全文获取类型
收费全文 | 18667篇 |
免费 | 2470篇 |
国内免费 | 3539篇 |
专业分类
测绘学 | 1516篇 |
大气科学 | 2483篇 |
地球物理 | 3079篇 |
地质学 | 7249篇 |
海洋学 | 2409篇 |
天文学 | 5302篇 |
综合类 | 1038篇 |
自然地理 | 1600篇 |
出版年
2024年 | 70篇 |
2023年 | 188篇 |
2022年 | 545篇 |
2021年 | 606篇 |
2020年 | 645篇 |
2019年 | 748篇 |
2018年 | 623篇 |
2017年 | 660篇 |
2016年 | 704篇 |
2015年 | 772篇 |
2014年 | 1062篇 |
2013年 | 1231篇 |
2012年 | 1209篇 |
2011年 | 1310篇 |
2010年 | 1223篇 |
2009年 | 1539篇 |
2008年 | 1454篇 |
2007年 | 1457篇 |
2006年 | 1320篇 |
2005年 | 1178篇 |
2004年 | 922篇 |
2003年 | 788篇 |
2002年 | 691篇 |
2001年 | 616篇 |
2000年 | 547篇 |
1999年 | 522篇 |
1998年 | 420篇 |
1997年 | 255篇 |
1996年 | 231篇 |
1995年 | 196篇 |
1994年 | 186篇 |
1993年 | 201篇 |
1992年 | 104篇 |
1991年 | 76篇 |
1990年 | 70篇 |
1989年 | 46篇 |
1988年 | 61篇 |
1987年 | 20篇 |
1986年 | 23篇 |
1985年 | 33篇 |
1984年 | 31篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 7篇 |
1980年 | 13篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 19篇 |
1976年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 49 毫秒
91.
92.
93.
We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae. 相似文献
94.
95.
96.
White mica from the Liassic black shales and slates in Central Switzerland was analysed by transmission electron microscopy (TEM) and electron microprobe to determine its textural and compositional evolution during very low-grade prograde metamorphism. Samples were studied from the diagenetic zone, anchizone and epizone (T ≈100°–450 °C). Phyllosilicate minerals analysed include illite/smectite (I/S), phengite, muscovite, brammallite, paragonite, margarite and glauconite. Textural evolution primarily is towards larger, more defect-free grains with compositions that approach those of their respective end-members. The smectite-to-illite transformation reduced the amounts of the exchange components SiK?1Al?1, MgSiAl?2, and Fe3+Al?1. These trends continue to a lesser degree in the anchizone and epizone. Correlations between the proportion of smectite in I/S and the composition of I/S indicate that smectite layers may contain a high layer charge. Illite in I/S bears a compositional resemblance to macrocrystalline phengite in some samples, but is different in others. Paragonite first appears in the upper diagenetic zone or lower anchizone as an interlayer-deficient brammallite, and it may be mixed with muscovite on the nanometre scale. Owing to the small calculated structure factor for paragonite-muscovite superstructures, conventional X-ray powder diffraction cannot distinguish between mixed-layer structures and a homogeneous compositionally intermediate solid solutions. However, indirect TEM evidence shows that irregularly shaped domains of Na- and K-rich mica exist below 10 nm. Subsequent coarsening of domains at higher grades produced discrete paragonite grains at the margins of muscovite crystals or in laths parallel to the basal plane of the host muscovite. Margarite appears in the epizone and follows a textural evolution similar to paragonite in that mixtures of margarite, paragonite, and muscovite may initially occur on the nanometre scale. However, no evidence of interlayer-poor margarite has been found. 相似文献
97.
98.
R. F. Griffin 《Journal of Astrophysics and Astronomy》1990,11(4):533-540
Photoelectric radial-velocity measurements show that HD 118670 is a double-lined spectroscopic binary in an orbit which is
not quite circular and whose period is about 48 days. Spectral types of K0 V and K7 V would satisfy the photometry and the
mass ratio; the mass function would then suggest the possibility of eclipses. However, actual spectral classification indicates
a luminosity somewhat above the main sequence 相似文献
99.
张赤军 《大地测量与地球动力学》2002,22(4):31-34
介绍当今确定天文大地垂偏差的新仪器-CCD自动天体测量仪和确定两点间高程异常差的新方法,该方法用单极坐标代替繁琐的双极坐标进行计算,讨论了天文重力水准的误差及垂线偏差非线性影响等问题,估计在不久的将来,用这一手段施测山区似大地水准面的精度可望达到厘米级。 相似文献
100.
IMPACTS OF FUTURE SEA LEVEL RISE ON SALT WATER INTRUSION IN THE CHANGJIANG RIVER ESTUARY 总被引:1,自引:0,他引:1
杨桂山 《中国地理科学(英文版)》1992,2(1):30-41
Sea level rise could increase the salinity of an estuary by altering the balance between fresh water and salt water. The implications of sea level rise for increasing salinity have been examined in the Changjiang (Yangtze) River estuary. By correlative analysis of chlorinity, discharge and tidal level and calculation of two-dimensional chlorinity, distribution of the Changjiang River estuary, the changes of the intensity and lasting hours of salt water intrusion at Wusong Station and the changes of chlorinity distribution in the South Branch of the Changjiang River estuary have been estimated when future sea level rises 50-100 cm. The intensity of salt water intrusion in the future will be far more serious than current trend. 相似文献