Researcher: Dr Rafael Guimarães de Sá

Supervisor: Professor Bill Lee

There is much interest in reducing C contents in refractory linings for production of low C steels to avoid C contamination of the melt. A potential method is use of well dispersed nanostructured forms of carbon such as carbon nanofibres and carbon nanotubes (CNFs/CNTs). A new in-situ technique, located in a pulsed-pressure catalyst-assisted chemical vapour infiltration (PP-CACVI) reactor, has been developed in this work to optimize incorporation of CNFs/CNTs into low-carbon (<5 wt%) MgO-carbon-metal porous nanocomposites. Such nanocomposites have the potential to replace conventional high carbon (>10 wt%) refractories.

Initial optimization of precursor catalysts for CNFs/CNTs growth on MgO substrate powder particle surfaces revealed that up to 30 wt% carbon could be deposited with the use of Fe catalysts. Co and Ni were also shown to catalyze CNF/CNT deposition, while only amorphous carbon was obtained when using Mo, Mg and Al. In-situ infiltration of CNFs/CNTS into porous MgO-metal compacts using Fe/Co/Mo and Fe/Ni/Mo at different infiltration pulse pressure amplitudes resulted in incorporation of up to 5.7 wt% optimally dispersed CNFs/CNTs at depths of up to 13mm covering not only the MgO surface but also all porosity and grain boundaries. Deposition yield was best for the Co-based system and increased with pressure, albeit at the expense of CNF/CNT quality. Analysis of deposition as a function of sample depth revealed a decrease in CNF/CNT quality towards the centre of the compact.

Optimization of multi-wall carbon nanotubes (MWCNTs) dispersion using an ultrasound technique for the ex-situ incorporation led to identification of dispersing parameters (40W, 960 seconds) which resulted in the optimum dispersion of 2 vol% MWCNT into the MgO-carbon-metal nanocomposites. Use of CNFs/CNTs in high temperature MgO-carbon-metal composites in oxidizing atmospheres was, however, shown to be impractical due to the oxidation tendency of these highly reactive forms of carbon, as revealed by comparative oxidation tests between 1100-1300oC. However, oxidation testing at 1500oC, typical of real life application of these materials revealed that, caused by the enhanced MgO dense-layer formation, composites with CNFs/CNTs had similar oxidation resistance as those containing standard forms of graphite.