全文获取类型
收费全文 | 5023篇 |
免费 | 980篇 |
国内免费 | 1382篇 |
专业分类
测绘学 | 377篇 |
大气科学 | 851篇 |
地球物理 | 1262篇 |
地质学 | 2448篇 |
海洋学 | 1017篇 |
天文学 | 249篇 |
综合类 | 653篇 |
自然地理 | 528篇 |
出版年
2024年 | 58篇 |
2023年 | 127篇 |
2022年 | 244篇 |
2021年 | 304篇 |
2020年 | 269篇 |
2019年 | 293篇 |
2018年 | 336篇 |
2017年 | 305篇 |
2016年 | 331篇 |
2015年 | 260篇 |
2014年 | 297篇 |
2013年 | 296篇 |
2012年 | 383篇 |
2011年 | 332篇 |
2010年 | 333篇 |
2009年 | 294篇 |
2008年 | 239篇 |
2007年 | 268篇 |
2006年 | 220篇 |
2005年 | 205篇 |
2004年 | 138篇 |
2003年 | 133篇 |
2002年 | 104篇 |
2001年 | 116篇 |
2000年 | 165篇 |
1999年 | 219篇 |
1998年 | 155篇 |
1997年 | 186篇 |
1996年 | 129篇 |
1995年 | 117篇 |
1994年 | 110篇 |
1993年 | 108篇 |
1992年 | 75篇 |
1991年 | 56篇 |
1990年 | 48篇 |
1989年 | 32篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 12篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1958年 | 2篇 |
1957年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有7385条查询结果,搜索用时 0 毫秒
51.
地图作为地图数据库、地理信息系统、遥感等最基础和最重要的数据源 ,其精度问题已成为各学科共同关注的问题。本文提出了地图数字化的数学基础 ,详细讨论了两种常用的数据采集模型 :并以世界地图数字化系统为实例 ,介绍了系统的构成 ;并重点讨论了破译地图数学基础的原理和方法。 相似文献
52.
Acute toxicities of potassium permanganate, formalin, and Lugol’s iodine solution to a commonly occurred marine ciliate Pleuronema coronatum (Ciliophora, Scuticociliatida) were measured. Linear regression analysis of the results highlighted the close relationships
between doses of the medicines and mortalities of the organisms, thus providing a capability to predict toxicity effects from
the dose. Toxic effects of the medicines on the ciliates were described in the present paper, and the median lethal concentrations
(LC50 values) were given. Results of measurements indicated that 2 h-LC50 and 12 h-LC50 values of formalin on P. coronatum were 59.00×10−6 and 43.57×10−6, while those of Lugol’s solutions were 90.13 and 67.84×10−6 respectively. The tolerance of P. coronatum to formalin is apparently lower than that to Lugol’s iodine solution and potassium permanganate is a suitable medicine to
kill ciliates in short time. 相似文献
53.
The ability of several ab initio models to predict experimental 29Si-NMR chemical shift is examined. The shielding values of trimethylsilyl chloride (A), t-butyldimethylsilyl chloride (B)
and allytrimethylsilane (C) are calculated by GIAO, CSGT and IGAIM methods, using HF/6–31G*, B3LYP/6-31G*, HF/6–311+G**, B3LYP/6–311+G** and MPW1PW91/6–311+G** models respectively. The 29Si chemical shifts calculated by GIAO method using HF/6–311+G** model are highly in agreement with those obtained experimentally. All of the models above reproduce the trends of chemical
shifts in all cases studied, suggesting that the models are of practical value. 相似文献
54.
55.
Abstract Variational problem for irrotational, incompressible inviscid fluid in finite water depth is considered. Based on the variational principle, a special solution of the problem is presented under the assumption that the dispersion /u and the nonlinearity ?satisfied e = O(fj2) as the Lagrange function is expanded up to O(//). It is shown that the elevation of the free surface should be expanded to // order to ensure the Lagrange function is in fj* order. Comparison the nonlinear free surface profiles obtained from the solution with the corresponding ones obtained from linear solutions showed that the wave crest of the nonlinear wave is steepened but the trough is flattened compared to the linear wave as expected. 相似文献
56.
