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   23199篇
  免费   1062篇
  国内免费   323篇
电工技术   477篇
综合类   545篇
化学工业   4161篇
金属工艺   531篇
机械仪表   765篇
建筑科学   908篇
矿业工程   182篇
能源动力   626篇
轻工业   2328篇
水利工程   262篇
石油天然气   183篇
武器工业   27篇
无线电   2463篇
一般工业技术   2983篇
冶金工业   4815篇
原子能技术   172篇
自动化技术   3156篇
  2023年   135篇
  2022年   413篇
  2021年   565篇
  2020年   322篇
  2019年   406篇
  2018年   465篇
  2017年   441篇
  2016年   498篇
  2015年   411篇
  2014年   606篇
  2013年   1144篇
  2012年   892篇
  2011年   1106篇
  2010年   812篇
  2009年   876篇
  2008年   863篇
  2007年   870篇
  2006年   758篇
  2005年   637篇
  2004年   702篇
  2003年   916篇
  2002年   1217篇
  2001年   1023篇
  2000年   595篇
  1999年   564篇
  1998年   1688篇
  1997年   1015篇
  1996年   762篇
  1995年   508篇
  1994年   380篇
  1993年   404篇
  1992年   199篇
  1991年   160篇
  1990年   158篇
  1989年   147篇
  1988年   149篇
  1987年   124篇
  1986年   123篇
  1985年   167篇
  1984年   86篇
  1983年   90篇
  1982年   86篇
  1981年   112篇
  1980年   106篇
  1979年   61篇
  1978年   53篇
  1977年   139篇
  1976年   275篇
  1975年   54篇
  1973年   52篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
Interest in methods that routinely and accurately measure and predict animal characteristics is growing in importance, both for quality characterization of livestock products and for genetic purposes. Mid-infrared spectroscopy (MIRS) is a rapid and cost-effective tool for recording phenotypes at the population level. Mid-infrared spectroscopy is based on crossing matter by electromagnetic radiation and on the subsequent measure of energy absorption, and it is commonly used to determine traditional milk quality traits in official milk laboratories. The aim of this review was to focus on the use of MIRS to predict new milk phenotypes of economic relevance such as fatty acid and protein composition, coagulation properties, acidity, mineral composition, ketone bodies, body energy status, and methane emissions. Analysis of the literature demonstrated the feasibility of MIRS to predict these traits, with different accuracies and with margins of improvement of prediction equations. In general, the reviewed papers underlined the influence of data variability, reference method, and unit of measurement on the development of robust models. A crucial point in favor of the application of MIRS is to stimulate the exchange of data among countries to develop equations that take into account the biological variability of the studied traits under different conditions. Due to the large variability of reference methods used for MIRS calibration, it is essential to standardize the methods used within and across countries.  相似文献   
993.
994.
995.
High‐rise buildings are extensively built in China, and the structure of these buildings is composed of different types of conventional system of which framed structures are more commonly employed. A new cassette structure with advantageous performance properties is proposed. Static and dynamic numerical simulations were applied to investigate the characteristics of this new structure. First, the components and other details are presented. A comparative analysis was conducted between the cassette structure and traditional structures using eight finite models with fiber elements in three different heights. A static pushover analysis and an incremental dynamic analysis were conducted based on 18 near‐ground motion records recommended by the Federal Emergency Management Agency. The seismic characteristics, deformation curve, interstory drift, roof displacement, and fragility curve are investigated. Based on the analysis of the models with three different heights and the variety of seismic records, the economic advantage and application of cassette structures for building industrialization are also discussed.  相似文献   
996.
Since eutomer and distomer show different biological effects, access to pure enantiomers is essential. Preferential enrichment is a relatively new process of chiral resolution developed in the 90s. Extension of its scope was attempted with tryptophan ethyl ester hydrochloride. This salt meets one of the most important requirements for preferential enrichment, namely, a larger solubility for the pure enantiomers with reference to the racemic compound. But, due to the absence of a solid solution, this salt cannot perform preferential enrichment. This impossibility was endorsed by semi‐empirical molecular modeling.  相似文献   
997.
In the human body, the black‐brown biopigment eumelanin blocks harmful ultraviolet (UV) radiation. In the plastics industry, additives are often added to polymers to increase their UV‐absorption properties. We herein report an assessment of the biopigment eumelanin as a nature‐inspired additive for plastics to enhance their UV absorption. Since eumelanin is produced by natural sources and is nontoxic, it is an interesting candidate in the field of sustainable plastic additives. In this work, the eumelanin‐containing films of commercial ethylene–vinyl acetate copolymer, a plastic used for packaging applications, were obtained by melt compounding and compression molding. The biopigment dispersion in the films was improved by means of the melanin free acid treatment. It was observed that eumelanin amounts as low as 0.8 wt% caused an increase of the UV absorption, up to one order of magnitude in the UVA range. We also evaluated the effect of eumelanin on the thermal stability and photostability of the films: the biopigment proved to be double‐edged, working both as UV‐absorption enhancer and photo‐prooxidant, as thermogravimetric analysis and infrared spectroscopy revealed. © 2019 Society of Chemical Industry  相似文献   
998.
There is a considerable attention for the development of inhibitors of tyrosinase (TYR) as therapeutic strategy for the treatment of hyperpigmentation disorders in humans. Continuing in our efforts to identify TYR inhibitors, we describe the design, synthesis and pharmacophore exploration of new small molecules structurally characterized by the presence of the 4-fluorobenzylpiperazine moiety as key pharmacophoric feature for the inhibition of TYR from Agaricus bisporus (AbTYR). Our investigations resulted in the discovery of the competitive inhibitor [4-(4-fluorobenzyl)piperazin-1-yl]-(3-chloro-2-nitro-phenyl)methanone 26 (IC50=0.18 μM) that proved to be ∼100-fold more active than reference compound kojic acid (IC50=17.76 μM). Notably, compound 26 exerted antimelanogenic effect on B16F10 cells in absence of cytotoxicity. Docking analysis suggested its binding mode into AbTYR and into modelled human TYR.  相似文献   
999.
This paper presents an agent-based model of an organization. The model is made of a social network—composed of the different organization workers—and a knowledge network. Workers are assigned tasks, for which they have to use information in the knowledge network. We have modeled the quality of the information by assigning each information item a probability of being wrong. Agents can interact with other agents, who can recommend to them new information items in the knowledge network for the task to be performed. Workers are assigned different information-seeking behavior (passive, active, and learning), governing the way in which they interact with each other. Moreover, indirect interaction is also possible, as a publicly accessible knowledge base contains each agent's preferred information items.

The model was implemented in SDML, and its simulation shows that agents quickly learn to discern the better information items for the given task. However, group formation (agents' collaborating by exchanging information) takes longer to stabilize. Additionally, when the quality of items is changed, agents can quickly select the better new knowledge items, and organization performance improves again to a maximum that is only randomly disturbed.  相似文献   
1000.
This paper extends the emerging literature on the value of trademarks for innovation studies and policy-making with the first empirical study at the trademark level. It gives a view on how companies use trademarks and interpret trademark activities. A sample of 660 new Benelux trademarks registered by small- and medium-sized enterprises reveals that 60 per cent of recently registered Benelux trademarks refer to innovation activity, predominantly to product or service innovation. The reference to innovation co-varies with various applicant and trademark characteristics unknown from previous studies. Finally, the sample reveals that most of the trademarks used to signal innovative offers are filed close to its market introduction without combining them with other intellectual property rights. This holds especially for trademarks related to service innovation.  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