全文获取类型
收费全文 | 25674篇 |
免费 | 1217篇 |
国内免费 | 2372篇 |
专业分类
林业 | 2082篇 |
农学 | 2424篇 |
基础科学 | 2022篇 |
2938篇 | |
综合类 | 12037篇 |
农作物 | 1172篇 |
水产渔业 | 753篇 |
畜牧兽医 | 3339篇 |
园艺 | 711篇 |
植物保护 | 1785篇 |
出版年
2024年 | 154篇 |
2023年 | 490篇 |
2022年 | 889篇 |
2021年 | 949篇 |
2020年 | 882篇 |
2019年 | 1103篇 |
2018年 | 611篇 |
2017年 | 1110篇 |
2016年 | 1372篇 |
2015年 | 1231篇 |
2014年 | 1484篇 |
2013年 | 1484篇 |
2012年 | 2128篇 |
2011年 | 2028篇 |
2010年 | 1611篇 |
2009年 | 1518篇 |
2008年 | 1333篇 |
2007年 | 1397篇 |
2006年 | 1210篇 |
2005年 | 955篇 |
2004年 | 735篇 |
2003年 | 640篇 |
2002年 | 528篇 |
2001年 | 444篇 |
2000年 | 414篇 |
1999年 | 350篇 |
1998年 | 312篇 |
1997年 | 279篇 |
1996年 | 266篇 |
1995年 | 217篇 |
1994年 | 183篇 |
1993年 | 167篇 |
1992年 | 167篇 |
1991年 | 128篇 |
1990年 | 128篇 |
1989年 | 119篇 |
1988年 | 103篇 |
1987年 | 49篇 |
1986年 | 34篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1962年 | 2篇 |
1956年 | 10篇 |
1955年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
信号交叉口是城市路网的节点和交通关键,其服务水平反映了交叉口所提供的道路及控制条件满足交通顺畅、安全等需要的程度。本文提出一种基于蒙特卡罗模拟的信号交叉口运行状态描述和服务水平评价方法.为优化信号交叉口控制参数提供依据. 相似文献
42.
43.
Influence of death struggle on the structural changes in chub mackerel muscle during chilled storage
Masashi Ando Masao Joka Satoshi Mochizuki Koh-Ichi Satoh Yasuyuki Tsukamasa Yasuo Makinodan 《Fisheries Science》2001,67(4):744-751
ABSTRACT: Chub mackerel (34–35 cm, approximately 500 g), which were caught by fishing with a rod and line at the Bungo Channel, Oita prefecture, were rested overnight in a fish preserve and either killed by decapitation (control group) or allowed to struggle in air for 30 min (struggled group). Muscle samples were excised every 4 h, and measurements on breaking strength and histological observations were done for both groups. The breaking strength of muscle in the control group was significantly higher than that in the struggled group, whereby a decrease in breaking strength was delayed for 12 h compared to the struggled group. Light microscopy showed space extension among muscle cells in association with a decrease in breaking strength. Especially in the struggled group, the extended area was larger and the difference in area was significant at the time when breaking strength showed a significant difference. Using electron microscopy, the extended area showed cut and/or disappeared collagen fibrils. From these results, it was demonstrated that struggling to death promoted the degradation of collagen fibrils and the weakening of connective tissue and, resultantly, led to the faster softening of muscle of chub mackerel. 相似文献
44.
本文针对建筑创作思维方法进行研究 ,提出了研究整合思维方法的目的、意义以及功能 ,论述了以“真、善、美”作为建筑创作的追求目标和应用整合思维方法进行建筑创作应遵循的基本原则 相似文献
45.
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
46.
本文通过对桥梁实例施工步骤的概略介绍,重点突出了逆作法施工工艺的特点、施工中要注意事项及与常规施工方案的比较。 相似文献
47.
压力实时监测法在管道泄漏检测中的应用 总被引:2,自引:0,他引:2
检测输油管道漏油的方法很多,介绍了通过压力实时监测确定输油管道漏油点方法,以及实际应用情况。实践证明,通过压力实时监视确定输油管道漏油点的方法在实际输油运行中是切实可行的。 相似文献
48.
49.
50.
研究了陆地棉主要性状对混选—混交育种体系的响应。两轮群体提高了产量、早熟性和降低了麦克隆值的平均数;扩大了纤维品质的变幅;增加了高产株率,但对改进抗枯黄萎病性不明显。一轮群体的效果低于二轮。两次混交扩大了产量和纤维品质的变幅,增加了正末端的个体频率。一次混交的作用不及两次。早代混选相对缩小了群体变幅。一次混交后再经两代开放授粉和对一轮群体再混选,均表现出有益的反响。 相似文献