全文获取类型
收费全文 | 3249篇 |
免费 | 230篇 |
国内免费 | 867篇 |
专业分类
化学 | 1814篇 |
晶体学 | 5篇 |
力学 | 159篇 |
综合类 | 29篇 |
数学 | 777篇 |
物理学 | 253篇 |
综合类 | 1309篇 |
出版年
2024年 | 6篇 |
2023年 | 14篇 |
2022年 | 41篇 |
2021年 | 50篇 |
2020年 | 63篇 |
2019年 | 75篇 |
2018年 | 78篇 |
2017年 | 71篇 |
2016年 | 86篇 |
2015年 | 98篇 |
2014年 | 113篇 |
2013年 | 261篇 |
2012年 | 178篇 |
2011年 | 206篇 |
2010年 | 176篇 |
2009年 | 177篇 |
2008年 | 230篇 |
2007年 | 257篇 |
2006年 | 235篇 |
2005年 | 277篇 |
2004年 | 196篇 |
2003年 | 222篇 |
2002年 | 194篇 |
2001年 | 177篇 |
2000年 | 137篇 |
1999年 | 114篇 |
1998年 | 90篇 |
1997年 | 72篇 |
1996年 | 69篇 |
1995年 | 80篇 |
1994年 | 48篇 |
1993年 | 51篇 |
1992年 | 36篇 |
1991年 | 34篇 |
1990年 | 19篇 |
1989年 | 33篇 |
1988年 | 24篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有4346条查询结果,搜索用时 15 毫秒
1.
2.
Caesium carbonate supported on hydroxyapatite‐encapsulated Ni0.5Zn0.5Fe2O4 nanocrystallites as a novel magnetically basic catalyst for the one‐pot synthesis of pyrazolo[1,2‐b]phthalazine‐5,10‐diones 下载免费PDF全文
Behrooz Maleki Samaneh Barat Nam Chalaki Samaneh Sedigh Ashrafi Esmail Rezaee Seresht Farid Moeinpour Amir Khojastehnezhad Reza Tayebee 《应用有机金属化学》2015,29(5):290-295
A novel nanomagnetic basic catalyst of caesium carbonate supported on hydroxyapatite‐coated Ni0.5Zn0.5Fe2O4 magnetic nanoparticles (Ni0.5Zn0.5Fe2O4@HAP‐Cs2CO3) was prepared. This new catalyst was fully characterized using Fourier transform infrared spectroscopy, transmission and scanning electron microscopy, X‐ray diffraction and vibrating sample magnetometry techniques, and then the catalytic activity of this catalyst was investigated in the synthesis of 1H‐pyrazolo[1,2‐b]phthalazine‐5,10‐dione derivatives. Also, Ni0.5Zn0.5Fe2O4@HAP‐Cs2CO3 could be reused at least five times without significant loss of activity and could be recovered easily by applying an external magnet. Thus, the developed nanomagnetic catalyst is potentially useful for the green and economic production of organic compounds. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
3.
An oriented tetrahedron defined on four vertices is a set of four cyclic triples with the property that any ordered pair of vertices is contained in exactly one of the cyclic triples. A tetrahedral quadruple system of order with index , denoted by , is a pair , where is an ‐set and is a set of oriented tetrahedra (blocks) such that every cyclic triple on is contained in exactly members of . A is pure if there do not exist two blocks with the same vertex set. When , the spectrum of a pure TQS has been completely determined by Ji. In this paper, we show that there exists a pure if and only if and . A corollary is that a simple also exists if and only if and . 相似文献
4.
Immobilized Catalysts for Iridium‐Catalyzed Allylic Amination: Rate Enhancement by Immobilization 下载免费PDF全文
Dr. Chandi C. Malakar Prof. Dr. Günter Helmchen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(19):7127-7134
The first immobilized catalyst for Ir‐catalyzed asymmetric allylic aminations is described. The catalyst is a cationic (π‐allyl)Ir complex bound by cation exchange to an anionic silica gel support. Preparation of the catalyst is facile, and the supported catalyst displayed considerably enhanced activity compared with the parent homogeneous catalyst. Up to 43 consecutive amination runs were possible in recycling experiments. 相似文献
5.
6.
A (w,r) cover‐free family is a family of subsets of a finite set such that no intersection of w members of the family is covered by a union of r others. A (w,r) superimposed code is the incidence matrix of such a family. Such a family also arises in cryptography as the concept of key distribution pattern. In the present paper, we give some new results on superimposed codes. First we construct superimposed codes from super‐simple designs which give us results better than superimposed codes constructed by other known methods. Next we prove the uniqueness of the (1,2) superimposed code of size 9 × 12, the (2,2) superimposed code of size 14 × 8, and the (2,3) superimposed code of size 30 × 10. Finally, we improve numerical values of upper bounds for the asymptotic rate of some (w,r) superimposed codes. © 2004 Wiley Periodicals, Inc. 相似文献
7.
A simple, efficient and highly chemoselective method for tetrahydropyranylation of alcohols and phenols has been developed by their reaction with 3,4-dihydro-2H-pyran at room temperature in the presence of a catalytic amount of polystyrene supported AlCl3. The method is also highly selective for monoprotection of symmetrical diols. 相似文献
8.
基于CSCW的协同编著系统的设计 总被引:3,自引:0,他引:3
讨论了计算机支持的协同工作(CSCW)的主要技术,其中重点讨论了协同编著系统设计中的并发控制技术,指出了其与传统的并发控制技术的区别。根据CSCW的基本原理设计出了一个基于Browse/Server结构的协同编著系统的系统模型。该系统模型比传统的模型更加方便了用户之间通过Internet的浏览器进行协作。 相似文献
9.
We show that the LF-algebra considered by Akkar and Nacir in [1] is topologically simple. 相似文献
10.
Cao H. P. Chen G. Grechkoseeva M. A. Mazurov V. D. Shi W. J. Vasil'ev A. V. 《Siberian Mathematical Journal》2004,45(6):1031-1035
The spectrum of a finite group is the set of its element orders. A finite group G is said to be recognizable by spectrum, if every finite group with the same spectrum as G is isomorphic to G. The purpose of the paper is to prove that for every natural m the finite simple Chevalley group F
4(2
m
) is recognizable by spectrum. 相似文献