全文获取类型
收费全文 | 1121篇 |
免费 | 15篇 |
国内免费 | 30篇 |
专业分类
化学 | 637篇 |
晶体学 | 1篇 |
力学 | 32篇 |
数学 | 370篇 |
物理学 | 126篇 |
出版年
2020年 | 7篇 |
2018年 | 7篇 |
2016年 | 18篇 |
2015年 | 9篇 |
2014年 | 17篇 |
2013年 | 71篇 |
2012年 | 32篇 |
2011年 | 39篇 |
2010年 | 28篇 |
2009年 | 33篇 |
2008年 | 41篇 |
2007年 | 38篇 |
2006年 | 42篇 |
2005年 | 35篇 |
2004年 | 46篇 |
2003年 | 43篇 |
2002年 | 36篇 |
2001年 | 21篇 |
2000年 | 18篇 |
1999年 | 9篇 |
1998年 | 7篇 |
1997年 | 19篇 |
1996年 | 23篇 |
1995年 | 24篇 |
1993年 | 20篇 |
1992年 | 8篇 |
1991年 | 17篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 10篇 |
1985年 | 25篇 |
1984年 | 17篇 |
1983年 | 17篇 |
1982年 | 25篇 |
1981年 | 26篇 |
1980年 | 29篇 |
1979年 | 28篇 |
1978年 | 19篇 |
1977年 | 18篇 |
1976年 | 19篇 |
1975年 | 13篇 |
1974年 | 14篇 |
1973年 | 16篇 |
1972年 | 11篇 |
1971年 | 8篇 |
1970年 | 7篇 |
1967年 | 7篇 |
1966年 | 7篇 |
排序方式: 共有1166条查询结果,搜索用时 15 毫秒
11.
Precision capacitance dilatometry provides a sensitive measure of the thermal strain developed in a sample undergoing a structural distortion with its varying temperature. The A15 structure compounds, V3Si and Nb3Sn, are well known to undergo distortion from their cubic structures at room temperature to tetragonal structures (c/a > 1 for V3Si and c/a < 1 for Nb3Sn) at low temperatures. In the past, highly anomalous thermal expansion behaviour recorded for these materials has been attributed to a strongly anharmonic lattice potential manifesting itself in unusually high, and strongly temperature-dependent, Grüneisen parameters. Further studies on polycrystalline material revealed this anomalous expansion to be highly anisotropic at temperatures for which, according to conventional diffraction data, the materials are cubic. This behaviour was linked to control of sample morphology by a residual stress field resulting from sample preparation. More recent experiments, in which the transformation morphology has been controlled by the application of external stresses to single crystal V3Si and polycrystalline samples of Nb3Sn and Nb3(Sn1-x Sb x ), have confirmed the occurrence of significant anisotropy in the thermal strain in the cubic phase, well above the structural transformation. We link this departure from cubic symmetry with the well-known soft-mode character of these materials and the associated “central peak” scattering which is also observed well above the transformation temperature. We are led to propose that the “central peak” is the precursor to a Bragg reflection for the transformation structure. This coincidence between “central peak” scattering and the reciprocal lattice for the transformed phase in Ti-Ni has been termed a “ghost lattice”. 相似文献
12.
M. Fred Hoover 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(6):1327-1418
This paper is a literature survey of quaternary ammonium, sulfonium, and phosphonium polymers. 相似文献
13.
Andrzej Chmura Sven Rustler Monica Paravidino Fred van Rantwijk Andreas Stolz Roger A. Sheldon 《Tetrahedron: Asymmetry》2013,24(19):1225-1232
Enantiomerically pure (S)-mandelic acid was synthesised from benzaldehyde by sequential hydrocyanation and hydrolysis in a bienzymatic cascade at starting concentrations up to 0.25 M. A cross-linked enzyme aggregate (CLEA) composed of the (S)-selective oxynitrilase from Manihot esculenta and the non-selective nitrilase from Pseudomonas fluorescens EBC 191 was employed as the biocatalyst. The nitrilase produces approx. equal amounts of (S)-mandelic acid and (S)-mandelic amide from (S)-mandelonitrile under standard conditions, but we surprisingly found that high (up to 0.5 M) concentrations of HCN induced a marked drift towards amide production. By including the amidase from Rhodococcus erythopolis in the CLEA we obtained (S)-mandelic acid as the sole product in 90% yield and >99% enantiomeric purity. 相似文献
14.
