全文获取类型
收费全文 | 84866篇 |
免费 | 11217篇 |
国内免费 | 5127篇 |
专业分类
电工技术 | 8875篇 |
技术理论 | 2篇 |
综合类 | 6847篇 |
化学工业 | 13688篇 |
金属工艺 | 2676篇 |
机械仪表 | 7126篇 |
建筑科学 | 4143篇 |
矿业工程 | 17008篇 |
能源动力 | 4012篇 |
轻工业 | 1733篇 |
水利工程 | 1443篇 |
石油天然气 | 2131篇 |
武器工业 | 988篇 |
无线电 | 7171篇 |
一般工业技术 | 7432篇 |
冶金工业 | 3692篇 |
原子能技术 | 1281篇 |
自动化技术 | 10962篇 |
出版年
2024年 | 233篇 |
2023年 | 847篇 |
2022年 | 2022篇 |
2021年 | 2652篇 |
2020年 | 3032篇 |
2019年 | 2105篇 |
2018年 | 2171篇 |
2017年 | 2729篇 |
2016年 | 3278篇 |
2015年 | 3736篇 |
2014年 | 5891篇 |
2013年 | 5477篇 |
2012年 | 7681篇 |
2011年 | 7162篇 |
2010年 | 5240篇 |
2009年 | 5284篇 |
2008年 | 5054篇 |
2007年 | 5764篇 |
2006年 | 4995篇 |
2005年 | 4128篇 |
2004年 | 3428篇 |
2003年 | 2997篇 |
2002年 | 2397篇 |
2001年 | 2233篇 |
2000年 | 1845篇 |
1999年 | 1541篇 |
1998年 | 1141篇 |
1997年 | 1169篇 |
1996年 | 881篇 |
1995年 | 716篇 |
1994年 | 620篇 |
1993年 | 449篇 |
1992年 | 365篇 |
1991年 | 223篇 |
1990年 | 211篇 |
1989年 | 148篇 |
1988年 | 131篇 |
1987年 | 108篇 |
1986年 | 60篇 |
1985年 | 232篇 |
1984年 | 273篇 |
1983年 | 199篇 |
1982年 | 179篇 |
1981年 | 17篇 |
1980年 | 15篇 |
1979年 | 20篇 |
1977年 | 8篇 |
1959年 | 17篇 |
1955年 | 7篇 |
1951年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Accurate and timely network traffic measurement is essential for network status monitoring, network fault analysis, network intrusion detection, and network security management. With the rapid development of the network, massive network traffic brings severe challenges to network traffic measurement. However, existing measurement methods suffer from many limitations for effectively recording and accurately analyzing big-volume traffic. Recently, sketches, a family of probabilistic data structures that employ hashing technology for summarizing traffic data, have been widely used to solve these problems. However, current literature still lacks a thorough review on sketch-based traffic measurement methods to offer a comprehensive insight on how to apply sketches for fulfilling various traffic measurement tasks. In this paper, we provide a detailed and comprehensive review on the applications of sketches in network traffic measurement. To this end, we classify the network traffic measurement tasks into four categories based on the target of traffic measurement, namely cardinality estimation, flow size estimation, change anomaly detection, and persistent spreader identification. First, we briefly introduce these four types of traffic measurement tasks and discuss the advantages of applying sketches. Then, we propose a series of requirements with regard to the applications of sketches in network traffic measurement. After that, we perform a fine-grained classification for each sketch-based measurement category according to the technologies applied on sketches. During the review, we evaluate the performance, advantages and disadvantages of current sketch-based traffic measurement methods based on the proposed requirements. Through the thorough review, we gain a number of valuable implications that can guide us to choose and design proper traffic measurement methods based on sketches. We also review a number of general sketches that are highly expected in modern network systems to simultaneously perform multiple traffic measurement tasks and discuss their performance based on the proposed requirements. Finally, through our serious review, we summarize a number of open issues and identify several promising research directions. 相似文献
3.
Today, utility meters for water are tested for measurement behavior at stable operating conditions at specified flow rates as part of the approval process. The measurement error that occurs during start and stop or when changing between flow rates may not be taken into account. In addition, there are new technologies whose measuring behavior under real-world conditions is only known to a limited extend. To take these facts into account, a new method has been developed and tested to determine the measurement behavior of water meters under dynamic load profiles as they occur in the real application. For this purpose, a test rig for flow rate measurement was extended by a cavitation nozzle apparatus and the generation of dynamic load profiles was validated. For the cavitation nozzles used, possible factors influencing the flow rate, such as temperature and purity of the water as well as the upstream pressure were investigated. Using different types of domestic water meters, the applicability of the dynamic test procedure was demonstrated and the measurement behavior of the meters was characterised. 相似文献
4.
