Nowadays, various alloys have been developed for special applications. Among these alloys, magnesium alloys have relatively superior mechanical properties, such as low density, good wear resistance, and high elastic modulus. However, the toughness and the strength of the magnesium alloys are not able to meet the increasing needs of the automotive and aerospace industry for tougher and stronger alloys. Tsinghua University scientists created a magnesium-based carbon nanotube composite material that is strong, tough, has a high density, and can be widely used in a variety of fields including the automotive and aerospace industries.
In the magnesium-based composite material, nanoscale reinforcements (e.g. carbon nanotubes and carbon nanofibers) are mixed with the magnesium metal or alloy. The most common methods for making the magnesium-based composite material have been thixomolding and die-casting. However, in die-casting, the magnesium or magnesium alloy is easily oxidized. In thixomolding, the nanoscale reinforcements are added to melted metal or alloy but are prone to aggregate. As such, the nanoscale reinforcements can't be well dispersed.
Tsinghua researchers Kam-Shau Chan, Cheng-Shi Chen, Guang-Liang Sheu, Qing-Chun Du And Wen-Zhen Li developed an apparatus for fabricating a magnesium-based carbon nanotube composite material, in which the problems of aggregation and settling are eliminated or at least alleviated. Tsinghua University’s method includes the steps of: (a) providing a magnesium-based melt and carbon nanotubes, mixing the carbon nanotubes with the magnesium-based melt to achieve a mixture; (b) injecting the mixture into at least one mold to achieve a preform; and (c) extruding the preform to achieve the magnesium-based carbon nanotube composite material.
The mixing device includes a container with a protective gas therein, a stirrer disposed in a center of the container, and a heater (e.g. hot wires) disposed on a outer wall of the container. Quite suitably, the protective gas can, beneficially, be made up of at least one of nitrogen, ammonia, and a noble gas. The heater heats the container to a predetermined temperature in the approximate range from 550 degree C. to 750 degree C. Carbon nanotubes are in the approximate range from 1% to 5% by weight in the alloy. The materials is extruded into performs where it cools quickly enough to prevent settling and aggregation of the carbon nanotubes.
In the extrusion step, the preforms experience a deformation process when extruded from the exit. In the deformation process, different parts of the preforms will be mixed together. The carbon nanotubes are redistributed in the preforms which enhances dispersion uniformity of the carbon nanotubes in the magnesium-based carbon nanotube composite material. The mixing apparatus is detailed in U.S. Patent Application 20090127743. It is one of 47 applications concerning carbon nanotubes filed by Tsinghua University which is among China's leaders in nanotechnology research and development. The University already hold 20 U.S. Patents related to nanotechnology.