Jungkyu Park

Sensitivity of thermal conductivity of carbon nanotubes to defect concentrations and heat-treatment

In the present work, we use reverse non-equilibrium molecular dynamics with adaptive intermolecular reactive empirical bond order interatomic potential to investigate sensitivity of thermal conductivity in (6, 6) single-walled carbon nanotubes (SWCNTs) to side-wall defects and high temperature heat- treatment. Effects of two side-wall defect types and their concentrations are evaluated: chemisorbed hydrogen adatoms on the SWCNT side wall and point vacancy defects. The results of the simulations indicate that the degree of hydrogenation and vacancy concentrations have very similar detrimental effects on the thermal conductivity of (6, 6) SWCNTs. Vacancy repair is evident with heat treatment, and heat-treatment temperatures of 3000 °C for up to 22 ns are found to transform point vacancies into various non-hexagonal side-wall defects. The vacancy repair is accompanied by an approximately 10% increase in thermal conductivity. In addition, thermal conductivity measurements in both heat-treated and non-heat tre...

Effects of heat treatment and contact resistance on the thermal conductivity of individual multiwalled carbon nanotubes using a Wollaston wire thermal probe

Thermal conductivity measurements in commercially available, chemical vapor deposition–grown, heat-treated and non-heat-treated multiwalled carbon nanotubes (MWCNTs) are reported. The thermal conductivity of individual samples is measured using a suspended platinum wire as a thermal resistance probe in a “T-type” configuration. Changes in third harmonic voltage are measured across the heated suspended platinum wire as a specimen is attached to the platinum wire’s midpoint. An analytic model is used to correlate the reduction in the average temperature of the probe wire to the thermal resistance (and thermal conductivity) of the attached sample. Experiments are implemented inside a scanning electron microscope equipped with nanomanipulators for sample selection, and a gas injection system for platinum based electron beam-induced deposition to improve thermal contact resistances. The results indicate a nearly 5-fold increase in the average thermal conductivity of MWCNT samples annealed with a 20-h 3000 °C a...

Phonon scattering and thermal conductivity of pillared graphene structures with carbon nanotube-graphene intramolecular junctions

We present results of a reverse non-equilibrium molecular dynamics study of thermal transport in single-walled carbon nanotube (SWCNT)-graphene junctions comprised of carbon-carbon (C-C) bonds with either sp2 or mixed sp2/sp3 hybridization. In both cases, a finite interfacial thermal resistance is observed at the SWCNT-graphene junctions for thermal transport in the out-of-plane direction. The interfacial thermal resistance at the junctions is attributed to the combined effects of scattering of the phonons at the SWCNT-graphene junctions due to the presence of distorted sp2 bonds in the junction region and the change in dimensionality of the medium along the phonon transport path as the phonons propagate from SWCNT pillars (quasi-1D) to graphene sheet (2D) and then again to SWCNTs. Moreover, the thermal resistance is found to depend on the C-C bond hybridization at the intramolecular junctions with mixed sp2/sp3 hybridization showing a higher interfacial resistance when compared to pure sp2 bonding. Therm...

Thermal resistance across interfaces comprising dimensionally mismatched carbon nanotube-graphene junctions in 3D carbon nanomaterials

In the present study, reverse nonequilibrium molecular dynamics is employed to study thermal resistance across interfaces comprising dimensionally mismatched junctions of single layer graphene floors with (6,6) single-walled carbon nanotube (SWCNT) pillars in 3D carbon nanomaterials. Results obtained from unit cell analysis indicate the presence of notable interfacial thermal resistance in the out-of-plane direction (along the longitudinal axis of the SWCNTs) but negligible resistance in the in-plane direction along the graphene floor. The interfacial thermal resistance in the out-of-plane direction is understood to be due to the change in dimensionality as well as phonon spectra mismatch as the phonons propagate from SWCNTs to the graphene sheet and then back again to the SWCNTs. The thermal conductivity of the unit cells was observed to increase nearly linearly with an increase in cell size, that is, pillar height as well as interpillar distance, and approaches a plateau as the pillar height and the interpillar distance approach the critical lengths for ballistic thermal transport in SWCNT and single layer graphene. The results indicate that the thermal transport characteristics of these SWCNT-graphene hybrid structures can be tuned by controlling the SWCNT-graphene junction characteristics as well as the unit cell dimensions.

