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121.
Karine L Marques Jo?o L M Santos Jo?o A Lopes José L F C Lima 《Analytical sciences》2008,24(8):985-991
An automated multi-pumping flow system is proposed for the simultaneous chemiluminometric determination of benserazide and levodopa using multivariate calibration methods. The developed methodology is based on chemiluminescence (CL) emission generated by the reaction of benserazide with luminol, and on a concurrent inhibiting effect of levodopa on this reaction. A multi-pumping flow system comprising multiple solenoid micro-pumps as the only active components was developed to implement a stopped-flow approach for signal acquisition and processing. Artificial neural networks were used to establish a relationship between the CL emission profile and the concentration of both drugs. The concentration values used to establish the experimental calibration samples were varied between 5 and 30 mg l(-1) for levodopa and between 2.5 and 20 mg l(-1) for benserazide. The proposed method was successfully applied to the simultaneous determination of levodopa and benserazide in pharmaceutical formulations combining both drugs. 相似文献
122.
J. M. Lopes F. Lemos E. Derouane F. Ramôa Ribeiro 《Reaction Kinetics and Catalysis Letters》1996,58(1):33-38
A series of HNaZSM-20 catalysts with a varying amount of sodium cations were studied. The samples were prepared by ion exchange starting with a NaZSM-20 sample. The observedammonium-sodium exchange efficiency was slightly higher for ZSM-20 than for Y zeolite. The HNaZSM-20 series presentedhigher cracking activities in then-heptane transformation than the corresponding HNaY series. A decrease in theselectivity for coke deposition with sodium removal was also observed. 相似文献
123.
124.
Alpha-furil [C(4)H(3)O-C(=O)-C(=O)-C(4)H(3)O] has been isolated in argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The obtained spectra were fully assigned and revealed the presence in the matrices of three different conformers, all of them exhibiting skewed conformations around the intercarbonyl bond with the two C(4)H(3)O-C(=O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relative to the carbonyl groups: the most stable conformer, I (C(2) symmetry; O=C-C=O intercarbonyl dihedral equal to 153.1 degrees), has both furan rings orientated in such a way that one of their beta-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H-C=C-C-C=O six-membered rings; the second most stable conformer, II (C(1) symmetry; O=C-C=O intercarbonyl dihedral equal to 126.9 degrees ), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180 degrees (resulting in an energetically less favourable H-C=C-C=O five-membered ring); in the third conformer, III (C(2) symmetry; O=C-C=O dihedral equal to 106.2 degrees ), both furan rings assume the latter orientation relative to the dicarbonyl group. The theoretical calculations predicted the two higher energy forms being 5.85 and 6.22 kJ mol(-1) higher in energy than the most stable form, respectively, and energy barriers for conformational interconversion higher than 40 kJ mol(-1). These barriers are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of alpha-furil were also found to be present in CCl(4) solution, as well as in a low temperature neat amorphous phase of the compound prepared from fast condensation of its vapour onto a suitable 10 K cooled substrate. On the other hand, in agreement with the available X-ray data [S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 541], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, alpha-furil exists uniquely in its most stable conformational state, I. 相似文献
125.
Guaratini T Lopes NP Pinto E Colepicolo P Gates PJ 《Chemical communications (Cambridge, England)》2006,(39):4110-4112
We report here a general mechanism for the elimination of aromatic molecules from polyene containing natural products of several compound classes in tandem mass spectrometry. 相似文献
126.
127.
It is shown that the space of infinitesimal deformations of 2k-Einstein structures is finite dimensional on compact non-flat space forms. Moreover, spherical space forms are shown to be rigid in the sense that they are isolated in the corresponding moduli space. 相似文献
128.
Renato P. Freitas Cristiane Calza Tania A. Lima Angela Rabello Ricardo Tadeu Lopes 《X射线光谱测定》2010,39(5):307-310
In this work, 102 fragments of Marajoara ceramics, belonging to the National Museum collection (Rio de Janeiro, Brazil), were analyzed using energy dispersive X‐ray fluorescence (EDXRF) and principal component analysis (PCA) in order to identify possible groups of samples that present similar behaviors or different characteristics. This information will give an important aid to a more accurate classification of these artifacts. The EDXRF measurements were carried out with a portable system developed in the Nuclear Instrumentation Laboratory consisting of an Oxford TF3005 X‐ray tube, with W anode, and an Si‐PIN XR‐100CR detector from Amptek, working at 25 kV and 100 µA, acquisition time of 600 s and a beam collimation of 2 mm. PCA applied to the X‐ray fluorescence results revealed a clear cluster separation to the samples. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
129.
Sequential Bayesian learning for stochastic volatility with variance‐gamma jumps in returns 下载免费PDF全文
In this work, we investigate sequential Bayesian estimation for inference of stochastic volatility with variance‐gamma (SVVG) jumps in returns. We develop an estimation algorithm that combines the sequential learning auxiliary particle filter with the particle learning filter. Simulation evidence and empirical estimation results indicate that this approach is able to filter latent variances, identify latent jumps in returns, and provide sequential learning about the static parameters of SVVG. We demonstrate comparative performance of the sequential algorithm and off‐line Markov Chain Monte Carlo in synthetic and real data applications. 相似文献
130.
Lopes SC Soares CM Baptista AM Goormaghtigh E Cabral BJ Castanho MA 《The journal of physical chemistry. B》2006,110(7):3385-3394
The analgesic dipeptide kyotorphin (L-Tyr-L-Arg) and an acylated kyotorphin derivative were studied by a combination of theoretical (molecular dynamics simulation and quantum mechanics methods) and experimental (fluorescence and infrared spectroscopies) approaches both in solution and in model systems of membranes. At biological pH the peptides have a neutral net charge. Nevertheless, their phenolic rings interact with phospholipid molecules (partition coefficient varies from 6 x 10(2) to 2 x 10(4), depending on the lipid and pH used) despite being exposed to the aqueous bulk medium. The lowest energy transition dipole moment is displaced from the normal to the lipid bilayer by 20 degrees on average. The observed extensive interaction, pK(a), precise location, and well-defined orientation in membranes combined with the ability to discriminate rigid raftlike membrane domains suggest that kyotorphin meets the structural constraints needed for receptor-ligand interaction. The acylated kyotorphin derivative mimics kyotorphin properties and represents a promising way for entrapment in a drug carrier and transport across the blood-brain barrier. 相似文献