全文获取类型
收费全文 | 113488篇 |
免费 | 1920篇 |
国内免费 | 1127篇 |
专业分类
测绘学 | 3388篇 |
大气科学 | 8020篇 |
地球物理 | 23145篇 |
地质学 | 39510篇 |
海洋学 | 9629篇 |
天文学 | 26556篇 |
综合类 | 351篇 |
自然地理 | 5936篇 |
出版年
2021年 | 975篇 |
2020年 | 1069篇 |
2019年 | 1200篇 |
2018年 | 2723篇 |
2017年 | 2518篇 |
2016年 | 3182篇 |
2015年 | 1824篇 |
2014年 | 3064篇 |
2013年 | 5689篇 |
2012年 | 3310篇 |
2011年 | 4389篇 |
2010年 | 3897篇 |
2009年 | 5233篇 |
2008年 | 4531篇 |
2007年 | 4563篇 |
2006年 | 4273篇 |
2005年 | 3288篇 |
2004年 | 3352篇 |
2003年 | 3122篇 |
2002年 | 3034篇 |
2001年 | 2729篇 |
2000年 | 2614篇 |
1999年 | 2272篇 |
1998年 | 2273篇 |
1997年 | 2218篇 |
1996年 | 1920篇 |
1995年 | 1827篇 |
1994年 | 1727篇 |
1993年 | 1472篇 |
1992年 | 1361篇 |
1991年 | 1356篇 |
1990年 | 1471篇 |
1989年 | 1372篇 |
1988年 | 1262篇 |
1987年 | 1485篇 |
1986年 | 1291篇 |
1985年 | 1572篇 |
1984年 | 1806篇 |
1983年 | 1636篇 |
1982年 | 1590篇 |
1981年 | 1430篇 |
1980年 | 1320篇 |
1979年 | 1237篇 |
1978年 | 1230篇 |
1977年 | 1135篇 |
1976年 | 1055篇 |
1975年 | 1050篇 |
1974年 | 1006篇 |
1973年 | 1077篇 |
1972年 | 714篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The aim of this paper is to investigate the thermal stability of a fluid layer with permeable boundaries and a variable gravitational field. It is observed that the principle of exchange of stabilities is valid when the layer is heated form below and the complex growth rate of an arbitrary oscillatory mode exists outside of a circle whose radius depends upon the permeability parameter, Prandtl number and wavelength of the mode. In the case of a layer heated from below, gravity increasing upward has a destabilizing effect whereas the permeability parameter has a stabilizing effect. 相似文献
72.
73.
Rostov State Pedagogical Institute; All-Union Scientific Research Institute Gradient. Translated from Astrofizika, Vol. 33, No. 3, pp. 379–393, November–December, 1990. 相似文献
74.
Faith Vilas David J. Tholen Larry A. Lebofsky Humberto Campins Glenn J. Veeder Richard P. Binzel Alan T. Tokunaga 《Icarus》1985,63(2):201-205
Visual and infrared observations were made of Amor asteroid 1982 DV during its discovery apparition. Broadband visual and near-infrared photometry shows that it is an S-class asteroid. Narrowband spectrophotometry shows an absorption feature due to olivine or pyroxene or both centered at 0.93 μm. Applying a nonrotating thermal model to 10-μm photometry, the geometric albedo is calculated to be approximately 0.27. The geometric albedo for a slowly rotating, rocky surface was calculated for 1 night to be 0.15, consistent with S-class asteroid albedos. Thus, 1982 DV is either one of the most reflective S-class asteroids known, or a significant amount of bare rock is exposed on the asteroid's surface. For the nonrotating model, ellipsoidal dimensions for 1982 DV are 3.5 × 1.4 × 1.4 km. 相似文献
75.
It is shown that the kinematical parameters associated with the congruence formed by tachyonic motion can be defined in the manner of Greenberg, but not that of Ehlers. The space-like counterpart of Raychaudhuri's equation has also been obtained. 相似文献
76.
77.
Electron impact excitation rates for transitions in the S v ion, calculated with theR-matrix code, are used to derive the electron temperature sensitive emission line ratiosR
1 =I(854.8 Å)/I(786.9 Å),R
2 =I(852.2 Å)/I(786.9 Å),R
3 =I(849.2 Å)/I(786.9 Å), andR
4 =I(1199.1 Å)/I(786.9 Å), which are found to be significantly different from previous estimates. A comparison of the present results with observational data for a sunspot obtained with the Harvard S-055 spectrometer on boardSkylab reveals generally good agreement between theory and experiment, except in the case ofR
1, which is probably due to blending in the 854.8 Å feature. The possible effects of Lyman continuum absorption on the observed line ratios is briefly discussed. 相似文献
78.
The results of a general theoretical investigation of three commonly used types of inductive conductivity sensors, i.e., the single transformer, the double transformer, and the double transformer with an additional loop, are presented. The resulting formulas describe the dependence of the sensor output signal not only on the conductivity of the seawater but also on the parameters of the electrical circuit, among them the permeability of the transformer core(s), which-unlike the other parameters-shifts considerably during oceanographic in situ measurements. A mathematical discussion of these formulas shows that for certain circuit configurations, the sensor output is independent of changes in permeability. Most of these configurations form the basis of existing oceanographical conductivity sensors, among them the "classical" sensors developed by H. Hinkelmann [3], [4], and by N. L. Brown [14], while some others make evident further possibilities for eliminating the unwanted effects of shifting permeability. In the era of microelectronics, the latter might lead to a reassessment, especially of the single transformer-type sensor. 相似文献
79.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm. 相似文献
80.