全文获取类型
收费全文 | 253077篇 |
免费 | 3625篇 |
国内免费 | 1485篇 |
专业分类
化学 | 126380篇 |
晶体学 | 3261篇 |
力学 | 8981篇 |
综合类 | 61篇 |
数学 | 25171篇 |
物理学 | 66250篇 |
无线电 | 28083篇 |
出版年
2020年 | 1963篇 |
2019年 | 2149篇 |
2018年 | 2721篇 |
2017年 | 2770篇 |
2016年 | 4073篇 |
2015年 | 2715篇 |
2014年 | 4009篇 |
2013年 | 9675篇 |
2012年 | 8435篇 |
2011年 | 10322篇 |
2010年 | 7417篇 |
2009年 | 7471篇 |
2008年 | 10199篇 |
2007年 | 10592篇 |
2006年 | 10100篇 |
2005年 | 9225篇 |
2004年 | 8318篇 |
2003年 | 7396篇 |
2002年 | 7239篇 |
2001年 | 7891篇 |
2000年 | 6144篇 |
1999年 | 4614篇 |
1998年 | 3944篇 |
1997年 | 3965篇 |
1996年 | 3831篇 |
1995年 | 3423篇 |
1994年 | 3560篇 |
1993年 | 3446篇 |
1992年 | 3558篇 |
1991年 | 3635篇 |
1990年 | 3454篇 |
1989年 | 3326篇 |
1988年 | 3133篇 |
1987年 | 2824篇 |
1986年 | 2827篇 |
1985年 | 3688篇 |
1984年 | 3704篇 |
1983年 | 3060篇 |
1982年 | 3328篇 |
1981年 | 3074篇 |
1980年 | 2837篇 |
1979年 | 2997篇 |
1978年 | 3218篇 |
1977年 | 3200篇 |
1976年 | 3230篇 |
1975年 | 2970篇 |
1974年 | 3055篇 |
1973年 | 3100篇 |
1972年 | 2418篇 |
1971年 | 1943篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
981.
The growth mechanism of the peritectic η phase involving the peritectic reaction and peritectic transformation in Cu-70%Sn alloy was investigated under directional
solidification. The results show that a major growth mechanism in thickening of the peritectic η-layer is not the peritectic reaction but the peritectic transformation. The transformation temperature and isothermal time
play crucial roles in determining the volume fraction and the thickness of the peritectic η phase. With the increase of the temperature and isothermal time, the volume fraction of the peritectic η phase increases. The regressed data show that the relationship between the thickness of η phase (Δx) and the transformation temperature (T) meets the following equation In Δx=6.5−1673 1 / T. Additionally, there exists a relationship between the thickness of the η phase (Δx) and the isothermal time (t) at the 9 mm solidification distance below the peritectic reaction interface, Δx=0.72t
1/2, which is consistent with the theoretical model.
Supported by the National Science Foundation of China (Grant No. 50395102) 相似文献
982.
Streit D.C. Hafizi M.E. Umemoto D.K. Velebir J.R. Tran L.T. Oki A.K. Kim M.E. Wang S.K. Kim C.W. Sadwick L.P. Hwu R.J. 《Electron Device Letters, IEEE》1991,12(5):194-196
The authors have fabricated n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBTs) with base doping graded exponentially from 5×1019 cm-3 at the emitter edge to 5×1018 cm-3 at the collector edge. The built-in field due to the exponentially graded doping profile significantly reduces base transit time, despite bandgap narrowing associated with high base doping. Compared to devices with the same base thickness and uniform base doping of 1×1019 cm-3 , the cutoff frequency is increased from 22 to 31 GHz and maximum frequency of oscillation is increased from 40 to 58 GHz. Exponentially graded base doping also results ill consistently higher common-emitter current gain than uniform base doping, even though the Gummel number is twice as high and the base resistance is reduced by 40% 相似文献
983.
Chawki M.J. Auffret R. Le Coquil E. Pottier P. Berthou L. Paciullo H. Le Bihan J. 《Lightwave Technology, Journal of》1992,10(10):1388-1397
Characteristics of a two-electrode DFB laser filter are studied both theoretically and experimentally. Using a matrix analysis of spontaneous emission, a continuous tuning range of 6.7 Å is achieved by changing both net field gains of the two electrodes. A total discontinuous tuning range of over 10 nm comprising alternating mode jumps and continuous tuning range of 4 Å are measured experimentally. The laser filter presents a FWHM bandwidth of 5 GHz which depends on the optical input power. In addition, it is demonstrated that a DFB laser filter can act as a frequency discriminator/photodetector, i.e., a narrow-band FM receiver, with a uniform bandwidth of 1.5 GHz. Using the two-electrode DFB laser for both transmitter and receiver, a two-channel FSK-WDM transmission system utilizing the discontinuous tuning range is reported. The advantage of such a device is the simplicity as compared to the heterodyne technique 相似文献
984.
985.
Zupac D. Baum K.W. Kosier S.L. Schrimpf R.D. Galloway K.F. 《Electron Device Letters, IEEE》1991,12(10):546-549
The effect of noncatastrophic positive human body model (HBM) electrostatic discharge (ESD) stress on n-channel power MOSFETs is radically different from that on p-channel MOSFETs. In n-channel transistors, the stress causes negative shifts of the current-voltage characteristics indicative of positive charge trapping in the gate oxide. In p-channel transistors, the stress increases the drain-to-source leakage current, probably due to localized avalanche electron injection from the p-doped drain 相似文献
986.
987.
Partially supported by NSF Grant DMS-9004123 and ARO through MSI Cornell (DAAG 29-85-C-0018) 相似文献
988.
989.
Zboril R. Mashlan M. Machala L. Walla J. Barcova K. Martinec P. 《Hyperfine Interactions》2004,156(1-4):403-410
Hyperfine Interactions - The natural garnets from almandine (Fe3Al2Si3O12)–pyrope (Mg3Al2Si3O12) series with the iron to magnesium atomic ratio ranging from 0.2 to 1 were characterised and... 相似文献
990.
This paper delineates the first steps in a systematic quantitative study of the spacetime fluctuations induced by quantum
fields in an evaporating black hole. We explain how the stochastic gravity formalism can be a useful tool for that purpose
within a low-energy effective field theory approach to quantum gravity. As an explicit example we apply it to the study of
the spherically-symmetric sector of metric perturbations around an evaporating black hole background geometry. For macroscopic
black holes we find that those fluctuations grow and eventually become important when considering sufficiently long periods
of time (of the order of the evaporation time), but well before the Planckian regime is reached. In addition, the assumption
of a simple correlation between the fluctuations of the energy flux crossing the horizon and far from it, which was made in
earlier work on spherically-symmetric induced fluctuations, is carefully analyzed and found to be invalid. Our analysis suggests
the existence of an infinite amplitude for the fluctuations of the horizon as a three-dimensional hypersurface. We emphasize
the need for understanding and designing operational ways of probing quantum metric fluctuations near the horizon and extracting
physically meaningful information.
Dedicated to Rafael Sorkin on the occasion of his 60th birthday. 相似文献