全文获取类型
收费全文 | 424篇 |
免费 | 21篇 |
国内免费 | 81篇 |
专业分类
林业 | 18篇 |
农学 | 20篇 |
基础科学 | 144篇 |
165篇 | |
综合类 | 114篇 |
农作物 | 12篇 |
水产渔业 | 4篇 |
畜牧兽医 | 19篇 |
园艺 | 2篇 |
植物保护 | 28篇 |
出版年
2024年 | 10篇 |
2023年 | 23篇 |
2022年 | 41篇 |
2021年 | 20篇 |
2020年 | 24篇 |
2019年 | 28篇 |
2018年 | 25篇 |
2017年 | 27篇 |
2016年 | 28篇 |
2015年 | 11篇 |
2014年 | 20篇 |
2013年 | 23篇 |
2012年 | 39篇 |
2011年 | 28篇 |
2010年 | 22篇 |
2009年 | 20篇 |
2008年 | 17篇 |
2007年 | 22篇 |
2006年 | 19篇 |
2005年 | 10篇 |
2004年 | 14篇 |
2003年 | 11篇 |
2002年 | 4篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1994年 | 1篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1987年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有526条查询结果,搜索用时 15 毫秒
481.
Yuqing Yang Shongming Huang Guillermo Trincado Shawn X. Meng 《European Journal of Forest Research》2009,128(4):415-429
Four variable-exponent taper equations and their modified forms were evaluated for lodgepole pine (Pinus contorta var. latifolia Engelm.) trees in Alberta, Canada. A nonlinear mixed-effects modeling approach was applied to account for within- and between-tree
variations in stem form. Even though a direct modeling of within-tree autocorrelation by a variance–covariance structure failed
to achieve convergence, most of the autocorrelation was accounted for when random-effects parameters were included in the
models. Using an independent data set, the best taper equation with two random-effects parameters was chosen based on its
ability to predict diameter inside bark, whole tree volume, and sectioned log volume. Diameter measurements from various stem
locations were evaluated for tree-specific calibrations by predicting random-effects parameters using an approximate Bayesian
estimator. It was found that an upper stem diameter at 5.3 m above ground was best suited for calibrating tree-specific predictions
of diameter inside bark, whole tree volume, and sectioned log volume. 相似文献
482.
483.
FAO Penman—Monteith及简化方法在西北适用性研究 总被引:1,自引:0,他引:1
为了实现气象资料缺失下参考作物蒸发蒸腾量(ET0)在西北地区的高精度预测,采用FAO 56 PenmanMonteith(P-M)公式作为对照,与气象资料缺失下P-M公式8种情况以及Pristley-Taylor(P-T)法、Makkink法、Hargreaves-Samani(H-S)法、Irmak法对西北5省区30个辐射站的逐日气象资料进行了统计比较,并对P-T法、Makkink法、H-S法、Irmak法重新进行了参数率定。结果表明,气象资料缺失时,总辐射资料缺失精度下降最小,基于日照资料的ngstrm-Presscott(A-P)法是该地区适宜的替代方法,其各站平均R2为0.983、RMSE小于0.4 mm/d。当相对湿度、风速或日照时数有一项缺失时,使用FAO推荐的P-M替代方法可以使多年平均ET0的RMSE小于0.47 mm/d,R2保持在0.94以上。当气象要素缺少风速和相对湿度时,使用率定后Makkink法是该地区适宜的替代选择,其RMSE为0.68 mm/d,R2为0.94,当仅有气温资料时,改进后的H-S法是该地区适宜的替代选择,其RMSE为0.68 mm/d,R2为0.94。P-T法在该地区精度低于Makkink法,其RMSE为0.71 mm/d,R2为0.88,30个站参数值率定后α值介于1.02~1.64之间。 相似文献
484.
485.
