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91.
Time-dependent density functional theory (TDDFT) calculations show that silicon nanoclusters (Si-NC) capped by linear silane groups have large oscillator strengths of the same magnitude as reported in recent experimental studies. We propose a mechanism where linear silanes attached to the Si-NC surface affect the optical properties enhancing the oscillator strengths and thereby accounting for the bright luminescence observed in the blue region of the visible spectrum. The anisotropic emission seen experimentally can also be explained by the presence of the silane groups on the cluster surface. The excitation energies are found to be only slightly affected by the silanes, whereas the oscillator strengths increase with the length of the silane chain and are significantly larger than obtained for unsubstituted Si-NCs. In TDDFT studies of Si-NC dimers interconnected by a linear silane bridge, we obtained large oscillator strengths indicating that such structures could be useful light sources for optical devices. 相似文献
92.
Olli Martiskainen Ryszard Gawinecki Borys Ośmiałowski Kirsti Wiinamäki Kalevi Pihlaja 《Rapid communications in mass spectrometry : RCM》2009,23(7):1075-1084
Tautomerism has been studied conventionally in solutions or in the solid state. However, the importance of mass spectrometry in the gas phase was realized relatively late. 2‐Phenacylquinolines are known to undergo ketimine‐enaminone tautomerism. The ratio of tautomers is dependent on the nature of the phenyl ring substituent and the Hammett substituent constants σ. Theoretical calculations indicate the presence of ketimine and enaminone tautomers in the gas phase. The electron ionization mass spectra of eight 2‐phenacylquinolines (ketimine form) were recorded at 70 eV in order to determine the fragmentation routes and to screen for the presence of their enaminone tautomers, (Z)‐2‐benzoylmethylene‐1,2‐dihydroquinolines, in the gas phase. The relative abundances or total ion currents of some ions correlated with the Hammett substituent constants and Hammett‐Brown constants. The product ions [M–CO]+ . and [M–HCO]+ were observed. A reaction mechanism is suggested for the formation of these ions, requiring skeletal rearrangements. The results furnish information relating to tautomerism in the gas phase. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
93.
Tero Taipale Monika Österberg Antti Nykänen Janne Ruokolainen Janne Laine 《Cellulose (London, England)》2010,17(5):1005-1020
Different types of microfibrillated cellulose (MFC) and fines suspensions were produced, characterized, and then added to
a papermaking pulp suspension. High and medium molar mass cationic polyelectrolytes were used as fixatives. The drainage behavior
of the pulp suspensions with additives were evaluated against the strength properties of hand sheets made thereof. The effects
of salt concentration, pH, fixative type, dosage and type of fibrillar material on drainage were examined. All the MFC and
fines samples produced had clearly different properties due to their dissimilar production methods, and they also introduced
specific responses on the measured drainage and paper strength. Generally, the addition of MFC decreased the drainage rate
of pulp suspension and increased the strength of paper. However, it was shown that by optimum selection of materials and process
conditions an enhancement of the strength properties could be achieved without simultaneously deteriorating the drainage. 相似文献
94.
Self-Adaptive Genetic Algorithm for Clustering 总被引:6,自引:0,他引:6
Clustering is a hard combinatorial problem which has many applications in science and practice. Genetic algorithms (GAs) have turned out to be very effective in solving the clustering problem. However, GAs have many parameters, the optimal selection of which depends on the problem instance. We introduce a new self-adaptive GA that finds the parameter setup on-line during the execution of the algorithm. In this way, the algorithm is able to find the most suitable combination of the available components. The method is robust and achieves results comparable to or better than a carefully fine-tuned non-adaptive GA. 相似文献
95.
96.
Laine M 《Physical review D: Particles and fields》1994,49(8):3847-3853
97.
98.
Laine O Osterholm H Järvinen H Wickström K Vainiotalo P 《Rapid communications in mass spectrometry : RCM》2000,14(6):482-495
Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was used for qualitative and quantitative end-group analysis of a small molecular weight polyester, poly(2-butyl-2-ethyl-1,3-propylene phthalate). The presence of carboxyl-terminated linear and cyclic polyester oligomers was confirmed with the help of simple sample preparation methods. The presence of carboxyl end-groups in the polyester chains was verified through their formation of carboxylate salts with alkali metal cations. Cyclic oligomers were identified through deuterium exchange of the exchangeable protons of the polyester. Various inorganic salts were tested for salt formation of the carboxyl end-groups, but only the alkali metal salts proved effective. The influence of the alkali metal salts on the results of the quantitative end-group analysis was also studied. The relative amounts of differently terminated and cyclic oligomers were calculated when the alkali metal salts were used with different matrices. The results showed that both the salts and the matrices used in sample preparation can have a marked effect on the quantitative results of the end-group analysis. The measurements were carried out using 2,5-dihydroxybenzoic acid (DHB), 1,8, 9-trihydroxyanthracene (dithranol), and 2-(4-hydroxyphenylazo)benzoic acid (HABA) as matrix compounds. Dithranol and HABA repeatably exhibited similar results, and these results differed from those obtained with DHB probably because of the different ionization mechanisms in the MALDI process. Copyright-Copyright 2000 John Wiley & Sons, Ltd. 相似文献
99.
Acid strength distributions of Mo–Al–Si and Ni–Mo–Al oxide systems have been found by means of a relationship that gives pKa values from amounts of ammonia adsorption. Molybdenum is suggested to cause a homogenization in the acid strength distribution of alumina but a heterogenization in silicaalumina, whereas nickel is found to neutralize strong acid sites inherent to molybdenum.
Mo–Al–Si Ni–Mo–Al , . , , .相似文献
100.
Xue J Song L Khaja SD Locke RD West CM Laine RA Matta KL 《Rapid communications in mass spectrometry : RCM》2004,18(17):1947-1955
Fixed-energy sequential tandem mass spectrometry (MS(n)) capabilities offered by quadrupole ion trap instruments have been explored in a systematic study of six isomers of Gal-Fucalpha-OBenzyl disaccharides. Under collision-induced dissociation (CID), sodiated molecular species generated in the positive-ion electrospray ionization mode yield simple and predictable mass spectra. Information on interglycosidic linkages and configurations can be deduced from the relative intensities of the selected diagnostic fragments arising from the glycosidic bond cleavages and corroborated by the fragments arising from cross-ring cleavages. As the CID patterns are not dependent on the number of prior tandem mass spectrometric steps, structures can be unambiguously assigned by matching the spectra with a library. The rules governing the fragmentation behavior of this class of oligosaccharides were tested for a representative isomeric disaccharide, Glcbeta1,3Fucalpha-OAllyl. The findings establish a basis for using MS(n) with a quadrupole ion trap instrument to elucidate structures of hexose-fucose subunits from more complicated oligosaccharides. Energy-resolved mass spectra were also acquired by CID tandem triple-quadrupole mass spectrometry. The breakdown behavior of the molecular ions revealed patterns which could differentiate stereoisomers of Gal-Fuc disaccharides over a range of collision energy from 20 to 50 eV. 相似文献