全文获取类型
收费全文 | 3014篇 |
免费 | 563篇 |
国内免费 | 837篇 |
专业分类
测绘学 | 369篇 |
大气科学 | 654篇 |
地球物理 | 702篇 |
地质学 | 1538篇 |
海洋学 | 314篇 |
天文学 | 121篇 |
综合类 | 328篇 |
自然地理 | 388篇 |
出版年
2024年 | 15篇 |
2023年 | 51篇 |
2022年 | 147篇 |
2021年 | 204篇 |
2020年 | 144篇 |
2019年 | 140篇 |
2018年 | 167篇 |
2017年 | 176篇 |
2016年 | 188篇 |
2015年 | 148篇 |
2014年 | 200篇 |
2013年 | 196篇 |
2012年 | 219篇 |
2011年 | 196篇 |
2010年 | 187篇 |
2009年 | 180篇 |
2008年 | 151篇 |
2007年 | 169篇 |
2006年 | 122篇 |
2005年 | 134篇 |
2004年 | 105篇 |
2003年 | 79篇 |
2002年 | 87篇 |
2001年 | 92篇 |
2000年 | 99篇 |
1999年 | 100篇 |
1998年 | 124篇 |
1997年 | 107篇 |
1996年 | 74篇 |
1995年 | 66篇 |
1994年 | 75篇 |
1993年 | 58篇 |
1992年 | 46篇 |
1991年 | 40篇 |
1990年 | 26篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 15篇 |
1986年 | 16篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1958年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有4414条查询结果,搜索用时 15 毫秒
51.
52.
志留系是塔里木盆地第一套砂岩储层广泛分布的沉积盖层,其沉积来源与成因对志留纪构造演化及周边造山带的研究具有重要意义。塔中地区3个志留系样品的碎屑锆石LA?ICP?MS U?Pb定年研究表明,志留系具有比较集中的二期物源年龄:南华纪中期、古元古代中期。碎屑锆石测定的年龄表明塔中志留系物源均来自前寒武纪,塔中东部源区方向的塔南隆起基底在奥陶纪已隆升成为蚀源区。大量的新元古代中期锆石年龄表明塔里木板块在新元古代时期可能与Rodinia超大陆具有相似的聚合与裂解演化史。 相似文献
53.
对土壤中14种有机氯农药的超声提取技术和磺化、固相萃取小柱两种净化方法进行系统研究。结果表明,超声提取节省提取时间,提高分析效率;磺化法和固相萃取小柱法都有净化作用,但磺化法使样品中的艾氏剂、狄氏剂和异狄氏剂全部损失;固相萃取小柱净化当丙酮-正己烷淋洗液体积比为1∶9、淋洗液收集8 mL时回收率>90%,干扰最小。方法检出限为0.63~2.59μg/kg,精密度(RSD,n=7)为1.17%~14.66%,基体加标平均回收率为82.78%~106.94%。方法用于生态地球化学调查评价批量土壤样品的实际分析,检测结果满足评价工作需要。 相似文献
54.
55.
Contributors to chromium isotope variation of meteorites 总被引:3,自引:0,他引:3
Liping Qin Conel M.O’D. Alexander Mary F. Horan 《Geochimica et cosmochimica acta》2010,74(3):1122-400
We report the results of a comprehensive, high precision survey of the Cr isotopic compositions of primitive chondrites, along with some differentiated meteorites. To ensure complete dissolution of our samples, they were first fused with lithium borate-tetraborate at 1050-1000 °C. Relative to the NIST Cr standard SRM 3112a, carbonaceous chondrites exhibit excesses in 54Cr/52Cr from 0.4 to 1.6ε (1ε = 1 part in 10,000), and ordinary chondrites display a common 54Cr/52Cr deficit of ∼0.4ε. Analyses of acid-digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54Cr-enriched carrier, which is characterized by a large excess in 54Cr/52Cr (up to 200ε) associated with a very small deficit in 53Cr/52Cr (<2ε). We did not find 54Cr anomalies in either bulk enstatite chondrites or in leachates of their acid-digestion residues. This either requires that the enstatite chondrite parent bodies did not incorporate the 54Cr anomaly carrier phase during their accretion, or the phase was destroyed by parent body metamorphism. Chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the NIST SRM 3112a Cr standard. The eucrite and Martian meteorites studied exhibit small deficits in 54Cr/52Cr. The 54Cr/52Cr variations among different meteorite classes suggest that there was a spatial and/or temporal heterogeneity in the distribution of a 54Cr-rich component in the inner Solar System.We confirm the correlated excesses in 54Cr/52Cr and 53Cr/52Cr for bulk carbonaceous chondrites, but the new data yield a steeper slope (∼6.6) than that reported in Shukolyukov and Lugmair (2006). The correlated excesses may affect the use of the Mn-Cr chronometer in carbonaceous chondrites. We could not confirm the bulk carbonaceous chondrite Mn-Cr isochron reported by Shukolyukov and Lugmair (2006) and Moynier et al. (2007), mostly because we find much smaller total variations in ε53Cr (∼0.2). All bulk chondrites have small ε53Cr excesses (up to 0.3) relative to the Earth, most likely reflecting the sub-chondritic Mn/Cr ratio of the Earth. The ε53Cr variations in chondrites do seem to grossly correlate with Mn/Cr and yield an initial Solar System 53Mn/55Mn value of 5.4(±2.4) × 10−6, corresponding to an absolute age of 4566.4 (±2.2) Ma.Nuclear interactions with cosmic rays result in coupled excesses in ε54Cr and ε53Cr with a ∼4:1 ratio in phases with high Fe/Cr. These are most dramatically demonstrated in the iron meteorite Carbo, showing excesses in ε54Cr of up to 140ε. These new results show that the Mn-Cr chronometer should be used with caution in samples/minerals with high Fe/Cr and long cosmic ray exposure ages. 相似文献
56.
