中国南水北调项目的可行性探讨

中国的南水北调工程，如果全部实施，每年可从长江流域调水400～500亿m^3到华北平原，以缓解华北地区的3～3．25亿人的水短缺问题。南水北调工程第一阶段(调水200亿m^3／年)建设，已于2002年或2003年开工(已在2002年12月开工。校译者注)，需投入170亿美元建设资金(包括70亿美元的附属项目资金)。世界银行最近研究认为“该项目在经济上有较大效益”。但世界自然基金会却反对该结论，争议较大。笔者认为该项目在经济上投资昂贵，并需要移民30万人，调水价格也很难为农业所承担；但是，中国在社会经济发展方面正发生着史无前例的变化，而华北平原严重水短缺已引发了一系列严重的问题。通过将当地灌溉用水重新配置并转让给生活和工业用户以及改善环境用水，实际操作是不可能的。而南水北调工程的实施可在很大程度上缓解这些矛盾，从这个意义(更多的是从政治上和实际需要出发)说，而不是严格从经济上或粮食安全的角度考虑，中国政府决策实施此项目是完全可以理解的。     

The Scope of Turing's Analysis of Effective Procedures

Turing's (1936) analysis of effective symbolic procedures is a model of conceptual clarity that plays an essential role in the philosophy of mathematics. Yet appeal is often made to the effectiveness of human procedures in other areas of philosophy. This paper addresses the question of whether Turing's analysis can be applied to a broader class of effective human procedures. We use Sieg's (1994) presentation of Turing's Thesis to argue against Cleland's (1995) objections to Turing machines and we evaluate her proposal to understand the effectiveness of procedures in terms of their reliability and precision. A number of conditions for effectiveness are identified and these are used to provide a general argument against the possibility of a Leibnizian decision procedure.     

A transient, multi-phase and multi-component model of a new passive DMFC

A 2-dimensional, transient multi-phase, multi-component fuel cell model is developed to model a passive fuel delivery system including the fuel cell itself for a direct methanol fuel cell (DMFC). This model captures evaporative effects, as water and fuel management are crucial issues. The evaporation/condensation rates are formulated in a manner to capture non-equilibrium effects between the phases. Also, the full kinetics are modeled at both the anode and cathode catalyst layers, along with the electric potential of the membrane, catalyst and gas diffusion layers. The fuel cell operation is examined by quantifying the fuel consumption due to chemical reaction and evaporation as a function of feed concentration. The passive delivery system utilizes a porous media to passively deliver methanol to the fuel cell while controlling the concentration of methanol at the anode side to limit the amount of methanol cross-over. The results illustrate the feasibility of the passive thermal-management system, and characterize the relevant transport phenomena.     

An octave-bandwidth negligible-loss radiofrequency metamaterial

Metamaterials provide an unprecedented ability to manipulate electromagnetic waves and are an enabling technology for new devices ranging from flat lenses that focus light beyond the diffraction limit to coatings capable of cloaking an object. Nevertheless, narrow bandwidths and high intrinsic losses arising from the resonant properties of metamaterials have raised doubts about their usefulness. New design approaches seek to turn the perceived disadvantages of dispersion into assets that enhance a device's performance. Here we employ dispersion engineering of metamaterial properties to enable specific device performance over usable bandwidths. In particular, we design metamaterials that considerably improve conventional horn antennas over greater than an octave bandwidth with negligible loss and advance the state of the art in the process. Fabrication and measurement of a metahorn confirm its broadband, low-loss performance. This example illustrates the power of clever implementation combined with dispersion engineering to bring metamaterials into their full potential for revolutionizing practical devices.     

Spin polarization measurement of homogeneously doped Fe(1-x)Co(x)Si nanowires by Andreev reflection spectroscopy

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x? = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.     

