首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2041篇
  免费   75篇
  国内免费   3篇
化学   1326篇
晶体学   20篇
力学   50篇
数学   179篇
物理学   544篇
  2023年   23篇
  2022年   36篇
  2021年   32篇
  2020年   47篇
  2019年   41篇
  2018年   42篇
  2017年   41篇
  2016年   64篇
  2015年   60篇
  2014年   81篇
  2013年   145篇
  2012年   152篇
  2011年   163篇
  2010年   107篇
  2009年   91篇
  2008年   119篇
  2007年   122篇
  2006年   101篇
  2005年   93篇
  2004年   58篇
  2003年   53篇
  2002年   37篇
  2001年   24篇
  2000年   22篇
  1999年   29篇
  1998年   16篇
  1997年   13篇
  1996年   11篇
  1995年   13篇
  1994年   17篇
  1993年   13篇
  1992年   21篇
  1991年   14篇
  1990年   11篇
  1989年   13篇
  1988年   13篇
  1987年   16篇
  1986年   6篇
  1985年   17篇
  1984年   16篇
  1983年   11篇
  1982年   16篇
  1981年   8篇
  1980年   9篇
  1979年   7篇
  1978年   8篇
  1976年   7篇
  1975年   9篇
  1974年   7篇
  1973年   6篇
排序方式: 共有2119条查询结果,搜索用时 78 毫秒
111.
112.
Abstract

An efficient, atom-economic, one-pot protocol has been developed for the synthesis of highly functionalized dihydropyridines via four-component reactions of aromatic aldehydes, arylamines, malononitrile, and dimethylacetylenedicarboxylate in the presence of sodium hydroxide in ethanol. This domino reaction proceeded smoothly in good to excellent yields and offered several advantages, including short reaction time, simple experimental procedure, and applicability to a broad range of substrates.

Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file.  相似文献   
113.
We report, herein, the results of an in depth study and concomitant analysis of the AC conduction [σ′(ω): f=20 Hz to 2 MHz] mechanism in a reduced graphene oxide–zinc sulfide (RGO–ZnS) composite. The magnitude of the real part of the complex impedance decreases with increase in both frequency and temperature, whereas the imaginary part shows an asymptotic maximum that shifts to higher frequencies with increasing temperature. On the other hand, the conductivity isotherm reveals a frequency‐independent conductivity at lower frequencies subsequent to a dispersive conductivity at higher frequencies, which follows a power law [σ′(ω)∝ωs] within a temperature range of 297 to 393 K. Temperature‐independent frequency exponent ′s′ indicates the occurrence of phonon‐assisted simple quantum tunnelling of electrons between the defects present in RGO. Finally, this sample follows the “time–temperature superposition principle”, as confirmed from the universal scaling of conductivity isotherms. These outcomes not only pave the way for increasing our elemental understanding of the transport mechanism in the RGO system, but will also motivate the investigation of the transport mechanism in other order–disorder systems.  相似文献   
114.
115.
116.
An analysis is made using a four-dimensional mathematical model where there is a constant rate of flow of input nutrient. An organism is introduced in the model which is growing by taking in that nutrient. Two other predators on that organism are also introduced. The predators at the second and third trophic levels belong to the same species, though of different age groups. The predator at the third trophic level exhibits a distinct cannibalistic attitude to the predator of the second trophic level. Our analysis leads to different thresholds in terms of the model parameters acting as conditions under which the species associated with the system cannot thrive even in the absence of competition. Local stability of the system is obtained when one or more of the predators go extinct. Under appropriate circumstances a positive rest point of the system is obtained. Computer simulations have been carried out to illustrate different analytical results.  相似文献   
117.
118.
In the present work zinc ferrite nanoparticles of different crystallite size were irradiated with 200 MeV Ag15+ ion beam. The structural and magnetic characterization performed for these samples indicate the presence of size dependent irradiation induced changes in the nanoparticles. The superparamgnetic nanoparticles do not alter their behavior after irradiation; however paramagnetic samples exhibit weak ferrimagnetism in the irradiated specimen. Results obtained from these measurements are in agreement with results obtained from the electron paramagnetic resonance spectroscopy.  相似文献   
119.
Dark currents n+/ν/p+ Hg0.69Cd0.31Te mid wave infrared photodiodes were measured at room temperature. The diodes exhibited negative differential resistance at room-temperature, but with increasing leakage currents as a function of reverse bias. The current–voltage characteristics were simulated and fitted by incorporating trap assisted tunneling via traps and Shockley–Read–Hall generation recombination process due to dislocations in the carrier transport equations. The thermal suppression of carriers was simulated by taking energy level of trap (Et), trap density (Nt) and the doping concentrations of n+ and ν regions as fitting parameters. Values of Et and Nt were 0.78Eg and ~6–9 × 1014 cm?3 respectively for most of the diodes. Variable temperature current voltage measurements on variable area diode array (VADA) structures confirmed the fact that variation in zero bias resistance area product (R0A) is related to gr processes originating from variation in concentration and kind of defects that intersect a junction area.  相似文献   
120.
The essential oil of Ageratina adenophora Spreng., growing in the Canary Islands was analyzed by GC-MS. A total of 78 volatile compounds was identified and p-cymene (11.6%) was the major component in the oil. The sesquiterpene fraction (44.3%) was higher than the monoterpene one (32.1%).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号