全文获取类型
收费全文 | 5410篇 |
免费 | 658篇 |
国内免费 | 195篇 |
专业分类
电工技术 | 228篇 |
综合类 | 284篇 |
化学工业 | 1790篇 |
金属工艺 | 539篇 |
机械仪表 | 87篇 |
建筑科学 | 168篇 |
矿业工程 | 333篇 |
能源动力 | 520篇 |
轻工业 | 116篇 |
水利工程 | 27篇 |
石油天然气 | 79篇 |
武器工业 | 14篇 |
无线电 | 189篇 |
一般工业技术 | 683篇 |
冶金工业 | 1129篇 |
原子能技术 | 52篇 |
自动化技术 | 25篇 |
出版年
2024年 | 22篇 |
2023年 | 132篇 |
2022年 | 202篇 |
2021年 | 218篇 |
2020年 | 234篇 |
2019年 | 200篇 |
2018年 | 157篇 |
2017年 | 216篇 |
2016年 | 181篇 |
2015年 | 178篇 |
2014年 | 327篇 |
2013年 | 296篇 |
2012年 | 305篇 |
2011年 | 414篇 |
2010年 | 288篇 |
2009年 | 305篇 |
2008年 | 274篇 |
2007年 | 328篇 |
2006年 | 277篇 |
2005年 | 247篇 |
2004年 | 225篇 |
2003年 | 164篇 |
2002年 | 166篇 |
2001年 | 140篇 |
2000年 | 116篇 |
1999年 | 118篇 |
1998年 | 92篇 |
1997年 | 77篇 |
1996年 | 63篇 |
1995年 | 57篇 |
1994年 | 50篇 |
1993年 | 37篇 |
1992年 | 37篇 |
1991年 | 23篇 |
1990年 | 26篇 |
1989年 | 15篇 |
1988年 | 14篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 1篇 |
1982年 | 9篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1951年 | 4篇 |
排序方式: 共有6263条查询结果,搜索用时 15 毫秒
61.
压电陶瓷变压器材料PMMN-PZT的研究 总被引:1,自引:0,他引:1
研究了一种四元系压电陶瓷变压器材料PMMN -PZT。XRD分析证明 ,采用二步法预合成的PMMN -PZT粉料不如一步法 ;SEM分析进一步证实了一步法预合成的粉料能制成性能更好的陶瓷样品。造粒工艺对陶瓷样品的显微结构和压电性能也有重要的影响。最后制得了性能较好的PMMN -PZT压电陶瓷材料 ,其Kp=0 .5 18,Qm=3887,tanδ =0 .71% ,εT3 3 /ε0 =70 1,d3 3 =2 0 3,基本上能满足压电变压器的要求 相似文献
62.
63.
P.M. lvarez F.J. Beltrn J.P. Pocostales F.J. Masa 《Applied catalysis. B, Environmental》2007,72(3-4):322-330
A series of Co/Al2O3 catalysts were prepared by the incipient wetness impregnation method using γ-Al2O3 support and (CH3COO)2Co·4H2O solutions, followed by calcination at 500–800 °C. Characterization of catalysts was accomplished by several techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), physisorption of nitrogen, mercury and helium-based pycnometries, Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and pH of zero charge (PZC). Impregnation of support produced a moderate decrease of its surface area and pore volume and also led to minor changes of its PZC. Depending on preparation conditions (i.e., calcination atmosphere and temperature and metal loading), one or more of the following Co-containing compounds were identified: CoO, Co3O4 and CoAl2O4. The support and prepared Co/Al2O3 catalysts were tested to catalyze the ozonation of aqueous pyruvic acid at pH 2.5. Pyruvic acid was shown refractory towards single ozonation but the use of γ-Al2O3 and Co/Al2O3 catalysts resulted in 56–96% pyruvic acid conversion and 41–78% decrease in DOC after 2 h of ozonation of phosphate-buffered solutions. In the absence of the buffer, conversion rate was enhanced likely as a result of pH increase during the course of the process thus giving rise to the indirect way of ozonation through hydroxyl radicals. Acetic acid was found as the main by-product of pyruvic acid ozonation. Depending on the catalyst used, yield of acetic acid varied from 32 to 49%, values noticeably lower that that obtained from the control non-catalytic ozonation experiment (73%). Differences in catalytic activity amongst the various Co/Al2O3 catalysts investigated were attributed to the different Co active phases deposited on the γ-Al2O3 surface. The following sequence of increasing activity can be inferred from experimental results: CoO, CoAl2O4 and Co3O4. All the Co/Al2O3 catalysts prepared showed good stability as the percentage of cobalt leached out was rather low. 相似文献
64.
