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   653821篇
  免费   7538篇
  国内免费   1114篇
电工技术   11753篇
综合类   552篇
化学工业   102977篇
金属工艺   27749篇
机械仪表   20919篇
建筑科学   14481篇
矿业工程   5158篇
能源动力   16047篇
轻工业   50902篇
水利工程   8341篇
石油天然气   18434篇
武器工业   42篇
无线电   68279篇
一般工业技术   135158篇
冶金工业   112212篇
原子能技术   17340篇
自动化技术   52129篇
  2021年   6048篇
  2019年   5824篇
  2018年   10243篇
  2017年   10422篇
  2016年   10940篇
  2015年   6732篇
  2014年   11486篇
  2013年   29339篇
  2012年   17658篇
  2011年   23654篇
  2010年   19005篇
  2009年   21212篇
  2008年   21532篇
  2007年   21187篇
  2006年   18454篇
  2005年   16777篇
  2004年   15955篇
  2003年   15568篇
  2002年   15128篇
  2001年   14738篇
  2000年   14137篇
  1999年   13864篇
  1998年   32334篇
  1997年   23354篇
  1996年   18120篇
  1995年   13877篇
  1994年   12514篇
  1993年   12334篇
  1992年   9580篇
  1991年   9363篇
  1990年   9263篇
  1989年   8937篇
  1988年   8627篇
  1987年   7881篇
  1986年   7652篇
  1985年   8556篇
  1984年   7800篇
  1983年   7534篇
  1982年   6801篇
  1981年   6990篇
  1980年   6720篇
  1979年   6912篇
  1978年   6904篇
  1977年   7519篇
  1976年   9454篇
  1975年   6288篇
  1974年   6094篇
  1973年   6203篇
  1972年   5397篇
  1971年   4976篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The requirements on an object-oriented DBMS for management of information in a large, complex enterprise are presented. These requirements aid in the achievement of an environment characterized by data sharing, open architectures, application and data portability, and assurance of data integrity. They were defined from the point of view of a user of the DBMS; therefore they describe the expected functionality of the DBMS and do not specify the method of implementation to achieve this functionality. They encompass requirements on the data model, query and data manipulation languages, the system architecure, interfaces to the system, change management, and transaction management.  相似文献   
992.
Surface tension as a controlled variable in mechanical dishwashing   总被引:1,自引:0,他引:1  
Using a continuous, in-line, nondestructive technique based on the maximum bubble pressure method, the surface tension of the wash water in a mechanical dishwashing machine has been monitored. This technique has enabled surface tension to be used as a controlled variable, providing feedback to regulate the amount of surfactant added. Assuming the mechanical force of the water spray is adequate to remove bound soil from the dish surface, the food soil may be titrated against surfactant, providing an indirect indication as to when the dishes are clean. This technique also could be applied to a recirculated wash solution for in-place cleaning.  相似文献   
993.
994.
995.
996.
Triton polymers are commercial surfactants whose molecular weight distributions are conventionally determined by means of high-performance liquid chromatography (HPLC). However, in the case of the important octylphenol ethoxylates [p-C8H17-C6H4-O-(CH2CH2O)n-H], HPLC cannot resolve individual oligomers of high molecular weight Triton surfactants (e.g., greater than 2000 u or so; u = unified atomic mass unit). In this paper, we show that laser desorption Fourier transform ion cyclotron resonance mass spectrometry (LD/FT/ICR/MS) provides a simple and accurate measure of such Triton surfactant molecular weight distributions up to at least 3500 u, based on a single-shot laser pulse measurement of a few seconds duration. Comparison of LD/FT/ICR/MS and HPLC molecular weight distributions of low molecular weight surfactants shows that laser desorption/ionization produces minimal fragmentation and thus offers an accurate measure of the relative abundances of the neutral oligomers, without the need for prior chromatographic separation of the components. Moreover, for all Triton polymer molecular weight distributions (700-3000 u), LD/FT/ICR/MS provides much more highly resolved profiles of oligomer relative abundances. Finally, LD/FT/ICR/MS reveals the presence of poly(ethylene oxide) side products of the polymerization process, which are not observed by HPLC with conventional ultraviolet absorption detection.  相似文献   
997.
This paper presents the quantified study of the electromagnetic radiation mechanism of the 20-H rule using a numerical approach that has not yet been systematically addressed. The 20-H rule is a rule-of-thumb layout technique recommended to minimize radiated fields propagating from the edges of a printed circuit board (PCB) coupling onto nearby structures. Propagating electromagnetic fields may corrupt adjacent cable assemblies, sheet metal enclosures, and aperture openings. The magnitude of this design rule is investigated using the full-wave finite-difference time-domain (FDTD) method. An analysis on whether benefits exist from use of this rule is examined and under what conditions the rule is valid when correctly implemented. The purpose of this paper is to provide insight into the validity of the 20-H rule, recognizing that every PCB will have different simulation results. FDTD is used to capture a snapshot view of field propagation. This view allows one to determine the validity of the 20-H rule at a single point of time within a dynamic structure and what may be expected when digital components are finally added to a PCB assembly, which generally negates simulated results.  相似文献   
998.
Increasing attention at oil refineries is being focused on solving environmental problems caused by stiffening of the requirements for emissions of harmful substances into air and water. Advanced technologies that reduce the effect of industrial enterprises on the environment to the minimum are becoming increasingly in demand. Treatment of process wastewaters to remove hydrogen sulfide and ammonia is one such problem of the modern refinery with cat crackers. __________ Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 3, pp. 52–55, May–June, 2006.  相似文献   
999.
Several coats, such as gray cast iron, cast iron alloyed with nickel and copper, nickel-aluminum, Nichrome (nickel-chromium alloy), bronze, etc., have been investigated. Corrosion resistance of coats in a medium based on drilling mud and sodium chloride and sulfate solutions has been checked. Alloyed cast iron admixed with tin-based compounds showed the best results. __________ Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 3, pp. 43–44, March, 2006.  相似文献   
1000.
In this paper, we propose low-power designs for the synchronizer and channel estimator units of the Inner Receiver in wireless local area network systems. The objective of the work is the optimization, with respect to power, area, and latency, of both the signal processing algorithms themselves and their implementation. Novel circuit design strategies have been employed to realize optimal hardware and power efficient architectures for the fast Fourier transform, arc tangent computation unit, numerically controlled oscillator, and the decimation filters. The use of multiple clock domains and clock gating reduces the power consumption further. These blocks have been integrated into an experimental digital baseband processor for the IEEE 802.11a standard implemented in the 0.25mum- 5-metal layer BiCMOS technology from Institute for High Performance Microelectronics.  相似文献   
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