全文获取类型
收费全文 | 845篇 |
免费 | 79篇 |
国内免费 | 135篇 |
专业分类
测绘学 | 33篇 |
大气科学 | 30篇 |
地球物理 | 189篇 |
地质学 | 315篇 |
海洋学 | 134篇 |
天文学 | 6篇 |
综合类 | 41篇 |
自然地理 | 311篇 |
出版年
2024年 | 2篇 |
2023年 | 11篇 |
2022年 | 22篇 |
2021年 | 23篇 |
2020年 | 38篇 |
2019年 | 29篇 |
2018年 | 26篇 |
2017年 | 23篇 |
2016年 | 30篇 |
2015年 | 51篇 |
2014年 | 46篇 |
2013年 | 46篇 |
2012年 | 39篇 |
2011年 | 64篇 |
2010年 | 46篇 |
2009年 | 57篇 |
2008年 | 49篇 |
2007年 | 57篇 |
2006年 | 54篇 |
2005年 | 50篇 |
2004年 | 49篇 |
2003年 | 46篇 |
2002年 | 32篇 |
2001年 | 33篇 |
2000年 | 24篇 |
1999年 | 21篇 |
1998年 | 25篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有1059条查询结果,搜索用时 15 毫秒
181.
为了考察铁锰氧化物对酚类污染物的氧化降解能力,采用天然以及合成的铁锰氧化物对苯酚的氧化降解进行对比实验研究。土壤中铁锰氧化物样品分别为天然针铁矿及氧化锰,合成铁锰氧化物样品分别为合成针铁矿及软锰矿。结果表明:苯酚与铁锰氧化物发生氧化还原作用时,还可能与土壤中杂质发生吸附等作用;铁锰氧化物还原反应强度随着反应介质pH值的升高而迅速下降;可用零级反应动力学方程拟合铁氧化物还原溶解反应,针铁矿溶解反应的强度与介质的pH值呈负相关关系;天然针铁矿对酚类污染物的氧化降解能力明显高于合成针铁矿,pH值对天然针铁矿溶解反应影响较大;可用一级指数衰减方程拟合锰氧化物还原溶解反应,锰氧化物溶解反应的强度与介质的pH值呈指数衰减关系;pH值对软锰矿还原溶解反应的影响大于对土壤中氧化锰的影响,pH值越小,影响越显著;对比pH值对铁和锰还原作用的影响发现,在pH=6.5时,锰氧化物仍有较强的氧化性能。 相似文献
182.
183.
铁锰氧化物对苯酚氧化降解的实验研究 总被引:1,自引:0,他引:1
为了考察铁锰氧化物对酚类污染物的氧化降解能力,采用天然以及合成的铁锰氧化物对苯酚的氧化降解进行对比实验研究.土壤中铁锰氧化物样品分别为天然针铁矿及氧化锰,合成铁锰氧化物样品分别为合成针铁矿及软锰矿.结果表明:苯酚与铁锰氧化物发生氧化还原作用时,还可能与土壤中杂质发生吸附等作用;铁锰氧化物还原反应强度随着反应介质pH值的... 相似文献
184.
In the last decade, there has been increasing interest in climate change, pasture degradation and its driving forces, and innovations in nomadic pastoralism on the Tibetan Plateau. However, little is known of indigenous strategies of adaptation to pasture degradation, which limits the effectiveness of adaptation strategies planned by local government. This paper analyzes nomads’ strategies of adaptation to pasture degradation on the basis of a field survey of three townships of Dalag County in the source regions of the Yangtze and Yellow rivers. Pastures there have evidently degraded, with pastures in Wasai mainly in a state of slight or medium degradation and those in Manzhang and Jianshe in a state of medium or severe degradation. With the degradation of pasture, the grazing time is reduced, which affects the livelihoods of nomads. Although the Four-Package Project has commenced in this region, there is still severe fodder shortage in winter and spring. The traditional hay storage strategy does not work because of pasture degradation, and few nomads establish fenced and artificial pastures. Therefore, nomads have employed other strategies, such as renting pasture, providing supplementary feed, and diversifying their livelihoods. Local strategies taken by nomads can provide valuable insights into ecological restoration and livelihood improvement in the region and suggest changes to means promoted by local government. It is necessary to seek new means that combine the best aspects of nomadic pastoralism with modern stockbreeding technologies to help nomads adapt to pasture degeneration and improve their livelihoods. 相似文献
185.
186.
187.
Jean de la Paix Mupenzi Lanhai Li Jiwen Ge Achal Varenyam Gabriel Habiyaremye Nzayisenga Theoneste Kamanzi Emmanuel 《地学前缘(英文版)》2011,2(4):599-607
This study has focused on the processes of soil degradation and chemical element concentration
in tea-growing regions of Rwanda, Africa. Soil degradation accelerated by erosion is caused not only
by topography but also by human activities. This soil degradation involves both the physical loss and
reduction in the amount of topsoil associated with nutrient decline. Soil samples were collected from
eleven tropical zones in Rwanda and from variable depth within each collecting site. Of these, Samples
from three locations in each zone were analyzed in the laboratory, with the result that the pH of all soil
samples is shown to be less than 5 (pH < 5) with a general average of 4.4. The elements such as iron (Fe),
copper (Cu), manganese (Mn), and zinc (Zn) are present in high concentration levels. In contrast calcium
(Ca) and sodium (Na) are present at low-level concentrations and carbon (C) was found in minimal concentrations. In addition, elements derived from fertilizers, such as nitrogen (N), phosphorous (P), and
potassium (K) which is also from minerals such as feldspar, are also present in low-level concentrations.
The results indicate that the soil in certain Rwandan tea plantations is acidic and that this level of pH may
help explain, in addition to natural factors, the deficiency of some elements such as Ca, Mg, P and N. The
use of chemical fertilizers, land use system and the location of fields relative to household plots are also
considered to help explain why tea plantation soils are typically degraded. 相似文献
188.
A three-dimensional advection-diffusion model coupled with the degradation process is established for describing the transport of chemical oxygen demand (COD). Comparison of the simulated distribution of COD at the surface in the Bohai Sea in August, 2001 with field observations, shows that the model simulates the dataset reasonably well. The Laizhou Bay, Bohai Bay, and Liaodong Bay were contaminated heavily near shore. Based on the optimal discharge flux method, the Environmental Capacity (EC) and allocate... 相似文献
189.
《Geomechanics and Geoengineering》2013,8(4):235-251
The bearing capacity of footing has been studied by both conventional and numerical methods by many researchers. However, degradation of the microstructure of material, that is, a change in the microstructure of the soil, has not been adequately taken into account. Degradation of microstructure causes strain softening of materials and it leads to strain localization such as shear bands and slip bands. From an engineering point of view the strain localization is crucial because it is a precursor of failure. In the present study, finite element analyses of the bearing capacity of a shallow foundation on homogeneous and inhomogeneous saturated clay strata have been conducted using an elasto-viscoplastic soil constitutive model of microstructure change. A series of analyses of footing on clay deposit with different microstructure parameters have been carried out. Numerical results show that strain localization can be predicted during the loading of rigid footing on highly structured soil and strain localization affects the footing–soil interaction. The effects of footing roughness on the failure mechanism are also discussed in the study. 相似文献
190.