全文获取类型
收费全文 | 96816篇 |
免费 | 1214篇 |
国内免费 | 1743篇 |
专业分类
测绘学 | 3180篇 |
大气科学 | 7188篇 |
地球物理 | 19245篇 |
地质学 | 36043篇 |
海洋学 | 7540篇 |
天文学 | 18874篇 |
综合类 | 2339篇 |
自然地理 | 5364篇 |
出版年
2021年 | 577篇 |
2020年 | 669篇 |
2019年 | 686篇 |
2018年 | 6005篇 |
2017年 | 5244篇 |
2016年 | 4306篇 |
2015年 | 1388篇 |
2014年 | 1900篇 |
2013年 | 3795篇 |
2012年 | 3014篇 |
2011年 | 5378篇 |
2010年 | 4359篇 |
2009年 | 5664篇 |
2008年 | 4789篇 |
2007年 | 5176篇 |
2006年 | 2838篇 |
2005年 | 2444篇 |
2004年 | 2611篇 |
2003年 | 2495篇 |
2002年 | 2219篇 |
2001年 | 1820篇 |
2000年 | 1798篇 |
1999年 | 1567篇 |
1998年 | 1552篇 |
1997年 | 1494篇 |
1996年 | 1289篇 |
1995年 | 1231篇 |
1994年 | 1082篇 |
1993年 | 961篇 |
1992年 | 941篇 |
1991年 | 918篇 |
1990年 | 985篇 |
1989年 | 867篇 |
1988年 | 801篇 |
1987年 | 944篇 |
1986年 | 856篇 |
1985年 | 1059篇 |
1984年 | 1188篇 |
1983年 | 1112篇 |
1982年 | 1003篇 |
1981年 | 1003篇 |
1980年 | 889篇 |
1979年 | 803篇 |
1978年 | 820篇 |
1977年 | 747篇 |
1976年 | 714篇 |
1975年 | 706篇 |
1974年 | 694篇 |
1973年 | 718篇 |
1972年 | 429篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
This paper presents a method that incorporates a non‐associated flow rule into the limit analysis to investigate the influence of the dilatancy angle on the factor of safety for the slope stability analysis. The proposed method retain's the advantage of the upper bound method, which is simple and has no stress involvement in the calculation of the energy dissipation and the factor of safety. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
42.
The Etna volcano is located in an apparently anomalous position on the hinge zone of the Apennines subduction and its Na-alkaline geochemistry does not favour a magma source from the deep slab as indicated for the Aeolian K-alkaline magmatism. The steeper dip of the regional foreland monocline at the front of the Apennines in the Ionian Sea than in Sicily, implies a larger rollback of the subduction hinge in the Ionian Sea. Moreover, the lengthening of the Apennines arc needs extension parallel to the arc. Therefore, the larger southeastward subduction rollback of the Ionian lithosphere with respect to the Hyblean plateau in Sicily, should kinematically produce right-lateral transtension and a sort of vertical 'slab window' which might explain (i) the Plio-Pleistocene alkaline magmatism of eastern Sicily (e.g. the Etna volcano) and (ii) the late Pliocene to present right lateral transtensional tectonics and seismicity of eastern Sicily. The area of transfer of different dip and rollback occurs along the inherited Mesozoic passive continental margin between Sicily and the oceanic Ionian Sea, i.e. the Malta escarpment. 相似文献
43.
This paper compares lead-acid batteries, sodium-sulfur batteries, solid polymer fuel cells and closed-cycle diesel engines for autonomous underwater vehicle (AUV) applications. The service is described in terms of a parametric mission and life cycle. A generic AUV is used as a basis for comparison. Power systems are evaluated by two criteria: (1) submerged endurance capability and (2) life cycle cost. This study determines categories of service for which each power system is preferred. The solid polymer fuel cell can provide greater submerged endurance than other power systems examined. For extremely long duration AUV missions, the fuel cell is the required system, indicating a possible market niche for today's fuel cell technology. Considering cost projections for each power system, the results also show that the SPFC can become cost-competitive with conventional technologies, particularly for services characterized by high levels of utilization 相似文献
44.
Swath bathymetric, gravity, and magnetic studies were carried out over a 55 km long segment of the Central Indian Ridge.
The ridge is characterized by 12 to 15 km wide rift valley bounded by steep walls and prominent volcanic constructional ridges
on either side of the central rift valley. A transform fault at 7°45′S displaces the ridge axis. A mantle Bouguer anomaly
low of −14 mGals and shallowing of rift valley over the middle of the ridge segment indicate along axis crustal thickness
variations. A poorly developed neovolcanic zone on the inner rift valley floor indicate dominance of tectonic extension. The
off-axis volcanic ridgs suggest enhanced magmatic activity during the recent past.
Received: 24 May 1996 / Rivision received: 13 January 1997 相似文献
45.
V. Garnier D. Ohnenstetter G. Giuliani P. Blanc D. Schwarz 《Mineralogy and Petrology》2002,76(3-4):179-193
Summary ?Mong Hsu rubies of the “trapiche” type are sporadically seen in the gem market. However, they have never been described in
the field. The study of the nature of solid inclusions, the variation of trace-element contents, as well as the cathodoluminescence
behaviour of six “trapiche” rubies permit the conclusion that these rubies crystallised in the same geological environment
(marble-type deposit) as the normal rubies from Mong Hsu: (1) Cr and V are the main chromophorous elements in both ruby types;
they act, together with Ti, as activators or quenchers for cathodoluminescence; (2) calcite, dolomite, rutile, mica, diaspore,
apatite, chlorite, and feldspar are solid inclusions found in both ruby types; (3) the presence of bastn?site in trapiche
ruby and fluorite in non-trapiche ruby indicates the circulation of F-bearing fluids during ruby deposition; (4) the distribution
of trace-element contents in the crystal is similar for both ruby types. In the Cr2O3 vs. Fe2O3 and Cr2O3 vs. Fe2O3/TiO2 diagrams, the population fields of Mong Hsu “trapiche” and non-“trapiche” rubies overlap. They are distinct from those of
rubies and sapphires hosted in basalts from South-east Asia.
Received October 30, 2001; revised version accepted March 25, 2002 相似文献
46.
Donald C. Gordon Peter J. Cranford Con Desplanque 《Estuarine, Coastal and Shelf Science》1985,20(2):205-227
The Cumberland Basin, a 118 km2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin. 相似文献
47.
K.A. MAASCH P.A. MAYEWSKI E.J. ROHLING J.C. STAGER W. KARLEN L.D. MEEKER E.A. MEYERSON 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):7-15
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14 C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale. 相似文献
48.
49.
Replicate portions of a Delaware salt marsh were enclosed in cylindrical microcosms and exposed to elevated levels of inorganic arsenic (arsenate). All biotic and abiotic components in dosed cylinders rapidly incorporated arsenic. Spartina blades showed the greatest arsenic enrichment, with dosed plants incorporating arsenic concentrations an order of magnitude higher than controls. Spartina detritus and sediments also exhibited greatly elevated arsenic concentrations. Virtually all of the arsenic was incorporated into plant tissue or strongly sorbed to cell surfaces. Thus, elevated arsenic concentrations in estuarine waters will be reflected in living and non-living components of a salt marsh ecosystem, implying that increased arsenic will be available to organisms within the marsh ecosystem. 相似文献
50.