Consider the magnesium oxide-aluminum oxide phase diagram; ss indicates a solid solution. The radii of Mg2 + and Al3 + are 0.072 and 0.053 nm, respectively. (a) There are two eutectic reactions on the diagram; describe them (temperatures, compositions and phases). (b) Spinel, at temperatures below 1000 °C, is a compound with a composition of 50 mol% Al2O3-50 mol% MgO. For higher temperatures, spinel is stable for a range of compositions. What is the temperature of the congruent fusion? (c) What are the main reasons for the low solubility of Al2O3 in MgO below 1400 °C?
Consider the magnesium oxide-aluminum oxide phase diagram; ss indicates a solid solution. The radii of Mg2 + and Al3 + are 0.072 and 0.053 nm, respectively. (a) There are two eutectic reactions on the diagram; describe them (temperatures, compositions and phases). (b) Spinel, at temperatures below 1000 °C, is a compound with a composition of 50 mol% Al2O3-50 mol% MgO. For higher temperatures, spinel is stable for a range of compositions. What is the temperature of the congruent fusion? (c) What are the main reasons for the low solubility of Al2O3 in MgO below 1400 °C? (d) Maximum non-stoichiometry on the Al2O3-rich side, in the spinel phase field, occurs approximately 2000 °C and corresponds to approximately 82 mol% (92% w) of Al2O3. Determine the type of gaps defect that is produced and the percentage of gaps that exists in this composition.
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