全文获取类型
收费全文 | 55571篇 |
免费 | 6741篇 |
国内免费 | 4341篇 |
专业分类
电工技术 | 3596篇 |
技术理论 | 5篇 |
综合类 | 5637篇 |
化学工业 | 3681篇 |
金属工艺 | 991篇 |
机械仪表 | 1898篇 |
建筑科学 | 6451篇 |
矿业工程 | 2920篇 |
能源动力 | 1328篇 |
轻工业 | 976篇 |
水利工程 | 2654篇 |
石油天然气 | 3561篇 |
武器工业 | 316篇 |
无线电 | 8153篇 |
一般工业技术 | 3404篇 |
冶金工业 | 1361篇 |
原子能技术 | 277篇 |
自动化技术 | 19444篇 |
出版年
2024年 | 247篇 |
2023年 | 899篇 |
2022年 | 1434篇 |
2021年 | 1671篇 |
2020年 | 1925篇 |
2019年 | 1642篇 |
2018年 | 1518篇 |
2017年 | 1993篇 |
2016年 | 2177篇 |
2015年 | 2496篇 |
2014年 | 3679篇 |
2013年 | 3646篇 |
2012年 | 4133篇 |
2011年 | 4438篇 |
2010年 | 3427篇 |
2009年 | 3691篇 |
2008年 | 3674篇 |
2007年 | 3857篇 |
2006年 | 3251篇 |
2005年 | 2759篇 |
2004年 | 2320篇 |
2003年 | 2021篇 |
2002年 | 1769篇 |
2001年 | 1474篇 |
2000年 | 1242篇 |
1999年 | 1012篇 |
1998年 | 833篇 |
1997年 | 702篇 |
1996年 | 562篇 |
1995年 | 474篇 |
1994年 | 365篇 |
1993年 | 301篇 |
1992年 | 232篇 |
1991年 | 131篇 |
1990年 | 126篇 |
1989年 | 87篇 |
1988年 | 70篇 |
1987年 | 38篇 |
1986年 | 46篇 |
1985年 | 71篇 |
1984年 | 51篇 |
1983年 | 48篇 |
1982年 | 49篇 |
1981年 | 26篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1978年 | 3篇 |
1959年 | 4篇 |
1956年 | 3篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The motional transition and heterogeneity of semi‐interpenetrating networks (SIPNs) based on polyurethane (PU) with carboxylic groups and methacrylic copolymer (PM) with tertiary amine groups were studied by the electron spin resonance (ESR) spin probe method. The concentration of functional groups in both prepolymers varied from 0 to 0.45 mmol g?1. Spin‐probed SIPNs show that the temperature‐dependent spectra are sensitive to polymer interactions imposed by functional groups. These interactions determine the free volume distribution in the matrix and temperature at which motional transition takes place. The fraction of free volume increases with functional group concentration and reaches its maximum at 0.25 mmol g?1. Further increases in the functional group concentration reduce the free volume. The results of the networks with strong interactions are discussed in terms of the interference of the plasticizing effect of the PU component and the formation of possible cluster cross‐links, which restricts segmental motions. Copyright © 2003 Society of Chemical Industry 相似文献
82.
J. Gegner 《Materialwissenschaft und Werkstofftechnik》2003,34(3):290-297
Chemical Composition and Microstructure of Polymer‐Derived Glasses and Ceramics in the Si–C–O System. Part 2: Characterization of microstructure formation by means of high‐resolution transmission electron microscopy and selected area diffraction Liquid or solid silicone resins represent the economically most interesting class of organic precursors for the pyrolytic production of glass and ceramics materials on silicon basis. As dense, dimensionally stable components can be cost‐effectively achieved by admixing reactive filler powders, chemical composition and microstructure development of the polymer‐derived residues must be exactly known during thermal decomposition. Thus, in the present work, glasses and ceramics produced by pyrolysis of the model precursor polymethylsiloxane at temperatures from 525 to 1550 °C are investigated. In part 1, by means of analytical electron microscopy, the bonding state of silicon was determined on a nanometre scale and the phase separation of the metastable Si–C–O matrix into SiO2, C and SiC was proved. The in‐situ crystallization could be considerably accelerated by adding fine‐grained powder of inert fillers, such as Al2O3 or SiC, which permits effective process control. In part 2, the microstructure is characterized by high‐resolution transmission electron microscopy and selected area diffraction. Turbostratic carbon and cubic β‐SiC precipitate as crystallization products. Theses phases are embedded in an amorphous matrix. Inert fillers reduce the crystallization temperature by several hundred °C. In this case, the polymer‐derived Si–C–O material acts as a binding agent between the powder particles. Reaction layer formation does not occur. On the investigated pyrolysis conditions, no crystallization of SiO2 was observed. 相似文献
83.
