全文获取类型
收费全文 | 26091篇 |
免费 | 289篇 |
国内免费 | 945篇 |
专业分类
测绘学 | 1472篇 |
大气科学 | 2104篇 |
地球物理 | 4862篇 |
地质学 | 12178篇 |
海洋学 | 1172篇 |
天文学 | 1955篇 |
综合类 | 2165篇 |
自然地理 | 1417篇 |
出版年
2023年 | 12篇 |
2022年 | 7篇 |
2021年 | 38篇 |
2020年 | 48篇 |
2019年 | 65篇 |
2018年 | 4812篇 |
2017年 | 4083篇 |
2016年 | 2655篇 |
2015年 | 306篇 |
2014年 | 154篇 |
2013年 | 169篇 |
2012年 | 1062篇 |
2011年 | 2827篇 |
2010年 | 2100篇 |
2009年 | 2412篇 |
2008年 | 1978篇 |
2007年 | 2432篇 |
2006年 | 136篇 |
2005年 | 254篇 |
2004年 | 467篇 |
2003年 | 461篇 |
2002年 | 288篇 |
2001年 | 79篇 |
2000年 | 89篇 |
1999年 | 40篇 |
1998年 | 48篇 |
1997年 | 15篇 |
1996年 | 22篇 |
1995年 | 14篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 30篇 |
1980年 | 23篇 |
1979年 | 7篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1974年 | 4篇 |
1972年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
291.
Shin-Ichi Machida Hisako Hirai Taro Kawamura Yoshitaka Yamamoto Takehiko Yagi 《Physics and Chemistry of Minerals》2007,34(1):31-35
High-pressure Raman studies of methane hydrate were performed using a diamond anvil cell in the pressure range of 0.1–86 GPa
at room temperature. Raman spectra of the methane molecules revealed that new softened intramolecular vibration mode of ν
1 appeared at 17 GPa and that the splitting of vibration mode of ν
3 occurred at 15 GPa. The appearance of these two modes indicates that an intermolecular attractive interaction increases between
the methane molecules and the host water molecules and between the neighboring methane molecules. These interactions might
result in the exceptional stability of a high-pressure structure, a filled ice Ih structure (FIIhS) for methane hydrate, up
to 40 GPa. At 40 GPa, a clear change in the slope of the Raman shift versus pressure occurred, and above 40 GPa the Raman
shift of the vibration modes increased monotonously up to 86 GPa. A previous XRD study showed that the FIIhS transformed into
another new high-pressure structure at 40 GPa. The change in the Raman spectra at 40 GPa may be induced by the transition
of the structure. 相似文献
292.
M. Tamzid Rahman T. Kameda S. Kumagai T. Yoshioka 《International Journal of Environmental Science and Technology》2018,15(2):263-272
Health hazards from heavy metal pollution in water systems are a global environmental problem. Of similar concern is sludge that results from wastewater treatment due to unsatisfactory sludge management technology. Therefore, the effectiveness of using Mg–Al-layered double hydroxide in the removal of heavy metals from mine wastewater was tested and compared with that of calcium hydroxide [Ca(OH)2], which is a common treatment method for heavy metal removal. Initially, the mine wastewater contained cations of the heavy metals iron (Fe), zinc (Zn), copper (Cu), and lead (Pb). The Mg–Al-layered double hydroxides were able to remove 371, 7.2, 121, and 0.4 mg/L of these pollutants, respectively, using the co-precipitation method. The removal of these metals is most effective using 0.5 g Mg–Al-layered double hydroxide (Mg/Al molar ratio 4) and 20 min of shaking. Zn was removed by the formation of Zn(NO3)(OH)·H2O and Zn5(NO3)2(OH)8 when LDH, Mg/Al molar ratios of 4 and 2, respectively, were used. Similarly, Fe, Cu, and Pb were removed by the formation of Fe–Al-layered double hydroxide, Cu2(OH)3·NO3 and Pb4(OH)4(NO3)4, respectively. While Ca(OH)2 is also capable of reducing the heavy metal concentrations below the Japanese recommended values, this analysis shows that using 0.5 g Mg–Al-layered double hydroxide is a better treatment condition for mine wastewater, because it generates lower sludge volumes than 0.1 g of Ca(OH)2. The measured sludge volume was 1.5 mL for Mg–Al-layered double hydroxide and 2.5 mL for Ca(OH)2, a nearly twofold further reduction. 相似文献
293.
Rainfall, throughfall, stemflow and spring were studied in a secondary forest during a wet season from April to August in
2006. Some of the chemicals in throughfall, stemflow and spring were increased in contrast with incident rainfall. Specifically,
Cl−, HCO3
−, Na+ and Ca2+ were leached negatively in throughfall, but K+ and Mg2+ were leached positively. In stemflow, Cl− and Na+ were leached negatively, the others were leached positively and their concentrations were higher than those in throughfall.
