全文获取类型
收费全文 | 89573篇 |
免费 | 1241篇 |
国内免费 | 917篇 |
专业分类
测绘学 | 2553篇 |
大气科学 | 6413篇 |
地球物理 | 16592篇 |
地质学 | 34116篇 |
海洋学 | 7660篇 |
天文学 | 19913篇 |
综合类 | 352篇 |
自然地理 | 4132篇 |
出版年
2022年 | 441篇 |
2021年 | 782篇 |
2020年 | 843篇 |
2019年 | 920篇 |
2018年 | 4154篇 |
2017年 | 3855篇 |
2016年 | 3526篇 |
2015年 | 1393篇 |
2014年 | 2294篇 |
2013年 | 3873篇 |
2012年 | 3206篇 |
2011年 | 5171篇 |
2010年 | 4610篇 |
2009年 | 5538篇 |
2008年 | 4583篇 |
2007年 | 5167篇 |
2006年 | 3024篇 |
2005年 | 2573篇 |
2004年 | 2528篇 |
2003年 | 2446篇 |
2002年 | 2216篇 |
2001年 | 1799篇 |
2000年 | 1741篇 |
1999年 | 1439篇 |
1998年 | 1466篇 |
1997年 | 1352篇 |
1996年 | 1136篇 |
1995年 | 1101篇 |
1994年 | 1023篇 |
1993年 | 858篇 |
1992年 | 843篇 |
1991年 | 797篇 |
1990年 | 897篇 |
1989年 | 723篇 |
1988年 | 718篇 |
1987年 | 764篇 |
1986年 | 687篇 |
1985年 | 883篇 |
1984年 | 973篇 |
1983年 | 866篇 |
1982年 | 844篇 |
1981年 | 729篇 |
1980年 | 721篇 |
1979年 | 667篇 |
1978年 | 649篇 |
1977年 | 555篇 |
1976年 | 507篇 |
1975年 | 530篇 |
1974年 | 465篇 |
1973年 | 505篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
953.
Despite the fact that BaP is a carcinogen, mammalian immunosuppressant, and ubiquitous aquatic pollutant, knowledge regarding the effects of BaP on the immune system of fish is still lacking. To begin to fill this gap, studies were conducted in medaka to examine the effects and mechanisms by which BaP exposure might alter host immunocompetence. Fish, exposed by IP injection of BaP (2-600 microg/g BW), were examined after 48 h for effects upon immune function and CYP1A expression/activity. Benzo[a]pyrene, at a concentration below that which increased levels of CYPIA expression/activity (2 microg BaP/g BW) suppressed lymphocyte proliferation. Concentrations of BaP at 20 and 200 microg/g BW. suppressed antibody-forming cell (AFC) numbers, superoxide production, and host resistance against bacteria. In contrast, exposure to the low affinity aryl hydrocarbon receptor (AhR) agonist, benzo[e]pyrene (BeP), neither induced CYP1A expression nor altered immune function. Given the lack of immunosuppressive effects produced by BeP, and the fact that exposure to the AhR antagonist (and CYP1A inhibitor) alpha-naphthoflavone (ANF) ameliorated the suppressive effects of BaP upon AFC numbers, the AhR pathway (including CYP1A-mediated production of reactive BaP metabolites) appears important in mediating BaP-induced immunotoxicity in fish, as in mammals. In the past, the medaka has proven a successful model for assessing carcinogenic agents. These studies have demonstrated its utility for also determining the immunosuppressive effects of an important aquatic contaminant. 相似文献
954.
Recent events concerning Atlantic salmon escapees in the Pacific 总被引:1,自引:0,他引:1
955.
The basic relations of the classical dyamic method have been complemented with the condition allowing for the non-monotonicity
of the vertical kinematic structure. The elements of the multi-layered system of geostrophic currents have been found to correlate
over depth with seawater density and hydrostatic pressure distribution. An example of the analysis of the vertical structure
of currents, involving specific hydrological data, is given.
Translated by Vladimir A. Puchkin. 相似文献
956.
The sway, heave and roll added masses of three uniform cylinders with semi-circular, rectangular and triangular cross-sectional shapes in shallow and narrow water are numerically analysed. The method is based on simulation of the potential flow induced by the cylinder's mode of motion. The effects of shallow and narrow water on added mass are analysed and presented. It is concluded that the shallow and narrow water effects on added mass depend on the different cross-section shapes of the cylinders. In particular, the water depth effect on sway added mass is stronger than that on heave added mass while the narrow water effect on sway is weaker than that on heave. The shallow water effect on added mass tends to weaken the narrow water effect. Lastly the effect of shallow and narrow water on added mass on a rectangular cylinder is the strongest while that on a triangular cylinder is the weakest. 相似文献
957.
958.
K. Nagendra Prasad S. Triveni T. Schanz Late T. S. Nagaraj 《Marine Georesources & Geotechnology》2007,25(3):181-197
Despite the sophisticated methods employed to obtain high quality samples, they are prone to disturbance, due to stress release, let apart, due to sampling tools and techniques adopted. Critical reappraisal of different methods suggest that any method of sample quality assessment involving strength and deformation parameters would enable easier implementation and correct other engineering parameters for estimated sample disturbance. In this technical paper, sample disturbance index, using merely the slopes of compression paths, (representing mechanical response), in the pre- and post-yield stress regimes under odeometric loading conditions is proposed. This method is based on single oedometer test results on samples of unknown degrees of disturbance during sampling and handling. The unique feature of this approach is that the ideal rigid plastic material behavior is taken as reference. This eliminates the need for any additional experimental data or any calculations from the relationships formulated with any set of assumptions. 相似文献
959.
A. G. Matul T. A. Khusid V. V. Mukhina M. P. Chekhovskaya S. A. Safarova 《Oceanology》2007,47(1):80-90
The study of diatoms and benthic foraminifers from the southeastern shelf of the Laptev Sea shows that their most diverse and abundant recent assemblages populate the peripheral underwater part of the Lena River delta representing the marginal filter of the sea. This area is characterized by an intense interaction between the fresh waters of the Siberian rivers and the basin seawater, the Atlantic one included. Local Late Holocene (~last 2300 years) environments reflect the main regional and global paleoclimatic changes, the Medieval Warm Period (~600–1100 years B.P.) and the Little Ice Age (~100–600 years B.P.) inclusive. In addition, the composition and distribution of planktonic foraminifers implies a strong influence of the Atlantic water during the Holocene optimum ~5100–6200 years B.P. 相似文献
960.