bluuebunny/crossref_metadata_2025 huggingface dataset:
Differential Modulation of Motor Cortical Plasticity and Excitability in Early and Late Phases of Human Motor Learning
Authors: Rosenkranz Karin , Kacar Aleksandra , Rothwell John C.
Published: October 2007

Different phases of motor skill learning appear to involve different physiological processes, with long-term potentiation (LTP) occurring at existing synapses in early and cortical reorganization involving synaptogenesis in later phases. Here, we test the evolution of skill learning-dependent changes in motor plasticity and excitability in six subjects trained to perform rapid thumb abductions over 5 d. Plasticity was examined using paired-associative stimulation (PAS) of the median nerve and motor cortex to induce LTP-like “PAS given with an interstimulus interval of 25 ms (PAS25)” or long-term depression (LTD)-like “PAS given with an interstimulus interval of 10 ms (PAS10)” plasticity. Excitability was tested by measuring recruitment of motor-evoked-potentials “input–output (IO) curve” and of short-latency intracortical inhibition (SICI curve), and sensorimotor organization (SMO). Task performance improved continuously over 5 d. After practice on day 1, the PAS25 effect reversed from facilitation to inhibition whereas the slope of the IO curve increased and the level of SICI decreased. These effects on IO curve and SICI were still present or even enhanced before the last practice on day 5, and were not changed by it. The effect of proprioceptive input from the trained muscle on SMO was also strengthened before practice on day 5. In contrast, PAS-induced plasticity was not influenced by motor practice on day 5, and had returned to prepractice values. The interference with PAS-induced plasticity suggests that the initial performance improvement relies on increasing the efficacy of existing synaptic connections. However, the long-lasting changes in the IO curve, SICI curve, and SMO suggest that continued practice enhances performance by changing Motor cortical organization. We hypothesize that new synaptic connections might have formed that allow LTP/LTD-susceptibility to be restored without reducing synaptic strength and performance skill.
Regulation of Long-Term Depression and Climbing Fiber Territory by Glutamate Receptor δ2 at Parallel Fiber Synapses through its C-Terminal Domain in Cerebellar Purkinje Cells
Authors: Uemura Takeshi , Kakizawa Sho , Yamasaki Miwako , Sakimura Kenji , Watanabe Masahiko , Iino Masamitsu , Mishina Masayoshi
Published: October 2007

Glutamate receptor (GluR) δ2 selectively expressed in cerebellar Purkinje cells (PCs) plays key roles in long-term depression (LTD) induction at parallel fiber (PF)–PC synapses, motor learning, the matching and connection of PF–PC synapses in developing and adult cerebella, the elimination of multiple climbing fibers (CFs) during development, and the regulation of CF territory on PCs. However, it remains unsolved how GluRδ2 regulates cerebellar synaptic plasticity, PF–PC synapse formation, and CF wiring. One possible signaling mechanism through GluRδ2 is signaling by protein–protein interactions. The C-terminal region of GluRδ2 contains at least three domains for protein–protein interactions. The PDZ (postsynaptic density-95/Discs large/zona occludens 1)-binding domain at the C terminal, named as the T site, interacts with several postsynaptic density proteins. Here, we generated GluRδ2ΔT mice carrying mutant GluRδ2 lacking the T site. There were no significant differences in the amount of receptor proteins at synapses, histological features, and the fine structures of PF–PC synapses between wild-type and GluRδ2ΔT mice. However, LTD induction at PF–PC synapses and improvement in the accelerating rotarod test were impaired in GluRδ2ΔT mice. Furthermore, CF territory expanded distally and ectopic innervation of CFs occurred at distal dendrites in GluRδ2ΔT mice, but the elimination of surplus CF innervation at proximal dendrites appeared to proceed normally. These results suggest that the C-terminal T site of GluRδ2 is essential for LTD induction and the regulation of CF territory but is dispensable for PF–PC synapse formation and the elimination of surplus CFs at proximal dendrites during development.
Cortical Acetylcholine Release Is Lateralized during Asymmetrical Slow-Wave Sleep in Northern Fur Seals
Authors: Lapierre Jennifer L. , Kosenko Peter O. , Lyamin Oleg I. , Kodama Tohru , Mukhametov Lev M. , Siegel Jerome M.
Published: October 2007

Fur seals are unique in that they display both bilateral slow-wave sleep (BSWS), as seen in all terrestrial mammals, and slow-wave sleep with interhemispheric electroencephalogram (EEG) asymmetry, resembling the unihemispheric slow waves of cetaceans. Little is known about the underlying mechanisms of this phenomenon, which is also termed asymmetrical slow wave sleep (ASWS). However, we may begin to understand the expression of ASWS by studying the neurotransmitter systems thought to be involved in the generation and maintenance of sleep–wake states in terrestrial mammals. We examined bilaterally the release of cortical acetylcholine (ACh), a neurotransmitter implicated in the regulation of cortical EEG and behavioral arousal, across the sleep–wake cycle in four juvenile northern fur seals (Callorhinus ursinus).In vivomicrodialysis and high-performance liquid chromatography coupled with electrochemical detection were used to measure cortical ACh levels during polygraphically defined behavioral states. Cortical ACh release was state-dependent, showing maximal release during active waking (AW), similar levels during quiet waking (QW), and rapid eye movement (REM) sleep, and minimal release during BSWS. When compared with BSWS, cortical ACh levels increased ∼300% during AW, and ∼200% during QW and REM sleep. During these bilaterally symmetrical EEG states, ACh was synchronously released from both hemispheres. However, during ASWS, ACh release was lateralized with greater release in the hemisphere displaying lower voltage activity, at levels approximating those seen in QW. These findings demonstrate that cortical ACh release is tightly linked to hemispheric EEG activation.
Topographic Organization in and near Human Visual Area V4
Authors: Hansen Kathleen A. , Kay Kendrick N. , Gallant Jack L.
Published: October 2007

