全文获取类型
收费全文 | 668篇 |
免费 | 33篇 |
国内免费 | 1篇 |
专业分类
化学 | 439篇 |
晶体学 | 3篇 |
力学 | 12篇 |
数学 | 127篇 |
物理学 | 121篇 |
出版年
2023年 | 12篇 |
2022年 | 7篇 |
2021年 | 26篇 |
2020年 | 19篇 |
2019年 | 18篇 |
2018年 | 10篇 |
2017年 | 6篇 |
2016年 | 34篇 |
2015年 | 27篇 |
2014年 | 27篇 |
2013年 | 32篇 |
2012年 | 56篇 |
2011年 | 57篇 |
2010年 | 31篇 |
2009年 | 26篇 |
2008年 | 49篇 |
2007年 | 40篇 |
2006年 | 41篇 |
2005年 | 35篇 |
2004年 | 24篇 |
2003年 | 17篇 |
2002年 | 21篇 |
2001年 | 8篇 |
2000年 | 10篇 |
1999年 | 3篇 |
1998年 | 9篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 2篇 |
排序方式: 共有702条查询结果,搜索用时 31 毫秒
631.
Eichner S Eichner T Floss HG Fohrer J Hofer E Sasse F Zeilinger C Kirschning A 《Journal of the American Chemical Society》2012,134(3):1673-1679
The amide synthase of the geldanamycin producer, Streptomyces hygroscopicus, shows a broader chemoselectivity than the corresponding amide synthase present in Actinosynnema pretiosum, the producer of the highly cytotoxic ansamycin antibiotics, the ansamitocins. This was demonstrated when blocked mutants of both strains incapable of biosynthesizing 3-amino-5-hydroxybenzoic acid (AHBA), the polyketide synthase starter unit of both natural products, were supplemented with 3-amino-5-hydroxymethylbenzoic acid instead. Unlike the ansamitocin producer A. pretiosum, S. hygroscopicus processed this modified starter unit not only to the expected 19-membered macrolactams but also to ring enlarged 20-membered macrolactones. The former mutaproducts revealed the sequence of transformations catalyzed by the post-PKS tailoring enzymes in geldanamycin biosynthesis. The unprecedented formation of the macrolactones together with molecular modeling studies shed light on the mode of action of the amide synthase responsible for macrocyclization. Obviously, the 3-hydroxymethyl substituent shows similar reactivity and accessibility toward C-1 of the seco-acid as the arylamino group, while phenolic hydroxyl groups lack this propensity to act as nucleophiles in the macrocyclization. The promiscuity of the amide synthase of S. hygroscopicus was further demonstrated by successful feeding of four other m-hydroxymethylbenzoic acids, leading to formation of the expected 20-membered macrocycles. Good to moderate antiproliferative activities were encountered for three of the five new geldanamycin derivatives, which matched well with a competition assay for Hsp90α. 相似文献
632.
Salinas M Jäger CM Amin AY Dral PO Meyer-Friedrichsen T Hirsch A Clark T Halik M 《Journal of the American Chemical Society》2012,134(30):12648-12652
We report a quantitative study that describes and correlates the threshold voltage of low-voltage organic field-effect transistors with the molecular structure of self-assembled monolayer dielectrics. We have observed that the component of the dipole moment of such self-assembled molecules perpendicular to the surface correlates linearly with the threshold voltage shift in devices. The model was validated using three different organic semiconductors (pentacene, α,α'-dihexylsexithiophene, and fullerene-C(60)) on six different self-assembled monolayers. The correlation found can help optimize future devices, by tuning the dipole moments of the molecules that constitute the self-assembled monolayer. 相似文献
633.
