全文获取类型
收费全文 | 25723篇 |
免费 | 1897篇 |
国内免费 | 1018篇 |
专业分类
电工技术 | 309篇 |
综合类 | 1159篇 |
化学工业 | 15405篇 |
金属工艺 | 396篇 |
机械仪表 | 68篇 |
建筑科学 | 185篇 |
矿业工程 | 268篇 |
能源动力 | 1927篇 |
轻工业 | 350篇 |
水利工程 | 29篇 |
石油天然气 | 6675篇 |
武器工业 | 86篇 |
无线电 | 116篇 |
一般工业技术 | 977篇 |
冶金工业 | 488篇 |
原子能技术 | 120篇 |
自动化技术 | 80篇 |
出版年
2024年 | 54篇 |
2023年 | 294篇 |
2022年 | 611篇 |
2021年 | 742篇 |
2020年 | 719篇 |
2019年 | 740篇 |
2018年 | 708篇 |
2017年 | 697篇 |
2016年 | 844篇 |
2015年 | 760篇 |
2014年 | 1357篇 |
2013年 | 1280篇 |
2012年 | 1475篇 |
2011年 | 1584篇 |
2010年 | 1258篇 |
2009年 | 1388篇 |
2008年 | 1211篇 |
2007年 | 1540篇 |
2006年 | 1503篇 |
2005年 | 1336篇 |
2004年 | 1179篇 |
2003年 | 1121篇 |
2002年 | 981篇 |
2001年 | 954篇 |
2000年 | 825篇 |
1999年 | 690篇 |
1998年 | 575篇 |
1997年 | 393篇 |
1996年 | 416篇 |
1995年 | 316篇 |
1994年 | 292篇 |
1993年 | 208篇 |
1992年 | 154篇 |
1991年 | 122篇 |
1990年 | 103篇 |
1989年 | 63篇 |
1988年 | 38篇 |
1987年 | 20篇 |
1986年 | 8篇 |
1985年 | 17篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 8篇 |
1981年 | 1篇 |
1974年 | 1篇 |
1951年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
991.
992.
993.
994.
A series of Li/CaO catalysts has been prepared by impregnating 0.5–5.0 wt% Li in CaO by wet chemical method. Prepared Li/CaO catalysts have been characterized by powder X-ray diffraction, scanning electron and transmission electron microscopy and Brunauer–Emmett–Teller (BET) surface area studies, in order to establish the structure and surface morphology of the catalyst. Hammett indicator test study was performed to determine the basic strength of the Li/CaO catalysts. The prepared Li/CaO catalysts have been employed as a heterogeneous catalyst for the transesterification of waste cottonseed oil (having 2.8 wt% free fatty acid contents) with ethanol. Under optimal reaction conditions viz., ethanol/oil molar ratio of 12:1, catalyst to oil weight fraction of 5% and 65 °C reaction temperature, 98% fatty acid ethyl ester yield was obtained in 2.5 h of reaction duration. Under the optimized reaction conditions, the pseudo first order constant and Arrhenius activation energy were found to be 0.03 min−1 and 70.0 kJ mol−1, respectively. Further Li/CaO catalyst was also found to be effective for the ethanolysis and methanolysis of vegetable oils having up to 3.4 wt% free fatty acids. The use of 3-Li/CaO catalyst is advantageous considering that it not only utilizes waste cottonseed oil as a feedstock, but also renewable and nontoxic alcohol, ethanol, for the biodiesel production. 相似文献
995.
In this work, diversified quaternary onium salt‐type triphase catalysts (TPC) were prepared based on crosslinked polystyrene (CPS) microspheres, and the relationship between their chemical structures and catalytic activities in liquid‐solid‐liquid reaction system were investigated in depth by using the esterification reaction of benzyl chloride with sodium acetate as a model system. The experimental results indicate that the chemical structures of the TPC affect their catalytic activity greatly and there are four basic points: (1) the quaternary phosphonium‐type TPC have higher activity than quaternary ammonium‐type catalyst; (2) the TPC with more lipophilic substitutes at N atom have higher catalytic activity; (3) the TPC with longer spacer arm, which links quaternary onium salt group to the matrix microsphere, have higher catalytic activity; (4) the bonding density of quaternary onium salt group on the polymeric carrier affects the hydrophilic and hydrophobic property of the TPC, and consequently, influences the catalytic activity significantly. For a given triphase catalysis system, there is an optimal bonding density of quaternary onium salt group on the solid catalysts. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011 相似文献
996.
Transesterification of Canola Oil to Biodiesel Using CaO/Talc Nanopowder as a Mixed Oxide Catalyst
下载免费PDF全文
![点击此处可从《化学工程与技术》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A series of heterogeneous catalysts including different molar ratios of CaO/talc was synthesized to study the transesterification reaction of canola oil and methanol under different reaction conditions. Characterization and kinetic results revealed that the activity of this catalyst was enhanced due to the increase of CaO/talc molar ratio value leading to an improvement in the biodiesel production. Moreover, the effect of various parameters on the activity of the undertaken catalysts was studied in order to determine the optimum process conditions. Leaching measurements and the durability of the CaO/talc catalyst under several reaction cycles were evaluated and proved it to be a stable catalyst. 相似文献
997.
In this paper, a series of PANI-CdS (PANI is the abbreviation of Polyaniline) composites were synthesized by changing the mass of PANI (molar ratio of PANI and CdS : 0.5, 0.7, 1, 1.5, 2). The PANI-CdS composites were characterized by XRD patterns, IR spectra and elemental analysis. The activities of as-prepared composites were also tested with regard to hydrogen evolution. It was found that PANI-CdS composites showed good activity for hydrogen evolution. With increment of PANI, the activity for hydrogen evolution of PANI-CdS reduced. But compared with pure CdS, PANI-CdS had a much higher activity for hydrogen evolution. 相似文献
998.
聚α-烯烃合成油是所有合成润滑油基础油中性能最优异的一种,具有黏度指数高、低温性能和高温氧化安定性好、抗燃性好及挥发性低等优点,是未来应用广泛的合成基础油,市场需求持续增加。综述聚α-烯烃基础油及其加氢工艺、加氢精制催化剂的研究现状。传统聚α-烯烃合成油生产工艺有乙烯齐聚法和石蜡裂解法等,潞安煤制油工艺和天然气制油工艺等新工艺也开始兴起。加氢精制工艺对提高聚α-烯烃合成油的质量起着至关重要的作用,北京燕化聚华工贸有限公司两段加氢精制工艺和中国石油兰州润滑油厂聚α-烯烃合成油加氢精制处理工艺均极大改善了聚α-烯烃合成油的质量。聚α-烯烃合成油加氢精制效果很大程度取决于加氢精制催化剂性能,要求加氢精制催化剂具有较高的选择性和较好的稳定性。 相似文献
999.
1000.