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   21960篇
  免费   2206篇
  国内免费   18篇
  24184篇
  2022年   214篇
  2021年   446篇
  2020年   243篇
  2019年   307篇
  2018年   395篇
  2017年   327篇
  2016年   479篇
  2015年   908篇
  2014年   894篇
  2013年   1230篇
  2012年   1555篇
  2011年   1509篇
  2010年   943篇
  2009年   857篇
  2008年   1204篇
  2007年   1224篇
  2006年   1129篇
  2005年   1185篇
  2004年   1104篇
  2003年   1038篇
  2002年   1045篇
  2001年   249篇
  2000年   196篇
  1999年   287篇
  1998年   281篇
  1997年   184篇
  1996年   176篇
  1995年   165篇
  1994年   155篇
  1993年   174篇
  1992年   173篇
  1991年   166篇
  1990年   159篇
  1989年   168篇
  1988年   165篇
  1987年   145篇
  1986年   134篇
  1985年   155篇
  1984年   148篇
  1983年   148篇
  1982年   193篇
  1981年   195篇
  1980年   149篇
  1979年   127篇
  1978年   142篇
  1977年   112篇
  1976年   111篇
  1975年   103篇
  1974年   103篇
  1973年   94篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
In contradistinction to the observation made before that iproniazid has a potentiating effect on certain amines in guinea pigs, human studies have shown that this effect has very little clinical importance, when both the amines and the amine oxidase inhibitor are given in the usual therapeutic doses. However, neither in man nor in the guinea pig pretreatment with iproniazid showed a potentiating effect on amines which are not substrates of monoamine oxidase (ephedrine and amphetamine). Patients with epilepsy and allergic disease may receive iproniazid.  相似文献   
52.
53.
The Nuclear Envelope after KMnO4 Fixation   总被引:2,自引:2,他引:0       下载免费PDF全文
  相似文献   
54.
55.
56.
57.
Summary A study was made of the fine structure of neurons in lumbar sympathetic ganglia which were surgically removed from a 52 year old male patient. Some samples were fixed by direct immersion into osmium tetroxide, others were first prefixed with an aldehyde solution. Large vesiculated pigment granules were described, compared with similar particles, previously seen in human and animal autonomic neurons, and their glycogen and lipid-soluble components were tentatively identified. These granules increase numerically during aging, are probably derivatives of lysosomes, and may incorporate both lipofuscin and melanin in the same particle. Mitochondrial changes (enlargement, disappearance of cristae and deposit of opaque central material) in sympathetic perikarya occurred under all conditions of preservation, and were interpreted as hypoxia-induced structures which developed during life. Abnormal aggregates of mitochondria were seen in nerve processes. Laminar bodies, chiefly associated with mitochondria were present, especially, in aldehyde prefixed material. It remained undecided whether these structures occurred during life or were artifacts. Compared with direct osmium tetroxide fixation, prefixation with aldehyde, followed by osmium tetroxide proved to be a superior, but not entirely adequate method of preserving deep layers of neuronal tissue. This investigation was supported (in whole) by United States Public Health Service Grant NB 1879-09, Institute for Nervous Diseases and Blindness.The author is greatly indebted to Dr. Jere W. Lord, Jr., Department of Surgery, New York University Medical Center, for the surgically removed tissues used in this study; he is grateful to Dr. Sanford L. Palay, Department of Anatomy, Harvard Medical School, for suggesting the use of the then unpublished method of prefixation with the aldehyde solution. It is a pleasure to acknowledge the expert skill with which Miss Carmen DeLemos and Mr. Arnlod Cianella assisted in completing this work.  相似文献   
58.
The uncontrolled transmission of hog cholera in a large animal isolation facility, designed to control the movement of aerosols within and between individual wings of a multiunit building, indicated the need for a critical study of aerosol behavior under existing conditions of operation. Studies with aerosols of Escherichia coli B T3 bacteriophage (T3 coliphage) conclusively demonstrated the impossibility of obtaining the desired control by means of a "static" air balance relationship between adjacent areas within the facility. Modifications needed to provide the desired control of the air-handling system are outlined and discussed.  相似文献   
59.
60.
Early flexible manufacturing system (FMS) production planning models exhibited a variety of planning objectives; typically, these objectives were independent of the overall production environment. More recently, some researchers have proposed hierarchical production planning and scheduling models for FMS. In this article, we examine production planning of FMS in a material requirements planning (MRP) environment. We propose a hierarchical structure that integrates FMS production planning into a closed-loop MRP system. This structure gives rise to the FMS/MRP rough-cut capacity planning (FMRCP) problem, the FMS/MRP grouping and loading (FMGL) problem, and the FMS/MRP detailed scheduling problem. We examine the FMRCP and FMGL problems in detail and present mathematical programming models for each of these problems. In particular, the FMRCP problem is modeled as a generalized assignment problem (GAP), and a GAP-based heuristic procedure is defined for the problem. We define a two-phase heuristic for the FMGL problem and present computational experience with both heuristics. The FMRCP heuristic is shown to solve problems that exhibit a dependent-demand relation within the FMS and with FMS capacity utilization as high as 99 percent. The FMGL heuristic requires very little CPU time and obtains solutions to the test problems that are on average within 1.5 percent of a theoretical lower bound. This FMS/MRP production planning framework, together with the resulting models, constitutes an important step in the integration of FMS technology with MRP production planning. The hierarchical planning mechanism directly provides for system-level MRP planning priorities to induce appropriate production planning and control objectives on the FMS while simultaneously allowing for necessary feedback from the FMS. Moreover, by demonstrating the tractability of the FMRCP and FMGL problems, this research establishes the necessary groundwork upon which to explore systemwide issues pertaining to the coordination of the hierarchical structure.  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