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   2163篇
  免费   81篇
  国内免费   4篇
电工技术   36篇
综合类   14篇
化学工业   539篇
金属工艺   49篇
机械仪表   35篇
建筑科学   63篇
矿业工程   9篇
能源动力   33篇
轻工业   247篇
水利工程   12篇
石油天然气   4篇
无线电   182篇
一般工业技术   345篇
冶金工业   411篇
原子能技术   30篇
自动化技术   239篇
  2023年   22篇
  2022年   37篇
  2021年   43篇
  2020年   33篇
  2019年   42篇
  2018年   44篇
  2017年   31篇
  2016年   48篇
  2015年   32篇
  2014年   65篇
  2013年   81篇
  2012年   73篇
  2011年   114篇
  2010年   71篇
  2009年   67篇
  2008年   87篇
  2007年   75篇
  2006年   61篇
  2005年   52篇
  2004年   55篇
  2003年   66篇
  2002年   57篇
  2001年   44篇
  2000年   34篇
  1999年   47篇
  1998年   104篇
  1997年   68篇
  1996年   61篇
  1995年   37篇
  1994年   34篇
  1993年   36篇
  1992年   27篇
  1991年   29篇
  1990年   24篇
  1989年   17篇
  1988年   32篇
  1987年   21篇
  1986年   28篇
  1985年   26篇
  1984年   23篇
  1983年   20篇
  1982年   22篇
  1981年   20篇
  1980年   16篇
  1978年   14篇
  1977年   21篇
  1976年   35篇
  1974年   17篇
  1968年   12篇
  1940年   11篇
排序方式: 共有2248条查询结果,搜索用时 15 毫秒
1.
Virtual Reality - It is generally accepted that natural environments reduce stress and improve mood. Since access to natural environments is sometimes limited, virtual natural environments,...  相似文献   
2.
3.
Dye‐loaded AlPO4‐5 single crystals were prepared by microwave‐assisted hydrothermal synthesis from a batch, containing a mixture of three chromophores (Coumarin 40, Rhodamine BE50, and Oxazine 1) differing in their absorption range, molecular dimensions, and solubilities. Confocal fluorescence images reveal a spatial separation effect of the dye molecules, where the slimmer, more‐soluble dye molecule (Coumarin 40) is uniformly distributed in the body of the single crystal, and the bulky and/or less‐soluble ones (Rhodamine BE50, Oxazine 1) are situated in distinct domains. Visible spectra show good panchromatic absorption of visible light. Fluorescence lifetime measurements indicate the presence of an energy transfer cascade of the entirely fixed dye molecules from Coumarin 40 to Oxazine 1. The transfer mechanism is predominantly radiative.  相似文献   
4.
5.
A new hourglassing control technique for quadrilateral and hexahedral elements used in dynamic finite element codes is presented. Based on expansion of the strees in a Taylor series and retention of additional terms beyond the usual constant stress term, this technique has the advantage that actual rather than artificial material properties are used. Simplifications that render this technique competitive with artificial viscosity and stiffness procedures in terms of additional storage and computation requirements are described.  相似文献   
6.
7.
Nascent high-molecular-weight (UHMW) polyethylene (PE) samples of different origins show a rather high crystallinity of about 70–75% and contain both a major portion of orthorhombic extended chain crystallites and a minor portion of triclinic crystallites. The triclinic content is greater the higher the molecular weight of the sample and the higher the activity of the used catalyser. A melting / recooling treatment results in a reduction of crystallinity by about 15–25% and disappearance of the triclinic phase. Further, an irreversible conversion of nascent orthorhombic extended chain crystallites to orthorhombic folded chain crystallites of increased lateral dimensions and crystalline perfections takes place during the melting / recooling treatment. The results are compared to those obtained for lower-molecular-weight PE samples and for high-strength / modulus PE fibers of different origins.  相似文献   
8.
9.
PURPOSE: This retrospective review of femorodistal vein grafts was analyzed to determine the usefulness of various graft surveillance criteria. METHOD: The surveillance schedule involved evaluations at 1 month, every 3 months the first year, and then every 6 months. Salvage intervention or graft occlusion occurring within the next follow-up interval defined surveillance end points. One hundred two grafts (329 surveillance visits) had an ankle/brachial index (ABI). A duplex scanning-determined midgraft peak systolic flow velocity (PSFV) was available for 81 grafts (262 visits). Forty-eight grafts (137 visits) had both a PSFV and entire graft duplex scanning (EGDS) to determine stenosis greater than 50%, whereas 40 grafts (91 visits) had simultaneous ABI and EGDS. RESULTS: When a greater than 15% decrease in ABI denoted an abnormal surveillance study result, a positive predictive value (PPV) of 24.3% and negative predictive value of 94.5% were noted. Similarly, a PSFV cutoff of less than 35 cm/sec demonstrated values of 26.3% and 94.2%, respectively. When an EGDS of greater than 50% stenosis or a PSFV of less than 35 cm/sec were the cutoff criteria, the PPV was 36.7% and negative predictive value 99.1%, whereas characterizing abnormal results further with ABI (> 15% decreases) increased the PPV to 83.3%. CONCLUSION: The combination of an EGDS, midgraft PSFV, and ABI provides optimal follow-up for our patients with a femorodistal vein graft.  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