50175 research outputs found
Sort by
On the Trap Assisted Stress Induced Safe Operating Area Limits of AlGaN/GaN HEMTs
No full textThis experimental study reports a systematic investigation of Safe Operating Area limits in AlGaN/GaN HEMT using sub-mu s pulse characterization with on the fly Raman and CV characterization to probe defect and stress evolution across the device. Influence of a recess depth on SOA boundary is analyzed. Post failure analysis corroborates well with the failure physics unveiled in this work
Hot Deformation and Dynamic Recrystallization in Titanium Aluminide
No full textThe hot workability of gamma titanium aluminide alloy, Ti-48Al-2V-2Nb, was assessed in the cast condition through a series of compression tests conducted over a range of temperatures (1000 to 1175 degrees C) and at the strain rate of 10 S-1. The mechanism of dynamics recrystallization has been investigated from SEM Z-contrast images and from the Electron backscattered diffraction EBSD as well. It has been observed that volume fraction of the recrystallized grains increases with increasing the deformation temperature. The major volume fraction of the recrystallized grains was observed in the shear band which was forming at an angle 45 degrees with respect to the compression direction. The mechanism of breaking of the laths and the region of the dynamic recrystallization were also investigated from the SEM Z-contrast image and EBSD. The dynamic recrystallization occurred in the region of the broken laths and shear bands. The breaking of the laths was because of the kinking of the lamellae. The shear band, kinked lamellae and dynamic recrystallized region where all investigated simultaneously
Performance of masonry units prepared using construction and demolition waste as fine aggregates
No full textConstruction activities demand huge quantity of materials like aggregates, masonry units, cement, steel, water etc. The energy consumed in production and procurement of these materials is considerable, leading to negative impacts on the environment. To have a sustainable solution, low cost alternatives and energy efficient materials need to be explored and adopted. The 3R's (reduce, re-use, recycle) of sustainability is one such means. The present study attempts to utilize construction and demolition (C&D) waste in the form of crushed demolished brick masonry (DBM) as an alternative to soil and fine aggregate in (a) stabilized adobe blocks and (b) concrete blocks respectively. Stabilized Adobe blocks were cast in various combinations; soil and fine aggregate were replaced by crushed C&D waste. Concrete blocks with varying percentage of C&D waste as fine aggregate of different sizes were cast. These masonry units were tested for their physical and mechanical properties and were compared with the properties of commercially procured burnt clay bricks and concrete blocks respectively for their suitability to be used as masonry units. Test results exhibited satisfactory engineering properties including strength characteristics, when soil was replaced in the order of 60-80 in stabilized adobe and about 50 replacement of manufactured sand (M sand) by crushed C&D waste in the form of demolished brick masonry. The experimental program indicates suitability and potential of utilizing demolished brick masonry waste as partial replacement of soil in stabilized adobe and fine aggregate in concrete blocks. The process outlined in preparing masonry units is one way of handling the wastes in an effective manner. © 2018 The International Masonry Society (IMS)
A Frugal Approach to Reduce RCU Grace Period Overhead
No full textGrace period computation is a core part of the Read-Copy-Update (RCU) synchronization technique that determines the safe time to reclaim the deferred objects' memory. We first show that the eager grace period computation employed in the Linux kernel is appropriate only for enterprise workloads such as web and database servers where a large amount of reclaimable memory awaits the completion of a grace period. However, such memory is negligible in High-Performance Computing (HPC) and mostly idling environments due to limited OS kernel activity. Hence an eager approach is not only futile but also detrimental as the CPU cycles consumed to compute a grace period leads to jitter in HPC and frequent CPU wake-ups in idle environments. We design frugal grace periods, an economical grace period computation for non-enterprise environments that consume fewer CPU cycles. In addition, we reduce the number of grace periods either by using heuristics or by letting the memory allocator to explicitly request for a grace period only when it is running out of free objects. Our implementation in the Linux kernel reduces the number of grace periods by 68% to 99%, reduces the CPU time consumed by grace periods by 39% to 99%, improves the throughput by up to 28% for NAS parallel benchmarks and increases the CPU time spent in low power states by 2.4x when the system is idle
ON-SUN TESTING OF A HIGH TEMPERATURE BLADED SOLAR RECEIVER AND TRANSIENT EFFICIENCY EVALUATION USING AIR
No full textPrior research at Sandia National Laboratories showed the potential advantages of using light-trapping features which are not currently used in direct tubular receivers. A horizontal bladed receiver arrangement showed the best potential for increasing the effective solar absorptance by increasing the ratio of effective surface area to the aperture footprint. Previous test results and models of the bladed receiver showed a receiver efficiency increase over a flat receiver panel of 5-7% over a range of average irradiances, while showing that the receiver tubes can withstand temperatures > 800 degrees C with no issues. The bladed receiver is being tested at various peak heat fluxes ranging 75-150 kW/m(2) under transient conditions using Air as a heat transfer fluid at inlet pressure 250 kPa (-36 psi) using a regulating flow loop. The flow loop was designed and tested to maintain a steady mass flow rate for 15 minutes using pressurized bottles as gas supply. Due to the limited flow -time available, a novel transient methodology to evaluate the thermal efficiencies is presented in this work. Computational fluid dynamics (CFD) models are used to predict the temperature distribution and the resulting transient receiver efficiencies. The CFD simulations results using air as heat transfer fluid have been validated experimentally at the National Solar Thermal Test Facility in Sandia National Labs
