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   357479篇
  免费   2099篇
  国内免费   643篇
化学   171037篇
晶体学   5434篇
力学   18538篇
综合类   9篇
数学   52795篇
物理学   112408篇
  2019年   2323篇
  2018年   11123篇
  2017年   11916篇
  2016年   8269篇
  2015年   3449篇
  2014年   4427篇
  2013年   10863篇
  2012年   12095篇
  2011年   21279篇
  2010年   13636篇
  2009年   13513篇
  2008年   18923篇
  2007年   22284篇
  2006年   9108篇
  2005年   13971篇
  2004年   10484篇
  2003年   9908篇
  2002年   7969篇
  2001年   7482篇
  2000年   5968篇
  1999年   4398篇
  1998年   3905篇
  1997年   3865篇
  1996年   3764篇
  1995年   3446篇
  1994年   3582篇
  1993年   3269篇
  1992年   3733篇
  1991年   3742篇
  1990年   3685篇
  1989年   3553篇
  1988年   3664篇
  1987年   3547篇
  1986年   3367篇
  1985年   4447篇
  1984年   4738篇
  1983年   3961篇
  1982年   4094篇
  1981年   4021篇
  1980年   3992篇
  1979年   3977篇
  1978年   4295篇
  1977年   4140篇
  1976年   4230篇
  1975年   3898篇
  1974年   3938篇
  1973年   4128篇
  1972年   2883篇
  1971年   2399篇
  1970年   2205篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Cognitive style refers to differences that can be perceived in individuals due to differing cognitive structures. There has been a large amount of research on how cognitive style affects managerial decision-making, particularly in the context of management information systems, but little or no attempt to relate these findings to operational research. This paper reviews the findings of cognitive-style research from the perspective of operational research, and draws a number of useful inferences concerning the matching of OR aids to managers, the presentation of information and recommendations, the use and acceptance of OR, and the structure of OR groups. Further, it is argued that cognitive-style findings help to give a justification to the move towards ‘soft’ approaches to analysis.  相似文献   
82.
83.
The objective of this research is to evaluate a new methodology, data envelopment analysis (DEA), as a tool for measuring and evaluating the operational efficiency of US Air Force organizations. This study involves the application of DEA to locate possible inefficiencies in the performance of US Air Force real-property maintenance activities. Dynamic behaviour, as exhibited via window analyses, is reviewed along with static counterparts. The testing was done in close coordination with Air Force officials, who reviewed the results for accuracy, validity and relevance. We conclude that this type of efficiency analysis does have value for the Air Force, where it can serve as a guide to auditors, budget programmers, managers and others in measuring, evaluating and enhancing operational efficiency.  相似文献   
84.
The first enantiospecific synthesis of allopupukeanones has been accomplished starting from 6-methylcarvone. A biogenetically patterned rearrangement of a pupukeanane to allopupukeanane was employed as the key step.  相似文献   
85.
86.
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   
87.
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   
88.
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   
89.
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   
90.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 4, pp. 581–587, April, 1991.  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