全文获取类型
收费全文 | 4362篇 |
免费 | 150篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 171篇 |
大气科学 | 589篇 |
地球物理 | 1021篇 |
地质学 | 1766篇 |
海洋学 | 203篇 |
天文学 | 575篇 |
综合类 | 15篇 |
自然地理 | 182篇 |
出版年
2021年 | 64篇 |
2020年 | 60篇 |
2019年 | 47篇 |
2018年 | 123篇 |
2017年 | 140篇 |
2016年 | 213篇 |
2015年 | 132篇 |
2014年 | 189篇 |
2013年 | 247篇 |
2012年 | 102篇 |
2011年 | 162篇 |
2010年 | 181篇 |
2009年 | 216篇 |
2008年 | 148篇 |
2007年 | 133篇 |
2006年 | 112篇 |
2005年 | 95篇 |
2004年 | 68篇 |
2003年 | 73篇 |
2002年 | 102篇 |
2001年 | 88篇 |
2000年 | 74篇 |
1999年 | 74篇 |
1998年 | 75篇 |
1997年 | 65篇 |
1996年 | 57篇 |
1995年 | 65篇 |
1994年 | 79篇 |
1993年 | 35篇 |
1992年 | 38篇 |
1991年 | 38篇 |
1990年 | 45篇 |
1989年 | 38篇 |
1988年 | 31篇 |
1987年 | 35篇 |
1985年 | 47篇 |
1984年 | 47篇 |
1983年 | 57篇 |
1982年 | 46篇 |
1981年 | 46篇 |
1980年 | 41篇 |
1979年 | 34篇 |
1978年 | 62篇 |
1977年 | 35篇 |
1976年 | 36篇 |
1975年 | 44篇 |
1974年 | 53篇 |
1973年 | 51篇 |
1968年 | 27篇 |
1962年 | 26篇 |
排序方式: 共有4522条查询结果,搜索用时 403 毫秒
241.
A method for calibration and measurement with a four-wire probe is described. For each of the wires a three dimensional calibration field is determined, thus no assumption like King's law or the cosine law need to be made. The velocity vector can then be detected in a fairly large angular range (± 40°) with a numerical search algorithm. First measurements in a free jet and a confined, strongly swirling flow are presented.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988 相似文献
242.
243.
244.
The molecules constituting the first hydrate layers on montmorillonite crystal platelets appear to be characterized by preferred orientation and, depending on the exchangeable cation, by an ice-like configuration. Such organized interlamellar water is concluded to contribute to the high swelling pressure and shear resistance, as well as to the very low hydraulic conductivity at high clay bulk densities. At low densities the parallel arrangement of the lamellae alters to edge-to-face associations and the fraction of free water increases. However, even at very low densities the homogeneity of the clay gel still offers very tortuous passages to flowing water, which is manifested by a very low hydraulic conductivity. 相似文献
245.
246.
Prof. Dr. Jörg Loeschke 《International Journal of Earth Sciences》1985,74(3):537-546
20 clasts of acidic volcanic rocks from Silurian conglomerates of the Midland Valley of Scotland are similar in their geochemical composition to rhyolithes, rhyodacites and dacites of recent island arcs and active continental margins. They differ from the geochemical composition of upper Ordovician peralkaline rhyolites of the Tweed Valley in the Southern Uplands (Scotland) because of their lower Zr, Nb, Ce and Y contents. It is argued that these volcanic clasts represent erosional remnants of an island arc or active continental margin which was situated in the Midland Valley during upper Ordovician or lower Silurian time. This has implications on palaeogeographic models which have been suggested for the Scottish Caledonides.
Zusammenfassung 20 Gerölle saurer Vulkanite aus silurischen Konglomeraten des Midland Valley in Schottland haben in ihrer geochemischen Zusammensetzung Ähnlichkeiten mit Rhyolithen, Rhyodaziten und Daziten von rezenten Inselbögen und aktiven Kontinentalrändern. Sie unterscheiden sich durch niedrigere Gehalte an Zr, Nb, Ce und Y von oberordovizischen Peralkali-Rhyolithen des Tweed Valley in den Southern Uplands (Schottland). Es wird vermutet, daß die analysierten Gerolle Erosionsreste eines Inselbogens oder aktiven Kontinentalrandes darstellen, der zur Zeit des oberen Ordoviziums oder unteren Silurs im Raum des Midland Valley gelegen hat. Es ergeben sich daraus Folgerungen für paläogeographische Modelle, die für die schottischen Kaledoniden entwickelt worden sind.
Résumé La présente note porte sur 20 galets de volcanites acides contenus dans des conglomérats siluriens du Midland Valley (Ecosse). Leurs compositions géochimiques sont analogues à celles de rhyolites, rhyo-dacites et dacites d'arcs insulaires et de marges continentales actives actuels. Leur faible teneur relative en Zr, Nb, Ce et Y les distinguent des rhyolites peralcalines d'âge ordovicien supérieur des Southern Uplands (Ecosse). Ces galets de volcanites sont interprétés comme des produits de l'érosion soit d'un arc insulaire, soit d'une marge active, situés à l'emplacement de l'actuelle Midland Valley au cours de l'Ordovicien supérieur ou du Silurien inférieur.Ceci entraîne certaines implications dans les modèles paléogéographiques qui ont été proposés pour les Calédonides d'Ecosse.
