全文获取类型
收费全文 | 420253篇 |
免费 | 6125篇 |
国内免费 | 1405篇 |
专业分类
电工技术 | 7783篇 |
综合类 | 356篇 |
化学工业 | 63336篇 |
金属工艺 | 14404篇 |
机械仪表 | 12004篇 |
建筑科学 | 12162篇 |
矿业工程 | 899篇 |
能源动力 | 11380篇 |
轻工业 | 44397篇 |
水利工程 | 3389篇 |
石油天然气 | 1863篇 |
武器工业 | 13篇 |
无线电 | 55938篇 |
一般工业技术 | 77978篇 |
冶金工业 | 76109篇 |
原子能技术 | 5598篇 |
自动化技术 | 40174篇 |
出版年
2021年 | 2830篇 |
2019年 | 2601篇 |
2018年 | 4019篇 |
2017年 | 4081篇 |
2016年 | 4322篇 |
2015年 | 3580篇 |
2014年 | 5955篇 |
2013年 | 19828篇 |
2012年 | 10603篇 |
2011年 | 15255篇 |
2010年 | 11676篇 |
2009年 | 13268篇 |
2008年 | 14338篇 |
2007年 | 14652篇 |
2006年 | 13101篇 |
2005年 | 12252篇 |
2004年 | 11816篇 |
2003年 | 11499篇 |
2002年 | 11122篇 |
2001年 | 11245篇 |
2000年 | 10398篇 |
1999年 | 10874篇 |
1998年 | 24175篇 |
1997年 | 17729篇 |
1996年 | 14025篇 |
1995年 | 11080篇 |
1994年 | 9844篇 |
1993年 | 9434篇 |
1992年 | 7181篇 |
1991年 | 6847篇 |
1990年 | 6464篇 |
1989年 | 6157篇 |
1988年 | 5897篇 |
1987年 | 4940篇 |
1986年 | 4885篇 |
1985年 | 6026篇 |
1984年 | 5649篇 |
1983年 | 4887篇 |
1982年 | 4534篇 |
1981年 | 4479篇 |
1980年 | 4241篇 |
1979年 | 4111篇 |
1978年 | 3804篇 |
1977年 | 4574篇 |
1976年 | 6121篇 |
1975年 | 3138篇 |
1974年 | 3028篇 |
1973年 | 2901篇 |
1972年 | 2324篇 |
1971年 | 2009篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The variation of current density with bias or temperature is examined for DNA molecules of different configuration. To this end, the DNA molecule is represented as an equivalent electrical network whose behavior is then simulated with PSPICE. The results are found to be in close agreement with ones obtained within a physical model. It is established that the electrical response of a DNA molecule to an applied electric field depends on the boundary conditions and the potential profile along the molecule. This finding should contribute to the creation of a complete library of DNA-molecule configurations with prescribed electrical properties. 相似文献
72.
This paper provides an overview of the main aspects of modern fluorescence microscopy. It covers the principles of fluorescence and highlights the key discoveries in the history of fluorescence microscopy. The paper also discusses the optics of fluorescence microscopes and examines the various types of detectors. It also discusses the signal and image processing challenges in fluorescence microscopy and highlights some of the present developments and future trends in the field. 相似文献
73.
Dynamics of liquid-filled spacecraft 总被引:1,自引:0,他引:1
A method is presented for simulating coupled liquid-solid dynamics. An important example of a coupled liquid-solid system is a satellite carrying fuel. The dynamics of the satellite and the onboard fuel influence each other, which may lead to satellite motion that is uncontrollable. For better understanding of the complex dynamics of coupled systems, a numerical model is developed. The model consists of two parts. The first part that solves the liquid motion is only briefly discussed here. The focus in this paper is on the way in which the dynamics of the liquid and the solid body are coupled. For this, the governing equations are presented in which terms appear that represent the force and torque on the solid body due to the sloshing liquid. The governing equations are rewritten such that the discrete approximation of these equations can be integrated in a stable manner for arbitrary liquid/solid mass ratios. Results are presented demonstrating the stability of the present model. A grid-refinement study and a time-step analysis are performed. Finally, the flat-spin motion of a satellite, partially filled with liquid, that flew in 1992 as part of the Wet Satellite Model experiment is studied. Results from the simulation are compared with the actual flight data. 相似文献
74.
