全文获取类型
收费全文 | 23160篇 |
免费 | 5338篇 |
国内免费 | 5590篇 |
专业分类
测绘学 | 2181篇 |
大气科学 | 4057篇 |
地球物理 | 6718篇 |
地质学 | 11444篇 |
海洋学 | 3118篇 |
天文学 | 1368篇 |
综合类 | 1977篇 |
自然地理 | 3225篇 |
出版年
2024年 | 122篇 |
2023年 | 379篇 |
2022年 | 1143篇 |
2021年 | 1351篇 |
2020年 | 1127篇 |
2019年 | 1475篇 |
2018年 | 1487篇 |
2017年 | 1465篇 |
2016年 | 1572篇 |
2015年 | 1610篇 |
2014年 | 1722篇 |
2013年 | 1861篇 |
2012年 | 1744篇 |
2011年 | 1782篇 |
2010年 | 1640篇 |
2009年 | 1590篇 |
2008年 | 1546篇 |
2007年 | 1338篇 |
2006年 | 1125篇 |
2005年 | 883篇 |
2004年 | 690篇 |
2003年 | 686篇 |
2002年 | 714篇 |
2001年 | 606篇 |
2000年 | 588篇 |
1999年 | 569篇 |
1998年 | 455篇 |
1997年 | 444篇 |
1996年 | 352篇 |
1995年 | 315篇 |
1994年 | 346篇 |
1993年 | 260篇 |
1992年 | 212篇 |
1991年 | 159篇 |
1990年 | 128篇 |
1989年 | 99篇 |
1988年 | 114篇 |
1987年 | 60篇 |
1986年 | 56篇 |
1985年 | 43篇 |
1984年 | 38篇 |
1983年 | 40篇 |
1982年 | 41篇 |
1981年 | 30篇 |
1980年 | 10篇 |
1979年 | 21篇 |
1978年 | 7篇 |
1958年 | 17篇 |
1957年 | 3篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
CHANGE OF NUTRIENT IMPORT AND EXPORT IN PROCESS OF RAINFALL IN AILAO MOUNTAIN OF YUNNAN PROVINCEGanJianmin(甘健民);XueJingyi(薛敬意... 相似文献
12.
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
13.
Non‐equilibrium concepts lead to a unified explanation of the formation of chondrules and chondrites
Milton BLANDER Arthur D. PELTON In‐Ho JUNG Richard WEBER 《Meteoritics & planetary science》2004,39(12):1897-1910
Abstract— Calculations of the formation of seven types of chondrules in Semarkona from a gas of solar composition were performed with the FACT computer program to predict the chemistries of oxides (including silicates), developed by the authors and their colleagues. The constrained equilibrium theory was used in the calculations with two nucleation constraints suggested by nucleation theory. The first constraint was the blocking of Fe and other metal gaseous atoms from condensing to form solids or liquids because of very high surface free energies and high surface tensions of the solid and liquid metals, respectively. The second constraint was the blocking of the condensation of solids and the formation of metastable liquid oxides (including silicates) well below their liquidus temperatures. Our laboratory experiments suggested subcooling of type IIA chondrule compositions of 400 degrees or more below the liquidus temperature. The blocking of iron leads to a supersaturation of Fe atoms, so that the partial pressure of Fe (pFe) is larger than the partial pressure at equilibrium (pFe(eq)). The supersaturation ratio S = pFe/pFe(eq) becomes larger than 1 and increases rapidly with a decrease in temperature. This drives the reaction Fe + H2O ? H2 + FeO to the right. With S = 100, the activity of FeO in the liquid droplet is 100 times as large as the value at equilibrium. The FeO activities are a function of temperature and provide relative average temperatures of the crystallization of chondrules. Our calculations for the LL3.0 chondrite Semarkona and our study of some non‐equilibrium effects lead to accurate representations of the compositions of chondrules of types IA, IAB, IB, IIA, IIAB, IIB, and CC. Our concepts readily explain both the variety of FeO concentrations in the different chondrule types and the entire process of chondrule formation. Our theory is unified and could possibly explain the formation of chondrules in all chondritic meteorites as well as provide a simple explanation for the complex chemistries of chondrites, and especially for type 3 chondrites. 相似文献
14.
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
15.
A numerical scheme is developed in order to simulate fluid flow in three dimensional (3‐D) microstructures. The governing equations for steady incompressible flow are solved using the semi‐implicit method for pressure‐linked equations (SIMPLE) finite difference scheme within a non‐staggered grid system that represents the 3‐D microstructure. This system allows solving the governing equations using only one computational cell. The numerical scheme is verified through simulating fluid flow in idealized 3‐D microstructures with known closed form solutions for permeability. The numerical factors affecting the solution in terms of convergence and accuracy are also discussed. These factors include the resolution of the analysed microstructure and the truncation criterion. Fluid flow in 2‐D X‐ray computed tomography (CT) images of real porous media microstructure is also simulated using this numerical model. These real microstructures include field cores of asphalt mixes, laboratory linear kneading compactor (LKC) specimens, and laboratory Superpave gyratory compactor (SGC) specimens. The numerical results for the permeability of the real microstructures are compared with the results from closed form solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
16.
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
17.
18.
首先指出目前星风吸积模型中理论上的不自洽,考虑到δr≠0,重新推导轨道参量变化方程,消去了理论上的不自洽.提出一个新模型:首次将星风吸积同内禀AGB星核合成模型结合起来计算钡(Ba)星的重元素超丰,并将计算结果与观测值进行了比较.各参量按标准情况取值时,计算结果不太理想.取Bondihoyle吸积率的0.5倍或取较大的星风速率时,对于较长轨道周期(P>1000天)的Ba星,计算结果与观测值基本符合;而较短轨道周期(P<600天)的Ba星,其重元素超丰机制可能是盘吸积 相似文献
19.
This paper deals with the formation processes and the palaeoenvironmental significance of relict slope deposits located on the uppermost part of the north Portugal mountains. For this purpose, seven key sites representative of the different lithofacies have been selected and analysed in detail. The data show that three main dynamic processes are responsible for the emplacement of regional fossil slope deposits: runoff, debris flows and dry grain flows. The ubiquity of these processes and the lack of frost‐related features or landforms do not support the existence of severe Pleistocene climates in this part of the lberian Peninsula as postulated by previous work. Pedological data gathered at one of the study sites show that a subalpine environment was probably present at 700–800 m altitude between 29 and 14 kyr. Using data from the Pyrenees Mountains, a 6.5 to 12°C depression in mean annual temperature has been tentatively postulated for this Pleniglacial period. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
20.
CARBON DYNAMICS OF WETLAND IN THE SANJIANG PLAIN 总被引:2,自引:0,他引:2
SONG Chang-chun WANG Yi-yong WANG Yue-si YAN Bai-xing WANG De-xuan ZHAO Zhi-chun LOU Yan-jing 《中国地理科学(英文版)》2003,13(3):228-231
1INTRODUCTIONWetlandsplayanimportant roleintheprocessofcar-bonstorage.Thetotalcarbonstoredindifferentkindsofwetlandsisabout15%-35%ofthetotalcarboninthegloballandsoils(POSTetal.,1982;GORHAM,1991).Inaddition,wetlandsaresignificantnaturalsources fortheatmospheric CH4 (MOORE,1994).It isestimatedthatabout110×1012gCH4 originates fromanaerobicdecompositioninthenaturalwetlands,CH4 emission fromthenaturalwetlandsis15%-30%oftheglobalCH4 emission andtheCH4 emission from thepeat land at hi… 相似文献