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71.
In a recent work, we constructed modular multisphere system which expands upon the design of an existing, commercially available Bonner sphere system by adding concentric shells of copper, tungsten, or lead. Our modular multisphere system is referred to as the Bonner Sphere Extension (BSE). The BSE was tested in a high energy neutron beam (thermal to 800 MeV) at Los Alamos Neutron Science Center and provided improvement in the measurement of the neutron spectrum in the energy regions above 20 MeV when compared to the standard BSS (Burgett, 2008 and Howell et al., 2009).However, when the initial test of the system was carried-out at LANSCE, the BSE had not yet been calibrated. Therefore the objective of the present study was to perform calibration measurements. These calibration measurements were carried out using monoenergetic neutron ISO 8529-1 reference beams at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany. The following monoenergetic reference beams were used for these experiments: 14.8 MeV, 1.2 MeV, 565 keV, and 144 keV. Response functions for the BSE were calculated using the Monte Carlo N-Particle Code, eXtended (MCNPX). The percent difference between the measured and calculated responses was calculated for each sphere and energy. The difference between measured and calculated responses for individual spheres ranged between 7.9 % and 16.7 % and the arithmetic mean for all spheres was (10.9 ± 1.8) %. These sphere specific correction factors will be applied for all future measurements carried-out with the BSE. 相似文献
72.
Howell JA 《Dalton transactions (Cambridge, England : 2003)》2006,(4):545-553
The mechanism of cyclopropanation catalysed by Cu(I) complexes has been investigated by calculation using a series of diazoalkanes containing inductive electron donating (methyl) and withdrawing (CF3) substituents and a range of metal fragments (Cu+, [(DAB)Cu]+, ClCu and (triflate)Cu). Copper-diazoalkane complexes exist as an equilibrium of C- and N-bonded isomers. Catalysis occurs through lowering of the activation energy for rate determining C-N bond cleavage of the C-bonded isomer; this is most marked for (triflate)Cu. Direct reaction of the copper-carbene complex occurs to yield stable cupracyclobutanes in all but one case. Associative substitution of the cupracyclobutane by diazoalkane completes the catalytic cycle. 相似文献
73.
Huang W Schopfer M Zhang C Howell RC Gee BA Francesconi LC Polenova T 《The journal of physical chemistry. B》2006,110(25):12340-12350
Paramagnetic Eu-substituted Keggin oxopolytungstates crystallize in different forms, determined by the nature of the counterions. The crystal packing is in turn responsible for the variations in the geometry of paramagnetic Eu sites with respect to the anion core. We probed the paramagnetic environments in a series of Eu-substituted Keggin solids, by 31P magic angle spinning NMR spectroscopy. 31P spinning sideband envelopes are dominated by the electron-nuclear dipolar interaction. For the compounds under investigation, both the magnitude and the asymmetry parameter of the electron-nuclear dipolar coupling tensor are sensitive to the mutual arrangements of paramagnetic Eu sites in the crystal lattice. and also report on the stoichiometry of the anion. The electron-nuclear dipolar coupling tensors were calculated from the crystallographic coordinates and the experimentally determined effective magnetic moments, assuming a point dipole approximation. The computed tensors are in very good agreement with the experimental spectra. Furthermore, the P-Eu distance estimates, accurate to within 0.06-0.12 A, can be obtained directly from the magnitude of the electron-nuclear dipolar coupling. This work demonstrates that 31P MAS NMR spectroscopy is a useful probe for investigating local environments in paramagnetic Keggin solids. 相似文献
74.
Fragments of wall-paintings from Roman villas in Easton Maudit, which date from ca 150 AD have been studied by Raman spectroscopy.
An intact ancient Roman paint pot discovered in the remains of a villa in Castor, Cambridgeshire, still containing a mixture
of white and red pigment was also analysed and the pigments identified as haematite and anatase. The discovery of anatase
in the intact artist’s paint pot, particularly, and also on fragments of broken paint pots from the Easton Maudit villa site,
is a unique contribution to current knowledge of ancient European pigment history, because the presence of this mineral has
not hitherto been recognised fully in an ancient artist’s palette. The relative spectral response of anatase and haematite
in the Raman data is compared with that of anatase and other red pigments such as minium, cinnabar, and litharge. 相似文献
75.
