全文获取类型
收费全文 | 314篇 |
免费 | 10篇 |
国内免费 | 1篇 |
专业分类
化学 | 152篇 |
晶体学 | 1篇 |
力学 | 2篇 |
数学 | 47篇 |
物理学 | 103篇 |
无线电 | 20篇 |
出版年
2022年 | 5篇 |
2021年 | 12篇 |
2020年 | 4篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 2篇 |
2015年 | 6篇 |
2014年 | 4篇 |
2013年 | 20篇 |
2012年 | 14篇 |
2011年 | 23篇 |
2010年 | 11篇 |
2009年 | 14篇 |
2008年 | 14篇 |
2007年 | 12篇 |
2006年 | 18篇 |
2005年 | 18篇 |
2004年 | 7篇 |
2003年 | 17篇 |
2002年 | 20篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1989年 | 3篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 3篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1961年 | 1篇 |
1956年 | 2篇 |
1955年 | 1篇 |
1940年 | 1篇 |
1937年 | 1篇 |
1928年 | 1篇 |
排序方式: 共有325条查询结果,搜索用时 15 毫秒
111.
The Cassini May 2000 solar conjunction 总被引:2,自引:0,他引:2
Morabito D.D. Shambayati S. Finley S. Fort D. 《Antennas and Propagation, IEEE Transactions on》2003,51(2):201-219
112.
113.
A recently developed theory of the effects of transport coherence of triplet excitons on the time dependence of delayed fluorescence in molecular crystals is applied to two- and three-dimensional crystals. Manifestations of anisotropy are studied for arbitrary coherence and it is shown that the mean-square-velocity tensor can be extracted explicitly in the coherent limit. 相似文献
114.
The reaction of 2-picoline at the methyl group with NDA and KDA followed by dimethyldisulfide trapping furnished, respectively, dithioacetals and trithioortho esters with high selectivity. The method was successfully applied to other methyl-substituted pyridines, quinolines, and pyrazines. Dithioketals were prepared by a one-pot procedure involving the reaction of metalated 2-picoline with 2 equiv of dimethyldisulfide followed by in situ trapping with a second electrophile. All of the generated thio-substituted compounds were efficiently transformed in presence of mercury salts or under oxidizing conditions to other functional groups comprising aldehydes, ketones, ketals, thiol esters, orthoesters, and esters. 相似文献
115.
We report the phase coherent splitting of Bose-Einstein condensates by means of a phase grating produced near the surface of a microelectronic chip. A lattice potential with a period of 4 mum is generated by the superposition of static and oscillating magnetic fields. Precise control of the diffraction is achieved by controlling the currents in the integrated conductors. The interference of overlapping diffraction orders is observed after 8 ms of propagation in a harmonic trap and subsequent ballistic expansion of the atomic ensemble. By analyzing the interference pattern we show a reproducible phase relation between the diffraction orders with an uncertainty limited by the resolution of the diffraction grating. 相似文献
116.
We have investigated the magnetoelastic nature of the dodecagonal anisotropy in the magnetic anisotropy energy (MAE) in the basal plane of the hcp crystalline structure in holmium single crystal. We have proved that the origin of the second harmonic of the hexagonal symmetry in MAE clearly lies on a sixth-order magnetoelastic coupling term. The appearance of a 12-fold anisotropy in MAE in a single crystal having hexagonal symmetry provides a new insight on how the magnetic anisotropy can be modified in a magnetic material with giant spin-lattice coupling. 相似文献
117.
Covalent Functionalization of HiPco Single‐Walled Carbon Nanotubes: Differences in the Oxidizing Action of H2SO4 and HNO3 during a Soft Oxidation Process 下载免费PDF全文
Dr. Xavier Devaux Dr. Brigitte Vigolo Dr. Edward McRae Dr. Fabrice Valsaque Naoual Allali Dr. Victor Mamane Prof. Yves Fort Prof. Alexander V. Soldatov Dr. Manuel Dossot Dr. Svetlana Yu. Tsareva 《Chemphyschem》2015,16(12):2692-2701
The results of a study on the evolution of HiPco single‐walled carbon nanotubes during the oxidizing action of H2SO4 and HNO3 are presented. The process conditions used have been chosen so as to avoid any significant damage to the nanotube structure. The type and level of functionalization, the location of the grafted functions on the surface of the nanotube and the changes in morphological characteristics of the samples were examined by using a wide and complementary range of analytical techniques. We propose an explanation for the differences in the oxidizing action of sulfuric and nitric acids. The combined results allow us to suggest possible reaction mechanisms that occur on the surface of the nanotube. 相似文献
118.
