全文获取类型
收费全文 | 46550篇 |
免费 | 8082篇 |
国内免费 | 12298篇 |
专业分类
测绘学 | 3550篇 |
大气科学 | 7487篇 |
地球物理 | 7917篇 |
地质学 | 25820篇 |
海洋学 | 6603篇 |
天文学 | 3397篇 |
综合类 | 3170篇 |
自然地理 | 8986篇 |
出版年
2024年 | 226篇 |
2023年 | 611篇 |
2022年 | 1709篇 |
2021年 | 1859篇 |
2020年 | 1813篇 |
2019年 | 2321篇 |
2018年 | 1741篇 |
2017年 | 1922篇 |
2016年 | 1955篇 |
2015年 | 2168篇 |
2014年 | 2855篇 |
2013年 | 2758篇 |
2012年 | 2954篇 |
2011年 | 3094篇 |
2010年 | 2548篇 |
2009年 | 3235篇 |
2008年 | 3169篇 |
2007年 | 3277篇 |
2006年 | 3279篇 |
2005年 | 3016篇 |
2004年 | 2746篇 |
2003年 | 2539篇 |
2002年 | 2199篇 |
2001年 | 1907篇 |
2000年 | 1914篇 |
1999年 | 1733篇 |
1998年 | 1433篇 |
1997年 | 1121篇 |
1996年 | 933篇 |
1995年 | 782篇 |
1994年 | 752篇 |
1993年 | 605篇 |
1992年 | 447篇 |
1991年 | 378篇 |
1990年 | 243篇 |
1989年 | 205篇 |
1988年 | 163篇 |
1987年 | 95篇 |
1986年 | 49篇 |
1985年 | 34篇 |
1984年 | 21篇 |
1983年 | 18篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 15篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
911.
About 30 samples representing major lithologies of Sulu ultrahigh-pressure (UHP) metamorphic rocks were collected from surface exposures and exploration wells, and compressional (Vp) and shear wave (Vs) velocities and their directional dependence (anisotropy) were determined over a range of constant confining pressures up to 600 MPa and temperatures ranging from 20 to 600 °C. Samples range in composition from acidic to ultramafic. P- and S-wave velocities measured at 600 MPa vary from 5.08 to 8.64 km/s and 2.34 to 4.93 km/s, respectively. Densities are in the range from 2.60 to 3.68 g/cm3. To make a direct tie between seismic measurements (refraction and reflection) and subsurface lithologies, the experimental velocity data (corresponding to shallow depths) were used to calculate velocity profiles for the different lithologies and profiles of reflection coefficients at possible lithologic interfaces across the projected 5000-m Chinese Continental Scientific Drilling Program (CCSD) crustal segment. Comparison of calculated in situ velocities with respective intrinsic velocities suggests that the in situ velocities at shallow depths are lowered by an increased abundance of open microcracks. The strongly reflective zone beneath the Donghai drill site can be explained by the impedance contrasts between the different lithologies. Contacts between eclogite/peridotite and felsic rocks (gt-gneiss, granitic gneiss), in particular, may give rise to strong seismic reflections. In addition, shear-induced (lattice preferred orientation (LPO)-related) seismic anisotropy can increase reflectivity. For the explanation of the high velocity bodies (>6.4 km/s) around 1000 m and below 3200-m depth, large proportions of eclogite/peridotite (about 40 and 30 vol.%, respectively) are needed. 相似文献
912.