Thermal Transport at Carbon Nanotube-Graphene Junction

We present results of a molecular dynamics study to analyze thermal transport at carbon nanotube (CNT)-graphene junctions comprising of single layer graphene and (6,6) armchair single-walled carbon nanotubes (SWCNTs). Two possible junction types with different degrees of sp2 and sp3 hybridization are investigated. Reverse Non-Equilibrium Molecular Dynamics (RNEMD) simulations are used to obtain the thermal conductivities in these hybrid structures and also analyze the role of the interfacial thermal resistance at the SWCNT-graphene junctions in limiting thermal transport. The highest out-of-plane (along the SWCNT axis) thermal conductivity of a hybrid structure with a CNT-graphene junction was obtained to be 158.9±1.2 W/m-K when the junction comprised of only sp2 bonds with an interpillar distance of 15 nm and a pillar height of 200 nm. The highest in-plane thermal conductivity (along the graphene layer plane) with two CNT-graphene junctions was found to be 392.2±9.9 W/m-K with junctions comprising of only sp2 bonds and an interpillar distance of 20 nm and a pillar height of 25 nm. In all cases, junctions with mixed sp2/sp3 hybridization showed higher interfacial thermal resistance than junctions with pure sp2 bonds, and the thermal interfacial resistance was found to be weakly dependent on the length of CNT and the interpillar distance. The highest interfacial thermal resistance measured across the CNT-graphene junction was 3.10×10−6 K-cm2/W when the junction comprised of mixed sp2/sp3 bonds and with 15 nm interpillar distance and 50 nm pillar height.Copyright

Thermal Transport in 3D Pillared CNT-Graphene Nanostructures

Thermal transport in two types of 3D pillared SWCNT-graphene nanostructures, which combine graphene floors and (6,6) armchair single-walled carbon nanotube (SWCNT) columns is studied by measuring both in-plane and out-of-plane thermal conductivity using molecular dynamics (MD) simulations with the AIREBO interatomic potential. Interpillar distance and pillar height dependency of thermal conductivity in 3D pillared SWCNT-graphene super-structure are examined at various temperatures (300K, 600K, 900K, and 1200K). It is shown that the thermal conductivity of these 3D nanostructures can be readily tuned: the in-plane thermal conductivity increases with increasing interpillar distance while the out of plane thermal conductivity increases with increasing pillar height and decreasing interpillar distance. The highest in-plane thermal conductivity obtained is 40 W/m-K for 3D super-structure with Type 1 unit cell with a 3.3 nm interpillar distance and 1.2 nm pillar height at room temperature. The highest out of plane thermal conductivity is 6.8 W/m-K for 3D super-structure with Type 1 unit cell which has 2.1 nm interpillar distance and 4.2 nm pillar height. Later, these values are compared with the thermal conductivity values of pure (6,6) SWCNT and single graphene layer, which are calculated using MD with the same interatomic potential.Copyright

Sensitivity of Thermal Conductivity of Carbon Nanotubes to Defect Concentrations and Heat-Treatment

In the present study, classical MD simulations using reverse non-equilibrium molecular dynamics with the AIREBO interatomic potential are used to investigate the sensitivity of thermal conductivity in SWCNTs to side-wall defect concentration and heat-treatment. Two types of defects are investigated. First, the thermal conductivity of (6,6) SWCNTs is obtained as a function of concentration of chemisorbed hydrogen adatoms. Secondly, the thermal conductivity is obtained as a function of point-vacancy concentrations. The results of the studies show that 2 atom% of hydrogenation and 1.5–2% vacancy concentrations have very similar detrimental effects on the thermal conductivity of SWCNT. Vacancy repair is evident with heat treatment, and heat-treatments at 3000°C for up to 22 ns are found to transform point vacancies into various types of non-hexagonal side-wall defects; this vacancy repair is accompanied by a ca. 10% increase in thermal conductivity. Thermal conductivity measurements in both heat-treated and non-heat treated chemical vapor deposition grown MWCNTs are also reviewed. The results suggest that CNT thermal conductivity can be drastically increased if measures are taken to remove common defects from the SWCNT side-walls.Copyright

Electronic structure changes with room‐temperature aging in supersaturated alloys of cadmium in zinc

The de Haas‐van Alphen effect is used to determine the change of the Zn Fermi surface parameters in dilute Zn Cd alloys containing up to 1.2 at.% Cd. Its application in observing the change of the matrix band structure during room‐temperature aging is demonstrated. The presence of local concentration gradients and strain fields in the matrix during precipitation produce strong phase‐smearing effects.