全文获取类型
收费全文 | 259322篇 |
免费 | 4362篇 |
国内免费 | 3388篇 |
专业分类
测绘学 | 7017篇 |
大气科学 | 19199篇 |
地球物理 | 54564篇 |
地质学 | 89449篇 |
海洋学 | 21446篇 |
天文学 | 56246篇 |
综合类 | 1020篇 |
自然地理 | 18131篇 |
出版年
2021年 | 2268篇 |
2020年 | 2626篇 |
2019年 | 2868篇 |
2018年 | 3332篇 |
2017年 | 3027篇 |
2016年 | 5646篇 |
2015年 | 4261篇 |
2014年 | 6961篇 |
2013年 | 14317篇 |
2012年 | 6496篇 |
2011年 | 7843篇 |
2010年 | 6740篇 |
2009年 | 9451篇 |
2008年 | 8395篇 |
2007年 | 7750篇 |
2006年 | 9765篇 |
2005年 | 7762篇 |
2004年 | 7702篇 |
2003年 | 7205篇 |
2002年 | 6818篇 |
2001年 | 6070篇 |
2000年 | 6000篇 |
1999年 | 5232篇 |
1998年 | 5247篇 |
1997年 | 5057篇 |
1996年 | 4711篇 |
1995年 | 4457篇 |
1994年 | 4134篇 |
1993年 | 3880篇 |
1992年 | 3673篇 |
1991年 | 3610篇 |
1990年 | 3792篇 |
1989年 | 3539篇 |
1988年 | 3319篇 |
1987年 | 3865篇 |
1986年 | 3416篇 |
1985年 | 4250篇 |
1984年 | 4762篇 |
1983年 | 4433篇 |
1982年 | 4345篇 |
1981年 | 3956篇 |
1980年 | 3652篇 |
1979年 | 3519篇 |
1978年 | 3489篇 |
1977年 | 3287篇 |
1976年 | 3061篇 |
1975年 | 2964篇 |
1974年 | 2934篇 |
1973年 | 3085篇 |
1972年 | 2034篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
181.
182.
Jorma J. Riihimaa 《Earth, Moon, and Planets》1985,32(1):9-19
Dynamic spectra of Jupiter's decametric emission often display narrow-band features, referred to as events of type N (Carr et al., 1983). The average bandwidth of these emissions is in the vicinity of 200 kHz, their durations are typically in the decasecond range, and their f-t slopes are small and random. Although the N-bursts can be described as narrow-band L-bursts, it seems that they are realted to S-bursts in their area of occurrence in the Io-B region, the durations of the emission envelopes, and their bandwidths. Possible implications are discussed. 相似文献
183.
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
184.
185.
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
186.
An HF radar called the Coastal Ocean Dynamics Applications Radar (CODAR) is presently being used in several forms to measure ocean surface parameters. The original version was developed by the National Oceanic and Atmospheric Administration (NOAA) and utilizes a four-element receive array. The array consists of four equally spaced elements arranged on a circle with a radius of 0.2151 wavelengths (at 25.4 MHz). It was designed to measure ocean currents using a direction-finding technique based on an extension to a simple two-element interferometer. The problem of determining the bearing of a radiating source can be readily shown to be equivalent to that incurred in spectral estimation. In an attempt to improve upon the processing of existing data, modern nonlinear spectral estimation techniques are applied in a beam-forming bearing estimation procedure and compared against several direction-finding algorithms. Enhancement of bearing estimators via analysis of the eigenstructure of a spatial correlation matrix is included. Antenna response patterns are calculated and used to investigate properties of direction-finding algorithms. Simulated data are used for a comparison of direction finding and beam forming. The asymmetrical bias of each method is investigated to determine its effect on the error in estimating the angle of arrival of a radar target. 相似文献
187.
Measurements of salinity perturbations in a partially mixed estuary have been used to evaluate the usefulness of an inductive salinometer and to determine some of the characteristics of the salinity perturbations. The salinometer performed satisfactorily under most conditions. Although internal wave like effects were present, the turbulence fluctuations were dominant. The salinity fluctuations and the turbulent fluxes and were found to behave in a manner similar to the density fluctuations in a thermally stratified atmospheric boundary layer and a laboratory open channel flow. A quadrant analysis suggested that the contribution of each quadrant to the turbulent flux changed with Ri. The turbulence parameters ν and cγ were found to decrease and increase respectively as Ri increases. 相似文献
188.
The paper considers the random phase errors in the phase shifters which are used in an antenna array to steer the beam in the look direction, and analyzes the effect of these errors on the performance of the optimal processor which maximizes the output SNR by deriving the expressions for the output signal power, residual interference power, output SNR, and the array gain as a function of the variance of these errors. The paper also considers the phase quantization error which arises in the digital phase shifters and shows how the performance of the optimal processor depends on the number of bits of the digital phase shifters. 相似文献
189.
190.
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