全文获取类型
收费全文 | 108298篇 |
免费 | 1617篇 |
国内免费 | 857篇 |
专业分类
测绘学 | 2721篇 |
大气科学 | 7114篇 |
地球物理 | 20946篇 |
地质学 | 40871篇 |
海洋学 | 9199篇 |
天文学 | 24175篇 |
综合类 | 404篇 |
自然地理 | 5342篇 |
出版年
2022年 | 692篇 |
2021年 | 1158篇 |
2020年 | 1263篇 |
2019年 | 1358篇 |
2018年 | 4506篇 |
2017年 | 4136篇 |
2016年 | 4108篇 |
2015年 | 1796篇 |
2014年 | 3149篇 |
2013年 | 5156篇 |
2012年 | 3896篇 |
2011年 | 5917篇 |
2010年 | 5066篇 |
2009年 | 6212篇 |
2008年 | 5463篇 |
2007年 | 5810篇 |
2006年 | 4093篇 |
2005年 | 3133篇 |
2004年 | 2991篇 |
2003年 | 2869篇 |
2002年 | 2621篇 |
2001年 | 2351篇 |
2000年 | 2177篇 |
1999年 | 1662篇 |
1998年 | 1750篇 |
1997年 | 1718篇 |
1996年 | 1357篇 |
1995年 | 1403篇 |
1994年 | 1206篇 |
1993年 | 1081篇 |
1992年 | 1056篇 |
1991年 | 983篇 |
1990年 | 1102篇 |
1989年 | 945篇 |
1988年 | 860篇 |
1987年 | 1017篇 |
1986年 | 814篇 |
1985年 | 1089篇 |
1984年 | 1176篇 |
1983年 | 1096篇 |
1982年 | 1047篇 |
1981年 | 918篇 |
1980年 | 878篇 |
1979年 | 764篇 |
1978年 | 793篇 |
1977年 | 716篇 |
1976年 | 674篇 |
1975年 | 647篇 |
1974年 | 646篇 |
1973年 | 642篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
501.
502.
Near the inner edge of the Edgeworth-Kuiper Belt (EKB) are Pluto and Charon, which are known to have N2- and H2O-dominated surface ices, respectively. Such non-polar and polar ices, and perhaps mixtures of them, also may be present on other trans-Neptunian objects. Pluto, Charon, and all EKB objects reside in a weak, but constant UV-photon and energetic ion radiation environment that drives chemical reactions in their surface ices. Effects of photon and ion processing include changes in ice composition, volatility, spectra, and albedo, and these have been studied in a number of laboratories. This paper focuses on ice processing by ion irradiation and is aimed at understanding the volatiles, ions, and residues that may exist on outer solar system objects. We summarize radiation chemical products of N2-rich and H2O-rich ices containing CO or CH4, including possible volatiles such as alcohols, acids, and bases. Less-volatile products that could accumulate on EKB objects are observed to form in the laboratory from acid-base reactions, reactions promoted by warming, or reactions due to radiation processing of a relatively pure ice (e.g., CO → C3O2). New IR spectra are reported for the 1–5 mu;m region, along with band strengths for the stronger features of carbon suboxide, carbonic acid, the ammonium and cyanate ions, polyoxymethylene, and ethylene glycol. These six materials are possible contributors to EKB surfaces, and will be of interest to observers and future missions. 相似文献
503.
Churyumov K. I. Klesachonok V. V. Mussaev F. A. Bikmaev I. F. Galazutdinov G. H. 《Earth, Moon, and Planets》1997,78(1-3):105-110
We present the results of the preliminary study of the comet Hale-Bopp spectrum obtained April 17, 1997 by K. Churyumov and F. Mussayev with the help of the 1-meter Zeiss reflector and the echelle spectrometer (spectral resolutionλ/Δ λ ≈ 50000), CCD and the long slit, oriented along the radius-vector(“Sun-comet direction”). Energy distributions for three selected regions including the C3, C2 (0-0) and CN(Δ ν = 0) molecules emissions of the comet Hale-Bopp spectrum were built. The rotational lines of the CN(Δ ν = 0) band were identified. The nature of the high emission peak near λ 4020 Å in the C3 band is discussed. The presence of the cometary continuum of the nonsolar origin is assumed. 相似文献
504.
