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   298725篇
  免费   3043篇
  国内免费   658篇
化学   163059篇
晶体学   4391篇
力学   12492篇
综合类   9篇
数学   34008篇
物理学   88467篇
  2020年   2584篇
  2019年   2784篇
  2018年   3767篇
  2017年   3717篇
  2016年   5655篇
  2015年   3603篇
  2014年   5270篇
  2013年   12689篇
  2012年   10249篇
  2011年   12270篇
  2010年   8462篇
  2009年   8043篇
  2008年   11302篇
  2007年   11428篇
  2006年   10562篇
  2005年   9580篇
  2004年   8652篇
  2003年   7677篇
  2002年   7535篇
  2001年   7907篇
  2000年   6204篇
  1999年   4762篇
  1998年   4158篇
  1997年   4079篇
  1996年   3933篇
  1995年   3570篇
  1994年   3723篇
  1993年   3459篇
  1992年   3905篇
  1991年   3832篇
  1990年   3747篇
  1989年   3595篇
  1988年   3705篇
  1987年   3589篇
  1986年   3408篇
  1985年   4535篇
  1984年   4791篇
  1983年   4002篇
  1982年   4118篇
  1981年   4033篇
  1980年   3993篇
  1979年   3973篇
  1978年   4285篇
  1977年   4196篇
  1976年   4252篇
  1975年   3924篇
  1974年   3956篇
  1973年   4134篇
  1972年   2892篇
  1971年   2418篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
1H-Pyrazoles, triazoles, and imidazoles in reaction with ethoxycarbonylimine and arylsulfonylimines of chloral yield addition products, corresponding 1-(1-amidotrichloroethyl)azoles. Derivatives of 1-alkylpyrazoles and pyrazolones react with chloral 4-chlorophenylsulfonylimine to furnish products of C-amidotrichloroethylation into position 4 of the azole ring.  相似文献   
83.
Fundamental processes in long air gap discharges   总被引:2,自引:0,他引:2  
The development of atmospheric lightning is initiated and sustained by the formation in virgin air of ‘streamer corona’ and ‘leader’ discharges, very similar to those observed in laboratory long sparks. Therefore, the experimental and theoretical investigations of these laboratory discharges have become of large interest to improve the physical knowledge of the lightning process and to develop self-consistent models that could be applied to new protection concepts.In the present paper the fundamental processes of the subsequent phases of long air gap discharges are analyzed, from the first corona inception and development to the leader channel formation and propagation. For all these processes simulations models are discussed that have been essentially derived and simplified by the authors, in order to develop sequential time-dependent simulation of the laboratory breakdown, with both positive and negative voltages. The possibility of extending these models to the case of natural lightning is discussed in the companion paper, presented in this same volume. To cite this article: I. Gallimberti et al., C. R. Physique 3 (2002) 1335–1359.  相似文献   
84.
Regioselective demethylation of 3,4-dihydropapaveraldine (2a) at 7 and 3' positions affords a properly substituted diphenolic key intermidiate (2d) for the synthesis of reticuline and N-norreticuline.  相似文献   
85.
We propose a simplified method of determining the integral characteristics of the opening of a parachute which makes it possible to compute the maximal stress on a parachute and the trajectory parameters of the motion of the load in the region of the opening of the parachute, and to determine the size of the portion of the canopy at the time of maximal stress.Translated fromDinamicheskie Sistemy, No. 6, 1987, pp. 38–41.  相似文献   
86.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 4, pp. 581–587, April, 1991.  相似文献   
87.
It was shown that the reaction of o-hyroxyphenyldiphenylmethanol with benzaldehyde labeled with 17O and 18O isotopes proceeds in such a way that oxygen atoms of the starting phenoloalcohol are retained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 632–633, May, 1991.  相似文献   
88.
It is proved that the variety of relevant disjunction lattices has the finite embeddability property. It follows that Avron's relevance logic RMI min has a strong form of the finite model property, so it has a solvable deducibility problem. This strengthens Avron's result that RMI min is decidable. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
89.
The kinetics of radical copolymerization of acrylamide with lithium, sodium, and potassium 2-acrylamido-2-methylpropanesulfonates in concentrated aqueous solutions in the presence of potassium persulfate as initiating agent at pH 9 and 50°C was studied by dilatometry.  相似文献   
90.
In this paper we suggest a unique model for estimating the operating cost of each of three waste-collection systems. Under the traditional system, which is widely used, waste is typically collected in plastic bags and a three-man crew is needed on each vehicle. The other two systems require a one-man crew for vehicle collecting street containers. The side-loader system with fixed body automatically empties street containers into the vehicle body and empties the load at the disposal site. The side-loader system with demountable body allows the separation of the waste collection phase from transport to the disposal site, since the vehicle body can be demounted. We also present two case studies and show how the estimation of operating costs is a critical issue in decisions regarding the type of system to be used for waste collection.  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