Load control in scalable distributed file structures

The paper presents a family of distributed file structures, coined DiFS, for record structured, disk resident files with key based exact or interval match access. The file is organized into buckets that are spread among multiple servers, where a server may hold several buckets. Client requests are serviced by mapping keys onto buckets and looking up the corresponding server in an address table. Dynamic growth, in terms of file size and access load, is supported by bucket splits and bucket migrations onto the existing or newly created servers.The major problem that we are addressing is achieving scalability in the sense that both the file size and the client throughput can be scaled up by linearly increasing the number of servers and dynamically redistributing the data. Unlike previous work with similar objectives, our data redistribution considers explicitly the cost/performance ratio of the system by aiming to minimize the number of servers that are used to provide the required performance. A new server is added only if the overall server load in the system does not drop below a pre-specified threshold. Simulation results demonstrate the scalability with controlled cost/performance and the importance of global load control. The impact of various tuning parameters on the effectiveness of the load control is studied in detail. Finally, we compare our approach with other approaches known to date and demonstrate that each of the previous approaches can be recast as a special case of our model. Recommended by: Mei HsuThis material is based in part upon work supported by a grant from Hewlett-Packard Corporation and by NSF under grant IRI-9221947.     

Accuracy and speed of real and complex interpolation

Because complex arithmetic takes only a little more time than real arithmetic on modern computers it is advisable to take a closer look at some of the known mathematical tools and compare the advantages in complex computations. This paper compares real and complex interpolation from the point of view of the application for larger problems. It shows that real interpolation breaks down very early whereas complex interpolation is superior by many orders of magnitude with regard to time as well as accuracy. It is shown theoretically that complex interpolation on the unit circle is the best that can be achieved.     

Transmission parameters of a class of coupled distributed networks

It is shown how the equations of a class of coupled distributed networks can be decoupled, and their transmission parameters expressed in terms of those of an equivalent scalar system.     

M.O.S. switched-capacitor amplifiers

Switched-capacitor voltage gain elements, in which the gain is precisely controlled by the ratio of two capacitors, are presented. The voltage gain is obtained by means of a transconductance element operating into a switched-capacitor resistor. Circuit configurations use a single operational amplifier or a unity-gain buffer. In some of these configurations, the offset voltage of the operational amplifier or the buffer is not amplified.     

An effect of iron(III) oxides crystallinity on their catalytic efficiency and applicability in phenol degradation—A competition between homogeneous and heterogeneous catalysis

Catalytic efficiency, stability and environmental applicability of five iron(III) oxide nanopowders differing in surface area and crystallinity were tested in degradation of concentrated phenolic aqueous solutions (100 g/L) at mild temperature (30 °C), initially almost neutral pH and equimolar ratio of hydrogen peroxide and phenol. The catalyst properties were easily controlled by varying in reaction time during isothermal treatment of ferrous oxalate dihydrate in air at 175 °C. Although the catalytic efficiency clearly increases with the surface area of the nanopowders, it is not due to the solely heterogeneous catalytic mechanism as would be expected. The amorphous Fe₂O₃ nanopowders possessing the largest surface areas (401 m² g⁻¹, 386 m² g⁻¹) are the most efficient catalysts evidently due to their highest susceptibility to leaching in acidic environment arising as a consequence of phenol degradation products. Thus, these amorphous samples act partially as homogeneous catalysts, which was confirmed by a high concentration of leached Fe(III) ions in the solution (19 ppm). The crystalline hematite (α-Fe₂O₃) samples, varying in surface area between 337 m² g⁻¹ and 245 m² g⁻¹, are generally less efficient when compared to the amorphous powders, however their catalytic action is almost exclusively heterogeneous as only 3 ppm of leached Fe(III) was found in the reaction systems catalyzed by nanohematite samples. A significant difference in relative contributions of heterogeneous and homogenous catalysis was definitely established in buffered reaction systems catalyzed by amorphous Fe₂O₃ and nanocrystalline hematite. The nanohematite sample exhibiting the highest heterogeneous action was tested at decreased initial phenol concentration (10 g/L), which is closer to the real contents of phenol in waste waters, and at different hydrogen peroxide/phenol molar ratios to consider its environmental applicability. At the hydrogen peroxide/phenol ratio equal to 5, no traces of the leached iron were detected and the phenol conversion of 84% was reached. Moreover, such a high degree of conversion is accompanied by a decrease of the chemical oxygen demand (COD) from the initial value of 11.23 g/L to 4.22 g/L after 125 min. This fact indicates that the considerable fraction of primary reaction products was totally degraded.     

