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   1441498篇
  免费   27206篇
  国内免费   6848篇
电工技术   33731篇
综合类   6343篇
化学工业   255870篇
金属工艺   63468篇
机械仪表   40534篇
建筑科学   43572篇
矿业工程   11371篇
能源动力   49921篇
轻工业   104125篇
水利工程   14663篇
石油天然气   36464篇
武器工业   133篇
无线电   194579篇
一般工业技术   276075篇
冶金工业   143573篇
原子能技术   33851篇
自动化技术   167279篇
  2021年   14848篇
  2020年   11797篇
  2019年   14578篇
  2018年   14948篇
  2017年   14284篇
  2016年   20972篇
  2015年   17283篇
  2014年   28705篇
  2013年   87912篇
  2012年   34494篇
  2011年   46449篇
  2010年   41886篇
  2009年   50456篇
  2008年   43487篇
  2007年   40569篇
  2006年   43923篇
  2005年   38472篇
  2004年   40761篇
  2003年   40645篇
  2002年   39848篇
  2001年   36290篇
  2000年   34617篇
  1999年   31180篇
  1998年   35750篇
  1997年   33117篇
  1996年   30341篇
  1995年   28648篇
  1994年   27138篇
  1993年   26996篇
  1992年   25345篇
  1991年   22352篇
  1990年   22817篇
  1989年   21740篇
  1988年   20316篇
  1987年   18696篇
  1986年   18090篇
  1985年   21428篇
  1984年   21766篇
  1983年   19716篇
  1982年   18812篇
  1981年   18898篇
  1980年   17530篇
  1979年   18132篇
  1978年   17376篇
  1977年   16877篇
  1976年   17465篇
  1975年   15717篇
  1974年   15245篇
  1973年   15318篇
  1972年   12855篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
With the prevalent food insecurity in Africa, there is a growing need to utilize the available crops to develop nutritious, affordable and palatable food for the populace. Millet is critical in this role, relative to its abundance in the continent and good nutritional composition. For ages, fermentation and malting have been traditionally used to transform millet into variety of produce. A paradigm shift has however occurred over the years, giving birth to new commercially available products. This review thus appraises and gives an overview of traditional and modern fermented and malted products. Although, millet has been diversified to several products, its major food uses are still restrained to traditional consumers and largely remains underutilized. Considering the potential embedded in this grain, it is important to explore this crop through the application of appropriate modern fermentation and malting technologies. This will ensure the availability of ready to eat (RTE) and ready to use (RTU) food products and to a large extent address the incessant food security challenges plaguing Africa.  相似文献   
992.
Zilpaterol is a β-adrenergic agonist feed additive approved in the United States to increase weight gain and improve feed efficiency of cattle. A zilpaterol immunochromatographic assay was developed as an economical and user-friendly rapid detection method for zilpaterol and validated using urine and tissue samples derived from animal studies. The assay sensitivity was 1.7–23.2 ng g?1 or mL?1 across a variety of feed and animal matrices and did not cross-react with clenbuterol or ractopamine. No sample pre-treatment of cattle and sheep urine was needed, but horse urine and feed required dilution; skeletal muscle required solvent extraction prior to testing. Of 32 incurred sheep urine samples tested, zilpaterol content was correctly identified in all but 2 samples. Horse urine containing >10 ng mL?1 of incurred zilpaterol residue (n = 48) was correctly identified as zilpaterol positive. The assay correctly identified 0-day withdrawal sheep muscle samples as zilpaterol positive and the control and longer withdrawal day sheep muscle samples as negative. Zilpaterol was demonstrated to be stable in horse urine when stored at ?20°C for 7 years.  相似文献   
993.
A novel, Nb- and Si-rich and Be-free Ni-based alloy was cast by two methods of investment casting and continuous casting to study the microstructure evolution during solidification and its mechanical properties. The solidification of the alloy started with the primary crystallization of FCC-γ, followed by a binary eutectic reaction, with the formation of a heterogeneous constituent: FCC-γ+G-phase, which replaced the low-melting eutectic (FCC-γ+NiBe) in the Be-bearing alloys. AlNi6Si3 and γ′ formed during the terminal stages of solidification by investment casting, while the formation of AlNi6Si3 was suppressed by continuous casting. The Scheil solidification model agreed very well with the experimental results.  相似文献   
994.
高熵合金Al_(0.3)Cr Fe_(1.5)Mn Ni_(0.5)(HEA)具有许多特殊性能。在大气环境、接合温度为250℃保持60 s的条件下,以Sn3.5Ag4Ti为活性焊料对HEA/HEA及HEA/6061-Al进行直接活性软焊接合,并对其进行显微组织及剪力强度分析。实验结果显示,在接合过程中,高熵合金中所有元素缓慢扩散进入连接区域。HEA/HEA和HEA/6061-Al样品的剪切强度分别为(14.20±1.63)和(15.70±1.35)MPa。HEA/6061-Al样品的断裂面呈显明显的半脆性断裂特征。  相似文献   
995.
