全文获取类型
收费全文 | 59866篇 |
免费 | 22441篇 |
国内免费 | 47092篇 |
专业分类
测绘学 | 3825篇 |
大气科学 | 25397篇 |
地球物理 | 18920篇 |
地质学 | 48523篇 |
海洋学 | 17974篇 |
天文学 | 2326篇 |
综合类 | 7249篇 |
自然地理 | 5185篇 |
出版年
2024年 | 259篇 |
2023年 | 776篇 |
2022年 | 1596篇 |
2021年 | 2044篇 |
2020年 | 2945篇 |
2019年 | 6311篇 |
2018年 | 6868篇 |
2017年 | 6449篇 |
2016年 | 6711篇 |
2015年 | 5749篇 |
2014年 | 5600篇 |
2013年 | 6025篇 |
2012年 | 5637篇 |
2011年 | 5880篇 |
2010年 | 5636篇 |
2009年 | 5164篇 |
2008年 | 4000篇 |
2007年 | 3999篇 |
2006年 | 3596篇 |
2005年 | 3475篇 |
2004年 | 3789篇 |
2003年 | 3349篇 |
2002年 | 2947篇 |
2001年 | 2705篇 |
2000年 | 2470篇 |
1999年 | 3012篇 |
1998年 | 2805篇 |
1997年 | 2808篇 |
1996年 | 2297篇 |
1995年 | 2122篇 |
1994年 | 1914篇 |
1993年 | 1912篇 |
1992年 | 1587篇 |
1991年 | 1149篇 |
1990年 | 962篇 |
1989年 | 860篇 |
1988年 | 713篇 |
1987年 | 538篇 |
1986年 | 443篇 |
1985年 | 343篇 |
1984年 | 340篇 |
1983年 | 205篇 |
1982年 | 258篇 |
1981年 | 179篇 |
1980年 | 134篇 |
1979年 | 129篇 |
1978年 | 64篇 |
1977年 | 57篇 |
1976年 | 48篇 |
1971年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
231.
Copper complexing ligand concentrations in the Daya Bay, Qingdao coast, Jiaozhou Bay, South China Sea and Huanghe Estuary waters were determined by the anodic stripping voltammetry technique. The distribution regularity and the relationship with other parameters were discussed. The results were as follows: Copper complexing ligand concentrations of the South China Sea were a little higher than those of other sea areas, and they were apparently higher than those of the ocean. Compared with the subsurface layer (SSL) in the sea surface microlayer copper complexing ligand concentrations showed an enrichment phenomenon, of which the mechanism is similar to dissolved organic matter. The metal complexing ligand concentration profiles of the South China Sea showed that the value in the sea surface was the highest, then it decreased with depth accruing, and a higher value appeared at the bottom. Copper complexing ligand concentrations were higher than those of cadmium and lead. Ligands in each sea area exhibited a complicated property. In short, the distribution regularity of copper complexing ligand concentrations in China' s coastal waters was consistent with that of other regions in the world. Meanwhile, the positive relationship between the copper complexing ligand concentrations and biological oxygen demand, chemical oxygen demand, dissolved organic carbon, and viscosity were found clearly. 相似文献
232.
采用随机克隆、功能筛选、逐次排除和同源比较的基因克隆新策略进行克隆假单胞菌(Pseudomonas sp.cn4902)磷酸甘油磷酸酯酶基因的研究。结果表明,该结构基因长819bp,与铜绿假单胞菌的磷酸甘油磷酸酯酶基因的核苷酸一致性达61.5%,氨基酸同源性为56.2%。该基因已输入GenBank数据库,收录号AF348165。将该基因转化大肠杆菌,受体菌在含NaCl 1.0mol/L的培养基中甘油含量升高2.9倍,最终菌浓度提高3.6倍。可见这是一个与生物耐盐性相关的主基因,以其转化、培育耐盐农作物的前景十分光明。 相似文献
233.
234.
Starting from the widespread phenomena of porous bottoms in the near shore region, considering fully the diversity of bottom topography and wave number variation, and including the effect of evanescent modes, a general linear wave theory for water waves propagating over uneven porous bottoms in the near shore region is established by use of Green‘s scond identity. This theory can be reduced to a number of the most typical mild-slope equations curreutly in use and provide a reliable research basis for follow-up development of nonlinear water wave theory involving porous bottoms. 相似文献
235.
More and more researches show that neither the critical downward acceleration nor the critical slope of water waves is a universal constant. On the contrary, they vary with particular wave conditions. This fact moders the models either for the probability of wave breaking B or for the whitecap coverage W based on these criteria difficult to apply. In this paper and the one which follows we seek to develop models for the prediction of both B and W based on the kinematical criterion. First, several joint probabihstic distribution functions (PDFs) of wave characteristics are derived, based on which the breaking properties B and W are estimated. The estimation is made on the assumption that a wave breaks ff the horizontal velocity of water particles at its crest exceeds the local wave celerity, and whitecapping occurs in regions of fluid where water particles travel faster than the waves. The consequent B and W depend on wave spectral moments of orders 0 to 4.Then the JONSWAP spectrum is used to represent the fetch-limited sea waves in deep water, so as to relate the probahility of wave breaking and the whitecap coverage with wind parameters. To this end, the time-averaging technique proposed by Glazman (1986) is applied to the estimation of the spectral moments involved, and furthermore, the theoretical models are compared with available observations collected from published literature. From the comparison, the averaging time scale is determined. The final models show that the probability of wave breaking as well as the whitecap coverage depends on the dimensionless fetch. The agreement between these models and the database is reasonable. 相似文献
236.
An optimization mathematical model of the pile forces for piled breasting dolphins in the open sea under various loading conditions is presented. The optimum layout with the well distributed pile forces and the least number of piles is achieved by the multiplier penalty function method. Several engineering cases have been calculated and compared with the result of the conventional design method. It is shown that the number of piles can be reduced at least by 10%—20% and the piles‘ bearing state is improved greatly. 相似文献
237.
科考船定点作业时会受到海洋风、涌、浪、流等外界环境因素影响,导致工作效率降低,原位测量精度下降,甚至影响作业安全。动力定位系统 (DP) 具有自动定位功能,能够抵抗外界环境因素的影响,可实现科考船高精度定点控位。
单波束测深仪不仅可以测量水深,也可反映水下设备深度信息,可以起到辅助监控水下设备功能。本文在介绍定点作业施工现状与局限性的基础上,分析 DP 系统与单波束测深仪工作原理,以“向阳红 01”船为载体,在定点作业时开启 DP 系统与单波束测深仪,发现该方法可以提高科考船定点作业工作效率、原位测量精度并保障作业安全,可为其他科考船定点作业提供参考。 相似文献
238.
239.
Tang Rongqing Wang Daoyan Zhang Xixian Egnineer Shanghai Jiao Tong University Shanghai Associate Professor Shanghai Jiao University Shanghai 《中国海洋工程》1993,(3)
- The authors have used a low-cost ROV of observation type for underice surveying of the Nen-Jiang Cross-river oil pipe line in the Da Qing oil field. The operation method, process and the safety measures for severe cold areas are presented. The fruitful tries and experience may be useful to ROV operators in severe cold areas. 相似文献
240.
Semidiurnal and diurnal tidal waves in the Taiwan Strait are computed simultaneously by using finite difference method with two-dimensional nonlinear hydrodynamic equations. The agreement between the computed and observed results is considered satisfactory. 相似文献