全文获取类型
收费全文 | 51326篇 |
免费 | 21846篇 |
国内免费 | 46027篇 |
专业分类
测绘学 | 5494篇 |
大气科学 | 23089篇 |
地球物理 | 15745篇 |
地质学 | 44309篇 |
海洋学 | 18241篇 |
天文学 | 1299篇 |
综合类 | 5931篇 |
自然地理 | 5091篇 |
出版年
2024年 | 242篇 |
2023年 | 717篇 |
2022年 | 1661篇 |
2021年 | 2178篇 |
2020年 | 2952篇 |
2019年 | 6281篇 |
2018年 | 6489篇 |
2017年 | 6052篇 |
2016年 | 6139篇 |
2015年 | 5858篇 |
2014年 | 5375篇 |
2013年 | 6056篇 |
2012年 | 6064篇 |
2011年 | 5934篇 |
2010年 | 5824篇 |
2009年 | 4966篇 |
2008年 | 4267篇 |
2007年 | 4157篇 |
2006年 | 3669篇 |
2005年 | 3348篇 |
2004年 | 3380篇 |
2003年 | 2981篇 |
2002年 | 2717篇 |
2001年 | 2465篇 |
2000年 | 2015篇 |
1999年 | 1919篇 |
1998年 | 1879篇 |
1997年 | 1916篇 |
1996年 | 1486篇 |
1995年 | 1421篇 |
1994年 | 1308篇 |
1993年 | 1222篇 |
1992年 | 1036篇 |
1991年 | 777篇 |
1990年 | 734篇 |
1989年 | 601篇 |
1988年 | 515篇 |
1987年 | 404篇 |
1986年 | 320篇 |
1985年 | 267篇 |
1984年 | 279篇 |
1983年 | 165篇 |
1982年 | 204篇 |
1981年 | 147篇 |
1980年 | 114篇 |
1979年 | 129篇 |
1978年 | 47篇 |
1977年 | 44篇 |
1971年 | 53篇 |
1970年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
93.
对在1981-2000年世界上所发表的和中国学者所发表的有关恒星与恒星系统的论文作统计发现:此期间世界上这一领域的发展较平稳,而我国的发展快速.这反映了改革开放后,我国基础学科研究大有进展.从各分支所占的比重和发展来看,我国在恒星与恒星系统的研究与世界同期有几乎相同的分布,因此总体上我国在这一领域的发展基本正常.当然有些分支发展较快,如有关超新星及其遗迹、星际介质和恒星形成区、化学丰度的研究等,这和一些较强的研究团组形成有关;在双星研究方面,我国则与世界发展一致,双星研究始终是恒星研究领域的重点;而在世界范围内较突出的关于银河系的研究,在我国却相对较弱.恒星和恒星系统这一研究领域20年的论文数统计显示,我国学者所发表的论文只占世界总论文数的1.3%,虽然在最后5年有大幅上升,但也只占2.0%,这与我国IAU会员数所占比例相比是偏少的.就世界整体而言,恒星领域的研究进展与整个天文学领域相比是较慢的,显然这与一批能做深空探测和高能波段观测的设备投入有关.因此,除了对恒星及恒星系统领域作统计分析外,对整个天文学领域各大分支作分析可能对制定今后我国天文学发展计划更有利。 相似文献
94.
1 INTRODUCTIONCompact Symmetric Objects (CSOs) are powerful and compact sources (overall size <1 bpc) with lobe emission on both sides of the central engine. The small size of these sourcesis almost certainly to be attributed to the youth of the sources themselves (ages < 104 yr) andnot due to a dense coallning medium (Readhead 1996). The unification scenario assumes thatCSOs evolve into compact steep spectrum (CSS) sotirces and then into Fanaroff-Riley type 11objects (Fanti 1995). … 相似文献
95.
Yanjun ZHANG Fenglong ZHANG Full QI Fuquan NI 《东北亚地学研究》2006,9(2):199-203
In view of the situation of excavation of open coal mine for the underground water disaster,we should carry out simulation studies for the numerical value of the water lowering project and improve the accuracy and the level of the water lowering project.On the basis of the hydrological geological conditions of certain open mine digging,a more reasonable seepage numerical model was built according to MODFLOW.It was simulated in advance that the process of the confined water level descending with the time,and combining with the actual observations to test the correctness of the model.The calculation showed that the results coincided well with the results of actual measurement.Based on this,different water lowering numerical simulations were built for the open coal mine digging.It could be simulated and forecast that the changes of the groundwater level in drainage process within and outside the mine pit,and it was quantitatively assessed that the possible water lowering result of the opencast water drainage process,which provide an important basis for the actual water lowering project and the possible project disposal. 相似文献
96.
The accuracy of the AE index as a measure of the joule heat production rate is examined for a typical substorm event on 18 March 1978 by estimating the global joule heat production rate, as a function of time, using data obtained from the IMS six meridian chains. In spite of the fact that the AE index had had an initial slow growth which was followed by a rapid growth, the joule heat production rate attained a high level during the slow growth and thus their variations were considerably different from those of the AE index. Therefore, although the AE index is statistically linearly related to the global joule heat production rate, one should be cautious in assuming that details of time variations of the AE index during individual events represent those of the joule heat production rate. 相似文献
97.
98.
Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment. 相似文献
99.
我们于1989年11月30日晚对PG0027+260进行了时间分辨率为108秒的高速CCD测光,得到了一条完整的光变曲线,从而确认其为激变食变星系统,轨道周期0.146~d 相似文献
100.