Arun Bodapati

Effect of chemical functionalization on thermal transport of carbon nanotube composites

We use molecular dynamics simulations to analyze the role of chemical bonding between the matrix and the fiber on thermal transport in carbon nanotube organic matrix composites. We find that chemical bonding significantly reduces tube-matrix thermal boundary resistance, but at the same time decreases intrinsic tube conductivity. Estimates based on the effective medium theory predict increase, by about a factor of two, of the composite conductivity due to functionalization of single-walled nanotubes with aspect ratios within 100–1000 range. Interestingly, at high degree of chemical functionalization, intrinsic tube conductivity becomes independent of the bond density.

Limits of localized heating by electromagnetically excited nanoparticles

Based on an analysis of the diffusive heat flow equation, we determine limits on the localization of heating of soft materials and biological tissues by electromagnetically excited nanoparticles. For heating by rf magnetic fields or heating by typical continuous wave lasers, the local temperature rise adjacent to magnetic or metallic nanoparticles is negligible. However, heat dissipation for a large number of nanoparticles dispersed in a macroscopic region of a material or tissue produces a global temperature rise that is orders of magnitude larger than the temperature rise adjacent to a single nanoparticle. One approach for producing a significant local temperature rise on nanometer length scales is heating by high-power pulsed or modulated lasers with low duty cycle.

Thermal transport and grain boundary conductance in ultrananocrystalline diamond thin films

Although diamond has the highest known room temperature thermal conductivity, k∼2200W∕mK, highly sp3 amorphous carbon films have k<15W∕mK. We carry out an integrated experimental and simulation study of thermal transport in ultrananocrystalline diamond (UNCD) films. The experiments show that UNCD films with a grain size of 3–5nm have thermal conductivities as high as k=12W∕mK at room temperature, comparable with that of the most conductive amorphous diamond films. This value corresponds to a grain boundary (Kapitza) conductance greater than 3000MW∕m2K, which is ten times larger than that previously seen in any material. Our simulations of both UNCD and individual diamond grain boundaries yield values for the grain boundary conductance consistent with the experimentally obtained value, leading us to conclude that thermal transport in UNCD is controlled by the intrinsic properties of the grain boundaries.

Predicting the thermal conductivity of inorganic and polymeric glasses: The role of anharmonicity

The thermal conductivity of several amorphous solids is numerically evaluated within the harmonic approximation from Kubo linear-response theory following the formalism developed by Allen and Feldman [Phys. Rev. B 48, 12581 (1993)]. The predictions are compared to the results of molecular dynamics (MD) simulations with realistic anharmonic potentials and to experimental measurements. The harmonic theory accurately predicts the thermal conductivity of amorphous silicon, a model Lennard-Jones glass, and a bead-spring Lennard-Jones glass. For polystyrene and amorphous silica at room temperature, however, the harmonic theory underestimates the thermal conductivity by a factor of about 2. This result can be explained by the existence of additional thermal transport via anharmonic energy transfer. More surprisingly, the thermal conductivity of polystyrene and amorphous silica at low temperature (MD and experimental) are significantly below the predictions of the harmonic theory. Potential reasons for the failur...

Crossover in thermal transport mechanism in nanocrystalline silicon

Using vibrational mode analysis, we demonstrate that lattice vibrations in small grain (≲3nm) structurally inhomogeneous nanocrystalline silicon are almost identical to those in homogeneous amorphous structures with the majority of the vibrations delocalized and unpolarized. As a consequence the principal thermal conductivity mechanism in such a nanocrystalline material is the same as in the amorphous material. With increasing grain size the ability of vibrations to homogenize over the nanocrystalline structure is gradually lost and the phonon spectrum becomes progressively more like that of a crystalline material; this is reflected in a crossover in the mechanism of thermal transport.