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   832篇
  免费   9篇
化学   491篇
晶体学   5篇
力学   41篇
数学   153篇
物理学   151篇
  2021年   6篇
  2020年   9篇
  2019年   13篇
  2016年   7篇
  2015年   10篇
  2014年   14篇
  2013年   38篇
  2012年   34篇
  2011年   29篇
  2010年   14篇
  2009年   12篇
  2008年   34篇
  2007年   23篇
  2006年   30篇
  2005年   31篇
  2004年   25篇
  2003年   19篇
  2002年   18篇
  2001年   22篇
  2000年   17篇
  1999年   9篇
  1998年   7篇
  1997年   21篇
  1996年   14篇
  1995年   17篇
  1994年   13篇
  1993年   12篇
  1992年   20篇
  1991年   11篇
  1990年   17篇
  1989年   15篇
  1988年   9篇
  1987年   8篇
  1986年   15篇
  1985年   16篇
  1984年   14篇
  1983年   12篇
  1982年   16篇
  1981年   10篇
  1980年   13篇
  1979年   15篇
  1978年   13篇
  1977年   7篇
  1976年   6篇
  1975年   11篇
  1974年   14篇
  1973年   11篇
  1972年   8篇
  1969年   4篇
  1929年   4篇
排序方式: 共有841条查询结果,搜索用时 62 毫秒
1.
2.
3.
A free-piston driver that employs entropy-raising shock processes with diaphragm rupture has been constructed, which promises significant theoretical advantages over isentropic compression. Results from a range of conditions with helium and argon driver gases are reported. Significant performance gains were achieved in some test cases. Heat losses are shown to have a strong effect on driver processes. Measurements compare well with predictions from a quasi-one-dimensional numerical code. Received 7 September 1996 / Accepted 5 October 1996  相似文献   
4.
This paper is concerned with the development of a customized circle packing algorithm for a manufacturer of sprockets for the motor cycle industry. Practical constraints mean that the problem differs somewhat from those tackled elsewhere in the literature. In particular, the layouts need to conform to a given structure. This is achieved by using a local search algorithm with an appropriate starting solution and a series of neighbourhoods designed to preserve the layout structure. Empirical evidence based on real data shows that the quality of the resulting solutions closely matches that of cutting patterns currently produced by human experts. Computation times average around 20–30?s per order as compared to several hours for an equivalent manual solution.  相似文献   
5.
Muonic hydrogen isotopes (μ p, μ d, and μt) are simple quantum mechanical systems ideally suited for studies of numerous fundamental phenomena in electroweak and strong interactions as well as in applied areas such as muon chemistry or muon catalyzed fusion. Emission of muonic hydrogen isotopes into vacuum helps to overcome the limitations which are normally imposed on conventional investigations with gaseous and liquid targets. A proof of principle experiment for this new technique was performed at TRIUMF last year. Negative muons with 30 MeV/c momentum were stopped in a thin film of solid hydrogen and produced very low energy μd in vacuum. The distribution center of the normal velocity components of emitted μd atoms was measured to be ∼1 cm/μs. The yield of μd in vacuum is an increasing function of H2 film thickness δ up to a value of δ≥1 mm.  相似文献   
6.
7.
8.
There are 2 n-1 ways in which a tree on n vertices can be oriented. Each of these can be regarded as the (Hasse) diagram of a partially ordered set. The maximal and minimal widths of these posets are determined. The maximal width depends on the bipartition of the tree as a bipartite graph and it can be determined in time O(n). The minimal width is one of [/2] or [/2]+1, where is the number of leaves of the tree. An algorithm of execution time O(n + 2 log ) to construct the minimal width orientation is given.This research was partially funded by the National Science and Engineering Research Council of Canada under Grant Number A4219.  相似文献   
9.
Processes that produce only ethanol from lignocellulosics display poor economics. This is generally overcome by constructing large facilities having satisfactory economies of scale, thus making financing onerous and hindering the development of suitable technologies. Lignol Innovations has developed a biorefining technology that employs an ethanol-based organosolv step to separate lignin, hemicellulose components, and extractives from the cellulosic fraction of woody biomass. The resultant cellulosic fraction is highly susceptible to enzymatic hydrolysis, generating very high yields of glucose (>90% in 12–24h) with typical enzyme loadings of 10–20 FPU (filter paper units)/g. This glucose is readily converted to ethanol, or possibly other sugar platform chemicals, either by sequential or simultaneous saccharification and fermentation. The liquor from the organosolv step is processed by well-established unit operations to recover lignin, furfural, xylose, acetic acid, and a lipophylic extractives fraction. The process ethanol is recovered and recycled back to the process. The resulting recycled process water is of a very high quality, low BOD5, and suitable for overall system process closure. Significant benefits can be attained in greenhouse gas (GHG) emission reductions, as per the Kyoto Protocol. Revenues from the multiple products, particularly the lignin, ethanol and xylose fractions, ensure excellent economics for the process even in plants as small as 100 mtpd (metric tonnes per day) dry woody biomass input—a scale suitable for processing wood residues produced by a single large sawmill.  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