Browsing by Subject "Transmission electron microscopy"
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Item Experimental investigations of thermal transport in carbon nanotubes, graphene and nanoscale point contacts(2011-05) Pettes, Michael Thompson, 1978-; Shi, Li, Ph. D.As silicon-based transistor technology continues to scale ever downward, anticipation of the fundamental limitations of ultimately-scaled devices has driven research into alternative device technologies as well as new materials for interconnects and packaging. Additionally, as power dissipation becomes an increasingly important challenge in highly miniaturized devices, both the implementation and verification of high mobility, high thermal conductivity materials, such as low dimensional carbon nanomaterials, and the experimental investigation of heat transfer in the nanoscale regime are requisite to continued progress. This work furthers the current understanding of structure-property relationships in low dimensional carbon nanomaterials, specifically carbon nanotubes (CNTs) and graphene, through use of combined thermal conductance and transmission electron microscopy (TEM) measurements on the same individual nanomaterials suspended between two micro-resistance thermometers. Through the development of a method to measure thermal contact resistance, the intrinsic thermal conductivity, [kappa], of multi-walled (MW) CNTs is found to correlate with TEM observed defect density, linking phonon-defect scattering to the low [kappa] in these chemical vapor deposition (CVD) synthesized nanomaterials. For single- (S) and double- (D) walled (W) CNTs, the [kappa] is found to be limited by thermal contact resistance for the as-grown samples but still four times higher than that for bulk Si. Additionally, through the use of a combined thermal transport-TEM study, the [kappa] of bi-layer graphene is correlated with both crystal structure and surface conditions. Theoretical modeling of the [kappa] temperature dependence allows for the determination that phonon scattering mechanisms in suspended bi-layer graphene with a thin polymeric coating are similar to those for the case of graphene supported on SiO₂. Furthermore, a method is developed to investigate heat transfer through a nanoscale point contact formed between a sharp silicon tip and a silicon substrate in an ultra high vacuum (UHV) atomic force microscope (AFM). A contact mechanics model of the interface, combined with a heat transport model considering solid-solid conduction and near-field thermal radiation leads to the conclusion that the thermal resistance of the nanoscale point contact is dominated by solid-solid conduction.Item Strengthening Mechanisms of Sputtered Copper, Cobalt and Their Nanocomposites(2014-04-28) Liu, YueLow energy planar defects such as twin boundaries have been employed to strengthen materials effectively with insignificant loss of the conductivity and ductility. High density growth twins can be formed in low stacking fault energy (SFE) metals, such as copper (Cu) and silver (Ag). However, low SFE metal cobalt (Co) received little attention due to the complex coexistence of hexagonal close-packed (HCP) and face-centered cubic (FCC) structure. The focus of this research is to identify the strengthening mechanisms of planar defects such as twin boundaries, stacking faults, and layer interfaces in epitaxial FCC/HCP Co, and Cu/Co multilayers. Our studies show that epitaxial Cu/Co multilayers with different texture have drastic different mechanical properties, dictated by the transmission of partial vs. full dislocations across layer interfaces. Furthermore the mechanical properties of epitaxial Co are dominated by high density stacking faults. Moreover, by applying advanced nanoindentation techniques, such as thermal-drift corrected strain-rate sensitivity measurement, the mechanical properties including strain-rate sensitivity is accurately determined. By using in situ nanoindentation under transmission electron microscope (TEM), we determined deformation physics of nanotwinned Cu, including detwinning, dislocation-twin interactions and work hardening. This project provides an important new perspective to investigate mechanical behavior of nanostructured metals with high density stacking faults.Item Transmission electron microscopy characterization of composite nanostructures(2006) García Gutiérrez, Domingo Ixcóatl; Yacaman, Miguel Jose