全文获取类型
收费全文 | 476篇 |
免费 | 25篇 |
专业分类
化学 | 370篇 |
晶体学 | 1篇 |
力学 | 3篇 |
数学 | 59篇 |
物理学 | 68篇 |
出版年
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 8篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 15篇 |
2015年 | 18篇 |
2014年 | 14篇 |
2013年 | 31篇 |
2012年 | 20篇 |
2011年 | 34篇 |
2010年 | 23篇 |
2009年 | 16篇 |
2008年 | 27篇 |
2007年 | 20篇 |
2006年 | 24篇 |
2005年 | 23篇 |
2004年 | 22篇 |
2003年 | 15篇 |
2002年 | 13篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 13篇 |
1991年 | 6篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 6篇 |
1974年 | 2篇 |
1932年 | 1篇 |
1924年 | 1篇 |
1907年 | 2篇 |
排序方式: 共有501条查询结果,搜索用时 0 毫秒
81.
82.
Kreidler B Baro A Frey W Christoffers J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(9):2660-2667
Regioselective enamine formation from cyclic β‐diketones 1 is obtained by the appropriate choice of activating agent: Brønsted acid catalyzed condensation gives endocyclic enamines 3 as the thermodynamically favored products. Activation with Lewis acid BF3 ? OEt2 affords betaines 8 as intermediate products, which can be reacted with L ‐valine diethylamide ( 2 ) to preferentially furnish exocyclic enamines 4 as kinetic products. Derivatives with quaternary stereocenters were accessible from both isomeric enamines by using asymmetric, copper(II )‐catalyzed Michael reactions at ambient temperature. Both regioisomers afford the triketones 7 with the same constitution but bearing the opposite absolute configuration at the quaternary stereocenter. Thus, both enantiomers of the product are prepared by using the same chiral auxiliary derived from L ‐valine. 相似文献
83.
Brückner A 《Chemical communications (Cambridge, England)》2005,(13):1761-1763
For the first time, three operando methods, namely EPR, UV-vis and laser-Raman spectroscopy have been applied in parallel on the same V/TiO2 catalyst and under identical reaction conditions during oxidative dehydrogenation of propane to elucidate structure-reactivity relationships in status operandi. 相似文献
84.
85.
J. Sebald S. Krohns P. Lunkenheimer S.G. Ebbinghaus S. Riegg A. Reller A. Loidl 《Solid State Communications》2010,150(17-18):857-860
In the present work we demonstrate that in addition to the well-known colossal-dielectric-constant material CaCu3Ti4O12 various members of the series Ln2/3Cu3Ti4O12 with Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm also exhibit giant values of the dielectric constant. Just as CaCu3Ti4O12, all these materials show a Maxwell–Wagner type relaxation process. For the best material, Pr2/3Cu3Ti4O12, we provide a detailed investigation of its dielectric properties in a broad frequency range up to 1 GHz. Polarization at internal barriers, most likely grain boundaries, seems to be the reason for the observed very high values of the dielectric constant. Taking into account the present results and those reported in the literature, we conclude that CaCu3Ti4O12 is not exceptional at all and there seem to be various isostructural materials with similar dielectric properties. 相似文献
86.
In the context of augmented Lagrangian approaches for solving semidefinite programming problems, we investigate the possibility of eliminating the positive semidefinite constraint on the dual matrix by employing a factorization. Hints on how to deal with the resulting unconstrained maximization of the augmented Lagrangian are given. We further use the approximate maximum of the augmented Lagrangian with the aim of improving the convergence rate of alternating direction augmented Lagrangian frameworks. Numerical results are reported, showing the benefits of the approach. 相似文献
87.
In this work we study properties of random graphs that are drawn uniformly at random from the class consisting of biconnected
outerplanar graphs, or equivalently dissections of large convex polygons. We obtain very sharp concentration results for the
number of vertices of any given degree, and for the number of induced copies of a given fixed graph. Our method gives similar
results for random graphs from the class of triangulations of convex polygons. 相似文献
88.
In the setting of high-dimensional linear models with Gaussian noise, we investigate the possibility of confidence statements connected to model selection. Although there exist numerous procedures for adaptive (point) estimation, the construction of adaptive confidence regions is severely limited (cf. Li in Ann Stat 17:1001–1008, 1989). The present paper sheds new light on this gap. We develop exact and adaptive confidence regions for the best approximating model in terms of risk. One of our constructions is based on a multiscale procedure and a particular coupling argument. Utilizing exponential inequalities for noncentral χ 2-distributions, we show that the risk and quadratic loss of all models within our confidence region are uniformly bounded by the minimal risk times a factor close to one. 相似文献
89.
90.
Tamás Hergert Bálint Varga Angelika Thurner Ferenc Faigl Béla Mátravölgyi 《Tetrahedron》2018,74(16):2002-2008
5-Formyl-2-thiopheneboronic acid is a widely used building block for the synthesis of diverse structures, however, due to a significant instability its transformation to 5-arylthiophene-2-carboxaldehydes by CC couplings could be performed only in low yields. Herein, as a solution, a convenient technique is described for the preparation of 1,3-dioxolane-protected 5-arylthiophene-2-carboxaldehydes via a one-pot borylation–copper(I) chloride co-catalyzed Suzuki-Miyaura coupling reaction. The utilization of the in situ prepared 2-thiopheneboronic ester derivative is allowed by the method developed. Elimination of the crucial, acidic liberation step of boronic acid species resulted in improved yields and purities. In addition, comparison of various aryl bromides is shown to demonstrate the high tolerance of the transformation to functional groups. 相似文献