Shreyas Rajasekhara

Monodisperse Porous LiFePO4 Microspheres for a High Power Li-Ion Battery Cathode

A novel solvothermal approach combined with high-temperature calcinations was developed to synthesize on a large scale LiFePO(4) microspheres consisting of nanoplates or nanoparticles with an open three-dimensional (3D) porous microstructure. These micro/nanostructured LiFePO(4) microspheres have a high tap density and, as electrodes, show excellent rate capability and cycle stability.

Hydrothermal Synthesis and Electrochemical Properties of Li3V2(PO4)3/C-Based Composites for Lithium-Ion Batteries

To improve performance at higher rates, we developed a hydrothermal method to prepare carbon-coated monoclinic lithium vanadium phosphate (Li(3)V(2)(PO(4))(3)) powder to be used as a cathode material for Li-ion batteries. The structural, morphological and electrochemical properties were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), and galvanostatic charge-discharge cycling. A superior cycle and rate behavior are demonstrated for Li(3)V(1.85)Sc(0.15)(PO(4))(3)/C and Li(2.96)Ca(0.02)V(2)(PO(4))(3)/C electrodes at charge-discharge current rates above 5C.

Effect of downscaling nano-copper interconnects on the microstructure revealed by high resolution TEM-orientation-mapping.

In this work, a recently developed electron diffraction technique called diffraction scanning transmission electron microscopy (D-STEM) is coupled with precession electron microscopy to obtain quantitative local texture information in damascene copper interconnects (1.8 µm-70 nm in width) with a spatial resolution of less than 5 nm. Misorientation and trace analysis is performed to investigate the grain boundary distribution in these lines. The results reveal strong variations in texture and grain boundary distribution of the copper lines upon downscaling. Lines of width 1.8 µm exhibit a strong <111> normal texture and comprise large micron-size grains. Upon downscaling to 180 nm, a {111}<110> bi-axial texture has been observed. In contrast, narrower lines of widths 120 and 70 nm reveal sidewall growth of {111} grains and a dominant <110> normal texture. The microstructure in these lines comprises clusters of small grains separated by high angle boundaries in the vicinity of large grains. The fraction of coherent twin boundaries also reduces with decreasing line width.

Microstructural evolution of nanocrystalline nickel thin films due to high-energy heavy-ion irradiation

This initial feasibility study demonstrates that recent advancements in precession electron diffraction microscopy can be applied to nanostructured metals exposed to high displacement damage from a Tandem accelerator. In this study, high purity, nanocrystalline, free-standing nickel thin films produced by pulsed laser deposition were irradiated with approximately 3 × 1014 ions/cm2 of 35 MeV Ni6+ ions resulting in an approximately uniform damage profile to approximately 16 dpa. Pristine and ionirradiated regions of the nanocrystalline Ni films were characterized by conventional transmission electron microscopy and precession electron diffraction microscopy. Precession electron diffraction microscopy provided additional insight into the texture, phase, and grain boundary distribution resulting from the displacement damage that could not be obtained from traditional electron microscopy techniques. For the nanocrystalline nickel film studied, this included the growth in number and percentage of a metastable h...

Rapid and automated grain orientation and grain boundary analysis in nanoscale copper interconnects

A combination of diffraction scanning transmission electron microscopy (D-STEM) and automated precession microscopy is used to obtain orientation information from 108 copper grains in 120 nm wide copper interconnect lines. Grain boundary analysis based on this orientation data reveals that Σ3n (n = 1, 2) boundaries are predominant in these lines. Finite element analysis reveals regions of high and low stresses within the copper microstructure.

Texture and Phase Analysis in Nanocrystalline Ni Thin Films by Precession Electron Diffraction Microscopy

Grain refinement has been considered a powerful method to obtain high strength in metals and alloys. However, in materials with nano grain sizes, rapid and abnormal grain growth may occur when they are subjected to an annealing treatment. In addition, as grain growth occurs, the local texture may evolve, which will also affect the mechanical properties. Therefore, to achieve nanocrystalline metal thin films with high strength, it is critical to correlate the film thickness with grain size and local texture as a function of annealing temperature.