The existence and location of a human counterpart of macaque visual area V4 are disputed. To resolve this issue, we used functional magnetic resonance imaging to obtain topographic maps from human subjects, using visual stimuli and tasks designed to maximize accuracy of topographic maps of the fovea and parafovea and to measure the effects of attention on topographic maps. We identified multiple topographic transitions, each clearly visible in ≥75% of the maps, that we interpret as boundaries of distinct cortical regions. We call two of these regions dorsal V4 and ventral V4 (together comprising human area V4) because they share several defining characteristics with the macaque regions V4d and V4v (which together comprise macaque area V4). Ventral V4 is adjacent to V3v, and dorsal V4 is adjacent to parafoveal V3d. Ventral V4 and dorsal V4 meet in the foveal confluence shared by V1, V2, and V3. Ventral V4 and dorsal V4 represent complementary regions of the visual field, because ventral V4 represents the upper field and a subregion of the lower field, whereas dorsal V4 represents lower-field locations that are not represented by ventral V4. Finally, attentional modulation of spatial tuning is similar across dorsal and ventral V4, but attention has a smaller effect in V3d and V3v and a larger effect in a neighboring lateral occipital region.
Processing of Odor Mixtures in theDrosophilaAntennal Lobe Reveals both Global Inhibition and Glomerulus-Specific Interactions
Authors: Silbering Ana F. , Galizia C. Giovanni
Published: October 2007

To understand how odor information is represented and processed in the antennal lobe (AL) ofDrosophila melanogaster, we have optically recorded glomerular calcium responses to single odors and odor mixtures from olfactory sensory neurons (OSNs) and projection neurons (PNs). Odor mixtures offer a good tool to analyze odor processing because experimental results can be tested against clear predictions. At the level of the OSNs, the representation of odor mixtures could be predicted from the response patterns of the components in most cases. PN responses to mixtures, however, provide evidences of interglomerular inhibition. Application of picrotoxin (PTX), an antagonist of GABAA-like receptors, enhanced odor responses, modified their temporal course, and eliminated mixture suppression at the PN level. Our results can be best explained by postulating the existence of at least two local networks in the fly AL: a glomerulus specific network, which includes excitatory and inhibitory connections and a PTX sensitive inhibitory global network that acts on all glomeruli with proportional strength to the global AL input.
Fructose-1,6-Bisphosphate Has Anticonvulsant Activity in Models of Acute Seizures in Adult Rats
Authors: Lian Xiao-Yuan , Khan Firdous A. , Stringer Janet L.
Published: October 2007

A variety of observations suggest that decreasing glycolysis and increasing levels of reduced glutathione, generated by metabolism of glucose through the pentose phosphate pathway, would have an anticonvulsant effect. Because fructose-1,6-bisphosphate (F1,6BP) shifts the metabolism of glucose from glycolysis to the pentose phosphate pathway, it was hypothesized to have anticonvulsant activity. The anticonvulsant activity of F1,6BP was determined in rat models of acute seizures induced by pilocarpine, kainic acid, or pentylenetetrazole. The efficacy of F1,6BP was compared with that of 2-deoxyglucose (2-DG; an inhibitor of glucose uptake and glycolysis), valproic acid (VPA), and the ketogenic diet. One hour before each convulsant, Sprague Dawley rats received either saline (as seizure controls), F1,6BP (0.25, 0.5 or 1 g/kg), 2-DG (0.25 or 0.5 g/kg), or VPA (0.3 g/kg). Additional animals received the ketogenic diet (starting at 20 or 60 d old). Time to seizure onset, seizure duration, and seizure score were measured in each group. F1,6BP had dose-dependent anticonvulsant activity in all three models, whereas VPA had partial efficacy. 2-DG was only effective in the pilocarpine model. The ketogenic diet had no effect in these models. F1,6BP was also partially effective when given at the first behavioral seizure after pilocarpine. Administration of sodium lactate, which bypasses the block in the glycolytic pathway, abolished the anticonvulsant activity of 2-DG in the pilocarpine model, but only decreased the efficacy of F1,6BP. These data demonstrate that F1,6BP has significant anticonvulsant efficacy.
Paucity of Pericytes in Germinal Matrix Vasculature of Premature Infants
Authors: Braun Alex , Xu Hongmin , Hu Furong , Kocherlakota Praneeth , Siegel Donald , Chander Praveen , Ungvari Zoltan , Csiszar Anna , Nedergaard Maiken , Ballabh Praveen
Published: October 2007