Leping coal is known for its high content of “barkinite”, which is a unique liptinite maceral apparently found only in the Late Permian coals of South China. “Barkinite” has previously identified as suberinite, but on the basis of further investigations, most coal petrologists conclude that “barkinite” is not suberinite, but a distinct maceral. The term “barkinite” was introduced by (State Bureau of Technical Supervision of the People's Republic of China, 1991, GB 12937-91 (in Chinese)), but it has not been recognized by ICCP and has not been accepted internationally.In this paper, elemental analyses (EA), pyrolysis-gas chromatography, Rock-Eval pyrolysis and optical techniques were used to study the optical features and the hydrocarbon-generating model of “barkinite”. The results show that “barkinite” with imbricate structure usually occurs in single or multiple layers or in a circular form, and no definite border exists between the cell walls and fillings, but there exist clear aperture among the cells.“Barkinite” is characterized by fluorescing in relatively high rank coals. At low maturity of 0.60–0.80%Ro, “barkinite” shows strong bright orange–yellow fluorescence, and the fluorescent colors of different cells are inhomogeneous in one sample. As vitrinite reflectance increases up to 0.90%Ro, “barkinite” also displays strong yellow or yellow–brown fluorescence; and most of “barkinite” lose fluorescence at the maturity of 1.20–1.30%Ro. However, most of suberinite types lose fluorescence at a vitrinite reflectance of 0.50% Ro, or at the stage of high volatile C bituminous coal. In particular, the cell walls of “barkinite” usually show red color, whereas the cell fillings show yellow color under transmitted light. This character is contrary to suberinite.“Barkinite” is also characterized by late generation of large amounts of liquid oil, which is different from the early generation of large amounts of liquid hydrocarbon. In addition, “barkinite” with high hydrocarbon generation potential, high elemental hydrogen, and low carbon content. The pyrolysis products of “barkinite” are dominated by aliphatic compounds, followed by low molecular-weight aromatic compounds (benzene, toluene, xylene and naphthalene), and a few isoprenoids. The pyrolysis hydrocarbons of “barkinite” are mostly composed of light oil (C6–C14) and wet gas (C2–C5), and that heavy oil (C15+) and methane (C1) are the minor hydrocarbon.In addition, suberinite is defined only as suberinized cell walls—it does not include the cell fillings, and the cell lumens were empty or filled by corpocollinites, which do not show any fluorescence. Whereas, “barkinite” not only includes the cell walls, but also includes the cell fillings, and the cell fillings show bright yellow fluorescence.Since the optical features and the hydrocarbon-generating model of “barkinite” are quite different from suberinite. We suggest that “barkinite” is a new type of maceral. 相似文献
913.
As a legacy of the centrally planned economy, the economies in transition of Central and Eastern Europe (CEE) have a unique
potential to reduce their greenhouse gas emissions through the improvement in their high energy intensities. Since much of
this `low-hanging fruit' in energy-efficiency improvements can be highly cost-effective, many developed countries facing difficulties
in meeting their greenhouse gas (GHG) emission targets domestically are eager to find such opportunities in the CEE region.
Therefore, studies analysing the potentials and costs of carbon dioxide reduction through technology improvement in the region
have come into the limelight. While there are a few excellent studies in the region aimed at analysing climate change abatement
potentials, they all embark on different assumptions, methodologies and boundary conditions. It is hence difficult, if not
impossible, to compare and analyse the results of these studies across different authors, countries or time horizons. Consequently,
the purpose of this paper is to place four leading studies on GHG mitigation through technology improvement from the CEE region
into an internationally comparable framework. Four studies were selected from three countries, Poland, Hungary and Estonia,
which are all the results of major national and international efforts to assess costs and potentials of GHG reduction. The
paper places their assumptions, methods and final results into a framework which enables policy-makers and project designers
to compare these across geographical and technological boundaries. Since other studies from around the globe have been analysed
in this framework in the literature, this paper provides a vehicle for the findings of these four studies to be compared to
others worldwide. In addition, the paper highlights a few areas where similar studies to be completed in the future in the
region may be enhanced by incorporating features used in GHG mitigation research in other parts of the world.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
914.
915.
Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton 总被引:40,自引:0,他引:40
Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle. 相似文献
916.
Authigenic albites in carbonate rocks typically grow in a high-grade diagenetic to low-grade metamorphic environment and often show Roc-Tourné-twinning sensu Füchtbauer. Based on an investigation of four Middle to South European occurrences, they show Mn2+- and Fe3+-activated cathodoluminescence (CL), as revealed by combined high resolution spectroscopy of cathodoluminescence emission (HRS-CL), electron paramagnetic resonance (EPR), and proton-induced X-ray emission (μPIXE). 相似文献
917.
Understanding the nature and impacts of tsunamis within the Aegean Sea region ofGreece is of importance to both the academic community and those organisationsconcerned with tsunami disaster management. In order to determine hazard and riskand consequently pre-plan mitigative strategies, it is necessary to analyse historical(documentary) and geological records of former tsunami events. Therefore, firstlythis paper provides a summary of the written sources of information on Aegeantsunamis paying particular attention to published catalogues. From the availabledata, it is noted that a large number of events have been reported during the last3500 years. Secondly, the paper provides a review of the published on-shore(terrestrial) geological records of tsunamis within the region. From this analysisit is seen that little geological evidence has been identified for the large numberof tsunamis reported in the catalogues. Thirdly, the paper considers the reliabilityof the written and geological records and how problems of accuracy, coverage,extent and reliability, may have potential implications for the estimation of hazardand risk. The paper concludes by making recommendations for disaster managers,geologists and historians to work closely together. 相似文献
918.
919.
920.