全文获取类型
收费全文 | 2625篇 |
免费 | 351篇 |
国内免费 | 671篇 |
专业分类
测绘学 | 30篇 |
大气科学 | 121篇 |
地球物理 | 518篇 |
地质学 | 1757篇 |
海洋学 | 726篇 |
天文学 | 11篇 |
综合类 | 131篇 |
自然地理 | 353篇 |
出版年
2024年 | 15篇 |
2023年 | 48篇 |
2022年 | 92篇 |
2021年 | 108篇 |
2020年 | 118篇 |
2019年 | 96篇 |
2018年 | 95篇 |
2017年 | 96篇 |
2016年 | 116篇 |
2015年 | 118篇 |
2014年 | 173篇 |
2013年 | 140篇 |
2012年 | 142篇 |
2011年 | 195篇 |
2010年 | 125篇 |
2009年 | 182篇 |
2008年 | 176篇 |
2007年 | 169篇 |
2006年 | 180篇 |
2005年 | 136篇 |
2004年 | 138篇 |
2003年 | 112篇 |
2002年 | 110篇 |
2001年 | 93篇 |
2000年 | 110篇 |
1999年 | 88篇 |
1998年 | 78篇 |
1997年 | 76篇 |
1996年 | 69篇 |
1995年 | 42篇 |
1994年 | 37篇 |
1993年 | 27篇 |
1992年 | 25篇 |
1991年 | 11篇 |
1990年 | 19篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1979年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有3647条查询结果,搜索用时 78 毫秒
991.
有机柱撑粘土(蒙脱石)的研究进展 总被引:10,自引:1,他引:10
有机柱撑蒙脱石是近年来受到广泛关注的一类有机-无机粘土复合材料,其优异的性能使其在许多领域都有巨大的应用潜力。本综述了有机柱撑蒙脱石的起源、发展、主要类型,探讨了有机柱撑蒙脱石的研究方向。 相似文献
992.
有机质在MVT铅锌矿床形成中的作用 总被引:8,自引:0,他引:8
MVT铅锌矿床与有机质密切相关。本以有机质生成世代为序,讨论了有机质低、中、高成熟阶段的产物在MVT铅锌矿床成矿过程中的作用。腐殖酸在矿床形成的早期阶段配合、吸附Pb、Zn等金属离子,起到促进成矿源层形成的作用。油田卤水可能成为铅锌多金属元素迁移的主要载体和成矿流体的重要组成部分,甚至就是形成密西西比河谷型矿床的一种很重要的潜在成矿流体。甲烷在矿床形成晚期充当还原剂。 相似文献
993.
994.
Many organic compounds or their precursorsfound in meteorites originated in the interstellar or circumstellarmedium and were later incorporated intoplanetesimals during the formation of thesolar system. There they either survivedintact or underwent further processing tosynthesize secondary products on themeteorite parent body.The most distinct feature of CI and CM carbonaceouschondrites, two typesof stony meteorites, is their high carbon content(up to 3% of weight), either in theform of carbonates or of organic compounds. The bulkof the organic carbon consistsof an insoluble macromolecular material with a complexstructure. Also present is asoluble organic fraction, which has been analyzedby several separation and analyticalprocedures. Low detection limits can be achievedby derivatization of the organicmolecules with reagents that allow for analysisby gas chromatography/massspectroscopy and high performance liquidchromatography. The CM meteoriteMurchison has been found to contain more than70 extraterrestrial amino acids andseveral other classes of compounds includingcarboxylic acids, hydroxy carboxylicacids, sulphonic and phosphonic acids, aliphatic,aromatic and polar hydrocarbons,fullerenes, heterocycles as well as carbonylcompounds, alcohols, amines and amides.The organic matter was found to be enriched indeuterium, and distinct organiccompounds show isotopic enrichments of carbon andnitrogen relative to terrestrialmatter. 相似文献
995.
David Dunkerley 《地球表面变化过程与地形》2002,27(1):45-58
This study investigates the contribution of surface tension forces to friction coefficients in shallow, laminar interrill flows. Friction coefficients in these flows are known to be increased greatly by organic litter and by stems. Fine litter provides extensive edges along which surface tension menisci can be drawn up, and evaluating the significance of this in the frictional retardation of flow was the primary objective of the experiments reported here. Using both standardized ‘litter particles’ (small wooden blocks of fixed dimensions) and natural plant litter, meniscus behaviour and the Darcy–Weisbach friction coefficient were evaluated in shallow flows on a laboratory sand board. For some tests, the surface tension of ordinary water was reduced by 40 per cent by the addition of a surfactant, and the friction coefficient redetermined. Results show that the presence of surface tension menisci flanking litter particles provides areas of deeper flow that are up to 7 mm in width and which can increase flow depths by 100–300 per cent. These zones support significantly higher flow speeds. Increased water depths within menisci are additionally associated with reduced depths beyond the menisci, so that an increase in the spatial variability of flow depths is a second consequence of meniscus formation. These modifications of flow depth by surface tension menisci are shown to reduce rather than increase the overall friction coefficient applicable to the flow. Consequently, additional frictional retardation does not arise within the menisci flanking litter particles and so cannot account for the greater drag arising from litter than from other surface features. Different factors, possibly the direct obstruction of flow paths, must therefore underlie the frictional drag. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
996.
