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   8275篇
  免费   140篇
  国内免费   14篇
电工技术   131篇
综合类   6篇
化学工业   1108篇
金属工艺   138篇
机械仪表   97篇
建筑科学   144篇
矿业工程   27篇
能源动力   239篇
轻工业   315篇
水利工程   21篇
石油天然气   20篇
无线电   731篇
一般工业技术   1082篇
冶金工业   3542篇
原子能技术   39篇
自动化技术   789篇
  2023年   48篇
  2022年   81篇
  2021年   123篇
  2020年   106篇
  2019年   104篇
  2018年   138篇
  2017年   134篇
  2016年   113篇
  2015年   91篇
  2014年   129篇
  2013年   369篇
  2012年   198篇
  2011年   260篇
  2010年   211篇
  2009年   212篇
  2008年   229篇
  2007年   191篇
  2006年   188篇
  2005年   186篇
  2004年   149篇
  2003年   135篇
  2002年   112篇
  2001年   114篇
  2000年   89篇
  1999年   177篇
  1998年   1036篇
  1997年   633篇
  1996年   413篇
  1995年   249篇
  1994年   247篇
  1993年   280篇
  1992年   121篇
  1991年   106篇
  1990年   87篇
  1989年   82篇
  1988年   111篇
  1987年   88篇
  1986年   82篇
  1985年   100篇
  1984年   60篇
  1983年   51篇
  1982年   52篇
  1981年   67篇
  1980年   64篇
  1979年   42篇
  1978年   42篇
  1977年   94篇
  1976年   201篇
  1975年   40篇
  1973年   35篇
排序方式: 共有8429条查询结果,搜索用时 11 毫秒
11.
12.
13.
We report isolating the Drosophila retinal degeneration E (rdgE) mutation. The hypomorphic rdgE1 allele causes rapid photoreceptor degeneration in light and a slower rate of degeneration when the flies are raised in constant darkness. The rdgE1 flies exhibited an electrophysiological light response that decreased with age, coinciding with the degeneration. This suggests that degeneration caused the loss of the light response. We determined that the ninaE (rhodopsin) mutation, but not norpA [phospholipase C (PLC)], slowed the rdgE-dependent degeneration. This was consistent with the light-enhanced degeneration, but revealed that the degeneration is independent of the PLC-mediated phototransduction cascade. Transmission electron microscopy revealed that rdgE1 photoreceptors exhibited a number of vesicular transport defects including unpacking/vesiculation of rhabdomeres, endocytosis of novel vesicles by photoreceptors, a buildup of very large multivesicular bodies, and an increased amount of rough endoplasmic reticulum. We determined that the rdgE null phenotype is a late embryonic lethality. Therefore, rdgE+ is required in cells outside of the retina, quite possibly in a large number of neurons. Thus, rdgE may define a mutational class that exhibits both light-enhanced retinal degeneration and a recessive null lethality by perturbing neuronal membrane biosynthesis and/or recycling.  相似文献   
14.
Lake Erie's food web has been dramatically modified by exotic species. Both exotic dreissenid mussels and the round goby Neogobius melanastomus have shifted the food web from a pelagicbased to a benthic-based one, potentially creating a new pathway for contaminant transfer to top predators. Before the invasion of round gobies, few predators of dreissenids occurred in Lake Erie, allowing contaminants to be confined to these benthic organisms. The invasion of the round goby has produced a new pathway through which these contaminants can enter the food web. To characterize heavy-metal transfer through this new food web and to assess risk to humans, water, surficial sediment, dreissenid, round goby, and smallmouth bass Micropterus dolomieui samples were collected at three sites during summers, 2002 and 2003, and analyzed for total lead (Pb), total mercury (Hg), and methyl mercury (MeHg). In addition, we compared smallmouth bass Pb and Hg concentrations to those measured in 1993/1994, before round gobies were prevalent. Pb biodiminished and MeHg biomagnified through the food web to smallmouth bass; patterns were similar among our three sites. Total Pb concentrations in smallmouth bass were higher before the incorporation of round gobies into their diet. We attributed this decline to changes in food web structure, changes in contaminant burdens in prey, or declines in sediment Pb concentrations in Lake Erie. By comparison, Hg concentrations in smallmouth bass changed little, before and after the round goby invasion, possibly due to a shift in diet that increased growth. Despite a decline in sediment Hg concentrations in Lake Erie, smallmouth bass continued to accumulate Hg at historical rates possibly because of their high consumption rates of benthivorous round gobies. As smallmouth bass continue to consume round gobies during their lives, their Hg concentrations may well continue to increase, potentially increasing the risk of Hg contamination to humans.  相似文献   
15.
