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   310414篇
  免费   2805篇
  国内免费   585篇
电工技术   5505篇
综合类   1179篇
化学工业   52143篇
金属工艺   17469篇
机械仪表   11841篇
建筑科学   6669篇
矿业工程   4585篇
能源动力   4868篇
轻工业   13233篇
水利工程   4774篇
石油天然气   14236篇
武器工业   39篇
无线电   27062篇
一般工业技术   71883篇
冶金工业   41379篇
原子能技术   11594篇
自动化技术   25345篇
  2022年   4560篇
  2021年   5622篇
  2020年   4244篇
  2019年   5098篇
  2018年   6677篇
  2017年   6624篇
  2016年   6782篇
  2015年   3655篇
  2014年   6224篇
  2013年   12338篇
  2012年   8643篇
  2011年   11442篇
  2010年   9268篇
  2009年   10278篇
  2008年   9911篇
  2007年   9517篇
  2006年   8046篇
  2005年   7291篇
  2004年   7848篇
  2003年   7385篇
  2002年   7265篇
  2001年   6520篇
  2000年   6236篇
  1999年   5519篇
  1998年   8426篇
  1997年   6953篇
  1996年   5390篇
  1995年   4561篇
  1994年   4412篇
  1993年   4498篇
  1992年   4050篇
  1991年   3949篇
  1990年   4192篇
  1989年   4138篇
  1988年   3930篇
  1987年   3901篇
  1986年   3823篇
  1985年   4013篇
  1984年   3898篇
  1983年   3751篇
  1982年   3557篇
  1981年   3492篇
  1980年   3498篇
  1979年   3724篇
  1978年   3961篇
  1977年   3928篇
  1976年   4286篇
  1975年   3625篇
  1974年   3630篇
  1973年   3666篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Clean Technologies and Environmental Policy - Food production and consumption is one of the major causes of global environmental degradation. One way to address environmental impacts in the food...  相似文献   
2.
3.
Abstract

Lung deposited surface area (LDSA) is a relatively new metric that has been argued to be more accurate at predicting health effects from aerosol exposure. For typical atmospheric aerosol, the LDSA concentration depends mainly on the concentration of ultrafine particles (e.g. vehicular exhaust emissions and residential wood combustion) and therefore optical methods cannot be used to measure and quantify it. The objective of this study was to investigate and describe typical characteristics of LDSA under different urban environments and evaluate how a diffusion charging-based Pegasor AQ Urban sensor (Pegasor Ltd., Finland) can be used as an alternative to optical sensors when assessing local combustion emissions and respective LDSA concentrations. Long-term (12?months) sensor measurements of LDSA were carried out at three distinctly different measurement sites (four sensor nodes) in the Helsinki metropolitan area, Finland. The sites were affected mainly by vehicular exhaust emission (street canyon and urban background stations) and by residential wood combustion (two detached housing area stations). The results showed that the accuracy of the AQ Urban was good (R2 = 0.90) for the measurement of LDSA when compared to differential mobility particle sizer. The mean concentrations of LDSA were more than twice as high at the street canyon (mean 22 µm2 cm?3) site when compared to the urban background site (mean 9.4 µm2 cm?3). In the detached housing area, the mean concentrations were 12 µm2 cm?3, and wood combustion typically caused high LDSA peaks in the evenings. High correlations and similar diurnal cycles were observed for the LDSA and black carbon at street canyon and urban background stations. The utilization of a small-scale sensor network (four nodes) showed that the cross-station variability in hourly LDSA concentrations was significant in every site, even within the same detached housing area (distance between the two sites ~670?m).  相似文献   
4.
This paper presents the results of ongoing research carried out by the author exploring methods to provide a more robust estimate of rock mass properties specifically for use in tunnel design. Data from various large-scale rock mass failures are introduced, including coal pillars. The damage-initiation,spalling-limit approach is compared to the coal pillar database. New comparisons of estimating the geological strength index(GSI) and relationships to estimate the Hoeke Brown failure criterion parameters, mb, s and a, are presented.  相似文献   
5.
6.
7.
8.

Stability constants and formation enthalpies of supramolecular complexes of crown ethers and their cyclic and acyclic analogues are determined on the basis of experimental data obtained by different physicochemical methods in the terms of a general approach developed and implemented in the ChemEqui software package. The established regularities of variation of stability of complexes are discussed as dependent on the ligand structure, nature of the cation, solvent, and anion. The applicability of the suggested method of determining complexation selectivity is shown for multicomponent equilibria in solutions.

  相似文献   
9.
A layer-by-layer analysis of the structures, the phase compositions, and the defect substructures of differentially hardened rails has been carried by optical and transmission electron microscopy. It is found that the material volume of the rail head fillet is cooled faster than the volume located along the central axis.  相似文献   
10.
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