全文获取类型
收费全文 | 300篇 |
免费 | 10篇 |
国内免费 | 17篇 |
专业分类
测绘学 | 9篇 |
大气科学 | 9篇 |
地球物理 | 83篇 |
地质学 | 137篇 |
海洋学 | 21篇 |
天文学 | 46篇 |
综合类 | 3篇 |
自然地理 | 19篇 |
出版年
2021年 | 3篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 16篇 |
2015年 | 9篇 |
2014年 | 17篇 |
2013年 | 15篇 |
2012年 | 21篇 |
2011年 | 15篇 |
2010年 | 14篇 |
2009年 | 21篇 |
2008年 | 16篇 |
2007年 | 13篇 |
2006年 | 10篇 |
2005年 | 6篇 |
2004年 | 8篇 |
2003年 | 6篇 |
2002年 | 5篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1995年 | 4篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1978年 | 4篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 4篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1967年 | 1篇 |
1965年 | 3篇 |
1964年 | 1篇 |
1958年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有327条查询结果,搜索用时 15 毫秒
321.
The purpose of this research is to investigate what factors influence the management of International Business Air Travel (IBAT). The researchers interviewed 34 business travellers, travel organisers and executives involved in IBAT from ten organisations in differing industries, in Australia. It was found that there are a multitude of factors influencing IBAT management, including professionalism and job satisfaction of travellers and the achievement of organisational goals such as carrying out successful international ventures or operating effectively in international markets. From the findings a strategic model was developed called the Threshold of Tolerance Model—which illustrates the potential point where both organisational goals are most likely to be met and where IBAT is most likely to be well managed for both travellers and organisations. 相似文献
322.
In order to reveal the temperature variation mechanism of a small and relatively closed cave, the surface temperature, the cave air temperature and the drip water temperatures were monitored with high-resolution recorders for a 7-year period. The results indicated that the cave temperature is mainly affected by the external seasonal oscillations. The temperature curve of the inner cave area (with standard deviation between 0.59 and 0.87) was much smoother in comparison with that of the surface (with SD as high as 8.46). However, the cave reacted to the temperature drifts with some delay, for example, at VP3 in 2006, the internal temperature increased for additional 49 days after the surface temperature dropped colder than inside of the cave. This seasonal variation and delayed response of the internal temperature were mainly affected by conduction from the epikarst rock mass. Air convection also occurred in the cave, especially when the surface temperature dropped lower than that of the inside cave (during the winter time) and in the outer section (close to the entrance), which caused an abrupt and irregular variation of the outer section. In addition, the influence from the percolating water was ephemeral and depended on its supplementary amount and temperature difference, e.g., there were more apparent responses at VP1 and VP3 when the intensive rain fell. The percolating water temperature could be a good indicator for water flow in epikarst. 相似文献
323.
Diversity,composition and host-species relationships of epiphytic orchids and ferns in two forests in Nepal 总被引:1,自引:0,他引:1
Yagya P. Adhikari Anton Fischer Hagen S. Fischer Maan B. Rokaya Prakash Bhattarai Axel Gruppe 《山地科学学报》2017,14(6):1065-1075
Epiphytic plant species are an important part of biological diversity. It is therefore essential to understand the distribution pattern and the factors influencing such patterns. The present study is aimed at observing the patterns of species richness, abundances and species composition of epiphytic orchids and ferns in two subtropical forests in Nepal. We also studied the relationship of host plants (Schima wallichii and Quercus lanata) and epiphyte species. Data were collected in Naudhara community forest (CF) and the national forest (NF) in Shivapuri Nagarjun National Park. The data were analyzed using univariate and multivariate tests. In total, we recorded 41 species of epiphytes (33 orchid and 8 fern species). Orchid species abundance is significantly higher in CF compared to NF. Orchid species richness and abundance increased with increasing southern aspect whereas it decreased with increasing canopy cover, and fern species richness increased with host bark roughness. Orchid abundance was positively correlated with increasing bark pH, stem size, tree age and tree height and negatively correlated with increasing steepness of the area. Likewise, fern abundances were high in places with high canopy cover, trees that were tall and big, but decreased with increasing altitude and southern aspect. The composition of the orchid and fern species was affected by altitude, aspect, canopy cover, DBH, number of forks and forest management types. We showed that the diversity of orchid and fern epiphytes is influenced by host characteristics as well as host types. The most important pre-requisite for a high epiphyte biodiversity is the presence of old respectively tall trees, independent of the recent protection status. This means: (i) for protection, e.g. in the frame of the national park declaration, such areas should be used which host such old tall trees; and (ii) also in managed forests and even in intensively used landscapes epiphytes can be protected by letting a certain number of trees be and by giving them space to grow old and tall. 相似文献
324.
Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry 总被引:1,自引:0,他引:1
Nancy Coe Anton le Roex John Gurney D. Graham Pearson Geoff Nowell 《Contributions to Mineralogy and Petrology》2008,156(5):627-652
Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton,
have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich
(~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible
elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised
La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). 87Sr/86Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and 176Hf/177Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic
differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a
combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There
is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated
by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE
enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich
(850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched
in incompatible elements (La/Ybn ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1)
and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from
calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue
for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during
the Proterozoic by calc-alkaline fluids/melts. 相似文献
325.
Experimental and numerical modelling of sedimentation in a rectangular shallow basin 总被引:9,自引:6,他引:3
Sameh A. KANTOUSH Erik BOLLAERT and Anton J. SCHLEISS Dr. Ecole Polytechnique Fédérale de Lausanne 《国际泥沙研究》2008,23(3):212-232
Numerical simulation of flows in shallow reservoirs has to be checked for its consistency in predicting real flow conditions and sedimentation patterns. Typical flow patterns may exhibit flow separation at the inlet, accompanied by several recirculation and stagnation areas all over the reservoir surface. The aim of the present research project is to study the influence of the geometry of a reservoir on sediment transport and deposition numerically and experimentally, focusing on a prototype reservoir depth between 5 and 15 m as well as suspended sediment transport.
A series of numerical simulations is presented and compared with scaled laboratory experiments, with the objective of testing the sensitivity to different flow and sediment parameters and different turbulence closure schemes. Different scenarios are analyzed and a detailed comparison of preliminary laboratory tests and some selected simulations are presented.
The laboratory experiments show that suspended sediment transport and deposition are determined by the initial flow pattern and by the upstream and downstream boundary conditions. In the experiments, deposition in the rectangular basin systematically developed along the left bank, although inflow and outflow were positioned symmetrically along the centre of the basin. Three major horizontal eddies developed influencing the sediment deposition pattern. Although asymmetric flow patterns are privileged, a symmetric pattern can appear from time to time. This particular behaviour could also be reproduced by a two-dimensional depth-averaged flow and sediment transport model (CCHE2D). The paper presents numerical simulations using different turbulence closure schemes (k-ε and eddy viscosity models). In spite of the symmetric setup, these generally produced an asymmetric flow pattern that can easily switch sides depending on the assumptions made for the initial and boundary conditions. When using the laboratory experiment as a reference, the most reliable numerical results have been obtai 相似文献
A series of numerical simulations is presented and compared with scaled laboratory experiments, with the objective of testing the sensitivity to different flow and sediment parameters and different turbulence closure schemes. Different scenarios are analyzed and a detailed comparison of preliminary laboratory tests and some selected simulations are presented.
The laboratory experiments show that suspended sediment transport and deposition are determined by the initial flow pattern and by the upstream and downstream boundary conditions. In the experiments, deposition in the rectangular basin systematically developed along the left bank, although inflow and outflow were positioned symmetrically along the centre of the basin. Three major horizontal eddies developed influencing the sediment deposition pattern. Although asymmetric flow patterns are privileged, a symmetric pattern can appear from time to time. This particular behaviour could also be reproduced by a two-dimensional depth-averaged flow and sediment transport model (CCHE2D). The paper presents numerical simulations using different turbulence closure schemes (k-ε and eddy viscosity models). In spite of the symmetric setup, these generally produced an asymmetric flow pattern that can easily switch sides depending on the assumptions made for the initial and boundary conditions. When using the laboratory experiment as a reference, the most reliable numerical results have been obtai 相似文献
326.
Jürgen Gose Esther Schmädicke Margit Markowitz Anton Beran 《Mineralogy and Petrology》2010,99(1-2):105-111
The infrared (IR) spectra of gem-quality olivine crystals from Pakistan, formed in serpentinised dunitic rocks, are characterised by strongly pleochroic absorption bands at 3,613, 3,597, 3,580 and 3,566 cm?1. These bands are assigned to O-H stretching vibrations of OH point defects corresponding to H2O concentrations of about 35 wt ppm. Unlike other olivine spectra, the dominating bands are strongly polarised parallel to the b-axis. The unusual spectra type, excludes the presence of planar defects. This finding is supported by transmission electron microscopy. The 3,613 cm?1 band is related to vacant Si sites, the slightly lower energetic bands preferentially to vacant M2 sites. The exclusive presence of these bands is not only a characteristic feature of olivines treated under high P,T conditions equivalent to mantle environment, the presence of these bands in untreated natural olivine also indicates formation conditions equivalent to crustal rocks. 相似文献
327.
微波消解法测定磷矿石中的主次量组分 总被引:1,自引:0,他引:1
奥地利安东帕有限公司 《岩矿测试》2007,26(6):512-512
磷元素是植物和动物营养的必需元素,所以全世界绝大多数的磷被作为一个主要成分用于粮食作物的氮-磷-钾肥料中。磷矿石是全球磷元素的最重要来源。 相似文献