全文获取类型
收费全文 | 412740篇 |
免费 | 5817篇 |
国内免费 | 1386篇 |
专业分类
电工技术 | 7704篇 |
综合类 | 349篇 |
化学工业 | 61994篇 |
金属工艺 | 14285篇 |
机械仪表 | 11871篇 |
建筑科学 | 11637篇 |
矿业工程 | 869篇 |
能源动力 | 11117篇 |
轻工业 | 43797篇 |
水利工程 | 3312篇 |
石油天然气 | 1835篇 |
武器工业 | 13篇 |
无线电 | 55389篇 |
一般工业技术 | 76617篇 |
冶金工业 | 74953篇 |
原子能技术 | 5568篇 |
自动化技术 | 38633篇 |
出版年
2021年 | 2669篇 |
2019年 | 2434篇 |
2018年 | 3838篇 |
2017年 | 3904篇 |
2016年 | 4144篇 |
2015年 | 3426篇 |
2014年 | 5714篇 |
2013年 | 19282篇 |
2012年 | 10203篇 |
2011年 | 14681篇 |
2010年 | 11284篇 |
2009年 | 12884篇 |
2008年 | 13914篇 |
2007年 | 14285篇 |
2006年 | 12778篇 |
2005年 | 11962篇 |
2004年 | 11575篇 |
2003年 | 11279篇 |
2002年 | 10913篇 |
2001年 | 11131篇 |
2000年 | 10271篇 |
1999年 | 10769篇 |
1998年 | 24010篇 |
1997年 | 17587篇 |
1996年 | 13903篇 |
1995年 | 10979篇 |
1994年 | 9771篇 |
1993年 | 9355篇 |
1992年 | 7121篇 |
1991年 | 6790篇 |
1990年 | 6397篇 |
1989年 | 6106篇 |
1988年 | 5860篇 |
1987年 | 4884篇 |
1986年 | 4834篇 |
1985年 | 5978篇 |
1984年 | 5599篇 |
1983年 | 4832篇 |
1982年 | 4496篇 |
1981年 | 4448篇 |
1980年 | 4208篇 |
1979年 | 4080篇 |
1978年 | 3771篇 |
1977年 | 4548篇 |
1976年 | 6097篇 |
1975年 | 3115篇 |
1974年 | 3011篇 |
1973年 | 2890篇 |
1972年 | 2318篇 |
1971年 | 2003篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
841.
BACKGROUND: A diaphragmatic phonomyogram (PMG) evoked by maximal phrenic nerve stimulation at end expiratory lung volume (FRC) has been previously described as a good index of changes in diaphragmatic contractility with fatigue. A study was undertaken to assess whether this conclusion could be extended to different lung volumes. METHODS: Diaphragmatic compound motor action potentials (CMAPs) were recorded on each side of the chest by the means of surface electrodes placed over the eight intercostal spaces in five healthy subjects. Diaphragmatic PMGs from both sides were recorded with condenser microphones fixed to the skin close to the CMAP recording electrodes. Oesophageal and gastric balloon tipped catheters were employed to measure transdiaphragmatic pressure twitches (TwPDI) which served as the standard measure of changes in diaphragmatic contractility. PMG and TwPDI responses were compared at different lung volumes over inspiratory capacity both before and after fatiguing inspiratory resistive loading. RESULTS: No consistent relationship was found in different subjects or on different days in the same subject between PMG and lung volume or between PMG and TwPDI. However, the PMG:CMAP ratio from both sides at any given lung volume decreased after fatigue in roughly the same proportion as the TwPDI. CONCLUSIONS: These results show that, although PMG can detect changes in diaphragmatic contractility caused by fatigue in normal subjects, lung volume changes need to be controlled and each subject should serve as his or her own control. 相似文献
842.
Microfibre reinforced cement composites reinforced with high volume fractions of carbon, steel and polypropylene fibres were tested in uniaxial tension. Composites investigated included those with only one type of fibre (mono-fibre composites) and those with two or more types of fibres (hybrid-fibre composites) in the same mix. Considerable strengthening, toughening and stiffening of the host matrix due to microfibre reinforcement were observed. In the hybrid-fibre composites, different fibres appear to act as additive phases; i.e., they maintain their individual reinforcing capabilities. The composites were also impact tested in uniaxial tension using a newly designed instrumented impact machine. When compared with static test results, considerable sensitivity to stress rate was noted; composites were found to be stronger and tougher under impact and the improvements were more pronounced at higher fibre volume fractions. The potential of these composites for use in thin sheet products and other similar applications is recognized, and the need for continued research is stressed. 相似文献
843.
