Sliding friction between an elastomer network and a grafted polymer layer: the role of cooperative effects

We study the friction between a flat solid surface where polymer chains have been end-grafted and a cross-linked elastomer at low sliding velocity. The contribution of isolated grafted chains' penetration in the sliding elastomer has been early identified as a weakly velocity dependent pull-out force. Recent experiments have shown that the interactions between the grafted chains at high grafting density modify the friction force by grafted chain. We develop here a simple model that takes into account those interactions and gives a limit grafting density beyond which the friction no longer increases with the grafting density, in good agreement with the experimental dataComment: Submitted to Europhys. Letter

A sub-critical barrier thickness normally-off AlGaN/GaN MOS-HEMT

A new high-performance normally-off gallium nitride (GaN)-based metal-oxide-semiconductor high electron mobility transistor that employs an ultrathin subcritical 3 nm thick aluminium gallium nitride (Al0.25Ga0.75N) barrier layer and relies on an induced two-dimensional electron gas for operation is presented. Single finger devices were fabricated using 10 and 20 nm plasma-enhanced chemical vapor-deposited silicon dioxide (SiO2) as the gate dielectric. They demonstrated threshold voltages (Vth) of 3 and 2 V, and very high maximum drain currents (IDSmax) of over 450 and 650 mA/mm, at a gate voltage (VGS) of 6 V, respectively. The proposed device is seen as a building block for future power electronic devices, specifically as the driven device in the cascode configuration that employs GaN-based enhancement-mode and depletion-mode devices

A novel AlGaN/GaN based enhancement-mode high electron mobility transistor with sub-critical barrier thickness

Power-switching devices require low on-state conduction losses, high-switching speed, high thermal stability, and high input impedance. Using gallium nitride (GaN) based field-effect transistors, these properties for switching devices can be satisfied. GaN-based High Electron Mobility Transistors (HEMTs) are emerging as promising candidates for high-temperature, high-power (power electronics) and radio-frequency (RF) electronics due to their unique capabilities of achieving higher current density, higher breakdown voltage, higher operating temperatures and higher cut-off frequencies compared to silicon (Si). Conventional GaN HEMTs with an aluminium gallium nitride (AlGaN) barrier are of depletion-mode (d-mode) or normally-on which require a negative polarity power supply to turn off. On the other hand, enhancement-mode (e-mode) or normally-off AlGaN/GaN HEMTs are attracting increasing interest in recent years because no negative gate voltage is necessary to turn off the devices. This leads to the advantage of simple circuit design and low stand-by power dissipation. For power electronics applications, power switches which incorporate e-mode devices provide the highly desirable essential fail-safe operation. In this research, a new high performance normally-off GaN-based metal-oxide-semiconductor (MOS) high electron mobility transistor (HEMT) that employs an ultrathin sub-critical 3nm Al_0.25Ga_0.75N barrier layer and relies on an induced two dimensional electron gas (2DEG) for operation was designed, fabricated and characterized. The device consists of source and drain Ohmic contacts nominally overlapped by the gate contact and employs a gate dielectric. With no or low gate-to-source voltage (V_GS), there is no two dimensional electron gas (2DEG) channel at the AlGaN/GaN interface to allow conduction of current between the drain and source contacts as the AlGaN barrier thickness is below the critical thickness required for the formation of such channel. However, if a large enough positive bias voltage V_GS is applied, it causes the formation of a quantum well at the AlGaN/GaN interface into which electrons from the source and drain Ohmic regions are attracted (by the positive gate voltage), effectively creating a 2DEG channel, and so the structure is a normally-off field effect transistor. Normally-off GaN MOS-HEMT devices were fabricated using plasma enhanced chemical vapour-deposited (PECVD) silicon dioxide (SiO_2) as the gate dielectric. They demonstrated positive threshold voltages (V_th) in the range of +1V to +3 V, and very high maximum drain currents (I_DSmax) in the range of 450mA/mm to 650mA/mm, at high gate voltage (V_GS) of around 6 V. The devices also exhibited breakdown voltages in the range of 9V and 17V depending on the gate dielectric thickness, making them suitable for realising high current low voltage power devices required, for instance, for buck converters for mobile phones, tablets, laptop chargers, etc

