全文获取类型
收费全文 | 49495篇 |
免费 | 11653篇 |
国内免费 | 17786篇 |
专业分类
测绘学 | 8857篇 |
大气科学 | 6939篇 |
地球物理 | 10142篇 |
地质学 | 32731篇 |
海洋学 | 8919篇 |
天文学 | 805篇 |
综合类 | 4036篇 |
自然地理 | 6505篇 |
出版年
2024年 | 574篇 |
2023年 | 1500篇 |
2022年 | 2808篇 |
2021年 | 3584篇 |
2020年 | 2843篇 |
2019年 | 3133篇 |
2018年 | 2789篇 |
2017年 | 2763篇 |
2016年 | 2851篇 |
2015年 | 3251篇 |
2014年 | 3138篇 |
2013年 | 3736篇 |
2012年 | 4017篇 |
2011年 | 4355篇 |
2010年 | 4127篇 |
2009年 | 4027篇 |
2008年 | 3870篇 |
2007年 | 3729篇 |
2006年 | 3851篇 |
2005年 | 3348篇 |
2004年 | 3060篇 |
2003年 | 2147篇 |
2002年 | 2006篇 |
2001年 | 1736篇 |
2000年 | 1369篇 |
1999年 | 776篇 |
1998年 | 489篇 |
1997年 | 426篇 |
1996年 | 333篇 |
1995年 | 236篇 |
1994年 | 210篇 |
1993年 | 378篇 |
1992年 | 369篇 |
1991年 | 187篇 |
1990年 | 111篇 |
1989年 | 125篇 |
1988年 | 75篇 |
1987年 | 107篇 |
1986年 | 55篇 |
1985年 | 48篇 |
1984年 | 32篇 |
1983年 | 43篇 |
1982年 | 30篇 |
1979年 | 32篇 |
1978年 | 17篇 |
1977年 | 26篇 |
1976年 | 19篇 |
1957年 | 16篇 |
1954年 | 25篇 |
1937年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Y.-B. WU S. GAO H.-J. GONG H. XIANG W.-F. JIAO S.-H. YANG Y.-S. LIU H.-L. YUAN 《Journal of Metamorphic Geology》2009,27(6):461-477
U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and 176 Lu/177 Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean 207 Pb/206 Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ LuN ≥2213; 14.7 ≤ LuN /SmN ≤ 354) and 176 Lu/177 Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean 207 Pb/206 Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T DM2 ) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust. 相似文献
2.
3.
This paper presents a method that incorporates a non‐associated flow rule into the limit analysis to investigate the influence of the dilatancy angle on the factor of safety for the slope stability analysis. The proposed method retain's the advantage of the upper bound method, which is simple and has no stress involvement in the calculation of the energy dissipation and the factor of safety. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
4.
Northern Hemispheric“polar lows”,i.e.maritime,small,but fairly intense cyclonic systems have been studied formore than two decades.Recently researchers have turned their attention towards the occurrence of polar lows and othersmall-scale cyclonic disturbances on the Southern Hemisphere.In this paper a short account of our present knowledgeof Northern Hemispheric polar lows will be given,followed by some preliminary results of an investigation of the natureand occurrence of polar lows on the Southern Hemisphere. 相似文献
5.
PROPAGATION OF 30—60 DAY LOW FREQUENCY OSCILLATIONS AND THEIR INFLUENCE UPON THE SUBTROPICAL WESTERLIES JET STREAM DURING NORTHERN HEMISPHERE WINTER 下载免费PDF全文
Based on daily ECMWF gridpoint data of two winters during 1981—1983 including an ENSOyear,propagation of low frequency oscillations(LFO)during Northern Hemisphere winters andtheir influences upon 30—60 day oscillations of the subtropical jet stream are studied with the sta-tistical methods as complex empirical orthogonal function(CEOF)and so on.Results show that inthe winter of a normal year(1981—1982),30—60 day oscillations in the subtropical zone aremainly in the northern and southern flanks of exit region of jet stream.In the ENSO year(1982—1983),they are mainly in the vicinity of entrance and exit regions of jet stream.Intraseasonalchanges of subtropical jet stream manifested themselves as latitudinal fluctuation or longitudinalprogression or regression of about 40 day period.There are marked differences between propagat-ing passages of low frequency modes responsible for changes of subtropical jet stream in the normalyear(1981—1982)and in the ENSO year(1982—1983).Changes of oscillation amplitude showobvious phases.In general,the one in late winter is stronger than that in early winter,strongestone occurs in February. 相似文献
6.
大气层对GPS信号的传播有时间延迟作用,时廷将影响GPS系统的定位和定时精度,且随气象因素的变化而变化.本文根据3年有关的气象资料,采用大气指数模型分析了广州地区大气层随昼夜变化、季节变化和太阳活动变化对GPS信号传播时延影响的变化规律.分析得知,在信号垂直传播时,大气层对GPS信号传播的影响,用距离表示.最大时为2.691m,最小时也会产生1.993m的误差;如果信号斜传播,误差因仰角因素将更大. 相似文献
7.
DunaIiella salina, a halotolerent unicellular green alga, can accurmulate a Iarge amount of β-caroteneunder environmental conditions. The isorners of β-carotene extIacted from D. salina culturedin medium with different nitrate and phosphate concentrations were analysed by HPLC with Alox-Talumins column. At least six isomers were found in different proportions depending on the culture me-dia's nitrate and/or phosphate concentrations. Nitrate and/or phosphae defidency was conducive tothe accumulation of totaI cis isomers but not of al1 trans isomer. lt is sUggeSted that 1 mmol/L KNO_3and 0.1 mmol/L KH_2, PO_4 are favourable for accumulation of total cis β-carotene. 相似文献
8.
9.
A THREE-DIMENSIONAL ELASTIC NESTED-GRID MESO-(β-γ)SCALE ATMOSPHERIC MODEL——PARTI:MODEL DESCRIPTION 下载免费PDF全文
A three-dimensional elastic nonhydrostatic mesoscale(β-γ)model with nested-grid is presented.It uses a set of fullequations in terrain-following coordinates as its basic dynamic frame,which is solved with a time-splitting algorithmfor acoustic and gravity waves.The model physical parameterization includes a K-theory subgrid eddy mixing for cloudand free atmosphere,a bulk planetary boundary layer parameterization,and three types of sofisticated cloudmicrophysics schemes with double-parameters for hail-bearing clouds,warm clouds and snowing clouds respectively.The model is designed to be used flexibly for simulations of a variety of meso-and small-scale atmospheric processes,and can be improved as a regional and local operational NWP system in future. 相似文献
10.
土壤微生物是陆地生态系统中分解者亚系统的主要组成部分, 参与了包括有机质降解、营养转化、 植物生长的促进或抑制以及各种土壤物理过程在内的一系列反应活动. 土壤微生物则是土壤质量重要的生物指标, 可以用来监控土壤质量的变化. 等温微量热法是一种简便、快速地测量微生物活性的方法, 在土壤微生物代谢热效应的研究领域中广泛应用. 就等温微量热法在土壤微生物活性中的研究进展进行综述: 等温微量热法的简介, 微量热方法与传统方法的比较, 等温微量热法在各种外界环境和土壤条件影响下的土壤微生物活性研究中的应用, 并对等温微量热法在土壤微生物和其它方面的研究进行了展望. 相似文献