全文获取类型
收费全文 | 1998篇 |
免费 | 143篇 |
国内免费 | 84篇 |
专业分类
化学 | 697篇 |
晶体学 | 3篇 |
力学 | 127篇 |
综合类 | 41篇 |
数学 | 876篇 |
物理学 | 481篇 |
出版年
2024年 | 3篇 |
2023年 | 25篇 |
2022年 | 67篇 |
2021年 | 42篇 |
2020年 | 68篇 |
2019年 | 62篇 |
2018年 | 31篇 |
2017年 | 59篇 |
2016年 | 83篇 |
2015年 | 67篇 |
2014年 | 95篇 |
2013年 | 163篇 |
2012年 | 97篇 |
2011年 | 102篇 |
2010年 | 84篇 |
2009年 | 103篇 |
2008年 | 121篇 |
2007年 | 120篇 |
2006年 | 79篇 |
2005年 | 77篇 |
2004年 | 78篇 |
2003年 | 60篇 |
2002年 | 64篇 |
2001年 | 62篇 |
2000年 | 37篇 |
1999年 | 38篇 |
1998年 | 47篇 |
1997年 | 41篇 |
1996年 | 25篇 |
1995年 | 19篇 |
1994年 | 32篇 |
1993年 | 24篇 |
1992年 | 16篇 |
1991年 | 14篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 17篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1980年 | 2篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有2225条查询结果,搜索用时 15 毫秒
161.
WANG WeiZhen 《中国科学 数学(英文版)》2010,(4)
It is common in statistical practice that one needs to make a choice among m + 1 mutually exclusive claims on distributions.When m=1,it is done by the (traditional) hypothesis test.In this paper,a generalization to the case m > 1 is proposed.The fundamental difference with the case m=1 is that the new alternative hypothesis is a partition of m multiple claims and is data-dependent.Data is used to decide which claim in the partition is to be tested as the alternative.Thus,a random alternative is involved.The... 相似文献
162.
163.
Francisco A. Macías José O. Guerra Ana M. Simonet Andy J. Pérez Clara Nogueiras 《Magnetic resonance in chemistry : MRC》2010,48(5):350-355
A careful NMR analysis, especially by 1D TOCSY and 1D ROESY, of a refined saponin fraction allowed us to determine the structure of three saponins from a polar extract of Agave brittoniana Trel. spp. Brachypus leaves. The use of 1D DOSY for the suppression of the solvent signal was useful to obtain the chemical shifts of anomeric signals. A full assignment of the 1H and 13C spectral data for the new saponins, agabrittonosides E–F (1–2) and the well‐known Karatavioside C (3) and their methoxyl derivatives, is reported. The structures were established using a combination of 1D and 2D (1H, 1H‐COSY, TOCSY, ROESY, g‐HSQC, g‐HMBC and g‐HSQC‐TOCSY) NMR techniques and ESI–MS. In addition, the methoxylation of these furostane saponins in the presence of MeOH was studied. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
164.
It is widely accepted that liquid water structure is comprised of two closely interweaved components; i.e. tetrahedral (low density) and hexagonal (high density) structures. The relative amount of these components is temperature and pressure dependent. We propose an order parameter, based on the radial distribution function, that quantifies the relative structural composition at any defined temperature and pressure, thus establishing the crossover point in structural dominance. At 300 K this point lies close to 2 kbar, pressure at which water looses most of its ‘anomalous’ properties. 相似文献
165.
Raji Heyrovska 《Electroanalysis》2010,22(9):903-907
Aqueous standard potentials, referred to that of the SHE as zero, were recently shown to vary linearly with gaseous ionization potentials, with the absolute potential of SHE as the intercept. This enabled arriving at the absolute redox potentials of elements. Here, the distances between the oxidized and reduced forms in aqueous solutions have been evaluated. From the linear dependence of these distances on the covalent radii of atoms, the radii of the redox components have been obtained. The latter also vary linearly with the aqueous ionic radii estimated earlier from ion‐water distances, and indicate the presence of aqueous molecular anions. 相似文献
166.
Chirped fibre Bragg grating based multiplexer and demultiplexer for DWDM applications 总被引:1,自引:0,他引:1
R. Romero O. Frazo F. Floreani L. Zhang P.V.S. Marques H.M. Salgado 《Optics and Lasers in Engineering》2005,43(9):987-994
A multiplexer/demultiplexer for 100 GHz channel spacing based on chirped fibre Bragg gratings with different bandwidths and optical circulators is presented. The spectral characteristics, specifications and operation of these passive devices are described, showing its potential use in dense wavelength division multiplexing (DWDM) applications. 相似文献
167.
Marco D. R. Gomes da Silva Higuinaldo J. Chaves das Neves 《Journal of separation science》1997,20(5):275-283
The aroma composition of three different Fragaria anannassa cul-tivars Selva, Chandler, and Osso Grande) is studied by purge-and-trap HRGC and the components identified by MRGC-MS and HRGC-FTIR. The chromatographic aroma profiles from four different maturation stages in each variety are compared and the similarities accessed by principal component analysis (PCA). The contents in n-hexanal and hex-2(Z)-enal are characteristic features of the green stage in the three varieties where the content differences in methyl, ethyl, and butyl acetates act as typifying variables among varieties. Ethyl acetate acts as an important differentiation variable for the Selva variety. In the mature stage, the three cultivars may be distinguished from each other through their relative contents of methyl and ethyl 2-methylbutyrate (Selva), methyl 2-methyl-butyrate, methyl butyrate (Chandler), methyl and ethyl butyrates, and methyl caproate (Osso Grande). 相似文献
168.
Fluorescence spectrum of Dy3+, Dy3+-Ho3+, Tb3+ and Tb3+-Er3+ doped in calibo glass have been studied using Ar+ and excimer lasers. Non-radiative energy transfer from trivalent dysprosium and terbium (donors) to holmium and erbium (acceptors)
respectively has been observed on the basis of decrease in the life time of the levels and reduction in fluorescence intensity
of Dy3+ and Tb3+ on increasing Ho and Er concentrations. The interaction mechanism of donor and acceptor ions is found to be dipole-dipole
in both cases. Various parameters such as donor-acceptor distances, non-radiative energy transfer efficiency (η) and energy transfer probability (P
da) have been computed. Stark splitting have also been marked in several intense transition of the two. 相似文献
169.
170.