Atomistic and mesoscopic simulations of heat transfer across heterogeneous material interfaces

The study of heat transfer and the associated thermal interface resistance at heterogeneous material interfaces is over 70 years old since the first measurements of thermal interface resistance by Kapitza in 1941. However, recent developments in experimental metrology techniques that enable spectrally-resolved phonon transport measurements at the nanoscale along with the development of high-fidelity simulation methods have provided a renewed interest in the fundamental physics of heat transfer across interfaces. Miniaturized electronic devices and nanostructured materials for energy applications are among technologies that would benefit from a fundamental understanding of interfacial thermal transport. This dissertation focuses on the study of problems in interfacial heat transfer that span the atomistic and mesoscopic length scales and have broad applications in electronic thermal management. The first part of this dissertation develops a mesoscale simulation framework to predict the mechanical and thermal performance of carbon nanotube (CNT) thermal interface materials (TIMs). CNT arrays have been widely studied for use as TIMs due to the high thermal conductivity and mechanical compliance of CNTs. However, modeling of CNT TIMs has been largely limited to semi-empirical methods that lack detailed consideration of CNT array microstructure. We develop a physics-based, microstructure-sensitive, thermo-mechanical simulation framework that can be used in the design and optimization of CNT TIMs. Coarse-grain mechanics simulations are used to predict the CNT array microstructure and the finite volume method is used to solve the Fourier conduction equations for CNTs embedded in a filler matrix. The simulations provide insights on the sensitivity of thermal resistance of the CNT array to microscopic CNT-CNT and CNT-substrate contact resistances. Microstructural parameters that are not readily accessible in experiments such as the contact areas and the fraction of CNTs in contact with the opposing substrate are reported to demonstrate the usefulness of the simulation approach. The latter part of this dissertation deals with the development of a first-principles atomistic simulation framework to study heat transfer across metal-semiconductor heterojunctions which form an important class of interfaces used in electronic devices. The silicides of transition metals such as titanium and cobalt (TiSi2, CoSi2) are commonly used as metal contacts to silicon in transistors; hence, TiSi2-Si and CoSi 2-Si interfaces are chosen here as model metal-semiconductor junctions for studies of thermal transport. All the atomistic simulations reported in this work use the atomistic Green\u27s function (AGF) method that is analogous to the non-equilibrium Green\u27s function (NEGF) method used in quantum transport calculations of electrons. We propose the use of Büttiker probe scattering models to develop a phenomenological but computationally efficient description of phonon-phonon and electron-phonon scattering within the AGF framework. First-principles calculations of electron-phonon coupling reveal that energy transfer between metal electrons and lattice vibrations in the semiconductor is mediated by interfacial phonon modes whose vibrational pattern is delocalized across the metal and semiconductor regions, and the coupling of metal electrons with phonon modes localized in the semiconductor is negligible. The transport simulations also help identify the contributions of various scattering mechanisms such as elastic interfacial scattering, inelastic phonon scattering, electron-phonon coupling within the metal, and direct electron-phonon coupling across the interface to the total thermal conductance of a CoSi 2-Si interface. The inclusion of the various transport processes in the simulation is found to be critical to obtain a good agreement with experimental data on thermal conductance of an epitaxial CoSi2-Si interface. The last part of this work develops an eigenspectrum formulation of the AGF method that enables the prediction of polarization- or branch-resolved contributions to the phonon transmission function and the thermal interface conductance. Unlike prior work in the literature, our approach makes a direct connection to the bulk phonon dispersion of materials forming an interface and is also computationally efficient. The essential idea behind the formulation is the use of bulk phonon eigenspectrum to obtain the surface Green\u27s functions used in the AGF method instead of the more commonly used Sancho-Rubio or decimation technique. The new approach is applied to study phonon transport across a Si-Ge interface with atomic intermixing. The computation of polarization-resolved transmission functions, which are not accessible within the conventional AGF method that groups different phonon branches together, provides insights on the microscopic mechanisms responsible for the increase in phonon transmission due to interfacial disorder

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Exxon, Chevron and Conocophillips: 25 Years of Rejecting Shareholder Concerns on Climate Change

