全文获取类型
收费全文 | 398099篇 |
免费 | 80130篇 |
国内免费 | 104725篇 |
专业分类
测绘学 | 65743篇 |
大气科学 | 75538篇 |
地球物理 | 70831篇 |
地质学 | 205535篇 |
海洋学 | 67799篇 |
天文学 | 6930篇 |
综合类 | 40481篇 |
自然地理 | 50097篇 |
出版年
2024年 | 5103篇 |
2023年 | 16912篇 |
2022年 | 19776篇 |
2021年 | 21564篇 |
2020年 | 17777篇 |
2019年 | 23000篇 |
2018年 | 21993篇 |
2017年 | 15175篇 |
2016年 | 17070篇 |
2015年 | 20170篇 |
2014年 | 26819篇 |
2013年 | 24522篇 |
2012年 | 26394篇 |
2011年 | 26741篇 |
2010年 | 25899篇 |
2009年 | 25270篇 |
2008年 | 24642篇 |
2007年 | 22888篇 |
2006年 | 21930篇 |
2005年 | 18913篇 |
2004年 | 17734篇 |
2003年 | 16317篇 |
2002年 | 14767篇 |
2001年 | 13608篇 |
2000年 | 11302篇 |
1999年 | 8918篇 |
1998年 | 6459篇 |
1997年 | 5624篇 |
1996年 | 5126篇 |
1995年 | 4848篇 |
1994年 | 4572篇 |
1993年 | 4328篇 |
1992年 | 4300篇 |
1991年 | 3899篇 |
1990年 | 3688篇 |
1989年 | 3876篇 |
1988年 | 3999篇 |
1987年 | 3878篇 |
1986年 | 3304篇 |
1985年 | 3351篇 |
1984年 | 3018篇 |
1983年 | 2827篇 |
1982年 | 2344篇 |
1981年 | 1781篇 |
1980年 | 1313篇 |
1979年 | 838篇 |
1978年 | 415篇 |
1959年 | 321篇 |
1958年 | 350篇 |
1957年 | 322篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
城市政府要实现对土地市场的高度垄断和集中统一管理,必须解决存量土地的控制问题。我国土地使用制度改革十几年的经验和教训证明,政府如果不能对存量土地进行有效控制,集中统一管理就是一句空话,控制建设用地供应总量就不可能落实,土地招、拍、挂制度就难于推行,国有资产流失就会堵而不止。土地收购储备制度的出现、发展和完善,使这个问题迎刃而解,政府对土地市场的调控管理能力得到了大大加强,并在推进城镇化进程中发挥着重要作用。 相似文献
172.
173.
From elements formed in interstellar furnaces to humans peering back at the stars, the evolution of life has been a long, intricate and perhaps inevitable process. Life as we know it requires a planet orbiting a star at just the right distance so that water can exist in liquid form. It needs a rich supply of chemicals and energy sources. On Earth, the combination of chemistry and energy generated molecules that evolved ways of replicating themselves and of passing information from one generation to the next. Thus, the thread of life began. This chart traces the thread, maintained by DNA molecules for much of its history, as it weaves its way through the primitive oceans, gaining strength and diversity along the way. Organisms eventually moved onto the land, where advanced forms, including humans, ultimately arose. Finally, assisted by a technology of its own making, life has reached back out into space to understand its own origins, to expand into new realms, and to seek other living threads in the cosmos. 相似文献
174.
Theoretical rotational excitation rates were computed for C3H2 in collisions with He atoms at temperatures from 30 to 120 K. The intermolecular forces were obtained from accurate self-consistent field and perturbation theory calculations, and collision dynamics were treated within the infinite-order sudden approximation. The accuracy of the latter was examined by comparing with the more exact coupled states approximation. 相似文献
175.
利用降水平均递增率求山地最大降水高度 总被引:6,自引:0,他引:6
本文引入降水平均递增率的概念,改进了文献中的计算方法,可以一次求出山地降水量与海拔的经验关系式中全部参数的最佳值,其中包括山地最大降水高度。并用实例说明了本法的步骤及特点。 相似文献
176.
This article deals with the interaction of zinc with δ-MnO2,γ-MnOOH and manganite existing in natural water systems. The mechanism of the reaction has been studied in detail. From the fact that the "ratio of ion exchange (%)-pH" graph is an "S shaped" curve, it is possible to deduce that the chemical reaction is of the nature of cation exchange. And since the pH range of ion exchange = 4, it is possible to further deduce that the reaction can be explained by the mechanism of monovalence cation exchange.The main result of this article is the discovery of a new type of isotherm which has not been mentioned in previous literature here and abroad. This isotherm cannot be represented by any presently available adsorption isotherm equations in marine chemistry. The characteristics of this new type of isotherm are as follows: the isotherm has two "knees" and three "plateaus", the heights of these three "plateaus" are in the ratio 1:2:3. In order to explain theoretically our new isotherm, this article suggest 相似文献
177.
178.
We have obtained velocity-resolved spectra of the 12C II 157.8 microns 2P3/2-2P1/2 fine structure line in the M42 region of Orion. Observations at 0.8 km s-1 resolution with a laser heterodyne spectrometer show multiple velocity components in some locations, with typical linewidths of 3-5 km s-1. Spectra of theta 1C and BN-KL also show weak emission from the F = 2-1 hyperfine component of the equivalent 13C II line. From the observed 12C II/13C II line intensity ratios, we deduce that the 12C II emission is optically thick with tau approximately 5 at both positions. Excitation temperatures of 128 K and 90 K, together with column densities of approximately 1 x 10(19) cm-2 and approximately 4 x 10(18) cm-2, are derived for theta 1C and BN-KL, respectively. 相似文献
179.
A total of 21 elements, Si, Al, K, Na, Ca, Mg, Fe, Mn, Ti, P, Cu, Co, Ni, Zn, Pb, Cr, Rb, Sr Ba, Cd and Corg(organic carbon), was determined in the trough. Average contents of most elements were found to be higher in the trough bottom than in the western and eastern slopes. We call these elements the indicator elements because of their "transition" property. This is in keeping with the "intermediate" geographical position of the trough where the continental crust turns to oceanic crust.Though the authigenic fraction of some elements here increases, the detrital fraction .of most elements is still dominant, indicating that the influences of terrigenous and volcanic detrital materials on the trough sediments are still dominant.The abundant biogenous component in the trough is characterized by the increase of Ca, Sr and Corg contents. Ca existes mainly in the form of CaCo3 that is mostly found in various shells. Sr often easily replaces Ca within the shells.A belt-shaped distribution pattern of elements wa 相似文献
180.
"An attempt is made in the light of an analysis of population changes occurring in several sub-districts of Greater Santiago in the period between the last population censuses of 1970 and 1982, based on small-area census units (distritos and communas) to draw attention to the effects on the spatial growth of the city resulting from changed conditions in the political and juridical sphere since the change of power in 1973." The effect of changes in government policy concerning the liberalization of the property market and the illegal occupation of land are noted. (summary in ENG) 相似文献