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   568087篇
  免费   6255篇
  国内免费   1004篇
电工技术   10584篇
综合类   467篇
化学工业   90400篇
金属工艺   23351篇
机械仪表   18516篇
建筑科学   12588篇
矿业工程   4314篇
能源动力   14447篇
轻工业   45998篇
水利工程   6944篇
石油天然气   14965篇
武器工业   39篇
无线电   60332篇
一般工业技术   116406篇
冶金工业   95782篇
原子能技术   14688篇
自动化技术   45525篇
  2021年   5497篇
  2019年   5277篇
  2018年   9217篇
  2017年   9429篇
  2016年   9853篇
  2015年   6034篇
  2014年   10269篇
  2013年   26140篇
  2012年   15803篇
  2011年   21154篇
  2010年   16991篇
  2009年   18852篇
  2008年   19115篇
  2007年   18806篇
  2006年   16317篇
  2005年   14777篇
  2004年   14030篇
  2003年   13717篇
  2002年   13276篇
  2001年   12865篇
  2000年   12351篇
  1999年   12030篇
  1998年   27690篇
  1997年   20066篇
  1996年   15660篇
  1995年   12031篇
  1994年   10882篇
  1993年   10631篇
  1992年   8354篇
  1991年   8117篇
  1990年   8008篇
  1989年   7777篇
  1988年   7493篇
  1987年   6766篇
  1986年   6541篇
  1985年   7408篇
  1984年   6716篇
  1983年   6449篇
  1982年   5789篇
  1981年   5909篇
  1980年   5636篇
  1979年   5746篇
  1978年   5676篇
  1977年   6191篇
  1976年   7703篇
  1975年   5119篇
  1974年   4923篇
  1973年   4996篇
  1972年   4286篇
  1971年   4054篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
992.
In this paper we describe a framework for analysing the creation and justification of Research & Development. The 4S framework is developed for analysing the scope, scale, skills and social network aspects of Research & Development value. The framework is based on social system theory, a process contingency model, and recent Research & Development metrics. We present a first empirical assessment based on a workshop using the 4S framework for leveraging Research & Development. Results that assist in the assessment of value creation utilising R & D within networks are very relevant in high tech industries. The multi–dimensional process approach of this framework seems promising for understanding and managing R&D value creation, but needs further operationalisation. Case studies are described and a Dutch network on leveraging R&D has been initiated.  相似文献   
993.
Intestinal calcium absorption has been shown to include two processes, a saturable transcellular movement and a non-saturable paracellular pathway. The potential utility of cell monolayers for studying transepithelial intestinal calcium transport has already been demonstrated; however, simultaneous evaluation of the contribution of the saturable transcellular and of the non-saturable paracellular processes to the total transepithelial transport has not yet been attempted. The aim of this study was to investigate the contribution both of transcellular and paracellular transport processes to the total transepithelial calcium transport in two cell culture monolayers. Caco-2 cells and a clone derived from HT-29 cells (HT29-Cl.19A), two cell lines derived from colon adenocarcinomas which are known to be able to exhibit typical enterocytic differentiation, were used. Cell monolayers were grown on a permeable support and used after 15 days of culture when these cells express enterocytic differentiation and high transepithelial resistance. Isotopic transport rate measurements were performed in the absence of a chemical gradient. The paracellular route was evaluated using [3H]mannitol. Calcium and [3H]mannitol transport rates across cell monolayers were not significantly different. Augmentation of calcium uptake by 200 mM sorbitol did not significantly increase calcium or mannitol transepithelial transport; however, calcium accumulation in the cells was increased by about 200%. Modulation of the monolayer permeability by addition of 10 nM vasoactive intestinal polypeptide (VIP) or 0.5 mM carbachol treatment, which respectively increased and decreased the transepithelial resistance, consequently modified calcium and mannitol transport in a parallel manner. Our results show that Caco-2 and HT29-Cl.19A cell monolayers are good models for studying the calcium paracellular transport pathway.  相似文献   
994.
