全文获取类型
收费全文 | 83139篇 |
免费 | 27059篇 |
国内免费 | 54195篇 |
专业分类
测绘学 | 12615篇 |
大气科学 | 25697篇 |
地球物理 | 20648篇 |
地质学 | 62775篇 |
海洋学 | 23370篇 |
天文学 | 1353篇 |
综合类 | 7900篇 |
自然地理 | 10035篇 |
出版年
2024年 | 813篇 |
2023年 | 2055篇 |
2022年 | 4068篇 |
2021年 | 5009篇 |
2020年 | 5020篇 |
2019年 | 7894篇 |
2018年 | 7592篇 |
2017年 | 7217篇 |
2016年 | 7399篇 |
2015年 | 7650篇 |
2014年 | 6971篇 |
2013年 | 8203篇 |
2012年 | 8568篇 |
2011年 | 8432篇 |
2010年 | 8405篇 |
2009年 | 7435篇 |
2008年 | 6865篇 |
2007年 | 6750篇 |
2006年 | 6174篇 |
2005年 | 5316篇 |
2004年 | 4809篇 |
2003年 | 3944篇 |
2002年 | 3874篇 |
2001年 | 3451篇 |
2000年 | 2852篇 |
1999年 | 2096篇 |
1998年 | 1836篇 |
1997年 | 1891篇 |
1996年 | 1459篇 |
1995年 | 1393篇 |
1994年 | 1249篇 |
1993年 | 1212篇 |
1992年 | 1032篇 |
1991年 | 770篇 |
1990年 | 742篇 |
1989年 | 608篇 |
1988年 | 531篇 |
1987年 | 424篇 |
1986年 | 352篇 |
1985年 | 278篇 |
1984年 | 301篇 |
1983年 | 181篇 |
1982年 | 230篇 |
1981年 | 166篇 |
1980年 | 113篇 |
1979年 | 138篇 |
1978年 | 57篇 |
1977年 | 50篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
211.
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
212.
以p-硝基苯酚-β-D-氨基葡萄糖苷(pNP-NAG)为底物,研究产物类似物:p-羧基苯酚和苯酚对青蟹N-乙酰-β-D-氨基葡萄糖苷酶(NAGase,Ec3.2.1.52)活力的影响.结果表明:p-羧基苯酚和苯酚对该酶有抑制的作用,IC50分别为20.0和100.0mmol/dm^3.p-羧基苯酚和苯酚对酶的抑制均表现为竞争性类型,其抑制常数K1分别为5.03和25.67mmol/dm^3.结合产物N-乙酰-β-D-氨基葡萄糖(NAG)抑制作用表现为反竞争性类型,判断NAGase水解pNP-NAG反应为有序双双反应(Ordered Bi Bi). 相似文献
213.
214.
215.
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
216.
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
217.
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
218.
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
219.
220.
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