全文获取类型
收费全文 | 71661篇 |
免费 | 7351篇 |
国内免费 | 2569篇 |
专业分类
电工技术 | 5937篇 |
技术理论 | 2篇 |
综合类 | 5573篇 |
化学工业 | 12403篇 |
金属工艺 | 5690篇 |
机械仪表 | 3267篇 |
建筑科学 | 8414篇 |
矿业工程 | 1588篇 |
能源动力 | 10790篇 |
轻工业 | 2943篇 |
水利工程 | 3916篇 |
石油天然气 | 2957篇 |
武器工业 | 413篇 |
无线电 | 2774篇 |
一般工业技术 | 6560篇 |
冶金工业 | 3795篇 |
原子能技术 | 1523篇 |
自动化技术 | 3036篇 |
出版年
2024年 | 238篇 |
2023年 | 945篇 |
2022年 | 1954篇 |
2021年 | 2232篇 |
2020年 | 2342篇 |
2019年 | 1772篇 |
2018年 | 1572篇 |
2017年 | 1870篇 |
2016年 | 2246篇 |
2015年 | 2341篇 |
2014年 | 4531篇 |
2013年 | 4227篇 |
2012年 | 5368篇 |
2011年 | 5570篇 |
2010年 | 4173篇 |
2009年 | 4242篇 |
2008年 | 3498篇 |
2007年 | 4641篇 |
2006年 | 4121篇 |
2005年 | 3607篇 |
2004年 | 3288篇 |
2003年 | 2831篇 |
2002年 | 2535篇 |
2001年 | 2038篇 |
2000年 | 1728篇 |
1999年 | 1402篇 |
1998年 | 1114篇 |
1997年 | 961篇 |
1996年 | 791篇 |
1995年 | 679篇 |
1994年 | 550篇 |
1993年 | 443篇 |
1992年 | 368篇 |
1991年 | 243篇 |
1990年 | 187篇 |
1989年 | 197篇 |
1988年 | 132篇 |
1987年 | 101篇 |
1986年 | 86篇 |
1985年 | 67篇 |
1984年 | 98篇 |
1983年 | 66篇 |
1982年 | 62篇 |
1981年 | 16篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1959年 | 30篇 |
1951年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
《International Journal of Hydrogen Energy》2020,45(57):32856-32864
The electrode ionomer is a key factor that significantly affects the catalyst layer morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-based electrode ionomers with polymers of various molecular weights and alcohol/water mixtures were prepared, and those comprising the alcohol/water mixture showed a higher performance than the ones prepared using higher boiling solvents, such as dimethylacetamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the sulfonation degree was investigated. Because the chain length of polymer varies with molecular weight and chain entanglement degree, its molecular weight affects the electrode morphology. As the ionomer covered the catalyst, the agglomerates formed were of different morphologies according to their molecular weight, which could be deduced indirectly through dynamic light scattering and scanning electron microscopy. Additionally, the fuel cell performance was confirmed in the current-voltage curve. 相似文献
32.
The new layered niobate Cu0.5Nb3O8 is synthesized by soft chemistry in aqueous electrolyte via Cu2+→H+ exchange between copper nitrate and HNb3O8·H2O. The characterization of the exchanged product is made by means of thermal gravimetry, chemical analysis, X-ray diffraction and IR spectroscopy. Thermal analysis shows a conversion to anhydrous compound above 500 °C. The oxide displays a semiconductor like behavior; the thermal variation of the conductivity shows that d electrons are strongly localized and the conduction is thermally activated with activation energy of 0.13 eV. The temperature dependence of the thermopower is indicative of an extrinsic conductivity; the electrons are dominant carriers in conformity with an anodic photocurrent. Indeed, the Mott–Schottky plot confirms n-type conduction from which a flat band potential of −0.82 VSCE, an electronic density of 8.72×1019 m−3 and a depletion width of 4.4 nm are determined. The upper valence band, located at ~5.8 eV below vacuum is made up predominantly of Cu2+: 3d with a small admixture of O2−: 2p orbitals whereas the conduction band consists of empty Nb5+: 5s level. The energy band diagram shows the feasibility of the oxide for the photocatalytic hydrogen production upon visible light (29 mW cm−2) with a rate evolution of 0.31 mL g−1 min−1. 相似文献
33.
不同折流板结构螺旋折流板换热器传热性能的比较(英文) 总被引:1,自引:0,他引:1
Numerical simulations were performed on flow and heat transfer performances of heat exchangers having six helical baffles of different baffle shapes and assembly configurations, i.e., two trisection baffle schemes, two quadrant baffle schemes, and two continuous helical baffle schemes. The temperature contour or the pressure contour and velocity contour plots with superimposed velocity vectors on meridian, transverse and unfolded concentric hexagonal slices are presented to obtain a full angular view. For the six helix baffled heat exchangers, the different patterns of the single vortex secondary flow and the shortcut leakage flow were depicted as wel as the heat transfer properties were compared. The results show that the optimum scheme among the six configurations is a circumferential overlap trisection helix baffled heat exchanger with a baffle incline angle of 20° (20°TCO) scheme with an anti-shortcut baffle structure, which exhibits the second highest pressure dropΔpo, the highest overal heat transfer coefficient K, shel-side heat transfer coefficient ho and shel-side average comprehensive index ho/Δpo. 相似文献
34.
