首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   479篇
  免费   48篇
  国内免费   2篇
测绘学   11篇
大气科学   35篇
地球物理   164篇
地质学   204篇
海洋学   38篇
天文学   39篇
综合类   4篇
自然地理   34篇
  2022年   3篇
  2021年   13篇
  2020年   20篇
  2019年   9篇
  2018年   35篇
  2017年   25篇
  2016年   61篇
  2015年   31篇
  2014年   42篇
  2013年   38篇
  2012年   28篇
  2011年   39篇
  2010年   23篇
  2009年   28篇
  2008年   27篇
  2007年   20篇
  2006年   8篇
  2005年   8篇
  2004年   10篇
  2003年   6篇
  2002年   3篇
  2001年   5篇
  2000年   3篇
  1999年   5篇
  1998年   5篇
  1997年   3篇
  1996年   2篇
  1995年   3篇
  1994年   1篇
  1993年   2篇
  1989年   3篇
  1987年   2篇
  1985年   1篇
  1982年   1篇
  1981年   2篇
  1978年   1篇
  1975年   1篇
  1974年   2篇
  1972年   1篇
  1971年   1篇
  1969年   1篇
  1968年   1篇
  1966年   1篇
  1965年   1篇
  1963年   2篇
  1962年   1篇
  1958年   1篇
排序方式: 共有529条查询结果,搜索用时 337 毫秒
41.
Andean grasslands ecosystems are fragile environments with rigorous climatologic conditions and low and variable food for the grazing. The Apolobamba area is located in the Bolivian Andean Mountains. Its high grasslands provide a natural habitat for wild and domestic camelids such as vicuna(Vicugna vicugna) and alpaca(Lama pacos). The botanical diversity plays an essential role in maintaining vital ecosystem functions. The objectives of this research were to determine the seasonal changes in soil properties, to study the vegetation changes during the wet and dry seasons and the influence of soil properties and camelid densities on the vegetation in the Apolobamba grasslands. Four zones with different vicuna populations were selected to be studied. The following soil parameters were determined: total organic carbon, total nitrogen, available phosphorous, cation exchange capacity, exchangeable cations, pH and texture. The vegetation season changes were studied through botanical identification, above-ground biomass, plant cover and species richness. Results showed that some soil properties such as C/N ratio, CEC, silt and clay percentages kept stable against the seasonal changes. Generally, soil nutrients were relatively higher during the dry season in the surface and subsurface. The results did not point out the predominant vegetation growth during the wet season. The seasonal vegetation growth depended on each species. Thegood soil fertility corresponded to the highest plant cover. Soil fertility presented no influence on the above-ground biomass of the collected species. The negative influence of camelid grazing on soil properties could not be assessed. However, overgrazing could affect some plant species. Therefore, protection is needed in order to preserve the biodiversity in the Andean mountain grasslands.  相似文献   
42.
The concentration of Si, Al, Mg, K, Ca, Na, Sr, and Ba in the subcolloidal fraction of sediments at the Razdol’naya River-Amur Bay cross-section is determined using the atomic emission spectroscopy method. Computed is the mean content of studied elements in the soil and river bottom sediments (to the mixing zone). Revealed are river bottom sediments with the high concentration of macroelements corresponding to the zone of mixing of fresh river water and salt sea water. It is demonstrated that the additional accumulation of macroelements (Ca, Na, and Sr) in these sediments is associated with the formation of hydroxides of Fe and Mn and processes of sorption (Mg, Ca, Na, K, Sr, and Ba) on clay minerals (hydromica and smectites).  相似文献   
43.
Conventional water treatments when downsized to cover the demands of small and scattered populations are costly and ineffective. In fact, many small towns cannot afford the high average treatment costs nor provide the skill and complex management requirements. Even when these facilities are available, waste water might not be properly treated. Land application system treatments (LAST) have been shown to be preferable on the basis of their technical effectiveness and financial viability. The recently approved Spanish regulations (RDL 1620/2007, pp 50369–50661) represent an opportunity for the implementation of well-designed LASTs. This paper presents an assessment methodology for the design of LASTs by combining technical, financial and location criteria. The method is applied to the design of LASTs to cover the water disposal demand of twelve municipalities located within the protected natural landscape, and surrounding areas of ‘El Rebollar’, Salamanca, Spain.  相似文献   
44.
