全文获取类型
收费全文 | 5536篇 |
免费 | 553篇 |
国内免费 | 163篇 |
专业分类
测绘学 | 248篇 |
大气科学 | 646篇 |
地球物理 | 2006篇 |
地质学 | 2249篇 |
海洋学 | 279篇 |
天文学 | 382篇 |
综合类 | 189篇 |
自然地理 | 253篇 |
出版年
2022年 | 7篇 |
2021年 | 26篇 |
2020年 | 11篇 |
2019年 | 15篇 |
2018年 | 441篇 |
2017年 | 383篇 |
2016年 | 264篇 |
2015年 | 160篇 |
2014年 | 119篇 |
2013年 | 147篇 |
2012年 | 656篇 |
2011年 | 446篇 |
2010年 | 139篇 |
2009年 | 149篇 |
2008年 | 136篇 |
2007年 | 126篇 |
2006年 | 138篇 |
2005年 | 834篇 |
2004年 | 885篇 |
2003年 | 670篇 |
2002年 | 188篇 |
2001年 | 80篇 |
2000年 | 49篇 |
1999年 | 17篇 |
1998年 | 11篇 |
1997年 | 20篇 |
1996年 | 13篇 |
1994年 | 4篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 7篇 |
1987年 | 6篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1965年 | 4篇 |
1963年 | 2篇 |
1961年 | 2篇 |
1959年 | 2篇 |
1955年 | 2篇 |
1954年 | 2篇 |
1951年 | 2篇 |
1948年 | 2篇 |
排序方式: 共有6252条查询结果,搜索用时 15 毫秒
21.
Axel Wittmann Randy L. Korotev Bradley L. Jolliff Kunihiko Nishiizumi A. J. Timothy Jull Marc W. Caffee Michael Zanetti Anthony J. Irving 《Meteoritics & planetary science》2019,54(10):2167-2188
Oued Awlitis 001 is a highly feldspathic, moderately equilibrated, clast‐rich, poikilitic impact melt rock lunar meteorite that was recovered in 2014. Its poikilitic texture formed due to moderately slow cooling, which judging from textures of rocks in melt sheets of terrestrial impact structures, is observed in impact melt volumes at least 100 m thick. Such coherent impact melt volumes occur in lunar craters larger than ~50 km in diameter. The composition of Oued Awlitis 001 points toward a crustal origin distant from incompatible‐element‐rich regions. Comparison of the bulk composition of Oued Awlitis 001 with Lunar Prospector 5° γ‐ray spectrometer data indicates a limited region of matches on the lunar farside. After its initial formation in an impact crater larger than ~50 km in diameter, Oued Awlitis 001 was excavated from a depth greater than ~50 m. The cosmogenic nuclide inventory of Oued Awlitis 001 records ejection from the Moon 0.3 Ma ago from a depth of at least 4 m and little mass loss due to ablation during its passage through Earth's atmosphere. The terrestrial residence time must have been very short, probably less than a few hundred years; its exact determination was precluded by a high concentration of solar cosmic ray‐produced 14C. If the impact that excavated Oued Awlitis 001 also launched it, this event likely produced an impact crater >10 km in diameter. Using petrologic constraints and Lunar Reconnaissance Orbiter Camera and Diviner data, we test Giordano Bruno and Pierazzo as possible launch craters for Oued Awlitis 001. 相似文献
22.
Conventions on thermal expansion modelling of radio telescopes for geodetic and astrometric VLBI 总被引:7,自引:6,他引:1
Axel Nothnagel 《Journal of Geodesy》2009,83(8):787-792
Thermal expansion of radio telescopes has long been recognized as an effect which cannot be neglected in geodetic and astrometric
VLBI data analysis if millimeter accuracy is desired. In this article, the author documents the conventions which are being
set by the International VLBI Service for Geodesy and Astrometry (IVS) for a consistent modelling of this effect in its routine
product generation. For the largest telescopes, the annual cycle of thermal expansion may change the height of the VLBI reference
point by as much as 20 mm. However, for telescopes which are used in present-day IVS operations, the variations rather range
from 4 to 6 mm. 相似文献
23.
