全文获取类型
收费全文 | 3333篇 |
免费 | 187篇 |
国内免费 | 83篇 |
专业分类
化学 | 2497篇 |
晶体学 | 16篇 |
力学 | 180篇 |
数学 | 399篇 |
物理学 | 511篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 53篇 |
2021年 | 88篇 |
2020年 | 118篇 |
2019年 | 148篇 |
2018年 | 203篇 |
2017年 | 150篇 |
2016年 | 275篇 |
2015年 | 167篇 |
2014年 | 239篇 |
2013年 | 453篇 |
2012年 | 289篇 |
2011年 | 257篇 |
2010年 | 205篇 |
2009年 | 172篇 |
2008年 | 184篇 |
2007年 | 139篇 |
2006年 | 74篇 |
2005年 | 70篇 |
2004年 | 74篇 |
2003年 | 52篇 |
2002年 | 41篇 |
2001年 | 18篇 |
2000年 | 20篇 |
1999年 | 17篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1963年 | 1篇 |
排序方式: 共有3603条查询结果,搜索用时 531 毫秒
131.
Mohammad Kazem Rofouei Mehdi Khodadadian Ali Reza Jalalvand Armin Beiza 《International journal of environmental analytical chemistry》2013,93(6):665-675
A new PVC-membrane electrode for Co2+ ions based on N,N′-di(thiazol-2-yl)formimidamide (TF) as membrane carrier has been developed. The electrode resulted in Nernstian response (29.5?±?0.4?mV decade?1) for Co2+ ion over a wide concentration range (2.5?×?10?7 ?1.0?×?10?1?M) with a detection limit of 6.1?×?10?8?M. The sensor has a response time of about 10?s, and can be used for at least 2 months without observing any deviation from the Nernstain response. The electrode revealed good selectivity towards cobalt(II) ion over a wide variety of alkali, alkaline earth, transition, and heavy metal ions and could be used in the pH range 2.0–7.0. The electrode was used for determination of Co2+ in real samples. 相似文献
132.
Mahdi Azarnoosh Ali Motie Nasrabadi Mohammad Reza Mohammadi Mohammad Firoozabadi 《Complexity》2012,17(6):7-16
This study investigates the behavioral indices of attention. A simple repetitive attentive task that resulted in mental fatigue was used consecutively in four trials. In the first step, reaction time and error responses were recorded to evaluate differences among trials. During the task, subjects showed different responses to stimulations. In the second part, to recognize the strategies, multiple clustering methods such as k‐means and fuzzy c‐means were performed in which behavioral indices and nonlinear features were used. In the last section, mental behavior was identified as a result of the chaotic properties of variations in reaction time. Therefore, the Lyapunov exponent of reaction times was evaluated. Results revealed that behavioral indices could distinguish attention from the occurrence of mental fatigue in trials. In addition, the three strategies used by subjects during the test protocol were assessed. Finally, variation of indices extracted from nonlinear analysis, that is, decrease in degree of chaotic behavior determined the transition from attention to mental fatigue. © 2012 Wiley Periodicals, Inc. Complexity, 2012 相似文献
133.
Using a cosmological black hole model proposed recently, we have calculated the quasi-local mass of a collapsing structure
within a cosmological setting due to different definitions put forward in the last decades to see how similar or different
they are. It has been shown that the mass within the horizon follows the familiar Brown–York behavior. It increases, however,
outside the horizon again after a short decrease, in contrast to the Schwarzschild case. Further away, near the void, outside
the collapsed region, and where the density reaches the background minimum, all the mass definitions roughly coincide. They
differ, however, substantially far from it. Generically, we are faced with three different Brown–York mass maxima: near the
horizon, around the void between the overdensity region and the background, and another at cosmological distances corresponding
to the cosmological horizon. While the latter two maxima are always present, the horizon mass maxima is absent before the
onset of the central singularity. 相似文献
134.
135.
Hamid Reza Shaterian Hassan Hadadzadeh 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5):1059-1071
A novel and simple method for the chemoselective deprotection of S,S- and S,O-acetals and ketals in the presence of their O,O-analogs with electrophilic halogens to their corresponding carbonyl compounds is described using N-bromosuccinimide, N-chlorosuccinimide, 2,4,4,6-tetrabromo-2,5-cyclohexen-1-one, trichlorocyanuric acid, or molecular bromine in aqueous acetonitrile. The use of these reagents in the presence of hydrated silica gel provide efficient, novel, and mild procedures for the deprotection of cyclic and acyclic O,O-, S,S-, and S,O-acetals and ketals in excellent yields in short reaction times. 相似文献
136.
Dadkhoda Ghazanfari Mohammed M. Hashemi Mohammad Reza Akhgar Mohammad Mehdi Foroughi Fariba Najafi-Zadeh 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):3018-3022
The ring-opening reaction of epoxides with thiols by SbCl 3 supported on Kieselguhr under solvent-free conditions, afforded high yields of β-hydroxy sulfides. Nucleophilic attack of the thiols occurs regioselectively at the less hindered side of the epoxides. 相似文献
137.
Hamid Reza Shaterian Asghar Hosseinian Majid Ghashang 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):3136-3144
Herein, we describe a green procedure for the one-pot preparation of 1,8-dioxo-octahydroxanthene derivatives by condensation of dimedone and substituted benzaldehydes in the presence of FeCl 3 -SiO 2 as an efficient and heterogeneous catalyst under microwave irradiation and thermal conditions. This method has the advantages of high yields, cleaner reactions, efficient and cost-effective method, simple methodology, short reaction times, easy workup, and greener conditions. 相似文献
138.
Ali Ramazani Ali Morsali Bijan Ganjeie Ali Reza Kazemizadeh Ebrahim Ahmadi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2439-2442
Abstract Selenourea reacts with dialkyl acetylenedicarboxylates in acetone to form 1:1 adducts, which undergo a cyclization reaction to produce alkyl Z-2-(2-amino-4-oxo-1,3-selenazol-5(4H)-yliden) acetates in fairly good yields. The reaction is completely stereoselective. 相似文献
139.
Ali Ramazani Ali Reza Kazemizadeh Bijan Ganjeie Ebrahim Ahmadi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2569-2572
Tiourea reacts with dialkyl acetylenedicarboxylates in solventless conditions to form 1:1 adducts, which undergo a cyclization reaction to produce alkyl Z-2-(2-amino-4-oxo-1,3-thiazol-5(4H)-yliden)acetates in fairly good yields. The stereochemistry of the ethyl Z-2-(2-amino-4-oxo-1,3-thiazol-5(4H)-yliden)acetate was established by the use of X-ray single crystal structure analysis. The reaction is completely stereoselective. 相似文献
140.
Abdolkarim Zare Alireza Hasaninejad Mohammad Hassan Beyzavi Ahmad Reza Moosavi-Zare Ali Khalafi-Nezhad Mehrnoosh Roshankar 《Phosphorus, sulfur, and silicon and the related elements》2013,188(7):1702-1712
KF/Al 2 O 3 efficiently catalyzes the microwave-assisted Michael addition of sulfonamides to α,β-unsaturated esters under solvent-free conditions to afford N-alkyl derivatives of sulfonamides as biologically interesting compounds in high yields and in short reaction times. In this reaction, N,N-dialkylsulfonamides are also produced, but in very low yields. 相似文献