Interdigitation between surface-anchored polymer chains and an elastomer : consequences for adhesion promotion

We study the adhesion between a cross-linked elastomer and a flat solid surface where polymer chains have been end-grafted. To understand the adhesive feature of such a system, one has to study both the origin of the grafted layer interdigitation with the network, and the end-grafted chains extraction out of the elastomer when it comes unstuck from the solid surface. We shall tackle here the first aspect for which we develop a partial interdigitation model that lets us analytically predict a critical surface grafting density $\sigma^{*} \simeq P^{{1/10}}N^{-{3/5}}$ beyond which the layer no longer interdigitates with the elastomer. We then relate this result with recent adhesion measurements

Stressful life-events exposure is associated with 17-year mortality, but it is health-related events that prove predictive

Objectives Despite the widely-held view that psychological stress is a major cause of poor health, few studies have examined the relationship between stressful life-events exposure and death. The present analyses examined the association between overall life-events stress load, health-related and health-unrelated stress, and subsequent all-cause mortality.\ud \ud Design This study employed a prospective longitudinal design incorporating time-varying covariates.\ud \ud Methods Participants were 968 Scottish men and women who were 56 years old. Stressful life-events experience for the preceding 2 years was assessed at baseline, 8–9 years and 12–13 years later. Mortality was tracked for the subsequent 17 years during which time 266 participants had died. Cox's regression models with time-varying covariates were applied. We adjusted for sex, occupational status, smoking, BMI, and systolic blood pressure.\ud \ud Results Overall life-events numbers and their impact scores at the time of exposure and the time of assessment were associated with 17-year mortality. Health-related event numbers and impact scores were strongly predictive of mortality. This was not the case for health-unrelated events.\ud \ud Conclusions The frequency of life-events and the stress load they imposed were associated with all-cause mortality. However, it was the experience and impact of health-related, not health-unrelated, events that proved predictive. This reinforces the need to disaggregate these two classes of exposures in studies of stress and health outcomes.\u

Genetic determinants of cerebral edema in severe traumatic brain injury: A pilot study of the role of CACNA1 and AQP4 gene mutations

Cerebral edema is the one of the most significant predictors of poor outcome after traumatic brain injury. It is still unclear what the pathophysiological and cellular mechanisms and predictors of post-traumatic edema are. The exponential growth in genetic information has opened an avenue for investigation in traumatic brain injury and implicated specific genes in the pathophysiology of post-traumatic injury edema. Two examples are the Aquaporin-4 and CACNA1 genes, which respectively encode water and calcium channels. The Aquaporin-4 gene on chromosome 18q11.2-12.1 encodes the Aquaporin-4 protein (AQP4) water channel. AQP4 is one of the bidirectional high capacity water channels that is primarily expressed in astrocytic foot processes in the central nervous system at the blood-brain barrier and is thought to be critical for brain water homeostasis. Experimental studies showed that AQP4 deficient mice had significantly reduced cerebral edema and better survival in a water intoxication model. The CACNA1 gene on chromosome 19p13 encodes the a1A subunit of a neuronal calcium channel. Patients with Familial Hemiplegic Migraine and delayed fatal cerebral edema and seizuresfrom minor trauma have been found to have mutations in CACNA1, which are hypothesized to enhance development of cytotoxic edema. A missense mutation is reported to enhance risk of delayed fatal cerebral edema. Hypothesis: The CACNA1 gene missense mutation S218L and AQP4 polymorphisms will be over-represented in patients with post-traumatic cerebral edema. Our Specific Aim is to perform full exon sequence analysis of these two genes in 20 well-defined cases of excessive cerebral edema. Our long term goal is to systematically investigate genetic variants as determinants of risk of excessive cerebral edema. It is hoped that this will further elucidate secondary mechanisms of injury specifically in the formation of post-traumatic edema and lead to targeted therapies in the future

