全文获取类型
收费全文 | 99354篇 |
免费 | 1892篇 |
国内免费 | 909篇 |
专业分类
测绘学 | 2464篇 |
大气科学 | 7041篇 |
地球物理 | 19967篇 |
地质学 | 35077篇 |
海洋学 | 8808篇 |
天文学 | 22511篇 |
综合类 | 267篇 |
自然地理 | 6020篇 |
出版年
2022年 | 506篇 |
2021年 | 960篇 |
2020年 | 1056篇 |
2019年 | 1128篇 |
2018年 | 2485篇 |
2017年 | 2288篇 |
2016年 | 2943篇 |
2015年 | 1701篇 |
2014年 | 2803篇 |
2013年 | 5248篇 |
2012年 | 3020篇 |
2011年 | 4102篇 |
2010年 | 3544篇 |
2009年 | 4758篇 |
2008年 | 4257篇 |
2007年 | 4102篇 |
2006年 | 3866篇 |
2005年 | 3153篇 |
2004年 | 3102篇 |
2003年 | 2926篇 |
2002年 | 2764篇 |
2001年 | 2483篇 |
2000年 | 2370篇 |
1999年 | 1923篇 |
1998年 | 2025篇 |
1997年 | 1948篇 |
1996年 | 1594篇 |
1995年 | 1595篇 |
1994年 | 1390篇 |
1993年 | 1294篇 |
1992年 | 1231篇 |
1991年 | 1145篇 |
1990年 | 1219篇 |
1989年 | 1102篇 |
1988年 | 1016篇 |
1987年 | 1233篇 |
1986年 | 1053篇 |
1985年 | 1303篇 |
1984年 | 1420篇 |
1983年 | 1309篇 |
1982年 | 1252篇 |
1981年 | 1170篇 |
1980年 | 1033篇 |
1979年 | 972篇 |
1978年 | 955篇 |
1977年 | 909篇 |
1976年 | 854篇 |
1975年 | 815篇 |
1974年 | 817篇 |
1973年 | 794篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
93.
94.
We analyzed data from 23 boreholes at 19 sites in central and eastern Canada, for the purpose of estimating ground surface temperature (GST) histories. These boreholes were logged down to at least 550 m depth with thermistor probes. Thermal conductivity measurements had been previously made at small depth intervals for the entire depth ranges of most of the boreholes. The temperature profiles of these boreholes do not indicate water disturbance. We estimated terrain effects for each borehole using a time dependent solid-angle method. The thermal perturbations caused by lakes or deforestation near the borehole sites are insignificant in most cases. However, four of the holes were found to be severely influenced by terrain effects. GSTs estimated from the borehole data less influenced by the terraineffects form two groups. The first group, which are generally from data of better quality, show a cold period near the end of the last century before the recent warming trend; the second show it 80–100 years earlier. We consider the former typical of the climate of the Boreal climatic region of Canada. The difference between the two groups may reflect the spacial variability of the climate. Four GST estimates do not belong to either type, and the reasons are discussed. 相似文献
95.
The dynamics of the vortex lattice in the inner crust of a neutron star is considered. A general equation of motion is obtained
and solved under the assumption that there are regions of pinned and of free vortices. By comparing these solutions with observational
data for the Vela pulsar, the relative moments of inertia of regions of relaxation with the corresponding characteristic times
are calculated for two model stars with different equations of state. It is shown that the theory can be reconciled with observations
of the relaxation of pulsar angular velocity only for model stars with extremely stiff equations of state.
Translated from Astrofizika, Vol. 40, No. 1, pp. 67–76, January–March, 1997. 相似文献
96.
Numerical simulations of the evolving activity of comet Hale-Bopp are presented, assuming a porous, spherical nucleus, 20
km in radius, made of dust and gas-laden amorphous ice. The main effects included are: crystallization of amorphous ice and
release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust
grains. The model parameters, such as initial pore size and porosity, emissivity, dust grain size, are varied in order to
match the observed activity. In all cases, a sharp rise in the activity of the nucleus occurs at a large heliocentric distance
pre-perihelion, marked by a few orders of magnitude increase in the CO and the CO2 fluxes and in the rate of dust emission. This is due to the onset of crystallization, advancing down to a few meters below
the surface, accompanied by release of the trapped gases. A period of sustained, but variable, activity ensues. The emission
of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. Thereafter the activity is largely determined
by the behaviour of the dust. If a dust mantle is allowed to build up, the water production rate does not increase dramatically
towards perihelion; if most of the dust is ejected, the surface activity increases rapidly, producing a very bright comet.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
97.
R. Sarmiento M. Gallardo A. Laquidara J. Reyna Almandos 《Astrophysics and Space Science》1997,256(1-2):359-363
This work presents preliminaries spectroscopic results about ionized xenon and xenon-helium mixture using a capillary pulsed discharge under several experimental results. 相似文献
98.
Donald D. BOGARD Daniel H. GARRISON Jozef MASARIK 《Meteoritics & planetary science》2001,36(1):107-122
Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The 39Ar‐40Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of 40Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic 3He, 21Ne, and 38Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic 21Ne and 38Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of 36Ar in the halite were produced during regolith exposure by neutron capture on 35Cl, followed by decay to 36Ar. The thermal neutron fluence seen by the halite was (2–4) × 1014 n/cm2. The thermal neutron flux during regolith exposure was ~0.4‐0.7 n/cm2/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the 36Arn/21Necos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded. 相似文献
99.
100.
Several temperature-depth profiles measured in Kasai and in Shaba provinces of Zaire using mining exploration boreholes exhibit a significant negative temperature gradient near the surface. This anomalous curvature which extends to 100–200 m depth could reflect the effect of variations in surface conditions. Applying the theory of heat conduction in a semi-infinite homogeneous medium, these profiles indicate a surface warming by 3–4°C. This warming is related to the effect of the environmental changes associated with the mining exploitation and the urbanization during the last 40–90 years. 相似文献