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.
The development of carbon-carbon (C-C) composites for aerospace applications has prompted the need for ways to improve the poor oxidation resistance of these materials, In order to evaluate and test materials to be used as thermal protection system (TPS) material the need for readily available and reliable testing methods are critical to the success of materials development efforts, With the purpose to evaluate TPS materials, three testing methods were used to assess materials at high temperatures (> 2000°C) and heat flux in excess of 200 Wcm-2. The first two methods are located at the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories, which are the Solar Furnace Facility and the Solar Tower Facility, The third method is an oxyacetylene torch set up according to ASTM E285-80 with oxidizing flame control and maximum achievable temperatures in excess of 2000°C In this study, liquid precursors to ultra high temperature ceramics (UHTCs) have been developed into multilayer coatings on C-C composites and evaluated using the oxidation testing methods. The tests will be discussed in detail and correlated with preliminary materials evaluation results with the aim of presenting an understanding of the testing environment on the materials evaluated for oxidation resistance.
Ceramic honeycombs based on homogeneous composites of zirconia and iron oxide are formed using polymer-based coextrusion for testing in thermochemical reactors to generate CO for renewable fuels. The honeycomb substrates possess controlled surface areas and are processed using zirconia with 3 and 8 mol % yttria additions to investigate the influence of surface area and oxygen conductivity of the substrate on the CO generation properties. CO generation was tested using a gas chromatography mass spectrometer and a laboratory scale thermochemical reactor capable of precisely controlling temperature and gas conditions. Results showed that reaction temperature and reactant gas flow rate effect CO generation. The yttria content of the zirconia support phase was also found to have a significant impact on the long-term CO generation, improving iron oxide conversion from 41 to 58%. Yttria content did not markedly impact the short-term reaction properties. Increasing the surface area of the substrates, from 2.6 up to 8.5 cm(2), did not result in improvements in CO generation within the resolution of the test equipment. The substrates reacted by two distinct mechanisms, an initial, spontaneous surface reaction that changed over time to a diffusion-based mechanism utilizing reaction material from the bulk. These substrate systems exhibit the high reactivity necessary for large-scale thermochemical reactors, while being based on common materials.