Session: Research Posters

Paper Number: 120065

120065 - Synthesis and Characterization of Sic-Mullite Thermal Material 

SiC-mullite is considered for high-temperature applications due to its high mechanical strength and thermal shock resistance. Mullite (3Al₂O₃.2SiO₂) has a similar coefficient of thermal expansion (5.3 x 10^-6 /K at 273-1273 K) to that of SiC (4.7 x 10^-6 /K at 293-973 K) and has good chemical compatibility with SiC which ensures a good interfacial bonding between SiC and mullite phase. Similar to SiC, mullite has high oxidation resistance due to its low oxygen diffusion coefficient. Therefore, many studies focused on the in-situ synthesis of mullite liquid phase at the joints of SiC particles to provide better adhesion strength among SiC particles and promote high mechanical strength and thermal shock resistance. Al₂O₃, Al, AlN, and Al(OH)₃ powders have been used in different studies to form mullite through the reaction with oxidation-derived SiO₂ on the surface of SiC particles. However, the in-situ formation of mullite using these agents required long duration of thermal treatment above 1500 °C which causes excessive oxidation of SiC and reduces the thermomechanical properties of the mullite-SiC composite due to the formation of access amount of cristobalite (SiO₂) phase. Cristobalite (SiO₂) has lower strength, and more importantly large coefficient of thermal expansion (CTE) mismatch between SiC (4.7 x 10^-6 /K at 293-973 K) and cristobalite phase (17.5 x 10^-6 /K at 273-1273 K) decreases thermal shock resistance significantly. Herein, we report on using coal fly ash as a source of alumina (Al₂O₃) that reacts in situ with the silica (SiO₂) produced by SiC oxidation during sintering to form mullite binder for SiC particles. The metal oxides from coal fly ash facilitated the instantaneous formation of mullite at the surface of SiC particles at relatively low temperature (1400 °C/1 hr) and promoted strong interfacial bond between SiC particles. Thus, 85 weight % SiC mixed with 15 % coal fly ash sintered at 1400 °C/1h produced a composite made of, in phase percent, 84.1 % SiC, 11.4 % mullite, and 4.5 % cristobalite solid solution. SEM-EDX revealed a concentration gradient of Al in the cristobalite which enhanced the formation of functionally graded bonding zones between phases. The compressive strength, nanoindentation elastic modulus, Vicker’s hardness were 434 ± 20 MPa, 370.9 ± 22.6 GPa, 11.5 ± 1.2 GPa respectively. The thermal shock resistance test showed high dimensional and mechanical stabilities after quenching in liquid nitrogen (−196 °C) from 1400 °C. The thermal expansion co-efficient showed a linear increase from 3.17 x 10^-7  /K to 5.615 x 10^-6 /K when the sample was heated from 182 K to 354 K. The specific heat capacity, thermal diffusivity, and thermal conductivity were 7.83 ± 0.0014 J/g.K, 1.04 ± 0.013 mm²/s, and 17 W/m.K at 100 °C, respectively. The SiC-mullite composite exhibited moderate electrical conductivity of 3.48 x 10^-2 S/m at 1000 °C. Results of the study suggest the as-prepared SiC-mullite composite can be used for many high-temperature applications including diesel motor parts, gas turbines, industrial heat exchangers, fusion reactor parts, and high-temperature energy exchanger systems. 

Presenting Author: FARJANA SULTANA University of North Carolina at Charlotte

Presenting Author Biography: I am a Ph.D. student in the mechanical engineering department of the University of North Carolina at Charlotte.

Authors:

FARJANA SULTANA University of North Carolina at Charlotte
Ahmed El-Ghannam University of North Carolina at Charlotte