全文获取类型
收费全文 | 49287篇 |
免费 | 479篇 |
国内免费 | 326篇 |
专业分类
测绘学 | 1298篇 |
大气科学 | 3626篇 |
地球物理 | 9258篇 |
地质学 | 20638篇 |
海洋学 | 3588篇 |
天文学 | 9200篇 |
综合类 | 189篇 |
自然地理 | 2295篇 |
出版年
2020年 | 136篇 |
2019年 | 128篇 |
2018年 | 3497篇 |
2017年 | 3306篇 |
2016年 | 2106篇 |
2015年 | 426篇 |
2014年 | 450篇 |
2013年 | 1178篇 |
2012年 | 1669篇 |
2011年 | 3598篇 |
2010年 | 3356篇 |
2009年 | 3624篇 |
2008年 | 2948篇 |
2007年 | 3597篇 |
2006年 | 868篇 |
2005年 | 1229篇 |
2004年 | 1128篇 |
2003年 | 1180篇 |
2002年 | 972篇 |
2001年 | 618篇 |
2000年 | 636篇 |
1999年 | 572篇 |
1998年 | 546篇 |
1997年 | 559篇 |
1996年 | 461篇 |
1995年 | 458篇 |
1994年 | 439篇 |
1993年 | 408篇 |
1992年 | 378篇 |
1991年 | 326篇 |
1990年 | 379篇 |
1989年 | 295篇 |
1988年 | 340篇 |
1987年 | 375篇 |
1986年 | 326篇 |
1985年 | 481篇 |
1984年 | 523篇 |
1983年 | 529篇 |
1982年 | 422篇 |
1981年 | 432篇 |
1980年 | 455篇 |
1979年 | 381篇 |
1978年 | 396篇 |
1977年 | 346篇 |
1976年 | 380篇 |
1975年 | 341篇 |
1974年 | 380篇 |
1973年 | 365篇 |
1972年 | 233篇 |
1971年 | 186篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
861.
Mathematical models have been developed which simulate both random and nonrandom thermal cracking of branched and straight-chain hydrocarbons. Application of these models to n-paraffins suggests that thermal cracking alone cannot be the dominant mechanism in formation of the n-paraffin distributions present in crude oils. Application to isoprenoid hydrocarbons indicates that nonrandom cracking could be important in producing the isoprenoid distributions found in oils.Results of the mathematical modeling show that methane formation should, as predicted from energy considerations, be kinetically disfavored. It therefore is likely that substantial quantities of methane are produced from saturated hydrocarbons only under thermal conditions more severe than those under which oil is produced.The mathematical models employed are adaptable for other geochemical applications, such as isotope fractionation. 相似文献
862.
863.
W.J. Stahl 《Geochimica et cosmochimica acta》1978,42(10):1573-1577
An isotopic type-curve has been defined based on the ratios of the saturated, aromatic, heterocomponent (NOSs), and asphaltene fractions of crude oils. These fractions show 13C enrichments with increasing polarity or polarizability. This systematic pattern can be used to estimate the ratio of the kerogen from which the oil had been generated. Genetically associated source rock oil pairs have been used to show that the difference between the measured and the estimated δ-values of kerogen is about ?0.5%., and between the δ-values of the kerogen and the asphaltene fraction is approximately +0.6%. 相似文献
864.
The chemistry of orthophosphate uptake from synthetic seawater onto the surfaces of synthetic calcite, aragonite and low-magnesium biogenic calcite has been studied, in order to elucidate the kinetics of the process (generally believed to be the major control of dissolved reactive phosphate in carbonate-rich marine sediments). Our results differ from those obtained by others, who have studied orthophosphate uptake in low ionic strength solutions and at much higher supersaturations relative to apatite.In both ‘free drift’ and chemostat experiments, Mg and F have only a minor effect on the reaction rate. Even at constant solution composition the rate of orthophosphate uptake was found to decrease by 106 over a two week period. The data from the ‘free drift’ experiments can be fitted to the Elovich equation. This indicates that the kinetics observed for this reaction can be explained by an exponential decrease in available surface reaction sites and/or a linear increase in the activation energy associated with chemisorption as the reaction proceeds. 相似文献
865.
Amino acids comprise from 15 to 36% by weight of humic substances from carbonate and non-carbonate sediments. Humic and fulvic acids extracted from carbonate sediments are characterized by an amino acid composition consisting primarily of the acidic amino acids, aspartic and glutamic acid. Humic substances from non-carbonate sediments have a distinctly different amino acid composition consisting primarily of glycine and alanine. Amino acid analyses of various molecular weight fractions of fulvic acids extracted from carbonates show that lower molecular weight fractions have appreciably higher relative abundances of the acidic amino acids compared to higher molecular weight fractions. Based on typical values for carboxyl group content in humic substances, acidic amino acids may be a significant contributor of these functional groups. Carbonate surfaces appear to selectively adsorb aspartic acid-enriched organic matter while non-carbonates do not have this property. 相似文献
866.
J. W. LAVELLE D. J. P. SWIFT P. E. GADD W. L. STUBBLEFIELD F. N. CASE† H. R. BRASHEAR† K. W. HAFF† 《Sedimentology》1978,25(6):823-842
Both spring-summer and fall-winter sand transport have been observed on the Long Island, New York, inner shelf at water depths of 20-22 m using a radio-isotope sand tracer system. The extent of dispersal of the tagged, fine sand was measured at 3 week intervals in two 70 day experiments. In the late spring and early summer, movement was primarily diffusive in nature, extending 100 m around the line of tracer injection, while late fall-winter patterns had strong advective features, including an ellipsoidal outline extending approximately 1500 m westward of the injection points after the passage of several storms with strong northeasterly winds. Near-bottom current observations made with Savonius rotor sensors identify the event responsible for the bulk of the transport over the 135 day observation period as a storm flow of 2 days duration. Tracer and current observations together suggest that westward winter storm flow along the Long Island shelf is the major mechanism of sand transport at these depths on a yearly time scale. A least-squares fit of several of the observed winter patterns with a plume model yields average sediment mass flux lower bounds of 3.2 × 10?3 gm/cm/sec and 1.7 × 10?1 gm/cm/sec for ‘typical’ and extreme winter storm activity. 相似文献
867.
868.
869.
870.
Absolute spectrophotometric measurements of the Swan bands of two comets have been compared with computed synthetic spectra using modern Franck-Condon and Hönl-London factors, and varying rotational, vibrational, and electronic excitation temperatures. Rotational and vibrational temperatures were obtained for the comets. Although the electronic excitation temperature and the molecular column density cannot be separated, a relationship is found between these two quantities. A review is made of recent determinations of column densities for CN in comets. 相似文献