全文获取类型
收费全文 | 2381篇 |
免费 | 105篇 |
国内免费 | 13篇 |
专业分类
化学 | 1727篇 |
晶体学 | 44篇 |
力学 | 79篇 |
数学 | 213篇 |
物理学 | 436篇 |
出版年
2024年 | 16篇 |
2023年 | 19篇 |
2022年 | 114篇 |
2021年 | 127篇 |
2020年 | 79篇 |
2019年 | 75篇 |
2018年 | 113篇 |
2017年 | 82篇 |
2016年 | 122篇 |
2015年 | 77篇 |
2014年 | 69篇 |
2013年 | 191篇 |
2012年 | 173篇 |
2011年 | 159篇 |
2010年 | 102篇 |
2009年 | 100篇 |
2008年 | 107篇 |
2007年 | 85篇 |
2006年 | 66篇 |
2005年 | 54篇 |
2004年 | 38篇 |
2003年 | 48篇 |
2002年 | 67篇 |
2001年 | 44篇 |
2000年 | 50篇 |
1999年 | 17篇 |
1998年 | 28篇 |
1997年 | 23篇 |
1996年 | 16篇 |
1995年 | 20篇 |
1994年 | 11篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 15篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 9篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 7篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1975年 | 6篇 |
1974年 | 7篇 |
排序方式: 共有2499条查询结果,搜索用时 421 毫秒
61.
Jia‐Xiang Yang Yu‐Peng Tian Qing‐Liang Liu Yong‐Shu Xie Hoong‐Kun Fun Suchada Chantrapromma Ibrahim Abdul Razak 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(1):m43-m44
In the title compound, [Fe(C5H5)(C16H12N3OS)], the 8‐aminoquinoline and acylthiourea moieties are almost planar. There are two perpendicular arrangements of the molecules in the crystal with slightly different conformations. The two cyclopentadienyl rings in each molecule are parallel and eclipsed. 相似文献
62.
An unprecedented one-pot reductive trimerization of aromatic aldehydes to 1,2,3-triaryl-2-propen-1-ones as well as tandem reductive coupling-rearrangements of aryl ketones to pinacolone analogues was efficiently achieved using tetrachlorosilane-zinc at room temperature. 相似文献
63.
The color of a complex number is defined as the number of vertices of the convex hull of powers of that number. This induces a coloring of the unit disk. The structure of the set of points where the color changes is investigated here. It is observed that there is a connection between this fractal set and some family of trinomial equations. Three algorithms for coloring the unit disk are described, the last one (related to the Farey sequence) arising out of a conjecture. This conjecture is formulated and proved in this presentation. 相似文献
64.
A novel zirconia-based hybrid organic-inorganic sol-gel coating was developed for capillary microextraction (CME) (in-tube SPME). High degree of chemical inertness inherent in zirconia makes it very difficult to covalently bind a suitable organic ligand to its surface. In the present work, this problem was addressed from a sol-gel chemistry point of view. Principles of sol-gel chemistry were employed to chemically bind a hydroxy-terminated silicone polymer (polydimethyldiphenylsiloxane, PDMDPS) to a sol-gel zirconia network in the course of its evolution from a highly reactive alkoxide precursor undergoing controlled hydrolytic polycondensation reactions. A fused silica capillary was filled with a properly designed sol solution to allow for the sol-gel reactions to take place within the capillary for a predetermined period of time (typically 15-30 min). In the course of this process, a layer of the evolving hybrid organic-inorganic sol-gel polymer got chemically anchored to the silanol groups on the capillary inner walls via condensation reaction. At the end of this in-capillary residence time, the unbonded part of the sol solution was expelled from the capillary under helium pressure, leaving behind a chemically bonded sol-gel zirconia-PDMDPS coating on the inner walls. Polycyclic aromatic hydrocarbons, ketones, and aldehydes were efficiently extracted and preconcentrated from dilute aqueous samples using sol-gel zirconia-PDMDPS coated capillaries followed by thermal desorption and GC analysis of the extracted solutes. The newly developed sol-gel hybrid zirconia coatings demonstrated excellent pH stability, and retained the extraction characteristics intact even after continuous rinsing with a 0.1 M NaOH solution for 24 h. To our knowledge, this is the first report on the use of a sol-gel zirconia-based hybrid organic-inorganic coating as an extraction medium in solid phase microextraction (SPME). 相似文献
65.
