Ishan Srivastava

Variable-cell method for stress-controlled jamming of athermal, frictionless grains

A method is introduced to simulate jamming of polyhedral grains under controlled stress that incorporates global degrees of freedom through the metric tensor of a periodic cell containing grains. Jamming under hydrostatic (isotropic) stress and athermal conditions leads to a precise definition of the ideal jamming point at zero shear stress. The structures of tetrahedra jammed hydrostatically exhibit less translational order and lower jamming-point density than previously described maximally random jammed hard tetrahedra. Under the same conditions, cubes jam with negligible nematic order. Grains with octahedral symmetry having s>0.5 (where s interpolates from octahedra [s=0] to cubes [s=1]) jam with an abundance of face-face contacts in the absence of nematic order. For sufficiently large face-face contact number, percolating clusters form that span the entire simulation box. The response of hydrostatically jammed tetrahedra and cubes to shear-stress perturbation is also demonstrated with the variable-cell method.

Thermal conduction in graphite flake-epoxy composites using infrared microscopy

Thermally conductive polymer composites, in particular those composed of polymers and carbon-based nanomaterials, are promising for thermal management in electronic devices because they offer high thermal conductivity at low filler loading. The effective thermal properties of these composites exhibit high variability that depend on the topological arrangements and morphological characteristics of the filler particles. In order to tailor the thermal conduction within these composites for use as an efficient heat dissipation material, careful control of the microstructural arrangement of the filler material is required. In this work, we use infrared (IR) microscopy to characterize thermal transport through epoxy composites containing sub-millimeter sized graphitic flakes as filler particles. Graphite flake-epoxy composites of two volume fractions (3%, 25%) are prepared and characterized using an infrared microscope with a temperature resolution of 0.1 K that images the temperature distribution at the top surface of the composite subject to a temperature gradient. The effective thermal conductivity of the composite with a 25% filler fraction was found to be 2.9 W/m-K, a factor of 16 higher than the neat epoxy. With the micron-scale resolution of the IR microscope, the steady-state particle-scale temperature fields within the composite are directly observed and highlight the non-uniform heat transfer pathways. This local temperature analysis reveals the impact of important microstructural features such as clustering of filler particles. Ultimately, this approach could be used to investigate percolation and anisotropic heat conduction in composites with shear aligned particles.

Microstructure and Thermal Conductivity Modeling of Granular Nanoplatelet Assemblies

Consolidation and sintering of bismuth telluride nanoplatelets is a cost-effective method of manufacturing high thermoelectric figure-of-merit materials. A structural optimization method is employed here to study the effects of columnar structures formed by nanoplatelet composites on thermal transport. The initially sparse and random distribution of nanoplatelets is compacted into a jammed state under an external hydrostatic stress, thereby simulating the compaction of nanopowders in experiments. The jammed morphology exhibits randomly oriented stacks of nanoplatelets and a discontinuous distribution of the pore phase within the microstructure. Grain and pore morphologies are statistically quantified using computational geometry techniques. High aspect ratio nanoplatelets exhibit large degree of stacking that results in large pore size heterogeneity. Quasi-ballistic heat conduction through the grain phase is modeled using a Landauer approach. The volume fraction and morphology of the discontinuous pore phase strongly influences thermal conductance by providing sites for phonon scattering at pore–grain interfaces. A thermal model incorporating interface scattering is employed to explore the relationship between microstructure and thermal transport.

Shear-induced failure in jammed nanoparticle assemblies

The state of stress during the bottom-up assembly of nanoparticles strongly correlates with the microstructure of dense nanoparticle aggregates therein. A range of interaction length scales exists in these dry granular systems spanning from particle-scale elastic repulsion to aggregate van der Waals cohesion; the competition among these interactions dominates athermal microstructural evolution under applied stress. In this work, structural optimization is employed to simulate the nano-mechanical physics of athermal densification and jamming. The translational and rotational motions of nanoparticles are optimized to static equilibrium. An initially sparse and random configuration of particles is compacted into a mechanically stable (i.e., jammed) state by densifying the system under various external-loading paths (e.g., hydrostatic, uniaxial, and shear). The resultant jammed structures and their responses to shear exhibit strong correlation with the strength of interactions in addition to particle shape [s...

Online Thermal Properties Database for Structure-Property Correlated Materials

New data for thermophysical properties of materials are being discovered and measured routinely throughout the world, but sometimes conflicts exist in the published data from various sources. We introduce an online thermal properties database intended for use as a repository for experimentally obtained thermophysical properties of materials by researchers across the globe. This online database is a part of www.thermalhub.org, which is a NSF-funded cyber infrastructure initiative aimed at serving the global heat transfer community. The relevant details associated with an experiment, such as the material type and composition, the thermophysical property measured, the test methods employed, the test conditions of the experiment, the results of the experiments, the uncertainty associated with the experimental results, and the literature where the results are published, are included as fields in the online database. It is known that one thermophysical property can be tested by various methods whose results may differ. Similarly, one thermophysical property can be experimentally measured under varying test conditions to produce different results.Copyright