全文获取类型
收费全文 | 271256篇 |
免费 | 3458篇 |
国内免费 | 858篇 |
专业分类
电工技术 | 5274篇 |
综合类 | 346篇 |
化学工业 | 38895篇 |
金属工艺 | 10481篇 |
机械仪表 | 8722篇 |
建筑科学 | 6538篇 |
矿业工程 | 1095篇 |
能源动力 | 6595篇 |
轻工业 | 24208篇 |
水利工程 | 2654篇 |
石油天然气 | 3934篇 |
武器工业 | 68篇 |
无线电 | 35828篇 |
一般工业技术 | 51648篇 |
冶金工业 | 49295篇 |
原子能技术 | 5559篇 |
自动化技术 | 24432篇 |
出版年
2021年 | 2208篇 |
2019年 | 2185篇 |
2018年 | 3654篇 |
2017年 | 3718篇 |
2016年 | 3946篇 |
2015年 | 2446篇 |
2014年 | 4291篇 |
2013年 | 11822篇 |
2012年 | 6954篇 |
2011年 | 9365篇 |
2010年 | 7383篇 |
2009年 | 8232篇 |
2008年 | 9130篇 |
2007年 | 9027篇 |
2006年 | 8156篇 |
2005年 | 7486篇 |
2004年 | 7140篇 |
2003年 | 6995篇 |
2002年 | 6604篇 |
2001年 | 6707篇 |
2000年 | 6405篇 |
1999年 | 6524篇 |
1998年 | 15099篇 |
1997年 | 10816篇 |
1996年 | 8512篇 |
1995年 | 6709篇 |
1994年 | 5960篇 |
1993年 | 5842篇 |
1992年 | 4686篇 |
1991年 | 4322篇 |
1990年 | 4165篇 |
1989年 | 3908篇 |
1988年 | 3744篇 |
1987年 | 3259篇 |
1986年 | 3164篇 |
1985年 | 3742篇 |
1984年 | 3481篇 |
1983年 | 3112篇 |
1982年 | 2918篇 |
1981年 | 3038篇 |
1980年 | 2840篇 |
1979年 | 2732篇 |
1978年 | 2541篇 |
1977年 | 3042篇 |
1976年 | 3699篇 |
1975年 | 2363篇 |
1974年 | 2343篇 |
1973年 | 2368篇 |
1972年 | 1878篇 |
1971年 | 1771篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Ng G.I. Pavlidis D. Tutt M. Oh J.-E. Bhattacharya P.K. 《Electron Device Letters, IEEE》1989,10(3):114-116
The DC and microwave properties of strained In0.65Ga 0.35As/In052Al0.48As HEMTs (high electron-mobility transistors) with double-heterojunction design are presented. The high sheet carrier density and good carrier confinement give rise to excellent device performance with very low output conductance. For 1×150-μm2 long-gate HEMTs, the measured cutoff frequency f T and maximum frequency of oscillation f max are as high as 37 and 66 GHz, respectively 相似文献
962.
D. Le Masne 《Geothermics》1985,14(5-6)
Within the framework of an EEC project involving ten European laboratories, research on the application of several electric and electromagnetic methods to a geothermal field (Travale, Tuscany), has been undertaken by the B.R.G.M. The objective was to refine the conceptual model of the Travale field and, therefore, to describe the morphology of the basement (down to 2 km depth) and of the Rhaetian reservoir covered by impermeable series.The method best suited to this type of prospection appeared to be electrical dipole — dipole profiling, combined with a 2-D interpretation. Contrasts at more than 2 km depth (500 m long dipoles) could be seen and compared to the depths measured in the boreholes. The EM Melos method (Syscal equipment) should be improved (lower frequencies) to increase the depth penetration. It is, however, a good complementary method for the surface layers. An attempt at a computational interpretation of the self potential profiles gave promising results. To make full advantage of the bipole — dipole measurements efforts should be devoted to their computational interpretation. 相似文献
963.
A first-order field theory for electromagnetic waves in moving ferrites and ferrite thin films is presented. The dominant effect of the motion is found to be the Doppler-shifted frequency observed in the moving frame. This gives rise to an anomalously large shift in wavenumber, due to the dispersive nature of the ferrite medium. Because of the large effect, it is suggested that a moving-medium experiment using magnetostatic waves could be used to distinguish between various competing forms for the dispersion term in the Fresnel-Fizeau coefficient. The results of the field theory are discussed with relation to relative and absolute rotation rate sensing. The author describes how magnetostatic waves could be used to measure relative rotation rates if confined to propagate around the perimeter of a rotating disk. Since the phase shift would be established in the time required to propagate around the disk, the response time could be significantly shorter than conventional tachometers. An experiment with counterpropagating magnetostatic waves is suggested to clarify the effect of a magnetic medium on the magnitude of the Sagnac effect 相似文献
964.
965.
Britain's new offshore oil industry might have generated much additional employment especially in the building, manning and supply of rigs. But there was a failure at first to take proper advantage; little was done to train people in new skills. Consequently the oil companies brought in equipment and skilled personnel from abroad. Eventually efforts were made to improve matters and the paper examines the steps taken to bring more of the employment benefits to Britain, but finds only partial success. 相似文献
966.
The throttling of the working fluid flow at the inlet and exhaust port openings of multi-vane expanders leads to power losses. Therefore, a mathematical model has been developed to analyse their breathing characteristics. In particular two existing designs of multi-vane expanders, using refrigerant 113 as the working fluid, were considered. The results obtained indicate that multi-vane expanders exhibit easy-breathing behaviours. Improved breathing characteristics can be achieved by the wise selection of operating conditions for any existing design or by optimising the design parameters for a specific application. 相似文献
967.
Stresses and strains near a rapidly propagating crack tip are affected by the mass density of the material. This paper starts with a brief summary of analytical results for near-tip dynamic fields as predicted by linear elastic fracture mechanics. Next, exact expressions are derived for dynamic crack-line strains, for mode-III crack propagation in a nonlinear elastic material and in an elastic perfectly-plastic material. These expressions are valid on the crack line from the moving crack tip to the moving boundary with the region of linearly elastic deformation. For steady-state crack growth, a critical strain criterion is used to compute the relation between external load and crack tip speed. The required external load increases with crack-tip speed. 相似文献
968.
Application-level quality of service (QoS) is the Achilles' heel of services offered overthe Internet. The articles in this special issue cover various aspects of this complex problem, while exposing the challenges we have yet to overcome. 相似文献
969.
Shear sensitivity measurements with variation of ingredients in a synthetic sludge can be related to biosolids' structural properties and particulate amount. The shear sensitivity is primarily controlled by the gel fraction, with alginate and the associated cations increasing the gel-like behavior. However the effect of particulate fraction on shear sensitivity should also be taken into account. Shear sensitivity is thus a complex parameter including two types of response: the resistance to dispersed mass concentration into centrate, and the maintenance of desired strength while the solids concentration increases in the shear field. 相似文献
970.