全文获取类型
收费全文 | 1417篇 |
免费 | 35篇 |
国内免费 | 2篇 |
专业分类
化学 | 882篇 |
晶体学 | 4篇 |
力学 | 57篇 |
数学 | 162篇 |
物理学 | 349篇 |
出版年
2022年 | 5篇 |
2021年 | 12篇 |
2020年 | 9篇 |
2019年 | 19篇 |
2018年 | 12篇 |
2017年 | 15篇 |
2016年 | 40篇 |
2015年 | 26篇 |
2014年 | 38篇 |
2013年 | 56篇 |
2012年 | 73篇 |
2011年 | 91篇 |
2010年 | 52篇 |
2009年 | 44篇 |
2008年 | 77篇 |
2007年 | 93篇 |
2006年 | 81篇 |
2005年 | 88篇 |
2004年 | 81篇 |
2003年 | 54篇 |
2002年 | 53篇 |
2001年 | 28篇 |
2000年 | 18篇 |
1999年 | 21篇 |
1998年 | 20篇 |
1997年 | 25篇 |
1996年 | 24篇 |
1995年 | 13篇 |
1994年 | 19篇 |
1993年 | 28篇 |
1992年 | 29篇 |
1991年 | 14篇 |
1990年 | 6篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 6篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1973年 | 6篇 |
排序方式: 共有1454条查询结果,搜索用时 31 毫秒
31.
Henry Holmstrand Maria Unger Daniel Carrizo Per Andersson Örjan Gustafsson 《Rapid communications in mass spectrometry : RCM》2010,24(14):2135-2142
The bromine isotope composition is potentially diagnostic in both degradation monitoring and source apportionment of organobromines in the environment. A method for compound‐specific bromine isotope analysis (δ81Br) based on gas chromatography multiple collector inductively coupled plasma mass spectrometry (GC/ICPMS) was developed for common brominated diaromatic compounds. Brominated diphenyl ethers (BDEs) in Bromkal 70‐5DE, a technical flame‐retardant mixture containing mainly BDEs #47, #99 and #100, were used as test substances, with standard bracketing for the samples achieved through co‐injected monobromobenzene (MBB) with a known δ81Br of ?0.39‰ vs. Standard Mean Ocean Bromine (SMOBr). Three different heated transfer lines were constructed and tested to achieve efficient conduction of the BDEs from the gas chromatograph to the ICPMS instrument. The MBB was analyzed with a precision of 0.4‰ (1 s, n = 18). The precision for BDEs was 1.4–1.8‰ (1 s, n = 10–12 depending on the congener). The lower precision for the BDEs than for MBB may reflect the heat required to prevent condensation of the analytes in ICP torch assembly. The use of an internal standard of similar chemical structure to the analytes alleviates this problem, as illustrated by a difference of 0.3 ± 0.7‰ (1 s, n = 6) between the δ81Br values of co‐injected methoxy BDE‐47 and BDE‐47 extracted from whale blubber. Improvements in precision and accuracy may be achieved by the use of a more efficient heating of the torch assembly in conjunction with a set of internal standards that match the target compounds. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
32.
Rahul B. Kawthekar Sai Kumar Chakka Vivian Francis Pher G. Andersson Hendrik G. Kruger Glenn E.M. Maguire Thavendran Govender 《Tetrahedron: Asymmetry》2010,21(7):846-852
A novel family of eleven new tetrahydroisoquinoline (TIQ)–oxazoline intermediates and five corresponding copper(II) catalysts has been developed and applied to the catalytic asymmetric Henry reaction of various aldehydes with nitromethane to provide β-hydroxy nitroalkanes in high conversion (>99%). This paper describes the synthesis of the TIQ compounds from l-dihydroxyphenylalanine (l-DOPA) as the starting material. The chiral ligands were complexed in situ with various transition metals such as Cu, Sc, Co, Zn, Ni and Mn and tested as a chiral catalyst for the Henry reaction. The reaction was optimized in terms of the metal, counter ion, solvent, temperature and over a range of substrates. The corresponding catalyst with copper(II) acetate and 2-propanol as the solvent provides the best enantioselectivities (up to 77% ee) of the corresponding nitroalcohol for 4-chlorobenzaldehyde. 相似文献
33.
