全文获取类型
收费全文 | 5611篇 |
免费 | 461篇 |
国内免费 | 544篇 |
专业分类
化学 | 5773篇 |
晶体学 | 36篇 |
力学 | 67篇 |
综合类 | 6篇 |
数学 | 3篇 |
物理学 | 731篇 |
出版年
2024年 | 4篇 |
2023年 | 102篇 |
2022年 | 104篇 |
2021年 | 151篇 |
2020年 | 131篇 |
2019年 | 152篇 |
2018年 | 134篇 |
2017年 | 169篇 |
2016年 | 317篇 |
2015年 | 315篇 |
2014年 | 310篇 |
2013年 | 451篇 |
2012年 | 402篇 |
2011年 | 443篇 |
2010年 | 378篇 |
2009年 | 440篇 |
2008年 | 380篇 |
2007年 | 343篇 |
2006年 | 291篇 |
2005年 | 235篇 |
2004年 | 205篇 |
2003年 | 100篇 |
2002年 | 344篇 |
2001年 | 102篇 |
2000年 | 53篇 |
1999年 | 74篇 |
1998年 | 80篇 |
1997年 | 48篇 |
1996年 | 62篇 |
1995年 | 55篇 |
1994年 | 38篇 |
1993年 | 24篇 |
1992年 | 32篇 |
1991年 | 25篇 |
1990年 | 12篇 |
1989年 | 18篇 |
1988年 | 15篇 |
1987年 | 12篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 3篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有6616条查询结果,搜索用时 484 毫秒
11.
Xanthenediones derivatives have attracted considerable interests in recent times because they constitute a structural unit in a number of natural products1 and have been used as versatile synthons due to the inherent reactivity of the inbuilt pyran ring2. The conventional syntheses of xanthenediones were acid or base catalyzed condensation of appropriate active methylene carbonyl compounds with aldehydes3. However, many of these procedures involved longer reaction times,low yields and side reactions of aldehydes. In recent years, room temperature ionic liquids (RTILs) have been used as novel green reaction media4. Considering that InCl3 is an efficient Lewis acid catalyst used in promoting many organic reactions, especially in several condensation processes, we herein wish to report a very simple and green method for the preparation of poly-hydrogenated xanthenediones through InCl3·4H2O promoted cascade reaction of aldehydes and 5,5-dimethyl-l,3-cyclohexanedione in ionic liquid,1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). The preparative process presented here is operationally simple, environmentally benign and has the advantage of enhanced atom utilization. Furthermore, the solvent and the catalyst used can be recovered easily and reused efficiently. 相似文献
12.
在水热晶化合成一系列NASICON型化合物的基础上,应用固体高分辨31P和29Si MAS NMR,研究了几种NASICON化合物的结构,观察其骨架原子P和Si在结构中的状态及分布,并解释了由于在结构中发生取代作用而引起,Si或P的化学位移的变化. 相似文献
13.
14.
R. Lü H. Pan J.-L. Zhu B.-L. Gu 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(4):479-491
The tunneling of a giant spin at excited levels is studied theoretically in mesoscopic magnets with a magnetic field at an
arbitrary angle in the easy plane. Different structures of the tunneling barriers can be generated by the magnetocrystalline
anisotropy, the magnitude and the orientation of the field. By calculating the nonvacuum instanton solution explicitly, we
obtain the tunnel splittings and the tunneling rates for different angle ranges of the external magnetic field ( θ
H = π/2 and π/2 < θ
H < π). The temperature dependences of the decay rates are clearly shown for each case. It is found that the tunneling rate
and the crossover temperature depend on the orientation of the external magnetic field. This feature can be tested with the
use of existing experimental techniques.
Received 12 March 2001 and Received in final form 18 October 2001 相似文献
15.
16.
We generalize a variational principle for the mean spherical approximation for a system of charged hard spheres in 3D to arbitrary
dimensions. We first construct a free energy variational trial function from the Debye-Hückel excess charging internal energy
at a finite concentration and an entropy obtained at the zero-concentration limit by thermodynamic integration. In three dimensions
the minimization of this expression with respect to the screening parameter leads to the mean spherical approximation, usually
obtained by solution of the Ornstein-Zernike equation. This procedure, which interpolates naturally between the zero concentration/coupling
limit and the high-concentration/ coupling limit, is extended to arbitrary dimensions. We conjecture that this result is also
equivalent to the MSA as originally defined, although a technical proof of this point is left for the future. The Onsager
limitT ΔS
MSA
/ΔE
MSA
→ 0 for infinite concentration/coupling is satisfied for all d ≠ 2, while ford=2 this limit is 1.
On leave from Department of Physics, University of Puerto Rico, Mayagüez Campus, Mayagüez, Puerto Rico, 00681. 相似文献
17.
Mária Meiarová Katarína Hubinská tefan Toma Burkhard Koch Albrecht Berkessel 《Monatshefte für Chemie / Chemical Monthly》2007,138(11):1181-1186
Summary.
N-Toluensulfonyl-l-prolin amide was tested as catalyst in the enantioselective Michael addition of carbonyl compounds to (E)-β-nitrostyrene in nine ionic liquids under different reaction conditions. The reaction rates and enantioselectivities were
strongly dependent on the ionic liquids. Change of enantioselectivity was observed too and it is attributed to both the cation
and the anion of ionic liquid. The best yields (up to 98%) and enantioselectivity (70% ee) of product were obtained in a basic ionic liquid [bmim]BF4 at room temperature. 相似文献
18.
本文提出一种利用材料Hugoniot数据研究高压下离子晶体中微观离子状态及其离子间排斥作用势的新方法。对氢化锂晶体进行研究时,我们得到Li~+和H~-离子之间排斥作用势函数,结果表明文献〔1〕中提出的离子压缩效应具有客观性。 相似文献
19.
We investigated the features of the glass transition relaxation of two room temperature ionic liquids using DSC. An important
observation was that the heat capacity jump, that is the signature of the glass transition relaxation, shows a particularly
strong value in this type of new and promising materials, candidates for a range of applications. This suggests a high degree
of molecular mobility in the supercooled liquid state. The study of the influence of the heating rate on the temperature location
of the glass transition signal, allowed the determination of the activation energy at the glass transition temperature, and
the calculation of the fragility index of these two ionic glass-formers. It was concluded that this kind of materials belong
to the class of relatively strong glass-forming systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
Yasuhiro Ishida 《Tetrahedron letters》2004,45(51):9455-9459
A planar-chiral imidazolium salt with a tris(oxoethylene) bridge was synthesized, and its potential application as a room temperature ionic liquid with a molecular-recognition ability was demonstrated. 相似文献