全文获取类型
收费全文 | 7531篇 |
免费 | 238篇 |
国内免费 | 24篇 |
专业分类
化学 | 5005篇 |
晶体学 | 112篇 |
力学 | 176篇 |
数学 | 1041篇 |
物理学 | 1459篇 |
出版年
2021年 | 72篇 |
2020年 | 113篇 |
2019年 | 90篇 |
2018年 | 58篇 |
2017年 | 54篇 |
2016年 | 184篇 |
2015年 | 165篇 |
2014年 | 164篇 |
2013年 | 299篇 |
2012年 | 288篇 |
2011年 | 372篇 |
2010年 | 238篇 |
2009年 | 224篇 |
2008年 | 317篇 |
2007年 | 318篇 |
2006年 | 312篇 |
2005年 | 284篇 |
2004年 | 293篇 |
2003年 | 235篇 |
2002年 | 225篇 |
2001年 | 147篇 |
2000年 | 148篇 |
1999年 | 119篇 |
1998年 | 114篇 |
1997年 | 111篇 |
1996年 | 130篇 |
1995年 | 112篇 |
1994年 | 110篇 |
1993年 | 110篇 |
1992年 | 101篇 |
1991年 | 85篇 |
1990年 | 73篇 |
1989年 | 118篇 |
1988年 | 112篇 |
1987年 | 118篇 |
1986年 | 90篇 |
1985年 | 83篇 |
1984年 | 81篇 |
1983年 | 89篇 |
1982年 | 91篇 |
1981年 | 107篇 |
1980年 | 90篇 |
1979年 | 93篇 |
1978年 | 86篇 |
1977年 | 81篇 |
1976年 | 66篇 |
1975年 | 59篇 |
1974年 | 52篇 |
1973年 | 62篇 |
1970年 | 44篇 |
排序方式: 共有7793条查询结果,搜索用时 15 毫秒
861.
The paper describes a new single-beam thermowave technique based on a two-frequency modulation of the laser beam used for both excitation and detection of the photothermal response in a sample. This technique provides frequency transformation to the low-frequency region and, to a certain degree, phase sensitivity without applying the lock-in detection method. This single-beam technique is proved to yield substantial evidence in the field of laser annealing of semiconductors such as the annealing threshold, the defects generated by the ultrafast solidification and the homogeneity of the laser beam applied for irradiation. 相似文献
862.
863.
Given an open bounded connected subset Ω of ℝn, we consider the overdetermined boundary value problem obtained by adding both zero Dirichlet and constant Neumann boundary
data to the elliptic equation −div(A(|∇u|)∇u)=1 in Ω. We prove that, if this problem admits a solution in a suitable weak sense, then Ω is a ball. This is obtained under
fairly general assumptions on Ω and A. In particular, A may be degenerate and no growth condition is required. Our method of proof is quite simple. It relies on a maximum principle
for a suitable P-function, combined with some geometric arguments involving the mean curvature of ∂Ω. 相似文献
864.
Upon crystalline solidification of one component in a homogeneously molten polymer blend, composition profiles develop outside (i.e., in the rest melt) and behind (i.e., within the spherulites) the crystal growth front. The present article is devoted to the detailed verification and the interpretation of these distributions and their temporal development inside growing spherulites. To this end, the energy dispersive X‐ray emission (EDX) of suitable elements has been recorded locally resolved in a scanning electron microscope and evaluated correspondingly. The investigations were performed at the melt homogeneous blend of poly(vinylidene fluoride) (PVDF) as crystallizing and poly(methyl methacrylate) (PMMA) as steadily amorphous component. If the spherulites are not volume filling, the mean PMMA content 〈?PMMA〉 inside the PVDF spherulites is for all blends about 0.2 below the starting composition. ?PMMA increases however slightly from the center of a spherulite to its border. That increase reflects the PMMA concentration in front of the spherulite surface, which increases likewise with time, and is clearly above the initial composition. There is at the spherulite surface, consequently, a remarkable jump in composition from the spherulite internal to its amorphous surroundings. It may amount up to 0.5. With volume filling spherulites, a slight variation of the composition from the center of a spherulite to its border is observed, too. This proves that also at these conditions composition profiles develop in the spherulite's surroundings. They remain however so weak that they do not inhibit crystallization even in its later stages. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 338–346, 2006 相似文献
865.
Hexa‐ and nonanuclear titanium complexes were obtained by self‐assembly of titanium(IV)‐tert‐butoxide and D ‐mandelic acid. Suitable single crystals of these complexes were characterized by X‐ray structure analysis. When used with these complexes, aldol adducts were isolated with a high degree of regioselectivity in direct aldol additions of aromatic and aliphatic aldehydes to functionalized unsymmetrical ketones. High syn‐diastereoselectivities were obtained in aldol additions of enolizable aldehydes with hydroxyacetone and methoxyacetone. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
866.
867.
868.
Juliana Nanclares Jesús Gil Benjamn Rojano Jairo Saez Bernd Schneider Felipe Otlvaro 《Tetrahedron letters》2008,49(24):3844-3847
4-Methoxy-1H-phenalen-1-one (4-methoxyperinaphthenone, 1), a subunit found in some Musa phytoalexins and related natural products from the Haemodoraceae, was synthesized starting from 2-methoxynaphthalene in five steps and an overall yield of 36%. A Heck–Fujiwara coupling between ethyl acrylate and 1-bromonaphthalene afforded the corresponding (E)-naphthylpropanoic acid which, after hydrogenolysis, was subjected to a one-pot Friedel–Crafts acylation–DDQ dehydrogenation procedure to furnish 1. 相似文献
869.
Reactions of [Re(NPh)Cl3(PPh3)2] with N‐[(N′,N′‐dialkylamino)(thiocarbonyl)]benzamidines (H2R2tcb) (R2 = Et2, (CH2)2O(CH2)2) in methanol give mono‐chelates of the composition [Re(NPh)Cl2(PPh3)(HR2tcb)] as the sole products independent of the amount of the added H2R2tcb. Addition of a supporting base such as NEt3 results in hydrolysis of the Re=NPh bonds and partial hydrolysis of the thiocarbamoylbenzamidines. Orange‐brown, cationic oxorhenium(V) compounds of the formula [ReO(HR2tcb)2]Cl were isolated from such reaction mixtures in good yields, and the formation of small amount of the unusual sulfido/persulfido‐bridged ReV dimer [{ReO(HEt2tcb)}2(μ‐S)(μ‐S2)] give evidence for a considerable degree of ligand decomposition under such conditions. The products have been characterized by spectroscopic methods and X‐ray crystallography. Acidification of orange‐brown solutions of the five‐coordinate ReV oxo complex [ReO(HEt2tcb)2]Cl causes an immediate change of the color and deep blue crystals of the neutral, six‐coordinate [ReOCl(HEt2tcb)2] can be isolated from the resulting mixture. Alternatively, the product can be prepared by a ligand‐exchange protocol starting from (NBu4)[ReOCl4] and H2Et2tcb. The pH‐dependent isomerization between [ReO(HEt2tcb)2]Cl and [ReOCl(HEt2tcb)2] is reversible. 相似文献
870.
Tom Wagner Elisa Lopez‐Capel David Manning 《Rapid communications in mass spectrometry : RCM》2008,22(11):1625-1625