全文获取类型
收费全文 | 57858篇 |
免费 | 14477篇 |
国内免费 | 21369篇 |
专业分类
测绘学 | 10628篇 |
大气科学 | 8246篇 |
地球物理 | 10673篇 |
地质学 | 38569篇 |
海洋学 | 11243篇 |
天文学 | 786篇 |
综合类 | 5055篇 |
自然地理 | 8504篇 |
出版年
2024年 | 478篇 |
2023年 | 1339篇 |
2022年 | 3580篇 |
2021年 | 4324篇 |
2020年 | 3532篇 |
2019年 | 4249篇 |
2018年 | 3736篇 |
2017年 | 3584篇 |
2016年 | 3471篇 |
2015年 | 4179篇 |
2014年 | 4091篇 |
2013年 | 4956篇 |
2012年 | 5429篇 |
2011年 | 5423篇 |
2010年 | 5389篇 |
2009年 | 4926篇 |
2008年 | 5036篇 |
2007年 | 4798篇 |
2006年 | 4518篇 |
2005年 | 3849篇 |
2004年 | 2909篇 |
2003年 | 1992篇 |
2002年 | 2025篇 |
2001年 | 1825篇 |
2000年 | 1528篇 |
1999年 | 748篇 |
1998年 | 337篇 |
1997年 | 201篇 |
1996年 | 174篇 |
1995年 | 153篇 |
1994年 | 123篇 |
1993年 | 99篇 |
1992年 | 105篇 |
1991年 | 63篇 |
1990年 | 84篇 |
1989年 | 44篇 |
1988年 | 31篇 |
1987年 | 35篇 |
1986年 | 38篇 |
1985年 | 36篇 |
1984年 | 32篇 |
1983年 | 25篇 |
1982年 | 18篇 |
1981年 | 12篇 |
1979年 | 24篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1957年 | 20篇 |
1954年 | 30篇 |
1933年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
161.
A method combining boundary and finite element approaches as well as modal analysis to solve the acoustical scattering from elastic obstacle in infinite fluid is described. An elastic cylinder with finite length immersed in water is chosen as an example for numerical demonstration. The computed frequency response and direction pattern of the backscattering target strength of the cylinder are conformable to those measured. 相似文献
162.
Up to now, available method of numerical forecasting and suitable wind field model for the Bohai Sea storm surge have been few. In this paper, through the analysis of the weather situation is presented a mathematical model for the wind fields involved mainly the deformation field of a high pressure matched with a low pressure, the temperate cyclone, the cold wave and the northword typhoon. Meanwhile, numerical computations combined with the nonlinear storm surge models are made by using "ADI" method. The computed results are generally coincident with the practical observations. It has showed a success in the simulated wind field and the feasibility of using "ADI" method to forecast the Bohai Sea storm surge. 相似文献
163.
164.
南海尖峰海山多金属结壳地球化学 总被引:4,自引:5,他引:4
南海尖峰海山多金属结壳富含30多种元素,其锰铁矿物主要由钡镁锰矿,δ-MnO_2和FeOOH·xH_2O组成。与其它海区的结壳相比,尖峰海山结壳富含Cu、Ni、Ba、Zn、Pb等元素,而Co、Ti、稀土元素(REE)、Sr等元素相对较贫。研究表明,HREE亏损,具明显的Ce正异常,较明显的Tb正异常和Yb负异常。这是氧化弱碱性海洋环境所致。结壳是水成作用的产物,它的形成受南海独特的古海洋环境所控制,海底火山热液作用,可能也是影响因素之一。 相似文献
165.
Using 203HgCl2 as a tracer, uptake and combination of inorganic mercury in Tilapia mossambica (petres) and its transfer between different tissues of the fish were studied. The results demonstrate that gill is the main tissue uptaking mercury directly from seawater and that the mercury in gill, skin, digestive tube and fin would be transferred to the other tissues. The result of gel chromatography with Sephadex G-75 shows that most of mercury in gill filaments, liver, and digestive tube are bound in high molecules (MW-70000) and not in metallothionein, but in kidney and bile mercury binding proteins are similar to metallothionein and are accounted for up to 60% and 46% respectively. 相似文献
166.
Chemical forms of Ba are determined in samples of suspension and sediment from the Zhujiang Estuary. Their interfacial geochemical processes are discussed. Total suspended Ba content is between 2. 4 and 40. 4 μg/L, and mostly exists in the crystalline form (43. 5%), secondly in the Fe-Mn oxidative form (23.1%). Percentages of organic, carbonate and exchangeable forms are 14. 8%, 11. l%and 7.4%, respectively. Total content of Ba in the sediment is between 158. 6 and 48. 0 ug/g. Percentages of crystalline form, Fe-Mn oxidative form and carbonate form are 78. 4% ,13. 5%and 8. 2%, respectively. Organic and exchangeable forms are not detected. The study on the mechanism of interfacial movement suggests that the salinity range of 10 is the turning point for the varied distribution of Ba. The subsidence of crystalline form affects the decrease of Ba content in suspension. The decrease mostly takes place in the salinity range lower than 10, which corresponds to the high value of Ba content in the sediment. The 相似文献
167.
Based on the shear effect of internal wave currents in stratified fluid, the necessary condition for the possible existence of an internal wave and the sufficient condition under which there will be no internal wave are deduced starting from the continuity equation of incompressible fluid. The above two conditions are verified by the measured results from vertical current meter arraies at some observation stations in the Bohai Sea and the South Huanghai Sea. In the treatment of measured results, first of all, the tidal period components are separated from the measured currents by Fourier-analysis method, and then the internal wave currents of tidal period from the tidal period components in the light of some considerations. The analytical results of observed currents are well consistent with theoretical analysis. Furthermore, one can also deduce some features of internal waves therefrom. Consequently, it is concluded that the observation from a vertical current meter array is an important means to verif 相似文献
168.
A new approach to high-order Boussinesq-type equations with ambient currents is presented. The current velocity is assumed to be uniform over depth and of the same magnitude as the shallow water wave celerity. The wave velocity field is expressed in terms of the horizontal and vertical wave velocity components at an arbitrary water depth level. Linear operators are introduced to improve the accuracy of the kinematic condition at the sea bottom. The dynamic and kinematic conditions at the free surface are expressed in terms of wave velocity variables defined directly on the free surface. The new equations provide high accuracy of linear properties as well as nonlinear properties from shallow to deep water, and extend the applicable range of relative water depth in the case of opposing currents. 相似文献
169.
170.