全文获取类型
收费全文 | 40539篇 |
免费 | 4934篇 |
国内免费 | 2395篇 |
专业分类
电工技术 | 12415篇 |
技术理论 | 1篇 |
综合类 | 3401篇 |
化学工业 | 2164篇 |
金属工艺 | 1479篇 |
机械仪表 | 3533篇 |
建筑科学 | 1977篇 |
矿业工程 | 1024篇 |
能源动力 | 1605篇 |
轻工业 | 1189篇 |
水利工程 | 1094篇 |
石油天然气 | 1559篇 |
武器工业 | 374篇 |
无线电 | 6978篇 |
一般工业技术 | 2752篇 |
冶金工业 | 1034篇 |
原子能技术 | 617篇 |
自动化技术 | 4672篇 |
出版年
2024年 | 104篇 |
2023年 | 566篇 |
2022年 | 1066篇 |
2021年 | 1423篇 |
2020年 | 1450篇 |
2019年 | 1110篇 |
2018年 | 1057篇 |
2017年 | 1410篇 |
2016年 | 1526篇 |
2015年 | 1775篇 |
2014年 | 2838篇 |
2013年 | 2705篇 |
2012年 | 3356篇 |
2011年 | 3531篇 |
2010年 | 2593篇 |
2009年 | 2568篇 |
2008年 | 2331篇 |
2007年 | 2771篇 |
2006年 | 2385篇 |
2005年 | 2027篇 |
2004年 | 1693篇 |
2003年 | 1330篇 |
2002年 | 1122篇 |
2001年 | 972篇 |
2000年 | 763篇 |
1999年 | 623篇 |
1998年 | 472篇 |
1997年 | 413篇 |
1996年 | 370篇 |
1995年 | 298篇 |
1994年 | 269篇 |
1993年 | 196篇 |
1992年 | 147篇 |
1991年 | 112篇 |
1990年 | 95篇 |
1989年 | 104篇 |
1988年 | 63篇 |
1987年 | 36篇 |
1986年 | 32篇 |
1985年 | 30篇 |
1984年 | 35篇 |
1983年 | 19篇 |
1982年 | 33篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1976年 | 3篇 |
1964年 | 3篇 |
1959年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Toshihisa Shimizu Keisuke Kakazu Koushi Takano Hitoshi Ishii 《Electrical Engineering in Japan》2015,192(4):51-63
Because of the improved performance of power devices, the volume of the ac filter inductors used in high‐frequency PWM inverters has been reduced. However, the temperature rise in the filter inductor due to this miniaturization has become more pronounced. Therefore, we have proposed an iron loss calculation method for the ac filter inductor. However, the accuracy of the value calculated via the loss map method cannot be verified, because the iron loss arising during each switching period cannot be measured with conventional power measuring instruments. In order to resolve this problem, we developed an inductor loss analyzer (ILA), which allows precise measurement of the iron loss in the inductor during each switching period. The accuracy of the calculation of iron loss in the filter inductor by the loss map method was verified with the ILA. We found that the value calculated by the loss map method differed slightly from the value measured with the ILA. However, these differences can be reduced if we take into account the accurate flux density calculation and the effect of the duty ratio of PWM pulses on the loss. Finally, we verified that the loss map method can provide accurate iron loss calculations. 相似文献
92.
This paper presents a fast distance relay for series compensated transmission lines based on the R–L differential-equation algorithm using the theory of equal transfer process of transmission lines. The measuring distances based on the proposed algorithm can fast approach the actual value of fault distance when a fault occurs in front of the series capacitor. When a fault occurs behind of the series capacitor, the fault loop, including the series capacitor, does not match the R–L transmission line model, so the measuring distances fluctuate severely. Based on this, the relative position of the fault with respect to the series capacitor can be judged effectively according to the fluctuation range of the measuring distances, and the accurate fault location can be obtained fast. A variety of PSCAD/EMTDC simulation tests show that the new relay has fast operating speed and high accuracy when applied to the long series compensated transmission lines. 相似文献
93.
94.
本文通过分析反恐视频监控的需求,研究了普通红外线摄像机和激光摄像机的性能、铁路通信传输网通道容量和视频占用的资源量,推荐反恐视频监控方案采用数字激光摄像机。 相似文献
95.