Shape‐Assisted 2D MOF/Graphene Derived Hybrids as Exceptional Lithium‐Ion Battery Electrodes

Herein, a novel polymer‐templated strategy is described to obtain 2D nickel‐based MOF nanosheets using Ni(OH)₂, squaric acid, and polyvinylpyrrolidone (PVP), where PVP has a dual role as a structure‐directing agent, as well as preventing agglomeration of the MOF nanosheets. Furthermore, a scalable method is developed to transform the 2D MOF sheets to Ni₇S₆/graphene nanosheet (GNS) heterobilayers by in situ sulfidation using thiourea as a sulfur source. The Ni₇S₆/GNS composite shows an excellent reversible capacity of 1010 mAh g^?1 at 0.12 A g^?1 with a Coulombic efficiency of 98% capacity retention. The electrochemical performance of the Ni₇S₆/GNS composite is superior not only to nickel sulfide/graphene‐based composites but also to other metal disulfide–based composite electrodes. Moreover, the Ni₇S₆/GNS anode exhibits excellent cycle stability (≈95% capacity retention after 2000 cycles). This outstanding electrochemical performance can be attributed to the synergistic effects of Ni₇S₆ and GNS, where GNS serves as a conducting matrix to support Ni₇S₆ nanosheets while Ni₇S₆ prevents restacking of GNS. This work opens up new opportunities in the design of novel functional heterostructures by hybridizing 2D MOF nanosheets with other 2D nanomaterials for electrochemical energy storage/conversion applications.     

Densely Functionalized Cyanographene Bypasses Aqueous Electrolytes and Synthetic Limitations Toward Seamless Graphene/β‐FeOOH Hybrids for Supercapacitors

Supercapacitors are a promising energy storage technology owing to their unparalleled power and lifetime. However, to meet the continuously rising demands of energy storage, they must be equipped with higher energy densities. For this purpose, the seamless integration of metal oxides on carbon matrices, such as iron oxides/oxyhydroxides, has been pursued through hydrothermal, atomic layer and electro‐deposition methods directly on current collectors. Nevertheless, such methods present limited compatibility with commercial paste‐coating processes on the current collectors. Furthermore, iron oxides/oxyhydroxides lack conductivity and are hydrophilic, operating with low‐voltage aqueous electrolytes, limiting their power and energy and requiring corrosion‐resistant H₂O current collectors. To mitigate these challenges, a seamless and paste‐ready material is successfully developed through a 15 min wet‐chemical method, via the coordination of ultrasmall β‐FeOOH (akaganéite) nanoparticles to the nitrile groups of a covalent graphene derivative. Endowed with graphene‐like impedance response and very high wettability in organic electrolytes, combined high power and energy densities are obtained, with respect to the total mass of both electrode materials and current collectors, overcoming the identified challenges. This offers future prospects for the exploration of alternative molecular handles for improved interfaces and their application in different energy‐storage chemistries.     

Dry versus hydrated collagen scaffolds: are dry states representative of hydrated states?

Collagen composite scaffolds have been used for a number of studies in tissue engineering. The hydration of such highly porous and hydrophilic structures may influence mechanical behaviour and porosity due to swelling. The differences in physical properties following hydration would represent a significant limiting factor for the seeding, growth and differentiation of cells in vitro and the overall applicability of such hydrophilic materials in vivo. Scaffolds based on collagen matrix, poly(DL-lactide) nanofibers, calcium phosphate particles and sodium hyaluronate with 8 different material compositions were characterised in the dry and hydrated states using X-ray microcomputed tomography, compression tests, hydraulic permeability measurement, degradation tests and infrared spectrometry. Hydration, simulating the conditions of cell seeding and cultivation up to 48?h and 576?h, was found to exert a minor effect on the morphological parameters and permeability. Conversely, hydration had a major statistically significant effect on the mechanical behaviour of all the tested scaffolds. The elastic modulus and compressive strength of all the scaffolds decreased by ~95%. The quantitative results provided confirm the importance of analysing scaffolds in the hydrated rather than the dry state since the former more precisely simulates the real environment for which such materials are designed.

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High‐Performance Supercapacitors Based on a Zwitterionic Network of Covalently Functionalized Graphene with Iron Tetraaminophthalocyanine

Graphene derivatives are promising candidates as electrode materials in supercapacitor cells, therefore, functionalization strategies are pursued to improve their performance. A scalable approach is reported for preparing a covalently and homogenously functionalized graphene with iron tetraaminophthalocyanine (FePc‐NH₂) with a high degree of functionalization. This is achieved by exploiting fluorographene's reactivity with the diethyl bromomalonate, producing graphene‐dicarboxylic acid after hydrolysis, which is conjugated with FePc‐NH₂. The material exhibits an ultrahigh gravimetric specific capacitance of 960 F g^?1 at 1 A g^?1 and zero losses upon charging–discharging cycling. The energy density of 59 Wh kg^?1 is eminent among supercapacitors operating in aqueous electrolytes with graphene‐based electrode materials. This is attributed to the structural and functional synergy of the covalently bound components, giving rise to a zwitterionic surface with extensive π–π stacking, but not graphene restacking, all being very beneficial for charge and ionic transport. The safety of the proposed system, owing to the benign Na₂SO₄ aqueous electrolyte, the high capacitance, energy density, and potential of preparing the electrode material on a large‐scale and at low cost make the reported strategy very attractive for development of supercapacitors based on the covalent attachment of suitable molecules onto graphene toward high‐synergy hybrids.