Matthew A. Panzer

Thermal Conduction in Aligned Carbon Nanotube–Polymer Nanocomposites with High Packing Density

Nanostructured composites containing aligned carbon nanotubes (CNTs) are very promising as interface materials for electronic systems and thermoelectric power generators. We report the first data for the thermal conductivity of densified, aligned multiwall CNT nanocomposite films for a range of CNT volume fractions. A 1 vol % CNT composite more than doubles the thermal conductivity of the base polymer. Denser arrays (17 vol % CNTs) enhance the thermal conductivity by as much as a factor of 18 and there is a nonlinear trend with CNT volume fraction. This article discusses the impact of CNT density on thermal conduction considering boundary resistances, increased defect concentrations, and the possibility of suppressed phonon modes in the CNTs.

Thickness and stoichiometry dependence of the thermal conductivity of GeSbTe films

Thermal conduction in GeSbTe films strongly influences the writing energy and time for phase change memory (PCM) technology. This study measures the thermal conductivity of Ge2Sb2Te5 between 25 and 340°C for layers with thicknesses near 60, 120, and 350nm. A strong thickness dependence of the thermal conductivity is attributed to a combination of thermal boundary resistance (TBR) and microstructural imperfections. Stoichiometric variations significantly alter the phase transition temperatures but do not strongly impact the thermal conductivity at a given temperature. This work makes progress on extracting the TBR for Ge2Sb2Te5 films, which is a critical unknown parameter for PCM simulations.

Thermal Properties of Ultrathin Hafnium Oxide Gate Dielectric Films

Thin-film HfO2 is a promising gate dielectric material that will influence thermal conduction in modern transistors. This letter reports the temperature dependence of the intrinsic thermal conductivity and interface resistances of 56-200-Aring-thick HfO2 films. A picosecond pump-probe thermoreflectance technique yields room-temperature intrinsic thermal conductivity values between 0.49 and 0.95 W/(mmiddotK). The intrinsic thermal conductivity and interface resistance depend strongly on the film-thickness-dependent microstructure.

Temperature-Dependent Phonon Conduction and Nanotube Engagement in Metalized Single Wall Carbon Nanotube Films

Interfaces dominate the thermal resistances in aligned carbon nanotube arrays. This work uses nanosecond thermoreflectance thermometry to separate interface and volume resistances for 10 microm thick aligned SWNT films coated with Al, Ti, Pd, Pt, and Ni. We interpret the data by defining the nanotube-metal engagement factor, which governs the interface resistance and is extracted using the measured film heat capacity. The metal-SWNT and SWNT-substrate resistances range between 3.8 and 9.2 mm(2)K/W and 33-46 mm(2)K/W, respectively. The temperature dependency of the heat capacity data, measured between 125 and 300 K, is in good agreement with theoretical predictions. The temperature dependence demonstrated by the metal-SWNT interface resistance data suggests inelastic phonon transmission.

Infrared Microscopy Thermal Characterization of Opposing Carbon Nanotube Arrays

Carbon nanotubes (CNTs) have received much recent research interest for thermal management applications due to their extremely high thermal conductivity. An advanced thermal interface structure made of two opposing, partially overlapped CNT arrays is designed for thermally connecting two contact surfaces. The performance of this interface structure is thermally characterized using diffraction-limited infrared microscopy. Significant temperature discontinuities are found at the CNT-CNT contact region, which indicates a large thermal resistance between CNTs. Due to this intertube resistance, the thermal performance of the CNT-based interface structure is far below expectation (with a thermal resistance value about 3.8 X 10 -4 K m 2 /W).

Thermal conductivity in porous silicon nanowire arrays

The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.

Thermal resistance between low-dimensional nanostructures and semi-infinite media

Nanostructured electronic and photonic devices include a high density of material interfaces, which can strongly impede heat conduction and influence performance and reliability. Thermal conduction through interfaces is a very mature discipline for the traditional geometry, in which the lateral interface dimensions are large compared to the phonon wavelength. In nanostructures, however, the localization of phonons in the directions parallel to the interface may strongly influence the effective thermal resistance. The present work investigates model problems of abrupt junctions between a harmonic one-dimensional (1D) and a three-dimensional (3D) fcc lattice and between a 1D and a two-dimensional square lattice. The abrupt change in geometry modifies the phonon modes participating in energy transmission and creates an additional thermal resistance that is comparable with that occurring due to the acoustic mismatch at the interface of bulk media. For both cases, varying the impedance mismatch at the junction...

