全文获取类型
收费全文 | 30510篇 |
免费 | 5056篇 |
国内免费 | 5989篇 |
专业分类
测绘学 | 7125篇 |
大气科学 | 3031篇 |
地球物理 | 3665篇 |
地质学 | 16091篇 |
海洋学 | 3621篇 |
天文学 | 847篇 |
综合类 | 3464篇 |
自然地理 | 3711篇 |
出版年
2024年 | 349篇 |
2023年 | 1267篇 |
2022年 | 1733篇 |
2021年 | 1792篇 |
2020年 | 1262篇 |
2019年 | 1725篇 |
2018年 | 1171篇 |
2017年 | 1231篇 |
2016年 | 1245篇 |
2015年 | 1391篇 |
2014年 | 2321篇 |
2013年 | 1738篇 |
2012年 | 2103篇 |
2011年 | 2027篇 |
2010年 | 1806篇 |
2009年 | 1768篇 |
2008年 | 1725篇 |
2007年 | 1545篇 |
2006年 | 1422篇 |
2005年 | 1391篇 |
2004年 | 1117篇 |
2003年 | 949篇 |
2002年 | 866篇 |
2001年 | 894篇 |
2000年 | 729篇 |
1999年 | 700篇 |
1998年 | 681篇 |
1997年 | 697篇 |
1996年 | 639篇 |
1995年 | 543篇 |
1994年 | 526篇 |
1993年 | 433篇 |
1992年 | 462篇 |
1991年 | 345篇 |
1990年 | 288篇 |
1989年 | 260篇 |
1988年 | 80篇 |
1987年 | 55篇 |
1986年 | 45篇 |
1985年 | 29篇 |
1984年 | 19篇 |
1983年 | 10篇 |
1982年 | 25篇 |
1981年 | 12篇 |
1980年 | 17篇 |
1979年 | 17篇 |
1977年 | 9篇 |
1961年 | 9篇 |
1957年 | 12篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
41.
本文讨论了利用LAMOST作银河系研究的科学意义及可能性;并提出具体实施LAMOST/GAIAXY计划的初步方案。 相似文献
42.
对广西岩溶区三个铅锌硫化物矿床风化带观测结果表明,中低山岩溶区硫化物矿床具有更强烈的风化淋滤作用,但次生晕面积较小;岩溶平原区硫化物矿床风化与元素表生迁移特征较相似于非岩溶区的情况,表现在可产生较宽的pH偏低带和较大面积的次生晕 相似文献
43.
Xiao-Hong Cui En-Wei Liang Rui-Jing Lu National Astronomical Observatories/Yunnan Observatory Chinese Academy of Sciences Kunming ciwei@.com Department of Physics Guangxi University Nanning Department of Astronomy Nanjing University Nanjing Graduate School of the Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(2):151-158
The narrowness of the distribution of the peak energy of the vFv spectrum of gamma-ray bursts (GRBs) and the unification of GRB populations are great puzzles yet to be solved. We investigate the two puzzles based on the global spectral behaviors of different GRB populations, the long GRBs, the short GRBs, and the X-ray flashes (XRFs), in the HR - Ep plane (HR the spectral hardness ratio) with BATSE and HETE-2 observations. It is found that the long GRBs and the XRFs observed by HETE-2 seem to follow the same sequence in the HR-Ep plane, with the XRFs at the low end of this sequence. We fit the sequence by a universal Band function, and find that this sequence is mainly defined by the low energy index α, and is insensitive to the high energy index,β. With fixed β=-5, a best fit is given by α=-1.00 with X^2min/dof=2.2. The long and short GRBs observed by BATSE follow significantly different sequences in the HR-Ep plane, with most of the short GRBs having a larger hardness ratio than the long GRBs at a given Ep. For the long GRBs a best-fit yields α=-0.30 and β=-2.05. For the short GRBs, a best fit gives α=-0.60 with x^2min=1.1 (withβfixed at-2.0 because it is numerically unstable). The α value for the short GRBs is significantly greater than that for the long GRBs. These results indicate that the global spectral behaviors of the long GRB sample and the XRF sample are similar, while that of the short GRBs is different. The short GRBs seem to be a unique subclass of GRBs, and they are not the higher energy extension of the long GRBs. 相似文献
44.
45.
Tao Kiang Yong-Feng Wu Xing-Fen Zhu Dunsink Observatory Dublin Institute for Advanced Studies Dublin Ireland Center for Astrophysics University of Science Technology of China Hefei 《中国天文和天体物理学报》2004,4(3):209-220
Rhombic cell analysis as outlined in the first paper of the present seriesis applied to samples of varying depths and liming luminosities of the IRAS/PSCzCatalogue. Numerical indices are introduced to summarize essential information.Because of the discrete nature of the analysis and of the space distribution of galax-ies, the indices for a given sample must be regarded as each having an irreduciblescatter. Despite the scatter, the mean indices show remarkable variations acrossthe samples. The underlying factor for the variations is shown to be the limitingluminosity rather than the sampling depth. As samples of more and more luminousgalaxies are considered over a range of some 2.5 magnitudes (a factor of some 75in space density), the morphology of the filled and empty regions defined by thegalaxies degrades steadily towards insignificance, and the degrading is faster for thefilled than the empty region. 相似文献
46.
47.
宣森 《广东海洋大学学报》1993,(1)
将专家系统技术与常规控制、模糊控制、超前控制、仿人智能操作控制以及计算机控制技术相结合,设计了一种新的基于专家系统决策的多模式控制(MCDBES)。经试验和实际运行结果表明,MCDBES系统对未知数学模型的被控对象,具有良好的控制性能。本文提出的MCDBES已成功地应用于燃油陶瓷窑炉计算机控制系统。 相似文献
48.
49.
Radio observations of some active regions (ARs) obtained with the Nobeyama radioheliograph at λ=1.76cm are used for estimating the magnetic field strength in the upper chromosphere, based on thermal bremsstrahlung. The results are compared with the magnetic field strength in the photosphere from observations with the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station of Beijing Astronomical Observatory. The difference in the magnetic field strength between the two layers seems reasonable. The solar radio maps of active regions obtained with the Nobeyama radioheliograph, both in total intensity (I-map) and in circular polarizations (V-map), are compared with the optical magnetograms obtained with the SMFT. The comparison between the radio map in circular polarization and the longitudinal photospheric magnetogram of a plage region suggest that the radio map in circular polarization is a kind of magnetogram of the upper chromosphere. The comparison of the radio map in total intensity with the photospheric vector magnetogram of an AR shows that the radio map in total intensity gives indications of magnetic loops in the corona, thus we have a method of defining the coronal magnetic structure from the radio I-maps at λ=1.76 cm. Analysing the I-maps, we identified three components: (a) a compact bright source; (b) a narrow elongated structure connecting two main magnetic islands of opposite polarities (observed in both the optical and radio magnetograms); (c) a wide, diffuse, weak component that corresponds to a wide structure in the solar active region which shows in most cases an S or a reversed S contour, which is probably due to the differential rotation of the Sun. The last two components suggest coronal loops on different spatial scales above the neutral line of the longitudinal photospheric magnetic field. 相似文献
50.
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