全文获取类型
收费全文 | 273237篇 |
免费 | 2316篇 |
国内免费 | 738篇 |
专业分类
化学 | 127296篇 |
晶体学 | 3991篇 |
力学 | 14546篇 |
综合类 | 7篇 |
数学 | 42174篇 |
物理学 | 88277篇 |
出版年
2021年 | 992篇 |
2020年 | 1112篇 |
2019年 | 1127篇 |
2018年 | 10441篇 |
2017年 | 11407篇 |
2016年 | 6542篇 |
2015年 | 2844篇 |
2014年 | 2610篇 |
2013年 | 7819篇 |
2012年 | 10755篇 |
2011年 | 20847篇 |
2010年 | 12438篇 |
2009年 | 12591篇 |
2008年 | 18260篇 |
2007年 | 21897篇 |
2006年 | 7210篇 |
2005年 | 13887篇 |
2004年 | 9559篇 |
2003年 | 8565篇 |
2002年 | 6396篇 |
2001年 | 6188篇 |
2000年 | 4867篇 |
1999年 | 3515篇 |
1998年 | 2671篇 |
1997年 | 2632篇 |
1996年 | 2743篇 |
1995年 | 2371篇 |
1994年 | 2244篇 |
1993年 | 2102篇 |
1992年 | 2406篇 |
1991年 | 2379篇 |
1990年 | 2090篇 |
1989年 | 2076篇 |
1988年 | 2108篇 |
1987年 | 2040篇 |
1986年 | 1945篇 |
1985年 | 2859篇 |
1984年 | 2850篇 |
1983年 | 2296篇 |
1982年 | 2474篇 |
1981年 | 2355篇 |
1980年 | 2312篇 |
1979年 | 2277篇 |
1978年 | 2355篇 |
1977年 | 2228篇 |
1976年 | 2195篇 |
1975年 | 2168篇 |
1974年 | 2068篇 |
1973年 | 2190篇 |
1972年 | 1232篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
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
52.
53.
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
54.
N. I. Golovina G. N. Nechiporenko G. G. Nemtsev G. P. Dolganova V. P. Roshchupkin D. B. Lempert G. B. Manelis 《Russian Journal of Applied Chemistry》2007,80(1):24-30
Methods for phase stabilization of ammonium nitrate were sought for in order to considerably expand the application area of this oxidizing agent in various-purpose self-combustible formulations, including that in a new generation of gas-generating formulations for automobile air bags. New methods for stabilization of ammonium nitrate were studied and, in particular, a search was made for organic compounds that can stabilize ammonium nitrate even at their low content. The mechanism of phase state stabilization of ammonium nitrate by compounds of this kind was examined. 相似文献
55.
Yu. V. Pokonova 《Russian Journal of Applied Chemistry》2007,80(12):2163-2164
Kinetic, sorption and selectivity properties of commercial adsorbents are improved by adding petroleum residues (tar asphaltene concentrates) 13% of whole mass. Obtained adsorbents can be used for selective extraction of nobel metals from multicomponent polymetallic solutions of heavy metals and fro sewages purification for removing arsenic. 相似文献
56.
G. Panina 《Central European Journal of Mathematics》2003,1(2):157-168
All 3-dimensional convex polytopes are known to be rigid. Still their Minkowski differences (virtual polytopes) can be flexible
with any finite freedom degree. We derive some sufficient rigidity conditions for virtual polytopes and present some examples
of flexible ones. For example, Bricard's first and second flexible octahedra can be supplied by the structure of a virtual
polytope. 相似文献
57.
58.
A. Foussats C. Repetto O. P. Zandron O. S. Zandron 《International Journal of Theoretical Physics》2002,41(6):1053-1082
In order to describe the dynamics of the t–J model, two different families of first-order Lagrangians in terms of the generators of the Hubbard algebra are found. Such families correspond to different dynamical second-class constrained systems. The quantization is carried out by using the path-integral formalism. In this context the introduction of proper ghost fields is needed to render the model renormalizable. In each case the standard Feynman diagrammatics is obtained and the renormalized physical quantities are computed and analyzed. In the first case a nonperturbative large-N expansion is considered with the purpose of studying the generalized Hubbard model describing N-fold-degenerate correlated bands. In this case the 1/N correction to the renormalized boson propagator is computed. In the second case the perturbative Lagrangian formalism is developed and it is shown how propagators and vertices can be renormalized to each order. In particular, the renormalized ferromagnetic magnon propagator coming from our formalism is studied in details. As an example the thermal softening of the magnon frequency is computed. The antiferromagnetic case is also analyzed, and the results are confronted with previous one obtained by means of the spin-polaron theories. 相似文献
59.
D. J. Plazek C. A. Bero S. Neumeister G. Floudas G. Fytas K. L. Ngai 《Colloid and polymer science》1994,272(11):1430-1438
By making creep and recoverable creep measurements of a nearly monodisperse low molecular weight poly(methyl phenyl siloxane) sample, we have found on decreasing temperature towardsT
g that there is continuously a change in the viscoelastic spectrum concomitant with a decrease of the steadystate recoverable compliance. This behavior is exactly the same as previously observed in low molecular weight poly(styrene), proving that this spectacular anomaly in the viscoelasticity of low molecular weight polymers is general and deserves an explanation. Photon correlation spectroscopic measurements performed on the same sample have extended the observation of the viscoelastic response to shorter times and the result corroborates the trend of variation established by the creep data.Dedicated to Prof.Dr. E. W. Fischer on his 65th Birthday. Prof.Dr. Fischer is known for his valuable contribution to fosterine, international collaboration of research in polymer science. This work is an example of his contribution because it would not be possible without him bringing us together. One of us (KLN) would like to take this opportunity to thank Prof. Dr. Fischer for his unwaiving support of the 1st (Crete) and the 2nd (Alicante) International Discussion Meeting on Relaxations in Complex Systems 相似文献
60.
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