全文获取类型
收费全文 | 14863篇 |
免费 | 2720篇 |
国内免费 | 3407篇 |
专业分类
测绘学 | 939篇 |
大气科学 | 3040篇 |
地球物理 | 3672篇 |
地质学 | 7406篇 |
海洋学 | 1785篇 |
天文学 | 957篇 |
综合类 | 1410篇 |
自然地理 | 1781篇 |
出版年
2024年 | 36篇 |
2023年 | 200篇 |
2022年 | 583篇 |
2021年 | 643篇 |
2020年 | 562篇 |
2019年 | 687篇 |
2018年 | 717篇 |
2017年 | 719篇 |
2016年 | 813篇 |
2015年 | 705篇 |
2014年 | 818篇 |
2013年 | 843篇 |
2012年 | 781篇 |
2011年 | 805篇 |
2010年 | 896篇 |
2009年 | 793篇 |
2008年 | 828篇 |
2007年 | 765篇 |
2006年 | 548篇 |
2005年 | 579篇 |
2004年 | 461篇 |
2003年 | 462篇 |
2002年 | 459篇 |
2001年 | 546篇 |
2000年 | 495篇 |
1999年 | 705篇 |
1998年 | 607篇 |
1997年 | 577篇 |
1996年 | 547篇 |
1995年 | 466篇 |
1994年 | 383篇 |
1993年 | 372篇 |
1992年 | 324篇 |
1991年 | 232篇 |
1990年 | 194篇 |
1989年 | 160篇 |
1988年 | 168篇 |
1987年 | 81篇 |
1986年 | 75篇 |
1985年 | 58篇 |
1984年 | 43篇 |
1983年 | 28篇 |
1982年 | 44篇 |
1981年 | 38篇 |
1980年 | 26篇 |
1979年 | 21篇 |
1978年 | 15篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1958年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Calculation and comparative study of the chlorophyll fluorescence ratio F685/F735 in brown algae (Laminaria japonica, Underia pinnatifida and Padina crassa) excited by blue and green light showed that the fluorescence ratios were higher when the algae were excited by blue light (440 nm), but reduced obviously under green light (540 nm) excitation. The values also reduced under dehydration but could recover during rehydration if the stress was not serious. The variation of the fluorescence ratio under dehydration was mainly because changes in fluorescence emission at 735 nm were always sharper than those at 685 nm. The ratio was sensitive to stress and has potential as a stress indicator in phycological research. Measurement of the fluorescence excitation spectra showed that the only peak at 540 hm changed apparently during dehydration. It meant that the function of the Chl a /Fucoxanthin protein complex for energy transfer was easily inhibited by water stress. However, no variation of the ratio was foun 相似文献
992.
During late February to mid-March, 1991, when mature N. japonica swarmed in the sea surface off Jimo County, Qingdao, seawater containing numerous early embryos was pumped into a nearby 2.7 ha. muddy-sand shrimp pond, where the embryos continued to develop. 800000 Penaeus chinensis seedlings were introduced into the pond on May 30, 1991. Monthly benthic samplings were carried out to determine the population dynamics and production of N. japonica in this atypical environ -ment.The density of N. japonica varied from 0 to 20400 ind./m2. The predation of P. chinensis was considered to be responsible for the mass mortality of N. japonica in June through July, when the worms were 2.2-3.3cm in length and lived in the top 2-4cm sediment. From August to September of 1991, the hottest period in the site, there was no substantial mortality of N. japonica as reported in earlier literature. This was explained by the deeper burrowing of the' worms that protected them from predation by shrimps.Monthly biomass ranged f 相似文献
993.
本文着重讨论了保持地球上某一条特定曲线等长的等角投影的存在性特点和数学模型,并在此基础上首次求出了保持等角航线等长的等角投影和斜椭圆柱等角投影。本文所述的理论和方法可做为建立等角空间投影的基础。 相似文献
994.
