全文获取类型
收费全文 | 1941篇 |
免费 | 60篇 |
国内免费 | 23篇 |
专业分类
测绘学 | 59篇 |
大气科学 | 161篇 |
地球物理 | 473篇 |
地质学 | 712篇 |
海洋学 | 164篇 |
天文学 | 310篇 |
综合类 | 12篇 |
自然地理 | 133篇 |
出版年
2021年 | 24篇 |
2020年 | 21篇 |
2019年 | 24篇 |
2018年 | 54篇 |
2017年 | 35篇 |
2016年 | 55篇 |
2015年 | 31篇 |
2014年 | 50篇 |
2013年 | 99篇 |
2012年 | 57篇 |
2011年 | 106篇 |
2010年 | 75篇 |
2009年 | 85篇 |
2008年 | 86篇 |
2007年 | 70篇 |
2006年 | 58篇 |
2005年 | 63篇 |
2004年 | 45篇 |
2003年 | 40篇 |
2002年 | 57篇 |
2001年 | 36篇 |
2000年 | 51篇 |
1999年 | 54篇 |
1998年 | 32篇 |
1997年 | 48篇 |
1996年 | 43篇 |
1995年 | 34篇 |
1994年 | 30篇 |
1993年 | 28篇 |
1992年 | 20篇 |
1991年 | 22篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 34篇 |
1986年 | 18篇 |
1985年 | 24篇 |
1984年 | 29篇 |
1983年 | 27篇 |
1982年 | 35篇 |
1981年 | 21篇 |
1980年 | 29篇 |
1979年 | 20篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 25篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1971年 | 12篇 |
排序方式: 共有2024条查询结果,搜索用时 15 毫秒
271.
The Chakradharpur Granite—Gneiss complex (CKPG) is exposed as an elliptical body within the arcuate metamorphic belt sandwiched between the Singhbhum Granite in the south and the Chotonagpur Granite—Gneiss to the north. It consists of an older bimodal suite of grey gneiss and amphibolites, intruded by a younger unit of pegmatitic granite. The bimodal suite represents the basement to the enveloping metasediments.The average major-element chemistry of the grey gneiss conforms to the definition of trondhjemite and includes both low-Al2O3 and high-Al2O3 types. The amphibolites can be grouped into a low-MgO and a high-MgO type. Rocks of the younger unit range in composition from granodiorite to quartz monzonite. The two granitic units also differ significantly in their Rb, Sr and Ba contents, and in the REE distribution pattern. The grey gneiss shows a highly fractionated REE pattern and a distinct positive Eu anomaly, with Eu/Eu* values increasing with increase in SiO2 %. In samples of the younger granite, the REE pattern is less fractionated, with higher HREE abundance relative to the grey gneiss and usually shows a negative Eu anomaly. The two types of REE patterns in amphibolites are interpreted to represent the two broad groups identified on the basis of major element chemistry.On the basis of chemical data, a two-stage partial melting model for the genesis of grey gneiss is suggested, involving separation of hornblende and varying amounts of plagioclase in the residual phase. Varying amounts of plagioclase in the residuum result in the wide range of Al2O3 content of the partial melt from which the trondhjemites crystallised. Residual hornblende produces the highly fractionated REE pattern, but a relatively higher HREE content of the trondhjemites compared to those produced by separation of garnet in the residual phase. The high level of Ba together with moderate levels of Sr in the trondhjemites can also be explained by plagioclase in the residue, whose effectiveness in partitioning Ba compared to Sr is lower. Of the two groups of amphibolites, the low-MgO type shows relative depletion of LREE compared to the high-MgO type. It contains varying amounts of plagioclase and is tentatively suggested to represent the residue. The other group, with a slightly fractionated REE pattern (CeN/ YbN = 2.01), is generally considered to represent the source material for the trondhjemites. This may have been generated by 5–15% partial melting of mantle peridotites, containing higher concentrations of LIL elements than those which produced average Precambrian tholeiites. This first phase of partial melting resulted in the slightly fractionated REE pattern of these amphibolites. Derivation of the younger granitic unit from the trondhjemites can be ruled out on the basis of REE data alone. REE data suggest partial melting of metasediments to be the origin of these rocks. It is also possible that deeply buried volcanic rocks, similar to calc-alkaline components of greenstone belts, are the parent for this component. 相似文献
272.
273.
The influence of temperature changes in circumstellar silicate-like envelopes upon the polarization effects is investigated. It is shown that under the assumption that ΔT g>50° and conductivity of silicate grains is indirectly proportional toT g this mechanism can be responsible for the observed dependence of intensity vs polarization in some late-type stars, e.g. V CVn. The same effects can be produced by dirty ices and graphite grains. It is suggested that irradiation by electrons and/or protons can affect the circumstellar envelopes in a similar way, especially those of early-type stars, and irradiation by neutrons can exert an influence on the envelopes of supernovae. 相似文献
274.
The probability that γ-ray bursts may be generated by the infall of comet-like objects on the neutron stars, as recently proposed by Harwit and Salpeter (1973), is reexamined. Although hypothetical cometary clouds around the parent star may survive the supernova outburst virtually untouched, the frequency of γ-outbursts due to the comet impact on the neutron star or white dwarf is only about 10?3 of the observed occurrence. A considerably higher rate of comets passing per year at critical periastron distance must be assumed if the γ-ray outbursts are to be due to the collision of coments with compact stars. 相似文献
275.
