全文获取类型
收费全文 | 5607篇 |
免费 | 1098篇 |
国内免费 | 1477篇 |
专业分类
测绘学 | 484篇 |
大气科学 | 1084篇 |
地球物理 | 1365篇 |
地质学 | 3068篇 |
海洋学 | 657篇 |
天文学 | 261篇 |
综合类 | 491篇 |
自然地理 | 772篇 |
出版年
2024年 | 70篇 |
2023年 | 176篇 |
2022年 | 359篇 |
2021年 | 416篇 |
2020年 | 306篇 |
2019年 | 401篇 |
2018年 | 364篇 |
2017年 | 331篇 |
2016年 | 337篇 |
2015年 | 334篇 |
2014年 | 362篇 |
2013年 | 362篇 |
2012年 | 346篇 |
2011年 | 313篇 |
2010年 | 277篇 |
2009年 | 315篇 |
2008年 | 267篇 |
2007年 | 256篇 |
2006年 | 259篇 |
2005年 | 216篇 |
2004年 | 156篇 |
2003年 | 157篇 |
2002年 | 153篇 |
2001年 | 141篇 |
2000年 | 160篇 |
1999年 | 220篇 |
1998年 | 149篇 |
1997年 | 163篇 |
1996年 | 144篇 |
1995年 | 103篇 |
1994年 | 134篇 |
1993年 | 93篇 |
1992年 | 66篇 |
1991年 | 39篇 |
1990年 | 60篇 |
1989年 | 33篇 |
1988年 | 34篇 |
1987年 | 22篇 |
1986年 | 16篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1975年 | 1篇 |
1965年 | 1篇 |
1958年 | 5篇 |
1954年 | 1篇 |
排序方式: 共有8182条查询结果,搜索用时 0 毫秒
241.
242.
243.
Using a theoretical model describing pulse shapes, we have clarified the relations between the observed pulses and their corresponding timescales, such as the angular spreading time, the dynamic time as well as the cooling time. We find that the angular spreading timescale caused by curvature effect of fireball surface only contributes to the falling part of the observed pulses, while the dynamic one in the co‐moving frame of the shell merely contributes to the rising portion of pulses provided the radiative time is negligible. In addition, the pulses resulted from the pure radiative cooling time of relativistic electrons exhibit properties of fast rise and slow decay (a quasi‐FRED) profile together with smooth peaks. Besides, we interpret the phenomena of wider pulses tending to be more asymmetric to be a consequence of the difference in emission regions. Meanwhile, we find the intrinsic emission time is decided by the ratios of lorentz factors and radii of the shells between short and long bursts. Based on the analysis of asymmetry, our results suggest that the long GRB pulses may occur in the regions with larger radius, while the short bursts could locate at the smaller distance from central engine. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
244.
A New Feature Vector Using Selected Line Spectra for Pulsar Signal Bispectrum Characteristic Analysis and Recognition 总被引:1,自引:0,他引:1
Zhen-Hua Xie Lu-Ping XU Guang-Ren Ni Yan Wang 《中国天文和天体物理学报》2007,7(4):565-571
Average pulse profiles of pulsar signals are analyzed using the bispectrum tech-nique. The result shows that there are nonlinear phase couplings between the two frequency axes of the bispectrum charts, which indicate nonlinear factors in the generation and prop-agation of pulsar signals. Bispectra can be used as feature vectors of pulsar signals because of their being translation invariant. A one-dimension selected line spectrum algorithm for ex-tracting pulsar signal characteristic is proposed. Compared with selected bispectra, the pro-posed selected line spectra have the maximum interclass separability measurements from the point of view of the whole one-dimension feature vector. Recognition experiments on several pulsar signals received at several frequency bands are carried out. The result shows that the selected line spectrum algorithm is suitable for extracting pulsar signal characteristics and has a good classification performance. 相似文献
245.
翡翠的结构特征及其对翡翠质量的影响 总被引:2,自引:0,他引:2
通过对市面上常见翡翠结构的研究表明.翡翠的结构类型主要为:变晶结构、交代结构和碎裂结构,其中以粒柱状变晶结构最具代表性。不同结构的翡翠质量具有一定差异:变晶结构中以显微纤状变晶结构为最佳.具此结构的翡翠可达半透明至全透明;具交代结构的翡翠中常因含有钠质闪石、钠长石或绿泥石,且使翡翠的粒度不均匀而使其质量变差;具碎裂结构的翡翠硬度将略有降低.因而也会影响翡翠的质量。 相似文献
246.
西昆仑北段玛尔坎苏地区探明的大型碳酸锰成矿带,是我国近年最重要的找矿成果之一。该锰矿带构造上属北昆仑晚古生代弧后伸展盆地,其构造动力学背景为古特提斯洋向北俯冲于塔里木地块之下形成的弧盆体系。锰矿体主要发育于晚石炭世喀拉阿特河组含炭泥质灰岩夹薄层灰岩中。矿石中主要金属矿物为菱锰矿(75%~95%),次为软锰矿、硫锰矿及少量黄铁矿等。含锰岩系岩性和岩相学研究表明,玛尔坎苏锰矿带属典型的海相沉积锰矿床,其矿床成因可能与晚古生代半局限盆地沉积和海底热液活动有关。海底热液活动可能为成矿提供了丰富的物质来源。含锰岩系元素和同位素地球化学特征表明,玛尔坎苏锰矿沉淀时的水体环境为常氧条件,而矿层下盘(部分)岩系的岩性及地球化学特征反映其沉积时的水体环境为低氧—贫氧条件。玛尔坎苏锰矿带锰矿石具有负的δ13C值(-23.3‰~-10.0‰),推测有机质导致的还原作用是该锰矿由原生氧化锰在成岩期转化为菱锰矿和形成富锰矿的重要机制。 相似文献
247.
248.
巴州农区气候变化及其对农业生产的影响 总被引:1,自引:1,他引:1
分析了1960—2001年巴州农区年、季平均温度、降水的变化,结果表明:各地气候变化并不一致,大部分地区有温度升高、降水偏多的趋势。冬季的升温幅度明显大于其它季节,年平均温度的上升主要由冬季升温所致。 相似文献
249.
Corrosion behavior of steel in the juncture area of sea clay and sand was studied. The results show that galvanic corrosion occurred between steel samples buried in sea clay and sand, and that the polarity of the galvanic cell reversed after some testing later. The cause of reversing of polarity is discussed. 相似文献
250.
Impact of land use covers upon karst processes in a typical Fengcong depression system of Nongla, Guangxi, China 总被引:2,自引:0,他引:2
Cheng Zhang Jianguo Pei Yunqiu Xie Jianhua Cao Lanling Wang 《Environmental Geology》2008,55(8):1621-1626
The direction and intensity of karst processes can be deeply affected by soil physical and chemical variations which were
resulted from land use. Taking Nongla Fengcong depression area, Mashan County, Guangxi as an example, authors discussed the
impact of land use on karst processes based on the data of field limestone tablet. The results showed that the corrosional
rates at varied soil depth are quite different. Corrosional rate in woodland and orchard is mostly bigger than 20 mg/a, which
is much higher than that in tilled land and shrub. Generally, corrosional rate decreased from orchard, woodland, tilled land,
fallow land and shrub successively, in which soil organic matter (OM) and soil pH are two major controlling factors: corrosion
process is controlled remarkably by soil OM in woodland and orchard. The higher the organic matter content is and the less
the pH value is, the higher the corrosional rate is. Owing to lower organic matter content, the corrosional rate is mainly
affected by soil CO2 in tilled land and shrub. 相似文献