全文获取类型
收费全文 | 7835篇 |
免费 | 206篇 |
国内免费 | 25篇 |
专业分类
化学 | 5854篇 |
晶体学 | 38篇 |
力学 | 75篇 |
数学 | 1158篇 |
物理学 | 941篇 |
出版年
2020年 | 78篇 |
2019年 | 74篇 |
2016年 | 149篇 |
2015年 | 150篇 |
2014年 | 116篇 |
2013年 | 276篇 |
2012年 | 250篇 |
2011年 | 333篇 |
2010年 | 188篇 |
2009年 | 202篇 |
2008年 | 301篇 |
2007年 | 317篇 |
2006年 | 279篇 |
2005年 | 312篇 |
2004年 | 295篇 |
2003年 | 234篇 |
2002年 | 213篇 |
2001年 | 117篇 |
2000年 | 113篇 |
1999年 | 111篇 |
1998年 | 116篇 |
1997年 | 134篇 |
1996年 | 134篇 |
1995年 | 131篇 |
1994年 | 116篇 |
1993年 | 129篇 |
1992年 | 100篇 |
1991年 | 101篇 |
1990年 | 85篇 |
1989年 | 108篇 |
1988年 | 115篇 |
1987年 | 105篇 |
1986年 | 109篇 |
1985年 | 138篇 |
1984年 | 110篇 |
1983年 | 108篇 |
1982年 | 113篇 |
1981年 | 102篇 |
1980年 | 111篇 |
1979年 | 95篇 |
1978年 | 90篇 |
1977年 | 85篇 |
1976年 | 85篇 |
1974年 | 96篇 |
1973年 | 90篇 |
1971年 | 74篇 |
1961年 | 141篇 |
1960年 | 195篇 |
1959年 | 102篇 |
1958年 | 116篇 |
排序方式: 共有8066条查询结果,搜索用时 328 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.
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
5.
6.
U. Kunze 《Zeitschrift für Physik B Condensed Matter》1989,76(4):463-472
Metal/insulator/semiconductor junctions are prepared on degeneratep-type InAs substrates with hole concentrations ranging from 2.3×1017 cm–3 to 2.7×1018 cm–3. The low work function of the top metal Yb, Al, or Au and charged interface states influence a two-dimensional (2D) electron inversion layer at the InAs surface. The insulator barrier that is formed by thermal oxidation is designed sufficiently thin, so that the bias voltage applied at the metal electrode mainly drops across the depletion layer separating the electron channel from the bulk. The current-voltage (I–V) characteristics exhibit strong negative differential conductance due to interband, tunneling from the 2D subband into the 3D valence band with peak-to-valley current ratios up to 3.1, 18, and 32 at 300 K, 77 K, and 4.2 K, respectively. In agreement with a theoretical model based on coherentelastic tunneling, the form of the I–V curves resembles those of double-barrier resonant tunnel devices rather than those of 3D Esaki diodes. The series resistance is obtained from the saturation of the differential conductance dI/dV at high forward bias and from the shift of structures in d2
I/dV
2 arising from phonon assisted tunneling.Dedicated to G. Lautz on the occasion of his 65th birthday 相似文献
7.
8.
9.
JPC – Journal of Planar Chromatography – Modern TLC - This paper reports the efficiency of the C30 phase for HPLC and TLC separation of a mixture of tocopherol homologs and... 相似文献
10.
Simplicial complexes X provide commutative rings A(X) via the Stanley- Reisner construction. We calculated the cotangent cohomology, i.e., T1 and T2 of A(X) in terms of X. These modules provide information about the deformation theory of the algebro geometric objects assigned to X. 相似文献