James Morris

Senior Staff, Materials Science & Technology Division, Oak Ridge National Laboratory
Associate Professor, Materials Science & Engineering , University of Tennessee

(865) 974-2484 (at UT)
(865) 576-7094 (at ORNL)
323C Dougherty
E-mail: morrisj at ornl.gov

Teaching:

  • Spring 2012: Materials Science 513 "Fundamentals of Materials Science III" (Thermodynamics, phase diagrams & kinetics)

    Research Areas

    Theoretical and computational modeling of materials at small length scales, particulary for energy applications. Most work is in close collaboration with experimentalists. These include the following projects:

    Recent publications for James R. Morris

    Size Effects and Stochastic Behavior of Nanoindentation Pop In
    J. R. Morris, H. Bei, G. M. George and E. P. George
    Phys. Rev. Lett. 106, 165502 (2011)
     A statistical model was developed to explain the size-dependent stresses needed to initiate plasticity in nano- and micro-scale volumes during nanoindentation. This marks an important step towards understanding and accurately predicting the strengths of small-scale structures which tend to be inherently stochastic unlike the well-behaved strengths of large-scale structures. Our model captures two essential ideas: (i) the larger the volume of stressed material, the greater the probability of finding a dislocation there, and (ii) pre-existing dislocations can be activated at a much lower stress than that needed to nucleate a new dislocation. These concepts were quantified utilizing the elastic stress fields underneath spherical indenters and the probabilities of finding dislocations, to derive the probability for plasticity to initiate at any given stress. The predictions of our model agree well with experiments: we find that the stress for incipient plasticity ranges from the ideal strength in small volumes, to one order of magnitude lower in larger volumes, with wide variability at intermediate sizes.

    Ginzburg-Landau-Type Multiphase Field Model for Competing fcc and bcc Nucleation
    G. I. Tóth, J. R. Morris, and L. Gránásy
    Phys. Rev. Lett. 106, 045701 (2011)
     We address crystal nucleation and fcc-bcc phase selection in alloys using a multiphase field model that relies on Ginzburg-Landau free energies of the liquid-fcc, liquid-bcc, and fcc-bcc subsystems, and determine the properties of the nuclei as a function of composition, temperature, and structure. With a realistic choice for the free energy of the fcc-bcc interface, the model predicts well the fcc-bcc phase-selection boundary in the Fe-Ni system.
    Viscosity, Shear Waves, and Atomic-Level Stress-Stress Correlations
    V. A. Levashov, J. R. Morris, and T. Egami
    Phys. Rev. Lett. 106, 115703 (2011)
     The Green-Kubo equation relates the macroscopic stress-stress correlation function to a liquid’s viscosity. The concept of the atomic-level stresses allows the macroscopic stress-stress correlation function in the equation to be expressed in terms of the space-time correlations among the atomic-level stresses. Molecular dynamics studies show surprisingly long spatial extension of stress-stress correlations and also longitudinal and transverse waves propagating in liquids over ranges which could exceed the system size. The results reveal that the range of propagation of shear waves corresponds to the range of distances relevant for viscosity. Thus our results show that viscosity is a fundamentally nonlocal quantity. We also show that the periodic boundary conditions play a nontrivial role in molecular dynamics simulations, effectively masking the long-range nature of viscosity.

    Structure and hydrogen adsorption properties of low density nanoporous carbons from simulations
    L.J. Peng and J. R. Morris
    Carbon 50, 1394-1406 (2012)
     We systematically model the hydrogen adsorption in nanoporous carbons over a wide range of carbon bulk densities (0.6–2.4 g/cm3) by using tight binding molecular dynamics simulations for the carbon structures and thermodynamics calculations of the hydrogen adsorption. The resulting structures are in good agreement with the experimental data of ultra-microporous carbon (UMC), a wood-based activated carbon, as indicated by comparisons of the microstructure at atomic level, pair distribution function, and pore size distribution. The hydrogen adsorption calculations in carbon structures demonstrate both a promising hydrogen storage capacity (excess uptake of 1.33 wt.% at 298 K and 5 MPa, for carbon structures at the lower range of densities) and a reasonable heat of adsorption (12–22 kJ/mol). This work demonstrates that increasing the heat of adsorption does not necessarily increase the hydrogen uptake. In fact, the available adsorption volume is as important as the isosteric heat of adsorption for hydrogen storage in nanoporous carbons.

    Graphene/graphane interface energy and implications for defects
    F. W. Averill and J. R. Morris
    Phys. Rev. B 84, 035411 (2011)
     Recent theoretical work has shown that electronic properties of graphene sheets can be systematically modified by the partial hydrogenation of the sheets. Two possible perfect and distinct graphene/graphane interfaces (called zigzag and armchair) have very different but potentially useful electronic properties, which are nevertheless likely to be affected by the presence of defects. In an effort to evaluate their relative energetics and their potential for defects, the structure and energies of the zigzag and armchair interfaces have been computed for infinite sheets of periodically alternating stripes of graphene and graphane ribbons of various widths. The presence of an interface causes significant strains in both the graphene and graphane regions, with both shear strains and area strains typically close to 1%. The associated large strain energies may lead to defects that relieve the strain but disrupt the lattice. The energies per unit length associated with the interfaces alone are approximately 0.12 eV/Å for the zigzag interface and 0.11 eV/Å for the armchair. The large misfit strains and energies suggest that formation of strain-relieving defects at the interface should be highly favorable.

    Selected older publications: