全文获取类型
收费全文 | 84679篇 |
免费 | 617篇 |
国内免费 | 128篇 |
专业分类
化学 | 41188篇 |
晶体学 | 1260篇 |
力学 | 3277篇 |
综合类 | 1篇 |
数学 | 7827篇 |
物理学 | 24680篇 |
无线电 | 7191篇 |
出版年
2022年 | 596篇 |
2021年 | 923篇 |
2020年 | 1054篇 |
2019年 | 1193篇 |
2018年 | 1703篇 |
2017年 | 1798篇 |
2016年 | 2282篇 |
2015年 | 1063篇 |
2014年 | 1840篇 |
2013年 | 3462篇 |
2012年 | 3040篇 |
2011年 | 3427篇 |
2010年 | 2813篇 |
2009年 | 2909篇 |
2008年 | 3302篇 |
2007年 | 3262篇 |
2006年 | 3084篇 |
2005年 | 2771篇 |
2004年 | 2544篇 |
2003年 | 2398篇 |
2002年 | 2396篇 |
2001年 | 2500篇 |
2000年 | 1893篇 |
1999年 | 1494篇 |
1998年 | 1368篇 |
1997年 | 1249篇 |
1996年 | 1068篇 |
1995年 | 986篇 |
1994年 | 968篇 |
1993年 | 942篇 |
1992年 | 986篇 |
1991年 | 1065篇 |
1990年 | 1017篇 |
1989年 | 1011篇 |
1988年 | 917篇 |
1987年 | 828篇 |
1986年 | 801篇 |
1985年 | 942篇 |
1984年 | 1014篇 |
1983年 | 860篇 |
1982年 | 878篇 |
1981年 | 777篇 |
1980年 | 729篇 |
1979年 | 797篇 |
1978年 | 914篇 |
1977年 | 945篇 |
1976年 | 834篇 |
1975年 | 798篇 |
1974年 | 817篇 |
1973年 | 788篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
71.
SAR-retrieved wind in polar regions-comparison with in situ data and atmospheric model output 总被引:1,自引:0,他引:1
Furevik B.R. Johannessen O.M. Sandvik A.D. 《Geoscience and Remote Sensing, IEEE Transactions on》2002,40(8):1720-1732
European remote sensing (ERS) satellites synthetic aperture radar (SAR) wind retrievals using CMOD-IFR2 are, for the first time, retrieved in the marginal ice zone (MIZ) and in Arctic coastal areas and compared with in situ observations from reseach vessels (RVs) and output from a high-resolution atmospheric model. The root mean squares (rms) of the comparisons were 1.6 m s/sup -1/ and 2 m s/sup -1/, respectively. The spatial variation of the SAR wind fields established a decrease in wind speed close to the ice edge for the late summer situations where the wind was along the ice edge with the ice to the left. This decrease is believed to be due to changes in atmospheric stability, possibly through development of an internal boundary layer caused by the cold ice cover and melt water. Lower wind speed near the ice edge is confirmed by the atmospheric model and the in situ observations. Furthermore, good results are obtained from SAR wind retrieval in leads when compared with model output during a cold-air outbreak. Routine measurements in the MIZ are useful for estimating the wind stress, and therefore SAR may play an important role in this region. Finally, the identification of a jet out from Hinlopen Strait in the Svalbard region and low wind wakes along the coast in the SAR-retrieved wind field is confirmed by in situ observations as the RV moves through the region. The jet is also confirmed by the atmospheric model, which is able to reproduce the situation. 相似文献
72.
Two novel configurations for digitally tunable optical filters based on arrayed-waveguide grating (AWG) multiplexers are described in detail with emphasis on the connection of the AWG multiplexer and optical switches. Performance comparisons show that conventional configurations are disadvantaged by the switch size required and loss imbalance among the optical frequency-division-multiplexed (FDM) channels; the proposed configurations require only O(√(N)) switch elements to select one of N FDM channels, and the loss imbalance is lower by up to 75% in decibel 相似文献
73.
Valenzuela R.A. Landron O. Jacobs D.L. 《Vehicular Technology, IEEE Transactions on》1997,46(1):203-212
We explore techniques for the measurement of local mean signal strength at 900 MHz and 2 GHz. In particular, we characterize the impact of transmitter and receiver antenna rotation on the estimated local mean. Then, we explore the collection of high resolution data while moving along a linear trajectory and using linear averaging techniques to estimate the local mean. With this information, the best measurement techniques can be chosen depending on the required speed versus accuracy tradeoff. Finally, we use a ray tracing propagation model to evaluate different methods of calculating the local mean signal strength for indoor environments 相似文献
74.
The changes in the magnetization of yttrium iron garnet (YIG) when irradiated by a pulsed neodymium laser beam with wavelength
λ=1.06 μm are investigated. Measurements are performed in the temperature range from 100 K to 600 K in various external magnetic
fields. YIG single crystals grown along the crystallographic (100), (110), and (111) directions are chosen so that the external
anisotropy of the indicated processes can be determined. Characteristic temperature intervals dominated by different mechanisms
of variation of the magnetization under the influence of a laser pulse are discussed.
Fiz. Tverd. Tela (St. Petersburg) 39, 1263–1266 (July 1997) 相似文献
75.
76.
Plastic encapsulated microcircuits (PEMs) are increasingly being used in applications requiring operation at temperatures lower than the manufacturer’s recommended minimum temperature, which is 0°C for commercial grade components and −40°C for industrial and automotive grade components. To characterize the susceptibility of PEMs to delamination at these extreme low temperatures, packages with different geometries, encapsulated in both biphenyl and novolac molding compounds, were subjected to up to 500 thermal cycles with minimum temperatures in the range −40 to −65°C in both the moisture saturated and baked conditions. Scanning acoustic microscopy revealed there was a negligible increase in delamination at the die-to-encapsulant interface after thermal cycling for the 84 lead PQFPs encapsulated in novolac and for both 84 lead PQFPs and 14 lead PDIPs encapsulated in biphenyl molding compound. Only the 14 lead novolac PDIPs exhibited increased delamination. Moisture exposure had a significant effect on the creation of additional delamination. 相似文献
77.
I. V. Ukrainets S. G. Taran O. V. Gorokhova P. A. Bezuglyi A. V. Turov 《Chemistry of Heterocyclic Compounds》1994,30(2):204-207
In a solution of DMF, 2-carbethoxymethyl-4H-3,1-benzoxazin-4-one reacts with primarily aromatic amines basically with the formation of the corresponding 2-carbethoxymetlzyl-3-arylquinazolin-4(3H)-ones. Possible mechanisms of these chemical transformations are reported and discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 225–228, February, 1994. Original article submitted June 28, 1993. 相似文献
78.
79.
The electron-impact mass spectra of a number of phenylpropanoids and iridoid glycosides containing a phenylethanoid fragment in their structure have been studied. This has revealed features of the formation of fragments of 4-hydroxyphenylethyl and 3,4-dihydroxyphenylethyl alcohols and also of fragments of phenylethylamine and indolylethylamine derivatives present in the structures of the compounds investigated. 相似文献
80.