全文获取类型
收费全文 | 2678篇 |
免费 | 339篇 |
国内免费 | 1108篇 |
专业分类
测绘学 | 171篇 |
大气科学 | 155篇 |
地球物理 | 302篇 |
地质学 | 2769篇 |
海洋学 | 311篇 |
天文学 | 58篇 |
综合类 | 133篇 |
自然地理 | 226篇 |
出版年
2024年 | 13篇 |
2023年 | 50篇 |
2022年 | 105篇 |
2021年 | 123篇 |
2020年 | 103篇 |
2019年 | 161篇 |
2018年 | 114篇 |
2017年 | 139篇 |
2016年 | 127篇 |
2015年 | 137篇 |
2014年 | 181篇 |
2013年 | 210篇 |
2012年 | 173篇 |
2011年 | 205篇 |
2010年 | 171篇 |
2009年 | 202篇 |
2008年 | 219篇 |
2007年 | 209篇 |
2006年 | 202篇 |
2005年 | 160篇 |
2004年 | 151篇 |
2003年 | 132篇 |
2002年 | 119篇 |
2001年 | 118篇 |
2000年 | 85篇 |
1999年 | 90篇 |
1998年 | 87篇 |
1997年 | 59篇 |
1996年 | 44篇 |
1995年 | 41篇 |
1994年 | 36篇 |
1993年 | 39篇 |
1992年 | 23篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1977年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有4125条查询结果,搜索用时 31 毫秒
891.
In this paper, the three-dimensional thermal structure and the two-dimensional thermal stress caused by the horizontal inhomogeneity
of the thermal structure in the crust of the Bohai Sea and its surroundings were analyzed using the geothermal and the crust
structural data by means of the finite element method. As resulted, the horizontal distribution of the temperature in the
upper crust is obviously different from that in the lower crust in most part of the region. But the horizontal distribution
of the heat flow is constant in the crust. There is a belt where the thermal structure of the crust is intensely changed around
Tangshan. There are some different characteristics of the thermal stress of the crust around Tangshan, in the North-China
plain, in the Bohai Sea gulf, and in the middle part of the Bohai Sea. Utilizing those results, the distribution features
of the seismic activity in the region were explained in this paper.
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 29–35, 1992.
This project is supported by the Chinese Joint Seismological Science Foundation. 相似文献
892.
893.
Concentration levels of the trace elements Al, As, B, Ba, Be, Cd, Co, Cr, Cu, Li, Mo, Ni, Pb, Sr, V, Y, and Zn in the shallow Dutch ground water were studied. Data were extracted from the data base of the Dutch National Ground Water Quality Monitoring Network, a network set up to monitor the diffuse contamination. The network contains over 350 sites at a low density of about 1 per 100 km2. The sites are sampled once a year at two depths (approx. 10 and 25 m below surface). A two-step multivariate statistical approach was used, in which the major element chemistry was used to define water types. Within each water type, trace element behavior could be coupled to distinct geochemical processes: dilution, acidification and weathering, carbonate dissolution, oxidation/reduction, and ion exchange. In recently infiltrated acid rain water in low buffering capacity sands, the anthropogenic influence indirectly caused mobilization of Al (median 430 g/l), Cd (0.6 g/l), Co (14 g/l), Cu (2.7 g/l), Ni (16 g/l), Y (11 g/l), and Zn (50 mg/l). In carbonate bearing sediments the acidification is neutralized, and the mentioned trace elements remain immobile. Arsenic and Cr have higher concentrations levels in ground waters with a slightly reducing character and are possibly governed by the dissolution of iron-manganese hydroxides. Boron, Li, and Sr have high concentrations (respectively 875, 80, 2700 g/l) in the water type related to a seawater source. Strontium is related to carbonate dissolution in all other water types (medians ranging from 100 to 1000 g/l). Barium shows a complex behaviour. It is concluded that the high Al, Cd, Co, Cr, Ni, and Zn concentrations are anthropogenically induced. High Ba and Cr concentrations are inferred to be due to natural processes. 相似文献
894.
895.
