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   38775篇
  免费   2846篇
  国内免费   35篇
电工技术   239篇
综合类   20篇
化学工业   10581篇
金属工艺   553篇
机械仪表   816篇
建筑科学   1264篇
矿业工程   74篇
能源动力   836篇
轻工业   8444篇
水利工程   410篇
石油天然气   184篇
武器工业   1篇
无线电   1512篇
一般工业技术   5961篇
冶金工业   5913篇
原子能技术   101篇
自动化技术   4747篇
  2024年   106篇
  2023年   450篇
  2022年   1207篇
  2021年   1741篇
  2020年   1058篇
  2019年   1090篇
  2018年   1676篇
  2017年   1670篇
  2016年   1749篇
  2015年   1392篇
  2014年   1663篇
  2013年   3137篇
  2012年   2554篇
  2011年   2464篇
  2010年   1964篇
  2009年   1737篇
  2008年   1728篇
  2007年   1596篇
  2006年   1091篇
  2005年   879篇
  2004年   806篇
  2003年   753篇
  2002年   667篇
  2001年   466篇
  2000年   354篇
  1999年   442篇
  1998年   2006篇
  1997年   1299篇
  1996年   833篇
  1995年   466篇
  1994年   359篇
  1993年   377篇
  1992年   151篇
  1991年   120篇
  1990年   98篇
  1989年   92篇
  1988年   114篇
  1987年   106篇
  1986年   101篇
  1985年   106篇
  1984年   88篇
  1983年   57篇
  1982年   51篇
  1981年   77篇
  1980年   79篇
  1979年   47篇
  1978年   50篇
  1977年   117篇
  1976年   215篇
  1975年   44篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
991.
992.
NiPcTSTNa(L) [L = ethylenediamine (EDA); 1,4-diaminobutane (BDA); and 2,6-diamineanthraquinone (AqDA)] thin films were deposited by thermal evaporation. Their surface morphology was studied by AFM and SEM, and their chemical composition determined by EDS. Optical absorption studies of NiPcTSTNa(L) films were performed in the 200?C1150?nm wavelength range. The optical bandgap of thin films was determined from the (??h ??)1/2 vs h?? plots for indirect allowed transitions. The temperature dependence of electrical conductivity shows a semiconducting behaviour. The amorphous semiconductor films show thermal activation energies of electrical conduction between 3·3 and 3·7?eV.  相似文献   
993.
Are institutional repositories mere warehouses for digital documents or are they in fact establishing themselves as a rigorous option for the spread of scientific knowledge? This study analyses the competitive environment of the Top100 university repositories, defined as leaders in terms of market participation and penetration. The study also analyses the basic functionalities of preservation and diffusion of academic production through factors related to the prestige of the repositories and of the institutions that operate them. The results show that repositories with a larger digital academic supply are associated with the production of demonstrated scientific rigor.  相似文献   
994.
The aim of this paper is to present experimental validation results of an energy management system for hybrid electrical vehicles based on type-2 fuzzy logic. The energy management system (EMS) is designed by extracting knowledge from several experts using surveys. The consideration of interval type-2 fuzzy sets enables modeling the uncertainty in the answers of the experts. The validation of the EMS is performed on a real-scale heavy duty vehicle equipped with different energy sources such as batteries, fuel cell system and ultracapacitors. Experimental results are strong evidence that type-2 fuzzy logic is wide adapted for performing the energy management in hybrid electrical vehicles.  相似文献   
995.
We solve the simultaneous closed‐loop identification and tracking‐control problems for fully‐actuated Euler–Lagrange systems under input constraints. We use a nonlinear adaptive controller reminiscent of computed‐torque‐type controllers in which linear correction terms are saturated in order to comply with the imposed bounds on the control inputs. Adaptation, reminiscent of gradient methods, is used also with saturation. With respect to related literature, our contribution consists in establishing uniform global asymptotic stability. Therefore, our control scheme ensures robustness with respect to bounded perturbations and uniform convergence of the estimation errors for any initial conditions. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
996.
