全文获取类型
收费全文 | 28584篇 |
免费 | 2179篇 |
国内免费 | 1892篇 |
专业分类
电工技术 | 2360篇 |
技术理论 | 9篇 |
综合类 | 2978篇 |
化学工业 | 4363篇 |
金属工艺 | 1985篇 |
机械仪表 | 622篇 |
建筑科学 | 3614篇 |
矿业工程 | 1265篇 |
能源动力 | 1218篇 |
轻工业 | 2342篇 |
水利工程 | 942篇 |
石油天然气 | 1075篇 |
武器工业 | 109篇 |
无线电 | 1549篇 |
一般工业技术 | 5223篇 |
冶金工业 | 1414篇 |
原子能技术 | 184篇 |
自动化技术 | 1403篇 |
出版年
2024年 | 98篇 |
2023年 | 364篇 |
2022年 | 767篇 |
2021年 | 921篇 |
2020年 | 927篇 |
2019年 | 863篇 |
2018年 | 691篇 |
2017年 | 846篇 |
2016年 | 926篇 |
2015年 | 978篇 |
2014年 | 1673篇 |
2013年 | 1769篇 |
2012年 | 1789篇 |
2011年 | 2065篇 |
2010年 | 1690篇 |
2009年 | 1686篇 |
2008年 | 1591篇 |
2007年 | 1728篇 |
2006年 | 1571篇 |
2005年 | 1385篇 |
2004年 | 1266篇 |
2003年 | 1062篇 |
2002年 | 987篇 |
2001年 | 704篇 |
2000年 | 749篇 |
1999年 | 557篇 |
1998年 | 476篇 |
1997年 | 400篇 |
1996年 | 345篇 |
1995年 | 256篇 |
1994年 | 226篇 |
1993年 | 172篇 |
1992年 | 196篇 |
1991年 | 188篇 |
1990年 | 158篇 |
1989年 | 104篇 |
1988年 | 88篇 |
1987年 | 44篇 |
1986年 | 47篇 |
1985年 | 50篇 |
1984年 | 61篇 |
1983年 | 33篇 |
1982年 | 27篇 |
1981年 | 21篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1977年 | 13篇 |
1976年 | 8篇 |
1975年 | 11篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
991.
H. Liu D.‐G. Shang Z.‐K. Guo Y.‐G. Zhao J.‐Z. Liu 《Fatigue & Fracture of Engineering Materials & Structures》2014,37(8):937-944
Fatigue crack growth behaviours in different welding zones of laser beam welded specimens were investigated using central crack tension specimens for 6156 aluminium alloy under constant amplitude loading at nominal applied stress ratio R = 0.5, 0.06, ?1. The experimental results showed that base metal (BM) exhibited superior fatigue crack resistance compared to weld metal (WM) and heat‐affected zone (HAZ). Crack growth resistance of WM was the lowest. The exponent m values for BM and HAZ at different stress ratios are close and around 2.6, while m for WM at different stress ratio is around 4.7. The discrepancy between crack growth rates for WM and BM is more evident with increasing stress ratio, while it is a little change for HAZ and BM. Change of the microstructure in WM deteriorates the resistance of fatigue crack growth compared to BM. It was mainly due to grain boundary liquation and dissolving of second‐phase particles in the weld region. It was also found that the variety of fatigue crack resistance for different welding zones is in conformity with the change of hardness. BM with the highest hardness exhibited the maximum resistance for fatigue crack, and WM with the lowest hardness exhibited the minimum fatigue crack resistance. 相似文献
992.
Jiaqi Li Xiaohui Chen Chengzhi Xu Kemei Wei 《Energy Sources, Part A: Recovery, Utilization, and Environmental Effects》2016,38(7):898-904
In this study, two continuous processes for biodiesel production, alkali catalyzed process and solid acid catalyzed process, are discussed with the help of Aspen Plus. By comparing some economic indicators, it can be found that solid acid catalysis could be a good choice for investors in the near future. 相似文献
993.
L. P. Pook 《Fatigue & Fracture of Engineering Materials & Structures》2013,36(8):699-723
This review is a brief survey of three‐dimensional effects at cracks and sharp notches. The overall aim is to review developments over the past 50 years leading up to the current state of the art. The review is restricted to linear elastic, homogeneous, isotropic materials, with any yielding confined to a small region at a crack or notch tip. It is also restricted to static loading and to constant amplitude fatigue loading. An enormous amount of theoretical and experimental information relevant to three‐dimensional effects has been published in the past five decades, so the review is, of necessity, highly selective. Theoretical topics covered are linear elastic fracture mechanics, including Volterra distorsioni, stress intensity factors, corner point singularities, crack front line tension, displacement analysis of cracks and notches, and through thickness distributions of stresses and stress intensity factors. Crack path topics covered are fatigue crack path constraints, determination of fatigue crack paths, oscillating crack fronts in thin sheets and the transition to slant crack propagation in thin sheets. Plane strain fracture toughness testing, including standards, is covered. Overall, it can be concluded that the existence of three‐dimensional effects at cracks and sharp notches has been known for many years, but understanding has been limited, and for some situations still is. Understanding improved when the existence of corner point singularities and their implications became known. Increasingly powerful computers made it possible to investigate three‐dimensional effects numerically in detail. Despite increased understanding, three‐dimensional effects are sometimes ignored in situations where they may be important. 相似文献
994.