Total carbon, organic carbon and inorganic carbon in throughfall and stemflow were increased as rainfall went through the
secondary forest. The concentration of free CO2 in rainfall was lower than both, throughfall and stemflow; the relationship between total acidity and free CO2 was linear. pH of throughfall and stemflow , such as maximum, minimum and mean, were lower than that of rainfall and the
extent of pH in spring was changed minimally. We came to a conclusion that rainfall via the secondary forest can lead to further
erosion, accelerate the biogeochemical cycle in epikarst zone, enhance the effective state of alkali elements in the soil,
supply vegetation with more nutrients and advance vegetation’s growth and succession, which are reasonably sufficient to form
a stable karst ecosystem. 相似文献
294.
Flow slides in municipal solid waste (MSW) landfills are common geoenvironmental issues in the urban environment and can pose a serious threat to the surrounding population and infrastructure. Prediction of the maximum run-out distance of flow slides in MSW landfills is therefore an essential part of hazard and risk assessment in engineering design. Based on the framework for simple analysis originally developed by Hungr (1995), we propose a simplified analytical model for calculating dam breaks in a plastic fluid along a single inclined base. In the proposed model, a quarter-elliptical shape is used to describe the approximate configuration of the flow slide. Following this step, the physical laws relating to the conservation of mass and energy are used to calculate the potential flow. Of additional note is a boundary condition in mathematics relating to this simplified analytical model, which is also reported in this study. Taking the obvious mobility characteristics of the MSW at point of failure into consideration, a three-phase simplified model along double inclined bases has been further developed for run-out prediction of the flow slide in MSW landfill. The proposed three-phase model is then applied to estimate the maximum run-out distance of two typical flow failures of landfills located in Sarajevo and Bandung, which demonstrate the capability of the proposed simplified analytical model for use in hazard assessments of landfills. 相似文献
295.
Successional stages of biological soil crusts and their microstructure variability in Shapotou region (China) 总被引:5,自引:1,他引:5
In order to investigate succession of biological soil crusts (BSCs) and their microstructure variability, we conducted this
work in Shapotou revegetation region at the southeast edge of Tengger Deser. The results showed that BSCs generally succeeded
as a pathway of “Algae crusts, algae–lichen crusts, lichen crusts, lichen–moss crusts and moss crusts”. Occasionally mosses
directly occurred on algae crusts, and BSCs succeeded from algae crusts to moss crusts. Crust vertical stratification was
a common phenomenon, from top to bottom an inorganic layer, algae-dense layer and algae-sparse layer were divided in algae
crusts; a thallus layer, rhizoid layer and sub-rhizoid layer in lichen crusts; a “stem-leaf” layer, rhizoid layer and sub-rhizoid
layer in moss crusts, respectively. The main crust binding organisms varied from filamental cyanobacteria (dominated by Microcoleus) in algae crusts to lichen rhizoids, free-living cyanobacterial filaments and fungal hyphaes in lichen crusts, and to moss
rhizoids and fungal hyphaes in moss crusts. The dominant phototrophic organisms varied from Microcoleus (algae) in algae crusts to Collema (lichens) in lichen crusts, and to Bryum (or Didymodon and Tortula; mosses) in moss crusts. Total phototrophic biomass increased while the free-living algal biomass decreased with the succession
of BSCs. In addition, exopolysaccharides and fine particles accumulated in the course of development and succession of BSCs,
all of which lead to a gradual increase in crust thickness and porosity, while decrease in the bulk density. 相似文献
296.
Adsorption characteristics of perchloroethylene in natural sandy materials with low organic carbon content 总被引:2,自引:0,他引:2
Laboratory batch tests were conducted to investigate the sorption isotherms and sorption kinetics of the chlorinated hydrocarbon
perchloroethylene (PCE) in five natural sandy materials with an organic carbon content (f
oc) in the range 0.080–0.540%. The amended non-linear dual-mode model can describe the sorption isotherms in materials with
f
oc in the range 0.080–0.090%. For a sample with a much higher f
oc of 0.54%, the absorption isotherm was found to fit a linear model. These results may indicate that organic carbon is not
the main factor influencing the sorption isotherm. The sorption kinetics of PCE in samples with f
oc in the range 0.080–0.090% are not first-order and are different from those observed in the samples with higher f
oc. The sorption process in the materials with lower f
oc involves fast sorption, fast desorption and an equilibrium stage. The results may imply that the factors affecting sorption
kinetics of PCE in low f
oc media are pore filling and capillary condensation rather than organic carbon content. 相似文献
297.
