全文获取类型
收费全文 | 66621篇 |
免费 | 1456篇 |
国内免费 | 502篇 |
专业分类
测绘学 | 1675篇 |
大气科学 | 5334篇 |
地球物理 | 13888篇 |
地质学 | 21699篇 |
海洋学 | 5764篇 |
天文学 | 15326篇 |
综合类 | 135篇 |
自然地理 | 4758篇 |
出版年
2020年 | 485篇 |
2019年 | 512篇 |
2018年 | 965篇 |
2017年 | 942篇 |
2016年 | 1387篇 |
2015年 | 1036篇 |
2014年 | 1430篇 |
2013年 | 3291篇 |
2012年 | 1526篇 |
2011年 | 2313篇 |
2010年 | 1968篇 |
2009年 | 2967篇 |
2008年 | 2705篇 |
2007年 | 2431篇 |
2006年 | 2501篇 |
2005年 | 2152篇 |
2004年 | 2260篇 |
2003年 | 2088篇 |
2002年 | 1979篇 |
2001年 | 1787篇 |
2000年 | 1763篇 |
1999年 | 1511篇 |
1998年 | 1501篇 |
1997年 | 1493篇 |
1996年 | 1280篇 |
1995年 | 1217篇 |
1994年 | 1099篇 |
1993年 | 999篇 |
1992年 | 948篇 |
1991年 | 801篇 |
1990年 | 1014篇 |
1989年 | 853篇 |
1988年 | 758篇 |
1987年 | 929篇 |
1986年 | 818篇 |
1985年 | 1022篇 |
1984年 | 1186篇 |
1983年 | 1131篇 |
1982年 | 1021篇 |
1981年 | 981篇 |
1980年 | 837篇 |
1979年 | 822篇 |
1978年 | 875篇 |
1977年 | 790篇 |
1976年 | 753篇 |
1975年 | 701篇 |
1974年 | 704篇 |
1973年 | 712篇 |
1972年 | 443篇 |
1971年 | 389篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
631.
Mineralogy and geochemistry of El Dorado epithermal gold deposit, El Sauce district, central-northern Chile 总被引:1,自引:0,他引:1
J. Carrillo-Rosúa S. Morales-Ruano D. Morata A. J. Boyce M. Belmar A. E. Fallick P. Fenoll Hach-Alí 《Mineralogy and Petrology》2008,92(3-4):341-360
Summary The El Dorado Au-Cu deposit is located in an extensive intra-caldera zone of hydrothermal alteration affecting Upper Cretaceous
andesites of the Los Elquinos Formation at La Serena (≈ 29°47′S Lat., 70°43′W Long., Chile). Quartz-sulfide veins of economic potential are hosted by N25W and N20E
fault structures associated with quartz-illite alteration (+supergene kaolinite). The main ore minerals in the deposit are
pyrite, chalcopyrite ± fahlore (As/(As + Sb): 0.06−0.98), with electrum, sphalerite, galena, bournonite-seligmanite (As/(As
+ Sb): 0.21−0.31), marcasite, pyrrhotite being accessory phases. Electrum, with an Ag content between 32 and 37 at.%, occurs
interstitial to pyrite aggregates or along pyrite fractures. Pyrite commonly exhibits chemical zonation with some zones up
to 1.96 at.% As. Electron probe microanalyses of pyrite indicate that As-rich zones do not exhibit detectable Au values. Fluid
inclusion microthermometry shows homogenization temperatures between 130 and 352 °C and salinities between 1.6 and 6.9 wt.%
NaCl eq. Isotope data for quartz, ankerite and phyllosilicates and estimated temperatures show that δ18O and δD for the hydrothermal fluids were between 3 and 10‰ and between −95 and −75‰, respectively. These results suggest
the mineralizing fluids were a mixture of meteoric and magmatic waters. An epithermal intermediate-sulfidation model is proposed
for the formation of the El Dorado deposit.
Author’s present address: J. Carrillo-Rosúa, Dpto. de Didáctica de las Ciencias Experimentales, Universidad de Granada, Campus de Cartuja, 18071, Granada, Spain 相似文献
632.
