299 research outputs found
Optimizing Average-Maximum TTR Trade-off for Cognitive Radio Rendezvous
In cognitive radio (CR) networks, "TTR", a.k.a. time-to-rendezvous, is one of
the most important metrics for evaluating the performance of a channel hopping
(CH) rendezvous protocol, and it characterizes the rendezvous delay when two
CRs perform channel hopping. There exists a trade-off of optimizing the average
or maximum TTR in the CH rendezvous protocol design. On one hand, the random CH
protocol leads to the best "average" TTR without ensuring a finite "maximum"
TTR (two CRs may never rendezvous in the worst case), or a high rendezvous
diversity (multiple rendezvous channels). On the other hand, many
sequence-based CH protocols ensure a finite maximum TTR (upper bound of TTR)
and a high rendezvous diversity, while they inevitably yield a larger average
TTR. In this paper, we strike a balance in the average-maximum TTR trade-off
for CR rendezvous by leveraging the advantages of both random and
sequence-based CH protocols. Inspired by the neighbor discovery problem, we
establish a design framework of creating a wake-up schedule whereby every CR
follows the sequence-based (or random) CH protocol in the awake (or asleep)
mode. Analytical and simulation results show that the hybrid CH protocols under
this framework are able to achieve a greatly improved average TTR as well as a
low upper-bound of TTR, without sacrificing the rendezvous diversity.Comment: Accepted by IEEE International Conference on Communications (ICC
2015, http://icc2015.ieee-icc.org/
Identification of Free and Bound Exciton States and Their Phase-Dependent Trapping Behavior in Lead Halide Perovskites
In this work we probe the sub-gap energy states within polycrystalline and
single crystal lead halide perovskites to better understand their intrinsic
photophysics behaviors. Through combined temperature and intensity-dependent
optical measurements, we reveal the existence of both free and bound exciton
contributions within the sub-gap energy state manifold. The trapping and
recombination dynamics of these excitons is shown to be strongly dependent on
the structural phase of the perovskite. The orthorhombic phase exhibits
ultrafast exciton trapping and distinct trap emission, while the tetragonal
phase gives low monomolecular recombination velocity and capture cross-sections
(~10-18 cm2). Within the multiphonon transition scenario, this suppression in
charge trapping is caused by the increase in the charge capture activation
energy due to the reduction in electron-lattice interactions, which can be the
origin for the unexpected long carrier lifetime in these material systems.Comment: 5 figure
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Micro-Crack Formation and Controlling of Inconel625 Parts Fabricated by Selective Laser Melting
Micro-crack is one of the most serious defects in selective laser melting (SLM), which
impair the mechanical properties of the fabricated parts. In this study, Inconel625
superalloy specimens were fabricated by SLM process with progressive alternative
scan strategy. The morphology of the cracks, elements distribution were detected by
optical microscope (OM), scanning electron microscope (SEM) and electron back
scattered diffraction (EBSD). The results showed that a large numbers of micro-cracks
occurred at room temperature, with the average length of approximately 100 µm. It
was found that crack formation was attribute to the local segregation of Nb and Mo
element in the process of rapid solidification, resulting in the generation of low
melting temperature eutectic solidification (γ+Laves). Micro-cracks grows along the
interface of (γ+Laves) under the thermal stress. Base-plate preheating shows an
efficient method to reduce the scales and number of cracks. The residual stress was
reduced by more than 50% when preheating at 300℃.Mechanical Engineerin
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Parameter Optimization for Preparing Carbon Fiber/Epoxy Composites by Selective Laser Sintering
Carbon fiber (CF) reinforced thermosetting resin composites offer a wide range
of high performance features including excellent strength, modulus and thermal
resistance and light weight. Consequently, they are increasingly demanded by
aerospace and automotive industries due to the tighter requirements of the transport
vehicles for lightweight as well as higher payloads. Although thermoplastics and their
composites have been widely used in additive manufacturing (AM), to date it is
difficult to manufacture carbon fibers reinforced thermosetting composite parts via
AM technologies. Therefore, this study developed a novel method based on selective
laser sintering (SLS) to fabricate high-performance carbon fiber/epoxy resin
composites. The response surface method was employed to study the processing
parameters affecting the quality of final parts, and an optimized processing condition
was obtained.Mechanical Engineerin
Hot isostatic pressing of in-situ TiB/Ti-6Al-4V composites with novel reinforcement architecture, enhanced hardness and elevated tribological properties
In this study, titanium borides reinforced Ti-6Al-4V composites have been successfully prepared by hot isostatic pressing (HIPing). The microstructure of the as-fabricated samples was investigated using X-ray diffraction technique, secondary electron microscopy and electron backscatter diffraction and the mechanical properties evaluated through micro-hardness and wear resistance measurements together with nano-indentation. It was found that during HIPing the additive particles TiB2 have transformed into TiB needles which tend to decorate at prior particle boundaries of the consolidated powder particles to form a network structure. Under the same HIPing condition, the needles became increasingly coarser and agglomerated with increased addition of TiB2. The micro-hardness of the synthesized materials increased with increased volume fraction of TiB. Nano-indentation measurement demonstrates that the TiB network structure shows much higher nanohardness than the surrounding matrix regions. The friction coefficient of the synthesized composites decreased continuously with increased volume fraction of TiB, indicating improved wear resistance. High resolution transmission electron microscopy analysis on wear debris revealed the formation of a series of oxides suggesting that chemical reaction between alloy elements and oxygen in air may have happened. It is thus believed that the wearing of the current samples is a result of both friction and chemical reaction
Recovery of absolute phases for the fringe patterns of three selected wavelengths with improved anti-error capability
Hot isostatic pressing of in-situ TiB/Ti-6Al-4V composites with novel reinforcement architecture, enhanced hardness and elevated tribological properties
In this study, titanium borides reinforced Ti-6Al-4V composites have been successfully prepared by hot isostatic pressing (HIPing). The microstructure of the as-fabricated samples was investigated using X-ray diffraction technique, secondary electron microscopy and electron backscatter diffraction and the mechanical properties evaluated through micro-hardness and wear resistance measurements together with nano-indentation. It was found that during HIPing the additive particles TiB2 have transformed into TiB needles which tend to decorate at prior particle boundaries of the consolidated powder particles to form a network structure. Under the same HIPing condition, the needles became increasingly coarser and agglomerated with increased addition of TiB2. The micro-hardness of the synthesized materials increased with increased volume fraction of TiB. Nano-indentation measurement demonstrates that the TiB network structure shows much higher nanohardness than the surrounding matrix regions. The friction coefficient of the synthesized composites decreased continuously with increased volume fraction of TiB, indicating improved wear resistance. High resolution transmission electron microscopy analysis on wear debris revealed the formation of a series of oxides suggesting that chemical reaction between alloy elements and oxygen in air may have happened. It is thus believed that the wearing of the current samples is a result of both friction and chemical reaction
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