全文获取类型
收费全文 | 113207篇 |
免费 | 6234篇 |
国内免费 | 6751篇 |
专业分类
测绘学 | 3328篇 |
大气科学 | 12495篇 |
地球物理 | 24196篇 |
地质学 | 44700篇 |
海洋学 | 9999篇 |
天文学 | 19897篇 |
综合类 | 3285篇 |
自然地理 | 8292篇 |
出版年
2022年 | 1217篇 |
2021年 | 1550篇 |
2020年 | 1478篇 |
2019年 | 1633篇 |
2018年 | 5258篇 |
2017年 | 4954篇 |
2016年 | 4395篇 |
2015年 | 2325篇 |
2014年 | 2958篇 |
2013年 | 4671篇 |
2012年 | 3880篇 |
2011年 | 6196篇 |
2010年 | 5918篇 |
2009年 | 6828篇 |
2008年 | 5834篇 |
2007年 | 6175篇 |
2006年 | 3479篇 |
2005年 | 3480篇 |
2004年 | 3328篇 |
2003年 | 3254篇 |
2002年 | 2878篇 |
2001年 | 2471篇 |
2000年 | 2676篇 |
1999年 | 2873篇 |
1998年 | 2589篇 |
1997年 | 2574篇 |
1996年 | 2258篇 |
1995年 | 2070篇 |
1994年 | 1900篇 |
1993年 | 1680篇 |
1992年 | 1473篇 |
1991年 | 1225篇 |
1990年 | 1320篇 |
1989年 | 1139篇 |
1988年 | 1020篇 |
1987年 | 1094篇 |
1986年 | 959篇 |
1985年 | 1122篇 |
1984年 | 1246篇 |
1983年 | 1176篇 |
1982年 | 1083篇 |
1981年 | 1040篇 |
1980年 | 875篇 |
1979年 | 842篇 |
1978年 | 877篇 |
1977年 | 792篇 |
1976年 | 759篇 |
1975年 | 697篇 |
1974年 | 704篇 |
1973年 | 708篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
912.
针对多传感器观测环境下带乘性噪声系统的逆向最优滤波与反褶积融合估计问题 ,本文提出了 1种基于极大似然准则的最优融合算法。该算法中各单传感器间并行计算 ,并且融合中心与单传感器处理中心间无反向通讯 ,因而执行效率较高。仿真表明 ,该融合算法产生的逆向滤波与反褶积比单传感器处理结果有较明显提高 相似文献
913.
The Karman Vortex Street generated by a circular cylinder is investigated by the numerical solution of the compressible Navier–Stokes equations in the incompressible Mach number range (Mach<0.3) using the Beam and Warming implicit scheme. The agreement with the fully incompressible projection method (Chorin, 1968) is fairly good while convergence time is very much better. The investigation suggests that the compressible Navier–Stokes equations may be used as an efficient alternative to study incompressible flows as well. Mach numbers just below 0.3 are enough to simulate incompressible flow behavior and at the same time do not cause numerical ill-conditioning in the solution. In addition, some relevant features of the vortices generated and carried by the wake of the cylinder could be fairly well captured. 相似文献
914.
Taxonomy and ultrastructure of five species of Tetraselmis (Prasinophyceae) isolated from China seas
INTRODUCTIONThegenusTetraselmisStein ( 1 878)isagreenflagellates (Prasinophyceae)thatconsistsofmanymarineaswellasafewfreshwaterspecies .Marinespeciesoftenoccurindensepopula tionscausingbloomsintidepoolsorbays ,therebybeingimportanttoanunderstandingofthedynam… 相似文献
915.
采用组化方法首次探讨碱性磷酸酶在文昌鱼体内的分布。结果发现 :碱性磷酸酶存在于腹褶部分表皮、围腮腔上皮、构成背鳍腔的内皮、肌膜、中肠腔和肝盲囊腔细胞纹状缘、生殖上皮和精原细胞中 相似文献
916.
917.
Ronald Osinga Arjen J. Kop Gerard C. A. Duineveld Rudolf A. Prins Fleur C. Van Duyl 《Journal of Sea Research》1996,36(3-4)
Benthic oxygen uptake, sulphate reduction and benthic bacterial production were measured at two contrasting locations in the southern North Sea: the shallow and turbulent Broad Fourteens area in the Southern Bight, and the deeper Oyster Grounds, a deposition area, where thermohaline stratification occurs during summer. Oxygen uptake and sulphate reduction showed a clear seasonal pattern in the Broad Fourteens area, indicating a supply of carbon to the benthic system that is closely related to the standing stock of carbon in the water column. This close benthic-pelagic coupling is probably due to the influence of the tide in this part of the North Sea, which keeps the water column permanently mixed. At the Oyster Grounds, no seasonal pattern was observed. Peaks in oxygen uptake and sulphate reduction were found in winter. Irregularly occurring events, such as storms and fishery-related activities, are likely to affect the benthic mineralization patterns in this area. Annual benthic carbon mineralization rates estimated from oxygen uptake rates were 44 gC·m−2 at the Broad Fourteens, and 131 gC·m−2 at the Oyster Grounds, of which 26 and 28%, respectively, could be attributed to sulphate reduction (assuming an annual sulphide reoxidation rate of 100%). Although sulphate reduction rates in the southern North Sea are higher than previously suggested, aerobic respiration is the most important pathway for benthic carbon mineralization at the stations visited. Production rates of benthic bacterial carbon measured with labelled leucine were much higher than carbon mineralization rates based on oxygen uptake or sulphate reduction. This may either imply a very high bacterial carbon conversion efficiency, or point to shortcomings in the accuracy of the techniques. A critical evaluation of the techniques is recommended. 相似文献
918.
根据大亚湾的自然条件认为,大亚湾沉积物的物质来源是其周围集水区的岩石风化壳,并计算了其风化壳中重金属元素的平均丰度值;又根据沉积物的粒度和深层沉积物重金属元素的含量资料,列方程计算出湾内沉积物中重金属元素的平均含量,根据这两组数据,确定该区域内重金属元素的平均背景值。再通过粒度关系,计算得出大亚湾内各站沉积物中重金属元素的背景值。应用此结果和大亚湾沉积物重金属元素实测结果,计算了由人为影响带入沉积物中的重金属元素含量,了解其受污染的程度。 相似文献
919.
920.
In this study the zinc exchange process and the reason for the non-additive exchange of δ-MnO2/montmorrillonite have been investigated. The results suggest that with pH increasing from 2 to 8, the stepwise exchange of zinc occurs on δ-MnO. and montmorrillonite in their mixture.At low pH zinc ions are bound on δ-MnO2, while at high pH, the exchange on montmorrillonite dominates the surface interaction in the mixture. The study demonstrates that the stepwise exchange and the heterogeneity of the binding sites on δ-MnO2 surface are the reason for the non-additive exchange. It is also shown that when δ-MnO2 amounts to 5 %, zinc exchange on sea sediments is controlled by 8-MnO2. 相似文献