20 Midland Valley, . , . Tweed Valley , . , , , , Midland Valley. .相似文献
247.
248.
Lake Pontchartrain., Louisiana is a 1,630 km2 shallow brackish lagoon with a mean salinity from 1.2‰ in the west to 5.4‰ in the east., The construction of a 120 km long deep-water connection to the Gulf of Mexico in 1963, was expected to cause a 5‰ increase in lagoon salinity. However, the actual increase was everywhere in the lagoon less than 2‰ Analysis of 31 years of daily salinity and discharge records indicates that discharge is the most important factor controlling salinity variations in Lake Pontchartrain., Seventy-four percent of low-frequency salinity variations are explained by freshwater discharge and the completion of the deep-water canal. Lake Pont.-chartrain experiences annual variations of salinity as high as 8‰ This salinity signal has remained constant since the beginning of the salinity records in 1946. It appears that the deep-water canal is not responsible for observed die-back of freshwater swamps and retreat of lagoon shorelines. 相似文献
249.
A general model for the structural state of iron in a variety of silicate and aluminosilicate glass compositions in the systems Na2O-Al2O3-SiO2-Fe-O, CaO-Al2O3-SiO2-Fe-O, and MgO-Al2O3-SiO2-Fe-O is proposed. Quenched melts with variable Al/Si and NBO/T (average number of nonbridging oxygens per tetrahedrally coordinated cation), synthesized over a range of temperatures and values of oxygen fugacity, are analyzed with57Fe Mössbauer spectroscopy. For oxidized glasses with Fe3+/∑Fe>0.50, the isomer shift for Fe3+ is in the range ~0.22–0.33 mm/s and ~0.36 mm/s at 298 K and 77 K, respectively. These values are indicative of tetrahedrally coordinated Fe3?. This assignment is in agreement with the interpretation of Raman, luminescence, and X-ray,K-edge absorption spectra. The values of the quadrupole splitting are ~0.90 mm/s (298 K and 77 K) in the Na-aluminosilicate glasses and compare with the values of 1.3 mm/s and 1.5 mm/s for the analogous Ca- and Mg-aluminosilicate compositions. The variations in quadrupole splittings for Fe3+ are due to differences in the degree of distortion of the tetrahedrally coordinated site in each of the systems. The values of the isomer shifts for Fe2+ ions in glasses irrespective of Fe3+/∑Fe are in the range 0.90–1.06 mm/s at 298 K and 1.0–1.15 mm/s at 77 K. The corresponding range of values of the quadrupole splitting is 1.75–2.10 mm/s at 298 K and 2.00–2.35 mm/s at 77 K. The temperature dependence of the hyperfine parameters for Fe2+ is indicative of noninteracting ions, but the values of the isomer shift are intermediate between those values normally attributable to tetrahedrally and octahedrally coordinated Fe2+. The assignment of the isomer-shift values of Fe2+ to octahedral coordination is in agreement with the results of other spectral studies. For reduced glasses (Fe3+/∑Fe≈<0.50), the value of the isomer shift for Fe3+ at both 298 K and 77 K increases and is linearly correlated with decreasing Fe3+/∑Fe in the range of \(f_{O_2 } \) between 10?3 and 10?6 atm when a single quadrupole-split doublet is assumed to represent the absorption due to ferric iron. The increase in value of the isomer shift with decreasing \(f_{O_2 } \) is consistent with an increase in the proportion of Fe3+ ions that are octahedrally coordinated. The concentration of octahedral Fe3+ is dependent on the \(T - f_{O_2 } \) conditions, and in the range of log \(f_{O_2 } \) between 10?2.0 and 10?5 a significant proportion of the iron may occur as iron-rich structural units with stoichiometry similar to that of inverse spinels such as Fe3O4, in addition to isolated Fe2+ and Fe3+ ions. 相似文献
250.
Significance of Europium anomalies in silicate melts and crystal-melt equilibria: a re-evaluation 总被引:1,自引:0,他引:1
Although europium speciation in silicate melts partly reflects prevailing oxygen fugacities, melt composition and structure play the major role in determining Eu2+/ Eu3+ ratios and europium partitioning into mineral phases. Experimental evidence by different investigators on the magnitude of the compositional effect on Eu2+/Eu3+ provides consistent results only if account is taken of the oxygen buffer system employed in the experiments. The medium-dependent reduction of europium can be understood in terms of the preferential stabilization of Eu2+ by a strong aluminosilicate complex in the melt phase, and to a much lesser degree by metasilicate complexes. The stability of these complexes increases as the field strength of the associated cation species decreases. Hence Eu2+-aluminosilicate complexes are preferentially stabilized relative to trivalent lanthanides in melts of appropriate composition and their presence minimizes the enthalpy of the melt. The influence of these complexes is particularly pronounced in melts with a high feldspathic component and a strongly polymerized structure. Their petrogenetic influence is best documented through the behaviour of europium relative to the other lanthanides during anhydrous anatexis in high-grade metamorphic terrains and in the anomalous europium partition coefficients of phenocryst phases which formed during the crystallization of highly silicic magmas. 相似文献