Germinal matrix (GM) is a richly vascularized collection of neuronal–glial precursor cells in the developing brain, which is selectively vulnerable to hemorrhage in premature infants. It has rapid angiogenesis associated with high levels of vascular endothelial growth factor (VEGF). Because pericytes provide structural stability to blood vessels, we asked whether pericytes were fewer in the GM than in the subjacent white matter and neocortex and, if so, whether angiogenic inhibition could increase the pericyte density in the GM. We found pericyte coverage and density less in the GM vasculature than in the cortex or white matter in human fetuses, premature infants, and premature rabbit pups. Notably, although VEGF suppression significantly enhanced pericyte coverage in the GM, it remained less than in other brain regions. Therefore, to further elucidate the basis of fewer pericytes in the GM vasculature, we examined expression of ligand–receptor systems responsible for pericyte recruitment. Transforming growth factor-β1 (TGF-β1) protein expression was lower, whereas sphingosine-1-phosphate1 (S1P1) and N-cadherin levels were higher in the GM than in the cortex or white matter. Low TGF-β1 may be involved in promoting endothelial proliferation, whereas elevated S1P1 with N-cadherin may assist vascular maturation. Hence, a paucity of pericytes in the GM vasculature may contribute to its propensity to hemorrhage, and a lower expression of TGF-β1 could be a basis of reduced pericyte density in its vasculature.
Steroid Hormones Act Transsynaptically within the Forebrain to Regulate Neuronal Phenotype and Song Stereotypy
Authors: Meitzen John , Moore Ignacio T. , Lent Karin , Brenowitz Eliot A. , Perkel David J.
Published: October 2007

Steroid sex hormones induce dramatic seasonal changes in reproductive related behaviors and their underlying neural substrates in seasonally breeding vertebrates. For example, in adult white-crowned sparrows, increased Spring photoperiod raises circulating testosterone, causing morphological and electrophysiological changes in song-control nuclei, which modify song behavior for the breeding season. We investigated how photoperiod and steroid hormones induce these changes in morphology, electrophysiology, and behavior. Neurons in a song premotor nucleus, the robust nucleus of the arcopallium (RA), show increased intrinsic spontaneous firing rate and soma size when birds are in breeding condition. Using combinations of systemic and unilateral local intracerebral hormonal manipulations, we show that long-day photoperiod accelerates the effects of systemic testosterone on RA neurons via the estradiol-synthesizing enzyme aromatase (CYP19A1); these changes require inputs from the afferent song control nucleus HVC (used as a proper name) and steroid receptor activation within HVC; local coactivation of androgen and estrogen receptors (ARs and ERs, respectively) within HVC, but not RA, is sufficient to cause neuronal changes in RA; activation of ARs in RA is also permissive. Using bilateral local intracerebral hormone-receptor blockade, we found that ARs and ERs in the song-control nucleus HVC mediate systemic testosterone-induced changes in song stereotypy but not rate. This novel transsynaptic effect of gonadal steroids on activity and morphology of RA neurons is part of a concerted change in key premotor nuclei, enabling stereotyped song.
Opioid-Mediated Placebo Responses Boost Pain Endurance and Physical Performance: Is It Doping in Sport Competitions?
Authors: Benedetti Fabrizio , Pollo Antonella , Colloca Luana
Published: October 2007

The neurobiological investigation of the placebo effect has shown that placebos can activate the endogenous opioid systems in some conditions. So far, the impact of this finding has been within the context of the clinical setting. Here we present an experiment that simulates a sport competition, a situation in which opioids are considered to be illegal drugs. After repeated administrations of morphine in the precompetition training phase, its replacement with a placebo on the day of competition induced an opioid-mediated increase of pain endurance and physical performance, although no illegal drug was administered. The placebo analgesic responses were obtained after two morphine administrations that were separated as long as 1 week from each other. These long time intervals indicate that the pharmacological conditioning procedure has long-lasting effects and that opioid-mediated placebo responses may have practical implications and applications. For example, in the context of the present sport simulation, athletes can be preconditioned with morphine and then a placebo can be given just before competition, thus avoiding administration of the illegal drug on the competition day. However, these morphine-like effects of placebos raise the important question whether opioid-mediated placebo responses are ethically acceptable in sport competitions or whether they have to be considered a doping procedure in all respects.
In VivoTracing of Neural Tracts in the Intact and Injured Spinal Cord of Marmosets by Diffusion Tensor Tractography
Authors: Fujiyoshi Kanehiro , Yamada Masayuki , Nakamura Masaya , Yamane Junichi , Katoh Hiroyuki , Kitamura Kazuya , Kawai Kenji , Okada Seiji , Momoshima Suketaka , Toyama Yoshiaki , Okano Hideyuki
Published: October 2007

In spinal cord injury, axonal disruption results in motor and sensory function impairment. The evaluation of axonal fibers is essential to assess the severity of injury and efficacy of any treatment protocol, but conventional methods such as tracer injection in brain parenchyma are highly invasive and require histological evaluation, precluding clinical applications. Previous advances in magnetic resonance imaging technology have led to the development of diffusion tensor tractography (DTT) as a potential modality to performin vivotracing of axonal fibers. The properties and clinical applications of DTT in the brain have been reported, but technical difficulties have limited DTT studies of the spinal cord. In this study, we report the effective use of DTT to visualize both intact and surgically disrupted spinal long tracts in adult common marmosets. To verify the feasibility of spinal cord DTT, we first performed DTT of postmortem marmosets. DTT clearly illustrated spinal projections such as the corticospinal tract and afferent fibers in control animals, and depicted the severed long tracts in the injured animals. Histology of the spinal cords in both control and injured groups were consistent with DTT findings, verifying the accuracy of DTT. We also conducted DTT in live marmosets and demonstrated that DTT can be performed in live animals to revealin vivonerve fiber tracing images, providing an essential tool to evaluate axonal conditions in the injured spinal cord. Taken together, these findings demonstrate the feasibility of applying DTT to preclinical and clinical studies of spinal cord injury.
Medial Prefrontal Theta Bursts Precede Rapid Motor Responses during Visual Selective Attention
Authors: Delorme Arnaud , Westerfield Marissa , Makeig Scott
Published: October 2007