45.
Spinel lherzolite and wehrlite xenoliths from the Cenozoic Calatrava volcanic field carry the geochemical imprint of metasomatic agents that have affected the subcontinental lithospheric mantle beneath Central Iberia. Some xenoliths (mainly wehrlites) were enriched in REE, Sr, P, and CO2 by silicic-carbonate-rich metasomatic melts/fluids, while others record the effects of subduction-related hydrous silicate fluids that have precipitated amphibole and induced high Ti/Eu in primary clinopyroxene. The petrographic observations and geochemical data suggest that interstitial glass in the xenoliths represent the quenched products of Si-rich melts that infiltrated the mantle peridotite shortly before the entrainment of the xenoliths in the host magmas that erupted ca 2 million years ago. During their infiltration, the metasomatic melts reacted with peridotite, resulting in silica enrichment, while remobilizing grains of iron-rich monosulfide solid solution (Fe-rich Mss) initially enclosed in, or intergranular to, primary olivine and pyroxenes. In situ laser ablation inductively coupled plasma-mass spectrometry analysis of single sulfide grains reveals that the Fe-rich Mss in glass shows platinum-group element (PGE) patterns and 187Os/188Os compositions identical to the Fe-rich Mss occurring as inclusions in, or at grain boundaries of primary silicates. Moreover, independent of its microstructural position, Fe-rich Mss exhibits PGE and 187Os/188Os signatures typical of Mss either residual after partial melting or crystallized directly from sulfide melts. Our findings reveal that young metasomatic melt(s)/fluid(s) may carry remobilized sulfides with PGE and Os-isotopic signatures identical to those of texturally older sulfides in the peridotite xenolith. These sulfides thus still provide useful information about the timing and nature of older magmatic events in the subcontinental mantle.  相似文献   
46.
We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars.
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples.  相似文献   
47.
We have monitored S Ori 45, a young, low-mass (20 M j up) brown dwarf of the σ Orionis cluster (~3 Myr, 352 pc), using optical and near-infrared filters. S Ori 45 (spectral type M8.5) is found to be multi-periodic with a dominant modulation at 2.5–3.5 h, and a short modulation at about 46 min. We ascribe the longer of these modulations to a rotation period. After comparing these results with observations of more massive cluster brown dwarfs and field brown dwarfs, we conclude that substellar objects present rotational and angular momentum evolution. We have also obtained intermediate-resolution near-infrared spectroscopy of S Ori 70, which is a T-class, free-floating planetary candidate member in the σ Orionis cluster. Its observed spectrum has been compared to data of field brown dwarfs of similar types and to theoretical spectra computed for different surface temperatures and gravities. We conclude that S Ori 70 has a significantly cool, low-gravity atmosphere. This supports the young age of this object and its membership in the cluster. From state-of-the-art evolutionary models, the mass of S Ori 70 is estimated at 3 times the Jovian mass (+5 ?2 M j up), challenging current stellar/substellar formation models. S Ori 70 remains the lowest mass object so far identified in any open cluster.  相似文献   
48.
49.
50.
The existing United Nations Framework Convention on Climate Change (UNFCCC) has failed to deliver the rate of low-carbon technology transfer (TT) required to curb GHG emissions in developing countries. This failure has exposed the limitations of universalism and renewed interest in bilateral approaches to TT. Gaps are identified in the UNFCCC approach to climate change TT: missing links between international institutions and the national enabling environments that encourage private investment; a non-differentiated approach for (developing) country and technology characteristics; and a lack of clear measurements of the volume and effectiveness of TTs. Evidence from econometric literature and business experience on climate change TT is reviewed, so as to address the identified pitfalls of the UNFCCC process. Strengths and weaknesses of different methodological approaches are highlighted. International policy recommendations are offered aimed at improving the level of emission reductions achieved through TT.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号