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排序方式: 共有120条查询结果,搜索用时 31 毫秒
61.
62.
Crystalline Ropes of Metallic Carbon Nanotubes 总被引:7,自引:0,他引:7
A Thess R Lee P Nikolaev H Dai P Petit J Robert C Xu YH Lee SG Kim AG Rinzler DT Colbert GE Scuseria D Tomanek JE Fischer RE Smalley 《Science (New York, N.Y.)》1996,273(5274):483-487
Fullerene single-wall nanotubes (SWNTs) were produced in yields of more than 70 percent by condensation of a laser-vaporized carbon-nickel-cobalt mixture at 1200degreesC. X-ray diffraction and electron microscopy showed that these SWNTs are nearly uniform in diameter and that they self-organize into "ropes," which consist of 100 to 500 SWNTs in a two-dimensional triangular lattice with a lattice constant of 17 angstroms. The x-ray form factor is consistent with that of uniformly charged cylinders 13.8 +/- 0.2 angstroms in diameter. The ropes were metallic, with a single-rope resistivity of <10(-4) ohm-centimeters at 300 kelvin. The uniformity of SWNT diameter is attributed to the efficient annealing of an initial fullerene tubelet kept open by a few metal atoms; the optimum diameter is determined by competition between the strain energy of curvature of the graphene sheet and the dangling-bond energy of the open edge, where growth occurs. These factors strongly favor the metallic (10,10) tube with C5v symmetry and an open edge stabilized by triple bonds. 相似文献
63.
Zhang K Wong JW Yang P Tech K Dibenedetto AL Lee NS Hayward DG Makovi CM Krynitsky AJ Banerjee K Jao L Dasgupta S Smoker MS Simonds R Schreiber A 《Journal of agricultural and food chemistry》2011,59(14):7636-7646
A multiresidue method analyzing 209 pesticides in 24 agricultural commodities has been developed and validated using the original Quick, Easy, Cheap, Effective, Rugged and Safe (QuEChERS) procedure and high performance liquid chromatography-positive electrospray ionization-tandem mass spectrometry (LC-MS/MS) analysis. Using solvent-only calibration standards (SOCSs) and matrix-matched calibration standards (MMCSs), it was demonstrated that a minimal concentration of 5-10 μg/kg (part per billion, ppb) of analytes in matrix is required for the consistent identification of targeted pesticides with two MRM transitions. Method performance was validated by the precision and accuracy results obtained from fortification studies at 10, 25, 100, and 500 ppb and MMCSs. The method was demonstrated to achieve an average recovery of 100 ± 20% (n = 4) for >75% of evaluated pesticides at the low fortification level (10 ppb) and improved to >84% at the higher fortification concentrations in all 24 matrices. Matrix effects in LC-MS/MS analysis were studied by evaluating the slope ratios of calibration curves (1.0-100 ng/mL) obtained from the SOCSs and MMCSs. Principal component analysis (PCA) of LC-MS/MS and method validation data confirmed that each matrix exerts its specific effect during the sample preparation and LC-MS/MS analysis. The matrix effect is primarily dependent on the matrix type, pesticide type and concentration. Some caution is warranted when using matrix matched calibration curves for the quantitation of pesticides to alleviate concerns on matrix effects. The QuEChERS method with LC-MS/MS was used to identify and quantitate pesticides residues, with concentrations ranging from 2.5 to >1000 ppb in a variety of agricultural samples, demonstrating fitness for screening and surveillance applications. 相似文献
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65.
The breeding chronology and reproductive attempts of the seabird community on Christmas Island in the central Pacific Ocean (2 degrees N, 157 degrees W) were interrupted by the 1982-1983 El Ni?o Southern Oscillation. The resultant reproductive failure and disappearance of the entire seabird community of this equatorial atoll represents the most dramatic interruption on record of a seabird community located distant from coastal upwelling. Our data indicate the effect that the abiotic and biotic aspects of a global atmospheric-oceanic anomaly have on marine birds. The 1982-1983 El Ni?o Southern Oscillation provides an example of selective pressures and a natural experiment in the study of vertebrate population dynamics. 相似文献
66.
SR Scofield CM Tobias JP Rathjen JH Chang DT Lavelle RW Michelmore BJ Staskawicz 《Science (New York, N.Y.)》1996,274(5295):2063-2065
Transient expression of the Pseudomonas syringae avirulence gene avrPto in plant cells resulted in a Pto-dependent necrosis. The AvrPto avirulence protein was observed to interact directly with the Pto resistance protein in the yeast two-hybrid system. Mutations in the Pto and avrPto genes which reduce in vivo activity had parallel effects on association in the two-hybrid assay. These data suggest that during infection the pathogen delivers AvrPto into the plant host cell and that resistance is specified by direct interaction of Pto with AvrPto. 相似文献
67.
68.
Schreiber SJ 《Science (New York, N.Y.)》2011,334(6060):1214-1215
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70.
Bengtson Simlick Binette LovettIV Alves Schreiber & Specker 《Aquaculture Nutrition》2000,6(3):193-198
Summer flounder is a new aquaculture species, the culture of which could be made simpler by the replacement, to the greatest extent possible, of live feed with formulated diets. Previous work has shown that development of the summer flounder stomach can be accelerated by treatment of the larvae with thyroid hormone. Two series of experiments were conducted. In the first experiment, 2-week feeding trials were conducted beginning with fish (which were not treated with thyroid hormone) 14, 21, 28, 35, or 42 days after hatch (DAH) to determine when they could begin to utilize commercially available formulated diets. In the second experiment, two groups of sibling larvae, one treated with thyroid hormone, the other not, were placed in similar 2-week feeding trials at 25, 30 or 35 DAH to test whether survival of thyroid hormone-treated larvae would surpass those of untreated larvae. The first series of experiments demonstrated that the larvae could not effectively utilize the formulated diets until 35 DAH. The second series of experiments indicated that thyroid hormone-treated larvae do not exhibit significantly greater survival than untreated larvae. 相似文献