全文获取类型
收费全文 | 498873篇 |
免费 | 8644篇 |
国内免费 | 2039篇 |
专业分类
电工技术 | 9462篇 |
综合类 | 508篇 |
化学工业 | 76411篇 |
金属工艺 | 20470篇 |
机械仪表 | 15994篇 |
建筑科学 | 12537篇 |
矿业工程 | 1358篇 |
能源动力 | 14304篇 |
轻工业 | 44623篇 |
水利工程 | 3833篇 |
石油天然气 | 4599篇 |
武器工业 | 19篇 |
无线电 | 66509篇 |
一般工业技术 | 95398篇 |
冶金工业 | 92482篇 |
原子能技术 | 8601篇 |
自动化技术 | 42448篇 |
出版年
2022年 | 2980篇 |
2021年 | 4837篇 |
2020年 | 3456篇 |
2019年 | 4130篇 |
2018年 | 6521篇 |
2017年 | 6471篇 |
2016年 | 7044篇 |
2015年 | 5113篇 |
2014年 | 8489篇 |
2013年 | 23885篇 |
2012年 | 13624篇 |
2011年 | 18543篇 |
2010年 | 14740篇 |
2009年 | 16420篇 |
2008年 | 17061篇 |
2007年 | 16431篇 |
2006年 | 14803篇 |
2005年 | 13379篇 |
2004年 | 12755篇 |
2003年 | 12883篇 |
2002年 | 12432篇 |
2001年 | 12446篇 |
2000年 | 11461篇 |
1999年 | 12337篇 |
1998年 | 30444篇 |
1997年 | 21292篇 |
1996年 | 16660篇 |
1995年 | 12608篇 |
1994年 | 11098篇 |
1993年 | 10809篇 |
1992年 | 7813篇 |
1991年 | 7655篇 |
1990年 | 7062篇 |
1989年 | 6850篇 |
1988年 | 6653篇 |
1987年 | 5643篇 |
1986年 | 5577篇 |
1985年 | 6281篇 |
1984年 | 5656篇 |
1983年 | 5276篇 |
1982年 | 4922篇 |
1981年 | 4825篇 |
1980年 | 4633篇 |
1979年 | 4355篇 |
1978年 | 4126篇 |
1977年 | 5130篇 |
1976年 | 7291篇 |
1975年 | 3500篇 |
1974年 | 3367篇 |
1973年 | 3296篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
Geoffrey M. Eggert Paul R. Dawson Kapil K. Mathur 《International journal for numerical methods in engineering》1991,31(6):1031-1054
A forming model based on a viscoplastic flow formulation is derived which includes the effects of small elastic strains. A significant feature of the formulation is its reliance on the dominant inelastic material characteristics in the formation of the stiffness matrix for large strain problems. The resultant non-linear system of equations is solved by an adaptive descent method which combines the rapid convergence of Newton's method near the solution with the robustness of a method of successive approximations. The use of the adaptive descent method effectively extends the viscoplastic flow formulations into the nearly rate-insensitive range of behaviours exhibited, for example, by metals at low temperature, where slow convergence of the non-linear solution algorithm has traditionally hampered their use. 相似文献
993.
MetaModel is a user-friendly program for calculating steady-state fluxes and metabolite concentrations of metabolic systems on the IBM PC and compatible computers. For any steady state that is obtained, one can then calculate a matrix of elasticity coefficients at that steady state, or a matrix of control and response coefficients. It thus offers a simple way to calculate the control structure of a pathway: it provides not only an educational tool that allows the student to verify empirically the classic summation relationships of metabolic control analysis but also a research tool for addressing 'what if?' questions about the behaviour of metabolic systems. Results can not only be printed or stored in a file, but can also be written to a special file that can be read by popular spreadsheet programs, thereby giving access to rapid, flexible and powerful methods for subsequent analysis and plotting of these results. 相似文献
994.
995.
Determination of the relative amounts of free and complexed manganese ions in aqueous solution by nuclear magnetic resonance. 总被引:1,自引:0,他引:1
The ratio of the transverse relaxation rate to the longitudinal relaxation rate of water protons (T1/T2) at 90 MHz can be used to determine the relative amounts of free Mn2+ ions (the hexaquo ion) and chelated manganese ions when both species are present in the same aqueous solution. This technique is demonstrated for simple aqueous solutions containing known concentrations of both the free Mn2+ ion and a manganese(III) porphyrin. The accuracy of the method is found to be +/- 7.3% for the solutions considered in this report. The determinations of the relative amounts of the free Mn2+ ion and the manganese(III) porphyrin by this T1/T2 method are shown to agree well with those determined by nuclear magnetic resonance dispersion profiles. The theoretical basis of this ratio method as well as the scope and limitations in its application to aqueous solutions containing both free Mn2+ ions and chelated manganese species other than manganese(III) porphyrins are also discussed. This work demonstrates a unique use of NMR as a means for the quantitative analysis of manganese in which the effects of the metal ion on the solvent, and not the metal itself, are observed. 相似文献
996.
