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   22874篇
  免费   794篇
  国内免费   84篇
电工技术   293篇
综合类   44篇
化学工业   5147篇
金属工艺   479篇
机械仪表   460篇
建筑科学   1719篇
矿业工程   160篇
能源动力   686篇
轻工业   2135篇
水利工程   182篇
石油天然气   56篇
武器工业   1篇
无线电   1483篇
一般工业技术   4166篇
冶金工业   2770篇
原子能技术   137篇
自动化技术   3834篇
  2023年   152篇
  2022年   308篇
  2021年   534篇
  2020年   312篇
  2019年   327篇
  2018年   493篇
  2017年   426篇
  2016年   530篇
  2015年   479篇
  2014年   689篇
  2013年   1506篇
  2012年   1081篇
  2011年   1521篇
  2010年   1092篇
  2009年   1043篇
  2008年   1265篇
  2007年   1098篇
  2006年   939篇
  2005年   865篇
  2004年   774篇
  2003年   688篇
  2002年   661篇
  2001年   392篇
  2000年   377篇
  1999年   412篇
  1998年   409篇
  1997年   371篇
  1996年   341篇
  1995年   294篇
  1994年   301篇
  1993年   298篇
  1992年   250篇
  1991年   214篇
  1990年   201篇
  1989年   230篇
  1988年   186篇
  1987年   177篇
  1986年   174篇
  1985年   239篇
  1984年   214篇
  1983年   184篇
  1982年   168篇
  1981年   195篇
  1980年   130篇
  1979年   147篇
  1978年   143篇
  1977年   127篇
  1976年   138篇
  1975年   120篇
  1974年   105篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
162.
Cloud computing is an elastic computing model where users can lease computing and storage resources on demand from a remote infrastructure.It is gaining popularity due to its low cost,high reliability,...  相似文献   
163.
Trying to identify the mechanism(s) responsible for isothermal oscillatory behavior of heterogeneous catalytic reaction systems, many investigators have assumed low conversions with respect to the limiting reactant(s) so that the bulk phase concentrations of the reactants may be considered constant. In this study, it is shown that even in a differential catalytic CSTR, in which conversion of the reactants may be less than %, the changes of the gas phase concentrations of the reacting.species may play a key role in the overall dynamic behavior of a reaction system; for instance, they may drive the oscillations observed. The reduction of nitric oxide by ammonia on polycrystalline platinum is discussed in detail within this context. Several other oxidation reactions are also discussed  相似文献   
164.
Phosphatidylcholine acyltransferase (lecithin:cholesterol acyltransferase or LCAT; EC 2.3.1.43) activity was found to be present in pig ovarian follicular fluid (POFF), in addition to pig serum (PS). The cholesterol esterification rate in both POFF and PS is linear with incubation time up to 2 hr. The mean absolute rate of POFF-cholesterol esterification was 8.1±0.4 nmoles per ml per hr approximately one-fourth of that in PS. However, the fractional rate (percent of labeled cholesterol esterified per hr) of POFF-cholesterol esterification was similar to that observed in PS. There was little variation of absolute rate of cholesterol esterification in the fluid obtained from different sizes of follicles. Fatty acid or triacylglycerol did not participate in the reaction of cholesterol esterification in POFF. No appreciable change in enzymatic activity was found from storing POFF at 4 C for periods of time up to 24 hr or at −70 C up to 2 months, but activity was lost thereafter. On the other hand, PS showed a much longer period of stability (5 days at 4 C and 9 months at −70 C). A discrepancy between the fatty acid composition of cholesteryl esters formed by the LCAT reaction and the fatty acid composition at the C-2 position of phosphatidylcholine led us to propose a two-step mechanism for the LCAT reaction. It is concluded that the LCAT of POFF, as well as that of plasma, is specific for individual fatty acids rather than for the fatty acid composition of phosphatidylcholine. The fatty acid concentration of lysophosphatidylcholine decreased during prolonged incubation times (6 to 21 hr) suggesting that the increased lysophosphatidylcholine formed as a product of the LCAT reaction may be reused as substrate for the LCAT reaction or for hydrolysis by lysophosphatidylcholine hydrolase. Presented at the AOCS Meeting, New York, May 1977.  相似文献   
165.
Peter H. Given  Ajay Sood 《Fuel》1982,61(10):968-971
Further experimentation with coals in a continuous-flow reactor system at 440 and 455°C confirms the significance of the previous classification of a set of coals by cluster analysis. The greatest yields of distillable products tend to be given by coals of high sulphur content in the middle of the range of the high-volatile bituminous rank classes. Yet the lowest yields of preasphaltene (toluene-insoluble material) tend to be given by coals of high rank and medium sulphur content. The group of low sulphur, low-rank coals (mostly Rocky Mountain province), in a solvent different from that used for the other groups, showed poor conversion and poor distillate yield at the higher temperature. It is concluded that this coal/solvent combination is particularly prone to retrogressive reactions, including coking.  相似文献   
166.
167.

Algorithmic decision-making plays an important role in financial markets. Current tools in trading focus on popular companies which are discussed in thousands of news items. However, it remains unclear whether methodologies from the field of data analytics relying on large samples can also be applied to small datasets of less popular companies or whether these methodologies lead to the discovery of meaningless patterns resulting in economic losses. We analyze whether the impact of media sentiment on financial markets is influenced by two levels of investor attention and whether this impacts algorithmic decision-making. We find that the influence differs substantially between news and companies with high and low investor attention. We apply a trading simulation to outline the practical consequences of these interrelations for decision support systems. Our results are of high importance for financial market participants, especially for algorithmic traders that consider sentiment for investment decision support.

  相似文献   
168.
International Journal of Information Security - The complexity of today’s integrated circuit (IC) supply chain, organised in several tiers and including many companies located in different...  相似文献   
169.
170.
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