Michael F. P. Bifano

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...

Multifunctional carbon nanotube–epoxy composites for thermal energy management:

This paper reports development and thermal characterization of tin-capped vertically aligned multiwalled carbon nanotube array composites for thermal energy management in load-bearing structural applications. Three-omega voltage measurements are used to characterize thermal conductivity in the vertically aligned multiwalled carbon nanotube-epoxy composites as well as in its individual constituents, i.e. bulk epon-862 (matrix) and tin thin film in the temperature range 240 K–300 K, and in individual multiwalled carbon nanotubes at room temperature taken from the same vertically aligned multiwalled carbon nanotube batch as the one used to fabricate the carbon nanotube-epoxy composites. A 1-D multilayer thermal model that includes effects of thermal interface resistance is developed to interpret the experimental results. The thermal conductivity of the carbon nanotube-epoxy composite is estimated to be ∼5.8 W/m-K and exhibits a slight increase in the temperature range of 240 K to 300 K. The study suggests that morphological structure/quality of the individual multiwalled carbon nanotubes as well as thin tin capping layer are dominating factors that control the overall thermal conductivity of the thermal interface materials. These results are encouraging in light of the fact that thermal conductivity of a vertically aligned multiwalled carbon nanotube array can be increased by an order of magnitude by using a standard high-temperature post-annealing step. In this way, multifunctional (load bearing) thermal interface materials with effective through-thickness thermal conductivities as high as 25 W/m-K can potentially be fabricated.

Application of elastic wave dispersion relations to estimate thermal properties of nanoscale wires and tubes of varying wall thickness and diameter

This paper reports dependency of specific heat and ballistic thermal conductance on cross-sectional geometry (tube versus rod) and size (i.e., diameter and wall thickness), in free-standing isotropic non-metallic crystalline nanostructures. The analysis is performed using dispersion relations found by numerically solving the Pochhammer-Chree frequency equation for a tube. Estimates for the allowable phonon dispersion relations within the crystal lattice are obtained by modifying the elastic acoustic dispersion relations so as to account for the discrete nature of the materials crystal lattice. These phonon dispersion relations are then used to evaluate the specific heat and ballistic thermal conductance in the nanostructures as a function of the nanostructure geometry and size. Two major results are revealed in the analysis: increasing the outer diameter of a nanotube while keeping the ratio of the inner to outer tube radius (gamma) fixed increases the total number of available phonon modes capable of thermal population. Secondly, decreasing the wall thickness of a nanotube (i.e., increasing gamma) while keeping its outer diameter fixed, results in a drastic decrease in the available phonon mode density and a reduction in the frequency of the longitudinal and flexural acoustic phonon modes in the nanostructure. The dependency of the nanostructures specific heat on temperature indicates 1D, 2D, and 3D geometric phonon confinement regimes. Transition temperatures for each phonon confinement regime are shown to depend on both the nanostructures wall thickness and outer radius. Compared to nanowires (gamma = 0), the frequency reduction of acoustic phonon modes in thinner walled nanotubes (gamma = 0.96) is shown to elevate the ballistic thermal conductance of the thin-walled nanotube between 0.2 and 150 K. At 20 K, the ballistic thermal conductance of the thin-walled nanotube (gamma = 0.96) becomes 300% greater than that of a solid nanowire. For temperatures above 150 K, the trend in ballistic thermal conductance inverts. The greater number of phonon modes in nanostructures with increased outer diameter and wall thickness is shown to have a larger contribution to ballistic thermal conductance when compared to the increased contribution from the frequency reduction of acoustic phonon modes in thinner walled nanotubes.

