A model for initial-stage sintering thermodynamics of an alumina powder

A model was proposed to calculate several thermodynamic parameters for the initial-stage sintering of an alumina powder obtained after calcinations at 900 degrees C for 2 h of a precursor. The precursor was synthesized by an alumina sulphate-excess urea reaction in boiling aqueous solution. The cylindrical compacts of the powder with a diameter of 14 mm were prepared under 32 MPa by uniaxial pressing using oleic acid (12% by mass) as binder. The compacts were fired at various temperatures between 900 and 1400 degrees C for 2 h. The diameter (D) of the compacts before and after firing was measured by a micrometer. The D Value after firing was taken as a sintering equilibrium parameter. An arbitrary sintering equilibrium constant (K,) was calculated for each firing temperature by assuming K(a) = (D(i) -D)/(D-D(f)), where D(i) is the largest value before sintering and D(f) is the smallest value after firing at 1400 degrees C. Also, an arbitrary change in Gibbs energy (Delta G(a)degrees) was calculated for each temperature using the K(a) value. The graphs of In K(a) vs. 1/T and Delta G(a)degrees vs. T were plotted, and the real change in enthalpy (Delta H degrees) and the real change in entropy (AS') were calculated from the slopes of the obtained straight lines, respectively. Inversely, real Delta G degrees and K values were calculated using the real Delta H degrees and Delta S degrees values in the Delta G degrees = -RT In K = Delta H degrees-T Delta S degrees relation. The best fitting Delta H degrees and Delta S degrees values satisfying this relation were found to be 157,301 J mol(-1) and 107.6 J K(-1) mol(- 1), respectively. (c) 2008 Elsevier B.V. All rights reserved.