全文获取类型
收费全文 | 367篇 |
免费 | 47篇 |
国内免费 | 180篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 57篇 |
地球物理 | 54篇 |
地质学 | 341篇 |
海洋学 | 78篇 |
天文学 | 2篇 |
综合类 | 20篇 |
自然地理 | 39篇 |
出版年
2023年 | 7篇 |
2022年 | 10篇 |
2021年 | 11篇 |
2020年 | 14篇 |
2019年 | 17篇 |
2018年 | 12篇 |
2017年 | 19篇 |
2016年 | 14篇 |
2015年 | 12篇 |
2014年 | 25篇 |
2013年 | 31篇 |
2012年 | 23篇 |
2011年 | 27篇 |
2010年 | 25篇 |
2009年 | 26篇 |
2008年 | 25篇 |
2007年 | 23篇 |
2006年 | 38篇 |
2005年 | 32篇 |
2004年 | 19篇 |
2003年 | 16篇 |
2002年 | 14篇 |
2001年 | 20篇 |
2000年 | 11篇 |
1999年 | 20篇 |
1998年 | 8篇 |
1997年 | 19篇 |
1996年 | 9篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 17篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1980年 | 1篇 |
1978年 | 4篇 |
排序方式: 共有594条查询结果,搜索用时 15 毫秒
21.
22.
The shear viscosities and 1 bar heat capacities of glasses and melts along the 67mol% silica isopleth in the system SiO2-Al2O3-Na2O-TiO2 have been determined in the temperature ranges 780-1140 K and 305-1090 K respectively. Anomalous behaviour of both these properties is observed for compositions rich in TiO2 and/or Al2O3, an observation attributed to liquid-liquid phase separation followed by anatase crystallization. For samples which do not show anomalous behaviour, it is found that the partial molar heat capacity of the TiO2 component previously determined in Al-free compositions reproduces our heat capacities to within 1.3%. Viscosity data show that addition of TiO2 tends to increase viscosity and melt fragility at constant temperature. Furthermore, heat capacity and viscosity data may be combined within the framework of the Adam-Gibbs theory to extract values of the configurational entropy of the liquids and qualitative estimates of the variation of the average energy barrier to viscous flow. Configurational entropy at 900K is inferred to decrease upon addition of TiO2, in contrast to previous results from Al-free systems. The compositional limit separating normal from anomalous behaviour, as well as the data for homogenous melts have been used to constrain the structural role of Ti in these samples. Our data are consistent with a majority of Ti in five-fold coordination associated with a titanyl bond, in agreement with previous spectroscopic studies. Furthermore, we find no evidence for a Ti-Al interaction in our samples, and we are led to the conclusion that Al and Ti are incompletely mixed, a hypothesis consistent with the observed reduction of configurational entropy upon addition of TiO2, suggesting an important role of medium range order in controlling the variations in thermodynamic properties. 相似文献
23.
Dissolved nitrous oxide (N2O) dynamics in agricultural field drains and headwater streams in an intensive arable catchment 下载免费PDF全文
Indirect nitrous oxide (N2O) emissions produced by nitrogen (N) leaching into surface water and groundwater bodies are poorly understood in comparison to direct N2O emissions from soils. In this study, dissolved N2O concentrations were measured weekly in both lowland headwater streams and subsurface agricultural field drain discharges over a 2‐year period (2013–2015) in an intensive arable catchment, Norfolk, UK. All field drain and stream water samples were found to have dissolved N2O concentrations higher than the water–air equilibrium concentration, illustrating that all sites were acting as a net source of N2O emissions to the atmosphere. Soil texture was found to significantly influence field drain N2O dynamics, with mean concentrations from drains in clay loam soils (5.3 μg N L?1) being greater than drains in sandy loam soils (4.0 μg N L?1). Soil texture also impacted upon the relationships between field drain N2O concentrations and other water quality parameters (pH, flow rate, and nitrate (NO3) and nitrite (NO2) concentrations), highlighting possible differences in N2O production mechanisms in different soil types. Catchment antecedent moisture conditions influenced the storm event mobilisation of N2O in both field drains and streams, with the greatest concentration increases recorded during precipitation events preceded by prolonged wet conditions. N2O concentrations also varied seasonally, with the lowest mean concentrations typically occurring during the summer months (JJA). Nitrogen fertiliser application rates and different soil inversion regimes were found to have no effect on dissolved N2O concentrations, whereas higher N2O concentrations recorded in field drains under a winter cover crop compared to fallow fields revealed cover crops are an ineffective greenhouse gas emission mitigation strategy. Overall, this study highlights the complex interactions governing the dynamics of dissolved N2O concentrations in field drains and headwater streams in a lowland intensive agricultural catchment. 相似文献
24.
