全文获取类型
收费全文 | 18449篇 |
免费 | 3551篇 |
国内免费 | 4890篇 |
专业分类
测绘学 | 1782篇 |
大气科学 | 3328篇 |
地球物理 | 4520篇 |
地质学 | 9821篇 |
海洋学 | 2698篇 |
天文学 | 834篇 |
综合类 | 1656篇 |
自然地理 | 2251篇 |
出版年
2024年 | 119篇 |
2023年 | 373篇 |
2022年 | 895篇 |
2021年 | 1005篇 |
2020年 | 864篇 |
2019年 | 1021篇 |
2018年 | 1078篇 |
2017年 | 1004篇 |
2016年 | 1108篇 |
2015年 | 1045篇 |
2014年 | 1162篇 |
2013年 | 1211篇 |
2012年 | 1240篇 |
2011年 | 1315篇 |
2010年 | 1305篇 |
2009年 | 1193篇 |
2008年 | 1198篇 |
2007年 | 1089篇 |
2006年 | 933篇 |
2005年 | 797篇 |
2004年 | 648篇 |
2003年 | 604篇 |
2002年 | 653篇 |
2001年 | 588篇 |
2000年 | 595篇 |
1999年 | 577篇 |
1998年 | 464篇 |
1997年 | 446篇 |
1996年 | 362篇 |
1995年 | 315篇 |
1994年 | 336篇 |
1993年 | 276篇 |
1992年 | 203篇 |
1991年 | 163篇 |
1990年 | 130篇 |
1989年 | 96篇 |
1988年 | 113篇 |
1987年 | 55篇 |
1986年 | 55篇 |
1985年 | 47篇 |
1984年 | 40篇 |
1983年 | 33篇 |
1982年 | 36篇 |
1981年 | 33篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1978年 | 8篇 |
1958年 | 9篇 |
1957年 | 6篇 |
1954年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
We discuss the determination of membership of 42 open clusters. Our analysis shows that Vasilevskis' mathematical model can be reasonably applied to this case. Our improved version of Sanders' method and our definition of cluster member based on the principles of discriminatory analysis effectively exclude stars of low probabilities. It is important in the study of open cluster to use only those with high probabilities. The effectiveness of the statistical method is closely related to the velocity distributions of the member and field stars. For fields where the error rate is high, it is better to combine other data than proper motion in determining membership. 相似文献
33.
低频扩频中伪随机序列产生方法的研究 总被引:2,自引:0,他引:2
伪随机序列的产生是实现BPC低频时码发播台扩展频谱技术升级的一个重要环节。针对BPC低频时码发播台的现有系统,提出了伪随机序列的产生方案,并对其性能作了分析。 相似文献
34.
35.
Teruyuki Maruoka Christian Koeberl Jun‐ichi Matsuda Yasuhiko Syono 《Meteoritics & planetary science》2003,38(8):1255-1262
Abstract— Carbon isotopic compositions were measured for shock‐produced diamond and shocked graphite formed at peak pressures ranging from 37 to 52 GPa. The δ13C values of diamonds produced in a sealed container were generally lower than that of the initial graphite. The differences in the carbon isotopic composition between initial graphite and shocked graphite/diamond may reflect kinetic isotopic fractionation during the oxidation of the graphite/diamond and/or analytical artifacts possibly induced by impurities in the samples. The pressure effect on the isotopic fractionations between graphite and diamond can be estimated from the δ13C values of impurity‐free diamonds produced using a vented container from which gases, including oxygen, in pore spaces escaped during or after the diamond formation (e.g., 0.039 ± 0.085‰ at a peak pressure of 52 GPa). Any isotopic fractionation induced by shock conversion of graphite to diamond is too small to be detected in natural shock‐induced diamond‐graphite systems related to terrestrial impact cratering processes. 相似文献
36.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
37.
Dong-Mei Qin Ping Guo Zhan-Yi Hu Yong-Heng ZhaoNational Laboratory of Pattern Recognition Laboratory Institute of Automation Chinese Academy of Sciences Beijing dmqin@nlpr.ia.ac.cnDepartment of Computer Sciences Beijing Normal University Beijing National Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2003,3(3)
For LAMOST, the largest sky survey program in China, the solution of the problem of automatic discrimination of stars from galaxies by spectra has shown that the results of the PSF test can be significantly refined. However, the problem is made worse when the redshifts of galaxies are not available. We present a new automatic method of star/(normal) galaxy separation, which is based on Statistical Mixture Modeling with Radial Basis Function Neural Networks (SMM-RBFNN). This work is a continuation of our previous one, where active and non-active celestial objects were successfully segregated. By combining the method in this paper and the previous one, stars can now be effectively separated from galaxies and AGNs by their spectra-a major goal of LAMOST, and an indispensable step in any automatic spectrum classification system. In our work, the training set includes standard stellar spectra from Jacoby's spectrum library and simulated galaxy spectra of EO, SO, Sa, Sb types with redshift ranging from 0 to 1 相似文献
38.
39.
In this paper, the equilibrium configurations of rapid rotating compact stars and some gravitational effects are studied within the general relativity by use of the Harrison-Wheeler equation of state and by the self-consistent field method. Numerical calculations show that the equilibrium configuration of a rotating star is a spheroid. For large spin velocities, say, ω > 3.0 × 102 sec−1 the eccentricity and mass increase very rapidly as the angular velocity increases, for the critical angular velocity of the rotating star, the eccentricity is about 0.7, the increase in mass is about 10–35%. The difference of the gravitational redshifts at the surface of the star caused by rotation, and the difference of the light bending when the beam moves in the direction of rotation or in the opposite direction are obvious. 相似文献
40.