全文获取类型
收费全文 | 371篇 |
免费 | 16篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 16篇 |
地球物理 | 80篇 |
地质学 | 203篇 |
海洋学 | 39篇 |
天文学 | 29篇 |
综合类 | 3篇 |
自然地理 | 14篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 2篇 |
2021年 | 10篇 |
2020年 | 6篇 |
2019年 | 11篇 |
2018年 | 22篇 |
2017年 | 15篇 |
2016年 | 20篇 |
2015年 | 13篇 |
2014年 | 18篇 |
2013年 | 20篇 |
2012年 | 18篇 |
2011年 | 23篇 |
2010年 | 26篇 |
2009年 | 23篇 |
2008年 | 13篇 |
2007年 | 16篇 |
2006年 | 15篇 |
2005年 | 21篇 |
2004年 | 11篇 |
2003年 | 9篇 |
2002年 | 10篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1990年 | 1篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1973年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有390条查询结果,搜索用时 109 毫秒
381.
Heather Xiaoquan Zhang P. Mick Kelly Catherine Locke Alexandra Winkels W. Neil Adger 《Geoforum》2006,37(6):1066-1081
Migration is frequently portrayed as a negative force in its relationship with economic and social development. This negative perception is exhibited through describing population movements as either ‘forced’ (e.g. political and environmental refugees) or ‘voluntary’ movements (e.g. economic and uninhibited relocation). This paper examines the limitations of this conceptual dualism. It points out that the dualistic approach, widely used in the context of developing countries, simplifies a highly complex phenomenon by ignoring its essential heterogeneity and spatial and temporal dynamics. As such, it is limited in explaining and understanding the globally diversified, historically and politically contextualised situations. Focusing on the migratory experiences of contemporary Vietnam, the study identifies major patterns and trends of population mobility in the country in the past fifty years. It shows that despite the state’s continued attempts to reshape the spatial distribution of population over recent historical periods, the policy outcomes with respect to population mobility have been swayed as much by individuals and their families in pursuit of their own aspirations and livelihoods as by state plans. The Vietnam case has provided evidence of a much more complicated relationship between migration and livelihoods than the conceptual dichotomy assumes, and the opportunity for a richer set of policy options. We argue that the evidence from Vietnam, and elsewhere, warrants an integrated approach to studying migration, combining analysis at the macro- and micro-levels with the nexus lying at the critical decision-making point of the individual or household. 相似文献
382.
Enthalpies of solution in molten 2 PbO · B2O3 at 974 K were measured for four spinelloids, phases I (0.75 NiAl2O4 · 0.25 Ni2SiO4), II (0.60 NiAl2O4 · 0.40 Ni2SiO4), III and IV (0.50 NiAl2O4 · 0.50 Ni2SiO4) in the system NiAl2O4 · Ni2SiO4. The enthalpies (in cal per 4-oxygen mol) of formation from NiAl2O4 and Ni2SiO4 spinels are: phase I, 945±366; phase II, 1072±360; phase III, 2253±390; phase IV, 3565±544. Using these enthalpy data in combination with phase relations at high pressure at 1373 K, positive entropies of formation of the spinelloids from NiAl2O4 and Ni2SiO4 spinels were estimated (in cal mol?1 K?1): phase I, 1.2; phase II, 1.5; phase III, 2.0–2.3; phase IV, 3.0–3.1. The thermochemical data obtained above suggest that the spinelloids are “entropy-stabilized” phases with partially disordered cation distributions. The configurational entropies of the spinelloids were calculated based on the observed cation distribution in each spinelloid phase. The positive entropies of formation of the spinelloids from the spinel endmembers are due primarily to the configurational entropies although small positive vibrational entropy changes may also exist. 相似文献
383.
Enthalpies of solution in 2PbO · B2O3 at 981 K have been measured for glasses in the system albite-orthoclase-silica and along the join Na1.6Al1.6Si2.4O8-K1.6Al1.6Si2.4O8. The join KAlSi3O8-Si4O8 shows zero heat of mixing similar to that found previously for NaAlSi3O8-Si4O8 glasses. Albite-orthoclase glasses show negative heats of mixing symmetric about Ab50Or50 (Wn = ? 2.4 ± 0.8 kcal). Negative heats of (Na, K) mixing are also found at Ternary excess enthalpies of mixing in the glassy system Ab-Or-4Q are positive but rarely exceed 1 kcal mol?1.Using earlier studies of the thermodynamic properties of the crystals, the present calorimetric data and the “two-lattice” entropy model, the albite-orthoclase phase diagram is calculated in good agreement with experimental data. Attempts to calculate albite-silica and orthoclase-silica phase diagrams reveal complexities probably related to significant (but unknown) mutual solid solubility between cristobalite and alkali feldspar and to the very small heat and entropy of fusion of SiO2. 相似文献
384.