The equivalent width calculations for the fundamental vibration rotation band lines of HeH+ have been carried out for a non-DA white dwarf model with an effective temperature of 12 000 K. BothP andR branch lines with rotational quantum numbersJ=3 to 18 were included in the calculations. A search for these lines in helium rich white dwarfs is suggested. 相似文献
505.
In laboratory investigations with model waters, the behaviour of selected cation-exchange resins (Amberlite 200 C, Duolite C-25, Lewatit S-100, Wofatit KPS and Wofatit KS-10) is tested. The usable volume capacity of the resins in the forms of H, Na and NH4 as well as the water content and the total volume capacity after 25 cycles of loading/regeneration are determined. It is recommended to use Amberlite 200 C and Wofatit KS-10. 相似文献
506.
507.
508.
Energetic protons haying ring type distributions are shown to generate low-frequency electrostatic waves, propagating nearly transverse to the geomagnetic field lines, in the ring current region by exciting Mode 1 arid Mode 2 nonresonant instabilities and a resonant instability. Mode 1 nonresonant instability has frequencies around ~4 Hz with transverse wavelengths of ~(8–80) km, and it is likely to occur in the region L = (7–8). Mode 2 nonresonant instability can generate frequencies ~(850–1450) Hz with transverse wavelengths ~(2–20) km. The typical frequencies and transverse wavelengths associated with the resonant instability are (950–1250) Hz and (30–65) km. Both the Mode 2 nonresonant instability and the resonant instability can occur in the ring current region with L = (4–6). The low-frequency modes driven by energetic protons could attain maximum saturation electric field amplitude varying from 0.8 mV/m to 70 mV/m. It is suggested that the turbulence produced by the low-frequency modes may cause pitch angle scattering of ring current protons in the region outside the plasmapause resulting in the ring current decay. 相似文献
509.
Vagle S. Foote K.G. Trevorrow M.V. Farmer D.M. 《Oceanic Engineering, IEEE Journal of》1996,21(3):298-305
A calibration technique has been adapted to render complete system calibrations of high-frequency acoustical instrumentation. This is based on standard targets; specifically, precisely manufactured spheres composed of tungsten carbide with 6% cobalt binder. The use of multiple sphere sizes was found to be advantageous, both as an independent check of the calibrations, and so that resonances in the sphere responses at certain frequencies could be avoided. Complete system gains and beam patterns, which include effects of bandpass filters and finite-pulse lengths, were determined by moving the spheres individually in the transducer far-fields. Use of this procedure ensures control over the acoustical characteristics of transducers, which may change from the time of manufacture and first testing due, for example, to platform mounting. It also provides a direct means of measuring the sampling volume at relatively high and constant signal-to-noise ratios. Implementation of this technique is discussed using a multifrequency sonar system as an example 相似文献
510.
V. I. Ferronsky S. A. Denisik S. V. Ferronsky 《Celestial Mechanics and Dynamical Astronomy》1996,64(3):167-183
The physical meaning of the terms of the potential and kinetic energy expressions, expanded by means of the density variation function for a nonuniform self-gravitating sphere, is discussed. The terms of the expansions represent the energy and the moment of inertia of the uniform sphere, the energy and the moment of inertia of the nonuniformities interacting with the uniform sphere, and the energy of the nonuniformities interacting with each other. It follows from the physical meaning of the above components of the energy structure, and also from the observational fact of the expansion of the Universe that the phase transition, notably, fusion of particles and nuclei and condensation of liquid and solid phases of the expanded matter accompanied by release of energy, must be the physical cause of initial thermal and gravitational instability of the matter. The released kinetic energy being constrained by the general motion of the expansion, develops regional and local turbulent (cyclonic) motion of the matter, which should be the second physical effect responsible for the creation of celestial bodies and their rotation. 相似文献