Beyond the Maxwell Limit: Thermal Conduction in Nanofluids with Percolating Fluid Structures

In a well-dispersed nanofluid with strong cluster-fluid attraction, thermal conduction paths can arise through percolating amorphous-like interfacial structures. This results in a thermal conductivity enhancement beyond the Maxwell limit of 3*phi, with phi being the nanoparticle volume fraction. Our findings from non-equilibrium molecular dynamics simulations, which are amenable to experimental verification, can provide a theoretical basis for the development of future nanofluids.Comment: 5 Pages, 3 Figures, In Review: APL, Accepted for presentation at "Nanofluids: Fundamentals and Applications", September 16-20, 2007, Copper Mountain, Colorad

Towards Understanding Cement Paste Creep: Implications from Glass Studies

Research briefThis research brief presents foundational work toward understanding the mechanics of the complex process of creep by evaluating the phenomena occurring at the nano-scale in metallic glass, a model material, to better understand what causes creep in C-S-H.This research was carried out by CSHub@MIT with sponsorship provided by the Portland Cement Association and the Ready Mixed Concrete Research & Education Foundation. CSHub@MIT is solely responsible for content

Small-Energy Rotational Transitions in Slow-Neutron Scattering by Water

A model which treats one rotational degree of freedom as hindered and the other as free and all translational degrees of freedom as hindered has been employed to calculate neutron differential scattering cross section of water in the region of small-energy transfers. The distribution is found to be sensitive to the presence of free-rotation transitions. It is suggested that such transitions present additional complexities in the study of molecular center-of-mass motions from high-resolution scattering data.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86122/1/PhysRev.131.2547-RKO.pd

Calculating Phase-Coherent Quantum Transport in Nanoelectronics with \u3cem\u3eab initio\u3c/em\u3e Quasiatomic Orbital Basis Set

We present an efficient and accurate computational approach to study phase-coherent quantum transport in molecular and nanoscale electronics. We formulate a Green’s-function method in the recently developed ab initio nonorthogonal quasiatomic orbital basis set within the Landauer-Büttiker formalism. These quasiatomic orbitals are efficiently and robustly transformed from Kohn-Sham eigenwave functions subject to the maximal atomic-orbital similarity measure. With this minimal basis set, we can easily calculate electrical conductance using Green’s-function method while keeping accuracy at the level of plane-wave density-functional theory. Our approach is validated in three studies of two-terminal electronic devices, in which projected density of states and conductance eigenchannel are employed to help understand microscopic mechanism of quantum transport. We first apply our approach to a seven-carbon atomic chain sandwiched between two finite crosssectioned Al(001) surfaces. The emergence of gaps in the conductance curve originates from the selection rule with vanishing overlap between symmetry-incompatible conductance eigenchannels in leads and conductor. In the second application, a (4,4) single-wall carbon nanotube with a substitutional silicon impurity is investigated. The complete suppression of transmission at 0.6 eV in one of the two conductance eigenchannels is attributed to the Fano antiresonance when the localized silicon impurity state couples with the continuum states of carbon nanotube. Finally, a benzene-1,4-dithiolate molecule attached to two Au(111) surfaces is considered. Combining fragment molecular orbital analysis and conductance eigenchannel analysis, we demonstrate that conductance peaks near the Fermi level result from resonant tunneling through molecular orbitals of benzene- 1,4-dithiolate molecule. In general, our conductance curves agree very well with previous results obtained using localized basis sets while slight difference is observed near the Fermi level and conductance edges

