全文获取类型
收费全文 | 46897篇 |
免费 | 1039篇 |
国内免费 | 381篇 |
专业分类
测绘学 | 1435篇 |
大气科学 | 3639篇 |
地球物理 | 9887篇 |
地质学 | 15945篇 |
海洋学 | 3894篇 |
天文学 | 10980篇 |
综合类 | 138篇 |
自然地理 | 2399篇 |
出版年
2021年 | 431篇 |
2020年 | 490篇 |
2019年 | 542篇 |
2018年 | 1221篇 |
2017年 | 1178篇 |
2016年 | 1518篇 |
2015年 | 919篇 |
2014年 | 1396篇 |
2013年 | 2457篇 |
2012年 | 1556篇 |
2011年 | 1922篇 |
2010年 | 1764篇 |
2009年 | 2315篇 |
2008年 | 1977篇 |
2007年 | 1965篇 |
2006年 | 1849篇 |
2005年 | 1404篇 |
2004年 | 1439篇 |
2003年 | 1289篇 |
2002年 | 1333篇 |
2001年 | 1168篇 |
2000年 | 1084篇 |
1999年 | 951篇 |
1998年 | 932篇 |
1997年 | 952篇 |
1996年 | 747篇 |
1995年 | 718篇 |
1994年 | 668篇 |
1993年 | 570篇 |
1992年 | 498篇 |
1991年 | 508篇 |
1990年 | 510篇 |
1989年 | 505篇 |
1988年 | 444篇 |
1987年 | 531篇 |
1986年 | 482篇 |
1985年 | 566篇 |
1984年 | 695篇 |
1983年 | 598篇 |
1982年 | 588篇 |
1981年 | 536篇 |
1980年 | 454篇 |
1979年 | 453篇 |
1978年 | 461篇 |
1977年 | 400篇 |
1976年 | 352篇 |
1975年 | 365篇 |
1974年 | 339篇 |
1973年 | 377篇 |
1972年 | 269篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Carbon isotopic fractionation between CO2 vapour,silicate and carbonate melts: an experimental study to 30 kbar 总被引:5,自引:0,他引:5
David P. Mattey W. R. Taylor D. H. Green C. T. Pillinger 《Contributions to Mineralogy and Petrology》1990,104(4):492-505
The carbon isotopic fractionation between CO2 vapour and sodamelilite (NaCaAlSi2O7) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag2C2O4 was the source of CO2 and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the 13C of coexisting vapour was determined directly by capsule piercing. CO2 solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO2 dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO2 reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO2 solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO2 vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2, favouring13C enrichment in CO2 vapour. The results are maxima and are independent of pressure and temperature. Similar values of 2 are obtained for the carbon isotopic fractionation between CO2 vapour and carbonate melts at 1300°–1400° C and 20–30 kbars. 相似文献
992.
P. R. Marchant 《Astrophysics and Space Science》1981,79(2):521-523
The motion of a large (100 Mpc radius) volume of local matter with respect to the frame in which the thermal cosmic background radiation (CBR) appears isotropic is shown to produce a contribution to anisotropy in the arrival directions of the highest energy (>1019 eV) cosmic rays. The magnitude of the effect predicted is 1%, below that currently observed and below the sensitivity of present experiments. 相似文献
993.
Accurate numerical continuation of families of plane symmetric direct periodic orbits around the large primary in the Sun-Jupiter case of the restricted problem of three bodies allows the determination of the vertical branching points where families of three-dimensional symmetric periodic orbits bifurcate from the planar ones. Three families of plane periodic orbits, and the initial segments of ten bifurcating families of three-dimensional ones are determined. The stability of these families is examined and examples of their orbits are illustrated. 相似文献
994.
We present statistical distributions of Jovian perturbations on short-period comet orbits resulting from accurate numerical integrations. Our sample of 60, 000 cometary orbits with low inclinations and random orientations is characterized by perihelia between 0 and 7 AU and aphelia between 4 and 13 AU. The perturbations considered are those experienced because of Jupiter's gravitation per orbital revolution by the comets. Regularization and accurate step-length control in the numerical integration gives statistical results appreciably different from those computed by Rickman and Vaghi (1978). Their use of a crude method of integration led to erroneous results for close encounters. Strong asymmetries of the distributions, in particular for the extreme tails, are observed for perihelion- or aphelion-tangent orbits. These orbits are also shown to experience the strongest energy perturbations on the average. Some results concerning the perturbations of Tisserand parameters are indicated. The perturbation distributions for the angular elements are described and discussed. The role of the minimum distance from Jupiter as an indicator of perturbations is investigated. 相似文献
995.