Diffusion dynamics of small molecules from mesoporous silicon films by real-time optical interferometry

Time-dependent laser reflectometry measurements are presented as a means to rigorously characterize analyte diffusion dynamics of small molecules from mesoporous silicon (PSi) films for drug delivery and membrane physics applications. Calculations based on inclusion of a spatially and temporally dependent solute concentration profile in a one-dimensional Fickian diffusion flow model are performed to determine the diffusion coefficients for the selected prototypical polar species, sucrose (340 Da), exiting from PSi films. The diffusion properties of the molecules depend on both PSi pore size and film thickness. For films with average pore diameters between 10-30?nm and film thicknesses between 300-900?nm, the sucrose diffusion coefficient can be tuned between approximately 100 and 550?μm2/s. Extensions of the real-time measurement and modeling approach for determining the diffusivity of small molecules that strongly interact with and corrode the internal surfaces of PSi films are also discussed.     

Role of conducting carbon in electrodes for electric double layer capacitors

Six electrodes with a varying amount (5, 10, and 15 wt.%) of conducting carbon nanotubes (CNT) and carbon nanofibers (CNF) were fabricated and their performance evaluated against a control sample that was devoid of any conducting material. The goal of this work was to determine the correlation between electrode conductivity and capacitance in 1 M tetraethyl ammonium tetrafluoroborate (TEABF₄) in propylene carbonate (PC) electrolyte. CNT electrodes exhibit the lowest electrical resistance, while CNF electrodes had the highest capacitance. The specific capacitance (120-140 F/g) increased monotonically up to 2.5 V. An inverse correlation between electrical resistance and capacitance was observed for various concentrations. The electrodes were characterized using CV, EIS, SEM, and BET analysis.     

Antibiofouling hybrid dendritic Boltorn/star PEG thiol-ene cross-linked networks

A series of thiol-ene generated amphiphilic cross-linked networks was prepared by reaction of alkene-modified Boltorn polyesters (Boltorn-ene) with varying weight percent of 4-armed poly(ethylene glycol) (PEG) tetrathiol (0-25 wt%) and varying equivalents of pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) (0-64 wt%). These materials were designed to present complex surface topographies and morphologies, with heterogeneity of surface composition and properties and robust mechanical properties, to serve as nontoxic antibiofouling coatings that are amenable to large-scale production for application in the marine environment. Therefore, a two-dimensional matrix of materials compositions was prepared to study the physical and mechanical properties, over which the compositions spanned from 0 to 25 wt% PEG tetrathiol and 0-64 wt% PETMP (the overall thiol/alkene (SH/ene) ratios ranged from 0.00 to 1.00 equiv), with both cross-linker weight percentages calculated with respect to the weight of Boltorn-ene. The Boltorn-ene components were prepared through the esterification of commercially available Boltorn H30 with 3-butenoic acid. The subsequent cross-linking of the Boltorn-PEG-PETMP films was monitored using IR spectroscopy, where it was found that near-complete consumption of both thiol and alkene groups occurred when the stoichiometry was ca. 48 wt% PETMP (0.75 equiv SH/ene, independent of PEG amount). The thermal properties of the films showed an increase in T(g) with an increase in 4-armed PEG-tetrathiol wt%, regardless of the PETMP concentration. Investigation of the bulk mechanical properties in dry and wet states found that the Young's modulus was the greatest at 48 wt% PETMP (0.75 equiv of SH/ene). The ultimate tensile strength increased when PETMP was constant and the PEG concentration was increased. The Young's modulus was slightly lower for wet films at constant PEG or constant PETMP amounts, than for the dry samples. The nanoscopic surface features were probed using atomic force microscopy (AFM), where it was observed that the surface of the amphiphilic films became increasingly rough with increasing PEG wt%. On the basis of the physicochemical data from the diverse sample matrix, a focused compositional profile was then investigated further to determine the antifouling performance of the cross-linked Boltorn-PEG-PETMP networks. For these studies, a low, constant PETMP concentration of 16 wt% was maintained with variation in the PEG wt% (0-35 wt%). Antifouling and fouling-release activities were tested against the marine alga Ulva. Spore settlement densities were low on these films, compared to that on standards of polydimethylsiloxane and glass.