全文获取类型
收费全文 | 3659篇 |
免费 | 617篇 |
国内免费 | 453篇 |
专业分类
化学 | 2095篇 |
晶体学 | 19篇 |
力学 | 165篇 |
综合类 | 23篇 |
数学 | 308篇 |
物理学 | 1128篇 |
无线电 | 991篇 |
出版年
2024年 | 20篇 |
2023年 | 100篇 |
2022年 | 132篇 |
2021年 | 103篇 |
2020年 | 148篇 |
2019年 | 148篇 |
2018年 | 145篇 |
2017年 | 123篇 |
2016年 | 173篇 |
2015年 | 182篇 |
2014年 | 210篇 |
2013年 | 244篇 |
2012年 | 285篇 |
2011年 | 294篇 |
2010年 | 219篇 |
2009年 | 203篇 |
2008年 | 268篇 |
2007年 | 188篇 |
2006年 | 203篇 |
2005年 | 206篇 |
2004年 | 144篇 |
2003年 | 139篇 |
2002年 | 135篇 |
2001年 | 105篇 |
2000年 | 79篇 |
1999年 | 88篇 |
1998年 | 65篇 |
1997年 | 46篇 |
1996年 | 48篇 |
1995年 | 48篇 |
1994年 | 45篇 |
1993年 | 41篇 |
1992年 | 30篇 |
1991年 | 27篇 |
1990年 | 23篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1936年 | 1篇 |
排序方式: 共有4729条查询结果,搜索用时 15 毫秒
101.
Asymmetric Michael Addition of Aldimino Esters with Chalcones Catalyzed by Silver/Xing‐Phos: Mechanism‐Oriented Divergent Synthesis of Chiral Pyrrolines 下载免费PDF全文
Xing‐Feng Bai Dr. Li Li Dr. Zheng Xu Dr. Zhan‐Jiang Zheng Prof. Dr. Chun‐Gu Xia Dr. Yu‐Ming Cui Prof. Dr. Li‐Wen Xu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(30):10399-10404
The mechanism‐oriented reaction design for the divergent synthesis of chiral molecules from simple starting materials is highly desirable. In this work, aromatic amide‐derived nonbiarylatropisomer/silver (silver/Xing‐Phos) complex was used to catalyze the Michael addition of glycine aldimino esters to chalcones and successfully applied to the subsequent cyclocondensation to afford substituted cis‐Δ(1)‐pyrroline derivatives with up to 98 % ee. Besides the inherent performance of the chiral Ag/Xing‐Phos catalyst system, it was found that the workup of such reactions played an important role for the stereoselective construction of stereodivergent Δ(1)‐pyrrolines, in which an epimerization of the cis‐Δ(1)‐pyrrolines to the trans‐isomers during was revealed. 相似文献
102.
Guo Ping Zhan 《数学学报(英文版)》2013,29(4):703-716
In this paper, we consider the dynamics of the map z→exp(z)/z on the punctured plane C*=C\{0}. We show that for almost every point z ∈ C*, the ω-limit set of z is equal to {0, ∞}. In particular, the map is not recurrent. 相似文献
103.
104.
Wei Zhao Yong Zhang Mingzhou Zhan Yongbo Chen Ruimin Xu 《Journal of Infrared, Millimeter and Terahertz Waves》2012,33(1):25-35
In this paper, a simple method for millimeter-wave finline balanced mixer design using three-dimensional field simulation
software has been proposed. The method can be widely used to design the diode-based circuits, especially for the circuit structures
with orthogonal field in some specific hybrid integrated circuits which are unavailable to be designed using the circuit simulator.
In these circuits, the power directly at diodes is correlated to the input reflection coefficient. The diodes mounted on the
finline circuits are defined as impedance boundary in the commercial computer-aided design (CAD) tool High Frequency Structure
Simulator (HFSS) model, and hence simulation with the use of HFSS can be implemented to optimize the input matching network
of the finline circuits for transferring maximum power to the diodes. Two finline balanced mixers at U-band using commercial
GaAs Schottky diodes have been designed and fabricated to validate this method. Matching structures at the radio frequency
(RF) port have been employed for a better return loss and a lower conversion loss. Experiment results are presented and show
good agreement with simulation data. The proposed method has proven to be useful for the design of millimeter-wave mixers
in finline technique. 相似文献
105.
