全文获取类型
收费全文 | 3410篇 |
免费 | 92篇 |
国内免费 | 38篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 296篇 |
地球物理 | 778篇 |
地质学 | 1208篇 |
海洋学 | 326篇 |
天文学 | 485篇 |
综合类 | 10篇 |
自然地理 | 387篇 |
出版年
2021年 | 44篇 |
2020年 | 40篇 |
2019年 | 46篇 |
2018年 | 57篇 |
2017年 | 62篇 |
2016年 | 68篇 |
2015年 | 73篇 |
2014年 | 69篇 |
2013年 | 179篇 |
2012年 | 92篇 |
2011年 | 139篇 |
2010年 | 146篇 |
2009年 | 163篇 |
2008年 | 129篇 |
2007年 | 114篇 |
2006年 | 119篇 |
2005年 | 106篇 |
2004年 | 90篇 |
2003年 | 82篇 |
2002年 | 82篇 |
2001年 | 72篇 |
2000年 | 43篇 |
1999年 | 52篇 |
1998年 | 53篇 |
1997年 | 43篇 |
1996年 | 66篇 |
1995年 | 43篇 |
1994年 | 48篇 |
1993年 | 44篇 |
1992年 | 52篇 |
1991年 | 55篇 |
1990年 | 49篇 |
1989年 | 46篇 |
1988年 | 50篇 |
1987年 | 50篇 |
1986年 | 37篇 |
1985年 | 65篇 |
1984年 | 83篇 |
1983年 | 81篇 |
1982年 | 66篇 |
1981年 | 46篇 |
1980年 | 52篇 |
1979年 | 50篇 |
1978年 | 49篇 |
1977年 | 44篇 |
1976年 | 38篇 |
1975年 | 30篇 |
1974年 | 28篇 |
1973年 | 33篇 |
1972年 | 24篇 |
排序方式: 共有3540条查询结果,搜索用时 15 毫秒
941.
Diagonal damping matrices were computed for three systems which have non-proportional damping matrices. These diagonal damping matrices were computed on three bases, as follows: 1. After normalizing the equations of motion by the modal matrix, the diagonal terms are retained ignoring the non-diagonal terms. 2. Diagonal damping matrix is established by the optimization algorithm which minimizes the mean square error of the frequency response. 3. Diagonal damping is determined from the normalized differential equation by matching the peaks of the coupled and uncoupled system. The frequency responses for the three cases of one of the three systems are presented together with a comparison of the energy dissipation. 相似文献
942.
William M. B. Roberts 《Mineralium Deposita》1968,3(4):364-367
Experiments are described in which pyrite is synthesized at room temperature and atmospheric pressure from hydrated iron oxides and hydrogen sulphide under conditions similar to those expected in the natural environment. This synthesis differs from earlier work in that oxygen was rigorously excluded from the system and the pH was 5.6. During a later ageing period a reaction appears to take place in which iron monosulphide is converted to pyrite at pH 5.85.
Published with the approval of the Director; Bureau of Mineral Resources, Geology and Geophysics. 相似文献
Zusammenfassung Versuche zur Synthese von Pyrit bei Raumtemperatur unter atmosphärischem Druck, ausgehend von Eisenhydroxiden und Schwefelwasserstoff und somit unter gleichen Verhältnissen, wie in natürlicher Umgebung, werden beschrieben.Die Synthese unterscheidet sich von früheren Arbeiten durch den strengen Ausschluß von Sauerstoff und den pH-Wert von 5.6. In einer darauffolgenden Alterungsperiode scheint eine Reaktion einzusetzen, in der Eisenmonosulfid bei pH 5.85 zu Pyrit umgewandelt wird.
Published with the approval of the Director; Bureau of Mineral Resources, Geology and Geophysics. 相似文献
943.
William P. Elliott 《Climatic change》1983,5(2):141-144
The historical record of CO2 emissions from industrial activity is reexamined. The overall annual growth rate has been about 3.5% but with wide variations due to economic fluctuations. It is unlikely that the total CO2 production would have been greatly different had the major wars of the Twentieth Century been avoided. 相似文献
944.
945.
Joseph A Curiale William E. Harrison Garmon Smith 《Geochimica et cosmochimica acta》1983,47(3):517-523
Solid bitumens (grahamite and impsonite) of southeastern Oklahoma have been shown to originate from near-surface alteration of crude oil (Curiale, 1981; Curiale and Harrison, 1981). Pyrolysis of these solids has been employed to compare the sterane distribution of geographically proximate oils to that of the bitumens. The ratio of rearranged to regular steranes is higher in the pyrolyzates than in the oils, a finding consistent with a bitumen origin due to biodegradation of oil. The remaining presence of steranes, particularly regular steranes, in the bitumens suggests that sterane occlusion may have occurred prior to or during the alteration process, thus removing tetracyclic compounds from the influence of microbial attack. These data suggest that pyrolysis- offers a viable approach to correlation problems involving solid bitumens. 相似文献
946.
Geotechnical property analyses on sediments recovered by DSDP drilling within select convergent margins were integrated to determine the variability in these properties along the trench inner-slope and to delineate the effects of convergence on these properties. Consolidation states range from very underconsolidated to highly overconsolidated, with preconsolidation pressures exceeding 46,000 kPa in Quaternary sediments. Underconsolidated sediments are attributed to:
- 1. (1) high sedimentation rates.
