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   1369753篇
  免费   26704篇
  国内免费   6798篇
电工技术   33234篇
综合类   6284篇
化学工业   248881篇
金属工艺   62084篇
机械仪表   38141篇
建筑科学   40560篇
矿业工程   11184篇
能源动力   49548篇
轻工业   95912篇
水利工程   13797篇
石油天然气   36230篇
武器工业   128篇
无线电   191776篇
一般工业技术   262610篇
冶金工业   116310篇
原子能技术   33623篇
自动化技术   162953篇
  2021年   14382篇
  2020年   11649篇
  2019年   14412篇
  2018年   13687篇
  2017年   12856篇
  2016年   20110篇
  2015年   17023篇
  2014年   28396篇
  2013年   87233篇
  2012年   32388篇
  2011年   42981篇
  2010年   40015篇
  2009年   49083篇
  2008年   40831篇
  2007年   37469篇
  2006年   42198篇
  2005年   36438篇
  2004年   39106篇
  2003年   38555篇
  2002年   36489篇
  2001年   32517篇
  2000年   30866篇
  1999年   29598篇
  1998年   27293篇
  1997年   27993篇
  1996年   26958篇
  1995年   26710篇
  1994年   25501篇
  1993年   25415篇
  1992年   24744篇
  1991年   21729篇
  1990年   22251篇
  1989年   21178篇
  1988年   19659篇
  1987年   18123篇
  1986年   17490篇
  1985年   20858篇
  1984年   21471篇
  1983年   19434篇
  1982年   18624篇
  1981年   18657篇
  1980年   17265篇
  1979年   17904篇
  1978年   17186篇
  1977年   16277篇
  1976年   16112篇
  1975年   15529篇
  1974年   15103篇
  1973年   15165篇
  1972年   12721篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
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   
992.
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   
993.
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   
994.
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   
995.
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   
996.
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   
997.
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   
998.
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   
999.
The present study deals with the estimation of toxic phorbol esters in Balanites aegyptiaca seed cake and to evaluate the performance and combustion characteristics of different blends of biodiesel in a diesel engine. The seeds were manually collected from arid regions of Rajasthan, India and the oil extracted from the powdered kernel with Solvent extraction process. The extracted oil processed by enzymatic catalytic reactions are carried out at optimum operating conditions such as the molar ratio of 8:1 (methanol:oil), reaction temperature at 40°C for 4?h reaction time and xylanase 1% based on oil weight. The total conversion rate of extracted oil to methyl ester by enzymatic reactions yields up to 98.3%. The toxic content of seed cake was determined by high-performance liquid chromatography (HPLC, Shimadzu). The performance and combustion parameters such as BP, BTE and CP decreased, whereas BSFC and DP increased with biodiesel blends.  相似文献   
1000.
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