Showing papers by "Beihang University published in 2013"

First result from the Alpha Magnetic Spectrometer on the International Space Station: precision measurement of the positron fraction in primary cosmic rays of 0.5-350 GeV.

Abstract: A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on 6.8 × 10(6) positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to ∼ 250 GeV, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena.

Naturalness Preserved Enhancement Algorithm for Non-Uniform Illumination Images

Abstract: Image enhancement plays an important role in image processing and analysis. Among various enhancement algorithms, Retinex-based algorithms can efficiently enhance details and have been widely adopted. Since Retinex-based algorithms regard illumination removal as a default preference and fail to limit the range of reflectance, the naturalness of non-uniform illumination images cannot be effectively preserved. However, naturalness is essential for image enhancement to achieve pleasing perceptual quality. In order to preserve naturalness while enhancing details, we propose an enhancement algorithm for non-uniform illumination images. In general, this paper makes the following three major contributions. First, a lightness-order-error measure is proposed to access naturalness preservation objectively. Second, a bright-pass filter is proposed to decompose an image into reflectance and illumination, which, respectively, determine the details and the naturalness of the image. Third, we propose a bi-log transformation, which is utilized to map the illumination to make a balance between details and naturalness. Experimental results demonstrate that the proposed algorithm can not only enhance the details but also preserve the naturalness for non-uniform illumination images.

Understanding the Role of Different Conductive Polymers in Improving the Nanostructured Sulfur Cathode Performance

Abstract: Lithium sulfur batteries have brought significant advancement to the current state-of-art battery technologies because of their high theoretical specific energy, but their wide-scale implementation has been impeded by a series of challenges, especially the dissolution of intermediate polysulfides species into the electrolyte. Conductive polymers in combination with nanostructured sulfur have attracted great interest as promising matrices for the confinement of lithium polysulfides. However, the roles of different conductive polymers on the electrochemical performances of sulfur electrode remain elusive and poorly understood due to the vastly different structural configurations of conductive polymer–sulfur composites employed in previous studies. In this work, we systematically investigate the influence of different conductive polymers on the sulfur cathode based on conductive polymer-coated hollow sulfur nanospheres with high uniformity. Three of the most well-known conductive polymers, polyaniline (PANI)...

Bioinspired multifunctional foam with self-cleaning and oil/water separation

Abstract: Oil/water separation is a worldwide challenge. Learning from nature provides a promising approach for the construction of functional materials with oil/water separation. In this contribution, inspired by superhydrophobic self-cleaning lotus leaves and porous biomaterials, a facile method is proposed to fabricate polyurethane foam with simultaneous superhydrophobicity and superoleophilicity. Due to its low density, light weight, and superhydrophobicity, the as-prepared foam can float easily on water. Furthermore, the foam demonstrates super-repellency towards corrosive liquids, self-cleaning, and oil/water separation properties, possessing multifunction integration. We expect that this low-cost process can be readily and widely adopted for the design of multifunctional foams for large-area oil-spill cleanup.

Fast, Robust and Accurate Digital Image Correlation Calculation Without Redundant Computations

