全文获取类型
收费全文 | 4358篇 |
免费 | 171篇 |
国内免费 | 32篇 |
专业分类
测绘学 | 70篇 |
大气科学 | 307篇 |
地球物理 | 1037篇 |
地质学 | 1634篇 |
海洋学 | 390篇 |
天文学 | 726篇 |
综合类 | 15篇 |
自然地理 | 382篇 |
出版年
2021年 | 55篇 |
2020年 | 59篇 |
2019年 | 77篇 |
2018年 | 120篇 |
2017年 | 66篇 |
2016年 | 137篇 |
2015年 | 96篇 |
2014年 | 109篇 |
2013年 | 216篇 |
2012年 | 148篇 |
2011年 | 161篇 |
2010年 | 170篇 |
2009年 | 203篇 |
2008年 | 182篇 |
2007年 | 179篇 |
2006年 | 192篇 |
2005年 | 146篇 |
2004年 | 175篇 |
2003年 | 147篇 |
2002年 | 135篇 |
2001年 | 87篇 |
2000年 | 93篇 |
1999年 | 62篇 |
1998年 | 73篇 |
1997年 | 56篇 |
1996年 | 66篇 |
1995年 | 57篇 |
1994年 | 55篇 |
1993年 | 51篇 |
1992年 | 56篇 |
1991年 | 36篇 |
1990年 | 45篇 |
1989年 | 49篇 |
1988年 | 45篇 |
1987年 | 54篇 |
1986年 | 44篇 |
1985年 | 68篇 |
1984年 | 68篇 |
1983年 | 75篇 |
1982年 | 68篇 |
1981年 | 64篇 |
1980年 | 66篇 |
1979年 | 62篇 |
1978年 | 55篇 |
1977年 | 44篇 |
1976年 | 34篇 |
1975年 | 38篇 |
1974年 | 43篇 |
1973年 | 48篇 |
1972年 | 18篇 |
排序方式: 共有4561条查询结果,搜索用时 14 毫秒
71.
Autumn Oczkowski Scott Nixon Kelly Henry Peter DiMilla Michael Pilson Stephen Granger Betty Buckley Carol Thornber Richard McKinney Joaquin Chaves 《Estuaries and Coasts》2008,31(1):53-69
Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one
of the more intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage
treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third
or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary
(macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (δ15N) and carbon (δ13C). The δ15N signatures of the macroalgae show a clear gradient of approximately 4‰ from north to south, i.e., high to low point source
loading. There is also evidence of a west to east gradient of heavy to light values of δ15N in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett
Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower δ15N values in macroalgae there reflected preferential uptake of 14N in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of
dissolved C species may control the observed δ13C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ15N (13.2 ± 0.54‰ SD) and δ13C (−16.76 ± 0.61‰ SD) throughout the bay, and the δ15N values were 2–5‰ heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a
food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate
that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources. 相似文献
72.
We have obtained infrared colors and limiting magnitudes from 1.25–4.8µm for a sample of 26 of the cm continuum radio sources located in the core of the Oph molecular cloud. Their colors demonstrate that the majority of the sources appear to be heavily reddened objects surrounded by circumstellar accretion disks. In these cases the radio emission most likely diagnoses accretion driven energetic outflow phenomena: either ionized winds or possibly synchrotron emission from shocked gas associated with stellar jets. 相似文献
73.
Positions of active regions estimated from observations of the whole solar disk in Caii
K
iv
during the period 1977–1989 at the Coimbra Astronomical Observatory are compared with the time-dependent latitudinal distribution of background solar magnetic fields and with the latitudinal shifts of boundaries of their polarities. We confirm that the sunspot groups are located near the zonal boundaries between the opposite polarities of the solar background magnetic field during different phases of the two recent consecutive cycles of activity. We demonstrate a probable connection between the increased number of groups and the commencement of poleward migration of zonal boundaries in both hemispheres. But the influence of the dominant convective rolls seems to he still unclear. A new problem of interrelation between the zonal and sector boundaries has also appeared. 相似文献
74.
