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71.
研究一类凹角区域双曲型外问题的数值方法.先用Newmark方法对时间进行离散化,在每个时间步求解一个椭圆外问题.然后引入人工边界,并获得精确的人工边界条件.给出半离散化问题的变分问题,证明了变分问题的适定性,并给出了误差估计.最后给出数值例子,以示该方法的可行性与有效性. 相似文献
72.
Mechanical Behavior of Polymer Gels for RDCs and RCSAs Collection: NMR Imaging Study of Buckling Phenomena 下载免费PDF全文
Erich Hellemann Dr. Rubens R. Teles Prof. Dr. Fernando Hallwass Prof. Dr. W. Barros Jr. Dr. Armando Navarro‐Vázquez Prof. Dr. Roberto R. Gil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(46):16632-16635
Anisotropic NMR parameters, such as residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs or ΔνQ), appear in solution‐state NMR when the molecules under study are subjected to a degree of order. The tunable alignment by reversible compression/relaxation of gels (PMMA and p‐HEMA) is an easy, user‐friendly, and very affordable method to measure them. When using this method, a fraction of isotropic NMR signals is observed in the NMR spectra, even at a maximum degree of compression. To explain the origin of these isotropic signals we decided to investigate their physical location inside the NMR tube using deuterium 1D imaging and MRI micro‐imaging experiments. It was observed that after a certain degree of compression the gels start to buckle and they generate pockets of isotropic solvent, which are never eliminated. The amount of buckling depends on the amount of cross‐linker and the length of the gel. 相似文献
73.
Daniel Francisco Palacios Juan A. Alfonso Haydn Barros John J. LaBrecque Karla Prez Marian R. Lossada 《Radiation measurements》2008,43(1):77-84
A semi-empirical method to determine radionuclide concentrations in large environmental samples without the use of reference material and avoiding the typical complexity of Monte-Carlo codes is proposed. The calculation of full-energy peak efficiencies was carried out from a relative efficiency curve (obtained from the gamma spectra data), and the geometric (simulated by Monte-Carlo), absorption, sample and intrinsic efficiencies for energies between 130 and 3000 keV. The absorption and sample efficiencies were determined from the mass absorption coefficients, whereas the intrinsic efficiency was approximated by an empirical function. The deviations between calculated and experimental efficiencies for a reference material in most cases are less than 10%. Radionuclide activities in marine sediment samples calculated by the proposed method and by the experimental comparative method were not significantly different. This new method can be used for routine environmental monitoring when uncertainties up to 10% are acceptable. 相似文献
74.
Room temperature Mössbauer spectra of Ga and Al substituted lithium ferrite Li[Fe0.9(AlxGa1?x)0.1]5O8 with x = 0.00, 0.25, 0.50, 0.75 and 1.00 are reported. It is shown that the varying covalence of bonds caused by gallium and aluminium ions can explain the observed values of hyperfine fields and isomer-shifts. 相似文献
75.
We have measured the luminescent properties of single crystals of LiAl5O8:Fe3+. In addition to a zero-phonon line due to Fe3+ in A-sites, we have observed another sharp fluorescent line at 699.2 nm which we assign to Fe3+ occupying B-sites. The excitation spectrum of the B-site Fe3+ shows characteristics similar to those of the A-site Fe3+ but are also shifted towards longer wavelengths. The spectra of the single crystals are compared with those of ordered and disordered powder samples. 相似文献
76.
Duarte MF Martins F Fernandez MT Langley GJ Rodrigues P Barros MT Costa ML 《Rapid communications in mass spectrometry : RCM》2003,17(9):957-962
A series of derivatives of 2-azidoacetic acid and 2-azidoacetone were synthesized and their behaviour under electron ionization conditions was investigated. This paper reports the electron ionization fragmentation mechanisms for five aliphatic alpha-carbonyl azides, which were clarified by accurate mass measurements and B/E linked scans. The substituent influences the abundance and the nature of the ions resulting from the molecular ion fragmentation. 相似文献
77.
[reaction: see text] The directing ability of an aziridine group for the epoxidation of adjacent double bonds is demonstrated. The aziridine group is also used to effectively protect a double bond in a cycloenone system for a short synthesis of the title compound. 相似文献
78.
Barros W de Sousa PL Engelsberg M 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,165(1):175-179
Intermolecular double-quantum coherence (i-DQC) signals in liquids are usually associated with high magnetic fields. We demonstrate that, in a magnetic field of only 16mT, i-DQC imaging of water protons is feasible thanks to the nuclear magnetization enhancement produced by the Overhauser effect. i-DQC images of a phantom containing an aqueous solution of a trityl free radical, with phase encoding in the DQC evolution period or in the acquisition period, are presented. Possible applications of low field i-MQC images are proposed. 相似文献
79.
80.
Fourier transform infrared (FTIR) spectroscopy has being emphasised as a widespread technique in the quick assess of food components. In this work, procyanidins were extracted with methanol and acetone/water from the seeds of white and red grape varieties. A fractionation by graded methanol/chloroform precipitations allowed to obtain 26 samples that were characterised using thiolysis as pre-treatment followed by HPLC-UV and MS detection. The average degree of polymerisation (DPn) of the procyanidins in the samples ranged from 2 to 11 flavan-3-ol residues. FTIR spectroscopy within the wavenumbers region of 1800-700 cm−1 allowed to build a partial least squares (PLS1) regression model with 8 latent variables (LVs) for the estimation of the DPn, giving a RMSECV of 11.7%, with a R2 of 0.91 and a RMSEP of 2.58. The application of orthogonal projection to latent structures (O-PLS1) clarifies the interpretation of the regression model vectors. Moreover, the O-PLS procedure has removed 88% of non-correlated variations with the DPn, allowing to relate the increase of the absorbance peaks at 1203 and 1099 cm−1 with the increase of the DPn due to the higher proportion of substitutions in the aromatic ring of the polymerised procyanidin molecules. 相似文献