全文获取类型
收费全文 | 10127篇 |
免费 | 265篇 |
国内免费 | 30篇 |
专业分类
化学 | 7602篇 |
晶体学 | 61篇 |
力学 | 117篇 |
数学 | 1264篇 |
物理学 | 1378篇 |
出版年
2020年 | 117篇 |
2019年 | 88篇 |
2016年 | 182篇 |
2015年 | 190篇 |
2014年 | 169篇 |
2013年 | 341篇 |
2012年 | 333篇 |
2011年 | 418篇 |
2010年 | 242篇 |
2009年 | 261篇 |
2008年 | 382篇 |
2007年 | 390篇 |
2006年 | 372篇 |
2005年 | 397篇 |
2004年 | 365篇 |
2003年 | 296篇 |
2002年 | 274篇 |
2001年 | 195篇 |
2000年 | 182篇 |
1999年 | 153篇 |
1998年 | 149篇 |
1997年 | 162篇 |
1996年 | 177篇 |
1995年 | 156篇 |
1994年 | 161篇 |
1993年 | 151篇 |
1992年 | 155篇 |
1991年 | 143篇 |
1990年 | 116篇 |
1989年 | 139篇 |
1988年 | 144篇 |
1987年 | 140篇 |
1986年 | 140篇 |
1985年 | 171篇 |
1984年 | 135篇 |
1983年 | 140篇 |
1982年 | 147篇 |
1981年 | 122篇 |
1980年 | 127篇 |
1979年 | 120篇 |
1978年 | 113篇 |
1977年 | 98篇 |
1976年 | 102篇 |
1974年 | 109篇 |
1973年 | 105篇 |
1971年 | 89篇 |
1961年 | 145篇 |
1960年 | 199篇 |
1959年 | 109篇 |
1958年 | 118篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Pu Duan Bo Zhi Luke Coburn Christy L. Haynes Klaus Schmidt-Rohr 《Magnetic resonance in chemistry : MRC》2020,58(11):1130-1138
The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave-assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid-state nuclear magnetic resonance (NMR) experiments. 13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15N NMR resonances near 80 and 160 ppm proved the presence of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross-peaks in a 1H─13C HETCOR spectrum with brief 1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13C and 15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13C chemical-shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation. 相似文献
3.
4.
5.
Let R be a discrete nonsingular equivalence relation on a standard probability space , and let V be an ergodic strongly asymptotically central automorphism of R. We prove that every V-invariant cocycle with values in a Polish group G takes values in an abelian subgroup of G.
The hypotheses of this result are satisfied, for example, if A is a finite set, a closed, shift-invariant subset, V is the shift, μ a shift-invariant and ergodic probability measure on X, the two-sided tail-equivalence relation on X, a shift-invariant subrelation which is μ-nonsingular, and a shift-invariant cocycle. 相似文献
6.
7.
Summary In this paper a Gauss-Jordan algorithm with column interchanges is presented and analysed. We show that, in contrast with Gaussian elimination, the Gauss-Jordan algorithm has essentially differing properties when using column interchanges instead of row interchanges for improving the numerical stability. For solutions obtained by Gauss-Jordan with column interchanges, a more satisfactory bound for the residual norm can be given. The analysis gives theoretical evidence that the algorithm yields numerical solutions as good as those obtained by Gaussian elimination and that, in most practical situations, the residuals are equally small. This is confirmed by numerical experiments. Moreover, timing experiments on a Cyber 205 vector computer show that the algorithm presented has good vectorisation properties. 相似文献
8.
M. Stangl V. Dittel J. Acker V. Hoffmann W. Gruner S. Strehle K. Wetzig 《Applied Surface Science》2005,252(1):158-161
At room temperature electroplated copper layers exhibit changes in resistivity, residual stress, and microstructure. This process, known as self-annealing, is intimately linked to the release of organic impurities, which stem from the incorporation of organic additives into the Cu layer in the course of the electroplating process. The behavior of these impurities during self-annealing, represented by the carbon content, could be detected by analytical radio frequency glow discharge optical emission spectrometry (GD-OES) and carrier gas hot extraction (CGHE). The precondition of a quantitative determination is a surface cleaning procedure to remove adsorbed organics from the copper surface. It was observed that at first almost all impurities have to leave the Cu metallization before an accelerated abnormal grain growth can start. The small amount of remaining organic species after self-annealing could be quantified by both examination techniques, GD-OES and CGHE. 相似文献
9.
10.