Studies on polymeric composites with Ni-Mn-Ga alloys and carbon nanotubes

Abstract

In the first part of the study, the temperature and stress dependencies on the damping of bulk Ni-Mn-Ga alloys were studied. The measurements showed a significant decrease in the storage modulus values in martensite region for all alloys with an increasing temperature. The loss modulus values were almost constant in the martensite phase, decreasing to ∼ zero at the martensite to austenite transformation. The damping of bulk Ni-Mn-Ga decreased sharply at the transformation temperature. The stress dependency on the damping was measured for a non-modulated Ni-Mn-Ga alloy, showing a certain level of stress, twinning stress, which must be overcome before the twins in an Ni-Mn-Ga alloy can begin to move, and the damping due to the twin movement can be activated. Polymer composites were manufactured using Ni-Mn-Ga as powder, ribbons, and thin bulk. The adhesion between Ni-Mn-Ga and the polymers was studied for epoxies, silicones, and polyurethanes. Epoxies with different mechanical strength levels were tested, the epoxies with strength levels comparable to the used Ni-Mn-Ga showed improvements in passive damping. A magnetic circuit was constructed for dynamic mechanical analysis, and an improvement in damping was found with an external magnetic field and with Ni-Mn-Ga powder orientation in epoxy matrix. In the second part of the study, methyl methacrylate was in situ polymerized in the presence of carbon nanotubes. Both the low molecular weight anionic surfactants and amphiphilic copolymers with a significantly higher molecular weight were used as dispersing agents. Both types of surfactants were able to effectively disperse the multi-walled carbon nanotubes into the polymer matrix. Carbon nanotubes, surfactants, and initiators all had an influence on polymerization and on the molecular weight of the resulting polymer. The modulus values of the composites were improved, and the stress at break values improved or remained stable up to high carbon nanotube loading (6 wt.%), which further confirmed a good dispersion of carbon nanotubes. The electrical and thermal conductivities were both improved by the addition of carbon nanotubes. The composites were also tested as masterbatches. The electrical percolation threshold was lower with the masterbatch-approach than with the direct extrusion of carbon nanotubes and poly(methyl methacrylate). The viscosities of the melts had a significant influence on the extrusion and thus percolation threshold.