Alexis R. Abramson

Thermal conductivity of Si/SiGe and SiGe/SiGe superlattices

The cross-plane thermal conductivity of four Si/Si0.7Ge0.3 superlattices and three Si0.84Ge0.16/Si0.76Ge0.24 superlattices, with periods ranging from 45 to 300 and from 100 to 200 A, respectively, were measured over a temperature range of 50 to 320 K. For the Si/Si0.7Ge0.3 superlattices, the thermal conductivity was found to decrease with a decrease in period thickness and, at a period thickness of 45 A, it approached the alloy limit. For the Si0.84Ge0.16/Si0.76Ge0.24 samples, no dependence on period thickness was found and all the data collapsed to the alloy value, indicating the dominance of alloy scattering. This difference in thermal conductivity behavior between the two superlattices was attributed to interfacial acoustic impedance mismatch, which is much larger for Si/Si0.7Ge0.3 than for Si0.84Ge0.16/Si0.76Ge0.24. The thermal conductivity increased slightly up to about 200 K, but was relatively independent of temperature from 200 to 320 K.

Fabrication and characterization of a nanowire/polymer-based nanocomposite for a prototype thermoelectric device

This paper discusses the design, fabrication and testing of a novel thermoelectric device comprised of arrays of silicon nanowires embedded in a polymer matrix. By exploiting the low-thermal conductivity of the composite and presumably high-power factor of the nanowires, a thermoelectric figure of merit, higher than the corresponding bulk value, should result. Arrays were first synthesized using a vapor-liquid-solid (VLS) process leading to one-dimensional (1-D) growth of single-crystalline nanowires. To provide structural support while maintaining thermal isolation between nanowires, parylene, a low thermal conductivity and extremely conformal polymer, was embedded within the arrays. Mechanical polishing and oxygen plasma etching techniques were used to expose the nanowire tips and a metal contact was deposited on the top surface. Scanning electron micrographs (SEMs) illustrate the results of the fabrication processes. Using a modification of the 3/spl omega/ technique, the effective thermal conductivity of the nanowire matrix was measured and 1 V characteristics were also demonstrated. An assessment of the suitability of this nanocomposite for high thermoelectric performance devices is given.

Interface and Strain Effects on the Thermal Conductivity of Heterostructures: A Molecular Dynamics Study

Molecular dynamics simulations are used to examine how thermal transport is affected by the presence of one or more interfaces. Parameters such as film thickness, the ratio of respective material composition, the number of interfaces per unit length, and lattice strain are considered. Results indicate that for simple nanoscale strained heterostructures containing a single interface, the effective thermal conductivity may be less than half the value of an average of the thermal conductivities of the respective unstrained thin films. Increasing the number of interfaces per unit length, however, does not necessarily result in a corresponding decrease in the effective thermal conductivity of the superlattice.

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...

Thermal Transport in Nanostructured Solid-State Cooling Devices

Low-dimensional nanostructured materials are promising candidates for high efficiency solid-state cooling devices based on the Peltier effect. Thermal transport in these low-dimensional materials is a key factor for device performance since the thermoelectric figure of merit is inversely proportional to thermal conductivity. Therefore, understanding thermal transport in nanostructured materials is crucial for engineering high performance devices. Thermal transport in semiconductors is dominated by lattice vibrations called phonons, and phonon transport is often markedly different in nanostructures than it is in bulk materials for a number of reasons. First, as the size of a structure decreases, its surface area to volume ratio increases, thereby increasing the importance of boundaries and interfaces. Additionally, at the nanoscale the characteristic length of the structure approaches the phonon wavelength, and other interesting phenomena such as dispersion relation modification and quantum confinement may arise and further alter the thermal transport. We discuss phonon transport in semiconductor superlattices and nanowires with regards to applications in solid-state cooling devices. Systematic studies on periodic multilayers called superlattices disclose the relative importance of acoustic impedance mismatch, alloy scattering, and crystalline imperfections at the interfaces. Thermal conductivity measurements of monocrystalline silicon nanowires of different diameters reveal the strong effects of phonon-boundary scattering

Thermal conductivity of graphene and graphene oxide nanoplatelets

The superior thermal transport in graphene has been a topic of great interest to the scientific community, for graphene is envisioned to be important in numerous applications such as thermal management of electronics. While single layer graphene exhibits high thermal conductivity, molecular and lattice dynamics simulations reveal that even in the presence of one or few additional layers, thermal conductivity can be significantly reduced. In fact, with increasing number of layers, thermal conductivity is expected to eventually approach the value of bulk graphite. The interlayer spacing is also known to have a significant influence on thermal conductivity, for it is the combination of the number of layers and the spacing between them that truly is responsible for the thermal conductivity of a multi-layer graphene platelet. Here, we report the experimentally obtained thermal conductivities for nanoplatelets of graphene oxide and reduced graphene exfoliated to differing degrees. Results show that the thermal conductivity measured for reduced graphene platelets with ~ 30 to 45 layers approaches the value of bulk graphite. The thermal conductivity of oxygen intercalated graphene nanoplatelets with ~ 3 layers and 7% oxygen is higher than bulk graphite with similar interlayer spacing. Despite the increased interlayer spacing and presence of the oxygen atoms, which typically enhances phonon scattering, the high value of thermal conductivity can be attributed to the increase in the interlayer coupling due to covalent interactions provided by the oxygen atoms.

