全文获取类型
收费全文 | 120808篇 |
免费 | 2562篇 |
国内免费 | 1073篇 |
专业分类
测绘学 | 3078篇 |
大气科学 | 8930篇 |
地球物理 | 25354篇 |
地质学 | 41963篇 |
海洋学 | 10392篇 |
天文学 | 26454篇 |
综合类 | 334篇 |
自然地理 | 7938篇 |
出版年
2021年 | 1020篇 |
2020年 | 1194篇 |
2019年 | 1244篇 |
2018年 | 2674篇 |
2017年 | 2520篇 |
2016年 | 3328篇 |
2015年 | 2133篇 |
2014年 | 3268篇 |
2013年 | 6358篇 |
2012年 | 3463篇 |
2011年 | 4789篇 |
2010年 | 4100篇 |
2009年 | 5541篇 |
2008年 | 5065篇 |
2007年 | 4685篇 |
2006年 | 4556篇 |
2005年 | 3769篇 |
2004年 | 3816篇 |
2003年 | 3547篇 |
2002年 | 3308篇 |
2001年 | 2982篇 |
2000年 | 2892篇 |
1999年 | 2422篇 |
1998年 | 2489篇 |
1997年 | 2383篇 |
1996年 | 2036篇 |
1995年 | 1981篇 |
1994年 | 1802篇 |
1993年 | 1635篇 |
1992年 | 1555篇 |
1991年 | 1419篇 |
1990年 | 1622篇 |
1989年 | 1393篇 |
1988年 | 1245篇 |
1987年 | 1524篇 |
1986年 | 1331篇 |
1985年 | 1664篇 |
1984年 | 1860篇 |
1983年 | 1775篇 |
1982年 | 1659篇 |
1981年 | 1524篇 |
1980年 | 1358篇 |
1979年 | 1263篇 |
1978年 | 1325篇 |
1977年 | 1211篇 |
1976年 | 1156篇 |
1975年 | 1089篇 |
1974年 | 1082篇 |
1973年 | 1087篇 |
1972年 | 691篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
301.
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
302.
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
303.
304.
305.
306.
The restricted problem of 2 + 2 bodies when one of the infinitesimal masses is further acted upon by the light pressure of the two primaries, is considered. The stationary solutions of this problem are found out. A short discussion is devoted to the stability of these solutions. 相似文献
307.
Perspectives for the Radio-Optical Telescope 54/32/2.6 are given for scientific research in radioastronomy. Its characteristics and potential scientific fields are summarized in the frame of the present French-Armenian collaboration. A 3-phase upgrade plan including a detailed technical evaluation of the antenna is presented.Published in Astrofizika, Vol. 38, No. 4, pp. 645–648, October–December, 1995. 相似文献
308.
We have studied the influence of the magnetic helicity on solar particle propagation using the IMF data observed by the HELIOS spacecraft in the range 0.31–0.95 AU, during eight solar proton events. For this, we have derived power and helicity spectra of the turbulence of the magnetic field during the time of the events. These are used to compute the particle pitch-angle scattering coefficients according to the quasi-linear theory (QLT) treatment of particle propagation in turbulent magnetic fields. The results show that in all the cases the helicity effects are negligible and the particle's mean free paths deduced from the pitch-angle diffusion coefficients are the same regardless of whether or not helicity effects are included in the calculations. The computed mean free paths are quite different in each case.Deceased 10 April, 1995. 相似文献
309.
Abstract— Platinum group elements (PGE) enrichments are found in Fe-Ni blebs (<1 μm) in Al- and Cr-rich objects in the ordinary chondrites Ste. Marguerite, Forest Vale, Montferré. In Ste. Marguerite, high ZrO2 concentrations (baddeleyite) are also present in a chromite inclusion. Iridium enrichment in this material compared to its metal content is confirmed by INAA. The widespread occurrence of PGE in Al- and Cr-rich objects, mostly present in H-group chondrites of nearly equilibrated types, must be taken into account to understand their conditions of formation. 相似文献
310.
Pittichovâ J. Sekenina Z. Birkle K. Boehnhardt J. Engels D. Keller P. 《Earth, Moon, and Planets》1997,78(1-3):329-338
The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images
of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring
outbursts with a periodicity of 11h21m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one
source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion.
The parent particles were subjected to a radiation pressure acceleration of βp ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized
to 1 AU from the Sun. Brief analysis of Watanabe et al.'s measurements of striae on their images from March 5–9, 1997 shows
even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献