全文获取类型
收费全文 | 97077篇 |
免费 | 1846篇 |
国内免费 | 1153篇 |
专业分类
测绘学 | 2421篇 |
大气科学 | 7182篇 |
地球物理 | 19639篇 |
地质学 | 34197篇 |
海洋学 | 8493篇 |
天文学 | 21853篇 |
综合类 | 306篇 |
自然地理 | 5985篇 |
出版年
2021年 | 843篇 |
2020年 | 966篇 |
2019年 | 1004篇 |
2018年 | 2313篇 |
2017年 | 2148篇 |
2016年 | 2840篇 |
2015年 | 1753篇 |
2014年 | 2718篇 |
2013年 | 5131篇 |
2012年 | 2959篇 |
2011年 | 4076篇 |
2010年 | 3390篇 |
2009年 | 4626篇 |
2008年 | 4227篇 |
2007年 | 4025篇 |
2006年 | 3753篇 |
2005年 | 3199篇 |
2004年 | 3028篇 |
2003年 | 2886篇 |
2002年 | 2663篇 |
2001年 | 2414篇 |
2000年 | 2393篇 |
1999年 | 2002篇 |
1998年 | 2039篇 |
1997年 | 1947篇 |
1996年 | 1622篇 |
1995年 | 1577篇 |
1994年 | 1366篇 |
1993年 | 1264篇 |
1992年 | 1240篇 |
1991年 | 1137篇 |
1990年 | 1213篇 |
1989年 | 1041篇 |
1988年 | 977篇 |
1987年 | 1162篇 |
1986年 | 1006篇 |
1985年 | 1255篇 |
1984年 | 1383篇 |
1983年 | 1308篇 |
1982年 | 1241篇 |
1981年 | 1089篇 |
1980年 | 1013篇 |
1979年 | 925篇 |
1978年 | 946篇 |
1977年 | 857篇 |
1976年 | 822篇 |
1975年 | 772篇 |
1974年 | 777篇 |
1973年 | 765篇 |
1972年 | 483篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Alan M Bond Julie R Bradbury Henry A Hudson John S Garnham Peter J Hanna Stanley Strother 《Marine Chemistry》1985,16(1):1-9
Evidence was found that uptake of lead from seawater in both model and natural systems by the leaves of the seagrass Zostera muelleri does occur for live, dead and scraped leaves at all the lead concentrations tested. Positive uptake of lead was measured using the three analytical techniques of radio-tracer, differential pulse anodic stripping voltammetry and atomic absorption spectrometry; similar uptake profiles were obtained for each technique. Profiles always showed an initial rapid uptake phase followed by a gradual transition to a plateau phase at which an approximate equilibrium between lead attached to seagrass and free ionic lead in seawater was reached. The presence of active lead uptake processes was indicated in experiments which attempted to remove lead from leaves by a chelating agent. 相似文献
182.
183.
Abstract— We report measurements of K isotope ratios in 28 Semarkona chondrules with a wide range of petrologic types and bulk compositions as well as the compositions of CPX‐mesostasis pairs in 17 type I Semarkona chondrules, including two chondrules with radial alkali zonation and 19 type II chondrules. Despite the wide range in K/Al ratios, no systematic variations in K isotopic compositions were found. Semarkona chondrules do not record a simple history of Rayleigh‐type loss of K. Experimentally determined evaporation rates suggest that considerable alkali evaporation would have occurred during chondrule formation. Nevertheless, based on Na CPX‐mesostasis distribution coefficients, the alkali contents of the cores of most chondrules in Semarkona were probably established at the time of final crystallization. However, Na CPX‐mesostasis distribution coefficients also show that alkali zonation in type I Semarkona chondrules was produced by entry of alkalis after solidification, probably during parent body alteration. This alkali metasomatism may have gone to completion in some chondrules. Our preferred explanation for the lack of systematic isotopic enrichments, even in alkali depleted type I chondrule cores, is that they exchanged with the ambient gas as they cooled. 相似文献
184.
The existence of prolonged periods of abnormally low solar activity (such as the Maunder minimum) is explained within the framework of Leighton's model of a solar cycle with a hypothetical internal magnetic field of the Sun taken into account. 相似文献
185.
186.
Wave propagation in anisotropic linear viscoelastic media: theory and simulated wavefields 总被引:6,自引:0,他引:6
J. M. Carcione 《Geophysical Journal International》1990,101(3):739-750
187.
Michael K. WEISBERG Harold C. CONNOLLY Denton S. EBEL 《Meteoritics & planetary science》2004,39(10):1741-1753
Abstract— Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine‐grained aggregates of olivine and Ca, Al‐rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn‐rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas‐solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn‐rich forsterite is proposed. The Mn‐rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum‐rich inclusion (CAI) with an AOA attached to its Wark‐Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark‐Lovering rim, revealing the crystallization of olivine at the end stages of Wark‐Lovering rim formation. The Ca, Al‐rich nodules in the AOAs may be petrogenetically related to the Ca, Al‐rich minerals in Wark‐Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark‐Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors. 相似文献
188.
We performed polarization observations of giant radio pulses from the millisecond pulsar B1937+21. The observations were carried out in July 2002 with the 64-m Kalyazin radio telescope at a frequency of 600 MHz in two polarization channels with left-and right-hand circular polarizations (RCP and LCP). We used the S2 data acquisition system with a time resolution of 125 ns. The duration of an observing session was 20 min. We detected twelve giant radio pulses with peak flux densities higher than 1000 Jy; five and seven of these pulses appeared in the RCP and LCP channels, respectively. We found no event that exceeded the established detection threshold simultaneously in the two polarization channels. Thus, we may conclude that the detected giant pulses have a high degree of circular polarization, with the frequency of occurrence of RCP and LCP pulses being the same. 相似文献
189.
190.
J. CAMPSIE M. H. RASMUSSEN L. C. KOVACS F. DITTMER J. C. BAILEY N. O. HANSEN J. LAURSEN L. JOHNSON 《Polar research》1990,8(2):237-243
New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone. 相似文献