Understanding the mechanisms of amorphous creep through molecular simulation

Molecular processes of creep in metallic glass thin films are simulated at experimental timescales using a metadynamics-based atomistic method. Space-time evolutions of the atomic strains and nonaffine atom displacements are analyzed to reveal details of the atomic-level deformation and flow processes of amorphous creep in response to stress and thermal activations. From the simulation results, resolved spatially on the nanoscale and temporally over time increments of fractions of a second, we derive a mechanistic explanation of the well-known variation of creep rate with stress. We also construct a deformation map delineating the predominant regimes of diffusional creep at low stress and high temperature and deformational creep at high stress. Our findings validate the relevance of two original models of the mechanisms of amorphous plasticity: one focusing on atomic diffusion via free volume and the other focusing on stress-induced shear deformation. These processes are found to be nonlinearly coupled through dynamically heterogeneous fluctuations that characterize the slow dynamics of systems out of equilibrium. Keywords: creep, molecular simulation, deformation mechanism, atomistic modeling, metallic glassUnited States. Department of Energy (Grant DE-NE0008450)National Science Foundation (U.S.) (CAREER Grant DMR-1654548)United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0002633

Dynamical Behavior of Heat Conduction in Solid Argon

Equilibrium molecular dynamics is performed to obtain the thermal conductivity of crystalline argon using the Green-Kubo formalism, which permits the study of dynamical details of the transport process. A large system run to longer times is used to derive the heat flux autocorrelation functions from the low temperature solid to the liquid state. The power spectrum of an autocorrelation function reveals the change in the nature of the underlying atomic motions across the temperature range

Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB\u3csub\u3e2\u3c/sub\u3e+SiC and Oxygen Transport Mecha

Refractory diboride with silicon carbide additive has a unique oxide scale microstructure with two condensed oxide phases (solid+liquid), and demonstrates oxidation resistance superior to either monolithic diboride or silicon carbide. We rationalize that this is because the silica-rich liquid phase can retreat outward to remove the high SiO gas volatility region, while still holding onto the zirconia skeleton mechanically by capillary forces, to form a solid pillars, liquid roof scale architecture and maintain barrier function. Basic assessment of the oxygen carriers in the borosilicate liquid in oxygen-rich condition is performed using first-principles calculations. It is estimated from entropy and mobility arguments that above a critical temperature Tc~1500°C, the dominant oxygen carriers should be network defects, such as peroxyl linkage or oxygen-deficient centers, instead of molecular O2* as in the Deal–Grove model. These network defects will lead to sublinear dependence of the oxidation rate with external oxygen partial pressure. The present work suggests that there could be significant room in improving the high-temperature oxidation resistance by refining the oxide scale microstructure as well as controlling the glass chemistry

Multiscale materials modeling at the mesoscale

The challenge to link understanding and manipulation at the microscale to functional behaviour at the macroscale defines the frontiers of mesoscale science. O ver the course of the past decade, the impact of computation on materials research has expanded dramatically. A number of panel reports In 2012, the Office of Science, part of the US Department of Energy, initiated a dialogue with the science community through a series of town-hall meetings, the purpose being to identify new science frontiers at the mesoscale 10 . A website (www.meso2012.com) 4 was established to solicit community input. A report, From Quanta to the Continuum, has been released 4 along with an overview of the findings relevant to the materials community 11 . (Reference 4 is particularly relevant in that it gives a complete account of the broad community discussions of strategic research that connects materials science and engineering to the science and technology community at large.) It seems that 'mesoscale science' (MSS) should be viewed as an open concept, the principles of which are not precisely specified until a problem context is established. In other words, MSS can be characterized in many different ways. An early approach looked for organizing principles governing certain phenomena, such as energy landscape descriptions of transition states, selforganization and dynamical feedback, and frustration (or localization) effects, known , and corresponding theoretical calculations (solid curves

Point defect concentrations in metastable Fe-C alloys

Point defect species and concentrations in metastable Fe-C alloys are determined using density functional theory and a constrained free-energy functional. Carbon interstitials dominate unless iron vacancies are in significant excess, whereas excess carbon causes greatly enhances vacancy concentration. Our predictions are amenable to experimental verification; they provide a baseline for rationalizing complex microstructures known in hardened and tempered steels, and by extension other technological materials created by or subjected to extreme environments