全文获取类型
收费全文 | 867篇 |
免费 | 71篇 |
国内免费 | 297篇 |
专业分类
测绘学 | 68篇 |
大气科学 | 124篇 |
地球物理 | 341篇 |
地质学 | 456篇 |
海洋学 | 100篇 |
天文学 | 41篇 |
综合类 | 29篇 |
自然地理 | 76篇 |
出版年
2023年 | 11篇 |
2022年 | 7篇 |
2021年 | 7篇 |
2020年 | 21篇 |
2019年 | 14篇 |
2018年 | 17篇 |
2016年 | 12篇 |
2015年 | 11篇 |
2014年 | 13篇 |
2013年 | 16篇 |
2012年 | 28篇 |
2011年 | 47篇 |
2010年 | 29篇 |
2009年 | 23篇 |
2008年 | 27篇 |
2007年 | 39篇 |
2006年 | 47篇 |
2005年 | 45篇 |
2004年 | 40篇 |
2003年 | 31篇 |
2002年 | 49篇 |
2001年 | 31篇 |
2000年 | 24篇 |
1999年 | 22篇 |
1998年 | 29篇 |
1997年 | 10篇 |
1996年 | 19篇 |
1995年 | 27篇 |
1994年 | 22篇 |
1993年 | 33篇 |
1992年 | 45篇 |
1991年 | 40篇 |
1990年 | 46篇 |
1989年 | 35篇 |
1988年 | 38篇 |
1987年 | 37篇 |
1986年 | 24篇 |
1985年 | 28篇 |
1984年 | 35篇 |
1983年 | 19篇 |
1982年 | 13篇 |
1981年 | 23篇 |
1980年 | 27篇 |
1978年 | 7篇 |
1936年 | 4篇 |
1935年 | 6篇 |
1934年 | 4篇 |
1933年 | 4篇 |
1932年 | 4篇 |
1924年 | 5篇 |
排序方式: 共有1235条查询结果,搜索用时 0 毫秒
211.
We present new statistical results on the spectral index-flux density relation for large samples of radio sources using archival data of the most sensitive surveys, such as 6C, Miyun, WENSS, B3, NVSS, GB87. Instrumental selection effects and the completeness of the catalogs are discussed. Based on the spectral indices calculated for about 200 000 sources from the WENSS (327 MHz) and NVSS (1.4 GHz) catalogs, we obtained (1) The median spectral index increases from amed --0.9 toαmed --0.8 (Svαvα), while S327 flux densities decrease from 0.1 Jy down to 25mJy. The median spectral indices nearly show no variation within the error bars when the flux density is larger than 0.1 Jy. (2) A dependence of the fraction of ultra-steep spectrum sources (USS, -1.5 <α<-1.0), steep spectrum sources (SSS, -1.0 <α< -0.5) and flat spectrum sources (FSS, -0.5≤α≤0.0) is partly responsible for the spectral flattening. Another contribution to the spectral flattening comes from the variation of αmed of steep spectrum so 相似文献
212.
Wdowiak TJ Lee W Cronin J Beegle LW Robinson MS 《Planetary and Space Science》1995,43(10-11):1175-1182
Processes resulting in the formation of hydrocarbons of carbonaceous chondrites and the identity of the interstellar molecular precursors involved are an objective of investigations into the origin of the solar system and perhaps even life on earth. We have combined the resources and experience of an astronomer and physicists doing laboratory simulations with those of a chemical expert in the analysis of meteoritic hydrocarbons, in a project that investigated the conversion of polycyclic aromatic hydrocarbons (PAHs) formed in stellar atmospheres into alkanes found in meteorites. Plasma hydrogenation has been found in the University of Alabama at Birmingham Astrophysics Laboratory to produce from the precursor PAH naphthalene, a new material having an IR absorption spectrum (Lee, W. and Wdowiak, T.J., Astrophys. J. 417, L49-L51, 1993) remarkably similar to that obtained at Arizona State University of the benzene-methanol extract of the Murchison meteorite (Cronin, J.R. and Pizzarello, S., Geochim. Cosmochim. Acta 54, 2859-2868, 1990). There are astrophysical and meteoritic arguments for PAH species from extra-solar sources being incorporated into the solar nebula, where plasma hydrogenation is highly plausible. Conversion of PAHs into alkanes could also have occurred in the interstellar medium. The synthesis of laboratory analogs of meteoritic hydrocarbons through plasma hydrogenation of PAH species is underway, as is chemical analysis of those analogs. The objective is to clarify this heretofore uninvestigated process and to understand its role during the origin of the solar system as a mechanism of production of hydrocarbon species now found in meteorites. Results have been obtained in the form of time-of-flight spectroscopy and chemical analysis of the lab analog prepared from naphthalene. 相似文献
213.