Mehmood A Malik A Anis I Khan PM Riaz M Makhmoor T Choudhary MI 《Natural product letters》2002,16(6):371-376
Two new oleanane triterpenes; 2alpha,3alpha,24-trihydroxyolean-12-ene-28,30-dioic acid ([structure: see text]) and 2alpha,3alpha,24,28-tetrahydroxyolean-12-ene ([structure: see text]) have been isolated from the roots of Atropa acuminata. Anti-oxidant p-hydroxyphenethyl trans-ferulate ([structure: see text]), beta-sitosterol-3-O-beta-D-glucopyranoside ([structure: see text]) and oleanolic acid ([structure: see text]) have also been reported for the first time from this species. The structures were determined by spectroscopic studies including 2D-NMR. 相似文献
66.
M. B. Alamin A. A. Mhapes A. M. Bejey A. Sadek R. H. Atweer K. Dubali Daw M. Saad 《Journal of Radioanalytical and Nuclear Chemistry》2007,271(1):247-250
Since there is no database in Libya defining the intake of the individuals from different essential (minor and trace) and
toxic elements provided through food, drinking water and aerosol, a project has been proposed with the cooperation of IAEA
to determine the concentration of a number of elements such as Cs, Fe, Cr, Rb, Sc, Se, Co, Zn in the three mentioned sources.
Emphasis was placed on the use of nuclear and nuclear-related techniques. In this paper, the primary results are presented
for the concentration of minor and trace elements in some vegetables, spices and other foods which are widely used in the
Libyan meals. Instrumental neutron activation analysis utilizing a 10 MW water pool reactor and a γ-ray spectroscopy facility
was employed. For quality control, certified reference materials were analyzed simultaneosly with the samples which show good
agreements compared with the certified data. Emphasis was given to both elements iron and zinc for their importance. 相似文献
67.
Coordination Chemistry of Functional Phosphorus Ylides. VII. Pentacarbonylmetal Complexes of Cyanomethylene Triphenylphosphorane, [Ph3P?CH(CN)] · M(CO)5 (M = Cr, Mo, W) The pentacarbonyl complexes [Ph3P?CH(CN)] · M(CO)5 ( 1a –c) are obtained by reaction of the irradiated metal hexacarbonyls M(CO)6 (M = Cr, Mo, W) with cyanomethylene triphenylphosphorane under exclusion of light. The IR and NMR spectra indicate N-coordination of the ligand, that means a nitrile complex. The chromium complex 1a crystallizes triclinic (space group P1 , Z = 2) with the lattice constant a = 1126.5(2), b = 1153.6(3), c = 951.4(3) pm; α = 103.47(3), β = 102.04(3), γ = 84.00(2)°. The linear array of the ligand atoms C7, C6, N forms an angle of 168.1(5)° with the metal-nitrogen bond. Significant bond distances are Cr? N = 206.2(6), N? C6 = 115.3(7), C6? C7 = 137.2(8) and P? C7 = 170.9(5) pm. 相似文献
68.
In this paper, we propose a novel all-optical 2R (re-amplification and re-shaping) regenerator based on inducing nonlinear chirp over a continuous wave probe. The regenerator also performs continuous wave to pulsed wavelength conversion. The chirp is induced in a highly nonlinear fiber by the data modulated pulsed signal that is to be regenerated. Offset filtering is employed at the output of the highly nonlinear fiber to extract the frequencies generated as a result of chirping. The regenerator performance has been evaluated with the help of bit-error-rate plots and eye diagrams at different values of optical signal to noise ratios. Apart from re-shaping and re-amplification, inclusion of the regenerator results in better resilience to amplified spontaneous emission noise. 相似文献
69.
Saad Zagloul Rida 《Applicable analysis》2013,92(1-2):93-102
We derive none some explicit formula for the power of fractional order (differential and integral) operators. 相似文献
70.