Abstract: High-efficiency and high-accuracy deformation analysis using digital image correlation (DIC) has become increasingly important in recent years, considering the ongoing trend of using higher resolution digital cameras and common requirement of processing a large sequence of images recorded in a dynamic testing. In this work, to eliminate the redundant computations involved in conventional DIC method using forward additive matching strategy and classic Newton–Raphson (FA-NR) algorithm without sacrificing its sub-pixel registration accuracy, we proposed an equivalent but more efficient DIC method by combining inverse compositional matching strategy and Gauss-Newton (IC-GN) algorithm for fast, robust and accurate full-field displacement measurement. To this purpose, first, an efficient IC-GN algorithm, without the need of re-evaluating and inverting Hessian matrix in each iteration, is introduced to optimize the robust zero-mean normalized sum of squared difference (ZNSSD) criterion to determine the desired deformation parameters of each interrogated subset. Then, an improved reliability-guided displacement tracking strategy is employed to achieve further speed advantage by automatically providing accurate and complete initial guess of deformation for the IC-GN algorithm implemented on each calculation point. Finally, an easy-to-implement interpolation coefficient look-up table approach is employed to avoid the repeated calculation of bicubic interpolation at sub-pixel locations. With the above improvements, redundant calculations involved in various procedures (i.e. initial guess of deformation, sub-pixel displacement registration and sub-pixel intensity interpolation) of conventional DIC method are entirely eliminated. The registration accuracy and computational efficiency of the proposed DIC method are carefully tested using numerical experiments and real experimental images. Experimental results verify that the proposed DIC method using IC-GN algorithm and the existing DIC method using classic FA-NR algorithm generate similar results, but the former is about three to five times faster. The proposed reliability-guided IC-GN algorithm is expected to be a new standard full-field displacement tracking algorithm in DIC.

Observation of a Charged Charmoniumlike Structure Z(c) (4020) and Search for the Z(c) (3900) in e(+)e(-) -> pi(+) pi(-)h(c)

Abstract: We study e(+)e(-) -> pi(+) pi(-)h(c) at center-of-mass energies from 3.90 to 4.42 GeV by using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections are measured at 13 energies and are found to be of the same order of magnitude as those of e(+)e(-) -> pi(+) pi(-) J/Psi but with a different line shape. In the pi(+/-)h(c) mass spectrum, a distinct structure, referred to as Z(c)(4020) is observed at 4. 02 GeV/c(2). The Z(c)(4020) carries an electric charge and couples to charmonium. A fit to the pi(+/-)h(c) invariant mass spectrum, neglecting possible interferences, results in a mass of (4022.9 +/- 0.8 +/- 2.7) MeV/c(2) and a width of (7.9 +/- 2.7 +/- 2.6) MeV for the Z(c)(4020), where the first errors are statistical and the second systematic. The difference between the parameters of this structure and the Z(c) (4025) observed in the D*(D) over bar* final state is within 1.5 sigma, but whether they are the same state needs further investigation. No significant Z(c)(3900) signal is observed, and upper limits on the Z(c)(3900) production cross sections in pi +/- h(c) at center-of-mass energies of 4.23 and 4.26 GeVare set.

Structured cone arrays for continuous and effective collection of micron-sized oil droplets from water

Abstract: Environmental protection agencies and the petroleum industry require effective methods to separate micron-sized oil droplets from water. However, for most existing separation methods, phase separation occurs in the oil-water mixture. The remaining micron-scale oil droplets, which are not affected by phase separation, are difficult to handle with conventional methods on a large scale because of either a lack of separation ability or drawbacks in throughput capacity. Here we develop an oleophilic array of conical needle structures for the collection of micron-sized oil droplets, inspired by the collection of similar sized water droplets on conical cactus spines. Underwater, these structures mimic cacti and can capture micron-sized oil droplets and continuously transport them towards the base of the conical needles. Materials with this structure show obvious advantages in micron-sized oil collection with high continuity and high throughput.

Zeolite-coated mesh film for efficient oil–water separation

Abstract: Oil–water separations are helping with urgent issues due to increasing industrial oily wastewater, as well as frequent oil spill accidents. Membrane-based materials with special wettability are desired to separate oils from water. However, fabrication of energy-efficient and stable membranes that are suitable for practical oil–water separation remains challenging. Zeolite films have attracted intense research interest due to their unique pore character, excellent chemical, thermal and mechanical stability, etc. Here we first demonstrate zeolite-coated mesh films for gravity-driven oil–water separation. High separation efficiency of various oils can be achieved based on the excellent superhydrophilicity and underwater superoleophobicity of the zeolite surface. Flux and intrusion pressure are tunable by simply changing the pore size, dependent on the crystallization time of the zeolite crystals, of the zeolite meshes. More importantly, such films are corrosion-resistant in the presence of corrosive media, which gives them promise as candidates in practical applications of oil–water separation.

