全文获取类型
收费全文 | 1350篇 |
免费 | 47篇 |
国内免费 | 28篇 |
专业分类
测绘学 | 61篇 |
大气科学 | 62篇 |
地球物理 | 252篇 |
地质学 | 823篇 |
海洋学 | 60篇 |
天文学 | 84篇 |
综合类 | 28篇 |
自然地理 | 55篇 |
出版年
2023年 | 6篇 |
2022年 | 43篇 |
2021年 | 45篇 |
2020年 | 48篇 |
2019年 | 35篇 |
2018年 | 96篇 |
2017年 | 108篇 |
2016年 | 89篇 |
2015年 | 52篇 |
2014年 | 90篇 |
2013年 | 123篇 |
2012年 | 66篇 |
2011年 | 72篇 |
2010年 | 57篇 |
2009年 | 66篇 |
2008年 | 42篇 |
2007年 | 32篇 |
2006年 | 36篇 |
2005年 | 31篇 |
2004年 | 30篇 |
2003年 | 27篇 |
2002年 | 24篇 |
2001年 | 18篇 |
2000年 | 13篇 |
1999年 | 18篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 7篇 |
1992年 | 13篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1973年 | 4篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1964年 | 2篇 |
排序方式: 共有1425条查询结果,搜索用时 31 毫秒
91.
Two-dimensional analytical elastic models are developed for evaluating dynamic characteristics, namely natural frequencies and modes of vibration of a wide class of earth dams in a direction parallel to the dam axis. In these models the non-homogeneity of the dam materials is taken into account by assuming a specific variation of the stiffness properties along the depth (due to the continuous increase in confining pressure). In addition, both shear and normal (axial) deformations are considered. Cases having constant elastic moduli, linear and trapezoidal variations of elastic moduli, and elastic moduli increasing as the one-half, one-third, two fifths, and a general (l/m)th powers of the depth are studied. Dynamic properties of three real earth dams in a seismically active area (Southern California) estimated from their earthquake records (input ground motion and crest response in the longitudinal direction) as well as results from full-scale dynamic tests on one of these dams (including ambient and forced vibration tests) are compared with those from the suggested models. It was found that the models in which the shear modulus and the modulus of elasticity of the dam material vary along the depth are the most appropriate representations for predicting the dynamic characteristics. The agreement between the experimental and earthquake data and the theoretical results from some of the models is reasonably good. 相似文献
92.
S.M. Ahmed 《International Journal of Mineral Processing》1978,5(2):163-174
The electrocatalytic activity of galena, pyrite and Co3S4 for oxygen reduction has been studied by potentiostatic methods. Open circuit potentials of the sulphide electrodes have also been measured as a function of pH in nitrogen, air and oxygen atmospheres and also in the presence of H2O2 and ethyl xanthate. The adsorption of xanthate on sulphides was followed by observing bubble attachment to the electrodes.The catalytic activity for oxygen (or H2O2) reduction (the cathodic currents), the electrode potentials and the xanthate adsorption as shown by bubble attachment within certain pH limits, all varied as indicating considerable dependence of the redox processes in flotation on the d - electron character of the sulphides.In the absence of oxygen, xanthate is probably bonded to the water structure of the surface through hydrogen-bonding, thus keeping the surface hydrophilic. Such adsorption reduces the electrode potential and inhibits oxygen reduction. 相似文献
93.
Gerhard Bart Mohammed Ikramuddin Michael E. Lipschutz 《Geochimica et cosmochimica acta》1980,44(5):719-730
We report data for Ag, Bi, Cd, Co, Cs, Ga, In, Se, Te, Tl and Zn determined by neutron activation in Allende samples heated for one week at 100° increments in the 1000–1400°C range in a low pressure (initially 10?5atmH2) environment using an apparatus of novel design. In the extremes, concentrations of these trace elements—initially present at ppm-ppb levels—in unheated material are lowered even farther by factors of 10?4–10?5 over a broad temperature span. Loss of some elements above 1000°C extends trends evident below 1000°C; loss of others is even more extreme. On Arrhenius diagrams some elements exhibit but one apparent activation energy over the entire temperature span of loss while others exhibit 2 or 3, each operative in a particular temperature region. These discontinuities seem related to mineralogic/petrologic alteration and probably reflect differences in diffusion mechanism rather than siting differences. The extension of previous experiments into the temperature regime postulated for chondritic differentiation should lead to a better understanding of the evolution of meteoritic parent bodies. 相似文献
94.