36.
Communication between organizations is formalized as process choreographies in daily business. While the correct ordering of exchanged messages can be modeled and enacted with current choreography techniques, no approach exists to describe and automate the exchange of data between processes in a choreography using messages. This paper describes an entirely model-driven approach for BPMN introducing a few concepts that suffice to model data retrieval, data transformation, message exchange, and correlation – four aspects of data exchange. For automation, this work utilizes a recent concept to enact data dependencies in internal processes. We present a modeling guideline to derive local process models from a given choreography; their operational semantics allows to correctly enact the entire choreography from the derived models only including the exchange of data. Targeting on successful interactions, we discuss means to ensure correct process choreography modeling. Finally, we implemented our approach by extending the camunda BPM platform with our approach and show its feasibility by realizing all service interaction patterns using only model-based concepts. 相似文献
37.
Social media-based screening is a well-known practice to both recruiters and job seekers. Little is known, however, about how job seekers present themselves on social media, i.e. ‘self-disclosure’, for employment purposes. This study builds on the theories of hyperpersonal computer-mediated communication, self-efficacy and social exchange to examine job seekers’ professional online image concerns, social media self-efficacy, and perceptions of social media effectiveness in the job search as predictors of inappropriate and career-oriented self-disclosures on these media. Findings from a sample of 3374 Italian respondents showed that career-oriented self-disclosure was predicted by all three factors, whereas inappropriate self-disclosure was only predicted by social media self-efficacy. Furthermore, the relationship between professional online image concerns and inappropriate self-disclosure was moderated by age, education and work experience, but not by gender. Theoretical and practical implications are discussed, and directions for future research are suggested. 相似文献
38.
39.
Tomoya Suzuki Kento Shiota Yu-ichiro Izato Masahiro Komori Koichi Sato Yasuyuki Takai Takayuki Ninomiya Atsumi Miyake 《International Journal of Hydrogen Energy》2021,46(11):8329-8343
Although hydrogen refueling stations (HRSs) are becoming widespread across Japan and are essential for the operation of fuel cell vehicles, they present potential hazards. A large number of accidents such as explosions or fires have been reported, rendering it necessary to conduct a number of qualitative and quantitative risk assessments for HRSs. Current safety codes and technical standards related to Japanese HRSs have been established based on the results of a qualitative risk assessment and quantitative effectiveness validation of safety measures over ten years ago. In the last decade, there has been much development in the technologies of the components or facilities used in domestic HRSs and much operational experience as well as knowledge to use hydrogen in HRSs safely have been gained through years of commercial operation. The purpose of the present study is to conduct a quantitative risk assessment (QRA) of the latest HRS model representing Japanese HRSs with the most current information and to identify the most significant scenarios that pose the greatest risks to the physical surroundings in the HRS model. The results of the QRA show that the risk contours of 10?3 and 10?4 per year were confined within the HRS boundaries, whereas the risk contours of 10?5 and 10?6 per year are still present outside the HRS. Comparing the breakdown of the individual risks (IRs) at the risk ranking points, we conclude that the risk of jet fire demonstrates the highest contribution to the risks at all of the risk ranking points and outside the station. To reduce these risks and confine the risk contour of 10?6 per year within the HRS boundaries, it is necessary to consider risk mitigation measures for jet fires. 相似文献
40.
Guangbo Zeng Dongqing Zhang Liuming Yan Baohua Yue Ting Pan Yidong Hu Shufa He Hongbin Zhao Jiujun Zhang 《International Journal of Hydrogen Energy》2021,46(39):20664-20677
Side-chain optimized poly (2,6-dimethyl-1,4-phenylene oxide)-g-poly (styrene sulfonic acid) (PPO-g-PSSA) is designed with balanced water-resistance and sulfonation degree. The PPO-g-PSSA is synthesized by controlled atom-transfer radical polymerization (ATRP) from brominated poly (2,6-dimethyl-1,4-phenylene oxide) (PPO-xBr) and ethyl styrene-4-sulfonate and followed by hydrolysis. A series of PPO-g-PSSA are prepared possessing different bromination degree (x) of PPO-xBr and polymerization degree (m) of the side-chains and the water-resistances of the fabricated membranes are investigated. The results show that a PPO-g-PSSA at relatively low x (x < 0.2) and high m (m > 4) exhibits good balance between the water-resistance and the sulfonation degree. Namely, it displays suitable proton conductivity with compromised water-resistance. Moreover, a maximum ion exchange capacity (IEC) of 3.24 mmol g?1 is reached without the sacrifice of water-resistance. In addition, PPO-g-0.08PSSA-13 and PPO-g-0.14PSSA-4 are chosen characterized by thermogravimetric analysis, proton conductivities and mechanical properties. At 90% RH, the optimized PPO-g-0.08PPSA-13 possesses a proton conductivity of 37.9 mS cm?1 at 40 °C and 45.5 mS cm?1 at 95 °C, respectively. 相似文献