全文获取类型
收费全文 | 2708篇 |
免费 | 80篇 |
国内免费 | 6篇 |
专业分类
化学 | 2184篇 |
晶体学 | 14篇 |
力学 | 22篇 |
数学 | 267篇 |
物理学 | 307篇 |
出版年
2023年 | 24篇 |
2022年 | 76篇 |
2021年 | 80篇 |
2020年 | 60篇 |
2019年 | 54篇 |
2018年 | 28篇 |
2017年 | 30篇 |
2016年 | 84篇 |
2015年 | 88篇 |
2014年 | 92篇 |
2013年 | 152篇 |
2012年 | 174篇 |
2011年 | 202篇 |
2010年 | 117篇 |
2009年 | 113篇 |
2008年 | 198篇 |
2007年 | 201篇 |
2006年 | 164篇 |
2005年 | 153篇 |
2004年 | 106篇 |
2003年 | 110篇 |
2002年 | 69篇 |
2001年 | 27篇 |
2000年 | 39篇 |
1999年 | 26篇 |
1998年 | 14篇 |
1997年 | 19篇 |
1996年 | 24篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 23篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1973年 | 7篇 |
1969年 | 5篇 |
1968年 | 8篇 |
1967年 | 7篇 |
1966年 | 5篇 |
排序方式: 共有2794条查询结果,搜索用时 15 毫秒
171.
Photo-excited arenes have been found to initiate a substitution reaction in which the sulfonyloxy group of α-sulfonyloxyketones is displaced, and the corresponding β-arylketones are formed. Evidence is presented that the electronically excited arene need not be involved directly in the substitution step. It would seem to be more likely that upon interaction with the sulfonyloxyketone a reactive species derived from the latter is produced which is capable of aromatic substitution. 相似文献
172.
Chitosan (CTS), a biocompatible, biodegradable, non-toxic polymer, dissolves in water only if pH is lowered under 6.5, when a substantial fraction of the amino groups is protonated. Its range of application has been much extended by partially depolymerising it or converting it to water-soluble derivatives. Working under high-intensity ultrasound at 17.8-18.5 kHz, using either a simple horn or a cup horn, we achieved a controlled depolymerization of CTS, also prepared in high yields several derivatives that can be useful intermediates for further chemical modification, as well as several water-soluble derivatives that lend themselves to a host of industrial applications. Compared to conventional methods, all these reactions went to completion in considerably shorter times at lower temperatures. 相似文献
173.
Bark T von Zelewsky A Rappoport D Neuburger M Schaffner S Lacour J Jodry J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(19):4839-4845
The hexadentate, and ditopic ligand 2,5-bis([2,2']bipyridin-6-yl)pyrazine yields a chiral, tetrameric, square-shaped, self-assembled species upon complexation with Fe2+ ions. The racemate of this complex was resolved with antimonyl tatrate as the chiral auxiliary. The purity of the enantiomer was determined by NMR spectroscopy, by using a chiral, diamagnetic shift reagent, and by circular dichroism (CD). The CD spectrum was also calculated by time-dependent density functional theory, and the correlation that was found between CD spectrum and configuration was confirmed by X-ray cristallography. When a "chiralised" version of the ligand was used instead, the corresponding iron complex was obtained in diastereomerically pure form. 相似文献
174.
Silvia Bertoluzza. 《Mathematics of Computation》2004,73(246):659-689
We study a class of preconditioners based on substructuring, for the discrete Steklov-Poincaré operator arising in the three fields formulation of domain decomposition in two dimensions. Under extremely general assumptions on the discretization spaces involved, an upper bound is provided on the condition number of the preconditioned system, which is shown to grow at most as ( and denoting, respectively, the diameter and the discretization mesh-size of the subdomains). Extensive numerical tests--performed on both a plain and a stabilized version of the method--confirm the optimality of such bound.
175.
Recent progress in somatic embryogenesis from selected mature trees of Quercus suber, has led to a demand for maintenance of a large number of selected embryogenic lines. To facilitate the management of this material a protocol for the long-term storage of this germplasm should be defined. This study reports on the use of a simple vitrification procedure for the successful cryopreservation of three cork oak embryogenic lines. High embryo recovery levels (88-93 percent) were obtained by first preculturing 2-4 mg clumps of two or three globular embryos on semisolid medium containing 0.3 M sucrose for three days, followed by incubation in PVS2 vitrification solution at 0 degree C for 60 min before direct immersion in liquid nitrogen. The mean number of embryos produced per explant was significantly greater for cryostored embryos than for untreated stock cultures, but the productivity of the latter was recovered in subsequent subcultures of the material produced by cryostored embryos. The germination and plant regeneration rates achieved by cultures derived from cryostored embryos, around 60 percent, were similar to those of non-cryopreserved stock cultures. 相似文献
176.
We present a novel model for calculating the bit error rate in optical communication systems in the case of on-off keying intensity modulation and optically preamplified direct detection. The model accounts for the intersymbol interference and is based on the Laguerre photon-count probability distribution predicted by photodetection theory. For a non-return-to-zero modulation format an accurate value of the sensitivity for a quantum-limited receiver of 33.9 photons/bit is obtained. 相似文献
177.
A mutant of aristolochene synthase, in which Phe 178 was replaced by Val, produced significant amounts of alpha-and beta-farnesene as well as alpha and beta-selinene and selina-4,11-diene, suggesting that Phe 178 is involved in the stabilisation of transition states preceding germacrene A and following eudesmane cation. 相似文献
178.
Szimtenings M Olt S Haase A 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,161(1):70-76
An NMR flow quantification technique applicable to metabolite flow in plants is presented. It combines flow sensitive magnetization preparation with slice selective spectroscopy. Flow encoded NMR spectroscopy is described to quantify, for the first time, flow velocities of metabolites in plants non-invasively. Flow sensitivity is introduced by magnetization preparation based on a stimulated echo experiment, prior to slice selective spectroscopy. For flow quantification eight different flow-weighted spectra are collected. With this flow preparation very slow flow velocities down to 0.1mm/s can be detected and small amounts of flowing metabolites can be observed despite the large background signal of stationary and flowing water. Important sequence optimization steps include appropriate choice of experimental parameters used for flow encoding as well as complete balancing of eddy currents from the flow encoding gradients. The method was validated in phantom experiments and applied in vivo. Examples of quantitative flow measurements of water and metabolites in phantoms and plants are provided to demonstrate the reliability and the performance of flow encoded spectroscopy. 相似文献
179.
180.
The study of the transport mechanisms connected to solid-liquid interactions is fundamental in the determination of the extension of the pollution of a site and in the evaluation of the best remediation process to be applied. The sorption of hydrophobic ionizable organic contaminants from the groundwaters is supervised not only by the physico-chemical properties of soil and pollutants, but also by the groundwaters pH, which deeply influences their solubility in the aqueous media, and consequently their transport mechanisms in the aquifer. In this work an experimental study of the sorption of phenol and 2-nitrophenol on two soils, different in particle-size distribution, CEC and organic carbon content, was realized. The sorption potential of the soils was evaluated by means of a physical, mineralogical and chemical characterization. The experimental data coming from some batch tests, performed at pH values equal to 4, 7 and 10 were fitted by means of linear, Freundlich and Langmuir isotherms models. The soil-contaminants interaction mechanisms that influence the isotherms shapes were then analyzed and discussed, and a comparison between the theoretical and experimental values of the partitioning coefficient KD was performed. 相似文献