全文获取类型
收费全文 | 86596篇 |
免费 | 1164篇 |
国内免费 | 534篇 |
专业分类
测绘学 | 1770篇 |
大气科学 | 5694篇 |
地球物理 | 16512篇 |
地质学 | 31170篇 |
海洋学 | 7957篇 |
天文学 | 20081篇 |
综合类 | 226篇 |
自然地理 | 4884篇 |
出版年
2022年 | 628篇 |
2021年 | 1062篇 |
2020年 | 1132篇 |
2019年 | 1263篇 |
2018年 | 2569篇 |
2017年 | 2378篇 |
2016年 | 2741篇 |
2015年 | 1376篇 |
2014年 | 2645篇 |
2013年 | 4540篇 |
2012年 | 2897篇 |
2011年 | 3730篇 |
2010年 | 3405篇 |
2009年 | 4319篇 |
2008年 | 3746篇 |
2007年 | 3876篇 |
2006年 | 3582篇 |
2005年 | 2562篇 |
2004年 | 2476篇 |
2003年 | 2337篇 |
2002年 | 2304篇 |
2001年 | 1997篇 |
2000年 | 1985篇 |
1999年 | 1560篇 |
1998年 | 1623篇 |
1997年 | 1481篇 |
1996年 | 1259篇 |
1995年 | 1263篇 |
1994年 | 1062篇 |
1993年 | 1016篇 |
1992年 | 958篇 |
1991年 | 978篇 |
1990年 | 952篇 |
1989年 | 829篇 |
1988年 | 763篇 |
1987年 | 903篇 |
1986年 | 788篇 |
1985年 | 966篇 |
1984年 | 1097篇 |
1983年 | 1047篇 |
1982年 | 973篇 |
1981年 | 908篇 |
1980年 | 808篇 |
1979年 | 766篇 |
1978年 | 737篇 |
1977年 | 630篇 |
1976年 | 633篇 |
1975年 | 621篇 |
1974年 | 598篇 |
1973年 | 669篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
741.
742.
Lei Chen Jian-Wei Li R. O. Rye W. M. Benzel H. A. Lowers Ming-Zhong He 《Mineralogy and Petrology》2013,107(4):487-499
Supergene jarosite-group minerals are widespread in weathering profiles overlying Pb-Zn sulfide ores at Xitieshan, northern Tibetan Plateau, China. They consist predominantly of K-deficient natrojarosite, with lesser amounts of K-rich natrojarosite and plumbojarosite. Electron microprobe (EMP) analyses, scanning electron microcopy (SEM) investigation, and X-ray mapping reveal that the jarosite-group minerals are characterized by spectacular oscillatory zoning composed of alternating growth bands of K-deficient and K-bearing natrojarosite (K2O >1 wt.%). Plumbojarosite, whenever present, occurs as an overgrowth in the outermost bands, and its composition can be best represented by K0.29Na0.19Pb0.31Fe2.66Al0.22(SO4)1.65(PO4)0.31(AsO4)0.04(OH)7.37. The substitution of monovalent for divalent cations at the A site of plumbojarosite is charge balanced by the substitution of five-valent for six-valent anions in XO4 at the X site. Thermogravimetric analysis (TGA) of representative samples reveal mass losses of 11.46 wt.% at 446.6 °C and 21.42 wt.% at 683.4 °C due to dehydroxylation and desulfidation, respectively. TGA data also indicate that the natrojarosite structure collapses at 446.6 °C, resulting in the formation of NaFe(SO4)2 and minor hematite. The decomposition products of NaFe(SO4)2 are hematite and Na2SO4. Powder X-ray diffraction (XRD) analyses show that the jarosite-group minerals have mean unit-cell parameters of a?=?7.315 Å and c?=?016.598 Å. XRD and EMP data support the view that substitutions of Na for K in the A site and full Fe occupancy in the B site can considerably decrease the unit-cell parameter c, but only slightly increase a. The results from this study suggest that the observed oscillatory zoning of jarosite-group minerals at Xitieshan resulted mainly from substitutions of K for Na at the A site and P for S at the X site. 相似文献
743.
