全文获取类型
收费全文 | 24163篇 |
免费 | 2728篇 |
国内免费 | 1344篇 |
专业分类
电工技术 | 1690篇 |
技术理论 | 1篇 |
综合类 | 1537篇 |
化学工业 | 3757篇 |
金属工艺 | 1621篇 |
机械仪表 | 1776篇 |
建筑科学 | 1898篇 |
矿业工程 | 754篇 |
能源动力 | 659篇 |
轻工业 | 1661篇 |
水利工程 | 575篇 |
石油天然气 | 1485篇 |
武器工业 | 262篇 |
无线电 | 2738篇 |
一般工业技术 | 2868篇 |
冶金工业 | 1241篇 |
原子能技术 | 388篇 |
自动化技术 | 3324篇 |
出版年
2024年 | 174篇 |
2023年 | 500篇 |
2022年 | 976篇 |
2021年 | 1274篇 |
2020年 | 1016篇 |
2019年 | 854篇 |
2018年 | 891篇 |
2017年 | 975篇 |
2016年 | 895篇 |
2015年 | 1324篇 |
2014年 | 1585篇 |
2013年 | 1749篇 |
2012年 | 1895篇 |
2011年 | 1746篇 |
2010年 | 1563篇 |
2009年 | 1405篇 |
2008年 | 1277篇 |
2007年 | 1245篇 |
2006年 | 1183篇 |
2005年 | 964篇 |
2004年 | 648篇 |
2003年 | 550篇 |
2002年 | 532篇 |
2001年 | 451篇 |
2000年 | 384篇 |
1999年 | 371篇 |
1998年 | 274篇 |
1997年 | 230篇 |
1996年 | 224篇 |
1995年 | 178篇 |
1994年 | 147篇 |
1993年 | 99篇 |
1992年 | 102篇 |
1991年 | 71篇 |
1990年 | 55篇 |
1989年 | 44篇 |
1988年 | 43篇 |
1987年 | 25篇 |
1986年 | 34篇 |
1985年 | 19篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1977年 | 10篇 |
1976年 | 23篇 |
1975年 | 10篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
为了研究氢化芳烃的微观结构,进一步认识和理解氢化芳烃在催化裂化过程中的反应特性。以四氢萘为模型化合物,利用基于密度泛函理论的量子化学从头计算方法,对四氢萘的几何结构和电子结构进行了系统研究,得到了四氢萘不同位置C-H键和C-C键的键长、键级、键能以及电子云密度、前线轨道分布等微观结构信息。发现四氢萘结构具有轴对称特性,苯环上的C-C键和C-H键要远比环烷环的C-C键和C-H键稳定。由于受苯环的影响,会使得其环烷环上不同位置的C-C键的键长、键级和键能均具有明显差异,也导致环烷环上不同位置C-H键的键长、键级和键能有较明显的差异,具体表现为:对C-C键而言,与苯环相连的C-C键的键长较短,键级较高,键能也明显较高,而苯环β位C-C键的键能则明显较低;对C-H键而言,与苯环β位环烷碳原子上的C-H键相比,苯环α位环烷碳原子上的C-H键的键能明显较低。电子云分布计算结果能够较好地给出的这些C-C键和C-H键结构特征差异的原因。前线轨道计算结果表明,在催化裂化过程中,苯环α位环烷碳原子上的C-H键和β位C-C键较易受到催化剂酸性中心的进攻,是酸催化反应的位点。这些计算结果对于认识与理解氢化芳烃的反应特性具有一定的基础性理论意义。 相似文献
52.
53.
Bong Ban S. Baker Curtis L. Long Dale D. 《IEEE transactions on bio-medical engineering》1976,(1):75-77
A stimulator circuit is presented which is capable of generating pulse train waveforms suitable for neurophysiological experiments are available, as well as special test conditions such as dishabituation. Digital and linear integrated circuits are used to provide precise control over the stimulus parameters. 相似文献
54.
The development, operation, and applications of two configurations of an integrated plasma-aided nanofabrication facility (IPANF) comprising low-frequency inductively coupled plasma-assisted, low-pressure, multiple-target RF magnetron sputtering plasma source, are reported. The two configurations of the plasma source have different arrangements of the RF inductive coil: a conventional external flat spiral “pancake” coil and an in-house developed internal antenna comprising two orthogonal RF current sheets. The internal antenna configuration generates a “unidirectional” RF current that deeply penetrates into the plasma bulk and results in an excellent uniformity of the plasma over large areas and volumes. The IPANF has been employed for various applications, including low-temperature plasma-enhanced chemical vapor deposition of vertically aligned single-crystalline carbon nanotips, growth of ultra-high aspect ratio semiconductor nanowires, assembly of optoelectronically important Si, SiC, and Al1-xInxN quantum dots, and plasma-based synthesis of bioactive hydroxyapatite for orthopedic implants. 相似文献
55.
Paul F. Mlakar Donald O. Dusenberry James R. Harris Gerald Haynes Long T. Phan Mete A. Sozen 《Canadian Metallurgical Quarterly》2005,19(3):197-205
On September 11, 2001, an airliner was intentionally crashed into the Pentagon. It struck at the first elevated slab on the west wall, and slid approximately 310?ft (94.5?m) diagonally into the building. The force of the collision demolished numerous columns and the fa?ade of the exterior wall, and induced damage to first-floor columns and the first elevated slab over an area approximately 90?ft (27.4?m) wide and 310?ft (94.5?m) long. None of the building collapsed immediately. The portion that remained standing, even after an intense fire, sustained substantial damage at the first-floor level. 相似文献
56.
Often quantitative data in the social sciences have only ordinal justification. Problems of interpretation can arise when least squares multiple regression (LSMR) is used with ordinal data. Two ordinal alternatives are discussed, dominance-based ordinal multiple regression (DOMR) and proportional odds multiple regression. The Q2 statistic is introduced for testing the omnibus null hypothesis in DOMR. A simulation study is discussed that examines the actual Type I error rate and power of Q2 in comparison to the LSMR omnibus F test under normality and non-normality. Results suggest that Q2 has favorable sampling properties as long as the sample size-to-predictors ratio is not too small, and Q2 can be a good alternative to the omnibus F test when the response variable is non-normal. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
57.
The stress-strain state of the structure is a matter of interest to designer. The strain measurement of superconducting magnets at cryogenic temperature is a specific technique. Based on strain measurement of TF coil case for EAST, this paper presents a measuring technique at cryogenic temperature and on intense magnetic field. The compensation methods for both temperature and magnetic field effects of the gauges, together with the measured results are involved, and the discussions of the measured results are given in the paper. 相似文献
58.
59.
60.