全文获取类型
收费全文 | 620篇 |
免费 | 10篇 |
专业分类
化学 | 358篇 |
晶体学 | 3篇 |
力学 | 20篇 |
数学 | 73篇 |
物理学 | 176篇 |
出版年
2020年 | 11篇 |
2019年 | 5篇 |
2018年 | 4篇 |
2017年 | 7篇 |
2016年 | 9篇 |
2015年 | 8篇 |
2014年 | 13篇 |
2013年 | 33篇 |
2012年 | 25篇 |
2011年 | 25篇 |
2010年 | 16篇 |
2009年 | 12篇 |
2008年 | 23篇 |
2007年 | 27篇 |
2006年 | 41篇 |
2005年 | 21篇 |
2004年 | 22篇 |
2003年 | 20篇 |
2002年 | 20篇 |
2001年 | 16篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1996年 | 10篇 |
1995年 | 13篇 |
1994年 | 17篇 |
1993年 | 8篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 11篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 9篇 |
1973年 | 6篇 |
1971年 | 4篇 |
1970年 | 4篇 |
1969年 | 4篇 |
1968年 | 9篇 |
1967年 | 7篇 |
排序方式: 共有630条查询结果,搜索用时 187 毫秒
621.
Stephen G. Davies Ai M. Fletcher Clément Lebée Paul M. Roberts James E. Thomson Jingda Yin 《Tetrahedron》2013,69(4):1369-1377
The most efficient and concise asymmetric synthesis of (?)-(1R,7aS)-absouline to date, which was accomplished in eight steps and 20% overall yield from commercially available starting materials, is described. The doubly diastereoselective conjugate addition of lithium (S)-N-benzyl-N-(α-methylbenzyl)-amide to an enantiopure α,β-unsaturated ester derived from l-proline was employed as the key step. Subsequent hydrogenolytic N-debenzylation and acid-promoted cyclisation of the resultant β-amino ester produced the 1-aminopyrrolizidin-3-one scaffold, then reduction with DIBAL-H was followed by DCC-mediated coupling with (E)-p-methoxycinnamic acid to complete the synthesis of (?)-(1R,7aS)-absouline. 相似文献
622.
Stephen Fletcher Victoria Jane Black Iain Kirkpatrick Thomas Stephen Varley 《Journal of Solid State Electrochemistry》2013,17(2):327-337
In many modern technologies (such as batteries and supercapacitors), there is a strong need for redox-stable ionic liquids. Experimentally, the stability of ionic liquids can be quantified by the voltage range over which electron tunneling does not occur, but so far, quantum theory has not been applied systematically to this problem. Here, we report the electrochemical reduction of a series of quaternary ammonium cations in the presence of bis(trifluoromethylsulfonyl)imide (TFSI) anions and use nonadiabatic electron transfer theory to explicate the results. We find that increasing the chain length of the alkyl groups confers improved chemical inertness at all accessible temperatures. Simultaneously, decreasing the symmetry of the quaternary ammonium cations lowers the melting points of the corresponding ionic liquids, in two cases yielding highly inert solvents at room temperature. These are called hexyltriethylammonium TFSI (HTE-TFSI) and butyltrimethylammonium TFSI (BTM-TFSI). Indeed, the latter are two of the most redox-stable solvents in the history of electrochemistry. To gain insight into their properties, very high precision electrical conductivity measurements have been carried out in the range +20 °C to +190 °C. In both cases, the data conform to the Vogel-Tammann-Fulcher (VTF) equation with “six nines” precision (R 2?>?0.999999). The critical temperature for the onset of conductivity coincides with the glass transition temperature T g. This is compelling evidence that ionic liquids are, in fact, softened glasses. Finally, by focusing on the previously unsuspected connection between the molecular degrees of freedom of ionic liquids and their bulk conductivities, we are able to propose a new theory of the glass transition. This should have utility far beyond ionic liquids, in areas as diverse as glassy metals and polymer science. 相似文献
623.
624.
Lee JL Gilmore IS Seah MP Fletcher IW 《Journal of the American Society for Mass Spectrometry》2011,22(10):1718-1728
Quantitative chemical characterization of surfaces with topography by secondary ion mass spectrometry (SIMS) remains a significant
challenge due to the lack of systematic and validated measurement methods. In this study, we combine an experimental approach
using a simple model system with computer simulation using SIMION, to understand and quantify the key factors that give rise
to unwanted topographic artefacts in SIMS images of conducting samples with microscale topography. Experimental data are acquired
for gold wires (diameters 33 to 125 μm) mounted onto silicon wafers. Significant loss of ion intensities and shadowing arise
from the distortion of the extraction field, and the chemical analysis over the whole of the sample surface is difficult.
