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   312645篇
  免费   4333篇
  国内免费   712篇
电工技术   6227篇
综合类   562篇
化学工业   47115篇
金属工艺   10373篇
机械仪表   8873篇
建筑科学   8034篇
矿业工程   867篇
能源动力   8678篇
轻工业   31395篇
水利工程   2574篇
石油天然气   3679篇
武器工业   8篇
无线电   39916篇
一般工业技术   59148篇
冶金工业   57596篇
原子能技术   5482篇
自动化技术   27163篇
  2021年   2335篇
  2019年   2191篇
  2018年   3696篇
  2017年   3517篇
  2016年   3616篇
  2015年   2598篇
  2014年   4528篇
  2013年   14513篇
  2012年   7547篇
  2011年   10346篇
  2010年   8075篇
  2009年   9311篇
  2008年   9799篇
  2007年   9772篇
  2006年   8656篇
  2005年   8043篇
  2004年   7913篇
  2003年   7614篇
  2002年   7462篇
  2001年   7620篇
  2000年   7267篇
  1999年   7601篇
  1998年   17733篇
  1997年   12899篇
  1996年   10157篇
  1995年   7932篇
  1994年   7264篇
  1993年   6961篇
  1992年   5334篇
  1991年   5094篇
  1990年   4960篇
  1989年   4824篇
  1988年   4715篇
  1987年   3923篇
  1986年   4048篇
  1985年   4767篇
  1984年   4308篇
  1983年   4047篇
  1982年   3625篇
  1981年   3790篇
  1980年   3472篇
  1979年   3444篇
  1978年   3239篇
  1977年   3833篇
  1976年   4858篇
  1975年   2791篇
  1974年   2667篇
  1973年   2679篇
  1972年   2221篇
  1971年   1968篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Fusion behavior of poly(vinyl chloride) (PVC) compounds plays an important role in the development of physical properties of processed material. The fusion characteristics in PVC processing are governed by material variables that affect the fusion with some interactions. In this research, the aim was to characterize the effects of formulation ingredients on fusion characteristics of PVC. Four material parameters, including the contents of nanoclay (NC), azodicarbonamide, calcium stearate, and processing aid, are proposed as affecting variables. The fusion time (FT) as well as fusion factor (FF) are considered fusion indicators and are experimentally determined in some different levels of affecting parameters. The multivariable regression analysis (MRA) and the Artificial Neural Network (ANN) modeling are considered as two analytical methods. The regression analysis result for the FT denotes, in part, significant linear and quadratic effects of NC and also its significant interactions with azodicarbonamide and calcium stearate, whereas that of FF indicates only a linear effect of NC. ANN modeling is performed with a three‐layer (input, hidden, and output) neural network. The results of the comparison of the MRA and ANN predictions with experimental values are reported as the correlation coefficient (R2), mean‐square error, and mean absolute percentage error for both FF and FT parameters. The obtained values clearly denote that the ANN results are more precise and especially more general than those of MRA. However, in the case of FT, improvement of the ANN modeling is much greater than that of FF. J. VINYL ADDIT. TECHNOL., 21:147–155, 2015. © 2014 Society of Plastics Engineers  相似文献   
82.
Neural Computing and Applications - With an increase in online longitudinal users’ interactions, capturing users’ precise preferences and giving accurate recommendations have become an...  相似文献   
83.
84.
Powder Metallurgy and Metal Ceramics - Multiphase La 2 MgNi 9 alloys are synthesized by induction melting. The crystalline structures of all phases in the alloys are determined with X-ray...  相似文献   
85.
86.
87.
88.
Anthropogenic influences, including climate change, are increasing river temperatures in northern and temperate regions and threatening the thermal habitats of native salmonids. When river temperatures exceed the tolerance levels of brook trout and Atlantic salmon, individuals exhibit behavioural thermoregulation by seeking out cold‐water refugia – often created by tributaries and groundwater discharge. Thermal infrared (TIR) imagery was used to map cold‐water anomalies along a 53 km reach of the Cains River, New Brunswick. Trout and salmon parr did not use all identified thermal anomalies as refugia during higher river temperature periods (>21°C). Most small‐bodied trout (8–30 cm) were observed in 80% of the thermal anomalies sampled. Large‐bodied trout (>35 cm) required a more specific set of physical habitat conditions for suitable refugia, that is, 100% of observed large trout used 30% of the anomalies sampled and required water depths >65 cm within or adjacent to the anomaly. Densities of trout were significantly higher within anomalies compared with areas of ambient river temperature. Salmon parr were less aligned with thermal anomalies at the observed temperatures, that is, 59% were found in 65% of the sampled anomalies; and densities were not significantly different within/ outside anomalies. Salmon parr appeared to aggregate at 27°C, and after several events over 27°C variability in aggregation behaviour was observed – some fish aggregated at 25°C, others did not. We stipulate this is due to variances of thermal fatigue. Habitat suitability curves were developed for velocity, temperature, depth, substrate, and deep water availability to characterize conditions preferred by fish during high‐temperature events. These findings are useful for managers as our climate warms, and can potentially be used as a tool to help conserve and enhance thermal refugia for brook trout and Atlantic salmon in similar systems.  相似文献   
89.
90.
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