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   46161篇
  免费   2550篇
  国内免费   48篇
电工技术   359篇
综合类   39篇
化学工业   8339篇
金属工艺   921篇
机械仪表   929篇
建筑科学   1454篇
矿业工程   95篇
能源动力   889篇
轻工业   5898篇
水利工程   396篇
石油天然气   178篇
武器工业   4篇
无线电   2672篇
一般工业技术   8101篇
冶金工业   12210篇
原子能技术   250篇
自动化技术   6025篇
  2023年   376篇
  2022年   373篇
  2021年   946篇
  2020年   799篇
  2019年   716篇
  2018年   1619篇
  2017年   1619篇
  2016年   1680篇
  2015年   1204篇
  2014年   1506篇
  2013年   3016篇
  2012年   2584篇
  2011年   2461篇
  2010年   1926篇
  2009年   1724篇
  2008年   1992篇
  2007年   1742篇
  2006年   1298篇
  2005年   1146篇
  2004年   1072篇
  2003年   973篇
  2002年   844篇
  2001年   570篇
  2000年   590篇
  1999年   718篇
  1998年   3940篇
  1997年   2360篇
  1996年   1526篇
  1995年   903篇
  1994年   775篇
  1993年   769篇
  1992年   261篇
  1991年   282篇
  1990年   263篇
  1989年   259篇
  1988年   244篇
  1987年   216篇
  1986年   219篇
  1985年   257篇
  1984年   222篇
  1983年   165篇
  1982年   204篇
  1981年   218篇
  1980年   193篇
  1979年   149篇
  1978年   144篇
  1977年   244篇
  1976年   510篇
  1975年   128篇
  1973年   103篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
111.
112.
The glass forming region in the B2O3-Al2O3-Y2O3 composition diagram has been determined by a melting and quenching procedure at temperatures up to 1800°C. Different physical characteristics (density, coefficient of thermal expansion, glass transition and crystallization peak temperatures) have been determined for a 35B2O3-40Al2O3-25Y2O3 glass composition (in mol.%). By using a predictive model and some NMR structural data, different elastic moduli (Young's modulus, bulk modulus, shear modulus and Poisson's ratio) have been calculated. The devitrification behaviour has also been studied. Internal crystallization is the dominant mechanism and a new (Y, Al)BO3 ternary phase has been characterized by X-ray powder diffraction. The temperature and time nucleation dependence have been determined from DTA experiments as well as the crystallization kinetics (i.e. the Avrami exponent and the activation energy for crystal growth).  相似文献   
113.
This letter presents a novel approach for organizing computational resources into groups within H.264/AVC motion estimation architectures, leading to reductions of up to 75% in the equivalent gate count with respect to state‐of‐the‐art designs.  相似文献   
114.
Copper slag is a by‐product generated during smelting to extract copper metal from the ore. The copper slag obtained may exhibit pozzolanic activity and may therefore be used in the manufacture of addition‐containing cements. In this paper the effect of the incorporation of the copper slag in cement is measured. Blends of copper slag with Portland cement generally possess properties equivalent to Portland cement containing fly ash, but very different to the silica fume incorporation. Copper slag and fly ash reduce the heat of hydration more effectively than silica fume in mortars. The replacement of 30% cement by copper slag reduces the flexural and compressive strength in a similar way to fly ash; however, after 28 days, the reduction is less than the percentage of substitution. Hydrated calcium aluminate phases were analysed using scanning electron microscopy (SEM) and X‐ray diffraction (XRD) techniques. The pozzolanic activity of copper slag is similar to that of fly ash and higher than silica fume. In the presence of low water/cement ratios, certain pozzolanic materials produce a very compact cement paste that limits the space available for hydration products, a determining factor in the formation of hydrated calcium aluminates. SEM was found to be a useful analytical technique when aluminates are formed and can be clearly detected by XRD. Copyright © 2008 Society of Chemical Industry  相似文献   
115.
An experimental study is made of the time decay of activity of the CO–NO reaction on a Pd/Al2O3 looking at the effect on reaction order and apparent activation energy. The optimum kinetics parameters fitting the steady state data at moderate pressures are determined. The time decay curves are analyzed through various catalyst deactivation models.  相似文献   
116.
Formaldehyde is used in ruminant feeding for different purposes including the protection of dietary proteins from ruminal degradation. The formaldehyde content of milk of goats fed various levels of formaldehyde-treated soybean oil-meal has been determined by using a sensitive HPLC method. Results showed a significant linear correlation between ingested formaldehyde and formaldehyde concentration in milk. About 0.02% of ingested formaldehyde was excreted in milk, as free formaldehyde.  相似文献   
117.
118.
The DNA molecule is modeled as an elastic rod with bending and twisting rigidities, subjected to external tension and twist applied at one end, the other end being clamped. We study the plectonemic equilibrium of such a rod, taking into account the impenetrability constraint. Numerical solutions of this boundary value problem have previously shown that purely elastic models can reproduce the supercoiling response of the DNA molecule. Using a variational approach, we derive analytical formulae for the elastic response of the filament, and extend former numerical results.  相似文献   
119.
Bioavailability and Toxicity of Metal Nutrients during Anaerobic Digestion   总被引:1,自引:0,他引:1  
This paper investigates the effect of chelating agents on the bioavailability of Fe and Cu during anaerobic digestion. The results on metal speciation and methane production in anaerobic serum bottles showed that biomass was able to grow in the presence of citrate 1?mM and nitrilotriacetic acid (NTA) 1?mM, suggesting that the binding sites at the cell surface competed efficiently for the metals with the chelating agents added. The presence of free ethylenediaminetetraacetic acid 1?mM inhibited methanogenesis, and this seemed to be related to a loss in metal uptake capacity. Although the addition of soluble microbial products (SMP) did not change metal distribution in anaerobic systems, it caused an increase in the rate of methane production, and it is believed that direct uptake of Cu-SMP complexes was responsible for this increase. The best protection against Cu toxicity occurred when stoichiometric amounts of NTA, which should complex and solubilize most of the Cu, was added, and it is likely that NTA prevented lethal concentrations of Cu from being adsorbed onto the cell and hence internalized.  相似文献   
120.
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