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   33555篇
  免费   872篇
  国内免费   66篇
电工技术   373篇
综合类   35篇
化学工业   6838篇
金属工艺   861篇
机械仪表   630篇
建筑科学   1856篇
矿业工程   249篇
能源动力   1029篇
轻工业   2785篇
水利工程   340篇
石油天然气   209篇
武器工业   2篇
无线电   2223篇
一般工业技术   5248篇
冶金工业   6825篇
原子能技术   327篇
自动化技术   4663篇
  2022年   198篇
  2021年   391篇
  2020年   333篇
  2019年   403篇
  2018年   494篇
  2017年   513篇
  2016年   523篇
  2015年   466篇
  2014年   742篇
  2013年   2149篇
  2012年   1249篇
  2011年   1606篇
  2010年   1180篇
  2009年   1268篇
  2008年   1450篇
  2007年   1480篇
  2006年   1269篇
  2005年   1192篇
  2004年   1066篇
  2003年   1032篇
  2002年   1045篇
  2001年   645篇
  2000年   628篇
  1999年   587篇
  1998年   645篇
  1997年   531篇
  1996年   625篇
  1995年   565篇
  1994年   549篇
  1993年   544篇
  1992年   494篇
  1991年   330篇
  1990年   444篇
  1989年   430篇
  1988年   388篇
  1987年   431篇
  1986年   388篇
  1985年   520篇
  1984年   496篇
  1983年   450篇
  1982年   467篇
  1981年   446篇
  1980年   356篇
  1979年   366篇
  1978年   333篇
  1977年   315篇
  1976年   274篇
  1975年   320篇
  1974年   246篇
  1973年   283篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Feedback provided to Ss and 3rd parties from intra- and extramural investigators following the completion of research involving an agency's clients or staff is described. Ss were from the division of a public health department that provides mental health and substance abuse services in a city with 740,000 people. The division's research requirements include submission of annual progress reports and final reports. Of 135 studies conducted over 12 yrs, annual reports were received from 37% of investigators, and final reports from 30%. There were no significant differences based on investigator discipline or affiliation. Professional and ethical implications are discussed, including adequacy of professional training, the need for mechanisms to ensure compliance with ethical standards, the obligations of clinic directors, and the impact on public academic liaison programs. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
102.
随着信息产业的飞速发展,高频信号设备,如高速计算机、移动电话、卫星通信等,都已从兆赫频段扩展到千兆赫,这就意味着对它们所用的微波电路基板提出了更高的要求.长期以来,高频微波基板几乎没有越出使用聚四氟乙烯的老传统,但是,它有若干缺点玻璃化温度低,故刚性差;加工复杂,故成本高;金属化孔镀层与孔壁的结合力弱,故可靠性不高. 多年来,美国GIL科技公司一直在研制替代聚四氟乙烯的材料,经过姐妹公司的合作,终于研制成功了TPA板材,这种材料克服了上述缺点,从而获得了美国专利.本文介绍了TPA层压板的电气特性、机械特性和热效应,给出了许多宝贵数据.  相似文献   
103.
104.
Many companies are using Just-In-Time (JIT) control systems in their mixed-model multi-level production facilities. When scheduling these facilities the most important objective is to keep a constant rate of usage for every part used by the system. In this paper a mathematical model for scheduling these facilities is studied and techniques for determining good schedules are developed. Procedures for solving very large problems are given.  相似文献   
105.
We present a characterization of some processes affecting the performance of solid state array cameras designed for ground based astronomical imaging in the 8–13μm atmospheric window. Our discussion includes a novel model for electron-hole generationrecombination noise based on the probable pathlength of an electron in a photoconductor. We use the Berkeley mid-IR Array Camera as an example. For this camera, the results show that the total optical system composed of the camera, a 3m telescope, and the atmosphere has an efficiency of about 3%, a 1σ noise equivalent flux density of 25 mJy min?1/2arcsec?2 measured over a Δλ/λ=10% band width, and a noise equivalent expressed as the ambient temperature thermal black body noise of 23%.  相似文献   
106.
Diamond and BN have common relatively soft precursor structures that are hexagonal. Electronic structure calculations have now revealed subtle energy differences between the two materials. Another hard material, stishovite, also has a relatively soft precursor phase, quartz, and here again electronic structure calculations have given insight into the nature of the phase transformation. On the other hand, with boron-related materials, the main focus is to connect basic icosahedra structures to produce a hard material. The structural aspects of these materials are examined and possible consequences for low- and high-coordination ultrahard nitrides considered.  相似文献   
107.
Aqueous extracts of the coralline red algaeBossiella orbigniana have been shown to catalyze the enzymatic oxidation of arachidonic acid to a previously unreported ω6 eicosapentaenoic acid product. This unique fatty acid contains a conjugated tetraene with absorption maxima at 293, 306, and 321 nm, and was identified by spectral methods as 5(Z),8(Z),10(E),12(E),14(Z)-eicosapentaenoic acid. The compound was given the trivial name bosseopentaenoic acid. Under experimental conditions, production of bosseopentaenoic acid was linear up to 2 hr and dioxygen was consumed. Bosseopentaenoic acid, along with several other conjugated tetraenes, was also present in the algae endogenously as revealed by a comparison of the ultraviolet (UV) spectra and the high-performance liquid chromatographic (HPLC) pattern of the purified product and the organic extract ofBossiella.  相似文献   
108.
This article presents a comparative analysis of contemporary printed wiring assembly (PWA) batch manufacturing systems employing surface mount technology (SMT), This analysis is accomplished by reviewing a typical case study of PWA manufacturing system in the USA. A previously developed model is then applied to the system to predict the cost and performance in the production of typical PWA designs. Various modifications to the system are then proposed and evaluated. From the case study, conclusions concerning optimum manufacturing system configuration for this industry can be drawn and are presented.  相似文献   
109.
110.
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