全文获取类型
收费全文 | 34956篇 |
免费 | 19646篇 |
国内免费 | 41910篇 |
专业分类
测绘学 | 2668篇 |
大气科学 | 21600篇 |
地球物理 | 12896篇 |
地质学 | 35090篇 |
海洋学 | 16220篇 |
天文学 | 871篇 |
综合类 | 4696篇 |
自然地理 | 2471篇 |
出版年
2024年 | 103篇 |
2023年 | 281篇 |
2022年 | 577篇 |
2021年 | 838篇 |
2020年 | 2058篇 |
2019年 | 5418篇 |
2018年 | 5708篇 |
2017年 | 5436篇 |
2016年 | 5533篇 |
2015年 | 4808篇 |
2014年 | 4324篇 |
2013年 | 4874篇 |
2012年 | 4582篇 |
2011年 | 4366篇 |
2010年 | 4321篇 |
2009年 | 3658篇 |
2008年 | 2917篇 |
2007年 | 2955篇 |
2006年 | 2590篇 |
2005年 | 2454篇 |
2004年 | 2786篇 |
2003年 | 2536篇 |
2002年 | 2236篇 |
2001年 | 2024篇 |
2000年 | 1677篇 |
1999年 | 1801篇 |
1998年 | 1879篇 |
1997年 | 1913篇 |
1996年 | 1511篇 |
1995年 | 1467篇 |
1994年 | 1316篇 |
1993年 | 1252篇 |
1992年 | 1066篇 |
1991年 | 781篇 |
1990年 | 756篇 |
1989年 | 615篇 |
1988年 | 521篇 |
1987年 | 409篇 |
1986年 | 320篇 |
1985年 | 261篇 |
1984年 | 291篇 |
1983年 | 172篇 |
1982年 | 213篇 |
1981年 | 151篇 |
1980年 | 115篇 |
1979年 | 112篇 |
1978年 | 49篇 |
1977年 | 43篇 |
1971年 | 54篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
油田抽油机的抽油杆幌动幅度过大,是引起抽油杆断裂的一个重要因素,检测这种幌动幅度是防止抽油杆断裂的一种有效手段。设计了一种基于面阵CCD和普通半导体激光器(LD)测量这种幌动的悬点投影测量方法,通过数字卷积滤波,达到了范围为0-40mm,误差<0.2mm的技术检测指标。 相似文献
92.
本文依据稳恒磁场中的高斯定理和安培环路定理,用迭代法推导出对称轴上的磁感应强度与空间任意一点磁感应强度的关系式,从而可得出轴对称稳恒磁场的无电流分布空间的解式。 相似文献
93.
Mercury (Hg) was investigated in bone tissues of skua ( Catharacta maccormick) and penguin (Pygoscelis adeliae) collected in the maritime Antarctic using atomic fluorescence spectrometry (AFS) and synchrotron radiation X-ray fluorescence (SR-XRF) method. The total levels of mercury in bone tissues of penguin and skua are much lower than those in other organs (e. g. , kidney, liver). The toxic effects of mercury in bone tissues of seabirds in polar region are not known. We have used SR- XRF method to map the distribution of trace levels of mercury in bones. The levels of mercury are found to be enriched somewhere near the periosteal surface and/or endosteal surface. The distribution of mercury shows strongly correlation with that of some essential elements and probably poses negative effect on the bone metabolism inferring from the relationship of mercury with the other elements. These studies represent a first step toward understanding the toxic effects of mercury on bone of polar animals by suggesting the possible microscopic investigation. 相似文献
94.
Xue-Fei Chen Zhan-Wen HanNational Astronomical Observatories Yunnan Observatory Chinese Academy of Sciences. Kunming Graduate School of the Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(1):65-76
We have studied the influence of different choices of core-envelope transition point on the final merger of contact binaries with two main-sequence components. A binary of 1.00 + 0.90M⊙ with an initial orbital period of 0.35d is examined. The mass fraction of the primary mixed with the matter of the secondary, qmix, determined by the chosen core-envelope transition point, ranges from 0.04 to 1.00 in our analysis. If as qmix< 0.8, none of the 相似文献
95.
1 IntroductionBacteriaandtheiractivitiesplayanimportantroleintheelementalbiogeochemicalcyclesandenergytransformingintheocean (Zhenetal.1 997) .DortchandPackard(1 989) proposedthatfoodwebsintheeutrophicwatersaredominatedbythebiomassofprimaryproducerswhilefoodwebsintheoligotrophicwatersaredominatedbythebiomassofmicrobes.Heterotrophicbacteriahadbeenshowntoplayanimportantroleinthedecompositionoflarge ,rapidlysinkingorganicparticleswithinandbelowtheeuphot iczone ,andfurthertoaffecttheelementaldyn… 相似文献
96.
Histological study on the ontogeny of the lymphoid organs, kidney, thymus and spleen of Japanese flounder, Paralichthys olivaceus, from hatching to 40 d was carried out. The pronephric kidney duct appeared early in hatching although the primordial haemopoietic
stem cells were observed within a week after hatching. The spleen was first seen after 8d of hatching. The thymus appeared
after 15d, situated near the pronephric kidney. Small lymphoid cells appeared during the later phase of the post-larval stage
in the sequence of thymus, kidney and spleen. During the 40d of observations, there were no distinct inner or outer zones
in thymus and no red or white pulp in spleen. These results suggest that the nonspecific defense immune system plays a very
important role in the early larval stage of Japanese flounder. 相似文献
97.
TANGYa XIEJiasui SUNHui 《山地科学学报》2004,1(1):38-45
Dry valleys are a striking geographic landscape in Hengduan Mountains Region and are characterized by low rainfall, desert type of vegetation and fragile environment. Past efforts and resources have been concentrated mainly on rehabilitation of degraded ecosystem and fragile environment, particularly reforestation, while socio-economic development has been largely overlooked. Despite successes in pocket areas, the overall trend of unsustainability and environmental deterioration are continuing. It is important to understand that uplift of the Tibetan Plateau is the root cause of development of dry valleys, and development and formation of dry valleys is a natural process. Human intervention has played a secondary role in development of dry valleys and degradation of dry valleys though human intervention in many cases has speeded up environmental degradation of the dry valleys. It is important to understand that dry valleys are climatic enclaves and an integrated approach that combines rehabilitation of degraded ecosystems and socio-economic development should be adopted if the overall goal of sustainable development of dry valleys is to be achieved. Promotion of niche-based cash crops, rural energy including hydropower, solar energy, biogas and fuelwood plantation is recommended as the priority activities. 相似文献
98.
本文论述了珠穆朗玛峯地区土壤中稀土元素的含量,以及它们在土壤中的分布模式,并发现土壤中稀土元素的含量水平在很大程度上受土壤母质的制约。 相似文献
99.
100.
地壳对海洋潮汐的响应 总被引:2,自引:2,他引:0
应用三维动态有限元方法研究了中国北部地区的地壳对邻近的渤海与黄海海平面变化的响应。虽然此应力场过于微弱不足以引发地震,但发现应力集中的位置及应力场变化较大的位置恰好与某些现代地震的震中一致。这一结果表明研究地壳对广泛分布的载荷的响应对研究区域地震构造是有帮助的。 相似文献