Jivtesh Garg

Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid

This study investigates the thermal conductivity and viscosity of copper nanoparticles in ethylene glycol. The nanofluid was prepared by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. Volume loadings of up to 2% were prepared. The measured increase in thermal conductivity was twice the value predicted by the Maxwell effective medium theory. The increase in viscosity was about four times of that predicted by the Einstein law of viscosity. Analytical calculations suggest that this nanofluid would not be beneficial as a coolant in heat exchangers without changing the tube diameter. However, increasing the tube diameter to exploit the increased thermal conductivity of the nanofluid can lead to better thermal performance.

Coherent Phonon Heat Conduction in Superlattices

Coherent Heat Flow Typically, heat in solids is transported incoherently because phonons scatter at interfaces and defects. Luckyanova et al. (p. 936) grew super-lattice films made from one to nine repeats of layers of GaAs and AlAs, each 12-nm thick. Thermal conductivity through this sandwich structure increased linearly with the number of superlattice repeats, which is consistent with theoretical simulations of coherent heat transport. Coherent phonon transport is evidenced by linear increases of thermal conductivity with total superlattice thickness. The control of heat conduction through the manipulation of phonons as coherent waves in solids is of fundamental interest and could also be exploited in applications, but coherent heat conduction has not been experimentally confirmed. We report the experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods. The measured thermal conductivity increased linearly with increasing total superlattice thickness over a temperature range from 30 to 150 kelvin, which is consistent with a coherent phonon heat conduction process. First-principles and Green’s function–based simulations further support this coherent transport model. Accessing the coherent heat conduction regime opens a new venue for phonon engineering for an array of applications.

Acoustic Phonon Lifetimes and Thermal Transport in Free-Standing and Strained Graphene

We use first-principles methods based on density functional perturbation theory to characterize the lifetimes of the acoustic phonon modes and their consequences on the thermal transport properties of graphene. We show that using a standard perturbative approach, the transverse and longitudinal acoustic phonons in free-standing graphene display finite lifetimes in the long-wavelength limit, making them ill-defined as elementary excitations in samples of dimensions larger than ∼1 μm. This behavior is entirely due to the presence of the quadratic dispersions for the out-of-plane phonon (ZA) flexural modes that appear in free-standing low-dimensional systems. Mechanical strain lifts this anomaly, and all phonons remain well-defined at any wavelength. Thermal transport is dominated by ZA modes, and the thermal conductivity is predicted to diverge with system size for any amount of strain. These findings highlight strain and sample size as key parameters in characterizing or engineering heat transport in graphene.

Gallium arsenide thermal conductivity and optical phonon relaxation times from first-principles calculations

In this paper, thermal conductivity of crystalline GaAs is calculated using first-principles lattice dynamics. The harmonic and cubic force constants are obtained by fitting them to the force-displacement data from density functional theory calculations. Phonon dispersion is calculated from a dynamical matrix constructed using the harmonic force constants and phonon relaxation times are calculated using Fermis Golden rule. The calculated GaAs thermal conductivity agrees well with experimental data. Thermal conductivity accumulations as a function of the phonon mean free path and as a function of the wavelength are obtained. Our results predict a significant size effect on the GaAs thermal conductivity in the nanoscale. Relaxation times of optical phonons and their contributions from different scattering channels are also studied. Such information will help the understanding of hot phonon effects in GaAs-based devices.

Anisotropy of the thermal conductivity in GaAs/AlAs superlattices.

We combine the transient thermal grating and time-domain thermoreflectance techniques to characterize the anisotropic thermal conductivities of GaAs/AlAs superlattices from the same wafer. The transient grating technique is sensitive only to the in-plane thermal conductivity, while time-domain thermoreflectance is sensitive to the thermal conductivity in the cross-plane direction, making them a powerful combination to address the challenges associated with characterizing anisotropic heat conduction in thin films. We compare the experimental results from the GaAs/AlAs superlattices with first-principles calculations and previous measurements of Si/Ge SLs. The measured anisotropy is smaller than that of Si/Ge SLs, consistent with both the mass-mismatch picture of interface scattering and with the results of calculations from density-functional perturbation theory with interface mixing included.

Transition from near-field thermal radiation to phonon heat conduction at sub-nanometre gaps

When the separation of two surfaces approaches sub-nanometre scale, the boundary between the two most fundamental heat transfer modes, heat conduction by phonons and radiation by photons, is blurred. Here we develop an atomistic framework based on microscopic Maxwells equations and lattice dynamics to describe the convergence of these heat transfer modes and the transition from one to the other. For gaps >1 nm, the predicted conductance values are in excellent agreement with the continuum theory of fluctuating electrodynamics. However, for sub-nanometre gaps we find the conductance is enhanced up to four times compared with the continuum approach, while avoiding its prediction of divergent conductance at contact. Furthermore, low-frequency acoustic phonons tunnel through the vacuum gap by coupling to evanescent electric fields, providing additional channels for energy transfer and leading to the observed enhancement. When the two surfaces are in or near contact, acoustic phonons become dominant heat carriers.

