全文获取类型
收费全文 | 3577篇 |
免费 | 98篇 |
国内免费 | 39篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 308篇 |
地球物理 | 821篇 |
地质学 | 1275篇 |
海洋学 | 336篇 |
天文学 | 504篇 |
综合类 | 10篇 |
自然地理 | 410篇 |
出版年
2021年 | 42篇 |
2020年 | 42篇 |
2019年 | 48篇 |
2018年 | 60篇 |
2017年 | 67篇 |
2016年 | 74篇 |
2015年 | 79篇 |
2014年 | 69篇 |
2013年 | 187篇 |
2012年 | 97篇 |
2011年 | 143篇 |
2010年 | 153篇 |
2009年 | 171篇 |
2008年 | 136篇 |
2007年 | 116篇 |
2006年 | 121篇 |
2005年 | 110篇 |
2004年 | 94篇 |
2003年 | 84篇 |
2002年 | 87篇 |
2001年 | 79篇 |
2000年 | 48篇 |
1999年 | 54篇 |
1998年 | 54篇 |
1997年 | 47篇 |
1996年 | 71篇 |
1995年 | 47篇 |
1994年 | 51篇 |
1993年 | 45篇 |
1992年 | 56篇 |
1991年 | 54篇 |
1990年 | 51篇 |
1989年 | 47篇 |
1988年 | 53篇 |
1987年 | 53篇 |
1986年 | 37篇 |
1985年 | 69篇 |
1984年 | 88篇 |
1983年 | 81篇 |
1982年 | 72篇 |
1981年 | 47篇 |
1980年 | 57篇 |
1979年 | 55篇 |
1978年 | 50篇 |
1977年 | 48篇 |
1976年 | 42篇 |
1975年 | 35篇 |
1974年 | 31篇 |
1973年 | 35篇 |
1972年 | 25篇 |
排序方式: 共有3714条查询结果,搜索用时 31 毫秒
311.
The relative density of bacterial colonization of small (5 to 40 μ diameter) suspended and sedimented detritus particles in salt marsh creeks was studied using an europium chelate-fluorescent brightener stain coupled with epifluorescent microscopy. Particles were obtained from four substations at each of three sites in salt marshes bordering the York River, Virginia. Coverage of detrital particles by bacteria was uniformly low, averaging 1.54%±1.83% of the particle surface area. Suspended particles larger than 10 μ diameter had lower bacterial densities than did sedimented particles in the same size ranges. It is speculated that microbial growth may be enhanced on sedimented particles both through lack of motion and access to greater nutrient concentrations. 相似文献
312.
William A. Bonner Neal E. Blair Richard M. Lemmon Jose J. Flores Glenn E. Pollock 《Geochimica et cosmochimica acta》1979,43(11):1841-1846
The optically pure d- and l-enantiomers of isovaline (I), which cannot be racemized by ordinary chemical mechanisms involving α-hydrogen removal, and which has been isolated in apparently racemic form from the Murchison meteorite, have been subjected to partial radiolysis by the ionizing radiation from a 3000 Ci 60Co γ-ray source. Both in the anhydrous and hydrated solid states and as solid sodium or hydrochloride salts each enantiomer suffered significant radioracemization of the undestroyed residue during its partial radiolysis. The sodium salt of isovaline in 0.1 M aqueous solution suffered extensive radiolysis with relatively small radiation doses, but showed no detectable radioracemization. The significance of these observations with respect to the primordial enantiomeric composition of the isovaline (and other amino acids) indigenous to meteroties is discussed. 相似文献
313.
314.
315.
David F. Reid Willard S. Moore William M. Sackett 《Earth and Planetary Science Letters》1979,43(2):227-236
The Mn-fiber technique for extracting radium from seawater has proved useful for studying the marine geochemistry of228Ra. In the Gulf of Mexico, this technique was used to measure the surface and near-surface distribution of226Ra and228Ra. The observed surface distribution of228Ra, and particularly the radium activity ratio (228/226) can be explained by known circulation patterns, or, when local surface currents are not well understood, may provide insight into their general characteristics.The radium activity ratio has increased from 0.5 in 1968 to 0.7 in 1973 in the surface Gulf of Mexico. This observed increase cannot be attributed to known anthropogenic or natural source perturbations within the Caribbean Sea-Gulf of Mexico system. Possible causes include a change in the residence time for near-surface water, or variations in the relative dominance of the two sources for water entering the eastern Caribbean; the North Equatorial Current and the Guiana Current.The temporal distribution of228Ra is unstable and naturally variable over a time period less than or equal to five years in the Gulf of Mexico and by extrapolation, the Caribbean Sea. Therefore, its usefulness in calculations of eddy diffusion coefficients for these regions is greatly diminished. 相似文献
316.
The non-linear response history and failure mechanism of coupled wall systems under dynamic loads and static loads are investigated through an analytical model. The walls and coupling beams are replaced by flexural elements. Axial and shear stiffnesses are included for the wall members. The stiffness characteristics of each member are determined by inelastic properties. The suitable hysteresis loops to each constituent member are established to include the specific characteristics of coupled wall systems. The computed results are compared with results obtained from tests using model structures statically and dynamically tested on the Illinois Earthquake Simulator. 相似文献
317.
318.
Petrogenesis of voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental,Chihuahua, Mexico 总被引:1,自引:0,他引:1
Maryellen Cameron William C. Bagby Kenneth L. Cameron 《Contributions to Mineralogy and Petrology》1980,74(3):271-284
The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico.In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains 1% K2O at 60% SiO2. Trace element abundances of a typical ignimbrite with 73% SiO2 are Sr 225 ppm, Rb 130 ppm, Y 32 ppm, Th 12 ppm, Zr 200 ppm, and Nb 15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation.Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma. 相似文献
319.
William A. Bassett John M. Bird Maura S. Weathers D.L. Kohlstedt 《Physics of the Earth and Planetary Interiors》1980,23(4):255-261
Josephinite, a complex, metal-bearing rock from the region of the Josephine Peridotite in southwest Oregon, contains FeNiCo metal alloy phases having exsolution textures. Scanning and transmission electron microscopy have revealed Widmanstaetten patterns in which the lamellae are polysynthetically twinned, ordered, face-centered-cubic FeNi3 surrounded by untwinned, ordered, face-centered-cubic FeNi3 and body-centered-cubic FeCo. These exsolution textures require a temperature in excess of 500°C for their formation. This is consistent with a mantle derivation of josephinite prior to obduction of the peridotite in one of the Klamath ophiolites. 相似文献
320.
William A. Bassett 《Physics of the Earth and Planetary Interiors》1980,23(4):337-340
The Wilson electron synchrotron at Cornell University has recently undergone modifications which enable it to produce a highly collimated X-ray beam of 1015 photons (s mrad. 10% bandwidth)?1 in the range from 1 to 40 keV (12.4 to 0.31 Å wavelength). It is expected that a monochromatic beam produced from this source will have at least a hundred times the intensity of the X-rays from a rotating anode source. It is hoped that the use of a position-sensitive detector will improve the detection efficiency by another factor of a hundred over photographic film. If these improvements are realized, it should be possible to conduct real-time or nearly real-time diffraction experiments in the diamond anvil cell. The technique will be used to collect data for experiments in which slower data collecting would mean missing transient phenomena. The technique will also be useful for making studies during conditions that cause deterioration of the experimental apparatus such as graphitization of the diamonds or creep of the metal parts. 相似文献