全文获取类型
收费全文 | 529篇 |
免费 | 3篇 |
国内免费 | 6篇 |
专业分类
化学 | 371篇 |
晶体学 | 15篇 |
力学 | 4篇 |
数学 | 92篇 |
物理学 | 56篇 |
出版年
2021年 | 3篇 |
2020年 | 7篇 |
2019年 | 4篇 |
2018年 | 5篇 |
2017年 | 3篇 |
2016年 | 5篇 |
2015年 | 4篇 |
2014年 | 11篇 |
2013年 | 17篇 |
2012年 | 33篇 |
2011年 | 32篇 |
2010年 | 24篇 |
2009年 | 18篇 |
2008年 | 22篇 |
2007年 | 20篇 |
2006年 | 17篇 |
2005年 | 14篇 |
2004年 | 20篇 |
2003年 | 17篇 |
2002年 | 9篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 10篇 |
1996年 | 14篇 |
1995年 | 6篇 |
1994年 | 10篇 |
1993年 | 4篇 |
1992年 | 8篇 |
1991年 | 10篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 9篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1973年 | 10篇 |
1972年 | 7篇 |
排序方式: 共有538条查询结果,搜索用时 31 毫秒
91.
We present a miniature fiber-optic electromagnetic field (EMF) sensor that is capable of simultaneously detecting the amplitude and phase of an EMF in the range of 0.1-6 GHz. We focus on magnetic field measurements, since the H-field is more significant in our target applications due its direct relation to the current. The sensor is based on an open optical platform to which various antennas can be attached and contains a radio-frequency amplifier for signal conditioning and a vertical-cavity surface-emitting laser as an electro-optic converter. The millimeter size and the full electrical isolation of the sensor allow EMF detection with minimal disturbance. We have characterized the sensor in the near field of a lambda/2 dipole, a rectangular waveguide, and a microstrip line, and we explain the experimental results with a simple theoretical model confirming the mapped near-field distribution of the investigated field source. 相似文献
92.
Shahid Ullah Tomas Alsberg Robin Vestergren Urs Berger 《Analytical and bioanalytical chemistry》2012,404(8):2193-2201
A sensitive and accurate method was developed and validated for simultaneous analysis of perfluoroalkyl carboxylic acids, sulfonic acids, and phosphonic acids (PFPAs) at low picograms per gram concentrations in a variety of food matrices. The method employed extraction with acetonitrile/water and cleanup on a mixed-mode co-polymeric sorbent (C8?+?quaternary amine) using solid-phase extraction. High-performance liquid chromatographic separation was achieved on a C18 column using a mobile phase gradient containing 5?mM 1-methyl piperidine for optimal chromatographic resolution of PFPAs. A quadrupole time-of-flight high-resolution mass spectrometer operating in negative ion mode was used as detector. Method detection limits were in the range of 0.002 to 0.02?ng?g?1 for all analytes. Sample preparation (extraction and cleanup) recoveries at a spiking level of 0.1?ng?g?1 to a baby food composite were in the range of 59 to 98?%. A strong matrix effect was observed in the analysis of PFPAs in food extracts, which was tentatively assigned to sorption of PFPAs to the injection vial in the solvent-based calibration standard. The method was successfully applied to a range of different food matrices including duplicate diet samples, vegetables, meat, and fish samples. Figure
Extracted high-resolution mass chromatograms of a PFPAs spiked at 0.06 ng g?–1 to baby food, b PFSAs spiked at 0.02 ng g?–1 to baby food, and c PFCAs spiked at 0.02 ng g?–1 to baby food 相似文献
93.
J Streuff M Feurer P Bichovski G Frey U Gellrich 《Angewandte Chemie (International ed. in English)》2012,51(34):8661-8664
Reduction, please! The title reaction affords α-hydroxyketones, a common structural motif in biologically active natural products, in good yields and high enantioselectivities at room temperature. The commercially available ansa-titanocene 1 was found to be an efficient catalyst for this process, which presumably proceeds by addition of a ketyl radical to a nitrile. 相似文献
94.
Silvio Dutz M.E. Hayden Allison Schaap Boris Stoeber Urs O. Häfeli 《Journal of magnetism and magnetic materials》2012
Magnetic microspheres (MMS) are useful tools for a variety of medical and pharmaceutical applications. Typically, commercially manufactured MMS exhibit broad size distributions. This polydispersity is problematic for many applications. Since the direct synthesis of monodisperse MMS is often fraught with technical challenges, there is considerable interest in and need associated with the development of techniques for size-dependent fractionation of MMS. In this study we demonstrated continuous size-dependent fractionation of sub-micron scale particles driven by secondary (Dean effect) flows in curved microfluidic channels. Our goal was to demonstrate that such techniques can be applied to MMS containing superparamagnetic nanoparticles. To achieve this goal, we developed and tested a microfluidic chip for continuous MMS fractionation. Our data address two key areas. First, the densities of MMS are typically in the range 1.5–2.5 g/cm3, and thus they tend be non-neutrally buoyant. Our data demonstrate that efficient size-dependent fractionation of MMS entrained in water (density 1 g/cm3) is possible and is not significantly influenced by the density mismatch. In this context we show that a mixture comprising two different monodisperse MMS components can be separated into its constituent parts with 100% and 88% success for the larger and smaller particles, respectively. Similarly, we show that a suspension of polydisperse MMS can be separated into streams containing particles with different mean diameters. Second, our data demonstrate that efficient size-dependent fractionation of MMS is not impeded by magnetic interactions between particles, even under application of homogeneous magnetic fields as large as 35 kA/m. The chip is thus suitable for the separation of different particle fractions in a continuous process and the size fractions can be chosen simply by adjusting the flow velocity of the carrier fluid. These facts open the door to size dependent fractionation of MMS. 相似文献
95.
