全文获取类型
收费全文 | 8194篇 |
免费 | 339篇 |
国内免费 | 95篇 |
专业分类
测绘学 | 199篇 |
大气科学 | 566篇 |
地球物理 | 1922篇 |
地质学 | 2972篇 |
海洋学 | 749篇 |
天文学 | 1213篇 |
综合类 | 19篇 |
自然地理 | 988篇 |
出版年
2021年 | 89篇 |
2020年 | 113篇 |
2019年 | 114篇 |
2018年 | 166篇 |
2017年 | 162篇 |
2016年 | 226篇 |
2015年 | 194篇 |
2014年 | 196篇 |
2013年 | 469篇 |
2012年 | 248篇 |
2011年 | 307篇 |
2010年 | 282篇 |
2009年 | 338篇 |
2008年 | 319篇 |
2007年 | 279篇 |
2006年 | 314篇 |
2005年 | 238篇 |
2004年 | 291篇 |
2003年 | 262篇 |
2002年 | 264篇 |
2001年 | 173篇 |
2000年 | 171篇 |
1999年 | 141篇 |
1998年 | 143篇 |
1997年 | 103篇 |
1996年 | 114篇 |
1995年 | 114篇 |
1994年 | 128篇 |
1993年 | 113篇 |
1992年 | 113篇 |
1991年 | 109篇 |
1990年 | 101篇 |
1989年 | 85篇 |
1988年 | 87篇 |
1987年 | 122篇 |
1986年 | 100篇 |
1985年 | 163篇 |
1984年 | 186篇 |
1983年 | 144篇 |
1982年 | 126篇 |
1981年 | 131篇 |
1980年 | 108篇 |
1979年 | 124篇 |
1978年 | 118篇 |
1977年 | 103篇 |
1976年 | 96篇 |
1975年 | 91篇 |
1974年 | 67篇 |
1973年 | 83篇 |
1972年 | 49篇 |
排序方式: 共有8628条查询结果,搜索用时 31 毫秒
101.
102.
103.
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
104.
John W. Larimer 《Icarus》1979,40(3):446-454
It has long been recognized that Cr, Mg, and Si are fractionated in chondritic material along with, but to a much lesser extent than, a large group of more refractory elements. Reasoning that this might imply some unique distribution at the time of fractionation, the patterns have been reexamined. It now appears as if two distinct fractionation patterns can be resolved: one involving ordinary and enstatite chondrites and the other involving carbonaceous chondrites, the Earth, the Moon, and the eucrite parent body. Significantly, the two trends inevitably intersect at C1 composition. Ordinary and enstatite chondrites appear to have evolved from C1 composition via the removal of about 40 and 56% of a high-temperature condensate. Another high-temperature condensate, with a distinctly different composition, appears to be enriched in the carbonaceous chondrites, the Moon, and possibly the Earth, but depleted in the eucrite parent body. The compositions of these two components are constrained to fall on the appropriate mixing lines. These lines intersect the condensation path at two points, one where Mg2SiO4 has just begun to condense (~20%) and a second where Mg2SiO4 was almost completely condensed (~90%). This represents about an 80° temperature difference. But it is within this range that the largest fraction of planetary matter (Mg, Si, and Fe) condenses. Conceivably the relatively sudden appearance of large amounts of condensed material is in some way related to the fractionation process, although the exact relationship cannot be specified. 相似文献
105.
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
106.
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
107.
Three representative numerical simulations of the growth of the terrestrial planets by accretion of large protoplanets are presented. The mass and relative-velocity distributions of the bodies in these simulations are free to evolve simultaneously in response to close gravitational encounters and occasional collisions between bodies. The collisions between bodies, therefore, arise in a natural way and the assumption of expressions for the relative velocity distribution and the gravitational collision cross section is unnecessary. These simulations indicate that the growth of bodies with final masses approaching those of Venus and the Earth is possible, at least for the case of a two-dimensional system. Simulations assuming an initial uniform distribution of orbital eccentricities on the interval from 0 to emax are found to produce final states containing too many bodies with masses which are too small when emax < 0.10, while simulations with emax > 0.20 result in too many catastrophic collisions between bodies thus preventing rapid accretion of planetary-size bodies. The emax = 0.15 simulation ends with a state surprisingly similar to that of the present terrestrial planets and, therefore, provides a rough estimate of the range of radial sampling to be expected for the terrestrial planets. 相似文献
108.
109.
John P. Vinti 《Celestial Mechanics and Dynamical Astronomy》1977,16(4):391-406
Newtonian cosmology is developed with the assumption that the gravitational constantG diminishes with time. The functional form adopted forG(t), a modification of a suggestion of Dirac, isG=A(k+t)
–1, wheret is the age of the Universe and a small constantk is inserted to avoid a singularity in the two-body problem. IfR is the scale factor, normalized to unity at an epoch time , the differential equation is then
. Here 0 is the mean density at the epoch time. With the above form forG(t), the solution is reducible to quadratures.The scale factorR either increases indefinitely or has one and only one maximum. LetH
0 be the present value of Hubble's constant
/R and 0c
the minimum density for a maximum ofR, i.e., for closure of the Universe. The conditions for a maximum lead to a boundary curve of 0c
versusH
0 and the numbers indicate strongly that thisG-variable Newtonian model corresponds to an open universe. An upward estimate of the age of the Universe from 1010 yr to five times such a value would still lead to the same conclusion.The present Newtonian cosmology appears to refute the statement, sometimes made, that the Dirac model forG necessarily leads to the conclusion that the age of the Universe is one-third the Hubble time. Appendix B treats this point, explaining that this incorrect conclusion arises from using all the assumptions in Dirac (1938). The present paper uses only Dirac's final result, viz,G(k+t)–1, superposing it on the differential equation
. 相似文献
110.