全文获取类型
收费全文 | 35974篇 |
免费 | 2421篇 |
国内免费 | 2118篇 |
专业分类
化学 | 12101篇 |
晶体学 | 361篇 |
力学 | 601篇 |
综合类 | 192篇 |
数学 | 14229篇 |
物理学 | 6483篇 |
无线电 | 6546篇 |
出版年
2024年 | 72篇 |
2023年 | 512篇 |
2022年 | 544篇 |
2021年 | 592篇 |
2020年 | 740篇 |
2019年 | 860篇 |
2018年 | 845篇 |
2017年 | 774篇 |
2016年 | 872篇 |
2015年 | 1028篇 |
2014年 | 1468篇 |
2013年 | 2291篇 |
2012年 | 1758篇 |
2011年 | 2593篇 |
2010年 | 2032篇 |
2009年 | 2535篇 |
2008年 | 2658篇 |
2007年 | 2491篇 |
2006年 | 2018篇 |
2005年 | 1616篇 |
2004年 | 1511篇 |
2003年 | 1351篇 |
2002年 | 1532篇 |
2001年 | 861篇 |
2000年 | 801篇 |
1999年 | 820篇 |
1998年 | 630篇 |
1997年 | 521篇 |
1996年 | 532篇 |
1995年 | 484篇 |
1994年 | 431篇 |
1993年 | 359篇 |
1992年 | 298篇 |
1991年 | 253篇 |
1990年 | 209篇 |
1989年 | 219篇 |
1988年 | 155篇 |
1987年 | 151篇 |
1986年 | 133篇 |
1985年 | 157篇 |
1984年 | 130篇 |
1983年 | 75篇 |
1982年 | 121篇 |
1981年 | 116篇 |
1980年 | 72篇 |
1979年 | 61篇 |
1978年 | 56篇 |
1977年 | 50篇 |
1976年 | 37篇 |
1971年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Barry M. Trost 《Angewandte Chemie (International ed. in English)》1989,28(9):1173-1192
The history of ring systems in organic chemistry parallels their synthetic accessibility. Transition-metal-catalyzed cyclizations offer a new opportunity to create carbo- and heterocyclic compounds with great facility. Among these methods, allylic alkylations catalyzed by palladium have proven unusually productive because of the extraordinary chemo-, regio-, and diastereoselectivity and the continuing possibility for the development of enantioselectivity. The rules for ring closure differ from those for non-transition-metal-catalyzed reactions. A major benefit is the ability to generate medium (eight-, nine-, ten-, and eleven-membered) and large rings in preference to normal (five-, six- and seven-membered) rings. With the appropriate substrate, efficient macrocyclizations are possible under conditions of normal concentrations. A second major benefit derives from the complementary stereochemistry of the metalcatalyzed substitution (net retention of configuration) compared to non-metal-catalyzed reactions (inversion of configuration). Further, the requirement for the substrate to conform to the transition-metal template may impose a stereochemical preference in the intermediate that ultimately translates into the thermodynamically less stable organic product regardless of the stereochemistry of the starting material. While more work has focused on carbocyclic synthesis, the possibilities for heterocyclic synthesis are just beginning to be tapped. In addition to forming heterocycles by C? C bond formation, use of a heteroatom as a nucleophile has already proven effective for oxygen and nitrogen, with other nucleophiles awaiting investigation. New dimensions for cyclization via allylic alkylation arise by generating the requisite π-allylpalladium intermediates by methods other than palladium(0)-initiated allylic ionizations. In addition, metals other than palladium will clearly expand the possibilities, but as yet remain untapped. 相似文献
102.
探针反应和FTIR法研究Pt与L型沸石的相互作用董家禄,朱建华,须沁华,张婕,刘大壮(南京大学化学系,南京,210008)(郑州工学院化工系,郑州,450002)关键词Pt/L沸石,沸石催化剂,异丙醇分解反应,Pt-载体相互作用Pt的分散度和电子状态... 相似文献
103.
The design and some properties of a new general-purpose isothermal microcalorimeter are reported. The instrument is a twin thermopile heat conduction calorimeter, which is designed for use up to 200 °C. The calorimetric units and surrounding heat sink are suspended inside a hollow aluminium construction, which is thermostated. Above that unit a second thermostated block is positioned and the whole assembly is suspended inside a Dewar vessel. When the instrument is used at room temperature and below, the thermostated units are cooled by use of an insertion Peltier effect cooler. The instrument can be used with a wide range of different reaction vessels (diameter 14 mm). Baseline experiments have been conducted in the temperature range 15-200 °C. Typical values obtained during 10 h periods at 200 °C are ±3 and ±10 nW for the baseline drift and baseline fluctuations, respectively. The heat detection limit, determined by release of electrical energy, is about 2 μJ. Preliminary stability measurements have been conducted at 100 °C on samples of stabilised and non-stabilised polyamide film. 相似文献
104.
K. Coolsaet 《Combinatorica》1995,15(4):481-487
Several properties of graphs with ==2,a
2=4 are studied. It is proved that such graphs are locally unions of triangles, hexagons or heptagons. As a consequence, a distance regular graph with intersection array (13,10,7;1,2,7) does not exist. 相似文献
105.
Summary We introduce a new Skorohod topology for functions of several variables. Since ann-variable function may be viewed as a one-variable function with values in the set of (n–1)-variable functions, this topology is defined by induction from the classical Skorohod topology for one-variable functions. This allows us to define the notion of completen-parameter symmetric Markov processes: Such processes are, for any 1pn, rawp-parameter Markov processes (in the sense of our previous paper [17]) with values in the space of (n–p)-variable functions. We prove, for these processes and their Bochner subordinates, a maximal inequality which implies the continuity of additive functionals associated with finite energy measures. We finally present several important examples. 相似文献
106.
Scott Sciffer 《Set-Valued Analysis》1995,3(4):375-380
The space of continuous functions on the double arrow space has long been of interest in differentiability theory since many convex functions on this space are densely but not generically Gâteaux differentiable. We show that this space has the property that minimal weak* cuscos into its dual take compact values at the points of a denseG
set. 相似文献
107.
M. Sorg 《Letters in Mathematical Physics》1995,33(2):113-125
An exact solution of Dirac's equation in an open Robertson-Walker universe is constructed such that the corresponding energy-momentum densityT
µv
obeys the cosmological principle. However, in anopen universe, this principlemust always be violated by the other physical densities of the cosmological solution (e.g. pseudoscalar, (axial-) current and polarization). If the current densityj
µ
=
isrequired to obey the cosmological principle, then the universe must beflat. 相似文献
108.
It is shown that with probability 1 on , resp. ongx the irrational rotation algebra
with respect to the CAT map and the generalized Price-Powers shiftA
X
are asymptotically highly anticommutative. 相似文献
109.
X. -D. Cao 《Periodica Mathematica Hungarica》1994,28(1):43-54
Let
, wheref(n) is the characteristic function of square-full integers. A formula is proved that can be used to obtain better upper bound estimate for (x). We get (x)=O(x
5/33+), which improves the exponent 11/72 obtained by R. Blasubramania and K. Ramachandra. 相似文献
110.
IfA is a -algebra on setX, thenl
0
(X,A) is a barrelled space of class 0. IfA is an algebra, there are conditions which imply thatl
0
(X,A) is suprabarelled. Here, wheneverA is an algebra, we give conditions forl
0
(X,A) to be not barrelled which are related with the existence of non-trivial convergent sequences.Supported in part by DGICYT, project PB91-0407 and by the Institució Valenciana d'Estudis i Investigació, project 023. 相似文献