全文获取类型
收费全文 | 3567篇 |
免费 | 87篇 |
国内免费 | 20篇 |
专业分类
化学 | 2602篇 |
晶体学 | 10篇 |
力学 | 37篇 |
数学 | 530篇 |
物理学 | 495篇 |
出版年
2021年 | 25篇 |
2020年 | 47篇 |
2019年 | 51篇 |
2016年 | 75篇 |
2015年 | 75篇 |
2014年 | 83篇 |
2013年 | 124篇 |
2012年 | 162篇 |
2011年 | 182篇 |
2010年 | 113篇 |
2009年 | 80篇 |
2008年 | 128篇 |
2007年 | 161篇 |
2006年 | 172篇 |
2005年 | 147篇 |
2004年 | 110篇 |
2003年 | 95篇 |
2002年 | 105篇 |
2001年 | 69篇 |
2000年 | 57篇 |
1999年 | 48篇 |
1998年 | 44篇 |
1997年 | 42篇 |
1996年 | 38篇 |
1995年 | 37篇 |
1994年 | 55篇 |
1993年 | 51篇 |
1992年 | 64篇 |
1991年 | 44篇 |
1990年 | 41篇 |
1989年 | 40篇 |
1988年 | 47篇 |
1987年 | 49篇 |
1986年 | 44篇 |
1985年 | 52篇 |
1984年 | 41篇 |
1983年 | 30篇 |
1982年 | 35篇 |
1981年 | 41篇 |
1980年 | 40篇 |
1979年 | 33篇 |
1978年 | 42篇 |
1977年 | 38篇 |
1976年 | 40篇 |
1975年 | 38篇 |
1974年 | 41篇 |
1973年 | 40篇 |
1971年 | 26篇 |
1968年 | 24篇 |
1967年 | 96篇 |
排序方式: 共有3674条查询结果,搜索用时 15 毫秒
91.
Harald Luschgy 《Probability Theory and Related Fields》1992,93(1):39-66
Summary Let
(Q) be the statistical experiment based on the observation of an unknown function in the presence of an additive noise process with distributionQ. The (possible) loss of information whenQ is replaced by some other noise distributionP is measured by the deficiency of
(P) relative to
(Q). This deficiency and its relation to the variational distance ofP andQ are studied mainly for Gaussian noise processes. Gaussian diffusion processes and special set-indexed processes are treated in detail.Research supported by a Heisenberg grant of the Deutsche Forschungsgemeinschaft 相似文献
92.
93.
Wack H France S Hafez AM Drury WJ Weatherwax A Lectka T 《The Journal of organic chemistry》2004,69(13):4531-4533
We detail the synthesis of a new C(2)-symmetric bis(cyclophane) ligand system that can be thought of as electronically analogous to binol, but which possesses the added "third dimension" of cyclophane chirality. The ligand synthesis involves a spontaneous (but unexpected) atropisomerization to the desired product. We have employed this ligand to form a metal complex that is an effective cocatalyst for the highly enantio- and diastereoselective catalytic asymmetric synthesis of a beta-lactam. 相似文献
94.
Oxidefluorides of Niobium and Tantalum The reaction of NbF5 with SiO2 (silica glass ampules) at 310°C or with SiO2 (Aerosil) at 180°C always leads to NbO2F. To the contrary the reaction with laboratory glass (Jenaer Geräteglas) at 130°C leads to NbOF3. TaF5 reacts in silica glass ampules at 400°C by formation of TaO2F, however at 300°C or 260°C by formation of TaOF3. Silica glass did not react with NbF5 at 130°C, however Nb2O5 and NbF5 gave at 130°C in silica glass ampoules NbOF3. Similarly, TaF5 and Ta2O5 or TaO2F formed at 260°C in nickel ampoules TaOF3. The chemical and the thermochemical behaviour of oxidefluorides have been investigated. The compounds NbOF3 and TaOF3 are isomorphic. Lattice constants are mentioned. 相似文献
95.