Ichiro Hirosawa Tetsuo Honma Kazuo Kato Naoto Kijima Yasuo Shimomura 《Journal of the Society for Information Display》2004,12(3):269-273
Abstract— We studied the influence of annealing in air on doped europium in BaMgAl10O17 by performing x‐ray absorption fine‐structure measurements. We determined the oxidation of doped divalent europium by annealing in air at over 500°C. The interatomic distance between the europium and the surrounding oxygen atoms was compressed by oxidation. It also appears that the oxidation process of europium is determined by the diffusion of oxygen into BaMgAl10O17. 相似文献
84.
85.
P. J. C. Skitt M. A. Javed S. A. Sanders A. M. Higginson 《Journal of Intelligent Manufacturing》1993,4(1):79-94
The potential of using artificially simulated neural networks as intelligent, adaptive process-monitoring devices is discussed. The investigation is considered as a method for automatic, intelligent exception reporting for quality control applications. The technique is also compared with the conventional statistical approaches of principal component analysis and Kohonen's feature map. The applications of the technique in aerospace and manufacturing environments are presented and a possible extension of the method to incorporate a diagnostic function is discussed.Seconded from Cheltenham and Gloucester College of Higher Education as a Royal Society/SERC Research Fellow at Smith's Industries Aerospace and Defence Systems, Bishop's Cleeve, Cheltenham, UK. 相似文献
86.
无线局域网安全问题研究 总被引:10,自引:0,他引:10
为了提供相当于有线局域网的数据安全,IEEE802.11定义了有线等价保密(WEP)协议。然而,最近的研究发现WEP存在严重的缺陷。介绍了无线局域网存在的安全隐患,分析了WEP的结构以及WEP协议的缺陷以及可能遭受的攻击,并探讨了改进方案。 相似文献
87.
Ying Tan 《The Journal of VLSI Signal Processing》2002,32(1-2):45-54
In this paper, we propose a new approach for signal detection in wireless digital communications based on the neural network with transient chaos and time-varying gain (NNTCTG), and give a concrete model of the signal detector after appropriate transformations and mappings. It is well known that the problem of the maximum likelihood signal detection can be described as a complex optimization problem that has so many local optima that conventional Hopfield-type neural networks fail to solve. By refraining from the serious local optima problem of Hopfield-type neural networks, the NNTCTG makes use of the time-varying parameters of the recurrent neural network to control the evolving behavior of the network so that the network undergoes the transition from chaotic behavior to gradient convergence. It has richer and more flexible dynamics rather than conventional neural networks only with point attractors, so that it can be expected to have much ability to search for globally optimal or near-optimal solutions. After going through a transiently inverse-bifurcation process, the NNTCTG can approach the global optimum or the neighborhood of global optimum of our problem. Simulation experiments have been performed to show the effectiveness and validation of the proposed neural network based method for the signal detection in digital communications. 相似文献
88.
Noel E. Sharkey 《Robotics and Autonomous Systems》1997,22(3-4):345-359
It is time to locate connectionist representation theory in the new wave of robotics research. The utility of representations developed in artificial neural networks (ANNs) during learning has been demonstrated in cognitive science research since the 1980s. The research reported here puts learned representations to work in a decentered control task, the disembodied arm problem, in which a mobile robot operates an arm fixed to a table to pick up objects. There is no physical linkage between the arm and the robot and so the robot's point of view must be decentered. This is done by developing a modular Artificial Neural Net system in three stages: (i) a classifier net is trained with laser scan data to output transformationally invariant position classes; (ii) an arm net is trained for picking up objects; (iii) an inter net is trained to communicate and coordinate the sensing and acting. The completed system is shown to create new nonsymbolic transformationally invariant representations in order to perform the effective generalization of decentered viewpoints. 相似文献
89.
本文以系统节能和提高综合经济效益的观点,分析了金属换热器传热系数的影响因素及提高途径;气流阻力与动力消耗;传热面积与投资的定量关系。为正确选用不同结构金属换热器及其性能提供了依据。 相似文献
90.