Phase equilibria in the PbO--Al/sub 2/O/sub 3/ system

Abstract

The use of the Knudsen effusion weight loss technique to measure the equilibrium vapor pressure of PbO in the Pb0-Al 2 o 3 system is described. The pressures are measured in two phase regions of the system to assess the applicability of this system as a packing powder to control thestoichiometry of lead zirconate-titanate ceramics during processing by fixing the PbO pressure in the sintering environme>Jt:. The need for revision of the current Pb0-Al 2 o 3 phase diagram is demonstrated and the revision is accomplished by calculating the decomposition temperatures of the three intermediate es. The calculations combine the measured PbO pressures \vith estimated pressures along the high Al 2 o 3 liquidus. The liquidus position and pressures are estimated using three approaches: the regular solution, the empiric.a.l and the modified ideal solution. The Gibb's free energies of formation of the intermediate phases are de term Lned and the extent of nonstoichiorr,etry is considered.

Effect of Lead Oxide Vapor on the Strength of Alumina

“…Three incongruent melting compounds, that is, Pb 2 Al 2 O 5 , PbAl 2 O 4 , and PbAl 12 O 19 , were included in the phase diagram. The calculated temperatures of the three peritectic reactions, that is, Pb 2 Al 2 O 5 →PbAl 2 O 4 +Liquid, PbAl 2 O 4 →PbAl 12 O 19 +Liquid, and PbAl 12 O 19 →Al 2 O 3 +Liquid, are 1230, 1370, and 1780 K, respectively, which reproduce the temperatures measured by Powell 12 very well. The modeled temperature and composition of the eutectic reaction (Liquid→Pb 2 Al 2 O 5 +PbO) are in perfect agreement with the measurement by Kuxmann and Fischer 13 .…”

“… Calculated phase diagram of the pseudo‐binary PbO–Al 2 O 3 system under one atmospheric pressure. Open triangles and circles are experimental data measured by Powell 12 and Kuxmann and Fischer, 13 respectively. …”

“…The Gibbs energy functions of PbO and Al 2 O 3 were taken from the SGTE database 11 . The enthalpies and entropies of formation of the three intermediate compounds, that is, Pb 2 Al 2 O 5 , PbAl 2 O 4 , and PbAl 12 O 19 , measured by Powell 12 were utilized to model these stoichiometric compounds. The liquid phase was modeled as a subregular solution phase.…”