全文获取类型
收费全文 | 8560篇 |
免费 | 1786篇 |
国内免费 | 2746篇 |
专业分类
测绘学 | 1036篇 |
大气科学 | 1588篇 |
地球物理 | 1945篇 |
地质学 | 4964篇 |
海洋学 | 1242篇 |
天文学 | 282篇 |
综合类 | 848篇 |
自然地理 | 1187篇 |
出版年
2024年 | 66篇 |
2023年 | 193篇 |
2022年 | 515篇 |
2021年 | 612篇 |
2020年 | 464篇 |
2019年 | 595篇 |
2018年 | 567篇 |
2017年 | 546篇 |
2016年 | 538篇 |
2015年 | 529篇 |
2014年 | 628篇 |
2013年 | 672篇 |
2012年 | 638篇 |
2011年 | 648篇 |
2010年 | 570篇 |
2009年 | 580篇 |
2008年 | 554篇 |
2007年 | 460篇 |
2006年 | 463篇 |
2005年 | 401篇 |
2004年 | 301篇 |
2003年 | 243篇 |
2002年 | 209篇 |
2001年 | 181篇 |
2000年 | 199篇 |
1999年 | 240篇 |
1998年 | 207篇 |
1997年 | 222篇 |
1996年 | 173篇 |
1995年 | 133篇 |
1994年 | 155篇 |
1993年 | 118篇 |
1992年 | 106篇 |
1991年 | 56篇 |
1990年 | 76篇 |
1989年 | 42篇 |
1988年 | 45篇 |
1987年 | 28篇 |
1986年 | 20篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1965年 | 1篇 |
1958年 | 9篇 |
1957年 | 1篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
We analyse conditions of the innermost portion of an accretion disk and establish a set of equations for this region. A stable innermost region may exist, which can probably explain the observed UV and X-ray spectra, avoiding the unstable emission. We then discuss the detailed radial structure of a disk around a black hole for typical AGN parameters and obtain different kinds of- relationships for different regions of a disk. On the basis of this, we discuss the stability. A new type of cycle is present, which we call a double S shaped cycle. In this cycle, the extent of accretion rate variability is much larger than that in dwarf nova cycles. This probably solves the problem of violent variability of AGN. In the meantime, the very high accretion rate at the hottest state in limit cycles in the unstable region may provide continuous injection of matter to the jet and power the relativistic motion of the jet. 相似文献
102.
Suppose that ¯(x1),...,¯Z(xn). are observations of vector-valued random function ¯(x). In the isotropic situation, the sample variogram γ*(h) for a given lag h is $$\bar \gamma ^ * (h) = \frac{1}{{2N(h)}}\mathop \sum \limits_{s(h)} (\overline Z (x_1 ) - \overline Z (x_1 )) \overline {(Z} (x_1 ) - \overline Z (x_1 ))^T $$ where s(h) is a set of paired points with distance h and N(h) is the number of pairs in s(h).. For a selection of lags h1, h2, .... hk such that N (h1) > O. we obtain a ktuple of (semi) positive definite matrices $\bar \gamma ^ * (h_{ 1} ),. . . ., \bar \gamma ^ * (h_{ k} )$ . We want to determine an orthonormal matrix B which simultaneously diagonalizes the $\bar \gamma ^ * (h_{ 1} ),. . . ., \bar \gamma ^ * (h_{ k} )$ or nearly diagonalizes them in the sense that the sum of squares of offdiagonal elements is small compared to the sum of squares of diagonal elements. If such a B exists, we linearly transform $\overline Z (x)$ by $\overline Y (x) = B\overline Z (x)$ . Then, the resulting vector function $\overline Y (x)$ has less spatial correlation among its components than $\overline Z (x)$ does. The components of $\overline Y (x)$ with little contribution to the variogram structure may be dropped, and small crossvariograms fitted by straightlines. Variogram models obtained by this scheme preserve the negative definiteness property of variograms (in the matrix-valued function sense). A simplified analysis and computation in cokriging can be carried out. The principles of this scheme arc presented in this paper. 相似文献
103.