After visual target stimuli presented infrequently at a covertly attended location, quicker speeded button presses immediately followed a larger positive (P3f) ramp in averaged electroencephalographic (EEG) recordings from the forehead. We show this peak in the mean response time locked to the button press to be principally composed of triphasic, primarily low-theta band (4.5 Hz) complexes preceding but only partially phase-locked to the button press, with larger complexes preceding quicker motor responses. For 10 of 15 subjects, independent component analysis of the unaveraged 31-channel data identified a temporally independent medial frontal EEG process contributing to these phenomena. Low-resolution tomographic modeling localized related components of two 253-channel data sets to medial frontal polar cortex (BA32/10). The far-frontal low-theta complexes and concomitant mean P3f positivity may index cortical activity induced by paralimbic processes involved in disinhibiting impulsive motor responses to rewarding or goal-fulfilling stimuli or events.
Neurobiology of Escalated Aggression and Violence
Authors: Miczek Klaus A. , de Almeida Rosa M. M. , Kravitz Edward A. , Rissman Emilie F. , de Boer Sietse F. , Raine Adrian
Published: October 2007

Psychopathological violence in criminals and intense aggression in fruit flies and rodents are studied with novel behavioral, neurobiological, and genetic approaches that characterize the escalation from adaptive aggression to violence. One goal is to delineate the type of aggressive behavior and its escalation with greater precision; second, the prefrontal cortex (PFC) and brainstem structures emerge as pivotal nodes in the limbic circuitry mediating escalated aggressive behavior. The neurochemical and molecular work focuses on the genes that enable invertebrate aggression in males and females and genes that are expressed or suppressed as a result of aggressive experiences in mammals. Thefruitlessgene, immediate early genes in discrete serotonin neurons, or sex chromosome genes identify sexually differentiated mechanisms for escalated aggression. Male, but not female, fruit flies establish hierarchical relationships in fights and learn from previous fighting experiences. By manipulating either thefruitlessortransformergenes in the brains of male or female flies, patterns of aggression can be switched with males using female patterns and vice versa. Work withStsorSrygenes suggests so far that other genes on the X chromosomes may have a more critical role in female mouse aggression. New data from feral rats point to the regulatory influences on mesocortical serotonin circuits in highly aggressive animals via feedback to autoreceptors and via GABAergic and glutamatergic inputs. Imaging data lead to the hypothesis that antisocial, violent, and psychopathic behavior may in part be attributable to impairments in some of the brain structures (dorsal and ventral PFC, amygdala, and angular gyrus) subserving moral cognition and emotion.
Neurotech for Neuroscience: Unifying Concepts, Organizing Principles, and Emerging Tools
Authors: Silver Rae , Boahen Kwabena , Grillner Sten , Kopell Nancy , Olsen Kathie L.
Published: October 2007

The ability to tackle analysis of the brain at multiple levels simultaneously is emerging from rapid methodological developments. The classical research strategies of “measure,” “model,” and “make” are being applied to the exploration of nervous system function. These include novel conceptual and theoretical approaches, creative use of mathematical modeling, and attempts to build brain-like devices and systems, as well as other developments including instrumentation and statistical modeling (not covered here). Increasingly, these efforts require teams of scientists from a variety of traditional scientific disciplines to work together. The potential of such efforts for understanding directed motor movement, emergence of cognitive function from neuronal activity, and development of neuromimetic computers are described by a team that includes individuals experienced in behavior and neuroscience, mathematics, and engineering. Funding agencies, including the National Science Foundation, explore the potential of these changing frontiers of research for developing research policies and long-term planning.
More Is Not Always Better: Increased Fractional Anisotropy of Superior Longitudinal Fasciculus Associated with Poor Visuospatial Abilities in Williams Syndrome
Authors: Hoeft Fumiko , Barnea-Goraly Naama , Haas Brian W. , Golarai Golijeh , Ng Derek , Mills Debra , Korenberg Julie , Bellugi Ursula , Galaburda Albert , Reiss Allan L.
Published: October 2007

We used diffusion tensor imaging to examine white matter integrity in the dorsal and ventral streams among individuals with Williams syndrome (WS) compared with two control groups (typically developing and developmentally delayed) and using three separate analysis methods (whole brain, region of interest, and fiber tractography). All analysis methods consistently showed that fractional anisotropy (FA; a measure of microstructural integrity) was higher in the right superior longitudinal fasciculus (SLF) in WS compared with both control groups. There was a significant association with deficits in visuospatial construction and higher FA in WS individuals. Comparable increases in FA across analytic methods were not observed in the left SLF or the bilateral inferior longitudinal fasciculus in WS subjects. Together, these findings suggest a specific role of right SLF abnormality in visuospatial construction deficits in WS.
The Roles of Kinases in Familial Parkinson's Disease
Authors: Cookson Mark R. , Dauer William , Dawson Ted , Fon Edward A. , Guo Ming , Shen Jie
Published: October 2007