FC-PACO-RM: A Parallel Method for Service Composition Optimal-Selection in Cloud Manufacturing System

Abstract: In order to realize the full-scale sharing, free circulation and transaction, and on-demand-use of manufacturing resource and capabilities in modern enterprise systems (ES), Cloud manufacturing (CMfg) as a new service-oriented manufacturing paradigm has been proposed recently. Compared with cloud computing, the services that are managed in CMfg include not only computational and software resource and capability service, but also various manufacturing resources and capability service. These various dynamic services make ES more powerful and to be a higher-level extension of traditional services. Thus, as a key issue for the implementation of CMfg-based ES, service composition optimal-selection (SCOS) is becoming very important. SCOS is a typical NP-hard problem with the characteristics of dynamic and uncertainty. Solving large scale SCOS problem with numerous constraints in CMfg by using the traditional methods might be inefficient. To overcome this shortcoming, the formulation of SCOS in CMfg with multiple objectives and constraints is investigated first, and then a novel parallel intelligent algorithm, namely full connection based parallel adaptive chaos optimization with reflex migration (FC-PACO-RM) is developed. In the algorithm, roulette wheel selection and adaptive chaos optimization are introduced for search purpose, while full-connection parallelization in island model and new reflex migration way are also developed for efficient decision. To validate the performance of FC-PACO-RM, comparisons with 3 serial algorithms and 7 typical parallel methods are conducted in three typical cases. The results demonstrate the effectiveness of the proposed method for addressing complex SCOS in CMfg.

Homogeneous CoO on Graphene for Binder‐Free and Ultralong‐Life Lithium Ion Batteries

Abstract: Ultralong cycle life, high energy, and power density rechargeable lithium-ion batteries are crucial to the ever-increasing large-scale electric energy storage for renewable energy and sustainable road transport. However, the commercial graphite anode cannot perform this challenging task due to its low theoretical capacity and poor rate-capability performance. Metal oxides hold much higher capacity but still are plagued by low rate capability and serious capacity degradation. Here, a novel strategy is developed to prepare binder-free and mechanically robust CoO/graphene electrodes, wherein homogenous and full coating of -Co(OH)(2) nanosheets on graphene, through a novel electrostatic induced spread growth method, plays a key role. The combined advantages of large 2D surface and moderate inflexibility of the as-obtained -Co(OH)(2)/graphene hybrid enables its easy coating on Cu foil by a simple layer-by-layer stacking process. Devices made with these electrodes exhibit high rate capability over a temperature range from 0 to 55 degrees C and, most importantly, maintain excellent cycle stability up to 5000 cycles even at a high current density.