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Significance Aligned carbon nanotube films promise the unusual combination of high thermal conductivity and mechanical compliance. Here, the mechanical compliance of single-walled nanotube films has been measured and linked to their morphology and microscopic motions, including zipping, unzipping, and entanglement. The physical mechanisms governing the mechanical response include bending forces or van der Waals interactions, with the dominant mechanism depending on the nanotube density and alignment. The dependence of film morphology on mechanical modulus explored here provides the foundation for modeling of a variety of other properties including thermal and electrical conductivity. Reliably routing heat to and from conversion materials is a daunting challenge for a variety of innovative energy technologies––from thermal solar to automotive waste heat recovery systems––whose efficiencies degrade due to massive thermomechanical stresses at interfaces. This problem may soon be addressed by adhesives based on vertically aligned carbon nanotubes, which promise the revolutionary combination of high through-plane thermal conductivity and vanishing in-plane mechanical stiffness. Here, we report the data for the in-plane modulus of aligned single-walled carbon nanotube films using a microfabricated resonator method. Molecular simulations and electron microscopy identify the nanoscale mechanisms responsible for this property. The zipping and unzipping of adjacent nanotubes and the degree of alignment and entanglement are shown to govern the spatially varying local modulus, thereby providing the route to engineered materials with outstanding combinations of mechanical and thermal properties.

Temperature-Dependent Thermal Properties of Phase-Change Memory Electrode Materials

The programming current required to switch a phase-change memory cell depends upon the thermal resistances in the device. In many designs, significant heat loss occurs through the electrode. This letter investigates the thermal properties of a multilayer electrode stack. This material offers greater thermal resistance than single-material electrodes due to the presence of multiple thermal boundary resistances (TBRs), reducing heat loss from the device and potentially lowering the programming current. Picosecond time-domain thermoreflectance interrogates the temperature-dependent thermal conductivity of three as-deposited and postannealed electrode materials: carbon, titanium nitride, and tungsten nitride. These data are used to extract the temperature-dependent, as-deposited, and postannealed TBR in two multilayer electrode stacks: carbon-titanium nitride and tungsten-tungsten nitride. The C-TiN stacks demonstrate an as-deposited TBR of 4.9 m2K/GW, increasing to 11.9 m2K/GW postanneal. The W-WNx stacks demonstrate an as-deposited TBR of 3.9 m2K/GW, decreasing to 3.6 m2 K/GW postanneal. These resistances are equivalent to electrode films with thickness on the order of tens of nanometers.

Towards Thermal Characterization of Pico-Liter Volumes Using the 3Omega Method

The continued efforts in the biological community to optimize methodologies such as PCR and to characterize biological reactions and processes are motivating reductions in sample volume. There is a growing need for the detection of thermal phenomena in these small volumes, such as the heat released by recombination and the effective conductivities and capacities in extremely small fluidic regions. While past work has focused largely on heat transport in essentially bulk fluid volumes, there is a need to scale these techniques to the much smaller volumes of interest for biological and biomedical research.”This work applies the 3ω measurement technique to μL volumes by using heaters with dimensions of 200–700μm in lengths and 2–5μm in widths. We investigate fluid samples of DI water, silicone oil, and a salt buffer solution to experimentally determine their temperature-dependent thermal properties from 25°C to 80°C. Validation is achieved through comparison of these values of thermal conductivity κ and volumetric heat capacity Cν to literature. The work also demonstrates the device capability to conduct temperature-dependent measurements down to pL droplet volumes by conducting a volume analysis given the dimensions of heaters used, independent of droplet boundary conditions. Sensitivity and uncertainty analyses based on these heater dimensions and surrounding material stack show the detection capabilities of these heaters, as they are optimally designed to maximize signal while accommodating the size restrictions of small volume droplets.© 2013 ASME