全文获取类型
收费全文 | 44289篇 |
免费 | 4392篇 |
国内免费 | 2190篇 |
专业分类
电工技术 | 2753篇 |
技术理论 | 3篇 |
综合类 | 2981篇 |
化学工业 | 7278篇 |
金属工艺 | 2525篇 |
机械仪表 | 2724篇 |
建筑科学 | 3320篇 |
矿业工程 | 1280篇 |
能源动力 | 1253篇 |
轻工业 | 2904篇 |
水利工程 | 866篇 |
石油天然气 | 2945篇 |
武器工业 | 425篇 |
无线电 | 5310篇 |
一般工业技术 | 5159篇 |
冶金工业 | 2028篇 |
原子能技术 | 471篇 |
自动化技术 | 6646篇 |
出版年
2024年 | 267篇 |
2023年 | 904篇 |
2022年 | 1668篇 |
2021年 | 2271篇 |
2020年 | 1638篇 |
2019年 | 1240篇 |
2018年 | 1471篇 |
2017年 | 1543篇 |
2016年 | 1294篇 |
2015年 | 1838篇 |
2014年 | 2356篇 |
2013年 | 2684篇 |
2012年 | 3006篇 |
2011年 | 3219篇 |
2010年 | 2855篇 |
2009年 | 2429篇 |
2008年 | 2554篇 |
2007年 | 2281篇 |
2006年 | 2292篇 |
2005年 | 1950篇 |
2004年 | 1307篇 |
2003年 | 1107篇 |
2002年 | 1063篇 |
2001年 | 956篇 |
2000年 | 970篇 |
1999年 | 1118篇 |
1998年 | 854篇 |
1997年 | 675篇 |
1996年 | 639篇 |
1995年 | 528篇 |
1994年 | 443篇 |
1993年 | 280篇 |
1992年 | 261篇 |
1991年 | 192篇 |
1990年 | 144篇 |
1989年 | 111篇 |
1988年 | 89篇 |
1987年 | 58篇 |
1986年 | 48篇 |
1985年 | 37篇 |
1984年 | 33篇 |
1983年 | 28篇 |
1982年 | 24篇 |
1981年 | 25篇 |
1980年 | 26篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1973年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Initialization speed is one of the most important factors in network real time kinematic (NRTK) performance. Owing to the low correlation among the error s 相似文献
22.
23.
The phase shift characteristics reflect the state change of electromagnetic wave in plasma sheath and can be used to reveal deeply the action mechanism between electromagnetic wave and plasma sheath. In this paper, the phase shift characteristics of electromagnetic wave propagation in plasma were investigated. Firstly, the impact factors of phase shift including electron density,collision frequency and incident frequency were discussed. Then, the plasma with different electron density distribution profiles were employed to investigate the influence on the phase shift characteristics. In a real case, the plasma sheath around the hypersonic vehicle will affect and even break down the communication. Based on the hypersonic vehicle model, we studied the electromagnetic wave phase shift under different flight altitude, speed, and attack angle. The results indicate that the phase shift is inversely proportional to the flight altitude and positively proportional to the flight speed and attack angle. Our work provides a theoretical guidance for the further research of phase shift characteristics and parameters inversion in plasma. 相似文献
24.
Xiaohu Ren Xilou Pu Hongfeng Yin Yun Tang Hudie Yuan Huiqing Fan 《Ceramics International》2021,47(9):12122-12129
Hexagonal barium ferries is a promising and efficient microwave (MW) absorbing material, but the low dielectric loss and poor conductivity have limited their extensive applications. In this work, a simple tactic of coating conductive polymer PANI on hexaferrite BaCo2Fe16O27 is presented, wherein the dielectric properties are customized, and more significantly, the electromagnetic loss is greatly enhanced. As displayed from structural characterizations, PANI were coated equably on the surface of hexaferrite grains by an in-situ polymerization process. The outcomes exhibit the as-prepared PANI@hexaferrite composite has remarkable electromagnetic wave absorption capacity. When the thickness is 6.0 mm, the minimal RL of ?40.4 dB was achieved at 2.9 GHz. The effective absorption bandwidth (RL < ?20 dB) of 0.65 GHz, 0.53 GHz, 0.65 GHz, 0.52 GHz, 0.46 GHz and 0.39 GHz was achieved separately when the thickness ranges from 4 to 9 mm. The highly efficient MW absorbing performance of PANI@hexaferrite composite were the consequence of multiple loss mechanisms and perfect impedance matching. It is demonstrated that the PANI@hexaferrite composite with excellent MW absorption performance is expected to be potential MW absorbers for extensive applications. 相似文献
25.
