全文获取类型
收费全文 | 3662篇 |
免费 | 526篇 |
国内免费 | 857篇 |
专业分类
测绘学 | 143篇 |
大气科学 | 263篇 |
地球物理 | 639篇 |
地质学 | 2510篇 |
海洋学 | 393篇 |
天文学 | 57篇 |
综合类 | 238篇 |
自然地理 | 802篇 |
出版年
2024年 | 9篇 |
2023年 | 42篇 |
2022年 | 120篇 |
2021年 | 117篇 |
2020年 | 138篇 |
2019年 | 160篇 |
2018年 | 123篇 |
2017年 | 131篇 |
2016年 | 142篇 |
2015年 | 157篇 |
2014年 | 181篇 |
2013年 | 219篇 |
2012年 | 224篇 |
2011年 | 213篇 |
2010年 | 171篇 |
2009年 | 218篇 |
2008年 | 217篇 |
2007年 | 242篇 |
2006年 | 272篇 |
2005年 | 220篇 |
2004年 | 198篇 |
2003年 | 173篇 |
2002年 | 174篇 |
2001年 | 169篇 |
2000年 | 150篇 |
1999年 | 135篇 |
1998年 | 110篇 |
1997年 | 118篇 |
1996年 | 79篇 |
1995年 | 86篇 |
1994年 | 61篇 |
1993年 | 52篇 |
1992年 | 39篇 |
1991年 | 25篇 |
1990年 | 24篇 |
1989年 | 35篇 |
1988年 | 26篇 |
1987年 | 17篇 |
1986年 | 19篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1979年 | 6篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有5045条查询结果,搜索用时 15 毫秒
1.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars. 相似文献
2.
根据印度洋西北部鸢乌贼(Sym plectoteuthis oualaniensis)作业渔场的现场调查数据.分析了作业水深、作业时段、摄食等级和月相等因子对手钓上钩率的影响。统计表明.晚上、午夜和凌晨3个不同时段和不同作业水深对手钓上钩率均有显著性影响。3个时段中,其摄食等级呈先低后高再低的现象。午夜和凌展,上钩率的变化趋势与摄食等级的变化趋势呈显著的负相关。在月相的影响下.离朔日三四天之前,手钓产量呈上升趋势;离望日五六天之前。手钓产量呈明显的下降趋势。 相似文献
3.
地震前后垂直形变场动态演化的量化指标 总被引:3,自引:2,他引:1
提出一种表达垂直形变场动态演化过程的量化指标——区域应变率、应变集中度。在此基础上,对南北地震带各水准监测区近30年的垂直形变资料进行了实际计算.并结合具体震例进行了对比研究。结果表明:量化指标在一定程度上能够反映地震前后形变场的动态演化过程和地壳运动状态,对地震的中长期预报有一定的积极意义。 相似文献
4.
A new technique designed to help quantify the degree of damage to the landscape from one area to another shows a close relationship between population density and the degree of landscape damage. The technique establishes a scale of damage from 0 to 5 (zero = no damage; 5 = severe damage) using data from aerial photographs, land-use maps, and field data. The related formula allows one to compare the relative degree of damage across regions using a combination of an absolute index, a theoretical index, a relative index, and population density. Xing'an County is used to demonstrate the technique. 相似文献
5.
Joachim Köppen 《Astrophysics and Space Science》2003,284(2):837-840
Representative results from a comparison of the chemical evolution of spherical collapse models without and with a intercloud
medium are presented. The hot metal-rich gas distributes quickly the metals produced in supernovae throughout the galaxy,
thus leading to a more homogeneous chemical evolution and flatter metallicity gradients in the gas and the stars. The stellar
population is somewhat less concentrated towards the centre. The strong outflow results in a substantial loss of metals from
the galaxy to its surroundings, and a lower effective yield in the galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
A constitutive relation is derived for describing the mechanical response of chalk. The approach is based on a phenomenological framework which employs chemo‐plasticity. The properties of the material are assumed to be affected by the physico‐chemical processes that occur through the interaction between the skeleton and the pore fluid. The underlying mechanism is discussed by invoking a micromechanical analysis. The performance of the framework is illustrated by examining the evolution of mechanical characteristics in the presence of different pore fluids. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
7.
The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
8.
This study examines the depletion of ferromagnesian silicate minerals from a sequence of thin, distal, mainly rhyolitic tephra layers of Holocene age preserved in an acid peat bog (Kopouatai), North Island, New Zealand. The rate of such depletion has been fast, as indicated by the complete loss of biotite from one tephra layer (Kaharoa Tephra), in which it is normally dominant, in only ca. 770 yr. Chemical dissolution is advocated as the likely cause for the depletion, with amphiboles and other mineral grains commonly showing etch pits, microcaves, and other characteristic surface solution features. Theoretical thermodynamic and kinetic models show a marked increase in the rate of dissolution of all ferromagnesian minerals under conditions of low pH (< 4), but that where silica concentrations in solution are high the relative proportions of minerals remaining are unaffected. However, where concentrations of dissolved silica are low, as in most bog environments, the relative proportions of ferromagnesian minerals are affected as well as absolute amounts being decreased. Amphiboles are depleted relative to pyroxenes, consistent with kinetic studies. The results show that the identification and correlation of tephras on the basis of relative abundances of ferromagnesian minerals alone may be unreliable, and emphasise the need to use multiple criteria in such studies. 相似文献
9.
I INTRODUCTIONWetland has very important effect on retainingwater and against drought, regUlating climate, controlling soil erosion and so on ac. Today men often attachimportance to economic effect of wetland reclamation,but neglect and destroy other functions and effectswhich result in blind wetland development, a lot ofwetland degeneration. The area of freshwater wetland inthe 1950s was 134 x 10'ha, but in the 1970s 70% hadchanged into farmland (MITSCH, 1993). The area ofwetland in d… 相似文献
10.
Olga Hadz̆ija Mladen Jurac̆ić Marija Luić Maja Tonković Biserka Jeric̆ević 《Estuarine, Coastal and Shelf Science》1985,21(5):701-709
The investigation of mineral, granulometric and chemical composition of sediments of the River Krka estuary (Yugoslavia) were performed in order to elucidate the origin of the sediments and the pattern of sedimentation. Estuarine surface sediments were found to be fine-grained with a bimodal distribution. Environmental conditions in estuarine sediments favour conservation of the organic matter (anoxic conditions). The carbohydrates in the sediments were investigated to determine whether they are of terrigenous or authigenous origin. Glucose, galactose, mannose, xylose, rhamnose, glucosamine and glucuronic acid were detected in the sediments. Their mutual relationship indicates a preferentially terrigenous source of sedimented organic material in estuarine sediments. 相似文献