Li, Ju
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Publication Dynamical Behavior of Heat Conduction in Solid Argon(2011-04-01) Kaburaki, Hideo; Li, Ju; Yip, Sidney; Kimizuka, HajimeEquilibrium 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.Publication Pressure-temperature phase diagram for shapes of vesicles: A coarse-grained molecular dynamics study(2009-10-06) Liu, Ping; Li, Ju; Zhang, Yong-weiCoarse-grained molecular dynamics simulations are performed to obtain the phase diagram for shapes of a vesicle with a variation in temperature and pressure difference across the membrane. Various interesting vesicle shapes are found, in particular, a series of shape transformations are observed for a vesicle with an initial spherical shape, which changes to a prolate shape, then an oblate shape, and then a stomatocyte shape, with either increasing temperature or decreasing pressure difference across the membrane.Publication Variable Nanoparticle-Cell Adhesion Strength Regulates Cellular Uptake(2010-09-21) Yuan, Hongyan; Li, Ju; Bao, Gang; Zhang, SulinIn receptor-mediated endocytosis, cells exercise biochemical control over the mechanics of adhesion to engulf foreign particles, featuring a variable adhesion strength. Here we present a thermodynamic model with which we elucidate that the variable adhesion strength critically governs the cellular uptake, yielding an uptake phase diagram in the space of ligand density and particle size. We identify from the diagram an endocytosed phase with markedly high uptake, encompassed by a lower and an upper phase boundary that are set, respectively, by the enthalpic and entropic limits of the adhesion strength. The phase diagram may provide useful guidance to the rational design of nanoparticle-based therapeutic and diagnostic agents.Publication Plastic flow and failure resistance of metallic glass: Insight from in situ compression of nanopillars(2008-04-14) Shan, Z. W; Li, Ju; Cheng, Y. Q; Minor, A. M.; Syed Asif, S. A; Warren, O. L.; Ma, E.We report in situ nanocompression tests of Cu-Zr-Al metallic glass (MG) pillars in a transmission electron microscope. This technique is capable of spatially and temporally resolving the plastic flow in MGs. The observations reveal the intrinsic ability of fully glassy MGs to sustain large plastic strains, which would otherwise be preempted by catastrophic instability in macroscopic samples and conventional tests. The high ductility in volume-limited MGs and the sample size effects in suppressing the rapid failure common to MGs are analyzed by modeling the evolution of the collectivity of flow defects toward localization.Publication Hydrostatic compression and high-pressure elastic constants of coesite silica(2008-03-06) Ogata, Shigenobu; Kimizuka, Hajime; Li, JuUsing density-functional theory, we computed all the independent elastic constants of coesite, a high-pressure polymorph of silica, as functions of pressure up to 15 GPa. The results are in good agreement with experimental measurements under ambient conditions. Also, the predicted pressure-dependent elastic properties are consistent with x-ray data in the literature concerning lattice strains at high pressures. We find that coesite, like quartz, exhibits a gradual softening of a shear modulus B44 with increasing pressure, in contrast to the rising bulk modulus.Publication Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB2+SiC and Oxygen Transport Mecha(2008-05-01) Li, Ju; Lenosky, Thomas J; Först, Clemens J; Yip, SidneyRefractory 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.Publication Lithium Fiber Growth on the Anode in a Nanowire Lithium Ion Battery During Charging(2011-05-04) Liu, Xiau Hua; Zhong, Li; Kushima, Akihiro; Zhang, Li Qiang; Li, Ju; Mao, Scott X.; Ye, Zhi Zhen; Sullivan, John P.; Huang, Jian YuLithium (Li) dendrite formation has been recognized as one of the major safety concerns for Li metal batteries but not for conventional Li ion batteries (LIBs) where Li metal is not used. With the advanced in situ transmission electron microscopy enabling direct observation of battery operation, we found that Li fibers with length up to 35 µm grew on nanowire tip after charging. The Li fibers growth were highly directional, i.e., nucleating from the nanowire tip, and extending along the nanowire axis, which was attributed to the strong electric field enhancement effect induced by the sharp nanowire tip. This study reveals a potential safety concern of short-circuit failure for LIBs using nanowire anodes.Publication Temperature and Strain-Rate Dependence of Surface Dislocation Nucleation(2008-05-05) Li, Ju; Zhu, Ting; Samanta, Amit; Leach, Austin; Gall, KenDislocation nucleation is essential to the plastic deformation of small-volume crystalline solids. The free surface may act as an effective source of dislocations to initiate and sustain plastic flow, in conjunction with bulk sources. Here, we develop an atomistic modeling framework to address the probabilistic nature of surface dislocation nucleation. We show the activation volume associated with surface dislocation nucleation is characteristically in the range of 1–10b3, where b is the Burgers vector. Such small activation volume leads to sensitive temperature and strain-rate dependence of the nucleation stress, providing an upper bound to the size-strength relation in nanopillar compression experiments.Publication Diffusive Molecular Dynamics and its Application to Nanoindentation and Sintering(2011-08-04) Li, Ju; Sarkar, Sanket; Cox, William; Lenosky, Thomas J; Bitzek, Erik; Wang, YunzhiThe interplay between diffusional and displacive atomic movements is a key to understanding deformation mechanisms and microstructure evolution in solids. The ability to handle the diffusional time scale and the structural complexity in these problems poses a general challenge to atomistic modeling. We present here an approach called diffusive molecular dynamics (DMD), which can capture the diffusional time scale while maintaining atomic resolution, by coarse-graining over atomic vibrations and evolving a smooth site-probability representation. The model is applied to nanoindentation and sintering, where intimate coupling between diffusional creep, displacive dislocation nucleation, and grain rotation are observed.Publication Calculating Phase-Coherent Quantum Transport in Nanoelectronics with ab initio Quasiatomic Orbital Basis Set(2010-11-23) Qian, Xiaofeng; Li, Ju; Yip, SidneyWe 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.