For large primary ion incidence angles of ≥55° to the surface normal, a fraction of the primary ions are scattered from the
target and impact the substrate, emitting secondary ions that may be mistaken as originating from the wire. For conducting
samples, topographic field effects may be reduced by the use of a smaller extraction voltage and an extraction delay. The
effects of an extraction delay on ion intensities, mass resolution and time-of-flight are studied, and its application is
demonstrated on an anisotropically etched silicon sample. The use of a simple sample holder with a V-shaped groove to reduce
topographic field effects for wires is also presented. Using these results, we provide clear guidance to analysts for the
diagnosis and identification of topography effects in SIMS, and present key recommendations to minimize them in practical
analysis. 相似文献
625.
Filopodia are long, fingerlike membrane tubes supported by cytoskeletal filaments. Their shape is determined by the stiffness of the actin filament bundles found inside them and by the interplay between the surface tension and bending rigidity of the membrane. Although one might expect the Euler buckling instability to limit the length of filopodia, we show through simple energetic considerations that this is in general not the case. By further analyzing the statics of filaments inside membrane tubes, and through computer simulations that capture membrane and filament fluctuations, we show under which conditions filopodia of arbitrary lengths are stable. We discuss several in vitro experiments where this kind of stability has already been observed. Furthermore, we predict that the filaments in long, stable filopodia adopt a helical shape. 相似文献
626.
We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high-voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (<20 micros), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of the magnetic field (up to 70 G). We observe that the expansion velocity scales as B(-1/2), explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions. 相似文献
627.
Luke Taylor Lili Sasky Tara Brodie Vytautas Remekevi
ius Hannah Jayne Moir James Barker John Fletcher Baljit Kaur Thatti Gavin Trotter Brian Rooney 《Molecules (Basel, Switzerland)》2022,27(15)
Ethanol is the most commonly used recreational drug worldwide. This study describes the development and validation of a headspace gas chromatography flame ionisation detection (HS-GC-FID) method using dual columns and detectors for simultaneous separation and quantitation. The use of a dual-column, dual-detector HS-GC-FID to quantitate ethanol is a common analytical technique in forensic toxicology; however, most analytical systems utilise pressure-balance injection rather than a simplified gas-tight syringe, as per this technique. This study is the first to develop and validate a technique that meets the specifications of the United Kingdom’s requirements for road traffic toxicology testing using a Shimadzu GC-2014 gas-tight syringe. The calibration ranged from 10 to 400 mg/100 mL, with a target minimum linearity of r2 > 0.999, using tertiary butanol as the internal standard marker. The method has an expanded uncertainty at 99.73% confidence of 3.64% at 80 mg/100 mL, which is the blood alcohol limit for drink driving in England and Wales. In addition, at 200 mg%—the limit at which a custodial sentence may be imposed on the defendant—the expanded uncertainty was 1.95%. For both the 80 mg% and 200 mg% concentrations, no bias was present in the analytical method. This method displays sufficient separation for other alcohols, such as methanol, isopropanol, acetaldehyde, and acetone. The validation of this technique complies with the recommended laboratory guidelines set out by United Kingdom and Ireland Association of Forensic Toxicologists (UKIAFT), the recently issued Laboratory 51 guidelines by the United Kingdom Accreditation Service (UKAS), and the criteria set out by the California Code of Regulations (CCR), 17 CCR § 1220.1. 相似文献
628.
629.
Dr. Inci Barut Dr. Xiulan He Assoc. Prof. Erol Sener Dr. Sanna Sämfors Prof. Andrew G. Ewing Prof. John S. Fletcher 《Angewandte Chemie (International ed. in English)》2023,62(15):e202217993
Aberrant functioning of the proteasome has been associated with crucial pathologic conditions including neurodegeneration. Yet, the complex underlying causes at the cellular level remain unclear and there are conflicting reports of neuroprotective to neurodegenerative effects of proteasomal inhibitors such as lactacystin that are utilised as models for neurodegenerative diseases. The conflicting results may be associated with different dose regimes of lactacystin and hence we have performed a dose dependent study of the effects of lactacystin to identify concurrent changes in the cell membrane lipid profile and the dynamics of exocytosis using a combination of surface sensitive mass spectrometry and single cell amperometry. Significant changes of negatively charged lipids were associated with different lactacystin doses that showed a weak correlation with exocytosis while changes in PE and PE−O lipids showed dose dependent changes correlated with initial pore formation and total release of vesicle content respectively. 相似文献
630.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option. 相似文献