全文获取类型
收费全文 | 643979篇 |
免费 | 8487篇 |
国内免费 | 1971篇 |
专业分类
电工技术 | 11750篇 |
综合类 | 597篇 |
化学工业 | 97552篇 |
金属工艺 | 25243篇 |
机械仪表 | 18559篇 |
建筑科学 | 15878篇 |
矿业工程 | 2957篇 |
能源动力 | 17584篇 |
轻工业 | 57229篇 |
水利工程 | 6294篇 |
石油天然气 | 11305篇 |
武器工业 | 39篇 |
无线电 | 72796篇 |
一般工业技术 | 123416篇 |
冶金工业 | 126685篇 |
原子能技术 | 13200篇 |
自动化技术 | 53353篇 |
出版年
2021年 | 5690篇 |
2020年 | 4063篇 |
2019年 | 5245篇 |
2018年 | 9009篇 |
2017年 | 8832篇 |
2016年 | 9282篇 |
2015年 | 6532篇 |
2014年 | 10814篇 |
2013年 | 29292篇 |
2012年 | 17361篇 |
2011年 | 23989篇 |
2010年 | 18972篇 |
2009年 | 21345篇 |
2008年 | 21704篇 |
2007年 | 21433篇 |
2006年 | 18721篇 |
2005年 | 17195篇 |
2004年 | 16461篇 |
2003年 | 16077篇 |
2002年 | 15345篇 |
2001年 | 15451篇 |
2000年 | 14417篇 |
1999年 | 15251篇 |
1998年 | 39511篇 |
1997年 | 27702篇 |
1996年 | 21162篇 |
1995年 | 15938篇 |
1994年 | 13755篇 |
1993年 | 13458篇 |
1992年 | 9728篇 |
1991年 | 9168篇 |
1990年 | 8956篇 |
1989年 | 8737篇 |
1988年 | 8186篇 |
1987年 | 7310篇 |
1986年 | 7150篇 |
1985年 | 8014篇 |
1984年 | 7484篇 |
1983年 | 6701篇 |
1982年 | 6314篇 |
1981年 | 6425篇 |
1980年 | 6056篇 |
1979年 | 5976篇 |
1978年 | 5723篇 |
1977年 | 6927篇 |
1976年 | 9063篇 |
1975年 | 4975篇 |
1974年 | 4734篇 |
1973年 | 4866篇 |
1972年 | 4002篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The bisymmetry property is studied for operators defined on a finite chain. In particular, a characterization of all bisymmetric, commutative and smooth binary operators is given. It is deduced from this characterization that bisymmetry and associativity are equivalent for finite chains, under these additional conditions. 相似文献
992.
This study examined whether objects are attended in serial or in parallel during a demanding visual search task. A component of the event-related potential waveform, the N2pc wave, was used as a continuous measure of the allocation of attention to possible targets in the search arrays. Experiment 1 demonstrated that the relative allocation of attention shifts rapidly, favoring one item and then another. In Experiment 2, a paradigm was used that made it possible to track the absolute allocation of attention to individual items. This experiment showed that attention was allocated to one object for 100-150 ms before attention began to be allocated to the next object. These findings support models of attention that posit serial processing in demanding visual search tasks. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
993.
994.
Using the improved expression of the defect pool model proposed by Powell and Deane we match the experimental current-voltage and the spectral response characteristic curves of hydrogenated amorphous silicon solar cells. We compare the electrical parameters resulting from using the different defect pool models published in the literature and from assuming a uniform density of dangling bond in every device layer. We discuss the applicability of the algorithm derived by Schumm for the stabilized state exploring its sensitivity to the sample history. Finally we propose an expression for stabilized cells adapting Schumm's ideas to the expression derived by Powell and Deane. 相似文献
995.
Published data on the preparation procedures, stability, and complexation of U(III) in aqueous solutions are summarized and correlated. Reactions with inorganic and organic free radicals studied by the flash radiolysis method, the spectroscopic properties, the extraction and ion-exchange behavior of U(III), and methods for isolation of solid U(III) compounds from aqueous solutions are discussed. 相似文献
996.
Using meta-analysis, randomized experiments in education that either clearly did or clearly did not experience student attrition were examined for the baseline comparability of groups. Results from 35 studies suggested that after attrition, the observed measures of baseline comparability of groups did not differ more than would be expected given sampling error. The degree of either overall or differential attrition did not relate to baseline comparability, a finding that held under sensitivity analyses. Also, both overall and differential attrition rates were unrelated to posttest effect sizes. All of these analyses, however, lacked sufficient statistical power to detect small but potentially meaningful effects. Results suggest caution is warranted when applying quality scales and other blanket rules pertaining to attrition that are meant to either serve as inclusion-exclusion criteria or in scoring study quality. Much greater attention is needed to both the reporting of attrition in primary studies and to the development of conceptual and empirical models of the attrition process. These developments would aid further investigation of the relation between attrition and study outcomes. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
997.
998.
Titanium Dioxide, TiO2, is a photocatalyst with a unique characteristic. A surface coated with TiO2 exhibits an extremely high affinity for water when exposed to UV light and the contact angle decreases nearly to zero. Inversely, the contact angle increases when the surface is shielded from UV. This superhydrophilic nature gives a self-cleaning effect to the coated surface and has already been applied to some construction materials, car coatings and so on. We applied this property to the enhancement of boiling heat transfer. An experiment involving the pool boiling of pure water has been performed to make clear the effect of high wettability on heat transfer characteristics. The heat transfer surface is a vertical copper cylinder of 17 mm in diameter and the measurement has been done at saturated temperature and in a steady state. Both TiO2-coated and non-coated surfaces were used for comparison. In the case of the TiO2-coated surface, it is exposed to UV light for a few hours before experiment and it is found that the maximum heat flux (CHF) is about two times larger than that of the uncoated surface. The temperature at minimum heat flux (MHF) for the superhydrophilic surface is higher by 100 K than that for the normal one. The superhydrophilic surface can be an ideal heat transfer surface. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
999.
1000.