全文获取类型
收费全文 | 55244篇 |
免费 | 21347篇 |
国内免费 | 45696篇 |
专业分类
测绘学 | 3702篇 |
大气科学 | 24668篇 |
地球物理 | 17622篇 |
地质学 | 45163篇 |
海洋学 | 17653篇 |
天文学 | 2201篇 |
综合类 | 6626篇 |
自然地理 | 4652篇 |
出版年
2024年 | 238篇 |
2023年 | 707篇 |
2022年 | 1442篇 |
2021年 | 1851篇 |
2020年 | 2816篇 |
2019年 | 6182篇 |
2018年 | 6714篇 |
2017年 | 6317篇 |
2016年 | 6618篇 |
2015年 | 5615篇 |
2014年 | 5464篇 |
2013年 | 5954篇 |
2012年 | 5505篇 |
2011年 | 5318篇 |
2010年 | 5222篇 |
2009年 | 4624篇 |
2008年 | 3799篇 |
2007年 | 3824篇 |
2006年 | 3334篇 |
2005年 | 3129篇 |
2004年 | 3264篇 |
2003年 | 3021篇 |
2002年 | 2708篇 |
2001年 | 2476篇 |
2000年 | 2318篇 |
1999年 | 2769篇 |
1998年 | 2573篇 |
1997年 | 2596篇 |
1996年 | 2156篇 |
1995年 | 1994篇 |
1994年 | 1808篇 |
1993年 | 1658篇 |
1992年 | 1376篇 |
1991年 | 1069篇 |
1990年 | 951篇 |
1989年 | 805篇 |
1988年 | 700篇 |
1987年 | 520篇 |
1986年 | 441篇 |
1985年 | 362篇 |
1984年 | 354篇 |
1983年 | 223篇 |
1982年 | 271篇 |
1981年 | 196篇 |
1980年 | 149篇 |
1979年 | 145篇 |
1978年 | 71篇 |
1972年 | 55篇 |
1971年 | 74篇 |
1970年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
In this paper, two factors — the redistribution of the density and the variation in the angular velocity of the Earth rotation, that affect the adopted value of the flattening for equidensity surface within the Earth, are discussed. The computational results show that the contribution of the redistribution of the density in the Earth interior (especially in the core) on the change of the flattening at the core-mantle boundary (CMB) is marginal, and that the calculated value of the flattening at the CMB can be in good agreement with the VLBI observed value so long as the fact that the angular velocity of the Earth rotation has undergone the tidal evolution is taken into account. As a result, this paper presents a set of recommended values of the dynamical parameters of the Earth (see Table III) for computing Earth's forced nutation series. 相似文献
92.
93.
LUO Kai-li LI Bao-sheng ZHU Yi-zhi JIN He-ling ZHANG David Dian YAN Man-cun LI Hou-xin YAO Chun-xia ZHANG Yu-hongf . Department of Geography South China Normal University Guangzhou P. R. China . State Key Labor 《中国地理科学(英文版)》2001,11(4):336-342
About 70years ago,Frenc卜卜alaeohdoglst回LL sc卜dars al卜ome and a卜road卜ave successively con-HARD de Chardin P.et al.Initiated the Quaternary ducted large amount ofwork on the Later Quaternarygeologlcal research In the >alawusu River Basin of strata(TEILHARD,1924; YUAN,1978; LI,1987;desert region of Northern China and established th,ZHENG,1989; SUN et al,1996; LI et al,1993),Salawusu Formation门EILHARD,1924).Sine,then,palaeobiology… 相似文献
94.
95.
Hong Lin Zhang Douglas H. Sampson 《Monthly notices of the Royal Astronomical Society》2001,322(3):433-437
Calculations are made of the resonance contribution to electron-impact excitation of H-like 13 C and Li-like 23 Na, 25 Mg, 27 Al and 29 Si to the upper hyperfine levels that produce millimetre (mm) lines of likely astrophysical interest. The resonance contribution is found to be very important for these Li-like ions, considerably more important than for Li-like 57 Fe considered previously. However, resonances are found to be rather unimportant for H-like 13 C. The effect of radiative decay on the resonance contribution is found to be insignificant in all of the present calculations. 相似文献
96.
