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Metastability of Spinel-type Solid Solutions in the SiO2-Al2O3 System

Summary: Metastability of Spinel-type Solid Solutions in the
SiO2-Al2O3 System
J. M. McHale,,§ K. Yu¨rekli,,§ D. M. Dabbs,,§ A. Navrotsky,*,,§
S. Sundaresan,,§ and I. A. Aksay,§
Departments of Geosciences and Chemical Engineering, and Princeton Materials Institute,
Princeton University, Princeton, New Jersey 08544-5263
Received June 24, 1997. Revised Manuscript Received August 19, 1997X
The addition of small amounts (2-10 wt %) of SiO2 to -Al2O3 increases the temperature
of heat treatment necessary for transformation to R-Al2O3 by 100 K. We have studied
this system using high-temperature solution calorimetry in molten 2PbO,B2O3 at 1043 K.
Our results indicate that the spinel-type Al2O3-SiO2 solid solutions with 2-10 wt % SiO2
are always energetically metastable by 30-35 kJ,mol-1
(on a 4 O2-
per mole basis) with
respect to R-Al2O3 and quartz. Calculation of the maximum configurational entropy of the
solid solutions allowed determination of the likely most negative value of the Gibbs free
energy of the materials. The solid solutions are somewhat entropy stabilized, but still
thermodynamically metastable by >10 kJ,mol-1 at 1400 K. Therefore, SiO2 addition appears
to provide mainly a kinetic hindrance to R-Al2O3 formation.


Source: Aksay, Ilhan A. - Department of Chemical Engineering, Princeton University


Collections: Materials Science