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   115970篇
  免费   8849篇
  国内免费   4237篇
电工技术   5802篇
综合类   4775篇
化学工业   19506篇
金属工艺   5372篇
机械仪表   7014篇
建筑科学   5685篇
矿业工程   1679篇
能源动力   4084篇
轻工业   8120篇
水利工程   1549篇
石油天然气   2508篇
武器工业   832篇
无线电   17959篇
一般工业技术   17904篇
冶金工业   8409篇
原子能技术   1426篇
自动化技术   16432篇
  2024年   312篇
  2023年   1332篇
  2022年   2423篇
  2021年   3709篇
  2020年   2729篇
  2019年   2466篇
  2018年   2897篇
  2017年   3149篇
  2016年   3373篇
  2015年   3815篇
  2014年   5133篇
  2013年   7508篇
  2012年   7396篇
  2011年   8429篇
  2010年   7231篇
  2009年   7350篇
  2008年   6958篇
  2007年   6274篇
  2006年   5862篇
  2005年   4917篇
  2004年   4119篇
  2003年   3776篇
  2002年   3742篇
  2001年   3142篇
  2000年   2579篇
  1999年   2188篇
  1998年   3018篇
  1997年   2053篇
  1996年   1738篇
  1995年   1371篇
  1994年   1085篇
  1993年   985篇
  1992年   717篇
  1991年   674篇
  1990年   568篇
  1989年   529篇
  1988年   447篇
  1987年   345篇
  1986年   325篇
  1985年   284篇
  1984年   247篇
  1983年   189篇
  1982年   185篇
  1981年   165篇
  1980年   158篇
  1979年   122篇
  1978年   113篇
  1977年   146篇
  1976年   193篇
  1974年   87篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
932.
OBJECTIVES: (1) To determine whether and how outcome measurements in the ECASS trial are influenced by a shorter time window (0-3 vs. 3-6 h) between onset of symptoms and start of thrombolytic therapy using recombinant tissue plasminogen activator (rt-PA) in acute ischemic stroke. (2) To discuss the results of the ECASS 0- to 3-hour cohort with the results of the National Institute of Neurological Disorders and Stroke Study (NINDSS). DESIGN AND ANALYSIS: Analysis of the 0- to 3-hour and the 3- to 6-hour cohort in accordance with the ECASS protocol. Comparative analysis of the ECASS and NINDSS results following the NINDSS protocol using dichotomized endpoints. MAIN OUTCOME MEASURES: Primary endpoints: modified Rankin Scale, Barthel Index; secondary endpoints: combined Barthel/Rankin, long-term Scandinavian Stroke Scale, National Institutes of Health Stroke Scale, mortality at 30 and 90 days, occurrence of intracranial hemorrhage. NINDS trial endpoint: favorable outcome as defined in the NINDS trial. RESULTS: In ECASS, 87 patients were randomized within 3 h of stroke onset. Differences in favor of rt-PA treatment can be found for all primary and secondary outcome measures in the ECASS 0- to 3-hour cohort, except for mortality at day 30, which is somewhat higher in the rt-pA-treated group. However, due to the small sample size, the differences do not reach statistical significance. Early infarct signs (as defined by the ECASS protocol) are found as early as 2 h after stroke onset. Parenchymal hemorrhages are found significantly more often among rt-PA-treated patients. The results in the ECASS 0- to 3-hour cohort fit well with the results in NINDSS. CONCLUSION: Data from the 3-hour ECASS cohort support the efficacy of early thrombolytic therapy in acute hemispheric stroke patients. Comparing bleeding complications between the ECASS and NINDSS is difficult because of differences in the definition and occurrence of hemorrhagic events.  相似文献   
933.
Circular groove guide is a kind of waveguide for millimeter waves with low loss, large dimensions, and low dispersion. In this paper, the dominant mode characteristics of it is analysed. The technique that mode macthing method combines with point matching method is used to derive the Nth-order characteristic equation of the guide. The 1st order, 2nd order and 3rd order approximate curves that the normalized cutoff wavelength of circular groove guide varies with c/a are given. The attenuation curves are given at 1mm, 1.5mm and 3mm wavelength. The numerical results are in well agreement with the experimental results published.  相似文献   
934.
