全文获取类型
收费全文 | 21895篇 |
免费 | 1007篇 |
国内免费 | 593篇 |
专业分类
测绘学 | 449篇 |
大气科学 | 238篇 |
地球物理 | 1235篇 |
地质学 | 3355篇 |
海洋学 | 582篇 |
天文学 | 16476篇 |
综合类 | 155篇 |
自然地理 | 1005篇 |
出版年
2024年 | 56篇 |
2023年 | 133篇 |
2022年 | 240篇 |
2021年 | 204篇 |
2020年 | 192篇 |
2019年 | 265篇 |
2018年 | 148篇 |
2017年 | 155篇 |
2016年 | 179篇 |
2015年 | 274篇 |
2014年 | 266篇 |
2013年 | 295篇 |
2012年 | 313篇 |
2011年 | 345篇 |
2010年 | 377篇 |
2009年 | 1689篇 |
2008年 | 1639篇 |
2007年 | 1923篇 |
2006年 | 1908篇 |
2005年 | 1706篇 |
2004年 | 1860篇 |
2003年 | 1564篇 |
2002年 | 1379篇 |
2001年 | 1213篇 |
2000年 | 1006篇 |
1999年 | 935篇 |
1998年 | 1129篇 |
1997年 | 266篇 |
1996年 | 163篇 |
1995年 | 321篇 |
1994年 | 326篇 |
1993年 | 158篇 |
1992年 | 99篇 |
1991年 | 103篇 |
1990年 | 82篇 |
1989年 | 153篇 |
1988年 | 100篇 |
1987年 | 90篇 |
1986年 | 81篇 |
1985年 | 45篇 |
1984年 | 32篇 |
1983年 | 25篇 |
1982年 | 5篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1977年 | 6篇 |
1954年 | 3篇 |
1905年 | 3篇 |
1900年 | 3篇 |
1897年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Hua-Wei Zhang Gang Zhao Department of Astronomy School of Physics Peking University Beijing National Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(3):238-244
The abundances of the wide binary pair HD 219175 A and B are determined and compared using a line-by-line differential analysis. No evidence for difference has been found in the abundances of Fe, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Ni, Cu and Ba. Our results support a physical relation between the two components of HD 219175. 相似文献
992.
993.
Liliana Formiggini Noah Brosch Elchanan Almoznino Stuart Bowyer Michael Lampton 《Monthly notices of the Royal Astronomical Society》2002,332(2):441-455
We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars.
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples. 相似文献
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples. 相似文献
994.
Dark energy has a dramatic effect on the dynamics of the Universe, causing the recently discovered acceleration of the expansion. The dynamics are also central to the behaviour of the growth of large-scale structure, offering the possibility that observations of structure formation provide a sensitive probe of the cosmology and dark energy characteristics. In particular, dark energy with a time-varying equation of state can have an influence on structure formation stretching back well into the matter-dominated epoch. We analyse this impact, first calculating the linear perturbation results, including those for weak gravitational lensing. These dynamical models possess definite observable differences from constant equation of state models. Then we present a large-scale numerical simulation of structure formation, including the largest volume to date involving a time-varying equation of state. We find the halo mass function is well described by the Jenkins et al. mass function formula. We also show how to interpret modifications of the Friedmann equation in terms of a time-variable equation of state. The results presented here provide steps toward realistic computation of the effect of dark energy in cosmological probes involving large-scale structure, such as cluster counts, the Sunyaev–Zel'dovich effect or weak gravitational lensing. 相似文献
995.
996.
Ramona Núñez-López Anton Lipovka Vladimir Avila-Reese 《Monthly notices of the Royal Astronomical Society》2006,369(4):2005-2012
Possible detection of signatures of structure formation at the end of the 'dark age' epoch ( z ∼ 40–20) is examined. We discuss the spectral–spatial fluctuations in the cosmic microwave background radiation (CMBR) temperature produced by elastic resonant scattering of CMBR photons on deuterated hydrogen (HD) molecules located in protostructures moving with peculiar velocity. Detailed chemical kinematic evolution of HD molecules in the expanding homogeneous medium is calculated. Then, the HD abundances are linked to protostructures at their maximum expansion, whose properties are estimated by using the top-hat spherical approach and the Λ cold dark matter (ΛCDM) cosmology. We find that the optical depths in the HD three lowest pure rotational lines for high-peak protohaloes at their maximum expansion are much higher than those in LiH molecule. The corresponding spectral–spatial fluctuation amplitudes, however, are probably too weak to be detected by current and forthcoming millimetre telescope facilities. We extend our estimates of spectral–spatial fluctuations to gas clouds inside collapsed CDM haloes by using results from a crude model of HD production in these clouds. The fluctuations for the highest peak CDM haloes at redshifts ∼20–30 could be detected in the future. Observations will be important to test model predictions of early structure formation in the Universe. 相似文献
997.
Most ground-penetrating radar (GPR) measurements are performed on nearly flat areas. If strongly dipping reflections and/or diffractions are present in the GPR data, a classical migration-processing step is needed in order to determine the geometries of shallow structures. Nevertheless, a standard migration routine is not suitable for GPR data collected on areas showing a variable and large topographic relief. To take into account topographic variations, the GPR data are, in general, corrected by applying static shifts instead of using an appropriate topographic migration that would place the reflectors at their correct locations with the right dip angle. In this article, we present an overview of Kirchhoff's migration and show the importance of topographic migration in the case where the depth of the target structures is of the same order as the relief variations. Examples of synthetic and real GPR data are shown to illustrate the efficiency of the topographic migration. 相似文献
998.
As to salty soil, salt migrates with water in freezing soils, assembles and crystallizes continuously. Consequently the swelling of the soil volume leads to the phenomenon of salt heaving. It has a practical significance for solving salt-heaving and frost-heaving damage in engineering to deepen the understanding of salt heaving mechanism. In this paper a general overview about new research results at this aspect was presented. And then the study of salt migration and salt heaving mechanism and present salt heaving models were summarized. For further researching the field of salt transfer it is urgent to continually strengthen the salt migration and the numerical simulation study of salt heaving mechanism to expect perfecting the general evaluation of salt heaving prediction models so as to have a better service for engineering. 相似文献
999.
E. Biémont P. Palmeri P. Quinet G. Paquin Z. G. Zhang G. Somesfalean S. Svanberg 《Monthly notices of the Royal Astronomical Society》2001,328(4):1085-1090
Lifetimes of six levels belonging to the 4f10 6p configuration of Ho iii have been measured for the first time using the time-resolved laser-induced fluorescence method. They are compared with multiconfigurational pseudo-relativistic Hartree–Fock calculations. A very good agreement is found. Using the experimental lifetimes and theoretical branching fractions, a first set of transition probabilities of astrophysical interest has been obtained for this ion. 相似文献