Pankaj B. Kaul

Application of the three omega method for the thermal conductivity measurement of polyaniline

The three omega method has proven to provide accurate and reliable measurements of thermal conductivity of thin films and other materials. However, if the films are soft and conductive, conventional methodologies to prepare samples for the measurement technique are challenging and often unachievable. Various modifications to the sample preparation to employ this technique for soft conducting films are reported in this paper including the use of shadow masks for metal heater deposition and a process for preparation of low temperature insulating films required between film and heater. In this work, thick (∼5μm) and ultrathin (∼110nm) films of polyaniline as well as a thin (∼300nm) film of low temperature plasma enhanced chemical vapor deposited SiO2 as a function of temperature were measured. Though not considered a soft material, the silicon dioxide film was utilized for comparison with previous data. Results indicate that the SiO2 film exhibits a thermal conductivity slightly lower than others’ data [S. M...

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.

Carrier interactions and porosity initiated reversal of temperature dependence of thermal conduction in nanoscale tin films

Recently, tin has been identified as an attractive electrode material for energy storage/conversion technologies. Tin thin films have also been utilized as an important constituent of thermal interface materials in thermal management applications. In this regards, in the present paper, we investigate thermal conductivity of two nanoscale tin films, (i) with thickness 500 ± 50 nm and 0.45% porosity and (ii) with thickness 100 ± 20 nm and 12.21% porosity. Thermal transport in these films is characterized over the temperature range from 40 K–310 K, using a three-omega method for multilayer configurations. The experimental results are compared with analytical predictions obtained by considering both phonon and electron contributions to heat conduction as described by existing frequency-dependent phenomenological models and BvK dispersion for phonons. The thermal conductivity of the thicker tin film (500 nm) is measured to be 46.2 W/m-K at 300 K and is observed to increase with reduced temperatures; the mechan...

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.

Thickness and Temperature Dependent Thermal Conductivity of Nanoscale Tin Films

Thin films in general exhibit different thermal properties compared to bulk due to size effect [1–3]. In this study, the thermal conductivity of sputtered Sn films of thickness 500 nm ± 50 nm and 100 nm ± 20 nm are obtained from 55K to 300K and from 40K to 310K, respectively, using the three omega method. The thermal conductivity of 500 nm thin film at room temperature is 46.2 ±4.2 W/m-K, which is lower when compared to its bulk value of 63 W/m-K, and increases gradually as the temperature is lowered to 55K. In contrast, the thermal conductivity of the 100 nm thin film exhibits even reduced thermal conductivity, 36 ± 2.88 W/m-K at 300K, when compared to the 500 nm film, and decreases as the temperature is lowered. The reduction in thermal conductivity of Sn thin film may be due to the pronounced effects of electron scattering at the grain boundaries as well as the twin boundaries in addition to the scattering from the boundary surface at lower temperatures. These experimentally determined thermal conductivities are compared to models that take into account size effects on thermal conductivity of metallic films based on electronic scattering as proposed by Fuchs-Sondheimer (FS), Mayadas-Shatzkes (MS) and Qiu and Tien (QT). The experimentally measured thermal conductivity of Sn films is in good agreement with the MS model indicating the importance of the grain boundary scattering. Thickness measurements are obtained by ellipsometry and profilometer. The estimation of the mean grain size in both films and the evidence of twin boundaries are obtained by Atomic Force Microscopy.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

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

Mechanical Behavior of Individual Micro/Nano-Fibers Using a Novel Characterization Device

The present paper reports the development of a novel mechanical testing device that enables highly reliable mechanical tensile testing on individual micro-/nano-structures. The device features independent measurement of both force and displacement histories in the specimen with nanoNewton force and sub-picometer displacement resolutions, respectively. Moreover, the device is well suited for in-situ testing of micro-/nano- structures within a high resolution scanning electron microscope (SEM), which permits continuous high resolution imaging of the specimen during straining. The device comprises of two main parts: (a) a three-plate capacitive transducer that doubles up both as an actuator and a force sensor, and (b) a commercially available nano-manipulator that facilitates transportation and positioning of nanoscale structures with nano-precision. In order to conduct the mechanical tests, the two ends of the specimen are attached to the probe tips at the nanomanipulator and the transducer ends, using either electron-beam or ion-beam induced deposition (EBID/IBID). The working and capabilities of the testing device are illustrated by presenting results of nanomechanical tensile tests on electrospun polyaniline microwires. The engineering stress versus engineering strain curves exhibit two very distinct Youngs moduli during the loading or the unloading segments of the applied displacement. Failure at the probe/sample weld junction occurred at ~ 67 MPa, suggesting that polyaniline microfibers exhibit a yield stress higher than most comparable bulk polymers.Copyright