全文获取类型
收费全文 | 65352篇 |
免费 | 17812篇 |
国内免费 | 42281篇 |
专业分类
测绘学 | 3107篇 |
大气科学 | 22417篇 |
地球物理 | 18486篇 |
地质学 | 46049篇 |
海洋学 | 19049篇 |
天文学 | 9457篇 |
综合类 | 4032篇 |
自然地理 | 2848篇 |
出版年
2023年 | 253篇 |
2022年 | 762篇 |
2021年 | 1227篇 |
2020年 | 2521篇 |
2019年 | 6036篇 |
2018年 | 7105篇 |
2017年 | 6691篇 |
2016年 | 6814篇 |
2015年 | 5172篇 |
2014年 | 5402篇 |
2013年 | 6584篇 |
2012年 | 5897篇 |
2011年 | 5721篇 |
2010年 | 5785篇 |
2009年 | 5312篇 |
2008年 | 4394篇 |
2007年 | 4438篇 |
2006年 | 3851篇 |
2005年 | 3148篇 |
2004年 | 3432篇 |
2003年 | 3180篇 |
2002年 | 2836篇 |
2001年 | 2619篇 |
2000年 | 2141篇 |
1999年 | 1942篇 |
1998年 | 2077篇 |
1997年 | 2187篇 |
1996年 | 1657篇 |
1995年 | 1627篇 |
1994年 | 1493篇 |
1993年 | 1399篇 |
1992年 | 1230篇 |
1991年 | 1003篇 |
1990年 | 1019篇 |
1989年 | 836篇 |
1988年 | 738篇 |
1987年 | 640篇 |
1986年 | 502篇 |
1985年 | 541篇 |
1984年 | 575篇 |
1983年 | 459篇 |
1982年 | 476篇 |
1981年 | 392篇 |
1980年 | 378篇 |
1979年 | 314篇 |
1978年 | 297篇 |
1977年 | 250篇 |
1976年 | 215篇 |
1975年 | 199篇 |
1973年 | 223篇 |
排序方式: 共有10000条查询结果,搜索用时 453 毫秒
11.
对青藏东北缘现今块体划分、运动及变形的初步研究 总被引:19,自引:9,他引:10
利用2维非连续变形分析方法(DDA),以位移代替围压作为边界约束力,研究青藏东北缘现今块体划分及其运动变形。根据该地区地质构造及地震活动,以GPS点测量位移作为模拟结果约束点,得出了较合理的块体划分模型和随时间演化的主应变分布图,并把应变高值区与近几年来发生的5级以上地震作对比,得出了研究区内地震危险性可能较大的区域。另外,对模拟的甘青块体与阿拉善块体的边缘带断裂左旋运动做了大概计算。 相似文献
12.
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
13.
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
14.
15.
1 IntroductionBacteriaandtheiractivitiesplayanimportantroleintheelementalbiogeochemicalcyclesandenergytransformingintheocean (Zhenetal.1 997) .DortchandPackard(1 989) proposedthatfoodwebsintheeutrophicwatersaredominatedbythebiomassofprimaryproducerswhilefoodwebsintheoligotrophicwatersaredominatedbythebiomassofmicrobes.Heterotrophicbacteriahadbeenshowntoplayanimportantroleinthedecompositionoflarge ,rapidlysinkingorganicparticleswithinandbelowtheeuphot iczone ,andfurthertoaffecttheelementaldyn… 相似文献
16.
V. Delouille J. De Patoul J. F. Hochedez L. Jacques J. P. Antoine 《Solar physics》2005,228(1-2):301-321
The extreme ultraviolet imaging telescope (EIT) of SOHO offers a unique record of the solar atmosphere for its sampling in
temperature, field of view, resolution, duration, and cadence. To investigate globally and locally its topology and evolution
during the solar cycle, we consider a multi-scale approach, and more precisely we use the wavelet spectrum.
We present three results among the applications of such a procedure. First, we estimate the typical dimension of the supergranules
as seen in the 30.4 nm passband, and we show that the evolution of the characteristic network scale is almost in phase with
the solar cycle. Second, we build pertinent time series that give the evolution of the signal energy present in the corona
at different scales. We propose a method that detects eruptions and post-flaring activity in EUV image sequences. Third, we
introduce a new way to extract active regions in EIT images, with perspectives in, e.g., long-term irradiance analysis. 相似文献
17.
18.
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
19.
20.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during
1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal
that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles.
The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease
of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles
of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed. 相似文献