Summary Boron-doped mullites were synthesized using aluminium nitrate-nonahydrate, tetraethoxysilane and boric acid in a sol–gel process
with subsequent annealing at 950 and 1300 °C for five hours. Two different bulk compositions with constant Al2O3 contents (60 and 70 mol%, respectively) and varying SiO2 plus B2O3 contents were investigated. X-ray powder diffraction analyses yielded a linear decrease of the lattice parameters with increasing
bulk B2O3 content, which was interpreted as to be due to boron incorporation. Related to the increasing boron content, corresponding
infrared spectra revealed a slight and continuous shift for most of the absorption bands. These data show that mullite is
able to incorporate large amounts of boron into its structure (up to about 20 mol% B2O3 depending on the bulk composition of the starting materials). Infrared analyses suggest that boron is incorporated into the
mullite structure in form of planar three-fold coordinated BO3 groups.
Author’s address: A. Beran, Institut für Mineralogie und Kristallographie, Universit?t Wien-Geozentrum, Althanstra?e 14, 1090 Wien, Austria 相似文献
633.
M. Mejías J. Garcia-Orellana J. L. Plata M. Marina E. Garcia-Solsona B. Ballesteros P. Masqué J. López C. Fernández-Arrojo 《Environmental Geology》2008,54(3):521-536
A methodology for the characterization of deep carbonate aquifers has been developed and applied to El Maestrazgo Jurassic
aquifer in Castellón, Spain. Characterization of these aquifer formations, located at more than 300 m deep, consisted of a
previous phase of compilation, analysis and synthesis of the existing information about the area, followed by a coordinated
combination of different speciality studies: geology, stratigraphy, structural analysis, hydrogeology, hydrochemistry, geophysics
and remote sensing. Geological studies included geological mapping, definition of stratigraphical units and facies and structural
analysis. The aim of the hydrogeology study was to define aquifer formations, recharge area, aquifer points inventory and
groundwater flow directions for the establishment of piezometric and water quality observation nets. Special techniques were
applied, like thermal infrared aerial images and the evaluation of submarine groundwater discharge by means of natural radium
isotopes. Hydrochemical techniques, including majority elements characterization and stable isotopes (18O, 2H and 3H) determination, allowed classifying hydrochemical facies and establishing a renewal pattern for water within the system.
Geophysics was useful in determining the aquifer geometry, the features of the basement and the petrophysical characteristics
of the geological formations. Preliminary results show an important tectonic complexity and the possibilities for groundwater
uses in the area of study. 相似文献
634.
635.
636.
A numerical analysis of cyclotron instabilities is carried out by computing the dispersion relation for a three component cold plasma-beam system. Rates of growth and damping for various values of the stream density are calculated from the dispersion relation. The rates of growth and damping increase monotonically as the number density of the proton stream increases. It is found that the frequencies at the rates of maximum growth and the damping decrease slightly to lower frequencies and a sharp peak at these frequencies becomes blunt. The minimum e-folding times of an ion cyclotron wave for (a) σs = 10−4, σi = 10−2 and (b) σs = 10−1, σi = 10−2 are about 3·84 and 0·16 sec respectively in the vicinity of the equatorial plane at 6 Re, where σs and σi are the ratios of the beam density Ns and the helium ion (H6+) density Ni to the total positive ions in the plasma-beam system. 相似文献
637.
638.
J. W. Norman 《The Photogrammetric Record》1968,6(32):133-149
The geologist's use of air photographs and the characteristics studied by photogeologists are briefly described. The influence of photographic factors such as scale, season, time of day, film type, processing, overlap and dimensional accuracy are considered from the geologist's viewpoint. 相似文献
639.
Photogrammetric methods using sequential aerial photography can provide the geomorphologist with comparative measurements of rapidly changing glacial landforms. Examples are given of eskers, kame and kettle areas, an ice-dammed lake and coastal features in south-east Iceland. 相似文献
640.