本文作者利用IOS近似模型,计算了星际分子云条件下A型CH3CN-H2含超精细能级的碰撞跃迁速率系数。温度范围是20K-140K。为研究分子云与恒星形成区的物理、化学性质提供了有用的基础分子数据。 相似文献
57.
A type of pulsation in a time scale of seconds superimposed on microwave burst at 9.375 GHz has been found during the twenty-second solar active maximum period by us. This phenomenon is quite different from radio spike emission at decimeter and long centimeter wavelengths. The flux level of the bursts rises as the repetition rate of pulsations increases, following an approximate linear relationship. This feature resembles that at mm wavelength, but some other features are different. Some mechanisms for interpretation have been proposed. 相似文献
58.
秦曾灏 《中国海洋大学学报(自然科学版)》1987,(3)
本文着重研究大地形对正压大气迭加在非均匀纬向基本气流上挑动演变的影响。用WKBJ方法推导了正压扰动的频率和频散关系以及波作用量所满足的方程。当基本流场定常且存在无限长的东西向山脉时,则有波作用量守恒。据此,获得了正压不稳定的必要条件:即在所考察的平面域内至少在某点或某些点上含地形效应的某一物理量等于零,扰动就可能得到发展。对波包动能的倚时变化的讨论得知,在无摩擦正压纬向非均匀基流的情形,地形对扰动动能并无直接影响,即导波和曳波分别在西风急流的南侧和北侧得到加强,并分别在西风急流的北侧和南侧强度减弱。扰动的发展由扰动的结构所决定。导波和曳波的波长缩短总伴同其轴线倾向于东西向相联系,而它们的波长伸长总是与其轴线倾向于南北向同时发生。还指出,正压扰动的加强(或减弱)与波长的伸长(或缩短)不是必然的联系,它的成立是有条件的。 相似文献
59.
Huan-Xin Weng Ya-Chao Qin Xiang-Wei Sun Hailiang Dong Xiang-Hua Chen 《Estuarine, Coastal and Shelf Science》2007,73(3-4):501-509
Based on lab-culture experiments analyzing limitation and combination of iron and phosphorus on the growth of Cryptomonas sp. (Cryptophyceae), and the study of accumulation and release of Fe-bound P in sediment cores collected from the marine region of the Pearl River Estuary, China, reasons for the high frequency of phytoplankton bloom therein are discussed. Results show that the combined effect of Fe and P can obviously accelerate algal development, and the optimum culture conditions maintaining maximum growth rate are 0.05 μM Fe and 50 μM P. Cellular contents of Fe and P is consistent and the P:Fe molar ratio is 159:1. The optimum range of the P:Fe molar ratio in culture experiments for cell incubation is 500–1400. The vertical trends of total Fe and total P variations in sediments are parallel. Fe-bound P is the main species of inorganic sedimentary P. Through continuous leaching with agitation, 34–80% of exchangeable P and 4–23% of exchangeable Fe are concurrently released from the surficial sediments. This is a possible way by which nutrients are made available to phytoplankton. These factors might be responsible for a high frequency of harmful algal blooms in the Pearl River Estuary. 相似文献
60.
城市空气质量数值预报的不确定性与可预报性 总被引:1,自引:0,他引:1
主要综述了数据误差、随机误差和模式物理误差所造成的城市空气质量数值预报的不确定性,简要介绍分析预报不确定性的统计方法。并对由内在随机性和外在误差引起的可预报性问题进行了分析讨论 相似文献