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   261571篇
  免费   4310篇
  国内免费   1830篇
电工技术   5467篇
技术理论   5篇
综合类   1193篇
化学工业   42178篇
金属工艺   11054篇
机械仪表   7801篇
建筑科学   6642篇
矿业工程   1678篇
能源动力   5625篇
轻工业   25895篇
水利工程   2914篇
石油天然气   6003篇
武器工业   254篇
无线电   27836篇
一般工业技术   49264篇
冶金工业   47243篇
原子能技术   5628篇
自动化技术   21031篇
  2021年   2499篇
  2019年   2142篇
  2018年   3410篇
  2017年   3381篇
  2016年   3711篇
  2015年   2836篇
  2014年   4620篇
  2013年   11539篇
  2012年   7741篇
  2011年   10246篇
  2010年   8176篇
  2009年   8711篇
  2008年   9500篇
  2007年   9620篇
  2006年   8463篇
  2005年   7408篇
  2004年   6671篇
  2003年   6213篇
  2002年   6214篇
  2001年   6330篇
  2000年   5916篇
  1999年   5880篇
  1998年   12855篇
  1997年   9535篇
  1996年   7278篇
  1995年   5527篇
  1994年   5082篇
  1993年   4947篇
  1992年   3943篇
  1991年   3751篇
  1990年   3824篇
  1989年   3773篇
  1988年   3530篇
  1987年   3021篇
  1986年   3051篇
  1985年   3410篇
  1984年   3321篇
  1983年   3075篇
  1982年   2700篇
  1981年   2903篇
  1980年   2643篇
  1979年   2841篇
  1978年   2737篇
  1977年   2841篇
  1976年   3694篇
  1975年   2457篇
  1974年   2291篇
  1973年   2320篇
  1972年   1981篇
  1971年   1786篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Four production steps, extrusion, blowing, stabilization, and finishing, are involved in the manufacture of biorented nylon 6 films. The films obtained after each step were studied by wide‐angle diffraction X‐rays, IR analysis, and density measurements. Orientation in the amorphous phase was evaluated by thermal retraction. Free volume was investigated by positron annihilation spectroscopy. Mechanical properties were tested by tensile tests, and permeability was tested with isopiestic permeameters. Quenching, after extrusion, generated an unstable γ crystalline phase and an amorphous phase. Blowing transformed the unstable γ phase into the thermodynamically stable α phase and increased the chain orientation; stabilization increased α phase crystals, favored hydrogen bond formation and, therefore, the mechanical properties, leaving unchanged the chain orientation; the final treatment increased the mechanical properties. The film permeabilities to O2, N2, and CO2 were practically unchanged after each production step; this result may be explained by considering that the nanoholes, present in the films and responsible for the diffusion inside the polymer, maintained practically constant their radius, whereas their number concentration decreased slowly after thermomechanical treatments. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 559–571, 2002  相似文献   
992.
Protective coatings based on metallic aluminum are developed that can be used to improve the resistance of the refractory lining for extra-furnace steelmaking facilities. By sintering a slurry sprayed on the refractory surface, a heat-resistant gas-impermeable coating containing -corundum is formed.__________Translated from Novye Ogneupory, No. 10, pp. 61 – 63, October, 2004.  相似文献   
993.
Understanding the dynamic phenomena of viability loss of shear sensitive cells and bubble breakage and coalescence within airlift reactors requires knowledge of local, liquid-phase hydrodynamics. The laser-Doppler velocimeter (LDV) is a non-invasive instrument which may be used to obtain this information. Experimental procedures and software were developed to detect and measure Doppler bursts in two-phase flow in a split-cylinder airlift reactor. Off-line analysis of the data indicated a detection rate approximately one order of magnitude greater than that observed using an available commercial frequency tracker. Approximately 400 to 500 observations are needed for the ensemble mean to characterize the local mean velocity to within ±5% for a superficial gas velocity of 10.4 cm/s, the highest superficial gas velocity used in these studies. The limitations, prospects, and signal-processing options for LDV in this application are also discussed  相似文献   
994.
995.
Slow growth and high seedling mortality limit direct seeding establishment of guayule (Parthenium argentatum G.). This study was conducted to assess seedling growth enhancement by the addition of different rates and forms of N fertilizers and Ca salts. Experiments were conducted in a greenhouse with cultivar 593 under both surface and subirrigated conditions using water low in salts (salinity of 0.8 dSm–1, SAR of 5.0 and 10 mg Ca L–1). Under surface-irrigated conditions, seedling height and fresh plant weight increased with N application to the irrigation water to 70 mg L–1. The best seedling growth was observed when (NH4)2SO4 was added in combination with CaCl2 or CaSO4. Progressively less growth was observed by addition of (NH4)2SO4 alone, CO(NH2)2 plus CaSO4, CO(NH2)2 alone and Ca(NO3)2. When seedlings were subirrigated, however, the best growth was observed with Ca(NO3)2. Intermediate growth was obtained with (NH4)2SO4 plus CaSO4 and lowest growth rates with (NH4)2SO4 alone. These differential responses may be explained by the differences in leaching and volatile characteristics of the N forms. Growth enhancement from N and Ca additions increased with time with significant increases 45 days after seeding. Nitrogen application with Ca may be effective amendment in promoting subsequent growth of direct seeded guayule.  相似文献   
996.
997.
998.
Materials made from yttrium silicates were synthesized by the sol-gel method. The effect of the amount of yttrium oxide, which varied from 10 to 50% (molar content), and the water:alcoholate molar ratio was investigated. Structure formation and the character of crystallization of gels in heat treatment in the 650–1300°C range were investigated. __________ Translated from Steklo i Keramika, No. 1, pp. 3–6, January, 2007.  相似文献   
999.
1000.
C.L. Choy  W.K. Luk  F.C. Chen 《Polymer》1981,22(4):543-548
Dynamic mechanical measurements between — 180°C and 180°C were made on both isotropic and drawn samples of polybutene-1 (PB-1) and poly-4-methylpentene-1 (P4MB1) over a wide frequency range by the use of a torsional pendulum (0.3–3 Hz), a viscoelastic spectrometer (5–90 Hz) and ultrasonic technique (3 MHz). The relaxation peaks were identified and the associated activation energies determined from Arrhenius plots. For PB-1 it was observed that orientation reduces the height and shifts up the temperature of the αa-peak associated with large scale main-chain motion in the amorphous regions, but has little effect on the β-peak associated with side-group motion. In addition to the αa and β relaxations a high-temperature crystalline relaxation (αc) is also observed in P4MP1. For both the αc and β relaxations the mechanical loss at 45° to the draw direction is much larger than that at 90°, which indicates that shear processes are involved in these relaxations.  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