全文获取类型
收费全文 | 22623篇 |
免费 | 3619篇 |
国内免费 | 5042篇 |
专业分类
测绘学 | 1938篇 |
大气科学 | 3961篇 |
地球物理 | 5229篇 |
地质学 | 10813篇 |
海洋学 | 3387篇 |
天文学 | 1438篇 |
综合类 | 1758篇 |
自然地理 | 2760篇 |
出版年
2024年 | 143篇 |
2023年 | 417篇 |
2022年 | 962篇 |
2021年 | 1151篇 |
2020年 | 966篇 |
2019年 | 1075篇 |
2018年 | 1151篇 |
2017年 | 1114篇 |
2016年 | 1306篇 |
2015年 | 1136篇 |
2014年 | 1352篇 |
2013年 | 1510篇 |
2012年 | 1452篇 |
2011年 | 1537篇 |
2010年 | 1468篇 |
2009年 | 1399篇 |
2008年 | 1291篇 |
2007年 | 1190篇 |
2006年 | 1010篇 |
2005年 | 902篇 |
2004年 | 708篇 |
2003年 | 646篇 |
2002年 | 639篇 |
2001年 | 632篇 |
2000年 | 606篇 |
1999年 | 702篇 |
1998年 | 557篇 |
1997年 | 551篇 |
1996年 | 501篇 |
1995年 | 450篇 |
1994年 | 421篇 |
1993年 | 369篇 |
1992年 | 314篇 |
1991年 | 258篇 |
1990年 | 200篇 |
1989年 | 195篇 |
1988年 | 164篇 |
1987年 | 110篇 |
1986年 | 89篇 |
1985年 | 89篇 |
1984年 | 70篇 |
1983年 | 75篇 |
1982年 | 70篇 |
1981年 | 63篇 |
1980年 | 47篇 |
1979年 | 38篇 |
1978年 | 28篇 |
1977年 | 25篇 |
1976年 | 21篇 |
1975年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Methane, a potential biosignature, has recently been detected in the martian atmosphere. This Note focuses on field investigations/operational simulations and laboratory studies which resulted in successful detection of methane within arid terrestrial soils, as distinct from the usual methanogen environment, but in at least partial analogy to martian conditions. 相似文献
62.
The characteristics of red karst weathering crust in the research region can be divided into three subtypes. The first one includes the silicon to aluminum ratio, the composition of the clay minerals, etc., and reflects the weak action of de-silicon and aluminum enrichment. The second one, for example the granularity, shows that the viscous action is strong. The third one, such as the CaCO3 content and pH value, has regional difference in a large scale, which is consistent with the condition of modern scenery, so it is not important as for the ancient environment.
相似文献63.
介绍了一种新颖的静态传感器网络实现方法,它利用可移动的中继节点来收集静态传感器网络的数据,有效的节约各传感器节点的能源,延长整个传感器网络的有效工作时间。也可以对该方法进行适当的改进,应用到某些实时性要求较高的场合中去。然后介绍了应用该原理实现的一个通讯协议。 相似文献
64.
65.
Gang Zhao Yonggheng Zhao Jtasheng Huang Lonlong Feng Keliang Huang Qiuhe Peng Tan Lu 《Astrophysics and Space Science》1988,141(1):169-176
In this paper, the neutrino mass has been determined from SN1987a observation in a manner that the simultaneity of neutrino emission is not regarded as the starting point, but is itself defined through the analysis by Monte-Carlo simulation. The result is that the neutrino mass lies in 3–4 eV, possiblym
v
3.6 eV. Neutrino luminosity variation and neutrino spectrum are also obtained. Comparison with theories gives further support to the mass determination, and also predicts the mass of progenitor star of SN1987a to be in the range of 12–25M
.The project supported by the National Natural Science Foundation of China. 相似文献
66.
近年我国区域水文研究的若干进展 总被引:1,自引:1,他引:0
我国区域水文学的研究是在解放后其正发展起来的。尽管迄今人们对区域水文学仍有不同的认识,例如区域水文学是否可以与水文地理学等同,或是区域水文学是否应隶属于区域地理学等等,但是作为水文学的分支学科是无疑的。区域水文学从整体的观点来研究区域水文情势。区城可以是功能区,如城市、出口加工中心区等,或为自然景观区,如干旱区、喀斯特区等,也可以是行政区。目前,许多水文水利工作者已在致力于区域水文学的研究。 相似文献
67.
贵州和四川盆地云量的气候研究 总被引:1,自引:1,他引:0
贵州和四川盆地是我国云量最多的地区,素有“天无三日晴”和“蜀犬吠日”之说,盆地西缘的雅安古时还有“雅州天漏”的谚语。从全球日照百分率分布推知,川黔地区也是世界上天气最阴沉的地区之一。川黔之阴一向为国内外气象界所注意,本文根据1951—1980 相似文献
68.
Energetic solar electrons in the interplanetary medium 总被引:3,自引:0,他引:3
R. P. Lin 《Solar physics》1985,100(1-2):537-561
ISEE-3 measurements extending down to 2 keV energy have provided a new perspective on energetic solar electrons in the interplanetary medium. Impulsive solar electron events are observed, on average, several times a day near solar maximum, with 40% detected only below 15 keV. The electron energy spectra have a nearly power-law shape extending smoothly down to 2 keV, indicating that the origin of these events is high in the corona. These coronal flare-like events often produced 3He-rich particle events.In large solar flares which accelerate electrons and ions to relativistic energies, the electron spectrum appears to be modified by a second acceleration which results in a double power-law shape above 10 keV with a break near 100 keV and flattening from 10–100 keV. Large flares result in long-lived (many days) streams of outflowing electrons which dominate the interplanetary fluxes at low energies. Even in the absence of solar activity, significant fluxes of low energy electrons flow out from the Sun.Solar type-III radio bursts are produced by the escaping 2–102 keV electrons through a beam-plasma instability. The detailed ISEE-3 measurements show that electron plasma waves are generated by the bump-on-tail distribution created by the faster electrons running ahead of the slower ones. These plasma waves appear to be converted into radio emission by nonlinear wave-wave interactions. 相似文献
69.
70.
R. Scott Anderson Renata B. Jass Jaime L. Toney Craig D. Allen Marcey Hess Julianna Fessenden 《Quaternary Research》2008,69(2):263-275
Chihuahueños Bog (2925 m) in the Jemez Mountains of northern New Mexico contains one of the few records of late-glacial and postglacial development of the mixed conifer forest in southwestern North America. The Chihuahueños Bog record extends to over 15,000 cal yr BP. An Artemisia steppe, then an open Picea woodland grew around a small pond until ca. 11,700 cal yr BP when Pinus ponderosa became established. C/N ratios, δ13C and δ15N values indicate both terrestrial and aquatic organic matter was incorporated into the sediment. Higher percentages of aquatic algae and elevated C/N ratios indicate higher lake levels at the opening of the Holocene, but a wetland developed subsequently as climate warmed. From ca. 8500 to 6400 cal yr BP the pond desiccated in what must have been the driest period of the Holocene there. C/N ratios declined to their lowest Holocene levels, indicating intense decomposition in the sediment. Wetter conditions returned after 6400 cal yr BP, with conversion of the site to a sedge bog as groundwater levels rose. Higher charcoal influx rates after 6400 cal yr BP probably result from greater biomass production rates. Only minor shifts in the overstory species occurred during the Holocene, suggesting that mixed conifer forest dominated throughout the record. 相似文献