Alexandra Navrotsky Hans D Zimmermann Richard L Hervig 《Geochimica et cosmochimica acta》1983,47(8):1535-1538
High temperature solution calorimetry of glasses in the system CaMgSi2O6 (Di)-CaAl2SiO6 (CaTs) show them to have negative enthalpies of mixing with a regular enthalpy parameter, WH, of -11.4 ± 0.7 kcal. Negative heats of mixing between alumina-rich and alumina-poor glasses seem to be a general phenomenon in aluminosilicates and are not confined only to glassy systems containing anorthite as a component. The thermodynamic behavior of glasses in the system SiO2-Ca0.5;AlO2-CaMgO2 appears to vary in a smooth fashion, with small positive heats of mixing near SiO2 and substantial negative heats of mixing for other compositions. The exothermic behavior with increasing may be related to local charge balance of M2+ and Al3+. The negative heats of mixing in MgCaSi2O6-CaAl2SiO6, MgCaSi2O6-CaAl2Si2O8 and NaAlSi3O8-CaAl2Si2O8 glasses are in contrast to the positive heats of mixing found in MgCaSi2O6-CaAl2SiO6 (pyroxene) and NaAlSi3O8-CaAl2Si2O8 (high plagioclase) crystalline solid solutions. 相似文献
385.
Summary The stilpnomelane minerals in the epimetamorphic series of the Romanian Carpathians are divided into four genetic groups. Some ferrostilpnomelanes from metamorphic iron-, manganese- and sulfide-ores have been studied optically, chemically, by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and IR absorption spectroscopy. Several conclusions on the chemical constitution and the widespread occurence of ferrostilpnomelane in the metamorphic ores of the Carpathians are given.
Mit 5 Figures 相似文献
Über Stilpnomelan in einigen kristallinen Formationen in den rumänischen Karpaten
Zusammenfassung Die Stilpnomelanvorkommen aus dem epimetamorphen Kristallin der rumänischen Karpaten werden in vier genetische Gruppen eingeteilt. Einige aus metamorphosierten Eisen-, Mangan- und Sulfiderzen stammende Ferrostilpnomelane wurden optisch, chemisch, mit Differentialthermoanalyse, Thermogravimetrie, Röntgenbeugung und Ultrarot-Absorptionsspektroskopie untersucht. Als Schlußfolgerungen ergeben sich einige allgemeine Hinweise über die Zusammensetzung des Ferrostilpnomelans in den Kristallinerzen der Karpaten.
Mit 5 Figures 相似文献
386.
High temperature solution calorimetry of synthetic quartz, coesite and stishovite provides enthalpies of transition. ΔH9750 for quartz-coesite and ΔH2980 for coesite-stishovite transition are 320 ± 70 and 11700 ± 410 cal mol?1, respectively. The present transformation enthalpy data represent a small but significant revision of those of Holm et al. Using the published phase equilibrium data, thermal expansivity, compressibility and heat capacity data, ΔS9750 for the quartz-coesite and ΔS2980 for the coesite-stishovite transition are ?1.2 ± 0.1 and ?1.0 ± 0.4 cal K?1 mol?1, respectively. These thermochemical data are used to calculate phase boundaries of the transitions. The calculated quartz-coesite transition boundary agrees well with the one determined experimentally by Bohlen and Boettcher. The calculated coesite-stishovite boundary is generally consistent with data by Yagi and Akimoto and by Suito. 相似文献
387.
Paul McMillan Bernard Piriou Alexandra Navrotsky 《Geochimica et cosmochimica acta》1982,46(11):2021-2037
Aluminosilicate glasses with compositions along the joins silica-calcium aluminate, silica sodium aluminate and silica-potassium aluminate have been prepared by conventional and solar melting techniques and studied by Raman spectroscopy. The Raman spectra of crystalline calcium aluminate, anorthite and silica polymorphs are discussed in relation to their crystal structures, and compared with the spectra of the corresponding glasses. The glass and crystal spectra are generally comparable, suggesting similar vibrational structures. These crystals have structures based on tetrahedral aluminosilicate frameworks, and a similar molecular structure is suggested for the glasses, although it is noted that the Raman spectra do not directly characterize the aluminate polyhedra. Within the three glass series, our interpretation of the unresolved high-frequency bands shows the appearance of discrete bands near 1120, 1000, 930 and 890 cm?1 as the silica content is decreased. This is compared with the behaviour of high-frequency bands in simple silicate systems, and used to suggest that the four bands in the aluminosilicate systems are due to stretching vibrations of silicate tetrahedra bound to one, two, three and four aluminium atoms. The spectra of calcium, sodium, potassium and lithium aluminosilicate glasses with similar silica contents are compared, and interpreted by the above model. This is used to construct a simple model for the effect of metal cation on aluminosilicate molecular groups in the glass structure, consistent with the results of calorimetric studies on similar systems. 相似文献
388.