997.
K. Mannschreck K. Bächmann K. H. Becker Th. Heil R. Kurtenbach M. Memmesheimer V. Mohnen A. Obermeier D. Poppe R. Steinbrecher Th. Schmitz A. Volz-Thomas F. Zabel 《Journal of Atmospheric Chemistry》2002,42(1):281-286
The database for volatile organic compounds (VOC data base) was created with the aim of providing an overview of tropospheric hydrocarbon measurements. The data base contains 202 substances, for which atmospheric and useful kinetic data such as rate coefficients, photolysis frequencies, mixing ratios, emission data and ozone formation potentials are compiled from available literature. The database file can be downloaded without charge from http://www.physchem.uni-wuppertal.de/voc-database. Registered users will be informed about the appearance of updates. 相似文献
998.
R. Kurtenbach R. Ackermann K. H. Becker A. Geyer J. A. G. Gomes J. C. Lörzer U. Platt P. Wiesen 《Journal of Atmospheric Chemistry》2002,42(1):395-411
In 1997 and 1998 several field campaigns for monitoring non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NOx) were carried out in a road traffic tunnel and in the city center of Wuppertal, Germany. C2–C10 aliphatic and aromatic hydrocarbons were monitored using a compact GC instrument. DOAS White and long path systems were used to measure aromatic hydrocarbons and oxygenated aromatic compounds. A formaldehyde monitor was used to measure formaldehyde. Chemiluminescence NO analysers with NO2 converter were used for measuring NO and NO2. The high mixing ratios of the NMVOCs observed in the road traffic tunnel, especially 2.9 ppbv phenol, 1.5ppbv para-cresol and 4.4 ppbv benzaldehyde, in comparison with themeasured background concentration clearly indicate that these compounds were directly emitted from road traffic. Para-Cresol was for the first timeselectively detected as primary pollutant from traffic. From the measured data a NMVOC profile of the tunnel air and the city air, normalised to benzene (ppbC/ppbC), was derived. For most compounds the observed city air NMVOC profile is almost identical with that obtained in the traffic tunnel. Since benzene originates mainly from road traffic emission, the comparison of the normalised emission ratios indicate that the road traffic emissions in Wuppertal have still the largest impact on the city air composition, which is in contrast to the German emission inventory. In both NMVOC profiles, aromatic compounds have remarkably large contributions of more than 40 ppbC%. In addtion, total NMVOC/NOx ratios from 0.6 up to 3.0ppbC/ppb in the traffic tunnel air and 3.4± 0.5 in the city air of Wuppertal were obtained. From the observed para-cresol/toluene and ortho-cresol/toluene ratios in the city air, evidence was found thatalso during daytime NO3 radical reactions play an important role in urban air. 相似文献
999.
Environmental records of carbon in recent lake sediments 总被引:4,自引:0,他引:4
Based on careful sampling and accurate analysis of recent sediments in Lake Chenghai, this paper discusses the staggered positive/negative
correlation between organic carbon (OC) concentration and inorganic carbon (IC) concentration. The result indicates that temperature
change, and its induced relative changes are the main factors affecting the relationship between IC concentration, δ13C of carbonate and OC concentration. When temperature and its induced photosynthesis strength change control autochthonous
calcite precipitation, OC concentration is positively correlated with IC concentration and δ13C of carbonate. When temperature and its induced physical/chemical changes dominate, OC concentration displays negative correlation
with IC concentration and δ13C of carbonate. IC concentration and δ13C of carbonate in sediments of Lake Chenghai are good proxies for climatic warm/cold changes. 相似文献
1000.
Yang Guifang Yin Hongfu Li ChanganFaculty of Earth Sciences China University of Geosciences Wuhan 《中国地质大学学报(英文版)》2002,(1)
The Hongzuisi Section in Lanzhou area is used to infer the paleoclimate since the late glacial epoch. Through combined analysis of organic carbon isotope and CaCO3 content, paleoenvironment evolution of Hongzuisi Section since the late glacial epoch is discussed. There were changes in both dry-moist and warm-cold. Especially the climate changes can be compared with synchronous sections of other areas, which provides useful information for our future forecast. 相似文献