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   96467篇
  免费   17276篇
  国内免费   9563篇
电工技术   2001篇
技术理论   84篇
综合类   3113篇
化学工业   18329篇
金属工艺   7044篇
机械仪表   5552篇
建筑科学   2915篇
矿业工程   1528篇
能源动力   6025篇
轻工业   7329篇
水利工程   1312篇
石油天然气   2341篇
武器工业   200篇
无线电   18587篇
一般工业技术   19139篇
冶金工业   2334篇
原子能技术   4463篇
自动化技术   21010篇
  2025年   627篇
  2024年   3303篇
  2023年   3908篇
  2022年   3916篇
  2021年   4459篇
  2020年   4690篇
  2019年   4055篇
  2018年   4253篇
  2017年   4999篇
  2016年   5310篇
  2015年   5530篇
  2014年   6657篇
  2013年   7135篇
  2012年   8043篇
  2011年   7888篇
  2010年   6053篇
  2009年   6123篇
  2008年   4775篇
  2007年   6342篇
  2006年   5775篇
  2005年   3068篇
  2004年   1714篇
  2003年   1636篇
  2002年   1643篇
  2001年   1284篇
  2000年   1181篇
  1999年   1163篇
  1998年   884篇
  1997年   719篇
  1996年   679篇
  1995年   535篇
  1994年   429篇
  1993年   318篇
  1992年   188篇
  1991年   181篇
  1990年   159篇
  1989年   144篇
  1988年   279篇
  1987年   718篇
  1986年   655篇
  1985年   159篇
  1984年   111篇
  1981年   86篇
  1979年   79篇
  1972年   95篇
  1971年   76篇
  1970年   84篇
  1967年   103篇
  1966年   99篇
  1965年   96篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
1 Introduction Along with the matureness of laser diode (LD) manufacturing technology, the performance of LD has been improved greatly since 1980s, so various kinds of laser devices based on LD have been developed rapidly, especially the all-solid state lasers. After early experiments and researches, the all-solid state lasers have been commercialized successfully.  相似文献   
2.
《Materials Letters》2007,61(11-12):2259-2261
We report the formation of silver nanoclusters of average size ∼ 1.1 nm in a single step ion-exchanged silica glass. Silver clusters are formed in a glass during ion-exchange by fine-tuning the ion-exchange parameters such as salt composition, temperature and time. Such nanocluster formation during ion-exchange preserves the waveguiding properties of the ion-exchanged surface of the glass making them suitable for nonlinear waveguide device fabrication.  相似文献   
3.
In this paper we design an experiment which can be depicted as a simple scenario, a very limited 'world'. In this world, there are an actor that can pursue a project and an observer that is keeping its eyes on the actor. We try to show in the experiment that the observer can to some degree understand the actor based on its knowledge and some metaphors, i.e. understand what the actor is doing and why. As the conclusion of this experiment, we try to show some features of 'understanding'. These are (1) that 'understanding' has to be based on some preliminary knowledge; (2) that 'understanding' is a process of incremental learning; (3) that, as for symbolic systems, some metaphors are necessary for mapping real entities into concepts in mind.  相似文献   
4.
5.
Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm−1 for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm−1 and 13.62 ± 2.11 μm−1 for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight.  相似文献   
6.
7.
The relatively simple to measure ‘time constant’ is presented as an index for characterizing the level of corrosion experienced by steel in concrete. This communication, largely in the form of a technical note, explicitly illustrates that the time constant determined from electrochemical testing has significant merit for monitoring corrosion of steel in concrete, and appears to be insensitive to the area of electrode being probed. In this work, the time constant, κ, is determined following a galvanostatic excitation signal, revealing a good correlation between the value of κ and corrosion status across specimens of widely varying geometry. Although this notion has been suggested in the past, this work presents both a survey and consolidated review to indicate the utility of the parameter as an index to corrosion rate on-site.  相似文献   
8.
The microscopic structures of PLZT(7.9/70/30 and x/65/35, x = 7 or 8) ceramics were studied by means of transmission electron microscopy. The presence of micro-regions in PLZTs was first verfied.  相似文献   
9.
Firstly, the compress experiment is undertaken to investigate the efficiency of repaired panels in this paper, and then modeling of the mechanical behavior of the repaired composite panel under compressive static load is conducted by using of the finite element method. The effect of geometric non-linearity on the stress–strain response is considered in the numeric analysis. Fatherly, the user material subroutine (UMAT) is integrated with the ABAQUS package with the geometric non-linearity effect for studying the damage initiation and its progression in the composite structure, and quadrilateral, linear, thick shell elements (S8R) are adopted. Finally, the predicted strain distribution, damage evolution and strength of the laminate are compared with the test results.  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