全文获取类型
收费全文 | 50762篇 |
免费 | 21157篇 |
国内免费 | 45141篇 |
专业分类
测绘学 | 3966篇 |
大气科学 | 23689篇 |
地球物理 | 16418篇 |
地质学 | 43097篇 |
海洋学 | 17415篇 |
天文学 | 1718篇 |
综合类 | 6111篇 |
自然地理 | 4646篇 |
出版年
2024年 | 231篇 |
2023年 | 599篇 |
2022年 | 1452篇 |
2021年 | 1838篇 |
2020年 | 2795篇 |
2019年 | 6208篇 |
2018年 | 6542篇 |
2017年 | 6153篇 |
2016年 | 6293篇 |
2015年 | 5504篇 |
2014年 | 5274篇 |
2013年 | 5781篇 |
2012年 | 5467篇 |
2011年 | 5367篇 |
2010年 | 5199篇 |
2009年 | 4487篇 |
2008年 | 3679篇 |
2007年 | 3677篇 |
2006年 | 3199篇 |
2005年 | 3011篇 |
2004年 | 3170篇 |
2003年 | 2882篇 |
2002年 | 2606篇 |
2001年 | 2440篇 |
2000年 | 2129篇 |
1999年 | 2359篇 |
1998年 | 2305篇 |
1997年 | 2287篇 |
1996年 | 1895篇 |
1995年 | 1746篇 |
1994年 | 1649篇 |
1993年 | 1533篇 |
1992年 | 1267篇 |
1991年 | 941篇 |
1990年 | 877篇 |
1989年 | 719篇 |
1988年 | 633篇 |
1987年 | 480篇 |
1986年 | 386篇 |
1985年 | 302篇 |
1984年 | 316篇 |
1983年 | 199篇 |
1982年 | 230篇 |
1981年 | 157篇 |
1980年 | 112篇 |
1979年 | 117篇 |
1978年 | 55篇 |
1977年 | 45篇 |
1971年 | 54篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
通过杂质对导电类型的影响的研究,得出了同种矿物的不同导电类型与矿物形成温度的规律:电子型(n型)导电矿物的形成温度大于电子与空穴混合型导电矿物的形成温度,又大于空穴型(p型)导电矿物的形成温度,绝缘体矿物的形成温度低于上述三种导电类型矿物。应用矿物热电图谱仪,天然金刚石半导体测试仪对闪锌矿、黄铁矿、金刚石等矿物的导电类型进行了测试,并探讨了导电类型成因,证实了上述规律的正确性。 相似文献
103.
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
104.
(姚建衢)(杨焕宗)COORDINATINGDEVELOPMENTOFAGRICULTURALRESOURCESANDENVIRONMENTOFTHETROPICREGIONINYUNNANPROVINCE¥YaoJianqu(Instituteof... 相似文献
105.
106.
107.
Simulation of double cold cores of the 35°N section in the Yellow Sea with a wave-tide-circulation coupled model 总被引:1,自引:0,他引:1
Based on the MASNUM wave-tide-circulation coupled numerical model, the temperature structure along 35°N in the Yellow Sea
was simulated and compared with the observations. One of the notable features of the temperature structure along 35°N section
is the double cold cores phenomena during spring and summer. The double cold cores refer to the two cold water centers located
near 122°E and 125°E from the depth of 30m to bottom. The formation, maintenance and disappearance of the double cold cores
are discussed. At least two reasons make the temperature in the center (near 123°E) of the section higher than that near the
west and east shores in winter. One reason is that the water there is deeper than the west and east sides so its heat content
is higher. The other is invasion of the warm water brought by the Yellow Sea Warm Current (YSWC) during winter. This temperature
pattern of the lower layer (from 30m to bottom) is maintained through spring and summer when the upper layer (0 to 30m) is
heated and strong thermocline is formed. Large zonal span of the 35°N section (about 600 km) makes the cold cores have more
opportunity to survive. The double cold cores phenomena disappears in early autumn when the west cold core vanishes first
with the dropping of the thermocline position.
Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No.
49736190). 相似文献
108.
109.
We discuss the determination of membership of 42 open clusters. Our analysis shows that Vasilevskis' mathematical model can be reasonably applied to this case. Our improved version of Sanders' method and our definition of cluster member based on the principles of discriminatory analysis effectively exclude stars of low probabilities. It is important in the study of open cluster to use only those with high probabilities. The effectiveness of the statistical method is closely related to the velocity distributions of the member and field stars. For fields where the error rate is high, it is better to combine other data than proper motion in determining membership. 相似文献
110.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献