High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO

Abstract: We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu2Se2)2− and insulating (Bi2O2)2+ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi3+ with Ca2+. The resulting materials exhibit a large positive Seebeck coefficient of ∼+330 μV K−1 at 300 K, which may be due to the ‘natural superlattice’ layered structure and the moderate effective mass suggested by both electronic density of states and carrier concentration calculations. After doping with Ca, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of ∼4.74 μW cm−1 K−2 at 923 K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature range of 300 (∼0.9 W m−1 K−1) to 923 K (∼0.45 W m−1 K−1). Such low thermal conductivity values are most likely a result of the weak chemical bonds (Young’s modulus, E∼76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, γ∼1.5). In addition to increasing the power factor, Ca doping reduces the thermal conductivity of the lattice, as confirmed by both experimental results and Callaway model calculations. The combination of optimized power factor and intrinsically low thermal conductivity results in a high ZT of ∼0.9 at 923 K for Bi0.925Ca0.075CuSeO. Li-Dong Zhao, Jiaqing He and co-workers have gained insight into the highly thermoelectric properties of a bismuth–copper oxyselenide (BiCuSeO), a polycrystalline, layered compound. BiCuSeO's ability to produce a significant electric potential from a temperature difference, and vice versa, arises from its intrinsically low thermal conductivity, and can be further improved by boosting the material's electrical conductivity through doping with strontium or barium, or introducing copper deficiencies. The researchers have now carried out an extensive characterization of the oxyselenide and propose that its conveniently low thermal conductivity results from the weak chemical bonds that exist between two different kinds of layers, and a particular bonding arrangement, in the material's lattice. Moreover, by substituting bismuth ions (Bi3+) with calcium ions (Ca3+) the thermal conductivity of the lattice could be lowered further, leading to an improvement in the oxyselenide's thermoelectric properties. We report on the promising thermoelectric performance of p-type polycrystalline BiCuSeO, which is a layered oxyselenide composed of conductive (Cu2Se2)2− layers that alternate with insulating (Bi2O2)2+ layers. Electrical transport properties can be optimized by substituting Bi3+ with Ca2+. Moreover, BiCuSeO shows very low thermal conductivity in the temperature ranges of 300 (∼0.9 W m−1K−1) to 923 K (∼0.45 W m−1 K−1). These intrinsically low thermal conductivity values may result from the weak chemical bonds of the material as well as the strong anharmonicity of the bonding arrangement. The combination of the optimized power factor and the intrinsically low thermal conductivity results in a high ZT of ∼0.9 at 923 K for Bi0.925Ca0.075CuSeO.

Nonlinear-Disturbance-Observer-Based Robust Flight Control for Airbreathing Hypersonic Vehicles

Abstract: The work presented here is concerned with the robust flight control problem for the longitudinal dynamics of a generic airbreathing hypersonic vehicles (AHVs) under mismatched disturbances via a nonlinear-disturbance-observer-based control (NDOBC) method. Compared with other robust flight control method for AHV, the proposed method obtains not only promising robustness and disturbance rejection performance but also the property of nominal performance recovery. The merits of the proposed method are validated by simulation studies.

Texturation boosts the thermoelectric performance of BiCuSeO oxyselenides

Abstract: We present a high ZT ∼ 1.4 in textured Bi0.875Ba0.125CuSeO obtained by a hot-forging process. The carrier mobility along the direction perpendicular to the pressing direction was significantly increased, resulting in increase in the electrical conductivity and maximization of the power factor at 923 K from 6.3 μW cm−1 K−2 for the sample before hot-forging to 8.1 μW cm−1 K−2 after the hot-forging process. Therefore, the maximum ZT was significantly increased from ∼1.1 to 1.4 through texturing for Bi0.875Ba0.125CuSeO, which is the highest ZT ever reported among oxygen containing materials.

Electrospinning of multilevel structured functional micro-/nanofibers and their applications

Abstract: Electrospinning is a straightforward and versatile way to fabricate multilevel structured ultrafine fibers down to the micro-/nanometer scale. In this review, recent achievements in electrospun fibers with multilevel surfaces and inner structures are presented. The multilevel surface structures include branched structures, porous structures, necklace structures and non-cylinder deformed structures. The multilevel inner structures are classified as peapod structures, multiwalled tube structures, wire-in-tube structures and multichannel structures. Furthermore, the outstanding properties of multilevel structured fibrous materials are discussed and highlighted. These electrospun multilevel structured materials have wide applications in sensing, filtration and adsorption, catalysis, energy storage, bio-field, and many other fields.

Patterning of controllable surface wettability for printing techniques

Abstract: Patterning of controllable surface wettability has attracted wide scientific attention due to its importance in both fundamental research and practical applications. In particular, it is crucial to form clear image areas and non-image areas in printing techniques based on wetting and dewetting. This review summarizes the recent research on and applications of patterning of controllable surface wettability for printing techniques, with a focus on the design and fabrication of the precise surface wettability patterning by enhancing the contrast of hydrophilicity and hydrophobicity, such as superhydrophilicity and superhydrophobicity. The selected topics mainly include patterned surface wettability for lithographic printing with different plate-making techniques, patterned surface wettability for microcontact printing with a patterned wetting stamp and special wettability mediated patterning microtransfer printing, patterned surface wettability for inkjet printing with controllable surface wettability of the substrate and printing head to ink, and patterned surface wettability by a combination of different printing techniques. A personal perspective on the future development and remaining challenges of this research is also briefly discussed.