Aaron T. Fricke Charles A. Nittrouer Andrea S. Ogston Daniel J. Nowacki Nils E. Asp Pedro W. M. Souza Filho 《地球表面变化过程与地形》2019,44(1):204-218
Depositional environments along the tidal river downstream of Óbidos have been proposed as important sinks for up to one third of the sediment discharge from the Amazon River. However, the morphology of the intertidal floodplain and the dynamics of sediment exchange along this reach have yet to be described. River-bank surveys in five regions along the Amazon tidal river reveal a distinct transition in bank morphology between the upper, central and lower reaches of the tidal river. The upper tidal-river floodplain is defined by prominent natural levees that control the transfer of water and sediment between the mainstem Amazon River and its floodplain. Greater tidal influence in the central tidal river suppresses levee development, and tidal currents increase sediment transport into the distal parts of the floodplain. In the lower tidal river, the floodplain morphology closely resembles marine intertidal environments (e.g. mud flats, salt marshes), with dendritic tidal channels incising elevated vegetated flats. Theory, morphology and geochronology suggest that the dynamics of sediment delivery to the intertidal floodplain of the Amazon tidal river vary along its length due to the relative influence and coupling of fluvial and tidal dynamics. © 2018 John Wiley & Sons, Ltd. 相似文献
744.
A careful survey of quarter-hourly ionograms for the year 1969 has shown that blanketing layer type ionization irregularities occur on many occasions in the E-region of the ionosphere over Trivandrum (dip ~ 0.6°S). It is shown that horizontal shears of horizontal neutral winds are the most likely sources of such layer type irregularities at the magnetic equator. The horizontal wind shears of required magnitude are shown to be provided by internal gravity waves of short period. The rarity of Es-layer occurrence is attributed to the stringent requirements on the amplitude and wave vector orientation of the relevant gravity waves generating the Es-layers. 相似文献
745.
746.
We analyze the data presented in a previous paper by Gyulzadyan and Petrosian, and discuss the results of a statistical investigation
of the relationship between SBS galaxies and Zwicky clusters. The main results are that SBS galaxies follow the overall galaxy
distribution in clusters and they do not avoid any type of Zwicky cluster. There is a significantly higher probability of
finding SBS galaxies occurring in medium compact clusters than in open clusters. They also follow the well-established morphology-density
relation. Earlier morphological type, higher luminosity, larger linear size, and redder SBS galaxies tend to be found in clusters
with higher compactness, or in more compact regions of the clusters. The number distribution of SBS galaxies in Zwicky open
clusters probably follows the distribution of normal galaxies. The number distribution of SBS galaxies in medium compact and
compact clusters shows two-maxima structure.
Published in Astrofizika, Vol. 52, No. 2, pp. 225–236 (May 2009). 相似文献
747.
748.
A broad sample of computed realistic equations of state of superdense matter with a quark phase transition is used to construct a series of models of neutron stars with a strange quark core. The integral characteristics of the stellar configurations are obtained: gravitational mass, rest mass, radius, relativistic moment of inertia, and red shift from the star's surface, as well as the mass and radius of the quark core within the allowable range of values for the central pressure. The parameters of some of the characteristic configurations of the calculated series are also given and these are studied in detail. It is found that a new additional region of stability for neutron stars with strange quark cores may exist for some models of the equation of state. 相似文献
749.
750.
Dating climatic episodes of the Holocene 总被引:2,自引:0,他引:2
Monitoring evidence indicates that the Holocene embraced a sequence of rather discrete climatic episodes. The transitions between these environmental episodes apparently were abrupt and globally synchronous. This paper reports on statistical analyses of radiocarbon dates associated with environmental change and cultural change.Over 800 14C dates associated with pollen maxima and minima, sea level maxima and minima, and top and bottom surfaces of peat beds were simultaneously analyzed to identify times of globally synchronous environmental discontinuities.Some 3700 14C dates associated with 155 cultural continua of the world were collectively analyzed to identify worldwide synchroneities in appearance and termination of the cultures.Significant globally synchronous discontinuities were identified in each independent analysis. The dates of environmental and cultural discontinuities are rather similar, particularly during the recent half of the Holocene. The fact that the cultural discontinuities mostly follow rather closely those of the paleobotanical record suggests that there has been a distinct climatic impact on the cultural history of man. 相似文献