Simulations of acoustic wave propagation in time-domain are presented. In the simulations, the discontinuous Galerkin method for spatial derivatives and the low-storage Runge–Kutta approach for time derivatives are used. Three different simulation cases are studied. First, the directivity of loudspeaker is simulated. In the second case, acoustic wave propagation in free space is studied using a short pulse. In the last case, acoustic wave scattering from a metallic cylinder is simulated. All simulation results are compared with measurement results. The measurements for the acoustic wave scattering from the metallic cylinder are made in 2D planes using an automated measurement system. Comparison between the simulation and measurement results are made both temporally and spatially and a good agreement between the simulation and measurement results is found. The results suggest that the discontinuous Galerkin method coupled with the low-storage Runge–Kutta approach is a viable tool for modeling acoustic wave propagation in the time-domain. 相似文献
634.
635.
We present an optofluidic nonlinear coupler fabricated by selective filling of two strands of a photonic crystal fiber with the liquid CCl4 which exhibits a large ultrafast Kerr nonlinearity. We demonstrate power dependent switching in this novel optofluidic device. The large thermo-optical effect of liquids enables us to tune the behavior of the nonlinear coupler by changing the coupling strength with temperature. This opens the road towards flexible designs and realization of a new class of tunable ultrafast nonlinear couplers with switching times below 1 ps. 相似文献
636.
We analyse the non-perturbative superpotential due to M5-brane instantons in F-theory compactifications on Calabi–Yau fourfolds. The M5 partition function is obtained via holomorphic factorisation by explicitly performing the sum over chiral 3-form fluxes. Comparison with the partition function of fluxed Euclidean D3-brane instantons in Type IIB orientifolds allows us to fix the spin structure on the intermediate Jacobian of the M5-instanton. We furthermore analyse the contribution of the M5-instanton to the superpotential in the presence of G4 gauge flux, where the superpotential is dressed with matter fields. We explicitly evaluate the pullback of G4 onto the M5-brane as a measure for the presence of charged instanton zero modes. This accounts for the M5 charge both under massless U(1)s, if present, and under what corresponds in Type II language to geometrically massive U(1)s. 相似文献
637.
Timo?SchultzEmail authorView authors OrcID profile 《Calculus of Variations and Partial Differential Equations》2018,57(5):139
We introduce a more restrictive version of the strict \(CD(K,\infty )\) -condition, the so-called very strict \(CD(K,\infty )\) -condition, and show the existence of optimal maps in very strict \(CD(K,\infty )\) -spaces despite the possible lack of uniqueness of optimal plans. 相似文献
638.
Heiko Dartsch Christian Tessarek Timo Aschenbrenner Stephan Figge Carsten Kruse Marco Schowalter Andreas Rosenauer Detlef Hommel 《Journal of Crystal Growth》2011,320(1):108-112
Thermodynamic analyses of halide vapor phase epitaxy (HVPE) for the growth of ZnO were conducted to investigate the effects of growth conditions against growth rates. The partial pressures of gaseous species in equilibrium with ZnO and the resultant driving force for ZnO deposition are calculated with respect to temperatures, input H2 pressures and input VI/II ratios. It has been revealed that the driving force is weakly dependent on the temperature and is still positive even at 1200 °C, while it greatly decreases at higher temperatures in the presence of H2. The driving force is also significantly influenced when the VI/II ratio is below 1000. The experimental growth rates substantially agree with the dispositions of the driving force expected from the thermodynamic analyses. 相似文献
639.
Timo J. Tolmunen Antti V. Räisänen 《International Journal of Infrared and Millimeter Waves》1987,8(10):1313-1336
The efficiency of millimeter wave doublers with a wide tunable bandwidth was studied. The efficiency depends on the varactor parameters and the embedding impedances seen by the diode at fundamental and harmonic frequencies. Millimeter wave doublers were simulated with a nonlinear analysis program to find optimum embedding impedances for a given diode. Also the sensitivity of the efficiency to various diode and circuit parameters was evaluated. A scaled model was constructed in order to experimentally optimize the impedances. For experimental verification a doubler from 40–58 GHz to 80–116 GHz was constructed. The highest efficiency measured was 45% at 94 GHz with 5 mW input power. The highest efficiency obtained with 20 mW input power was 38%. 相似文献