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   671篇
  免费   198篇
  国内免费   109篇
化学   261篇
晶体学   4篇
力学   47篇
综合类   24篇
数学   285篇
物理学   357篇
  2023年   8篇
  2022年   23篇
  2021年   19篇
  2020年   20篇
  2019年   30篇
  2018年   29篇
  2017年   34篇
  2016年   33篇
  2015年   36篇
  2014年   55篇
  2013年   91篇
  2012年   45篇
  2011年   44篇
  2010年   42篇
  2009年   42篇
  2008年   45篇
  2007年   47篇
  2006年   28篇
  2005年   33篇
  2004年   26篇
  2003年   31篇
  2002年   25篇
  2001年   22篇
  2000年   19篇
  1999年   16篇
  1998年   14篇
  1997年   10篇
  1996年   12篇
  1995年   14篇
  1994年   13篇
  1993年   10篇
  1992年   11篇
  1991年   3篇
  1990年   2篇
  1989年   1篇
  1988年   2篇
  1987年   4篇
  1986年   3篇
  1985年   7篇
  1984年   8篇
  1983年   2篇
  1982年   2篇
  1981年   3篇
  1980年   4篇
  1979年   2篇
  1978年   4篇
  1977年   3篇
  1975年   1篇
排序方式: 共有978条查询结果,搜索用时 11 毫秒
61.
A simple Ni(dppe)Cl2/Ag2O combination has been established as an efficient catalyst system for the homo-coupling of aromatic terminal alkynes. This reaction proceeds under mild conditions and can be applied to alkynone substrates. Various aromatic 1,3-diynes were obtained in good to excellent yields.  相似文献   
62.
We have synthesized a homologous series of azoesters consisting of a coumaryl moiety as the end group. Eleven derivatives of this series exhibit mesomorphism. The nematic mesophase is exhibited from ethyl homologue onward, while the smectic phase commences at the pentyl derivative and is exhibited along with the nematic phase up to the hexadecyl derivative. The N-I transition temperatures curves show the usual odd-even effect. All the compounds in this series are thermally stable and exhibit a wide mesomorphic range of nearly 120°C. Their thermal stabilities and other characteristics are discussed.  相似文献   
63.
Reduced graphene oxide (RGO)‐supported Cu–Cu2O nanocomposite material (Cu‐Cu2O@RGO) was prepared through a one‐pot reflux synthesis method. The morphology, crystal structure and composition of the prepared Cu‐Cu2O@RGO were characterized using transmission electron microscopy, X‐ray diffraction, and X‐ray photoelectron, infrared and Raman spectroscopies. Cu‐Cu2O@RGO as a heterogeneous catalyst was applied to tandem reactions of halides and sodium azide with terminal alkynes to synthesize effectively 1,4‐disubstituted 1,2,3‐triazoles. Moreover, the catalyst showed excellent recyclability performance with very little leaching of the metal. Compared with homogeneous catalysts, Cu‐Cu2O@RGO as a green and efficient catalyst was recoverable, easy to separate and highly stable in the tandem method for the synthesis of 1,2,3‐triazole compounds.  相似文献   
64.
When the Fe(CO)4 and M(CO)5 (M=Cr, W) groups are co-ordinated in C3v and C4v fashion, respectively, in transition metal carbonyl cluster species they contain two sets of non-symmetry related carbonyl groups. In the application of the spherical harmonic model (SHM) to the interpretation of the infrared spectra of these compounds it proves necessary first to treat these as for a normal, isolated, M(CO)4 or M(CO)5 group and then apply the SHM. This recognition gives insights into the general application of the SHM.  相似文献   
65.
唐平生  王博 《应用化学》2017,34(6):664-670
为了合成selaginellin家族天然产物,我们开发了炔锂和对亚甲基苯醌的1,6-加成反应。研究了此反应的反应条件及底物适用范围和限制。结果表明,对亚甲基苯醌苯环上的取代基对反应速率和产率有显著的影响:吸电子基团增加其亲电性,使反应变快,并且可以得到较高的产率;与此相反,给电子基团则降低了对亚甲基苯醌的亲电性,反应变慢,并且表现出可逆性,产率变低。端炔对反应影响很小,均能得到高产率的产物。本方法可用于快速构建selaginellin家族天然产物的骨架结构。  相似文献   
66.
The critical aggregation concentration (CAC) of four with three kinds of conventional surfactants, namely, two cationic surfactants [hexadecyltrimethyl ammonium bromide (CTAB) and tetradecyltrimethyl ammonium bromide (TTAB)], one anionic surfactant [sodium dodecyl sulfate (SDS)], and a nonionic surfactant [Triton X-100 (TX-100)], were determined by variation of 1H chemical shifts with surfactant concentrations. Results show that the CAC values of protons at different positions of the same molecule are different, and those of the terminal methyl protons are the lowest, respectively, which suggests that the terminal groups of the alkyl chains aggregates first during micellization. Measurement of the transverse relaxation time (T2) of different protons in SDS also show that the terminal methyl protons start to decrease with the increase in concentration first, which supports the above mentioned tendency.  相似文献   
67.
A new strategy for a large scale synthesis is developed and evaluated by the improvement of the current synthesis for fenleuton, a drug with important applications in leukotriene-mediated inflammatory diseases that acts through inhibition of the 5-lipoxygenase enzyme.  相似文献   
68.
In this paper,we develop a new algorithm to find the exact solutions of the Einstein's field equations.Time-periodic solutions are constructed by using the new algorithm.The singularities of the time-periodic solutions are investigated and some new physical phenomena,such as degenerate event horizon and time-periodic event horizon,are found.The applications of these solutions in modern cosmology and general relativity are expected.  相似文献   
69.
自然灾害情境造成部分决策信息模糊和应急决策者的消极情绪。考虑消极情绪在自然灾害应急决策的影响,构建了考虑情绪下的前景理论的价值函数。运用反函数原理,根据应急决策者对模糊信息的心理感知价值,构建了其实际值的估计函数,用于解决决策信息模糊下的终端供电设施应急抢修决策规划问题,提出了相应的决策模型和求解算法。采用自主开发的在线实验系统,完成了信息模糊下的电网应急抢修决策实验,验证了决策模型和消极情绪下基于决策者心理感知对模糊信息估值方法的有效性。  相似文献   
70.
The focus of this article is on the analysis of repairable systems that are subject to multiple sources of recurrence. The event of interest at the system level is assumed to be caused by the earliest occurrence of a source, thereby conforming to a series system competing risks framework. Parametric inference is carried out under the power law process model that has found significant attention in industrial applications. Dependence among the cause‐specific recurrent processes is induced via a shared frailty structure. The theoretical inference results are implemented to a warranty database for a fleet of automobiles, for which the warranty repair is triggered by the failure of one of many components. Extensive finite‐sample simulation is carried out to supplement the asymptotic findings. Copyright © 2014 John Wiley & Sons, Ltd.  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