Pierre Etore Gersende Fort Benjamin Jourdain Eric Moulines 《Annals of Operations Research》2011,189(1):127-154
This paper investigates the use of stratified sampling as a variance reduction technique for approximating integrals over
large dimensional spaces. The accuracy of this method critically depends on the choice of the space partition, the strata, which should be ideally fitted to the subsets where the functions to integrate is nearly constant, and on the allocation
of the number of samples within each strata. When the dimension is large and the function to integrate is complex, finding
such partitions and allocating the sample is a highly non-trivial problem. In this work, we investigate a novel method to
improve the efficiency of the estimator “on the fly”, by jointly sampling and adapting the strata which are hyperrectangles
and the allocation within the strata. The accuracy of estimators when this method is used is examined in detail, in the so-called
asymptotic regime (i.e. when both the number of samples and the number of strata are large). It turns out that the limiting
variance depends on the directions defining the hyperrectangles but not on the precise abscissa of their boundaries along
these directions, which gives a mathematical justification to the common choice of equiprobable strata. So, only the directions
are adaptively modified by our algorithm. We illustrate the use of the method for the computation of the price of path-dependent
options in models with both constant and stochastic volatility. The use of this adaptive technique yields variance reduction
by factors sometimes larger than 1000 compared to classical Monte Carlo estimators. 相似文献
119.
Kirschner R Varga-Szemes A Brott BC Litovsky S Elgavish A Elgavish GA Simor T 《Magnetic resonance imaging》2011,29(5):650-658
Introduction
A substantial, common shortcoming of the currently used semiautomated techniques for the quantification of myocardial infarct with delayed enhancement magnetic resonance imaging is the assumption that the whole myocardial slab that corresponds to the hyperenhanced tomographic area is 100% nonviable. This assumption is, however, incorrect. To resolve this conflict, we have recently proposed the signal intensity percent-infarct mapping method and validated it in an ex vivo, canine experiment. The purpose of the current study has been the validation of the signal intensity percent-infarct mapping method in vivo, using a porcine model of reperfused myocardial infarct.Methods
In swines (n=6), reperfused myocardial infarct was generated occluding for 90 min by an angioplasty balloon either the left anterior descending or the left circumflex coronary artery. To obtain DE images, Gd(DTPA) enhanced inversion-recovery fast gradient-echo acquisitions were carried out on day 28 after myocardial infarction. Scanning started 15 min after intravenous injection of 0.2 mmol/kg Gd(DTPA). At the end of the MRI session, the animal was sacrificed and 2,3,5-triphenyltetrazolium chloride staining was used to validate the existence and to determine the accurate size of the myocardial infarct. Tissue samples were taken and stained with hematoxylin-eosin and Masson's trichrome for histological assessment of the infarct and the periinfarct zone. The signal intensity percent-infarct mapping data were compared with corresponding data from the delayed enhancement images analyzed with SIremote+2S.D. thresholding, and with corresponding triphenyltetrazolium-chloride staining data using Friedman's repeated measure analysis of variance on ranks.Results
The infarct volume determined by the triphenyltetrazolium chloride, SIremote+2S.D. and signal intensity percent-infarct mapping methods was 3.04 ml [2.74, 3.45], 13.62 ml [9.06, 18.45] and 4.27 ml [3.45, 6.33], respectively. Median infarct volume determined by SIremote+2S.D. significantly differed from that determined by triphenyltetrazolium chloride (P<.05). The Bland-Altman overall bias was 12.49% of the volume of the left ventricle. Median infarct volume determined by signal intensity percent-infarct mapping, however, did not differ significantly (NS) from that obtained by triphenyltetrazolium chloride. Signal intensity percent-infarct mapping yielded only a 1.99% Bland-Altman overall bias of the left ventricular volume.Conclusions
This in vivo study in the porcine reperfused myocardial infarct model demonstrates that signal intensity percent-infarct mapping is a highly accurate method for the determination of the extent of myocardial infarct. MRI images for signal intensity percent-infarct mapping are obtained with the pulse sequence of conventional delayed enhancement imaging and are acquired within clinically acceptable scanning time. This makes signal intensity percent-infarct mapping a practical method for clinical implementation. 相似文献120.
Espa D Pilia L Marchiò L Mercuri ML Serpe A Barsella A Fort A Dalgleish SJ Robertson N Deplano P 《Inorganic chemistry》2011,50(6):2058-2060
The synthesis and full characterization of the redox-active nickel triad mixed-ligand dithiolene complexes based on Bz(2)pipdt = 1,4-dibenzylpiperazine-3,2-dithione and dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate ligands are reported. These complexes show a reversibly bleacheable solvatochromic peak and a remarkably high negative molecular first hyperpolarizability, whose values depend on the metal being highest for the platinum(II) compound. 相似文献