F. Kneer G. Scharmer W. Mattig A. Wyller G. Artzner P. Lemaire J. C. Vial 《Solar physics》1981,69(2):289-300
Observations with the French (L.P.S.P.) experiment on board OSO-8 of a sunspot and nearby plage region are described. The behaviour of the emission cores of the Ca II H and K and Mg II h and k resonance lines is very similar and the correspondence in intensity between the four lines persists in all observed features. In contrast, the Lyman lines show little correlation with the other lines. Their emission regions appear broader in the spectroheliograms than the underlying sunspot structure and must not necessarily possess a counterpart in lower layers. From the central intensity of L above the umbra an electron density of 4.3 × 1010 cm-3 n
e
* 2.3 × 1011 cm-3 at 20 000 K is estimated.Mitteilungen aus dem Kiepenheuer-Institut Nr. 186.Stockholm Observatorium, S-13300 Saltsjöbaden, Sweden.Laboratoire de Physique Stellaire et Planétaire, CNRS, P.O. Box 10, F-91370 Verrières-le-Buisson, France. 相似文献
996.
A time dependent model for the flow of gas in a spicule is studied. In this model, the flow occurs in a magnetic flux sheath. Starting from hydrostatic equilibrium, the flux sheath is allowed to collapse normal to itself. The collapse induces a flow of gas along the magnetic field and this flow is identified as a spicule. A variety of sheath geometries and velocity patterns for the normal flow have been studied. It is observed that a large curvature in the field geometry and a large initial value for the normal flow are necessary to achieve spicule-like velocities. The duration for which a large velocity of normal flow is required is much shorter than the average lifetime of a spicule. It is proposed that the initial rapid collapse occurs during an impulsive spicule phase and it is the subsequent gradual relaxation of the flow which is observed as a spicule. 相似文献
997.
Curson P 《Population geography : a journal of the Association of Population Geographers of India》1981,3(1-2):77-95
"This paper comments upon the role of remittances in a variety of emigrant situations, traces the way remittance patterns change over time and evaluates the importance of remittance payments for personal and national economies. It considers the concept of 'remittance-societies" in the South Pacific and elsewhere, and briefly examines the various mechanisms involved in the emigration-remittance cycle, including the expectation of interactions and interdependence that exists between migrants and their home-based kin, and the sacrifices and hardships often endured by many migrants to meet obligations back home." 相似文献
998.
999.
Part I gives a survey of the drastic revision of cosmic plasma physics which is precipitated by the exploration of the magnetosphere throughin situ measurements. The pseudo-plasma formalism, which until now has almost completely dominated theoretical astrophysics, must be replaced by an experimentally based approach involving the introduction of a number of neglected plasma phenomena, such as electric double layers, critical velocity, and pinch effect. The general belief that star light is the main ionizer is shown to be doubtful; hydromagnetic conversion of gravitational and kinetic energy may often be much more important.In Part II the revised plasma physics is applied to dark clouds and star formation. Magnetic fields do not necessarily counteract the contraction of a cloud; they may just as well pinch the cloud. Magnetic compression may be the main mechanism for forming interstellar clouds and keeping them together.Part III treats the formation of stars in a dusty cosmic plasma cloud. Star formation is due to an instability, but it is very unlikely that it has anything to do with the Jeans instability. A reasonable mechanism is that the sedimentation of dust (including solid bodies of different size) is triggering off a gravitationally assisted accretion. A stellesimal accretion analogous to the planetesimal accretion leads to the formation of a star surrounded by a very low density hollow in the cloud. Matter falling in from the cloud towards the star is the raw material for the formation of planets and satellites.The study of the evolution of a dark cloud leads to a scenario of planet formation which is reconcilable with the results obtained from studies based on solar system data. This means that the new approach to cosmical plasma physics discussed in Part I logically leads to a consistent picture of the evolution of dark clouds and the formation of solar systems. 相似文献
1000.
P. Brosche 《Astrophysics and Space Science》1978,57(2):463-467
The empirical evidence for a connection between type and relative angular momentum of galaxies is reviewed and some constraints for the theoretical explanation are discussed.
.相似文献