全文获取类型
收费全文 | 3263篇 |
免费 | 93篇 |
国内免费 | 92篇 |
专业分类
测绘学 | 70篇 |
大气科学 | 402篇 |
地球物理 | 812篇 |
地质学 | 1006篇 |
海洋学 | 648篇 |
天文学 | 302篇 |
综合类 | 46篇 |
自然地理 | 162篇 |
出版年
2023年 | 12篇 |
2022年 | 22篇 |
2021年 | 48篇 |
2020年 | 47篇 |
2019年 | 63篇 |
2018年 | 134篇 |
2017年 | 124篇 |
2016年 | 139篇 |
2015年 | 87篇 |
2014年 | 171篇 |
2013年 | 220篇 |
2012年 | 140篇 |
2011年 | 203篇 |
2010年 | 190篇 |
2009年 | 190篇 |
2008年 | 164篇 |
2007年 | 176篇 |
2006年 | 146篇 |
2005年 | 122篇 |
2004年 | 105篇 |
2003年 | 96篇 |
2002年 | 98篇 |
2001年 | 76篇 |
2000年 | 79篇 |
1999年 | 55篇 |
1998年 | 45篇 |
1997年 | 41篇 |
1996年 | 25篇 |
1995年 | 37篇 |
1994年 | 19篇 |
1993年 | 17篇 |
1992年 | 21篇 |
1991年 | 18篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 24篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 28篇 |
1983年 | 31篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 25篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1977年 | 16篇 |
1975年 | 15篇 |
1974年 | 10篇 |
1973年 | 7篇 |
排序方式: 共有3448条查询结果,搜索用时 0 毫秒
991.
In this work, we apply multichannel singular spectrum analysis (MSSA), a data-adaptive, multivariate, non-parametric technique that simultaneously exploits the spatial and temporal correlations of the input data to extract common modes of variability, to investigate the intermediate quasi-periodicities of the Fe xiv green coronal emission line at 530.3 nm for the period between 1944 and 2008. Our analysis reveals several significant mid-term periodicities in a range from about one to four years that are consistent with the so-called quasi-biennial oscillations (QBOs), which have been detected by several authors using different data sets and analysis methods. These QBOs display amplitudes varying significantly with time and latitude over the six solar cycles (18 to 23) covered by this study. A clear North–South asymmetry is detected both in their intensity and period distribution, with a net predominance of spectral power in the active-region belt of the northern hemisphere. On the other hand, while the QBOs with periods \(\gtrsim1.7\) years are particularly intense around the polar regions and therefore related to the global magnetic field, the ones with shorter periods are mainly generated at mid-latitudes, in correspondence with the emergence of active regions. Our findings indicate that the North–South asymmetry manifested in the uneven latitudinal distribution of QBOs is a fundamental, albeit puzzling, characteristic of solar activity. 相似文献
992.
This paper investigates the effect of atomic disorder on the electronic structure, magnetism, and half-metallicity of full-Heusler Co2FeSi alloy by using the full-potential linearized augmented plane wave method within the generalized gradient approximation (GGA) and GGA+U schemes. It considers three types of atomic disorders in Co2FeSi alloy: the Co--Fe, Co--Si, and Fe--Si disorders. Total energy calculations show that of the three types of disorders, the Fe--Si disorder is more likely to occur. It finds that for the Co--Si disorder, additional states appear in the minority band-gap at the EF and the half-metallcity is substantially destroyed, regardless of the disorder level. On the other hand, the Co--Fe and Fe--Si disorders have little effect on the half-metallicity at a low disorder level. When increasing the disorder levels, the half-metallcity is destroyed at about 9 % of the Co--Fe disorder level, while that stays at 25 % of the Fe--Si disorder level. 相似文献
993.
