全文获取类型
收费全文 | 1128篇 |
免费 | 47篇 |
国内免费 | 10篇 |
专业分类
化学 | 658篇 |
晶体学 | 14篇 |
力学 | 32篇 |
数学 | 230篇 |
物理学 | 251篇 |
出版年
2023年 | 6篇 |
2022年 | 11篇 |
2021年 | 22篇 |
2020年 | 19篇 |
2019年 | 20篇 |
2018年 | 23篇 |
2017年 | 27篇 |
2016年 | 61篇 |
2015年 | 50篇 |
2014年 | 55篇 |
2013年 | 79篇 |
2012年 | 74篇 |
2011年 | 100篇 |
2010年 | 77篇 |
2009年 | 60篇 |
2008年 | 68篇 |
2007年 | 58篇 |
2006年 | 50篇 |
2005年 | 48篇 |
2004年 | 52篇 |
2003年 | 25篇 |
2002年 | 32篇 |
2001年 | 15篇 |
2000年 | 18篇 |
1999年 | 13篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1996年 | 12篇 |
1995年 | 6篇 |
1994年 | 11篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1978年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1941年 | 2篇 |
排序方式: 共有1185条查询结果,搜索用时 0 毫秒
101.
Diego Oliveira Cordeiro Amanda Duarte Gondim Antonio Souza Araújo Marta Maria da Conceição Antonio Gouveia de Souza Valter José FernandesJr 《Journal of Thermal Analysis and Calorimetry》2017,127(2):1253-1260
The aim of this work was to evaluate the influence of the purification process on the stability of Jatropha curcas biodiesel. The biodiesel was obtained using a variety of purification processes: three wet methods with different drying processes (vacuum oven, conventional oven and anhydrous sodium sulfate) and one dry method (purification with adsorbent magnesium silicate). Biodiesel was characterized through the analysis of carbon residue, acidity index, infrared and gas chromatography. The composition J. curcas oil indicated 56.3 % of unsaturated fatty acids and 43.7 % of saturated fatty acids. Jatropha oil presented high quantity of saturated acids, which are less susceptible at oxidation. The biodiesel sample that was chemically purified (PUsq) presented better purity, indicating be the process more efficient in remove the residues of synthesis. Thermogravimetric curves of purified biodiesel by wet method, PUsq, with chemical drying using anhydrous sodium sulfate, and PUsv, with vacuum drying, showed the highest initial decomposition temperatures, indicating higher thermal stability. The carbon residue and infrared analyses suggested that contamination by catalyst residue is a determining factor in reduction of the oxidative stability of biodiesel. The oxidative stability was evaluated using Rancimat and pressure differential scanning calorimetry. Biodiesel samples showing better oxidative stability were purified using PUsq and PUsv, which obtained stability of 6 h using the Rancimat technique, the minimum limit set by Brazilian legislation, without the addition of antioxidant, suggesting that these methods least influenced the stability of biodiesel. 相似文献
102.
103.
Reza Ghiasi Taherh Jahan Ara Amir Hossein Hakimyoun 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(4):616-624
The electronic structure and properties of the platinanaphthalenes, and ring-fused B-N platinanaphthalenes isomers have been explored using the hybrid density functional mpw1pw91 theory. The energetic aspect shows that stability of Ptb and Pta isomers are isoenergetic in platinanaphthalenes. On the other hand, BNa1 isomer is the most stable isomer of ring-fused B-N platinanaphthalenes. This is compatible with principles of minimum energy and minimum polarizability in ring-fused B-N platinanaphthalenes. Molecular orbital analysis shows increasing of hardness in ring-fused B-N platinanaphthalenes isomers. Also, electronic spectra analysis indicates that, in all the molecules HOMO-1 → LUMO transition makes the major contribution in most intense electronic transition. 相似文献
104.
Vanfleteren D Ghillemijn D Van Neck D Bultinck P Waroquier M Ayers PW 《Journal of computational chemistry》2011,32(16):3485-3496
For the Hirshfeld-I atom in the molecule (AIM) model, associated single-atom energies and interaction energies at the Hartree-Fock level are efficiently determined in one-electron Hilbert space. In contrast to most other approaches, the energy terms are fully consistent with the partitioning of the underlying one-electron density matrix (1DM). Starting from the Hirshfeld-I AIM model for the electron density, the molecular 1DM is partitioned with a previously introduced double-atom scheme (Vanfleteren et al., J Chem Phys 2010, 132, 164111). Single-atom density matrices are constructed from the atomic and bond contributions of the double-atom scheme. As the Hartree-Fock energy can be expressed solely in terms of the 1DM, the partitioning of the latter over the AIM naturally leads to a corresponding partitioning of the Hartree-Fock energy. When the size of the molecule or the molecular basis set does not grow too large, the method shows considerable computational advantages compared with other approaches that require cumbersome numerical integration of the molecular energy integrals weighted by atomic weight functions. 相似文献
105.
