全文获取类型
收费全文 | 42058篇 |
免费 | 758篇 |
国内免费 | 323篇 |
专业分类
测绘学 | 977篇 |
大气科学 | 3033篇 |
地球物理 | 8192篇 |
地质学 | 14485篇 |
海洋学 | 3899篇 |
天文学 | 9843篇 |
综合类 | 74篇 |
自然地理 | 2636篇 |
出版年
2021年 | 414篇 |
2020年 | 414篇 |
2019年 | 459篇 |
2018年 | 957篇 |
2017年 | 869篇 |
2016年 | 1118篇 |
2015年 | 633篇 |
2014年 | 1024篇 |
2013年 | 2121篇 |
2012年 | 1159篇 |
2011年 | 1657篇 |
2010年 | 1507篇 |
2009年 | 2014篇 |
2008年 | 1689篇 |
2007年 | 1761篇 |
2006年 | 1623篇 |
2005年 | 1339篇 |
2004年 | 1294篇 |
2003年 | 1274篇 |
2002年 | 1221篇 |
2001年 | 1074篇 |
2000年 | 1000篇 |
1999年 | 819篇 |
1998年 | 824篇 |
1997年 | 840篇 |
1996年 | 684篇 |
1995年 | 677篇 |
1994年 | 622篇 |
1993年 | 577篇 |
1992年 | 533篇 |
1991年 | 503篇 |
1990年 | 510篇 |
1989年 | 502篇 |
1988年 | 487篇 |
1987年 | 559篇 |
1986年 | 494篇 |
1985年 | 617篇 |
1984年 | 654篇 |
1983年 | 599篇 |
1982年 | 534篇 |
1981年 | 584篇 |
1980年 | 479篇 |
1979年 | 465篇 |
1978年 | 437篇 |
1977年 | 436篇 |
1976年 | 387篇 |
1975年 | 379篇 |
1974年 | 382篇 |
1973年 | 385篇 |
1971年 | 226篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
We present solutions for the effective stress induced by gas flow through a porous solid into a borehole resulting from sudden pressure reduction. Tensile effective stress that exceeds the strength of the solid will lead to borehole failure. This has applications to the intentional creation of cavities, relevant to the efficient recovery of coalbed methane, and the avoidance of borehole stability problems in conventional gas production. 相似文献
103.
The sub-iron-iron flux ratio in cosmic rays at an atmospheric depth of 9.8 g cm–2 has been estimated using a balloon-borne CR-39 (HCB) stack launched from Alice Springs for 32 hours. The recovered and chemically etched plates were analysed optically and the measured etch pit diameters yielded the sub-iron-iron flux ratio at the flight altitude. The sub-iron-iron flux ratio has been corrected for the top of the atmosphere by considering the loss of heavy ions due to nuclear interaction and fragmentation. The present result has been compared with the result expected from the source composition derived by Protheroeet al. as well as other authors. 相似文献
104.
Arnaud M. Aubourg E. Bareyre P. Br';ehin S. Caridroit R. de Kat J. Dispau G. Djidi K. Gros M. Lachièze-Rey M. Laigneau Y. Laurent B. Lesquoy E. Lavocat Ph. Magneville C. Mazeau B. Milsztajn A. Moscoso L. Pasquaud J. Paul B. Perrin P. Petibon J. Piret Y. Queinnec F. Rich J. Spiro M. de Trogoff J. Vigroux L. Zylberajch S. Ansari R. Cavalier F. Moniez M. Beaulieu J. P. Ferlet R. Grison Ph. Vidal-Madjar A. Adrianzyk G. Berger J. P. Burnage R. Delclite J. C. Kohler D. Magnan R. Richaud A. Guibert J. Moreau O. Tajahmady F. Baranne A. Maurice E. Prévôt L. Gry C. 《Experimental Astronomy》1994,4(3-4):265-278
A 20cm
2
CCD mosaic camera has been especially built to search for dark galactic halo objects by the gravitational microlensing effect. The sensitive area is made of 16 edge-buttable CCDs developped by Thomson-CTS, with 23×23 µm
2
pixels. The 35 kg camera housing and mechanical equipment is presented. The associated electronics and data acquisition system are described in a separate paper. The camera resides at the focal plane of a 40 cm, f/10, Ferson reflector. The instrument has been in operation since December 1991 at the La Silla Observatory (ESO). 相似文献
105.