Edwards HG Mohsin MA Sadooni FN Nik Hassan NF Munshi T 《Analytical and bioanalytical chemistry》2006,385(1):46-56
The Raman spectroscopic biosignatures of halotrophic cyanobacterial extremophiles from sabkha evaporitic saltpans are reported for the first time and ideas about the possible survival strategies in operation have been
forthcoming. The biochemicals produced by the cyanobacteria which colonise the interfaces between large plates of clear selenitic
gypsum, halite, and dolomitized calcium carbonates in the centre of the salt pans are identifiably different from those which
are produced by benthic cyanobacterial mats colonising the surface of the salt pan edges in the intertidal zone. The prediction
that similar geological formations would have been present on early Mars and which could now be underlying the highly peroxidised
regolith on the surface of the planet has been confirmed by recent satellite observations from Mars orbit and by localised
traverses by robotic surface rovers. The successful adoption of miniaturised Raman spectroscopic instrumentation as part of
a scientific package for detection of extant life or biomolecular traces of extinct life on proposed future Mars missions
will depend critically on interpretation of data from terrestrial Mars analogues such as sabkhas, of which the current study is an example. 相似文献
76.
77.
B. A. Howell 《Journal of Thermal Analysis and Calorimetry》2006,85(1):165-167
Thermal
analysis has a long and prominent role in the characterization of materials,
including polymeric materials. Kinetic studies in one form or another have
often been employed in an attempt to assess stability, predict lifetime, establish
degradation pathway, or project suitable processing conditions. The results
of such studies have often formed the basis for the proposal of the ‘mechanism’
of reaction. This despite the fact that the reaction being observed is often
unknown or is not a single process but rather several parallel or consecutive
events. This latter is particularly true for ‘variable temperature kinetics’.
The utility/value of such exercises is marginal at best and contributes nothing
to an understanding of the mechanism of any of the reactions involved. 相似文献
78.
In this paper we investigate the linear and affine mappings of near-vector spaces. 相似文献
79.
Lenize F. Maia Vanessa E. de Oliveira Maria E. R. Oliveira Felipe D. Reis Beatriz G. Fleury Howell G. M. Edwards Luiz F. C. de Oliveira 《Journal of Raman spectroscopy : JRS》2013,44(4):560-566
Polyenic pigments in octocorals have been investigated by Raman spectroscopy using laser excitation at 532, 785 and 1064 nm. The spectral features suggest the structural nature of carotenoids from Phyllogorgia dilatata, Leptogorgia punicea, Muricea atlantica, Carijoa riisei and conjugated polyenals from L. punicea, L. setacea, Muricea flamma and Renilla muelleri. The observed vibrational bands at ca. 1540–1520 ν1(C=C), 1159 ν2(C–C) and 1005 cm−1 ρ3(C–CH3) were assigned to carotenoids, whereas the identification of non‐methylated conjugated polyenals have been proposed due to two major Raman bands at ca. 1500 and 1120 cm‐1, assigned to ν(C=C) and ν(C–C), respectively. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
80.
Sakr Akmal A. Ghaly Mohamed F. Edwards Howell G. M. Elbashar Y. H. 《Journal of Radioanalytical and Nuclear Chemistry》2019,319(1):263-269
In order to comprehensively understanding the metabolism process of fluvastatin, the stable isotope-labeled (3R, 5S)-fluvastatin and (3S, 5R)-fluvastatin were required. Both of (3R, 5S)-fluvastatin and (3S, 5R)-fluvastatin were synthesized via a seven steps procedure starting from aniline and [2H6] 2-bromopropane. Two versions of deuterium labeled compound revealed over 98% deuterium enrichment, using as an internal standard.
相似文献