研究铜添加量(0.5%、1%、1.5%、2%、3%,质量分数)对A356合金的品质系数(Q_i)和热裂敏感性(HTS)的影响。结果显示,当铜添加量达到1.5%时,Q_i增加了约10%,这可能是由于富铜的Al_2Cu相和AlMgCuSi化合物的固溶强化和弥散硬化。然而,继续提高Cu添加量到3%,Q_i反而降低了近12%,这很可能是因为显微组织中脆性富Cu金属间化合物和微孔体积的增加导致抗拉强度和伸长率降低。用约束杆铸造法测试A356合金的热裂敏感性,结果显示随着铜含量的增加,其HTS也增加。热分析结果表明,铜拓宽了合金的凝固范围,进而降低了其流动性,增加了糊状合金暴露在热裂易感区的时间。高铜合金热裂表面的SEM也表明,其表面粗糙,且存在枝晶间/晶粒间微裂纹,这也证明了在凝固后期,合金萌生了热裂,没有足够的裂纹修复时间。  相似文献   
996.
Theoretical Foundations of Chemical Engineering - The operation of a linear crystallizer, which performs crystallization extraction of uranium from a nitric acid solution of uranyl nitrate during...  相似文献   
997.
In the present study, we investigate the fundamental properties of CeO2 by selecting La3+ (57), and Dy3+ (66) as dopants with optimized average atomic number of 61.5, which lies in between Pm3+ (62) and Sm3+ (62) in accordance with the criteria for optimum doping. A system of co-doped ceria ceramics Ce1–x–yLaxDyyO2-δ ((x, y) = (0.00, 0.00), (0.025, 0.025), (0.05, 0.05), (0.075, 0.075), (0.10, 0.10), (0.00, 0.20) and (0.20, 0.00)) as electrolytes for intermediate temperature solid oxide fuel cells were successfully prepared by a well-known sol-gel auto-combustion route. In order to obtain dense samples, the prepared pellets were sintered in air at 1300 °C for 4 h using conventional furnace and relative densities of all the samples were found to be higher than 95%. Single phase cubic structure, microstructural density and elemental composition analysis of all the samples were studied by powder X-ray diffraction, scanning electron microscope and energy dispersive spectroscopy techniques, respectively. Raman spectroscopy analysis confirmed the formation of concentrated O2-–vacancies in the co-doped ceria system. Impedance spectroscopy measurements revealed the high value of total ionic conductivity and low activation energy for the composition Ce0.85La0.075Dy0.075O2?δ i.e., 2.08 × 10–2 S cm–1 and 0.58 eV, respectively. Linear thermal expansion analyses of all the samples revealed the matched thermal expansion coefficients. Finally, these results recommend that the Ce0.85La0.075Dy0.075O2?δ sample can be useful as a solid electrolyte in IT-SOFC applications.  相似文献   
998.
Wireless Personal Communications - In this paper, we propose novel equalization schemes for compensating the adverse effects caused by carrier frequency offsets (CFOs), timing errors and doubly...  相似文献   
999.
Gurinovich  L. I.  Trotsiuk  L. L.  Kulakovich  O. S.  Sushko  N. I.  Demir  H. V.  Gaponenko  S. V. 《Semiconductors》2018,52(16):2054-2056
Semiconductors - Anisotropic polymer films containing aligned Au nanorods and semiconductor nanoparticles of various shape were fabricated. The photoluminescence of semiconductor nanoparticles in...  相似文献   
1000.
Data on the water uptake, proton conductivity, diffusion permeability, cation transport selectivity, and mechanical properties of short side chain Aquivion sulfonated perfluoropolymer membranes with an equivalent weight of 870 and 965 have been described. Properties of the membranes have been compared with those of a long side chain Nafion 212 membrane (equivalent weight of 1100). An increase in the equivalent weight leads to an increase in the sorption exchange capacity and water uptake of the membranes and a decrease in their proton conductivity. The conductivity of the Aquivion membrane with an equivalent weight of 870 is 1.4–1.5 times higher than that of the Nafion 212 membrane; it reaches 13.6 mS/cm in contact with water and 1.0 mS/cm at a relative humidity of 32% at 25°C. Diffusion permeability and cation transport selectivity exhibit a nonmonotonic dependence on the equivalent weight of the material. The lowest diffusion permeability, the highest Na+ cation transport selectivity (99.5%), and the best mechanical properties have been found for the Aquivion membrane with an equivalent weight of 965, which is characterized by the highest degree of crystallinity.  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