全文获取类型
收费全文 | 47187篇 |
免费 | 1048篇 |
国内免费 | 1110篇 |
专业分类
测绘学 | 1162篇 |
大气科学 | 3493篇 |
地球物理 | 9527篇 |
地质学 | 17601篇 |
海洋学 | 4348篇 |
天文学 | 10131篇 |
综合类 | 285篇 |
自然地理 | 2798篇 |
出版年
2022年 | 412篇 |
2021年 | 632篇 |
2020年 | 613篇 |
2019年 | 681篇 |
2018年 | 1340篇 |
2017年 | 1217篇 |
2016年 | 1444篇 |
2015年 | 871篇 |
2014年 | 1400篇 |
2013年 | 2373篇 |
2012年 | 1725篇 |
2011年 | 2234篇 |
2010年 | 2020篇 |
2009年 | 2534篇 |
2008年 | 2182篇 |
2007年 | 2221篇 |
2006年 | 2080篇 |
2005年 | 1495篇 |
2004年 | 1398篇 |
2003年 | 1268篇 |
2002年 | 1224篇 |
2001年 | 1046篇 |
2000年 | 1048篇 |
1999年 | 808篇 |
1998年 | 874篇 |
1997年 | 826篇 |
1996年 | 671篇 |
1995年 | 674篇 |
1994年 | 581篇 |
1993年 | 501篇 |
1992年 | 479篇 |
1991年 | 466篇 |
1990年 | 540篇 |
1989年 | 429篇 |
1988年 | 422篇 |
1987年 | 527篇 |
1986年 | 420篇 |
1985年 | 526篇 |
1984年 | 618篇 |
1983年 | 545篇 |
1982年 | 530篇 |
1981年 | 476篇 |
1980年 | 504篇 |
1979年 | 436篇 |
1978年 | 422篇 |
1977年 | 412篇 |
1976年 | 361篇 |
1975年 | 351篇 |
1974年 | 378篇 |
1973年 | 401篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
An experimental study on reduction of U (Ⅵ) by anaerobic bacteria, Shewane//a putrefaciens, is first reported here in China. The experimental conditions were: 35℃ and pH =7.0-7.4, corresponding to a physicochemical environments in which the sandstone-hosted interlayer oxidation-zone type uranium deposit formed in Northwest China's Xinjiang. Bacteria adopted in the present experiment, Shewanella putrefaciens, occur extensively in natural environment. Our study shows that nano-crystal precipitates of uraninite quickly occurred on the surface of the cells within one week. It was found that the pitchblende was characterized by a random arrangement of uraninite nanocrystals (2-4 nm) in it, significantly different from natural pitchblende in which uraninite nanocrystals are arranged in order. Finally, a possible mechanism of uranium biomineralization by microorganisms in the deposits is discussed. Our investigation may supply a technical train of thoughts for bioremediation of nuclear-contaminated water environments and for underground dissolving extraction of the sandstone-hosted uranium ores. 相似文献
72.
Production of a molybdophore during metal-targeted dissolution of silicates by soil bacteria 总被引:1,自引:0,他引:1
Laura J. Liermann Robin L. Guynn Ariel Anbar Susan L. Brantley 《Chemical Geology》2005,220(3-4):285-302
Although many bioessential metals are scarce in natural water and rock systems, microbial secretion of high-affinity ligands for metal extraction from solid phases has only been documented for Fe. However, we have discovered that Mo is extracted from a silicate by a high-affinity ligand (a possible “molybdophore”) secreted by an N2-fixing soil bacterium. The putative molybdophore, aminochelin, is secreted as a siderophore under Fe-depleted conditions, but is also secreted under Fe-sufficient, Mo-depleted conditions. Presumably, molybdophore production facilitates uptake of Mo for use in Mo enzymes. In contrast, an Fe-requiring soil bacterium without a special Mo requirement only enhances the release of Fe from the silicate. Fractionation of Mo stable isotopes during uptake to cells may provide a “fingerprint” for the importance of chelating ligands in such systems. Many such metal-specific ligands secreted by prokaryotes for extraction of bioessential metals, their effects on Earth materials, and their possible utility in the recovery of economic metals remain to be discovered. 相似文献
73.