Profiles of spectral lines emitted from an accretion ring around an object with strong gravitational field should be affected by Doppler shift, gravitational redshift, and deflection of light. Taking these effects into account, precise line profile of a Keplerian ring around a Kerr black hole for a distant observer is obtained by solving the kinetic equation of photons. 相似文献
97.
98.
BIAN Jian-min LIN Nian-feng TANG Jie College of Environment Resources Jilin University Changchun Jilin China 《东北亚地学研究》2005,(Z1)
Desertification is an environmental issue in the world. The salt-alkalization desertification land area formed by both primary and secondary salt-alkalization has extended in a large scale, which has become a significant eco-environmental problem. Based on the characteristics of eco-environment and the situation of desertification in western Songnen plain, this paper reports the analyzes of its formation in background and cause. An early warning system on the salt-alkalization desertification is established and the GIS technology is used to abstract the information of desertification evaluation index. Supported by the integrated technology of the GIS and ANN, the orientation and quantitative result of desertification are gained, which is helpful to the eco-environment protection and resource development in western Songnen Plain. 相似文献
99.
Silicon limitation on primary production and its destiny in Jiaozhou Bay, China——Ⅳ:Study on cross-bay transect from estuary to ocean 总被引:1,自引:0,他引:1
The authors analyzed the data collected in the Ecological Station Jiaozhou Bay from May 1991 to November 1994, including 12
seasonal investigations, to determine the characteristics, dynamic cycles and variation trends of the silicate in the bay.
The results indicated that the rivers around Jiaozhou Bay provided abundant supply of silicate to the bay. The silicate concentration
there depended on river flow variation. The horizontal variation of silicate concentration on the transect showed that the
silicate concentration decreased with distance from shorelines. The vertical variation of it showed that silicate sank and
deposited on the sea bottom by phytoplankton uptake and death, and zooplankton excretion. In this way, silicon would endlessly
be transferred from terrestrial sources to the sea bottom. The silicon took up by phytoplankton and by other biogeochemical
processes led to insufficient silicon supply for phytoplankton growth. In this paper, a 2D dynamic model of river flow versus
silicate concentration was established by which silicate concentrations of 0.028–0.062 μmol/L in seawater was yielded by inputting
certain seasonal unit river flows (m3/s), or in other words, the silicate supply rate; and when the unit river flow was set to zero, meaning no river input, the
silicate concentrations were between 0.05–0.69 μmol/L in the bay. In terms of the silicate supply rate, Jiaozhou Bay was divided
into three parts. The division shows a given river flow could generate several different silicon levels in corresponding regions,
so as to the silicon-limitation levels to the phytoplankton in these regions. Another dynamic model of river flow versus primary
production was set up by which the phytoplankton primary production of 5.21–15.55 (mgC/m2·d)/(m3/s) were obtained in our case at unit river flow values via silicate concentration or primary production conversion rate.
Similarly, the values of primary production of 121.98–195.33 (mgC/m2·d) were achieved at zero unit river flow condition. A primary production conversion rate reflects the sensitivity to silicon
depletion so as to different phytoplankton primary production and silicon requirements by different phytoplankton assemblages
in different marine areas. In addition, the authors differentiated two equations (Eqs. 1 and 2) in the models to obtain the
river flow variation that determines the silicate concentration variation, and in turn, the variation of primary production.
These results proved further that nutrient silicon is a limiting factor for phytoplankton growth.
This study was funded by NSFC (No. 40036010), and the Director's Fund of the Beihai Sea Monitoring Center, the State Oceanic
Administration. 相似文献
100.
The absolute magnitudeM
v
of the hydrogen deficient binary υ Sgr has been estimated as -4.8 ± 1.0 from the distribution of the interstellar reddening,
polarization and interstellar lines of the surrounding stars. From the ANS observations obtained at the time of the secondary
eclipse, it appears that the hotter secondary is surrounded by a disc with colours of a B8-B9 star. The λ 1550 CIv absorption
line arising in the stellar wind does not show any change in strength during the secondary minimum. The upper limit to the
mass-loss rate from the high temperature wind is estimated as ≤ 5 × 10-7 M⊙ yr-1 from the 2 cm and 6 cm radio observations.
Based on observations obtained with the Astronomical Netherlands Satellite and VLA. The National Radio Astronomy Observatory’s
Very Large Array at Socorro, New Mexico is operated by Associated Universities Inc. under contract with the National Science
Foundation. 相似文献