Cristian Bodin Fredrik Mauritzson Rob Horsefield Anna Aagaard Hongwei Guo Linda Öster Lisa Wissler Margareta Ek 《Journal of synchrotron radiation》2012,19(2):288-289
Currently there is no rack system for the long‐term storage of SPINE pucks in spite of their commercial availability and heavy usage at the ESRF. The only way to store pucks is in transport dewar canisters which presents a number of limitations and drawbacks. Here a simple affordable rack for storing SPINE pucks is described, which we believe is accessible to not only synchrotrons but also both academic and industrial research laboratories. 相似文献
34.
Emil Byström Anna Nordborg Fredrik Limé Ngoc Phuoc Dinh Knut Irgum 《Journal of separation science》2010,33(11):1563-1570
A combined surface activation and “grafting to” strategy was developed to convert divinylbenzene particles into weak cation exchangers suitable for protein separation. The initial activation step was based on plasma modification with bromoform, which rendered the particles amenable to further reaction with nucleophiles by introducing Br to a surface content of 11.2 atom‐%, as determined by X‐ray photoelectron spectroscopy. Grafting of thiol‐terminated glydicyl methacrylate telomers to freshly plasma activated surfaces was accomplished without the use of added initiator, and the grafting was verified both by reduction in bromine content and the appearance of sulfur‐carbon linkages, showing that the surface grafts were covalently bonded. Following grafting the attached glydicyl methacrylate telomer tentacles were further modified by a two‐step procedure involving hydrolysis to 2,3‐hydroxypropyl groups and conversion of hydroxyl groups to carboxylate functionality by succinic anhydride. The final material was capable of baseline separating four model proteins in 3 min by gradient cation exchange chromatography in a fully aqueous eluent. 相似文献
35.
Mustafa Barri George K. El Khoury Helge I. Andersson Bjrnar Pettersen 《国际流体数值方法杂志》2010,64(7):777-792
Direct numerical simulation (DNS) has been performed to study the channel flow over a backward‐facing step at a Reynolds number Reb=5600 based on the step height h and the inflow bulk velocity Ub. A dynamic method has been used in order to generate realistic turbulent inflow conditions. The results upstream of the step compared well with the fully developed channel flow. Downstream of the step our results show excellent agreement with experimental data. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
36.
37.
38.
Byron K. Peters Sai Kumar Chakka Tricia Naicker Glenn E.M. Maguire Hendrik G. Kruger Pher G. Andersson Thavendran Govender 《Tetrahedron: Asymmetry》2010,21(6):679-687
The use of the tetrahydroisoquinoline scaffold is well documented in biologically active compounds. However, reports of the utilisation of tetrahydroisoquinoline compounds in asymmetric catalysis are limited. The synthesis of novel diamine ligands possessing the tetrahydroisoquinoline (tetrahydroisoquinoline) backbone and evaluation of their activity in the asymmetric transfer hydrogenation of acetophenone are presented. The diamine ligands in conjunction with i-PrOH as the hydrogen source and [RhCl2(Cp1)]2 as the metal precursor proved to be the most effective of the tetrahydroisoquinoline derivatives for this catalytic system. Water was found to have a profound influence on the enantioselectivity of the reaction. Optimisation of the amount water, i-PrOH and catalytic loading content rendered the best result of 70% enantioselectivity for the (S)-1-phenylethanol isomer product. 相似文献
39.
Allan Carlsson Karl Håkansson Mathias Kvick Fredrik Lundell L. Daniel Söderberg 《Experiments in fluids》2011,51(4):987-996
Steerable filters are concluded to be useful in order to determine the orientation of fibers captured in digital images. The
fiber orientation is a key variable in the study of flowing fiber suspensions. Here, digital image analysis based on a filter
within the class of steerable filters is evaluated for suitability of finding the position and orientation of fibers suspended
in flowing suspensions. In sharp images with small noise levels, the steerable filter succeeds in determining the orientation
of artificially generated fibers with well-defined angles. The influence of reduced image quality on the orientation has been
quantified. The effect of unsharpness and noise is studied and the results show that the error in orientation is less than
1° for moderate levels. Images from two flow cases, one laminar shear flow and one turbulent, are also analyzed. The fiber
orientation distribution is determined in the flow-vorticity plane. For the laminar case a comparison is made to a robust,
but computationally more expensive, method involving convolutions with an oriented elliptic filter. A good agreement is found
when comparing the resulting fiber orientation distributions obtained with the two methods. For the turbulent case, it is
demonstrated that correct results are obtained and that the method can handle overlapping fibers. 相似文献
40.