全文获取类型
收费全文 | 72670篇 |
免费 | 6034篇 |
国内免费 | 11667篇 |
专业分类
化学 | 54412篇 |
晶体学 | 2078篇 |
力学 | 1252篇 |
综合类 | 478篇 |
数学 | 15620篇 |
物理学 | 16531篇 |
出版年
2024年 | 136篇 |
2023年 | 1205篇 |
2022年 | 1602篇 |
2021年 | 2034篇 |
2020年 | 2226篇 |
2019年 | 2196篇 |
2018年 | 1834篇 |
2017年 | 2108篇 |
2016年 | 2250篇 |
2015年 | 2101篇 |
2014年 | 2899篇 |
2013年 | 6054篇 |
2012年 | 4201篇 |
2011年 | 5141篇 |
2010年 | 4223篇 |
2009年 | 5060篇 |
2008年 | 5153篇 |
2007年 | 5056篇 |
2006年 | 4540篇 |
2005年 | 3819篇 |
2004年 | 3611篇 |
2003年 | 3063篇 |
2002年 | 2931篇 |
2001年 | 2092篇 |
2000年 | 1937篇 |
1999年 | 1698篇 |
1998年 | 1410篇 |
1997年 | 1202篇 |
1996年 | 1168篇 |
1995年 | 1157篇 |
1994年 | 1028篇 |
1993年 | 800篇 |
1992年 | 743篇 |
1991年 | 571篇 |
1990年 | 417篇 |
1989年 | 397篇 |
1988年 | 307篇 |
1987年 | 245篇 |
1986年 | 206篇 |
1985年 | 251篇 |
1984年 | 206篇 |
1983年 | 107篇 |
1982年 | 171篇 |
1981年 | 191篇 |
1980年 | 124篇 |
1979年 | 106篇 |
1978年 | 79篇 |
1977年 | 92篇 |
1976年 | 68篇 |
1973年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
Yu-Soo Lee Hyun-Ju Chung Jong-Han Joung Eung-Jo Kim Hee-Je Kim 《Optics & Laser Technology》2004,36(1):461-61
We describe the pulse forming of pulsed CO2 laser using multi-pulse superposition technique. Various pulse shapes, high duty cycle pulse forming network (PFN) are constructed by time sequence. This study shows a technology that makes it possible to make various long pulse shapes by activating SCRs of three PFN modules consecutively at a desirable delay time with the aid of a PIC one-chip microprocessor. The power supply for this experiment consists of three PFN modules. Each PFN module uses a capacitor, a pulse forming inductor, a SCR, a high voltage pulse transformer, and a bridge rectifier on each transformer secondary. The PFN modules operate at low voltage by driving the primary of HV pulse transformer. The secondary of the transformer has a full-wave rectifier, which passes the pulse energy to the load in a continuous sequence.We investigated various long pulse shapes as different trigger time intervals of SCRs among three PFN modules. As a result, we could obtain laser beam with various pulse shapes and durations from about 250 to 1000 μs. 相似文献
102.
[Na(18-C-6)]2[Cu(i-mnt)2]的合成与结构分析 总被引:2,自引:2,他引:0
研究了18-冠-6与Na2[Cu(i-mnt)2][i-mnt=异丁二腈烯二硫醇阴离子,S2CC(CN)2-2]的反应,得到的配合物[Na(18-C-6)]2[Cu(i-mnt)2](1)通过元素分析、红外光谱、X射线单晶衍射进行了结构分析.配合物为单斜晶系,空间群P2(1)/c.晶体学结构数据a=1.2819(11),b=1.1793(10),c=1.4928(13)nm,β=99.121(16)°,V=2.228(3)nm3,Z=2,Dcaled.=1.369g/cm3,F(000)=958,R1=0.0521,wR2=0.1003.1中的[Cu(i-mnt)2]基团通过配体i-mnt的氮原子与两个[Na(18-C-6)]基团中的钠原子成键,形成稳定的中性配合物. 相似文献
103.
104.
Let M be a Riemannian m-dimensional manifold with m ≥ 3, endowed with non zero parallel p-form. We prove that there is no minimal isometric immersions of M in a Riemannian manifold N with constant strictly negative sectional curvature. Next we show that, under the conform flatness of the manifold N and some assumptions on the Ricci curvature of N, there is no α-pluriharmonic isometric immersion. 相似文献
105.
What Is Its Real Existing Form?——Theoretical and Experimental Studies on 2-Mercaptobenzothiazole 总被引:1,自引:0,他引:1
1 INTRODUCTION The study of heterocyclic molecules with aroma- tic rings and their absorptions on metals is of con- siderable importance from both theoretical and te- chnological points of view. These molecules are in- teresting because of their applications as corrosion inhibitors and flotation collectors, and their abilities to form self-assembly layers[1]. 2-Mercaptobenzo- thiazole (C7H5NS2, in the following MBT) is an important heterocyclic molecule, which can be used as corrosion … 相似文献
106.
Kunyu Guo 《Journal of Functional Analysis》2004,213(2):380-411
This paper mainly concerns defect operators and defect functions of Hardy submodules, Bergman submodules over the unit ball, and Hardy submodules over the polydisk. The defect operator (function) carries key information about operator theory (function theory) and structure of analytic submodules. The problem when a submodule has finite defect is attacked for both Hardy submodules and Bergman submodules. Our interest will be in submodules generated by polynomials. The reason for choosing such submodules is to understand the interaction of operator theory, function theory and algebraic geometry. 相似文献
107.
108.
Various 2-substituted indoles were prepared by heteroannulation of o-iodoanilines and terminal alkynes in a one-pot reaction with a Pd(II)-NaY zeolite catalyst. The product formation largely depended on the solvent, base, and reaction temperature. The recycled catalyst showed good reusability in the heteroannulation reaction. 相似文献
109.
Sauer, Shelah, Vapnik and Chervonenkis proved that if a set system on n vertices contains many sets, then the set system has full trace on a large set. Although the restriction on the size of the
groundset cannot be lifted, Frankl and Pach found a trace structure that is guaranteed to occur in uniform set systems even
if we do not bound the size of the groundset. In this note we shall give three sequences of structures such that every set
system consisting of sufficiently many sets contains at least one of these structures with many sets. 相似文献
110.
The graph-theoretic operation of rooted subtree prune and regraft is increasingly being used as a tool for understanding and modelling reticulation events in evolutionary biology. In this paper, we show that computing the rooted subtree prune and regraft distance between two rooted binary phylogenetic trees on the same label set is NP-hard. This resolves a longstanding open problem. Furthermore, we show that this distance is fixed parameter tractable when parameterised by the distance between the two trees.Received March 16, 2004 相似文献