六足制孔机器人三自由度并联机械腿的误差模型及验证 总被引:4,自引:2,他引:2
为了研制六足制孔机器人,提出了一种基于(U+UPS)P+UPS机构的三自由度并联机械腿,建立了机械腿机构的误差模型与评价方法,并通过误差分析制造出机械腿试验样机,对试验样机进行了误差标定试验研究。首先,采用矢量链法建立了机械腿机构的误差矢量约束方程,得到了机械腿机构的误差传递模型。接着,定义了一组误差敏感性评价指标,并绘制了误差敏感性评价指标在机械腿机构工作空间内的分布曲面。然后,基于误差敏感性评价指标及机构其他机构学性能,采用蒙特卡罗法对机械腿进行了结构参数设计,选取了一组结构参数,制造了机械腿试验样机。最后,采用一套高精度机器人标定系统对机械腿试验样机进行了误差标定。试验表明:机械腿试验样机的位置误差实测值与理论值之间偏差均小于0.003 mm,姿态误差实测值与理论值之间偏差均小于0.05°,误差敏感性评价指标的实测值与理论值的差值均小于0.03。机械腿试验样机的误差均在合理范围之内,基本达到了设计要求。 相似文献
486.
棉花生长模拟模型GOSSYM及其在北疆棉区的应用 总被引:3,自引:1,他引:2
利用1996~1998年在北疆3个试点进行的试验,对GOSSYM模型进行了全面的验证。结果表明,在对模型针对地膜覆盖特点进行了修改的基础上,通过校正品种参数和土壤参数,GOSSYM的模型基本能模拟北疆主栽品种新陆早6号和新陆早7号的生长发育和产量形成过程。 相似文献
487.
Skinner JG 《Veterinary clinical pathology / American Society for Veterinary Clinical Pathology》2001,30(1):2-7
Acute phase proteins (APPs) such as haptoglobin, serum amyloid A and pig major acute phase protein are plasma proteins that increase in concentration following infection, inflammation, or trauma. The circulating concentrations of these proteins in pigs and cattle can provide an objective measure of the health status of an animal and are increasingly being used as markers of animal health and welfare. Plasma concentrations of APPs are related to the severity of the underlying condition, and provide a ready means of evaluating both the presence and extent of disease. Haptoglobin, for example, has been used to identify both clinical and subclinical disease in animals, and for objectively monitoring antibiotic therapy in experimentally infected animals. Interpretative benefit can be further enhanced by the "acute phase index", derived from a mathematical formula that uses both positive- and negative-reacting APPs. Research suggests that in the future, assays for APPs will be used routinely to assess animal health, optimize production rates, monitor antibiotic therapy, detect diseases such as mastitis in dairy cows, and assess the health of animals at slaughter. These applications have considerable benefit for human food safety. Before APP assays can be applied in animal production on a worldwide basis, however, the calibration of assay methods must be harmonized to ensure that results obtained in the laboratory or on the farm are universally comparable and of consistent quality. In February 2000 the European Commission Directorate General Research Concerted Action was established to fulfill the task of international standardization of APPs. The Concerted Action Group consists of a network of 14 institutions representing 9 European countries. In this report, the background and goals of the Concerted Action Group, and scientific presentations from the group's first colloquium are described. In addition, the progress of the group to date and the standardization plan for the full 30-month duration of the Concerted Action are summarized. 相似文献
488.
In the view of reliability calibration, this paper analyzes the probabilistic sense of the provisions of "Strong Column and Weak Beam' and axial compression ratio in aseismic criteria (GBJ 11 89), which demonstrates that there is much difference between different aseismic ranks of RC frame. The authors suggest that it is possible to make axial compression ratio larger by adjusting the moment capacity ratio between column and beam. Finally, the direction and method of further studies are proposed. 相似文献
489.
水果输送过程中定向问题的关键所在是寻找影响定向成功率的主要因素,需将各影响因素经过科学的组合对比,找到最佳组合,以提高水果在实际生产过程中的定向成功率。为此,利用正交试验和KMT动应变测量系统相结合的方法确定影响定向主要相关因素,利用KMT动应变系统测试的夹持片动应变大小与计算分析所得数据基本吻合。同时,确定了影响水果输送定向的主要因素有:入口通道宽度(D)、夹持带的速度大小(V)、杏子尺寸大小(I)。通过正交试验,得到较优的试验因素组合是:D为25mm,V为13.2m/min,I为34~35mm。利用高速摄像观测统计发现:水果大部分在定向通道的第4区域达到定向稳定。考虑夹持片对输送带振动影响的情况下,统计出杏子的定向稳定区域主要集中于输送通道的第4区域,为杏子切刀的位置选择提供了依据。 相似文献
490.