In order to improve the Zn homogeneity along the axial direction of CdZnTe boule, we have employed a modified Bridgman technique using a (Cd, Zn) alloy source in communication with the melt, whose temperature has been gradually changed from 800 to 840°C during growth. Electron probe microanalysis (EPMA) measurements of Zn composition in the boule shows an excellent homogeneity of Zn along the axis of the CdZnTe boule compared with results in a boule grown by using a fixed source temperature. We have performed a numerical simulation to obtain the approximate temperatures of additional heating and cooling needed to improve the radial Zn homogeneity. CdZnTe boule has been grown by seeded vertical Bridgman furnace with two zones of heater and cooler. Ultraviolet/visible spectroscopic measurements of Zn composition over the length of the boule indicate that the radial distribution of Zn composition is very homogeneous in the body region of the boule, where the radial variation of Zn composition is ±0.0005.  相似文献   
935.
The year 1996 marks the 50th anniversary of the public revelation of the ENIAC, the 50th anniversary of the establishment of the Large Scale Computing Subcommittee of the AIEE under the chairmanship of Charles Concordia, and the beginning of the 50th anniversary celebrations of the IEEE Computer Society and the ACM. Within those years, the computer field has not only developed but has had added to it new concepts and ideas that have transmogrified it into an almost unrecognizable entity. We are reaching the stage of development where each new generation of participants is unaware both of their overall technological ancestry and the history of the development of their specialty, and have no past to build upon. In this article, we look at the study of the history of computing and its applicability to today's technological challenges, and conclude with the recommendation that we need to know enough about our history to protect ourselves from it and not be condemned to repeat it, but also to use it to our advantage  相似文献   
936.
Insensitive current/voltage-mode filters using FTFNs   总被引:1,自引:0,他引:1  
A new configuration for realising insensitive current/voltage-mode filters using two four-terminal floating nullors (FTFNs) is presented. It can provide the curren/voltage-mode filters with reduced sensitivities. Moreover, the current-mode filters can have high output impedance and only grounded capacitors. The proposed universal voltage-mode filter with dual outputs can realise lowpass, bandpass, highpass, notch and allpass filtering functions. Simulation results are given to verify the theoretical analysis  相似文献   
937.
A multiway merge sorting network is presented, which generalizes the technique used in the odd-even merge sorting network. The merging network described here is composed of m k-way mergers and a combining network. It arranges k ordered lists of length n each into one ordered lists in T(k)+[log2k] [log2m] [log2m] steps, where T(k) is the number of steps needed to sort k keys in order; and k and m are any integers no longer restricted to 2  相似文献   
938.
The authors propose an efficient traceback scheme for the parallel hardware implementation of the Viterbi algorithm. Compared to the conventional Viterbi algorithm, where output is selected arbitrarily when multiple survivor paths exist, the proposed algorithm decides decoding output by analysing the survivor paths of consecutive tracebacks. Experimental results show that the proposed algorithm exhibits improved error-correction capability compared to the existing algorithms  相似文献   
939.
940.
OBJECTIVE: Spontaneous spinal epidural hematoma (SSEH) is an idiopathic accumulation of blood in the vertebral epidural space without identifiable predisposing factors. First reported in 1869, the clinical outcome in children younger than 18 years old has not been clearly delineated. DESIGN: A comprehensive review of the English language literature revealed 26 patients younger than 18 years old with reported clinical outcomes. The 27th case is presented. RESULTS: Complete neurologic recovery occurred in 14 of 27 (52%) patients, partial recovery in 12 of 27 (44%) patients, and death in 1 of 27 (4%) patients. CONCLUSION: There is an overall good prognosis for neurologic recovery in children who experience SSEH.  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