全文获取类型
收费全文 | 4626篇 |
免费 | 245篇 |
国内免费 | 410篇 |
专业分类
林业 | 606篇 |
农学 | 426篇 |
基础科学 | 616篇 |
650篇 | |
综合类 | 1876篇 |
农作物 | 192篇 |
水产渔业 | 211篇 |
畜牧兽医 | 379篇 |
园艺 | 78篇 |
植物保护 | 247篇 |
出版年
2024年 | 42篇 |
2023年 | 117篇 |
2022年 | 158篇 |
2021年 | 187篇 |
2020年 | 216篇 |
2019年 | 197篇 |
2018年 | 106篇 |
2017年 | 177篇 |
2016年 | 210篇 |
2015年 | 186篇 |
2014年 | 280篇 |
2013年 | 265篇 |
2012年 | 408篇 |
2011年 | 358篇 |
2010年 | 279篇 |
2009年 | 269篇 |
2008年 | 238篇 |
2007年 | 241篇 |
2006年 | 216篇 |
2005年 | 191篇 |
2004年 | 159篇 |
2003年 | 112篇 |
2002年 | 80篇 |
2001年 | 72篇 |
2000年 | 52篇 |
1999年 | 43篇 |
1998年 | 59篇 |
1997年 | 48篇 |
1996年 | 57篇 |
1995年 | 50篇 |
1994年 | 45篇 |
1993年 | 34篇 |
1992年 | 26篇 |
1991年 | 21篇 |
1990年 | 37篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1956年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有5281条查询结果,搜索用时 15 毫秒
1.
2.
杜仲研究的历史,现状和展望 总被引:13,自引:1,他引:12
本文讨论了杜仲研究的历史和现状,并根据现在研究的成果和存在的问题,对将来应该开展的研究课题提出了一些建议。 相似文献
3.
Mangroves as indicators of coastal change 总被引:2,自引:0,他引:2
In view of the unique biological characteristics of mangroves, it is interesting to assess the extent to which these ecosystems can be used as indicators of coastal change or sea-level rise. From recent studies of mangrove mortality at several locations (including Guiana, Gambia, Côte d'Ivoire, Kenya, India and Bangladesh), it appears that these coastal ecosystems are so specialized that any minor variation in their hydrological or tidal regimes causes noticeable mortality. Each species of mangrove (but particularly those belonging to the genera Rhizophora, Bruguiera, Sonneratia, Heritiera and Nypa) occurs in ecological conditions that approach its limit of tolerance with regard to salinity of the water and soil, as well as the inundation regime. If the duration of daily immersion were to be modified by tectonic, sedimentological or hydrological events, the species either readjusts to the new conditions or succumbs to unsuitable conditions. Consequently, the use of remote sensing data for mangrove ecosystems offers excellent potential as a tool for monitoring coastal change. 相似文献
4.
ABSTRACT: An application of the acoustic Doppler current profiler (ADCP, 153.6 kHz) in combination with a scientific echo sounder (EK60, 38 and 200 kHz) was investigated to estimate the density of krill Euphausia pacifica . The acoustic backscattering strength from sound scattering layers was compared with biomass estimates from midwater trawls. Euphausia pacifica was targeted among mixed species populations in the sound scattering layer in the offshore Funka Bay area of Hokkaido, Japan. The frequency characteristics of acoustic backscattering by krill were calculated using a distorted wave Born approximation scattering model at three frequencies. Krill aggregations identified from the EK60 data were extracted as the mean volume backscattering strength difference between two frequencies. They were then used to identify similar aggregations in the ADCP data by matching observation times and depths for the two methods, which were applied simultaneously. Results from the comparison of the mean volume backscattering strength and the density calculated from the ADCP and EK60 showed that ADCP can be used to measure density and spatial–temporal distribution of krill aggregations. Current speed and direction at the study site were found to be 16.1 cm/s and 187.0°, respectively, and krill speed and direction (including the current component) were found to be 19.8 cm/s and 172.2°, respectively. Based on the ADCP data, the net speed and direction of the krill aggregations were found to be 5.9 cm/s and 128.0°, respectively. 相似文献
5.
本文运用知识工程语言 M·1构造了一个用于小麦条锈病流行程度趋势预测的专家系统雏型。专家的知识用产生式规则来表示。知识库中的知识规则可相对独立,便于知识库的扩充和完善。 相似文献
6.
Improvements in human patient monitoring despite their development in animals, do not always find their way into veterinary clinical use due to financial constraints. Gastrointestinal intraluminal CO2 partial pressure (Gip1 CO2 ) monitoring, however, is not only proving very beneficial in human trauma and critical patient care but is also very likely to become relatively inexpensive. By providing information on the perfusion adequacy of a high risk, critically important tissue, the GI mucosa, GI P1 CO2 monitoring offers an easily accesible indicator of the efficacy and adequacy of resuscitative interventions. The potential for decreasing morbidity and mortality is enormous. Therefore, the practicing veterinarian should become familiar with GI P1 CO2 monitoring theory and technology so he or she can be better prepared to incorporate it into practice when in becomes available. 相似文献
7.
从实际工程的角度出发,对电路电测方法进行了论述。在微通信工程中,需要获得障碍物的位置和高度,最合理的天线高度组合和障碍物绕射损耗等参数,为此论述了微波电路电测方法及技术要求。在大庆-秦皇岛输油管道通信工程中采用了A,B测试法进行测试,得到高度-损耗曲线。这种曲线给出相对自由空间接收电平与收,发天线高度之间的关系。在测试所得的高度-损耗曲线与断面图相结合的基础上,由绕射损耗计算公式推出了几种常见障碍 相似文献
8.
9.
10.
本文在分析CT的暂态特性及其动态磁化过程的基础上,建立了计及铁芯饱和、局部磁滞回环、动态剩磁影响的CT单、复励磁暂态过程数字仿真模型,提出了继电保护用P、TPX、TPY、TPz级CT的电磁优化设计方法,编制了计算机程序,用于CT的电磁优化设计及数字仿真分析. 相似文献