全文获取类型
收费全文 | 734655篇 |
免费 | 9359篇 |
国内免费 | 2365篇 |
专业分类
电工技术 | 13597篇 |
综合类 | 623篇 |
化学工业 | 110723篇 |
金属工艺 | 27386篇 |
机械仪表 | 22701篇 |
建筑科学 | 17997篇 |
矿业工程 | 2890篇 |
能源动力 | 20218篇 |
轻工业 | 63357篇 |
水利工程 | 6772篇 |
石油天然气 | 9390篇 |
武器工业 | 30篇 |
无线电 | 91217篇 |
一般工业技术 | 142249篇 |
冶金工业 | 138222篇 |
原子能技术 | 13472篇 |
自动化技术 | 65535篇 |
出版年
2021年 | 5977篇 |
2020年 | 4412篇 |
2019年 | 5686篇 |
2018年 | 9364篇 |
2017年 | 9438篇 |
2016年 | 10157篇 |
2015年 | 7050篇 |
2014年 | 11484篇 |
2013年 | 35513篇 |
2012年 | 19142篇 |
2011年 | 26592篇 |
2010年 | 20782篇 |
2009年 | 23651篇 |
2008年 | 24629篇 |
2007年 | 24644篇 |
2006年 | 22143篇 |
2005年 | 20079篇 |
2004年 | 19376篇 |
2003年 | 19033篇 |
2002年 | 18276篇 |
2001年 | 18152篇 |
2000年 | 16995篇 |
1999年 | 17930篇 |
1998年 | 43250篇 |
1997年 | 30846篇 |
1996年 | 23988篇 |
1995年 | 18317篇 |
1994年 | 16312篇 |
1993年 | 16024篇 |
1992年 | 11782篇 |
1991年 | 11173篇 |
1990年 | 10866篇 |
1989年 | 10324篇 |
1988年 | 9927篇 |
1987年 | 8549篇 |
1986年 | 8419篇 |
1985年 | 9899篇 |
1984年 | 9210篇 |
1983年 | 8133篇 |
1982年 | 7538篇 |
1981年 | 7586篇 |
1980年 | 7086篇 |
1979年 | 6883篇 |
1978年 | 6618篇 |
1977年 | 7795篇 |
1976年 | 10353篇 |
1975年 | 5613篇 |
1974年 | 5340篇 |
1973年 | 5295篇 |
1972年 | 4362篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Nonlinear dynamics of crack propagation are investigated experimentally and theoretically with the goal of clarifying the nature of limiting crack velocity, the transition from steady state to branching regimes of crack dynamics, and the dynamics of crack arrest. The theoretical explanation of limiting steady-state crack velocity and the transition to a branching regime was proposed due to the study of collective behavior of a microcrack ensemble at the crack tip area. The experimental study of crack dynamics was carried out in a preloaded plate PMMA specimen using the high-speed camera coupled with the photo-elasticity method, the point stress recording with a laser system, and the failure surface roughness measurement. 相似文献
62.
The Boundary Element Method (BEM) incorporating the Embedded Cell Approach (ECA) has been used to analyse the effects of constituent material properties, fibre spatial distribution and microcrack damage on the localised behaviour of transversely fractured, unidirectional fibre-reinforced composites. Three specific composites, i.e., glass fibre reinforced polyester, carbon fibre reinforced epoxy and a glass-carbon hybrid, are considered. The geometrical structures examined were perfectly periodic, uniformly spaced fibre arrangements in square and hexagonal embedded cells. In addition, numerical simulations were also conducted using embedded cells containing randomly distributed fibres. The models involve both elastic fibres and matrix, with the interfaces between the different phases being fully bonded. The results indicate that the constituent material properties (two phase composite) and spatial distribution have a significant effect on the localised stress distributions around the primary crack tip. However, the strain energy release rate associated with crack propagation is predominantly influenced by the material composition. The three-phase hybrid composite exhibited an apparent intermediate fracture toughness value, compared to the all-glass and all-carbon models. Furthermore, the strain energy release rate for the macrocrack lowers as it enters a zone of localised damage (microcracking). The presence of microcracks relaxes the stress field, which can result in a significant reduction in the energetics of the primary crack. 相似文献
63.
Yu. V. Baldokhin V. V. Vavilova P. Ya. Kolotyrkin Yu. K. Kovneristyi N. A. Palii A. S. Solomatin 《Inorganic Materials》2003,39(6):562-567
Mössbauer effect measurements and physicochemical analysis demonstrate that annealing of amorphous Fe–P–Mn alloys leads to the formation of a nanocrystalline structure. 相似文献
64.
Mantooth B.A. Weiss P.S. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2003,91(11):1785-1802
One of the ultimate miniaturizations in nanotechnology is molecular electronics, where devices will consist of individual molecules. There are many complications associated with the use of molecules in electronic devices, such as the electronic perturbations in the molecule associated with being bonded to an electrode, how electrons traverse the metal-molecule junction, and the difficulty of macroscopically addressing single to very few molecules. Whether fabricating a test structure or a usable device, the use of self-assembly is fundamental to the fabrication of molecular electronic devices. We will discuss how to fabricate self-assembled monolayers into test assemblies and how to use intermolecular interactions to direct molecules into desired positions to create nanostructures and to connect functional molecules to the outside world. These assemblies serve as test structures for measurements on single or bundled molecules. The development of several experimental techniques, including various scanning probes, mercury drop junctions, break junctions, nanopores, crossed wires, and other techniques using nanoparticles have enabled the ability to test these structures and make reproducible measurements on single molecules. Many of these methods have been developed to test molecules with potential for integration into devices such as oligo (phenylene-ethynylene) molecules and other /spl pi/-conjugated molecules, in ensemble or single-molecule measurements. 相似文献
65.
H. L. Du S. R. Rose Z. D. Xiang P. K. Datta X. Y. Li 《Materialwissenschaft und Werkstofftechnik》2003,34(4):421-426
The oxidation/sulphidation behaviour of a Ti‐46.7Al‐1.9W‐0.5Si alloy with a TiAl3 diffusion coating was studied in an environment of H2/H2S/H2O at 850oC. The kinetic results demonstrate that the TiAl3 coating significantly increased the high temperature corrosion resistance of Ti‐46.7Al‐1.9W‐0.5Si. The SEM, EDX, XRD and TEM analysis reveals that the formation of an Al2O3 scale on the surface of the TiAl3‐coated sample was responsible for the enhancement of the corroison resistance. The Ti‐46.7Al‐1.9W‐0.5Si alloy was also modified by Nb ion implantation. The Nb ion implanted and as received sampels were subjected to cyclic oxidation in an open air at 800oC. The Nb ion implantation not only increased the oxidation resistance but also substantially improved the adhesion of scale to the substrate. 相似文献
66.
67.
68.
This paper presents the evolution of low-frequency, high-power transcontinental transmission, starting from the first successful transatlantic link from Poldhu up to the Italian transcontinental station of Coltano. The trend toward higher and higher transmitting power, and the dead end into which wireless was heading if it were not for research in the higher frequency ranges, are pointed out. 相似文献
69.
This article presents a graph-theoretic method for constructing low-density parity-check (LDPC) codes from connected graphs without the requirement of large girth. This method is based on finding a set of paths in a connected graph, which satisfies the constraint that any two paths in the set are either disjoint or cross each other at one and only one vertex. Two trellis-based algorithms for finding these paths are devised. Good LDPC codes of practical lengths are constructed and they perform well with iterative decoding. 相似文献
70.