CuBO2 nanonetwork: a novel and significant candidate for photocatalytic dye degradation
No full textCuBO2 is a novel material in the research field of transparent conducting oxide. In this study, CuBO2 nanostructures have been synthesized via sol-gel method. The phase formation is confirmed using an X-ray diffractometer. Detailed morphological analysis is performed by field emission scanning electron microscopy and transmission electron microscopy. Anovel uniform nanonetwork-like structure is obtained and its band gap is found to be 4.24eV. In ultraviolet light irradiation, this as-synthesized sample shows efficient photocatalytic activity for degradation of organic dye Rhodamine B. The degradation efficiency and the rate constant were calculated as similar to 70% and 1.32 x 10 -3 min -1, respectively. This nanonetwork-like structure can be a potential candidate as the base material to attach various metals and metal oxide nanostructures to get highly efficient future photocatalysts. As a result, this study opens up a new gateway to fabricate novel environment-friendly nanocatalysts with high performance
Effect of inhomogeneous mesoporosity and defects on the luminescent properties of slanted silicon nanowires prepared by facile metal-assisted chemical etching
No full textSlanted silicon nanowires show an improved optical absorption and better electrical contact than the vertical silicon nanowires. High aspect ratio mesoporous slanted silicon nanowires oriented along the (100) direction are fabricated by a facile two-step metal-assisted chemical etching process. Inhomogeneous porosity with a pore diameter of 2-10 nm is identified by the analysis of transmission electron microscopy, angle dependent Raman spectroscopy, and Brunauer-Emmett-Teller measurements. Slanted silicon nanowires possess a core/shell structure, and the porosity varies from top to bottom of the slanted silicon nanowires. The presence of neutral oxygen defects, self-trapped excitons, and surface defects is identified by photoluminescence spectroscopy, and the results are correlated with Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy data. In addition to mesoporosity, defects such as self-trapped excitons, oxygen vacancies, and surface defects on Si/SiOx interface contribute to the luminescence of slanted silicon nanowires. Red shift in the photoluminescence with increasing etching time is explained using quantum confinement luminescent center model. Understanding the role of defects and porosity in slanted silicon nanowires is highly desirable to increase the efficiency of silicon nanowires based optoelectronic devices. Published by AIP Publishing
Defect Unbinding in Active Nematics
No full textWe formulate the statistical dynamics of topological defects in the active nematic phase, formed in two dimensions by a collection of self-driven particles on a substrate. An important consequence of the nonequilibrium drive is the spontaneous motility of strength +1/2 disclinations. Starting from the hydrodynamic equations of active nematics, we derive an interacting particle description of defects that includes active torques. We show that activity, within perturbation theory, lowers the defect-unbinding transition temperature, determining a critical line in the temperature-activity plane that separates the quasilong-range ordered (nematic) and disordered (isotropic) phases. Below a critical activity, defects remain bound as rotational noise decorrelates the directed dynamics +1/2 defects, stabilizing the quasi-long-range ordered nematic state. This activity threshold vanishes at low temperature, leading to a reentrant transition. At large enough activity, active forces always exceed thermal ones and the perturbative result fails, suggesting that in this regime activity will always disorder the system. Crucially, rotational diffusion being a two-dimensional phenomenon, defect unbinding cannot be described by a simplified one-dimensional model
Chiral Assemblies of Achiral Dielectric Nanoparticles: Semianalytical Approach
No full textWe report a theoretical modeling on the excitation of chiro-optical activity in chiral assemblies comprising of dielectric nanoparticles using a semianalytical method based on coupled dipole approximation. In stark contrast to plasmonic counterparts where electric dipolar coupling dominates chiro-optical response, our analysis reveals a magnetic-dominated response, which originates from the dynamic electromagnetic coupling between the magnetic dipole moments of the individual high-index dielectric nanoparticles. The calculated circular dichroism response practically remains insensitive to interparticle spacing but shows strong dependence on the size of nanoparticle and refractive index, in close agreement with numerical simulations. Such a chiral assembly represents a novel material platform where individual achiral dielectric nanoparticles with strong magnetic resonances can be exploited as building blocks for engineering new type of chiral light-matter interaction, and being low in light absorption loss, this could be promising for applications in nanophotonics
Upper layer diapycnal mixing and nutrient flux in the subtropical frontal region of the Indian sector of the Southern Ocean
No full textUpper layer diapycnal mixing in the Subtropical Front (STF) was estimated using microstructure shear profiles collected from the Indian sector of the Southern Ocean (ISSO) during the austral summer of 2012. Observations were made in the northern and southern boundary of the highly mesoscale turbulent STF, which is characterized by the presence of the dynamic Agulhas Return Current. During the observational period, the STF was populated with alternating cyclonic and anticyclonic eddies. In this mesoscale turbulent region, the average eddy diffusivity at the base of the euphotic zone was 5.5 x 10(-5) m(2) s(-1). The average diapycnal nitrogen flux at the base of the euphotic zone, calculated using direct turbulence measurements, and nitrate (NO3) and nitrite (NO2) concentrations, was 6.4 x 10-5 mu mol m(-2) s(-1). The satellite-derived primary production in the STF was similar to 1000 mg C m(-2) day(-1). The observed diapycnal nutrient flux could only sustain < 1% of the production observed in the region. Analysis of satellite-derived ocean currents, sea level anomalies, and thermohaline distribution further shows that despite the study area is a highly mesoscale turbulent region, the primary supply of nutrients is a result of advection (vertical or zonal) rather than vertical mixing