Final LDRD report

We present the results of a three-year LDRD project focused on understanding microstructural evolution and related property changes in Zr-based nuclear cladding materials towards the development of high fidelity predictive simulations for long term dry storage. Experiments and modeling efforts have focused on the effects of hydride formation and accumulation of irradiation defects. Key results include: determination of the influence of composition and defect structures on hydride formation; measurement of the electrochemical property differences between hydride and parent material for understanding and predicting corrosion resistance; in situ environmental transmission electron microscope observation of hydride formation; development of a predictive simulation for mechanical property changes as a function of irradiation dose; novel test method development for microtensile testing of ionirradiated material to simulate the effect of neutron irradiation on mechanical properties; and successful demonstration of an Idaho National Labs-based sample preparation and shipping method for subsequent Sandia-based analysis of post-reactor cladding.

Characterizing Texture and Grain Boundaries in Nanoscale Cu Interconnects by Precession Electron Diffraction

The constant downscaling of back-end of line Cu interconnects (CIs) has resulted in changes to their microstructure [1]. Among these changes, any variation in local texture and grain boundary types could strongly affect reliability issues like stress migration and electromigration [2, 3]. In the current work, we couple precession electron microscopy and D-STEM [4] using the ASTAR system from NanoMEGAS to obtain texture information in 180 nm and 120 nm wide damascene Cu lines with a spatial resolution of 1-2 nm. Furthermore, we perform misorientation and trace analysis using the TSL OIM software to investigate the presence of 3 boundaries, which are typically predominant in Cu, and non-CSL high angle boundaries [5].

The Effect of Particle Size on Pt/C Electrocatalyst Degradation in Proton Exchange Membrane Fuel Cells

Pt nanoparticle electrocatalysts in Proton Exchange Membrane Fuel Cells (PEMFCs) have been shown to undergo irreversible degradation through coalescence and Pt dissolution, the degree of which vary with initial particle size. Such degradation leads to losses in the performance and operating hours of the PEMFC [1]. In this work, we study the relationship between Pt nanocatalyst size and degradation mechanisms for PEMFC cathodes cycled between 0.6 and 1.0 V in triangular wave with 32s time period. Furthermore, to address the nanoparticle morphology, we use for the first time roundness and circularity parameters as novel metrics. Two cycled membrane electrode assemblies (MEAs), S1 and S2, whose cathodes were initially loaded with Pt nanoparticles of sizes 2 nm and 12 nm, respectively were microtomed for crosssection transmission electron microscopy (TEM) analysis. Subsequently, TEM images from three separate regions of the cathode, 3 μm in length, thereby covering the approximate 10 μm electrode thickness (Fig. 1) were acquired in a JEOL 2010F operated at 200 kV and analyzed. In addition to individual spherical particles, the TEM observations identified nanoparticles with two distinct morphologies: (i) coalesced (Figs. 2a and 3a) and (ii) dendritic (Figs. 2b and 3b). As expected, coalesced and dendritic morphologies were more prevalent in sample S1 because smaller nanocatalysts have higher dissolution and mobility rates [1]. For example in region A, the frequency of coalesced nanoparticles was found to be 9.0% for sample S1 and 4.5% for sample S2, while the dendritic particles were 3.5% for sample S1 and 2.5% for sample S2. However, sample S1 exhibits a more pronounced dissolution than what is suggested by the dendritic frequency, as observed by the dense agglomeration of Pt nanoparticles along the cathode membrane interface, which was completely absent in the sample S2. The two morphologies were subsequently defined by a ratio R of roundness to circularity, for which R > 1 for dendritic particles and R < 1 for coalesced particles. The ratio R provides a means to quantify the severity and the mechanism of degradation for a particular nanoparticle. Roundness is more sensitive to elongation caused by coalesced particles along a central axis. Circularity is more sensitive to sharp discontinuities caused by variations in concavity and convexity, such as branching. The presence of these morphologies provides insight into specific degradation mechanisms. In particular, the dendritic particles arise from the dissolution and redeposition of Pt, whereas the coalesced particles are the result of nanoparticles migrating and coalescing on the carbon support [2].