全文获取类型
收费全文 | 411篇 |
免费 | 23篇 |
国内免费 | 1篇 |
专业分类
化学 | 321篇 |
晶体学 | 1篇 |
力学 | 6篇 |
数学 | 43篇 |
物理学 | 64篇 |
出版年
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 11篇 |
2020年 | 10篇 |
2019年 | 15篇 |
2018年 | 9篇 |
2017年 | 9篇 |
2016年 | 15篇 |
2015年 | 13篇 |
2014年 | 15篇 |
2013年 | 16篇 |
2012年 | 40篇 |
2011年 | 40篇 |
2010年 | 19篇 |
2009年 | 23篇 |
2008年 | 35篇 |
2007年 | 30篇 |
2006年 | 31篇 |
2005年 | 21篇 |
2004年 | 26篇 |
2003年 | 13篇 |
2002年 | 10篇 |
2001年 | 4篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1989年 | 1篇 |
1985年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1932年 | 1篇 |
排序方式: 共有435条查询结果,搜索用时 15 毫秒
31.
32.
Computational method for phase space transport with applications to lobe dynamics and rate of escape
Lobe dynamics and escape from a potential well are general frameworks introduced to study phase space transport in chaotic dynamical systems.While the former approach studies how regions of phase space get transported by reducing the flow to a two-dimensional map, the latter approach studies the phase space structures that lead to critical events by crossing certain barriers. Lobe dynamics describes global transport in terms of lobes, parcels of phase space bounded by stable and unstable invariant manifolds associated to hyperbolic fixed points of the system. Escape from a potential well describes how the critical events occur and quantifies the rate of escape using the flux across the barriers. Both of these frameworks require computation of curves, intersection points, and the area bounded by the curves. We present a theory for classification of intersection points to compute the area bounded between the segments of the curves. This involves the partition of the intersection points into equivalence classes to apply the discrete form of Green’s theorem. We present numerical implementation of the theory, and an alternate method for curves with nontransverse intersections is also presented along with a method to insert points in the curve for densification. 相似文献
33.
Finn ST Strnad JA Barreto PD Fox ME Torres J Sweeney JD Barreto JC 《Photochemistry and photobiology》2011,87(5):1184-1188
We describe a screening methodology that can be used to quickly determine the effectiveness of newly synthesized photocatalysts. We were particularly interested in measuring the destruction of organic molecules painted onto a photocatalytic surface by spraying, with destruction proceeding in ambient air (as a model for airborne toxin destruction). Our method can utilize photocatalysts that are synthesized as powders (such as doped and undoped titanium oxide) and which are then calcined onto a glass substrate disk at 600°C. Herein, we used UV illumination of Aeroxide P-25 TiO(2), but the method is general and can accommodate any region of the light spectrum. 相似文献
34.
Strotman NA Baxter CA Brands KM Cleator E Krska SW Reamer RA Wallace DJ Wright TJ 《Journal of the American Chemical Society》2011,133(21):8362-8371
The first example of an intramolecular asymmetric reductive amination of a dialkyl ketone with an aliphatic amine has been developed for the synthesis of Suvorexant (MK-4305), a potent dual Orexin antagonist under development for the treatment of sleep disorders. This challenging transformation is mediated by a novel Ru-based transfer hydrogenation catalyst that provides the desired diazepane ring in 97% yield and 94.5% ee. Mechanistic studies have revealed that CO(2), produced as a necessary byproduct of this transfer hydrogenation reaction, has pronounced effects on the efficiency of the Ru catalyst, the form of the amine product, and the kinetics of the transformation. A simple kinetic model explains how product inhibition by CO(2) leads to overall first-order kinetics, but yields an apparent zero-order dependence on initial substrate concentration. The deleterious effects of CO(2) on reaction rates and product isolation can be overcome by purging CO(2) from the system. Moreover, the rate of ketone hydrogenation can be greatly accelerated by purging of CO(2) or trapping with nucleophilic secondary amines. 相似文献
35.
36.
A new dialkylthio benzo[1,2-b:4,5-b']dithiophene (S-BDT) was designed and synthesized and the polymer S-PBDT was prepared by a Stille coupling reaction. A high open-circuit voltage (V(oc)) of up to 0.99 V was achieved in polymer solar cells with PCBM. 相似文献
37.
Shane O’Malley 《Tetrahedron letters》2007,48(51):9003-9007
We report both the preparation and sensor evaluation of a new metal-free calixarene bridged binuclear phthalocyanine. 相似文献
38.
Shane A. Dunn 《Experimental Mechanics》1993,33(1):32-36
Stress measurement by means of the thermoelastic technique has been growing in popularity since the mid-1980s. This paper
looks in detail at the effects of thermal conduction on such measurements. It is shown that for accurate quantitative stress
determination, on all but the simplest of geometries, thermal-conduction effects may have to be considered. Such effects can
be experimentally observed and modifications which can be made to an experiment affected by thermal conduction are suggested. 相似文献
39.
D. Zhao J. J. Shane W. Daniell P. G. Harrison D. Goldfarb 《Applied magnetic resonance》1996,10(4):539-557
The role of the Cu(II) in the catalytic oxidation of CO over Cu/SnO2 with low Cu(II) content was studied by continuous wave EPR, electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectroscopes. Three methods were employed for introducing the copper: (i) by coprecipitation, (ii) impregnation onto SnO2 gel and (iii) impregnation onto calcined SnO2. Two types of Cu(II) species were identified in these calcined Cu/SnO2 materials. Those belonging to the first type, termed B and C, exhibit highly resolved EPR spectra with well defined EPR parameters and are located within the bulk of the oxide. The other group comprises a distribution of surface Cu(II) species with unresolved EPR features and are referred to as S. While the latter were readily reduced by CO the former required long exposures at high temperatures (> 673 K). The specific interactions of the different Cu(II) species with CO were investigated through the determination of the13C hyperfine coupling of enriched13CO. The ESEEM spectra of calcined samples, generated either by coprecipitation or impregnation, show after the adsorption of CO signals at the Larmor frequencies of117, 119Sn and13C and at twice these Larmor frequencies. Although these signals indicate that117, 119Sn and13C are in the close vicinity of Cu(II), they cannot provide the hyperfine couplings of these nuclei. This problem was overcome by the application of the HYSCORE experiment. The 2D HYSCORE spectra show well resolved cross peaks which provide the hyperfine interaction of these nuclei. Simulations of the HYSCORE spectra yield for117, 119Sn an isotropic hyperfine constant,a iso, of ±4.0 MHz and an anisotropic component,T ?, of ±2.0 MHz. Pulsed ENDOR spectra also showed117, 119Sn signals which agree with the above values. The13C cross peaks yielda iso=±1.0 MHz andT ?=±2.0 MHz. Similar C cross peaks were observed in spectra of calcined Cu/SnO2 after the adsorption of CO2. Based on the same hyperfine couplings in the samples exposed to13CO and13CO2 the signals were assigned to surface carbonate species generated by part of the Cu(II) S type species rather then by species B and the role of the Cu(II) in the oxidation process is discussed. 相似文献
40.
Motivated by a problem arising in the regenerative analysis of discrete-event system simulation, we ask whether a certain class of random variate generation schemes exists or not. Under very reasonable conditions, we prove that such variate generation schemes do not exist. The implications of this result for regenerative steady-state simulation of discrete-event systems are discussed. 相似文献