This study examined the effects of instructional match and content overlap on students' ability to generalize from passage reading instruction. Four students with mild disabilities served as participants. Using a multielement design, students were instructed with passages at two levels of text difficulty (instructionally matched vs. instructionally mismatched), and generalization was assessed with passages at two levels of similarity to those instructed (low vs. high content overlap). Results indicated that students' oral reading accuracy and fluency showed the greatest degree of generalization when instructional materials were matched to the students' skill level and assessment materials were similar to those used during instruction. Moreover, these results were maintained at 1-month follow-up. The implications of these findings for classroom reading instruction and the assessment of students' reading skills are discussed.  相似文献   
16.
Evaluation of wear of turning carbide inserts using neural networks   总被引:2,自引:0,他引:2  
Recent trends, being towards mostly unmanned automated machining systems and consistent system operations, need reliable on-line monitoring processes. A proper on-line cutting tool condition monitoring system is essential for deciding when to change the tool. Many methods have been attempted in this connection.Recently, artificial neural networks have been tried for this purpose because of its inherent simplicity and reasonably quick data-processing capability. The present work uses the back propagation algorithm for training the neural network of 5-3-1 structure. The technique shows close matching of estimation of average flank wear and directly measured wear value. Thus the system developed demonstrates the possibility of successful tool wear monitoring on-line.  相似文献   
17.
A simple electronic circuit is proposed as an analog of the excitable membrane. It is based on the Hodgkin-Huxley model and their squid axon voltage clamp data. The simulated potassium and sodium conductances are reproduced satisfactorily and the electronic action potentials are very similar to the experimentally recorded ones. Since this analog contains only a few electronic elements, it is small and inexpensive to build. It could be very useful as a means of simulating a wide variety of membrane conductances and different types of action potentials. Also, the simplicity of the circuit makes it an ideal unit to build complex neuron networks.  相似文献   
18.
The secure operation of autonomous vehicle networks in the presence of adversarial observation is examined, in the context of a canonical double-integrator-network (DIN) model. Specifically, we study the ability of a sentient adversary to estimate the full network’s state, from noisy local measurements of vehicle motions. Algebraic, spectral, and graphical characterizations are provided, which indicate the critical role of the inter-vehicle communication topology and control scheme in achieving security.  相似文献   
19.
The role of solid state epitaxy in the crystallization of nanocomposite cordierite glass to glass ceramic was investigated. The use of isostructural (-cordierite) seeds in cordierite glass led to a lowering in the crystallization temperature to form glass ceramic by about 50 °C compared to the unseeded glass. The use of non-isostructural seeds such as ZrO2 and TiO2 did not lower the crystallization temperature of cordierite glass to glass ceramic, and in the case of the TiO2-seeded glass the crystallization temperature increased by about 50 °C compared to the unseeded-cordierite glass. The lowering in crystallization temperature by-cordierite seeding can be attributed to the nucleation and epitaxial growth mechanism.  相似文献   
20.
Dutta  Joy  Roy  Sarbani 《Multimedia Tools and Applications》2021,80(13):19989-20018
Multimedia Tools and Applications - Indoor air pollutants e.g., Carbon dioxide (CO2), Particulate Matter(PM)2.5, PM10, Total Volatile Organic Compounds (TVOC), etc. have a serious impact on human...  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