Major and trace elements in groundwater from basaltic aquifers in pristine conditions were investigated in a volcanic island to evaluate sources, sinks, and mobility of elements over a wide range of mineralization conditions with total dissolved solids from 50 mg/L to 3400 mg/L. Groundwater was highly undersaturated with respect to primary silicate minerals, indicating that dissolution of basaltic rocks may continue under conditions with precipitation of calcite and secondary silicates. Evolution of B/Cl ratio in groundwater from marine aerosols to basaltic rocks showed that the ratio could be used as a conservative tracer for interactions between water and basaltic rocks. Relative mobility (RM) of elements calculated using the concentrations of elements in the local basaltic rocks and those in groundwater showed that mobility decreased in the order of B > Rb > Na > K > Mg > Ca > Mo > V > Si > Sr > Sc > P > U > Zn > Pb > Cr > Cu > Ba > Ni > Ti > (Mn, Al, Fe, Co, Th) indicating that oxyanion-forming elements and alkali metals had the highest mobility. Compared to average RM, V had decreased mobility, and Fe and Mn had increased mobility in anoxic groundwater while V, Mo, and U had higher mobility in oxic-alkaline water. The sources of V, Cr, Cu, and Zn in rocks were estimated using the partition coefficients between minerals and basaltic melt, and the disparity between sources and mobility indicated that sinks are more important for controlling the concentrations of these elements in groundwater than the contents in the rocks. Principal component analysis (PCA) of hydrogeochemical parameters in groundwater produced three principal components (PC) which represent dissolution of basaltic rocks without significant attenuation of released solutes, higher degree of water–rock interactions resulting in oxic-alkaline conditions, and attenuation of Zn and Cu in higher pH, respectively. Spatial distribution of PCs revealed that groundwater with elevated concentrations of mobile elements was concentrated in the southwestern area and that concentrations of V and Cr were more scattered, which is likely to be controlled by pH and redox states of groundwater as well as degree of water–rock interactions. 相似文献
896.
In this study, eight organic-rich rivers that flow through the Brazilian craton in the southwestern Amazon rainforest are investigated. This investigation is the first of its type in this area and focuses on the effects of lithology, long-term weathering, thick soils, forest cover and hydrological period on the dissolved load compositions in rivers draining cratonic terrain. The major dissolved ion concentrations, alkalinity (TAlk), SiO2, trace element concentrations, and Sr isotope contents in the water were determined between April 2009 and January 2010. In addition, the isotopic values of oxygen and hydrogen were determined between 2011 and 2013. Overall, the river water is highly dilute and dominated by the major dissolved elements TAlk, SiO2 and K+ and the major dissolved trace elements Al, Fe, Ba, Mn, P, Zn and Sr, which exhibit large temporal and spatial variability and are closely correlated with the silicatic bedrock and hydrology. Additionally, rainwater and recycled water vapor and the size of the basin contribute to the geochemistry of the waters. The total weathering flux estimated from our results is 2–4 t km−2.yr−1, which is one of the lowest fluxes in the world. The CO2 consumption rate is approximately 21–61 103 mol km−2 yr−1, which is higher than expected given the stability of the felsic to basic igneous and metamorphic to siliciclastic basement rocks and the thick tropical soil cover. Thus, weathering of the cratonic terrain under intertropical humid conditions is still an important consumer of CO2. 相似文献
897.
玻璃固化是一种常用的高放废液固化方法,其优点在于具有较高的抗化学介质侵蚀的能力和很好的辐照稳定性、热稳定性和机械稳定性,其不足之处在于抗水浸出等性能有所下降而使其安全性需要进一步通过抗浸出实验来进行衡量和确认。使用二次离子质谱(SIMS)分析玻璃固化体中的放射性成分(如铀元素)的分布及浸出行为等各项指标,是一种评估玻璃固化体抗浸出性能的分析手段。本文应用SIMS测试模拟玻璃固化体,以碳作为镀膜材料通过真空蒸发镀碳的方法优化样品制备条件,有效地解决了样品导电性差的问题,~(235)U/~(238)U同位素测定结果约为7.9‰±0.395‰,基本符合制作模拟样品时所使用的天然铀的同位素特征(~(235)U/~(238)U参考值约7.3‰)。研究表明,建立的方法实现了铀元素同位素丰度的测量,能直接显示铀的分布情况,该方法可为研究玻璃固化体中放射性元素的浸出行为提供一定的技术支持。 相似文献
898.