844.
Experiments were carried out in a two pack system; polyvinyl butyral, zinc tetroxychromate in one pack, and phosphoric acid in the other with an isopropanol solvent in both packs. This study examines the functions of the phosphoric acid and the chromate containing pigment separately by utilising the feature that the alcohol dispersed components have sufficient conductivity to carry out potential-time measurements in the wet components before and during mixing. 相似文献
845.
846.
R Gouider E LeGuern J Emile S Tardieu F Cabon M Samid J Weissenbach Y Agid P Bouche A Brice 《Canadian Metallurgical Quarterly》1994,44(12):2250-2252
Fistulas secondary to Crohn's disease occur in about 33% of patients. A colobronchial fistula complicating Crohn's disease is very rare, having been reported only twice previously. We present an unusual fistula secondary to Crohn's colitis that originated from the splenic flexure and crossed the diaphragm to involve the bronchial tree. 相似文献
847.
This paper traces the development of engineering technology from bifurcation to the present time. They assert that ET desires to be distinguished more than it seeks to be distinct. This analysis of the successes and failures of bifurcation lead to a differentiation between engineering technology's being distinct as a discipline and becoming distinguished in its own right. They offer some conjecture about the bright future of ET as a partner with science and engineering in an atmosphere of increased technological development but intense international competition. 相似文献
848.
Stanisic B.R. Verghese N.K. Rutenbar R.A. Carley L.R. Allstot D.J. 《Solid-State Circuits, IEEE Journal of》1994,29(3):226-238
This paper describes new techniques for the simulation and power distribution synthesis of mixed analog/digital integrated circuits considering the parasitic coupling of noise through the common substrate. By spatially discretizing a simplified form of Maxwell's equations, a three-dimensional linear mesh model of the substrate is developed. For simulation, a macromodel of the fine substrate mesh is formulated and a modified version of SPICE3 is used to simulate the electrical circuit coupled with the macromodel. For synthesis, a coarse substrate mesh, and interconnect models are used to couple linear macromodels of circuit functional blocks. Asymptotic Waveform Evaluation (AWE) is used to evaluate the electrical behavior of the network at every iteration in the synthesis process. Macromodel simulations are significantly faster than device level simulations and compare accurately to measured results. Synthesis results demonstrate the critical need to constrain substrate noise and simultaneously optimize power bus geometry and pad assignment to meet performance targets 相似文献
849.
Blumenthal D.J. Prucnal P.R. Sauer J.R. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1994,82(11):1650-1667
Photonic packet switches offer high speed, data rate and format transparency, and flexibility required by future computer communications and cell-based telecommunications networks. In this paper, we review experimental progress in state-of-the-art photonic packet switches with an emphasis on all-optical guided-wave systems. The term all-optical implies that the data portion of a packet remains in optical format from the source to the destination. While the data remain all-optical, both optical and optoelectronic techniques have been used to process packet routing functions based on extremely simple routing protocols. An overview of the design issues for all-optical photonic packet switching is given and contrasted with electronic packet switch implementations. Low-level functions that have been experimentally implemented include routing, contention resolution, synchronization, and header regeneration. System level demonstrations, including centralized photonic switching and distributed all-optical multihop networks, will be reviewed 相似文献
850.
Cook R.D. Saulnier G.J. Gisser D.G. Goble J.C. Newell J.C. Isaacson D. 《IEEE transactions on bio-medical engineering》1994,41(8):713-722
Presents the design, implementation, and performance of Rensselaer's third-generation adaptive current tomograph, ACT3. This system uses 32 current sources and 32 phase-sensitive voltmeters to make a 32-electrode system that is capable of applying arbitrary spatial patterns of current. The instrumentation provides 16 b precision on both the current values and the real and reactive voltage readings and can collect the data for a single image in 133 ms. Additionally, the instrument is able to automatically calibrate its voltmeters and current sources and adjust the current source output impedance under computer control. The major system components are discussed in detail and performance results are given. Images obtained using stationary agar targets and a moving pendulum in a phantom as well as in vivo resistivity profiles showing human respiration are shown 相似文献