全文获取类型
收费全文 | 929篇 |
免费 | 42篇 |
国内免费 | 11篇 |
专业分类
化学 | 517篇 |
晶体学 | 20篇 |
力学 | 16篇 |
数学 | 273篇 |
物理学 | 156篇 |
出版年
2023年 | 4篇 |
2022年 | 9篇 |
2021年 | 10篇 |
2020年 | 10篇 |
2019年 | 17篇 |
2018年 | 23篇 |
2017年 | 11篇 |
2016年 | 29篇 |
2015年 | 20篇 |
2014年 | 22篇 |
2013年 | 45篇 |
2012年 | 50篇 |
2011年 | 75篇 |
2010年 | 39篇 |
2009年 | 50篇 |
2008年 | 62篇 |
2007年 | 66篇 |
2006年 | 48篇 |
2005年 | 49篇 |
2004年 | 45篇 |
2003年 | 38篇 |
2002年 | 33篇 |
2001年 | 12篇 |
2000年 | 15篇 |
1999年 | 18篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 14篇 |
1994年 | 4篇 |
1993年 | 10篇 |
1992年 | 5篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1973年 | 4篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1967年 | 5篇 |
排序方式: 共有982条查询结果,搜索用时 15 毫秒
41.
Diantha R. Kelman Lisa F. Szczepura Karen I. Goldberg Werner Kaminsky Anne K. Hermetet Lily J. Ackerman John K. Swearingen Douglas X. West 《Journal of Molecular Structure》2002,610(1-3):143-150
N-2-(4-picolyl)-N′-2-chlorophenylthiourea, 4PicTu2Cl, monoclinic, P21/c, a=10.068(5), b=11.715(2),
β=96.88(4)°,
and Z=4; N-2-(6-picolyl)-N′-2-chlorophenylthiourea, 6PicTu2Cl, triclinic, P-1, a=7.4250(8), b=7.5690(16), c=12.664(3) Å, =105.706(17), β=103.181(13), γ=90.063(13)°, V=665.6(2) Å3 and Z=2 and N-2-(6-picolyl)-N′-2-bromophenylthiourea, 6PicTu2Br, triclinic, P-1, a=7.512(4), b=7.535(6), c=12.575(4) Å, a=103.14(3), β=105.67(3), γ=90.28(4)°, V=665.7(2) Å3 and Z=2. The intramolecular hydrogen bonding between N′H and the pyridine nitrogen and intermolecular hydrogen bonding involving the thione sulfur and the NH hydrogen, as well as the planarity of the molecules, are affected by the position of the methyl substituent on the pyridine ring. The enthalpies of fusion and melting points of these thioureas are also affected. 1H NMR studies in CDCl3 show the NH′ hydrogen resonance considerably downfield from other resonances in their spectra. 相似文献
42.
David John Parsons Andrew Angus Martyn Brawn Joe Morris 《The Journal of the Operational Research Society》2014,65(9):1387-1395
Local Government Authorities (LGA) in England and Wales have statutory responsibility for the maintenance of Public Rights of Way (PROW), such as pathways and byways open to non-motorised traffic. The departments responsible have to compete for budgets and justify their expenditure in terms of councils’ priorities, such as well-being and environment. A need was identified for a simple decision support tool to provide a consistent and transparent framework for assessing the range of possible social and economic benefits from expenditure on PROW. The tool uses the Analytic Hierarchy Process to elicit weights forming the links from path attributes to users and usage to benefits, with a final stage to combine the benefits according to LGA priorities. It was successfully tested through case studies, where improving signage was generally found to be the most cost effective option, giving moderate benefits at low cost, whereas improving the physical conditions of the surface gave greater benefits at relatively high cost. 相似文献
43.
44.
45.
Joe Gildea 《Czechoslovak Mathematical Journal》2011,61(2):531-539
The structure of the unit group of the group algebra of the group A4 over any finite field of characteristic 2 is established
in terms of split extensions of cyclic groups. 相似文献
46.
We present a quantum-mechanical theory of the cooling of a cantilever coupled via radiation pressure to an illuminated optical cavity. Applying the quantum noise approach to the fluctuations of the radiation pressure force, we derive the optomechanical cooling rate and the minimum achievable phonon number. We find that reaching the quantum limit of arbitrarily small phonon numbers requires going into the good-cavity (resolved phonon sideband) regime where the cavity linewidth is much smaller than the mechanical frequency and the corresponding cavity detuning. This is in contrast to the common assumption that the mechanical frequency and the cavity detuning should be comparable to the cavity damping. 相似文献
47.
We report the measurement of the surface conductivity of biological macromolecules by dielectrophoretic trapping at the tip of a glass nanopipet. We find that the threshold voltage for trapping is a function of salt concentration and can be directly linked to the effective conductivity of the biomolecule and its solvation shell. The surface conductivities obtained for 20-mer single-stranded DNA, 40-mer double-stranded DNA, and yellow fluorescent protein are 7.9+/-1.9 nS, 5.3+/-0.7 nS, and 21.5+/-1.6 nS, respectively. 相似文献
48.