Christian Tamburini Madeleine Goutx Catherine Guigue Marc Garel Dominique Lefvre Bruno Charrire Richard Sempr Stphane Pepa Michael L. Peterson Stuart Wakeham Cindy Lee 《Deep Sea Research Part II: Topical Studies in Oceanography》2009,56(18):1533-1546
We used a new experimental device called PASS (PArticle Sinking Simulator) during MedFlux to simulate changes in in situ hydrostatic pressure that particles experience sinking from mesopelagic to bathypelagic depths. Particles, largely fecal pellets, were collected at 200 m using a settling velocity NetTrap (SV NetTrap) in Ligurian Sea in April 2006 and incubated in high-pressure bottles (HPBs) of the PASS system under both atmospheric and continuously increasing pressure conditions, simulating the pressure change experienced at a sinking rate of 200 m d−1. Chemical changes over time were evaluated by measuring particulate organic carbon (POC), carbohydrates, transparent exopolymer particles (TEP), amino acids, lipids, and chloropigments, as well as dissolved organic carbon (DOC) and dissolved carbohydrates. Microbial changes were evaluated microscopically, using diamidinophenylindole (DAPI) stain for total cell counts and catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) for phylogenetic distinctions. Concentrations (normalized to POC) of particulate chloropigments, carbohydrates and TEP decreased under both sets of incubation conditions, although less under the increasing pressure regime than under atmospheric conditions. By contrast, dissolved carbohydrates (normalized to DOC) were higher after incubation and significantly higher under atmospheric conditions, suggesting they were produced at the expense of the particulate fraction. POC-normalized particulate wax/steryl esters increased only under pressure, suggesting biochemical responses of prokaryotes to the increasing pressure regime. The prokaryotic community initially consisted of 43% Bacteria, 12% Crenarchaea and 11% Euryarchaea. After incubation, Bacteria dominated (90%) the prokaryote community in all cases, with γ-Proteobacteria comprising the greatest fraction, followed by the Cytophaga–Flavobacter cluster and α-Proteobacteria group. Using the PASS system, we obtained chemical and microbial evidence that degradation by prokaryotes associated with fecal pellets sinking through mesopelagic waters is limited by the increasing pressure they experience. 相似文献
994.
We measured the time-series variations in 222Rn activity at a fixed station in the Yeongsan River Estuary (YRE), Korea, where the upstream water is obstructed by an artificial dam, during November 18–24, 2008, and conducted a horizontal 222Rn survey in the Seomjin River Estuary (SRE), Korea, during December 16–18, 2008. In the YRE, we observed constantly low levels of 222Rn activity over the sampling period, relative to the summer in 2008. The 222Rn variations and 222Rn mass-balance modeling results indicate that the submarine groundwater discharge (SGD) in the YRE was relatively low and constant over the sampling period and that the temporal SGD variations were not related to tides or to the water elevation in the dam upstream. This low and constant SGD appears to be due to the lower water level in the dam relative to the local sea level during the study period, resulting in the SGD mostly from the surrounding land areas rather than from the dam. In the SRE, higher 222Rn activities were observed at upstream stations of the river (salinity: <5) and decreased as the salinity increased, but there were marked increases of 222Rn, together with dissolved inorganic silicate (DSi), at stations close to the river mouth (salinity: ~33). This suggests that the river has major groundwater inputs from the two extreme upstream and downstream areas. Because most chemical flux estimations via river discharge do not account for groundwater inputs from the downstream high-salinity zone, our results suggest that there is an important unaccounted source of river-driven chemical fluxes. Overall, our study shows that the continuous 222Rn monitoring system provides high-resolution information on SGD over different locations and times. 相似文献
995.
996.
T. J. Lee 《Astrophysics and Space Science》1989,160(1-2):249-253
There are a number of parallels between the UKIRT and the JNLT both of which are major astronomical facilities of nations which do not have premier observing sites in their own country. Some elements of experience with UKIRT relevant to the JNLT are described. These include matters related to personnel and to instruments.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988. 相似文献
997.
Groundwater flows and travel times were analyzed under a landfill site characterized by a thick unsaturated zone, in an arid environment, using two-dimensional models. Transport in the unsaturated zone is important in arid environments with thick vadose zones and usually determines the travel time of groundwater from the landfill site to the regulatory compliance surface. The combined use of a parallel flow (Dupuit-Forchheimer) areal model and vertical cross-sectional model is explored at a site typical in the utility industry. Flow paths and travel times to the compliance zone are obtained for use in landfill site design or environmental impact assessments. Sensitivity analysis shows that boundary conditions are important factors influencing the entire analysis and thus need to be confirmed with field measurements. 相似文献
998.
A fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden. It is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on Europa. Such penetration is found to be very unlikely in the absence of an ocean. A cycloidal crack would then form as a sequence of near instantaneous discrete failures, each extending roughly the brittle layer thickness in range, linked with a much lower apparent propagation speed set by the moving tidal stress field. The implications of this porous ice fracture model for ice-penetrating radar scattering loss and seismic activity are quantified. 相似文献
999.
H. Bruntt D. Stello J. C. Suárez T. Arentoft T. R. Bedding M. Y. Bouzid Z. Csubry T. H. Dall Z. E. Dind S. Frandsen R. L. Gilliland A. P. Jacob H. R. Jensen Y. B. Kang S.-L. Kim L. L. Kiss H. Kjeldsen J.-R. Koo J.-A. Lee C.-U. Lee J. Nuspl C. Sterken R. Szabó 《Monthly notices of the Royal Astronomical Society》2007,378(4):1371-1384
1000.