全文获取类型
收费全文 | 3328篇 |
免费 | 552篇 |
国内免费 | 704篇 |
专业分类
测绘学 | 132篇 |
大气科学 | 674篇 |
地球物理 | 739篇 |
地质学 | 1806篇 |
海洋学 | 409篇 |
天文学 | 168篇 |
综合类 | 335篇 |
自然地理 | 321篇 |
出版年
2024年 | 15篇 |
2023年 | 67篇 |
2022年 | 165篇 |
2021年 | 178篇 |
2020年 | 120篇 |
2019年 | 126篇 |
2018年 | 167篇 |
2017年 | 155篇 |
2016年 | 180篇 |
2015年 | 150篇 |
2014年 | 210篇 |
2013年 | 176篇 |
2012年 | 165篇 |
2011年 | 177篇 |
2010年 | 199篇 |
2009年 | 187篇 |
2008年 | 159篇 |
2007年 | 144篇 |
2006年 | 102篇 |
2005年 | 91篇 |
2004年 | 55篇 |
2003年 | 85篇 |
2002年 | 80篇 |
2001年 | 93篇 |
2000年 | 104篇 |
1999年 | 158篇 |
1998年 | 115篇 |
1997年 | 142篇 |
1996年 | 129篇 |
1995年 | 113篇 |
1994年 | 104篇 |
1993年 | 84篇 |
1992年 | 76篇 |
1991年 | 62篇 |
1990年 | 51篇 |
1989年 | 44篇 |
1988年 | 40篇 |
1987年 | 13篇 |
1986年 | 25篇 |
1985年 | 16篇 |
1984年 | 11篇 |
1983年 | 16篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1969年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有4584条查询结果,搜索用时 15 毫秒
991.
992.
报道一种合成Methomyl的两相法。此法能使甲硫基乙醛肟与甲胺基甲酰氯之间的缩合反应在两相界面上进行,从而有效抑制酰氯的水解,并使收率明显提高。 相似文献
993.
Groundwater simulation using a numerical model under different water resources management scenarios in an arid region of China 总被引:1,自引:1,他引:1
Shaoyuan Feng Zailin Huo Shaozhong Kang Zejun Tang Fengxin Wang 《Environmental Earth Sciences》2011,62(5):961-971
Groundwater plays an important role in the economic development and ecological balance of the arid area of northwest China.
Unfortunately, human activity, for example groundwater extraction for irrigation, have resulted in excessive falls in groundwater
level, and aquifer overdraft in the oasis, disrupting the natural equilibrium of these systems. A groundwater numerical model
for Minqin oasis, an arid area of northwest China, was developed using FEFLOW software to simulate regional groundwater changes
under transient conditions. The vertical recharge and discharge (source/sink terms) of the groundwater models were determined
from land-use data and irrigation systems for the different crops in the different sub-areas. The calibrated model was used
to predict the change for the period from 2000 to 2020 under various water resources management scenarios. Simulated results
showed that under current water resources management conditions groundwater levels at Minqin oasis are in a continuous drawdown
trend and groundwater depth will be more than 30 m by 2020. Reducing the irrigation area is more effective than water-saving
irrigation to reduce groundwater decline at Minqin oasis and the annual groundwater budget would be −0.978 × 108 m3. In addition, water-diversion projects can also reduce the drawdown trend of groundwater at Minqin oasis, and the groundwater
budget in the Huqu sub-area would be in zero equilibrium if the annual inflow into the oasis was enhanced to 2.51 × 108 m3. Furthermore, integrative water resources management including water-diversion projects, water-saving irrigation, and reducing
the irrigation area are the most effective measures for solving groundwater problems at Minqin oasis. 相似文献
994.
995.
This Mar. 1993 to Aug. 1994 study on suspended conchocelis ofPorphyra haitanensis showed that there were three patterns for development of vegetative filaments: filaments to filaments by “budding”; filaments
to sporangial branchlets by “budding”, or cell swelling. There were also three patterms for sporangial branchlet development:
vegetatively propagating, changing into conchospores, or dying. Each developmental stage had one or more different developmental
directions between vegetative filaments and sporangial branchlets. Developments from conchosporangial branchlets to conchospores
were sequential and irreversible. Although sporangial branchlets formed at 29°C could give rise to filaments, they could not
propagate as healthily under the same conditions as those formed at 25°C did. Probably the crucial period of plant cell differentiation
is in the late stage of sporangial branchlets. In line with the developmental directions of different stages, the authors
regulated the development of conchocelis to get ideal different developmental stages materials to obtain very developmentally
homogeneous stages, including filaments and sporangial branchlets.