David J. Barber John R. Beckett Julie M. Paque Edward Stolper 《Meteoritics & planetary science》1994,29(5):682-690
Abstract The crystallography and crystal chemistry of a new calcium-titanium-aluminosilicate mineral (UNK) observed in synthetic analogs to calcium-aluminum-rich inclusions (CAIs) from carbonaceous chondrites was studied by electron diffraction techniques. The unit cell is primitive hexagonal or trigonal, with a = 0.790 ± 0.002 nm and c = 0.492 ± 0.002 nm, similar to the lattice parameters of melilite and consistent with cell dimensions for crystals in a mixer furnace slag described by Barber and Agrell (1994). The phase frequently displays an epitactic relationship in which melilite acts as the host, with (0001)UNK | (001)mel and <10T0>UNK | <100>mel. If one of the two space groups determined by Barber and Agrell (1994) for their sample of UNK is applicable (P3ml or P31m), then the structure is probably characterized by puckered sheets of octahedra and tetrahedra perpendicular to the c-axis with successive sheets coordinated by planar arrays of Ca. In this likely structure, each unit cell contains three Ca sites located in mirror planes, one octahedrally coordinated cation located along a three-fold axis and five tetrahedrally coordinated cations, three in mirrors and two along triads. The octahedron contains Ti but, because there are 1.3–1.9 cations of Ti/formula unit, some of the Ti must also be in tetrahedral coordination, an unusual but not unprecedented situation for a silicate. Tetrahedral sites in mirror planes would contain mostly Si, with lesser amounts of Al while those along the triads correspondingly contain mostly Al with subordinate Ti. The structural formula, therefore, can be expressed as with Si + Ti = 4. Compositions of meteoritic and synthetic Ti-bearing samples of the phase can be described in terms of a binary solid solution between the end-members Ca3TiAl2Si3O14 and Ca3Ti(AlTi)(AlSi2)O14. A Ti-free analog with a formula of Ca3Al2Si4O14 synthesized by Paque et al. (1994) is thought to be related structurally but with the octahedral site being occupied by Al, that is 相似文献
75.
76.
"This paper establishes and discusses briefly regional patterns and trends in crude birth, death, and natural increase rates in Russia and the USSR from the turn of the century to 1989. The study is based on a comparable regional framework and placed in the context of the demographic transition theory. It discusses sources, collection, and quality of data, as well as necessary estimating procedures. An attempt also is made to examine briefly regional age-standardized rates." 相似文献
77.
Chemical equilibrium calculations on the stability of pure and dissolved graphite and cohenite (Fe3C), several other carbides, and several carbonates have been carried out for a system with solar elemental abundances over a very wide range of temperature and pressure. The calculated abundances of condensed carbon compounds are similar to the observed inventories on Earth and Venus, but fully 10 times smaller than the minimum carbon abundance found in ordinary chondrites. The total carbon content of most iron meteorites is compatible with their origin as a cooling FeNiCSP solution which was saturated with dissolved carbon at the solidus, such as would be produced by melting an ordinary chondrite, not by direct condensation from or equilibrium with the primitive solar nebula. It is argued that the carbon content of Mars need not be appreciably greater than that of the Earth. Material with even lower formation temperatures than Mars, such as the primitive material in the asteroid belt, may retain substantially more carbon as disequilibrium polymeric organic matter, possibly by the Fischer-Tropsch mechanism favored by Anders. Carbonates are not found as equilibrium products in a solar-composition system, and are probably secondary alteration products. CaCO3 might, however, persist in a solar-composition gas at temperatures below 460°K and pressures below 10?6.6 bar. The most stable condensed carbon compounds are found to be graphite, Fe3C, and possibly TiC, all in solid solution in the metal phase. 相似文献
78.
The thermochemistry of several hundred compounds of twelve selected trace elements (Ge, Se, Ga, As, Te, Pb, Sn, Cd, Sb, Tl, In, and Bi) has been investigated for solar composition material along a Jupiter adiabat. The results indicate that AsF3, InBr, TlI, and SbS, in addition to CO, PH3, GeH4, AsH3, H2Se, HCl, HF, and H3BO3 proposed by Barshay and Lewis (1978), may be potential chemical tracers of atmospheric dynamics. The reported observations of GeH4 is interpreted on the basis of new calculations as implying rapid vertical transport from levels where T ? 800°K. Upper limits are also set on the abundances of many gaseous compounds of the elements investigated. 相似文献
79.
Impacts into an icy surface could produce significant amounts of high pressure forms of water ice. Due to the relatively low ambient surface temperatures on satellites in the outer solar system and the modest temperature rises accompanying the impact pressures required for water ice metamorphism, high-pressure polymorphs will be created by and may remain after large cratering events. If so, those high-pressure ices should be ubiquitous. Low-pressure cubic ice may be abundant as well. Impacts into an icy regolith may both produce high-pressure polymorphs from ice I and destroy high-pressure polymorphs already present. The result will be an (unknown) equilibrium concentration of high pressure polymorphs in the regolith. Polymorphs may be detectable and mappable by reflection spectroscopy at vacuum ultraviolet and mid-infrared wavelengths. 相似文献
80.
A new estimate of Pluto's mass within the range of possible masses considered in an earlier work has enabled us to refine our model of Pluto's interior. 相似文献