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971.
S. S. Sankar S. V. Lonikar R. D. Gilbert R. E. Fornes E. O. Stejskal 《Journal of Polymer Science.Polymer Physics》1990,28(3):293-302
It has been shown that incorporation of masked isocyanates in the MY720/DDS epoxy significantly reduces the equilibrium moisture absorption by blocking of residual functional groups ( oxirane group) by the isocyanates released in the deblocking reaction of the masked isocyanates. We have now used high resolution 13C-CP/MAS NMR to follow the reactions in the solid state and to identify intermediates and by-products. The deblocking reaction of the masked isocyanates also releases the corresponding alcohol, part of which may evaporate during the curing reaction. The resolution in the solid-state spectra is good enough to identify all the reactants and the intermediates involved in the curing reaction. Difference spectra are used to emphasize changes between systems that differ in treatment or composition. 相似文献
972.
Shinozaki and Sibuya have shown that the Moore-Penrose inverse (AB)+ can always be expressed as B-A- for generalized inverses A- and B- of matrices A and B, respectively. In this paper, explicit solutions B-mr and A-lr to (AB)+ = B-mrA-lr are given. A class of solutions is obtained which is related to an equation of Greville, and expressions for the general solutions are presented. 相似文献
973.
Philippe Flajolet Gilbert Labelle Louise Laforest Bruno Salvy 《Random Structures and Algorithms》1995,7(2):117-144
Several characteristic parameters of randomly grown quadtrees of any dimension are analyzed. Additive parameters have expectations whose generating functions are expressible in terms of generalized hypergeometric functions. A complex asymptotic process based on singularity analysis and integral representations akin to Mellin transforms leads to explicit values for various structure constants related to path length, retrieval costs, and storage occupation. 相似文献
974.
A kinetic scheme is presented to rationalize recent observations of delayed fluorescence ODMR in isotopically mixed naphthalene crystals, in which the sign of the signal changes from negative to positive as the critical concentration for triplet-triplet transport among the perprotonated traps is approached. The effects of chopped optical excitation, prolonged microwave pumping, temperature and light intensity are considered. The observation of positive DF-ODMR in the high concentration case is used to obtain a lower limit for the ratio of the annihilation constantk 2 to the pair dissociation constantk ?1. 相似文献
975.
976.
Gilbert Helmberg 《Monatshefte für Mathematik》1964,68(3):218-223
Ohne Zusammenfassung 相似文献
977.
978.
979.
A. J. Chalk A. R. Gilbert 《Journal of polymer science. Part A, Polymer chemistry》1972,10(7):2033-2043
The monofunctional acetylenes, phenylacetylene and m-ethynyltoluene, were each copolymerized with diethynylbenzene in a 1:1 mole ratio by using TiCl4/3AlEt2Cl as cyclotrimerization catalyst. The aromatic polymers which were produced were polydisperse with a molecular weight of ~10,000. Variation of catalyst concentration had no effect on the molecular weight profile. An excess of bifunctional acetylene produced some insoluble gel but when some of the monofunctional acetylene was withheld and added only after an initial molecular weight build-up by the excess bifunctional acetylene, soluble polymers with molecular weights of approximately 50,000 were obtained in high yield, provided the overall ratio of mono- to bifunctional acetylene was maintained at 1:1. The resulting polyphenylenes were highly soluble in benzene and chlorinated solvents but gave brittle films. This was attributed to a highly branched structure resulting from a lack of specificity by the catalyst. Thermogravimetric analysis showed the polymers to have high thermal stability. 相似文献
980.
The ionisation potentials and the appearance potentials for ions arising from the loss of carbonyl groups as well as certain other low-energy fragmentation ions have been measured for a series of substituted arene chromium tricarbonyls. The values obtained are found to correlate very well, for the loss of one and two carbonyl groups, with both the CO force constant k and the Hammett σp function and substituent on the benzene ring. The appearance potentials of the ions arising by the loss of three carbonyl groups do not exhibit a high correlation. Arene+ ions are found to occur at energies intermediate between those for the loss of two and three carbonyl groups.The various effects observed are discussed in terms of a model which involves definite contributions from both σ- and π- bonding of carbonyl groups in the gaseous cations. 相似文献