The resolutions included in this list are focused on climate change, including carbon reductions, climate expertise on boards, related market shifts that could hurt companies' stock value and lobbying/political efforts that work against climate solutions. We excluded several kinds of proposals that were not explicitly climate related, even though they were relevant to climate change. Among the exclusions: resolutions on hydraulic fracturing, drilling in the Arctic election contributions, and the ability of shareholders to nominate board members

A Novel Ant based Clustering of Gene Expression Data using MapReduce Framework

Genes which exhibit similar patterns are often functionally related. Microarray technology provides a unique tool to examine how a cells gene expression pattern chang es under various conditions. Analyzing and interpreting these gene expression data is a challenging task. Clustering is one of the useful and popular methods to extract useful patterns from these gene expression data. In this paper multi colony ant based clustering approach is proposed. The whole processing procedure is divided into two parts: The first is the construction of Minimum spanning tree from the gene expression data using MapReduce version of ant colony optimization techniques. The second part is clustering, which is done by cutting the costlier edges from the minimum spanning tree, followed by one step k - means clustering procedure. Applied to different file sizes of gene expression data over different number of processors, the proposed approach exhibits good scalability and accuracy

Characterization of silver-kaolinite (AgK): an adsorbent for long-lived 129I species

Bentonite is a preferred buffer and backfill material for deep geological disposal of high-level nuclear waste (HLW). Bentonite does not retain anions by virtue of its negatively charged basal surface. Imparting anion retention ability to bentonite is important to enable the expansive clay to retain long-lived I-129 (iodine-129; half-life = 16 million years) species that may escape from the HLW geological repository. Silver-kaolinite (AgK) material is prepared as an additive to improve the iodide retention capacity of bentonite. The AgK is prepared by heating kaolinite-silver nitrate mix at 400 degrees C to study the kaolinite influence on the transition metal ion when reacting at its dehydroxylation temperature. Thermo gravimetric-Evolved Gas Detection analysis, X-ray diffraction analysis, X-ray photo electron spectroscopy and electron probe micro analysis indicated that silver occurs as AgO/Ag2O surface coating on thermally reacting kaolinite with silver nitrate at 400 degrees C

X-RAY DOSE DEPENDENCE OF DARK CURRENT IN AMORPHOUS SELENIUM-ALLOY X-RAY PHOTOCONDUCTORS

The dark current is an important characteristic of a photoconductive X-ray detector, and can impact the dynamic range of the detector and its detective quantum efficiency. It is therefore essential that the dark current and its behavior with time and x-ray irradiation are well characterized and understood in amorphous selenium (a-Se) X-ray detectors for the future enhancement of these detectors. Throughout the course of this work, the dark current in practical a-Se multilayer photoconductors were studied as function of time and x-ray dose delivered to the detector material. The dark current in these multilayer structures has been measured as a function of different rest time periods, sample structure, single X-ray irradiation on the sample and multiple irradiation on the sample. Experiments were performed by resting the sample in dark for a period of time (24 hours) and then samples were exposed to X-ray radiation. It has been observed that most of the trapped charge carriers in the bulk of the material are discharged after resting the sample in dark for 24 hours. It was observed that multilayer sample structures p-i-n and n-i-p exhibit much less dark current compared to other samples with single layer and double layer structures, that is, i-layer only, n-i and p-i structures. The experiments support that the dark current is controlled by injection of charge carriers from contacts. Single X-ray irradiation and multiple irradiation experiments were performed on multilayer a-Se photoconductors at a dose rate of 0.51 Gy s-1 with an exposure duration of 3 s. Samples were exposed to single irradiation at 100 s and 400 s. The dark current following the photocurrent was recorded. Multiple irradiation experiments were also performed on these multilayer samples. With different reverse bias voltages, samples were irradiated 10 times from 200s to 2000s. It was found that the dark current tends to increase with repeated X-ray irradiation but the increase depends on the applied reverse bias; the increase is negligible at a field of 10 V μm-1. After the cessation of the irradiation, the dark current decays and tends to reach a steady state value at t = 4000s. After 24 hr of resting in the dark, the dark current was nearly as low as the original dark current before the X-ray irradiatio