A surface-modified amorphous manganese oxide (SM-AMO) was prepared to increase the stability of a previously studied promising stabilizing agent and to compare its immobilizing efficiency with respect to contaminating metals with the original material. To synthesize the SM-AMO, the AMO surface was synthetically covered with a coating of MnCO3 because newly formed rhodochrosite precipitates were previously found to increase the stability of AMO particles in soils. A preliminary experiment evaluating the long-term stability of both materials in pure water suggested higher stability for the SM-AMO particles, showing a smaller release of Mn compared to the original AMO. An adsorption kinetics study focused on As, Cd, Pb and Zn showed lower adsorption rates and adsorption capacity for Zn, probably as a result of partial surface passivation. In comparison to these results for simple controlled systems, different effects were recorded when the two materials were applied to contaminated soils. When incubated in soil, a constantly lower mass loss was recorded in the case of SM-AMO. There were no significant differences in the release of Mn and DOC into the soil solution or in the stabilizing efficiency with regard to contaminating metal(loid)s between the original and surface-modified materials. Concerning the potential solid phase transformations in soil conditions, we observed a gradual equilibration between the surface composition of both materials. While the newly formed rhodochrosite precipitated on the AMO surface, the MnCO3 coatings on SM-AMO gradually dissolved. Both amendments also effectively supported microbial activity, especially in the more contaminated soil sample. Thus, despite the smaller mass loss, the effectiveness of both materials is comparable in the long term. 相似文献
25.
通过在水基钻井液中添加纳米氧化物颗粒,来改善其润滑性能。选用了Al2O3、TiO2及SiO2三种纳米氧化物颗粒作为钻井液改性添加剂,利用极压润滑仪及销盘摩擦磨损实验仪研究了添加不同种类和浓度纳米颗粒氧化物对水基钻井液的润滑性能及减摩效果的影响。结果表明,所添加的3种球形纳米颗粒均有润滑减阻减摩的效果,其中SiO2颗粒的钻井液的润滑性能、滤失性能以及减摩效果都是最好的。当纳米SiO2颗粒加量为0.4%时,滤失量最小,润滑系数降低率达到15.6%,摩擦系数降低率为32.0%,磨痕轨迹平滑,减摩效果最为明显。当纳米颗粒浓度超过0.5%时,减摩效果降低,磨盘脱层磨损明显。 相似文献
26.
The Three Gorges are considered to be critical to understand the formation of Yangtze River. Recent research results suggest that the Yangtze Three Gorges was created during the Quaternary but the exact time is debatable. Fe–Ti oxide minerals are seldom used to study sediment provenance, expecially using scanning electron microscopy(SEM), and energy dispersive spectrometer(EDS). In this study, the provenance of Quaternary sediments in Yichang area, which is located to the east of the Yangtze Three Gorges, was investigated by using SEM and EDS to research Fe–Ti oxides. The Panzhihua vanadium titanomagnetite and Emeishan basalt outcrop are located to the west of the Three Gorges. Further, the materials from them are observed in the Quaternary sediments of Yichang area. Fe–Ti oxide minerals from the Huangling granite are observed in the Yunchi and Shanxiyao Formations, which were formed before 0.75 Ma B.P., whereas Fe–Ti oxide minerals from the Huangling granite, Panzhihua vanadium titanomagnetite, and Emeishan basalt are observed in the riverbed and fifth-terrace sediments of the Yangtze River, which were formed after 0.73 Ma B.P.. Thus, we can infer that the Three Gorges formed after the deposition of the Shanxi Formation and before the fifth-terrace; i.e., 0.75–0.73 Ma B.P.. 相似文献
27.
28.
邻菲啰啉助溶保护-容量法测定电气石中的氧化亚铁 总被引:1,自引:0,他引:1
电气石是一种含硼的铝硅酸盐矿物,是成岩成矿作用的灵敏示踪剂,通过测试围岩电气石中氧化亚铁的含量,可以鉴定电气石所赋存岩床的成因。但是,国内外目前尚无系统的电气石化学成分标准分析方法及电气石化学成分标准物质,针对电气石中氧化亚铁分析方法的研究更是罕有报道。该文通过在溶矿过程中加入邻菲啰啉,与亚铁形成络合物,对亚铁能够起到较好的助溶和保护效果,建立了在聚四氟乙烯坩埚中加入邻菲啰啉、氢氟酸及硫酸,中高温电热板加热溶矿,重铬酸钾容量法测定电气石中的亚铁的方法。该方法的精密度(RSD,n=12)为0.59%,样品加标回收率为99.3%~100.9%。经过多家实验室间的比对实验,证明该方法准确度高,具有较好的应用效果。 相似文献
29.
利用Minolta CM-2002光谱光度计对南黄海陆架136个短柱样品20~25cm段的沉积物进行了颜色反射率数据测量,通过化学元素、粒度、磁化率等来确定影响沉积物颜色反射率变化的成分和因素,介绍了反射率光谱的一阶导数和因子分析的方法。分析结果表明,南黄海陆架沉积物颜色反射率受控于沉积物中的铁氧化物、有机质以及粘土矿物和钙质生物碎屑含量。主因子F1指示氧化环境,并与陆源物质相关,F2指示弱氧化环境,F3指示弱还原环境,主因子的波长范围分别是405~445 nm和495~595 nm,605~695 nm,445~485 nm。 相似文献
30.