全文获取类型
收费全文 | 40760篇 |
免费 | 894篇 |
国内免费 | 1271篇 |
专业分类
电工技术 | 1216篇 |
综合类 | 1344篇 |
化学工业 | 4493篇 |
金属工艺 | 1413篇 |
机械仪表 | 2059篇 |
建筑科学 | 2373篇 |
矿业工程 | 1626篇 |
能源动力 | 540篇 |
轻工业 | 8406篇 |
水利工程 | 1238篇 |
石油天然气 | 1633篇 |
武器工业 | 168篇 |
无线电 | 2946篇 |
一般工业技术 | 9011篇 |
冶金工业 | 1204篇 |
原子能技术 | 563篇 |
自动化技术 | 2692篇 |
出版年
2024年 | 110篇 |
2023年 | 394篇 |
2022年 | 379篇 |
2021年 | 403篇 |
2020年 | 390篇 |
2019年 | 496篇 |
2018年 | 525篇 |
2017年 | 304篇 |
2016年 | 381篇 |
2015年 | 443篇 |
2014年 | 963篇 |
2013年 | 816篇 |
2012年 | 3081篇 |
2011年 | 3853篇 |
2010年 | 1442篇 |
2009年 | 1151篇 |
2008年 | 2777篇 |
2007年 | 2899篇 |
2006年 | 2611篇 |
2005年 | 2613篇 |
2004年 | 2460篇 |
2003年 | 1966篇 |
2002年 | 1502篇 |
2001年 | 1522篇 |
2000年 | 1251篇 |
1999年 | 891篇 |
1998年 | 707篇 |
1997年 | 587篇 |
1996年 | 571篇 |
1995年 | 560篇 |
1994年 | 604篇 |
1993年 | 295篇 |
1992年 | 423篇 |
1991年 | 378篇 |
1990年 | 373篇 |
1989年 | 346篇 |
1988年 | 208篇 |
1987年 | 225篇 |
1986年 | 283篇 |
1985年 | 261篇 |
1984年 | 269篇 |
1983年 | 245篇 |
1982年 | 209篇 |
1981年 | 161篇 |
1980年 | 140篇 |
1979年 | 96篇 |
1978年 | 62篇 |
1976年 | 33篇 |
1975年 | 29篇 |
1974年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Daniel T. Payne Mandeep K. Chahal V clav B&rcaron ezina Whitney A. Webre Katsuhiko Ariga Francis D Souza Jan Labuta Jonathan P. Hill 《Frontiers of Chemical Science and Engineering》2020,14(1):28
Chiral 1,1’-binaphthyl-linked diporphyrin ‘tweezers’ (R)-1/(S)-1 and the corresponding zinc(II) complexes (R)-2/(S)-2 were prepared as chiral host molecules, and their utility for chiral analyses (especially enantiomeric excess (ee) determinations) were evaluated. Tris(1-n-dodecyl)porphyrins were used for the first time as the interacting units. Host capabilities of the diporphyrin tweezers were investigated by titrations with (R,R)- and (S,S)-cyclohexane-1,2-diamine (CHDA). The host molecules could be used as multichannel probes of ee by using UV-vis, circular dichroism (CD), fluorescence emission and 1H nuclear magnetic resonance (1H-NMR) methods. Chiral configurations could also be differentiated using CD or 1H-NMR spectroscopy. All three optical techniques give good resolution of ee with reasonable sensitivity considering the low concentrations used (ca. 10−6 mol·L−1). The ee determination of CHDA enantiomers using NMR spectroscopy is also possible because of the reasonably well separated resonances in the case of (R,R)- and (S,S)-CHDA. Non-metallated (R)-1/(S)-1 hosts could not be used to detect chiral information in a strongly acidic chiral guest. This work demonstrates the utility of 1,1’-binapthyl-linked chiral hosts for chiral analysis of ditopically interacting enantiomers. 相似文献
4.
5.
6.
以中国石油冀东油田原油为研究对象,筛选并评价了具有不同界面张力及乳化效果的4组驱油体系,研究了界面张力与乳化效果对提高原油采收率的贡献。实验结果表明,油水界面张力与原油乳化效果之间没有明显的一致性,当以0.1%(w)的Na_2CO_3或0.5%(w)Na_2CO_3+0.3%(w)JS-33复配体系为驱油剂时,界面张力均可达到超低,但乳化效果较差,析水率分别高达91.7%和83.3%,而原油采收率提高值只有8.9%和12.4%;当以0.5%(w)的Na_2CO_3或0.5%(w)Na_2CO_3+0.5%(w)JS-33复配体系为驱油剂时,界面张力分别为10~(-2)和10~(-3)数量级,乳化效果较好,析水率分别为1.7%和0,原油采收率提高值达27.5%和24.4%。乳化效果好的驱油体系原油采收率提高值要高于超低界面张力体系,乳化效果相比界面张力而言对提高采收率的贡献占主要部分。 相似文献
7.
8.
John F.MOXNES Anne K.PRYTZ ?yvind FR?YLAND Stian SKRIUDALEN Steinar B?RVE Gard ?DEG?RDSTUEN 《兵工学报(英文版)》2015,(1):1-9
For the characterization of the behaviors of a metal material in events like expanding warheads, it is necessary to know its strength and ductility at high strain rates, around 104e105/s. The flyer plate impact testing produces the uniform stress and strain rates but the testing is expensive. The Taylor test is relatively inexpensive but produces non-uniform stress and strain fields, and the results are not so easily inferred for material modeling. In the split-Hopkinson bar(SHB), which may be used in compression, tension and torsion testing, the strain rates never exceeds 103/s. In the present work, we use the expanding ring test where the strain rate is 104e105/s. A streak camera is used to examine the expanding ring velocity, and a water tank is used to collect the fragments. The experimental results are compared with the numerical simulations using the hydrocodes AUTODYN, IMPETUS Afea and a regularized smooth particle(RSPH) software. The number of fragments increases with the increase in the expansion velocity of the rings. The number of fragments is similar to the experimental results. The RSPH software shows much the same results as the AUTODYN where the Lagrangian solver is used for the ring. The IMPETUS Afea solver shows a somewhat different fragmentation characteristic due to the node splitting algorithm that induces pronounced tensile splitting. 相似文献
9.
Pratik P. BHATT Venkatesh K. R. KODUR Anuj M. SHAKYA Tarek ALKHRDAJI 《Frontiers of Structural and Civil Engineering》2021,15(1):177
This paper presents the results of fire resistance tests on carbon fiber-reinforced polymer (CFRP) strengthened concrete flexural members, i.e., T-beams and slabs. The strengthened members were protected with fire insulation and tested under the combined effects of thermal and structural loading. The variables considered in the tests include the applied load level, extent of strengthening, and thickness of the fire insulation applied to the beams and slabs. Furthermore, a previously developed numerical model was validated against the data generated from the fire tests; subsequently, it was utilized to undertake a case study. Results from fire tests and numerical studies indicate that owing to the protection provided by the fire insulation, the insulated CFRP-strengthened beams and slabs can withstand four and three hours of standard fire exposure, respectively, under service load conditions. The insulation layer impedes the temperature rise in the member; therefore, the CFRP–concrete composite action remains active for a longer duration and the steel reinforcement temperature remains below 400°C, which in turn enhances the capacity of the beams and slabs. 相似文献
10.