微波消解-双浊点萃取ICP-MS测定地球化学样品中的痕量铂钯钌铑 总被引:2,自引:2,他引:0
传统浊点萃取技术是将待测元素富集在黏稠的表面活性剂相中,溶液的黏度会对等离子体检测信号产生影响,通常使用甲醇作为稀释剂降低有机相黏度,有机成分也对等离子体的稳定性产生影响,同时有机物在进样管路上的吸附还会提高待测元素的记忆效应,因而限制了ICP-MS在浊点萃取中的应用。本文建立了双浊点萃取技术ICP-MS测定地球化学样品中铂钯钌铑的分析方法。样品用微波消解处理后,以DDTP为螯合剂,Triton X-114为表面活性剂,对消解溶液第一次浊点萃取,再在有机相中加入硝酸,通过加热完成第二次浊点萃取,使铂钯钌铑由有机相进入水相,铂钯钌铑的富集因子分别为45、33、18和35,高于单次浊点萃取的富集因子,ICP-MS检出限分别为0.05、0.02、0.10和0.03 μg/L。本方法通过两次浊点萃取过程实现了基体复杂的地球化学样品中痕量铂族元素的同时富集,提高了ICP-MS的稳定性。 相似文献
899.
S. Saminpanya 《Australian Journal of Earth Sciences》2016,63(7):873-884
Thai silicified woods were examined using electron probe microanalysis, yielding chemical data that characterised the samples into two groups: low and high silica contents (82—94 wt% and 94—98 wt%). The elements analysed in order of abundance include Si > Fe > Ca > Na > Al > Ti > K > Mg > Mn > Zr. Iron plays a major role in the colour range (red, orange, yellow, brown, grey and black) of the samples. Calcium is associated with Fe in the darker colours of the wood. Pseudo-crystallochemistry has been used for the substitution of trace elements for Si4+ in silica polymorphs. The atomic channels that run parallel to the c-axis of silica polymorphs or lattice defects, or even the charge balance for trivalent-ion substitution for Si4+, can accommodate monovalent ions (K+ and Na+). Vacant and atomic cavities, which are charged balanced by trivalent ions [Al3+ or Fe3+ substituting for Si4+], are commonly occupied by divalent ions (Ca2+, Mg2+ and Mn2+). Quadrivalent ions, Ti4+ and Zr4+ are non-structurally incorporated but form clusters of mineral inclusions in the samples. Several other trace-element contents are also in the form of mineral/fluid inclusions hosted in the woods. 相似文献
900.
A Comparison of In Situ Analytical Methods for Trace Element Measurement in Gold Samples from Various South African Gold Deposits
下载免费PDF全文
![点击此处可从《Geostandards and Geoanalytical Research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Christoph Gauert Mathias Schannor Lutz Hecht Martin Radtke Uwe Reinholz 《Geostandards and Geoanalytical Research》2016,40(2):267-289
Trace element concentrations in gold grains from various geological units in South Africa were measured in situ by field emission‐electron probe microanalysis (FE‐EPMA), laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and synchrotron micro X‐ray fluorescence spectroscopy (SR‐μ‐XRF). This study assesses the accuracy, precision and detection limits of these mostly non‐destructive analytical methods using certified reference materials and discusses their application in natural sample measurement. FE‐EPMA point analyses yielded reproducible and discernible concentrations for Au and trace concentrations of S, Cu, Ti, Hg, Fe and Ni, with detection limits well below the actual concentrations in the gold. LA‐ICP‐MS analyses required larger gold particles (> 60 μm) to avoid contamination during measurement. Elements that measured above detection limits included Ag, Cu, Ti, Fe, Pt, Pd, Mn, Cr, Ni, Sn, Hg, Pb, As and Te, which can be used for geochemical characterisation and gold fingerprinting. Although LA‐ICP‐MS measurements had lower detection limits, precision was lower than FE‐EPMA and SR‐μ‐XRF. The higher variability in absolute values measured by LA‐ICP‐MS, possibly due to micro‐inclusions, had to be critically assessed. Non‐destructive point analyses of gold alloys by SR‐μ‐XRF revealed Ag, Fe, Cu, Ni, Pb, Ti, Sb, U, Cr, Co, As, Y and Zr in the various gold samples. Detection limits were mostly lower than those for elements measured by FE‐EPMA, but higher than those for elements measured by LA‐ICP‐MS. 相似文献