The microstructure and electrical properties of mechanically-alloyed copper-polymer composites

Using conventional melt compounding techniques it is difficult to mix high concentrations of filler materials such as metal powders with polymers. This is particularly true when the melt viscosity of the polymer concerned is high. In solid state synthesis performed by mechanical alloying it is possible to get homogenous mixtures of high molecular weight polymers and high contents of filling material. In this work a planetary type high energy mill was used to mix high-density polyethylene (HDPE) and copper at different concentrations. The aim was to produce electrically conductive polymers. The results obtained showed that during the mechanical alloying process the high-density polyethylene and copper formed a homogeneous powder. The metal particles were homogeneously distributed within the matrix polymer particles. The shape of the polymer particles was spherical or flake-like depending on the process parameters. The shape of the copper particles inside the polymer particles was flake like and the saturation particle diameter was under 10 mu m independent of the initial particle size. The microstructure of the high-density polyethylene became amorphous during the milling process. The milling time required for the formation of the amorphous phase was found to depend on the copper content. After ten hours of milling with a copper content of 50 vol.% the structure of polyethylene was completely amorphous. At copper contents of 10 vol.% even 320 hours of milling was not sufficient to produce fully amorphous HDPE. After the milling process the powders were cold compacted. The volume resistivity of the compacted polymer metal mixtures was ca. 1.0*10(11) Omega cm at a copper content of 10 vol.% and 4.0*10(-5) Omega cm at a copper content of 50 vol.%. The results show that using the mechanical alloying method it is possible to produce electrically-conductive polymers over a wide range of process and material parameters.