<正> 1.前言测量人员总希望角度测量仪器不受度盘分划不可避免的剩余误差的影响。实现此目的最明显的观点认为,在每次角度测量中应计及所有的分划。这样所得平均值中就可不含任何分划误差,因为这时度盘分划误差的总和显然为零。利用几条分划线来求得一个读数,并非新鲜想法。早在17世纪,Hedraeus就第一次应用了这种方法,他在水平度盘对径位置上 相似文献
214.
Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia 总被引:2,自引:0,他引:2
Kaufman AJ Knoll AH Semikhatov MA Grotzinger JP Jacobsen SB Adams W 《Geological magazine》1996,133(5):509-533
Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykal Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved. 相似文献
215.
216.
近5ka温度的金川泥炭δ~(18)O记录 总被引:45,自引:1,他引:45
根据吉林金川泥炭纤维素20a分辨率的氧同位素组成时间序列及其与有关文献资料对比,论述了中国东部过去5000a历史时期温度的变化过程,提供了大暖期、新冰期、中世纪温暖期和小冰期在该区存在的δ~(18)O证据;揭示了在约4590~3600 aBP期间存在1次大幅度自然升温过程,并讨论了此气候自然变暖对干旱、陆地植被和文明发展的影响. 相似文献
217.
We download the Large sky Area Multi-Object fiber Spectroscopic Telescope Data Release 8(LAMOST DR8)low resolution catalog for 6,478,063 AFGK type stars and plot the figures of effective temperature,gravitational acceleration and metal abundance.Some small and medium mass stars are evolved from pre-main sequence or main sequence stage to planetary nebula stage or white dwarf stage by the stellar evolution code MESA.We analyze the observed statistical data and model calculation results,and then o... 相似文献
218.
Distribution and sources of polycyclic aromatic hydrocarbons (PAHs) in sediments from Zhanjiang Bay and Leizhou Bay, South China 总被引:4,自引:0,他引:4
The concentrations and spatial distribution of polycyclic aromatic hydrocarbons have been investigated in two adjacent bays of Zhanjiang and Leizhou, China. The total concentrations of the 16 USEPA priority PAHs were ranged from 41.96 to 933.90ng/g dry weight with an average concentration of 315.98ng/g and ranged from 21.72 to 319.61ng/g with an average concentration of 103.91ng/g in Zhanjiang and Leizhou Bays, respectively. The spatial distribution of PAHs was site-specific and appeared to be somewhat positively correlated with TOC and negatively correlated with sediment grain size in the two bays. The values of Phe/Ant, BaA/228 and InP/276 were higher than 10, less than 0.2, and from 0.2 to 0.5, respectively, indicating that the sources of PAHs in the two bays were mainly from petroleum and its combustion, which predominantly originated from those ships and boats coming and going in the two bays. 相似文献
219.
220.
A statistical study of the dependence between various critical fusion temperatures of a certain kind ofcoal and its chemical components is carried out.As well as using classical dependence techniques(multiple,stepwise and PLS regression,principal components,canonical correlation,etc.)together withthe corresponding inference on the parameters of interest,non-parametric regression and bootstrapinference are also performed. 相似文献