全文获取类型
收费全文 | 77810篇 |
免费 | 1346篇 |
国内免费 | 1003篇 |
专业分类
测绘学 | 1928篇 |
大气科学 | 6354篇 |
地球物理 | 15770篇 |
地质学 | 28139篇 |
海洋学 | 6741篇 |
天文学 | 16412篇 |
综合类 | 303篇 |
自然地理 | 4512篇 |
出版年
2021年 | 558篇 |
2020年 | 663篇 |
2019年 | 720篇 |
2018年 | 1944篇 |
2017年 | 1793篇 |
2016年 | 2130篇 |
2015年 | 1299篇 |
2014年 | 2011篇 |
2013年 | 3848篇 |
2012年 | 2379篇 |
2011年 | 3068篇 |
2010年 | 2660篇 |
2009年 | 3492篇 |
2008年 | 3107篇 |
2007年 | 2939篇 |
2006年 | 2834篇 |
2005年 | 2691篇 |
2004年 | 2670篇 |
2003年 | 2445篇 |
2002年 | 2046篇 |
2001年 | 1797篇 |
2000年 | 1822篇 |
1999年 | 1573篇 |
1998年 | 1477篇 |
1997年 | 1490篇 |
1996年 | 1301篇 |
1995年 | 1192篇 |
1994年 | 1059篇 |
1993年 | 963篇 |
1992年 | 923篇 |
1991年 | 924篇 |
1990年 | 907篇 |
1989年 | 803篇 |
1988年 | 809篇 |
1987年 | 920篇 |
1986年 | 883篇 |
1985年 | 1020篇 |
1984年 | 1146篇 |
1983年 | 1132篇 |
1982年 | 1026篇 |
1981年 | 947篇 |
1980年 | 912篇 |
1979年 | 867篇 |
1978年 | 867篇 |
1977年 | 781篇 |
1976年 | 691篇 |
1975年 | 709篇 |
1974年 | 756篇 |
1973年 | 741篇 |
1972年 | 467篇 |
排序方式: 共有10000条查询结果,搜索用时 625 毫秒
71.
Observations of the brightness, color, and polarization of the fuor V1057 Cyg over more than 30 years are presented and briefly discussed. Variability of the linear polarization was found. 相似文献
72.
73.
C.A. Mendoza-Briceño 《Astrophysics and Space Science》1997,256(1-2):387-392
The thermal evolution of structures is investigated for different values of the size of structure. A simplified cooling function and a constant heating mechanism are assumed. Analytical criteria for thermal instability are obtained. It is found that the response of the thermal structure not only depends on the amplitude of the disturbances, but also on whether the disturbance increases or decreases the initial steady temperature. Additionally, the evolution of the structure is examined numerically by using a time-dependent code under the assumption that the inertia terms are small. In particular, the analytical criteria obtained for thermal instability are verified. 相似文献
74.
75.
U. C. Joshi A. K. Sen M. R. Deshpande J. S. Chauhan 《Journal of Astrophysics and Astronomy》1992,13(3):267-277
Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope
(VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the
1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990.
The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a
sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of
0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C2 bands are quite strong; C3 emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C2 emission. Molecular band polarization for CN, C3, C2 and H2 O+ have been calculated. It has been found that emission polarization in CN, C2 and C3 is between 1–7% (phase angle between 107.4–109 degrees). For CN and C2 the polarization values are close to the theoretically predicted values, but for C3 the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley. 相似文献
76.
77.
J.H. Krolik 《Astrophysics and Space Science》1997,248(1-2):207-215
A short survey is presented of dynamical (and thermodynamical) issues relevant to outflows in Seyfert galaxies like NGC 1068.
Numerical simulations incorporating both realistic radiative heating and cooling, and angular momentum, show that the fraction
of injected gas which escapes in a wind is very sensitive to both the injection pressure and the gas's angular momentum. Radiation
pressure acting through a variety of atomic opacity processes may also be important.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
78.
Donald D. BOGARD Daniel H. GARRISON Jozef MASARIK 《Meteoritics & planetary science》2001,36(1):107-122
Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The 39Ar‐40Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of 40Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic 3He, 21Ne, and 38Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic 21Ne and 38Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of 36Ar in the halite were produced during regolith exposure by neutron capture on 35Cl, followed by decay to 36Ar. The thermal neutron fluence seen by the halite was (2–4) × 1014 n/cm2. The thermal neutron flux during regolith exposure was ~0.4‐0.7 n/cm2/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the 36Arn/21Necos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded. 相似文献
79.
80.
T. H. BELL S. E. JOHNSON B. DAVIS A. FORDE N. HAYWARD C. WILKINS 《Journal of Metamorphic Geology》1992,10(3):295-307
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens. 相似文献