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   538838篇
  免费   8689篇
  国内免费   1471篇
电工技术   9730篇
综合类   538篇
化学工业   81254篇
金属工艺   20229篇
机械仪表   15487篇
建筑科学   13576篇
矿业工程   2317篇
能源动力   14947篇
轻工业   48398篇
水利工程   5309篇
石油天然气   9384篇
武器工业   38篇
无线电   61153篇
一般工业技术   104243篇
冶金工业   105916篇
原子能技术   10700篇
自动化技术   45779篇
  2021年   5261篇
  2020年   3778篇
  2019年   4867篇
  2018年   8164篇
  2017年   7949篇
  2016年   8366篇
  2015年   5924篇
  2014年   9686篇
  2013年   24980篇
  2012年   15347篇
  2011年   21114篇
  2010年   16455篇
  2009年   18393篇
  2008年   18829篇
  2007年   18415篇
  2006年   16094篇
  2005年   14726篇
  2004年   13977篇
  2003年   13749篇
  2002年   12991篇
  2001年   13059篇
  2000年   12112篇
  1999年   12856篇
  1998年   33373篇
  1997年   23257篇
  1996年   17758篇
  1995年   13253篇
  1994年   11487篇
  1993年   11187篇
  1992年   8052篇
  1991年   7551篇
  1990年   7366篇
  1989年   7124篇
  1988年   6573篇
  1987年   5857篇
  1986年   5747篇
  1985年   6377篇
  1984年   6001篇
  1983年   5267篇
  1982年   4971篇
  1981年   5050篇
  1980年   4776篇
  1979年   4692篇
  1978年   4483篇
  1977年   5430篇
  1976年   7090篇
  1975年   3863篇
  1974年   3611篇
  1973年   3717篇
  1972年   3022篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Grass pea (Lathyrus sativus) is a leguminous plant of outstanding tolerance to abiotic stress. The aim of the presented study was to describe the mechanism of grass pea (Lathyrus sativus L.) photosynthetic apparatus acclimatisation strategies to salinity stress. The seedlings were cultivated in a hydroponic system in media containing various concentrations of NaCl (0, 50, and 100 mM), imitating none, moderate, and severe salinity, respectively, for three weeks. In order to characterise the function and structure of the photosynthetic apparatus, Chl a fluorescence, gas exchange measurements, proteome analysis, and Fourier-transform infrared spectroscopy (FT-IR) analysis were done inter alia. Significant differences in the response of the leaf and stem photosynthetic apparatus to severe salt stress were observed. Leaves became the place of harmful ion (Na+) accumulation, and the efficiency of their carboxylation decreased sharply. In turn, in stems, the reconstruction of the photosynthetic apparatus (antenna and photosystem complexes) activated alternative electron transport pathways, leading to effective ATP synthesis, which is required for the efficient translocation of Na+ to leaves. These changes enabled efficient stem carboxylation and made them the main source of assimilates. The observed changes indicate the high plasticity of grass pea photosynthetic apparatus, providing an effective mechanism of tolerance to salinity stress.  相似文献   
32.
Glass and Ceramics - An analysis of the specific mechanisms of the influence of the state of the grain structure on the thermoelectric properties (electrical resistivity and total thermal...  相似文献   
33.
The solid solutions based on the pyrochlore-type system Bi2MgNb2-xTaxO9 were formed in the compositional range х = 0–2.0 (Bi1·6Mg0·8Nb1.6-tTatO7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi2MgNb2-xTaxO9 (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi1·6Mg0·8Ta1·6O7.2). The increase in the Ta5+ concentration led to the decrease in the dielectric permeability from 104 (Bi1·6Mg0·8Nb1·6O7.2) to 20 (Bi1·6Mg0·8Ta1·6O7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme.  相似文献   
34.
Multimedia Tools and Applications - In this paper, a hashing based watermarking technique for the protection and authentication of document image is proposed. Message Digest 5 (MD5) hashing is...  相似文献   
35.
Ferroptosis is gaining followers as mechanism of selective killing cancer cells in a non-apoptotic manner, and novel nanosystems capable of inducing this iron-dependent death are being increasingly developed. Among them, polydopamine nanoparticles (PDA NPs) are arousing interest, since they have great capability of chelating iron. In this work, PDA NPs were loaded with Fe3+ at different pH values to assess the importance that the pH may have in determining their therapeutic activity and selectivity. In addition, doxorubicin was also loaded to the nanoparticles to achieve a synergist effect. The in vitro assays that were performed with the BT474 and HS5 cell lines showed that, when Fe3+ was adsorbed in PDA NPs at pH values close to which Fe(OH)3 begins to be formed, these nanoparticles had greater antitumor activity and selectivity despite having chelated a smaller amount of Fe3+. Otherwise, it was demonstrated that Fe3+ could be released in the late endo/lysosomes thanks to their acidic pH and their Ca2+ content, and that when Fe3+ was co-transported with doxorubicin, the therapeutic activity of PDA NPs was enhanced. Thus, reported PDA NPs loaded with both Fe3+ and doxorubicin may constitute a good approach to target breast tumors.  相似文献   
36.
Marrone  Mauricio  Lemke  Sascha  Kolbe  Lutz M. 《Scientometrics》2022,127(7):3857-3878
Scientometrics - Computer-assisted methods and tools can help researchers automate the coding process of literature reviews and accelerate the literature review process. However, existing...  相似文献   
37.
Journal of Materials Science: Materials in Electronics - Nano Mn0.95M0.05S (M ≡ Cu, Mg) samples were produced using molten salt solid state reaction method. Rietveld analysis of X-ray...  相似文献   
38.
Journal of Engineering Physics and Thermophysics - Results of investigation into the influence of a corrosion process on thermotechnical characteristics of heat pipes of low-temperature range are...  相似文献   
39.
Deshmukh  P.  Sar  S. K.  Smječanin  N.  Nuhanović  M.  Lalwani  R. 《Radiochemistry》2022,64(4):532-542
Radiochemistry - Magnetically modified waste bark of the Aegle marmelos tree was prepared by using green synthesis method and was used in a batch system for U(VI) removal from aqueous solution. The...  相似文献   
40.
Kamar  M. S.  Shalan  A. S.  Youssef  W. M.  Hussein  A. E. M.  Khawassek  Y. M.  Taha  M. H. 《Radiochemistry》2022,64(2):244-256
Radiochemistry - Ferrugination, sericitization, desulfitization, chloritization, and kaolinitization of hydrothermal origin are the main wall rock alterations occurring within the NNW shear zones...  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