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   8480篇
  免费   379篇
  国内免费   19篇
电工技术   99篇
综合类   3篇
化学工业   1943篇
金属工艺   148篇
机械仪表   193篇
建筑科学   388篇
矿业工程   8篇
能源动力   288篇
轻工业   604篇
水利工程   60篇
石油天然气   24篇
无线电   925篇
一般工业技术   1605篇
冶金工业   968篇
原子能技术   94篇
自动化技术   1528篇
  2023年   42篇
  2022年   105篇
  2021年   170篇
  2020年   122篇
  2019年   129篇
  2018年   198篇
  2017年   201篇
  2016年   238篇
  2015年   182篇
  2014年   251篇
  2013年   471篇
  2012年   426篇
  2011年   525篇
  2010年   398篇
  2009年   443篇
  2008年   416篇
  2007年   401篇
  2006年   352篇
  2005年   260篇
  2004年   270篇
  2003年   235篇
  2002年   226篇
  2001年   154篇
  2000年   154篇
  1999年   140篇
  1998年   270篇
  1997年   216篇
  1996年   170篇
  1995年   132篇
  1994年   130篇
  1993年   112篇
  1992年   85篇
  1991年   61篇
  1990年   67篇
  1989年   61篇
  1988年   68篇
  1987年   60篇
  1986年   50篇
  1985年   100篇
  1984年   75篇
  1983年   66篇
  1982年   58篇
  1981年   90篇
  1980年   63篇
  1979年   64篇
  1977年   49篇
  1976年   65篇
  1975年   45篇
  1974年   42篇
  1973年   36篇
排序方式: 共有8878条查询结果,搜索用时 406 毫秒
61.
62.
The efficacy of corticosteroids in asthma has been recognized over 40 years ago. Since that time, the advent of inhaled forms has further improved the therapeutic of these drugs which are now recognized as the fundamental treatment for asthma, and described in detail by national and international consensus. Based on a large body of literature, it can now be recommended to prescribe inhaled corticosteroids for symptomatic asthma patients. Long-term treatment is required and dosage not exceeding 1000 micrograms/d (beclometasone dipropionate equivalent) in adults are safe. Differences in the pharmacological characteristics of the various systematic and inhaled corticosteroids can be used to adapt treatment and administration route to each patient and achieve good patient compliance with optimal therapeutic efficacy.  相似文献   
63.
The role of nitrogen dioxide in the selective reduction of NO by propane over a Cu-MFI zeolite is investigated. NO2 and NO reductions were carried out under similar conditions of reaction. In the presence of oxygen, the reduction of NO by C3H8 does not differ significantly from that of NO2. In the absence of oxygen, the reduction of NO2 by propane occurs with a partial decomposition of the nitric dioxide molecule. Such a decomposition leads to the formation of oxygen, which is responsible for the increase in catalytic activity by comparison with the same reaction performed with NO. NO2 formed and released in the gas phase during the reduction of NO by propane in the presence of oxygen does not play a predominant role in the catalytic process.  相似文献   
64.
Utilizing cultured lenses from normal and homozygous glutathione peroxidase (GSHPx-1) knockout mice and inhibitors for GSSG Reductase (GSSG Red), 1,3-bis(2-chlorethyl)-1-nitrosourea (BCNU) and catalase (Cat), 3-aminotriazole (3-AT), the ability to degrade H2O2 was examined at two H2O2 concentrations, 300 microM and 80 microM. It was found that GSHPx-1 contributed about 15% to the H2O2 degradation. The Cat contribution was concentration dependent being about 30% at 300 microM H2O2 and approximately 8% to 15% at 80 microM H2O2. GSH loss measured as nonprotein thiol (NP-SH) was shown to be linked to most of the remaining H2O2 degradation accounting for about 54% to 72% of the H2O2 degradation at 300 microM and 80 microM, respectively. However, based on evaluation of the ability of GSH to nonenzymatically degrade H2O2, it can only account for about 36% at 300 microM and 19% at 80 microM H2O2 of the observed lens H2O2 degradation. It is, therefore, concluded that lens GSH must be involved in other reactions either directly or indirectly related to H2O2 degradation.  相似文献   
65.
Air trench structures for reduced-size bends in low-index contrast waveguides are proposed. To minimize junction loss, the structures are designed to provide adiabatic mode shaping between low- and high-index contrast regions, which is achieved by the introduction of "cladding tapers." Drastic reduction in effective bend radius is predicted. We present two-dimensional (2-D) finite-difference time-domain/effective index method simulations of bends in representative silica index contrasts. We also argue that substrate loss, while present, can be controlled with such air trenches and reduced to arbitrarily low levels limited only by fabrication capabilities. The required trench depth, given an acceptable substrate loss, is calculated in three dimensions using an approximate equivalent current sheet method and also by a numerical solver for full-vector leaky modes. A simple, compact waveguide T-splitter using air trench bends is presented.  相似文献   
66.
The radiotracer technique as a means to investigate the corrosion of zirconium, tantalum, and a Ta-40Nb alloy in fluoride containing azeotropic nitric acid Zirconium and tantalum as well as the tantalum 40% niobium alloy are of considerable technical importance due to their high corrosion resistance against numerous corrosive media. With respect to corrosion testing in analytically pure azeotropic nitric acid in the temperature range between 20 and 121°C, corrosion rates were determined for zirconium: 7 · 10?6 to 5 · 10?4 mm/y, for tantalum: 10?8 to 4 · 10?6 mm/y, and for the Ta-40Nb alloy: 2 · 10?7 to 8 · 10?6 mm/y [1]. These corrosion rates will be markedly increased by adding small amounts of fluorides or by fluoride impurities. The radiotracer method after neutron activation was applied to determine the corrosion rates in azeotropic fluoride containing nitric acid. Even minute additions of fluorides strongly affect the corrosion resistance of zirconium. In the range between 0.15 and 10 ppm F? and at a temperature of 108°C, corrosion rates between 5.3 · 10?3 and 3.1 mm/y were measured. It was impossible to establish a limit for the fluoride concentration, below which the corrosion rate of zirconium will not be adversely influenced. The corrosion rates of tantalum and the Ta-40Nb alloy are considerably increasing above a fluoride concentration of 10 ppm. The highest corrosion rates measured were between 8.4 · 10?3 mm/y at 50°C/280 ppm F? and 1.4 · 10?2 mm/y at 110°C/320 ppm F?. Within the range of this investigation, the corrosion resistance of tantalum was higher than that of the Ta-40Nb alloy by one order of magnitude. The corrosion resistance of zirconium and tantalum was not influenced by any treatment of the samples before testing.  相似文献   
67.
68.
A key feature of modern optimal planners such as graphplan and blackbox is their ability to prune large parts of the search space. Previous Partial Order Causal Link (POCL) planners provide an alternative branching scheme but lacking comparable pruning mechanisms do not perform as well. In this paper, a domain-independent formulation of temporal planning based on Constraint Programming is introduced that successfully combines a POCL branching scheme with powerful and sound pruning rules. The key novelty in the formulation is the ability to reason about supports, precedences, and causal links involving actions that are not in the plan. Experiments over a wide range of benchmarks show that the resulting optimal temporal planner is much faster than current ones and is competitive with the best parallel planners in the special case in which actions have all the same duration.1  相似文献   
69.
70.
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