4Fc-DNA probe on a gold electrode microsystem results in the appearance of the ferrocene redox couple in cyclic voltammetry. The probe sequence is a stem-loop structure that folds efficiently on the electrode, thus optimizing electron transfer. Such architecture serves as sensor for DNA detection which is based on hybridization. The resulting disposable voltammetric sensor allowed direct, reagentless DNA detection in a small volume (20 μL). Electrochemical response upon hybridization with complementary short sequence (30-base length) and long sequence (50-base length) strands was observed by differential pulse voltammetry. Current variations were compared. The longer the sequence, the greater the decrease in current. The system's detection limit was estimated at 3.5 pM (0.07 fmol in 20 μL) with the 50-base length target and provided a dynamic detection range between 3.5 pM and 5 nM. Single mismatch detection showed a good level of sensitivity. The system was regenerated twice with no significant loss of Fc signal. Finally, 1 pM sensitivity was reached with a long chain analog of DNA PCR products of Influenza virus.     

Modifications in the AA5083 Johnson-Cook Material Model for Use in Friction Stir Welding Computational Analyses

Johnson-Cook strength material model is frequently used in finite-element analyses of various manufacturing processes involving plastic deformation of metallic materials. The main attraction to this model arises from its mathematical simplicity and its ability to capture the first-order metal-working effects (e.g., those associated with the influence of plastic deformation, rate of deformation, and the attendant temperature). However, this model displays serious shortcomings when used in the engineering analyses of various hot-working processes (i.e., those utilizing temperatures higher than the material recrystallization temperature). These shortcomings are related to the fact that microstructural changes involving: (i) irreversible decrease in the dislocation density due to the operation of annealing/recrystallization processes; (ii) increase in grain-size due to high-temperature exposure; and (iii) dynamic-recrystallization-induced grain refinement are not accounted for by the model. In this study, an attempt is made to combine the basic physical-metallurgy principles with the associated kinetics relations to properly modify the Johnson-Cook material model, so that the model can be used in the analyses of metal hot-working and joining processes. The model is next used to help establish relationships between process parameters, material microstructure and properties in friction stir welding welds of AA5083 (a non-age-hardenable, solid-solution strengthened, strain-hardened/stabilized Al-Mg-Mn alloy).     

Spall-Fracture Physics and Spallation-Resistance-Based Material Selection

Spallation is a fracture mode commonly observed in ballistically/blast-wave-loaded structures. The interaction between decompression waves generated within the target structure produces tensile stresses which, if of a sufficient magnitude, may cause material damage and ultimate fracture (spallation). In this study, the phenomenon of spall-fracture is analyzed within a one-dimensional Lagrangian framework. Two distinct analyses are carried out. Within the first analysis, decompression waves are treated as decompression shocks, which simplified the analysis and enabled the formation of spallation-strength-based material index. In the second analysis, decompression waves are treated as smooth (centered simple) waves. This increased the fidelity of the computational analysis, but the material-selection procedure could be done only numerically and an explicit formulation of the spallation-strength-based material-selection index could not be carried out. Overall, the two analyses yielded similar results for the spallation-strength-based material-selection criterion suggesting that the simpler (decompression shock based) one is still adequate for use in the material-selection process.     

Shock-Wave Attenuation and Energy-Dissipation Potential of Granular Materials

The propagation of uniaxial-stress planar shocks in granular materials is analyzed using a conventional shock-physics approach. Within this approach, both compression shocks and decompression waves are treated as (stress, specific volume, particle velocity, mass-based internal energy density, temperature, and mass-based entropy density) propagating discontinuities. In addition, the granular material is considered as being a continuum (i.e., no mesoscale features like grains, voids, and their agglomerates are considered). However, while the granular material is treated as a (smeared-out) continuum, it is recognized that it contains a solid constituent (parent matter), and that the structurodynamic properties (i.e., Equations of State (EOS) and Hugoniot relations) of the granular material are related to its parent matter. Three characteristic shock loading regimes of granular material are considered and, in each case, an analysis is carried out to elucidate shock attenuation and energy dissipation processes. In addition, an attempt is made to identify a metric (a combination of the material parameters) which quantifies the intrinsic ability of a granular material to attenuate a shock and dissipate the energy carried by the shock. Toward that end, the response of a typical granular material to a flat-topped compressive stress pulse is analyzed in each of the three shock loading regimes.     

Reliability-Based Design Optimization for Durability of Ground Vehicle Suspension System Components

The effect of materials processing- and component manufacturing-induced uncertainties in material properties and component shape and size on the reliability of component performance is investigated. Specifically, reliability of a suspension system component from a high-mobility multipurpose wheeled vehicle which typically can fail under low-cycle strain-based fatigue conditions is analyzed. Toward that end, the most advanced reliability-based design optimization methods available in the literature were combined with the present understanding of low-cycle fatigue durability and applied to the component in question. This entailed intricate integration of several computational tools such as multibody vehicle dynamics, finite-element simulations, and fatigue strain-life assessment/prediction techniques. The results obtained clearly revealed the importance of consideration of material property uncertainties in attaining vehicle performance of critical structural components in complex systems (e.g., a vehicle).     

Multidisciplinary Design Optimization for Glass-Fiber Epoxy-Matrix Composite 5 MW Horizontal-Axis Wind-Turbine Blades

A multi-disciplinary design-optimization procedure has been introduced and used for the development of cost-effective glass-fiber reinforced epoxy-matrix composite 5 MW horizontal-axis wind-turbine (HAWT) blades. The turbine-blade cost-effectiveness has been defined using the cost of energy (CoE), i.e., a ratio of the three-blade HAWT rotor development/fabrication cost and the associated annual energy production. To assess the annual energy production as a function of the blade design and operating conditions, an aerodynamics-based computational analysis had to be employed. As far as the turbine blade cost is concerned, it is assessed for a given aerodynamic design by separately computing the blade mass and the associated blade-mass/size-dependent production cost. For each aerodynamic design analyzed, a structural finite element-based and a post-processing life-cycle assessment analyses were employed in order to determine a minimal blade mass which ensures that the functional requirements pertaining to the quasi-static strength of the blade, fatigue-controlled blade durability and blade stiffness are satisfied. To determine the turbine-blade production cost (for the currently prevailing fabrication process, the wet lay-up) available data regarding the industry manufacturing experience were combined with the attendant blade mass, surface area, and the duration of the assumed production run. The work clearly revealed the challenges associated with simultaneously satisfying the strength, durability and stiffness requirements while maintaining a high level of wind-energy capture efficiency and a lower production cost.