Contribution No 3283 from the Institute of Oceanology, Chinese Academy of Sciences.
Study supported by the Climbing Project B PD-B6-4-2 of the State Science and Technology Commission of China. 相似文献
996.
997.
FanAiwu TangJiaxiang LiLi YangJun 《中国地质大学学报(英文版)》2005,16(1):79-83
The investigation of sliding isolation is mainlyfocused on the response of a structure subjected to asingle directional earthquake excitation. Fan andTang (2001) studied the slide limited friction (S LF)base isolation system, using a sequential exponentialfriction model. Zhang and Cheng (2001) studied theisolated effect of a sliding bearing as a friction damp er. Xiong and Yu (2002) studied the numerical meth od of sliding isolation system. But studying a singlecomponent of e… 相似文献
998.
Contribution of subducted Pacific slab to Late Cretaceous mafic magmatism in Qingdao region, China: A petrological record 总被引:2,自引:0,他引:2
Abstract The occurrence of the Pishikou mafic dike in the Qingdao region, China provides important constraints on the origin of Late Cretaceous (86–78 Ma) mafic magmatism on the eastern North China craton. The Pishikou mafic dike is distributed in the Cretaceous Laoshan granitoid body, Qingdao region and contains peridotitic and granulitic xenoliths, xenocrysts, and megacrysts. Rocks from the Pishikou mafic dike are basanites and have low SiO2 (< 42 wt%) and Al2O3 (12.5 wt%) contents, and high MgO (> 8 wt%), total alkalis (Na2O + K2O > 4.8 wt%, Na2O/K2O > 1), TiO2 (> 2.5 wt%), CaO (> 9 wt%) and P2O5 (> 1 wt%). In trace element abundances, they are highly enriched in large ion lithophile elements (LILEs) and light rare‐earth elements (LREEs) (ΣREE = 339–403 ppm, (La/Yb)N = 39–42) without high field strength element (HFSE) depletion. These rocks have radiogenic Sr and Pb, and less radiogenic Nd isotopic compositions [(87Sr/86Sr)i > 0.7059, εNd ≈ 2.7–3.8 (206Pb/204Pb)i ≈ 18.0 ± 0.1]. The diagnostic elemental ratios, such as Nb/La, Nb/U, and Nb/Th, are compatible with those of mid‐oceanic ridge basalts (MORBs) and oceanic island basalts (OIBs). Therefore, the Pishikou mafic dike has a geochemical feature completely different from those of the Early Cretaceous mafic dikes from the Qingdao region, but similar to those of back‐arc basalts from the Japan Sea. This geochemical feature suggests that the Pishikou mafic dike was derived from an asthenosphere source, but contaminated by materials from the subducted Pacific slab. The discovery of this mafic dike thus provides a petrological evidence for the contribution of subducted Pacific slab to the Late Cretaceous magmatism in the Qingdao region of the eastern North China craton. 相似文献
999.
介绍了G.P.Gregori提出的关于日地关系的新因果链:太阳→太阳风(及月球引潮力)→地幔内地磁感应々→“发电机”感应电流→焦耳热→火山(及地热流)→水或其它微量元素进入大气圈→控制长期气候变化。对这种因果链提出的背景知识、资料证明及应用前景作了详细说明。介绍了国内一些学者关于日地关系的看法,并将二者加以比较。提出了对日地关系研究的看法及急需解决的问题。 相似文献
1000.
Northeast Sichuan basin is a Mesozoic-Cenozoic basin in the northern margin of the Yangtze plate which has a complex tectonic
background and hydrocarbon accumulation history. By means of the analysis method of wave processes, major cycles of sedimentary
wave process in this basin have been studied. The results show that there are five-order cycles corresponding to 760, 220,
100, 35 and 20 Ma respectively, two first-order sedimentary cycles (220 Ma) and four secondorder sedimentary cycles (100 Ma).
The authors find that the second-order sedimentary cycles are well matched with reservoir formation cycles through studying
the relationship between second-order sedimentary cycles and reservoir formation cycles, and divide three reservoir formation
cycles in this basin.
__________
Translated from Journal of Jilin University (Earth Science Edition), 2007, 37(3): 513–518 [译自: 吉林大学学报(地球科学版)] 相似文献