全文获取类型
收费全文 | 97201篇 |
免费 | 20308篇 |
国内免费 | 2996篇 |
专业分类
电工技术 | 4483篇 |
技术理论 | 4篇 |
综合类 | 4256篇 |
化学工业 | 26334篇 |
金属工艺 | 4108篇 |
机械仪表 | 4538篇 |
建筑科学 | 6844篇 |
矿业工程 | 1976篇 |
能源动力 | 2531篇 |
轻工业 | 11383篇 |
水利工程 | 1435篇 |
石油天然气 | 3446篇 |
武器工业 | 506篇 |
无线电 | 13738篇 |
一般工业技术 | 18384篇 |
冶金工业 | 3156篇 |
原子能技术 | 728篇 |
自动化技术 | 12655篇 |
出版年
2024年 | 322篇 |
2023年 | 1176篇 |
2022年 | 2205篇 |
2021年 | 3246篇 |
2020年 | 3737篇 |
2019年 | 4914篇 |
2018年 | 4992篇 |
2017年 | 5494篇 |
2016年 | 5713篇 |
2015年 | 6631篇 |
2014年 | 6980篇 |
2013年 | 8651篇 |
2012年 | 6688篇 |
2011年 | 6336篇 |
2010年 | 6248篇 |
2009年 | 5894篇 |
2008年 | 5366篇 |
2007年 | 5061篇 |
2006年 | 4681篇 |
2005年 | 4039篇 |
2004年 | 3300篇 |
2003年 | 3008篇 |
2002年 | 3140篇 |
2001年 | 2626篇 |
2000年 | 2412篇 |
1999年 | 1828篇 |
1998年 | 1099篇 |
1997年 | 885篇 |
1996年 | 833篇 |
1995年 | 669篇 |
1994年 | 570篇 |
1993年 | 423篇 |
1992年 | 336篇 |
1991年 | 247篇 |
1990年 | 175篇 |
1989年 | 154篇 |
1988年 | 108篇 |
1987年 | 88篇 |
1986年 | 70篇 |
1985年 | 31篇 |
1984年 | 27篇 |
1983年 | 21篇 |
1982年 | 21篇 |
1981年 | 19篇 |
1980年 | 17篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1959年 | 4篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
In this article, two novel kinds of focusing elements as reflectors are analyzed and compared. One is the grooved Fresnel zone plate reflector with continuous phase‐correcting. The other called subzone paraboloid reflector, has the profile that consists of a series of paraboloids. Their diffraction efficiencies and bandwidths are described. The two elements still preserve the advantages of Fresnel zone plates, namely, low profile, high efficiency, and simple fabrication. Two dual‐reflector antennas using the proposed focusing elements as the main reflectors are simulated and the results show that these antennas have good radiation performances. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:101–108, 2015. 相似文献
52.
53.
54.
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
55.
Journal of Porous Materials - In this work, a trifluoromethanesulfonic acid (TFOH) modified clay (TFOH-Clay) was developed for the removal of trace olefins in heavy naphtha. 5%TFOH-Clay can... 相似文献
56.
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
57.
Matthew O'Sullivan Daire
hUallachin Patrícia Oliveira Antunes Eleanor Jennings Mary Kelly‐Quinn 《河流研究与利用》2019,35(2):146-158
Cattle access to streams has been linked globally with degradation of stream water quality, driven largely by bank erosion and resultant instream, fine sediment deposition. The majority of evidence on such effects is however based in arid and semiarid regions of the United States and Australia, with few studies relating to cool temperate climates such as Northwest Europe. In this study, “Quorer” resuspendable sediment samples were taken from riffle geomorphic units upstream (control) and at two points downstream (pressure and recovery) of cattle access points in headwater streams in agricultural catchments in Ireland to assess levels of deposited stream sediment. Samples were taken in April/May (2016) prior to the grazing season and in October (2016) at the end of the grazing season. Sites in good‐high ecological status catchments and less than good ecological status catchments were included in the study. Higher levels of sediment were found downstream of cattle access points in both good‐high status and less than good status catchments; however, the impacts of access points were spatially confined to, in most cases, the area immediately downstream of the point of access. There was a strong correlation between deposited sediment mass and organic matter (OM) mass, with levels of OM increasing linearly with deposited sediment mass. Levels of measured sediment were negatively correlated with riparian habitat health (measured using a qualitative habitat assessment). The results of this study highlight the need for measures to prevent cattle access to headwater streams where access points can be many in order to manage local habitat quality and downstream water quality issues. 相似文献
58.
Xiaopo Wu Yangming Shi Weibo Meng Xiaofei Ma Nian Fang 《International Journal of Satellite Communications and Networking》2019,37(3):283-291
Electromagnetic signal emitted by satellite communication (satcom) transmitters are used to identify specific individual uplink satcom terminals sharing the common transponder in real environment, which is known as specific emitter identification (SEI) that allows for early indications and warning (I&W) of the targets carrying satcom furnishment and furthermore the real time electromagnetic situation awareness in military operations. In this paper, the authors are the first to propose the identification of specific transmitters of satcom by using probabilistic neural networks (PNN) to reach the goal of target recognition. We have been devoted to the examination by exploring the feasibility of utilizing the Hilbert transform to signal preprocessing, applying the discrete wavelet transform to feature extraction, and employing the PNN to perform the classification of stationary signals. There are a total of 1000 sampling time series with binary phase shift keying (BPSK) modulation originated by five types of satcom transmitters in the test. The established PNNs classifier implements the data testing and finally yields satisfactory accuracy at 8 dB(±1 dB) carrier to noise ratio, which indicates the feasibility of our method, and even the keen insight of its application in military. 相似文献
59.
60.
Chuangang Yao Jixing Yang Haixia Zhang Sigeng Chen Jian Meng Kedi Cai 《Ceramics International》2021,47(17):24589-24596
Bismuth doped La2-xBixNiO4+δ (x = 0, 0.02 and 0.04) oxides are investigated as SOFC cathodes. The effects of Bi doping on the phase structure, thermal expansion, electrical conduction behavior as well as electrochemical performance are studied. All the samples exist as a tetragonal Ruddlesden-Popper structure. Bi-doped LBNO-0.02 and LBNO-0.04 have good chemical and thermal compatibility with LSGM electrolyte. The average TEC over 20–900°С was 13.4 × 10?6 and 14.2 × 10?6 K?1 for LBNO-0.02 and LBNO-0.04, respectively. The electrical conductivity was decreasing with the rise of Bi doping content. EIS measurement indicates Bi doping can decrease the ASR values. At 750 °C, the obtained ASR for LBNO-0.04 is 0.18 Ωcm2, which is 56% lower than that of the sample without Bi doping, suggesting Bi doping is beneficial to the electrochemical catalytic activity of LBNO cathodes. 相似文献