Sascha?SandmannEmail author Thorsten?J.?Nagel Daniel?Herwartz Raúl?O.?C.?Fonseca Robert?M.?Kurzawski Carsten?Münker Nikolaus?Froitzheim 《Contributions to Mineralogy and Petrology》2014,168(5):1075
The Adula Nappe in the Central Alps is a mixture of various pre-Mesozoic continental basement rocks, metabasics, ultrabasics, and Mesozoic cover rocks, which were pervasively deformed during Alpine orogeny. Metabasics, ultrabasics, and locally garnet–mica schists preserve eclogite-facies assemblages while the bulk of the nappe lacks such evidence. We provide garnet major-element data, Lu profiles, and Lu–Hf garnet geochronology from eclogites sampled along a north–south traverse. A southward increasing Alpine overprint over pre-Alpine garnets is observed throughout the nappe. Garnets in a sample from the northern Adula Nappe display a single growth cycle and yield a Variscan age of 323.8 ± 6.9 Ma. In contrast, a sample from Alpe Arami in the southernmost part contains unzoned garnets that fully equilibrated to Alpine high-pressure (HP) metamorphic conditions with temperatures exceeding 800 °C. We suggest that the respective Eocene Lu–Hf age of 34.1 ± 2.8 Ma is affected by partial re-equilibration after the Alpine pressure peak. A third sample from the central part of the nappe contains separable Alpine and Variscan garnet populations. The Alpine population yields a maximum age of 38.8 ± 4.3 Ma in line with a previously published garnet maximum age from the central nappe of 37.1 ± 0.9 Ma. The Adula Nappe represents a coherent basement unit, which preserves a continuous Alpine high-pressure metamorphic gradient. It was subducted as a whole in a single, short-lived event in the upper Eocene. Controversial HP ages and conditions in the Adula Nappe may result from partly preserved Variscan assemblages in Alpine metamorphic rocks. 相似文献
389.
R. M. Fonseca O. Leeuwenburgh P. M. J. Van den Hof J. D. Jansen 《Computational Geosciences》2014,18(3-4):449-461
In an earlier study, two hierarchical multi-objective methods were suggested to include short-term targets in life-cycle production optimization. However, this earlier study has two limitations: (1) the adjoint formulation is used to obtain gradient information, requiring simulator source code access and an extensive implementation effort, and (2) one of the two proposed methods relies on the Hessian matrix which is obtained by a computationally expensive method. In order to overcome the first of these limitations, we used ensemble-based optimization (EnOpt). EnOpt does not require source code access and is relatively easy to implement. To address the second limitation, we used the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm to obtain an approximation of the Hessian matrix. We performed experiments in which water flood was optimized in a geologically realistic multilayer sector model. The controls were inflow control valve settings at predefined time intervals. Undiscounted net present value (NPV) and highly discounted NPV were the long-term and short-term objective functions used. We obtained an increase of approximately 14 % in the secondary objective for a decrease of only 0.2–0.5 % in the primary objective. The study demonstrates that ensemble-based hierarchical multi-objective optimization can achieve results of practical value in a computationally efficient manner. 相似文献
390.
L.?C.?BaldwinEmail author F.?Tomaschek C.?Ballhaus A.?Gerdes R.?O.?C.?Fonseca R.?Wirth T.?Geisler T.?Nagel 《Contributions to Mineralogy and Petrology》2017,172(6):43
Megacrystic sapphires are frequently associated with alkaline basalts, most notably in Asia and Australia, although basalt is not generally normative in corundum. Most of these sapphire occurrences are located in alluvial or eluvial deposits, making it difficult to study the enigmatic relationship between the sapphires and their host rocks. Here, we present detailed petrological and geochemical investigations of in situ megacrystic sapphires within alkaline basalts from the Cenozoic Siebengebirge Volcanic Field (SVF) in Germany. Markedly, the sapphires show several micrometer thick spinel coronas at the contact with the host basalt, indicating chemical disequilibrium between the sapphire and the basaltic melt, supporting a xenogenetic relationship. However, in situ U–Pb dating of a Columbite Group inclusion within one Siebengebirge sapphire using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) indicates a close genetic relationship between sapphire crystallization and alkaline mafic volcanism in the SVF. The syngenetic mineral inclusion suite including carbonates, members of the Pyrochlore, Betafite and Columbite Groupe minerals, as well as a high abundance of HFSE and of gaseous low-density CO2 inclusions support a parentage of a highly evolved, MgO and FeO deficient carbonatitic melt. We identified CO2 to be the link between alkaline basaltic volcanism and the xenocrystic sapphires. Only alkaline volcanic suites can build up enough CO2 in this magma chamber upon fractionation so that at high degrees of fractionation a carbonatitic melt exsolves which in turn can crystallize sapphires. 相似文献