Abstract:
Si3N4 nanocomposites reinforced with 1-, 2-, and 6-vol% single-walled carbon nanotubes (SWNTs) were processed using spark plasma sintering (SPS) in order to control the thermal and electrical properties of the ceramic. Only 2-vol% SWNTs additions were used to decrease the room temperature thermal conductivity by 62% over the monolith and 6-vol% SWNTs was used to transform the insulating ceramic into a metallic electrical conductor (92Sm-1). We found that densification of the nanocomposites was inhibited with increasing SWNT concentration however, the phase transformation from α- to β-Si3N4 was not. After SPS, we found evidence of SWNT survival in addition to sintering induced defects detected by monitoring SWNT peak intensity ratios using Raman spectroscopy. Our results show that SWNTs can be used to effectively increase electrical conductivity and lower thermal conductivity of Si3N4 due to electrical transport enhancement and thermal scattering of phonons by SWNTs using SPS. © 2010 Elsevier Ltd.
Carbon-carbon (C-C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high-temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high-temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600 degrees C. Oxidation protection for C-C composites can be achieved by duplicating the self-generating oxide chemistry of bulk UHTCs formed by a composite effect upon oxidation of ZrB2-SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE-TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600 degrees C. Pure SiC-based fillers are ineffective at protecting C-C from oxidation, whereas ZrB2-SiC filled C-C composites retain up to 90% initial mass. B2O3 in SiO2 scale reduces initial viscosity of self-generating coating, allowing oxide layer to spread across C-C surface, forming a protective oxide layer. Formation of a ZrO2-SiO2 glass-ceramic coating on C-C composite is believed to be responsible for enhanced oxidation protection. The glass-ceramic coating compares to bulk monolithic ZrB2-SiC ceramic oxide scale formed during DNE-TGA where a comparable glass-ceramic chemistry and surface layer forms, limiting oxygen diffusion.
