全文获取类型
收费全文 | 1279篇 |
免费 | 50篇 |
国内免费 | 149篇 |
专业分类
林业 | 135篇 |
农学 | 80篇 |
基础科学 | 154篇 |
378篇 | |
综合类 | 458篇 |
农作物 | 125篇 |
水产渔业 | 18篇 |
畜牧兽医 | 78篇 |
园艺 | 19篇 |
植物保护 | 33篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 26篇 |
2021年 | 30篇 |
2020年 | 33篇 |
2019年 | 34篇 |
2018年 | 25篇 |
2017年 | 46篇 |
2016年 | 51篇 |
2015年 | 54篇 |
2014年 | 82篇 |
2013年 | 64篇 |
2012年 | 90篇 |
2011年 | 120篇 |
2010年 | 87篇 |
2009年 | 90篇 |
2008年 | 84篇 |
2007年 | 85篇 |
2006年 | 95篇 |
2005年 | 74篇 |
2004年 | 55篇 |
2003年 | 43篇 |
2002年 | 25篇 |
2001年 | 19篇 |
2000年 | 17篇 |
1999年 | 27篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 17篇 |
1995年 | 12篇 |
1994年 | 14篇 |
1993年 | 6篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有1478条查询结果,搜索用时 171 毫秒
11.
12.
13.
连杆类锻件成形分析的三维网格生成方法 总被引:1,自引:0,他引:1
采用双三次均匀B样条曲面方法,研究了连杆类锻件模过程有限元分析的三维网格自动生成,提出了一种基于参数曲面事的三维网格自动生成方法,研究结果表明,该方法可以同锻件边界曲面几何描述相结合,简便易行,可实现三维网格的自动剖分。 相似文献
14.
15.
对滚珠丝杠的温升及热变形的对策进行了探讨,提出解决温升和热变形的方法,为滚珠丝杠的设计、安装、使用和维修提供参考. 相似文献
16.
滚珠丝杠的温升及热变形对策 总被引:1,自引:0,他引:1
对滚珠丝杠的温升及热变形的对策进行了探讨,提出解决温升和热变形的方法,为滚珠丝杠的设计、安装、使用和维修提供参考。 相似文献
17.
利用常规资料、多普勒雷达及自动站等资料,对秦巴山区一次致灾冰雹过程综合分析。结果表明,此次强对流天气是在高空前倾槽东移过程中产生的,中低层切变线为对流的发生提供了动力条件,地面辐合线是此次强冰雹天气的触发机制;适宜的0℃和-20℃层高度、地面暖低压和特殊的“喇叭口”地形等有利于强冰雹天气的发生;冰雹发生在低空急流左前方、高空急流出口区的左侧和地面辐合线附近。较强不稳定层结和垂直风切变是导致对流快速发展并维持的原因之一;多普勒雷达反射率因子图上对流单体中心强度大于68 dBZ且维持5个体扫,有超级单体生成;反射率因子剖面图上有有界弱回波、回波悬垂等出现;速度图上具有低层辐合和中气旋等特征;在反射率因子和速度图上均可看到三体散射;VIL跃增和骤降的时间与冰雹发生的时间相对应。 相似文献
18.
为筛选对胡杨幼苗生长有效的地表覆盖物。以1年生胡杨幼苗为试材,设置覆膜(T1)、覆草苫子(T2)、覆沙(T3)和裸地(CK)共4种处理,利用相关性分析和隶属函数值法全面评价3种地表覆盖物对植株产生的效果。结果表明:T1、T2、T3处理均显著提高了不同月份的土壤含水率(P<0.05),平均土壤含水率较CK分别提高了26.1%、27.9%和23.5%;各覆盖均显著提高了成活率、株高及地径的生长,T2增长幅度最大;各覆盖均显著提高了叶片净光合速率(P n)、气孔导度(G s)、蒸腾速率(T r)(P<0.05),生长初期T1值最大,生长中后期T2值最大;T2处理的最大净光合速率(P max)、光饱和点(L SP)最大,光补偿点(L CP)和暗呼吸速率(R d)最低;各覆盖对水分利用效率(W UE)和表观量子效率(A QY)影响较小;植株生长与土壤含水率、P n、G s、P max、L SP呈极显著正相关(P<0.01),综合效果由强到弱顺序为T2、T3、T1、CK。覆草苫子较适宜在胡杨幼苗栽植过程中应用推广。 相似文献
19.
In Ethiopia vertisols cover about 10% of the total land area and is the fourth most important soil used for crop production, accounting for nearly 23% of the total arable land used for crop production. More than half of the vertisols are found in the Central Highlands of Ethiopia, with an altitude of more than 1500 m above mean sea level. The unique physical and chemical properties of these soils and the high rainfall during the main cropping season create severe surface waterlogging problems which hinder crop production activities. Severe surface waterlogging affects the growth of plants by impeding nutrient uptake and creating oxygen deficiency around the root zone. To address this crop production problem, three surface water drainage methods, namely broad bed and furrow (BBF), ditch, and flat (traditional) methods were evaluated using the water balance of the plant root zone and wheat as a test crop. The experiment was conducted at the Ginchi Research Station in the central highlands of Ethiopia over two consecutive seasons (2000 and 2001). The results showed that both the BBF and the ditch drainage methods gave about 33% and 22% more grain yield than the flat treatment, respectively. However, there were no significant differences between BBF and ditch for both grain and biomass yield during both experimental seasons. During both seasons the total water balance (ΔWr) at the root zone especially, in the months of June, July and August on all the treatments was higher than the crop water requirement (ETc) and showed no significant difference between the treatments. Thus, the results of this study indicated that the soil water in the root zone was not significantly altered by surface drainage systems and therefore implies the need of further improvement of the different surface drainage methods regarding improving the waterlogging condition and hence the productivity of the vertisols in the Central Highlands of Ethiopia. 相似文献
20.
The objective of this study is to present a new application of optical and radar remote sensing with high spatial (∼10 m) and temporal (a few days) resolutions for the detection of tillage and irrigation operations. The analysis was performed for irrigated wheat crops in the semi-arid Tensift/Marrakech plain (Central Morocco) using three FORMOSAT-2 images and two ASAR images acquired within one week at the beginning of the 2005/2006 agricultural season.The approach we developed uses simple mapping algorithms (band thresholding and decision tree) for the characterisation of soil surface states. The first images acquired by FORMOSAT and ASAR were processed to classify fields into three main categories: ploughed (in depth), prepared to be sown (harrowed), and not ploughed-not harrowed. This information was combined with a change detection analysis based on multitemporal images to identify harrowing and irrigation operations which occurred between two satellite observations.The performance of the algorithm was evaluated using data related to land use and agricultural practices collected on 124 fields. The analysis shows that drastic changes of surface states caused by ploughing or irrigation are detected without ambiguity (consistency index of 96%). This study provided evidence that optical and radar data contain complementary information for the detection of agricultural operations at the beginning of agricultural season. This information could be useful in regional decision support systems to refine crop calendars and to improve prediction of crop water needs over large areas. 相似文献