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31.
中国硒研究历史回顾(上) 总被引:1,自引:0,他引:1
秦俊法 《广东微量元素科学》2014,(11):44-57
过去50年中,中国微量元素科学和营养科学中最重要的发现,就是认识了硒对人体健康的重要性。在诸多微量元素中,没有哪一个元素能像硒那样对人类产生如此深刻而又广泛的影响。从硒的生物必需性、硒的生物效应两面性、硒的抗癌性、硒的延寿作用及富硒产品开发五个方面回顾了中国的硒研究历程。 相似文献
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Emilio Di Giacomo Walter Didimo Giuseppe Liotta Fabrizio Montecchiani 《Computational Geometry》2013,46(8):909-916
In this paper we study how to compute compact straight-line drawings of planar graphs with a limited number of crossings per edge. We prove that every outerplanar graph can be drawn in area using a sub-linear number of crossings per edge, and that for any given number , every outerplanar graph admits an area drawing with crossing per edge. The drawing algorithms run in linear time and can be also generalized to another meaningful sub-family of series–parallel graphs with bounded vertex-degree. 相似文献
35.
Satish Babu Kodali Narendar Reddy Jakku Chinna Rajanna Kamatala Rajeshwar Rao Yerraguntla 《国际化学动力学杂志》2020,52(3):167-177
Bronsted acid catalyzed oxidation of certain sugar alcohols (polyols) has been studied by quinolinium dichromate (QDC) using aqueous sulfuric, perchloric, and hydrochloric acids at different temperatures. At constant acidity, reaction kinetics revealed the second-order kinetics with a first order in [Alcohol] and [QDC]. Zucker-Hammett, Bunnett, and Bunnett-Olsen criteria were used to analyze acid-dependent rate accelerations. Bunnett-Olsen plots of (log k + Hν) versus (Hν + log [H+]), and (log k) versus (Hν + log [H+]) afforded slope values (ϕ and ϕ*, respectively) > 0.47, suggesting that a water molecule acts as a prton transfer agent in the slow step of the mechanism in the oxidation of alcohols by QDC in the presence of aqueous sulfuric, perchloric, and hydrochloric acids. 相似文献
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巧妙地设计家庭化学小实验,并指导学生记录实验日记、组织实验沙龙,对激发学生学习化学的兴趣、提高实验教学效果有着积极的作用。 相似文献
37.
介绍了选择产品标准的技术要求的基本原则和依据。阐述了低温产品应根据实际需要从环境适应性、使用性能、理化性能、稳定性、健康安全和环境保护、耗能指标、外观和感观要求、材料要求和工艺要求等方面规定产品标准的性能要求。 相似文献
38.
竖向均布荷载作用在地基内部时的土中应力公式 总被引:18,自引:2,他引:18
建筑物基础一般都是埋入地基中,且有一定深度,而目前土中应力计算所常依据的布西奈斯克解却是假定荷载地表面导出的。本文以半无限体内受竖向力作用的明德林公式为根据,通过积分而首次完整地推导出竖向矩形均布荷载作用在地基内部上的土中应力分量解析表达式,并与布氏公式进行比较,说明其应用范围,以便于工程设计人员在设计时使用。 相似文献
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韩中庚 《数学建模及其应用》2017,6(2):42-48
本文针对数学建模竞赛论文的评阅要求和近年来参赛论文的实际情况,首先简要介绍了科技论文的写作规范,然后详细说明了竞赛论文的写作方法和注意事项,可供拟参加数学建模竞赛的学生和指导老师参考。 相似文献
40.
Lift and power requirements of hovering insect flight 总被引:6,自引:0,他引:6
Lift and power requirements for hovering flight of eight species of insects are studied by solving the Navier-Stokes equation numerically. The solution provides velocity and pressure fields, from which unsteady aerodynamic forces and moments are obtained. The inertial torque of wing mass are computed analytically. The wing length of the insects ranges from 2 mm (fruit fly) to 52 mm (hawkmoth); Reynolds numbers Re (based on mean flapping speed and mean chord length) ranges from 75 to 3850. The primary findings are shown in the following: (1) Either small (R = 2mm, Re = 75), medium (R ≈ 10 mm, Re ≈ 500) or large (R ≈ 50 mm, Re ≈ 4 000) insects mainly employ the same high-lift mechanism, delayed stall, to produce lift in hovering flight. The midstroke angle of attack needed to produce a mean lift equal to the insect weight is approximately in the range of 25° to 45°, which is approximately in agreement with observation. (2) For the small insect (fruit fly) and for the medium and large insects with relatively small wingbeat frequency (cranefly, ladybird and hawkmoth), the specific power ranges from 18 to 39W·kg^-1 , the major part of the power is due to aerodynamic force, and the elastic storage of negative work does not change the specific power greatly. However for medium and large insects with relatively large wingbeat frequency (hover fly, dronefly, honey bee and bumble bee), the specific power ranges from 39 to 61 W·kg^-1 , the major part of the power is due to wing inertia, and the elastic storage of negative work can decrease the specific power by approximately 33%. (3) For the case of power being mainly contributed by aerodynamic force (fruit fly, cranefly, ladybird and hawkmoth), the specific power is proportional to the product of the wingbeat frequency, the stroke amplitude, the wing length and the drag-to-lift ratio. For the case of power being mainly contributed by wing inertia (hoverfly, dronefly, honey bee and bumble bee), the specific power (without elastic storage) is proportional to the product of the cubic of wingbeat frequency, the square of the stroke amplitude, the square of the wing length and the ratio of wing mass to insect mass. 相似文献