全文获取类型
收费全文 | 1625篇 |
免费 | 68篇 |
国内免费 | 241篇 |
专业分类
电工技术 | 70篇 |
综合类 | 116篇 |
化学工业 | 217篇 |
金属工艺 | 565篇 |
机械仪表 | 45篇 |
建筑科学 | 1篇 |
矿业工程 | 16篇 |
能源动力 | 14篇 |
轻工业 | 28篇 |
石油天然气 | 4篇 |
武器工业 | 19篇 |
无线电 | 112篇 |
一般工业技术 | 570篇 |
冶金工业 | 136篇 |
原子能技术 | 10篇 |
自动化技术 | 11篇 |
出版年
2024年 | 5篇 |
2023年 | 16篇 |
2022年 | 17篇 |
2021年 | 25篇 |
2020年 | 22篇 |
2019年 | 17篇 |
2018年 | 41篇 |
2017年 | 30篇 |
2016年 | 37篇 |
2015年 | 45篇 |
2014年 | 55篇 |
2013年 | 68篇 |
2012年 | 94篇 |
2011年 | 95篇 |
2010年 | 73篇 |
2009年 | 102篇 |
2008年 | 83篇 |
2007年 | 147篇 |
2006年 | 165篇 |
2005年 | 128篇 |
2004年 | 131篇 |
2003年 | 106篇 |
2002年 | 95篇 |
2001年 | 43篇 |
2000年 | 72篇 |
1999年 | 51篇 |
1998年 | 45篇 |
1997年 | 32篇 |
1996年 | 20篇 |
1995年 | 26篇 |
1994年 | 24篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
排序方式: 共有1934条查询结果,搜索用时 15 毫秒
991.
真空偏压控制系统由计算机系统、真空调节阀、真空偏压泵等部件构成,其中真空调节阀是设备的关键部件之一。根据纳米晶合金磁粉真空连续片铸工艺要求和真空比例调节阀的性能,确定了积分项分离的位置型数字比例-积分-微分(PID)控制算法。基于监控构成(MonitorandControlGeneratedSystem,MCGS)组态软件系统,设计了真空调节阀控制算法程序及人机界面,工程法整定PID参数。运行后真空度控制精度达0.35%,符合生产要求。 相似文献
992.
In this work we elaborate the effect of grain size on the sensitivity of chemo-resistive metal-oxide gas sensors with nanosized grains. The effective carrier concentration in nanocrystalline SnO2 sensors with various grain sizes is calculated as a function of the surface state density. This involves numerical computation of the charge balance equation (i.e., the electroneutrality condition) using approximated analytical solutions of Poissons equation for small spherical crystallites. The calculations demonstrate a sharp decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with the gas phase, these calculations enable to simulate the response curves of nanocrystalline SnO2 gas sensors. The simulations show that the conductivity increases linearly with decreasing trapped charge densities, and that the sensitivity to the gas-induced variations in the trapped charge density is proportional to 1/D, where D is the average grain size. 相似文献
993.
Preferred growth of nanocrystalline silicon (nc-Si) was first found in boron-doped hydrogenated nanocrystalline (nc-Si:H) films prepared using plasma-enhanced chemical vapor deposition system. The films were characterized by high-resolution transmission electron microscope, X-ray diffraction (XRD) spectrum and Raman Scattering spectrum. The results showed that the diffraction peaks in XRD spectrum were at 2θ≈47° and the exponent of crystalline plane of nc-Si in the film was (2 2 0). A considerable reason was electric field derived from dc bias made the bonds of Si-Si array according to a certain orient. The size and crystalline volume fraction of nc-Si in boron-doped films were intensively depended on the deposited parameters: diborane (B2H6) doping ratio in silane (SiH4), silane dilution ratio in hydrogen (H2), rf power density, substrate's temperature and reactive pressure, respectively. But preferred growth of nc-Si in the boron-doped nc-Si:H films cannot be obtained by changing these parameters. 相似文献
994.
The nanocrystalline structure and mechanical properties of TaSi2 films deposited by sputtering of TaSi2 target have been investigated by x-ray diffraction, cross-sectional transmission electron microscopy (TEM), four-point electrical resistance measurement, and cyclic depth-sensitive nanoindentation. The purpose of this work is to study the formation of nanocrystalline structure in TaSi2 films on a silicon substrate. As revealed, a decrease in the deposition rate leads to an increase in the O and C impurity content in the films. Contamination of the film by O and C atoms during a low-rate deposition causes the formation of an amorphous phase in the deposited films. Upon annealing, the amorphous structures crystallize into mixtures of disilicide and a small amount of polysilicide, i.e. TaSi2 and Ta5Si3, respectively. After annealing at 970 K, the formation of a nanocrystalline structure with a grain size about 10 nm takes place in the film produced at a deposition rate of 0.2 nm/sec. The formation of a nanocrystalline structure changes drastically the mechanical properties of the film. The nanohardness and elastic modulus increase significantly, and the film becomes brittle and overstressed. After deposition in the film produced at the 1 nm/sec deposition rate mainly Ta disilicide and the amorphous phase are observed. After annealing, the amorphous phase near the Si substrate coexists with column-shape grains of Ta disilicide of size 150 × 500 nm. The annealed thin film becomes nonuniform in thickness. The nanohardness and elastic modulus increase. 相似文献
995.
996.
997.
以工程实际中应用较广的Al-Cu-Mg铝合金作为研究对象, 用Al-Cu-Mg铝合金气体雾化粉末作为原材料, 通过低温液氮球磨获得纳米晶后, 再经真空热压和热挤压制备了致密的大块体纳米材料. 通过力学性能测试, 挤压态的纳米晶Al-Cu-Mg块体材料抗拉强度达470 MPa, 经过T4处理后, 抗拉强度达到590 MPa, 远远超过常规方法制备的Al-Cu-Mg铝合金抗拉强度. 对纳米晶Al-Cu-Mg块体材料进行微观组织观察, 分析了材料强度提高的原因. 相似文献
998.
研究了460~640℃等温退火后纳米晶(FexCo1-x)73.5Cu1Nb3Si13.5B9(x=0.5,1)合金的初始磁导率麒随温度变化。与双相纳米晶Fe73.5Nb3Si13.5B9合金相比,(Fe0.5Co0.5)73.5Cu1Nb3Si13.5B9合金室温下的磁导率降低,但晶化相和非晶相居里温度明显升高,并显著提高了合金在高温下的软磁性能。初步探讨了改善纳米晶合金高温磁性的机理。 相似文献
999.
1000.