全文获取类型
收费全文 | 85837篇 |
免费 | 985篇 |
国内免费 | 819篇 |
专业分类
测绘学 | 2120篇 |
大气科学 | 5691篇 |
地球物理 | 16180篇 |
地质学 | 33224篇 |
海洋学 | 7423篇 |
天文学 | 18809篇 |
综合类 | 344篇 |
自然地理 | 3850篇 |
出版年
2022年 | 565篇 |
2021年 | 888篇 |
2020年 | 991篇 |
2019年 | 1065篇 |
2018年 | 4245篇 |
2017年 | 3843篇 |
2016年 | 3469篇 |
2015年 | 1289篇 |
2014年 | 2208篇 |
2013年 | 3809篇 |
2012年 | 3198篇 |
2011年 | 5027篇 |
2010年 | 4399篇 |
2009年 | 5250篇 |
2008年 | 4496篇 |
2007年 | 5023篇 |
2006年 | 3045篇 |
2005年 | 2302篇 |
2004年 | 2145篇 |
2003年 | 2159篇 |
2002年 | 1948篇 |
2001年 | 1675篇 |
2000年 | 1569篇 |
1999年 | 1265篇 |
1998年 | 1204篇 |
1997年 | 1241篇 |
1996年 | 1019篇 |
1995年 | 959篇 |
1994年 | 857篇 |
1993年 | 760篇 |
1992年 | 767篇 |
1991年 | 745篇 |
1990年 | 785篇 |
1989年 | 673篇 |
1988年 | 641篇 |
1987年 | 684篇 |
1986年 | 612篇 |
1985年 | 806篇 |
1984年 | 867篇 |
1983年 | 829篇 |
1982年 | 772篇 |
1981年 | 708篇 |
1980年 | 688篇 |
1979年 | 587篇 |
1978年 | 636篇 |
1977年 | 546篇 |
1976年 | 496篇 |
1975年 | 502篇 |
1974年 | 496篇 |
1973年 | 521篇 |
排序方式: 共有10000条查询结果,搜索用时 569 毫秒
251.
252.
The first year of sodium nightglow observations from Natal (6°S, 35°W) are examined. Time variations appear to follow a pattern of their own, different from low latitude results. The major seasonal peak occurs in September-October and the average variation during the night decreases from dusk to dawn. Statistics on cloud coverage show that Natal has roughly only about 3 clear hours per night. The best observing period is April with an average of 5 clear hours per night. 相似文献
253.
254.
We investigate the evolution of rotation period and spindown age of a pulsar whose surface magnetic field undergoes a phase
of growth. Application of these results to the Crab pulsar strongly indicates that its parameters cannot be accounted for
by the field growth theories. 相似文献
255.
256.
S. Casertano M. G. Lattanzi M. A. C. Perryman A. Spagna 《Astrophysics and Space Science》1996,241(1):89-104
The proposed baseline GAIA mission will be able to detect the astrometric signature of Jupiter-size planets around of the order of a million stars, using either global or narrow-angle astrometry. If the mission can realize the higher astrometric accuracy that photon statistics allows for bright stars, lower-mass planets (from Earth size to ten times larger) can be found around ten to a few hundred stars. 相似文献
257.
V. Yu. Terebizh 《Astrophysics and Space Science》1992,193(2):269-288
Image restoration, computerized tomography, and other similar problems are considered as a unified class of stochastic inverse problems. The conventional approach to these problems that proceeds from some integral or functional equations suffers from three main shortcomings: (i) subjectivity, (ii) inability to account for the inner (radiational) noise, and (iii) inability to include the fundamental concept of the natural limit of solution accuracy. A general approach is developed, the Statistical Parameterization of Inverse Problems (SPIPR), that takes into account both the inner and external random noise and gives an explicit form of the above-mentioned natural limit. Applications of the SPIPR to various problems show that the maximum likelihood method as the concrete way to obtain an object estimate has practically limiting efficiency.Two new fields of applications of the SPIPR are outlined along with the image restoration problem: the elimination of blurring due to atmosphere turbulence and reconstruction of an object structure in the computerized tomography. The expressions for the main distribution function in all these problems are found. The corresponding real examples and model cases are considered as well. 相似文献
258.
259.
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
260.
Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment. 相似文献