Diploptene: an indicator of terrigenous organic carbon in Washington coastal sediments 总被引:1,自引:0,他引:1
The pentacyclic triterpene 17 beta(H),21 beta(H)-hop-22(29)-ene (diploptene) occurs in sediments throughout the Columbia River drainage basin and off the southern coast of Washington state in concentrations comparable to long-chain plantwax n-alkanes. The same relationship is evident for diploptene and long-chain n-alkanes in soils from the Willamette Valley. Microorganisms indigenous to soils and soil erosion are indicated as the biological source and physical process, respectively, for diploptene in coastal sediments. Similarity between the stable carbon isotopic composition (delta 13CPDB) of diploptene isolated from soil in the Willamette Valley (-31.2 +/- 0.3%) and from sediments deposited throughout the Washington coastal environment (-31.2 +/- 0.5%) supports this argument. Values of delta for diploptene in river sediments are variable and 8-17% lighter, indicating that an additional biological source such as methane-oxidizing bacteria makes a significant contribution to the diploptene record in river sediments. Selective biodegradation resulting from a difference in the physicochemical association within eroded particles can explain the absence of the more-13C-depleted form of diploptene in Washington coastal sediments, but this mechanism remains unproven. 相似文献
104.
105.
106.
流域水文模型的研制是水文科学中最重要的分支之一。其中包括集总式和分布式流域水文模型,分布式流域水文模型的种类很多,在我国的研究和应用比较普遍。那棱格勒河是柴达木盆地最大的一条河流,正确评价该河流的水文情况,对开发柴达木盆地盐湖资源具有十分重要的意义。应用分布式流域水文模型理论,对适用于那棱格勒河流域的水文循环动力学模型的类型进行选择,通过分析对比,选择出最适合本地区特殊条件的TOPKAPI模型,以便于探讨该地区水文循环的特点与规律,从而阐明盐湖区各类水体间的相互补给关系。 相似文献
107.
The Xiangxi River is the first middling tributary of the Changjiang River near the Three Gorges Dam. The River is subject
to phosphorus pollution mainly from industrial wastewater. As the water quality of the Xiangxi River could directly influence
the water quality of the Three Gorges Reservoir, the research on phosphorus levels and its change in the sediment profile
of the Xiangxi River could provide useful information in the dynamic changes in the system, thereby offering options for mitigative
measures. Water and sediment samples from lower reaches of Xiangxi River were collected and the different forms of phosphorus
in sediments of the Xiangxi River were studied. The concentrations of total phosphorus in sediment ranged from 757.67 to 1438.54 mg/kg.
Inorganic phosphorus concentrations ranged from 684.63 to 1055.58 mg/kg. Phosphorus contamination was serious in some parts
of the Xiangxi River. With an average concentration of 635.17 mg/kg, calcium-bound phosphorus is the main form among different
inorganic phosphorus forms. Labile phosphorus and iron/aluminum-bound phosphorus measured 3.40, 0.05and 35.28 mg/kg, respectively.
The mobilization potential of phosphorus of sediments was studied through adsorption and release experiments. The equilibrium
concentration of phosphorus adsorption and release was around 0.1 mg/L. The initial concentrations of phosphorus in the overlying
water and the sediments have obvious effect on phosphorus mobilization potential. In addition, the release rate of phosphorus
in sediment increased with water depth. 相似文献
108.
基于微电子系统(MEMS)加速度地震勘探三分量数字检波器简介 总被引:4,自引:0,他引:4
基于微电子机械系统(MEMS)制成的微加速度三分量数字地震检波器。集数据采集、处理及传输功能干一体,在结构模型、工作原理、性能指标控制等方面不同于常规检波器,其信号幅值在低于自然频率范围内没有衰减。因其独特的制造技术,MEMS检波器具有超低噪声,大动态范围和极高的向量保真度性能。 相似文献
109.
合成并表征了新螯合树脂———邻苯二酚螯合树脂(XAD-2-Catechol),研究了XAD-2-Catechol吸附铝的特性和茜素红-铝的显色反应,在pH4.5的HAc-NaAc缓冲介质中,茜素红和铝(Ⅲ)反应生成红色络合物,λm ax=500 nm,铝的含量在0-50μg/25 mL内符合比耳定律。建立了邻苯二酚螯合树脂分离/富集-茜素红分光光度法测定天然水样中铝的新方法,对水样中铝形态进行测定,结果满意。 相似文献
110.
1IntroductionQuite a number of porphyry copper deposits havebeen found in the Cenozoic alkali-intrusions along theJinshajiang-Ailaoshan-Red River fault zone,whichconstitute the Cenozoic ore belt in China.The Cenozo-ic alkali-intrusive rocks are similar in… 相似文献