全文获取类型
收费全文 | 66769篇 |
免费 | 689篇 |
国内免费 | 1212篇 |
专业分类
测绘学 | 2341篇 |
大气科学 | 4990篇 |
地球物理 | 12851篇 |
地质学 | 28445篇 |
海洋学 | 4437篇 |
天文学 | 10190篇 |
综合类 | 2232篇 |
自然地理 | 3184篇 |
出版年
2020年 | 180篇 |
2019年 | 162篇 |
2018年 | 7014篇 |
2017年 | 6331篇 |
2016年 | 4099篇 |
2015年 | 633篇 |
2014年 | 595篇 |
2013年 | 1313篇 |
2012年 | 2318篇 |
2011年 | 5347篇 |
2010年 | 4668篇 |
2009年 | 5090篇 |
2008年 | 4188篇 |
2007年 | 5115篇 |
2006年 | 948篇 |
2005年 | 1417篇 |
2004年 | 1467篇 |
2003年 | 1492篇 |
2002年 | 1168篇 |
2001年 | 669篇 |
2000年 | 692篇 |
1999年 | 601篇 |
1998年 | 574篇 |
1997年 | 567篇 |
1996年 | 468篇 |
1995年 | 465篇 |
1994年 | 448篇 |
1993年 | 416篇 |
1992年 | 393篇 |
1991年 | 335篇 |
1990年 | 385篇 |
1989年 | 300篇 |
1988年 | 343篇 |
1987年 | 379篇 |
1986年 | 336篇 |
1985年 | 490篇 |
1984年 | 530篇 |
1983年 | 533篇 |
1982年 | 431篇 |
1981年 | 443篇 |
1980年 | 461篇 |
1979年 | 383篇 |
1978年 | 399篇 |
1977年 | 347篇 |
1976年 | 386篇 |
1975年 | 344篇 |
1974年 | 382篇 |
1973年 | 368篇 |
1972年 | 233篇 |
1971年 | 187篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
92.
S. Baes-Fischlmair W.W. Zeilinger J.-C. Vega-Beltran J.E. Beckman 《Astrophysics and Space Science》2003,284(2):735-738
It is assumed that the two-fold disc-wide symmetry of spirals is caused by density waves, but also the potential of a bar
component may have a significant influence on structural properties. The strength of the bar component appears to be anti-correlated
with the degree of symmetry of star-forming regions in the spiral arms (Rozas et al., 1998). We present new results of R and
Hα surface photometry of a sample of bright barred spirals. A photometric decompositon of the galaxy components is carried
out in order to make a more accurate measurement of the strength of the bar and its interrelation to gas and stars in the
disc.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
93.
DharmalingamVENUGOPAL 《山地科学学报》2004,1(1):74-80
The Nilgiri Mountains of south India is considered unique by anthropologists, geologists, climatologists, botanists as well as tourists. It has remained a subject of constant study and research over the last two centuries. Man-nature balance had continued undisturbed in the Nilgiris for thousands of years until the early 19th century when it became a British colony attracting, in due course, various developmental activities. Subsequently, the Nilgiris and its popular hill stations emerged as favourite places for the British population in India for rest and recuperation, game and for raising commercial plantations. In the process, the traditional indigenous crops were replaced by “English“ vegetables and the natural forests gave way to commercial plantations of coffee, tea and other exotic species of trees. 相似文献
94.
高从堦 《中国海洋湖沼学报》2003,21(1):40-45
Seawater desalination has been people‘s fond dream since ancient times,the dream is now becoming a reality.This paper presents a brief development history of reverse osmosis.Much attention was paid to innovative development in mebranes,modules.equipments and applied technology,including asymmetric and composite membranes,spiral-wound element and hollow fiber module,energy recovery equipments and different technological processes.The extension of reverse osmosis,such as desalination,pre-concentration,integrated processes and nanofiltration,is also briefly mentioned. 相似文献
95.
P. G. Jonker M. van der Klis C. Kouveliotou M. Méndez W. H. G. Lewin T. Belloni 《Monthly notices of the Royal Astronomical Society》2003,346(2):684-688
We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of 1.7 ± 0.2 h , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc. 相似文献
96.
M. G. Revnivtsev 《Astronomy Letters》2003,29(6):383-386
Analysis of the RXTE slew data in October 1996 revealed a weak X-ray burst from the millisecond pulsar SAX J 1808.4-3658. The 3–20-keV energy spectrum of the source can be described by a power law with an index of 2.0 and a(3-to 20 keV) luminosity of ~1.4×1035 erg s?1 (the distance to the source was taken to be 2.5 kpc). Because of the short exposure time, we failed to detect weak pulsations at a frequency of 401 Hz in the source. The (2σ) upper limit of the pulse fraction is ~13%. 相似文献
97.
98.
The paper provides a new stereo‐analytical method, which is a combination of the stereographic method and analytical methods, to separate finite removable blocks from the infinite and tapered blocks in discontinuous rock masses. The methodology has applicability to both convex and concave blocks. Application of the methodology is illustrated through examples. Addition of this method to the existing block theory procedures available in the literature improves the capability of block theory in solving practical problems in rock engineering. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
99.
Infiltration experiments have been performed at three sites along a well-known catena under virgin tropical rain forest using a portable sprinkling infiltrometer. Experimentally determined infiltration curves are presented. Infiltration curves are also simulated on the basis of the Mein-Larson equation. The parameters for this model have been obtained from the infiltration curves (saturated conductivity) and simple soil moisture determinations (fillable porosity). The agreement between experimentally determined and modelled infiltration is reasonable, provided (a) saturated conductivity as derived from the experimental data is corrected, (b) a storage parameter, also derived from the experimental data, is added to the Mein-Larson model, and (c) the decline in soil porosity with depth is either small or occurs abruptly at shallow depth. Comparison of observed infiltration rates with rainfall intensity shows that Horton Overland Flow has to occur naturally at least on the middle and lower section of the catena. Despite the fact that most parameters can be estimated in principle from basic soil data, it remains advisable to obtain sprinkling infiltrometer field measurements, because of soil variability due to dynamic surface conditions, macroporosity, air entrapment, and irregularity of the wetting front. 相似文献
100.