For rechargeable wireless sensor networks, limited energy storage capacity, dynamic energy supply, low and dynamic duty cycles cause that it is unpractical to maintain a fixed routing path for packets delivery permanently from a source to destination in a distributed scenario. Therefore, before data delivery, a sensor has to update its waking schedule continuously and share them to its neighbors, which lead to high energy expenditure for reestablishing path links frequently and low efficiency of energy utilization for collecting packets. In this work, we propose the maximum data generation rate routing protocol based on data flow controlling technology. For a sensor, it does not share its waking schedule to its neighbors and cache any waking schedules of other sensors. Hence, the energy consumption for time synchronization, location information and waking schedule shared will be reduced significantly. The saving energy can be used for improving data collection rate. Simulation shows our scheme is efficient to improve packets generation rate in rechargeable wireless sensor networks. 相似文献
26.
Tong Le Fan Ronggui Yang Shuangchun Zhang Qiushi Pan Yi 《Chemistry and Technology of Fuels and Oils》2022,57(6):991-999
Chemistry and Technology of Fuels and Oils - Application of water flooding technologies in the oilfield causes intensive heterogeneity of the oil reservoir and improper circulation of the injected... 相似文献
27.
以转基因水稻中最常用的CaMV35S启动子、NOS终止子、Cry1Ab/Ac基因、HPT基因及SPS水稻内标基因为研究对象,利用5 种不同的荧光信号(FAM、HEX、Taxas Red、Cy5、Cy5.5)进行多重实时聚合酶链式反应(real-time polymerase chain reaction,real-time PCR)检测方法的研究。通过引物组合筛选、反应体系优化、特异性测试、灵敏度测试、适用性测试等一系列实验,建立了5 重real-time PCR方法,灵敏度可达0.032%。此方法具有灵敏度高、结果准确、通量大等优点,可实现水稻中转基因成分的快速、高效检测。 相似文献
28.
A novel AlSiMgAl2O4Al2O3 composite brick was prepared and evaluated in the low vessel of an RH (the initials of Ruhrstahl and Hereaeus) secondary refining furnace; it was characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The results show that after use, the AlSiMgAl2O4Al2O3 composite has a functional gradient with an erosion zone–reinforced zone–original zone phase distribution, in which the phases in the erosion zone (0–1.8?cm) are a Mg-hercynite spinel solid solution, α-Al2O3, and minor amount of Al3Fe5O12. Furthermore, the phases in the reinforced zone (1.8–5.0?cm) are γ-AlON, 21RSiAlON, SiC, Mg0.388Al2.408O4, and α-Al2O3; i.e., the Al and Si in the composite are completely converted into non-oxide reinforced phases. Finally, the phases in the original zone (>5.0?cm) show no change. The reaction mechanism is as follows. During operation, a Mg-hercynite spinel solid solution is formed in the erosion zone due to a reaction between MgAl2O4 and FeO from a refinery operation. Therefore, the slag erosion of the material is improved. The Al and Si metals undergo active oxidation, and 21RSiAlON flakes are subsequently formed from the products of the metastable Al2O(g), SiO(g), and N2(g) in the ambient. The γ-AlON is formed by a carbothermal reduction nitridation of the α-Al2O3 and residual active carbon from the resin binder. The 21RSiAlON and γ-AlON reinforce the composite brick and improve its high temperature performance accordingly. Its service life is 110% that of the magnesia-chrome bricks used in the same period. The reaction model was also established. 相似文献
29.
Zhuo Wang Haonan Chen Tian Wang Yujia Xiao Wenwen Nian Jiahao Fan 《Journal of the European Ceramic Society》2018,38(11):3847-3852
The appearance of colossal permittivity materials broadened the choice of materials for energy-storage applications. In this work, colossal permittivity in ceramics of TiO2 co-doped with niobium and europium ions ((Eu0.5Nb0.5)xTi1-xO2 ceramics) was reported. A large permittivity (εr ~ 2.01?×?105) and a low dielectric loss (tanδ ~ 0.095) were observed for (Eu0.5Nb0.5)xTi1-xO2 (x?=?1%) ceramics at 1?kHz. Moreover, two significant relaxations were observed in the temperature dependence of dielectric properties for (Eu, Nb) co-doped TiO2 ceramics, which originated from defect dipoles and electron hopping, respectively. The low dielectric loss and high relative permittivity were ascribed to the electron-pinned defect-dipoles and electrons hopping. The (Eu0.5Nb0.5)xTi1-xO2 ceramic with great colossal permittivity is one of the most promising candidates for high-energy density storage applications. 相似文献
30.