全文获取类型
收费全文 | 3063篇 |
免费 | 288篇 |
国内免费 | 116篇 |
专业分类
电工技术 | 176篇 |
综合类 | 189篇 |
化学工业 | 539篇 |
金属工艺 | 177篇 |
机械仪表 | 137篇 |
建筑科学 | 225篇 |
矿业工程 | 81篇 |
能源动力 | 64篇 |
轻工业 | 163篇 |
水利工程 | 63篇 |
石油天然气 | 201篇 |
武器工业 | 12篇 |
无线电 | 445篇 |
一般工业技术 | 407篇 |
冶金工业 | 180篇 |
原子能技术 | 41篇 |
自动化技术 | 367篇 |
出版年
2024年 | 13篇 |
2023年 | 47篇 |
2022年 | 91篇 |
2021年 | 118篇 |
2020年 | 87篇 |
2019年 | 68篇 |
2018年 | 96篇 |
2017年 | 106篇 |
2016年 | 103篇 |
2015年 | 125篇 |
2014年 | 127篇 |
2013年 | 189篇 |
2012年 | 193篇 |
2011年 | 210篇 |
2010年 | 153篇 |
2009年 | 147篇 |
2008年 | 173篇 |
2007年 | 143篇 |
2006年 | 153篇 |
2005年 | 149篇 |
2004年 | 108篇 |
2003年 | 90篇 |
2002年 | 69篇 |
2001年 | 83篇 |
2000年 | 75篇 |
1999年 | 89篇 |
1998年 | 76篇 |
1997年 | 53篇 |
1996年 | 74篇 |
1995年 | 42篇 |
1994年 | 34篇 |
1993年 | 36篇 |
1992年 | 36篇 |
1991年 | 24篇 |
1990年 | 26篇 |
1989年 | 14篇 |
1988年 | 7篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1976年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有3467条查询结果,搜索用时 15 毫秒
71.
72.
Sen Zeng Guoqi Ni Xiaojun Kuang 《Journal of Materials Science: Materials in Electronics》2016,27(10):10078-10081
High-Q dielectric materials ilmenite MgTiO3, columbite MgNb2O6 and cubic perovskite Ba3NiTa2O9 with negative temperature coefficient of resonant frequency (τ f ) were selected as candidates for compensating the τ f of hexagonal perovskite Ba8ZnTa6O24. X-ray diffraction data shows that Ba8ZnTa6O24 coexists with Ba3NiTa2O9 but is not compatible with MgTiO3 and MgNb2O6 at high temperature. The τ f for the mixed hexagonal/cubic perovskite Ba8ZnTa6O24–Ba3NiTa2O9 system is tunable via the temperature compensation effect and its quality factor may be improved via annealing the ceramics at high temperature to enhance the cation ordering in the cubic component. Permittivity ε r ~ 22–25, Q×f > 30,000 GHz and tunable τ f within ±10 ppm/°C were achieved in the range of about 50–80 wt% Ba3NiTa2O9 for the hexagonal/cubic perovskite composite Ba8ZnTa6O24–Ba3NiTa2O9 ceramics, which is suitable for the application as dielectric resonators and filters. 相似文献
73.
Daojin Zhou Zhao Cai Yongmin Bi Weiliang Tian Ma Luo Qian Zhang Qian Zhang Qixian Xie Jindi Wang Yaping Li Yun Kuang Xue Duan Michal Bajdich Samira Siahrostami Xiaoming Sun 《Nano Research》2018,11(3):1358-1368
Nickel-iron layered double hydroxide (NiFe-LDH) nanosheets have shown optimal oxygen evolution reaction (OER) performance; however, the role of the intercalated ions in the OER activity remains unclear. In this work, we show that the activity of the NiFe-LDHs can be tailored by the intercalated anions with different redox potentials. The intercalation of anions with low redox potential (high reducing ability), such as hypophosphites, leads to NiFe-LDHs with low OER overpotential of 240 mV and a small Tafel slope of 36.9 mV/dec, whereas NiFe-LDHs intercalated with anions of high redox potential (low reducing ability), such as fluorion, show a high overpotential of 370 mV and a Tafel slope of 80.8 mV/dec. The OER activity shows a surprising linear correlation with the standard redox potential. Density functional theory calculations and X-ray photoelectron spectroscopy analysis indicate that the intercalated anions alter the electronic structure of metal atoms which exposed at the surface. Anions with low standard redox potential and strong reducing ability transfer more electrons to the hydroxide layers. This increases the electron density of the surface metal sites and stabilizes their high-valence states, whose formation is known as the critical step prior to the OER process. 相似文献
74.