In this article we consider a variant of Rabinowitz Floer homology in order to define a homological count of discriminant points for paths of contactomorphisms. The growth rate of this count can be seen as an analogue of Givental’s nonlinear Maslov index. As an application we prove a Bott–Samelson type obstruction theorem for positive loops of contactomorphisms. 相似文献
96.
E. González-Peñas M. López-Alvarez F. Martínez de Narvajas A. Ursúa 《Chromatographia》2000,52(3-4):245-248
Summary A gas chromatographic assay has been developed for the simultaneous quantitative determination of turpentine, camphor, methol
and methyl salicylate in a topical formualtion (Dologex?). Eucalyptol was used as the internal standard. Compounds were separated on a cross-linked 5% phenyl polydimethylsiloxane
capillary column and detected by FID. The method was validated on the basis of selectivity, recovery, linearity, precision
and accuracy. Calibration graphs were linear for each analyte in the range studied. Intra and inter-day precision (coefficient
of variation,CV) did not exceed 2%. The applicability of the method was demonstrated in a stability study of these compounds in samples of
the formulation which had been maintained under different temperature conditions for six months. 相似文献
97.
Urs Achim Wiedemann 《Nuclear Physics A》2001,690(4):607-751
We study the gluon radiation spectrum off a hard in-medium produced quark in the multiple soft-rescattering formalism of Baier–Dokshitzer–Mueller–Peigné–Schiff and of Zakharov (BDMPS-Z). Its dependence on the quark and gluon energy, on the gluon transverse momentum, on the in-medium pathlength and on the rescattering properties of the nuclear medium is analyzed quantitatively. The two components of gluon radiation, the hard vacuum radiation associated to the quark production vertex, and the medium-induced rescattering contribution interfere destructively. For small spatial extensions of the medium, this destructive interference overcompensates the hard vacuum radiation, and the total medium-induced radiative energy loss decreases as ΔE∝− L3. Medium-induced gluon production dominates only above a finite critical length L>Lcrit which varies between 3 and more than 6 fm depending on the rescattering properties of the medium. Deviations from the BDMPS-L2-behaviour persist above Lcrit. The medium-dependence of the angular gluon distribution is dominated by transverse brownian k-broadening. This results in a depletion of the low transverse momentum part of both the hard and the medium-induced contribution. As a consequence, the medium-induced energy loss outside a finite angular cone size Θ can be more than a factor two larger than the total medium-induced radiative energy loss. We discuss implications of these results for the jet quenching signal in relativistic heavy-ion collisions at RHIC and LHC. 相似文献
98.
Urs T. Hartl 《manuscripta mathematica》2003,110(3):365-380
Let R be a complete discrete valuation ring with field of fractions K and let X
K
be a smooth, quasi-compact rigid-analytic space over Sp K. We show that there exists a finite separable field extension K' of K, a rigid-analytic space X'
K'
over Sp K' having a strictly semi-stable formal model over the ring of integers of K', and an étale, surjective morphism f : X'
K'
→X
K
of rigid-analytic spaces over Sp K. This is different from the alteration result of A.J. de Jong [dJ] who does not obtain that f is étale. To achieve this property we have to work locally on X
K
, i.e. our f is not proper and hence not an alteration.
Received: 26 October 2001 / Revised version: 14 August 2002 Published online: 14 February 2003 相似文献
99.
100.
Urs Wuthier Hng Viêt Pham Ern Pretsch Daniel Ammann Albert K. Beck Dieter Seebach Wilhelm Simon 《Helvetica chimica acta》1985,68(7):1822-1827
Anion-Selectivity of Distannyl Derivatives in Membranes A series of distannyl derivatives (2,2-bis(trimethylstannyl)-1,3-dithiane, 2,2-bis(tributylstannyl)-1,3-dithiane, hexamethyldistannane, hexabutyldistannane, hexaphenyldistannane, bis(triphenylstannyl)sulfide, o-bis(trimethylstannyl)benzene) has been studied in view of their applicability as anion-selective ionophores in solvent polymeric membranes. None of these compounds induces significant changes in the anion-selectivity pattern as compared with the membranes containing no organotin compound. Representatives with tributylstannyl groups, however, undergo chemical reactions leading to highly active anion ionophores of the type Bu3SnX, several of which (e.g. Bu3SnCl and Bu3SnOH) may be present in equilibrium in the membrane phase depending on the measuring conditions. 相似文献