全文获取类型
收费全文 | 3879篇 |
免费 | 216篇 |
国内免费 | 315篇 |
专业分类
林业 | 513篇 |
农学 | 337篇 |
基础科学 | 567篇 |
705篇 | |
综合类 | 1618篇 |
农作物 | 107篇 |
水产渔业 | 106篇 |
畜牧兽医 | 224篇 |
园艺 | 118篇 |
植物保护 | 115篇 |
出版年
2024年 | 13篇 |
2023年 | 55篇 |
2022年 | 92篇 |
2021年 | 89篇 |
2020年 | 107篇 |
2019年 | 139篇 |
2018年 | 87篇 |
2017年 | 111篇 |
2016年 | 170篇 |
2015年 | 134篇 |
2014年 | 183篇 |
2013年 | 211篇 |
2012年 | 267篇 |
2011年 | 314篇 |
2010年 | 246篇 |
2009年 | 239篇 |
2008年 | 222篇 |
2007年 | 270篇 |
2006年 | 212篇 |
2005年 | 189篇 |
2004年 | 151篇 |
2003年 | 110篇 |
2002年 | 99篇 |
2001年 | 64篇 |
2000年 | 80篇 |
1999年 | 84篇 |
1998年 | 61篇 |
1997年 | 56篇 |
1996年 | 47篇 |
1995年 | 61篇 |
1994年 | 48篇 |
1993年 | 45篇 |
1992年 | 32篇 |
1991年 | 31篇 |
1990年 | 23篇 |
1989年 | 33篇 |
1988年 | 23篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 2篇 |
排序方式: 共有4410条查询结果,搜索用时 15 毫秒
51.
对落叶松胶合木用层板进行目测分级并测定其弹性模量和静曲强度,研究各等级层板的弹性模量和静曲强度及其相互之间的关系,探讨层板目测分级和机械弹性模量分级的科学性、合理性。结果表明:根据层板缺陷进行目测分级,等级高的层板其弹性模量和静曲强度的平均值和5%分位值高,目测分级能够基本反映其材质性能,具有一定的科学性与合理性;但目测分级层板弹性模量和静曲强度值的离散度很大,目测分级所能达到的准确与可靠程度决定其不能替代机械弹性模量分级。机械弹性模量分级层板的静曲强度与弹性模量呈正相关性,但各等级层板的静曲强度与弹性模量的相关系数大不相同,其静曲强度值的离散度也比较大,说明该分级方法还有改善的余地。更加科学、可靠的胶合木层板分级方法还有待进一步的探索研究。 相似文献
52.
The Brazil nut tree(Bertholletia excelsa) is a frequent component of agroforestry systems in the Amazon because of its adaptation to nutrient-poor upland soils and multiple uses.We investigated the aboveground biomass production(kg dry weight),nutrient uptake and requirements(N,P,Ca,Mg,K) of Brazil nut trees of different sizes grown under agroforestry conditions and fertilized at different levels.Eight of 70 experimental trees with different size were harvested and stem,branches and leaves were separated.Nutrient contents were determined for three trees of varying size.Average tree growth was fast,but variability was high,suggesting considerable potential for the improvement of this economically important species.The trees responded to increased levels of fertilizer and lime with significantly increased foliar nutrient contents and growth,probably because of the improved availability of Mg and Ca for which the species seems to have a relatively high demand.In contrast to Brazil nut trees grown in forest or dense plantations,the agroforestry trees invested a substantial part of their biomass and nutrients in large branches and developed spreading crowns.To improve stem form,reduce competition with associated crops for light and recycle nutrients,regular pruning of lower branches or planting arrangements that favor self-pruning are recommended.These measures would also increase the recycling of Ca and Mg,large quantities of which are contained in the branches. 相似文献
53.
介绍了在三维机械CAD系统Solid Edge中,建立常用零件参数化特征库的技术以及参数化特征的使用方法,着重探讨了将驱动尺寸与变量表结合起来提高常用零件建模效率的技术。 相似文献
54.
55.
张锡滨 《林业机械与木工设备》2004,32(11):36-37
提出了一种基于成组技术思想,针对不同类型双颚板散货抓斗工作过程计算机仿真的综合建模方法。用同一模型同时描述三种不同类型的抓斗,通过改变初始输入条件,来反映不同抓斗的工作规律。 相似文献
56.
A modified rate equation, in which the diffusion of reagent was taken into consideration, was proposed and applied to results reported previously for the acetylation of wood. The equation was fitted more successfully to the prolonged as well as the early stage of the reaction than a common first-order rate equation. Although the conclusions were virtually unchanged, the reliability was enhanced because reaction parameters such as the rate constant, ultimate extent of the reaction, and activation energy calculated for the modified equation were based on much more data than those reported previously. The equation proposed here may be applicable not only to acetylation but to other chemical modifications of cellulosic materials in heterogeneous systems. 相似文献
57.
58.
59.
ABSTRACT Growth Simulation, an analytical modeling technique, has been increasingly used in ecological studies and practical forestry applications where dendrochronology is not applicable. The technique uses randomly sampled diameter increments from tagged trees over a known time interval to assemble a statistical sample of lifetime growth trajectories. We carried out a validation of Growth Simulation using a temperate species in order to compare indirect model outputs with direct tree ring analysis. Rings were measured on sample disks cut from 55 pine branches ranging in age from 8–36 years. Assessments included lifetime growth rates, growth rate with respect to diameter and age, periodic annual increment (PAI), cross-referencing of rings by date, and autocorrelation of growth over successive periods. Tree ring analysis and Growth Simulation showed close correspondence for all parameters tested (maximum, median, and minimum growth rates; longevity estimates). Growth Simulation is found to be a robust and informative technique for studies of tropical tree growth, and is especially useful where analysis of tree rings is not feasible or when bootstrapping analysis of ring data is of interest. 相似文献
60.
lntroductionInordertosolvetheseriousecologicaIproblemwefacedandimprovethephenomenonoflanddesertifi-cationinthenorthareaofChina.ChinahasbeguntoconstructtheThree-NorthProtectiveForestSystemsince1978.Withthehardworkoftwentyyears,itistold"Thegreatestecologicalprojectoftheworld"and"ThegreengreatwaIl".AIthoughthecountryinvestedIotsofcapitaItobuiIdtheThree-Northshelter-forestsystemthroughmanywayssuchasThree-Northspecialinvestment,financeappropriatefunds,finan-cialloansandtakeoutfundsforbringup… 相似文献