Sakineh?Mashjoor Fatemeh?Heidary?Jamebozorgi Ehsan?KamraniEmail author 《Ocean Science Journal》2018,53(4):655-665
Ecosystem-based management is one of the most important approaches that may lead to reducing the impacts of fishing on ecosystems. In this context, we have assessed the impact of Iranian coastal fishing (using landing data of 49 exploited species) on the ecosystem of the North Sea of Oman (Sistan and Baluchestan Province), during the last decade (2002–2011), with emphasis on testing the occurrence of the “fishing down? phenomenon. The Mean Trophic Level (MTL) and Fishing-in-Balance (FiB) index are two indicators that we used for analysis. The data indicated that the increased total landings in this region might be related to the exploitation of marine fishery resources especially with regard to large pelagic fish. Over the past decade, moderate decreasing trends in MTL and an increasing trend in the FiB-index were observed. In this regard, an upward trend in the spatial expansion factor and also a downward trend in the piscivory index and in Primary Production Required (PPR) in the time period could all indicate a spatial expansion toward deep waters, the catching of the large pelagic piscivorous species, such as tuna, and a sign of fishing pressures on the ecosystem. The results suggest a range of fishery exploitation patterns throughout the food web but it seems that these patterns are not a consequence of ?fishing down?. We suggest that the monitoring research be continued in this region and these indicators should be used to make fisheries management decisions and to prevent the continuance of this trend in future. 相似文献
24.
垃圾夯扩短桩在邯郸市的工程实践 总被引:3,自引:0,他引:3
本文阐述垃圾夯扩短桩在邯郸新近堆积软土场地上的工程尝试,分析了其加固地基的机理,结合工程实例论述了夯扩短桩在邯郸的适用性及其推广应用前景 相似文献
25.
PAR E.GHERZI 《地质学报》1925,4(2):81-86
Hest bien clair que toute reponse a cette question, sera du plus haut interet pour les geologues de la Chine, vu que les resultats de la geodesie semblent bien indiquer que la croute terrestre est divisee en plusieurs blocs independants, juxtaposes et equilibres isostatiquement. 相似文献
26.
An Example of Island-Arc Petrogenesis: Geochemistry and Petrology of the Southern Luzon Arc, Philippines 总被引:6,自引:1,他引:6
DEFANT MARC J.; MAURY REN? C.; RIPLEY EDWARD M.; FEIGENSON MARK D.; JACQUES DARIO 《Journal of Petrology》1991,32(3):455-500
Volcanism throughout the Luzon arc is associated with eastwardsubduction of the South China Sea floor along the Manila Trench.The southern section of the arc, the focus of this study, extendsfrom the Lingayen-Dingalan fault to the small islands just southof Luzon. Two segments appear to exist along this section ofthe arc the northern Bataan and southern Mindoro segments whichare separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies:olivine, clinopyroxene, plagioclase, and titanomagnetite inthe most mafic rocks and clinopyroxene, plagioclase, orthopyroxene,titanomagnetite, ? amphibole in the more felsic samples. Complexzoning, sieve textures, and decoupling of incompatible traceelements suggest that processes such as assimilation have takenplace. The rocks from the study area range from basalts to rhyolitesand show typical calc-alkaline features. The rocks of the MacolodCorridor and Mindoro segment are particularly enriched in largeion lithophile elements (LILE), light rare earth elements (LREE),and radiogenic Sr compared with the Bataan segment. The datafall within the mantle array on Sr-Nd isotopic diagrams andgrade toward higher Sr and lower Nd isotopic values from northto south. A likely source for the volcanics of this study is either amid-ocean ridge basalt (MORB)-type mantle that undergoes higherdegrees of partial melting than regions involved in MORB generationor a previously depleted source. We suggest that the high fieldstrength element (HFSE) anomalies have been derived throughdifferential element partitioning during fluid transport fromthe subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant roleparticularly in the Macolod Corridor and the Mindoro segment,based on the high LILE, La/Sm ratios, radiogenic Sr isotopes,and 18O values. The Macolod Corridor and the Mindoro segmenthave undergone source contamination by crustal material fromthe North Palawan-Mindoro crustal block either during the collisionof this block with the Manila Trench or by subduction of sedimentsrich in this crustal material. A similar component has alsobeen detected in the Bataan segment but in minor amounts. Thetrace element and isotopic differences between the northernand southern sections of the arc are interpreted in terms ofvariable composition (i. e., variable amounts of a crustal componentintroduced from the Palawan-Mindoro crustal terrain) of themetasomatic fluids released into the source. 相似文献
27.
该文笔者多年研究大骨节病原始原因的一项新说。即病区外环境低氟。引起体内以低氟为中心的代谢紊乱,从而导致以骨骼系统为主的病变-大骨节病。防治该病,应向体外补氟。药疗这一进补方式故然必然,但最有效又生活化的办法是食疗。食疗又分氟盐防治和饮茶防治两种。而最适用且简便的方法是提倡患区人群适度饮茶,并且应从儿时做起。关于补氟和饮茶等防治大骨节病的实践,笔者已浓缩于下面四则拾趣之中……。 相似文献
28.
