全文获取类型
收费全文 | 4802篇 |
免费 | 980篇 |
国内免费 | 1191篇 |
专业分类
测绘学 | 89篇 |
大气科学 | 270篇 |
地球物理 | 509篇 |
地质学 | 4434篇 |
海洋学 | 383篇 |
天文学 | 228篇 |
综合类 | 268篇 |
自然地理 | 792篇 |
出版年
2024年 | 33篇 |
2023年 | 166篇 |
2022年 | 193篇 |
2021年 | 165篇 |
2020年 | 141篇 |
2019年 | 176篇 |
2018年 | 126篇 |
2017年 | 138篇 |
2016年 | 145篇 |
2015年 | 177篇 |
2014年 | 272篇 |
2013年 | 254篇 |
2012年 | 262篇 |
2011年 | 273篇 |
2010年 | 239篇 |
2009年 | 246篇 |
2008年 | 256篇 |
2007年 | 254篇 |
2006年 | 221篇 |
2005年 | 248篇 |
2004年 | 211篇 |
2003年 | 210篇 |
2002年 | 272篇 |
2001年 | 313篇 |
2000年 | 234篇 |
1999年 | 197篇 |
1998年 | 182篇 |
1997年 | 195篇 |
1996年 | 206篇 |
1995年 | 170篇 |
1994年 | 146篇 |
1993年 | 115篇 |
1992年 | 147篇 |
1991年 | 125篇 |
1990年 | 105篇 |
1989年 | 83篇 |
1988年 | 14篇 |
1987年 | 7篇 |
1986年 | 11篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1958年 | 3篇 |
1952年 | 2篇 |
1948年 | 3篇 |
1946年 | 1篇 |
1944年 | 1篇 |
1927年 | 1篇 |
排序方式: 共有6973条查询结果,搜索用时 15 毫秒
11.
基于可靠度理论的黄土高边坡优化设计 总被引:11,自引:2,他引:9
对黄土高边坡进行可靠性分析,应用数学原理和优化原理,建立了黄土高边坡的优化模型,对铜黄一级公路黄土高边坡进行分析验证。结果表明,该方法计算的结果与实际较接近.应用中易操作。 相似文献
12.
素土桩挤密地基由桩间挤密土和回填素土夯实的桩体组成,它是一种人工“复合地基”。其作用是通过素土桩挤密法提高地基土的密实度,从而对湿陷性黄土达到消除浅部或深部的部分或全部湿陷性的作用,并改善地基承载能力,减少地基变形。本文通过现场单桩、群桩单元体轻便触探、小环刀深层取样、开剖取样等方法对场地地基土挤密前、后干重度和压实系数进行了对比分析,判定用素土桩挤密法处理陇东湿陷性黄土地基的可行性。 相似文献
13.
14.
GiovanniB.Piccardo 《《幕》》2003,26(3):193-199
Mantle peridotites were early exposed at the sea-floor of the Jurassic Tethys derived from the subcontinental mantle of the Europe-Adria system. During continental rifting and oceanic spreading, these lithospheric peri-dotites were percolated via diffuse reactive porous flowby melt fractions produced by near-fractional melting of the upwelling asthenosphere. Ascending melts inter-acted with the lower lithosphere, dissolving pyroxenes and precipitating olivine, and crystallized at shallower levels in the mantle column causing melt impregnation.Subsequent focused porous flow formed replacive dunitechannels, cutting the impregnated oeridotites, which were conduits for upward migration of MORB-type liq-uids. Melt migration produced depletionlrefertilization and significant heating of the percolatedlimpregnated mantle, i.e the thermochemical erosion of the litho-sphere. Impregnated and thermally modified lithos-pheric mantle was cooled by conductive heat loss dur-ing progressive lithosphere thinning and was intrudeaby MORB magmas, which formed Mg-rich and Fe-richgabbroic dykes and bodies. Alpine-Apennine ophiolitic peridotites record the deep-seated migration of melts which changed their compositions and dynamics during the rift evolution. The thermochemical erosion of the lithospheric mantle by the ascending asthenospheric melts, which induces significant compositional and rhe-ological changes in the lower lithosphere, is a major process in the evolution of the continent-ocean transi-tion towards a slow spreading oceanic system. 相似文献
15.
16.
本文论述了保安地区火山岩的地质、岩石、副矿物、岩石化学、微量元素、稀土元素、稳定同位素、火山岩相及火山机构等特征。并对其演化规律、形成机制与成矿作用的关系,作了初步探讨。 相似文献
17.
本文研究了在1993年3月28日之前发现的899颗超新星(SNe)的样本。其中277颗SNe被用来研究超新星在其母星系中的径向分布。我们研究了四个星系样本中超新星在产生单个超新星(称为一般超新星)的星系及在产生多个超新星(称为多重超新星)的星系中的径向分布。这四个星系样本为:总旋涡星系样本,Sb ̄Sbc星系样本,Sc星系样本,Sc ̄Sd星系样本。研究的结果表明:多重超新星比一般超新星在其母星系中具 相似文献
18.
19.
TL ages of 56 loess samples collected from Xinjiang and the middle reaches of the Yellow River were determined by using fine-grained quartz and man-made light source bleaching techniques.The results indicate that :1.The well-preserved loess sections in Xinjiang began depositing 400000 a ago.The loess/paleosol series may be comparable with that observed in the middle reaches of the Yellow River but the depositing time was probably delayed by 30000-50000a.2.The bottom boundary age of the Malan loess varies from one place to another.The TL dates can be divided into two group:70000-130000 a and 30000-50000 a.The TL data are con-sistent with the appearance of two warmer climate periods since the Pleistocene,indicating obvious regional differences in stratigraphic development of loess,i.e.,paleosol occurred in the warm climate period in the same location,but loess was deposited in other locations.The initial age of loess deposition tends to become older from west to east and from south to north .In contrast ,the cold cli-matic belt showed an opposite trend.The TL ages of loess provide strong evidence for the Quaternary stratigraphy throughout geological times and the shift tendency of climatic belt.3.The depositing rate of the Malan loess along the middle reaches of the Yellow River varies from place to place but the TL age shows a linear relationship with the depositing thickness.The de-positing rate of loess in the north is higher than that in the south,suggesting that loess materials are derived from desert areas of northern China. 相似文献
20.