全文获取类型
收费全文 | 64933篇 |
免费 | 10277篇 |
国内免费 | 10185篇 |
专业分类
系统科学 | 2337篇 |
丛书文集 | 1209篇 |
教育与普及 | 868篇 |
理论与方法论 | 157篇 |
现状及发展 | 553篇 |
研究方法 | 118篇 |
综合类 | 80141篇 |
自然研究 | 12篇 |
出版年
2024年 | 327篇 |
2023年 | 618篇 |
2022年 | 959篇 |
2021年 | 1171篇 |
2020年 | 1518篇 |
2019年 | 3705篇 |
2018年 | 3905篇 |
2017年 | 4160篇 |
2016年 | 4376篇 |
2015年 | 4586篇 |
2014年 | 4999篇 |
2013年 | 4346篇 |
2012年 | 3913篇 |
2011年 | 3833篇 |
2010年 | 3939篇 |
2009年 | 3562篇 |
2008年 | 3018篇 |
2007年 | 3038篇 |
2006年 | 2228篇 |
2005年 | 2133篇 |
2004年 | 2360篇 |
2003年 | 1829篇 |
2002年 | 1650篇 |
2001年 | 1226篇 |
2000年 | 1183篇 |
1999年 | 1873篇 |
1998年 | 1791篇 |
1997年 | 1875篇 |
1996年 | 1553篇 |
1995年 | 1303篇 |
1994年 | 1284篇 |
1993年 | 1328篇 |
1992年 | 1139篇 |
1991年 | 1132篇 |
1990年 | 870篇 |
1989年 | 723篇 |
1988年 | 720篇 |
1987年 | 425篇 |
1986年 | 351篇 |
1985年 | 130篇 |
1984年 | 46篇 |
1983年 | 23篇 |
1982年 | 20篇 |
1980年 | 9篇 |
1978年 | 9篇 |
1977年 | 25篇 |
1972年 | 66篇 |
1969年 | 9篇 |
1968年 | 14篇 |
1967年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
The present study mainly probed the relationships between ERP components and the degrees of difficulty in visual search. Three experimental and one controlled conditions were randomly selected. Results showed that the P1, N1, and P2 amplitudes and latencies had no significant differences among the three experimental and one controlled conditions, and the ERP waveforms of low perceptual load of visual search were different from that of high perceptual load. These results might indicate that the latencies and amplitudes of initial P1, N1 and P2 in high and low perceptual load conditions were almost the same. The ERP waveforms of parallel search were very different from that of serial search, and the ERP waveforms of serial search were very similar. The P3 amplitudes increased as the degrees of the difficulty in visual search decreased. 相似文献
112.
113.
Zhang ping School of Information Electrical Engineering of Hunan University of Science Technology Hunan Xiangtan 《科技信息》2008,(17)
介绍了以三菱FX0NPLC为核心的温度控制系统在电镀生产线中的应用。简述了温度控制原理,论述了控制系统的设计方案及其软硬件的实现方法。实践表明该方法简单易行,适应性好,可靠性高。 相似文献
114.
The interleukin 1β (IL-1β) cDNA was cloned from the red seabream (Pagrus major) by homology cloning strategy. A cDNA fragment was amplified by PCR using two degenerated primers, which were designed according to the conserved regions of other known IL-1β sequences, and elongated by 3' ends and 5' ends RACE PCR to get the full length coding sequence of red seabream IL-1β (RS IL-1β). The sequence contained 1252 nucleotides that included a 5' untranslated region (UTR) of 84bp, a 3' UTR of 410 bp and an open reading frame (ORF) of 759 nucleotides which could be translated into a putative peptide of 253 amino acids with molecular weight of 28.6 kD and putative isoelectric point pI of 5.29. The deduced peptide contained two potential N-glycosylation sites and an identifiable IL1 family signature, but lacked the signal peptide and the clear ICE cut site, which were common in other nonmammalian IL-1β genes. The RS IL-1β had the highest homology with piscine IL-1β according to phylogenetic tree analysis. 相似文献
115.
To meet the demands of rapid development of pipeline constructions in China, a complete set of full automatic external welding machine for annular welds of pipeline and welding procedure have been developed thereof. The system has been applied in the “West-East Pipeline Project” and its performance is proven. With this welding machine the welding parameters can be either preset before or adjusted in real time during the welding process, which avoids the blemishes of same type domestic and international products. Thus it is best propitious to in-situ construction conditions and workers‘ technical level in China. In this article the development of mechanical and control systems of this product is introduced and typical welding procedure parameters are provided. 相似文献
116.
制备介孔复合环境材料的酸洗刻蚀工艺 总被引:2,自引:1,他引:1
探索了一种制备介孔复合环境材料的方法--破坏式造孔与有机复合相结合,使最终产品可大量吸附污染水体中的各种污染物并具有一定的杀菌功能.该方法通过酸洗刻蚀,达到了去除原材料中的杂质和刻蚀孔径的目的,所制备的中间产品24h吸湿率为原材料的2倍,染料吸附量为原材料的1.6倍. 相似文献
117.
118.
极低硫钢的精炼脱硫动力学模型 总被引:2,自引:1,他引:1
通过对影响脱硫反应限制性环节的分析,将持续接触和瞬时接触两种反应模式综合考虑,建立了精炼脱硫动力学模型,并通过实验室"极低硫钢精炼脱硫"实验数据对模型进行验证.结果表明,本模型的预测结果与实验结果比较吻合. 相似文献
119.
120.
ZHANG Huijuan FENG Juan AI Xicheng ZHANG Xingkang YU Zhongheng & ZHANG Jianping State Key Laboratory for Structural Chemistry of Unstable StableSpecies Center of Molecular Science Institute of Chemistry Chinese Academy of Sciences Beijing China 《科学通报(英文版)》2003,(17)
As the structural analogues of (bacterio)chlorophylls in photosynthetic organisms, porphyrins play an important role in building up artificial photosynthetic systems[13]. In natural photosynthetic pigment-protein complexes,certain amino acid residuals modulate the reactions ofcharge transfer and energy transfer via coordination andother types of interactions with chlorophyll molecules.For instance, in higher-plant photosynthetic reaction cen-ter PSⅡ, the oxidization-reduction system consistin… 相似文献