Jeffrey W. Kysar

Bio: Jeffrey W. Kysar is an academic researcher from Columbia University. The author has contributed to research in topics: Deformation (engineering) & Electron backscatter diffraction. The author has an hindex of 35, co-authored 139 publications receiving 21356 citations. Previous affiliations of Jeffrey W. Kysar include Columbia University Medical Center & Harvard University.

Continuum simulations of directional dependence of crack growth along a copper/sapphire bicrystal interface. Part I: experiments and crystal plasticity background

Abstract: Cracks that exhibit relative amounts of ductility along a copper/sapphire bicrystal interface are simulated within the context of continuum mechanics. The specimen in question exhibits a directional dependence of fracture; that is a crack oriented in one direction along the copper/sapphire interface propagates much more during a given load increment than does the crack oriented to propagate in the opposite direction along the interface. This phenomenon had previously been explained on the basis of an energetic competition between dislocation nucleation and cleavage failure at the two crack tips using both the Rice and Thomson (Philos. Mag. 29 (1974) 73) model as well as the more recent type of dislocation nucleation analysis by Rice (J. Mech. Phys. Solids 40 (1992) 239) based on a Peierls-like stress vs. displacement relationship on the slip plane. However, recent experiments by Kysar (Acta Mater. 48 (2000) 3509) have shown that the orientation of the directional dependence of fracture in the copper/sapphire bicrystal is opposite to that predicted on the basis of dislocation nucleation arguments. The goal of the present work is to attempt to explain the directional dependence of fracture solely on the basis of continuum mechanics. In Part I of this pair of papers we review the main results of the experiments and then set the stage for a series of finite element analyses of the bicrystal specimen by reviewing the fundamentals of single crystal plasticity and the general features of crack tip fields in single crystals. We then discuss two different constitutive hardening models for single crystals and predict that, depending on the hardening model, regions of single-slip around the crack tip may degenerate into regions of triple slip. This leads to a discussion of how the near-tip displacement field can change dramatically with constitutive models. Next the constitutive properties used in the simulations are fit to experimental data. Finally we describe the finite element meshes and procedures for simulating the stationary and quasistatically growing cracks. The simulation results are reported in Part II.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Atomically thin MoS2: a new direct-gap semiconductor

Abstract: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material

Graphene: Status and Prospects

Abstract: Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

Large-scale pattern growth of graphene films for stretchable transparent electrodes

Abstract: Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of approximately 280 Omega per square, with approximately 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V(-1) s(-1) and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

Graphene and Graphene Oxide: Synthesis, Properties, and Applications

Abstract: There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.