74.
75.
76.
An investigation of the influence of humate on the mobility of copper(II) ions in a kaolinite soil using leaching tests and electrokinetic experiments is reported. The data are interpreted in terms of humate–copper–clay interactions and humate electrical charge. Humate is mostly immobile below pH8 but is more mobile in alkaline conditions (sorption to kaolinite reduces its mobility in neutral conditions). Copper humate complexes are mobile in both acidic and alkaline conditions, but not in neutral conditions where they are sorbed. The dissolved copper humate complexes that form in acidic conditions are positively charged. The net effect of humate is to increase cupric ion mobility in kaolinite soil, especially in alkaline conditions. 相似文献
77.
J.Aramendia L.Gomez-Nubla M.L.Tuite K.H.Williford K.Castro J.M.Madariaga 《地学前缘(英文版)》2021,12(5):421-426
Nitrogen-containing heterocyclic compounds are fundamental biochemical components of all life on Earth and, presumably, life elsewhere in our solar system. Dete... 相似文献
78.
R. L. Jennings 《地球物理与天体物理流体动力学》2013,107(1-4):183-204
Abstract To model penetrative convection at the base of a stellar convection zone we consider two plane parallel, co-rotating Boussinesq layers coupled at their fluid interface. The system is such that the upper layer is unstable to convection while the lower is stable. Following the method of Kondo and Unno (1982, 1983) we calculate critical Rayleigh numbers Rc for a wide class of parameters. Here, Rc is typically much less than in the case of a single layer, although the scaling Rc~T2/3 as T → ∞ still holds, where T is the usual Taylor number. With parameters relevant to the Sun the helicity profile is discontinuous at the interface, and dominated by a large peak in a thin boundary layer beneath the convecting region. In reality the distribution is continuous, but the sharp transition associated with a rapid decline in the effective viscosity in the overshoot region is approximated by a discontinuity here. This source of helicity and its relation to an alpha effect in a mean-field dynamo is especially relevant since it is a generally held view that the overshoot region is the location of magnetic field generation in the Sun. 相似文献
79.
It is shown that the chronology of flare star discoveries in the Pleiades cluster and the Orion association can be described satisfactorily by various distribution functions (gamma, binomial, decreasing exponential, and delta) for the mean frequencies of stellar flares. However, it has been found that this is due to the uncertainty in the observationally derived distribution function for the mean frequency of stellar flares. The most likely function is that derived by Ambartsumian, which has a physical basis.Translated fromAstrofizika, Vol. 38, No. 1, pp. 25–32, January–March, 1995. 相似文献
80.
—The Tien-Shan orogene is a region in which the earth’s crust undergoes considerable thickening and tangential compression. Under these conditions the lithosphere heterogeneities (composi tion, rheological) create the prerequisites for the development of various phenomena of tectonic layering (lateral shearing, different deformation of layers). To study the distribution of velocity, density and other elastic parameters, the results from a seismic tomography study on P-wave as well as S-wave velocities were used. Using empirical as well as theoretical formulas on the relationship between velocity, density and silica content in rocks, their distribution in the Tien-Shan’s lithosphere has been calculated. In addition, other elastic parameters, such as Young’s modulus, shear modulus, Poisson’s ratio and coefficient of general compressions have been determined. Zoning of different types of crust was carried out for the region investigated. The characteristics of the "crust-mantle" transition have been investi gated. Large blocks with different types of the earth’s crust were distinguished. Layers with inverse values of velocity, density and shear and Young modulus are revealed in the Tien-Shan lithosphere. All of the above described features open new ways to solve geodynamics problems. 相似文献