Adhesively bonded joints have been extensively employed in the aeronautical and automotive industries to join thin-layer materials for developing lightweight components. To strengthen the structural integrity of joints, it is critical to estimate and improve joint failure loads effectually. To accomplish the aforementioned purpose, this paper presents a novel deep neural network (DNN) model-enabled approach, and a single lap joint (SLJ) design is used to support research development and validation. The approach is innovative in the following aspects: (i) the DNN model is reinforced with a transfer learning (TL) mechanism to realise an adaptive prediction on a new SLJ design, and the requirement to re-create new training samples and re-train the DNN model from scratch for the design can be alleviated; (ii) a fruit fly optimisation (FFO) algorithm featured with the parallel computing capability is incorporated into the approach to efficiently optimise joint parameters based on joint failure load predictions. Case studies were developed to validate the effectiveness of the approach. Experimental results demonstrate that, with this approach, the number of datasets and the computational time required to re-train the DNN model for a new SLJ design were significantly reduced by 92.00% and 99.57% respectively, and the joint failure load was substantially increased by 9.96%. 相似文献
96.
WeiJia Luo LingXia Li Shihui Yu Qianyu Guo Bowen Zhang Zheng Sun 《Ceramics International》2018,44(11):12414-12419
Low-loss (Zn1-xNix)ZrNbTaO8 (0.02?≤?x?≤?0.10) ceramics possessing single wolframite structure are initiatively synthesized by solid-state route. Based on the results of Rietveld refinement, complex chemical bond theory is used to establish the correlation between structural characteristics and microwave performance in this ceramic system. A small amount of Ni2+ (x?=?0.06) in A-site with the fixed substitution of Ta5+ in B-site can effectually raise the Q?×?f value of ZnZrNb2O8 ceramic, embodying a dense microstructure and high lattice energy. The dielectric constant and τf are mainly affected by bond ionicity and the average octahedral distortion. The (Zn0.94Ni0.06)ZrNbTaO8 ceramic sample sintered at 1150?°C for 3?h exhibits an outstanding combination of microwave dielectric properties: εr =?27.88, Q?×?f?=?128,951?GHz, τf =?–39.9?ppm/°C. Thus, it is considered to be a candidate material for the communication device applications at high frequency. 相似文献
97.
Bingcheng Luo Xiaohui Wang Enke Tian Hongzhou Song Qiancheng Zhao Ziming Cai Wei Feng Longtu Li 《Journal of the European Ceramic Society》2018,38(4):1562-1568
Fe doped BaTiO3 ceramics with giant permittivity and low dielectric loss were synthesized in N2/H2 atmosphere started with BaTiO3 powders and iron powders. XRD analysis exhibited the tetragonal-pseudocubic phase transition when the Fe content is 3 mol%. XPS spectra confirmed the iron oxides with mixed-valence structure of Fe2+/Fe3+, while Ti-ions maintain Ti4+3d0 states without any oxidization-reduction. For the case of ceramics with 5 mol% Fe, the dielectric constant was 66,650 at 1000 Hz at room temperature, 19 times higher than that of pure BaTiO3 ceramics, while the dielectric loss tangent was 0.13. Comparison with other giant-permittivity materials demonstrated the superior potential of present ceramics. First-principles calculations investigated the interfacial interaction of Fe-[TiO2] interface and Fe-[BaO] interface. Giant dielectric constant was induced by the interfacial polarization between insulating ferroelectrics and semiconducting iron oxides with mixed-valence states, as well as the contribution from the generated electron hopping conduction. 相似文献
98.
Yang Luo Shi-li Zheng Shu-hua Ma Chun-li Liu Xiao-hui Wang 《Journal of the European Ceramic Society》2018,38(16):5282-5293
A novel mullite-bonded SiC-whisker-reinforced SiC matrix composite (SiCw/SiC, SiC whisker-to-SiC powder mass ratio of 1:9) was designed and successfully prepared. Before preparing the composite, the inexpensive lab-made SiCw was first modified by an oxidation/leaching process and then coated with Al2O3. The kinetics results indicate that the oxidation process can be described by improved shrinking-cylinder models. The aspect ratio of SiCw improved after modification. Subsequently, raw materials with a SiC–SiO2–Al2O3 triple-layered structure were obtained after the Al2O3-coating process and used as feedstocks during the subsequent hot-pressing sintering. Finally, the characterization of the composites indicates that the mullite-bonded sample performs better (relative density of 93.8?±?1.4%, flexural strength of 533.3?±?18.2?MPa, fracture toughness of 13.6?±?2.1?MPa?m1/2, and Vickers hardness of 20.6?±?2.5?GPa) than the reference sample without the mullite interface. The improved toughness could essentially be attributed to the moderately strong interface bonding and effective load transfer effects of the mullite interface. 相似文献
99.
100.