The purpose of this mini-symposium is to discuss some of the inherited forms of Parkinson's disease (PD) in view of recent data suggesting that some of the proteins affect cellular signaling pathways. As an illustration, we shall focus on two different kinases associated with recessive and dominant forms of PD. Mutations in the mitochondrial kinase PTEN (phosphatase and tensin homolog)-induced kinase 1 (PINK1) are loss-of-function mutations in a normally neuroprotective protein. Loss-of-function mutations in model organisms have variable effects, from dramatic muscle and spermatid defects inDrosophilato more subtle neurophysiological abnormalities in mice. Several lines of evidence relate these to the action of a second gene for familial PD, parkin, an E3 ubiquitin ligase shown recently to have effects on Akt signaling. Mutations in leucine-rich repeat kinase 2 (LRRK2), a cytosolic kinase, are dominant and have the opposite effect of causing neuronal damage. The mechanism(s) involved are uncertain at this time because LRRK2 is a large and complex molecule with several domains. Increased kinase activity accounts for the action of at least some of the mutations, suggesting that hyperactive or misregulated kinase activity may lead to the damaging effects of LRRK2 in neurons. For both PINK1 and LRRK2, the following key question that needs to be answered: what are the physiological substrates that mediate effects in cells? Here, we will discuss some of the recent thinking about physiological and pathological roles for signaling in PD and how these may have therapeutic implications for the future.
Parieto-Frontal Connectivity during Visually Guided Grasping
Authors: Grol Meike J. , Majdandz̆ić Jasminka , Stephan Klaas E. , Verhagen Lennart , Dijkerman H. Chris , Bekkering Harold , Verstraten Frans A. J. , Toni Ivan
Published: October 2007

Grasping an object requires processing visuospatial information about the extrinsic features (spatial location) and intrinsic features (size, shape, orientation) of the object. Accordingly, manual prehension has been subdivided into a reach component, guiding the hand toward the object on the basis of its extrinsic features, and a grasp component, preshaping the fingers around the center of mass of the object on the basis of its intrinsic features. In neural terms, this distinction has been linked to a dedicated dorsomedial “reaching” circuit and a dorsolateral “grasping” circuit that process extrinsic and intrinsic features, linking occipital areas via parietal regions with the dorsal and ventral premotor cortex, respectively. We have tested an alternative possibility, namely that the relative contribution of the two circuits is related to the degree of on-line control required by the prehension movement.

We used dynamic causal modeling of functional magnetic resonance imaging time series to assess how parieto-frontal connectivity is modulated by planning and executing prehension movements toward objects of different size and width. This experimental manipulation evoked different movements, with different planning and execution phases for the different objects. Crucially, grasping large objects increased inter-regional couplings within the dorsomedial circuit, whereas grasping small objects increased the effective connectivity of a mainly dorsolateral circuit, with a degree of overlap between these circuits. These results argue against the presence of dedicated cerebral circuits for reaching and grasping, suggesting that the contributions of the dorsolateral and the dorsomedial circuits are a function of the degree of on-line control required by the movement.
Disparity Channels in Early Vision
Authors: Roe Anna W. , Parker Andrew J. , Born Richard T. , DeAngelis Gregory C.
Published: October 2007

The past decade has seen a dramatic increase in our knowledge of the neural basis of stereopsis. New cortical areas have been found to represent binocular disparities, new representations of disparity information (e.g., relative disparity signals) have been uncovered, the first topographic maps of disparity have been measured, and the first causal links between neural activity and depth perception have been established. Equally exciting is the finding that training and experience affects how signals are channeled through different brain areas, a flexibility that may be crucial for learning, plasticity, and recovery of function. The collective efforts of several laboratories have established stereo vision as one of the most productive model systems for elucidating the neural basis of perception. Much remains to be learned about how the disparity signals that are initially encoded in primary visual cortex are routed to and processed by extrastriate areas to mediate the diverse capacities of three-dimensional vision that enhance our daily experience of the world.
Illustrative analysis of load-displacement curves in nanoindentation
Authors: Fischer-Cripps A.C.
Published: October 2007

The nature of the elastic unloading after an elastic-plastic contact with a conical or Berkovich indenter is studied. Three representative specimens having different mechanical properties were tested. Finite-element results for the pressure distribution beneath the indenter during unloading suggest that the effective indenter is in fact very closely approximated by a sphere in the case of fused silica (a material with a relatively low value of E/H) and a more uniform pressure distribution in the case of silicon and sapphire (materials with higher values of E/H). The proposed reason for these observations is the extent and influence of an elastic enclave directly beneath the indenter as revealed by finite-element analysis. The results also show that the pressure distribution retains its form during the entire unloading. The work seeks to provide a physical reason for the value of the fitting exponent m as used in popular nanoindentation data analysis procedures.
Origin of the plasticity in bulk amorphous alloys
Authors: Lee Jae-Chul , Park Kyoung-Won , Kim Kyou-Hyun , Fleury Eric , Lee Byeong-Joo , Wakeda Masato , Shibutani Yoji
Published: October 2007