Fred L. Tobiason Duane D. Swank Gérard Vergoten Pierre Legrand 《Journal of carbohydrate chemistry》2013,32(7):959-974
ABSTRACT The flexible ring structures of α- and β-D-idopyranose have been investigated by conformational analysis using structures generated by MacroModel and GMMX search protocols. The lowest energy structures found during the conformer search for the 4 C 1, 1 C 4, O S 2 and the 3 S 1 structures were then examined by AM1 and Gaussian ab initio methods at the HF/6-311G** and HF/6-31+G* levels. The B 2,5 conformer found for β-D-idopyranose at 14 kJ/mol by GMMX and 29.5 kJ/mol for α-D-idopyranose by MacroModel would not contribute to Boltzmann-averaged 1H NMR coupling constants. The Merck MMFF force field tends to overweight the 1 C 4 structures, making these the lowest energy conformers for both anomers. Boltzmann-averaged coupling constants are heavily weighted by this structure in the MMFF search conformer ensemble. Averaged proton coupling constants determined using MMFF fit very well for α-D-idopyranose compared to the observed values, but fit poorly for the β-anomer. Ab initio results place the 1 C 4 conformer at lowest energy for the α-anomer and place the 4 C 1 conformer at lowest energy for the β-anomer. The GMMX and MM3* force fields find the 4 C 1 conformer to have the lowest energies for both anomers. 相似文献
15.
Ricardo ABREU-BLAYA Juan BORY-REYES Fred BRACKX Hennie DE SCHEPPER Frank SOMMEN 《数学学报(英文版)》2012,(11)
A Hilbert transform for Hlder continuous circulant (2 × 2) matrix functions, on the d-summable (or fractal) boundary Γ of a Jordan domain Ω in R2n , has recently been introduced within the framework of Hermitean Clifford analysis. The main goal of the present paper is to estimate the Hlder norm of this Hermitean Hilbert transform. The expression for the upper bound of this norm is given in terms of the Hlder exponents, the diameter of Γ and a specific d-sum (d d) of the Whitney decomposition of Ω. The result is shown to include the case of a more standard Hilbert transform for domains with left Ahlfors-David regular boundary. 相似文献
16.
Fred U. Nnamdi Dr. Colin Diner Prof. Pier Alexandre Champagne Prof. Michael G. Organ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(11):3855-3860
Two similar tridentate directing groups derived from glycine and 8-aminoquinoline were shown to enable the palladium-catalyzed anti-Markovnikov hydrofunctionalization of 4-pentenylamine with drastically different efficiencies. A computational investigation into the origin of the reactivity difference between these isomeric, carbonyl-transposed auxiliaries suggests that protonation state, thus charge of the substrate-metal complex prior to nucleopalladation is key. These investigations have culminated in a directing group design that can undergo Pd-catalyzed hydrofunctionalization under relatively mild conditions, as low as room temperature. 相似文献
17.
18.
Jeffery M. Tharp Omer Ad Kazuaki Amikura Fred R. Ward Emma M. Garcia Jamie H. D. Cate Alanna Schepartz Dieter Sll 《Angewandte Chemie (International ed. in English)》2020,59(8):3122-3126
By transplanting identity elements into E. coli tRNAfMet, we have engineered an orthogonal initiator tRNA (itRNATy2) that is a substrate for Methanocaldococcus jannaschii TyrRS. We demonstrate that itRNATy2 can initiate translation in vivo with aromatic non‐canonical amino acids (ncAAs) bearing diverse sidechains. Although the initial system suffered from low yields, deleting redundant copies of tRNAfMet from the genome afforded an E. coli strain in which the efficiency of non‐canonical initiation equals elongation. With this improved system we produced a protein containing two distinct ncAAs at the first and second positions, an initial step towards producing completely unnatural polypeptides in vivo. This work provides a valuable tool to synthetic biology and demonstrates remarkable versatility of the E. coli translational machinery for initiation with ncAAs in vivo. 相似文献
19.
Dr. Jeffery M. Tharp Dr. Omer Ad Dr. Kazuaki Amikura Fred R. Ward Emma M. Garcia Prof. Dr. Jamie H. D. Cate Prof. Dr. Alanna Schepartz Prof. Dr. Dieter Söll 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(8):3146-3150
By transplanting identity elements into E. coli tRNAfMet, we have engineered an orthogonal initiator tRNA (itRNATy2) that is a substrate for Methanocaldococcus jannaschii TyrRS. We demonstrate that itRNATy2 can initiate translation in vivo with aromatic non-canonical amino acids (ncAAs) bearing diverse sidechains. Although the initial system suffered from low yields, deleting redundant copies of tRNAfMet from the genome afforded an E. coli strain in which the efficiency of non-canonical initiation equals elongation. With this improved system we produced a protein containing two distinct ncAAs at the first and second positions, an initial step towards producing completely unnatural polypeptides in vivo. This work provides a valuable tool to synthetic biology and demonstrates remarkable versatility of the E. coli translational machinery for initiation with ncAAs in vivo. 相似文献
20.
James B Whitney Saied Mirshahidi So-Yon Lim Lauren Goins Chris C Ibegbu Daniel C Anderson Richard B Raybourne Fred R Frankel Judy Lieberman Ruth M Ruprecht 《Journal of immune based therapies and vaccines》2011,9(1):2