"Solid solution formation in spinel based systems proved to be a viable approach to decreasing thermal conductivity. Samples with systematically varied additions of MgGa₂O₄ to MgAl₂O₄ were prepared and thermal diffusivity was measured using the laser ash technique. Additionally, heat capacity was measured using differential scanning calorimetry and modeled for the MgAl₂O₄-MgGa₂O₄ system. At 200⁰C thermal conductivity decreased 24% with a 5 mol% addition of MgGa₂O₄ to the system. The solid solution continued to decrease the thermal conductivity by 13% up to 1000⁰C with 5 mol% addition. The decrease in thermal conductivity ultimately resulted in a decrease in heat flux when applied to a theoretical furnace lining, which could lead to energy savings in industrial settings.

The MgAl₂O₄-Al₂O₃ phase equilibria was investigated to fully understand the system and the thermal properties at elevated temperatures. The solvus line between MgAl₂O₄ and Al₂O₃ has been defined at 79.6 wt% Al₂O₃ at 1500⁰C, 83.0 wt% Al₂O₃ at 1600⁰C, and 86.5 wt% Al₂O₃ at 1700⁰C. A metastable region has been identified at temperatures up to 1700⁰C which could have significant implications for material processing and properties. The spinel solid solution region has been extended to form an infinite solid solution with Al₂O₃ at elevated temperatures. A minimum in melting at 1975⁰C and a chemistry of 96 wt% Al₂O₃ rather than a eutectic is present.

Thermal properties in the MgAl₂O₄-Al₂O₃ system were investigated in both the single phase solid solution region and the two phase region. The thermal diffusivity decreased through the MgAl₂O₄ solid solution region and was at a minimum through the entire metastable (nucleation and growth) region. As Al₂O₃ became present as a second phase the thermal diffusivity increased with Al₂O₃ content. There was an 11.7% increase in thermal diffusivity with a change in overall chemistry of 85.20 wt% Al₂O₃ to 87.71 wt% Al₂O₃, due to the drastic change in final chemistry (38.3 wt% Al₂O₃) caused by the nucleation and growth region in the system"--Abstract, page iv.