M. Isabel Osendi

Carbon nanotubes functionalization process for developing ceramic matrix nanocomposites

The carbon nanotubes (CNTs) functionalization for developing ceramic matrix composites with an optimum interface between the matrix and nanotubes is presented. The functionalization processes were successfully performed by means of oxidation and boron nitride and silicon oxide coatings, which were characterized, among others, by O2/N2 adsorption-desorption method, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. In all cases, the functionalized CNTs exhibited an enhanced thermal stability compared to the original nanotubes. Silicon nitride (Si3N4) nanocomposites having 5.3 vol% of multi-walled CNTs, both pristine and functionalized, were developed. Dense nanocomposites having well dispersed nanotubes were attained under all circumstances, although the nanocomposites containing the coated CNTs required the use of a dispersant agent to prevent nanotubes agglomeration. Besides, slightly higher sintering temperatures were needed to densify the composites containing the coated CNTs. A stronger mechanical interlocking between the matrix and the nanotubes was achieved by the functionalization processes, which led to some improvement of the mechanical properties of these nanocomposites, compared to those containing pristine CNTs; and actually values that were close or even higher than those measured for the blank Si3N4 were achieved.

Modelling thermal conductivity of biphasic ceramic materials by the finite element method

The thermal conductivity of two-phase ceramics has been modelled by finite element method for a wide range of second phase contents, aspect ratios and thermal conductivities of the filler phase. The finite element method model has been validated with the experimental thermal conductivity data from exemplary ceramics to show the applicability of this model, namely alumina/silicon carbide composites and highly porous mullite materials, which show second phases of very different thermal conductivity (SiC and pores). The selection of the mesh element dimensions has been based on a schematic picture of the microstructure. For the first time, the finite element method simulation of a large number of random biphasic meshes has been used for estimating errors in the predicted thermal conductivity values that were comparable with the experimental error. The model can be very helpful in designing new materials with specific thermal conduction, limiting the more expensive and time consuming material preparation work.