全文获取类型
收费全文 | 68240篇 |
免费 | 13771篇 |
国内免费 | 22121篇 |
专业分类
化学 | 49933篇 |
晶体学 | 2752篇 |
力学 | 5170篇 |
综合类 | 2524篇 |
数学 | 10362篇 |
物理学 | 33391篇 |
出版年
2024年 | 198篇 |
2023年 | 825篇 |
2022年 | 2563篇 |
2021年 | 2552篇 |
2020年 | 2523篇 |
2019年 | 2479篇 |
2018年 | 2224篇 |
2017年 | 3168篇 |
2016年 | 2635篇 |
2015年 | 3622篇 |
2014年 | 4408篇 |
2013年 | 5735篇 |
2012年 | 5650篇 |
2011年 | 5896篇 |
2010年 | 5823篇 |
2009年 | 5921篇 |
2008年 | 6661篇 |
2007年 | 5872篇 |
2006年 | 5775篇 |
2005年 | 5039篇 |
2004年 | 3886篇 |
2003年 | 2809篇 |
2002年 | 2744篇 |
2001年 | 2781篇 |
2000年 | 2780篇 |
1999年 | 1642篇 |
1998年 | 961篇 |
1997年 | 757篇 |
1996年 | 684篇 |
1995年 | 612篇 |
1994年 | 667篇 |
1993年 | 622篇 |
1992年 | 565篇 |
1991年 | 401篇 |
1990年 | 386篇 |
1989年 | 399篇 |
1988年 | 334篇 |
1987年 | 265篇 |
1986年 | 209篇 |
1985年 | 173篇 |
1984年 | 146篇 |
1983年 | 158篇 |
1982年 | 132篇 |
1981年 | 116篇 |
1980年 | 85篇 |
1979年 | 77篇 |
1978年 | 27篇 |
1977年 | 19篇 |
1974年 | 20篇 |
1959年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
71.
72.
73.
Jie Shen 《Numerische Mathematik》1992,62(1):49-73
Summary This paper is a continuation of our previous work [10] on projection methods. We study first existing higher order projection schemes in the semidiscretized form for the Navier-Stokes equations. One error analysis suggests that the precision of these schemes is most likely plagued by the inconsistent Neumann boundary condition satisfied by the pressure approximations. We then propose a penalty-projection scheme for which we obtain improved error estimates.This work is partially supported by NSF grant MS-8802596. 相似文献
74.
Synthesis, Structure and Conductivity of the New Charge-transfer Salt (ET)_2(CH_2=CH-CH_2-SO_3)·H_2O
Zhi LIU Qi FANG* Wen Tao YU Min Hua JIANG De Qing ZHANG Dao Ben ZHU State Key Laboratory of Crystal Materials Shandong University Jinan Institute of Chemistry The Chinese Academy of Sciences Beijing 《中国化学快报》2003,14(4)
ET is one of the most famous electron-donor molecules, which forms charge-transfer complexes (abbr. CT-complexes) with various types of counterions. These complexes have received intense attention because a wide range of physical properties such as conductivity and superconductivity1, ferromagnetism2-4 and nonlinear optical properties5 was found in these materials. Although the majority of the ET-based CT-complexes were prepared by combining with inorganic counterions, CT-complexes with o… 相似文献
75.
A. S. Aynacioglu A. -H. Meißner F. Walachowicz G. v. Oppen 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,18(1):85-91
The magnetic depolarization of the impact radiation at λ(41 D?21 P)=492 nm and λ(31 D?21 P) = 668 nm as well as the splitting of the magnetic depolarisation signals in electric fields were investigated for 10–25 keV H 2 + (D 2 + )-He collisions (only 492 nm line) and for 5-22.5 keV He-He collisions (both lines). The results are compared with analogous measurements for He+- He and Ne+ - He collisions and the dynamics of the excitation process are qualitatively discussed. In particular, we emphasize the importance of inertia for the evolution of the electron cloud during the final phase of the collision process. 相似文献
76.
Olefins are prevalent substrates and functionalities. The synthesis of olefins from readily available starting materials such as alcohols, amines and carboxylic acids is of great significance to address the sustainability concerns in organic synthesis. Metallaphotoredox-catalyzed defunctionalizations were reported to achieve such transformations under mild conditions. However, all these valuable strategies require a transition metal catalyst, a ligand or an expensive photocatalyst, with the challenges of controlling the region- and stereoselectivities remaining. Herein, we present a fundamentally distinct strategy enabled by electron donor–acceptor (EDA) complexes, for the selective synthesis of olefins from these simple and easily available starting materials. The conversions took place via photoactivation of the EDA complexes of the activated substrates with alkali salts, followed by hydrogen atom elimination from in situ generated alkyl radicals. This method is operationally simple and straightforward and free of photocatalysts and transition-metals, and shows high regio- and stereoselectivities.A visible-light-induced defunctionalization strategy for the synthesis of olefins by using easily available alcohols, amines and carboxylic acids as starting materials is demonstrated. 相似文献
77.
Four 2-(α-hydroxyalkyl)-2-oxazolines and a 2-(α-hydroxy-α,α-diphenyl)-2-benzooxazole were synthesized from β-amimoalcohols and α-hydroxy carboxylic acids under microwave irradiation in the absence of a solvent. 相似文献
78.
79.
利用二维光子晶体的等频线分析原理和平面波展开方法,得到了使二维光子晶体产生全入射角负折射(All-Angle Negative Refraction,AANR)现象时,入射电磁波的频率取值范围.同时,分析了AANR频率范围随着结构参量(晶格类型、介质棒半径与晶格周期的比值)和电磁参量(介质柱介电常量、本底介电常量、入射电磁波偏振方向)变化的行为.结果表明:固定组成光子晶体的一种介质的介电常量,另一介质的介电常量只有达到一定阈值,才有可能使光子晶体出现AANR现象.在给定两种介质介电常量的条件下,存在使AANR频率范围最大化的结构参量和电磁参量. 相似文献
80.
自然界中砷和砷的化合物一般可通过水、大气和食物等途径进入人体,危害人体健康,原子荧光光谱法测定砷含量被广泛应用。为准确测定样品中砷的含量,以原子荧光光谱法(AFS)测定土壤中总砷含量为例,重点研究高价态砷(Ⅴ)还原为低价态砷(Ⅲ)的实验条件,采用正交实验进行条件优化,建立适合的实验方法。通过设计L9(34)正交实验,高价态砷的还原反应与四个因素有关,即与反应温度、反应时间、盐酸加入量和硫脲的用量呈一定的正相关,结果表明,四个因素中硫脲用量的影响最为显著,其次是盐酸加入量、反应时间和反应温度。通过对四个因素的多水平检验,优化后的实验条件为:硫脲用量2.0 mL、盐酸加入量10%、反应时间20 min、温度20~35 ℃(即普通室温)。采用土壤标准物质验证优化后条件的可行性,得出砷的测定值在标准值范围内,RSD在2.2%~4.2%,精密度和准确性均满足质控要求。优化后的实验条件使得操作简便易行,提高了工作效率。 相似文献