Mohammed Mamdouh Ashry 《Contributions to Mineralogy and Petrology》1964,11(1):49-90
Core samples from the “Praezechstein” formation of the bore “Dransfeld 1” were investigated by optical, X-ray, and chemical methods. The rock formation consists of graywackes interbedded with Slates. The graywackes contain mineral.components and rock fragments which are also common in the graywackes of Harz and “Rheinisches Schiefergebirge”, so that a common source of derivation may be concluded, namely the “Mitteldeutsche Schwelle”. The distinctive features of the Dransfeld-graywackes however as compared to those of Harz and “Rheinisches Schiefergebirge” are:
- 1.A higher content of rock fragments specially of volcanic origin. 相似文献
95.
Fe-Cr-Ni particles and veinlets have been discovered in the top 15 m of the compressed zone with abundant shatter cones below the bottom of the Ries crater. The metallic particles are less than a few microns across. They occur in various minerals along healed intergranular and locally in intragranular microfractures in quartz diorite, amphibolite and chloritized granite of the basement crystalline rocks.The particles consist of major Fe, Cr, and Ni with minor Si and Ca. Origin due to contamination is absolutely ruled out. We believe that these Fe-Cr-Ni particles are probably condensed from the vaporized impacting body which produced the Ries crater. These particles were injected with high velocity into microfractures near the top of the compressed zone, implanted in and across various minerals before these microfractures were resealed. The presence of Si and Ca as well as the fact that the Cr content is nearly twice that of Ni, led us to conclude that the Ries impacting body is very likely not an iron meteorite but a stony meteorite. 相似文献
96.
R. Mohammed 《水文科学杂志》2013,58(10):1558-1573
ABSTRACTA baseflow separation methodology combining the outcomes of the flow–duration curve and the digital filtering algorithms to cope with the restrictions of traditional procedures has been assessed. Using this methodology as well as the monitored and simulated hydro-climatological data, the baseflow annual variations due to climate change and human-induced activities were determined. The outcomes show that the long-term baseflow index at the upstream sub-basin is nearly half of that at the downstream from October to April, whereas they are close to each other for the remaining months. Some of the groundwater reacts to precipitation and an evident rise in the groundwater contribution was detected for the hydrological years 1998–2001 and 2006–2008. The contrary was recorded for1987. The water released from the reservoir in the dry periods led to distinctions in the detected baseflow index between the pre-damming and post-damming periods of the river. 相似文献
97.
98.
Hassan Er-Raioui Saida Bouzid Mohammed Marhraoui Alain Saliot 《Ocean & Coastal Management》2009,52(2):124-129
Organic geochemical study of hydrocarbon pollution of the Mediterranean coastline of Morocco (Tangier-Nador) was performed during March–April 2002. It corresponds to a preliminary work representing an initial assessment of oil contamination of Moroccan coast.Three coastal sites corresponding to Tangier, Tetouan and Nador, representing the main built-up area of this marine region, were selected for superficial sediments, water and organism sampling. Results showed high contamination levels in these three selected sites, known as the most exposed areas to urban and industrial discharges. The maximal contamination corresponding to the sum of non-aromatic (NAH) and polycyclic aromatic hydrocarbons (PAH) encountered in sediment samples was about 553 μg/g (dw). Nevertheless, even a small city agglomeration without any industrial activity showed high values reaching 370 μg/g (dw), which confirm the important hydrocarbons input coming from discharges of fishing ships and oil transporting tankers.The obtained results from this survey reflecting in part the important amounts drained via effluents toward the Moroccan Mediterranean coasts, presented to the concerned authorities drove to the realization of the project of treatment of the wastewater.They later expect to lower rates by 50%, which is still insufficient for a fragile ecosystem such as the Mediterranean and sensitive to anthropogenic effects. 相似文献
99.
Ahmed Abdel-Latif 《Climate Policy》2015,15(1):103-126
The role of intellectual property rights (IPRs) in the development and transfer of climate change technologies has been a contentious issue in negotiations under the United Nations Framework Convention on Climate Change (UNFCCC). Irreconcilable differences seem to oppose those who believe IPRs are an inherent barrier to the transfer of climate change technologies and those who argue they are an essential incentive to innovation. After providing an overview of the polarized debate on this issue, this article reviews the existing literature and empirical evidence and looks into some practical initiatives and recent developments. Finally, it seeks to identify a way forward that could help overcome the current stalemate in international deliberations by suggesting a number of parameters to structure the discussion on this complex issue. 相似文献
100.