全文获取类型
收费全文 | 3120篇 |
免费 | 126篇 |
国内免费 | 81篇 |
专业分类
测绘学 | 96篇 |
大气科学 | 262篇 |
地球物理 | 726篇 |
地质学 | 1001篇 |
海洋学 | 296篇 |
天文学 | 721篇 |
综合类 | 29篇 |
自然地理 | 196篇 |
出版年
2022年 | 18篇 |
2021年 | 36篇 |
2020年 | 56篇 |
2019年 | 48篇 |
2018年 | 75篇 |
2017年 | 84篇 |
2016年 | 103篇 |
2015年 | 80篇 |
2014年 | 81篇 |
2013年 | 165篇 |
2012年 | 104篇 |
2011年 | 130篇 |
2010年 | 138篇 |
2009年 | 152篇 |
2008年 | 153篇 |
2007年 | 133篇 |
2006年 | 113篇 |
2005年 | 112篇 |
2004年 | 125篇 |
2003年 | 131篇 |
2002年 | 156篇 |
2001年 | 149篇 |
2000年 | 105篇 |
1999年 | 72篇 |
1998年 | 55篇 |
1997年 | 52篇 |
1996年 | 42篇 |
1995年 | 45篇 |
1994年 | 45篇 |
1993年 | 31篇 |
1992年 | 22篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 19篇 |
1988年 | 13篇 |
1987年 | 16篇 |
1986年 | 22篇 |
1985年 | 30篇 |
1984年 | 36篇 |
1983年 | 25篇 |
1982年 | 21篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1978年 | 20篇 |
1976年 | 20篇 |
1975年 | 19篇 |
1974年 | 28篇 |
1972年 | 22篇 |
1971年 | 17篇 |
1970年 | 13篇 |
排序方式: 共有3327条查询结果,搜索用时 15 毫秒
131.
Wild mushrooms are recognized as important non-wood forest products in mountainous ecosystems, but their real potential for generating rural economies has not been fully evaluated due to the difficulties in obtaining reliable productivity data, minimizing their true potential as contributor to rural economies. Mushroom yield models based on large data series from Pinus forest ecosystems in the region of Catalonia(Spain), combined with data from the Spanish National Forest Inventory allow us to estimate the potential mushroom productivity by forest ecosystems. The results of 24,500 tons/yr of mushrooms of which 16,300 tons are classified as edible and 7,900 tons are commonly marketed demonstrate the importance of mushroom productions in Catalonian pine forests, mostly located in mountainous areas where the development of agricultural activities is limited. Economic mushroom value is estimated at 48 million € for the edible mushroom and 32 million € for those corresponding to marketable yields, confirming the potential of this non-wood forest product. These production results and corresponding economic values provide a basis for the incorporation of wild mushrooms as significant non-wood forest products in the development of forest policies in mountainous areas. 相似文献
132.
以综合催熟措施和移植精荚手术相结合的技术方法,解决了鱼塭产斑节对虾催熟效应期长,产卵率低,易蜕壳失去精荚等难题。使用 278 尾雌虾,催熟率达90% 以上,在40 d 内共产卵608 尾次,卵量18 530 万粒,孵出无节幼体11 998 万尾,平均孵化率 64.7% ,平均每尾雌虾孵出无节幼体 43.1 万尾,育成商品虾苗 1 028.5 万尾,育苗平均成活率 13.6% ,试养成活率57.1% 相似文献
133.
N. Van der Putten C. Verbruggen R. Ochyra S. Spassov J.-L. de Beaulieu M. De Dapper J. Hus N. Thouveny 《Quaternary Science Reviews》2009,28(1-2):65-79
Botanical macrofossil analysis of a more than 9000 years old, radiocarbon dated peat sequence of a moss peat bank from South Georgia, shows a clear evolution in the vegetation. Seven ecological phases could be distinguished and they can be interpreted in terms of climate development during the Holocene. Until 2200 years ago, Warnstorfia fontinaliopsis was the dominant moss species pointing to a wet environment. Lower numbers of this species in association with the presence of drier species are assumed to indicate drier periods, such as occurring between ca 6000–5200 and 4400–3400 cal yr BP. The most prominent and definitive vegetation change took place around 2200 cal yr BP. A Polytrichum–Chorisodontium moss peat bank was formed, which is still growing there today. The forcing mechanism for this vegetation change is thought to be a temperature decrease, rather than a precipitation decrease. This conclusion is mainly based on the fact that, today, moss peat banks have their optimal occurrence range in the maritime Antarctic, a region were the mean annual temperature is ca 4 °C lower than on South Georgia. The remarkable change in the moss bank vegetation at 2200 cal yr BP raises the question whether this moment was only a short climatic deterioration, or a definitive change to a cooler and wetter climate after a Holocene climatic optimum period. 相似文献
134.