Unlike the dislocation-based plasticity in crystalline metals, which can be readily explained by their crystal structure and the presence of defects, the nature of the plasticity in amorphous alloys is not completely understood. Experiments have shown that the plasticity in amorphous alloys is strongly dependent on their atomic packing density. This study, based on the combination of experimental and computational techniques, examines the origin of the plasticity in amorphous alloys considering characteristics of the inherent atomic-scale structure as defined by short-range ordered (SRO) clusters. The role of various SRO atomic clusters in creating free volume during shear deformation is discussed. We report that the plasticity exhibited by amorphous alloys is very sensitive to the characteristics of the atomic packing state, which can be described by various SRO atomic structures and quantified by the effective activation energy for crystallization.
Surface nanodeformation of discontinuously reinforced Ti composite by in situ atomic force microscope observation
Authors: Tanaka Y. , Yang J.-M. , Liu Y.F. , Kagawa Y.
Published: October 2007

The surface nanodeformation of a discontinuously reinforced Ti–6Al–4V composite during tensile loading was investigated by in situ atomic force microscope (AFM) observation. The material used was a TiB whisker and TiC particle reinforced Ti–6Al–4V composite. The evolution of surface roughness and slip band spacing was quantified as a function of applied strain. The microstructural damage during tensile loading was also studied. The formation of slip bands within a grain of the Ti–6Al–4V matrix was clearly observed when the applied strain above was 1.3%. The amount of slip bands and surface roughness increase with increasing applied strain. The rupture of TiC particle and multiple cracking of TiB whiskers were also observed at the applied strain above 1.3%. The interaction of slip bands with the reinforcements and mechanisms of deformation and fracture of the composite were elucidated.
Enhanced high-field performance in Fe-sheathed MgB2 tapes using inexpensive stearic acid additives
Authors: Ma Yanwei , Gao Zhaoshun , Zhang Xianping , Wang Dongliang , Yu Zhengguang , Watanabe K. , Wen Haihu , Mossang E.
Published: October 2007

High performance Fe-cladded MgB2 taps were prepared by the in situ powder-in-tube method utilizing very cheap stearic acid (C18H36O2) as dopants. The amount of stearic acid was varied from 0 to 30 wt%. We found that a significant enhancement of Jc, Hirr, and Hc2 in comparison with undoped samples was easily achieved. At 4.2 K, the transport Jc for the best doped tapes (10 wt%) reached 2 × 104 A/cm2 at 10 T and 3.7 × 103 A/cm2 at 14 T, respectively, an order of magnitude higher than for the pure tapes. In particular, at 20 K, the irreversibility field of the 10 wt% doped tape was around 10 T, which is comparable to the upper critical field of the commercial NbTi at 4.2 K. The results demonstrate great potential of MgB2 tapes for superconducting magnets.
Effects of anisotropic pore structure and fiber texture on fatigue properties of lotus-type porous magnesium
Authors: Seki H. , Tane M. , Nakajima H.
Published: October 2007

We studied the effects of anisotropic pores and fiber texture on the fatigue strength and fracture surface of lotus-type porous magnesium fabricated through unidirectional solidification in pressurized hydrogen and argon atmospheres. The fatigue strength in the direction parallel to the longitudinal axis of pores is higher than that in the perpendicular direction. Not only anisotropic pores but also fiber texture grown along the pore direction contributes to the anisotropy in the fatigue strength. The fatigue strength at finite life of lotus magnesium is closely related to the ultimate tensile strength; the fatigue strength is proportional to the ultimate tensile strength for both loadings parallel and perpendicular to the pore direction. The fracture surface of lotus magnesium is not flat, which originates from porous structure. For parallel loading, fiber texture in lotus magnesium also contributes to the irregular surface, i.e., a combination of texture and pore structure affects fracture surfaces.
Structures and magnetic properties of Sm-Fe-N bulk magnets produced by the spark plasma sintering method
Authors: Saito Tetsuji
Published: October 2007

Sm-Fe-N powders were successfully consolidated at 873 K and below by the spark plasma sintering (SPS) method. Although the decomposition temperature of the hard magnetic Sm2Fe17N3 phase has been reported to be 873 K, partial decomposition of the Sm2Fe17N3 phase was noted in the bulk materials obtained by sintering at below that temperature. The resultant bulk materials showed a coercivity of around 0.24 MAm−1, significantly lower than that of the original Sm-Fe-N powder. It was found that decomposition of the Sm2Fe17N3 phase in the SPS method was significantly lowered by the addition of a small amount of Zn powder to the Sm-Fe-N powder. The bulk material obtained by sintering a mixture of Sm-Fe-N and Zn powder (10%Zn) at 723 K exhibited high coercivity, comparable with that of the original Sm-Fe-N powder.
Hydriding kinetics of ball-milled nanocrystalline MgH2 powders
Authors: Révész Á. , Fátay D. , Spassov T.
Published: October 2007

The kinetics of hydride formation and decomposition described by semiempirical models generally do not involve particle and grain-size dependence. However, ball-milled nanocrystalline powders usually exhibit log-normal grain-size and particle-size distribution. Considering size dependence, a total reacted function for a multiparticle system has been developed. We show that the shape of the measured reaction fraction curves do not determine unambiguously the rate-controlling mechanism of hydrogen sorption, since the kinetics are strongly affected by the microstructure. With the application the convolutional multiple whole profile fitting procedure for nanocrystalline MgH2, the parameters, e.g., the median and variance of the log-normal grain-size distribution have been determined. Taking these values into account, the reaction constants corresponding to different sorption states are considerably modified compared with values obtained from classical single-particle models.
Microstructural effect on the antimicrobial efficacy of arc ion plated TiO2
Authors: Chung C.J. , Lin H.I. , He J.L.
Published: October 2007

