全文获取类型
收费全文 | 5516篇 |
免费 | 889篇 |
国内免费 | 732篇 |
专业分类
测绘学 | 1342篇 |
大气科学 | 613篇 |
地球物理 | 1119篇 |
地质学 | 2406篇 |
海洋学 | 714篇 |
天文学 | 73篇 |
综合类 | 336篇 |
自然地理 | 534篇 |
出版年
2024年 | 5篇 |
2023年 | 50篇 |
2022年 | 147篇 |
2021年 | 192篇 |
2020年 | 179篇 |
2019年 | 245篇 |
2018年 | 164篇 |
2017年 | 225篇 |
2016年 | 214篇 |
2015年 | 212篇 |
2014年 | 352篇 |
2013年 | 343篇 |
2012年 | 400篇 |
2011年 | 384篇 |
2010年 | 346篇 |
2009年 | 321篇 |
2008年 | 303篇 |
2007年 | 431篇 |
2006年 | 370篇 |
2005年 | 318篇 |
2004年 | 293篇 |
2003年 | 289篇 |
2002年 | 205篇 |
2001年 | 202篇 |
2000年 | 187篇 |
1999年 | 158篇 |
1998年 | 137篇 |
1997年 | 83篇 |
1996年 | 88篇 |
1995年 | 59篇 |
1994年 | 60篇 |
1993年 | 35篇 |
1992年 | 38篇 |
1991年 | 15篇 |
1990年 | 18篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有7137条查询结果,搜索用时 515 毫秒
31.
There is a low nutrient level in the Aoshan Bay. In June 1999, the chemical adjustmentand control experiment was made in the Aoshan Bay. Following tracts investigation was carried out before the experiment and on the 1st, 2nd, 4th, 5th, 6th and 45th day/after the experiment. While the variance of amount of phytoplankton, the replacement of superior species and the species composition of phytoplankton were researched. The results show that the amount of phytoplankton in the Aoshan Bay rises gradually after the experiment. Ceratium macroceros Cleve of pyrophyta was the dominant species before the experiment, its dominant index was 37.7%. Six days after the experiment, its dominant index dropped to 17.6% . Meanwhile the dominant index of Asterionella japanics Cleve rose from 7.1% to 39.2% , it became the first dominant species. Forty-five days after the experiment, the amount of phytoplankton in the Aoshan Bay was 5.15 to 137.32 times more than that in 1997. 相似文献
32.
探讨淡水养殖尤其是大水域网箱养鱼中由于投饵方式的不当和投饵量过多导致过剩饵料沉积引起水域环境污染的解决方法。本研究利用鱼类的听觉生物学特性,参考音响驯化技术的有关参数,结合投饵用400 Hz正弦波连续音对鲤(Cyprinus carpioLinnaeus)进行了音响驯化实验。结果表明,最初放声时,实验鱼产生惊愕反应,迅速离声源而去;但驯化6 d后,实验鱼在放声后迅速游向声源,摄食时间由最初的150 s缩短到实验后期的60 s。整个实验阶段放声组与对照组摄食时间差异极显著(P<0.01),摄食率和特定生长率无显著差异(P>0.05)。根据实验鱼的生长参数,求得了参考投饵量曲线和方程。本实验结果为音响驯化技术在淡水鱼养殖中的推广应用提供了基础数据。 相似文献
33.
34.
35.
36.
Petroleum hydrocarbons were investigated in Mytilus edulis and Macoma balthica at six intertidal sites in Port Valdez, Alaska after three to five years of oil terminal operation. Information about the concentrations of total hydrocarbons, unresolved complex mixture, normal alkanes, isoprenoids, olefins, hopanes and polycyclic aromatic hyrocarbons shows that petroleum accumulation in these bivalve mollusks is greatest at the Valdez municipal boat harbor and a crude oil terminal which has been permitted to discharge 170 kg oil day−1. Accumulation two- to ten-fold less were observed at 3 km from the terminal and boat harbor. The temporal trends in petroleum hydrocarbon concentrations in M. edulis and M. balthica reflect the effects of selective retention and depuration of different hydrocarbon classes as well as variation in hydrocarbon sources. It is not clear whether the petroleum concentrations in Port Valdez bivalves had reached steady state by 1982, the final sampling time reported here. 相似文献
37.
38.
39.
Generally, the underwater flight vehicle (UFV) depth control system operates with the following problems: it is a multi-input multi-output (MIMO) system, it requires robustness, a continuous control input, and further, it has the speed dependency of controller parameters. To solve these problems, an expanded adaptive fuzzy sliding mode controller (EAFSMC), which is based on the decomposition method designed by using an expert knowledge and the decoupled sub-controllers and composition method designed by using the fuzzy basis function expansions (FBFEs), is proposed. To verify the performance of the EAFSMC, the depth control of UFV in various operating conditions is performed. Simulation results show that the EAFSMC solves all problems experienced in the UFV depth control system online. 相似文献
40.
本方案采用850、700、500hPa三个层次的环境平均流场作控制点法。综合考虑这些层次上的引导气流在不同时、空域中的相对重要性。试验结果表明,其效果比单一取某层作引导为佳。且对用来预测台风的未来24小时路径是否异常具有一定的参考价值。 相似文献