TASI Lectures on the Cosmological Constant

The energy density of the vacuum, Lambda, is at least 60 orders of magnitude smaller than several known contributions to it. Approaches to this problem are tightly constrained by data ranging from elementary observations to precision experiments. Absent overwhelming evidence to the contrary, dark energy can only be interpreted as vacuum energy, so the venerable assumption that Lambda=0 conflicts with observation. The possibility remains that Lambda is fundamentally variable, though constant over large spacetime regions. This can explain the observed value, but only in a theory satisfying a number of restrictive kinematic and dynamical conditions. String theory offers a concrete realization through its landscape of metastable vacua.Comment: 39 pages, 3 figure

A survey of carbon nanotube interconnects for energy efficient integrated circuits

This article is a review of the state-of-art carbon nanotube interconnects for Silicon application with respect to the recent literature. Amongst all the research on carbon nanotube interconnects, those discussed here cover 1) challenges with current copper interconnects, 2) process & growth of carbon nanotube interconnects compatible with back-end-of-line integration, and 3) modeling and simulation for circuit-level benchmarking and performance prediction. The focus is on the evolution of carbon nanotube interconnects from the process, theoretical modeling, and experimental characterization to on-chip interconnect applications. We provide an overview of the current advancements on carbon nanotube interconnects and also regarding the prospects for designing energy efficient integrated circuits. Each selected category is presented in an accessible manner aiming to serve as a survey and informative cornerstone on carbon nanotube interconnects relevant to students and scientists belonging to a range of fields from physics, processing to circuit design

Microcystin Prevalence throughout Lentic Waterbodies in Coastal Southern California.

Toxin producing cyanobacterial blooms have increased globally in recent decades in both frequency and intensity. Despite the recognition of this growing risk, the extent and magnitude of cyanobacterial blooms and cyanotoxin prevalence is poorly characterized in the heavily populated region of southern California. Recent assessments of lentic waterbodies (depressional wetlands, lakes, reservoirs and coastal lagoons) determined the prevalence of microcystins and, in some cases, additional cyanotoxins. Microcystins were present in all waterbody types surveyed although toxin concentrations were generally low across most habitats, as only a small number of sites exceeded California's recreational health thresholds for acute toxicity. Results from passive samplers (Solid Phase Adsorption Toxin Tracking (SPATT)) indicated microcystins were prevalent throughout lentic waterbodies and that traditional discrete samples underestimated the presence of microcystins. Multiple cyanotoxins were detected simultaneously in some systems, indicating multiple stressors, the risk of which is uncertain since health thresholds are based on exposures to single toxins. Anatoxin-a was detected for the first time from lakes in southern California. The persistence of detectable microcystins across years and seasons indicates a low-level, chronic risk through both direct and indirect exposure. The influence of toxic cyanobacterial blooms is a more complex stressor than presently recognized and should be included in water quality monitoring programs

Advances in Above- and In-Water Radiometry, Volume 3: Hybridspectral Next-Generation Optical Instruments

This publication documents the scientific advances associated with new instrument systems and accessories built to improve above- and in-water observations of the apparent optical properties (AOPs) for a diversity of water masses, including optically complex waters. The principal objective is to be prepared for the launch of next-generation ocean color satellites with the most capable commercial off-the-shelf (COTS) instrumentation in the shortest time possible. The technologies described herein are entirely new hybrid sampling capabilities, so as to satisfy the requirements established for next-generation missions. Both above- and in-water instruments are documented with software options for autonomous control of data collection activities as applicable. The instruments were developed for the Hybridspectral Alternative for Remote Profiling of Optical Observations for NASA Satellites (HARPOONS) vicarious calibration project. The state-of-the-art accuracy required for vicarious calibration also led to the development of laboratory instruments to ensure the field observations were within uncertainty requirements. Separate detailed presentations of the individual instruments provide the hardware designs, accompanying software for data acquisition and processing, and examples of the results achieved