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1.
Michel Hébert 《Archive for Mathematical Logic》1991,31(2):133-143
Summary We give a syntactic characterization of (finitary) theories whose categories of models are closed under the formation of connected limits (respectively the formation of pullbacks and substructures) in the category of all structures. They are also those theories whose consistent extensions by new atomic facts admit in each component an initial structure (respectively an initial term structure), and also thoseT for whichM(T) is locally finitely multi-presentable in a canonical way. We also show that these two properties of theories are nonuniform. 相似文献
2.
Constraint Programming and Operations Research: Comments from an Operations Researcher 总被引:4,自引:0,他引:4
Michel Gendreau 《Journal of Heuristics》2002,8(1):19-24
This paper relates the author's personal experience with constraint programming and gives a personal assessment of the relationships between constraint programming and operations research. 相似文献
3.
Franois Lavaud Michel Fontanille Yves Gnanou 《Journal of polymer science. Part A, Polymer chemistry》2004,42(19):4964-4975
Studies on the anionic polymerization of methyl methacrylate in tetrahydrofuran and in the presence of sparteine have revealed a beneficial effect due to this additive, resulting in a decrease in the extent of termination. Better control of the definition of the polymers formed can thus be achieved in the presence of this additive. On the other hand, macromolecular engineering requires a range of active species concentrations lower than 10?3 mol L?1 and particularly the synthesis of polymers of high molar masses. For a better understanding of the mechanism of chain growth under such concentration conditions, the kinetics of polymerization have been investigated with a technique based on adiabatic calorimetry. Sparteine has been found to lack sufficient cation‐binding power to prevent the propagating enolate ion pairs from aggregating. The rate constant of propagation of nonaggregated species has been estimated, as well as the aggregation constant of equilibrium. For very low initiator concentrations, termination reactions have been shown to profoundly alter the control of the polymerization and to prevent a quantitative monomer conversion. Theoretical maximal conversions have been calculated from kinetic data and compare well with the experimental values. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4964–4975, 2004 相似文献
4.
B. Hornetz H.-J. Michel J. Halbritter 《Fresenius' Journal of Analytical Chemistry》1994,349(1-3):233-235
ARXPS (angle resolved X-ray photoelectron spectroscopy) measurements are used to obtain informations about surfaces and grain boundaries. Data acquired from nanocrystalline carbidic hard coatings have been employed to establish structural models. Magnetron-sputtered coatings of TiC, SiC and TiC/SiC were examined. In such coatings, partly defective TiC nanocrystallites are surrounded by interfacial carbide. This excess carbon shows a binding state similar to that of doped graphite or fullerenes. X-ray amorphous SiC is found in the residue. On top of sputtered SiC coatings, less oxide and graphite is found as compared to TiC/SiC or TiC coatings. 相似文献
5.
The kinetics and the equilibrium constant of the chlorine transfer reaction between monochloramine NH2Cl and the amines: C2H5NH2, (CH3)2CHNH2, (CH3)2NH, and (C2H5)2NH are investigated by spectrophotometry in aqueous medium at 25°C, in the pH range from 8 to 13 and for an ionic strength equal to 1.03 ± 0.05M. For a concentration of total ammonia equal to 1M, the observed rate constant is pH independent below 8 and above 12.8 and reaches a maximum located between the pKas of NH4+ and RR'NH2+. From these results and those obtained earlier for NH2Cl and CH3NH2, the reaction is shown to involve an interaction between neutral molecules NH2Cl and RR'NH, subject to general acid catalysis. The ability of an interaction corresponding to a specific catalysis and involving NH3Cl+ and RR'NH rather than NH2Cl and RR'NH2+ is also discussed. The activation parameters are given for each reaction. 相似文献
6.
J. C. Favier P. Sigwalt M. Fontanille 《Journal of polymer science. Part A, Polymer chemistry》1977,15(10):2373-2379
A kinetic study in tetrahydrofuran of the addition of 1,1-diphenylethylene on para-substituted styrylcarbanions (with Cs+ as counterion) has given the reactivities of free ions and ion-pairs. For both species, the addition rate constant increased in the following sequence for the substituent of the phenyl ring: pH < p-CH3 <p-CH3O. It was also shown that in the same solvent, the dissociation constants of the polystyryl ion-pairs decreased with the electron-donating power of the substituent. 相似文献
7.
A study of the influence of tertiary polyamines on the reactivity of polystyryllithium ion-pairs in hydrocarbon solvents has been performed. The stability of active centres has been studied and allows the processing of kinetic data as for living systems. Rate constants of propagation have been measured in terms of the structure of the polyamine, its relative concentration, the concentration in organometallic species, the temperature and the nature of solvent; in all cases, they are absolute rate constants. The values show the important effect of the number of nitrogen atoms in the additive and especially the strong influence of its geometry: the cyclic polyamine studied leads to an activation of the reaction much greater than that of the corresponding linear substance. These results may be interpreted in terms of different solvation states for the polystyryllithium ion-pairs. 相似文献
8.
A. Parlier H. Rudler N. Platzer M. Fontanille A. Soum 《Journal of organometallic chemistry》1985,287(1):c8-c12
Treatment of VO(acac)2 with the facial-tridentate organometallic ligand [η-CpCo{P(O)(OEt)2}3]? affords a new binuclear compound [η-CpCo{P(O)(OEt)2}3VO(acac)] (I). This compound undergoes protonation with HPF6 in the presence of 1,10-phenanthroline (phen), or 2,2′-bipyridyl (bipy), to yield binuclear cationic derivatives [η-CpCo{P(O)(OEt)2}3VO(phen))]+PF6? (II), and [η-CpCo{P(O)(OEth)2}3VO(bipy)]+PF6? (III). The X-ray crystal structure determination and full characterization of I has been performed. The catalytic oxygenation and oxygen transfer to 3,5-di-t-butylcatechol in the presence of I, II+, or III+ complexes is reported. 相似文献
9.
A pulsed excimer-pumped dye laser was used to excite atomic flourescence in graphite tube electrothermal atomizer. A 60-Hz ac magnitude field was applied around the atomizer and parallel to the excitation beam, for Zeeman background correction. The correction system was found to degrade the detection limits for silver, cobalt, indium, manganese, lead, and thallium by a factor of between 1 and 10. An increase in magnetic field strength, or a decrease in laser linewidth, should improve the detection limits, but was not possible here. For copper, the application of Zeeman background correction was unsuccessfull because the instrumentation was unable to resolve the sigma components from the laser emission profile sufficiently during the background correction measurement. For elements that exhibit sufficient Zeeman splitting, the linear dynamic range was the same with or without background correction Zeeman background correction was used to correct for scatter, in the resonance flourescence determination of manganese in a zinc chloride matrix and in mouse brain tissue. 相似文献
10.
Designs, Codes and Cryptography - The problem of classifying linear systems of conics in projective planes dates back at least to Jordan, who classified pencils (one-dimensional systems) of conics... 相似文献