Based on the ECMWF data(1980-1983) and others, a further inquiry on the activities and the structure feature of 30-60 day oscillation in the tropical atmosphere has been completed. The following results are obtained:There is stronger perturbation kinetic energy of 30-60 day atmospheric oscillation(AO) in the equatorial eastern Pacific. This means the equatorial eastern Pacific is a stronger activity region of 30-60 day AO in the tropics. Analyses also show that the AO system with the time scale of 30-60 days might consist of various spatial scale disturbances. The zonal propagation of 30-60 day oscillation in the tropical atmosphere is not all eastward. Some differences are found for different spatial scales, and for propagations in upper and lower tropospheres. The meridional propagation of the oscillation is even more different in the various regions and might be related to the low-frequency wave train in the atmosphere. The stronger activities of 30-60 day AO in the equatorial middle-western Pacific are related to the El Nino events and the weaker ones are correspondent to the inverse El Nino phenomena. 相似文献
995.
大气水汽的输送和收支及其对副热带干旱的影响 总被引:15,自引:0,他引:15
本文利用1979年9月至1984年8月欧洲中期天气预报中心(ECMWF)每日四次的十三层分析资料,研究不同尺度的大气运动对水汽输送和收支的贡献。指出Hadley环流对水汽从副热带向赤道输送和从冬半球向夏半球的输送中起着决定性作用。热带行星波则把水汽从热带输向副热带。在中高纬的水汽向极输送中,天气尺度的波动比行星尺度的波动更重要。在南半球,天气尺度的波动支配着水汽的输送过程。分析和数值试验表明,副热带干旱受到nadley环流和热带行星波异常的强烈影响。 相似文献
996.
Warm and humid climate and gentle hilly topography have provided favourable conditions for the devel-opment of the weathering crust of the granite intrusions in the Longnan area, Jiangxi Province. REE is mostlyconcentrated in an adsorption state in clay in the wholly weathered zone. The rare-earth minerals enriched inthe parent rocks provided the source material for the REE enrichment. Exchangeable REE accounts for48-86%. Extraction experiments and stable isotopic study of clay minerals suggest that the downward infiltra-tion of meteoric water and increasing gradient of pH values have played an important role in the enrichment ofREE during the progressive weathering. Slight fractionation of individual REE can not change their distribu-tion patterns in the profiles. which are inherited from the parent rocks. 相似文献
997.
地质样品中砷、镓、钴、镍、溴、氯、硫和氟的X射线荧光光谱法测定 总被引:4,自引:0,他引:4
本文叙述了XRF测定地质样品中微量As,Ga,Co,Ni,Br,Cl,S和F的分析方法。采用压片法制样。对As,Ga,Co,Ni和Br元素,用散射内标法校正基体效应,而对Cl,S和F元素则用经验系数法进行基体校正。各元素的探测限(ppm)分别为:As 2.7,Ga 2.4,Co 2.8,Ni 2.5,Br 1.0,Cl,S 5.0,F 500。方法的精密度和准确度与其它化学方法相当,是一种简便快速和较灵敏的分析方法。 相似文献
998.
999.
陕西地区岩浆岩Nd、Sr同位素特征及其与大地构造发展的联系 总被引:31,自引:7,他引:31
岩浆岩Nd、Sr同位素特征表明,扬子地块基底有着和华北地块很不相同的早期演变历史。这意味着华北和扬子两地块在历史上曾经是相互独立的,后来才拼合在一起。Nd、Sr同位素研究还表明,华北和扬子两地块的会聚在泥盆纪以前就已经在进行中,它是通过位於两地块之间的古秦岭海洋壳俯冲消亡在华北地块之下实现;华北和扬子两陆块最终相碰撞至少发生在二叠纪以前。 相似文献
1000.
Land utilization in China: Its problems and prospect 总被引:1,自引:0,他引:1
Prof. Dr. Chuan-jun Wu 《GeoJournal》1990,20(4):347-352