Key mechanistic features of swelling and blistering of helium-ion-irradiated tungsten

Helium-ion-induced swelling and blistering of single-crystal tungsten is investigated using a Helium Ion Microscope for site-specific dose-controlled irradiation (at 25 keV) with analysis by Helium Ion Microscopy, Atomic Force Microscopy and Transmission Electron Microscopy (cross-sectioning by Focused Ion Beam milling). We show that the blister cavity forms at a depth close to the simulated helium peak and that nanobubbles coalesce to form nanocracks within the envelope of the ion stopping range, swelling the blister shell. These results provide the first direct experimental evidence for the interbubble fracture mechanism proposed in the framework of the gas pressure model for blister formation

In situ nanocompression testing of irradiated copper.

Increasing demand for energy and reduction of carbon dioxide emissions has revived interest in nuclear energy. Designing materials for radiation environments necessitates a fundamental understanding of how radiation-induced defects alter mechanical properties. Ion beams create radiation damage efficiently without material activation, but their limited penetration depth requires small-scale testing. However, strength measurements of nanoscale irradiated specimens have not been previously performed. Here we show that yield strengths approaching macroscopic values are measured from irradiated ~400 nm-diameter copper specimens. Quantitative in situ nanocompression testing in a transmission electron microscope reveals that the strength of larger samples is controlled by dislocation-irradiation defect interactions, yielding size-independent strengths. Below ~400 nm, size-dependent strength results from dislocation source limitation. This transition length-scale should be universal, but depends on material and irradiation conditions. We conclude that for irradiated copper, and presumably related materials, nanoscale in situ testing can determine bulk-like yield strengths and simultaneously identify deformation mechanisms

Cryogenic Stress-Driven Grain Growth Observed via Microcompression with in situ Electron Backscatter Diffraction

The deformation of materials at cryogenic temperature is of interest for space, arctic, and fundamental science applications. In this work, a custom-built cooling system attached to a commercial picoindenter was used for in situ cryogenic microcompression testing of equal-channel angular-pressed copper with real-time electron backscatter diffraction. Stress-driven grain growth at cryogenic temperatures was observed during a series of elastic and plastic deformations. These results provide direct evidence for the previously predicted phenomenon, whereas previous ex situ examinations demonstrated coarsening after cryogenic loading when samples were not maintained at cryogenic temperatures between deformation and characterization

Nearly defect-free dynamical models of disordered solids: The case of amorphous silicon

It is widely accepted in the materials modeling community that defect-free realistic networks of amorphous silicon cannot be prepared by quenching from a molten state of silicon using classical or ab initio molecular-dynamics (MD) simulations. In this work, we address this long-standing problem by producing nearly defect-free ultra-large models of amorphous silicon, consisting of up to half-a-million atoms, using classical molecular-dynamics simulations. The structural, topological, electronic, and vibrational properties of the models are presented and compared with experimental data. A comparison of the models with those obtained from using the modified Wooten-Winer-Weaire bond-switching algorithm shows that the models are on par with the latter, which were generated via event-based total-energy relaxations of atomistic networks in the configuration space. The MD models produced in this work represent the highest quality of amorphous-silicon networks so far reported in the literature using molecular-dynamics simulations.Comment: 8 pages, 8 figure

OncoLog Volume 51, Number 12, December 2006

Curable But Not Conquered: Hodgkin’s Lymphoma Today House Call: Big Benefits for Smokers Who Quithttps://openworks.mdanderson.org/oncolog/1187/thumbnail.jp

Computationally-efficient stochastic cluster dynamics method for modeling damage accumulation in irradiated materials