135.
Cornelis De Jager 《Solar physics》1987,114(2):387-388
136.
From radio spectra between 160 and 320 MHz of chains of type I bursts it appears that their duration distributions allow an exponential fit, and that those of samples containing long and short chains respectively, taken from the same storm, have virtually the same characteristic time (logarithmic slope). On the average this figure decreases - as a function of the frequency - at about 1 s per 10 MHz. The high frequency cut-off of chain activity (noise storms) is mainly a consequence of the frequency dependence of the probability for the first burst of a chain to appear. Given the density of type I bursts in a chain, it is concluded that the probability of a type I burst to be followed by another one is at least 90% below 250 MHz and 70–80% at 300 MHz, which makes it essential for type I theories to include a mechanism to this effect. The drift rate distribution for chains is symmetrical with a peak at-10 MHz/s. The statistics is indicative of a correlation between drift rate and duration. No evidence has been found for the occurrence of chain pairs or frequency splitting in chains, nor for an association between chains and type III bursts. 相似文献
137.
城市街道网络是承载人们日常行为的空间载体,街道网络及其中人的日常行为共同构成了主要的街道网络空间环境,并影响着犯罪行为的发生。既有研究表明街道空间中多发“两抢一盗”侵财类案件,并且街道网络形态和日常行为主体与犯罪存在联系,但对这一问题的深入探究较少。本文基于HS市原DP区的派出所辖区单元,使用路网数据、人口统计数据、手机信令数据等相关资料,采用空间分析和统计分析方法,对街道网络的几何形态和拓扑形态、动态人口和静态人口等因素与犯罪分布的关系展开分析,尝试探索街道网络空间环境对“两抢一盗”犯罪的影响。结果表明,街道网络空间环境中影响盗窃和两抢犯罪的要素存在较大差异。“X形单元”式街道网络、路网密度、低速交通的路网渗透性、静态人口密度和动态人口密度均对盗窃犯罪产生重要影响,与两抢犯罪相关的仅有“T形单元”式街道网络、静态人口中的实际居住户籍人员和外来人员密度,街道网络形态没有表现出与两抢犯罪的显著相关。最后提出,由于街道网络的通达性和危险性往往并存,如何通过调控或改善整体的街道网络空间环境来尽可能减少犯罪风险,是未来规划建设安全的街道网络空间需要思考的话题。 相似文献
138.
We present the analysis of X‐ray spectral variability made on a sample of 7 Seyfert 1 bright galaxies, using XMM‐Newton data. From the “XMM‐Newton Science Archive” we selected those bright Seyfert 1 showing one or more prominent flares in their 2–10 keV light curves. For each of them we extracted spectra in 3 different time intervals: before, during and after the flare. We fitted them with a simple power law and then shifted a narrow emission and absorption line template across the 2.5–10 keV data, in order to investigate the presence of line‐like features with a confidence level greater than 99%. Some highly significant features were detected in 3 out of 7 sources studied. In particular, the 3 sources, namely PG 1211+143, NGC 4051 and NGC 3783, showed the presence of a variable emission feature in the 4.5–5.8 keV band, characterized by an increase of its intensity after the flare peak. Because of the observed variability pattern, this feature seems to be ascribable to a reverbered redshifted relativistic component of the Fe K line. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
139.
M. Shyam Prasad N. G. Rudraswami Agnelo Alexandre De Araujo V. D. Khedekar 《Meteoritics & planetary science》2019,54(2):290-299
Metal in various forms is common in almost all meteorites but considerably rare among micrometeorites. We report here the discovery of two metal micrometeorites, i.e., (1) an awaruite grain similar to those found in the metal nodules of CV chondrites and (2) a metal micrometeorite of kamacite composition enclosing inclusions of chromite and merrillite. This micrometeorite appears to be a fragment of H5/L5 chondrite. These metal micrometeorites add to the inventory of solar system materials that are accreted by the Earth in microscopic form. They also strengthen the argument that a large proportion of material accreted by the Earth that survives atmospheric entry is from asteroidal sources. 相似文献
140.