全文获取类型
收费全文 | 3166篇 |
免费 | 85篇 |
国内免费 | 13篇 |
专业分类
化学 | 2166篇 |
晶体学 | 20篇 |
力学 | 77篇 |
数学 | 496篇 |
物理学 | 505篇 |
出版年
2021年 | 28篇 |
2020年 | 37篇 |
2019年 | 26篇 |
2018年 | 28篇 |
2017年 | 32篇 |
2016年 | 60篇 |
2015年 | 54篇 |
2014年 | 58篇 |
2013年 | 125篇 |
2012年 | 169篇 |
2011年 | 202篇 |
2010年 | 90篇 |
2009年 | 84篇 |
2008年 | 169篇 |
2007年 | 159篇 |
2006年 | 172篇 |
2005年 | 174篇 |
2004年 | 138篇 |
2003年 | 121篇 |
2002年 | 122篇 |
2001年 | 59篇 |
2000年 | 38篇 |
1999年 | 40篇 |
1998年 | 50篇 |
1997年 | 31篇 |
1996年 | 43篇 |
1995年 | 42篇 |
1994年 | 65篇 |
1993年 | 40篇 |
1992年 | 39篇 |
1991年 | 28篇 |
1990年 | 31篇 |
1989年 | 24篇 |
1988年 | 26篇 |
1987年 | 25篇 |
1986年 | 30篇 |
1985年 | 44篇 |
1984年 | 39篇 |
1983年 | 28篇 |
1982年 | 33篇 |
1981年 | 44篇 |
1980年 | 37篇 |
1979年 | 42篇 |
1978年 | 38篇 |
1977年 | 34篇 |
1976年 | 39篇 |
1975年 | 33篇 |
1974年 | 20篇 |
1973年 | 25篇 |
1972年 | 19篇 |
排序方式: 共有3264条查询结果,搜索用时 31 毫秒
991.
Posnansky O Huang R Shah NJ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,173(1):1-9
The free random walk approach has been used to analyze the attenuation of the NMR signal due to spin dephasing in the presence of a constant and pulsed parabolic magnetic field. The spin echo sequence was chosen to examine the attenuation of the NMR signal resulting from self-diffusion. In the framework of the gaussian approach, the long-time limit calculations predict more pronounced diffusion weighting for the parabolic field than for linear magnetic field. Analytical results were obtained and compared with those from other approaches based on a variety of different of approximations. 相似文献
992.
Signal overlap in the NMR dimension significantly complicates the construction and analysis of 2D diffusion-ordered (DOSY) spectra. Such problems can often be reduced or even eliminated by extending the NMR domain of a DOSY experiment into two dimensions, giving a 3D-DOSY spectrum. To date such experiments have generally sacrificed some signal-to-noise ratio and have required extensive and time-consuming phase cycling. A new family of pulse sequences with internal diffusion encoding (IDOSY) has been introduced which avoids both of these problems. It is often straightforward to incorporate convection compensation in such sequences at no cost in signal-to-noise ratio. Here, some of the problems caused by convection in DOSY are described and illustrated, and the efficacy of convection compensation in the 2DJ-IDOSY and COSY-IDOSY experiments is demonstrated. 相似文献
993.
994.
[reaction: see text] Two inhibitors of FOXO1a-mediated nuclear export, psammaplysenes A and B, have been synthesized by a flexible and efficient route. A common starting material, 4-iodophenol, was used to prepare both halves of these pseudosymmetric dibromotyrosine-derived metabolites. 相似文献
995.
This work is an experimental response to an intriguing paper recently published by Catlow and co-workers, which looked at the computational feasibility of fluorine location in three different all-silica zeotypes (Attfield, M. P.; Catlow, C. R. A.; Sokol, A. A. Chem. Mater. 2001, 13, 4708). The materials were chosen as representative of three unique host locations. Our present work examined the synthesis of zeotypes AST, IFR, and MTT using organo-cations with a strong preference for crystallizing these structures. We studied the effect of reaction time and the H(2)O/SiO(2) reactant ratio. The latter is probably the most important function in these zeolite crystallizations that use HF. As reaction conditions became more dilute, AST gave way to SGT and IFR to MTW as host structures, while the MTT synthesis was invariant. Our reactions were studied in terms of product yield vs time, product organo-cation content, fluorine content, and the representative (29)Si and (19)F NMR spectra for certain samples. A single crystal study was carried out for a sample of MTT. Our results showed that, consistent with other recent studies, low H(2)O/SiO(2) reactant ratios lead to more open framework host structures (i.e., IFR vs MTW), and there is typically a higher uptake of organo-cation and fluorine. The structure may well contain a higher population of 4-rings within the silicate substructure. While MTT that contains no 4-rings was chosen as the best possible candidate to achieve an ion-pair for the organo-cation and fluoride anion within the silicate host, both NMR and single crystal work confirm that fluoride is bonded to a 5-coordinate silica center within the lattice. 相似文献
996.
997.
Moser KW Raguin LG Harris A Morris HD Georgiadis J Shannon M Philpott M 《Magnetic resonance imaging》2000,18(2):199-207
A new magnetic resonance imaging technique was applied to the Taylor-Couette and spiral Poiseuille (Taylor-Couette with superposed mean axial flux) flows for the first time. The experimental technique is a combination of spatial tagging methods and a snapshot FLASH imaging sequence, which allows the full-field visualization of 2-D slices of the flow field, with image acquisition times approximately half a second. By acquiring images every few seconds, direct visualization of flow patterns can be obtained in the form of cinematography. Tagged images of the Taylor-Couette flow were acquired in both the axial and transverse planes and confirmed previously reported numerical predictions of Taylor cell size. Tagged images of the spiral Poiseuille flows verified that the cells in this flow propagate at a higher velocity than the mean axial flow. In addition, intermittent cell formation was observed as the axial flow was increased. 相似文献
998.
Anthony J. Pearson Tapan Ray Ian C. Richards Jon Clardy Linda Silveira 《Tetrahedron letters》1983,24(52):5827-5830
Use of the title reactions for the regioselective, and stereoselective functionalization of a range of dienylacetic acids, available from simple organoiron chemistry, is presented. 相似文献
999.
U. H. W. Lammers R. A. Marr J. B. Morris 《International Journal of Infrared and Millimeter Waves》1990,11(3):367-382
Highly correlated transmit and local oscillator signals for use in a narrowband heterodyne system are derived from a single submillimeter source. One component is Doppler shifted in frequency by a mechanically rotating device before transmission. The frequency translation does not lead to significant spectral deterioration. Under the condition of only a small differential delay between transmit and local oscillator paths, most phase and frequency instabilities of the submillimeter source are eliminated at the intermediate frequency. The paper describes the frequency shifter's mode of operation and a practical 637GHz design. Based on this design we simulate the effect of various imperfections in the frequency translation process on the output spectrum. Preliminary measurements at 140GHz are included. 相似文献
1000.
We describe an ordinalh(A) which plays a key role in the model theory of the admissible fragmentL
A
. In particular, the Hanf number ofL
A
is ℶh(A) IfL
A
isL
κ=ω wherecf(k)>ω thenh(A) can be characterized as the least ordinal which is notH(k
+)-recursive.
Financial support received from NSF Grant GP-23114
Financial support received from NSF Grant GP-23114 and the A. P. Sloan Foundation 相似文献