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   321001篇
  免费   5605篇
  国内免费   842篇
电工技术   5262篇
综合类   183篇
化学工业   51510篇
金属工艺   12495篇
机械仪表   9249篇
建筑科学   7988篇
矿业工程   1901篇
能源动力   7425篇
轻工业   30410篇
水利工程   3495篇
石油天然气   7214篇
武器工业   20篇
无线电   33385篇
一般工业技术   62959篇
冶金工业   59830篇
原子能技术   7677篇
自动化技术   26445篇
  2021年   2861篇
  2019年   2554篇
  2018年   4984篇
  2017年   4964篇
  2016年   5292篇
  2015年   3451篇
  2014年   5474篇
  2013年   14408篇
  2012年   8897篇
  2011年   11160篇
  2010年   9069篇
  2009年   9957篇
  2008年   10495篇
  2007年   10144篇
  2006年   8827篇
  2005年   8153篇
  2004年   7583篇
  2003年   7258篇
  2002年   7233篇
  2001年   6866篇
  2000年   6547篇
  1999年   6712篇
  1998年   17411篇
  1997年   12627篇
  1996年   9575篇
  1995年   7131篇
  1994年   6459篇
  1993年   6297篇
  1992年   4645篇
  1991年   4614篇
  1990年   4465篇
  1989年   4461篇
  1988年   4401篇
  1987年   3726篇
  1986年   3737篇
  1985年   4321篇
  1984年   4106篇
  1983年   3716篇
  1982年   3549篇
  1981年   3686篇
  1980年   3534篇
  1979年   3460篇
  1978年   3535篇
  1977年   4078篇
  1976年   5369篇
  1975年   3221篇
  1974年   3051篇
  1973年   3088篇
  1972年   2682篇
  1971年   2511篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
An experimental methodology for inferring brine dissolution rate in monoethylene glycol (MEG) solutions at different temperatures using a webcam combined with a mathematical model is presented. The measurement system is designed to track the RGB (red, green, and blue) colour variations during the dissolution process. A dynamic model augmented with the population balance equation is applied to describe the dissolution process. Moreover, the dissolution rate is consistently related to the temperature and MEG concentration through the driving force based on the Gibbs energy and chemical affinity. The applied low-cost measurement apparatus proved to be a useful resource for tracking the dissolution dynamics in a wide range of undersaturation.  相似文献   
162.
Pyrethroid esters are widely used as insecticides worldwide. In this study, we aimed to evaluate the harmful effect of deltamethrin on the male reproductive system through the assessment of reproductive hormones, inflammatory markers, and testicular function. To achieve our aim, eighty male 7-9-week-old, Wistar rats were taken, weighed, and divided into four experimental groups. The first group was kept as a control group, and the other three groups were given deltamethrin orally at different concentrations (0.87, 8.7, and 17.4 mg/kg body weight) for nine weeks. The results indicated that deltamethrin administration associated with a significant decrease in reproductive hormones, especially FSH, LH, and significant elevation in the interleukin 2 (IL2), interleukin 6 (IL6), histamine, and cortisol levels. Also, the significance of inhibition of sperm motility and viability, decreased testis weights, sperm count, and fructose in semen were noted. These findings clarify the harmful effect of deltamethrin on the male reproductive system by producing a significant alteration in reproductive hormones, inflammatory markers as well as testicular function.  相似文献   
163.
164.
165.
166.

This paper presents a PVS development of relevant results of the theory of rings. The PVS theory includes complete proofs of the three classical isomorphism theorems for rings, and characterizations of principal, prime and maximal ideals. Algebraic concepts and properties are specified and formalized as generally as possible allowing in this manner their application to other algebraic structures. The development provides the required elements to formalize important algebraic theorems. In particular, the paper presents the formalization of the general algebraic-theoretical version of the Chinese remainder theorem (CRT) for the theory of rings, as given in abstract algebra textbooks, proved as a consequence of the first isomorphism theorem. Also, the PVS theory includes a formalization of the number-theoretical version of CRT for the structure of integers, which is the version of CRT found in formalizations. CRT for integers is obtained as a consequence of the general version of CRT for the theory of rings.

  相似文献   
167.
168.
The central nervous system (CNS) is the most complex structure in the body, consisting of multiple cell types with distinct morphology and function. Development of the neuronal circuit and its function rely on a continuous crosstalk between neurons and non-neural cells. It has been widely accepted that extracellular vesicles (EVs), mainly exosomes, are effective entities responsible for intercellular CNS communication. They contain membrane and cytoplasmic proteins, lipids, non-coding RNAs, microRNAs and mRNAs. Their cargo modulates gene and protein expression in recipient cells. Several lines of evidence indicate that EVs play a role in modifying signal transduction with subsequent physiological changes in neurogenesis, gliogenesis, synaptogenesis and network circuit formation and activity, as well as synaptic pruning and myelination. Several studies demonstrate that neural and non-neural EVs play an important role in physiological and pathological neurodevelopment. The present review discusses the role of EVs in various neurodevelopmental disorders and the prospects of using EVs as disease biomarkers and therapeutics.  相似文献   
169.
In order to improve the process effectiveness and joint quality, ultrasonic vibrations were integrated with friction stir lap welding. Effect of ultrasonic exertion on the process and joint quality of AA 6061-T6 were investigated. Upon ultrasonic exertion, joints owned larger effective lap width, shorter hooks and improved strength. Weld fracture mode changed from a ductile–brittle mixed mode to a more ductile mode while the fracture path shifted from lap interface to beyond the stir zone. Material flow and interface defects were characterised using lap welded dissimilar aluminium alloy joints. Ultrasonic vibration improved the material flow and reduced the interfacial defects. Variations in failure load of joints were found in accordance with the variations in material flow and interfacial defects.  相似文献   
170.
Nano crystalline pure and Mg doped ceriaparticles were synthesized by simple chemical co-precipitation method using cerium nitrate hexahydrate as a source material and magnesium nitrate as doping precursor at room temperature. The effect of doping were investigated by X-ray diffraction pattern(XRD), FT-Raman,fourier transform infrared spectroscopy(FTIR), Ultraviolet spectroscopy(UV), photoluminescence spectroscopy(PL), field emission scanning electron microscope(FESEM) and high resolution transmission electron microscopy with energy dispersive spectroscopy (HRTEM &EDS). The X-ray diffraction pattern and FT-Raman studies showed that the prepared samples were nano particulates with cubic fluorite structure. The XRD pattern analysis showed that the size of the particles ranged from 13 to 20?nm, however 4?wt% Mg doping results in reduction of particle size compared with other doping concentrations. The effects of Mg concentration on various structural parameters of the prepared samples were also determined. The slight blue shift observed upon doping in UV–Vis absorption region around 330–360nmrecorded for reduction in particle size. The FTIR unveils the presence of Metal oxygen bonds below 700?cm?1in the prepared samples. All samples showed a broad emission band at 430?nm with linearly increasing intensity with respect to dopant concentrations. The Spherical morphology with weak agglomeration was identified through FESEM and HRTEM analysis. The elemental analysis of Ce, O and Mg were confirmed through EDS analysis.  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