全文获取类型
收费全文 | 65279篇 |
免费 | 22898篇 |
国内免费 | 47294篇 |
专业分类
测绘学 | 5239篇 |
大气科学 | 26143篇 |
地球物理 | 19340篇 |
地质学 | 49730篇 |
海洋学 | 18638篇 |
天文学 | 2295篇 |
综合类 | 7732篇 |
自然地理 | 6354篇 |
出版年
2024年 | 305篇 |
2023年 | 880篇 |
2022年 | 1992篇 |
2021年 | 2522篇 |
2020年 | 3281篇 |
2019年 | 6712篇 |
2018年 | 7136篇 |
2017年 | 6724篇 |
2016年 | 7068篇 |
2015年 | 6105篇 |
2014年 | 6018篇 |
2013年 | 6518篇 |
2012年 | 6093篇 |
2011年 | 5978篇 |
2010年 | 5929篇 |
2009年 | 5264篇 |
2008年 | 4427篇 |
2007年 | 4377篇 |
2006年 | 3801篇 |
2005年 | 3618篇 |
2004年 | 3529篇 |
2003年 | 3282篇 |
2002年 | 2990篇 |
2001年 | 2819篇 |
2000年 | 2640篇 |
1999年 | 3083篇 |
1998年 | 2829篇 |
1997年 | 2815篇 |
1996年 | 2362篇 |
1995年 | 2190篇 |
1994年 | 1951篇 |
1993年 | 1788篇 |
1992年 | 1471篇 |
1991年 | 1168篇 |
1990年 | 995篇 |
1989年 | 863篇 |
1988年 | 740篇 |
1987年 | 531篇 |
1986年 | 448篇 |
1985年 | 345篇 |
1984年 | 356篇 |
1983年 | 228篇 |
1982年 | 241篇 |
1981年 | 177篇 |
1980年 | 151篇 |
1979年 | 128篇 |
1978年 | 58篇 |
1977年 | 54篇 |
1971年 | 53篇 |
1958年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
51.
本矿床是我国金红石矿床重要的成因及工业类型,为具有特定化学成分的岩石在特定变质条件下形成的。本文通过对大量数据的处理,查明了化学成分与成矿的关系:高Al2O3、MgO、Fe2O3+FeO、K2O、P2O5、H2O+、MnO,低SiO2、CaO、Na2O、CO2、S等,利于钛富集;高TiO2、MgO、P2O5、S,低SiO2、CaO、K2O、Al2O3、Fe2O3/FeO,利于金红石形成;高Sc、Zr、Ni、Zn及La/Yb,低Y、Nb、Th、Cd、Ba、Be、ΣREE,利于金红石纯度提高。对研究矿床成因及寻找新的同类矿床等有重要意义 相似文献
52.
严格论证了在对空间或地面目标的定位中,利用伪距测量观测模型的几何因子大于用台站间一次差分观测模型的几何因子.此外,台站间完全差分测量观测的定位精度优于不完全测量情形的定位精度.这些结果给人们在实际工作中采取何种定位方式提供了可靠的理论依据. 相似文献
53.
Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment. 相似文献
54.
Quasi-electrostatic electron and ion-cyclotron instabilities are studied. The result indicates that the higher harmonic ion cyclotron instabilities (ICI) can be excited while the fast ions produced from reconnection are injected into a coronal loop. Part of the energetic ions can be dragged out of the magnetic mirror turning points and a negative plasma potential is generated. The plasma potential may directly accelerate the electrons up to the relativistic velocity within a short time. This acceleration is similar to the processes occurring in the magnetic mirror devices of controlled thermonuclear fusion. The spectrum and flux of accelerated electrons have also been obtained. Some observational results during the solar flare might be explained by this acceleration mechanism. 相似文献
55.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
56.
57.
The purpose of this work was to reinvestigate the existing hydrogeological conceptual model of the basin of Madrid, Spain. A cumulative chemical isotopic diagram which enabled the distinction between different groups of water as well as calculation of the mode of their blending was used for this investigation. It was found that the groups of discharge were lighter in their isotopic composition than that of recharge. The previous explanation of this fact, backed by carbon-14 dating, was the long residence time due to flow lines going down to depths of more than 1000 m. This flow model assumes homogenous conditions to these depths. This assumption can not be supported by evidence from deep wells. Thus a modified model is suggested which maintains homogenous conditions only to about 300 m and a deep confined aquifer below containing paleowater. The higher degree of depletion of this water has been explained by a colder climate on top of an altitude effect. Another interesting observation was the correlation between the isotopic composition of the rains, the month of the rain event and the composition of the recharge group groundwater. It could be seen that the winter rains resemble the groundwater composition, which shows that practically all the spring and summer rains were evapotranspirated. 相似文献
58.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献
59.
The sand–loess transition zone in north China is sensitive to climate change, and is an ideal place to investigate past environmental changes. However, past climate change at millennial–centennial timescales in this region has not been well reconstructed because of limited numerical dating. Alternations of sandy loam soils with aeolian sand layers in the Mu Us and Otindag sand fields, which lie along the sand–loess transition zone, indicate multiple intervals of dune activity and stability. This change is probably a response to variations of the East Asian monsoon climate during the late Quaternary. The single aliquot regeneration (SAR) optically stimulated luminescence (OSL) dating protocol, which has been successfully applied to aeolian deposits worldwide, is applied to these two sand fields in this study. The OSL ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale. Sections in both sand fields contain aeolian sand beds recording millennial‐scale episodes of dry climate and widespread dune activation, including episodes at about the same time as Heinrich Event 5 and the Younger Dryas in the North Atlantic region. These results demonstrate the potential of aeolian sediments in semi‐arid north China to record millennial‐scale climatic events, and also suggest that dry–wet climate variation at the desert margin in China may be linked to climatic change elsewhere in the Northern Hemisphere, through atmospheric circulation. This article was published online on 27 November 2008. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected (16 December 2008). Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
60.
Technique for Controlling Spread of Limnotic Oncomelania 总被引:1,自引:0,他引:1
Schistosomiasis is a parasitic disease mostly found in areas along the Changjiang River of China. The disease is spread solely through an intermediary named oncomelania, so its spread of schistosomiasis can be controlled by properly designing water intakes which prevent oncomelania from entering farming land or residential areas. This paper reports a successful design process and a new oncomelania-free intake device. The design of the new intake is based on a sound research program in which extensive experimental studies were carried out to gain knowledge of oncomelania eco-hydraulic behavior and detailed flow field information through CFD simulation. 相似文献