全文获取类型
收费全文 | 705篇 |
免费 | 16篇 |
国内免费 | 4篇 |
专业分类
化学 | 419篇 |
晶体学 | 3篇 |
力学 | 27篇 |
数学 | 162篇 |
物理学 | 114篇 |
出版年
2023年 | 5篇 |
2022年 | 17篇 |
2021年 | 26篇 |
2020年 | 21篇 |
2019年 | 15篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 30篇 |
2015年 | 23篇 |
2014年 | 26篇 |
2013年 | 40篇 |
2012年 | 48篇 |
2011年 | 48篇 |
2010年 | 35篇 |
2009年 | 27篇 |
2008年 | 37篇 |
2007年 | 39篇 |
2006年 | 35篇 |
2005年 | 36篇 |
2004年 | 27篇 |
2003年 | 19篇 |
2002年 | 19篇 |
2001年 | 10篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1921年 | 2篇 |
1919年 | 2篇 |
1915年 | 4篇 |
1909年 | 2篇 |
1901年 | 2篇 |
1875年 | 1篇 |
1868年 | 2篇 |
排序方式: 共有725条查询结果,搜索用时 15 毫秒
21.
An oscillator-amplifier XeCl laser system has been used to experimentally investigate the effectiveness of a liquid stimulated Brillouin scattering (SBS) mirror for correcting the spatial aberrations of broad-band laser radiation in a double-pass amplifier. It has been found that the SBS amplifier performance is strongly dependent on the intensity I
p exciting the Brillouin medium. A good beam reconstruction has been attained at I
p1GW/cm2, whereas highly aberrated output beams have been delivered by the SBS amplifier for I
p>1 GW/cm2. By comparing the broad-band SBS amplifier performance to that of the same amplifier with a dielectrically coated flat mirror at one end, it has been found that the use of a SBS mirror is advantageous to obtain lower divergence output beams only for low energy pump beams (<1 mJ). 相似文献
22.
Alberto Bargagna Pietro Schenone Filippo Evangelisti 《Journal of heterocyclic chemistry》1979,16(1):93-96
The dipolar 1,4-cycloaddition of dichloroketene to N,N-disubstituted 3-amino-1-phenyl-2-propene-1-onesled directly to N,N-disubstituted 4-amino-3-chloro-6-phenyl-2H-pyran-2-ones only in the case of an usual aliphatic N,N-disubstitution. In the case of partial or full aromatic N-substitution, N,N-disubstituted 4-amino-3,3-dichloro-3,4-dihydro-6-phenyl-2H-pyran-2-ones were instead obtained, which were dehydrochlorinated with DBN to the corresponding 4-amino-3-chloro-6-phenyl-2H-pyran-2-ones. 相似文献
23.
This paper discusses the partitioning of metals (K, Na, Ca, Mg, Al, Cu, Fe, Pb and Zn) between the aqueous phase and the suspended insoluble material in fog samples collected in the Po Valley during two extensive fields campaigns. Metals represent on average 11% of the mass of suspended insoluble matter, while the main component is carbon (both organic carbon, OC = 35%, and black carbon, BC = 8%). The unaccounted suspended matter mass is very high, on average 46%, and is attributable to non metallic species, such as O and N and of Si. The principal metals in the insoluble suspended fraction are Fe and Al (2-5%), while the contributions of other metals (Na, Mg, Cu, Pb and Zn) are lower than 1%. Ca and K exhibited high blank values and could not be detected above blank detection limit threshold. The main components in the aqueous phase are NO3- (34%), WSOC (23%), SO4(2-) (18%) and NH4+ (19%), while trace metals and remaining cations and anions accounted for less than 1% of solute mass. The main dissolved trace metals in fog droplets are Zn, Al and Fe, while the main metallic cations are Na and Ca. Fe and Al are the only metals preferentially distributed in the suspended insoluble matter of fog droplets (partitioning ratio respectively 37% and 33%). All other metals are mostly dissolved in the aqueous phase (mean partitioning ratios of Mg, Pb, Zn, Cu and Na are 69%, 70%, 77%, 81% and 87%). These findings are in agreement with literature data on metal speciation in cloud and rain samples. The dependence of partitioning ratios on pH is investigated for the different metals, with only Al showing a clear partitioning ratio decrease with increasing pH. Conversely, the other metals show no dependence or a complex and highly variable behaviour. The partitioning ratio of iron (mean 37%) observed in the Po Valley fog samples is much higher than the water extractable iron in aerosol particles (typically 1-2 %): this fact can be explained by differences in the aerosol sources and composition among sites and by chemical processes in the aqueous phase, such as complexation and redox reactions involving organic ligands (oxalate, or other organic acids as humic-like organic matter) which may promote Fe solubility. 相似文献
24.