The influences of pH, contact time, solid-liquid ratio, temperature and C60(C(COOH)2)n on Th(IV) adsorption onto the magnetic multi-walled carbon nanotubes(MMWCNTs) were studied by batch technique. The dynamic process showed that the adsorption of Th(IV) onto MMWCNTs could reach equilibrium in 40 h and matched the pseudo-second-order kinetics model. The adsorption of Th(IV) onto MMWCNTs was significantly dependent on pH values, the adsorption ratio increased markedly at pH 3.0–5.0, and then maintained a steady state as pH values increased. At low pH, different C60(C(COOH)2)n content could enhance the adsorption content of Th(IV) onto MMWCNTs, but restrained it at higher pH. Through simulating the adsorption isotherms by Langmuir, Freundlich and Dubini-Radushkevich models, it could be seen respectively that the adsorption pattern of Th(IV) onto MMWCNTs was mainly surface complexation, and that the adsorption process was endothermic and irreversible. 相似文献
106.
Jin Wang HongLei Zhan LiPing He Kun Zhao HuiBin Lü KuiJuan Jin GuoZhen Yang 《中国科学:物理学 力学 天文学(英文版)》2016,59(11):114221
正Dear Editors,The evaluation of the oil-gas resources’potential depends on the characterization of reservoirs.The geometry of the pore-crack structures and the compositions of the pore walls have influence on hydrocarbon production about capillarity and permeability[1].Besides,it has been known that the adsorption phenomenon in micro-structure plays a significant role in oil-gas reservoirs.Some techniques have been applied to characterize conventional and unconventional reservoirs, 相似文献
107.
Genggeng Yang Zeying He Xueke Liu Chang Liu Jing Zhan Donghui Liu Peng Wang Zhiqiang Zhou 《Mikrochimica acta》2016,183(3):1187-1194
108.
Zhijian Fu Xingzhou Zhan Lin Luo Andreas Schadschneider Junmin Chen 《Physics letters. A》2019,383(16):1897-1906
Fatigue even increases the complexity of the pedestrian dynamics which is regarded as a kind of nonlinear system, and might have a significant negative impact on the crowd evacuation. However, it has never been investigated completely and properly. Thus, the fine discrete floor field cellular automata model is modified by integrating the fatigue function to explore the influence of fatigue on the crowd ascending evacuation. The simulation fits well with the empirical data and the observations quantitatively and qualitatively, indicating the model captures the main features of evacuation considering fatigue impact. As a prediction, without merging streams, compared with the case of walking in constant speed, when fatigue is considered, it takes 71.4% longer for all persons to enter the stairs and 87.2% longer to evacuate. With merging streams, fatigue has little impact on the inflow, while it makes the total evacuation time 84.2% longer. 相似文献
109.
Our group has developed a series of molecular electrocatalysts for hydrogen generation based on triazenido–metal complexes (cobalt, copper, etc.). In this paper, we first present the electrocatalytic performance of a new dinuclear silver complex, [Ag2(L)2], formed by the reaction of the triazenido ligand 1‐[(2‐carboxymethyl)benzene]‐3‐[(2‐methoxy)benzene]triazene (HL) with AgNO3. At room temperature, the silver complex shows photoluminescence at 653 nm. The electrocatalytic systems based on this silver complex can afford 106.57 and 1536.36 moles of hydrogen per mole of catalyst per hour from acetic acid at an overpotential (OP) of 991.6 mV and from a neutral aqueous buffer (pH = 7.0) at an OP of 837.6 mV, respectively. Electrochemical investigations show that both silver ion and triazenido ligand play a role in determining the catalytic activities of the electrocatalytic system. 相似文献
110.
Hong-Hua Fang Shi-Yang Lu Xue-Peng Zhan Jing Feng Qi-Dai Chen Hai-Yu Wang Xing-Yuan Liu Hong-Bo Sun 《Organic Electronics》2013,14(3):762-767
Herein, a low threshold, wavelength-tunable, compact, two-photon pumped upconversion laser is presented. The surface emitting lasers are composed of melt-processed 1,4-bis[2-[4-[N,N-di(p-totyl)amino]phenyl]vinyl]benzene (DADSB) as active media and two designed distributed bragg reflectors. The melting fabrication process is very simple, and the lasing threshold is as low as 150 μJ cm?2 pulse?1, when pumped by a Ti:sapphire amplifier operating at 800 nm with a 150 fs pulse width. To the best of our knowledge, it is one of the lowest values for two-photon lasers. Lasing from multimode to single-mode oscillation is demonstrated. Tunable single mode oscillation was obtained at wavelength from 514 nm to 523 nm with a spectral width of less than 0.2 nm. 相似文献