Guided by the empirical observation that real-time MPEG video streams exhibit both multiple time scale and subexponential characteristics, we construct a video model that captures both of these characteristics and is amenable to queueing analysis. We investigate two fundamental approaches for extracting the model parameters: using sample path and second-order statistics-based methods. The model exhibits the following two canonical queueing behaviors. When strict stability conditions are satisfied, i.e., the conditional mean of each scene is smaller than the capacity of the server, precise modeling of the interscene dynamics (long-term dependency) is not essential for the accurate prediction of small to moderately large queue sizes. In this case, the queue length distribution is determined using quasistationary (perturbation theory) analysis. When weak stability conditions are satisfied, i.e., the conditional mean of at least one scene type is greater than the capacity of the server, the dominant effect for building a large queue size is the subexponential (long-tailed) scene length distribution. In this case, precise modeling of intrascene statistics is of secondary importance for predicting the large queueing behavior. A fluid model, whose arrival process is obtained from the video data by replacing scene statistics with their means, is shown to asymptotically converge to the exact queue distribution. Using the transition scenario of moving from one stability region to the other by a change in the value of the server capacity, we synthesize recent queueing theoretic advances and ad hoc results in video modeling, and unify a broad range of seemingly contradictory experimental observations found in the literature. As a word of caution for the widespread usage of second-order statistics modeling methods, we construct two processes with the same second-order statistics that produce distinctly different queueing behaviors  相似文献   
995.
996.
997.
Textural evolution in electrodeposits is strongly affected by foreign species in the electrolyte and also by the co-deposition of hydrogen. The effects of foreign species on textural development in electrodeposits were studied by employing a Monte Carlo model. This model is based on physical principles and incorporates parameters which are used to control electrodeposition processes. An attempt was made in the simulation to reflect the basic process of the deposit growth, and to identify the factors responsible for the texture formation and its variation with the deposition condition. It is assumed that the texture of the deposit results from the minimization of the free energy of the system. Based on this assumption, it was demonstrated that the surface-energy anisotropy played an important role in the formation of fibre texture. In particular, the model can be used to simulate textural evolution under the influence of adsorbed foreign species, because the surface-energy anisotropy is modified by the adsorbed foreign species. Part I of the paper presents results of our studies on iron electrodeposition with the emphasis on the effect of hydrogen adsorption on iron deposit texture. Relevant experimental results have also been presented to corroborate the simulation study. Part II of the paper presents simulation studies of textural evolution in copper deposits under the influence of adsorbed potassium and chloride ions. This revised version was published online in November 2006 with corrections to the Cover Date.  相似文献   
998.
All leaders in health care today are charged with the responsibility of transforming present practices into new and different ones that are needed for the future. The structural side of transforming is ultimately easier than the human side. However, the most frequent failures come from not concentrating sufficiently on the behavioral side of the change. Structural and psychological change must occur simultaneously and embrace each other for best results.  相似文献   
999.
This paper describes a prototype video coding platform meant for the conception and testing of multimedia products such as next-generation videophones. The platform is largely based on ITU-T Recommendation H.263, with a number of additional object-oriented quality enhancement features which make it especially well suited for very low bit-rate coding of “head-and-shoulders” video material typical of real-time multimedia applications, video teleconferencing, and video telephony. These features consist of: (1) segmentation into objects of interest, (2) segmentation-based prefiltering, (3) model-assisted rate control, (4) adaptive vector quantization, and finally (5) segmentation-based postfiltering. In the spirit of Recommendation H.263, these enhancements are modular and can be selectively turned on or off, thereby enabling a wide variety of coding modes  相似文献   
1000.
The authors discuss the modeling of zero impedance branches (short circuits) in power system state estimation. The concept of state variable in power system state estimation is generalized, and an extended version of the numerical observability method is proposed to cope with complex situations involving multiple short circuit branches, as they occur in some station arrangements. The impact of short circuit exact representation on bad data identification is also discussed  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