全文获取类型
收费全文 | 41269篇 |
免费 | 9669篇 |
国内免费 | 1409篇 |
专业分类
化学 | 44450篇 |
晶体学 | 369篇 |
力学 | 571篇 |
数学 | 2876篇 |
物理学 | 4081篇 |
出版年
2021年 | 201篇 |
2020年 | 1285篇 |
2019年 | 2621篇 |
2018年 | 1050篇 |
2017年 | 679篇 |
2016年 | 3452篇 |
2015年 | 3583篇 |
2014年 | 3513篇 |
2013年 | 4263篇 |
2012年 | 3080篇 |
2011年 | 2299篇 |
2010年 | 2926篇 |
2009年 | 2901篇 |
2008年 | 2480篇 |
2007年 | 1880篇 |
2006年 | 1589篇 |
2005年 | 1751篇 |
2004年 | 1523篇 |
2003年 | 1417篇 |
2002年 | 2111篇 |
2001年 | 1487篇 |
2000年 | 1393篇 |
1999年 | 433篇 |
1998年 | 137篇 |
1997年 | 140篇 |
1996年 | 142篇 |
1995年 | 141篇 |
1994年 | 113篇 |
1993年 | 127篇 |
1992年 | 119篇 |
1991年 | 101篇 |
1990年 | 81篇 |
1989年 | 107篇 |
1988年 | 113篇 |
1987年 | 98篇 |
1986年 | 89篇 |
1985年 | 144篇 |
1984年 | 117篇 |
1983年 | 95篇 |
1982年 | 108篇 |
1981年 | 113篇 |
1979年 | 106篇 |
1978年 | 101篇 |
1977年 | 94篇 |
1976年 | 86篇 |
1975年 | 90篇 |
1973年 | 96篇 |
1968年 | 105篇 |
1967年 | 107篇 |
1966年 | 121篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
41.
42.
43.
Tricyclic skeletons have been generated from acyclic enyne precursors by using an intramolecular Pauson-Khand reaction in combination with aldol, Michael and alkylation reactions. 相似文献
44.
45.
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
46.
5‐Amino‐4‐methyl‐2‐phenyl‐6‐substitutedfuro[2,3‐d]pyrimidines ( 2a‐c ) were reacted with 2,5‐dimethoxytetrahydrfuran to afford the pyrrolyl derivatives 3a‐c . Compound 3a was chosen as intermediate for the synthesis of poly fused heterocycles incorporated furopyrimidines moiety 4–11 . Some of the synthesized compounds were screened for their antibacterial and antifungal activities. 相似文献
47.
Thin Ca films were evaporated on Si(1 1 1) under UHV conditions and subsequently annealed in the temperature range 200–650 °C. The interdiffusion of Ca and Si was examined by ex situ Auger depth profiling. In situ monitoring of the Si 2p core-level shift by X-ray photoemission spectroscopy (XPS) was employed to study the silicide formation process. The formation temperature of CaSi2 films on Si(1 1 1) was found to be about 350 °C. Epitaxial growth takes place at T≥400 °C. The morphology of the films, measured by atomic force microscopy (AFM), was correlated with their crystallinity as analyzed by X-ray diffraction (XRD). According to measurements of temperature-dependent I–V characteristics and internal photoemission the Schottky-barrier height of CaSi2 on Si(1 1 1) amounts to qΦBn=0.25 eV on n-type and to qΦBp=0.82 eV on p-type silicon. 相似文献
48.
Johannes Elschner Rainer Hinder Gunther Schmidt 《Advances in Computational Mathematics》2002,16(2-3):139-156
This paper is devoted to the numerical study of diffraction by periodic structures of plane waves under oblique incidence. For this situation Maxwell's equations can be reduced to a system of two Helmholtz equations in R
2 coupled via quasiperiodic transmission conditions on the piecewise smooth interfaces between different materials. The numerical analysis is based on a strongly elliptic variational formulation of the differential problem in a bounded periodic cell involving nonlocal boundary operators. We obtain existence and uniqueness results for discrete solutions and provide the corresponding error analysis. 相似文献
49.
50.
Wen Zhang Hui Huang Yu Ding Margherita Gavagnin Ernesto Mollo Guido Cimino Yue‐Wei Guo 《Helvetica chimica acta》2006,89(4):813-820
Three new polyoxygenated steroids, muricesteroid ( 1 ), and menellsteroids A ( 2 ) and B ( 3 ), were isolated from two species of the South China Sea gorgonian Muricella flexuosa and Menella verrucosa Brundin , respectively. The structures of these new compounds were elucidated on the basis of extensive spectroscopic analysis, chemical methods and comparison with known related compounds. 相似文献