A Novel Melt Processing for Mg Matrix Composites Reinforced by Multiwalled Carbon Nanotubes

Carbon nanotubes（CNTs） reinforced Mg matrix composites were fabricated by a novel melt processing.The novel processing consisted of two courses：CNTs pre-dispersion and ultrasonic melt processing.Mechanical ball-milling was employed to pre-disperse CNTs on Zinc（Zn） flakes.Serious CNT entanglements were well dispersed to single CNT or tiny clusters on Zn flakes.The ultrasonic melt processing further dispersed CNTs in the Mg melt,especially tiny CNT clusters.Thus,a uniform dispersion of CNTs was achieved in the as-cast composites.Hot extrusion further improved the distribution of CNTs.CNTs increased both the strength and elongation of the matrix alloy.Notably,the elongation of the matrix alloy was enhanced by 40%.Grain refinement and the pulling-out of CNTs resulted in the evident improvement of ductility for the composites.

A Novel Method to Fabricate CNT/Mg–6Zn Composites with High Strengthening Efficiency

A novel method was developed to fabricate carbon nanotubes （CNTs）-reinforced Mg matrix composites. The method consists of two steps： CNTs pre-dispersion by ball-milling and the ultrasonic melt processing. Mechanical ball- milling effectively pre-dispersed CNTs on Zn flakes with suitable rotational speed and ball-milling time. Serious CNT entanglements were dispersed by the ball-milling. However, ball-milling for a long time at high speed would damage the morphology of CNTs. The ultrasonic overcame the poor wettability between Mg melt and CNTs and then dispersed pre-dispersed CNTs in the Mg melt. CNTs were distributed well in the composites and maintained integrated structure. CNTs significantly improved the mechanical properties of the matrix. The strengthening efficiency reached to 37.1, which proves the superiority of this novel method. Besides grain refinement, load transfer may make a great contribution to the improvement of the strength for the composites.

Capitulum density-dependent effects generate peak seed yield at an intermediate density of a Tibetan lotus

Aims Theory suggests that species perform best at intermediate densities,where density-dependent facilitation and antagonism are balanced,but empirical evidence is scarce,particularly in plants.In a selfincompatible perennial herb(Saussurea nigrescens),whose recruitment heavily relies on seed output,we test whether both intraspecific facilitation and antagonism significantly affect seed production,resulting in highest seed yield at an intermediate capitulum density.Methods Plots with different S.nigrescens densities were sampled in an Eastern Tibetan meadow during the growing season of 2012 to investigate the relationships between capitulum density and pollinator visitation rate,seed set ratio,parasite ratio,seed damage ratio,and capitulum size.Both simple linear and quadratic models were employed to determine the shape of relationships.Important Findings In line with general theory,hump-shaped relationships of capitulum density versus seed set ratio and number of florets per capitulum indicate intraspecific facilitation in sparse populations,which can be attributed to positive density-dependent pollinator visitation and the amelioration of detrimental physical factors.However,the proportion of seeds damaged by pre-dispersal predators increased monotonically with capitulum density,which may have—in combination with increased intraspecific competition for light and soil nutrients—resulted in density-dependent antagonism.Both positive and negative density-dependent agents acted simultaneously throughout the density range investigated and led to the highest seed yield at intermediate density levels in the Tibetan lotus.More efforts concurrently exploring the two effects are needed to facilitate understanding species abundance and community structure.