《Journal of the European Ceramic Society》2017,37(14):4347-4350
Subcritical crack growth is a phenomenon which limits service time of a ceramic material. It is especially prevalent for oxides, because this phenomenon is attributed to the activity of water at the crack tip of the material and can be caused even by water present as a humidity in the air (Salem and Jenkins, 2002; Michalske and Freiman, 1983). It is very important to determine its lifetime at the setting loads with high probabilities of survival. The Constant Stress Rate method gives results that are sufficient for lifetime predictions. Estimations are based on n parameter which results from the slope of the strength vs. stress rate dependence. Only a conversion from dynamic to static conditions has to be done (Wojteczko et al., 2016). The attempts were made at different stress rates on sintered samples with pre-existing flaws and in two environments − air and water. Tetragonal zirconia was the tested material. Biaxial loading method was used for strength measurements. Microstructural and fractographic observations were made using the scanning electron microscope. 相似文献
995.
Future bottlenecks in international river basins: where transboundary institutions,population growth and hydrological variability intersect 总被引:1,自引:0,他引:1
Using global data, this article examines the nexus of transboundary flood events and future social vulnerability. Which international river basins are forecast to experience an increase in both hydrological variability and population in the future, but currently lack institutional provisions to deal with these shared events? Concentrations of elevated risk are found in several basins in Central Asia, Central America and Central Africa. The article ends by highlighting transboundary basins that merit further investigation and possibly additional institution building to reduce urban flood risk. 相似文献
996.
The combined effects of different overload ratios(OLRs)and nitriding process on fatigue crack growth(FCG)were analyzed.Steel compact tension specimens made of AISI 4140 steel were subjected to the plasma nitrification process using agas mixture of 50% N2 and 50% H2 at 500℃for 2h.Structural properties of the samples were studied by scanning electron microscopy,optical microscopy,microhardness testing and digital imaging.Plasma nitriding slowed down FCG,but the crack growth retardation caused by the overload was reduced when the OLR was 2.0and 2.5.Nitriding did not take effect when the OLR was 1.5and 3.0. 相似文献
997.
《Energy Policy》2014
This paper examines the impact of natural gas consumption, real gross fixed capital formation and trade on the real GDP in the case of Tunisia over the period 1980–2010. We use an Autoregressive Distributed Lag (ARDL) bounds testing approach to test for cointegration between the variables. The Toda–Yamamoto approach is then used to test for causality. Our findings indicate the existence of a long-term relationship between the variables. Natural gas consumption, real gross fixed capital formation and trade add in economic growth. Natural gas consumption, real gross fixed capital formation and real trade cause real GDP in Tunisia. These findings open up new insights for policymakers to formulate a comprehensive energy policy to sustain economic growth in the long-term. 相似文献
998.
通过对PLC系统、密度测量系统、密度控制系统的详细介绍,阐明了TD-20密度控制系统的的组成、功能及理论依据,系统PLC采用西门子S7 300系列,CPU选用适于中等程序处理量的314型号,控制程序采用PID控制理论,通过阿姆德尔同位素密度计在线测量悬浮液密度。该系统在鸿源选煤厂应用后,悬浮液密度控制精度达到了0.5%,实现了对悬浮液密度高精度控制的同时,优化了其它各选煤工艺参数,确保了产品的灰分及回收率等指标,实现了良好的经济效益。 相似文献
999.
北京建工集团创新强企,与清华大学联手,成立了北京市首个集产、学、研于一体的污染场地修复联合研究中心。充分利用了企业与学校的各自强项,优势互补。实践表明,校企结合是企业创新经营模式的有益尝试,为企业带来了主营业务经济效益的提高、社会影响力的提升以及企业创新能力的升华。 相似文献
1000.
The traditional view that the role of the composite template that is adopted in the preparation of nano-TiO2 is only for forming a pore structure and increasing the material's specific surface area is challenged by this study. The complex impact of the composite template and La-doping on the microdynamic behavior of photo-generation free charge carriers (FCCs) in the mesoporous nano-TiO2 is investigated using the transient photovoltaic (TPV) technique, supplemented by electric field-induced surface photovoltaic spectroscopy and a computer simulation method. The experimental results reveal that utilizing an appropriate composite template, such as polyethylene glycol and octadecylamine with a molar ratio of 1:1, may result in the rapid separation and prolonged diffusion distance of electron–hole pairs that were excited by a 355 nm and 50 μJ laser pulse. These may be responsible for the stronger and broader TPV response in the microsecond and millisecond regions of the La-doped nano-TiO2 in TPV spectroscopy, as compared with that for materials that used other composite templates. This response is closely related to the reduced content of the surface state located at 367 nm. A suitable level of La-doping, however, was only responsible for the stronger and broader TPV response in the microsecond region of the spectrum of the nano-TiO2. The computer simulation results confirm that the photo-generation FCCs microdynamic characteristics in the microsecond region may partially originate from the state density distribution of both the d-electron in the antibonding orbital and the p-electron in the bonding orbital moving up to higher levels, and from a broader band-gap after La-doping in the anatase lattice cell. 相似文献