全文获取类型
收费全文 | 27901篇 |
免费 | 4871篇 |
国内免费 | 6330篇 |
专业分类
测绘学 | 1219篇 |
大气科学 | 6009篇 |
地球物理 | 7449篇 |
地质学 | 13453篇 |
海洋学 | 2932篇 |
天文学 | 1752篇 |
综合类 | 3047篇 |
自然地理 | 3241篇 |
出版年
2024年 | 82篇 |
2023年 | 391篇 |
2022年 | 1106篇 |
2021年 | 1240篇 |
2020年 | 1036篇 |
2019年 | 1164篇 |
2018年 | 1506篇 |
2017年 | 1349篇 |
2016年 | 1629篇 |
2015年 | 1282篇 |
2014年 | 1610篇 |
2013年 | 1565篇 |
2012年 | 1426篇 |
2011年 | 1480篇 |
2010年 | 1558篇 |
2009年 | 1581篇 |
2008年 | 1427篇 |
2007年 | 1360篇 |
2006年 | 1106篇 |
2005年 | 950篇 |
2004年 | 829篇 |
2003年 | 877篇 |
2002年 | 775篇 |
2001年 | 767篇 |
2000年 | 978篇 |
1999年 | 1430篇 |
1998年 | 1167篇 |
1997年 | 1152篇 |
1996年 | 1041篇 |
1995年 | 902篇 |
1994年 | 861篇 |
1993年 | 720篇 |
1992年 | 568篇 |
1991年 | 443篇 |
1990年 | 330篇 |
1989年 | 301篇 |
1988年 | 279篇 |
1987年 | 177篇 |
1986年 | 152篇 |
1985年 | 111篇 |
1984年 | 76篇 |
1983年 | 61篇 |
1982年 | 74篇 |
1981年 | 54篇 |
1980年 | 30篇 |
1979年 | 28篇 |
1978年 | 12篇 |
1977年 | 5篇 |
1976年 | 9篇 |
1958年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
21.
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
22.
23.
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
24.
A formula for the thickness of a shear band formed in saturated soils under a simple shear or a combined stress state has been proposed. It is shown that the shear band thickness is dependent on the pore pressure properties of the material and the dilatancy rate, but is independent of the details of the combined stress state. This is in accordance with some separate experimental observations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
25.
26.
Recent Doppler velocity measurements have revealed the existence of two planets orbiting the star HD 12661 on medium-eccentricity orbits. The inner planet has a period of 263.6 d and a mass of 2.3 Jupiter masses, and the outer planet has a period of 1444.5d and a mass of 1.57 Jupiter masses. The stability of this system requires the two planets to be in a state of mean-motion orbit resonances. By numerical method we have studied the orbit migration and stability of the system in its early ages under the action of the proto-stellar disk, and calculated the probabilities of the planets being captured into the mean -motion resonances during their migrations. It is found that at present the two planets are probably situated at the edge of the 11:2 mean-motion resonance and are in chaotic motions. This result may be helpful to clarify the arguments on the present configuration. Besides, it is indicated that very probably, after the formation of the system, the gaseous disk has almost disappeared before the planets migrated to the present configuration. 相似文献
27.
Stress wave attenuation across fractured rock masses is a great concern of underground structure safety. When the wave amplitude is large, fractures experience nonlinear deformation during the wave propagation. This paper presents a study on normal transmission of P‐wave across parallel fractures with nonlinear deformational behaviour (static Barton–Bandis model). The results show that the magnitude of transmission coefficient is a function of incident wave amplitude, nondimensional fracture spacing and number of fractures. Two important indices of nondimensional fracture spacing are identified, and they divide the area of nondimensional fracture spacing into three parts (individual fracture area, transition area and small spacing area). In the different areas, the magnitude of transmission coefficient has different trends with nondimensional fracture spacing and number of fractures. In addition, the study reveals that under some circumstances, the magnitude of transmission coefficient increases with increasing number of fractures, and is larger than 1. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
28.
29.
O.V.Chumak 《中国天文和天体物理学报》2003,3(2):175-182
We present a study of the relationship between integral area and corresponding total magnetic flux for solar active regions. It is shown that some of these relationships are satisfied to simple power laws. Fractal examination showed that some of these power laws can not be justified inside the simple models of stationary magnetic flux tube aggregation. All magnetic fluxes and corresponding areas were calculated using the data measured with the Solar Magnetic Field Telescope of the Huairou Solar Observing Station in Beijing. 相似文献
30.
对1995年至2001年《Natue》和《Science》上发表的天文学论文的统计表明,行星天文学领域的论文数量明显超过天文学的其它分支学科,占天文学总论文数的1/3左右。从这个角度来看,行星天文学是天文学最活跃和重要的分支学科之一。对这些论文具体内容的分析可以给出当前行星天文学领域的若干热点问题。相比之下,我国对这一重要领域的关注和投入还远远不够。 相似文献