全文获取类型
收费全文 | 42378篇 |
免费 | 19666篇 |
国内免费 | 43974篇 |
专业分类
测绘学 | 3374篇 |
大气科学 | 22588篇 |
地球物理 | 14357篇 |
地质学 | 38824篇 |
海洋学 | 16828篇 |
天文学 | 1458篇 |
综合类 | 5262篇 |
自然地理 | 3327篇 |
出版年
2024年 | 175篇 |
2023年 | 423篇 |
2022年 | 913篇 |
2021年 | 1212篇 |
2020年 | 2313篇 |
2019年 | 5754篇 |
2018年 | 6005篇 |
2017年 | 5715篇 |
2016年 | 5858篇 |
2015年 | 5121篇 |
2014年 | 4685篇 |
2013年 | 5235篇 |
2012年 | 4921篇 |
2011年 | 4742篇 |
2010年 | 4697篇 |
2009年 | 3972篇 |
2008年 | 3274篇 |
2007年 | 3276篇 |
2006年 | 2797篇 |
2005年 | 2745篇 |
2004年 | 2982篇 |
2003年 | 2758篇 |
2002年 | 2569篇 |
2001年 | 2376篇 |
2000年 | 1990篇 |
1999年 | 2151篇 |
1998年 | 2095篇 |
1997年 | 2083篇 |
1996年 | 1728篇 |
1995年 | 1635篇 |
1994年 | 1464篇 |
1993年 | 1411篇 |
1992年 | 1186篇 |
1991年 | 897篇 |
1990年 | 818篇 |
1989年 | 680篇 |
1988年 | 590篇 |
1987年 | 439篇 |
1986年 | 361篇 |
1985年 | 284篇 |
1984年 | 305篇 |
1983年 | 177篇 |
1982年 | 229篇 |
1981年 | 162篇 |
1980年 | 118篇 |
1979年 | 116篇 |
1978年 | 52篇 |
1977年 | 48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 109 毫秒
11.
初轨计算中的病态分析 总被引:5,自引:0,他引:5
本文对现有初轨计算方法进行病态性分析与误差分析;研究结果表明:病态对现有初轨算法的影响,主要来源于法方程系数中包含观测误差.系数行列式愈大,定轨精度的损失愈多,当■被随机误差项△μ淹盖时,现有初轨算法将失效.此外,仿真结果还显示:■与△μ的大小还极大地依赖观测弧段的空间位置,当观测弧段包含近站点作为中点时,■最大,而■小,此时定轨精度较高;当观测弧段位于近站点的某一侧时,■小,而■大,此时定轨精度较低,观测弧段愈偏离近站点,病态影响愈大;因而在观测时,应尽量使观测弧段与近站点对称(此时μ值较大),这是提高短弧定轨的一种有效途径. 相似文献
12.
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
13.
Vertical drains are usually installed in subsoil consisting of several layers. Due to the complex nature of the problem, over the past decades, the consolidation properties of multi‐layered ground with vertical drains have been analysed mainly by numerical methods. An analytical solution for consolidation of double‐layered ground with vertical drains under quasi‐equal strain condition is presented in this paper. The main steps for the computation procedure are listed. The convergence of the series solution is discussed. The comparisons between the results obtained by the present analytical method and the existing numerical solutions are described by figures. The orthogonal relation for the system of double‐layered ground with vertical drains is proven. Finally, some consolidation properties of double‐layered ground with vertical drains are analysed. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
14.
1 IntroductionBacteriaandtheiractivitiesplayanimportantroleintheelementalbiogeochemicalcyclesandenergytransformingintheocean (Zhenetal.1 997) .DortchandPackard(1 989) proposedthatfoodwebsintheeutrophicwatersaredominatedbythebiomassofprimaryproducerswhilefoodwebsintheoligotrophicwatersaredominatedbythebiomassofmicrobes.Heterotrophicbacteriahadbeenshowntoplayanimportantroleinthedecompositionoflarge ,rapidlysinkingorganicparticleswithinandbelowtheeuphot iczone ,andfurthertoaffecttheelementaldyn… 相似文献
15.
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
16.
17.
Starfish oocytes with intact germinal vesicles (GVs) were cut along desired planes with glass needles or ligated using silk
thread loops into two parts and allowed to mature in vitro, and inseminated. The experimental results showed that (1) only
the parts with GVs or partial GV contents (PGVCs) cleaved, those without any GV materials did not; but nucleated and non-nucleated
fragments cut from mature eggs were able to divide; (2) the development of animal parts of oocytes containing GVs or PGVCs
was like that of animal fragments of matured oocytes with female pronuclei; most of them gave rise to permanent blastulae,
and just a few formed ectodermal vesicles with a little primary mesenchyme; (3) a large part of vegetal fragments with GVs
or PGVCs, and the vegetal parts of mature eggs without female pronuclei developed into small but normal embryos; (4) the fragments
containing GVs or PGVCs obtained from the oocytes along a plane parallel to the animal-vegetal (A-V) axis developed as normally
as the halves (with or without female pronuclei) severed from mature eggs along the same axis. Based on the data above, it
was concluded that (1) the non-chromatin materials in the oocyte GVs are indispensable for successful fertilization and cleavage
of starfish eggs; (2) some factor (s) located asymmetrically in the vegetal hemispheres of starfish oocytes is (are) responsible
for formation of the archenteron and primary mesenchyme. It is evident from the above findings that the oocyte cytoplasm of
the starfish had already regionalized before the GV break-down.
Contribution No. 1722 from the Institute of Oceanology, Academia Sinica 相似文献
18.
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
19.
青藏块体东北缘断层形变与中强地震 总被引:7,自引:4,他引:3
对祁连山-海原断裂带近期断层形变特征进行了初步研究,发现多场地,大范围的断层活动异常是中等强度地震发生的显著背景,并且往往与大陆地震活动的阶段性总体状况相呼应;区域形变存在明显的特征量,包括特征形态和特征时间,同一场地在不同地震前的异常特征具有重复性,但会受到背景差异显著的不同地震的影响,目前形变状况表明研究区仍具有发生中强地震的地壳运动背景。 相似文献
20.
在对嘉祥县石灰岩矿进行地质灾害危险性评估、矿山环境地质问题评价的基础上,建立了一套评价体系,其中地质灾害危险性评估分为2个评价指标,矿山环境地质问题评价分为2种要素3个指标,同时将矿山恢复治理难易程度也作为一个评价因子。评价过程中对各指标危害性大、中、小程度均单独赋值,然后将各指标的分值进行叠加,确定出矿山环境地质问题的等级,将概划出的13个评价单元分为极差、差、一般3个区。提出了强化矿山管理、植树造林、科学避让、修建拦水坝等保护与治理措施及建议。 相似文献