Biodiesel is a renewable bio-fuel derived from natural fats or vegetable oils, and it is considered as a promising alternative to substitute diesel fuels. Cetane number, viscosity, density, and higher heating value are important properties to affect the utilization of biodiesel fuels, because they are involved in the definition of fuel quality and are required as input data for predictive engine combustion models. This work presents the characterization of two biodiesel samples made from beef tallow and soybean oil through their fatty acid methyl esters (FAMEs) profile. Empirical equations were developed to estimate four physical properties of methyl esters; and an average absolute deviation (AAD) of 5.95%, 2.57%, 0.11% and 0.21% for the cetane number, kinematic viscosity, density, and higher heating value were founded. Cetane number, viscosity, and higher heating value increases because of the increase of molecular weight and these physical properties decrease as the number of double bonds increases. Unlike that of above properties, density decreases as molecular weight increases and density increases as the degree of unsaturation increases. Two general mixing rules and five biodiesel samples were used to study the influence of FAMEs over the physical properties of biodiesel. The prediction of the cetane number, kinematic viscosity, density and higher heating value of biodiesel is very close to the experimental values.  相似文献   
997.
998.
Different solutions for the integration of high-speed rail (HSR) and air transport could be implemented, ranging from very basic integration to more sophisticated systems that include ticket and handling integration. This paper uses two statistical techniques, name cluster analysis and discrete choice models in order to investigate how different market segments have a determining influence in being more proactive to change to HSR for the second leg in multimodal trips. A discrete choice experiment is conducted to better understand passengers’ preferences. We obtain a number of clusters and estimate flexible choice models, taking into account the panel nature of stated preference data. We obtain a range of willingness-to-pay values for service quality attributes, finding results that can be used to infer policy conclusions about the real attractiveness of the Air-HSR integrated alternative. In this respect, clusters and schedule coordination, which reduces connecting time, are crucial to explain HSR attractiveness.  相似文献   
999.
Automatic segmentation of multiple sclerosis (MS) lesions in brain MRI has been widely investigated in recent years with the goal of helping MS diagnosis and patient follow-up. However, the performance of most of the algorithms still falls far below expert expectations. In this paper, we review the main approaches to automated MS lesion segmentation. The main features of the segmentation algorithms are analysed and the most recent important techniques are classified into different strategies according to their main principle, pointing out their strengths and weaknesses and suggesting new research directions. A qualitative and quantitative comparison of the results of the approaches analysed is also presented. Finally, possible future approaches to MS lesion segmentation are discussed.  相似文献   
1000.
The main chemical and biochemical constituents of nine edible mushrooms were evaluated in this study. An enzymatic method was used to determine the α‐ and β‐glucan contents, and the amino acid (AA) profiles were evaluated by RP‐HPLC‐DAD. The recommended dietary allowance (RDA) of the nutrients was evaluated. Finally, principal component analysis (PCA) was used to group the mushrooms according to their AA, glucans and chemical composition. All mushrooms evaluated can be considered as a potential and alternative source of dietary fibre (24.4–46.62%) and protein (16.47–36.96%) in a diet, with low contents of fat (1.40–2.08%). Phosphorus, potassium, iron, copper and zinc were the main minerals found in the mushrooms, whereas sodium concentration was negligible. All essential AA were detected in the mushrooms, with Pleurotus ostreatus (black oyster) representing the main source of essential amino acids among the samples. The contents of β‐glucan ranged from 1.58 to 16.91 mg g?1 of dry matter among the mushrooms and Pleurotus eryngii presented the highest amount of this component. Pleurotus ostreatus (black oyster) and P. eryngii were also grouped together due to similarities in their phosphorous and arginine contents. Among the main results, it can highlight the high concentration of dietary fibre and essential amino acids of the edible mushrooms.  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