Ultratough Artificial Nacre Based on Conjugated Cross-linked Graphene Oxide

Abstract: As the water-soluble derivative of graphene,graphene oxide (GO), with many functional groups on thesurface, is one of the best candidates for fabricating artificialnacre, because functional surface groups allow for chemicalcross-linking to improve the interfacial strength of theadjacent GO layers. Until now, several methods have beendeveloped to functionalize individual GO sheets and enhancethe resultant mechanical properties, including divalent ion(Mg

Ultratough artificial nacre based on conjugated cross-linked graphene oxide

Abstract: As the water-soluble derivative of graphene,graphene oxide (GO), with many functional groups on thesurface, is one of the best candidates for fabricating artificialnacre, because functional surface groups allow for chemicalcross-linking to improve the interfacial strength of theadjacent GO layers. Until now, several methods have beendeveloped to functionalize individual GO sheets and enhancethe resultant mechanical properties, including divalent ion(Mg

Nanostructured scaffolds for bone tissue engineering.

Abstract: It has been demonstrated that nanostructured materials, compared with conventional materials, may promote greater amounts of specific protein interactions, thereby more efficiently stimulating new bone formation. It has also been indicated that, when features or ingredients of scaffolds are nanoscaled, a variety of interactions can be stimulated at the cellular level. Some of those interactions induce favorable cellular functions while others may leads to toxicity. This review presents the mechanism of interactions between nanoscaled materials and cells and focuses on the current research status of nanostructured scaffolds for bone tissue engineering. Firstly, the main requirements for bone tissue engineering scaffolds were discussed. Then, the mechanism by which nanoscaled materials promote new bone formation was explained, following which the current research status of main types of nanostructured scaffolds for bone tissue engineering was reviewed and discussed.

Pearson's Principle Inspired Generalized Strategy for the Fabrication of Metal Hydroxide and Oxide Nanocages

Abstract: Designing a general route for rational synthesis of a series or families of nanomaterials for emerging applications has become more and more fascinating and vital in the view of nanoscience and nanotechnology. Herein, we explore a general strategy for fabricating uniform nanocages of metal hydroxides (MHs) and metal oxides (MOs). A template-assisted route inspired by Pearson’s hard and soft acid–base (HSAB) principle was employed for synthesizing MH nanocages via meticulous selection of the coordinating etchant as well as optimization of the reaction conditions. The concept of “coordinating etching” is successfully achieved in this work. This unique route shows potential in designing well-defined and high-quality MH nanocages with varying components, shell thicknesses, shapes, and sizes at room temperature. Consequently, porous MO nanocages can be obtained readily just through appropriate thermal treament of the respective MH nanocages. The overall strategy present in this work extends the application of ...

Surface structure dependent electrocatalytic activity of Co 3 O 4 anchored on graphene sheets toward oxygen reduction reaction

Abstract: Catalytic activity is primarily a surface phenomenon, however, little is known about Co3O4 nanocrystals in terms of the relationship between the oxygen reduction reaction (ORR) catalytic activity and surface structure, especially when dispersed on a highly conducting support to improve the electrical conductivity and so to enhance the catalytic activity. Herein, we report a controllable synthesis of Co3O4 nanorods (NR), nanocubes (NC) and nano-octahedrons (OC) with the different exposed nanocrystalline surfaces ({110}, {100}, and {111}), uniformly anchored on graphene sheets, which has allowed us to investigate the effects of the surface structure on the ORR activity. Results show that the catalytically active sites for ORR should be the surface Co2+ ions, whereas the surface Co3+ ions catalyze CO oxidation, and the catalytic ability is closely related to the density of the catalytically active sites. These results underscore the importance of morphological control in the design of highly efficient ORR catalysts.

