全文获取类型
收费全文 | 455289篇 |
免费 | 6185篇 |
国内免费 | 1947篇 |
专业分类
电工技术 | 8442篇 |
综合类 | 429篇 |
化学工业 | 66884篇 |
金属工艺 | 17171篇 |
机械仪表 | 12875篇 |
建筑科学 | 12048篇 |
矿业工程 | 959篇 |
能源动力 | 12438篇 |
轻工业 | 48031篇 |
水利工程 | 3551篇 |
石油天然气 | 3048篇 |
武器工业 | 8篇 |
无线电 | 58008篇 |
一般工业技术 | 82459篇 |
冶金工业 | 91089篇 |
原子能技术 | 6520篇 |
自动化技术 | 39461篇 |
出版年
2021年 | 3094篇 |
2020年 | 2260篇 |
2019年 | 2865篇 |
2018年 | 4473篇 |
2017年 | 4414篇 |
2016年 | 4923篇 |
2015年 | 3720篇 |
2014年 | 6197篇 |
2013年 | 21001篇 |
2012年 | 11043篇 |
2011年 | 15771篇 |
2010年 | 12463篇 |
2009年 | 14016篇 |
2008年 | 14885篇 |
2007年 | 14908篇 |
2006年 | 13837篇 |
2005年 | 12608篇 |
2004年 | 12160篇 |
2003年 | 12123篇 |
2002年 | 11609篇 |
2001年 | 12138篇 |
2000年 | 10921篇 |
1999年 | 12107篇 |
1998年 | 30687篇 |
1997年 | 21327篇 |
1996年 | 16260篇 |
1995年 | 12181篇 |
1994年 | 10731篇 |
1993年 | 10480篇 |
1992年 | 7418篇 |
1991年 | 7139篇 |
1990年 | 6589篇 |
1989年 | 6336篇 |
1988年 | 6226篇 |
1987年 | 5116篇 |
1986年 | 4997篇 |
1985年 | 6106篇 |
1984年 | 5479篇 |
1983年 | 4946篇 |
1982年 | 4594篇 |
1981年 | 4664篇 |
1980年 | 4342篇 |
1979年 | 4015篇 |
1978年 | 3834篇 |
1977年 | 4688篇 |
1976年 | 6520篇 |
1975年 | 3246篇 |
1974年 | 3060篇 |
1973年 | 3110篇 |
1972年 | 2441篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
R. C. Pennefather D. H. Boone 《International Journal of Pressure Vessels and Piping》1996,66(1-3):351-358
Cyclic oxidation tests were performed on a large variety of commercially available overlay coatings. The results of cyclic oxidation tests confirmed that the composition of the coating as well as the processing method of the coating can affect the life of the system. Coating life was determined by the time to coating penetration and initiation of substrate attack. As this work was part of a effort to evaluate the relative protectiveness of available coating systems for Industrial Gas Turbine applications at relatively high temperature, an extensive testing programme was undertaken. The different coatings were tested to visual failure. The results presented in this paper concentrate on the surface instability of the coating. Apart from usual oxidation of the coating an additional degradation mechanism was observed. A mechanical effect caused by the instability of the coating as a result of the difference in the thermal coefficient of expansion and mechanical properties between the substrate and coating. This effect, the so-called “rumpling” effect, resulted in a significant reduction in time to penetration and was in some systems the life controlling factor. 相似文献
952.
Zinc sulphide thin film electroluminescent devices doped with Mn or Tb have been produced on p-type Si substrates using a process in which doped zinc oxide films are deposited by a sol-gel drain coating method from a solution of zinc acetate containing a manganese or terbium dopant. The films are then converted to ZnS by heating them in an atmosphere containing hydrogen sulphide which replaces the oxygyn with sulphur. The composition, crystalline structure and optical properties of films have shown that complete conversion from the oxide to the sulphide takes place. The luminescent characteristics of the devices so produced have been measured as a function of the doping concentrations, film thickness, insulator thickness and driving voltage and frequency. It has been found that yellow or green luminescence can be obtained using Mn or Tb doping respectively. 相似文献
953.
Kepko E.L. Khurana K.K. Kivelson M.G. Elphic R.C. Russell C.T. 《IEEE transactions on magnetics》1996,32(2):377-385
Cluster introduces a new generation of spacecraft that will measure the spatial gradients of the magnetic field in the Earth's magnetosphere. As gradients require knowledge of differences, small errors resulting from an inadequate knowledge of the orientations, zero levels and the scale factors of the magnetometer sensors affect the calculation of field gradients disproportionately and must be removed with high accuracy. We show that twelve calibration parameters are required for each of the spacecraft (for a total of 48 for the four spacecraft) to correctly infer the measured magnetic fields at each of the spacecraft. By application of a Fourier transform technique, some of the parameters can be recovered. We show that errors in eight of the twelve calibration parameters generate coherent monochromatic signals at the first and the second harmonics of the spin frequency in the despun data. We relate the real and the imaginary parts of the monochromatic signals to the eight calibration parameters. We then present a least squares scheme that improves the eight calibration parameters by iteration until the power of the coherent signal superimposed above the broad-band background is minimized 相似文献
954.