Thermal properties of nanotubes and nanowires with acoustically stiffened surfaces

A multilayer elasticity model is developed to investigate the effects of acoustically stiffened surfaces (increased surface moduli) on the specific heat and thermal conductivity of typical nanowire and nanotubes as a function of temperature. Changes in phonon dispersion are analyzed using approximated phonon dispersion relations that result from the solutions to the frequency equation of a vibrating elastic tube or rod. The results of the investigation indicate a 10% reduction in specific heat and a 2% decrease in lattice thermal conductivity at 50 K for a 10 nm outer diameter crystalline nanotube with an inner diameter of 5 nm when the average Young’s modulus of the first three atomic layers on both the inner and outer free surfaces are increased by a factor of 1.87. In contrast, a 10 nm outer diameter nanowire composed of the same material and with an acoustically stiffened outer shell shows an approximate 30% increase in thermal conductivity and specific heat near 50 K. Our simplified model can potenti...

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

Thermal Conductivity of Heat Treated and Non-Heat Treated Individual Multiwalled Carbon Nanotubes

Thermal conductivity measurements of commercially available CVD grown individual multiwalled carbon nanotubes (MWCNTs) are reported. The measurements are performed using the three-omega-based Wollaston T-Type probe method inside a scanning electron microscope (SEM). An average 385% increase in thermal conductivity is measured for those MWCNTs samples which undergo a 20 hour 3000°C post annealing heat treatment. However, in most samples qualitatively characterized defects are found to negate any advantage of the heat treatment process. The highest thermal conductivity measured is 893.0 W/mK and is of a heat-treated sample. These results will help to improve the quality of MWCNT production and aid in the development of highly efficient CNT-structured thermal management devices and engineering materials.Copyright

Thermal Properties of Nanotubes and Nanowires With Acoustically Stiffened Surfaces

A core-shell elasticity model is employed to investigate the effect of a nanowire and nanotube’s increased surface moduli on specific heat, ballistic thermal conductance, and thermal conductivity as a function of temperature. Phonon confinement is analyzed using approximated phonon dispersion relations that result from solutions to the frequency equation of a vibrating rod and tube. The results indicate a maximum 10% decrease in lattice thermal conductivity and ballistic thermal conductance near 160 K for a 10 nm outer diameter nanotube with an inner diameter of 5 nm when the average Young’s Modulus of both the inner and outer free surfaces is increased by a factor of 1.53. In the presence of the acoustically stiffened surfaces, the specific heat of the nanotube is found to decrease by up to 20% at 160 K. Near room temperature, changes in thermal properties are less severe. In contrast, a 10 nm outer diameter nanowire composed of similar material exhibits up to a 12% maximum increase in thermal conductivity at 600 K, a 25% increase in ballistic thermal conductance at 400 K, and a 48% increase in specific heat at 470 K when its outer free surface is acoustically stiffened to the same degree. Our simplified model may be extended to investigate the acoustic tuning of nanowires and nanotubes by inducing surface stiffening or softening via appropriate surface chemical functionalization and coatings.Copyright

Application of Elastic Dispersion Relations to Estimate Thermal Properties of Nano-Scale Rods and Tubes of Varying Wall Thickness and Diameter

This paper reports the dependency of specific heat and ballistic thermal conductance on geometry and size in freestanding isotropic non-metallic crystalline nanowires and nanotubes having varying wall thicknesses and outer diameters. The analysis is performed using real dispersion relations found by numerically solving the Pochhammer-Chree frequency equation of a tube. The frequency equation is derived from the 3D cylindrical elastic wave model with stress free boundary conditions on both the inner and outer wall surfaces. Dimensional dependencies are distinctly noticeable and vary with specimen geometry and temperature. Trends in dimensional transition points are seen by varying the ratio of inner to outer nanotube radius (γ) for a 5 nm fixed outer diameter. With increasing γ, heat capacity and ballistic thermal conductance is shown to collapse onto that of a solid nanowire. Additionally, thermal properties of thick-walled nanotubes (γ = 0.5) having diameters of 5 nm, 10 nm, 15 nm, and 20 nm, are also investigated in this study. Increasing the diameter of a nanotube with a fixed γ is shown to have a similar mechanistic effect as fixing the outer diameter and thinning the tube wall.Copyright