《International Journal of Hydrogen Energy》2022,47(56):23762-23771
Limiting current density at different temperatures, backpressures, and balance gases can be used to separate molecular diffusion resistance, Knudsen diffusion resistance and local transport resistance of membrane electrode assembly (MEA). However, the measurement of limiting current density has no unified protocol. The diverse choices in the literature, either in the control of current or voltage or in the atmosphere like relative humidity and O2 concentrations, make it difficult to compare the results and identify the true bottleneck hindering the mass transport. In this work, the current-voltage curves obtained by current scanning/stepping and voltage scanning/stepping methods under dilute O2 of different concentrations and a wide range of relative humidity were measured and analyzed systematically. It is found that the voltage stepping method is superior to the other three ways of control for the reliable determination of the limiting current density. Aided with simultaneous electrochemical impedance spectroscopy measurement, the limiting current density can be determined with pinpoint accuracy. When the limiting current density is just used to qualitatively evaluate different MEA, the voltage scanning method can be used instead for its high time efficiency. The selection of the atmosphere also plays an important role in suppressing the distortion from excessive water and reducing the spurious contribution from proton conduction resistance. It is found that O2 concentrations at 0.5 vol% and relative humidity at 90% can give the best estimation of O2 transport resistance in membrane electrode assembly. 相似文献
5.
《Advanced Powder Technology》2022,33(12):103872
We investigate the transition evolution from the initial state with the random packing of the particles to the stable state in which successive avalanches exhibit consistent characteristics under the slumping regime. It is found that there exist three distinct stages in the transition evolution, considering the change of the volume fraction. The coordination number is almost unchanged during the transition evolution, which indicates the particle contact form is consistent in the three stages. The pause phenomena are discovered in some avalanches, and the probability of pause occurrence increases continuously in the three consecutive stages. We also explore the distribution of particles in the passive layer at the stable state. The particles in the middle region of the passive layer have the closest packing status, and the deeper the region is located, the later it reaches the stable state. 相似文献
6.
Michael Lommel Vera Froese Moritz Sieber Marvin Jentzsch Tim Bierewirtz Ümit Hasirci Tim Rese Josef Seefeldt Sebastian Schimek Ulrich Kertzscher Christian Oliver Paschereit 《Indoor air》2021,31(6):1860-1873
The SARS-CoV-2 pandemic has created a great demand for a better understanding of the spread of viruses in indoor environments. A novel measurement system consisting of one portable aerosol-emitting mannequin (emitter) and a number of portable aerosol-absorbing mannequins (recipients) was developed that can measure the spread of aerosols and droplets that potentially contain infectious viruses. The emission of the virus from a human is simulated by using tracer particles solved in water. The recipients inhale the aerosols and droplets and quantify the level of solved tracer particles in their artificial lungs simultaneously over time. The mobile system can be arranged in a large variety of spreading scenarios in indoor environments and allows for quantification of the infection probability due to airborne virus spreading. This study shows the accuracy of the new measurement system and its ability to compare aerosol reduction measures such as regular ventilation or the use of a room air purifier. 相似文献
7.
8.
Mitigating gibbsite particle cracking and breakage during industrial alumina production can increase the quality of smelter grade alumina product by reducing the ultrafine particle content. Therefore, it is essential to investigate the particle cracking during static calcination and the breakage of calcined gibbsite particles under external force. In this work, we investigated the impact of the calcination ramping rate and the crystallite size on gibbsite particle cracking during static calcination. A slow ramping rate and a large pristine crystallite size tend to increase particle cracking. Apart from the study of particle cracking behaviour, we also investigated the breakage of calcined gibbsite particle under external force. Cracks on the particle surface can initiate breakage within the crystallite and along the grain boundary under external force. The breakage within crystallite occurs as the cleavage of the crystallite, while the breakage along the grain boundary leads to the shedding of a whole crystallite. We further explored the factors influencing the strength of calcined gibbsite particles. With increasing calcination temperature, the strength of particle increases when gibbsite converts to boehmite, and then decreases when boehmite converts into amorphous alumina. Particles containing smaller crystallites and calcined with fast ramping rates exhibit higher resistance to breakage. 相似文献
10.
针对煤矿井下高瓦斯软煤顺层长钻孔排渣困难、成孔率低、施工困难等问题,通过数值模拟实验研究了井下深部软煤体变形破坏特征,分析了顺层长钻孔孔周松软煤体变形特征及应力变化,以揭示顺层长钻孔孔周松软煤体变形产渣规律。研究表明:深部高瓦斯软煤顺层钻孔孔周煤体的应力平衡临界条件破坏后将发生大体积突然垮落;钻孔水平最大变形位移为1.22mm,垂直方向最大变形位移为10.7mm;径向孔周煤体垂向变形呈现逐渐减小趋势,且垂向变形明显大于钻孔水平变形。在水平方向上,钻孔孔周煤体应力分布呈现先增大再逐渐减小的变化规律,径向距离对水平应力分布的影响逐渐减小;随着径向距离的增加,钻孔孔周煤体应力分布逐渐降低,钻孔孔壁处煤体的应力出现最大值,且垂直方向处应力值最大。 相似文献