Electrohydrodynamic Microfabricated Ionic Wind Pumps for Thermal Management Applications

This work demonstrates an innovative microfabricated air-cooling technology that employs an electrohydrodynamic (EHD) corona discharge (i.e., ionic wind pump) for electronics cooling applications. A single, micro fabricated ionic wind pump element consists of two parallel collecting electrodes between which a single emitting tip is positioned. A grid structure on the collector electrodes can enhance the overall heat-transfer coefficient and facilitate an IC compatible batch process. The optimized devices studied exhibit an overall device area of 5.4 mm × 3.6 mm, an emitter-to-collector gap of ~0.5 mm, and an emitter curvature radius of ~12.5 μm. The manufacturing process developed for the device uses glass wafers, a single mask-based photolithography process, and a low-cost copper-based electroplating process. Various design configurations were explored and modeled computationally to investigate their influence on the cooling phenomenon. The single devices provide a high heat-transfer coefficient of up to ~3200 W/m 2 K and a coefficient of performance (COP) of up to ~47. The COP was obtained by dividing the heat removal enhancement, ΔQ by the power consumed by the ionic wind pump device. A maximum applied voltage of 1.9 kV, which is equivalent to approximately 38 mW of power input, is required for operation, which is significantly lower than the power required for the previously reported devices. Furthermore, the microfabricated single device exhibits a flexible and small form factor, no noise generation, high efficiency, large heat removal over a small dimension and at low power, and high reliability (no moving parts); these are characteristics required by the semiconductor industry for next generation thermal management solutions.

Application of the thermal flash technique for low thermal diffusivity micro/nanofibers

The thermal flash method was developed to characterize the thermal diffusivity of micro/nanofibers without concern for thermal contact resistance, which is commonly a barrier to accurate thermal measurement of these materials. Within a scanning electron microscope, a micromanipulator supplies instantaneous heating to the micro/nanofiber, and the resulting transient thermal response is detected at a microfabricated silicon sensor. These data are used to determine thermal diffusivity. Glass fibers of diameter 15 microm had a measured diffusivity of 1.21x10(-7) m(2)/s; polyimide fibers of diameters 570 and 271 nm exhibited diffusivities of 5.97x10(-8) and 6.28x10(-8) m(2)/s, respectively, which compare favorably with bulk values.

Thermal and electrical characterization of nanocomposites for thermoelectrics

Nanostructures have been shown theoretically, and to a certain extent experimentally, to exhibit enhanced thermoelectric properties. While the use of thermoelectric devices is not widespread today, even marginal improvements in performance could lead to a revolution in small-scale energy conversion and generation and heat removal applications. The potential integration of nanostructures into actual thermoelectric devices has not been fully realized since these devices generally require larger scale materials for the thermoelectric elements. Therefore, a focused investigation into thermal, electrical and thermoelectric characteristics of nanocomposites comprised of nanostructures that, in their bulk form, exhibit good thermoelectric characteristics, was conducted. The presented research explores the thermal and electrical properties of nanocomposites comprised of bismuth nanoparticles that were embedded in a conducting polymer matrix. The thermal and electrical conductivities as well as the thermopower of thin films of polyaniline (the conducting polymer) with different volume fractions of nanoparticles were measured. Results demonstrate that the thermal and electrical properties of polyaniline may be significantly influenced by the presence of the nanoparticles

Mechanical characterization device for in situ measurement of nanomechanical properties of micro/nanostructures

A characterization device was developed for nanomechanical testing on one-dimensional micro/nanostructures. The tool consists of a nanomanipulator, a three-plate capacitive transducer, and associated probes, and is operated inside a scanning electron microscope. The transducer independently measures force and displacement with micronewton and nanometer sale resolutions, respectively. Tensile testing of a polyaniline microfiber (diameter ∼1μm) demonstrated the capabilities of the system. Engineering stress versus strain curves exhibited two distinct regions with different Young’s moduli. Failure at the probe-sample weld occurred at ∼67MPa, suggesting that polyaniline microfibers exhibit a yield stress that is higher than most comparable bulk polymers.