全文获取类型
收费全文 | 118314篇 |
免费 | 2403篇 |
国内免费 | 1061篇 |
专业分类
测绘学 | 3011篇 |
大气科学 | 8828篇 |
地球物理 | 24225篇 |
地质学 | 41088篇 |
海洋学 | 10354篇 |
天文学 | 26144篇 |
综合类 | 326篇 |
自然地理 | 7802篇 |
出版年
2021年 | 945篇 |
2020年 | 1137篇 |
2019年 | 1204篇 |
2018年 | 2558篇 |
2017年 | 2416篇 |
2016年 | 3194篇 |
2015年 | 2031篇 |
2014年 | 3149篇 |
2013年 | 6201篇 |
2012年 | 3320篇 |
2011年 | 4638篇 |
2010年 | 3988篇 |
2009年 | 5419篇 |
2008年 | 4981篇 |
2007年 | 4599篇 |
2006年 | 4515篇 |
2005年 | 3726篇 |
2004年 | 3748篇 |
2003年 | 3491篇 |
2002年 | 3273篇 |
2001年 | 2957篇 |
2000年 | 2872篇 |
1999年 | 2405篇 |
1998年 | 2455篇 |
1997年 | 2350篇 |
1996年 | 2011篇 |
1995年 | 1962篇 |
1994年 | 1765篇 |
1993年 | 1608篇 |
1992年 | 1529篇 |
1991年 | 1399篇 |
1990年 | 1597篇 |
1989年 | 1373篇 |
1988年 | 1228篇 |
1987年 | 1510篇 |
1986年 | 1311篇 |
1985年 | 1647篇 |
1984年 | 1845篇 |
1983年 | 1751篇 |
1982年 | 1630篇 |
1981年 | 1515篇 |
1980年 | 1344篇 |
1979年 | 1254篇 |
1978年 | 1315篇 |
1977年 | 1195篇 |
1976年 | 1140篇 |
1975年 | 1069篇 |
1974年 | 1071篇 |
1973年 | 1067篇 |
1972年 | 683篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
942.
Tensioned foundations are common in civil engineering applications such as transmission towers, harbors, offshore structures, basement slabs under pressure, industrial equipment, etc. Procedures for the design of tensioned foundations are discussed in this paper, including specific recommendations for more common transmission tower foundations. Starting from a distinction between shallow and deep modes of failure, the paper presents the most common failure mechanisms for shallow failure in tension, including procedures for calculation of foundation tensile capacity under vertical and inclined loading. Emphasis is given to the influence of the strength of the compacted backfill compared to the strength of the natural soil, including presentation of results of full-scale loading tests. 相似文献
943.
The paper [Wang, J.-J., Zhu, J.-G., Chiu, C.F., Zhang, H., 2007. Experimental study on fracture toughness and tensile strength of a clay. Engineering Geology 94, 64–75.] focuses on two important fracture parameters of soils: tensile strength and fracture toughness. These parameters control the behaviour of soils in a wide range of situations, from the design of a simple footing to much complicated fracture behaviour of clay liners or covers. The authors have done extensive laboratory work to determine these two parameters and their laborious and complicated experimental program needs praise. However, some of the points raised in their conclusions, based on the analysis and comparison with the data from the literature, need to be discussed. 相似文献
944.
945.
946.
A. A. Krylov O. M. Khlystov T. I. Zemskaya H. Minami A. Hachikubo H. Shoji M. Kida T. P. Pogodaeva L. Naudts J. Poort 《Geochemistry International》2008,46(10):985-995
This paper presents data on authigenic siderite first found in surface sediments from mud volcanoes in the Central (K-2) and Southern (Malen’kii) basins of Lake Baikal. Ca is the predominant cation, which substitutes Fe in the crystalline lattice of siderite. The enrichment of the carbonates in the 13C isotope (from +3.3 to +6.8‰ for the Malen’kii volcano and from +17.7 to +21.9‰ for K-2) results from the crystallization of the carbonates during methane generation via the bacterial destruction of organic matter (acetate). The overall depletion of the carbonates in 18O is mainly inherited from the isotopic composition of Baikal water. 相似文献
947.