Photocatalytic titanium dioxide coatings prepared via various processes have been developed for antimicrobial purposes. Among them is arc ion plating, which may provide more advantages than other processes such as high growth rate, strong film adhesion, as well as the ability to obtain anatase phase at a low deposition temperature. This research involves an arc ion plating method to produce TiO2film on stainless steel. Antimicrobial efficacy is examined as a function of coating parameters. The experimental results show that the deposited film mainly consists of a rutile phase at an initial growth stage, followed by the growth of an anatase phase at a later stage. By increasing oxygen partial pressure, an increased volume of anatase phase is obtained. The volume of anatase phase is found to strongly and positively affect antimicrobial efficiency. Such an arc ion plated TiO2coating can be potentially served for antimicrobial treatment of medical equipment.
Semiempirical method for calculation of secondary electron emission coefficients of insulating materials using their spectra of x-ray photoelectron spectroscopy
Authors: Heo Tae Wook , Moon Sung Hwan , Park Sun Young , Kim Jae Hyuk , Kim Hyeong Joon
Published: October 2007

The importance of the secondary electron emission of the protective layer in the alternating current plasma display panels is widely known. However, the difficulty in measuring the secondary electron emission coefficients (γ) of insulating materials has hampered efforts to obtain accurate estimates of their values. To overcome the difficulty of direct measurement, we devised a calculating γ using the spectra of x-ray photoelectron spectroscopy (XPS) and a well-defined theory. The XPS spectra of the valence and core bands of MgO, Al2O3, and Y2O3 films, which were deposited by e-beam evaporation, were measured. Calculations of the γ values were conducted for the case when gas ions such as He, Ne, Ar, and Xe slowly approach the solid surface.
Novel organic light-emitting transistors with PN-heteroboundary carrier recombination sites fabricated by lift-off patterning of organic semiconductor thin films
Authors: Suganuma Naotoshi , Shimoji Noriyuki , Oku Yoshiaki , Matsushige Kazumi
Published: October 2007

The authors have devised a novel organic light-emitting transistor (OLET) with a PN-heteroboundary combined with hole and electron transport materials (in other words, p-type and n-type organic semiconductors) along carrier channels. In this device, a clear modulation of the current and luminance with the gate voltage was observed. A luminance of 100 cd/m2 or more has been observed at the source–source voltage of 15 V with the turn-on voltage of 10 V or less, which is lower than that of OLETs based on a single organic material. The horizontal PN-heteroboundary structure has been implemented for the first time by using the photolithographic patterning of organic semiconductor thin films. This patterning technique can be applied to the fabrication of not only the OLETs reported in this work, but also to organic integrated circuits or organic displays.
Surface detection in nanoindentation of soft polymers
Authors: Deuschle Julia , Enders Susan , Arzt Eduard
Published: October 2007

In this work, we performed nanoindentation studies on polymers with different moduli in the range of several millipascals up to several gigapascals. The focus was on the initial contact identification during indentation testing. Surface-detection methods using quasi-static loading as well as methods employing the dynamic forces associated with the continuous stiffness measurement technique were compared regarding their practicability and accuracy for the testing of polymeric materials. For the most compliant material with a modulus of 1 MPa, where contact identification is most critical, we used load-displacement curves obtained from finite element modeling analysis as a reference for the evaluation of experimental techniques. The results show how crucial the precise surface detection is for achieving accurate indentation results, especially for compliant materials. Further, we found that surface detection by means of dynamic testing provides mechanical-property values of higher accuracy for all polymers used in this study. This was due to smaller errors in surface detection, thus avoiding a significant underestimation of the contact area.
A determination of structural evolution during the processing of glycol-based, sol-gel derived ceramics through the study of ferrimagnetic interactions
Authors: Donahue Edward J. , Ng Michael , Li Patrick
Published: October 2007

This work studies the chemical and structural changes that occur in sols upon heating to form ceramics. Ferrimagnetic Y3Fe5O12 (YIG) was chosen because the geometric and structural constraint of ferrimagnetic interactions allow for a direct measurement of the degree of well-defined structure present within the sol at various stages of development. Glycolate sols of 8% mol total metal were prepared using Y(NO3)3 and Fe(NO3)3 hydrates in stoichiometric ratios. Terminal straight-chain diols were used, ranging from 1,2-ethanediol to 1,6-hexanediol. The temperatures at which mass change occurred during heating were determined by thermogravimetric analysis. Samples were heated to these temperatures and examined by Fourier transform infrared spectroscopy (FTIR), x-ray diffraction, and magnetometry to determine chemical, structural, and magnetic changes. Ferrimagnetic ordering was present after the first heating step. Defined structure, determined by x-ray, occurred in the penultimate step. Analysis of FTIR spectra, in conjunction with the results of thermogravimetric analysis, revealed a predictable decomposition pathway.
Contact fatigue in an alumina microcontact: A confocal laser scanning microscope study
Authors: Chen Yun , Koynov Kaloian , Butt Hans-Jürgen
Published: October 2007

