全文获取类型
收费全文 | 588408篇 |
免费 | 61148篇 |
国内免费 | 78903篇 |
专业分类
测绘学 | 49589篇 |
大气科学 | 66423篇 |
地球物理 | 124546篇 |
地质学 | 259636篇 |
海洋学 | 71303篇 |
天文学 | 70165篇 |
综合类 | 24729篇 |
自然地理 | 62068篇 |
出版年
2023年 | 5102篇 |
2022年 | 14769篇 |
2021年 | 18793篇 |
2020年 | 16532篇 |
2019年 | 19319篇 |
2018年 | 22556篇 |
2017年 | 21423篇 |
2016年 | 23669篇 |
2015年 | 20377篇 |
2014年 | 25834篇 |
2013年 | 36856篇 |
2012年 | 31068篇 |
2011年 | 34355篇 |
2010年 | 33115篇 |
2009年 | 35044篇 |
2008年 | 33852篇 |
2007年 | 32538篇 |
2006年 | 31007篇 |
2005年 | 25256篇 |
2004年 | 20518篇 |
2003年 | 16947篇 |
2002年 | 16182篇 |
2001年 | 14818篇 |
2000年 | 15263篇 |
1999年 | 12140篇 |
1998年 | 9700篇 |
1997年 | 8838篇 |
1996年 | 8835篇 |
1995年 | 8313篇 |
1994年 | 7483篇 |
1993年 | 5615篇 |
1992年 | 5243篇 |
1991年 | 4838篇 |
1990年 | 4780篇 |
1989年 | 4412篇 |
1988年 | 4010篇 |
1987年 | 4461篇 |
1986年 | 4076篇 |
1985年 | 4906篇 |
1984年 | 5368篇 |
1983年 | 4911篇 |
1982年 | 4807篇 |
1981年 | 4320篇 |
1980年 | 3980篇 |
1979年 | 3830篇 |
1978年 | 3715篇 |
1977年 | 3473篇 |
1976年 | 3223篇 |
1975年 | 3108篇 |
1973年 | 3262篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Familiar since antiquity, and subject in contemporary times to various characterization schemes, the exact nature of solid bitumen is not yet fully known. Bitumens have ‘random polymer-like’ molecular structures, are mobile as highly viscous fluids or were once fluids but have since turned into solids. Solid bitumens consist mainly of large moieties, of polyclyclic aromatic hydrocarbons, occasionally with finely admixed, fine-grained cryptocrystalline graphite. Solid bitumens are distinguished from kerogen, which is the syngenetic and generally finely dispersed particulate organic matter in sedimentary rock that virtually does not migrate following its deposition. Occurrences of solid bitumens are relevant to petroleum exploration as well as the search for, and evaluation of, a variety of metallic mineral deposits. Genesis of bitumen is in many cases linked to the thermal and hydrothermal history of organic matter in sedimentary rock. Apparently bitumen, or more specifically organic acids generated along with bitumen during diagenesis, may alter porosity of reservoir rocks or otherwise prepare the ground for ore deposition. Bitumen is also relatively sensitive to alteration processes, some of which, such as oxidative weathering, water leaching, biodegradation (contact) metamorphism and ionizing radiation may likewise affect its nature. Elemental composition of bitumen commonly reflects the nature of mineral deposits. Is is possible that in petroleum exploration, trace metal abundances of bitumen may eventually allow prediction of crude oil types and volumes anticipated from a given source rock? Beside transition elements, notably Ni and V, highly anomalous concentrations of U, Pt and Au occur in some solid bitumens. During the generation of petroleum from kerogen, the trend in δ13C is toward lighter values. The opposite seems to occur when liquid petroleum is subjected to thermal cracking (and /or related processes) yielding solid bitumen enriched in 13C, and isotopically light methane. In fact, except for deasphalting and possibly some irradiation processes, the result of thermal cracking, oxidation, water leaching, inspissation (drying) and bacterial degradation of crude oil is that lower molecular weight hydrocarbons are removed leaving bitumen residues enriched in aromatic hydrocarbons, heteroatomic compounds (NSO) and 13C. Such phenomena are relevant to bitumen paragenesis in petroleum reservoir rocks, to certain Phanerozoic occurrences of multiple generations of bitumens, and to bitumens in mineral deposits. 相似文献
992.
Historical Changes in the Flood Hazard at Khartoum, Sudan: Lessons and Warnings for the Future 总被引:1,自引:0,他引:1
The 1988 floods at Khartoum were frequently described as "unprecedented". However, an examination of evidence for floods caused by the Nile and rain storms during the nineteenth and twentieth centuries leads to the conclusion that neither the Nile flood nor the rain storms nor their coincidence in one season were unprecedented and that similar situations will recur.We argue that most problems were created by complacency, lack of planning and mismanagement. We conclude with some simple suggestions for flood amelioration in the future and warn that complacency is the single most important phenomenon likely to recur to the detriment of Greater Khartoum in the future. 相似文献
993.
994.
Stephen A. Royle 《Singapore journal of tropical geography》1997,18(1):89-98
Indonesia is one of the most rapidly developing nations in the world. One important component of its transformation is an industrialisation policy which sees its people utilised by transnational corporations as sources of cheap labour for export goods processing. This has both positive and negative outcomes. This paper explores these issues in a review of Indonesian development strategy with particular reference to Batam Island in Riau Province. 相似文献
995.
996.
997.
998.
PALEOSOLSOFSANDYLANDSANDENVIRONMENTALCHANGESINTHEWESTERNPARTOFNORTHEASTPLAINOFCHINAWESTERNDURINGHOLOCENE¥QiuShanwen(裘善文)LiQus... 相似文献
999.
David A. Williams 《Planetary and Space Science》1992,40(12):1683-1693
We review the present understanding of interstellar CH+. We show that the essential chemistry was correctly identified by Bates and Spitzer in 1951. More recent work has been concerned with defining appropriate venues within which this chemistry can function efficiently. This process had led to a much fuller understanding of the nature of the interstellar medium. 相似文献
1000.