全文获取类型
收费全文 | 59538篇 |
免费 | 22176篇 |
国内免费 | 46808篇 |
专业分类
测绘学 | 4361篇 |
大气科学 | 25389篇 |
地球物理 | 18306篇 |
地质学 | 47804篇 |
海洋学 | 18271篇 |
天文学 | 2140篇 |
综合类 | 7065篇 |
自然地理 | 5186篇 |
出版年
2024年 | 281篇 |
2023年 | 871篇 |
2022年 | 1812篇 |
2021年 | 2343篇 |
2020年 | 3219篇 |
2019年 | 6474篇 |
2018年 | 6964篇 |
2017年 | 6605篇 |
2016年 | 6865篇 |
2015年 | 5855篇 |
2014年 | 5769篇 |
2013年 | 6186篇 |
2012年 | 5855篇 |
2011年 | 5668篇 |
2010年 | 5538篇 |
2009年 | 4932篇 |
2008年 | 3992篇 |
2007年 | 4006篇 |
2006年 | 3549篇 |
2005年 | 3295篇 |
2004年 | 3375篇 |
2003年 | 3083篇 |
2002年 | 2817篇 |
2001年 | 2625篇 |
2000年 | 2412篇 |
1999年 | 2803篇 |
1998年 | 2621篇 |
1997年 | 2726篇 |
1996年 | 2161篇 |
1995年 | 2096篇 |
1994年 | 1863篇 |
1993年 | 1720篇 |
1992年 | 1428篇 |
1991年 | 1077篇 |
1990年 | 919篇 |
1989年 | 813篇 |
1988年 | 707篇 |
1987年 | 550篇 |
1986年 | 443篇 |
1985年 | 330篇 |
1984年 | 351篇 |
1983年 | 210篇 |
1982年 | 259篇 |
1981年 | 176篇 |
1980年 | 148篇 |
1979年 | 122篇 |
1978年 | 61篇 |
1977年 | 51篇 |
1972年 | 45篇 |
1971年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Wang Yanhui 《水文研究》1992,6(2):241-251
Black locust (Robina pseudoacacia) has become one of the most important shelter species in the loess area of northwest China. This paper summarizes recent research concerning its hydrological influence, including canopy interception, litter absorption capacity, its effect on rainfall kinetic energy, infiltration rates, surface runoff, soil moisture, and evapotranspiration, and its role in soil conservation. Several predictive models are listed. on the basis of existing results, optimum characteristics for an effective plantation are defined, and problems requiring further research are identified. 相似文献
22.
Zhang Qing Wang Li-fan Hu Jing-yao P. A. Mazzali Wang Zhen-ru 《Chinese Astronomy and Astrophysics》1995,19(4):426-431
We present the modeling of the ultraviolet and optical spectra obtained simultaneously on 1993 April 15 with the HST and at Lick Observatory. A Monte Carlo code is employed in the modeling and a comparison is made between models reported by different groups. With an atmosphere similar to the Sun in chemical composition, the observed spectral lines are well reproduced by a power law density structure of index around 20 except the strong H and HeI λ5876 lines which have peculiar absorption profiles. The photospheric velocity is found to be 9500 km/s and the blackbody temperature of the spectrum is 7990 K. For H and HeI λ5876, we suggest a two-component density structure which has a smoother layer located immediately outside the steeply decreasing inner envelope. The power law indices are most probably 20 and 3, respectively, with the transition point at about 13 000 km/s. In addition, this outer smooth layer serves to flatten the far UV spectrum as observed. 相似文献
23.
Aimed at promoting regional coalitions and expanding the approach to economic links, this paper puts forward some new concepts
such as link intensity and receiving coefficient, expounds the indexes of quantitative analysis of economic links and establishes
the quantitative-analysis model of economic links. With help of the model, this paper calculates the values of the link intensities
between Su-Xi-Chang (Suzhou-Wuxi-Changzhou) region and Shanghai, and analyzes the regional difference of the economic links. 相似文献
24.
25.
26.
锥形海山是马里亚纳弧前区非火山成因的海山。它是由俯冲的太平洋板块对仰冲板块的上推作用,地壳下的上地幔物质底辟侵入的结果。海山顶部的岩石是由蛇纹石化超镁铁质方辉橄榄岩组成。其主要矿物成分为斜方辉石、橄榄石、尖晶石、磁铁矿、蛇纹石和碳酸盐。手标本呈显晶质块状构造,薄片中呈粒状、残留状、筛网状及斑状变晶结构。岩石化学特征表现为:低Si和Al,高M/F值,含水,亏损了所有过渡金属。岩石的成因属超基性岩浆直接结晶,后经海水的参入,在热液作用下普遍发生蛇纹石化和碳酸盐化。 相似文献
27.
Paraconchoeciadiacanthusn.sp.,anewspeciesofplanktonicOstracoda¥ChenRuixiangandLinJinghong(ReceivedApril16,19931acceptedJuly20... 相似文献
28.
Aquasi-three-dimensionalnumericalpredictionmodelofsalinitystructureinBohaiSeaandHuanghaiSea¥SunWeiyangandWangZongshan(Receive... 相似文献
29.
Chemical forms of Ba are determined in samples of suspension and sediment from the Zhujiang Estuary. Their interfacial geochemical processes are discussed. Total suspended Ba content is between 2. 4 and 40. 4 μg/L, and mostly exists in the crystalline form (43. 5%), secondly in the Fe-Mn oxidative form (23.1%). Percentages of organic, carbonate and exchangeable forms are 14. 8%, 11. l%and 7.4%, respectively. Total content of Ba in the sediment is between 158. 6 and 48. 0 ug/g. Percentages of crystalline form, Fe-Mn oxidative form and carbonate form are 78. 4% ,13. 5%and 8. 2%, respectively. Organic and exchangeable forms are not detected. The study on the mechanism of interfacial movement suggests that the salinity range of 10 is the turning point for the varied distribution of Ba. The subsidence of crystalline form affects the decrease of Ba content in suspension. The decrease mostly takes place in the salinity range lower than 10, which corresponds to the high value of Ba content in the sediment. The 相似文献
30.
Euniphysa, originally a genus of the family Eunicidae, was erected by Wesenberg-Lund in 1949, and for a longtime there has been only one species in this genus. The present report is based on our samples collected from the South China Sea, and we consider that the differences between Euniphysa and Eunice are very significant: Five occipital tentacles distributed unequidistantly; the jaw consists of 6 pairs of maxillary plates; the body may be divided into anterior region and posterior region, etc. Therefore we suggest that Euniphysa should be separated from Eunicidae, and become a new family Euniphysi-dae. At present this family includes 3 genera, namely, Euniphysa , Paraeuniphysa and 1 new genira Heterophysa; Totally there are 7 species, including 3 new species and 1 new combination species. 相似文献