全文获取类型
收费全文 | 298327篇 |
免费 | 5525篇 |
国内免费 | 1967篇 |
专业分类
电工技术 | 5895篇 |
技术理论 | 5篇 |
综合类 | 1764篇 |
化学工业 | 46089篇 |
金属工艺 | 11555篇 |
机械仪表 | 8574篇 |
建筑科学 | 7394篇 |
矿业工程 | 2045篇 |
能源动力 | 5982篇 |
轻工业 | 27605篇 |
水利工程 | 3411篇 |
石油天然气 | 5993篇 |
武器工业 | 247篇 |
无线电 | 29446篇 |
一般工业技术 | 52248篇 |
冶金工业 | 65514篇 |
原子能技术 | 5792篇 |
自动化技术 | 26260篇 |
出版年
2021年 | 2542篇 |
2019年 | 2142篇 |
2018年 | 3807篇 |
2017年 | 3984篇 |
2016年 | 4674篇 |
2015年 | 3542篇 |
2014年 | 4960篇 |
2013年 | 11712篇 |
2012年 | 9638篇 |
2011年 | 12473篇 |
2010年 | 8739篇 |
2009年 | 9379篇 |
2008年 | 10032篇 |
2007年 | 10160篇 |
2006年 | 8957篇 |
2005年 | 10563篇 |
2004年 | 9099篇 |
2003年 | 8154篇 |
2002年 | 7017篇 |
2001年 | 7023篇 |
2000年 | 6171篇 |
1999年 | 6389篇 |
1998年 | 17994篇 |
1997年 | 12744篇 |
1996年 | 9403篇 |
1995年 | 6780篇 |
1994年 | 6015篇 |
1993年 | 5902篇 |
1992年 | 4141篇 |
1991年 | 3990篇 |
1990年 | 4060篇 |
1989年 | 4002篇 |
1988年 | 3787篇 |
1987年 | 3205篇 |
1986年 | 3211篇 |
1985年 | 3550篇 |
1984年 | 3381篇 |
1983年 | 3143篇 |
1982年 | 2810篇 |
1981年 | 3058篇 |
1980年 | 2817篇 |
1979年 | 2899篇 |
1978年 | 2820篇 |
1977年 | 3390篇 |
1976年 | 4869篇 |
1975年 | 2545篇 |
1974年 | 2326篇 |
1973年 | 2364篇 |
1972年 | 2014篇 |
1971年 | 1828篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
W. Sjoerd Kijlstra Joop C.M.L. Daamen Jolinde M. van de Graaf Bart van der Linden Eduard K. Poels Alfred Bliek 《Applied catalysis. B, Environmental》1996,7(3-4):337-357
The effect of water on the selective catalytic reduction (SCR) of nitric oxide with ammonia over alumina supported with 2–15 wt.-% manganese oxide was investigated in the temperature range 385–600 K, with the emphasis on the low side of this temperature window. Studies on the effect of 1–5 vol.-% water vapour on the SCR reaction rate and selectivity were combined with TPD experiments to reveal the influence of water on the adsorption of the single SCR reactants. It turned out that the activity decrease due to water addition can be divided into a reversible inhibition and an irreversible deactivation. Inhibition is caused by molecular adsorption of water. TPD studies showed that water can adsorb competitively with both ammonia and nitric oxide. Additional kinetic experiments revealed that adsorbed ammonia is present in excess on the catalyst surface, even in the presence of water. Reduced nitric oxide adsorption is responsible for the observed reversible decrease in the reaction rate; the fractional reaction order changes from 0.79 in the absence of water to 1.07 in its presence. Deactivation is probably due to the dissociative adsorption of water, resulting in the formation of additional surface hydroxyls. As the amount of surface hydroxyls formed is limited to a saturation level, the deactivating effect on the catalyst is limited too. The additional hydroxyls condense and desorb in the temperature range 525–775 K, resulting in a lower degree of deactivation at higher temperature. A high temperature treatment at 775 K results in a complete regeneration. The amount of surface hydroxyls formed per unit surface area decreases at increasing MnOx-loading. The selectivity to the production of nitrogen is enhanced significantly by the presence of gas phase water. 相似文献
993.
A. Andreev L. Prahov M. Gabrovska A. Eliyas V. Ivanova 《Applied catalysis. B, Environmental》1996,8(4):365-373
The catalytic activity of a wide range of transition metal oxides in oxidation of sulphide ions by air in aqueous medium was studied. Some specific features of the reaction mechanism on some of the studied oxides were considered. The transition metal oxides are promising catalysts for practical application. Some of these oxides will allow the preparation of catalysts possessing activity comparable to that of the cobalt phthalocyanine based catalysts, popular in industrial practice. 相似文献
994.
995.
996.
997.
998.
Schmidt Frank L.; Law Kenneth; Hunter John E.; Rothstein Hannah R.; Pearlman Kenneth; McDaniel Michael 《Canadian Metallurgical Quarterly》1993,78(1):3
Using a large database, this study examined 3 refinements of validity generalization procedures: (1) a more accurate procedure for correcting the residual standard deviation (SD) for range restriction to estimate SDp, (2) use of r? instead of study-observed rs in the formula for sampling error variance, and (3) removal of non-Pearson rs. The 1st procedure does not affect the amount of variance accounted for by artifacts. The addition of the 2nd and 3rd procedures increased the mean percentage of validity variance accounted for by artifacts from 70 to 82%, a 17% increase. The cumulative addition of all 3 procedures decreased the mean SDp estimate from .150 to .106, a 29% decrease. Six additional variance-producing artifacts were identified that could not be corrected for. In light of these it was concluded that the obtained estimates of mean SDp and mean validity variance accounted for were consistent with the hypothesis that the true mean SDp value is close to zero. These findings provide further evidence against the situational specificity hypothesis. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
999.
Growth hormone (GH) secretion of the neonatal pituitary is stimulated by tau-aminobutyric acid (GABA) (1,2). Since in most cases GABA is known to act by increasing postsynaptic membrane permeability to chloride ions we tested the importance of chloride channel activation in the GH stimulatory effect of GABA in the neonatal pituitary. In the absence of chloride in the superfusion medium GABA was without effect on GH secretion of the neonatal pituitaries and its effect was attenuated by chloride channel inhibitors. The effect of growth hormone releasing hormone (GHRH) on GH secretion was attenuated in the chloride-free media, but it was not affected by simultaneous administration of chloride channel blockers. The present study indicates that GH stimulatory effect of GABA in the neonatal pituitaries might involve chloride channel activation probably resulting in secondary activation of calcium channels. 相似文献
1000.
The availability of low-cost VLSI (very large scale integration) design software and fabrication services for design, simulation, and layout of integrated circuits has become readily accessible to universities for classroom instruction. The design and fabrication of a CMOS integrated circuit is described, which converts an eight-bit digital signal to a pulse-width modulated (PWM) signal and vice versa for radio control hobbyist transceivers and motor servos. This example design serves to describe a subset of VLSI design tools from the University of California, Berkeley, the University of Washington, and Microelectronics Center of North Carolina and MOSIS fabrication services available for classroom instruction 相似文献