Mark D. Schwartz 《The Professional geographer》1991,43(1):77-91
Both subjective and objective schemes for air mass classification offer advantages to climatological studies. An integrated approach is developed and applied to 850 mb level data in the north central United States. Each air mass type was initially delimited by a subjective analysis. Final limits were then determined with an objective technique that separates temperature curves into normal components based on interpretation from the subjective method. Continental air was most frequent in all seasons except summer. The methodology also allows estimation of classification error. 相似文献
106.
107.
This paper considers the transient response of a pressurized long cylindrical cavity in an infinite poroelastic medium. To obtain transient solutions, Biot's equations for poroelastodynamics are specialized for this problem. A set of exact general solutions for radial displacement, stresses, pore pressure and discharge are derived in the Laplace transform space by using analytical techniques. Solutions are presented for three different types of prescribed transient radial pressures acting on the surface of a permeable as well as an impermeable cavity surface. Time domain solutions are obtained by inverting Laplace domain solutions using a reliable numerical scheme. A detailed parametric study is presented to illustrate the influence of poroelastic material parameters and hydraulic boundary conditions on the response of the medium. Comparisons are also presented with the corresponding ideal elastic solutions to portray the poroelastic effects. It is noted that the maximum radial displacement and hoop stress at the cavity surface are substantially higher than the classical static solutions and differ considerably from the transient elastic solutions. Time histories and radial variations of displacement, hoop stress, pore pressure and fluid discharge corresponding to a cavity in two representative poroelastic materials are also presented. 相似文献
108.
Several measurements have been performed to find antiprotons in the primary cosmic radiation. Because it is difficult to get completely separated secondary produced antiprotons primary ones, calculations based on accelerator results have been performed for the flux of secondary produced antideuterons and antihelium. 相似文献
109.
Ralph D. Lorenz A. J. Timothy Jull Timothy D. Swindle Jonathan I. Lunine 《Meteoritics & planetary science》2002,37(6):867-874
Abstract— We explore the likely production and fate of 14C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of 14C might place on Titan's photochemical, atmospheric transport and surface‐atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic‐ray flux limits the production of 14C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ?9 atom/cm2/s, ?4x higher than Earth. The fate and detectability of 14C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the 14C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are “freshest”. 相似文献
110.
C. Caroubalos D. Maroulis N. Patavalis J.-L. Bougeret G. Dumas C. Perche C. Alissandrakis A. Hillaris X. Moussas P. Preka-Papadema A. Kontogeorgos P. Tsitsipis G. Kanelakis 《Experimental Astronomy》2001,11(1):23-32
We present the new solar radiospectrograph of the University of Athensoperating at the Thermopylae Station since 1996. Observations cover thefrequency range from 110 to 688 MHz. The radiospectrograph has a 7-meterparabolic antenna and two receivers operating in parallel. One is a sweepfrequency receiver and the other a multichannel acousto-optical receiver.The data acquisition system consists of a front-end VME based subsystem anda Sun Sparc-5 workstation connected through Ethernet. The two subsystems areoperated using the VxWorks real-time package. The daily operation is fullyautomated: pointing of the antenna to the sun, starting and stopping theobservations at pre-set times, data acquisition, data compression by`silence suppression', and archiving on DAT tapes. The instrument can beused either by itself to study the onset and evolution of solar radio bursts or in conjunction with other instruments including theNançay Decametric Array and the WIND/WAVES RAD1 and RAD2 low frequencyreceivers to study associated interplanetary phenomena. 相似文献