Using a confocal laser scanning microscope the development of stress in a micromechanical contact could be measured for ruby with a resolution of ∼1 μm. Ruby (α-Al2O3: Cr3+) spheres with radii of 75 μm were compressed quasi-statically between two sapphire (α-Al2O3) plates. While applying an increasing uniaxial load, confocal microscopy was used to record the fluorescence spectra at the contact region. The peak positions of the fluorescence spectrum shift to longer wavelengths with increasing stress. By detecting the shift in wavelength the local stress could be measured. We adopted two-photon excitation process (800 nm wavelength) to reduce background fluorescence. Loading–unloading cycles were applied with the maximal loading force increased subsequently for each of the next cycle. Progressive fatigue was observed when the load exceeded 1.1 N. As long as the load did not exceed 4 N stress-versus-load curves were still continuous and could be described by Hertz’s law with a reduced Young’s modulus or increasing damage. Once the load exceeded 4 N, spikelike decreases of the stress were observed. This indicates the formation of microcracks on the 10 μm length scale.
Annealing behavior of a ferritic stainless steel subjected to large-strain cold working
Authors: Belyakov A. , Tsuzaki K. , Kimura Y. , Mishima Y.
Published: October 2007

Mechanisms of microstructure evolution during annealing after cold working were studied in an Fe-15%Cr ferritic stainless steel, which was processed by bar rolling/swaging to various total strains ranging from 1.0 to 7.3 at ambient temperature. Two types of recrystallization behavior were observed depending on the cold strain. An ordinary primary (discontinuous) recrystallization developed in the samples processed to conventional strains of 1.0–2.0. On the other hand, rapid recovery at early annealing resulted in ultrafine-grained microstructures in the larger strained samples that continuously coarsened on further annealing. Such annealing behavior was considered as continuous recrystallization.
Ordered and twinned multidomain structure in highly Al-rich mullite
Authors: Wang H.Z. , Kulkarni T. , Sarin V.K. , Basu S.N.
Published: October 2007

The structure of a region of a highly Al-rich mullite (Al4+2xSi2–2xO10–x with x ∼ 0.82; 47Al2O3-6SiO2; Al/Si ∼16) in a chemically vapor-deposited, functionally graded mullite coating was studied by high-resolution transmission electron microscopy. The region consisted of a fully ordered and twinned multidomain structure. Within each domain, well-ordered oxygen vacancies resulted in antiphase boundaries (APBs) characterized by a shift vector of [001]. The APBs were oriented parallel to (601) and (601). Along [001], the structure consisted of alternating layers of (601)- and (601)-domains with a spacing of 17d601, separated by (001) twinning planes. The (001) twinning did not generate any additional spots in the [010] mullite diffraction pattern. The lattice parameters of this ultrahigh Al-rich mullite phase were calculated from selected-area electron-diffraction patterns to be: a = 0.762 nm; b = 0.754 nm; and c = 0.291 nm.
Lead-free solders: Enthalpies of mixing of liquid Ag–Cu–Ni–Sn alloys
Authors: Saeed U. , Flandorfer H. , Ipser H.
Published: October 2007

Partial and integral enthalpies of mixing of liquid Ag–Cu–Ni–Sn alloys were determined at 1000 °C by a drop calorimetric technique using a Calvet type microcalorimeter. They were obtained by adding Ni to the ternary Ag–Cu–Sn alloys with different composition. The data were evaluated by means of an extended Redlich–Kister–Muggianu polynomial fit for substitutional solutions. The minimum and maximum in the quaternary system were also calculated. It was found that the maximum integral enthalpy of mixing (13,310 J/mol at 41 at.% Ag) occurs in the binary Ag–Ni system while the minimum integral enthalpy of mixing (−21,390 J/mol at 61 at.% Ni) occurs in the binary Ni–Sn system. Moreover the experimental data were compared to values calculated by different extrapolation models based on binary data.
The origin of driving force for the formation of Sn whiskers at room temperature
Authors: Li Shi-Bo , Bei Guo-Ping , Zhai Hong-Xiang , Zhang Zhi-Li , Zhou Yang , Li Cui-Wei
Published: October 2007

Sn whiskers can form at room temperature on the agglomerated flakes produced by mechanical alloying (MA) of Ti, Sn, and C powders, whether the flakes are stored in air or water. The Sn whiskers forming in air are tens of micrometers to several centimeters in length and 0.5 to ∼10 μm in diameter. Whereas a large amount of Sn polyhedra forms on the flakes that are stored in water, a small amount of Sn whiskers forms on the polyhedra. The driving force for Sn whisker formation is the compressive stress induced by mechanical alloying (MA) and oxidation. The mechanism about the spontaneous growth of metal whiskers is discussed.
Microstructural evolution during the heat treatment of nanocrystalline alloys
Authors: Detor A.J. , Schuh C.A.
Published: October 2007

Nanocrystalline alloys often show exceptional thermal stability as a consequence of kinetic and thermodynamic impediments to grain growth. However, evaluating the various contributions to stability requires detailed investigation of the solute distribution, which is challenging within the fine structural-length-scales of nanocrystalline materials. In the present work, we use a variety of techniques to assess changes in the grain size, chemical ordering, grain-boundary segregation, and grain-boundary structure during the heat treatment of Ni–W specimens synthesized over a wide range of grain sizes from 3 to 70 nm. A schematic microstructural evolution map is also developed based on analytical models of the various processes activated during annealing, highlighting the effects of alloying in nanocrystalline materials.