Stephanie M. Barbon Pauline A. Reinkeluers Dr. Jacquelyn T. Price Prof. Dr. Viktor N. Staroverov Prof. Dr. Joe B. Gilroy 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(36):11340-11344
The straightforward synthesis of a series of 3‐cyanoformazanate boron difluoride dyes is reported. Phenyl, 4‐methoxyphenyl and 4‐cyanophenyl N‐substituted derivatives were isolated and characterized by single‐crystal X‐ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. The compounds were demonstrated to possess tunable, substituent‐dependent absorption, emission, and electrochemical properties, which were rationalized through electronic structure calculations. 相似文献
49.
Comparative Studies of Thermally Induced Homolytic CarbonCarbon Bond Cleavage Reactions of Strained Dicarba[2]ferrocenophanes and Their Ring‐Opened Oligomers and Polymers
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Andrew D. Russell Prof. Joe B. Gilroy Prof. Ian Manners 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(14):4077-4085
Reactivity studies of dicarba[2]ferrocenophanes and also their corresponding ring‐opened oligomers and polymers have been conducted in order to provide mechanistic insight into the processes that occur under the conditions of their thermal ring‐opening polymerisation (ROP) (300 °C). Thermolysis of dicarba[2]ferrocenophane rac‐[Fe(η5‐C5H4)2(CHPh)2] (rac‐ 14 ; 300 °C, 1 h) does not lead to thermal ROP. To investigate this system further, rac‐ 14 was heated in the presence of an excess of cyclopentadienyl anion, to mimic the postulated propagating sites for thermally polymerisable analogues. This afforded acyclic [(η5‐C5H5)Fe(η5‐C5H4)‐CH2Ph] ( 17 ) through cleavage of both a Fe?Cp bond and also the C?C bond derived from the dicarba bridge. Evidence supporting a potential homolytic C?C bond cleavage pathway that occurs in the absence of ring‐strain was provided through thermolysis of an acyclic analogue of rac‐ 14 , namely [(η5‐C5H5)Fe(η5‐C5H4)(CHPh)2‐C5H5] ( 15 ; 300 °C, 1 h), which also afforded ferrocene derivative 17 . This reactivity pathway appears general for post‐ROP species bearing phenyl substituents on adjacent carbons, and consequently was also observed during the thermolysis of linear polyferrocenylethylene [Fe(η5‐C5H4)2(CHPh)2]n ( 16 ; 300 °C, 1 h), which was prepared by photocontrolled ROP of rac‐ 14 at 5 °C. This afforded ferrocene derivative [Fe(η5‐C5H4CH2Ph)2] ( 23 ) through selective cleavage of the ?H(Ph)C?C(Ph)H? bonds in the dicarba linkers. These processes appear to be facilitated by the presence of bulky, radical‐stabilising phenyl substituents on each carbon of the linker, as demonstrated through the contrasting thermal properties of unsubstituted linear trimer [(η5‐C5H5)Fe(η5‐C5H4)(CH2)2(η5‐C5H4)Fe(η5‐C5H4)(CH2)2(η5‐C5H4)Fe(η5‐C5H5)] ( 29 ) with a ?H2C?CH2? spacer, which proved significantly more stable under analogous conditions. Evidence for the radical intermediates formed through C?C bond cleavage was detected through high‐resolution mass spectrometric analysis of co‐thermolysis reactions involving rac‐ 14 and 15 (300 °C, 1 h), which indicated the presence of higher molecular weight species, postulated to be formed through cross‐coupling of these intermediates. 相似文献
50.
A gelatin matrix was simultaneously doped with nine equimolar, homologous, tetraalkylammonium salts ranging in mass from 210 to 700 Da. Bombardment of the sample with kiloelectronvolt ions resulted in a nonidentical distribution of relative cation intensities with a maximum at m/z 242 for samples with a total salt concentration of 0.004 g of salt/g of gelatin. A rapid increase in relative intensities with increasing mass is observed for the low mass salts and is believed to be linked to changes in the ionization efficiencies. The changes in ionization efficiencies are likely related to decreasing coulombic attractive forces between the organic cation and the counterion. Disappearance cross-sections, determined from decay curves, indicate that sputter-induced damage increases with increasing mass of the cation. Fragment-to-intact cation ratios also suggest that damage accumulates fastest in the heaviest salts. These observations indicate that desorption yields of the organic salts in a gelatin matrix decrease with increasing mass. In addition, suppression of lower mass tetraalkylammonium salt intact cation intensities was observed for salt-in-gelatin concentrations greater than 10−3 g/g. 相似文献