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   1530047篇
  免费   26452篇
  国内免费   6994篇
电工技术   34344篇
综合类   6495篇
化学工业   272650篇
金属工艺   64232篇
机械仪表   43030篇
建筑科学   47180篇
矿业工程   11377篇
能源动力   50388篇
轻工业   118385篇
水利工程   15519篇
石油天然气   37292篇
武器工业   136篇
无线电   197211篇
一般工业技术   288857篇
冶金工业   170923篇
原子能技术   33851篇
自动化技术   171623篇
  2021年   15688篇
  2020年   11935篇
  2019年   14723篇
  2018年   14990篇
  2017年   14160篇
  2016年   21230篇
  2015年   17520篇
  2014年   28997篇
  2013年   88265篇
  2012年   36507篇
  2011年   48830篇
  2010年   42465篇
  2009年   50651篇
  2008年   45625篇
  2007年   42824篇
  2006年   45819篇
  2005年   40058篇
  2004年   42577篇
  2003年   42424篇
  2002年   41480篇
  2001年   38486篇
  2000年   36726篇
  1999年   35727篇
  1998年   47214篇
  1997年   40735篇
  1996年   36449篇
  1995年   31575篇
  1994年   29614篇
  1993年   29161篇
  1992年   26563篇
  1991年   23652篇
  1990年   23898篇
  1989年   22973篇
  1988年   21467篇
  1987年   19741篇
  1986年   19204篇
  1985年   22587篇
  1984年   22699篇
  1983年   20580篇
  1982年   19430篇
  1981年   19497篇
  1980年   18101篇
  1979年   18744篇
  1978年   17925篇
  1977年   17994篇
  1976年   19551篇
  1975年   16110篇
  1974年   15646篇
  1973年   15721篇
  1972年   13209篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
An antenna array for wideband operation (up to 70%) is presented. The structure has low windloading area and consists of parallel printed circuit boards (PCB) with microstrip dipoles, feed network and metal fences placed between the PCBs. The low profile, low weight antenna array forms the main beam and three difference patterns for sidelobe cancellation. Experimental results are compared with calculations for both microstrip dipole and array  相似文献   
952.
Under the Mojave configurable computing project, we have developed a system for achieving high performance on an automatic target recognition (ATR) application through the use of configurable computing technology. The ATR system studied here involves real-time image acquisition from a synthetic aperture radar (SAR). SAR images exhibit statistical properties which can be used to improve system performance. In this paper, the Mojave configurable computing system uses field programmable gate arrays (FPGA's) to implement highly specialized circuits while retaining the flexibility of programmable components. A controller sequences through a set of specialized circuits in response to real-time events. Computer-aided design (CAD) tools have been developed to support the automatic generation of these specialized circuits. The resulting configurable computing system achieves a significant performance advantage over the existing solution, which is based on application specific integrated circuit (ASIC) technology  相似文献   
953.
954.
Thin microtomed samples from hot-tool butt welds of polypropylene pipes are subjected to uniaxial tensile stress using a specially built instrumented microtesting machine. The deformation of the interface between the weld and the bulk polymer is measured by an optical method. An analysis of these measurements is carried out using the finite element method, and contours of the effective stress are obtained. There is a steep stress gradient at the junction of the weld flash and the bulk polymer, although the stress concentration factor is relatively low. The stress is essentially constant in the bulk polymer apart from the region near the weld zone. Tests on samples without the weld flash show that the maximum stress occurs within the weld zone. This is consistent with long-term tests on larger samples, where the fracture is found to initiate within the weld. The method of analysis enables the stress-strain response of the weld material to be determined.  相似文献   
955.
956.
Dissolved organic carbon (DOC) dynamics were investigated over a two year period in the Upper Rhǒne River to examine the role of side-arms in providing DOC to the main channel in relation to discharge fluctuations, especially floods. Concentrations of DOC are shown to remain low in space and in time (average 1.5 mg ?1) and to be more related to global hydrological events (precipitation) than to any local flushing for backwaters during floods. The results do not support the general assumption that side-arms are the providers of DOC to the main channel.  相似文献   
957.
Estramustine is an estradiol-based agent that has been shown to accumulate in human glioma cells, resulting in a concentration-dependent alteration in cell size and shape within minutes and an inhibition of proliferation over 3 to 6 days. We evaluated human glioblastoma cultures with [3H]thymidine incorporation assays to determine estramustine's early effects on deoxyribonucleic acid synthesis in these tumors. Because estramustine shares a common structural motif with other antimicrotubule drugs, we synthesized four A-ring conjugates of estrone that contained a carbamate moiety but lacked nitrogen mustard. These analogs were examined by [3H]thymidine incorporation and compared with vinblastine. Greater than 70% inhibition of [3H]thymidine incorporation occurred within 1 hour of treatment with estramustine at 10(-5) mol/L, which increased to 80% inhibition at 4 hours. Ethyl carbamate JE208 was nearly as effective as estramustine in inhibiting deoxyribonucleic acid synthesis, and both were more effective than vinblastine. The inhibitory effects of estramustine and estrone analogs were reversible; vinblastine was not reversible. Although estramustine and JE208 induced similar antiproliferative and morphological changes in glioblastoma cells that persisted for at least 4 days, there was a modest recovery of morphology and thymidine incorporation with JE208 after prolonged treatment. The common findings with estramustine and JE208 suggest that these agents may have a similar mechanism of action and form the basis for the investigation of new agents that may rapidly and reversibly inhibit glioblastoma.  相似文献   
958.
We report a 20 month old female patient with diploid-triploid mixoploidy (46,XX/69,XXX) syndrome with hypothyroidism and precocious puberty. The triploid cell line was only expressed in the fibroblast culture and comprised the majority (95%) of the cells. Chromosome analysis of the fetal blood sample and peripheral blood sample were normal. The patient shows typical features of full triploidy (growth and severe mental retardation, cranial and facial dysmorphism, complete syndactyly of fingers 3/4, partial syndactyly of toes 2/3) and facial but no body asymmetry. At the age of 5 months central hypothyroidism and precocious puberty were diagnosed. Thin pigmented streaks were visible on the wrists and legs of the patient at the age of 16 months. This is the first patient reported so far with 46,XX/69,XXX mixoploidy suffering from hypothyroidism and precocious puberty.  相似文献   
959.
960.
36%渗乐乳油     
陈洪超  简锡贤 《农药》1995,34(9):13-14
36%渗乐乳油田HP-1增效剂与乐果及其他助剂调配而成。五年田间小区和大田药效试验证明:36%渗乐乳油高效,安全,用量少,对各类蚜虫防效优异。  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