全文获取类型
收费全文 | 35359篇 |
免费 | 4787篇 |
国内免费 | 2377篇 |
学科分类
工业技术 | 42523篇 |
出版年
2024年 | 181篇 |
2023年 | 795篇 |
2022年 | 1724篇 |
2021年 | 2330篇 |
2020年 | 1733篇 |
2019年 | 1281篇 |
2018年 | 1432篇 |
2017年 | 1721篇 |
2016年 | 1431篇 |
2015年 | 2128篇 |
2014年 | 2568篇 |
2013年 | 2843篇 |
2012年 | 3221篇 |
2011年 | 3202篇 |
2010年 | 2637篇 |
2009年 | 2236篇 |
2008年 | 2179篇 |
2007年 | 1849篇 |
2006年 | 1436篇 |
2005年 | 1143篇 |
2004年 | 786篇 |
2003年 | 624篇 |
2002年 | 617篇 |
2001年 | 517篇 |
2000年 | 364篇 |
1999年 | 252篇 |
1998年 | 212篇 |
1997年 | 191篇 |
1996年 | 157篇 |
1995年 | 136篇 |
1994年 | 118篇 |
1993年 | 86篇 |
1992年 | 71篇 |
1991年 | 42篇 |
1990年 | 36篇 |
1989年 | 46篇 |
1988年 | 27篇 |
1987年 | 18篇 |
1986年 | 24篇 |
1985年 | 15篇 |
1984年 | 9篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1973年 | 5篇 |
1951年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Bo Wang Edison Huixiang Ang Yang Yang Yufei Zhang Hongbo Geng Minghui Ye Cheng Chao Li 《Advanced functional materials》2020,30(28)
Orthorhombic molybdenum trioxide (MoO3) is one of the most promising anode materials for sodium‐ion batteries because of its rich chemistry associated with multiple valence states and intriguing layered structure. However, MoO3 still suffers from the low rate capability and poor cycle induced by pulverization during de/sodiation. An ingenious two‐step synthesis strategy to fine tune the layer structure of MoO3 targeting stable and fast sodium ionic diffusion channels is reported here. By integrating partially reduction and organic molecule intercalation methodologies, the interlayer spacing of MoO3 is remarkably enlarged to 10.40 Å and the layer structural integration are reinforced by dimercapto groups of bismuththiol molecules. Comprehensive characterizations and density functional theory calculations prove that the intercalated bismuththiol (DMcT) molecules substantially enhanced electronic conductivity and effectively shield the electrostatic interaction between Na+ and the MoO3 host by conjugated double bond, resulting in improved Na+ insertion/extraction kinetics. Benefiting from these features, the newly devised layered MoO3 electrode achieves excellent long‐term cycling stability and outstanding rate performance. These achievements are of vital significance for the preparation of sodium‐ion battery anode materials with high‐rate capability and long cycling life using intercalation chemistry. 相似文献
22.
Xi Chen Zhengkun Yu Chen Yang Yao Chen Chao Jin Yuejiao Ding Wenbin Li Zhongmin Wan 《International Journal of Hydrogen Energy》2021,46(19):11127-11139
Flow field structure can largely determine the output performance of Polymer electrolyte membrane fuel cell. Excellent channel configuration accelerates electrochemical reactions in the catalytic layer, effectively avoiding flooding on the cathode side. In present study, a three-dimensional, multi-phase model of PEMFC with a 3D wave flow channel is established. CFD method is applied to optimize the geometry constructions of three-dimensional wave flow channels. The results reveal that 3D wave flow channel is overall better than straight channel in promoting reactant gases transport, removing liquid water accumulated in microporous layer and avoiding thermal stress concentration in the membrane. Moreover, results show the optimal flow channel minimum depth and wave length of the 3D wave flow channel are 0.45 mm and 2 mm, respectively. Due to the periodic geometric characteristics of the wave channel, the convective mass transfer is introduced, improving gas flow rate in through-plane direction. Furthermore, when the cell output voltage is 0.4 V, the current density in the novel channel is 23.8% higher than that of conventional channel. 相似文献
23.
24.
This paper considers a novel distributed iterative learning consensus control algorithm based on neural networks for the control of heterogeneous nonlinear multiagent systems. The system's unknown nonlinear function is approximated by suitable neural networks; the approximation error is countered by a robust term in the control. Two types of control algorithms, both of which utilize distributed learning laws, are provided to achieve consensus. In the provided control algorithms, the desired reference is considered to be an unknown factor and then estimated using the associated learning laws. The consensus convergence is proven by the composite energy function method. A numerical simulation is ultimately presented to demonstrate the efficacy of the proposed control schemes. 相似文献
25.
We investigate the effects of doping and annealing on the dielectric properties of metal ions doped TiO2 ceramics. Colossal permittivity (CP) above 104 was observed in single Nb ion doped TiO2, which was dominated by electron transport related interfacial polarization. Moreover, the CP can be dropped to 120 when simultaneously introducing Mn ion into the sample. The disappearance of CP behaviors maybe due to the multivalence of Mn which would inhibit the reduction of Ti4+ to Ti3+, and thus reduce delocalized electrons. Interestingly, the CP was recovered for the (Nb+Mn) co-doped TiO2 after post-sintering heat treatment in N2 atmosphere. The recovery of CP in the sample after annealing can be ascribed to the semiconducting grain and the insulating grain boundary, according to impedance spectroscopy. We therefore believe that this work can help us understand the mechanism of CP from a new perspective. 相似文献
27.
28.
四川盆地作为国内页岩气开发的重点区域,通过近几年的不断攻关和实践,机械钻速得到了提高,但由于受地层复杂、可钻性差、非均质性强等地质因素的影响,导致井下钻柱系统不良振动剧烈,容易出现钻头损坏严重、钻速较低等问题,严重影响了钻井时效。为了解决上述难题,以该盆地涪陵工区上二叠统龙潭组—中二叠统茅口组为例,采用井下振动高频测量工具的实测手段,测量了钻头—钻柱系统的动态振动加速度参数,结合地层的岩性和矿物组分分析,研究钻头失效原因与对策,并开展了现场试验。研究结果表明:①在非均质地层中钻进的钻头—钻柱系统产生了大于40 m/s~2的高幅值瞬时冲击振动,高幅值的瞬时冲击是导致钻头先期失效的主要原因;②提出了抑制高幅值的瞬时冲击振动采用"减振+增压"工具组合和避免井下工具共振的钻井参数;③采用钻井新参数的试验井比邻井的高幅值瞬时振动降低了17%,单只钻头进尺增加24%,钻头工作环境得到了较大的改善,钻头使用数量减少。结论认为,该研究成果能够有效地改善钻头—钻柱系统的振动状态,有利于达成延长钻头使用寿命的目标。 相似文献
29.
回顾了异质结光晶体管(HPT)在近年来的重要进展,综合分析了HPT的工作原理以及影响其性能的主要参数。综述了不同材料制作的HPT的研究现状,得出了目前限制HPT发展的主要因素及当前应重点解决由于基区表面复合等效应导致的增益下降和由于结电容的充放电限制的响应带宽等问题的结论。 相似文献
30.