全文获取类型
收费全文 | 1944篇 |
免费 | 49篇 |
国内免费 | 15篇 |
专业分类
化学 | 1327篇 |
晶体学 | 8篇 |
力学 | 42篇 |
数学 | 338篇 |
物理学 | 293篇 |
出版年
2021年 | 16篇 |
2020年 | 23篇 |
2019年 | 10篇 |
2018年 | 18篇 |
2017年 | 23篇 |
2016年 | 31篇 |
2015年 | 37篇 |
2014年 | 44篇 |
2013年 | 72篇 |
2012年 | 110篇 |
2011年 | 142篇 |
2010年 | 53篇 |
2009年 | 49篇 |
2008年 | 113篇 |
2007年 | 91篇 |
2006年 | 104篇 |
2005年 | 113篇 |
2004年 | 97篇 |
2003年 | 80篇 |
2002年 | 76篇 |
2001年 | 27篇 |
2000年 | 27篇 |
1999年 | 30篇 |
1998年 | 33篇 |
1997年 | 22篇 |
1996年 | 27篇 |
1995年 | 22篇 |
1994年 | 31篇 |
1993年 | 31篇 |
1992年 | 24篇 |
1991年 | 18篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 28篇 |
1984年 | 27篇 |
1983年 | 17篇 |
1982年 | 21篇 |
1981年 | 31篇 |
1980年 | 21篇 |
1979年 | 25篇 |
1978年 | 12篇 |
1977年 | 14篇 |
1976年 | 20篇 |
1975年 | 13篇 |
1974年 | 9篇 |
1973年 | 7篇 |
1972年 | 7篇 |
排序方式: 共有2008条查询结果,搜索用时 250 毫秒
11.
The syntenic distance between two species is the minimum number of fusions, fissions, and translocations required to transform one genome into the other. The linear syntenic distance, a restricted form of this model, has been shown to be close to the syntenic distance. Both models are computationally difficult to compute and have resisted efficient approximation algorithms with non-trivial performance guarantees. In this paper, we prove that many useful properties of syntenic distance carry over to linear syntenic distance. We also give a reduction from the general linear synteny problem to the question of whether a given instance can be solved using the maximum possible number of translocations. Our main contribution is an algorithm exactly computing linear syntenic distance in nested instances of the problem. This is the first polynomial time algorithm exactly solving linear synteny for a non-trivial class of instances. It is based on a novel connection between the syntenic distance and a scheduling problem that has been studied in the operations research literature. 相似文献
12.
A WDM compatible Edge-to-Edge Self-Routed optical packet switched network that simplifies the optical processing is proposed. The system employs all-optical packet label generation and recognition using coded superstructured Fiber Bragg gratings. 相似文献
13.
Summary This paper describes a simplified model which links the sedimentary concentration of selected biomarkers to their export primary production, their stability during transit to the anoxic sediment layer and the sediment bulk sedimentation rate. Manipulation of the biomarker data permits the individual effects of these processes to be separated. Examples demonstrate the use of this strategy in the evaluation of environmentally significant parameters. 相似文献
14.
Jon Samseth Richard J. Spontak Kell Mortensen 《Journal of Polymer Science.Polymer Physics》1993,31(4):467-474
Poly(siloxaneimide) (PSI) segmented copolymers exhibit organized microdomains if the blocks are sufficiently incompatible. As with neat diblock and triblock copolymers, the processing route employed to prepare films of PSI materials is expected to influence the dimensions and/or morphology of the resultant microstructure. In this work, small-angle neutron scattering (SANS) is utilized to characterize the disordered microstructure found in films of a series of PSI copolymers which are subjected to solvent casting and various thermal treatments. Microstructural dimensions such as the periodicity and correlation length are deduced from the Teubner-Strey (TS) model for disordered microemulsions. The scattering intensity of each copolymer up to q = 5.0 nm?1, where q is the scattering vector, is found to scale as q?2.8+?0.1. Results indicate that processing the materials as cast films or as melt-pressed films allowed to cool slowly has a small, but discernible, effect on microstructural characteristics. SANS profiles of films quenched from elevated temperatures reveal a clear transition in microdomain periodicity, which correlates well with the glass transition temperature of the imide microphase in these and other materials of similar chemical structure. © 1993 John Wiley & Sons, Inc. 相似文献
15.
16.
Band broadening related to laminar flow and spray chamber dead volume is a potential problem in flow injection (FI)-inductively coupled plasma-mass spectrometry (ICP-MS). We studied these two dispersion effects with a sheath flow capillary electrophoresis (CE)-ICP-MS interface. A numerical model was used to simulate advection diffusion processes in the CE-capillary and dispersion in the spray chamber. Experimental results of FI with this CE-ICP-MS interface agree well with numerical modeling results. Dispersion due to laminar flow depends strongly on capillary diameter and analyte diffusion coefficient and to a lesser extent on laminar velocity and capillary length and typically amounts to one order of magnitude peak width increase. Three spray chambers of 5, 20 and 150 ml dead volume showed an increase in band broadening and peak tailing with increasing dead volume. The use of standard Scott-type spray chambers (>90 ml volume) increases peak widths by 5-10 s regardless of injection time. The use of a low dead volume spray chamber is recommended for experiments where resolution is critical. The modeling approach can be extended to the coupling of other flow injection techniques, like micro-LC and nano-LC with ICP-MS. 相似文献
17.
Jocelyne Hellou Raymond J. Andersen Shahin Rafii Edward Arnold Jon Clardy 《Tetrahedron letters》1981,22(42):4173-4176
The structure of luteone (), a twenty three carbon terpenoid, has been solved by performing a single crystal x-ray analysis on a 3,4-dinitrophenylhydrazone derivative. 相似文献
18.
EXTRARETINAL PHOTORECEPTION IN CHICKENS: ENTRAINMENT OF THE CIRCADIAN LOCOMOTOR ACTIVITY RHYTHM 总被引:2,自引:0,他引:2
Abstract— The Circadian locomotor activity rhythm of blinded chickens entrained to, or was synchron ized by, light in light dark cycles. 相似文献
19.
The thermodynamic model of inorganic arsenic was validated by comparing the predicted As(III) concentration with the experimentally determined one in several river waters samples of the Basque Country (Spain) collected in two sampling campaigns: spring and autumn 2000. This model takes into account the acid-base equilibria of As(III) and As(V) together with the redox equilibria between the H3AsO3 and H3AsO4 species. A correct prediction of As(III) concentration requires the knowledge of the total concentration of arsenic, pH, redox potential (referred to hydrogen electrode), and ionic strength values of the solution. The estimation of the activity coefficients of the arsenic species was performed by means of the Modified Bromley’s Methodology (MBM).In order to perform the experimental As(III) determination, an analytical method was implemented by using an ion exchange separation of As(III)/As(V) on a continuous FIA-IE-HG-AAS system. The total arsenic concentration was determined together with total concentration of the main alkaline or alkaline-earth metals and anions in the natural waters. Temperature compensated measurements of the pH and redox potentials were made in-situ at the sampling sites.For both seasonal campaigns, the agreement between predicted and experimental As(III) is really high for those samples belonging to non polluted river waters. 相似文献