全文获取类型
收费全文 | 57148篇 |
免费 | 950篇 |
国内免费 | 611篇 |
专业分类
测绘学 | 1581篇 |
大气科学 | 4090篇 |
地球物理 | 10711篇 |
地质学 | 20610篇 |
海洋学 | 5243篇 |
天文学 | 13513篇 |
综合类 | 199篇 |
自然地理 | 2762篇 |
出版年
2022年 | 398篇 |
2021年 | 672篇 |
2020年 | 710篇 |
2019年 | 752篇 |
2018年 | 1696篇 |
2017年 | 1614篇 |
2016年 | 2003篇 |
2015年 | 1071篇 |
2014年 | 1860篇 |
2013年 | 3047篇 |
2012年 | 2009篇 |
2011年 | 2581篇 |
2010年 | 2231篇 |
2009年 | 2889篇 |
2008年 | 2475篇 |
2007年 | 2494篇 |
2006年 | 2370篇 |
2005年 | 1790篇 |
2004年 | 1884篇 |
2003年 | 1735篇 |
2002年 | 1598篇 |
2001年 | 1390篇 |
2000年 | 1308篇 |
1999年 | 1058篇 |
1998年 | 1114篇 |
1997年 | 994篇 |
1996年 | 846篇 |
1995年 | 818篇 |
1994年 | 718篇 |
1993年 | 632篇 |
1992年 | 626篇 |
1991年 | 613篇 |
1990年 | 651篇 |
1989年 | 503篇 |
1988年 | 518篇 |
1987年 | 547篇 |
1986年 | 503篇 |
1985年 | 624篇 |
1984年 | 692篇 |
1983年 | 607篇 |
1982年 | 572篇 |
1981年 | 520篇 |
1980年 | 476篇 |
1979年 | 484篇 |
1978年 | 480篇 |
1977年 | 387篇 |
1976年 | 358篇 |
1975年 | 374篇 |
1974年 | 325篇 |
1973年 | 356篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
S. Evren 《Astrophysics and Space Science》1985,108(1):113-123
The RS CVn-type eclipsing binary star MM Her has been observed in two colours,B andV, in 1979, 1980, and 1983. Several minima times were obtained during the observations and new light elements calculated. The light curves of the system obtained in blue and yellow lights show a significant wave-like distortion which migrates towards the decreasing orbital phases. Its migration period was estimated to be about 3.5 yr. The amplitudes of the wave-like distortion inB andV appear to change each year. The primary minimum of the system is a total eclipse with a duration of 0d.08. 相似文献
172.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers. 相似文献
173.
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
174.
175.
176.
177.
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
178.
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
179.
Michael E. ZOLENSKY Andrei V. IVANOV S. Vincent YANG David W. MITTLEFEHLDT Kazumasa OHSUMI 《Meteoritics & planetary science》1996,31(4):484-493
Abstract Kaidun is a breccia of disparate enstatite and carbonaceous chondrite clasts that continues to provide real surprises. Many Kaidun clasts have been intensely altered by aqueous fluids, as evidenced by the widespread occurrence of ferromagnesian phyllosilicates and by the presence of carbonate- and phyllo-silicate-filled veins. In this report, we describe an unusual CM lithology containing many mineralogical features not previously reported from any meteorite, including pyrrhotite, with exclusive needlelike morphologies and thick mantles of phyllosilicate, and complex aggregates of phyllosilicate, melanite garnet, crosscut by pentlandite veins. The latter features appear to be due in large part to extensive hydrothermal alteration at temperatures on the order of 450 °C, which is significantly higher than that attained during secondary processing from other known CM material. 相似文献
180.
Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x
l
). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t). 相似文献