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.

Recently, a new manufacturing process for the production of metallic matrix composite materials reinforced with carbon nanotubes, known as sandwich technique has been proposed. This technique produces a material comprised of a metallic matrix and a banded structures-layers of multi-walled carbon nanotubes. However, among other issues, the matrix-reinforcement interface and the reinforcement dispersion degree are still open questions. The present study uses field emission scanning electron microscopy and high resolution transmission electron microscopy to probe that the method is capable to achieve a good dispersion of the multi-walled carbon nanotubes with no evidence of carbon nanotubes’ damage. The mechanical properties were measured by tensile and nanoindentation tests; improvements in the elastic modulus, yield and ultimate strengths were found, with respect to the unreinforced material.

Since their discovery by Ijiima [1], carbon nanotubes have been considered as an ideal reinforcing material to improve the mechanical properties in polymeric, ceramic and metallic materials [2][3][4]. The need to find high-strength lightweight materials in transportation industries has driven the development of new materials and manufacturing methods to reduce CO2 emissions into the atmosphere.Recently, a new manufacturing process for the production of metal matrix composites reinforced with carbon nanotubes, known as sandwich technique, has been proposed [5][6][7]. This technique produces materials comprised of a metallic matrix and banded structured-layers of multiwalled carbon nanotubes (MWCNTs).The composites fabricated, both pure aluminum and AZ31 magnesium reinforced with MWCNTs, were characterized and studied by FESEM, TEM and HRTEM techniques for identifying the interaction between aluminum or magnesium matrix and MWCNTs. Figure 1a shows the MWCNTs used in this study, which have outer diameters of 10-40 nm and inner diameters of 10-20 nm, with a length of 30-50 µm. Some imperfections, including variable numbers of carbon layers and partial interior filling, amorphous carbon, and "bamboo" defects, are showed. These defects consist of several transverse, internal walls segmenting the interior of MWCNTs into independent pods or isolated volumes.In both aluminum and magnesium composites a good dispersion of the MWCNTs into the metal matrix is clearly seen (Figures 1b and 1c), which proves the success of the sandwich technique to synthesize these kind of composites. Figures 2 shows TEM and HRTEM images for both composites, where the MWCNTs can be observed embedded into the aluminum matrix (Figure 2a) and the magnesium alloy matrix (Figure 2b). The details in Figure 2 let see a good and homogeneous interface between both metal matrices and the MWCNTs, with no evidence of the formation of carbides or any other phase.