Y. G. Wang H. Q. Ye K. H. Kuo X. J. Feng S. Z. Long G. L. Lao 《Journal of Materials Science》1991,26(23):6325-6330
The microstructures of unhydrated calcium aluminosulphate Ca4Al6SO16 and Ca3SrAl6SO16 have been studied by high-resolution electron microscopy (HREM). The results showed that twinning and twinned slabs could be introduced taking the [1 1 2] direction as the twin axis so that it seems to be coincident with the law of twinning formed in body-centred cubic structures. A previously reported superlattice with a repeat period twice that of the fundamental structure along the 〈1 1 0〉 direction has also been found in both matrix and twin variants. The close intergrowth of Ca3SrAl6SO16 and another phase, possibly Sr3Al2O6 existing as an inclusion between these two twin variants, was determined and clearly revealed by electron diffraction and HREM images. The coherent interphase boundaries and orientation relationship between them can also be deduced. 相似文献
997.
The mechanical properties of Si3N4-SiC, SiNx and SiCy films prepared at a low temperature of 400 °C by plasma chemical vapour deposition are reported. Microhardness, internal stress of the film and adhesive strength between the film and glass or stainless steel substrate were evaluated as principal mechanical properties. Microhardness was measured to be about 10 to 20 G Pa dependent on the film composition in each system. Internal stress of the films on borosilicate glass substrates extensively varied from tensile to compressive with the film composition change from Si3N4 to SiC. Adhesive strength, as ascertained by the scratch test, was about 580 to 800 MPa for crown glass substrates, and about 210 to 310 M Pa for 316 stainless steel substrates. It is pointed out that tensile stress in these films brought about more abrupt decreases of the adhesive strength than did compressive stress. 相似文献
998.
T. Torounidis M. Westlund H. Sunnerud B.-E. Olsson P.A. Andrekson 《Photonics Technology Letters, IEEE》2005,17(2):312-314
Short return-to-zero pulses (/spl sim/2 ps) are generated at bit rates of 40, 80, and 160 Gb/s using a fiber-optical parametric amplifier. The performance of the parametric pulse source is evaluated both back-to-back and in a 110-km transmission link. A receiver sensitivity of -33 dBm back-to-back was achieved after demultiplexing from 160 to 10 Gb/s. The power penalty at 160 Gb/s due to 110-km transmission was less than 2 dB. Very short pulses (0.5 ps) were also achieved when using the parametric amplifier as a compressor. 相似文献
999.
Uncooled DBR laser directly Modulated at 3.125 gb/s as athermal transmitter for low-cost WDM systems
Yiran Liu Davies A.R. Ingham J.D. Penty R.V. White I.H. 《Photonics Technology Letters, IEEE》2005,17(10):2026-2028
An uncooled three-section tunable distributed Bragg reflector laser is demonstrated as an athermal transmitter for low-cost uncooled wavelength-division-multiplexing (WDM) systems with tight channel spacing. A /spl plusmn/0.02-nm thermal wavelength drift is achieved under continuous-wave operation up to 70/spl deg/C. Dynamic sidemode suppression ratio of greater than 35 dB is consistently obtained under 3.125-Gb/s direct modulation over a 20/spl deg/C-70/spl deg/C temperature range, with wavelength variation of as low as /spl plusmn/0.2 nm. This indicates that more than an order of magnitude reduction in coarse WDM channel spacing is possible using this source. 相似文献
1000.
Waveband-switchable SOA ring laser constructed with a phase modulator loop mirror filter 总被引:1,自引:0,他引:1
M.P. Fok K.L. Lee C. Shu 《Photonics Technology Letters, IEEE》2005,17(7):1393-1395
An electrically tunable multiwavelength source has been developed using a birefringence-based optical comb filter in a semiconductor optical amplifier ring laser. The filter is constructed with an electrooptic phase modulator placed inside a fiber loop mirror. By controlling the birefringence of the modulator through the applied bias, the radio-frequency power, or the modulation frequency, we achieve a continuous shift of the spectral comb to access different interleaved wavebands. Electrical waveband switching has been successfully demonstrated for 21 wavelengths at 100-GHz grid spacing with an optical signal-to-noise ratio over 40 dB. 相似文献