全文获取类型
收费全文 | 207227篇 |
免费 | 3311篇 |
国内免费 | 766篇 |
专业分类
电工技术 | 3630篇 |
综合类 | 135篇 |
化学工业 | 34058篇 |
金属工艺 | 8593篇 |
机械仪表 | 5701篇 |
建筑科学 | 6108篇 |
矿业工程 | 1116篇 |
能源动力 | 5656篇 |
轻工业 | 18904篇 |
水利工程 | 2014篇 |
石油天然气 | 3314篇 |
武器工业 | 3篇 |
无线电 | 21888篇 |
一般工业技术 | 38122篇 |
冶金工业 | 39254篇 |
原子能技术 | 4578篇 |
自动化技术 | 18230篇 |
出版年
2021年 | 1667篇 |
2019年 | 1622篇 |
2018年 | 2562篇 |
2017年 | 2554篇 |
2016年 | 2675篇 |
2015年 | 1999篇 |
2014年 | 3407篇 |
2013年 | 9782篇 |
2012年 | 5695篇 |
2011年 | 7852篇 |
2010年 | 6122篇 |
2009年 | 7077篇 |
2008年 | 7193篇 |
2007年 | 7195篇 |
2006年 | 6158篇 |
2005年 | 5764篇 |
2004年 | 5467篇 |
2003年 | 5105篇 |
2002年 | 5063篇 |
2001年 | 4716篇 |
2000年 | 4503篇 |
1999年 | 4588篇 |
1998年 | 10931篇 |
1997年 | 7862篇 |
1996年 | 6228篇 |
1995年 | 4795篇 |
1994年 | 4109篇 |
1993年 | 4066篇 |
1992年 | 3063篇 |
1991年 | 2833篇 |
1990年 | 2823篇 |
1989年 | 2825篇 |
1988年 | 2727篇 |
1987年 | 2508篇 |
1986年 | 2415篇 |
1985年 | 2850篇 |
1984年 | 2620篇 |
1983年 | 2416篇 |
1982年 | 2314篇 |
1981年 | 2349篇 |
1980年 | 2124篇 |
1979年 | 2141篇 |
1978年 | 2078篇 |
1977年 | 2385篇 |
1976年 | 2934篇 |
1975年 | 1846篇 |
1974年 | 1761篇 |
1973年 | 1851篇 |
1972年 | 1493篇 |
1971年 | 1364篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
V. Elliott Smith John M. Spurr John C. Filkins Jody J. Jones 《Journal of Great Lakes research》1985,11(3):231-246
Organochlorine analysis was performed on carcasses of 13 diving ducks from a 1981 wintering population that foraged on contaminated sediments in the lower Detroit River. Mean total PCB concentrations were 10 mg/kg for seven lesser scaups (Aythya affinis), 11 mg/kg for three greater scaups (A. marila), and 7.6 mg/kg for three goldeneyes (Bucephala clangula). Highest mean levels of other residues were measured for hexachlorobenzene (1.7 mg/kg) in goldeneyes, and transnonachlor (0.33 mg/kg) and 4,4’-DDE (1.3 mg/kg) in greater scaup. Quantitative analysis of 72 PCB congeners also was applied to water, seston, sediment, benthie oligochaetes, and carp from the same site. Principal congeners in most of the samples included some of the more toxic and persistent PCBs. Results of a multivariate analysis indicated that ratios of more conservative to less conservative PCBs did not vary significantly among ducks (α = .05), but differed from those in carp, oligochaetes, and sediment. For the February through March period of fat mobilization, concentrations of total PCB in lipid were inversely correlated with percent lipid (r = 0.76) in ducks. The percentage of conservative PCBs increased slightly. The distribution and partitioning of organochlorines, including toxic PCB congeners, varied considerably within this water column-sediment-fauna ecosystem. Estimates of toxic exposure based on total PCB values may be unreliable. 相似文献
962.
G. F. Molokanov 《Journal of Computer and Systems Sciences International》2006,45(2):331-335
The year 2006 marks the ninetieth anniversary of the air navigation (later called navigation) service of the Air Forces of this country. The history of air navigation is investigated, and an opinion about its modern state and directions of further development is presented. 相似文献
963.
964.
965.
This article presents a detailed investigation on the influence of the phosphorus element upon the laser sintering of a multicomponent
Cu-based metal powder system consisting of Cu, Cu-10Sn, and Cu-8.4P. Powder systems containing 0, 10, 15, and 20 wt pct CuP
were sintered in atmosphere at room temperature using the following optimal processing parameters: laser power of 350 W, scan
speed of 0.04 m/s, scan line spacing of 0.15 mm, and layer thickness of 0.25 mm. It was found that the relative density of
the sintered sample with 15 wt pct CuP increased by 24,4 pct as compared with the sample without phosphorus addition. A further
increase in the CuP content (≥20 wt pct), however, resulted in a poor densification with a serious delamination. The exact
metallurgical roles of the phosphorus element in the laser sintering process were addressed as follows. First, the phosphorus
could prevent the sintering system from oxidation by forming CuPO3, thereby improving the wetting characteristics and the sintering kinetics. Second, the phosphorus could decrease the surface
tension of molten materials, leading to a successive transition from highly discontinuous sintered tracks to fairly coherent
ones with increasing the phosphorus content. Third, the phosphorus could lower the melt viscosity, thereby improving the microstructural
homogeneity of the laser-sintered samples. 相似文献
966.
967.
968.
A synthesis procedure for generating current conveyor active filters is presented, and a novel network which realises the general 2nd-order voltage transfer function is given.<> 相似文献
969.
970.