Scale Up from Small Oven-Drying Tests of Mineral Concentrate to Pilot-Scale Drying with a Heated Pad
While Fickian diffusion models are commonly used in other applications, there are few reports of them being applied to the batch drying of a mineral concentrate. Diffusion coefficients estimated from small-scale oven-drying tests were used to predict the drying behavior of a concentrate sample 1 m × 1 m in area and 50 cm deep, with a heated bottom pad. These pilot-scale tests included both daily turning of the sample and turning every three days. The excellent quantitative agreement between the predicted and observed pilot-scale behavior gives a high level of confidence in the model predictions and suggests that a Fickian diffusion model is adequate to predict the behavior of mineral concentrates at the low moisture contents used here. 相似文献
75.
Examination of the wreckage of a light aircraft revealed that approximately 20 cm was missing from one tip of the aluminum
alloy propeller. Fractographic and metallographic examination of the remaining portion of the propeller revealed extensive
grain-boundary separation in the vicinity of the fracture, and grain edges and corners rounded by corrosion on the fracture
surface. Energy dispersive X-ray analysis (EDXA) revealed fluorine on, and in the vicinity of, the fracture surface. In the
ensuing litigation, it was asserted that the crash occurred because the propeller fractured in flight as the result of intergranular
attack caused by the use of a fluorine-bearing cleaner. 相似文献
76.
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
77.
Parameterisation of slant-Haar transforms 总被引:1,自引:0,他引:1
Agaian S. Tourshan K. Noonan J.P. 《Vision, Image and Signal Processing, IEE Proceedings -》2003,150(5):11
A parameterisation of the slant-Haar transform is presented, which includes an existing version of the slant-Haar transform. An efficient algorithm for the slant-Haar transform is developed and its computational complexity is estimated. The parametric slant-Haar transforms are compared to the Karhunen-Loeve transform. The parametric slant-Haar is shown to perform better than the commonly used slant-Haar and slant-Hadamard transforms for the first-order Markov model and also performs better than the discrete cosine transform for images approximated by the generalised image correlation model 相似文献
78.
J. B. Quinn G. E. Schumacher L. W. Schultheis 《Journal of Failure Analysis and Prevention》2004,4(1):41-46
Several days after heart surgery, a patient discovered his upper right canine tooth had broken at the root. Such tooth damage,
recognized post-operatively, is usually assumed to be caused by blunt mechanical force from an instrument used by the anesthesiologist
during placement of a breathing tube at the start of surgery.
In this case, the patient had saved the crown portion of the broken tooth, and it was possible to examine the root fracture
characteristics. The curvature and direction of the crack path and natural tooth situation suggested that failure could be
described through a cantilever beam model. This was confirmed when a whole extracted sample tooth was embedded and broken
by a measured force in a manner consistent with the model. The resulting fracture surface matched that of the patient’s broken
canine tooth. However, the high load and force direction necessary to fracture the root was inconsistent with forces applied
during the anesthesia procedure. The failure analysis and further investigation indicated tooth clenching on the breathing
tube during recovery was the likely cause of fracture.
This paper presents an alternate explanation for intubation-related dental injury, demonstrates the practicality of fractographic
analysis of biological materials, and introduces a methodology for simulating in vitro tooth settings for mechanical testing. 相似文献
79.
80.
M. Riad Manaa Laurence E. Fried Evan J. Reed 《Journal of Computer-Aided Materials Design》2003,10(2):75-97
In this article, we review recent atomistic computational techniques to study the electronic structure aspects and chemistry
of energetic materials at high-pressure and/or high temperature. While several mechanisms have been proposed for the initial
events of energetic materials at high-pressure, we explore the validity of a proposed shear-induced local metallization via
molecular bond bending in the insensitive explosive TATB. We study the effect of high-stress (both uniform and uniaxial) on
the electronic energy band-gap and the first chemical event of a prototypical energetic material, that of nitromethane. We
also determine chemical reactions rate laws and decomposition mechanisms from a quantum-based molecular dynamics simulation
of HMX, a widely used explosive material, at conditions of high density and temperature similar to that encounter under detonation.
Finally, we review a new multi-scale computational tool recently developed to model the shock-induced chemistry of energetic
materials at the atomistic level, and report its applicability to shocked solid nitromethane.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献