Juan Meza

Mechanical and thermal behavior of polyvinyl alcohol reinforced with aligned carbon nanotubes

In recent years, due to their high specific mechanical properties, the polymer matrix composites have been widely used. In particular, carbon nanotubes (CNTs) have been used as reinforcement due to its exceptional mechanical and electrical properties. In this work the mechanical behavior of polyvinyl alcohol (PVA) reinforced with different percentages of mechanically oriented CNTs was studied. The composite material was made starting with a solution of PVA+ distilled water, and then the CNTs were added to the solution and dispersed by magnetic stirring and ultrasonic agitation. During this step, a surfactant was added to help with the CNTs dispersion. This mixture was poured into a container and dried using a heat source to accelerate its polymerization and curing. The layers obtained were subjected to mechanical stretching at controlled temperature to align the CNTs in the loading direction. The mechanical properties were measured by tensile testing and nanoindentation. The results show a large increase in modulus and hardness compared with non-reinforced polymeric material, it was also found a significant increase in the tensile strength. The composite with the best properties was subjected to thermal stabilization by ethanol, obtaining an increase in the degradation temperature of about 100°C approximately, although in this case the mechanical stiffness and hardness were decreased compared to unstabilized material, the usefulness of this material is enhanced.

Dispersion and alignment quantification of carbon nanotubes in a polyvinyl alcohol matrix

In recent years, due to their high specific mechanical properties, polymer matrix composites have been widely used. In particular, carbon nanotubes have been used as reinforcement because of their exceptional properties. In this work, the degree of dispersion and alignment of multiwalled carbon nanotubes into polyvinyl alcohol was quantified, which is a key information to optimize the mechanical properties of composites. For the dispersion of the multiwalled carbon nanotubes into the solution, a magnetic stirring and ultrasonic agitation were used. Finally, the mixture was dried to obtain a multiwalled carbon nanotubes reinforced polymer. The composites were mechanically stretched to obtain sheets with multiwalled carbon nanotubes aligned in the stretching direction. The layers obtained were prepared for transmission electron microscopy analysis. A dispersion quantification method based on the statistical distribution of horizontal and vertical separation distance between carbon nanotubes was used; a lognormal distribution was obtained. The angle of carbon nanotubes with respect to the stretching direction was used to quantify the alignment degree of carbon nanotubes. The bulk mechanical properties of the composites were measured by nanoindentation test; tensile test allowed to measure the mechanical properties of the composites in both the stretching and perpendicular directions to the stretching direction. Multiwalled carbon nanotubes showed a good dispersion and alignment degree and this, in conjunction with stretching, produced a high increase of both the stiffness and strength in the stretching direction, which allowed an increment of the mechanical properties measured by nanoindentation test; the best properties of the composites were reached with 0.5 wt% of MWCNTs.

Comportamiento microestructural de una fundición de magnesio puro con control en la temperatura de solidificación

The study of the microstructural behavior of pure magnesium (Mg) after a casting process conducted in a furnace of own manufacture with controlled atmosphere and solidification temperature control is presented. This study aimed to determine the change in the composition of the Mg by SEM/EDS, owing to the protective atmosphere used. Besides, the change in the microstructure of Mg was studied by performing solidification in a mold at temperature of 200°C within the casting system, in which a poor oxygen atmosphere was used with a mixture of CO2 and SF6. Through the compositional evaluation was evident a change in the material owing to the gases used and the elements that interacted with the Mg. It was possible to see a change in the microstructure of the Mg by optical microscopy analysis and SEM, observing differences in the amount and distribution of particles and inclusions present in the Mg. After casting process, the generation of a dendritic structure was observed, but not in the base material. Finally, using the manufactured casting system was possible to obtain an ingot of Mg with similar conditions to the base material, in addition to obtain a microstructure with finer grain and better distribution of particles in the matrix.