An improved version of a recently developed stochastic cluster dynamics (SCD) method {[}Marian, J. and Bulatov, V. V., {\it J. Nucl. Mater.} \textbf{415} (2014) 84-95{]} is introduced as an alternative to rate theory (RT) methods for solving coupled ordinary differential equation (ODE) systems for irradiation damage simulations. SCD circumvents by design the curse of dimensionality of the variable space that renders traditional ODE-based RT approaches inefficient when handling complex defect population comprised of multiple (more than two) defect species. Several improvements introduced here enable efficient and accurate simulations of irradiated materials up to realistic (high) damage doses characteristic of next-generation nuclear systems. The first improvement is a procedure for efficiently updating the defect reaction-network and event selection in the context of a dynamically expanding reaction-network. Next is a novel implementation of the $\tau$-leaping method that speeds up SCD simulations by advancing the state of the reaction network in large time increments when appropriate. Lastly, a volume rescaling procedure is introduced to control the computational complexity of the expanding reaction-network through occasional reductions of the defect population while maintaining accurate statistics. The enhanced SCD method is then applied to model defect cluster accumulation in iron thin films subjected to triple ion-beam ($\text{Fe}^{3+}$, $\text{He}^{+}$ and $$\text{H\ensuremath{{}^{+}}}$$) irradiations, for which standard RT or spatially-resolved kinetic Monte Carlo simulations are prohibitively expensive

Three event-related potential studies on phonological, morpho-syntactic, and semantic aspects

Sign languages have often been the subject of imaging studies investigating the underlying neural correlates of sign language processing. To the contrary, much less research has been conducted on the time-course of sign language processing. There are only a small number of event-related potential (ERP) studies that investigate semantic or morpho-syntactic anomalies in signed sentences. Due to specific properties of the manual-visual modality, sign languages differ from spoken languages in two respects: On the one hand, they are produced in a three-dimensional signing space, on the other hand, sign languages can use several (manual and nonmanual) articulators simul¬taneously. Thus, sign languages have modality-specific characteristics that have an impact on the way they are processed. This thesis presents three ERP studies on different linguistic aspects processed in German Sign Language (DGS) sentences. Chapter 1 investigates the hypothesis of a forward model perspec¬tive on prediction. In a semantic expectation mismatch design, deaf native signers saw videos with DGS sentences that ended in semantically expected or unexpected signs. Since sign languages entail relatively long transition phases between one sign and the next, we tested whether a prediction error of the upcoming sign is already detectable prior to the actual sign onset. Unexpected signs engendered an N400 previous to the critical sign onset that was thus elicited by properties of the transition phase. Chapter 2 presents a priming study on cross-modal cross-language co-activation. Deaf bimodal bilingual participants saw DGS sentences that contained prime-target pairs in one of two priming conditions. In overt phonological priming, prime and target signs were phonologically minimal pairs, while in covert orthographic priming, German translations of prime and target were orthographic minimal pairs, but there was no overlap between the signs. Target signs with overt phonological or with covert orthographic overlap engendered a reduced negativity in the electrophysiological signal. Thus, deaf bimodal bilinguals co-activate their second language (written) German unconsciously during processing sentences in their native sign language. Chapter 3 presents two ERP studies investigating the morpho-syntactic aspects of agreement in DGS. One study tested DGS sentences with incorrect, i.e. unspecified, agreement verbs, the other study tested DGS sentences with plain verbs that incorrectly inflected for 3rd person agreement. Agreement verbs that ended in an unspecified location engen¬dered two independent ERP effects: a positive deflection on posterior electrodes (220-570 ms relative to trigger nonmanual cues) and an anterior effect on left frontal electrodes (300-600 ms relative to the sign onset). In contrast, incorrect plain verbs resulted in a broadly distributed positive deflection (420-730 ms relative to the mismatch onset). These results contradict previous findings of agreement violation in sign languages and are discussed to reflect a violation of well-formedness or processes of context-updating. The stimulus materials of all four studies were consistently presented in continuously signed sentences presented in non-manipulated videos. This methodological innovation enabled a distinctive perspective on the time-course of sign language processing

OncoLog Volume 50, Number 07/08, July-August 2005

Getting Proactive About Prevention New Techniques in Tumor Ablation House Call: Calling for Help: Cancer Helplines and What They Offerhttps://openworks.mdanderson.org/oncolog/1138/thumbnail.jp

OncoLog Volume 49, Number 02, February 2004

Colorectal Cancer Screening: Encouraging Compliance Today and Looking to the Future Despite Its Drawbacks, Mammography Is Still Recommended Communication Is Essential When Guiding Patients through the Maze of Genetic Breast Cancer Screening House Call: Cancer Epidemiology, Step-by-Step DiaLog: On Being a Volunteer, by Anderson Network volunteerhttps://openworks.mdanderson.org/oncolog/1126/thumbnail.jp