Kanai H. Sato M. Koiwa Y. Chubachi N. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1996,43(5):791-810
For the noninvasive diagnosis of heart disease based on the acoustic and elastic characteristics of the heart muscle, it is necessary to transcutaneously measure small vibration signals, including components with an amplitude of less than 100 μm, from various parts of the heart wall continuously for periods of more than several heartbeats in a wide frequency range up to 1 kHz. Such measurement, however, has not been realized by any ultrasonic diagnostic methods or systems to date. By introducing the constraint least-square approach, this paper proposes a new method for accurately tracking the movement of the heart wall based on both the phase and magnitude of the demodulated signal to determine the instantaneous position of the object so that the vibration velocity of the moving object can be accurately estimated. By this method, small vibrations of the heart wall with small amplitudes less than 100 μm on the motion resulting from a heartbeat with large amplitude of 10 mm can be successfully detected with sufficient reproducibility in the frequency range up to several hundred Hertz continuously for periods of about 10 heartbeats. The resultant small vibration is analyzed not only in the time domain, but also in the frequency domain. As confirmed by the preliminary experiments herein reported, the new method offers potential for research in acoustical diagnosis of heart disease 相似文献
955.
J. H. Edgar C. A. Carosella C. R. Eddy Jr D. T. Smith 《Journal of Materials Science: Materials in Electronics》1996,7(4):247-253
The effects of nitrogen-beam voltage on the structure, stress, energy band gap and hardness of AIN thin films deposited on Si (111), Si (100) and sapphire (0001) by ion beam assisted deposition (IBAD) are reported. As the nitrogen-beam voltage was increased from 50 to 200 V, the stress and disorder in the AIN films increased as determined by X-ray diffraction, FTIR and Raman spectroscopy. The preferred orientation of the film's c-axis changed from completely normal to the film at 100 V, to a mixture of normal and in the plane of the film at 200 V. For AIN films deposited under the same conditions, the films were more highly oriented on sapphire (0001) than in Si (111). The hardness of the films increased from 18.2 to 23.7 GPa with the nitrogen-beam voltage, and possible reasons for this change in hardness are considered. 相似文献
956.
957.
E. M. Abdel-Bary H. H. Hassan A. M. El-Lawindy M. K. Abu-Assy F. K. El-Tantawy 《Polymer International》1993,30(3):371-374
Butyl rubber mixtures loaded with 70 phr general purpose furnace black (GPF) and tetramethyl thiuram disulphide (TMTD)/S as vulcanizing system were prepared. The kinetics of their electrical conductivity development during the vulcanization process were followed by using an especially devised system. It was found that the increase in the electrical conductivity during vulcanization obeys an exponential growth function with time constant τ, which markedly decreases with increasing vulcanization temperature as well as with the efficiency of the vulcanizing system. After completion of the vulcanization process, about 80 min, the samples obtained possess reasonable stability and reproducibility of electrical conductivity. 相似文献
958.
The authors have implemented chirped distributed feedback (DFB) grating lasers with phase shifts [λ/8, 2λ/8 and 3λ/8] distributed continuously over various axial distances, using their recently proposed method for producing bent optical waveguides. The singlemode stability of the lasers with distributed phase shifts was found to be considerably higher than for lasers with abrupt phase shifts 相似文献
959.
N. Ravishankar H. I. Aaronson K. Chattopadhyay 《Metallurgical and Materials Transactions A》1994,25(12):2631-2637
The early stages in the formation of α1 plates in the ordered β′ matrix of a Cu-39 pct Zn alloy have been studied with transmission electron microscopy (TEM). The
early stage plates were found to be free of stacking faults. Their contrast features are indeed those normally expected from
small coherent plates. Electron microdiffraction confirms that these plates have the 9R structure characteristic of α1 with the same lattice parameters and orientation relationships reported by previous investigators at much later stages of
growth. Similarly, the 9R structure was disordered, even though it was formed in an ordered matrix, again repeating previous
results obtained at a later stage of growth. These results further support the view that the α1 Cu-Zn plates form by a diffusion-controlled mechanism.
This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation
of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S
Phase Transformations Committee. 相似文献
960.