全文获取类型
收费全文 | 54498篇 |
免费 | 5203篇 |
国内免费 | 2666篇 |
专业分类
电工技术 | 3495篇 |
技术理论 | 4篇 |
综合类 | 3638篇 |
化学工业 | 8714篇 |
金属工艺 | 3279篇 |
机械仪表 | 3707篇 |
建筑科学 | 4173篇 |
矿业工程 | 1750篇 |
能源动力 | 1686篇 |
轻工业 | 3861篇 |
水利工程 | 1157篇 |
石油天然气 | 2833篇 |
武器工业 | 553篇 |
无线电 | 6159篇 |
一般工业技术 | 6509篇 |
冶金工业 | 2636篇 |
原子能技术 | 795篇 |
自动化技术 | 7418篇 |
出版年
2024年 | 267篇 |
2023年 | 966篇 |
2022年 | 1692篇 |
2021年 | 2355篇 |
2020年 | 1786篇 |
2019年 | 1565篇 |
2018年 | 1753篇 |
2017年 | 1877篇 |
2016年 | 1752篇 |
2015年 | 2329篇 |
2014年 | 2907篇 |
2013年 | 3373篇 |
2012年 | 3901篇 |
2011年 | 4076篇 |
2010年 | 3564篇 |
2009年 | 3422篇 |
2008年 | 3237篇 |
2007年 | 3035篇 |
2006年 | 2858篇 |
2005年 | 2407篇 |
2004年 | 1808篇 |
2003年 | 1646篇 |
2002年 | 1716篇 |
2001年 | 1501篇 |
2000年 | 1200篇 |
1999年 | 1165篇 |
1998年 | 790篇 |
1997年 | 632篇 |
1996年 | 595篇 |
1995年 | 517篇 |
1994年 | 371篇 |
1993年 | 314篇 |
1992年 | 244篇 |
1991年 | 174篇 |
1990年 | 139篇 |
1989年 | 89篇 |
1988年 | 102篇 |
1987年 | 47篇 |
1986年 | 58篇 |
1985年 | 27篇 |
1984年 | 24篇 |
1983年 | 16篇 |
1982年 | 13篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1977年 | 7篇 |
1975年 | 5篇 |
1970年 | 6篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
为了将高纯锗(HPGe)探测器用于快定时的物理实验中,本实验研究厂影响高纯锗探测器定时谱仪时间性能的各种因素,找出了获得谱仪最小时间分辨的各种最佳参数。并建立了分辨时间为4.19ns,能量分辨率为20keV,微分非线性好于±2.0%,计效率可承受120×10 ̄3s ̄(-1)的时间微分扰动角关联谱仪。利用此谱仪,以 ̄(204)Bi为探针核,研究了Bi系高温超导的微观机理,取得了较好的结果。 相似文献
22.
23.
G. Dong 《Annals of Mathematics and Artificial Intelligence》1993,7(1-4):107-127
LDL is one of the recently proposed logical query languages, which incorporate set, for data and knowledge base systems. Since LDL programs can simulate negation, they are not monotonic in general. On the other hand, there are monotonic LDL programs. This paper addresses the natural question of “When are the generally nonmonotonic LDL programs monotonic?” and investigates related topics such as useful applications for monotonicity. We discuss four kinds of monotonicity, and examine two of them in depth. The first of the two, called “ω-monotonicity”, is shown to be undecidable even when limited to single-stratum programs. The second, called “uniform monotonicity”, is shown to implyω-monotonicity. We characterize the uniform monotonicity of a program (i) by a relationship between its Bancilhon-Khoshafian semantics and its LDL semantics, and (ii) with a useful property called subset completion independence. Characterization (ii) implies that uniformly monotonie programs can be evaluated more efficiently by discarding dominated facts. Finally, we provide some necessary and/or sufficient, syntactic conditions for uniform monotonicity. The conditions pinpoint (a) enumerated set terms, (b) negations of membership and inclusion, and (c) sharing of set terms as the main source for nonuniform monotonicity. 相似文献
24.
25.
Summary Styrene polymerization was initiated by methylaluminoxane in the presence of haloalkane. The propagating species are shown by trapping experiments and electron paramagnetic resonance to be both radicals and cations. 相似文献
26.
Tian-lin Dong 《Journal of Infrared, Millimeter and Terahertz Waves》1992,13(8):1163-1174
Rigorous analysis of planar waveguides with two-dimensionally periodic (2DP) layer is presented. Characteristic solutions in periodic layer are exactly represented by a double Fourier series known as the Floquet solutions. The vector amplitudes of space harmonics are proved to be governed by five-term recurrence relation in the operator-vector form for sinusoidal modulation. A general transmission-line network representation of two-dimensionally periodic medium and multiport network representation of uniform-periodic-medium interface are introduced. Any layered structures cascading with periodic ones can be analyzed. Numerical results based on the established theory are also presented to illustrate characteristics of 2DP medium and structure. 相似文献
27.
28.
悬浮床加氢裂化油溶性催化剂与沥青质的作用 总被引:1,自引:1,他引:0
在高压釜反应器中对添加了油溶性 Ni 催化剂(UPC-O)和水溶性 Ni 催化剂(UPC-W)的辽河稠油常压渣油(LHAR)进行了悬浮床加氢裂化实验。通过元素分析和傅里叶变换红外光谱(FTIR)分析对抑制反应生焦效果好的 UPC-O 与 LHAR 沥青质的作用进行了研究。元素分析结果显示,在 LHAR 中加入 UPC-O1000μg/g,沥青质和脱沥青质油中 Ni 含量分别出476μg/g和110μg/g 增加到7 559μg/g 和1 024μg/g,表明 UPC-O 在沥青质中有富集的趋势。FTIR 表征结果显示,UPC-O 和沥青质发生了相互作用。UPC-O 可在沥青质原位硫化为活性组分,对沥青质在悬浮床加氢裂化反应中的缩合起到好的抑制作用。 相似文献
29.
30.
The surface of polyethyleneterephthalate (PET) nonwoven fabric was modified by He/O2 atmospheric pressure plasma treatment, varying plasma exposure time. The plasma treated PET surfaces have been analyzed to investigate the chemical nature and morphology of surface by X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. The change of wettability was measured depending on plasma exposure time. XPS results indicated the presence of oxygen‐based functional groups on the PET nonwoven fabric surface after plasma treatment and oxygen content increased as exposure time increased. The mean roughness increased after 30 s exposure and further increase in exposure to 60 s led to decrease of the roughness and then again increase. The root mean square roughness followed the similar trend to mean roughness. The average difference in height, Rz, increased after plasma exposure for 30 s, while it slightly decreased after 60 s exposure. Despite of redeposition, the Rz of 90 s exposed sample increased more than two times compared with those of 30 and 60 s exposed. Wettability increased progressively up to 10 times after 90 s exposure compared with the untreated. It is attributed to the increases of hydrophilicity and surface roughness. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 相似文献