全文获取类型
收费全文 | 102964篇 |
免费 | 1601篇 |
国内免费 | 1033篇 |
专业分类
测绘学 | 2745篇 |
大气科学 | 7344篇 |
地球物理 | 20970篇 |
地质学 | 36204篇 |
海洋学 | 8909篇 |
天文学 | 23164篇 |
综合类 | 368篇 |
自然地理 | 5894篇 |
出版年
2022年 | 569篇 |
2021年 | 969篇 |
2020年 | 1100篇 |
2019年 | 1195篇 |
2018年 | 2686篇 |
2017年 | 2516篇 |
2016年 | 3173篇 |
2015年 | 1848篇 |
2014年 | 3044篇 |
2013年 | 5405篇 |
2012年 | 3264篇 |
2011年 | 4366篇 |
2010年 | 3785篇 |
2009年 | 4971篇 |
2008年 | 4512篇 |
2007年 | 4373篇 |
2006年 | 4082篇 |
2005年 | 3221篇 |
2004年 | 3123篇 |
2003年 | 2895篇 |
2002年 | 2791篇 |
2001年 | 2550篇 |
2000年 | 2417篇 |
1999年 | 1988篇 |
1998年 | 2082篇 |
1997年 | 2024篇 |
1996年 | 1617篇 |
1995年 | 1614篇 |
1994年 | 1402篇 |
1993年 | 1293篇 |
1992年 | 1195篇 |
1991年 | 1148篇 |
1990年 | 1209篇 |
1989年 | 1107篇 |
1988年 | 990篇 |
1987年 | 1186篇 |
1986年 | 1023篇 |
1985年 | 1268篇 |
1984年 | 1422篇 |
1983年 | 1315篇 |
1982年 | 1275篇 |
1981年 | 1126篇 |
1980年 | 1022篇 |
1979年 | 971篇 |
1978年 | 974篇 |
1977年 | 858篇 |
1976年 | 822篇 |
1975年 | 800篇 |
1974年 | 771篇 |
1973年 | 783篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Elemental ratio data from archaeological and paleontological bone have often been used for paleoecological reconstruction, but recent studies have shown that, even when solubility profiling techniques are employed in an attempt to recover biogenic signals, bone is an unreliable material. As a result, there has been renewed interest in using enamel for such studies, as it is known to be less susceptible to diagenesis. Nevertheless, enamel is not immune from diagenetic processes, and several studies have suggested that paleoecologically relevant elements may be altered in fossil enamel. Here, we investigate Sr, Ba, Zn, and Pb compositions of enamel from South African karstic cave sites in an effort to ascertain whether or not this material provides reliable paleoecological information. We compared enamel data for mammals from three fossil sites aged 1.8-3.0 Ma, all of which are on dolomites, with data from modern mammals living on dolomitic and granitic substrates. Sr/Ca and Ba/Ca are about three times higher in enamel from modern mammals on granites than those living on dolomites, stressing the need for geologically appropriate modern/fossil comparisons. After pretreatment with dilute acid, we found no evidence of increased Sr/Ca, Ba/Ca, or Pb/Ca in fossil enamel. In contrast, Zn/Ca increased by over five times at one site (Makapansgat), but much more subtly elsewhere. Ecological patterning in Sr/Ca, Ba/Ca, and Sr/Ba ratios was also retained in fossil enamel. This study suggests that Sr/Ca, Ba/Ca, and Pb/Ca data likely preserve paleoecological information from these sites, but also demonstrates that geologically similar sites can differ in the degree to which they impart certain elements (Zn in this case) to fossils. Thus, screening is probably necessary on a site-by-site basis. Lastly, further investigation of elemental distributions in modern foodwebs is necessary before elemental ratio analysis can become a common tool for paleoecological reconstruction. 相似文献
992.
A. Bolourchi F. Atabi F. Moattar M. A. Ehyaei 《International Journal of Environmental Science and Technology》2018,15(12):2569-2580
The purpose of this paper was to perform the experimental and numerical analyses of PM10 and PM2.5 concentrations in Imam Khomeini (IKH) underground subway station in Tehran. The aim was to provide fundamental data in order to fulfill workers and passengers respiratory health necessities. Experimental measurements was done at three different locations (entrance, middle and exit) inside the platform and also outdoor ambient of the station. The Dust-Trak was applied to measure continuous PM2.5 and PM10 concentrations at a logging interval of 30 s. The measurements were recorded during rush hours (8:00 am–12:00 pm) for one week per each season from June 2015–June 2016.Moreover, computational fluid dynamic (CFD) simulation was done for the platform of the above station and the necessary boundary conditions were provided through field measurements. Those basic parameters which were considered for numerical analysis of particulate matters concentrations included air velocity, air pressure and turbulence. Furthermore, the piston effect caused by train movement inside the station provided natural ventilation in the platform. The results showed that seasonal measured PM2.5 and PM10 indoor concentrations had a variety range from 40–98 µg/m3 to 33–102 µg/m3, respectively, and were much higher than national indoor air quality standard levels. Meanwhile, PM2.5 and PM10 concentrations in the IKH underground subway station were approximately 2.5–2.9 times higher than those in outdoor ambient, respectively. Numerical simulation indicated that the predicted concentrations were underestimated by a factor of 8% in comparison with the measured ones. 相似文献
993.
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
994.
M. Deloncle 《Pure and Applied Geophysics》1961,50(1):169-175
Résumé On décrit und méthode qui permet de connaitre rapidement les dimensions des gouttes composant un brouillard naturel ou artificiel, ainsi que leur répartition, et qui peut être étendue au domaine des fines particules telles que brumes et noyaux de condensation. Cette méthode, basée sur la mesure photoélectrique de la lumiére diffusée par les gouttes individuelles libres en suspension dans l'air, présente ainsi l'avantage de permettre leur observation sans modifier leur état.
Summary A rapid method is described for finding the sizes of drops between 0.3 and 100 diameter, such as occur in fog and mist, which can be extended to the study of haze and large condensation nuclei. It is based upon photoelectric measurement of the light scattered by individual particles and has the advantage of allowing them to remain freely suspended in air during observation.相似文献
995.
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
996.
The convective stability of a simple model chromosphere, consisting of protons, electrons and hydrogen atoms in the ground state, has been studied in the presence of a vertically upward uniform magnetic field to include the effects of FLR, Hall-currents, finite conductivity and ionization. The ionization in the chromosphere is collisional and the recombination is radiative. It is found that the Schwarzchild criterion should necessarily be satisfied for the stability together with the condition thatv > 2v
0, where is kinematic viscosity andv
0 is gyroviscosity. Some special cases have also been investigated. 相似文献
997.
998.
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
999.
Klauea historical summit lavas have a wide range in matrix 18OVSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, 18O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine 18O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma 18O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine 18O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination 相似文献
1000.