Characterisation of Soot Emitted by Domestic Heating,Aircraft and Cars Using Diesel or Biodiesel 总被引:1,自引:0,他引:1
Anne?Smekens Ricardo?Henrique?Moreton?Godoi Patrick?Berghmans René?Van?GriekenEmail author 《Journal of Atmospheric Chemistry》2005,52(1):45-62
The characterisation of aggregates, like soot, firstly requires the determination of the size distribution of the primary
particles. The primary particle size of combustion generated aggregates depends upon the combustion environment and the formation
conditions, such as temperature, pressure and fuel-to-air ratio, among others. Since the combustion characteristics are different
in the different types of burners, the characterisation of primary particles may offer the possibility to distinguish soot
from different sources. In this paper, we present the signature of the primary particles and the aggregates of soot emitted
by cars using diesel or biodiesel, by domestic heating, and by aircraft exhausts, which can be considered as the major sources
as derived from measurements on transmission electron micrographs. The size distributions of all aggregates types with different
aerodynamic diameter were log-normal and quasi-monodisperse. The size distribution of the primary particles for soot emitted
by different sources showed minor differences. However, a comparison between the diameter of the primary particles and those
obtained using a standard method for carbon black revealed discrepancies. The median diameter of the primary particles was
combined with the median number of primary particles in an aggregate to calculate the relative particle surface area available
for adsorption. In a similar way, the relative specific surface area was determined. The surface area was measured using the
Brunauer-Emmett-Teller (B.E.T.) nitrogen adsorption method and the relative surface area available for adsorption was calculated. 相似文献
276.
W. Thomas T. Ruppert M. Van Roozendael J. Verdebout D. Balis C. Meleti C. Zerefos 《Journal of Atmospheric Chemistry》2005,50(3):295-320
We focus on the retrieval of volcanic sulfur dioxide (SO2) emissions from an analysis of atmospheric UV backscatter spectra obtained by the Global Ozone Monitoring Experiment (GOME) spectrometer on board the ESA European Remote Sensing Satellite (ERS-2). Here, the last major eruptions of Mt. Etna on Sicily (Italy) in July/August 2001 and October/November 2002 provided an excellent opportunity to study the retrieval of SO2 columnar amounts from ground-based, LIDAR and satellite measurements. Our study shows that the bulk of emitted SO2 was confined in the troposphere, mainly between 700 hPa and 400 hPa which is confirmed by trajectory analysis, by LIDAR observations and AVHRR observations. The area of influence of Mt. Etna eruptions ranges from the Western Saharan Desert to Greece and the near east states and even down to the basin of Tschad, Africa. Our analysis revealed that information about the plume height of volcanic eruptions and aerosol parameters is necessary for a reliable quantitative retrieval of SO2 from space-borne sensor data at periods perturbed by volcanic eruptions. 相似文献
277.
Thomas C. Van Flandern 《Celestial Mechanics and Dynamical Astronomy》1969,1(2):163-166
Based primarily upon the formation of new conditional equations using analytical partial derivatives of the moon's mean elements, meridian circle observations of the moon from 1952–67 have been examined to determine corrections to the constants of lunar theory and to the fundamental coordinate system (FK4). With certain exceptions, the new corrections are in agreement with those published earlier by the author. Systematic corrections to FK4 are surprisingly large, although in agreement with some other recent determinations. New corrections to the lunar ephemeris, resulting from the discussion, are also presented. 相似文献
278.
Under the assumption that type II bursts are excited by strong shock waves it is shown that the velocity determined from dynamic spectrograms should be corrected by a term proportional to the gradient of the shock strength. The magnitude of the correction strongly depends on the properties of the pre-shock atmosphere and the geometry of the shock propagation. 相似文献
279.
R. Thompson T. D. J. Cameron C. Schwarz K. A. Jensen V. Van Maenhaut Lemberge L. P. Sha 《第四纪科学杂志》1992,7(4):319-334
Cores from boreholes penetrating late Quaternary, glacial, interglacial and postglacial sediments and the underlying late Cenozoic delta complex of the southern North Sea have been examined for their magnetic properties. A magnetic polarity stratigraphy has been established as an aid to biostratigraphic dating of the sediments; the Kaena-Gauss and Gauss—Matuyama transitions and the base and top of the Olduvai subchron have been identified. The strength and stability of laboratory-induced isothermal remanent magnetisation display clear magneto-petrological variations, which match lithostratigraphic changes in the cores. Principal component analysis has picked out a basin-wide and palaeoenvironmental consistency in the magnetic data. Large, multi-domain magnetite grains predominate in the post-deltaic and fluvio-deltaic sediments, whereas smaller greigite or titanomagnetite grains are concentrated in the intertidal and marine deltaic facies. Since heavy mineral analysis indicates that most of the deltaic detritus derived from common source areas, the differences in magnetic mineralogy have probably been caused by the sediment transport processes operating within the delta complex. 相似文献
280.
There are many aspects of observational evidence that cometary nuclei have irregular or nonspherical shape. The triaxial figure of the Halley's Comet nucleus is a well known fact. Therefore, the nucleus shape plays a significant role in consideration of the formation and evolution of comets and several attempts have been made to explain their nonsphericity. These studies were mainly based on the random-walk schemes for the aggregation processes. Although some results indeed lead to irregularities and deviation from sphericity, the spherical or irregular shape seem to be prevailing results. On the other hand the triaxial figure can be formed by the tidal and rotational forces. Thus, the assumption that the shape of the cometary nucleus due to some of these effects is in principle acceptable. In here assumed scenario already evolved cometary nucleus is situated as a satellite in the gravitation field of a planetary-like body. Since the rigidity of the nucleus is low, it may be easily transferred in the state of a synchronous satellite and in its shape could be imprinted the dynamical effects from this epoch. Here presented results indicate, that such a possibility should be seriously considered. The theory of this process is applied to the nucleus of comet Halley. It is shown, that the nucleus might be synchronously orbiting around a planetary-like hypothetical body with a period of 0.7 days. The minimal bulk tensile strength of the cometary material of about 102 N m–2 is estimated. 相似文献