Enhanced wave absorption of nanocomposites based on the synthesized complex symmetrical CuS nanostructure and poly(vinylidene fluoride)

Abstract: Complex symmetrical CuS nanostructures were synthesized in large scale by a simple wet chemical method at low temperature. As a semiconductor material with superstructure, CuS was well characterized and firstly introduced into PVDF to form nanocomposites. The substantial enhancement of wave absorption (−102 dB at 7.7 GHz) was observed by addition of CuS with a low filler loading (5 wt%). The mechanism for the enhanced wave absorbing properties was explained in detail.

Understanding and Enhancement of Internal Clustering Validation Measures

Abstract: Clustering validation has long been recognized as one of the vital issues essential to the success of clustering applications. In general, clustering validation can be categorized into two classes, external clustering validation and internal clustering validation. In this paper, we focus on internal clustering validation and present a study of 11 widely used internal clustering validation measures for crisp clustering. The results of this study indicate that these existing measures have certain limitations in different application scenarios. As an alternative choice, we propose a new internal clustering validation measure, named clustering validation index based on nearest neighbors (CVNN), which is based on the notion of nearest neighbors. This measure can dynamically select multiple objects as representatives for different clusters in different situations. Experimental results show that CVNN outperforms the existing measures on both synthetic data and real-world data in different application scenarios.

Controllable fabrication of mono-dispersed RGO–hematite nanocomposites and their enhanced wave absorption properties

Abstract: Novel RGO–hematite nanocomposites have been successfully fabricated using a surfactant-governed approach in the presence of polyvinylpyrrolidone (PVP) under mild wet chemical conditions (105 °C). A series of characterization methods including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated that the as-prepared single-crystal hematite particles with relatively uniform size are embedded in RGO layers to form unique quasi shell–core nanostructures. The microwave absorption properties of the RGO–hematite composites were studied in detail; as an absorber, the RGO–hematite nanocomposites possess excellent microwave absorbing properties. The enhanced microwave absorbing properties were also explained based on the structures of the nanocomposites.

A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength

Abstract: Freestanding nanowires have ultrahigh elastic strain limits (4 to 7%) and yield strengths, but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. We exploited the intrinsic mechanical properties of nanowires in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the true elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we developed an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Young's modulus of ~28 gigapascals, and a high yield strength of ~1.65 gigapascals. Our elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be exploited in bulk materials.

Understanding latent interactions in online social networks

Abstract: Popular online social networks (OSNs) like Facebook and Twitter are changing the way users communicate and interact with the Internet. A deep understanding of user interactions in OSNs can provide important insights into questions of human social behavior and into the design of social platforms and applications. However, recent studies have shown that a majority of user interactions on OSNs are latent interactions, that is, passive actions, such as profile browsing, that cannot be observed by traditional measurement techniques.In this article, we seek a deeper understanding of both active and latent user interactions in OSNs. For quantifiable data on latent user interactions, we perform a detailed measurement study on Renren, the largest OSN in China with more than 220 million users to date. All friendship links in Renren are public, allowing us to exhaustively crawl a connected graph component of 42 million users and 1.66 billion social links in 2009. Renren also keeps detailed, publicly viewable visitor logs for each user profile. We capture detailed histories of profile visits over a period of 90 days for users in the Peking University Renren network and use statistics of profile visits to study issues of user profile popularity, reciprocity of profile visits, and the impact of content updates on user popularity. We find that latent interactions are much more prevalent and frequent than active events, are nonreciprocal in nature, and that profile popularity is correlated with page views of content rather than with quantity of content updates. Finally, we construct latent interaction graphs as models of user browsing behavior and compare their structural properties, evolution, community structure, and mixing times against those of both active interaction graphs and social graphs.