超临界流体干燥技术制备纳米二氧化锰 总被引:1,自引:0,他引:1
采用溶胶法和超临界流体干燥技术,在533K,7.6MPa的条件下,用超临界乙醇作为干燥剂制备纳米级二氧化锰,采用X射线衍射,透射电镜确定了二氧化锰粒子的行貌和粒径,结果表明,用超临界流体干燥技术可以得到颗粒细,大小均匀,抗团聚非常好的纳米粒子。 相似文献
65.
66.
67.
The homogeneous decomposition of ozone in the presence of a Co(II) catalyst has been investigated in aqueous solution. Under the conditions investigated (Co(II) concentration: 0.0 – 2.0?ppm, pH: 1.6- 8.4, O3 concentration: 5 10?5 – 2.0 104?M) the process can be assumed to follow pseudo first order kinetics with respect to ozone. Cobalt concentration dependency also obeys first order kinetics although it may be considered to reach a steady state concentration. pH exerts a positive influence on the decomposition rate from 1.6 to 7.1, the process leveling off at pH 8.4. An Arrhenius analysis of the temperature effect gave a moderate activation energy of the global reaction (E=10917?cal mol?1). A more detailed radical mechanism than a simple pseudo first order reaction can be postulated for simulation purposes. By numerical optimization of some unknown kinetic constants the influence of several operating variables can be adequately predicted. 相似文献
68.
在同成分铌酸锂晶体中掺入0.03?2O3和0.1%MnO2(质量分数),分别掺入0,1%,3%,4.5%,6%的MgO(摩尔分数),用提拉法生长了一系列Mg:Mn:Fe:LiNbO3晶体.检测了Mg:Mn:Fe:LiNbO3晶体的红外光谱和抗光损伤能力.掺0,1%,3%,4.5%Mg的Mg:Mn:Fe:LiNbO3晶体的OH-红外振动峰位于3484cm-1,而掺6%Mg的Mg:Mn:Fe:LiNbO3晶体红外振动峰移到3 535gm-1.采用波长为632nm的He-Ne激光器作为光源,通过二波耦合方法测试晶体的全息存储性能.结果表明:Mg:Mn:Fe:LiNbO3晶体的写入时间和动态范围随掺镁量的增加而显著减小,而光折变灵敏度略有上升,抗光损伤性能增强,其中掺镁量为3%Mg:Mn:Fe:LiNbO3晶体更适合作为全息存储介质. 相似文献
69.
70.
Dewi Tristantini Sara Lögdberg Börje Gevert Øyvind Borg Anders Holmen 《Fuel Processing Technology》2007
The Fischer–Tropsch synthesis over Co/γ-Al2O3 and Co–Re/γ-Al2O3 was investigated in a fixed-bed reactor at 20 bar and 483 K using feed gases with molar H2/CO ratios of 2.1, 1.5 and 1.0 simulating synthesis gas derived from biomass. With lower H2/CO ratios in the feed, the CO conversion and the CH4 selectivity decreased, while the C5+ selectivity and olefin/paraffin ratio for C2–C4 increased slightly. The water–gas shift activity was low for both catalysts, resulting in high molar usage ratios of H2/CO (close to 2.0), even at the lower inlet ratios (i.e. 1.5 and 1.0). For both catalysts, the drop in the production rate of hydrocarbons when shifting from an inlet ratio of 2.1 to 1.5 was significant mainly because the H2/CO usage ratio did not follow the change in the inlet ratio. The hydrocarbon selectivities were rather similar for inlet H2/CO ratios of 2.1 and 1.5, while significantly deviating from those for an inlet ratio of 1.0. With the studied catalysts, it is possible to utilize the advantages of an inlet ratio of 1.0 (higher selectivity to C5+, lower selectivity to CH4, no water–gas shifting of the bio-syngas needed prior to the FT reactor) if a low syngas conversion is accepted. 相似文献