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   261631篇
  免费   3274篇
  国内免费   824篇
电工技术   5136篇
综合类   334篇
化学工业   37481篇
金属工艺   10280篇
机械仪表   8509篇
建筑科学   6148篇
矿业工程   1078篇
能源动力   6441篇
轻工业   22958篇
水利工程   2585篇
石油天然气   3909篇
武器工业   68篇
无线电   34909篇
一般工业技术   50364篇
冶金工业   46907篇
原子能技术   5425篇
自动化技术   23197篇
  2021年   2070篇
  2019年   2081篇
  2018年   3520篇
  2017年   3580篇
  2016年   3778篇
  2015年   2363篇
  2014年   4105篇
  2013年   11368篇
  2012年   6647篇
  2011年   8991篇
  2010年   7057篇
  2009年   7926篇
  2008年   8749篇
  2007年   8685篇
  2006年   7853篇
  2005年   7189篇
  2004年   6932篇
  2003年   6751篇
  2002年   6400篇
  2001年   6486篇
  2000年   6185篇
  1999年   6270篇
  1998年   14294篇
  1997年   10315篇
  1996年   8137篇
  1995年   6430篇
  1994年   5731篇
  1993年   5607篇
  1992年   4568篇
  1991年   4224篇
  1990年   4063篇
  1989年   3791篇
  1988年   3635篇
  1987年   3181篇
  1986年   3070篇
  1985年   3618篇
  1984年   3395篇
  1983年   3039篇
  1982年   2858篇
  1981年   2958篇
  1980年   2769篇
  1979年   2655篇
  1978年   2506篇
  1977年   2957篇
  1976年   3565篇
  1975年   2322篇
  1974年   2317篇
  1973年   2320篇
  1972年   1852篇
  1971年   1747篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
Cooperative diversity is a rapidly emerging topic for wireless communications, with ad hoc and hybrid/relay networks as two main applications so far. In this paper, we investigate the cooperative diversity concept for MIMO multicell networks, where the processing must be optimized to account for the variability of the channel conditions across the cooperative devices. This can be done via distributed preceding and is realistically based on channel statistics (average gains, correlations, etc.). We give a new approach to the previously coined equal diversity spread principle, through minimization of an approximated SER expression. Next, we focus on a low-complexity approach to minimizing a PEP-based performance measure. Gains are evaluated in a multicell scenario with collaborating base stations.  相似文献   
992.
Density-Induced Support Vector Data Description   总被引:6,自引:0,他引:6  
  相似文献   
993.
In catalytic cracking of vacuum gasoils, the yields of products are correlated with their hydrogen content. A correlation that allows predicting the yields of products of this process was elaborated based on the results of cracking of 11 gasoils of different origin and data on their density, refractive index, and 50% distillation temperature. However, it is not possible to predict the octane number of the naphtha obtained in cracking with the group hydrocarbon composition of the feedstock. __________ Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 4, pp. 31–34, July–August, 2007.  相似文献   
994.
Thin-film polysilicon solar cells are a promising low-cost alternative for bulk silicon solar cells due to their reduced material thickness. Recently, we showed that the use of an amorphous silicon/polycrystalline silicon heterojunction emitter instead of a diffused homojunction emitter led to a boost in the open-circuit voltage by 90 mV. Now, we present a full evidence that shows that this improvement is related to the absence of dopant smearing along the grain boundaries. By using scanning spreading resistance microscopy, we found an enlargement of the junction area by a factor of five in case of a homojunction. The tips of the dopant spikes represent lowly doped areas with an enhanced recombination.  相似文献   
995.
996.
The infrastructure renewal program at MIT consists of a large number of projects with an estimated budget that could approach $1 billion. Infrastructure renewal at the Massachusetts Institute of Technology (MIT) is the process of evaluating and investing in the maintenance of facility systems and basic structure to preserve existing campus buildings. The selection and prioritization of projects must be addressed with a systematic method for the optimal allocation of funds and other resources. This paper presents a case study of a prioritization method utilizing multi-attribute utility theory. This method was developed at MIT's Department of Nuclear Engineering and was deployed by the Department of Facilities after appropriate modifications were implemented to address the idiosyncrasies of infrastructure renewal projects and the competing criteria and constraints that influence the judgment of the decision-makers. Such criteria include minimization of risk, optimization of economic impact, and coordination with academic policies, programs, and operations of the Institute. A brief overview of the method is presented, as well as the results of its application to the prioritization of infrastructure renewal projects. Results of workshops held at MIT with the participation of stakeholders demonstrate the feasibility of the prioritization method and the usefulness of this approach.  相似文献   
997.
Hydrogen peroxide is a potent, relatively inexpensive oxidant that chemically degrades chromophoric components in pulps and textiles. Oxidation of cellulose is a byproduct of this process step that decreases the tensile strength of individual fibres. The residence time of pulp in the bleaching reactor must be optimized to achieve the desired brightness and minimizing fibre degradation. To evaluate the impact of peroxide bleaching at the microfibrillar level, a single black spruce tree was chosen and kraft pulped. Peroxide bleaching was conducted via benchtop polyethylene bag bleaching in a temperature-controlled waterbath. Atomic force microscopy (AFM) topographical images acquired before and after the bleaching step show dramatic changes in fibre structure consistent with delignification and defects in the surface topography. This was further verified by X-ray work at Brookhaven National Laboratory, NY, U.S.A.  相似文献   
998.
The technical and economic evaluation of a project should involve not only the technological aspects but also the factors related to the interference of the construction site with the population and roads or to the interaction of the operation with the environment. This paper, through the use of the Analytic Hierarchy Process, compares two different excavation alternatives, microtunnelling and trench excavation, for an urban sewer construction project.  相似文献   
999.
An axisymmetrical model of a rapid nonisobaric heavily laden gas‐particle jet is developed. The model is based on the general functions for both axial and radial pressure distributions, which were discovered by a numerical study of rapid gas‐particle jets by the CFD code. The model equations were solved analytically. The results obtained by the model developed are in good agreement with those computed by the CFD code for a wide range of flow parameters.  相似文献   
1000.
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