Heterogenous photocatalysis is regarded as a holy grail in relation to the energy and environmental issues with which our society is currently struggling. In this context, the characterization of titanium dioxide nanostructures and the relationships between structural/electronic parameters and chemical/physical–chemical properties is a primary target, whose achievement is in high demand. Theoretical simulations can strongly support experiments to reach this goal. While the bulk and surface properties of TiO2 materials are quite well understood, the field of nanostructures still presents a few unexplored areas. Here we consider possible approaches for the modeling of reduced and extended TiO2 nanostructures, and we review the main outcomes of the investigation of the structural, electronic, and optical properties of TiO2 nanoparticles and their relationships with the size, morphology, and shape of the particles. Further investigations are highly desired to fill the gaps still remaining and to allow improvements in the efficiencies of these materials for photocatalytic and photovoltaic applications.The latest findings from theoretical investigations into TiO2 nanoparticles are reviewed, including both realistic models from a bottom-up approach (1–3 nm diameter) and cut from bulk models (>3 nm diameter) in a top-down approach. 相似文献
25.
Giuseppe Micalizzi Filippo Alibrando Federica Vento Emanuela Trovato Mariosimone Zoccali Paolo Guarnaccia Paola Dugo Luigi Mondello 《Molecules (Basel, Switzerland)》2021,26(6)
A microwave distillation method was optimized for the extraction and isolation of cannabis essential oil from fresh and dried hemp inflorescences. The developed method enabled us to obtain a distilled product rich in terpenes and terpenoid compounds, responsible of the typical and unique smell of the cannabis plant. The distillate from different hemp cultivars, including Kompolti, Futura 75, Carmagnola, Felina 32 and Finola were characterized by using a gas chromatograph equipped with both mass spectrometer and flame ionization detectors. In a single chromatographic run, the identity and absolute amounts of distilled compounds were determined. Peak assignment was established using a reliable approach based on the usage of two identification parameters, named reverse match, and linear retention index filter. Absolute quantification (mg g−1) of the analytes was performed using an internal standard method applying the flame ionization detector (FID) response factors according to each chemical family. An enantio-GC-MS method was also developed in order to evaluate the enantiomeric distribution of chiral compounds, an analytical approach commonly utilized for establishing the authenticity of suspicious samples. 相似文献
26.
27.
Filippo Mangani Michela Maione Luciano Lattanzi Jgor Arduini 《International journal of environmental analytical chemistry》2013,93(4):273-282
Abstract Chlorofluorocarbons are man-made long lasting atmospheric pollutants of great environmental concern, responsible for important global change phenomena. Recently, they were replaced by hydrogenated halocarbons that, even if less persistent, do not lack in environmental impact. Atmospheric concentrations of these compounds were measured in Antarctica by gas chromatography. The extremely low atmospheric mixing ratios of these compounds require a pre-concentration step of the air sample on suitable adsorbent in order to meet the sensitivity of the analytical method Results obtained analyzing air samples collected in Antarctica since 1988 for the determination of CFC-12 and CFC-11 are reported, together with data concerning the less abundant species. 相似文献
28.
We give sufficient conditions for local solutions to some fourth order semilinear ordinary differential equations to blow up in finite time with wide oscillations, a phenomenon not visible for lower order equations. The result is then applied to several classes of semilinear partial differential equations in order to characterize the blow up of solutions including, in particular, its applications to a suspension bridge model. We also give numerical results which describe this oscillating blow up and allow us to suggest several open problems and to formulate some related conjectures. 相似文献
29.
30.
Didier Lesesvre Paul Pegon Filippo Santambrogio 《Set-Valued and Variational Analysis》2013,21(3):541-556
The main result of this paper is the existence of an optimal transport map T between two given measures μ and ν, for a cost which considers the maximal oscillation of T at scale δ, given by ω δ (T) :?=??sup|x???y|?<?δ |T(x)???T(y)|. The minimization of this criterion finds applications in the field of privacy-respectful data transmission. The existence proof unfortunately only works in dimension one and is based on some monotonicity considerations. 相似文献