High Thermal Conductivity in Short-Period Superlattices

The thermal conductivity of ideal short-period superlattices is computed using harmonic and anharmonic force constants derived from density-functional perturbation theory and by solving the Boltzmann transport equation in the single-mode relaxation time approximation, using silicon-germanium as a paradigmatic case. We show that in the limit of small superlattice period the computed thermal conductivity of the superlattice can exceed that of both the constituent materials. This is found to be due to a dramatic reduction in the scattering of acoustic phonons by optical phonons, leading to very long phonon lifetimes. By variation of the mass mismatch between the constituent materials in the superlattice, it is found that this enhancement in thermal conductivity can be engineered, providing avenues to achieve high thermal conductivities in nanostructured materials.

Micro Fluidic Jets for Thermal Management of Electronics

Micro fluidics devices are conventionally used for boundary layer control in many aerospace applications. Synthetic Jets are intense small scale turbulent jets formed from entrainment and expulsion of the fluid in which they are embedded. The idea of using synthetic jets in confined electronic cooling applications started in late 1990s. These micro fluidic devices offer very efficient, high magnitude direct air-cooling on the heated surface. A proprietary synthetic jet designed in General Electric Company was able to provide a maximum air velocity of 90 m/s from a 1.2 mm hydraulic diameter rectangular orifice. An experimental study for determining the thermal performance of a meso scale synthetic jet was carried out. The synthetic jets are driven by a time harmonic signal. During the experiments, the operating frequency for jets was set between 3 and 4.5 kHz. The resonance frequency for a particular jet was determined through the effect on the exit velocity magnitude. An infrared thermal imaging technique was used to acquire fine scale temperature measurements. A square heater with a surface area of 156 mm2 was used to mimic the hot component and extensive temperature maps were obtained. The parameters varied during the experiments were jet location, driving jet voltage, driving jet frequency and heater power. The output parameters were point wise temperatures (pixel size = 30 μm), and heat transfer enhancement over natural convection. A maximum of approximately 8 times enhancement over natural convection heat transfer was measured. The maximum coefficient of cooling performance obtained was approximately 6.6 due to the low power consumption of the synthetic jets.Copyright

First-Principles Determination of Phonon Lifetimes, Mean Free Paths, and Thermal Conductivities in Crystalline Materials: Pure Silicon and Germanium

The thermal properties of insulating, crystalline materials are essentially determined by their phonon dispersions, the finite-temperature excitations of their phonon populations-treated as a Bose-Einstein gas of harmonic oscillators-and the lifetimes of these excitations. The conceptual foundations of this picture are now a well-established cornerstone in the theory of solids. However, only in recent years our theoretical and algorithmic capabilities have reached the point where we can now determine all these quantities from first-principles, i.e. from a quantum-mechanical description of the system at hand without any empirical input. Such advances have been largely due to the development of density-functional perturbation theory that allows to calculate second-and third-order perturbations of a system of interacting electrons with a cost that is independent of the wavelength of the perturbation. Here we present an extensive case study for the phonon dispersions, phonon lifetimes, phonon mean free paths, and thermal conductivities for isotopically pure silicon and germanium, showing excellent agreement with experimental results, where available, and providing much needed microscopic insight in the fundamental atomistic processes giving rise to thermal conductivity in crystals.

Impingement Air Cooling With Synthetic Jets Over Small and Large Heated Surfaces

Increasing heat loads and the need for improving natural convection heat transfer has brought micro fluidic technology into the thermal management of electronics. A maximum air velocity of 90 m/s from an 800 μm hydraulic diameter orifice was achieved through specially designed synthetic jets. The resonance frequency for a particular jet was determined through the effect of exit velocity magnitude, as well as power consumption. During the experiments, the operating frequency for the jet was varied between 3000 and 5000 Hz. Heat transfer augmentation experiments were performed on two heaters with different sizes to understand the size effect. A microscopic infrared thermal imaging technique was used to acquire fine scale temperature measurements. While an earlier study focused on a heater with a surface area of 160 mm2 , the current study used a heater with a surface area of 1450 mm2 , which is approximately 9 times larger. The effect of jet-location, driving-voltage, frequency and heater-power were studied during the experiments. A heat transfer coefficient enhancement over natural convection of 10 times, with the smaller heater, and 5 times, with the larger heater was measured. This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.Copyright