Composition and origin of organic matter in surface sediments of Lake Sarbsko: A highly eutrophic and shallow coastal lake (northern Poland) 总被引:1,自引:0,他引:1
Micha? Woszczyk Achim Bechtel Reinhard Gratzer Maciej J. Kotarba Miko?aj Kokociński Jens Fiebig Roman Cie?liński 《Organic Geochemistry》2011,42(9):1025-1038
We present an organic geochemical study of surface sediments of Lake Sarbsko, a shallow coastal lake on the middle Polish Baltic coast. The aim was to provide evidence concerning the origin of the organic matter (OM) and its compositional diversity in surface deposits of this very productive, highly dynamic water body. The content and composition of the OM in the bottom sediments were investigated at 11 sampling stations throughout the lake basin. OM sources were assigned on the basis of bulk indicators [total organic carbon (TOC), total nitrogen (TN), δ13CTOC and δ15N and extractable OM yield], biomarker composition of extractable OM and compound-specific C isotope signatures. The source characterization of autochthonous compounds was verified via phytoplankton analysis. The distribution of gaseous hydrocarbons in the sediments, as well as temporal changes in lake water pH, the concentration of DIC (dissolved inorganic carbon) and δ13CDIC were used to trace OM decomposition.The sedimentary OM is composed mainly of well preserved phytoplankton compounds and shows minor spatial variability in composition. However, the presence of CH4 and CO2 in the bottom deposits provides evidence for microbial degradation of sedimentary OM. The transformation of organic compounds in surface, bottom and pore waters via oxidative processes influences carbonate equilibrium in the lake and seasonally favours precipitation or dissolution of CaCO3.The data enhance our understanding of the relationships between the composition of sedimentary OM and environmental conditions within coastal ecosystems and shed light on the reliability of OM proxies for environmental reconstruction of coastal lakes. 相似文献
29.
N. V. Chukanov A. A. Mukhanova R. K. Rastsvetaeva D. I. Belakovsky S. M?ckel O. V. Karimova S. N. Britvin S. V. Krivovichev 《Geology of Ore Deposits》2011,53(7):583-590
Oxyphlogopite is a new mica-group mineral with the idealized formula K(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2. The holotype material came from a basalt quarry at Mount Rothenberg near Mendig at the Eifel volcanic complex in Rhineland-Palatinate,
Germany. The mineral occurs as crystals up to 4 × 4 × 0.2 mm in size encrusting cavity walls in alkali basalt. The associated
minerals are nepheline, plagioclase, sanidine, augite, diopside, and magnetite. Its color is dark brown, its streak is brown,
and its luster is vitreous. D
meas = 3.06(1) g/cm3 (flotation in heavy liquids), and D
calc = 3.086 g/cm3. The IR spectrun does not contain bands of OH groups. Oxyphlogopite is biaxial (negative); α = 1.625(3), β = 1.668(1), and
γ = 1.669(1); and 2V
meas = 16(2)° and 2V
calc = 17°. The dispersion is strong; r < ν. The pleochroism is medium; X > Y > Z (brown to dark brown). The chemical composition is as follows (electron microprobe, mean of 5 point analyses, wt %; the ranges
are given in parentheses; the H2O was determined using the Alimarin method; the Fe2+/Fe3+ was determined with X-ray emission spectroscopy): Na2O 0.99 (0.89–1.12), K2O 7.52 (7.44–7.58), MgO 14.65 (14.48–14.80), CaO 0.27 ((0.17–0.51), FeO 4.73, Fe2O3 7.25 (the range of the total iron in the form of FeO is 11.09–11.38), Al2O3 14.32 (14.06–14.64), Cr2O3 0.60 (0.45–0.69), SiO2 34.41 (34.03–34.66), TiO2 12.93 (12.69–13.13), F 3.06 (2.59–3.44), H2O 0.14; O=F2 −1.29; 99/58 in total. The empirical formula is (K0.72Na0.14Ca0.02)(Mg1.64Ti0.73Fe0.302+ Fe0.273+Cr0.04)Σ2.98(Si2.59Al1.27Fe0.143+ O10) O1.20F0.73(OH)0.07. The crystal structure was refined on a single crystal. Oxyphlogopite is monoclinic with space group C2/m; the unit-cell parameters are as follows: a = 5.3165(1), b = 9.2000(2), c = 10.0602(2) ?, β = 100.354(2)°. The presence of Ti results in the strong distortion of octahedron M(2). The strongest lines of the X-ray powder diffraction pattern [d, ? (I, %) [hkl]] are as follows: 9.91(32) [001], 4.53(11) 110], 3.300(100) [003], 3.090(12) [112], 1.895(21) [005], 1.659(12) [−135], 1.527(16)
[−206, 060]. The type specimens of oxyphlogopite are deposited at the Fersman Mineralogical Museum in Moscow, Russia; the
registration numbers are 3884/2 (holotype) and 3884/1 (cotype). 相似文献
30.