全文获取类型
收费全文 | 395605篇 |
免费 | 5897篇 |
国内免费 | 1361篇 |
专业分类
化学 | 202493篇 |
晶体学 | 4797篇 |
力学 | 18279篇 |
综合类 | 5篇 |
数学 | 71839篇 |
物理学 | 105450篇 |
出版年
2021年 | 2582篇 |
2020年 | 2953篇 |
2019年 | 3220篇 |
2018年 | 13882篇 |
2017年 | 13320篇 |
2016年 | 12095篇 |
2015年 | 5689篇 |
2014年 | 6546篇 |
2013年 | 16532篇 |
2012年 | 16685篇 |
2011年 | 24569篇 |
2010年 | 14893篇 |
2009年 | 14710篇 |
2008年 | 19408篇 |
2007年 | 21312篇 |
2006年 | 12477篇 |
2005年 | 12806篇 |
2004年 | 11611篇 |
2003年 | 10670篇 |
2002年 | 9415篇 |
2001年 | 8827篇 |
2000年 | 7026篇 |
1999年 | 5369篇 |
1998年 | 4487篇 |
1997年 | 4413篇 |
1996年 | 4419篇 |
1995年 | 3863篇 |
1994年 | 3778篇 |
1993年 | 3811篇 |
1992年 | 4117篇 |
1991年 | 4008篇 |
1990年 | 3882篇 |
1989年 | 3764篇 |
1988年 | 3778篇 |
1987年 | 3600篇 |
1986年 | 3459篇 |
1985年 | 4754篇 |
1984年 | 4875篇 |
1983年 | 3940篇 |
1982年 | 4402篇 |
1981年 | 4273篇 |
1980年 | 4058篇 |
1979年 | 4075篇 |
1978年 | 4299篇 |
1977年 | 4055篇 |
1976年 | 4326篇 |
1975年 | 3852篇 |
1974年 | 3986篇 |
1973年 | 4286篇 |
1972年 | 2580篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Thomas Wittmann Dr. Renée Siegel Nele Reimer Dr. Wolfgang Milius Prof. Dr. Norbert Stock Prof. Dr. Jürgen Senker 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(1):314-323
The resistance of metal–organic frameworks towards water is a very critical issue concerning their practical use. Recently, it was shown for microporous MOFs that the water stability could be increased by introducing hydrophobic pendant groups. Here, we demonstrate a remarkable stabilisation of the mesoporous MOF Al‐MIL‐101‐NH2 by postsynthetic modification with phenyl isocyanate. In this process 86 % of the amino groups were converted into phenylurea units. As a consequence, the long‐term stability of Al‐MIL‐101‐URPh in liquid water could be extended beyond a week. In water saturated atmospheres Al‐MIL‐101‐URPh decomposed at least 12‐times slower than the unfunctionalised analogue. To study the underlying processes both materials were characterised by Ar, N2 and H2O sorption measurements, powder X‐ray diffraction, thermogravimetric and chemical analysis as well as solid‐state NMR and IR spectroscopy. Postsynthetic modification decreased the BET equivalent surface area from 3363 to 1555 m2 g?1 for Al‐MIL‐101‐URPh and reduced the mean diameters of the mesopores by 0.6 nm without degrading the structure significantly and reducing thermal stability. In spite of similar water uptake capacities, the relative humidity‐dependent uptake of Al‐MIL‐101‐URPh is slowed and occurs at higher relative humidity values. In combination with 1H‐27Al D ‐HMQC NMR spectroscopy experiments this favours a shielding mechanism of the Al clusters by the pendant phenyl groups and rules out pore blocking. 相似文献
72.
Thermodynamic study of transthyretin association (wild‐type and senile forms) with heparan sulfate proteoglycan: pH effect and implication of the reactive histidine residue 下载免费PDF全文
Ambre Geneste Claire André Nadine Magy‐Bertrand Lydie Lethier Gharbi Tijani Yves Claude Guillaume 《Biomedical chromatography : BMC》2015,29(4):514-522
The tetramer destabilization of transthyretin into monomers and its fibrillation are phenomena leading to amyloid deposition. Heparan sulfate proteoglycan (HSPG) has been found in all amyloid deposits. A chromatographic approach was developed to compare binding parameters between wild‐type transthyretin (wtTTR) and an amyloidogenic transthyretin (sTTR). Results showed a greater affinity of sTTR for HSPG at pH 7.4 compared with wtTTR owing to the monomeric form of sTTR. Analysis of the thermodynamic parameters showed that van der Waals interactions were involved at the complex interface for both transthyretin forms. For sTTR, results from the plot representing the number of protons exchanged vs pH showed that the binding mechanism was pH‐dependent with a critical value at a pH 6.5. This observation was due to the protonation of a histidine residue as an imidazolium cation, which was not accessible when TTR was in its tetrameric structure. At pH >6.5, dehydration at the binding interface and several contacts between nonpolar groups of sTTR and HSPG were also coupled to binding for an optimal hydrogen‐bond network. At pH <6.5, the protonation of the His residue from sTTR monomer when pH decreased broke the hydrogen‐bond network, leading to its destabilization and thus producing slight conformational changes in the sTTR monomer structure. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
73.
Highly Selective Copper‐Catalyzed Asymmetric [3+2] Cycloaddition of Azomethine Ylides with Acyclic 1,3‐Dienes 下载免费PDF全文
María González‐Esguevillas Ana Pascual‐Escudero Dr. Javier Adrio Prof. Dr. Juan C. Carretero 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4561-4565
The first examples of the catalytic asymmetric 1,3‐dipolar cycloaddition of azomethine ylides with acyclic activated 1,3‐dienes (and 1,3‐enynes) are described. Under copper catalysis, a selective cycloaddition at the terminal γ,δ‐C?C bond is observed. In addition, depending on the ligand used, either the exo or the endo adduct can be obtained with high selectivity. Under appropriate reaction conditions, the acyclic 1,6‐addition product is detected, suggesting a stepwise mechanism. The resulting C4‐alkenyl‐substituted pyrrolidines are suitable substrates for further access to polycyclic systems, as highlighted by the preparation of hexahydrochromeno[4,3‐b]pyrrole and the tetracyclic core of the alkaloid gracilamine. 相似文献
74.
75.
76.
77.
78.
The structure, chemical composition, and magnetic properties of electrochemically deposited nanocrystalline Co-Ni-Fe films
were investigated using a number of techniques. A high saturation magnetic induction up to B
s
= 21 kG was attained. An enhancement of the saturation magnetization compared to the ideal anticipated one was revealed,
which correlated with the nonlinear behavior of the structural phase composition and lattice parameters with the change of
the composition.
The text was submitted by the authors in English. 相似文献
79.
R. Notoya 《Russian Journal of Electrochemistry》2002,38(3):255-260
It is found that the density (number of the component ion or molecule in 1 cm3) of a molten salt could be expressed as a function of the product of the electron donating and accepting properties of the component anion and cation. This equation is very useful for estimating the density of any molten salt at high temperature with the standard deviation accurately. 相似文献
80.
Investigated is the dependency of the crack tip opening displacement (CTOD) on the local microstructure of the heat affected zone in high-strength low-alloy (HSLA) microalloyed steel. Since the initiation of the crack tip location could not be controlled in fatigue, any possible correlation between heat iput in welding and fracture toughness could be smeared. Modified CTOD data are defined; they show that the fracture resistance of the weld joint decreased as the heat input increased. 相似文献