N. E. Kozlov E. V. Martynov N. O. Sorokhtin A. A. Ivanov N. M. Kudryashov N. E. Kozlova 《Geochemistry International》2008,46(6):608-613
Rock associations characterized by heterogeneous sets of petrogeochemical parameters were compared by quantifying the degree of their similarity-dissimilarity and searching for discrimination trends between them. Using procedures specially developed for this purpose, it was demonstrated for the first time that the lithotectonic complexes of the Murmansk domain are fundamentally different from those of typical granulite-gneiss terrains and resemble the granite-greenstone terrains of the Baltic shield, Greenland, and Canada. Based on the whole data set, the Murmansk domain can be considered as a deeply eroded Archean granite-greenstone terrain retaining only the tonalite-trondhjemite-gneiss basement with abundant supracrustal enclaves. A trend of the compositional difference between the older and younger rock associations is similar to that between the tholeiitic and boninitic volcanic series. It was suggested that the petrogeochemical “age” trend reflects the initial stage of the compositional evolution of the metamagmatic rocks of the region from metamorphic rocks similar to tholeiites at the early stages to the Paleoproterozoic boninite-like rocks, which are believed to be linked to the unique PGE-bearing province of the northeastern Baltic shield. This implies that the specific metallogenic features of the region emerged already in the Archean, which supported the suggestion on the long duration of geological processes in the Early Precambrian. 相似文献
948.
N. M. Sushchevskaya A. N. Evdokimov B. V. Belyatsky V. A. Maslov D. V. Kuz’min 《Geochemistry International》2008,46(1):1-16
Petrological and geochemical data obtained on the Quaternary lavas of volcanoes at Spitsbergen Island indicate that the rocks were produced via the deep-seated crystallization of parental alkaline magmas at 8–10 kbar. The character of clinopyroxene enrichment in incompatible elements indicates that the mineral crystallized from more enriched melts than those inferred from the composition of the host lavas. These melts were close to the parental melts previously found as veinlets in mantle hyperbasite xenoliths in the lavas. According to the character of their enrichment in Pb and Sr radiogenic isotopes and depletion in Nd, the basalts from Spitsbergen Island define a single trend with the weakly enriched tholeiites of the Knipovich Ridge, a fact suggesting the closeness of the enriched sources beneath the continental margin of Spitsbergen and beneath the spreading zone. Magmatic activity at Spitsbergen was related to the evolution of the Norwegian-Greenland basin, which evolved in pulses according to the shift of the spreading axes. The most significant of the latter events took place in the Neogene, when the Knipovich Ridge obtained its modern position near the western boundary of Spitsbergen. Early in the course of the evolution, the emplacement of alkaline melts generated at Spitsbergen into the oceanic mantle could form the enriched mantle, which was later involved in the melting process beneath the spreading zone. 相似文献
949.
Jürgen Konzett Daniel J. Frost Alexander Proyer Peter Ulmer 《Contributions to Mineralogy and Petrology》2008,155(2):215-228
Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO2–TiO2–Al2O3–Fe2O3–FeO–MnO–MgO–CaO–Na2O–K2O–P2O5 and SiO2–TiO2–Al2O3–MgO–CaO–Na2O to investigate the Ca-Eskola (CaEs Ca0.5□0.5AlSi2O6) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO2 ± TiO2 ± kyanite as a function of P, T, and bulk composition. The results show that CaEsss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative
anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEsss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison,
no significant increase in CaEsss with increasing P could be observed. If the formation of oriented SiO2-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these
textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites
cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEsss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes
are not necessarily restricted to ultrahigh pressure metamorphic conditions.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
950.
Anthony C. Withers Marc M. Hirschmann 《Contributions to Mineralogy and Petrology》2008,156(5):595-605
Olivine crystals were grown in the presence of a hydrous silicate fluid during multi-anvil experiments at 8 GPa and 1,000–1,600°C.
Experiments were conducted both in a simple system (FeO–MgO–SiO2–H2O) and in a more complex system containing additional elements (CaO–Na2O–Al2O3–Cr2O3–TiO2–FeO–MgO–SiO2–H2O). Silica activity was buffered by the presence of either pyroxene (high a
SiO2) or ferropericlase (low a
SiO2), and was buffered by the presence of Ni + NiO or Fe + FeO, or constrained by the presence of Fe2O3. Raman spectroscopy was used to identify pyroxene polymorphs in the run products. Clinoenstatite was present in the 1,000°C
experiment, and enstatite in experiments at 1,400–1,520°C. The H2O content of olivine was measured using secondary ion mass spectroscopy, and infrared spectroscopy was used to investigate
the nature of hydrous defects. The H2O storage capacity of olivine decreases with increasing temperature at 8 GPa. In contrast to previous experimental results
at ≤2 GPa, no significant effect of varying oxygen fugacity is evident, but H2O storage capacity is enhanced under conditions of low silica activity. No significant growth of low wavenumber (<3,400 cm−1) peaks, generally associated with high at low pressure, was observed in the FTIR spectra of olivine from the high experiments. Our experiments show that previous high pressure H2O storage capacity measurements for olivine synthesized under more oxidizing conditions than the Earth’s mantle are not likely
to be compromised by the of the experiments. However, the considerable effect of temperature on H2O storage capacity in olivine must be taken into account to avoid overestimation of the bulk upper mantle H2O storage capacity. 相似文献