Qixian Xie Zhao Cai Pengsong Li Daojin Zhou Yongmin Bi Xuya Xiong Enyuan Hu Yaping Li Yun Kuang Xiaoming Sun 《Nano Research》2018,11(9):4524-4534
Atomic composition tuning and defect engineering are effective strategies toenhance the catalytic performance of multicomponent catalysts by improvingthe synergetic effect; however, it remains challenging to dramatically tune the active sites on multicomponent materials through simultaneous defect engineeringat the atomic scale because of the similarities of the local environment. Herein,using the oxygen evolution reaction (OER) as a probe reaction, we deliberatelyintroduced base-soluble Zn(II) or Al(III) sites into NiFe layered double hydroxides(LDHs), which are one of the best OER catalysts. Then, the Zn(II) or Al(III) siteswere selectively etched to create atomic M(II)/M(III) defects, which dramaticallyenhanced the OER activity. At a current density of 20 mA·cm?2, only 200 mV overpotential was required to generate M(II) defect-rich NiFe LDHs, which is the best NiFe-based OER catalyst reported to date. Density functional theory(DFT) calculations revealed that the creation of dangling Ni–Fe sites (i.e., unsaturated coordinated Ni–Fe sites) by defect engineering of a Ni–O–Fe site at the atomic scale efficiently lowers the Gibbs free energy of the oxygen evolutionprocess. This defect engineering strategy provides new insights into catalysts atthe atomic scale and should be beneficial for the design of a variety of catalysts. 相似文献
75.
结合三峡工程实际,对坝内埋管结构进行了非线性有限元分析,着重研究了不同管顶混凝土厚度和配筋对管道结构应力和强度的影响.计算结果表明,增大管顶混凝土厚度,可以明显提高坝内埋管的抗裂安全,确保混凝土大坝的整体稳定性,但是配筋多少对管周混凝土的抗裂能力影响较小. 相似文献
76.
We present a non-intrusive molecular dye based method, i.e., laser-induced fluorescence photobleaching anemometer (LIFPA),
to significantly increase temporal resolution (TR) for velocity measurement of fast transient electrokinetic flows. To our
knowledge, the TR has been for the first time achieved to 5–10 μs, about 100 times better than that published from state-of-the-art
micro particle image velocimetry (μPIV), which is currently the most widely used velocimetry in the microfluidics community.
The new method provides us with new opportunities to study experimentally the fundamental phenomena of unsteady electrokinetics
(EK) and to validate relevant theoretical models. One application of the new method is demonstrated by measuring the rise
time of DC electroosmotic flows (EOFs) in a microcapillary of 10 μm in diameter. 相似文献
77.
入侵检测系统是在网络系统遭受攻击之前进行拦截和响应的一种积极主动的安全防护系统。分布式智能入侵检测系统DIIDS(Distributed Intelligent Intrusion Detection System)就是在分布式环境中将人工智能技术应用于入侵检测中,从而可以提高IDS(Intrusion Detection System)的检测的实时性、准确性和容错性。本文设计了一种基于免疫赦免机制的DIIDS。该方法以降低入侵检测系统的时延性与提高入侵检测系统的智能性、恢复能力为核心,改善入侵检测系统的实时性。为实现用户信息的安全转移和检测系统自身的免疫进化,构建了一种免疫赦免智能Agent植入机制(IPAT),从而提高了系统的可用性。 相似文献
78.
This paper presents an automatic stock portfolio selection system. In the proposed approach, 53 financial indices are collected
for each stock item and are consolidated into six financial ratios [Grey relational grades (GRGs)] using a Grey relational
analysis model. The GRGs are processed using a modified form of the PBMF index method (designated as the Huang index function)
to determine the optimal number of clusters per GRG. The resulting cluster indices are then processed using rough set theory
to identify the stocks within the lower approximate sets. Finally, the GRGs of each stock item in the lower approximate sets
are consolidated into a single GRG, indicating the ability of the stock item to maximize the rate of return. It is demonstrated
that the proposed stock selection mechanism yields a higher rate of return than several existing portfolio selection systems. 相似文献
79.
通过吡啶-3-甲酸与双三苄基氧化锡反应,合成三苄基锡吡啶-3-羧酸酯.经IR、1H NMR和元素分析表征.并用X-射线单晶衍射仪测定其晶体结构,属单斜晶系,空间群P2(1)/c,晶胞参数为:a=1.06160(7)nm,b=1.46940(10)nm,c=1.60558(11)nm,β=106.4830(10)°,Z=4,V=2.401.6(3)nm3,Dc=1.422 g·cm-3,F(000)=1040,μ(Mo Kα)=1.086 mm-1.R1=0.0247,ωR2=0.0544.中心锡原子与周围原子形成五配位三角双锥结构,N与Sn之间的弱作用形成一维链配聚合物.经分子结构的量子化学计算,探讨化合物的稳定性、分子轨道能量、原子净电荷布居规律以及一些前沿分子轨道的组成特征. 相似文献
80.
苯与羰基钼相互作用的密度泛函研究 总被引:1,自引:1,他引:0
用密度泛函理论在B3LYP/LANL2DZ基组水平上自由优化(η~x-C_6H_6)Mo(CO)_n(x=1-6;n=1-5)复合物体系的可能构型及计算相互作用能,探索不同羰基数对复合物稳定性、苯和羰基钼相互作用的影响,并分析苯和羰基钼相互作用的NBO。结论(1)苯以η~6与Mo(CO)_n(n=1-3)配位形成的复合物比较稳定,但η~6配位复合物CO的个数越多,则越不稳定;(2)复合物1、2、3和10中,Mo(CO)_n与苯的相互作用拉动电荷由苯的π键电子向Mo(CO)_n的σ_(Mo-CO)~*键转移,而在复合物7中,苯的π键电子向Mo(CO)_n中Mo的孤对电子轨道d转移。 相似文献