Biogenic silicate accumulation in sediments, Jiaozhou Bay 总被引:1,自引:0,他引:1
1 INTRODUCTION Silicate, or silicic acid (H4SiO4), is a very im- portant nutrient in the ocean. Unlike other major nu- trients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical req… 相似文献
57.
A two-dimensional, depth-integrated model proposed by Lynett and Liu (2002) was checked carefully, and several misprints in the model were corrected after detailed examination on both the theory and the numerical program. Several comparisons were made on wave profile, system energy and maximum wave amplitude. It is noted that the modified model can simulate the propagation of the internal solitary waves over variable bathymetry more reasonably to a certain degree, and the wave profiles obtained based on the modified model can better fit the experiment data reported by Helfrich (1992) than those from original model. 相似文献
58.
HAN Liang LI Bo SONG Tao TONG Lian-jun 《中国地理科学(英文版)》2006,16(2):148-153
1INTRODUCTION The circular-economyisan efficient resources metabolic process, based on principles of ecology and system engi- neering.Withcoupledstructure andthe complex ecolo- gical and economic evolution, it has the characteristics of overall integrity, coordination, and circulation (WANG, 2005). Compared with the traditionalecono- mic model, the circular-economy requires the technical model of production process to be changed from the opening flow pattern that is "resourcesconsumption… 相似文献
59.
Heesu Yang Jongchul Chae Eun-Kyung Lim Hyungmin Park Kyuhyoun Cho Ram Ajor Maurya Donguk Song Yeon-Han Kim Philip R. Goode 《Solar physics》2013,288(1):39-53
We investigated the velocity and temperature characteristics of an Ellerman bomb (EB) and its associated features based on observations made with the Fast Imaging Solar Spectrograph (FISS) and a broadband TiO filter of the 1.6 meter New Solar Telescope at Big Bear Solar Observatory. In the TiO images of the photospheric level, we found a granular cell expanding in two opposite directions near the site of the EB. When one end of this granule reached the EB site, the transverse speed of the tip of the expanding granule rapidly decreased and the EB brightened. The wings of the Hα profile of the EB indicated that the EB was blueshifted up to 7 km?s?1. About 260 s after the EB brightening, a surge was seen in absorption and varied from a blueshift of 20 km?s?1 to a redshift of 40 km?s?1 seen in the Hα and Ca ii 8542 Å lines. From the Doppler absorption width of the two lines determined by applying the cloud model, we estimated the mean temperature of the surge material to be about 29000 K and the mean speed of nonthermal motion to be about 11 km?s?1. We discuss the physical implications of our results in terms of magnetic reconnection and processes related to it. 相似文献
60.
依据1971-2010年地面观测气象数据,计算了三江源地区湿润指数。利用经验正交函数分解(EOF)和偏相关系数,对近40 a三江源地区干湿状况变化的时空特征及其影响因素进行了分析。结果表明:三江源地区干湿状况的变化在其北部与南部、东部与西部间存在明显反相位变化特征。北部和东部的部分区域分别在20世纪90年代和21世纪后表现出气候湿润化趋势,其余大部地区的持续干旱化趋势始于20世纪80年代初,其中南部与西部变干趋势显著,其湿润指数线性趋势率达到-8%/10 a。虽然三江源地区干湿状况主要决定于降水量和相对湿度的变化,但20世纪90年代中期后气温的显著上升,使得气温也成为关键的影响因子之一,即使在降水明显增加的背景下,也引起三江源主体区域湿润指数的明显下降。气候变暖情景下,北部和东部地区在近十几年暖湿化趋势明显,其余大部地区表现为不同程度的暖干化趋势。 相似文献