Land subsidence caused by groundwater exploitation in Suzhou City,China 总被引:10,自引:2,他引:10
Suzhou City, located at the lower reaches of the Yangtze River in southeastern Jiangsu Province, is one of the few cities
in China which suffer from severe ground settlement. A research project was carried out to investigate this problem. Geological
and hydrogeological studies show that there is a multi-layered aquifer system with three distinct, soft mud layers of marine
and lagoonal origins. An examination of historical records of groundwater extraction, water levels, and ground settlement
shows that the ground subsidence is associated with the continuously increasing groundwater extraction in the deep, confined
aquifer. It is believed that the consolidation of the soft mud layers, especially the third layer which is thick and close
to the main pumped aquifer, contributes to the ground settlement. A three-dimensional finite difference numerical model representing
the multi-layered aquifer system was developed to study the ground settlement in response to groundwater extraction. By calibrating
the model with both the measured groundwater level and ground settlement, the aquifer parameters were estimated. The model
outputs fit reasonably well with the observed results, which indicates that the numerical model can reproduce the dynamic
processes of both groundwater flow and soil consolidation. The hydraulic conductivity of the third mud layer near the center
of the ground settlement has been reduced by over 30% in the last 14 years. The gradual deterioration in the hydraulic conductivity
of the mud may have significant adverse effect on the sustainable groundwater resource of the deep confined aquifer, since
the recharge from the shallow aquifers through the mud layer is the only source of water to the deep aquifer. An analysis
of the spatial distributions of groundwater drawdown and ground settlement shows that the area with maximum drawdown is not
necessarily the area with maximum ground settlement due to the occurrence of the soft mud layer. A simple reallocation in
pumping rates on the basis of the spatial distribution of the thick mud layer could significantly reduce the ground settlement.
Electronic Publication 相似文献
298.
It is generally accepted that the compositions and properties of soil organic matter (SOM) are influenced by many factors. In order to reveal the effects of soil texture on characteristics and dynamics of SOM and its sub-fraction, humic acid (HA), along two soil profiles, a yellow soil profile and a purplish soil profile, under the same climate and vegetation conditions were determined. Results indicate that the decomposition and humification degrees of SOM and HA of the purplish soils are higher than those of the corresponding yellow soils indicated by A/O–A ratios of HAs, TOCs and HA yields of bulk soil samples, nevertheless, the development degree of the purplish soil is lower than that of the yellow soil. The variations of E4/E6 ratios of HAs along the soil profiles indicate the overall molecular sizes of HAs decreased downward along the soil profiles. A/O–A ratios of HAs decreased downward along both the soil profiles indicate that humification processes decrease downward along both the soil profiles. Leaching of SOM shows significant effects on the distribution and characteristics of HAs in the yellow soil profile but the purplish soil profile, which is consistent with the higher hydrophobicity of HAs in purplish soils, shows that the distribution characteristics of SOM along the soil profiles are a complex result of the combination of soil texture and characteristics of SOM itself. The remarkably different sand contents are concluded tentatively as one of reasons to the different distributions and dynamics of HAs along the soil profiles, however, to profoundly understand the evolution and transport of SOM along soil profiles needs more researches. 相似文献
299.
We report for the first time the occurrence of rare phosphate wagnerite as a stable phase from the Mg–Al granulites of Sonapahar. The wagnerite bearing assemblages consist of the spinel, phlogopite, brucite and corundum. The wagnerite appears in the Mg–Al granulites due to the break-down of spinel and fluorapatite. The mineral chemistry of the phases has been discussed from the EPMA data, which reveals that the fluorine content of the wagnerite is relatively low due to the exchange of F to coexisting phases. The major oxide analysis of the rocks show the low content of Ca, which is the requisite for the occurrences of wagnerite. 相似文献
300.
Nishi Kant Prasoon Kumar Singh Bijendra Kumar 《Journal of the Geological Society of India》2018,91(1):67-75
This paper reports petrography, geochemistry and Rb-Sr age data on the rare metal bearing Neoarchean fertile (Nb-Ta) granite at Allapatna and elucidates its petrogenesis and role in Nb-Ta-Li-Be mineralization. The Allapatna granite (AG) intrudes the Tonalitic-Trondhjemitic - Granodioritic (TTG) Peninsular Gneiss and analysed SiO2 (72.3-75.6 wt%), K2O (4.0-5.7wt%), Na2O (3.0-4.4wt%), CaO (0.7-1wt%), MgO (0.13-0.25wt%) and K2O/Na2O (>1) indicating evolved nature. The presence of muscovite, biotite and garnet in the mode, peraluminous nature and high initial 87Sr/86Sr ratio (0.7284±0.0083) attest to their S-type characteristics. Varying Nb/Ta ratio and high Li with moderate abundance of Cs further indicate affinity to Li-Cs-Ta (LCT) type granite-pegmatite system. TheAG showing whole rock Rb-Sr isochron age of 2803± 68 Ma, is the oldest reported fertile granite in India parental to rare metal pegmatites hosting Nb-Ta, Be, and Li resources. Partial melting of a mixed source consisting of both basement TTG rocks and metapelites has generated such type of granitic magma. Fractionation of such granitic magma possibly has given rise to the rare metal (Ta-Nb-Li-Be) bearing pegmatites intruding the nearby schist belt. 相似文献