- 2. (2) low sediment permeability relative to the length of the drainage path; (3) laterally applied stresses; and (4) induced pore water pressures resulting from the subduction of pelagic sediments with high water content. Factors contributing to the state of overconsolidation include: (1) tectonically induced overpressures; (2) removal of overburden by mass movement processes.
- 3. (3) low sediment accumulation rates.
- 4. (4) age.
947.
948.
A transmission electron microscope study of intracrystalline boundaries between two perthites of markedly different composition in composite crystals, one a tenary mesoperthite (Or26Ab52An22, initially a homogeneous potassian monalbite) the other a more potassic cryptoperthite (Or61Ab33An6, initially a homogeneous sodian sanidine), shows that the two perthites are in nearly parallel intergrowth. Most boundaries examined were of (hkO) type; (010) boundaries are straight, whereas other (hkO) boundaries are curved or stepped. Exsolution occurred first in the potassian monalbite (mesoperthite) and was unaffected by the boundary. Subsequent exsolution in the sodian sanidine (cryptoperthite) was affected by the boundary, but for up to only a few micrometers. Exsolution occurred by heterogeneous nucleation and growth of oligoclase on and from the intracrystalline boundary. At almost the same time the rest of the volume of sanidine exsolved by spinodal decomposition. 1–2 μm from the boundary in the intervening K-rich matrix of the sodian sanidine, further exsolution occurred by homogeneous nucleation. Time — temperature — transition curves for continuous cooling have been devised to account for the unusual complexity of the exsolution texture. Except in such exceptional circumstances as the example studied, the initial exsolution in high-temperature alkali feldspars of intermediate composition, unlike other minerals, probably does not occur by nucleation, but only by spinodal decomposition. 相似文献
949.
Richard?P.?EsserEmail author Philip?R.?Kyle William?C.?McIntosh 《Bulletin of Volcanology》2004,66(8):671-686
Mt. Erebus, a 3,794-meter-high active polygenetic stratovolcano, is composed of voluminous anorthoclase-phyric tephriphonolite and phonolite lavas overlying unknown volumes of poorly exposed, less differentiated lavas. The older basanite to phonotephrite lavas crop out on Fang Ridge, an eroded remnant of a proto-Erebus volcano and at other isolated locations on the flanks of the Mt. Erebus edifice. Anorthoclase feldspars in the phonolitic lavas are large (~10 cm), abundant (~30–40%) and contain numerous melt inclusions. Although excess argon is known to exist within the melt inclusions, rigorous sample preparation was used to remove the majority of the contaminant. Twenty-five sample sites were dated by the 40Ar/39Ar method (using 20 anorthoclase, 5 plagioclase and 9 groundmass concentrates) to examine the eruptive history of the volcano. Cape Barne, the oldest site, is 1,311±16 ka and represents the first of three stages of eruptive activity on the Mt. Erebus edifice. It shows a transition from sub-aqueous to sub-aerial volcanism that may mark the initiation of proto-Erebus eruptive activity. It is inferred that a further ~300 ky of basanitic/phonotephritic volcanism built a low, broad platform shield volcano. Cessation of the shield-building phase is marked by eruptions at Fang Ridge at ~1,000 ka. The termination of proto-Erebus eruptive activity is marked by the stratigraphically highest flow at Fang Ridge (758±20 ka). Younger lavas (~550–250 ka) on a modern-Erebus edifice are characterized by phonotephrites, tephriphonolites and trachytes. Plagioclase-phyric phonotephrite from coastal and flank flows yield ages between 531±38 and 368±18 ka. The initiation of anorthoclase tephriphonolite occurred in the southwest sector of the volcano at and around Turks Head (243±10 ka). A short pulse of effusive activity marked by crustal contamination occurred ~160 ka as indicated by at least two trachytic flows (157±6 and 166±10 ka). Most anorthoclase-phyric lavas, characteristic of Mt. Erebus, are less than 250 ka. All Mt. Erebus flows between about 250 and 90 ka are anorthoclase tephriphonolite in composition.Editorial responsibility: J. Donelly-Nolan 相似文献
950.
Bernard?BingenEmail author H?kon?Austrheim Martin?J.?Whitehouse William?J.?Davis 《Contributions to Mineralogy and Petrology》2004,147(6):671-683
Secondary-ion mass spectrometry (SIMS) U–Pb and trace element data are reported for zircon to address the controversial geochronology of eclogite-facies metamorphism in the Lindås nappe, Bergen Arcs, Caledonides of W Norway. Caledonian eclogite-facies overprint in the nappe was controlled by fracturing and introduction of fluid in the Proterozoic—Sveconorwegian—granulite-facies meta-anorthosite-norite protolith. Zircon grains in one massive eclogite display a core–rim structure. Sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith, i.e. 0.31Th/U0.89, heavy rare earth element (HREE) enrichment, and negative Eu anomaly. Weakly-zoned to euhedral oscillatory-zoned Caledonian rims are characterized by Th/U0.13, low LREE content (minimum normalized abundance for Pr or Nd), variable enrichment in HREE, and no Eu anomaly. A decrease of REE towards the outermost rim, especially HREE, is documented. This signature reflects co-precipitation of zircon with garnet and clinozoisite in a feldspar-absent assemblage, and consequently links zircon to the eclogite-facies overprint. The rims provide a mean 206Pb/238U crystallization age of 423±4 Ma. This age reflects eclogite-forming reactions and fluid–rock interaction. This age indicates that eclogite-facies overprint in the Lindås nappe took place at the onset of the Scandian (Silurian) collision between Laurentia and Baltica. 相似文献