Vinícius D. Araújo Waldir Avansi Elaine C. Paris Lauro J. Q. Maia Maria I. B. Bernardi 《Journal of Sol-Gel Science and Technology》2011,59(3):488-494
Pb2CrO5 nanoparticles were embedded in an amorphous SiO2 matrix by the sol–gel process. The pH and heat treatment effects were evaluated in terms of structural, microstructural and
optical properties from Pb2CrO5/SiO2 compounds. X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), energy dispersive spectroscopy
(EDS), and diffuse reflectance techniques were employed. Kubelka–Munk theory was used to calculate diffuse reflectance spectra
that were compared to the experimental results. Finally, colorimetric coordinates of the Pb2CrO5/SiO2 compounds were shown and discussed. In general, an acid pH initially dissolves Pb2CrO5 nanoparticles and following heat treatment at 600 °C crystallized into PbCrO4 composition with grain size around 6 nm in SiO2 matrix. No Pb2CrO5 solubilization was observed for basic pH. These nanoparticles were incorporated in silica matrix showing a variety of color
ranging from yellow to orange. 相似文献
106.
Soares SF Galvão RK Araújo MC da Silva EC Pereira CF de Andrade SI Leite FC 《Analytica chimica acta》2011,(1):30-28
This work proposes a modification to the successive projections algorithm (SPA) aimed at selecting spectral variables for multiple linear regression (MLR) in the presence of unknown interferents not included in the calibration data set. The modified algorithm favours the selection of variables in which the effect of the interferent is less pronounced. The proposed procedure can be regarded as an adaptive modelling technique, because the spectral features of the samples to be analyzed are considered in the variable selection process. The advantages of this new approach are demonstrated in two analytical problems, namely (1) ultraviolet–visible spectrometric determination of tartrazine, allure red and sunset yellow in aqueous solutions under the interference of erythrosine, and (2) near-infrared spectrometric determination of ethanol in gasoline under the interference of toluene. In these case studies, the performance of conventional MLR-SPA models is substantially degraded by the presence of the interferent. This problem is circumvented by applying the proposed Adaptive MLR-SPA approach, which results in prediction errors smaller than those obtained by three other multivariate calibration techniques, namely stepwise regression, full-spectrum partial-least-squares (PLS) and PLS with variables selected by a genetic algorithm. An inspection of the variable selection results reveals that the Adaptive approach successfully avoids spectral regions in which the interference is more intense. 相似文献
107.
The radical anion resulting from electron capture by diacetonyl molecule has been characterized by EPR and optical absorption spectroscopy in glassy ether matrices at 77 K. In non-polar alkane glasses this species was not observed under the same conditions, which confirms the crucial role of matrix interactions in stabilizing this species. Calculations at the MP2 level show the vertical detachment energy to increase gradually from roughly zero for a bare anion to ~1 eV for the complex involving six ether molecules. 相似文献
108.
Paulina V. F. Santos Ilanna C. Lopes Victor C. Diculescu Mário César U. de Araújo Ana Maria Oliveira‐Brett 《Electroanalysis》2011,23(10):2310-2319
The electrochemical behaviour of Nodularin (NOD), a hepatotoxic cyclic pentapeptide, was studied at a glassy carbon electrode. NOD electrochemical oxidation is an irreversible, pH‐independent process, involving the transfer of one electron. Upon incubation in different pH electrolytes, chemical degradation of NOD was electrochemically detected by the appearance of a new oxidation peak. The chemically degraded NOD (cdNOD), undergoes an irreversible, pH‐dependent oxidation, and its redox products are reversibly oxidised. The charge transfer properties of cdNOD as well as of its redox metabolites were investigated. Mechanisms for NOD oxidation, NOD chemical degradation and oxidation of cdNOD and its metabolites were proposed. 相似文献
109.
Dimitri Chikhladze 《Journal of Pure and Applied Algebra》2011,215(9):2148-2153
We generalize Barr’s embedding theorem for regular categories to the context of enriched categories. 相似文献
110.
Ono RJ Suzuki Y Khramov DM Ueda M Sessler JL Bielawski CW 《The Journal of organic chemistry》2011,76(9):3239-3245
Coupling of various 4-substituted phenyl azides with two distinct quinone-containing N-heterocyclic carbenes (NHCs) afforded the respective mono- and ditopic 1,3-disubstituted acyclic triazenes in moderate to excellent yields (38-92%). Depending on their pendant substituents (derived from the azides), the acyclic triazenes exhibited intense absorptions in the visible spectrum (359-428 nm), which were bathochromically shifted by up to Δλ=68 nm upon reduction of the quinone moiety on the component derived from the NHC. Cyclic voltammetry confirmed that the aforementioned redox processes were reversible, and a related set of UV-vis spectroelectrochemical experiments revealed that bulk electrolysis may also be used to switch reversibly the colors exhibited by these triazenes. 相似文献