全文获取类型
收费全文 | 44760篇 |
免费 | 4578篇 |
国内免费 | 2385篇 |
专业分类
电工技术 | 2915篇 |
技术理论 | 4篇 |
综合类 | 3185篇 |
化学工业 | 6883篇 |
金属工艺 | 3033篇 |
机械仪表 | 2916篇 |
建筑科学 | 3754篇 |
矿业工程 | 1395篇 |
能源动力 | 1220篇 |
轻工业 | 3137篇 |
水利工程 | 894篇 |
石油天然气 | 2251篇 |
武器工业 | 426篇 |
无线电 | 5502篇 |
一般工业技术 | 5393篇 |
冶金工业 | 1839篇 |
原子能技术 | 529篇 |
自动化技术 | 6447篇 |
出版年
2024年 | 271篇 |
2023年 | 811篇 |
2022年 | 1548篇 |
2021年 | 2297篇 |
2020年 | 1693篇 |
2019年 | 1254篇 |
2018年 | 1475篇 |
2017年 | 1590篇 |
2016年 | 1411篇 |
2015年 | 1928篇 |
2014年 | 2474篇 |
2013年 | 2838篇 |
2012年 | 3131篇 |
2011年 | 3273篇 |
2010年 | 2893篇 |
2009年 | 2659篇 |
2008年 | 2682篇 |
2007年 | 2573篇 |
2006年 | 2374篇 |
2005年 | 2091篇 |
2004年 | 1427篇 |
2003年 | 1155篇 |
2002年 | 1139篇 |
2001年 | 925篇 |
2000年 | 942篇 |
1999年 | 909篇 |
1998年 | 741篇 |
1997年 | 583篇 |
1996年 | 572篇 |
1995年 | 467篇 |
1994年 | 400篇 |
1993年 | 311篇 |
1992年 | 238篇 |
1991年 | 153篇 |
1990年 | 101篇 |
1989年 | 97篇 |
1988年 | 76篇 |
1987年 | 59篇 |
1986年 | 40篇 |
1985年 | 21篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 18篇 |
1979年 | 4篇 |
1974年 | 3篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1959年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Huizhu Cai Bingbing Chen Xiao Zhang Yuchen Deng Dequan Xiao Ding Ma Chuan Shi 《Nano Research》2021,14(1):122-130
Over recent years,catalytic materials of Fe-N-C species have been recognized being active for oxygen reduction reaction(ORR).However,the identification of active site remains challenging as it generally involves a pyrolysis process and mixed components being obtained.Herein Fe3C/C and Fe2N/C samples were synthesized by temperature programmed reduction of Fe precursors in 15%CH4/H2and pure NH3,respectively.By acid leaching of Fe2N/C sample,only single sites of FeN4species were presented,providing an ideal model for identification of catalytic functions of the single sites of FeN4in ORR.A correlation was conducted between the concentration of FeⅡN4in low spin state by Mossbauer spectra and the kinetic current density at 0.8 V in alkaline media,and such a structure-performance correlation assures the catalytic roles of low spin FeⅡN4 species as highly active sites for the ORR. 相似文献
12.
13.
14.
In this study, 30 subjects were exposed to different combinations of air temperature (Ta: 24, 27, and 30°C) and CO2 level (8000, 10 000, and 12 000 ppm) in a high-humidity (RH: 85%) underground climate chamber. Subjective assessments, physiological responses, and cognitive performance were investigated. The results showed that as compared with exposure to Ta = 24°C, exposure to 30°C at all CO2 levels caused subjects to feel uncomfortably warm and experience stronger odor intensity, while increased mental effort and greater intensity of acute health symptoms were reported. However, no significant effects of Ta on task performance or physiological responses were found. This indicated that subjects had to exert more effort to maintain their performance in an uncomfortably warm environment. Increasing CO2 from 8000 to 12 000 ppm at all Ta caused subjects to report higher rates of headache, fatigue, agitation, and feeling depressed, although the results were statistically significant only at 24 and 27°C. The text typing performance and systolic blood pressure (SBP) decreased significantly at this exposure, whereas diastolic blood pressure (DBP) and thermal discomfort increased significantly. These effects suggest higher arousal/stress. No significant interaction effect of Ta and CO2 concentration on human responses was identified. 相似文献
15.
Process object is the instance of process. Vertexes and edges are in the graph of process object. There are different types of the object itself and the associations between object. For the large-scale data, there are many changes reflected. Recently, how to find appropriate real-time data for process object becomes a hot research topic. Data sampling is a kind of finding c hanges o f p rocess o bjects. There i s r equirements f or s ampling to be adaptive to underlying distribution of data stream. In this paper, we have proposed a adaptive data sampling mechanism to find a ppropriate d ata t o m odeling. F irst o f all, we use concept drift to make the partition of the life cycle of process object. Then, entity community detection is proposed to find changes. Finally, we propose stream-based real-time optimization of data sampling. Contributions of this paper are concept drift, community detection, and stream-based real-time computing. Experiments show the effectiveness and feasibility of our proposed adaptive data sampling mechanism for process object. 相似文献
16.
Xi Chen Zhengkun Yu Chen Yang Yao Chen Chao Jin Yuejiao Ding Wenbin Li Zhongmin Wan 《International Journal of Hydrogen Energy》2021,46(19):11127-11139
Flow field structure can largely determine the output performance of Polymer electrolyte membrane fuel cell. Excellent channel configuration accelerates electrochemical reactions in the catalytic layer, effectively avoiding flooding on the cathode side. In present study, a three-dimensional, multi-phase model of PEMFC with a 3D wave flow channel is established. CFD method is applied to optimize the geometry constructions of three-dimensional wave flow channels. The results reveal that 3D wave flow channel is overall better than straight channel in promoting reactant gases transport, removing liquid water accumulated in microporous layer and avoiding thermal stress concentration in the membrane. Moreover, results show the optimal flow channel minimum depth and wave length of the 3D wave flow channel are 0.45 mm and 2 mm, respectively. Due to the periodic geometric characteristics of the wave channel, the convective mass transfer is introduced, improving gas flow rate in through-plane direction. Furthermore, when the cell output voltage is 0.4 V, the current density in the novel channel is 23.8% higher than that of conventional channel. 相似文献
17.
18.
19.
20.
This paper is concerned with the problem of joint input and state estimation for linear stochastic systems with direct feedthrough. Based on the fact that each unknown input between any two time steps is always bounded, a novel improved algorithm is proposed. Compared with existing results, this algorithm can effectively enhance estimation accuracy. Moreover, the stability of the algorithm is also discussed. Finally, an illustrative example is given to demonstrate the effectiveness of the proposed approach. 相似文献