Stereochemical effects in supramolecular self-assembly at surfaces: 1-D versus 2-D enantiomorphic ordering for PVBA and PEBA on Ag(111) 总被引:2,自引:0,他引:2
Barth JV Weckesser J Trimarchi G Vladimirova M De Vita A Cai C Brune H Günter P Kern K 《Journal of the American Chemical Society》2002,124(27):7991-8000
We present investigations on noncovalent bonding and supramolecular self-assembly of two related molecular building blocks at a noble metal surface: 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]benzoic acid (PEBA). These rigid, rodlike molecules comprising the same complementary moieties for hydrogen bond formation are comparable in shape and size. For PVBA, the ethenylene moiety accounts for two-dimensional (2-D) chirality upon confinement to a surface; PEBA is linear and thus 2-D achiral. Molecular films were deposited on a Ag(111) surface by organic molecular beam epitaxy and characterized by scanning tunneling microscopy. At low temperatures (around 150 K), both species form irregular networks of flat lying molecules linked via their endgroups in a diffusion-limited aggregation process. In the absence of kinetic limitations (adsorption or annealing at room temperature), hydrogen-bonded supramolecular assemblies form which are markedly different. With PVBA, enantiomorphic twin chains in two mirror-symmetric species running along a high-symmetry direction of the substrate lattice form by diastereoselective self-assembly of one enantiomer. The chirality signature is strictly correlated between neighboring twin chains. Enantiopure one-dimensional (1-D) supramolecular nanogratings with tunable periodicity evolve at intermediate coverages, reflecting chiral resolution in micrometer domains. In contrast, PEBA assembles in 2-D hydrogen-bonded islands, which are enantiomorphic because of the orientation of the supramolecular arrangements along low-symmetry directions of the substrate. Thus, for PVBA, chiral molecules form 1-D enantiomorphic supramolecular structures because of mesoscopic resolution of a 2-D chiral species, whereas with PEBA, the packing of an achiral species causes 2-D enantiomorphic arrangements. Model simulations of supramolecular ordering provide a deeper understanding of the stability of these systems. 相似文献
96.
The First Polyiodo Complex – Triethylsulfoniumtriiodomercurate(II)-tris(diiodine), (Et3S)[Hg2I6]1/2 · 3 I2 After Raman spectroscopic investigation of the system HgI2/Et3SIx, x = 3, 5, 7, triethylsulfoniumtriiodomercuratetris(diiodine), (Et3S)[Hg2I6]1/2 · 3 I2 was synthesized by reacting of HgI2 and liquid Et3SI7. The compound crystallizes at room temperature triclinically in the space group P1 with a = 879.4(7), b = 1 209.1(5), c = 1 291.5(5) pm, α = 96.16(3)°, β = 103.82(6)°, γ = 99.05(5)° and Z = 2. The crystal structure is composed of disordered Et3S+ cations, the centrosymmetric complex anion [HgI2/2I2]22? and three connecting iodine molecules I2. 相似文献
97.
Monika Schuster Claudia Turecek Helga Lichtenegger Robert Liska 《Applied Surface Science》2007,254(4):1131-1134
Continuously increasing life expectancy results in a rising number of bone diseases and fractures. Replacements from natural sources are not only limited in their availability, they also have some serious disadvantages such as possible immunological reactions or transmission of diseases. New synthetic biodegradable materials based on photopolymers could be an alternative solution. In these investigations an acrylate-based monomer formulation has been developed, consisting of a biodegradable basis monomer which is derived from gelatin, different reactive diluents, an appropriate photoinitiator and filler materials. For the three-dimensional shaping process stereolithography is the method of choice because of its capability to produce cellular structures with high resolutions. 相似文献
98.
99.
100.
Christopher Hubrich Axel Schulz Harald Brand Alexander Villinger 《Phosphorus, sulfur, and silicon and the related elements》2013,188(9):2179-2196
The silyl ethers (siloxanes) Me4? xSi(OC6H5CN)x (x = 1–4) (1–4), O(Si(OC6H4CN) (Me)2)2 (5), and Me3Si–O–C6F4CN (6) have been synthesized by the reaction of the respective p-hydroxybenzonitriles and chlorosilanes in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA) as hydrogen chloride acceptor. All compounds have been fully characterized by CHN-analysis, melting point, IR, Raman, mass spectroscopy, and 1H, 13C, 29Si NMR spectroscopy. Furthermore, the crystal structures of these compounds—with the exception of Me2Si(OC6H5CN)2, which is a liquid—were determined by X-ray diffractometry. 相似文献